news feature: lighting the way for dark matter · · 2017-11-20a glass window. entities with...
TRANSCRIPT
Correction
NEWS FEATURECorrection for ldquoNews Feature Lighting the way for dark matterrdquo byAdam Mann which was first published October 31 2017 101073pnas1716618114 (Proc Natl Acad Sci USA 11411557ndash11560)The editors note that on page 11559 right column third full
paragraph lines 4ndash5 ldquocooled to 42 Kelvinrdquo should have ap-peared as ldquocooled to around 100 milliKelvinrdquo The article hasbeen updated online
Published under the PNAS license
wwwpnasorgcgidoi101073pnas1719449114
E10504 | PNAS | November 28 2017 | vol 114 | no 48 wwwpnasorg
NEWS FEATURE
Lighting the way for dark matterA nagging lack of evidence for weakly interacting massive particles has spurred physicists to
start searching for a range of lightweight dark particles and even new dark forces
Adam Mann Science Writer
In 2015 a team of nuclear physicists in Hungary reportedan anomalous bump in the signal from radioactive decaysof unstable beryllium-8 corresponding to a putative newparticle with 34 times the mass of an electron (1) It waslargely overlooked at the time but a year later US theo-rists suggested that this might point to a new force felt bydarkmatter hinting that themysterious substance ismorecomplex than previously believed (2) Ideas about darkmatter are evolving
Since the late 1990s most researchers have positedthat dark matter is probably made of weakly interactingmassive particles (WIMPs) ghostly hypothetical objectsthat would pass through normal matter like light througha glass window Entities with perfectly WIMP-like prop-erties happen to appear in supersymmetry a popular
theory extending the Standard Model the current frame-work describing the interactions of all known particlesand forces But experiments searching for these WIMPshave so far turned up empty and the Large HadronCollider has failed to reveal any signs of supersymmetry
Although supersymmetric WIMPs remain the most-favored candidate for dark matter their nonappear-ance has led some scientists to begin doubting theirexistence and explore numerous new models A fewphysicists are turning to another kind of particle anultralight entity known as the axion Others suggestthat there might be many distinct dark matter parti-cles each with unique properties which could com-bine into dark atoms and dark molecules and emitdark photons ldquoThis generalization from a particle to a
Using observations of gravitational lensing of galaxies astronomers have mapped out dark matter in the Bullet Clusterwhich is formed by two enormous colliding clusters of galaxies Composite image courtesy of NASACXCM Weiss
Published under the PNAS license
wwwpnasorgcgidoi101073pnas1716618114 PNAS | October 31 2017 | vol 114 | no 44 | 11557ndash11560
NEW
SFEATURE
sector which has particles and forces in it has reallyopened up the floodgatesrdquo says particle physicistJonathan Feng of the University of California Irvine
New technology is creating ways to find theseelusive entities In March 2017 physicists attending aworkshop at the University of Maryland College Parklisted more than 100 ideas for such experiments (3)Some are already running others should begin takingdata in the next few years
Birth of DarknessDark matter is known to exist because of its gravita-tional effectsmdashstars in the outer reaches of galaxiesseem to be moving faster than they should given thevisible material present as if they were being tuggedby some huge unseen mass In the late 1970s a fewresearchers realized that as-yet-undiscovered stableparticles could have been created in the fiery condi-tions after the Big Bang which would account for thisenigma (4) Around the same time theorists de-veloped the idea of supersymmetry and realized thatthe lightest supersymmetric particle known as aneutralino happened to be an ideal fit
In the particle soup of the early universe neu-tralinos would have constantly collided with and an-nihilated one another producing decay products thatinclude ordinary matter At first the process would alsorun in reverse with ordinary matter particles crashingand creating dark matter But then as the universeexpanded and cooled ordinary matter particles wouldhave had too little energy to create heavy neutralinosThe neutralinos meanwhile would continue to meetand annihilate But if the neutralinos have a lowprobability of finding each other they could remain inlarge numbers today
Dark matter is currently thought to outweigh ordi-nary matter in the universe by a ratio of five to oneUsing this value theorists could ask themselves whatthe rate of interaction of neutralinos would have beenin the early universe By adding in the neutralinorsquosproposed mass which is between 50 and a fewthousand times that of a proton the calculationsshowed that a particle interacting through only theweak force and not electromagnetism or the strongforce would exactly produce the current dark matterdensitymdasha coincidence known as the WIMP miracle
Following this revelation researchers began to in-vent clever ways to look for the prospective particleThey seized on the possibility of a chance encounteralthough a neutralino should typically sail rightthrough ordinary matter without leaving a trace thereis always some extremely tiny probability that it willinteract with an atom via the weak force
Beginning in the 1980s experimentalists built darkmatter detection devices and placed them deep un-derground to protect them from interfering cosmicradiation in the hopes that a neutralino or other similarWIMP-like particle would eventually run into one ofthe particles in their detector producing a measurablerecoil signal
Coming Up EmptyRecoil detection works best if the target nucleus andthe projectile have roughly the same mass so many ofthe worldrsquos leading dark matter direct-detection ex-periments use xenon which has 131 times the mass ofa proton In January 2017 the Large UndergroundXenon (LUX) collaboration whose experiment ran atthe Sanford Underground Research Facility in SouthDakota released its final results (5) They showed nodark matter collisions Researchers working on theXENON 1-Ton (XENON1T) project in Gran SassoNational Laboratory near LrsquoAquila Italy the largestdark matter experiment of its kind unveiled their lat-est data analysis on May 18 2017 placing even moresensitive constraints on how readily WIMPs interactwith regular matter (6) The Particle and AstrophysicalXenon Detector team at the China Jinping Un-derground Laboratory in Sichuan China presentedfindings from their second-generation detector at aconference in August 2017 (7) Again they saw nothing
Perhaps some suggest these researchers have notbeen looking in the right place Between 2008 and2011 results from a few direct-detection experimentsseemed to suggest the existence of dark matter par-ticles between 1 and 10 times a protonrsquos mass belowthe threshold where xenon-based devices would seethem Since then many of these results have beendiscounted But this was a wake-up call to manyphysicists might dark matter particles be a little lighterthan originally thought If such particles interactedwith normal matter using the weak force they wouldhave already shown up in accelerator experiments notspecifically designed to look for them So theoristsbegan to wonder if perhaps dark matter was evenfurther removed from our ordinary world of protonsneutrons and electrons than previously suspected
The LUX experiment its light sensor shown here searches for a type of darkmatter known as a WIMP But WIMPs have failed to turn up in recent yearsleading physicists to consider alternative dark matter models Image courtesy ofMatt KapustSanford Underground Research Facility
11558 | wwwpnasorgcgidoi101073pnas1716618114 Mann
Into the Dark SectorAstronomers meanwhile have shown that dwarf gal-axies seem to have puffy halos of dark matter ratherunlike the dense dark matter cores predicted fromcosmological simulations assuming ordinary WIMPdark matter This puffiness could happen if the in-visible substance can interact with itself using forceslike ordinary gas moleculesmdashprompting some theoriststo suggest that dark matter could be combining intonuclear states like normal atoms and moleculesPerhaps it lives in a world with its own bevy of par-ticles and forces almost entirely tangential to theuniverse we inhabit what particle physicists calla hidden sector
If we have any hope of discovering this dark sectorthese new forces must have some interaction withordinary matter A few researchers think theyrsquove al-ready seen hints of such interactions in a short-livedparticle called the muon whose magnetic momentdoes not entirely line up with predictions from theStandard Model a dark sector force could account forthis discrepancy (8) Forces are carried by particlesand the particle carrying this presumed force wouldact much like the force-carrier of electromagnetismthe photon but have a mass between several and afew hundred times that of an electron It has beendubbed the dark photon
Physicists at Jefferson National Laboratory havebeen hunting for these since 2010 when the Aprime Ex-periment began slamming a high-intensity beam ofelectrons into a thin tungsten target hoping to pro-duce a few dark photons Other projects at Jeffersonsearching at different energy ranges include theHeavy Photon Search which first ran in 2015 and theDarkLight experiment which fires its electrons at ahydrogen gas target and took some initial data thisyear DarkLight will also be able to look into theHungarian beryllium resultmdashwhich is in the right massrange but would be an especially weird version of adark photon needing to interact with neutrons but notprotons to explain the data thus far Because of thiscontrivance some physicists remain skeptical thatwhat the Hungarians saw is truly a new force Addi-tional information should be forthcoming in 2018when the electron beam at Jefferson is upgraded andDarkLight is able to investigate the anomaly further
Althoughmost dark matter proposals have focusedon relatively heavy particles some theorists are ex-ploring the possibility that it is actually extremely lightA major puzzle in modern physics has to do with theneutron Being neutral it has no interaction withelectric fields But curiously it reacts to magneticfields almost like a little compass needle Why shouldthe neutron interact with only one component of theelectromagnetic force In 1977 physicists RobertoPeccei and Helen Quinn proposed that a hiddenmechanism tunes down the neutronrsquos interactions withelectric fields (9) Shortly thereafter other theoristsrealized that this mechanism gave rise to a particlethey called the axion
The Lighter SideAxions would be neutral ghostly and extremely lightless than a millionth or even a billionth the mass ofan electron With such low masses they would beproduced at much higher densities in the early uni-verse Every cubic centimeter of the cosmos would bepacked with axions This would make axions behaveless like particles and more like a field and if the ax-ionsrsquo energy field oscillates just around zero it wouldproduce the gravitational effects necessary to be darkmatter ldquoHerersquos a particle that people thought oughtto exist anyway and it just happened to solve darkmatterrdquo says physicist Gray Rybka of the University ofWashington in Seattle ldquoIt killed two birds with onestone and so itrsquos really worth looking forrdquo
Should an individual axion happen to run into axenon-based dark matter direct-detection experiment itwould barely budge the ordinary atoms ldquokind of like aspit wad hitting a trainrdquo says physicist Jeffrey Hutchinsonof Florida Gulf Coast University in Fort Myers So mostaxion-hunting experiments take advantage of the puta-tive particlersquos lightweight nature According to quantummechanics all particles behave like waves vibratingwith a frequency that depends on their mass The axionrsquosuncertain mass could correspond to a frequency any-where from 250 hertz up to 25 terahertz
The longest-running of these axion hunts is theAxion DarkMatter Experiment (ADMX) at the Universityof Washington which consists of a metallic cavityplaced inside a powerful magnet and cooled to around100 milliKelvin Axions vibrating at the resonant
frequency of the chamber can be converted into de-tectable microwave photons As ADMX runs rodsmove within the cavity to change its effective size andscan through different potential axion masses like alistener tuning through radio stations
ADMX finally hit the sensitivity necessary to detectlikely axion masses in January 2017 The collaborationexpects to examine some of the most-favored axionmass ranges between 500 megahertz and 10 giga-hertz over the next five or six years and is researchingtechnology that can search even higher frequencies
But the axion might be too light for ADMX soother projects are now getting off the ground TheDark Matter Radio consists of an electric circuit sur-rounded by a superconducting shield that blocks or-dinary electromagnetic waves but would produce ameasurable voltage if an axion penetrated it Apathfinder version of this experiment began takingdata in August 2017 Perusing a similar low massrange is the A BroadbandResonant Approach toCosmic Axion Detection with an Amplifying B-fieldRing Apparatus experiment which generates an im-mense magnetic field in hopes of detecting the
ldquoHerersquos a particle that people thought ought to existanyway and it just happened to solve dark matterrdquo
mdashGray Rybka
Mann PNAS | October 31 2017 | vol 114 | no 44 | 11559
signature of an axionmdashnamely a tiny wavering in thefield Another pathfinder experiment under construc-tion is the Cosmic Axion Spin Precession Experiment(CASPEr) which will use nuclear MRI technology tosearch for an oscillating electric charge inside theneutron to try to tease out the axion CASPEr will besensitive to axion masses far below the other detec-tors all the way down to around 200 Hz
WIMPs Still in PlayThe supersymmetric WIMP is not out of the runningyet and most of the dark matter community remainsfocused on xenon-based detectors XENON1Trsquos mostrecent data analysis was from only its first 34 daysldquoNow we are really starting to look into new territorywhere somebody has never probedrdquo says physicistElena Aprile of Columbia University in New York Citywho leads the project ldquoWe are all very hopeful andwe will keep exploring uncharted watersrdquo
Both Aprilersquos collaboration and the LUX team areworking on next-generation devices which will be anorder of magnitude larger and far more sensitive thanthe current generation Aprile says it will be at leastanother five years before WIMPs would be ruled outmdashand they might show up at any point before that
Physicists keep pushing to discover dark matter inas many ways as they can imagine Data from oldparticle accelerator experiments are being reanalyzedto look for anomalies and many particle acceleratorsare being retrofitted to search for dark photonsResearch and development have proliferated ontechnology that could detect lightweight dark matterparticles Upcoming experiments include the SubElectron Noise Skipper-CCD Experimental Instrumentand Dark Matter in CCDs project which would searchfor dark matter slamming into silicon-charged cou-pling devices and the Princeton Tritium Observatoryfor Light Early-Universe Massive-Neutrino Yield pro-gram which would use a graphene detector Researchersare also developing devices utilizing electrons crystallinegermanium and superfluid helium and many otherdirect-detection methods
As long as dark matterrsquos influence can be seen inthe universe it will remain a tantalizing target for ex-periments ldquoItrsquos really one of the deepest mysteries inparticle physics and we have so many good ideas forhow to detect itrdquo says particle physicist Jesse Thalerof the Massachusetts Institute of Technology in Cam-bridge ldquoIf human ingenuity has anything to say aboutit there will be an excellent chance of detectionhopefully in my lifetimerdquo
1 Krasznahorkay AJ et al (2016) Observation of anomalous internal pair creation in 8Be A possible indication of a light neutral bosonPhys Rev Lett 116042501ndash042506
2 Feng JL et al (2016) Protophobic fifth-force interpretation of the observed anomaly in 8Be nuclear transitions Phys Rev Lett117071803ndash071809
3 Battagleri M (2017) US cosmic visions New ideas in dark matter 2017 Community report arXiv170704591v14 Lee BW Weinberg S (1977) Cosmological lower bound on heavy neutrino masses Phys Rev Lett 39165ndash1685 Akerib DS et al LUX Collaboration (2017) Results from a search for dark matter in the complete LUX exposure Phys Rev Lett118021303ndash021311
6 Aprile E et al (2017) First dark matter search results from the XENON1T experiment arXiv170506655v27 Ji X (2017) New world-leading upper limits for spin independent WIMP-nucleon cross section from PandaX Available at httpspandaxsjtueducnnode300 Accessed August 18 2017
8 Bennett GW et al Muon (g - 2) Collaboration (2004) Measurement of the negative muon anomalous magnetic moment to 07 ppmPhys Rev Lett 92161802ndash161807
9 Peccei RD Quinn HR (1977) CP conservation in the presence of pseudoparticles Phys Rev Lett 381440ndash1443
11560 | wwwpnasorgcgidoi101073pnas1716618114 Mann
NEWS FEATURE
Lighting the way for dark matterA nagging lack of evidence for weakly interacting massive particles has spurred physicists to
start searching for a range of lightweight dark particles and even new dark forces
Adam Mann Science Writer
In 2015 a team of nuclear physicists in Hungary reportedan anomalous bump in the signal from radioactive decaysof unstable beryllium-8 corresponding to a putative newparticle with 34 times the mass of an electron (1) It waslargely overlooked at the time but a year later US theo-rists suggested that this might point to a new force felt bydarkmatter hinting that themysterious substance ismorecomplex than previously believed (2) Ideas about darkmatter are evolving
Since the late 1990s most researchers have positedthat dark matter is probably made of weakly interactingmassive particles (WIMPs) ghostly hypothetical objectsthat would pass through normal matter like light througha glass window Entities with perfectly WIMP-like prop-erties happen to appear in supersymmetry a popular
theory extending the Standard Model the current frame-work describing the interactions of all known particlesand forces But experiments searching for these WIMPshave so far turned up empty and the Large HadronCollider has failed to reveal any signs of supersymmetry
Although supersymmetric WIMPs remain the most-favored candidate for dark matter their nonappear-ance has led some scientists to begin doubting theirexistence and explore numerous new models A fewphysicists are turning to another kind of particle anultralight entity known as the axion Others suggestthat there might be many distinct dark matter parti-cles each with unique properties which could com-bine into dark atoms and dark molecules and emitdark photons ldquoThis generalization from a particle to a
Using observations of gravitational lensing of galaxies astronomers have mapped out dark matter in the Bullet Clusterwhich is formed by two enormous colliding clusters of galaxies Composite image courtesy of NASACXCM Weiss
Published under the PNAS license
wwwpnasorgcgidoi101073pnas1716618114 PNAS | October 31 2017 | vol 114 | no 44 | 11557ndash11560
NEW
SFEATURE
sector which has particles and forces in it has reallyopened up the floodgatesrdquo says particle physicistJonathan Feng of the University of California Irvine
New technology is creating ways to find theseelusive entities In March 2017 physicists attending aworkshop at the University of Maryland College Parklisted more than 100 ideas for such experiments (3)Some are already running others should begin takingdata in the next few years
Birth of DarknessDark matter is known to exist because of its gravita-tional effectsmdashstars in the outer reaches of galaxiesseem to be moving faster than they should given thevisible material present as if they were being tuggedby some huge unseen mass In the late 1970s a fewresearchers realized that as-yet-undiscovered stableparticles could have been created in the fiery condi-tions after the Big Bang which would account for thisenigma (4) Around the same time theorists de-veloped the idea of supersymmetry and realized thatthe lightest supersymmetric particle known as aneutralino happened to be an ideal fit
In the particle soup of the early universe neu-tralinos would have constantly collided with and an-nihilated one another producing decay products thatinclude ordinary matter At first the process would alsorun in reverse with ordinary matter particles crashingand creating dark matter But then as the universeexpanded and cooled ordinary matter particles wouldhave had too little energy to create heavy neutralinosThe neutralinos meanwhile would continue to meetand annihilate But if the neutralinos have a lowprobability of finding each other they could remain inlarge numbers today
Dark matter is currently thought to outweigh ordi-nary matter in the universe by a ratio of five to oneUsing this value theorists could ask themselves whatthe rate of interaction of neutralinos would have beenin the early universe By adding in the neutralinorsquosproposed mass which is between 50 and a fewthousand times that of a proton the calculationsshowed that a particle interacting through only theweak force and not electromagnetism or the strongforce would exactly produce the current dark matterdensitymdasha coincidence known as the WIMP miracle
Following this revelation researchers began to in-vent clever ways to look for the prospective particleThey seized on the possibility of a chance encounteralthough a neutralino should typically sail rightthrough ordinary matter without leaving a trace thereis always some extremely tiny probability that it willinteract with an atom via the weak force
Beginning in the 1980s experimentalists built darkmatter detection devices and placed them deep un-derground to protect them from interfering cosmicradiation in the hopes that a neutralino or other similarWIMP-like particle would eventually run into one ofthe particles in their detector producing a measurablerecoil signal
Coming Up EmptyRecoil detection works best if the target nucleus andthe projectile have roughly the same mass so many ofthe worldrsquos leading dark matter direct-detection ex-periments use xenon which has 131 times the mass ofa proton In January 2017 the Large UndergroundXenon (LUX) collaboration whose experiment ran atthe Sanford Underground Research Facility in SouthDakota released its final results (5) They showed nodark matter collisions Researchers working on theXENON 1-Ton (XENON1T) project in Gran SassoNational Laboratory near LrsquoAquila Italy the largestdark matter experiment of its kind unveiled their lat-est data analysis on May 18 2017 placing even moresensitive constraints on how readily WIMPs interactwith regular matter (6) The Particle and AstrophysicalXenon Detector team at the China Jinping Un-derground Laboratory in Sichuan China presentedfindings from their second-generation detector at aconference in August 2017 (7) Again they saw nothing
Perhaps some suggest these researchers have notbeen looking in the right place Between 2008 and2011 results from a few direct-detection experimentsseemed to suggest the existence of dark matter par-ticles between 1 and 10 times a protonrsquos mass belowthe threshold where xenon-based devices would seethem Since then many of these results have beendiscounted But this was a wake-up call to manyphysicists might dark matter particles be a little lighterthan originally thought If such particles interactedwith normal matter using the weak force they wouldhave already shown up in accelerator experiments notspecifically designed to look for them So theoristsbegan to wonder if perhaps dark matter was evenfurther removed from our ordinary world of protonsneutrons and electrons than previously suspected
The LUX experiment its light sensor shown here searches for a type of darkmatter known as a WIMP But WIMPs have failed to turn up in recent yearsleading physicists to consider alternative dark matter models Image courtesy ofMatt KapustSanford Underground Research Facility
11558 | wwwpnasorgcgidoi101073pnas1716618114 Mann
Into the Dark SectorAstronomers meanwhile have shown that dwarf gal-axies seem to have puffy halos of dark matter ratherunlike the dense dark matter cores predicted fromcosmological simulations assuming ordinary WIMPdark matter This puffiness could happen if the in-visible substance can interact with itself using forceslike ordinary gas moleculesmdashprompting some theoriststo suggest that dark matter could be combining intonuclear states like normal atoms and moleculesPerhaps it lives in a world with its own bevy of par-ticles and forces almost entirely tangential to theuniverse we inhabit what particle physicists calla hidden sector
If we have any hope of discovering this dark sectorthese new forces must have some interaction withordinary matter A few researchers think theyrsquove al-ready seen hints of such interactions in a short-livedparticle called the muon whose magnetic momentdoes not entirely line up with predictions from theStandard Model a dark sector force could account forthis discrepancy (8) Forces are carried by particlesand the particle carrying this presumed force wouldact much like the force-carrier of electromagnetismthe photon but have a mass between several and afew hundred times that of an electron It has beendubbed the dark photon
Physicists at Jefferson National Laboratory havebeen hunting for these since 2010 when the Aprime Ex-periment began slamming a high-intensity beam ofelectrons into a thin tungsten target hoping to pro-duce a few dark photons Other projects at Jeffersonsearching at different energy ranges include theHeavy Photon Search which first ran in 2015 and theDarkLight experiment which fires its electrons at ahydrogen gas target and took some initial data thisyear DarkLight will also be able to look into theHungarian beryllium resultmdashwhich is in the right massrange but would be an especially weird version of adark photon needing to interact with neutrons but notprotons to explain the data thus far Because of thiscontrivance some physicists remain skeptical thatwhat the Hungarians saw is truly a new force Addi-tional information should be forthcoming in 2018when the electron beam at Jefferson is upgraded andDarkLight is able to investigate the anomaly further
Althoughmost dark matter proposals have focusedon relatively heavy particles some theorists are ex-ploring the possibility that it is actually extremely lightA major puzzle in modern physics has to do with theneutron Being neutral it has no interaction withelectric fields But curiously it reacts to magneticfields almost like a little compass needle Why shouldthe neutron interact with only one component of theelectromagnetic force In 1977 physicists RobertoPeccei and Helen Quinn proposed that a hiddenmechanism tunes down the neutronrsquos interactions withelectric fields (9) Shortly thereafter other theoristsrealized that this mechanism gave rise to a particlethey called the axion
The Lighter SideAxions would be neutral ghostly and extremely lightless than a millionth or even a billionth the mass ofan electron With such low masses they would beproduced at much higher densities in the early uni-verse Every cubic centimeter of the cosmos would bepacked with axions This would make axions behaveless like particles and more like a field and if the ax-ionsrsquo energy field oscillates just around zero it wouldproduce the gravitational effects necessary to be darkmatter ldquoHerersquos a particle that people thought oughtto exist anyway and it just happened to solve darkmatterrdquo says physicist Gray Rybka of the University ofWashington in Seattle ldquoIt killed two birds with onestone and so itrsquos really worth looking forrdquo
Should an individual axion happen to run into axenon-based dark matter direct-detection experiment itwould barely budge the ordinary atoms ldquokind of like aspit wad hitting a trainrdquo says physicist Jeffrey Hutchinsonof Florida Gulf Coast University in Fort Myers So mostaxion-hunting experiments take advantage of the puta-tive particlersquos lightweight nature According to quantummechanics all particles behave like waves vibratingwith a frequency that depends on their mass The axionrsquosuncertain mass could correspond to a frequency any-where from 250 hertz up to 25 terahertz
The longest-running of these axion hunts is theAxion DarkMatter Experiment (ADMX) at the Universityof Washington which consists of a metallic cavityplaced inside a powerful magnet and cooled to around100 milliKelvin Axions vibrating at the resonant
frequency of the chamber can be converted into de-tectable microwave photons As ADMX runs rodsmove within the cavity to change its effective size andscan through different potential axion masses like alistener tuning through radio stations
ADMX finally hit the sensitivity necessary to detectlikely axion masses in January 2017 The collaborationexpects to examine some of the most-favored axionmass ranges between 500 megahertz and 10 giga-hertz over the next five or six years and is researchingtechnology that can search even higher frequencies
But the axion might be too light for ADMX soother projects are now getting off the ground TheDark Matter Radio consists of an electric circuit sur-rounded by a superconducting shield that blocks or-dinary electromagnetic waves but would produce ameasurable voltage if an axion penetrated it Apathfinder version of this experiment began takingdata in August 2017 Perusing a similar low massrange is the A BroadbandResonant Approach toCosmic Axion Detection with an Amplifying B-fieldRing Apparatus experiment which generates an im-mense magnetic field in hopes of detecting the
ldquoHerersquos a particle that people thought ought to existanyway and it just happened to solve dark matterrdquo
mdashGray Rybka
Mann PNAS | October 31 2017 | vol 114 | no 44 | 11559
signature of an axionmdashnamely a tiny wavering in thefield Another pathfinder experiment under construc-tion is the Cosmic Axion Spin Precession Experiment(CASPEr) which will use nuclear MRI technology tosearch for an oscillating electric charge inside theneutron to try to tease out the axion CASPEr will besensitive to axion masses far below the other detec-tors all the way down to around 200 Hz
WIMPs Still in PlayThe supersymmetric WIMP is not out of the runningyet and most of the dark matter community remainsfocused on xenon-based detectors XENON1Trsquos mostrecent data analysis was from only its first 34 daysldquoNow we are really starting to look into new territorywhere somebody has never probedrdquo says physicistElena Aprile of Columbia University in New York Citywho leads the project ldquoWe are all very hopeful andwe will keep exploring uncharted watersrdquo
Both Aprilersquos collaboration and the LUX team areworking on next-generation devices which will be anorder of magnitude larger and far more sensitive thanthe current generation Aprile says it will be at leastanother five years before WIMPs would be ruled outmdashand they might show up at any point before that
Physicists keep pushing to discover dark matter inas many ways as they can imagine Data from oldparticle accelerator experiments are being reanalyzedto look for anomalies and many particle acceleratorsare being retrofitted to search for dark photonsResearch and development have proliferated ontechnology that could detect lightweight dark matterparticles Upcoming experiments include the SubElectron Noise Skipper-CCD Experimental Instrumentand Dark Matter in CCDs project which would searchfor dark matter slamming into silicon-charged cou-pling devices and the Princeton Tritium Observatoryfor Light Early-Universe Massive-Neutrino Yield pro-gram which would use a graphene detector Researchersare also developing devices utilizing electrons crystallinegermanium and superfluid helium and many otherdirect-detection methods
As long as dark matterrsquos influence can be seen inthe universe it will remain a tantalizing target for ex-periments ldquoItrsquos really one of the deepest mysteries inparticle physics and we have so many good ideas forhow to detect itrdquo says particle physicist Jesse Thalerof the Massachusetts Institute of Technology in Cam-bridge ldquoIf human ingenuity has anything to say aboutit there will be an excellent chance of detectionhopefully in my lifetimerdquo
1 Krasznahorkay AJ et al (2016) Observation of anomalous internal pair creation in 8Be A possible indication of a light neutral bosonPhys Rev Lett 116042501ndash042506
2 Feng JL et al (2016) Protophobic fifth-force interpretation of the observed anomaly in 8Be nuclear transitions Phys Rev Lett117071803ndash071809
3 Battagleri M (2017) US cosmic visions New ideas in dark matter 2017 Community report arXiv170704591v14 Lee BW Weinberg S (1977) Cosmological lower bound on heavy neutrino masses Phys Rev Lett 39165ndash1685 Akerib DS et al LUX Collaboration (2017) Results from a search for dark matter in the complete LUX exposure Phys Rev Lett118021303ndash021311
6 Aprile E et al (2017) First dark matter search results from the XENON1T experiment arXiv170506655v27 Ji X (2017) New world-leading upper limits for spin independent WIMP-nucleon cross section from PandaX Available at httpspandaxsjtueducnnode300 Accessed August 18 2017
8 Bennett GW et al Muon (g - 2) Collaboration (2004) Measurement of the negative muon anomalous magnetic moment to 07 ppmPhys Rev Lett 92161802ndash161807
9 Peccei RD Quinn HR (1977) CP conservation in the presence of pseudoparticles Phys Rev Lett 381440ndash1443
11560 | wwwpnasorgcgidoi101073pnas1716618114 Mann
sector which has particles and forces in it has reallyopened up the floodgatesrdquo says particle physicistJonathan Feng of the University of California Irvine
New technology is creating ways to find theseelusive entities In March 2017 physicists attending aworkshop at the University of Maryland College Parklisted more than 100 ideas for such experiments (3)Some are already running others should begin takingdata in the next few years
Birth of DarknessDark matter is known to exist because of its gravita-tional effectsmdashstars in the outer reaches of galaxiesseem to be moving faster than they should given thevisible material present as if they were being tuggedby some huge unseen mass In the late 1970s a fewresearchers realized that as-yet-undiscovered stableparticles could have been created in the fiery condi-tions after the Big Bang which would account for thisenigma (4) Around the same time theorists de-veloped the idea of supersymmetry and realized thatthe lightest supersymmetric particle known as aneutralino happened to be an ideal fit
In the particle soup of the early universe neu-tralinos would have constantly collided with and an-nihilated one another producing decay products thatinclude ordinary matter At first the process would alsorun in reverse with ordinary matter particles crashingand creating dark matter But then as the universeexpanded and cooled ordinary matter particles wouldhave had too little energy to create heavy neutralinosThe neutralinos meanwhile would continue to meetand annihilate But if the neutralinos have a lowprobability of finding each other they could remain inlarge numbers today
Dark matter is currently thought to outweigh ordi-nary matter in the universe by a ratio of five to oneUsing this value theorists could ask themselves whatthe rate of interaction of neutralinos would have beenin the early universe By adding in the neutralinorsquosproposed mass which is between 50 and a fewthousand times that of a proton the calculationsshowed that a particle interacting through only theweak force and not electromagnetism or the strongforce would exactly produce the current dark matterdensitymdasha coincidence known as the WIMP miracle
Following this revelation researchers began to in-vent clever ways to look for the prospective particleThey seized on the possibility of a chance encounteralthough a neutralino should typically sail rightthrough ordinary matter without leaving a trace thereis always some extremely tiny probability that it willinteract with an atom via the weak force
Beginning in the 1980s experimentalists built darkmatter detection devices and placed them deep un-derground to protect them from interfering cosmicradiation in the hopes that a neutralino or other similarWIMP-like particle would eventually run into one ofthe particles in their detector producing a measurablerecoil signal
Coming Up EmptyRecoil detection works best if the target nucleus andthe projectile have roughly the same mass so many ofthe worldrsquos leading dark matter direct-detection ex-periments use xenon which has 131 times the mass ofa proton In January 2017 the Large UndergroundXenon (LUX) collaboration whose experiment ran atthe Sanford Underground Research Facility in SouthDakota released its final results (5) They showed nodark matter collisions Researchers working on theXENON 1-Ton (XENON1T) project in Gran SassoNational Laboratory near LrsquoAquila Italy the largestdark matter experiment of its kind unveiled their lat-est data analysis on May 18 2017 placing even moresensitive constraints on how readily WIMPs interactwith regular matter (6) The Particle and AstrophysicalXenon Detector team at the China Jinping Un-derground Laboratory in Sichuan China presentedfindings from their second-generation detector at aconference in August 2017 (7) Again they saw nothing
Perhaps some suggest these researchers have notbeen looking in the right place Between 2008 and2011 results from a few direct-detection experimentsseemed to suggest the existence of dark matter par-ticles between 1 and 10 times a protonrsquos mass belowthe threshold where xenon-based devices would seethem Since then many of these results have beendiscounted But this was a wake-up call to manyphysicists might dark matter particles be a little lighterthan originally thought If such particles interactedwith normal matter using the weak force they wouldhave already shown up in accelerator experiments notspecifically designed to look for them So theoristsbegan to wonder if perhaps dark matter was evenfurther removed from our ordinary world of protonsneutrons and electrons than previously suspected
The LUX experiment its light sensor shown here searches for a type of darkmatter known as a WIMP But WIMPs have failed to turn up in recent yearsleading physicists to consider alternative dark matter models Image courtesy ofMatt KapustSanford Underground Research Facility
11558 | wwwpnasorgcgidoi101073pnas1716618114 Mann
Into the Dark SectorAstronomers meanwhile have shown that dwarf gal-axies seem to have puffy halos of dark matter ratherunlike the dense dark matter cores predicted fromcosmological simulations assuming ordinary WIMPdark matter This puffiness could happen if the in-visible substance can interact with itself using forceslike ordinary gas moleculesmdashprompting some theoriststo suggest that dark matter could be combining intonuclear states like normal atoms and moleculesPerhaps it lives in a world with its own bevy of par-ticles and forces almost entirely tangential to theuniverse we inhabit what particle physicists calla hidden sector
If we have any hope of discovering this dark sectorthese new forces must have some interaction withordinary matter A few researchers think theyrsquove al-ready seen hints of such interactions in a short-livedparticle called the muon whose magnetic momentdoes not entirely line up with predictions from theStandard Model a dark sector force could account forthis discrepancy (8) Forces are carried by particlesand the particle carrying this presumed force wouldact much like the force-carrier of electromagnetismthe photon but have a mass between several and afew hundred times that of an electron It has beendubbed the dark photon
Physicists at Jefferson National Laboratory havebeen hunting for these since 2010 when the Aprime Ex-periment began slamming a high-intensity beam ofelectrons into a thin tungsten target hoping to pro-duce a few dark photons Other projects at Jeffersonsearching at different energy ranges include theHeavy Photon Search which first ran in 2015 and theDarkLight experiment which fires its electrons at ahydrogen gas target and took some initial data thisyear DarkLight will also be able to look into theHungarian beryllium resultmdashwhich is in the right massrange but would be an especially weird version of adark photon needing to interact with neutrons but notprotons to explain the data thus far Because of thiscontrivance some physicists remain skeptical thatwhat the Hungarians saw is truly a new force Addi-tional information should be forthcoming in 2018when the electron beam at Jefferson is upgraded andDarkLight is able to investigate the anomaly further
Althoughmost dark matter proposals have focusedon relatively heavy particles some theorists are ex-ploring the possibility that it is actually extremely lightA major puzzle in modern physics has to do with theneutron Being neutral it has no interaction withelectric fields But curiously it reacts to magneticfields almost like a little compass needle Why shouldthe neutron interact with only one component of theelectromagnetic force In 1977 physicists RobertoPeccei and Helen Quinn proposed that a hiddenmechanism tunes down the neutronrsquos interactions withelectric fields (9) Shortly thereafter other theoristsrealized that this mechanism gave rise to a particlethey called the axion
The Lighter SideAxions would be neutral ghostly and extremely lightless than a millionth or even a billionth the mass ofan electron With such low masses they would beproduced at much higher densities in the early uni-verse Every cubic centimeter of the cosmos would bepacked with axions This would make axions behaveless like particles and more like a field and if the ax-ionsrsquo energy field oscillates just around zero it wouldproduce the gravitational effects necessary to be darkmatter ldquoHerersquos a particle that people thought oughtto exist anyway and it just happened to solve darkmatterrdquo says physicist Gray Rybka of the University ofWashington in Seattle ldquoIt killed two birds with onestone and so itrsquos really worth looking forrdquo
Should an individual axion happen to run into axenon-based dark matter direct-detection experiment itwould barely budge the ordinary atoms ldquokind of like aspit wad hitting a trainrdquo says physicist Jeffrey Hutchinsonof Florida Gulf Coast University in Fort Myers So mostaxion-hunting experiments take advantage of the puta-tive particlersquos lightweight nature According to quantummechanics all particles behave like waves vibratingwith a frequency that depends on their mass The axionrsquosuncertain mass could correspond to a frequency any-where from 250 hertz up to 25 terahertz
The longest-running of these axion hunts is theAxion DarkMatter Experiment (ADMX) at the Universityof Washington which consists of a metallic cavityplaced inside a powerful magnet and cooled to around100 milliKelvin Axions vibrating at the resonant
frequency of the chamber can be converted into de-tectable microwave photons As ADMX runs rodsmove within the cavity to change its effective size andscan through different potential axion masses like alistener tuning through radio stations
ADMX finally hit the sensitivity necessary to detectlikely axion masses in January 2017 The collaborationexpects to examine some of the most-favored axionmass ranges between 500 megahertz and 10 giga-hertz over the next five or six years and is researchingtechnology that can search even higher frequencies
But the axion might be too light for ADMX soother projects are now getting off the ground TheDark Matter Radio consists of an electric circuit sur-rounded by a superconducting shield that blocks or-dinary electromagnetic waves but would produce ameasurable voltage if an axion penetrated it Apathfinder version of this experiment began takingdata in August 2017 Perusing a similar low massrange is the A BroadbandResonant Approach toCosmic Axion Detection with an Amplifying B-fieldRing Apparatus experiment which generates an im-mense magnetic field in hopes of detecting the
ldquoHerersquos a particle that people thought ought to existanyway and it just happened to solve dark matterrdquo
mdashGray Rybka
Mann PNAS | October 31 2017 | vol 114 | no 44 | 11559
signature of an axionmdashnamely a tiny wavering in thefield Another pathfinder experiment under construc-tion is the Cosmic Axion Spin Precession Experiment(CASPEr) which will use nuclear MRI technology tosearch for an oscillating electric charge inside theneutron to try to tease out the axion CASPEr will besensitive to axion masses far below the other detec-tors all the way down to around 200 Hz
WIMPs Still in PlayThe supersymmetric WIMP is not out of the runningyet and most of the dark matter community remainsfocused on xenon-based detectors XENON1Trsquos mostrecent data analysis was from only its first 34 daysldquoNow we are really starting to look into new territorywhere somebody has never probedrdquo says physicistElena Aprile of Columbia University in New York Citywho leads the project ldquoWe are all very hopeful andwe will keep exploring uncharted watersrdquo
Both Aprilersquos collaboration and the LUX team areworking on next-generation devices which will be anorder of magnitude larger and far more sensitive thanthe current generation Aprile says it will be at leastanother five years before WIMPs would be ruled outmdashand they might show up at any point before that
Physicists keep pushing to discover dark matter inas many ways as they can imagine Data from oldparticle accelerator experiments are being reanalyzedto look for anomalies and many particle acceleratorsare being retrofitted to search for dark photonsResearch and development have proliferated ontechnology that could detect lightweight dark matterparticles Upcoming experiments include the SubElectron Noise Skipper-CCD Experimental Instrumentand Dark Matter in CCDs project which would searchfor dark matter slamming into silicon-charged cou-pling devices and the Princeton Tritium Observatoryfor Light Early-Universe Massive-Neutrino Yield pro-gram which would use a graphene detector Researchersare also developing devices utilizing electrons crystallinegermanium and superfluid helium and many otherdirect-detection methods
As long as dark matterrsquos influence can be seen inthe universe it will remain a tantalizing target for ex-periments ldquoItrsquos really one of the deepest mysteries inparticle physics and we have so many good ideas forhow to detect itrdquo says particle physicist Jesse Thalerof the Massachusetts Institute of Technology in Cam-bridge ldquoIf human ingenuity has anything to say aboutit there will be an excellent chance of detectionhopefully in my lifetimerdquo
1 Krasznahorkay AJ et al (2016) Observation of anomalous internal pair creation in 8Be A possible indication of a light neutral bosonPhys Rev Lett 116042501ndash042506
2 Feng JL et al (2016) Protophobic fifth-force interpretation of the observed anomaly in 8Be nuclear transitions Phys Rev Lett117071803ndash071809
3 Battagleri M (2017) US cosmic visions New ideas in dark matter 2017 Community report arXiv170704591v14 Lee BW Weinberg S (1977) Cosmological lower bound on heavy neutrino masses Phys Rev Lett 39165ndash1685 Akerib DS et al LUX Collaboration (2017) Results from a search for dark matter in the complete LUX exposure Phys Rev Lett118021303ndash021311
6 Aprile E et al (2017) First dark matter search results from the XENON1T experiment arXiv170506655v27 Ji X (2017) New world-leading upper limits for spin independent WIMP-nucleon cross section from PandaX Available at httpspandaxsjtueducnnode300 Accessed August 18 2017
8 Bennett GW et al Muon (g - 2) Collaboration (2004) Measurement of the negative muon anomalous magnetic moment to 07 ppmPhys Rev Lett 92161802ndash161807
9 Peccei RD Quinn HR (1977) CP conservation in the presence of pseudoparticles Phys Rev Lett 381440ndash1443
11560 | wwwpnasorgcgidoi101073pnas1716618114 Mann
Into the Dark SectorAstronomers meanwhile have shown that dwarf gal-axies seem to have puffy halos of dark matter ratherunlike the dense dark matter cores predicted fromcosmological simulations assuming ordinary WIMPdark matter This puffiness could happen if the in-visible substance can interact with itself using forceslike ordinary gas moleculesmdashprompting some theoriststo suggest that dark matter could be combining intonuclear states like normal atoms and moleculesPerhaps it lives in a world with its own bevy of par-ticles and forces almost entirely tangential to theuniverse we inhabit what particle physicists calla hidden sector
If we have any hope of discovering this dark sectorthese new forces must have some interaction withordinary matter A few researchers think theyrsquove al-ready seen hints of such interactions in a short-livedparticle called the muon whose magnetic momentdoes not entirely line up with predictions from theStandard Model a dark sector force could account forthis discrepancy (8) Forces are carried by particlesand the particle carrying this presumed force wouldact much like the force-carrier of electromagnetismthe photon but have a mass between several and afew hundred times that of an electron It has beendubbed the dark photon
Physicists at Jefferson National Laboratory havebeen hunting for these since 2010 when the Aprime Ex-periment began slamming a high-intensity beam ofelectrons into a thin tungsten target hoping to pro-duce a few dark photons Other projects at Jeffersonsearching at different energy ranges include theHeavy Photon Search which first ran in 2015 and theDarkLight experiment which fires its electrons at ahydrogen gas target and took some initial data thisyear DarkLight will also be able to look into theHungarian beryllium resultmdashwhich is in the right massrange but would be an especially weird version of adark photon needing to interact with neutrons but notprotons to explain the data thus far Because of thiscontrivance some physicists remain skeptical thatwhat the Hungarians saw is truly a new force Addi-tional information should be forthcoming in 2018when the electron beam at Jefferson is upgraded andDarkLight is able to investigate the anomaly further
Althoughmost dark matter proposals have focusedon relatively heavy particles some theorists are ex-ploring the possibility that it is actually extremely lightA major puzzle in modern physics has to do with theneutron Being neutral it has no interaction withelectric fields But curiously it reacts to magneticfields almost like a little compass needle Why shouldthe neutron interact with only one component of theelectromagnetic force In 1977 physicists RobertoPeccei and Helen Quinn proposed that a hiddenmechanism tunes down the neutronrsquos interactions withelectric fields (9) Shortly thereafter other theoristsrealized that this mechanism gave rise to a particlethey called the axion
The Lighter SideAxions would be neutral ghostly and extremely lightless than a millionth or even a billionth the mass ofan electron With such low masses they would beproduced at much higher densities in the early uni-verse Every cubic centimeter of the cosmos would bepacked with axions This would make axions behaveless like particles and more like a field and if the ax-ionsrsquo energy field oscillates just around zero it wouldproduce the gravitational effects necessary to be darkmatter ldquoHerersquos a particle that people thought oughtto exist anyway and it just happened to solve darkmatterrdquo says physicist Gray Rybka of the University ofWashington in Seattle ldquoIt killed two birds with onestone and so itrsquos really worth looking forrdquo
Should an individual axion happen to run into axenon-based dark matter direct-detection experiment itwould barely budge the ordinary atoms ldquokind of like aspit wad hitting a trainrdquo says physicist Jeffrey Hutchinsonof Florida Gulf Coast University in Fort Myers So mostaxion-hunting experiments take advantage of the puta-tive particlersquos lightweight nature According to quantummechanics all particles behave like waves vibratingwith a frequency that depends on their mass The axionrsquosuncertain mass could correspond to a frequency any-where from 250 hertz up to 25 terahertz
The longest-running of these axion hunts is theAxion DarkMatter Experiment (ADMX) at the Universityof Washington which consists of a metallic cavityplaced inside a powerful magnet and cooled to around100 milliKelvin Axions vibrating at the resonant
frequency of the chamber can be converted into de-tectable microwave photons As ADMX runs rodsmove within the cavity to change its effective size andscan through different potential axion masses like alistener tuning through radio stations
ADMX finally hit the sensitivity necessary to detectlikely axion masses in January 2017 The collaborationexpects to examine some of the most-favored axionmass ranges between 500 megahertz and 10 giga-hertz over the next five or six years and is researchingtechnology that can search even higher frequencies
But the axion might be too light for ADMX soother projects are now getting off the ground TheDark Matter Radio consists of an electric circuit sur-rounded by a superconducting shield that blocks or-dinary electromagnetic waves but would produce ameasurable voltage if an axion penetrated it Apathfinder version of this experiment began takingdata in August 2017 Perusing a similar low massrange is the A BroadbandResonant Approach toCosmic Axion Detection with an Amplifying B-fieldRing Apparatus experiment which generates an im-mense magnetic field in hopes of detecting the
ldquoHerersquos a particle that people thought ought to existanyway and it just happened to solve dark matterrdquo
mdashGray Rybka
Mann PNAS | October 31 2017 | vol 114 | no 44 | 11559
signature of an axionmdashnamely a tiny wavering in thefield Another pathfinder experiment under construc-tion is the Cosmic Axion Spin Precession Experiment(CASPEr) which will use nuclear MRI technology tosearch for an oscillating electric charge inside theneutron to try to tease out the axion CASPEr will besensitive to axion masses far below the other detec-tors all the way down to around 200 Hz
WIMPs Still in PlayThe supersymmetric WIMP is not out of the runningyet and most of the dark matter community remainsfocused on xenon-based detectors XENON1Trsquos mostrecent data analysis was from only its first 34 daysldquoNow we are really starting to look into new territorywhere somebody has never probedrdquo says physicistElena Aprile of Columbia University in New York Citywho leads the project ldquoWe are all very hopeful andwe will keep exploring uncharted watersrdquo
Both Aprilersquos collaboration and the LUX team areworking on next-generation devices which will be anorder of magnitude larger and far more sensitive thanthe current generation Aprile says it will be at leastanother five years before WIMPs would be ruled outmdashand they might show up at any point before that
Physicists keep pushing to discover dark matter inas many ways as they can imagine Data from oldparticle accelerator experiments are being reanalyzedto look for anomalies and many particle acceleratorsare being retrofitted to search for dark photonsResearch and development have proliferated ontechnology that could detect lightweight dark matterparticles Upcoming experiments include the SubElectron Noise Skipper-CCD Experimental Instrumentand Dark Matter in CCDs project which would searchfor dark matter slamming into silicon-charged cou-pling devices and the Princeton Tritium Observatoryfor Light Early-Universe Massive-Neutrino Yield pro-gram which would use a graphene detector Researchersare also developing devices utilizing electrons crystallinegermanium and superfluid helium and many otherdirect-detection methods
As long as dark matterrsquos influence can be seen inthe universe it will remain a tantalizing target for ex-periments ldquoItrsquos really one of the deepest mysteries inparticle physics and we have so many good ideas forhow to detect itrdquo says particle physicist Jesse Thalerof the Massachusetts Institute of Technology in Cam-bridge ldquoIf human ingenuity has anything to say aboutit there will be an excellent chance of detectionhopefully in my lifetimerdquo
1 Krasznahorkay AJ et al (2016) Observation of anomalous internal pair creation in 8Be A possible indication of a light neutral bosonPhys Rev Lett 116042501ndash042506
2 Feng JL et al (2016) Protophobic fifth-force interpretation of the observed anomaly in 8Be nuclear transitions Phys Rev Lett117071803ndash071809
3 Battagleri M (2017) US cosmic visions New ideas in dark matter 2017 Community report arXiv170704591v14 Lee BW Weinberg S (1977) Cosmological lower bound on heavy neutrino masses Phys Rev Lett 39165ndash1685 Akerib DS et al LUX Collaboration (2017) Results from a search for dark matter in the complete LUX exposure Phys Rev Lett118021303ndash021311
6 Aprile E et al (2017) First dark matter search results from the XENON1T experiment arXiv170506655v27 Ji X (2017) New world-leading upper limits for spin independent WIMP-nucleon cross section from PandaX Available at httpspandaxsjtueducnnode300 Accessed August 18 2017
8 Bennett GW et al Muon (g - 2) Collaboration (2004) Measurement of the negative muon anomalous magnetic moment to 07 ppmPhys Rev Lett 92161802ndash161807
9 Peccei RD Quinn HR (1977) CP conservation in the presence of pseudoparticles Phys Rev Lett 381440ndash1443
11560 | wwwpnasorgcgidoi101073pnas1716618114 Mann
signature of an axionmdashnamely a tiny wavering in thefield Another pathfinder experiment under construc-tion is the Cosmic Axion Spin Precession Experiment(CASPEr) which will use nuclear MRI technology tosearch for an oscillating electric charge inside theneutron to try to tease out the axion CASPEr will besensitive to axion masses far below the other detec-tors all the way down to around 200 Hz
WIMPs Still in PlayThe supersymmetric WIMP is not out of the runningyet and most of the dark matter community remainsfocused on xenon-based detectors XENON1Trsquos mostrecent data analysis was from only its first 34 daysldquoNow we are really starting to look into new territorywhere somebody has never probedrdquo says physicistElena Aprile of Columbia University in New York Citywho leads the project ldquoWe are all very hopeful andwe will keep exploring uncharted watersrdquo
Both Aprilersquos collaboration and the LUX team areworking on next-generation devices which will be anorder of magnitude larger and far more sensitive thanthe current generation Aprile says it will be at leastanother five years before WIMPs would be ruled outmdashand they might show up at any point before that
Physicists keep pushing to discover dark matter inas many ways as they can imagine Data from oldparticle accelerator experiments are being reanalyzedto look for anomalies and many particle acceleratorsare being retrofitted to search for dark photonsResearch and development have proliferated ontechnology that could detect lightweight dark matterparticles Upcoming experiments include the SubElectron Noise Skipper-CCD Experimental Instrumentand Dark Matter in CCDs project which would searchfor dark matter slamming into silicon-charged cou-pling devices and the Princeton Tritium Observatoryfor Light Early-Universe Massive-Neutrino Yield pro-gram which would use a graphene detector Researchersare also developing devices utilizing electrons crystallinegermanium and superfluid helium and many otherdirect-detection methods
As long as dark matterrsquos influence can be seen inthe universe it will remain a tantalizing target for ex-periments ldquoItrsquos really one of the deepest mysteries inparticle physics and we have so many good ideas forhow to detect itrdquo says particle physicist Jesse Thalerof the Massachusetts Institute of Technology in Cam-bridge ldquoIf human ingenuity has anything to say aboutit there will be an excellent chance of detectionhopefully in my lifetimerdquo
1 Krasznahorkay AJ et al (2016) Observation of anomalous internal pair creation in 8Be A possible indication of a light neutral bosonPhys Rev Lett 116042501ndash042506
2 Feng JL et al (2016) Protophobic fifth-force interpretation of the observed anomaly in 8Be nuclear transitions Phys Rev Lett117071803ndash071809
3 Battagleri M (2017) US cosmic visions New ideas in dark matter 2017 Community report arXiv170704591v14 Lee BW Weinberg S (1977) Cosmological lower bound on heavy neutrino masses Phys Rev Lett 39165ndash1685 Akerib DS et al LUX Collaboration (2017) Results from a search for dark matter in the complete LUX exposure Phys Rev Lett118021303ndash021311
6 Aprile E et al (2017) First dark matter search results from the XENON1T experiment arXiv170506655v27 Ji X (2017) New world-leading upper limits for spin independent WIMP-nucleon cross section from PandaX Available at httpspandaxsjtueducnnode300 Accessed August 18 2017
8 Bennett GW et al Muon (g - 2) Collaboration (2004) Measurement of the negative muon anomalous magnetic moment to 07 ppmPhys Rev Lett 92161802ndash161807
9 Peccei RD Quinn HR (1977) CP conservation in the presence of pseudoparticles Phys Rev Lett 381440ndash1443
11560 | wwwpnasorgcgidoi101073pnas1716618114 Mann