1. Measurements of atmospheric neutrinos

M. Spurio

Università e INFN Bologna

XXVIII SEMINARIO* NAZIONALE di FISICA NUCLEARE E

SUBNUCLEARE "Francesco Romano"

OTRANTO (Serra degli Alimini 1) 3-10 giugno 2016

* 1 Istituto per la formazione religiosa e la preparazione culturale dei giovani aspiranti al sacerdozio 2 In ambito universitario, lezione in cui gli studenti partecipano attivamente con relazioni e interventi; esercitazione tenuta da un docente per un ristretto numero di studenti

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Avvertenze • Argomenti scelti in coordinazione con l’altro docente (FV) • Non intendo passare in rassegna tutte le ricerche e i risultati

sperimentali connessi con la fisica del neutrini • Ho selezionato alcuni argomenti, che tratto col taglio che ritengo

adeguato ad una scuola avanzata e per studenti con master in Fisica • Grafica delle slides spartana (come a lezione) • Le slides sono in inglese (possibilità di riutilizzo) • Come in ogni corso, ci sono delle domande che possono stimolare

la discussione tra studenti, e tra studenti e docenti • Come in ogni corso, c’e’ un libro di testo per gli

approfondimenti e ulteriori stimoli (io tratterò i capitoli 10, 11 e 12 del libro)

• Le figure sono tratte per la maggior parte dal libro; in tal caso i riferimenti sono omessi

• Come per ogni libro, c’e’ la possibilità di avere la copia piratata (chiavetta USB)

• Ovviamente, questa slide scompare

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Neutrini from the Cosmos

• Flux of neutrinos at the surface of the Earth.

• The three arrows near the x-axis indicate the energy thresholds for CC production of the charged lepton

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Once upon a time… • GUT theories predicted the proton decay with measurable livetime • The proton was thought to decay in (for instance) pe+π0

• Detector size: 103 m3, and mass 1kt (=1031 p) • The main background for the detection of proton decay were

atmospheric neutrinos interacting inside the experiment

Proton decay

γγ

e Neutrino Interaction

Water Cerenkov Experiments (IMB, Kamiokande)

Tracking calorimeters (NUSEX, Frejus, KGF)

Result: NO p decay ! But some anomalies on the neutrino measurement!

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The importance of atmospheric ν’s • “Yestelday’s signal is today’s backglound and

tomollow’s caliblation”

• It is not always true! Atmospheric neutrinos…

• < 1998 Background to nucleon Decay • > 1998 Signal of neutrino oscillations • > 2013 Background and calibration to HE neutrino astronomy • > 20xy Signal of Earth matter effects and of ν mass hierarchy • > 20xz Background to diffuse SN neutrino signal • >20yz Signal of nonstandard neutrino states or interaction? • >20wx Background to proton decay signals?

Adapted from E. Lisi 6

General problems for ν detectors

• Low cross section Large detector volume/mass • Particle identification • Energy/momentum measurement • Direction measurement • No magnetic field (ν =ν) • Backgrounds

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The recipes for the evaluation of the atmospheric neutrino flux

ee ννµ

νµπ

µ

µ

++→

+→++

++Independently from the details of the computation of Φνμ (E), Φνe(E), one can obtain two very robust properties: 1. At energies below few GeV, the flux of νμ is approximately twice as large as the νe, i.e.: Φ(νμ)= 2Φ(νe) 2. The νμ, νe fluxes are up-down symmetric in zenith θ, i.e.: Φ ν (Eν, θ) = Φ ν (Eν, π − θ)

Question 1: why 1. does not hold at higher (>> GeV) energies? 8

Direct measurements (satellites) Indirect measurements (Extensive Air Shower Arrays)

i) The primary CR spectrum

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i) The primary CR spectrum

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• Primary CR attenuation as function of X (g cm-2) and E

• Boundary condition:

• From Feyman scaling:

• The dependence on X depends on an effective attenuation length ΛN:

i) The role of primary CRs

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ii) p-air cross section

AUGER Coll. PRL 109 (2012) 062002 12

iii) Secondary charged multiplicity

Average number of charged hadrons produced in pp (andpp), e+e-, ep collisions versus center of mass energy

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ii+iii) The meson production • The pion propagation in atmosphere is described by:

• Competition between interaction and decay . The decay length:

• The pion decay constant επ=115 GeV. • Pions start to increase with increasing depth X, reach a maximum and

then decrease 14

• High-energy limit (E>> επ ): the decay term dπ can be neglected:

(the Z are the the spectrum weighted moments) • Low-energy limit (E cosθ << επ ): we neglect the term λIπ.

• Similar equations hold for other particles with different decay constants:

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ii+iii) The neutrino production • Conventional muons are produced by π and K decays:

• Prompt muons by the decay of charmed mesons. • The muon flux is thus described by the equation:

• The muon neutrino flux follows similarly.

}

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The conventional ν flux: π and K Solid lines: vertical, dashed lines: zenith 60o

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iv) Model of the atmosphere solar effects +geomagnetic field

•high precision 3D calculations, •refined geomagnetic cut-off treatment (also geomagnetic field in atmosphere) •elevation models of the Earth •different atmospheric profiles •geometry of detector effects

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• One ν flux prediction (Honda) from MC simulations;

• Different models exist;

Question 2: Explain qualitatively the (νµ/νe) ratio Question 3: Explain qualitatively why the (νµ/νµ) ratio increases with energy

The conventional ν flux (Honda) M. Honda, et al. Phys. Rev. D 92, 023004 (2015)

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Measurement of atmospheric ν‘s

νe= cascade

νµ=track

Example: Icarus@ Gran Sasso

Example: Soudan II@ USA

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• Tracking calorimeter: Frejus, Nusex, Soudan. • Water Cherenkov: IMB, Kamiokande • Measured the number of neutrino interaction in the detector,

separating tracks (=νµ) from showers (=νe)

(Early) measurement of atmo. ν‘s T. Kajita, N

ew J. Phys. 6 (2004) 194.

Intergral flux of atmo ν’s vs energy

Question 4: Evaluate the number of interaction/kton year for E> 1 GeV for kton of fiducial mass detector, assuming detection efficiency=1 21

The golden age: SK and MACRO

• SuperKamiokande (SK) is located in Japan, 1000 m Underground

• Active since 1996 • Filled 50.000 ton water • 11000 large PMTs +2000 PMTs

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1996.4 Start data taking

1999.6 K2K started

2001.7 data taking was stopped for detector upgrade 2001.11 Accident

partial reconstruction 2002.10 data taking was resumed

2005.10 data taking stopped for full reconstruction

2006.7 data taking was resumed

2001 Evidence of solar n oscillation (SNO+SK)

1998 Evidence of atmospheric n oscillation (SK)

2005 Confirm ν oscillation by accelerator ν (K2K)

SK-I

SK-II

SK-III

SK-IV 2009 data taking

20 years of Super-Kamiokande

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SuperKamiokande: νe

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SuperKamiokande: νµ

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As a charged particle travels, it disrupts the local electromagnetic field (EM) in a medium.

Electrons in the atoms of the medium will be displaced and polarized by the passing EM field of a charged particle.

Photons are emitted as an insulator's electrons restore themselves to equilibrium after the disruption has passed.

In a conductor, the EM disruption can be restored without emitting a photon.

In normal circumstances, these photons destructively interfere with each other and no radiation is detected.

However, when the disruption travels faster than light is propagating through the medium, the photons constructively interfere and intensify the observed Cerenkov radiation.

Cherenkov Radiation

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• Threshold velocity βT = 1/n θT ~ 0 • Angle of emission (β=1): θmax= arcos(1/n) • Distribution of emitted photons:

lpart=βc∆t

llight=(c/n)∆tθ

wave front

1)(with1cos ≥== λβ

θ nnnC

θC

dN/dλ

λ

dN/dE

Ε

.sin

sin2112

222

22

2

222

22

c

C

cz

dxdENd

zn

zdxd

Nd

ϑα

θλ

απβλ

απλ

=

⋅=

−=

Question 5: Evaluate the number of Cherenkov photons in water in the λ=300-600 nm interval for a relativistic single charged particle 28

ν energy: event topology Fully Contained

ν Partially Contained

ν

µ

Through going µ Stopping µ

ν

µ

ν

µ

Energy spectrum (Monte Carlo) of atmospheric ν seen with different event topologies (Super-Kamiokande, MACRO)

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SuperKamiokande I-IV: results

cosΘ<0

cosΘ>0

Θ

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MACRO @ Gran Sasso

• Liquid scintillator counters, (3 planes) for the measurement of time and dE/dx. • Streamer tubes (14 planes), for the measurement of the track position (<1o); • Detector mass: 5.3 kton • Downward going muons ~ 106 upward going muons • Different neutrino topologies

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+1 µ -1 µ

T1

T2 Streamer tube track

Neutrino induced events are upward throughgoing muons, Identified by the time-of-flight method

Atmospheric µ: downgoing

µ from ν: upgoing

( )=

⋅−=

LcTT 211

β

( )=

⋅−=

LcTT 211

β

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• Completely different topology w.r.t. SK • Different experimental technique • Deformation of the angular distribution w.r.t. expectation • Missing events from the vertical direction • Interpretation: oscillations • The same oscillation parameters!

The MACRO neutrino deficits

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The Soudan II neutrino deficit

• Iron tracking calorimeter • 770 t fiducial mass • Active from 1989 to 2001

in the Soudan Mine (USA) • (P)contained events • µ-like deficit from below

e-like µ-like

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|νe> , |νµ> , |ντ> =Weak Interactions (WI) eigenstats

|ν1> , |ν2> , |ν3> =Mass (Hamiltonian) eigenstats

Idea of neutrinos being massive was first suggested by B. Pontecorvo Prediction came from proposal of neutrino oscillations

Neutrino oscillations…

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⋅∆⋅−=

ννν θ

µµ ELm

P2

22 27.1sin2sin1

• ∆m2, sin22θ from Nature; • Eν = experimental parameter (energy distribution of neutrino giving a particular configuration of events) • L = experimental parameter (neutrino path length from production to interaction)

..with atmospheric neutrinos

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Discovery of neutrino oscillations

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Why not νμ→νe ?

Apollonio et al., CHOOZ Coll., Phys.Lett.B466 (1999) 415

PDG value: 0.095±0.010 38

Measurement of energy spectra

• See: neutrino telescopes (part 3)

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The 2015 Nobel Prize

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41

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Now and next.. Fr

om E

. Lisi

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From E. Lisi 44

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End of part1

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• At low energy, neutrinos exceed antineutrinos due to the fact that CR protons are more aboundant than neutrons

• Above few hundreds GeV, neutrinos from K decay are more abundant than from pions. Thus, conservation of the strangeness (S) and baryon (B) quantum numbers are responsible for the difference

• Consider the production of charged kaons from pp interactions: – Κ+ (B=0, S=1) is produced in association with Λ (B=1, S=-1); – Κ- (B=0, S=-1) requires at least one associated baryon (B=1) and an

additional strange meson (S=1).

• Κ+ are generated much more frequently than Κ- because of the associated production with the Λ.

Solution of question 3)

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1. Flux: Φν ~ 1 cm-2 s-1

2. Cross section (@ 1GeV): σν~0.5 10-38 cm2

3. Targets M= 6 1032 (nucleons/kton) 4. Time t= 3.1 107 s/y

Nint = Φν (cm-2 s-1) x σν (cm2)x M (nuc/kton) x t (s/y) ~ ~ 100 interactions/ (kton y)

Solution of question 4)

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• In water (n=1.33) for z=1 and β=1 the number of photons for energy interval and unith path length is:

• With: • The ∆λ=(600-300) nm corresponds to ∆E=2 eV, thus

Solution of question 5)

.sin222

ccz

dxdENd ϑα

=

occ n 4275.0)/1(cos =→== ϑϑ

cm330

cm10eV101027)67.0(eV2sin

)fm MeV197()137/1(

1363

22

2

=××⋅

×=⋅∆⋅= −cE

dxNd ϑ

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