16.451 lecture 14: two key experiments 21/10/2003 news flash from indiana university: tour-de-force...
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16.451 Lecture 14: Two Key Experiments 21/10/2003
News flash from Indiana University:
Tour-de-force experiment: http://www.cerncourier.com/main/article/43/5/4
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???He o4 dd
• isospin-forbidden reaction since T = 0 for the d, 4He, and T=1 for °: “textbook case”
(technically speaking, this reaction breaks “charge symmetry” which is the symmetry under reversal of all up and down quarks in a wave function, or equivalently a quark “isospin flip”. The pion wave function is CS – odd; the others are CS even)
• Charge symmetry is broken by the electromagnetic interaction: up-down quark mass difference, and their electric charge differences
• reaction could proceed with very low cross section compared to isospin-allowed cases, but there was never any convincing evidence published until a couple of weeks ago
• compare similar cross-sections at reaction threshold:
p + d 3He + ° = 13 b (Isospin allowed)
d + d 4He + ° = 13 2 pb (forbidden, new result)
• Rough estimate of cross section ratio :
?102~~ 432
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12
2
coemV
sV
forbidden
allowedif rdV
Comparison of precise measurement and theory, accounting for all known CSBeffects, tests our understanding of CS as a symmetry of the strong interaction
2
0
Cooler CSB
the search for d+d0
Ed StephensonPhysics Colloquium9/24/03
slides courtesy of Dr. E. Stephenson, Indiana University
full set: http://www.iucf.indiana.edu/Experiments/COOLCSB
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CHARGE SYMMETRY BREAKING
atomicnucleus proton
neutron
Simple notion: charge symmetryThe proton and neutron are the sameexcept for electromagnetic properties.
Isospin: the quantum number for CSProton and neutron have T(I) = 1/2
But they are different: mN – mP = 1.3 MeV(The neutron decays in 887 s: n p + e– + νe)
u u
u
d
d d
quarksinsidenucleons:CS saysup anddown arethe sameexceptfor charge
z = 2/3
z = –1/3
Nuclear charge symmetry breaking comes from:
electromagnetic interactions among quarks
md > mu
How much does each contribute?
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Observation of the Isospin-forbiddend+d 4He+π0 Reaction near Threshold
d + d 4He + π0
isospin: 0 0 0 1
CHARGE SYMMETRYsays that the physics isunchanged when protonsand neutrons are swapped,or when up and downquarks are swapped.
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2uu dd
The pionwavefunctionis notsymmetric under up-down exchange.Deuterons and helium reverseexactly. Thus, an observation of thisprocess is also an observation ofcharge symmetry breaking.
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For a reaction to occur in a fixed target experiment, m1 has to hit m2 with enough energy to make the particles in the final state. Theminimum kinetic energy required is called the threshold energy:
Threshold Energy:
m1m2
m3 - mn
Tthr = - Q 2m2
m1 + mfm2 + Q = m1 + mfm2 -
dd + +
Examples:
3Hedp + +
Tthr = 225.4 MeV
Tthr = 198.7 MeV
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Experimental approach:
Search just above threshold (225.5 MeV) (No other π channel open for d+d.) Capture forward-going 4He. Pb-glass arrays for π0 γγ. Efficiency on two sides ~ 1/3. Insensitive to other products (γbeam = 0.51)
6 bend in Cooler straight section Target upstream, surrounded by Pb-glass Magnetic channel to catch 4He (~100 MeV) Reconstruct kinematics from channel time of flight and position.
Target density = 3.1 x 1015
Stored current = 1.4 mALuminosity = 2.7 x 1031 /cm2/s
Expected rate ~ 5 /day
Pb-glass measures photonenergy via Cerenkov lightfrom high energy e- producedin a ‘shower’ initiated by highenergy photon collisions
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d
d
For a fixed target experiment just above threshold,
dd + + in the lab close to threshold:
.
coneangle
particles emerge within a narrow cone about the 0-degree line. (Spectrometer with small forward acceptance will catch every .)
low-energy quickly decays into two photons which emerge nearly back to back in the lab.
Therefore, the apparatus must identify a forward in coincidence with two photons that have a large opening angle between them.
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Target
D2 jet
Pb-glass array
256 detectorsfrom IUCF andANL (Spinka) +scintillators forcosmic trigger
228.5 or 231.8 MeVdeuteron beam
Separation Magnet
removes 4He at 12.5from beam at 6
20 Septum Magnet
FocussingQuads
MWPCs
Scintillator
E-1 MWPC
COOLER-CSB MAGNETIC CHANNELand Pb-GLASS ARRAYS
•separate all 4He for total cross section measurement•determine 4He 4-momentum (using TOF and position)•detect one or both decay ’s from 0 in Pb-glass array
Scintillators
E-2EVeto-1Veto-2
Note: MWPC = multiwire proportional chamber – gives tracking information for trajectory angles
scintillators measure flight time (ToF) velocity momentum
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Pb-glass Hit Patterns
beam goesinto X
0 from p+d 3He+0LEFT RIGHT
cosmic ray muon
color scale: red > pink > blue
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p + d 3He +
Mass (Mev/c2 * 100)
`Missing Mass’ measured with proton beam:
Mass of :
134.98 MeV/c2
Resolution is~ 100 KeV.
conservation of energy:
W Ep + Ed – E(3He) = m
• Ep from beam energy
• deuteron at rest in target
• E(3He) from energy and momentum measured with the magnetic channel
• calculate W from data, should find a peak at the pion mass for reaction at threshold.
• then check in Pb glass array to see if pion was observed
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SEPARATION OF 0 AND EVENTS
MWPC1 X-position (cm)
Y-p
osi
tion
(c
m)
Time of Flight (ΔE1 - ΔE2) (ns)
needed TOFresolution
GAUSS = 100 ps
MWPCspacing= 2 mm
IDEA: Calculate missing mass from the four-momentum measured in the magnetic channel, using time-of-flight for z-axis momentum and MWPC X and Y for transverse momentum. Should see a peak for ° reaction and a broad background from
[Monte Carlo simulation for illustration. Experimental errors included.] 0 peak
TOT = 10 pb
background(16 pb)
missing mass (MeV)
.Background: d + d + 2
Need very good resolution so thatthe peak is detectable!
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RESULTS (measured at two different beam energies)
231.8 MeV50 events
σTOT = 15.1 ± 3.1 pb
228.5 MeV66 events
σTOT = 12.7 ± 2.2 pb
missing mass (MeV)
Events in these spectra must satisfy: correct pulse height in channel scintillators usable wire chamber signals good Pb-glass pulse height and timing
Peaks give the correctπ0 mass with 60 keVerror.
First ever convincingobservation of both the° and reactions!
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Bottom line: time to revise all the textbooks!!!
Back to the neutron:
Lifetime: 885.7 0.8 sec (world average)
eepn
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isospin
Case study: “state of the art” neutron lifetime measurement 15
Outline of the method:
N
dt
dNdecay rate:
eepn
measure rate by counting decay protons in a given time interval (dN/dt) and normalizing to the neutron beam flux (N)
incoming n beam
decay volume, length L
decay proton, to detector
transmitted beam
neutron detector
Ideally done with “cold neutrons”, e.g. from a reactor, moderated in liquid hydrogen...
Issues: 1. precise decay volume ? 2. proton detection ? 3. beam normalization ? ...
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Neutron beam distribution – definitely not monoenergetic:
• ~ MeV neutrons from a reactor are “moderated” by scattering in a large tank of water (“thermal”) or liquid hydrogen (“cold”)
• after many scatterings, they come to thermal equilibrium with the moderator and are extracted down a beamline to the experiment
• velocity distribution is “Maxwellian”: energies in the meV range (kT = 26 meV @ 293K)
Krane, Fig 12.4
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Neutron detection at low energy: (Krane, ch. 12)
• several light nuclei have enormous neutron capture cross sections at low energy: (recall, cross sectional area of a nucleus, e.g. 10B is about 0.2 barns, lecture 4)
• key feature: cross sections scale as 1/velocity at low energy
(barn
s) 10B + n + 7Li + 2.79 MeV
= ovo/v ; o = 3838 b, vo = 2.2 km/s
kinetic energy of ionizedfragments can be convertedinto an electrical signal detector
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Put this together for detector counting rates as shown:
incoming n beam
decay volume, length L
decay proton, to detector
transmitted beam
10B neutron detector
beamdetdet ~
)v(
vN
NN
oo
dt
dNN beam
decay decay
beam
/ N
N
dtdN
N
LN
Nconst
T
LNconstN
decay
detdetbeam )()(
TL v
Decays and transmitted detector counts accumulated for time T
beamN
(DC beam, thermalenergy spread)
neutrons must be captured tobe detected!
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Experimental details (all in vacuum, at ILL reactor, France):
• use Penning trap to confine decay protons
• let them out of the trap after accumulation interval T
• measure the ratio Ndet/Ndecay as a function of trap length L slope gives
protons spiralaround field lineswhen let out of the trap
protondetectordecayN
thin 10B foilto capture beam n’s
particledetectorsfor captureproducts
detN
variable lengthPenning trap(16 electrodes)
L
LN
N
decay
det~
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Amazing results:
proton energyfrom timing:about 34 keV(after kick –raw energy isonly 0.75 keV)
Rate versus trap length L
Result: 893.6 5.3 seconds (1990)
PDG average: 885.7 0.8 (2003)
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