Upload
lamtuong
View
214
Download
0
Embed Size (px)
Citation preview
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin 98
Lecture 7
Experimental Nuclear Physics PHYS 741
References and Figures from:- Basdevant et al., “Fundamentals in Nuclear Physics”- Henley et al., “Subatomic Physics”
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Scattering Topics• scattering/cross-sections in QM (perturbation theory)
– elastic scattering– quasi-elastic scattering
• particle-particle scattering– two free particles– particles on bound particle (form factors)– scattering on charge distribution– electron - nucleus scattering– electron - nucleon scattering– resonances – nucleon-nucleus scattering– coherent scattering
99
+ scattering with polarized particles
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Review
• Rutherford/Mott Scattering• charge distributions and form factors• electrons as a probe of nuclei and nucleons• charge densities, magnetic moment densities• internal structure of nucleons
100
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Charge Distributions and Form Factors
101
charge distributions form factor
form factors equal at low q
charge distributions have same mean square radius <r2>
data
elastic scattering of e- on Ca
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Charge and Magnetic Moment Densities of Protons and Neutrons
102
Derived Charge and Magnetic Moment Densitiesproton: most charge within < 0.8 fm
neutron: positively charged core < 0.3fmsurrounded by neg charge 0.3-2fm
proton form factor - prediction for exponential charge distribution with mean charge radius of 0.8 fm- experimental data
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Nucleon-Nucleus Scattering
103
neutron emission
photon emissions
fission
Resonances, Elastic, Inelastic Scattering
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
SN1987A
104
observed 10 events in Kamiokande II detector
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Scattering of Waves on Target
105
forward scattering
plane wave
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Ultracold Neutrons at ILL
106
this could also be a topic for a course project
storage of neutrons with very low energies because of reflection of UCN under any angle of incidence
-> reflection caused by coherent strong interaction of neutrons with nuclei
neutrons diffuse into water moderator where they are thermalized
deuterium flask
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin 107
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin 108
Fermi pseudo potential (neutron optical potential)
v < critical velocity of material for reflections of neutrons from surface
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin 109
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Neutron Guides
110
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Coherent Neutrino Nucleus Scattering
111
- neutral current, flavor blind- coherent up to Eν ~ 50 MeV- important in SN processes
cross-section easily calculable
coherent weak interaction
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Neutrino Cross-Sections & New Physics
112
D.Z. Freedman PRD 9 (1974)A.Drukier & L. Stodolsky, PRD 30, 2295 (1984)Horowitz et al. astro-ph/0302071
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Coherent ν-A scattering has never been observed
113
recoil energies are tiny
CLEARat Spallation Neutron Source
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
One experimental approach: Cryogenic Detectors
114
Squid measures magnetic field by coil
At low temperatures of about 10 - 20 mK the heat capacity of solids is very low (∼ T3). Thus a small amount of deposited energy (→ recoil energy of the target nucleus) leads to a measurable change in temperature. This change in temperature is measured with a transition edge sensor.
A neutrino scatters off a nucleus. The recoil energy is converted into phonons. The phonons are reflected at the surface of the crystal and can only leave the detector through the transition edge sensor, which is the only part which is thermally connected to a heat sink.
Because of the increased temperature the resistance of the superconducting film increases. The resistance of the superconducting film is measured with a SQUID.
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin 115
Spin Polarized Scattering
Spins & Parity
Parity Violation
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Particles with Intrinsic Spin and Spin Polarization
116
if parity is conserved, probability that proton is scattered by target should be independent of spin direction (if target nuclei are random)
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Parity Transformation
117
handedness = relation between spin direction and direction of motion
right-handed=polarization along direction of motion
Note: - forces that depend on relationship of spin rotation to direction of motion violate parity conservation.
- weak interaction is only interaction in SM that violates parity
mirror reflection = one type of parity transformation
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Parity (Non)Conservation
118
Since 1925, physicists had accepted the principle that the parity is conserved in all types of interactions. During the 1950's, however, phenomena were found in high-energy physics that could not be explained by existing theories.
The K meson seemed to arise in two distinct versions, one decaying into two, the other decaying into three mesons, the two versions being identical in all other characteristics.
History
A mathematical analysis showed that the two-pion and the three-pion systems have opposite parity; hence, according to the prevalent theory, these two versions of the K meson had to be different particles.
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Yang and Lee
119
In the summer of 1956, T. D. Lee of Columbia University and C. N. Yang of the Institute for Advanced Study made a survey of experimental information on the question of parity. They concluded that the evidence then existing neither supported nor refuted parity conservation in the ``weak interactions'' responsible for the emission of beta particles, K-meson decay and such
They proposed that the K-meson itself may have definite parity, and the observed opposite parity of the two systems of decay products may be the manifestation of parity non-conservation in its decay.
They also proposed a number of experiments on beta decays and hyperon and meson decays that would provide the necessary evidence for or against parity conservation in weak interactions.
One of the proposed experiments involved measuring the directional intensity of beta radiation from oriented cobalt-60 nuclei
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Discovery of Parity Violation in 1956
120
beta-decay of 60Co nucleiC. S. Wu of Columbia University and Ernest Ambler, Raymond W. Hayward, Dale D. Hoppes, and Ralph P. Hudson.
The assembly is then placed between poles of a magnet for magnetic cooling to about 0.003K
After cooling, the cobalt-60 nuclei were polarized by the magnetic field from a solenoid
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Observation of Parity Non-Conservation
121
The magnetic polarity of the nucleus is determined by its direction of spin, and, under the influence of a magnetic field, most of the cobalt-60 nuclei align themselves so that their spin axes are parallel to the field.
If parity is conserved in such interactions, then the intensity of the beta emission should be the same in either direction along the axis of spin.
Measure the intensity of beta emission in both these directions. Used a beta scintillation counter inside experimental setup and a gamma counter outside.
Result: more electrons emitted preferentially in one direction.
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
December 27, 1956
122
An initially high counting rate of particles (emitted by the cobalt-60 nuclei as polarized by this field) was observed to decrease to the value for randomly oriented nuclei as the polarization decreased because of the gradual warming of the cobalt-60 nuclei
After again cooling the crystal and then polarizing the cobalt-60 nuclei in the opposite direction, the physicists observed the opposite behavior of the particle counts with time.
A second experiment was then performed using cobalt-58, which is a positron emitter. In this case the opposite effect was observed, namely that + particles are preferentially emitted along the direction of the nuclear spins.
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
The Discoverers
123
Ernest Ambler
C. S. Wu
Raymond W. Hayward
Dale D. Hoppes Ralph P. Hudson
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Experiments with Polarized Protons and Neutrons
124
1970s- parity violation in scattering of protons-protons
- proton has intrinsic spin but no intrinsic handedness -> spin can be changed relative to its direction of motion
- hydrogen suitable p target (average spin of protons in target is zero)
1980s - neutron experiments at LANL, Europe, and in the USSR
neutrons with opposite spin are scattered out of beam
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Scattering and Absorption of Neutrons by 232Th
125
parity violating effects expected for l=1 but not for l=0
- neutrons carry same amount of intrinsic spin as protons do
- spin can be polarized along or opposite direction of motion
- cross-sections differed depending on the polarization of incident neutrons
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Parity Violation in Neutron Resonance of 232Th
126
l=1, J=1/2- resonance
neutron transmission data
polarization along direction of motion
resonance exhibits parity violation
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Nucleon-Nucleon Weak Interaction at Quark Level
127
exchange of meson
nucleon diagram at quark level
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
E158 Experiment at SLAC - Moller Scattering
128
• http://www.slac.stanford.edu/exp/e158/
- first observation of Parity Violation in electron-electron (Møller) scattering
- measurement of weak electric charge
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Moller Scattering
129
bahbha scattering(electron-positron scattering)
electron-electron scattering electron-electron scattering
photon is symmetricZ boson prefers left-handed particles
thus cross-sections for left-handed electrons and right-handed ones differ
+ +
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Extracting the weak charge at low
Møller scattering :
- Sensitive to: e, Qw
Parity violation asymmetry :
Tree level Moller asymmetry :
Qw
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Running of the Weak Mixing Angle
131
θW = Weinberg angle/weak angle- parameter in electroweak force- relationship between W and Z masses- ratio of Z-mediated interactions to photon mediated interactions
θW varies as a function of momentum transfer Q = “running”
is a key prediction of electroweak theorymost precise measurements at mass of Z, Q =91.2 GeV/c
+ +
• Electroweak radiative corrections → sin2θW varies with Q
Running coupling constants in QED and QCDQED (running of α)
αs
QCD(running of αs)
137 →
Q2, GeV2
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Running Coupling Constants
133
strong force strength
(at tree level)
Qpweak: Extract from Parity-Violating Electron Scattering
measures Qp – proton’s electric charge measures Qpweak
– proton’s weak charge
MEM MNC
As Q2 → 0
• Qpweak is a well-defined experimental observable
• Qpweak has a definite prediction in the electroweak Standard Model
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Weak Charge Phenomenology
135
This accidental suppression of the proton weak charge in the SM makes it more sensitive to new physics (all other things being equal).
Note how the roles of the proton and neutron are become almost reversed(ie, neutron weak charge is dominant, proton weak charge is almost zero!)
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Radiative corrections
• 1-loop corrections change the relation between Aee
and :
3% corrections to
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Experiment principle
LH24-7 mrad
N+,N-• Ee= 45 GeV
•Fast polarization reversal 120 Hz
• High Polarization Pe=85% Aee=PeAexp
• High intensity 5x1011 e-/pulse
BEAM TARGET
DETECTOR 2,7 GHz scattered Møller
High density target , σee=12 µb L ~ 1038 cm-2s-1
Raw Asymmetry =1.3x10-7 (130 ppb) Δ(Apv) = 10-8 (10 ppb) Need 1016 electrons
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Polarized beam• Optical pumping :
Wavelength (nm)
Pola
rizat
ion
(%)
QE (%
)
Very high-charge polarized electron beams are possible (Pe~85%)
Beam helicity is chosen pseudo-randomly at 120 Hz
•Data analyzed as “pulse-pairs”
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Liquid Hydrogen target
Length 1.54 m Refrigeration capacity 1 kWBeam heat deposit 800WOperating temperature 20KFlow rate 5 m/s
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Moller Physics Asymmetry (unblinded; with corrections and normalization)
APV(e-e- at Q2 = 0.027 GeV2): -151.9 ± 29.0 (stat) ± 32.5 (syst)
parts per billion(preliminary)
Significance of parity nonconservation in Møller scattering: 3.6σ
140
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
E158 Results
141
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Q-Weak Experiment (e-p scattering)
142
A Precision Test of the Standard Model and Determination of the Weak Charges of the Quarks through Parity-Violating Electron Scattering
proton weak chargeQPW=1 - 4sin2θW
elastic e-p scattering at Q2=0.03 (GeV/c)2 employing 180 A of 85% polarized beam on a 35 cm liquid Hydrogen target
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
Q-Weak
143
A Precision Test of the Standard Model and Determination of the Weak Charges of the Quarks through Parity-Violating Electron Scattering
Polarized Electron Beam
35cm Liquid Hydrogen Target
Collimator with 8 openingsθ= 8° ± 2°
Region IGEM Detectors
Region IIDrift Chambers
Toroidal Magnet
Region IIIDrift Chambers
Elastically Scattered Electron
Eight Fused Silica (quartz)Čerenkov Detectors
Luminosity Monitors
JLab Qweak
Run I + II + III ±0.006
(proposed)-
• Qweak measurement will provide a stringent stand alone constraint on Lepto-quark based extensions to the SM.• Qp
weak (semi-leptonic) and E158 (pure leptonic) together make a powerful program to search for and identify new physics.
SLAC E158
Qpweak & Qe
weak – Complementary Diagnostics for New Physics
Erler, Kurylov, Ramsey-Musolf, PRD 68, 016006 (2003)
Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin
New Concepts
Scattering experiments with polarized n,p, e beams can tell us something about the fundamental forces and interactions
– spins & parity
– parity violation
– weak mixing angle
– weak charge
145