lecture 7 experimental nuclear physics phys...

Experimental Nuclear Physics - PHYS741Karsten Heeger, Univ. Wisconsin 98

Lecture 7

Experimental Nuclear Physics PHYS 741

[email protected]

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


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


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


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


Nucleon-Nucleus Scattering

103

neutron emission

photon emissions

fission

Resonances, Elastic, Inelastic Scattering


SN1987A

104

observed 10 events in Kamiokande II detector


Scattering of Waves on Target

105

forward scattering

plane wave


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


Fermi pseudo potential (neutron optical potential)

v < critical velocity of material for reflections of neutrons from surface


Neutron Guides

110


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


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


Coherent ν-A scattering has never been observed

113

recoil energies are tiny

CLEARat Spallation Neutron Source


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.


Spin Polarized Scattering

Spins & Parity

Parity Violation


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)


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


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.


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


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


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.


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.


The Discoverers

123

Ernest Ambler

C. S. Wu

Raymond W. Hayward

Dale D. Hoppes Ralph P. Hudson


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


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


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


Nucleon-Nucleon Weak Interaction at Quark Level

127

exchange of meson

nucleon diagram at quark level


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


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

+ +


Extracting the weak charge at low

Møller scattering :

- Sensitive to: e, Qw

Parity violation asymmetry :

Tree level Moller asymmetry :

Qw


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


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


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!)


Radiative corrections

• 1-loop corrections change the relation between Aee

and :

3% corrections to


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


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”


Liquid Hydrogen target

Length 1.54 m Refrigeration capacity 1 kWBeam heat deposit 800WOperating temperature 20KFlow rate 5 m/s


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


E158 Results

141


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


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)


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

lecture 7 experimental nuclear physics phys...

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