Text. References and Figures from: - Basdevant et al., Fundamentals in Nuclear Physics - Henley et al., Subatomic Physics

Transcription

1 Lecture 7 Experimental Nuclear Physics PHYS 741 Text heeger@wisc.edu References and Figures from: - Basdevant et al., Fundamentals in Nuclear Physics - Henley et al., Subatomic Physics 98

2 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 + scattering with polarized particles 99

3 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

4 Charge Distributions and Form Factors charge distributions form factor form factors equal at low q elastic scattering of e- on Ca charge distributions have same mean square radius <r 2 > data 101

5 Charge and Magnetic Moment Densities of Protons and Neutrons proton form factor - prediction for exponential charge distribution with mean charge radius of 0.8 fm - experimental data Derived Charge and Magnetic Moment Densities proton: most charge within < 0.8 fm neutron: positively charged core < 0.3fm surrounded by neg charge 0.3-2fm 102

6 Nucleon-Nucleus Scattering Resonances, Elastic, Inelastic Scattering neutron emission fission photon emissions 103

7 SN1987A observed 10 events in Kamiokande II detector 104

8 Scattering of Waves on Target forward scattering plane wave 105

9 Ultracold Neutrons at ILL storage of neutrons with very low energies because of reflection of UCN under any angle of incidence neutrons diffuse into water moderator where they are thermalized deuterium flask -> reflection caused by coherent strong interaction of neutrons with nuclei this could also be a topic for a course project 106

10 107

11 Fermi pseudo potential (neutron optical potential) v < critical velocity of material for reflections of neutrons from surface 108

12 109

13 Neutron Guides 110

14 Coherent Neutrino Nucleus Scattering coherent weak interaction - neutral current, flavor blind - coherent up to Eν ~ 50 MeV - important in SN processes cross-section easily calculable 111

15 Neutrino Cross-Sections & New Physics D.Z. Freedman PRD 9 (1974) ( 1984 ) A.Drukier & L. Stodolsky, PRD 30, 2295 Horowitz et al. astro-ph/

16 Coherent ν-a scattering has never been observed recoil energies are tiny CLEAR at Spallation Neutron Source 113

17 One experimental approach: Cryogenic Detectors At low temperatures of about mk the heat capacity of solids is very low ( T 3 ). 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. Squid measures magnetic field by coil 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. 114

18 Spin Polarized Scattering Spins & Parity Parity Violation 115

19 Particles with Intrinsic Spin and Spin Polarization if parity is conserved, probability that proton is scattered by target should be independent of spin direction (if target nuclei are random) 116

20 Parity Transformation mirror reflection = one type of parity transformation 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 117

21 Parity (Non)Conservation History 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. 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. 118

22 Yang and Lee 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 nonconservation 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 119

23 Discovery of Parity Violation in 1956 beta-decay of 60 Co nuclei C. 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 120

24 Observation of Parity Non-Conservation 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. 121

25 December 27, 1956 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. 122

26 The Discoverers Ernest Ambler Raymond W. Hayward C. S. Wu Dale D. Hoppes Ralph P. Hudson 123

27 Experiments with Polarized Protons and Neutrons 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 neutrons with opposite spin are scattered out of beam - hydrogen suitable p target (average spin of protons in target is zero) 1980s - neutron experiments at LANL, Europe, and in the USSR 124

28 Scattering and Absorption of Neutrons by 232 Th - 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 violating effects expected for l=1 but not for l=0 125

29 Parity Violation in Neutron Resonance of 232 Th neutron transmission data polarization along direction of motion resonance exhibits parity violation l=1, J=1/2- resonance 126

30 Nucleon-Nucleon Weak Interaction at Quark Level nucleon diagram at quark level exchange of meson 127

31 E158 Experiment at SLAC - Moller Scattering - first observation of Parity Violation in electronelectron (Møller) scattering - measurement of weak electric charge 128

32 Moller Scattering electron-electron scattering electron-electron scattering photon is symmetric Z boson prefers left-handed particles thus cross-sections for left-handed electrons and right-handed ones differ bahbha scattering (electron-positron scattering)

33 Extracting the weak charge at low Møller scattering : - Sensitive to: e, Q w Parity violation asymmetry : Tree level Moller asymmetry : Q w

34 Running of the Weak Mixing Angle θw = Weinberg angle/weak angle - parameter in electroweak force - relationship between W and Z masses - ratio of Z-mediated interactions to photon mediated interactions + + Electroweak radiative corrections sin 2 θ W varies with Q θw varies as a function of momentum transfer Q = running is a key prediction of electroweak theory most precise measurements at mass of Z, Q =91.2 GeV/c 131

35 Running coupling constants in QED and QCD QED (running of α) QCD(running of α s ) 137 α s Q 2, GeV 2

36 Running Coupling Constants strong force strength 133

37 Q p weak : Extract from Parity-Violating Electron Scattering As Q 2 0 M EM measures Q p proton s electric charge M NC measures Q p weak proton s weak charge (at tree level) Q p weak is a well-defined experimental observable Q p weak has a definite prediction in the electroweak Standard Model

38 Weak Charge Phenomenology 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!) This accidental suppression of the proton weak charge in the SM makes it more sensitive to new physics (all other things being equal). 135

39 Radiative corrections 1-loop corrections change the relation between A ee and : 3% corrections to

40 Experiment principle Raw Asymmetry =1.3x10-7 (130 ppb) Δ(Apv) = 10-8 (10 ppb) Need electrons BEAM E e = 45 GeV High Polarization Pe=85% A ee =P e A exp TARGET LH2 High intensity 5x10 11 e - /pulse Fast polarization reversal 120 Hz DETECTOR 2,7 GHz scattered Møller N+,N- 4-7 mrad High density target, σ ee =12 µb L ~ cm -2 s -1

41 Polarized beam Optical pumping : Polarization (%) QE (%) Wavelength (nm) Very high-charge polarized electron beams are possible (Pe~85%) Beam helicity is chosen pseudo-randomly at 120 Hz Data analyzed as pulsepairs

42 Liquid Hydrogen target Length 1.54 m Refrigeration capacity 1 kw Beam heat deposit 800W Operating temperature 20K Flow rate 5 m/s

43 Moller Physics Asymmetry (unblinded; with corrections and normalization) A PV (e - e - at Q 2 = GeV 2 ): ± 29.0 (stat) ± 32.5 (syst) parts per billion (preliminary) Significance of parity nonconservation in Møller scattering: 3.6σ 140

44 E158 Results 141

45 Q-Weak Experiment (e-p scattering) A Precision Test of the Standard Model and Determination of the Weak Charges of the Quarks through Parity-Violating Electron Scattering proton weak charge QPW=1-4sin 2 θw elastic e-p scattering at Q 2 =0.03 (GeV/c) 2 employing 180 A of 85% polarized beam on a 35 cm liquid Hydrogen target 142

46 Q-Weak A Precision Test of the Standard Model and Determination of the Weak Charges of the Quarks through Parity-Violating Electron Scattering Elastically Scattered Electron Luminosity Monitors Region III Drift Chambers Toroidal Magnet Region II Drift Chambers Region I GEM Detectors Eight Fused Silica (quartz) Čerenkov Detectors Collimator with 8 openings = 8 ± 2 35cm Liquid Hydrogen Target Polarized Electron Beam 143

47 Q p weak & Qe weak Complementary Diagnostics for New Physics JLab Qweak SLAC E158 (proposed) - Run I + II + III ±0.006 Erler, Kurylov, Ramsey-Musolf, PRD 68, (2003) Qweak measurement will provide a stringent stand alone constraint on Lepto-quark based extensions to the SM. Q p weak (semi-leptonic) and E158 (pure leptonic) together make a powerful program to search for and identify new physics.

48 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