Transcript Slide 1
Lead ( 208 Pb) Radius Experiment : E = 850 MeV, 6 0 electrons on lead PREX Elastic Scattering Parity Violating Asymmetry 0 Z of Weak Interaction : Clean Probe Couples Mainly to Neutrons ( T.W. Donnelly, J. Dubach, I Sick ) In PWIA (to illustrate) : d d GF Q 2 d d R L A 2 2 d d d R d L F n (Q2 ) 2 1 4 sin W FP (Q 2 ) 0 208Pb w/ Coulomb distortions (C. J. Horowitz) : dA 3% A PREX UVa Seminar, Nov 2005 dRn 1% Rn R. Michaels Jefferson Lab A piece of the weak interaction violates parity (mirror symmetry) which allows to isolate it. Incident electron Positive longitudinal spin S (spin) Target P (momentum) 208 Pb Negative longitudinal spin PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Parity Violating Asymmetry APV R L ~ 106 R L 2 e + Z0 e Applications of PV : • Nucleon Structure (strangeness) -- HAPPEX / G0 • Standard Model Tests ( sin2 W ) -- e.g. Qweak • Nuclear Structure (neutron density) : PREX PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Measured Asymmetry PREX Physics Impact Correct for Coulomb Distortions Weak Density at one Q 2 Mean Field & Other Models Small Corrections for Atomic Parity Violation G n E s GE MEC 2 Neutron Density at one Q Assume Surface Thickness Good to 25% (MFT) Neutron Stars Rn PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab PREX in Hall A at JLab Spectometers Lead Foil Target Pol. Source Hall A CEBAF PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Impact on Nuclear Physics: What is the size of a nucleus ? Is the size of a heavy nucleus determined by neutrons or by protons ? PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Reminder: Electromagnetic Scattering determines r (charge distribution) 208 Pb r d m b d str q fm 1 PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab 0 Z of weak interaction : sees the neutrons Analysis is clean, like electromagnetic scattering: 1. Probes the entire nuclear volume 2. Perturbation theory applies proton neutron Electric charge 1 0 Weak charge 0.08 1 PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Electron - Nucleus Potential Vˆ (r ) V (r ) 5 A(r ) axial electromagnetic / V (r ) d r Z (r ) | r r | 3 / / A(r ) FP (Q 2 ) 1 4 3 d r j0 (qr ) P (r ) 2 2 (1 4 sin 2 W ) Z P ( r ) N N ( r ) A(r ) is small, best observed by parity violation d d | FP (Q 2 ) | 2 d d Mott Proton form factor GF 1 4 sin 2 W 1 neutron weak charge >> proton weak charge Neutron form factor FN (Q 2 ) 1 4 3 d r j0 (qr ) N (r ) Parity Violating Asymmetry d d GF Q 2 d R d L A 2 2 d d d R d L PREX UVa Seminar, Nov 2005 FN (Q 2 ) 2 1 4 sin W 2 F ( Q ) P 0 R. Michaels Jefferson Lab Neutron Densities • Proton-Nucleus Elastic Involve strong probes • Pion, alpha, d Scattering • Pion Photoproduction Most spins couple to zero. • Magnetic scattering • Theory Predictions Fit mostly by data other than neutron densities Therefore, PREX is a powerful check of nuclear theory. PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Example : Recent Pion Photoproduction B. Krusche arXiv:nucl-ex/0509003 Sept 2005 This paper obtains RN RP !! Proton – Nucleus Elastic: 0.083 RN RP 0.111 fm Mean Field Theory 0.05 RN RP 0.35 fm PREX accuracy RN 0.05 fm PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab PREX: 2 Measurement at one Q is sufficient to measure R N ( R.J. Furnstahl ) Why only one parameter ? (next slide…) PREX error bar PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab PREX: pins down the symmetry energy E N Z av a 4 A A ( R.J. Furnstahl ) 2 as / A 1/ 3 ... (1 parameter) energy cost for unequal # protons & neutrons PREX error bar 208 Pb PREX PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Impact on Atomic Parity Measures atomic overlap with weak charge. Neutrons carry most weak charge PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Atomic Parity Violation 2 • Low Q test of Standard Model Isotope Chain Experiments e.g. Berkeley Yb • Needs R N to make further progress. H PNC GF 2 2 N N (r ) Z (1 4 sin 2 W ) P (r ) e/ 5 e d 3 r 0 APV PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Impact on Neutron Stars What is the nature of extremely dense matter ? Do collapsed stars form “exotic” phases of matter ? PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Inputs: Eq. of state (EOS) P( ) PREX constraint Hydrostatics (Gen. Rel.) Typ. Astro. Observations Luminosity L Temp. T Mass M from pulsar timing L 4 B R 2 T 4 (with corrections … ) Mass - Radius relationship Fig. from J.M. Lattimer & M. Prakash, Science 304 (2004) 536. PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab PREX & Neutron Stars ( C.J. Horowitz, J. Piekarweicz ) R N calibrates EOS of Neutron Rich Matter Crust Thickness Explain Glitches in Pulsar Frequency ? Combine PREX R N with Obs. Neutron Star Radii Phase Transition to “Exotic” Core ? Strange star ? Quark Star ? Some Neutron Stars seem too Cold Cooling by neutrino emission (URCA) Crab Pulsar PREX UVa Seminar, Nov 2005 Rn Rp 0.2 fm URCA probable, else not R. Michaels Jefferson Lab Neutron Star Crust vs Pb Neutron Skin Liquid/Solid Transition Density Liquid FP Neutron Star 208Pb PREX calibrates the EOS at subnuclear densities. PREX UVa Seminar, Nov 2005 Solid TM1 • Thicker neutron skin in Pb means energy rises rapidly with density Quickly favors uniform phase. • Thick skin in Pb low transition density in star. R. Michaels Jefferson Lab Pb Radius vs Neutron Star Radius • The 208Pb radius constrains the pressure of neutron matter at subnuclear densities. • The NS radius depends on the pressure at nuclear density and above. • Most interested in density dependence of equation of state (EOS) from a possible phase transition. • Important to have both low density and high density measurements to constrain density dependence of EOS. – If Pb radius is relatively large: EOS at low density is stiff with high P. If NS radius is small than high density EOS soft. – This softening of EOS with density could strongly suggest a transition to an exotic high density phase such as quark matter, strange matter, color superconductor, kaon condensate… PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab PREX Constrains Rapid Direct URCA Cooling of Neutron Stars • Proton fraction Yp for matter in beta equilibrium depends on symmetry energy S(n). • Rn in Pb determines density dependence of S(n). • The larger Rn in Pb the lower the threshold mass for direct URCA cooling. • If Rn-Rp<0.2 fm all EOS models do not have direct URCA in 1.4 M¯ stars. • If Rn-Rp>0.25 fm all models do have URCA in 1.4 M¯ stars. Rn-Rp in 208Pb If Yp > red line NS cools quickly via direct URCA reaction n p+e+ PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab PREX: Experiment Design Spokespersons: P.A. Souder, G.M. Urciuoli, R. Michaels Hall A Collaboration Experiment PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Hall A at Jefferson Lab Polarized eSource PREX UVa Seminar, Nov 2005 Hall A R. Michaels Jefferson Lab Hall A Cherenkov cones PMT Polarimeters Compton Moller Target PREX UVa Seminar, Nov 2005 Spectro: SQQDQ R. Michaels Jefferson Lab High Resolution Spectrometers Spectrometer Concept: Resolve Elastic Elastic detector Inelastic Quad target Dipole PREX UVa Seminar, Nov 2005 Q Q R. Michaels Jefferson Lab Optimum Kinematics for Lead Parity: <A> = 0.5 ppm. E = 850 MeV, Accuracy in Asy 3% Fig. of merit Min. error in Rn maximize: 1 month run 1% in R n PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Corrections to the Asymmetry are Mostly Negligible Horowitz, et.al. PRC 63 025501 • Coulomb Distortions ~20% = the biggest correction. • Strangeness • Electric Form Factor of Neutron • Parity Admixtures • Dispersion Corrections • Meson Exchange Currents • Shape Dependence • Isospin Corrections • Radiative Corrections • Excited States • Target Impurities PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Septum Magnets (INFN) •Superconducting magnets •Commissioned 2003-4 Electrons scattered at 6 deg sent to the HRS at 12.5 deg. PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Integrating Detection • Integrate in 30 msec helicity period. • Deadtime free. • 18 bit ADC with < 10 • -4 But must separate backgrounds & inelastics ( HRS). Integrator Calorimeter (for lead, fits in palm of hand) PMT PREX UVa Seminar, Nov 2005 nonlinearity. ADC R. Michaels Jefferson Lab Lead Target 208 Pb Successful ly tested at 80 A Liquid Helium Coolant 12 beam C Diamond Backing: • High Thermal Conductivity • Negligible Systematics PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Polarized Electron Source Laser GaAs Crystal Gun Pockel Cell Halfwave plate flips helicity (retractable, reverses helicity) e - beam • Rapid, random helicity reversal • Electrical isolation from rest of lab • Feedback on Intensity Asymmetry PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab PITA Effect at Polarized Source Polarization Induced Transport Asymmetry Intensity Asymmetry AI sin( ) Tx Ty where Tx Ty (G. D. Cates) Laser at Pol. Source Transport Asymmetry drifts, but slope is ~stable. Feedback on PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab Beam Asymmetries Araw = Adet - AQ + E+ ixi Slopes from PREX UVa Seminar, Nov 2005 •natural beam jitter (regression) •beam modulation (dithering) R. Michaels Jefferson Lab Helicity Correlated Differences: Position, Angle, Energy Scale +/- 10 nm BPM X1 slug Position Diffs avg ~ 1 nm Redundant Monitors • Stripline Monitors • Resonant Cavities BPM X2 slug BPM Y1 slug BPM Y2 slug Negligible Systematic Error PREX UVa Seminar, Nov 2005 “Energy” BPM “slug” = ~1 day running R. Michaels Jefferson Lab Polarimetry Møller : Pe/Pe ~ 3 % Compton 2 analyses based on either electron or photon detection e (limit: foil polarization) : 2% syst. at present PREX: 1 % desirable 2 % required Electron only Photon only Superlattice: Pe=86% ! PREX UVa Seminar, Nov 2005 Preliminary: 2.5% syst ( only) R. Michaels Jefferson Lab Upgrade of Compton Polarimeter (Nanda, Lhuillier) To reach 1% accuracy: • Green Laser (increased sensitivity at low E) • Integrating Method PREX UVa Seminar, Nov 2005 (removes some systematics of analyzing power) R. Michaels Jefferson Lab Moller Polarimetry with Atomic Hydrogen Target ( E. Chudakov, V. Luppov, D. Crabb) H atoms Ultra Cold Traps • Polarization ~ 100% • Density 3 1015 cm3 Solenoid 8T • Lifetime > 10 min Polarimetry beam Trap • 1% stat. err. in 30 min at 30 A • Low background • High beam currents allowed (100 A) • Goal: ~ 0.5 % systematic error PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab PREX : Summary • Fundamental Nuclear Physics • HAPPEX to demonstrate most technical aspects • Polarimetry Upgrade needed • PREX test run Nov 2005 (this weekend !) • Experiment Runs in 2007 ? PREX UVa Seminar, Nov 2005 R. Michaels Jefferson Lab