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 PAVI06 May 2006 dRn 1% Rn 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 PAVI06 May 2006 R. Michaels Jefferson Lab Reminder: Electromagnetic Scattering determines r (charge distribution) 208 Pb r d m b d str 1 2 3 q fm 1 PREX PAVI06 May 2006 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 PREX PAVI06 proton neutron Electric charge 1 0 Weak charge 0.08 1 May 2006 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 PAVI06 May 2006 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 PAVI06 May 2006 FN (Q 2 ) 2 1 4 sin W 2 F ( Q ) P 0 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 ( 1 ) PREX PAVI06 May 2006 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 ( 1 ) 208 Pb PREX PREX PAVI06 May 2006 R. Michaels Jefferson Lab Nuclear Structure: Neutron density is a fundamental observable that remains elusive. Reflects poor understanding of symmetry energy of nuclear matter = the energy cost of N Z E(n, x) E(n, x 1/ 2) S (n) (1 2 x 2 ) n n.m. density x ratio proton/neutrons • Slope unconstrained by data 208 • Adding R N from Pb will eliminate the dispersion in plot. PREX PAVI06 May 2006 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 PAVI06 May 2006 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 PAVI06 May 2006 Rn Rp 0.2 fm URCA probable, else not R. Michaels Jefferson Lab Neutron EOS and Neutron Star Crust Liquid/Solid Transition Density Liquid FP Solid Fig. from PREX PAVI06 J.M. Lattimer & M. Prakash, Science 304 (2004) 536. May 2006 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 ( C.J. Horowitz, J. Piekarweicz ) • 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 PAVI06 May 2006 R. Michaels Jefferson Lab PREX Constrains Rapid Direct URCA Cooling of Neutron Stars ( C.J. Horowitz, J. Piekarweicz ) • 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 PAVI06 May 2006 R. Michaels Jefferson Lab Impact on Atomic Parity Measures atomic overlap with weak charge. Neutrons carry most weak charge PREX PAVI06 May 2006 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 PAVI06 May 2006 R. Michaels Jefferson Lab PREX Physics Impact Measured Asymmetry 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) Heavy Ions Neutron Stars Rn PREX PAVI06 May 2006 R. Michaels Jefferson Lab Corrections to the Asymmetry are Mostly Negligible • Coulomb Distortions ~20% = the biggest correction. • Transverse Asymmetry (to be measured) • Strangeness • Electric Form Factor of Neutron • Parity Admixtures • Dispersion Corrections • Meson Exchange Currents • Shape Dependence Horowitz, et.al. PRC 63 025501 • Isospin Corrections • Radiative Corrections • Excited States • Target Impurities PREX PAVI06 May 2006 R. Michaels Jefferson Lab PREX: Experimental Issues Spokespersons: P.A. Souder, G.M. Urciuoli, R. Michaels Hall A Collaboration Experiment PREX PAVI06 May 2006 R. Michaels Jefferson Lab PREX in Hall A at JLab Spectometers Lead Foil Target Pol. Source Hall A CEBAF PREX PAVI06 May 2006 R. Michaels Jefferson Lab Hall A at Jefferson Lab Polarized eSource PREX PAVI06 May 2006 Hall A R. Michaels Jefferson Lab High Resolution Spectrometers Spectrometer Concept: Resolve Elastic Elastic detector Inelastic Left-Right symmetry to control transverse polarization systematic Quad target Dipole PREX PAVI06 Q Q May 2006 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 ( PMT May 2006 HRS). Integrator Calorimeter (for lead, fits in palm of hand) PREX PAVI06 nonlinearity. ADC 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 PAVI06 May 2006 R. Michaels Jefferson Lab Optimization for Barium -- of possible direct use for Atomic PV 1 GeV optimum PREX PAVI06 May 2006 R. Michaels Jefferson Lab Beam Asymmetries Araw = Adet - AQ + E+ ixi Slopes from PREX PAVI06 May 2006 •natural beam jitter (regression) •beam modulation (dithering) R. Michaels Jefferson Lab Helicity Correlated Differences: Position, Angle, Energy Scale +/- 10 nm BPM X1 slug Spectacular results from HAPPEX-H show we can do BPM X2 PREX. Position Diffs average to ~ 1 nm • Good model for controlling laser systematics at source • Accelerator setup (betatron matching, phase advance) PREX PAVI06 May 2006 slug BPM Y1 slug BPM Y2 slug “Energy” BPM “slug” = ~1 day running R. Michaels Jefferson Lab Redundant Position Measurements at the ~1 nm level Y (cavity) X (cavity) nm nm (Helicity – correlated differences averaged over ~1 day) X (stripline) PREX PAVI06 May 2006 nm Y (stripline) nm 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 PAVI06 May 2006 R. Michaels Jefferson Lab Num. events 208 Pb Elastic Detector 1st Excited State (2.6 MeV) Momentum (MeV) Data taken Nov 2005 • Check rates • Backgrounds (HRS is clean) • Sensitivity to beam parameters • Width of asymmetry A 1 Num. events PREX PAVI06 Lead Target Tests May 2006 • HRS resolution • Detector resolution I R. Michaels Jefferson Lab Polarimetry Møller : dPe/Pe ~ 3 % (limit: foil polarization) (a high field target ala Hall C being considered) Compton 2 analyses based on either electron or photon detection e : 2% syst. at present PREX: 1 % desirable 2 % required Electron only Photon only Superlattice: Pe=86% ! PREX PAVI06 May 2006 Preliminary: 2.5% syst ( only) R. Michaels Jefferson Lab Upgrade of Compton Polarimeter in ~ 1.5 years (Nanda, Lhuillier) To reach 1% accuracy: • Green Laser (increased sensitivity at low E) laser on-hand, being tested • Integrating Method (removes some systematics of analyzing power) developed during HAPPEX & in 2006 • New Photon Detector PREX PAVI06 May 2006 R. Michaels Jefferson Lab PREX : Summary • Fundamental Nuclear Physics with many applications • HAPPEX & test runs have demonstrated technical aspects • Polarimetry Upgrade needed • Will run 1 month, perhaps in 2008 PREX PAVI06 May 2006 R. Michaels Jefferson Lab Neutron Skin and Heavy – Ion Collisions • Impact on Heavy - Ion physics: constraints and predictions • Imprint of the EOS left in the flow and fragmentation distribution. Danielewicz, Lacey, and Lynch, Science 298 (2002) 1592. PREX PAVI06 May 2006 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 d RN 0.05 fm PREX PAVI06 May 2006 R. Michaels Jefferson Lab Transverse Polarization Part I: Left/Right Asymmetry Transverse Asymmetry AT AT0 PT sin Systematic Error for Parity Theory est. (Afanasev) A 5 1 ppm 0 T Transverse polarization PT P sin 3o HRS-Left P d A d AT0 PT “Error in” Left-right apparatus asymmetry Control w/ slow feedback on polarized source solenoids. d AT0 1 ppm HRS-Right Need PT << 103 103 correction PREX PAVI06 May 2006 measure in ~ 1 hr (+ 8 hr setup) syst. err. R. Michaels Jefferson Lab Transverse Polarization Part II: Up/Down Asymmetry Vertical misalignment cos 0 Systematic Error for Parity d A d AT0 PT cos Horizontal polarization e.g. from (g-2) • Measured in situ using 2-piece detector. PT P sin up/down misalignment HRS-Left P PREX PAVI06 • Study alignment with tracking & M.C. • Wien angle feedback ( Need HRS-Right PT cos << 103 103 ( Note, beam width is very tiny May 2006 ) ~ 100 m ) R. Michaels Jefferson Lab Noise • Need 100 ppm per window pair • Position noise already good enough • New 18-bit ADCs Will improve BCM noise. • Careful about cable runs, PMTs, grounds. Will improve detector noise. • Plan: Tests with Luminosity Monitor to demonstrate capability. PREX PAVI06 May 2006 R. Michaels Jefferson Lab Warm Septum Existing superconducting septum won’t work at high L Warm low energy (1 GeV) magnet designed. Grant proposal in preparation (~100 k$) [Syracuse / Smith College] TOSCA design P resolution ok PREX PAVI06 May 2006 R. Michaels Jefferson Lab 2 Measurement at one Q is sufficient to measure R N Pins down the symmetry energy (1 parameter) PREX accuracy PREX accuracy ( R.J. Furnstahl ) PREX PAVI06 May 2006 R. Michaels Jefferson Lab