Lead ( Pb) Radius Experiment : PREX

Transcription

1 Results from 08 Lead ( Pb) Radius Experiment : PREX Elastic Scattering Parity Violating Asymmetry E = 1 GeV, θ = electrons on lead 5 0 Spokespersons Paul Souder, Krishna Kumar Guido Urciuoli, Robert Michaels (speaker) Graduate Students Ahmed Zafar, Chun Min Jen, Abdurahim Rakham (Syracuse) Jon Wexler (UMass) Kiadtisak Saenboonruang (UVa) 08 Pb Ran March June 010 in Hall A at Jefferson Lab

2 Standard Electroweak Model The Glashow-Weinberg-Salam Theory unifies the electromagnetic and weak interactions. Left handed fermion fields (quarks & leptons) = doublets under SU() Right-handed fields = singlets under SU() Parity Violation β decay p, n Weak charge of 08 Pb

3 A piece of the weak interaction violates parity (mirror symmetry) which allows to isolate it. p Pb Pb p Pb Pb Pb p Pb rotation Positive spin Negative spin

4 Parity Violating Asymmetry A PV σ σ σ + σ R L 4 = ~ 10 Q R L ~ 10 6 σ e γ 0 08 Pb + e Z 08 Pb A PV from interference Applications of A PV at Jefferson Lab Nucleon Structure Strangeness s s in proton (HAPPEX, G0 expts) Test of Standard Model of Electroweak e e (MOLLER) or e q (PVDIS) elastic e p at low Q (QWEAK) sin θ W This talk Nuclear Structure (neutron density) : PREX

5 = Ω + Ω Ω Ω = ) ( 4sin 1 Q F Q G d d d d d d d d A P W F L R L R θ πα σ σ σ σ ) ( Q F n 1% % 3 = = n n R dr A da Idea behind PREX Z of Weak Interaction : Clean Probe Couples Mainly to Neutrons ( T.W. Donnelly, J. Dubach, I Sick 1989 ) 0 In PWIA (to illustrate) : w/ Coulomb distortions (C. J. Horowitz) : 0 5

6 Hall A at Jefferson Lab Hall A

7 PREX Physics Output Measured Asymmetry Correct for Coulomb Distortions Weak Density at one Q Atomic Parity Violation Small Corrections for G n E G s E MEC Neutron Density at one Q Mean Field & Other Models Slide adapted from C. Horowitz Assume Surface Thickness Good to 5% (MFT) R n Neutron Stars

8 Fundamental Nuclear Physics : What is the size of a nucleus? Neutrons are thought to determine the size of heavy nuclei like 08 Pb. Can theory predict it?

9 ( ) Reminder: Electromagnetic Scattering determines ρ r 08 Pb (charge distribution) dσ mb dω str ρ ( r) 1 3 q ( fm) 1

10 Z 0 of weak interaction : sees the neutrons proton neutron T.W. Donnelly, J. Dubach, I. Sick Nucl. Phys. A 503, 589, 1989 Electric charge 1 0 C. J. Horowitz, S. J. Pollock, P. A. Souder, R. Michaels Phys. Rev. C 63, 05501, 001 Weak charge Neutron form factor F 1 3 ) = d r j ( qr) ρ N ( r) 4π N ( Q 0 Parity Violating Asymmetry A = G F πα Q 1 4sin 0 θ W F F N P ( Q ( Q ) ) C.J. Horowitz 10

11 How to Measure Neutron Distributions, Symmetry Energy Proton-Nucleus Elastic Pion, alpha, d Scattering Pion Photoproduction Heavy ion collisions Rare Isotopes (dripline) Magnetic scattering Involve strong probes Most spins couple to zero. PREX (weak interaction) Theory MFT fit mostly by data other than neutron densities

12 Example: Heavy Ions (adapted from Betty Tsang, PREX Workshop, 008) Isospin Diffusion (NSCL) Probe the symmetry energy in 14 Sn + 11 Sn

13 ) ( ) ( ) ˆ( 5 r A r V r V γ + = = Ω + Ω Ω Ω = ) ( ) ( 4sin 1 Q F Q F Q G d d d d d d d d A P N W F L R L R θ πα σ σ σ σ neutron weak charge >> proton weak charge is small, best observed by parity violation ) ( ) ( / / / 3 r r r Z r d r V = ρ [ ] ) ( ) ( ) 4sin (1 ) ( r N r Z G r A N P W F ρ ρ θ = ) ( Q F d d d d P Ω Mott = Ω σ σ = ) ( ) ( 4 1 ) ( 0 3 r qr j r d Q F P P ρ π = ) ( ) ( 4 1 ) ( 0 3 r qr j r d Q F N N ρ π Electron - Nucleus Potential electromagnetic axial Neutron form factor Parity Violating Asymmetry ) A ( r 1 4sin 1 << W θ Proton form factor 0 Pb is spin 0 08 Using Parity Violation

14 PREX: Measurement at one Q is sufficient to measure R Skyrme covariant meson covariant point coupling N ( R.J. Furnstahl ) F n (Q )/N Why only one parameter? (next slide ) 0.6 proposed error r n in 08 Pb (fm)

15 Slide adapted from J. Piekarewicz 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/ ) + Sυ ( n) (1 x ) n = n.m. density x = ratio proton/neutrons Slope unconstrained by data 08 Adding R N from Pb will significantly reduce the dispersion in plot. 15

16 Thanks, Alex Brown PREX Workshop 008 Skx-s15 E/N ρ N

17 Thanks, Alex Brown PREX Workshop 008 Skx-s0 E/N ρ N

18 Thanks, Alex Brown PREX Workshop 008 Skx-s5 E/N ρ N 8

19 Application: Atomic Parity Violation Low Q test of Standard Model Needs R N (or APV measures R N ) Isotope Chain Experiments e.g. Berkeley Yb H PNC G F r r / 5 3 [ N ρ ( ) + Z (1 4sin θ ) ρ ( )] ψ γ ψ d r N W P e e 0 APV Momentum transfer 19

20 Application : Neutron Stars What is the nature of extremely dense matter? Do collapsed stars form exotic phases of matter? (strange stars, quark stars) Crab Nebula (X-ray, visible, radio, infrared)

21 Inputs: Eq. of state (EOS) P(ρ) PREX helps here Hydrostatics (Gen. Rel.) Astrophysics Observations Luminosity L Temp. T Mass M from pulsar timing L = 4πσ B R T 4 (with corrections ) Mass - Radius relationship Fig from: Dany Page. J.M. Lattimer & M. Prakash, Science 304 (004)

22 PREX & Neutron Stars C.J. Horowitz, J. Piekarewicz R N calibrates equation of state (pressure vs density) of Neutron Rich Matter Combine PREX R N with Observed Neutron Star Radii Phase Transition to Exotic Core? Strange star? Quark Star? Some Neutron Stars seem too cold Explained by Cooling by neutrino emission (URCA process)? Crab Pulsar R n R p > 0. fm URCA probable, else not

23 PREX Setup Parity: The entire lab is the experiment Spectometers Lead Foil Target Hall A JLAB Pol. Source CEBAF

24 How to do a Parity Experiment (integrating method) Flux Integration Technique: HAPPEX: MHz PREX: 500 MHz Example : HAPPEX

25 Polarized Electron Source GaAs Crystal Gun Pockel Cell flips helicity Laser Halfwave plate (retractable, reverses helicity) e - beam Based on Photoemission from GaAs Crystal Polarized electrons from polarized laser Need : Rapid, random helicity reversal Electrical isolation from the rest of the lab Feedback on Intensity Asymmetry

26 Important Systematic : P I T A Effect (Gordon Cates) Polarization Induced Transport Asymmetry Intensity Asymmetry where ε = z T T x x T + T Transport Asymmetry y y Asymmetric Transport System y' x' θ AI = ε sin(θ ) Polarization Ellipses = 0 L R Pockels Cell (Imperfect λ/4 Plate) Fast/ Slow Axes Initial Linear Polarization y 45 o x Laser at Pol. Source drifts, but slope is ~ stable. Feedback on 6

27 Intensity Asymmetry (ppm) Methods to Reduce Systematics Perfect DoCP Pockels cell voltage offset (V) (Gordon Cates, Kent Paschke, Mark Dalton, Rupesh Silwal ) Scanning the Pockels Cell voltage = scanning the residual linear polarization (DoLP) A simplified picture: asymmetry=0 corresponds to minimized DoLP at analyzer A rotatable λ/ waveplate downstream of the P.C. allows arbitrary orientation of the ellipse from DoLP

28 Intensity Feedback Adjustments for small phase shifts to make close to circular polarization ~ hours Low jitter and high accuracy allows sub-ppm cumulative charge asymmetry in ~ 1 hour 8

29 Two Wien Spin Manipulators in series Solenoid rotates spin +/-90 degrees (spin rotation as B but focus as B ). Flips spin without moving the beam! Electron Beam SPIN Double Wien Filter Crossed E & B fields to rotate the spin 9

30 Beam Asymmetries A raw = A det - A Q + α E + Σβ i x i Slopes from natural beam jitter (regression) beam modulation (dithering) PAVI 09 31

31 Points: Not sign corrected Parity Quality Beam! ( why we love Jlab! ) Helicity Correlated Position Differences Average with signs = what exp t feels < ~ 3 nm < X R X L> for helicity L, R Units: microns Slug # ( ~ 1 day)

32 Compton Polarimeter e γ scattering γ to measure electron beam s polarization (needed to normalize asymmetry) Upgrade for 1% accuracy at 1 GeV Green Laser (increased sensitivity at low E) Integrating Method (removes some systematics of analyzing power) New Photon & Electron Detectors

33 PREX Compton Polarimeter Results

34 Upgraded for PREX Moller Polarimeter e e scattering Superconducting Magnet from Hall C Saturated Iron Foil Targets 1 % Accuracy in Polarization Magnet and Target Electronics/DAQ Upgrade (FADC)

35 Hall A High Resolution Spectrometers Resolve Elastic Scattering Discriminate Excited States Elastic Inelastic detector Pure, Thin 08 Pb Target.6 MeV target Quads Dipole DETECTOR footprint Scattered Electron s Momentum (GeV/c) 35

36 Measure θfrom Nuclear Recoil ( Nilanga Liyanage, Kiadtisak Saenboonruang) δe=energy loss E=Beam energy M A =Nuclear mass θ=scattering angle (these data taken during HAPPEX) δ θ E E E M A Scattered Electron Energy (GeV) Recoil is large for H, small for nuclei (3X better accuracy than survey)

37 Detector cutoff Backgrounds that might re-scatter into the detector? Run magnets down: measure inelastic region Run magnets up : measure probability to rescatter No inelastics observed on top of radiative tail. Small systematic for tail.

38 Detector Package in HRS PREX Integrating Detectors DETECTORS UMass / Smith

39 Lead / Diamond Target Diamond LEAD Three bays Lead (0.5 mm) sandwiched by diamond(0.15 mm) Liquid He cooling (30 Watts)

40 Performance of Lead / Diamond Targets melted melted Last 4 days at 70 ua NOT melted Targets with thin diamond backing (4.5 % background) degraded fastest. Thick diamond (8%) ran well and did not melt at 70 ua. Solution: Run with 10 targets.

41 Beam-Normal Asymmetry in elastic electron scattering i.e. spin transverse to scattering plane A T σ σ σ + σ S e ( k e k' e) Possible systematic if small transverse spin component New results PREX x k r y A T > 0 means S r + - z 08 Pb: AT = ± 0.19 ± ppm 1 C: AT = 6.5 ± 0.36 ± ppm Small A T for 08 Pb is a big (but pleasant) surprise. A T for 1 C qualitatively consistent with 4 He and available calculations (1) Afanasev ; () Gorchtein & Horowitz 41

42 PREX-I Result Systematic Errors Error Source Absolute (ppm) Polarization (1) Beam Asymmetries () Detector Linearity BCM Linearity Rescattering Transverse Polarization Q (1) Target Thickness C Asymmetry () Inelastic States 0 0 TOTAL (1) Normalization Correction applied Relative ( % ) () Nonzero correction (the rest assumed zero) Physics Asymmetry A = ± ppm 0.060( stat) ± 0.014( syst) Statistics limited ( 9% ) Systematic error goal achieved! (%) A physics letter was recently accepted by PRL. arxiv [nucl-ex] 4

43 PREX Asymmetry (P e x A) ppm Slug ~ 1 day

44 Asymmetry leads to R N Establishing a neutron skin at ~95 % CL * Neutron Skin = R N - R P = fm fig from C.J. Horowitz PREX data * Interpretation requires the acceptance function for spectrometer: ) ε (θ

45 PREX-I Result, cont. A = ppm ± 0.060( stat) ± 0.014( syst) r N - r P (fm) Neutron Skin = R N - R P = fm DATA theory: P. Ring DATA r N = r P Atomic Number, A A physics letter was recently accepted by PRL. arxiv [nucl-ex] 46

46 PREX-II Approved by PAC (Aug 011) A Rating 35 days to run in 013 or 014 PREX-II: Kent Paschke, Krishna Kumar, Paul Souder, Guido Urciuoli, Robert Michaels

47 Recent Rn Predictions Can Be Tested By PREX at Full Precision PREX could provide an electroweak complement to Rn predictions from a wide range of physical situations and model dependencies Recent Rn predictions: Hebeler et al. Chiral EFT calculation of neutron matter. Correlation of pressure with neutron skin by Brown. Threeneutron forces! Hebeler Steiner Tamii Tsang These can be tested with δ(a PV )/A PV ~ 3% δ(r n )/R n ~ 1% Steiner et al. X-Ray n-star mass and radii observation + Brown correlation. (Ozel et alfinds softer EOS, would suggest smaller Rn). Tamii et al. Measurement of electric dipole polarizability of 08 Pb + model correlation with neutron skin. Tsang et al. Isospin diffusion in heavy ion collisions, with Brown correlation and quantum molecular dynamics transport model.

48 Improvements for PREX-II Tungsten Collimator & Shielding Septum Magnet Region downstream of target HRS-L Q1 target HRS-R Q1 Location of ill-fated O-Ring which failed & caused significant time loss during PREX-I Collimators PREX-II to use all-metal seals

49 After PREX R N Other Nuclei? Surface thickness and Shape Dependence? each point 30 days Parity Violating Electron Scattering Measurements of Neutron Densities Shufang Ban, C.J. Horowitz, R. Michaels J. Phys. G R N Surface thickness

50 Possible Future PREX Program? Each point 30 days stat. error only Nucleus E (GeV) dr N / R N comment 08 Pb 1 1 % PREX-II (approved by Jlab PAC, A rating) 48 Ca. (1-pass) 0.4 % natural 1 GeV exp t will next PAC 48 Ca.6 % surface thickness 40 Ca. (1-pass) 0.6 % basic check of theory tin isotope % apply to heavy ion tin isotope % surface thickness Not proposed Shufang Ban, C.J. Horowitz, R. Michaels J. Phys. G

51 UVa Participants in Jlab Parity-Violation & PREX Gordon Cates Kent Paschke Nilanga Liyanage Xiaochao Zheng PREX-II spokesperson Mark Dalton Diancheng Wang Rupesh Silwal Kiadtisak Saenboonruang Thesis on PREX-I Also: Chao Gu, Xiaoyan Deng, Ge Jin, Richard Lindgren, Vladimir Nelyubin, Seamus Riordan, Ramesh Subedi, Al Tobias

52 PREX : Summary Fundamental Nuclear Physics with many applications PREX-I achieved a 9% stat. error in Asymmetry (original goal : 3 %) Systematic Error Goals Achieved!! Significant time-losses due to O-Ring problem and radiation damage PREX-II approved (runs in 013 or 014 )

53 Extra Slides

54 scattering chamber Geant 4 Radiation Calculations PREX-II shielding strategies shielding Number of Neutrons per incident Electron 0-1 MeV beamline Strategy Tungsten ( W ) plug < < θ Shield the W MeV MeV Energy (MeV) --- PREX-I --- PREX-II, no shield --- PREX-II, shielded Energy (MeV) x 10 reduction in 0. to 10 MeV neutrons Energy (MeV) 49

55 Pull Plot (example) PREX Data ( A A ) / σ

56 Corrections to the Asymmetry are Mostly Negligible Coulomb Distortions ~0% = the biggest correction. Transverse Asymmetry (to be measured) Strangeness Electric Form Factor of Neutron Parity Admixtures Dispersion Corrections Meson Exchange Currents Shape Dependence Isospin Corrections Radiative Corrections Excited States Target Impurities Horowitz, et.al. PRC

57 Optimum Kinematics for Lead Parity: E = 1 GeV if <A> = 0.5 ppm. Accuracy in Asy 3% Fig. of merit Min. error in R n maximize: 1 month run 1% in R n PAVI 09 ( months x 100 ua 0.5% if no systematics)

58 ~ 000 ppm ~ 0.5 um Charge Asymmetry θ of Hel. Correl. Diff (X) waveplate Source Studies Kent Paschke, Gordon Cates, Mark Dalton, Rupesh Silwal Optimizing laser optics to minimize helicitycorrelated systematics. ~ 0.5 um Hel. Correl. Diff (Y) Delta X (um) Transmission of Helicity-Correlated Position DIffs Hel. Correl. Diff (X) Delta Y (um) BPMs in Injector Region Hel. Correl. Diff (Y)

59 Water Cell : Measure < θ > Nilanga Liyanage, Seamus Riordan, Kiadtisak Saenboonruang, (agrees with survey) Hydroge n 16 O

60 Pockel Cell Related Systematic Error wait integrate wait An instability in Pockel Cell bleeds into the itegration gate. It depends on helicity. Beam Current Detector (1 of 4) Response to pulsed beam time time Want small time constants, and same for detectors and bcm

61 PREX: E A pins down the symmetry energy (1 parameter) a v N Z 1/ 3 + a4 + as / A +... A energy cost for unequal # protons & neutrons ( R.J. Furnstahl ) PREX error bar ( 1σ ) 08 Pb Actually, it s the density dependence of a 4 that we pin down. PREX

62 Collimators inside Q1 Symmetry in all dimensions to 1 mm

63 (slide from C. Horowitz) Pb Radius vs Neutron Star Radius The 08 Pb 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 phasesuch as quark matter, strange matter, color superconductor, kaon condensate

64 (slide from C. Horowitz) PREX Constrains Rapid Direct URCA Cooling of Neutron Stars Proton fraction Y p for matter in beta equilibrium depends on symmetry energy S(n). R n in Pb determines density dependence of S(n). The larger R n in Pb the lower the threshold mass for direct URCA cooling. If R n -R p <0. fm all EOS models do not have direct URCA in 1.4 M stars. If R n -R p >0.5 fm all models do have URCA in 1.4 M stars. R n -R p in 08 Pb If Y p > red line NS cools quickly via direct URCA reaction n p+e+ν

65 Neutron Star Crust vs Pb Neutron Skin C.J. Horowitz, J. Piekarawicz FP Liquid/Solid Transition Density Liquid Neutron Star 08 Pb 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.

66 Weak Interaction 1930 s - The weak nuclear interaction was needed to explain nuclear beta decay Contact interaction with charge exchanged or, mediated by a heavy, charged boson 1950 s - Discovery of parity-violation by the weak interaction 60 Ni 60 Co Weak decay of 60 Co Nucleus V-A theory described W s as only interacting with left-handed particles! W Charge Left T = ± 1 Right zero 60 Co 60 Ni 60 Co 60 Ni observed L R right-handed handed anti-neutrino neutrino not observed R L left-handed anti-neutrino neutrino