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Lead ( Pb) Radius Experiment : PREX

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1 Lead ( Pb) Radius Experiment : PREX
Results from Lead ( Pb) Radius Experiment : PREX 208 Elastic Scattering Parity Violating Asymmetry E = 1 GeV, electrons on lead 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) 208Pb Ran March – June in Hall A at Jefferson Lab

2 Standard Electroweak Model
Left –handed fermion fields (quarks & leptons) = doublets under SU(2) Right-handed fields = singlets under SU(2) The Glashow-Weinberg-Salam Theory unifies the electromagnetic and weak interactions. Parity Violation p, n decay Weak charge 208 Pb of

3 Parity Transformation
A piece of the weak interaction violates parity (mirror symmetry) which allows to isolate it. Parity Transformation Pb p 1800 rotation Positive spin Negative spin

4 Parity Violating Asymmetry
2 + APV from interference 208Pb 208Pb Applications of APV at Jefferson Lab Nucleon Structure Test of Standard Model of Electroweak Nuclear Structure (neutron density) : PREX Strangeness s s in proton (HAPPEX, G0 expts) e – e (MOLLER) or e – q (PVDIS) elastic e – p at low Q2 (QWEAK) This talk

5 Idea behind PREX Clean Probe Couples Mainly to Neutrons
Z of Weak Interaction : Clean Probe Couples Mainly to Neutrons ( T.W. Donnelly, J. Dubach, I Sick ) In PWIA (to illustrate) : w/ Coulomb distortions (C. J. Horowitz) : 5

6 Hall A at Jefferson Lab Hall A

7 PREX Physics Output Mean Field & Other Atomic Parity Violation Models
Measured Asymmetry Physics Output Correct for Coulomb Distortions Weak Density at one Q 2 Mean Field Small Corrections for n s & Other Atomic Parity Violation G G MEC E E Models 2 Neutron Density at one Q Assume Surface Thickness Good to 25% (MFT) Neutron Stars Slide adapted from C. Horowitz R n

8 Fundamental Nuclear Physics : What is the size of a nucleus ?
Neutrons are thought to determine the size of heavy nuclei like 208Pb. Can theory predict it ?

9 Reminder: Electromagnetic Scattering determines
(charge distribution) 208 Pb 1 2 3

10 Z0 of weak interaction : sees the neutrons
T.W. Donnelly, J. Dubach, I. Sick proton neutron Electric charge 1 Weak charge 0.08 Nucl. Phys. A 503, 589, 1989 C. J. Horowitz, S. J. Pollock, P. A. Souder, R. Michaels Phys. Rev. C 63, , 2001 Neutron form factor C.J. Horowitz Parity Violating Asymmetry 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 PREX (weak interaction) Theory Involve strong probes Most spins couple to zero. MFT fit mostly by data other than neutron densities

12 Isospin Diffusion (NSCL)
Example: Heavy Ions (adapted from Betty Tsang, PREX Workshop, 2008) Isospin Diffusion (NSCL) Probe the symmetry energy in 124Sn + 112Sn

13 Using Parity Violation
Electron - Nucleus Potential electromagnetic axial is small, best observed by parity violation 208 Pb is spin 0 neutron weak charge >> proton weak charge Proton form factor Neutron form factor Parity Violating Asymmetry

14 PREX: Measurement at one Q is sufficient to measure R
2 Measurement at one Q is sufficient to measure R N ( R.J. Furnstahl ) Why only one parameter ? (next slide…) proposed error

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 ratio proton/neutrons n.m. density Slope unconstrained by data Adding R from Pb will significantly reduce the dispersion in plot. 208 N 15

16 Thanks, Alex Brown PREX Workshop 2008 Skx-s15 E/N

17 Thanks, Alex Brown PREX Workshop 2008 Skx-s20 E/N

18 Thanks, Alex Brown PREX Workshop 2008 Skx-s25 E/N 8

19 APV Application: Atomic Parity Violation Low Q test of Standard Model
Needs RN (or APV measures RN ) 2 Isotope Chain Experiments e.g. Berkeley Yb APV Momentum transfer 19

20 Neutron Stars What is the nature of extremely dense matter ?
Application : 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) PREX helps here Hydrostatics (Gen. Rel.)
pressure density Inputs: Eq. of state (EOS) PREX helps here Hydrostatics (Gen. Rel.) Astrophysics Observations Luminosity L Temp. T Mass M from pulsar timing (with corrections … ) Mass - Radius relationship Fig from: Dany Page. J.M. Lattimer & M. Prakash, Science 304 (2004) 536. 21

22 PREX & Neutron Stars C.J. Horowitz, J. Piekarewicz RN calibrates equation of state (pressure vs density) of Neutron Rich Matter Combine PREX RN 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) ? 0.2 fm URCA probable, else not Crab Pulsar

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

24 How to do a Parity Experiment
(integrating method) Flux Integration Technique: HAPPEX: 2 MHz PREX: MHz Example : HAPPEX

25 Polarized Electron Source
Laser GaAs Crystal Pockel Cell flips helicity Gun 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 P I T A Effect Intensity Asymmetry
Important Systematic : (Gordon Cates) Polarization Induced Transport Asymmetry Intensity Asymmetry Laser at Pol. Source where Transport Asymmetry drifts, but slope is ~ stable Feedback on 26

27 Methods to Reduce Systematics
(Gordon Cates, Kent Paschke, Mark Dalton, Rupesh Silwal ) A simplified picture: asymmetry=0 corresponds to minimized DoLP at analyzer Perfect DoCP Intensity Asymmetry (ppm) Pockels cell voltage D offset (V) Scanning the Pockels Cell voltage = scanning the residual linear polarization (DoLP) A rotatable l/2 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 Low jitter and high accuracy allows sub-ppm cumulative charge asymmetry in ~ 1 hour ~ 2 hours 28

29 Flips spin without moving the beam !
Double Wien Filter Crossed E & B fields to rotate the spin Two Wien Spin Manipulators in series Solenoid rotates spin +/-90 degrees (spin rotation as B but focus as B2). Flips spin without moving the beam ! Electron Beam SPIN 29

30 Araw = Adet - AQ + E+ ixi
Beam Asymmetries Araw = Adet - AQ + E+ ixi Slopes from natural beam jitter (regression) beam modulation (dithering) PAVI 09 31

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

32 Compton Polarimeter Upgrade for 1% accuracy at 1 GeV electrons
to measure electron beam’s polarization (needed to normalize asymmetry) electrons 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 Compton Polarimeter Results
PREX Compton Polarimeter Results

34 1 % Accuracy in Polarization
Upgraded for PREX Moller Polarimeter 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 detector Inelastic Pure, Thin Pb Target 2.6 MeV target Dipole DETECTOR footprint Quads Scattered Electron’s Momentum (GeV/c) 35

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

37 Backgrounds that might re-scatter into the detector ?
Detector cutoff 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 PREX Integrating Detectors
Detector Package in HRS PREX Integrating Detectors UMass / Smith DETECTORS

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

41 + - Beam-Normal Asymmetry in elastic electron scattering
i.e. spin transverse to scattering plane y z x + - AT > 0 means Possible systematic if small transverse spin component New results PREX Preliminary ! Publication in preparation Small AT for 208Pb is a big (but pleasant) surprise. AT for 12C qualitatively consistent with 4He and available calculations (1) Afanasev ; (2) Gorchtein & Horowitz 41

42 PREX-I Result Statistics limited ( 9% )
Systematic Errors Error Source Absolute (ppm) Relative ( % ) Polarization (1) 0.0083 1.3 Beam Asymmetries (2) 0.0072 1.1 Detector Linearity 0.0076 1.2 BCM Linearity 0.0010 0.2 Rescattering 0.0001 Transverse Polarization 0.0012 Q2 (1) 0.0028 0.4 Target Thickness 0.0005 0.1 12C Asymmetry (2) 0.0025 Inelastic States TOTAL 0.0140 2.1 Physics Asymmetry Statistics limited ( 9% ) Systematic error goal achieved ! (2%) A physics letter was recently accepted by PRL. arXiv [nucl-ex] (1) Normalization Correction applied (2) Nonzero correction (the rest assumed zero) 42

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

44 * * Asymmetry leads to RN
Establishing a neutron skin at ~95 % CL * Neutron Skin = RN - RP = fm fig from C.J. Horowitz PREX data * Interpretation requires the acceptance function for spectrometer:

45 PREX-I Result, cont. Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm
DATA rN - rP (fm) theory: P. Ring rN = rP Atomic Number, A DATA A physics letter was recently accepted by PRL. arXiv [nucl-ex] 46

46 PREX-II Approved by PAC (Aug 2011)
“A” Rating days to run in 2013 or 2014 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. Three- neutron forces! Steiner et al. X-Ray n-star mass and radii observation + Brown correlation. (Ozel et al finds softer EOS, would suggest smaller Rn). Tamii et al. Measurement of electric dipole polarizability of 208Pb + model correlation with neutron skin. Tsang et al. Isospin diffusion in heavy ion collisions, with Brown correlation and quantum molecular dynamics transport model. PREX-II proposed Hebeler Steiner Tamii Tsang d(APV)/APV ~ 3% d(Rn)/Rn ~ 1% These can be tested with

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

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

50 Possible Future PREX Program ?
Each point 30 days stat. error only Nucleus E (GeV) dRN / RN comment 208Pb 1 1 % PREX-II (approved by Jlab PAC, A rating) 48Ca 2.2 (1-pass) 0.4 % natural 12 GeV exp’t will next PAC 2.6 2 % surface thickness 40Ca 0.6 % basic check of theory tin isotope 1.8 apply to heavy ion 1.6 % 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 or )

53 Extra Slides

54 Geant 4 Radiation Calculations PREX-II shielding strategies
scattering chamber shielding Number of Neutrons per incident Electron MeV beamline Energy (MeV) PREX-I PREX-II, no shield PREX-II, shielded MeV Strategy Tungsten ( W ) plug Shield the W x 10 reduction in 0.2 to 10 MeV neutrons Energy (MeV) MeV Energy (MeV) 49

55 Pull Plot (example) PREX Data

56 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 Isospin Corrections Radiative Corrections Excited States Target Impurities Horowitz, 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 maximize: n 1 month run 1% in R n (2 months x uA  0.5% if no systematics) 5 PAVI 09

58 Source Studies Charge Asymmetry ~ 2000 ppm Kent Paschke, Gordon Cates, Mark Dalton, Rupesh Silwal Optimizing laser optics to minimize helicity-correlated systematics. ~ 0.5 um Hel. Correl. Diff (X) Transmission of Helicity-Correlated Position DIffs Hel. Correl. Diff (X) Hel. Correl. Diff (Y) Delta X (um) ~ 0.5 um BPMs in Injector Region Hel. Correl. Diff (Y) Delta Y (um)

59 16O Water Cell : Measure Hydrogen (agrees with survey)
Nilanga Liyanage, Seamus Riordan, Kiadtisak Saenboonruang, 16O Hydrogen

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 Pb PREX: pins down the symmetry energy (1 parameter) PREX error bar
energy cost for unequal # protons & neutrons PREX error bar ( R.J. Furnstahl ) Actually, it’s the density dependence of a4 that we pin down. 208 Pb PREX

62 Collimators inside Q1 Symmetry in all dimensions to 1 mm

63 Pb Radius vs Neutron Star Radius
(slide from C. Horowitz) 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…

64 PREX Constrains Rapid Direct URCA Cooling of Neutron Stars
(slide from C. Horowitz) 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+

65 Liquid/Solid Transition Density
Neutron Star Crust vs Pb Neutron Skin FP TM1 Solid Liquid C.J. Horowitz, J. Piekarawicz Neutron Star 208Pb 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 observed not observed
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 Weak decay of 60Co Nucleus 60Co 60Ni Left Right W Charge zero V-A theory described W’s as only interacting with left-handed particles! observed 60Co 60Ni L R right-handed anti-neutrino left-handed not observed

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