Electron Scattering at Jefferson Lab and The Lead

Electron Scattering at Jefferson Lab and The Lead Radius Experiment PREX Robert Michaels Jefferson Lab. (JLab) R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Hall A at Jefferson Lab Hall A R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 B C

Jefferson Lab At-A-Glance • Created to build and operate the Continuous Electron Beam Accelerator Facility (CEBAF), world-unique user facility for Nuclear Physics : – Mission is to gain a deeper understanding of the structure of matter • Through advances in fundamental research in nuclear physics – In operation since 1995 – 1, 356 Active Users – 172 Completed Experiments to date; 4 remaining in 6 Ge. V program – US Ph. Ds in Nuclear Physics : 419 Ph. Ds granted, 204 more in progress • Managed for DOE by Jefferson Science Associates, LLC (JSA) • Human Capital: – 763 full-time employees – 23 Joint faculty, 25 Post docs • Site is 169 Acres, operating cost $135 M R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Electron Scattering Known interaction (electromagnetic or weak) at Jefferson Lab Structure of the Nuclear Building Blocks How does the NN Force arise from the underlying quark and gluon structure of hadronic matter ? Understanding of confinement Nucleon or Nucleus to probe The Structure of Nuclei What is the structure of nuclear matter ? At what distance and energy scale does the underlying quark and gluon structure of nuclear matter become evident? Beyond the Standard Model (Electroweak Theory) High-precision frontier, complementary to high-energy. R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 4/53

Remainder of this talk: Lead ( 208 Pb) Radius Experiment : PREX Elastic Scattering Parity Violating Asymmetry E = 1 Ge. V, electrons on lead Spokespersons Paul Souder, Krishna Kumar Guido Urciuoli, Robert Michaels (speaker) Graduate Students Ahmed Zafar, Chun Min Jen, Abdurahim Rakham (Syracuse) 208 Pb Jon Wexler (UMass) Kiadtisak Saenboonruang (UVa) Ran March – June 2010 in Hall A at Jefferson Lab R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Standard Electroweak Model The Glashow-Weinberg-Salam Theory unifies the electromagnetic and weak interactions. Left –handed fermion fields (quarks & leptons) = doublets under SU(2) Right-handed fields decay = singlets under SU(2) p, n Weak charge of R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Parity Violation 208 Pb

A piece of the weak interaction violates parity (mirror symmetry) which allows to isolate it. p. Pb Pb Pb p p Pb 1800 rotation Positive spin R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Negative spin

Parity Violating Asymmetry 2 + 208 Pb APV from interference 208 Pb Applications of APV at Jefferson Lab • Nucleon Structure Strangeness s s in proton (HAPPEX, G 0 expts) • Test of Standard Model of Electroweak e – e (MOLLER) or e – q (PVDIS) elastic e – p at low Q 2 (QWEAK) This talk R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 • Nuclear Structure (neutron density) : PREX 8/53

Idea behind Z 0 PREX of Weak Interaction : Clean Probe Couples Mainly to Neutrons ( T. W. Donnelly, J. Dubach, I Sick 1989 ) In PWIA (to illustrate) : w/ Coulomb distortions (C. J. Horowitz) : R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

PREX Physics Output 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) Slide adapted from C. Horowitz R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Neutron Stars Rn

Fundamental Nuclear Physics : What is the size of a nucleus ? Neutrons are thought to determine the size of heavy nuclei like 208 Pb. Can theory predict it ? R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Reminder: Electromagnetic Scattering determines (charge distribution) 208 1 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Pb 2 3 12/53

Z 0 of weak interaction : sees the neutrons proton neutron Electric charge 1 0 Weak charge 0. 08 1 Neutron form factor T. W. Donnelly, J. Dubach, I. Sick Nucl. Phys. A 503, 589, 1989 C. J. Horowitz, S. J. Pollock, P. A. Souder, R. Michaels Phys. Rev. C 63, 025501, 2001 C. J. Horowitz Parity Violating Asymmetry R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 10

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 • Theory R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Involve strong probes Most spins couple to zero. (weak interaction) MFT fit mostly by data other than neutron densities

Using Parity Violation Electron - Nucleus Potential electromagnetic 208 Pb is spin 0 axial is small, best observed by parity violation neutron weak charge >> proton weak charge Proton form factor Parity Violating Asymmetry R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Neutron form factor

PREX: 2 Measurement at one Q is sufficient to measure R N ( R. J. Furnstahl ) Why only one parameter ? (next slide…) proposed error R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 16/53

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. m. density ratio proton/neutrons • Slope unconstrained by data 208 • Adding R N from Pb will significantly reduce the dispersion in plot. R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 15

Thanks, Alex Brown PREX Workshop 2008 E/N R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Skx-s 15

Thanks, Alex Brown PREX Workshop 2008 E/N R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Skx-s 20

Thanks, Alex Brown PREX Workshop 2008 E/N R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Skx-s 25

Application: Atomic Parity Violation • Low Q 2 test of Standard Model • Needs RN (or APV measures RN ) Isotope Chain Experiments e. g. Berkeley Yb APV R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Momentum transfer 21/53

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 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 (X-ray, visible, radio, infrared)

re ity su de ns es pr 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. R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

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 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Crab Pulsar URCA probable, else not

PREX Setup Parity: “The entire lab is the experiment” Spectometers Lead Foil Target Hall A JLAB R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Pol. Source CEBAF 25/53

How to do a Parity Experiment (integrating method) Flux Integration Technique: HAPPEX: 2 MHz PREX: 500 MHz R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Example : HAPPEX

Polarized Electron Source Ga. As Crystal Gun Laser Pockel Cell flips helicity Halfwave plate (retractable, reverses helicity) e - beam • Based on Photoemission from Ga. As Crystal • Polarized electrons from polarized laser • Need : • Rapid, random helicity reversal • Electrical isolation from the rest of the lab • Feedback on Intensity Asymmetry R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Important Systematic : P I T A Effect Polarization Induced Transport Asymmetry Intensity Asymmetry Laser at Pol. Source where Transport Asymmetry drifts, but slope is ~ stable. Feedback on R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 28/53

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 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Methods to Reduce Systematics Intensity Asymmetry (ppm) Perfect Do. CP Scanning the Pockels Cell voltage = scanning the residual linear polarization (Do. LP) Pockels cell voltage D offset (V) A rotatable l/2 waveplate downstream of the P. C. allows arbitrary orientation of the ellipse from Do. LP R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 A simplified picture: asymmetry=0 corresponds to minimized Do. LP at analyzer

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 B 2). Flips spin without moving the beam ! Electron Beam SPIN R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 31/53

Beam Asymmetries Araw = Adet - AQ + E+ i xi Slopes from R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 • natural beam jitter (regression) • beam modulation (dithering) PAVI 09 31

Parity Quality Beam ! Points: Not sign corrected ( why we love Jlab ! ) Helicity – Correlated Position Differences Average with signs = what exp’t feels < ~ 3 nm Units: microns R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Slug # ( ~ 1 day)

Compton Polarimeter to measure electron beam’s polarization (needed to normalize asymmetry) ns tro c e l e Upgrade for 1% accuracy at 1 Ge. V • Green Laser (increased sensitivity at low E) • Integrating Method • New Photon R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 (removes some systematics of analyzing power) & Electron Detectors 34/53

PREX R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Compton Polarimeter Results

Upgraded for PREX Moller Polarimeter Superconducting Magnet from Hall C Saturated Iron Foil Targets 1 % Accuracy in Polarization Magnet and Target R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Electronics/DAQ Upgrade (FADC)

Hall A High Resolution Spectrometers • Resolve Elastic Scattering • Discriminate Excited States Elastic Inelastic detector Pure, Thin 208 Pb Target 2. 6 Me. V target Dipole DETECTOR footprint Quads Scattered Electron’s R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Momentum (Ge. V/c)

Measure θ from Nuclear Recoil δE=Energy loss E=Beam energy MA=Nuclear mass θ=Scattering angle (these data taken during HAPPEX) Scattered Electron Energy (Ge. V) Recoil is large for H, small for nuclei (3 X better accuracy than survey) R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 38/53

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. R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Detector Package in HRS PREX Integrating Detectors UMass / Smith DETECTORS R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Lead / Diamond Target Diamond • Three bays • Lead (0. 5 mm) sandwiched by diamond (0. 15 mm) R. Michaels, Jlab • Seminar Liquid He @ TAMU Mar 6, 2012 cooling (30 Watts) LEAD

Performance of Lead / Diamond Targets melted NOT melted Last 4 days at 70 u. A R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 melted Targets with thin diamond backing (4. 5 % background) degraded fastest. Thick diamond (8%) ran well and did not melt at 70 u. A. Solution: Run with 10 targets.

Beam-Normal Asymmetry in elastic electron scattering i. e. spin transverse to scattering plane y Possible systematic if small transverse spin component AT > 0 means + x - z New results PREX ry P at c bli Pu n tio ra a in prepa limn in e r io ! • Small AT for • AT for R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 208 Pb is a big (but pleasant) surprise. 12 C qualitatively consistent with 4 He and available calculations (1) Afanasev ; (2) Gorchtein & Horowitz 43/53

PREX-I Result 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 0 Transverse Polarization 0. 0012 0. 2 Q 2 (1) 0. 0028 0. 4 Target Thickness 0. 0005 0. 1 12 C 0. 0025 0. 4 Inelastic States 0 0 TOTAL 0. 0140 2. 1 Asymmetry (2) (1) Normalization Correction applied (2) Nonzero correction (the rest assumed zero) R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Physics Asymmetry Statistics limited ( 9% ) Systematic error goal achieved ! (2%) A physics letter was recently accepted by PRL. ar. Xiv 1201. 2568 [nucl-ex]

PREX Asymmetry (Pe x A) ppm R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Slug ~ 1 day

Asymmetry leads to RN * Establishing a neutron skin at ~95 % CL Neutron Skin = RN - RP = 0. 33 + 0. 16 - 0. 18 fm fig from C. J. Horowitz PREX data R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 * Interpretation requires the acceptance function for spectrometer:

Neutron Skin = R N - RP = 0. 33 + 0. 16 - 0. 18 fm cont. DATA r. N - r P (fm) PREX-I Result, theory: P. Ring r N = r. P Atomic Number, A DATA A physics letter was recently accepted by PRL. ar. Xiv 1201. 2568 [nucl-ex] R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 47/53

PREX-II Approved by PAC “A” Rating 35 days R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 (Aug 2011) to run in 2013 or 2014

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: PR pr EX op os II ed Hebeler et al. Chiral EFT calculation of neutron matter. Correlation of pressure with neutron skin by Brown. Threeneutron forces! Hebeler Steiner Tamii Tsang Tamii et al. Measurement of electric dipole polarizability of 208 Pb + model correlation with neutron skin. These can be tested with d(APV)/APV ~ 3% R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Steiner et al. X-Ray n-star mass and radii observation + Brown correlation. (Ozel et al finds softer EOS, would suggest smaller Rn). d(Rn)/Rn ~ 1% Tsang et al. Isospin diffusion in heavy ion collisions, with Brown correlation and quantum molecular dynamics transport model.

Improvements for PREX-II Region downstream of target Tungsten Collimator & Shielding HRS-L Septum Magnet Q 1 target HRS-R Q 1 Location of ill-fated O-Ring which failed & caused significant time loss during PREX-I R. Michaels, Jlab PREX-II Seminar @ TAMU Mar 6, 2012 Collimators to use all-metal seals 50/53

After PREX … Other Nuclei ? RN 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 39 014104 2012 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 RN Surface thickness

Possible Future PREX Program ? Each point 30 days Nucleus 208 Pb stat. error only E (Ge. V) d. RN / RN comment 1 1% PREX-II (approved by Jlab PAC, A rating) 48 Ca 2. 2 (1 -pass) 0. 4 % natural 12 Ge. V exp’t will propose @ next PAC 48 Ca 2. 6 2% surface thickness 40 Ca 2. 2 (1 -pass) 0. 6 % basic check of theory tin isotope 1. 8 0. 6 % apply to heavy ion tin isotope 2. 6 1. 6 % surface thickness R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Shufang Ban, C. J. Horowitz, R. Michaels J. Phys. G 39 014104 2012 Not proposed

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 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 (runs in 2013 or 2014 )

Extra Slides R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Geant 4 Radiation Calculations scattering chamber PREX-II shielding strategies shielding Number of Neutrons per incident Electron 0 - 1 Me. V beamline Energy (Me. V) 1 - 10 Me. V Strategy ------- PREX-II, no shield PREX-II, shielded • Tungsten ( W ) plug Energy (Me. V) 10 - 1200 Me. V • Shield the W • x 10 reduction in R. 0. 2 Michaels, Jlab neutrons to 10 Me. V Seminar @ TAMU Mar 6, 2012 Energy (Me. V) 49

Pull Plot (example) R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 PREX Data

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 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Horowitz, et. al. PRC 63 025501

Optimum Kinematics for Lead Parity: <A> = 0. 5 ppm. E = 1 Ge. V if Accuracy in Asy 3% Fig. of merit Min. error in R n maximize: 1 month run 1% in R 5 PAVI 09 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 n (2 months x 100 u. A 0. 5% if no systematics)

Source Studies Kent Paschke, Gordon Charge Asymmetry ~ 2000 ppm Cates, Mark Dalton, Rupesh Silwal ~ 0. 5 um Hel. Correl. Diff (X) R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Delta X (um) Hel. Correl. Diff (Y) Transmission of Helicity-Correlated Position DIffs Hel. Correl. Diff (X) Delta Y (um) ~ 0. 5 um Optimizing laser optics to minimize helicitycorrelated systematics. BPMs in Injector Region Hel. Correl. Diff (Y)

Water Cell : Measure (agrees with survey) Nilanga Liyanage, Seamus Riordan, Kiadtisak Saenboonruang, 16 O Hydroge n R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

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 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 time Want small time constants, and same for detectors and bcm

PREX: pins down the symmetry energy (1 parameter) energy cost for unequal # protons & neutrons ( R. J. Furnstahl ) 208 PREX error bar Actually, it’s the density dependence of a 4 that we pin down. Pb PREX R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

Collimators inside Q 1 R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Symmetry in all dimensions to 1 mm

(slide from C. Horowitz) Pb Radius vs Neutron Star Radius • The 208 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 phase such as quark matter, strange matter, color superconductor, kaon condensate… R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012

(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. R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 Rn-Rp in 208 Pb If Yp > red line NS cools quickly via direct URCA reaction n p+e+

Neutron Star Crust vs Pb Neutron Skin Liquid/Solid Transition Density C. J. Horowitz, J. Piekarawicz Liquid FP Neutron Star 208 Pb Solid • 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, Jlab Seminar @ TAMU Mar 6, 2012 TM 1

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! 60 Co observed R. Michaels, Jlab Seminar @ TAMU Mar 6, 2012 60 Ni zero W Charge 60 Co 60 Ni left-handed right-handed L anti-neutrino R Right Left not observed R anti-neutrino L