The neutron skin in neutronrich nuclei at Jefferson Lab

Transcription

1 The neutron skin in neutronrich nuclei at Jefferson Lab Mark Dalton, University of Virginia For the PREX and CREX Collaborations Low Energy Workshop Boston 15 March

2 Weak Charge Distribution of Heavy Nuclei Nuclear theory Neutron distribution is not accessible predicts a neutron to the charge-sensitive photon. skin on heavy nuclei knowledge of neutron densities comes primarily from hadron scattering => model-dependent interpretation Parity Violation can measure weak form factor model independently proton neutron Electric charge 1 0 γ M EM = 4πα Q 2 F p Q 2 ( ) [ ( ) F ( n Q 2 )] M NC PV = G F ( 2 1 4sin2 θ W )F p Q 2 Weak charge ~ A PV G F Q2 4πα 2 ( ) ( ) F n Q 2 F p Q 2 2

3 A crucial calibration point for nuclear theory The single measurement of Fn translates to a measurement of Rn via mean-field nuclear models and measuring R N pins down the symmetry energy Skyrme covariant meson covariant point coupling 0.35 F n (Q 2 )/N ( R.J. Furnstahl ) r n in 208 Pb (fm) Rn calibrates the EOS of neutron rich matter - provides an important calibration point for nuclear theory and description of neutron stars r n! r p (fm) Skyrme relativistic meson relativistic point coupling symmetry energy a 4 (MeV)

4 Parity Violating Electron Scattering / A + A weak 2 A 2 +2A A weak interference between neutral weak and electromagnetic amplitudes Polarized e - Source Hall A pseudo-random Change helicity of beam - equivalent to changing parity 4

5 triggered plots. The S0 paddles are much bigger than the detector and covers the entire detector plane. The detector x/y distribution plots with detector triggers look identical to the S0 triggered plots, indicating that the detector does not impose any geometric cuts on the acceptance. Detection technique LHRS Detector Plane Dist. -3! Detector Plane y (m) Detector Plane y (m) S0 Trigger Detector Plane x (m) Detector Plane x (m) ! Detector Plane y (m) Detector Plane x (m) Detector Plane x (m) RHRS Detector Plane Dist.! Elastic Inelastic 102 No PID required Detector Plane x (m) Detector Plane x (m) detector 50 1 det Trigger S0 Trigger LHRS Detector Plane Dist. -3 Detector Plane y (m) RHRS Detector Plane Dist. -3!10 1 det Trigger Detector Plane x (m) Detector Plane x (m) Quad 1.4 target Figure 3.1: Detector acceptance plots with S0 and detector triggers. The bounding box is the outlinedipole of the detector with the PMT located at about 1.2m in x. Q Q Low Energy Workshop Neutron skin at JLab L.tr.n==1 && abs(extgtcor_l.th)<0.07 The cuts used to generate these plots are Mark Dalton && LHRS::"P.hapadcL>550 5

6 High Resolution Spectrometer Carbon Carbon Ground State Lead p (GeV/c) 2.6 MeV p (GeV/c) C 1st excited state C Pb Momentum (GeV/c) Pb excited states Ground States Detector integrates elastic peak. Backgrounds from inelastics are suppressed. Negligible contributions from inelastic events rescattering in spectrometer 6

7 Figure of Merit Optimization Use septum magnet to increase reach. Can not realistically go to θ < 4º or Ebeam > 3 GeV. Q 2 ~ 0.01 GeV 2 5º scattering angle A PV ~ 0.6 ppm Rate ~1.5 GHz Sensitivity quantifies how A varies with Rn 7

8 Lead / Diamond Target Diamond LEAD Three bays of: Lead (0.5 mm) sandwiched by diamond (0.15 mm) Liquid He cooling (30 Watts) 8

9 PREX Detector 1 cm thick quartz detector Size to match Pb elastic locus in focal plane Designed to maximize photoelectrons and resolution Remote controlled position to precisely control flux from excited states 9

10 Determining Q 2 Q 2 measured using standard HRS tracking package, with reduced beam current Water cell optics target for central angle Example: HAPPEx III Q 2 distributions Q 2 = Q 2 = δp between elastic and inelastic peaks reduces systematic error from spectrometer calibration δθ ~ 0.55 mrad (0.23%) 10

11 Hall A Compton Polarimeter Resonant cavity photon target, up to 2kW intensity Calibration of the analyzing power is usually the leading uncertainty measure asymmetry independently in: momentum analyzed electrons photons in calorimeter Electron detector achieved 1% accuracy for HAPPEX-2, but system was broken for HAPPEX-3 11

12 Integrating Analysis Electron detector achieved 1% accuracy for HAPPEX-2, but system was broken for HAPPEX-3 Photon self-triggered analysis has been limited in accuracy, and required electron coincidence measurements for calibration Integrating photon detection: immune to calibration, pile-up, deadtime, response function New DAQ, with SIS 2230 Flash ADC read out in two modes Triggered mode: triggered snap shot of fixed time interval (for calibration) 12

13 Detector Linearity Studied in situ and on bench with LED system optimized to linearity for differential rates of similar pulses -1600V, 18KHz, KHz PMT % change LEDs Normalized Baseline (DIFF OFF) -1200V, 18KHz, KHz DIFF ENABLE BASELINE ENABLE Flexio Board ADC Board % change Pulser Electronics Data Acquisition System Measurements taken in short deviations from high rate, to maintain consistent thermal properties Phototube and readout non-linearity bounded at the 0.5% level Normalized Baseline (DIFF OFF) 13

14 Controlling Beam Asymmetries False (non parity violating) asymmetries can arise from helicity correlated beam properties. Sources of differences diminished through careful setup and alignment of source polarization optics. A meas = A unreg i x i x i = x, y, x 0,y 0,E Sensitivities measured by deliberately modulating the beam. Position, angle and energy differences are measured and correct for using measured sensitivities. System worked very well for PREX 14

15 PREX Issues Runtime limited by experimental issues. Teething problems related to faster flip rate (240 Hz.) Scattering chamber vacuum failure - o-ring at downstream connection to collimator box Very high radiation flux in hall (particularly LE neutrons) - damage to electronics - activation of target area Target damage and lifetime of the individual lead targets (1 week). Run again with improved Solved during experiment. Qweak achieved 960 Hz. Redesign collimator box and vacuum connections Careful design of tungsten collimator and low-z neutron shielding. Run with 10 targets. Synchronize raster 15

16 Runtime limited by experimental issues. Teething problems related to faster flip rate (240 Hz.) Scattering chamber vacuum failure - o-ring at downstream connection to collimator box Very high radiation flux in hall (particularly LE neutrons) - damage to electronics - activation of target area Target damage and lifetime of the individual lead targets (1 week). PREX Issues Run again with improved Solved during experiment. Qweak achieved 960 Hz. Redesign collimator box and vacuum connections Careful design of tungsten collimator and low-z neutron shielding. All addressed! Run with 10 targets. Synchronize raster 15

17 Lead Target Damage Diamond foils used to conduct heat from beam spot to cryo-cooled frame Similar design used in E Rate patterns from damaged targets thickness ok, but became non-uniform Target #3 Target #2 Thickness differences significantly increased the statistical width. Solution is to precisely synchronize raster with the helicity flip rate. 16

18 Performance of Lead / Diamond Targets melted melted Did NOT melt Final 4 days at 70 ua Targets with thin diamond backing (4.5 % background) degraded fastest. Thick diamond (8%) ran well and did not melt at 70 ua. 17

19 PREX Result First electroweak observation of the neutron skin of a heavy nucleus (CL=95%) 18

20 Future Studies Complimentary measurements PREX II 208 Pb at E = 1.0 GeV and θ = 5 Q 2 = GeV 2 Rn measured to 0.06 fm (1.0%) APV ~ 0.6 ppm better approximation of infinite nuclear matter CREX 48 Ca at E = 2.2 GeV and θ = 4 Q 2 = GeV 2 Rn measured to 0.03 fm (0.9%) APV ~ 2 ppm larger asymmetry can use higher Q 2 and energy ab initio calculations feasible density dependence of the symmetry energy of neutron rich nuclear matter data as input for: neutron star structure, heavy ion collisions and atomic parity violation 19

21 Future Studies PREX II Complimentary measurements C-REX Workshop, JLab CREX March Pb at E = 1.0 GeV and θ = 5 Q 2 = GeV 2 Rn measured to 0.06 fm (1.0%) APV ~ 0.6 ppm better approximation of infinite nuclear matter 48 Ca at E = 2.2 GeV and θ = 4 Q 2 = GeV 2 Rn measured to 0.03 fm (0.9%) APV ~ 2 ppm larger asymmetry can use higher Q 2 and energy ab initio calculations feasible density dependence of the symmetry energy of neutron rich nuclear matter data as input for: neutron star structure, heavy ion collisions and atomic parity violation 19

22 Recent R n PredicFons 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 Hebeler Steiner Tamii Tsang PREX-II proposal 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 208 Pb + model correlation with neutron skin. Tsang et al. Isospin diffusion in heavy ion collisions, with Brown correlation and quantum molecular dynamics transport model. These can be tested with δ(a PV )/A PV ~ 3% δ(r n )/R n ~ 1%

23 Nuclear Parameter Correlations strong correlation between RN and the pressure of neutron matter densities near 0.1 fm 3 constrains the equation of state of neutron matter Combined experiments reduce uncertainty EOS 21

24 Transverse Asymmetries Beam-normal single-spin asymmetry. Forbidden in Born scattering. Test of multi-photon calculations. Calculations agree well with the data for H, He and C. World transverse asymmetry data for θ < 10º Curves from M. Gorchtein et al. Phys. Rev. C77, (2008) A n [ppm] H HAPPEX 4 He HAPPEX 12 C PREX 208 Pb PREX 1 H G Pb GeV 1 1 C GeV H GeV H 1.16 GeV He GeV H QWeak (prelim.) Q [GeV] 22

25 Z dependence of agreement The trend suggests that Coulomb distortions are playing a significant role at large Z. potential new Ca measurements Theory disagreement Coulomb distortions grow rapidly with Z E. D. Cooper, C. J. Horowitz, Phys. Rev. C72, (2005) 23

26 Potential Nuclei for AT Study All stable nuclei from Ca to Pb with first excited state more than 1 MeV above ground state. 18 in total, all spin-parity 0+. nucleus abundance 1st (kev) nucleus abundance 1st (kev) 40 Ca % Zr % Ca 2.09 % Mo % Ca 0.19 % Sn % Ti 5.4 % Sn % Ni % Sn % Ni % Ba 71.7 % Kr 17.3 % Ce % Sr 9.86 % Sm 3.07 % Sr % Pb 52.4 %

27 Summary PREX ran successfully and demonstrated the first electroweak confirmation of the neutron skin in a heavy nucleus. A more precise measurement of the 208 Pb neutron skin would have profound implications in many fields. PREX-II is feasible, approved to run and scheduled for A precise measurement of the 48 Ca neutron skin would be useful for many fields. C-REX is easier and conditionally approved. 208 Pb has a surprisingly small transverse asymmetry, presumably from multi-photon exchange, which may open up a unique experimental window into these effects. 25