Status of the PREX Experiment R n through PVeS at JLab

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1 Status of the PREX Experiment R n through PVeS at JLab Seamus Riordan University of Massachusetts, Amherst sriordan@physics.umass.edu for the PREX Collaboration June 18, 2011 Seamus Riordan NuSym11 PREX 1/31

2 Outline Motivation Parity at JLab PREX and Results Future Plans Seamus Riordan NuSym11 PREX 2/31

3 Nucleon Radii in Heavy Nuclei Measurements are important to understanding the strong nuclear force Calculations are difficult due to non-pqcd regime complicated by many-body physics Interesting for Fundamental nuclear structure Isospin dependence and nuclear symmetry Dense nuclear matter and neutron stars Proton radius is relatively easy - electromagnetic probes Neutron radius is difficult Weakly couples to electroweak probes Hadronic probes have considerable uncertainty Theory has range of R n R p for Pb of fm Seamus Riordan NuSym11 PREX 3/31

4 What do we learn from R n? What do we learn from R n? Constraints on EOS and symmetry energy B. Alex Brown, PRL 85, 5296 (2000) Slope of EOS can be used to constrain potential models Seamus Riordan NuSym11 PREX 4/31

5 Neutron Stars A. W. Steiner et al., Phys Rep 411, 325 (2005) Neutron star structure is also better understood with measurements on R n Larger R n correlates with larger pressure Additionally, symmetry energy governs proton fraction Direct Urca cooling depends on processes n p + e + ν e + p n + ν Larger symmetry energy gives larger proton fraction, need 11% Seamus Riordan NuSym11 PREX 5/31

6 How do we measure? Methods used to extract R n Hadronic Probes Elastic pn, pn, nn, π ± N π 0 photoproduction (Kruche, et al.) GDR Antiproton scattering Have theoretical uncertainty Electroweak Probes Parity violating electron scattering Atomic parity violation Clean measurements, fewer systematics Technically challenging Seamus Riordan NuSym11 PREX 6/31

7 Non-Parity Violating Electron Scattering Electron scattering γ exchange provides R p through nucleus FFs, spin 0: dσ dω = α2 cos 2 θ 2 4E 2 sin 4 θ F 2 (Q 2 ) 2 q [fm 1 ] Seamus Riordan NuSym11 PREX 7/31

8 Non-Parity Violating Electron Scattering Electron scattering γ exchange provides R p through nucleus FFs, spin 0: dσ dω = α2 cos 2 θ 2 4E 2 sin 4 θ F 2 (Q 2 ) 2 In limit of small Q 2 F (Q 2 ) F (0) + df dq = ρ( x)d 3 x 1 Q 2 =0 6 Q2 rcharge 2 So small Q 2 measurements give RMS radius (R n/p ) Seamus Riordan NuSym11 PREX 7/31

9 Parity Violating Electron Scattering e also exchange Z, which is parity violating Primarily couples to neutron: Q proton weak 1 4 sin 2 θ W 0.076, Qweak neutron 1 Detectable in parity violating asymmetry of electrons with different helicity In Born approximation, Q 2 MZ 2, from γ Z interference: A PV = σ+ σ σ + + σ = G F Q 2 [ 4πα 1 4 sin 2 θ W F n(q 2 ] ) 2 F p (Q 2 ) For fixed target exp., typical A PV Seamus Riordan NuSym11 PREX 8/31

10 Extraction A PV Polarization Backgrounds Coulomb Distortions Neutron ρ R n G n E MEC G s E Weak Density PV experiments are challenging for several reasons: Asymmetries are small, need lots of statistics Important control of systematics e polarimetry ( 1%) Good understanding and control of beam parameters Q 2 must be accurately known ( 1%) Seamus Riordan NuSym11 PREX 9/31

11 Parity Violation at JLab Jefferson Lab is an excellent facility for such measurements Two RF superconducting linacs - E e = 1 6 GeV High quality polarized beam, P e 85 90% PV expts. need quiet beam parameters over helicity windows: x < 10 µm x < 2 µrad E < 10 3 Seamus Riordan NuSym11 PREX 10/31

12 Typical Experiment Stolen from R. Michaels Seamus Riordan NuSym11 PREX 11/31

13 Parity Violation at JLab Experimental History Completed G 0, HAPPEX - G s E,G s M in proton, 4 He PVDIS PREX - R n in 208 Pb Running Qweak - proton weak charge (1 sin 2 θ W ) from elastic ep Proposed PVDIS SoLID - large acceptance PVDIS Moller - sin 2 θ W from ee Seamus Riordan NuSym11 PREX 12/31

14 PREX PREX measures R n of 208 Pb Lead is nice because Excess of neutrons Doubly-magic nucleus Nearest excited state 2.6 MeV from elastic Ran in Spring 2010 (approved 30 PAC days) E e = GeV, θ e 5, Q GeV 2 I e µa Expected uncertainty on A PV of 3%, R n 1% Seamus Riordan NuSym11 PREX 13/31

15 PREX Layout Experimental Layout Standard Hall A HRS spectrometers Detector huts well shielded against backgrounds Run dual arms - cancels out transverse asymmetry, addnl systematics Septum magnet bends 5 to 12.5 Seamus Riordan NuSym11 PREX 14/31

16 PREX Layout Inelastic Elastic PREX Optics Schematic VDCs Quartz Q3 Target Septum Q1 Q2 Dipole Seamus Riordan NuSym11 PREX 14/31

17 PREX Equipment Several pieces of instrumentation were important Upgrades in polarimetry Non-invasive Compton, 1% Invasive Moller, 1% Pb/D targets Quartz Cerenkov detectors Integrating ADCs Beamline monitoring components Seamus Riordan NuSym11 PREX 15/31

18 Lead/Diamond Targets 0.15 mm thick diamond, 0.5 mm thick Pb Cryogenically cooled frame (30 W) Beam is rastered by two fast magnets upstream to diffuse beam on surface Seamus Riordan NuSym11 PREX 16/31

19 Data Quality and Analysis All asymmetries are blinded approximately 1σ Widths are determined by statistics of photo-electrons, changes in beam parameters, etc. Integrated helicity pair-wise asymmetries are corrected for beam fluctuations Seamus Riordan NuSym11 PREX 17/31

20 Modulation Correction Modulation corrections provide narrower asymmetry widths (σ 180 ppm, I = 75 µa) Seamus Riordan NuSym11 PREX 18/31

21 Experiment Issues Several issues prevented full experimental program Large amounts of radiation were dumped in the experimental hall damaging electronics Mistune of septum field - loss of some small angle statistics Destruction of scattering chamber rubber O-rings Seamus Riordan NuSym11 PREX 19/31

22 Experiment Issues - Target Targets were destroyed over periods of time by beam Loss of material 10% Thicker diamond targets were more successful - Lasted 4 days at 70 µa Thickest diamond contributes 8% background - manageable Seamus Riordan NuSym11 PREX 20/31

23 Data Quality Measured asymmetries relatively stable over run Seamus Riordan NuSym11 PREX 21/31

24 Data Quality - Helicity Reversal Slow helicity reversal with HWP and double-wien successful in controlling systematics Seamus Riordan NuSym11 PREX 22/31

25 Backgrounds Inelastic Pb and C12 excitations C12 elastic Rescattering within the spectrometer Seamus Riordan NuSym11 PREX 23/31

26 Results Set 95% CL on existence of neutron skin R n R p = fm Each model neutron density is folded into numerical solution of Dirac eqn with Coulomb and weak axial potential Full acceptance (apertures, septum propagation, detectors) applied to A PV PRL forthcoming Seamus Riordan NuSym11 PREX 24/31

27 Results Set 95% CL on existence of neutron skin R n R p = fm Each model neutron density is folded into numerical solution of Dirac eqn with Coulomb and weak axial potential Full acceptance (apertures, septum propagation, detectors) applied to A PV PRL forthcoming Seamus Riordan NuSym11 PREX 24/31

28 Result and Error Budget A PV = ± ± ppm ± 9.22% (stat) ± 1.98% (sys) abs (ppm) rel (%) Polarization Detector Lin Beam Corrections Q C Asymmetry Transverse Pol BCM Lin Target Thick Rescattering Inelastic Cont Systematic of 2% achieved! Completely statistics dominated Seamus Riordan NuSym11 PREX 25/31

29 Future Plans New proposal to complete measurements to be submitted to August PAC Measurement of A PV to 3% (combined with PREX-I) with 35 days Several improvements to prior experiment Improved metal O-rings Additional radiation mitigation Must run at start of 12 GeV commissioning ? Separate proposal for similar measurement on 48 Ca likely in future Seamus Riordan NuSym11 PREX 26/31

30 Summary PREX experiment ran Spring 2010 to measure R n on 208 Pb Established existence of neutron skin with 95% CL despite experimental difficulties PREX-II proposal to be considered by PAC in upcoming months Seamus Riordan NuSym11 PREX 27/31

31 Target Degradation Thicker diamond targets were more successful Lasted 4 days at 70 mua Thickest diamond contributes 8% background - manageable Seamus Riordan NuSym11 PREX 28/31

32 Optics and Q 2 Measurements Q 2 fixed by elastic scattering of H 2 O target Don t use integrating detectors - do tracking through HRS VDCs Sieve placed between target and quad aperture for angular reconstruction over whole acceptance Q 2 determined to about 1.0% Seamus Riordan NuSym11 PREX 29/31

33 Compton Polarimetry Non-invasive polarization monitoring Upgraded from infrared to green laser, total power increase by 2. Allows running at lower beam energies (E e = 1 GeV) Polarization uncertainty 1% at E e = 1 GeV Seamus Riordan NuSym11 PREX 30/31

34 Möller Polarimetery Invasive polarization measurements Upgrade to DAQ to reduce deadtime systematics Brute force increase in Fe foil pol. by 3 4 T superconducting solenoid Abs. systematic uncertainty from 2 3% to 1.1% Seamus Riordan NuSym11 PREX 31/31