Status of the PREX Experiment Rn through PVeS at JLabPV to 3% (combined with PREX-I) with 35 days Several improvements to prior experiment Improved metal O-rings Additional radiation

Status of the PREX ExperimentRn through PVeS at JLab

Seamus RiordanUniversity of Massachusetts, [email protected]

for the PREX Collaboration

June 18, 2011

Seamus Riordan — NuSym11 PREX 1/31

Outline

Motivation

Parity at JLab

PREX and Results

Future Plans


Nucleon Radii in Heavy Nuclei

Measurements are important to understanding the strongnuclear force

Calculations are difficult due to non-pQCD regimecomplicated by many-body physics

Interesting for

Fundamental nuclear structureIsospin dependence and nuclear symmetryDense nuclear matter and neutron stars

Proton radius is relatively easy - electromagnetic probes

Neutron radius is difficult

Weakly couples to electroweak probesHadronic probes have considerable uncertaintyTheory has range of Rn − Rp for Pb of 0− 0.4 fm


What do we learn from Rn?

What do we learn from Rn?

Constraints on EOS and symmetry energy

B. Alex Brown, PRL 85, 5296 (2000)

Slope of EOS can be used to constrain potential models


Neutron Stars

Neutron star structure is alsobetter understood withmeasurements on Rn

Larger Rn correlates with largerpressure

A. W. Steiner et al.,

Phys Rep 411, 325 (2005)

Additionally, symmetry energy governs proton fractionDirect Urca cooling depends on processes

n → p + e− + ν

e− + p → n + ν

Larger symmetry energy gives larger proton fraction, need 11%


How do we measure?

Methods used to extract Rn

Hadronic Probes

Elastic pN, ~pN, nN, π±Nπ0 photoproduction (Kruche, et al.)GDRAntiproton scatteringHave theoretical uncertainty

Electroweak Probes

Parity violating electron scatteringAtomic parity violation“Clean” measurements, fewer systematicsTechnically challenging


Non-Parity Violating Electron Scattering

Electron scattering γ exchange provides Rp through nucleus FFs,spin 0:

dσ

dΩ=

α2 cos2 θ24E 2 sin4 θ

2

F 2(Q2)

q [fm−1]

→


Non-Parity Violating Electron Scattering

Electron scattering γ exchange provides Rp through nucleus FFs,spin 0:

dσ

dΩ=

α2 cos2 θ24E 2 sin4 θ

2

F 2(Q2)

→

In limit of small Q2

F (Q2) ≈ F (0) +dF

dQ2

∣∣∣∣Q2=0

+ ... =

∫ρ(~x)d3x − 1

6Q2〈r2charge〉

So small Q2 measurements give RMS radius (Rn/p)


Parity Violating Electron Scattering

e− also exchange Z , which is parity violating

Primarily couples to neutron:

Qprotonweak ∝ 1− 4 sin2 θW ≈ 0.076, Qneutron

weak ∝ −1

Detectable in parity violating asymmetry of electrons withdifferent helicity

In Born approximation, Q2 M2Z , from γ − Z interference:

APV =σ+ − σ−

σ+ + σ−=

GFQ2

4πα√

2

[1− 4 sin2 θW −

Fn(Q2)

Fp(Q2)

]For fixed target exp., typical APV ∼ 10−8 − 10−4


Extraction

Rn

Neutron ρ EGn

G sE

A PV Polarization

Backgrounds

DensityMEC

Coulomb

Distortions

Weak

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 parametersQ2 must be accurately known (∼ 1%)


Parity Violation at JLab

Jefferson Lab is an excellent facility for such measurements

Two RF superconducting linacs - Ee = 1− 6 GeVHigh quality polarized beam, Pe ∼ 85− 90%PV expts. need quiet beam parameters over helicity windows:

∆x < 10 µm∆x ′ < 2 µrad∆E < 10−3


Typical Experiment

Stolen from R. Michaels


Parity Violation at JLab

Experimental History

Completed

G 0, HAPPEX - G sE ,G s

M in proton, 4HePVDISPREX - Rn in 208Pb

Running

Qweak - proton weak charge (1− sin2 θW ) from elastic ~ep

Proposed

PVDIS SoLID - large acceptance PVDISMoller - sin2 θW from ~ee


PREX

PREX measures Rn of 208Pb

Lead is nice because

Excess of neutronsDoubly-magic nucleusNearest excited state 2.6 MeV from elastic

Ran in Spring 2010 (approved 30 PAC days)

Ee = 1.063 GeV, θe ≈ 5, Q2 ≈ 0.009 GeV2

Ie ∼ 50− 75 µA

Expected uncertainty on APV of 3%, Rn ∼ 1%


PREX Layout

Experimental Layout

Standard Hall A HRSspectrometers

Detector huts wellshielded againstbackgrounds

Run dual arms -cancels out transverseasymmetry, addnlsystematics

Septum magnetbends 5 to 12.5


PREX Layout

Target

Septum Q1 Q2

Dipole

Q3

VDCs

Quartz

Elastic

Inelastic

PREX Optics Schematic


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


Lead/Diamond Targets

0.15 mm thickdiamond, 0.5 mmthick Pb

Cryogenically cooledframe (30 W)

Beam is rastered bytwo fast magnetsupstream to diffusebeam on surface


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 forbeam fluctuations


Modulation Correction

Modulation corrections provide narrower asymmetry widths(σ ∼ 180 ppm, I = 75 µA)

→


Experiment Issues

Several issues prevented full experimental programLarge amounts of radiation were dumped in the experimentalhall damaging electronicsMistune of septum field - loss of some small angle statisticsDestruction of scattering chamber rubber O-rings


Experiment Issues - Target

Targets were destroyedover periods of time bybeam

Loss of material ∼ 10%

Thicker diamond targetswere more successful -Lasted 4 days at 70 µA

Thickest diamondcontributes 8%background -manageable


Data Quality

Measured asymmetries relatively stable over run


Data Quality - Helicity Reversal

Slow helicity reversal with HWP and double-Wien successfulin controlling systematics


Backgrounds

Inelastic Pb and C12 excitations

C12 elastic

Rescattering within the spectrometer


Results

Set 95% CL on existence of neutron skinRn − Rp = 0.34 + 0.15− 0.17 fm

Each model neutron density is folded into numerical solution ofDirac eqn with Coulomb and weak axial potentialFull acceptance (apertures, septum propagation, detectors)applied to APV

PRL forthcomingSeamus Riordan — NuSym11 PREX 24/31

Results

Set 95% CL on existence of neutron skinRn − Rp = 0.34 + 0.15− 0.17 fm

Each model neutron density is folded into numerical solution ofDirac eqn with Coulomb and weak axial potentialFull acceptance (apertures, septum propagation, detectors)applied to APV

PRL forthcomingSeamus Riordan — NuSym11 PREX 24/31

Result and Error Budget

APV = 0.6571 ± 0.0604 ± 0.0130 ppm± 9.22% (stat) ± 1.98% (sys)

abs (ppm) rel (%)Polarization 0.0071 1.1Detector Lin. 0.0071 1.1Beam Corrections 0.0072 1.1Q2 0.0028 0.412C Asymmetry 0.0025 0.4Transverse Pol. 0.0012 0.2BCM Lin. 0.0010 0.1Target Thick 0.0006 0.1Rescattering 0.0001 0.0Inelastic Cont. 0.0000 0.0

Systematic of ∼ 2% achieved!

Completely statistics dominated


Future Plans

New proposal to completemeasurements to be submitted toAugust PAC

Measurement of APV to 3%(combined with PREX-I) with 35days

Several improvements to priorexperiment

Improved metal O-ringsAdditional radiation mitigation

Must run at start of 12 GeV commissioning - 2014?

Separate proposal for similar measurement on 48Ca likely infuture


Summary

PREX experiment ran Spring 2010 to measure Rn on 208Pb

Established existence of neutron skin with 95% CL despiteexperimental difficulties

PREX-II proposal to be considered by PAC in upcomingmonths


Target Degradation

Thicker diamond targetswere more successful

Lasted 4 days at 70 muA

Thickest diamondcontributes 8% background- manageable


Optics and Q2 Measurements

Q2 fixed by elastic scattering of H2O target

Don’t use integrating detectors - do tracking through HRSVDCs

Sieve placed between target and quad aperture for angularreconstruction over whole acceptance

Q2 determined to about ∼1.0%


Compton Polarimetry

Non-invasive polarization monitoring

Upgraded from infrared to green laser,total power increase by ×2.

Allows running at lower beam energies (Ee = 1 GeV)

Polarization uncertainty ∼ 1% at Ee = 1 GeV


Moller Polarimetery

Invasive polarization measurements

Upgrade to DAQ to reduce deadtimesystematics

“Brute force” increase in Fe foil pol. by3− 4 T superconducting solenoid

Abs. systematic uncertainty from 2− 3%to 1.1%


Documents

Status of the PREX Experiment Rn through PVeS at JLabPV to 3% (combined with PREX-I) with 35 days Several improvements to prior experiment Improved metal O-rings Additional radiation