C-REX : Parity-Violating Measurement of the Weak Charge of 48Ca to an accuracy of 0.03 fm

Spokespersons:

Juliette MammeiRobert Michaels presenting todayKent PaschkeSeamus Riordan (contact person)Paul Souder

Elastic scattering -- parity-violating asymmetry -- from 48Caat 1-pass energy using HRS + septum 04,2.2 GeVE

R. Michaels, Hall A Collaboration Meeting, June 8, 2012 1/18

Parity Violating Asymmetry 2410~ QALR

LRPV

0Ze e

+2

APV from interference

48Ca 48Ca

Parameter of Proposal Value

Measured asymmetry (Pe x A) 2 ppm (“big”)

Beam Energy 2.2 GeV (1-pass)

New Equipment 40 septum, 48Ca target

Rates per HRS 80 MHz (“small”)

Stat. Error in Asy 2.8 % (requires 1% polarimetry)

Systematic Error in Asy 1.8 % (includes polarimetry)

Error in RN 0.03 fm 0.9 %

Beam current 100 uA

Beam Time Request 40 days (includes 10 days overhead)2/18

How to do a Parity Experiment

Flux Integration Technique:HAPPEX: 2 MHzPREX: 500 MHz

(integrating method)

Example : HAPPEX

3/18

• Offline asymmetries nearly identical to online.

• Corrections tiny (here, 3 ppb)

• Errors are statistical only

Parity Violating Asymmetry

Asym

met

ry (p

pm)

Slug

Small beam-related Systematics -- thanks to Jlab beam quality

(HWP = optical element used to flip beam helicity, helps suppress some systematics)

HAPPEX-II data

HAPPEX-II data

Araw = -1.58 ppm 0.12 (stat) 0.04 (syst)

(~1 day)

D. Lhuillier, K. Kumar spokespersons

4/18

Slug # ( ~ 1 day)

Uni

ts:

mic

rons

RLhelicityforXX LR ,

Parity Quality Beam : Unique Strength of JLab

Helicity – Correlated Position Differences

Sign flips provide further suppression : Average with signs = what experiment feels

achieved < 5 nm

Points: Not sign-corrected. 20-50 nm diffs. with pol. source setup & feedback

Araw = Adet - AQ + E+ i xi

Plotted below

Measured separately

Sign flips using ½ wave plate & Wien filter ++ -+ +- --

This BPM, Average = 2.4 3.1 nm

PREX data

5/18

C-REX (& PREX) : Z0 of weak interaction : sees the neutrons

proton neutron

Electric charge 1 0

Weak charge 0.08 1

Neutron form factor

)()(41)( 0

32 rqrjrdQF NN

Parity Violating Asymmetry

)()(sin41

22 2

22

2

QFQFQGA

P

NW

F

0

T.W. Donnelly, J. Dubach, I. Sick

C.J. Horowitz

Nucl. Phys. A 503, 589, 1989

C. J. Horowitz, S. J. Pollock, P. A. Souder, R. Michaels

Phys. Rev. C 63, 025501, 2001

6/18

C-REX, PREX & Neutron Stars

pn RR

Crab Pulsar

C.J. Horowitz, J. Piekarewicz

RN calibrates equation of state (pressure vs density) of Neutron Rich Matter

Combine C-REX / PREX RN with Observed Neutron Star Radii

Some Neutron Stars seem too cold

Strange star ? Quark Star ?

Explained by Cooling by neutrino emission (URCA process) ?

0.2 fm URCA probable, else not

Phase Transition to “Exotic” Core ?

7/18

Physics Asymmetry

CEBAFHall AJLAB

Pol. Source Statistics limited ( 9% ) Systematic error goal achieved ! (2%)

)(014.0)(060.0656.0

syststatppmA

Septum Magnet

HRS + septum

Pb target

HRS

Pb target 50

Reminder: PREX-I PRL 108 (2012) 112502

Results

8/18

Neutron Skin = RN - RP = 0.33 + 0.16 - 0.18 fm

Establishing a neutron skin at ~95 % CL Asymmetry leads to RN

Proposed at last PAC

published

Next, presenting a new 48Ca proposal …

Spokespersons K. Kumar R. Michaels K. Paschke P. A. Souder G. Urciuoli

• Isotopically pure 48 Ca

• Vacuum seal to trap atoms if beam destroys target

• Higher thermal conductivity and melting point than lead : should take 100 uA.

• Similar in concept to target used in E08014 (at 40 uA)

C-REX Target

10/18

C-REX PREX “Light” Nucleus 48Ca Heavy Nucleus 208PbDoubly magic, 1st excited state at 3.84 MeV use HRS to isolate elastic

Doubly magic, 1st excited state at 2.6 MeV use HRS to isolate elastic

relatively big neutron excess more sensitive to RN

44 extra neutrons, thick N skin looks like a neutron star

“state-of-the-art” microscopic calculations now feasible test 3N forces N-stars

sensitive to dynamics of bulk neutron matter N-stars

Smaller nucleus higher Q2 fits 12 GeV larger Asymmetry

Consensus first choice for parity-violating neutron density measurement.

Experimentally, want > 1 nucleus measured with this technique.

Why 48Ca ? Why 208Pb ? Why both ?

11/18

C-REX: Optimization of Kinematics (part I)

04

2.2 GeV = 1-pass

Rate = 80 MHz / HRS easy

Sensitivity of Asymmetry (A) to change in RN

04

A ~ 2 ppm “big” (easy)

~2x more sensitive than PREX

12/18

Optimization of FOM (part II)

equivalent to

minimum error in RN

E = 2.2 GeV (1-pass)

Erro

r in

RN

(uni

ts:

fm)

C-REX

Perc

ent

Erro

r in

RN

04,2.2 GeVE

04,2.2 GeVE

2.8% stat. err.

0.9% total err.

0.03 fm total error

13/18

Neutron Skin vs Mass Number AR N

– R

P (f

m)

A

PREX-I PREX-IIpublished Proposed at last PAC

C-REXProposed here

14/18

Reminder :

PREX-I result : rskin = rn-rp = 0.33 + 0.16 – 0.18 fm

PREX-II expects : +/- 0.05 fm

C-REX (this proposal): +/- 0.03 fm

C-REX and PREX results expected to be correlated.

Similar sensitivity to nuclear structure.

15/18

C-REX

PREX-II

credit Piekarewicz, et al., PRC 85, 041302 (2012)

I

Septum Magnet

A 50 septum (ala PREX) would work, but need a non-standard energy (1.8 GeV)

Room temperature

Need a good dipole hardware resolution

04

16/18

Radiation Load in Hall A C-REX much lower than PREX

Geant4 simulationsJ. Mammei

x10 fewer neutrons or photons, comparable electrons

C-REX vs PREX

Conclusions : C-REX

Fundamental Nuclear Physics with Applications to Neutron Stars and other fields

which need to know about neutrons (heavy ions, atomic parity violation)

48Ca: the next nucleus for Parity-Violating Neutron Density Measurements.

An accuracy of 0.03 fm will have a big impact on nuclear structure theory.

R. Michaels, Hall A Collaboration Meeting, June 8, 2012