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C-REX : Parity-Violating Measurement of the Weak Charge of 48 Ca to an accuracy of 0.03 fm. Spokespersons: Juliette Mammei Robert Michaels  presenting today Kent Paschke Seamus Riordan (contact person) Paul Souder.

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slide1

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

Spokespersons:

Juliette Mammei

Robert Michaels  presenting today

Kent Paschke

Seamus Riordan (contact person)

Paul Souder

Elastic scattering -- parity-violating asymmetry -- from 48Ca

at 1-pass energy using HRS + septum

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

1/18

slide2
Parity Violating Asymmetry

2

+

APV from interference

48Ca

48Ca

2/18

how to do a parity experiment

Flux Integration Technique:

HAPPEX: 2 MHz

PREX: 500 MHz

How to do a Parity Experiment

(integrating method)

Example : HAPPEX

3/18

slide4

Asymmetry (ppm)

Slug

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

Parity Violating Asymmetry

  • Offline asymmetries nearly identical to online.
  • Corrections tiny (here, 3 ppb)
  • Errors are statistical only

HAPPEX-II data

D. Lhuillier, K. Kumar spokespersons

(~1 day)

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

HAPPEX-II data

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

4/18

slide5

Parity Quality Beam : Unique Strength of JLab

Helicity – Correlated Position Differences

Plotted below

Araw = Adet - AQ +  E+ i xi

Measured separately

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

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

This BPM, Average = 2.4 3.1 nm

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

achieved

< 5 nm

Units: microns

PREX data

Slug # ( ~ 1 day)

5/18

c rex prex z 0 of weak interaction sees the neutrons
C-REX (&PREX): Z0 of weak interaction : sees the neutrons

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

Neutron form factor

C.J. Horowitz

Parity Violating Asymmetry

6/18

c rex prex neutron stars
C-REX, PREX & Neutron Stars

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

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 URCA probable, else not

7/18

Crab Pulsar

slide8

Pol. Source

Hall A

JLAB

CEBAF

Reminder: PREX-I

Results

PRL 108 (2012) 112502

HRS + septum

Physics Asymmetry

Pb target

  • Statistics limited ( 9% )
  • Systematic error goal achieved ! (2%)

HRS

Septum Magnet

50

Pb target

8/18

slide9

Asymmetry leads to RN

Establishing a neutron skin at ~95 % CL

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

published

Proposed at last PAC

Spokespersons

K. Kumar

R. Michaels

K. Paschke

P. A. Souder

G. Urciuoli

Next, presenting a new 48Ca proposal …

slide10

C-REX Target

  • 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)

10/18

slide12

C-REX: Optimization of Kinematics (part I)

Rate = 80 MHz / HRS  easy

Sensitivity of Asymmetry (A) to change in RN

2.2 GeV = 1-pass

~2x more sensitive than PREX

A ~ 2 ppm

 “big” (easy)

12/18

slide13

C-REX

Optimization of FOM (part II)

equivalent to

minimum error in RN

2.8% stat. err.

E = 2.2 GeV (1-pass)

0.03 fm total error

Error in RN(units: fm)

Percent Error in RN

0.9% total err.

13/18

slide14

Neutron Skin vs Mass Number A

C-REX

PREX-I

Proposed here

PREX-II

published

Proposed at last PAC

RN – RP(fm)

A

14/18

slide15

C-REXandPREX results expected to be correlated.

Similar sensitivity to nuclear structure.

PREX-II

C-REX

I

Reminder :

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

credit

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

PREX-II expects : +/- 0.05 fm

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

15/18

slide16

Septum Magnet

Room temperature

Need a good dipole  hardware resolution

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

16/18

slide17

Radiation Load in Hall A

C-REX much lower than PREX

Geant4 simulations

J. Mammei

C-REX vs PREX

x10 fewer neutrons or photons, comparable electrons

slide18

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