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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