PREX UVa Seminar, Nov 2005 R. Michaels ) ( sin 4 1 2 2 2 2 2 Q F Q G d d d d d d d d A P W F L R L R ) ( 2 Q F n % 1 % 3 n n R dR A dA Lead ( Pb) Radius Experiment : PREX Z of Weak Interaction : Clean Probe Couples Mainly to Neutrons ( T.W. Donnelly, J. Dubach, I Sick ) 0 In PWIA (to illustrate) : w/ Coulomb distortions (C. J. Horowitz) : 208 208 P b E = 850 MeV, electrons on lead 0 6 Elastic Scattering Parity Violating Asymmetry 0
Transcript
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
)(
sin4122 2
22
QF
QG
d
d
d
d
d
d
d
d
AP
WF
LR
LR
)( 2QF n
%1%3 n
n
R
dR
A
dA
Lead ( Pb) Radius Experiment : PREX
Z of Weak Interaction :
Clean Probe Couples Mainly to Neutrons( T.W. Donnelly, J. Dubach, I Sick )
0
In PWIA (to illustrate) :
w/ Coulomb distortions (C. J. Horowitz) :
208
208Pb
E = 850 MeV, electrons on lead
06 Elastic Scattering Parity Violating Asymmetry
0
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
A piece of the weak interaction violates parity (mirror symmetry) which allows to isolate it.
Negative longitudinal spin
Positive longitudinal spin
Pb208
P
S (spin)
(momentum)
Incident electron
Target
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Parity Violating Asymmetry 610~
LR
LRPVA
0Z
e e
+
2
Applications of PV :
• Nucleon Structure (strangeness) -- HAPPEX / G0
• Standard Model Tests ( ) -- e.g. Qweak
• Nuclear Structure (neutron density) : PREX
W2sin
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Measured Asymmetry
Weak Density at one Q2
Neutron Density at one Q2
Correct for CoulombDistortions
Small Corrections forG
nE G
sE MEC
Assume Surface Thickness Good to 25% (MFT)
Atomic Parity Violation
Mean Field & Other
Models
Neutron
Stars
R n
PREX Physics Impact
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX in Hall A at JLab
CEBAF
Hall A
Pol. Source
Lead Foil Target
Spectometers
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Impact on Nuclear Physics:
What is the size of a nucleus ?
Is the size of a heavy nucleus determined by neutrons or by protons ?
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
str
mb
d
d
1fmq
Reminder: Electromagnetic Scattering determines
r
r
Pb208
(charge distribution)
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Z of weak interaction : sees the neutrons
0
proton neutron
Electric charge 1 0
Weak charge 0.08 1
Analysis is clean, like electromagnetic scattering:
1. Probes the entire nuclear volume
2. Perturbation theory applies
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
neutron weak charge >> proton weak charge
is small, best observed by parity violation
)()()(ˆ5 rArVrV
||)()(/
//3 rrrZrdrV )()()sin41(22
)( 2 rNrZG
rA NPWF
22 |)(| QFd
d
d
dP
Mott
)()(4
1)( 0
32 rqrjrdQF PP )()(
4
1)( 0
32 rqrjrdQF NN
)(
)(sin41
22 2
22
2
QF
QFQG
d
d
d
d
d
d
d
d
AP
NW
F
LR
LR
Electron - Nucleus Potential
electromagnetic axial
Neutron form factor
Parity Violating Asymmetry
)(rA
1sin41 2 W
Proton form factor
0
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Neutron Densities
• Proton-Nucleus Elastic• Pion, alpha, d Scattering• Pion Photoproduction• Magnetic scattering
• Theory Predictions Fit mostly by data other than neutron densities
Involve strong probes
Most spins couple to zero.
Therefore, PREX is a powerful check of nuclear theory.
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
B. Krusche arXiv:nucl-ex/0509003 Sept 2005
Example : Recent Pion Photoproduction
This paper obtains
PN RR
Mean Field Theory fmRR PN 35.005.0
PREX accuracy
fmRN 05.0
!!
fmRR PN 111.0083.0 Proton – Nucleus Elastic:
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
( R.J. Furnstahl )
Measurement at one Q is sufficient to measure R
2
N
PREX error bar
Why only one parameter ?
(next slide…)
PREX:
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX: pins down the symmetry energy (1 parameter)
( R.J. Furnstahl )
energy cost for unequal # protons & neutrons.../ 3/1
2
4
AaA
ZNaa
A
Esv
PREX
PREX error bar
Pb208
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Impact on Atomic Parity
Measures atomic overlap with weak charge.
Neutrons carry most weak charge
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Atomic Parity Violation• Low Q test of Standard Model
• Needs R to make further progress.
2
N
rdrZrNG
H eePWNF
PNC35/2 )()sin41()(
22
0
APV
Isotope Chain Experiments e.g. Berkeley Yb
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Impact on Neutron Stars
What is the nature of extremely dense matter ?
Do collapsed stars form “exotic” phases of matter ?
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Fig. from J.M. Lattimer & M. Prakash, Science 304 (2004) 536.
)(PInputs:
Eq. of state (EOS)
Hydrostatics (Gen. Rel.)
Typ. Astro. Observations
Luminosity LTemp. TMass M from pulsar timing
PREX constraint
424 TRL B(with corrections … )
Mass - Radius relationship
pres
sure
dens
ity
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX & Neutron Stars
Crab Pulsar
( C.J. Horowitz, J. Piekarweicz )
R calibrates EOS of Neutron Rich Matter
Combine PREX R with Obs. Neutron Star Radii
Some Neutron Stars seem too Cold
N
N
Crust ThicknessExplain Glitches in Pulsar Frequency ?
Strange star ? Quark Star ?
Cooling by neutrino emission (URCA)
0.2 fm URCA probable, else not pn RR
Phase Transition to “Exotic” Core ?
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
FP
TM1Solid
Liquid
Liquid/Solid Transition Density
• Thicker neutron skin in Pb means energy rises rapidly with density Quickly favors uniform phase.
• Thick skin in Pb low transition density in star.
Neutron Star Crust vs Pb Neutron Skin
PREX calibrates the EOS at subnuclear densities.
208PbNeutron Star
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Pb Radius vs Neutron Star Radius• The 208Pb radius constrains the pressure of neutron
matter at subnuclear densities.• The NS radius depends on the pressure at nuclear
density and above.• Most interested in density dependence of equation of
state (EOS) from a possible phase transition.• Important to have both low density and high density
measurements to constrain density dependence of EOS.– If Pb radius is relatively large: EOS at low density is stiff with
high P. If NS radius is small than high density EOS soft.– This softening of EOS with density could strongly suggest a
transition to an exotic high density phase such as quark matter, strange matter, color superconductor, kaon condensate…
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX Constrains Rapid Direct URCA Cooling of Neutron Stars
• Proton fraction Yp for matter in beta equilibrium depends on symmetry energy S(n).
• Rn in Pb determines density dependence of S(n).
• The larger Rn in Pb the lower the threshold mass for direct URCA cooling.
• If Rn-Rp<0.2 fm all EOS models do not have direct URCA in 1.4 M¯ stars.
• If Rn-Rp>0.25 fm all models do have URCA in 1.4 M¯ stars.
Rn-Rp in 208Pb
If Yp > red line NS cools quickly via direct URCA reaction n p+e+
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
PREX: Experiment Design
Spokespersons:
P.A. Souder, G.M. Urciuoli, R. Michaels
Hall A Collaboration Experiment
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Hall A at Jefferson Lab
Polarized e-
SourceHall A
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Target Spectro: SQQDQ
Hall ACherenkovcones
PMT
Compton Moller Polarimeters
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
High Resolution Spectrometers
Elastic
Inelastic
detector
Q Q
Dipole
Quad
Spectrometer Concept:
Resolve Elastic
target
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Optimum Kinematics for Lead Parity: E = 850 MeV, <A> = 0.5 ppm. Accuracy in Asy 3%
n
Fig. of merit
Min. error in R
maximize:
1 month run
1% in R
n
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Corrections to the Asymmetry are Mostly Negligible
• Coulomb Distortions ~20% = the biggest correction.
• Strangeness
• Electric Form Factor of Neutron
• Parity Admixtures
• Dispersion Corrections
• Meson Exchange Currents
• Shape Dependence
• Isospin Corrections
• Radiative Corrections
• Excited States
• Target Impurities
Horowitz, et.al. PRC 63 025501
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Septum Magnets (INFN)
Electrons scatteredat 6 deg sent to theHRS at 12.5 deg.
•Superconducting magnets
•Commissioned 2003-4
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Integrating Detection
PMT
Calorimeter (for lead, fits in palm of hand)
ADC
Integrator
electrons
• Integrate in 30 msec helicity period.
• Deadtime free.
• 18 bit ADC with < 10 nonlinearity.
• But must separate backgrounds & inelastics ( HRS).
- 4
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Lead Target
Liquid Helium Coolant
Pb
C
208
12
Diamond Backing:
• High Thermal Conductivity• Negligible Systematics
Beam, rastered 4 x 4 mm
A 80at ly testedSuccessful
beam
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Halfwave plate (retractable, reverses helicity)
Laser
Pockel Cell flips helicity
Gun
GaAs Crystal
e beam-
• Rapid, random helicity reversal
• Electrical isolation from rest of lab
• Feedback on Intensity Asymmetry
Polarized Electron Source
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
P I T A Effect at Polarized Source
)sin( IA Laser at Pol. Source
Polarization Induced Transport Asymmetry
yx
yx
TT
TT
where
Transport Asymmetry
Intensity Asymmetry
drifts, but slope is ~stable. Feedback on
(G. D. Cates)
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Beam Asymmetries
Araw
= Adet
- AQ +
E+
ix
i
•natural beam jitter (regression) •beam modulation (dithering)Slopes from
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
“Energy” BPM
BPM Y2
BPM Y1
BPM X1
BPM X2
Scale +/- 10 nm
Position Diffs avg
~ 1 nm
Redundant Monitors
• Stripline Monitors
• Resonant Cavities
Negligible
Systematic Error
Helicity Correlated Differences: Position, Angle, Energy
slug
slug
slug
slug
“slug” = ~1 day running
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Polarimetry
Electron onlyPhoton only
Preliminary: 2.5% syst ( only)
Møller : Pe/Pe ~ 3 % (limit: foil polarization)
Compton : 2% syst. at present
2 analyses based on either electron
or photon detection
Superlattice:
Pe=86% !
PREX:
1 % desirable
2 % requirede
PREXUVa Seminar, Nov 2005
R. Michaels
Jefferson Lab
Upgrade of Compton Polarimeter (Nanda, Lhuillier)
To reach 1% accuracy:
• Green Laser (increased sensitivity at low E)
• Integrating Method (removes some systematics of analyzing power)
