Strange form factors: an update on G0

1
Strange form factors an update on G0
E. Beise University of Maryland and National
Science Foundation

• motivation
• parity violation observables
• recap of G0 forward results
• update on G0 backangle

courtesy of JLab
Support from U.S. NSF and DOE gratefully
acknowledged
2
Views of the Nucleon
quark model
look explicitly at strange quarks
QCD/parton description
p
pion cloud
p
p
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Spatial distribution of s-quarks

• Access via form factors contribution to nucleon
  charge and magnetism

Electromagnetic
charge symmetry
and use
sin2qW 0.23113 0.00015
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how to interpret the signs
and
If ms is negative, then s and sbar would make ()
overall contribution to mp.
Hannelius, Riska Glozman, Nucl. Phys. A 665
(2000) 353
(Jaffe convention) negative charge radius ?
positive rs ? GEs gt 0 ?
sbar on the outside (on average) see R. Jaffe,
PLB 229 (1989) 275 or Geiger Isgur, PRD 55
(1997) 299
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Lattice QCD inspired prediction
quenched LQCD chiral extrapolation
lattice
s/d in loop, assume 0 ? 1
assumes u and d quark loop contributions to
proton are same (true if mu md )
result
Leinweber etal, PRL 94, 212001 (2005)
from R. Young
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heavy quark contribution
X. Ji D. Toublan, hep-ph/0605055
For mQ gtgt LQCD can show
and
If s-quark heavy, then generally
mQ?
Is there a critical value of the mass at which
dmQ would change sign?
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another example Chiral Soliton Model
constituent quarks in meson field
A. Silva, et al., hep-ph/0210189, PRD 65
(2001) 014016 Kim, et al., NPA 616 (1997) 606
Mq 400 MeV ms 140 MeV
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Parity Violating elastic e-N scattering
polarized electrons, unpolarized target
t Q2/4M2 e 12(1t)tan2(q /2)-1 e
t(t1)(1-e2)1/2
Neutral Weak ffs contain explicit contributions
from strange sea
GZA(0) 1.2695 0.0035 (from b decay)
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Axial form factor seen by PV electron scattering
(hFAg Re) at Q20 computed by Zhu etal, PRD 62
(2000) 033008 SAMPLE deuterium data agree
well FAg(0) is likely small
Maekawa, Viega, van Kolck, PLB 488 (2000) 167
GA - like
but Q2 behavior is not well constrained
G0
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Quasielastic PV (ee) in Deuterium
Use Quasielastic scattering from deuterium as
lever arm for GAe(Q2)
vector s-quark contributions in p and n largely
cancel.
But there is NN physics
(Parity conserving) nuclear corrections 1-3
at backward angles
(Diaconescu, Schiavilla van Kolck, PRC 63
(2001) 044007) See also Hadjimichael, Poulis
Donnelly, PRC45 (1992) 2666 Schramm Horowitz,
PRC 49 (1994) 2777 Kuster Arenhovel, NPA 626
(1997) 911
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Ebeam 680 MeV, qe 100
thanks to R. Schiavilla
red includes 2-body terms
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Ebeam 360 MeV, qe 100
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Jefferson Laboratory
G0
HAPPEX
A
C
B
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The G0 experiment at JLAB

• Forward and backward angle PV e-p elastic and
  e-d (quasielastic) in JLab Hall C
• superconducting toroidal magnet

• scattered particles detected in segmented
  scintillator arrays in spectrometer focal plane
• custom electronics count and process scattered
  particles at gt 1 MHz
• forward angle run completed
• backward angle ? March 06

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G0 Forward Configuration in Hall C at JLAB
superconducting magnet (SMS)
G0 beam girder
detectors (Ferris wheel)
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G0 Forward Detection Scheme
Detect scattered Protons Magnet sorts protons by
Q2 at one setting Beam bunches 32 nsec
apart Flight time separates p and p
Beam spin flipped every 30 ms - -
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JLab Parity-Quality beam (circa 2004)

• G0 forward running beam
• strained GaAs (PB 73)
• 32 ns pulse spacing
• 40 mA beam current

• HAPPEX-II beam
• superlattice (PB gt 85)
• 2 ns pulse spacing
• 35 mA beam current

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G0 Forward angle Results
EM form factors J.J.Kelly, PRC 70, 068202 (2004)
Examining full data set, probability
that GEshGMs ? 0 is 89
D.S. Armstrong etal, PRL 95 (2005) 092001
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Results of Strange Form Factor Measurements
March 2006

• Measurements at
• Q2 0.1 GeV2
• SAMPLE
• HAPPEx
• PVA4
• G0 (extrapolated downward)

7 3 contribution to mp
D. Armstrong and K. Carter, CERN Courier
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Results of Strange Form Factor Measurements
May 2006

• Measurements at
• Q2 0.1 GeV2
• SAMPLE
• HAPPEx
• PVA4
• G0 (extrapolated downward)
• ADD HAPPEX 05

New HAPPEX results ? Gordon Cates is next
figure from K. Paschke
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Data at higher Q2
Q2 0.23 GeV2
Q2 0.5 GeV2
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Strange Form Factor Measurements Q2 dependence
Knowledge of Q2 dependence will require separated
form factors.
With current data, use very simple ideas to gain
qualitative insight.
(from M. Pitt)
Try dipole form for GsM constrained to agree
with world data at Q2 0.1 GeV2
0.61
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New Global Analysis to Extract GsE ,GsM and GeA
R. D. Young, J. Roche, R. D. Carlini, and A. W.
Thomas, nucl-ex/0604010

• uses data up to Q2 0.3 GeV2
• parameterizes s-quark form factors with Taylor
  expansions at low Q2
• uses only data (not the theory prediction) for
  GeA

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G0 Backward Angle
Electron detection q 108 Both LH2 and LD2
targets Add Cryostat Exit Detectors (CED) to
define electron trajectory 1 scaler per channel
FPD/CED pair Aerogel Cerenkov detector for p/e
separation (pp lt 380 MeV/c)
Common Q2 with HAPPEX-III and PVA4
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Superconducting Magnet (SMS)
Target module
G0 beam girder
FPDs
CEDCherenkov
Spokesman
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G0 First Backangle Run

• March 15 May 1 687 MeV
• 200 hours LH2, 50 hours LD2 (at 10 mA)
• 80 hours parity quality data w/ LH2 at 60 mA
• May 15-18 362 MeV
• first look at LD2 at low beam current
• outstanding beam delivery
• July 19-Sept 1 (360 MeV) / Sept 22-Dec 22 (687)
  production

• Synopsis from commissioning
• (blinded) H elastic asymmetry near expected
• pion asymmetry smaller than elastic, same sign
• H ? good elastic/inelastic electron separation
• coincidence rates not far from simulation
• D ? high singles rates in Cerenkov PM tubes,
  improvements under study (we think its
  neutrons)

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Particle ID CED-FPD plane
60 mA, LH2
10 mA, LD2
(rates in Hz/mA per octant)
Electrons
Pions
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Polarized Beam Performance
Polarized source superlattice GaAs w/ new
Fiber laser Beam polarization 85 Moller
modified to accommodate lower beam energy
Helicity Correlated Beam properties
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Target and Luminosity Monitors
LH2 and LD2 tested up to 60 mA Very linear rate
behavior Ratio LD2LH212C as expected

• Typical LUMI asymmetry width
• 250 ppm --
• 25 times better statistics than all detector
  octants combined
• boiling contribution negligible

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Asymmetries Electron and LH2 target
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Asymmetries Electron and LH2
(very!) preliminary and blinded
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-1/3
-1/3
world fit 06
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Anapole form factor FA(Q2)
Three scenarios
1. FA(Q2) is like GA(Q2)
2. FA(Q2) is flat (Riska, NPA 678 (2000) 79)
3. FA(Q2) 1 Q2 (Maekawa, Viega, van Kolck,
PLB 488 (2000) 167) most extreme set of model
parameters
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The G0 Collaboration
College of William and Mary, Institut de
Physique Nucléaire d'Orsay, Yerevan Physics
Institute, Laboratoire de Physique Subatomique et
de Cosmologie-Grenoble, University of Illinois,
University of Maryland, Thomas Jefferson National
Accelerator Facility, University of Manitoba,
Carnegie Mellon University, California Institute
of Technology, University of Kentucky, TRIUMF,
Louisiana Tech University, Virginia Tech,
University of Northern British Columbia, New
Mexico State University, University of Winnipeg,
Hampton University, 19Grinnell College Univ
Zagreb (for backward angle runs)
U.S. G0 participation jointly supported by DOE
and NSF. Significant additional contributions to
G0 from NSERC (Canada), CNRS (FR), and our
Armenian colleagues
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• backups

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G0 Apparatus
One octants scintillator array
20 cm LH2 Target
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G0 Backward angle configuration

• Particle detection and identification
• Turn-around of the magnet
• Detection of the electrons (qe 110)
• Extensive simulation of background
• Need for new shielding around the target cell
• and at the magnet exit window
• Cryostat Exit Detectors (CED)
• to separate elastic and inelastic electrons
• Cherenkov detector
• p/e separation (both are recorded)
• 5 cm aerogel, n 1.03 (OK for pp lt 380 MeV/c),
• 90 efficient for e- and p rejection factor ?
  100
• From cosmic muons and test beam 5-8 p.e.
• Special design for Magnetic shielding (SMS
  fringe field)

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Pion Asymmetries in LH2 687 MeV
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N-D axial transition form factor
S. Wells et al., LaTech
expected precision
D(1) 2(1-sin2qW) 1 D(2) non-resonant
contrib. (small) D(3) 2(1-4sin2qW) F(Q2,s)
at tree-level

• First measurement in neutral current process
• sensitive to hadronic radiative corrections

• data comes for free w/ back-angle G0 elastic
  measurement

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Hadronic PV contributions to the deuteron
Schiavilla, Carlson Paris, PRC 67 (2003) 032501
Liu, Prezeau, Ramsey-Musolf, PRC 67 (2003)
035501
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World Data at Q20.1 GeV2 Rosenbluth Plot

• Post-HAPPEX 05
• GEs 0.006 /- 0.016
• GMs 0.28 /- 0.20
• New HAPPEX results yield
• smaller GMs
• Even with this shift in the
• central value, world data still
• remarkably self-consistent

SAMPLE
h1.67

• G0
• HAPPEX 04
• HAPPEX 05
• Mainz A4

Thanks to Kent Paschke for figure