Transcript No Slide Title

Strangeness in the Proton:
The G0 Forward-Angle Measurement
Sarah K.
1The
1
Phillips ,
Benoit
2
Guillon ,
College of William and Mary,
Lars
3
Hannelius ,
2Grenoble, 3California
Jianglai
4
Liu ,
Kazutaka
Institute of Technology,
4University
G0 in Hall C
Spokesperson: Doug Beck (U. Illinois)
Deputy Spokesperson: Phil Roos
(U. Maryland)
Caltech, Carnegie-Mellon, William & Mary,
Grinnell, Hampton, IPN-Orsay, ISN-Grenoble,
Jefferson Lab, Kentucky, LaTech, NMSU,
TRIUMF, UIUC, U. Manitoba, U. Maryland,
UNBC, Virginia Tech, Winnipeg, Yerevan
What role do strange quarks play in nucleon properties?
Standard Model and QCD
5
Nakahara ,
of Maryland,
Target and Spectrometer
For the G0 forward-angle
measurement, a dedicated
spectrometer was installed in Hall C
• Measures asymmetries of a few
ppm from parity-violating e-p
scattering
•Takes measurement over full range
of Q2 (0.12 – 1.0 (GeV/c)2) in one
energy setting (3 GeV)
•Took 701 hours of parity-quality
beam (101 Coulombs) from
November 2003 to May 2004
• 20 cm LH2, Aluminum cell,
unpolarized
• Located inside magnet
• 250 W heat load from 40 µA
beam
• High flow rate to minimize
target density fluctuations
→ density change < 1.5%
Above: A schematic of the G0 target
The G0 Experiment in Hall C at Jefferson Lab
Below: A drawing of the particle trajectories from
the target to the detectors
The G0 Collaboration
5University
For G0, the background under the elastic
peak is almost entirely protons
• Inelastics from the LH2
• Quasi-elastic and inelastics from
aluminum target cell
To determine the background yield, data
were taken
• With gaseous H2 targets (27K, 37K)
• With dummy aluminum entrance and exit windows
• With Fastbus measurements for particle ID.
Valence quarks – determine
nucleon properties such as
overall spin and charge
G0 Beam Properties
u
d
u
Non-strange sea quarks – do
not change nucleon properties
but contribute to rest energy
Strange sea quarks – contribute
to rest energy, more massive
than u and d quarks, could
contribute to nucleon properties!
u
s
s
The Goal of G0: To determine the contribution of the
strange quarks to the electromagnetic properties of the
nucleon!
Why is this interesting? If one thinks of the nucleon as a hydrogen
atom, then strange quarks contributing to nucleon properties is like
a lamb shift due to virtual muons in the QED sea. It is the nonperturbative nature of QCD that makes this effect possibly sizable!
Parity-Violating Electron Scattering:
The Probe of Neutral Weak Form
Factors
Electron-proton elastic scattering →
e
Polarized electrons on an unpolarized target


p e

p
Achieved
Spec
Charge
Asymmetry
-0.14±0.32
ppm
1 ppm
x position
difference
3 ± 4 nm
20 nm
y position
difference
4 ± 4 nm
20 nm
x angle
difference
1 ± 1 nrad
2 nrad
y angle
difference
1.5 ± 1 nrad
energy
difference
29 ± 4 eV
Target Module
Magnet
Detectors
• Works well for detectors 1 – 14; modified for
detector 15
• Superconducting toroidal magnet sorts recoil
protons by Q2 into the focal plane scintillation
detectors
Asymmetry Fit
Results
• 8 octants: 4 French, 4 North American
• 16 scintillator pairs per octant → Q2 bins
2 nrad
75 eV
The G0 data are consistent with other
experiments and have an intriguing Q2
dependence!
– Detectors 1 to 15 sensitive to different
Q2 range
– Detector 16 measures background
Data Characteristics and Analysis
Construction of the NA octants
took place at Jefferson Lab
• Two different systems of
electronics: French and NA
(custom and commercial)
A typical ToF spectrum (detector 8)
• High counting rate (~ 1 MHz
per detector)
Above is a table of the helicity-correlated beam
properties that were measured in the
experiment. On the left is a plot of the beam
polarization measured through the data-taking
period, by IHWP state. The measurements are
divided into periods of stable polarization.
Proton/neutron charge symmetry
Strange electric
and magnetic form
factors, and axial
form factor
Forward angle result of measurement off LH2 is
(GEs+ηGMs) for a range of 18 Q2 values between
0.12 and 1.0 (GeV/c)2
During the data-taking, it was found that about
50 nA of beam from the Hall A & B lasers was
leaking into the 40 µA Hall C beam
• Had large, positive asymmetry
• Different time structure: 499 MHz
• Caused ToF and current dependent false asymmetry!
Asymmetries corrected by
• Measuring false asymmetry in signal-free regions of ToF spectra
• Studies with lasers to the other halls turned off
• Cross-checks against low-current runs
• Leakage asymmetry is +570 ppm
• Correction of order 0.71 ± 0.14 ppm
• Fit elastic peak of ToF spectrum with a
Gaussian, YBack with a 4th order polynomial to
extract bin-by-bin dilution factor
• Then fit ABack with a 2nd order polynomial
using constant AElastic and the dilution factors
from the yield fits
• ToF separates the protons
from the π+
• Over the full range, the data disagrees
with the no-strange hypothesis at the
89% confidence level
• The Q2 dependence, when combined
with HAPPEX and A4, suggests that
GsM at low Q2 is positive, and that GsE in
the range of Q2 ~0.3 may be negative
World Data at Q2 = 0.1 (GeV/c)2
Upper Figure: GsE + ηGsM for this
measurement. The gray bands indicate
the systematic uncertainties.
Lower Figure: The experimental
asymmetries versus Q2. The line is the
“no vector strange” asymmetry.
• ToF resolution: 0.25 ns
French, 1 ns NA
Beam Leakage Correction
 R   L   GF Q  AE  AM  AA
A


 R   L  4 2  2 unpol
 G Es
 GMs
 G Ae
• 40 µA beam current, 3 GeV beam energy
• Higher bunch charge complicated transport
• High polarization: 73.7 ± 1.3 %
• Excellent helicity correlated beam
properties
• Low beam halo
• Electron spin flipped in a pseudorandom
sequence every 30 ms with Pockels cell →
4 helicity flips = asymmetry computing unit!
Beam
Parameter
Yield Fit
2
2
AE   ( ) GEZ (Q 2 )GE (Q 2 )
AM   (Q 2 ) GMZ (Q 2 )GM (Q 2 )
AA  (1  4 sin 2  W )  G Ae (Q 2 )GM (Q 2 )
G0 had challenging beam specifications, all
successfully met by the accelerator division!
• Unusual time structure of 31 MHz (32 ns
between pulses for ToF)
• Required a new Ti:Sapphire laser for the
polarized source (shown on left).
We used an insertable half-wave
plate (IHWP) as a systematic check
→ reverses laser circular polarization
and sign of physics asymmetries
• “In” and “Out” asymmetries agree
• No evidence of electronic false
asymmetries
From this measurement, we determined
the combination GsE+ηGsM. For the
separation of the form factors, stay tuned
for the backangle measurements!
GEs  0.013 0.028 GMs  0.62  0.31 0.62 2
The asymmetries for the elastic
proton cut under IHWP reversal
Corrections to the raw asymmetry
• Beam leakage
• Helicity-correlated beam
properties
• Deadtime
Background Composition
For a typical detector (Detector 8), the
background correction is a two-step fitting
procedure:
The new G0 Tiger laser
u
of Illinois
• Background and its asymmetry
• Beam polarization
• Radiative effects
• Transverse polarization
References
1. D. S. Armstrong et al. (G0 Collaboration), nucl-ex/0506021
Acknowledgements
We gratefully give our thanks to Julie Roche (our Analysis Coordinator) and all of
our other collaborators. This work is supported in part by CNRS (France), DOE
(USA), NSERC (Canada) and NSF (USA).