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Strangeness in the Proton:
The G0 Forward-Angle Measurement
Sarah K. Phillips 1, Benoit Guillon2, Lars Hannelius 3, Jianglai Liu4, Kazutaka Nakahara 5, The G0 Collaboration
1The College of William and Mary, 2Grenoble, 3California Institute of Technology, 4University of Maryland, 5University of Illinois
For G0, the background under the elastic
Spokesperson: Doug Beck (U. Illinois) G0 in Hall C Target and Spectrometer peak is almost entirely protons
Deputy Spokesperson: Phil Roos For the G0 forward-angle • Inelastics from the LH2
• 20 cm LH2, Aluminum cell, • Quasi-elastic and inelastics from
(U. Maryland) measurement, a dedicated
unpolarized aluminum target cell
spectrometer was installed in Hall C
Caltech, Carnegie-Mellon, William & Mary, • Located inside magnet To determine the background yield, data
• Measures asymmetries of a few
Grinnell, Hampton, IPN-Orsay, ISN-Grenoble, • 250 W heat load from 40 µA were taken
Jefferson Lab, Kentucky, LaTech, NMSU, ppm from parity-violating e-p
TRIUMF, UIUC, U. Manitoba, U. Maryland, scattering beam • With gaseous H2 targets (27K, 37K)
UNBC, Virginia Tech, Winnipeg, Yerevan • High flow rate to minimize • With dummy aluminum entrance and exit windows
•Takes measurement over full range Above: A schematic of the G0 target Background Composition
target density fluctuations • With Fastbus measurements for particle ID.
of Q2 (0.12 – 1.0 (GeV/c)2) in one Below: A drawing of the particle trajectories from
energy setting (3 GeV) The G0 Experiment in Hall C at Jefferson Lab the target to the detectors → density change < 1.5%
What role do strange quarks play in nucleon properties? For a typical detector (Detector 8), the
•Took 701 hours of parity-quality background correction is a two-step fitting
Standard Model and QCD beam (101 Coulombs) from procedure:
November 2003 to May 2004
G0 Beam Properties • Fit elastic peak of ToF spectrum with a
The new G0 Tiger laser
Gaussian, YBack with a 4th order polynomial to
u
Valence quarks – determine G0 had challenging beam specifications, all
Yield Fit extract bin-by-bin dilution factor
u nucleon properties such as
successfully met by the accelerator division! • Then fit ABack with a 2nd order polynomial
d overall spin and charge
• Unusual time structure of 31 MHz (32 ns using constant AElastic and the dilution factors
u Non-strange sea quarks – do between pulses for ToF) Target Module Magnet Detectors from the yield fits
not change nucleon properties • Required a new Ti:Sapphire laser for the
u but contribute to rest energy • Superconducting toroidal magnet sorts recoil • Works well for detectors 1 – 14; modified for
polarized source (shown on left). detector 15
s Strange sea quarks – contribute protons by Q2 into the focal plane scintillation
Asymmetry Fit
to rest energy, more massive Beam Achieved Spec detectors
than u and d quarks, could • 40 µA beam current, 3 GeV beam energy Parameter Results
s • 8 octants: 4 French, 4 North American
contribute to nucleon properties! • Higher bunch charge complicated transport Charge -0.14±0.32 1 ppm
• High polarization: 73.7 ± 1.3 % Asymmetry ppm • 16 scintillator pairs per octant → Q2 bins The G0 data are consistent with other
The Goal of G0: To determine the contribution of the • Excellent helicity correlated beam x position 3 ± 4 nm 20 nm
– Detectors 1 to 15 sensitive to different
difference experiments and have an intriguing Q2
strange quarks to the electromagnetic properties of the properties Q2 range dependence!
y position 4 ± 4 nm 20 nm
nucleon! • Low beam halo
difference – Detector 16 measures background
• Electron spin flipped in a pseudorandom • Over the full range, the data disagrees
x angle 1 ± 1 nrad 2 nrad Construction of the NA octants
sequence every 30 ms with Pockels cell → with the no-strange hypothesis at the
Why is this interesting? If one thinks of the nucleon as a hydrogen difference took place at Jefferson Lab
4 helicity flips = asymmetry computing unit! Data Characteristics and Analysis 89% confidence level
atom, then strange quarks contributing to nucleon properties is like y angle 1.5 ± 1 nrad 2 nrad
a lamb shift due to virtual muons in the QED sea. It is the non- difference • Two different systems of • The Q2 dependence, when combined
perturbative nature of QCD that makes this effect possibly sizable! energy 29 ± 4 eV 75 eV A typical ToF spectrum (detector 8) electronics: French and NA with HAPPEX and A4, suggests that
difference (custom and commercial) GsM at low Q2 is positive, and that GsE in
Parity-Violating Electron Scattering: Above is a table of the helicity-correlated beam • High counting rate (~ 1 MHz the range of Q2 ~0.3 may be negative
properties that were measured in the per detector)
The Probe of Neutral Weak Form
experiment. On the left is a plot of the beam • ToF separates the protons World Data at Q2 = 0.1 (GeV/c)2 Upper Figure: GsE + ηGsM for this
Factors polarization measured through the data-taking from the π+ measurement. The gray bands indicate
e p e p period, by IHWP state. The measurements are the systematic uncertainties.
Electron-proton elastic scattering → divided into periods of stable polarization. • ToF resolution: 0.25 ns Lower Figure: The experimental
2 French, 1 ns NA asymmetries versus Q2. The line is the
Polarized electrons on an unpolarized target “no vector strange” asymmetry.
Beam Leakage Correction
R L GF Q AE AM AA
2
From this measurement, we determined
A During the data-taking, it was found that about We used an insertable half-wave the combination GsE+ηGsM. For the
R L 4 2 2 unpol 50 nA of beam from the Hall A & B lasers was plate (IHWP) as a systematic check separation of the form factors, stay tuned
leaking into the 40 µA Hall C beam → reverses laser circular polarization for the backangle measurements!
Proton/neutron charge symmetry • Had large, positive asymmetry and sign of physics asymmetries
Strange electric • Different time structure: 499 MHz • “In” and “Out” asymmetries agree GE 0.013 0.028 GM 0.62 0.31 0.62 2
s s
AE ( ) GE (Q 2 )GE (Q 2 )
Z
GE
s
and magnetic form • Caused ToF and current dependent false asymmetry! • No evidence of electronic false
The asymmetries for the elastic
AM (Q 2 ) GM (Q 2 )GM (Q 2 )
Z
GM
s
factors, and axial asymmetries proton cut under IHWP reversal References
AA (1 4 sin 2 W ) G A (Q 2 )GM (Q 2 )
e
GA
e
form factor Asymmetries corrected by 1. D. S. Armstrong et al. (G0 Collaboration), nucl-ex/0506021
• Measuring false asymmetry in signal-free regions of ToF spectra Corrections to the raw asymmetry
• Studies with lasers to the other halls turned off • Beam leakage • Background and its asymmetry Acknowledgements
Forward angle result of measurement off LH2 is
• Cross-checks against low-current runs • Helicity-correlated beam • Beam polarization
(GEs+ηGMs) for a range of 18 Q2 values between • Leakage asymmetry is +570 ppm
We gratefully give our thanks to Julie Roche (our Analysis Coordinator) and all of
properties • Radiative effects our other collaborators. This work is supported in part by CNRS (France), DOE
0.12 and 1.0 (GeV/c)2 • Correction of order 0.71 ± 0.14 ppm • Deadtime • Transverse polarization (USA), NSERC (Canada) and NSF (USA).
