G 0 Forward Angle Measurement and the Strange Sea of the Proton. Kazutaka Nakahara. Strangeness (briefly) Parity violation G0 Experiment Physics results. SLAC Seminar 1/19/06. Proton Structure. 3 valence quarks not a bad approx. at high energies
SLAC Seminar 1/19/06
3 valence quarks not a bad approx. at high energies
At low energy, things get messy. Sea includes gluons and “sea quarks”, and can contribute to proton structure
Strange quarks give
exclusive insight into the sea
and come in pairs no net strangeness
EM coupling to pointlike fermion
Represents internal structure of the proton
Sach’s FF ~ p, rp at q2=0
~ Fourier transform of the charge and magnetization distribution of the proton.
= Neutron EM form factor
Measure the neutral weak proton form factor
Access the neutral weak sector of elastic e-p scattering
Want to know and
= Proton EM form factor
can be determined through parity-violating elastic e-p scattering.
Parity Conserving Parity Violating
Measure asymmetry in elastic e-p cross section for + and – helicity incident electrons.
-3 to -40 ppm measurement!!
Measure at forward angles
Measure at backward angles
(elastic e-p and quasi-elastic e-d)
Can QCD tell us these things? In principle, yes. But hard to calculate.
Most calculations attempt to calculate s.
Maybe negative, but not much consensus...try measuring.
Ebeam = 3.03 GeV, 0.33 - 0.93 GeV
Ibeam = 40 mA, 80 mA
Pbeam = 75%, 80%
q = 52 – 760, 104 - 1160
DW = 0.9 sr, 0.5 sr
ltarget = 20 cm
L = 2.1, 4.2 x 1038 cm-2 s-1
A ~ -1 to -50 ppm, -12 to -70 ppm
- Gives different linear combination of u, d, and s contributions.
- Forward angles recoil protons
- Backward angles elastic and quasi-elastic electrons
Separates electric and magnetic contributions.
cryogenic target ‘service module’
3 endstations. One laser for each hall shining a common GaAs cathode
1497 MHz SRF cavities
Each hall receives beam at 499 MHz simultaneous 3 hall delivery possible.
~ 600 MeV / linac, 2 linacs per “pass” with up to 5 passes possible.
RF separator + Lambertson kicks beam into the correct hall.
Feedback shows convergence of HC beam properties
701 h at 40 A (101C)
19 x 106 quartets
76 x 106 MPS
Integrate yield over elastic region for + and – helicities
...done? Not so fast
Various systematic effects must be corrected.
Raw Asymmetries, Ameas
Leakage beam asymmetry
Helicity-correlated beam properties
EM form factors
- 100x to 1000x smaller than the smallest physics asymmetry measured in G0 - Significantly smaller than the 5-10% total uncertainty expected for G0.
2ns “background” spectra under the G0 spectra.
32 ns G0 beamOnly 1 out of 16 buckets should be filled!!
~ 1.6 pC G0
Use cut0 region to determine the leakage asymmetry. Agrees with leakage-only runs.
Aleak = -0.71 0.14 ppm (global uncertainty)
2 step fitting procedure:
Gaussian Yel, constant Ael
Pol’4 Ybkg, Pol’2 Abkg
No “vector strange” asymmetry, ANVS, is A(GEs,GMs=0)
Inner error band is stat., outer band is stat. + pt-pt. Global error band dominated by leakage and background corrections.
Forward angle results: http://www.npl.uiuc.edu/exp/G0/Forward
“Kelly form factors”: Kelly PRC 70 (2004) 068202
Forward angle results over 0.12 < Q2 < 1.0 GeV2. Model uncertainty from EW radiative corrections.
3 types of nucleon form factors shift in baseline
No-vector-strange hypothesis disfavored at 89%.
= -0.013 0.028
= +0.62 0.31
World data at Q2 = 0.1 including G0.
GEs and GMs:
(G0 and PVA4)
1. Show chi2 of background fits.
p6. Too cluttered. Try creating animation.
Proton is the only known stable hadron. Extensively studied, but the details of its composition is not well known.
Measure through Parity-Violation
Each was at different angular kinematics.
Minimize through active IA (charge) and PZT (position) feedback.
Particle identification through TOF separation.
Detect four-fold coincidence hits.
Fast counting electronics (~1MHz per detector).
Interpolate asymetries and yields over detectors 12 through 16 (for each timebin).
Linear interpolation, ±0.5 “detector” as uncertainty.
Smooth interpolation from lower detectors, ±1 “detector” and ± 0.5 ns time shift as uncertainty
Result shows good agreement with sideband asymmetries.
Fit suggests a positive and a negative bump in . Is this model realistic or too simplistic? There are some preliminary results that may support this claim. Otherwise, stay tuned for more results!