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The Strange Vector Current of the Nucleon Forward Angle Experiment

The Strange Vector Current of the Nucleon Forward Angle Experiment. Jianglai Liu University of Maryland. Strangeness, very briefly Parity violation as an experimental probe The G 0 experiment Emerging physics picture. Fermi Lab seminar, 11-15-2005.

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The Strange Vector Current of the Nucleon Forward Angle Experiment

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  1. The Strange Vector Current of the Nucleon Forward Angle Experiment Jianglai Liu University of Maryland • Strangeness, very briefly • Parity violation as an experimental probe • The G0 experiment • Emerging physics picture Fermi Lab seminar, 11-15-2005

  2. Quark-antiquark pairs and gluons make up the QCD vacuum (“sea”). • ss are “virtual pairs”, so the net strangeness is zero. • s and s might not have identical distributions. So strangeness might manifest locally. Analogous to the charge distribution in neutron! Strangeness in the Nucleon? Quark models: Only u and d quarks in nucleons. No strangeness! “Full QCD description”

  3. Nucleons are the “hydrogen atom” of QCD. ss gives direct access to the “loops” of QCD sea. Recall the QED loops and the famous Lamb shift, g-2 … What’s the Big Deal of Strangeness? Different from QED, however, QCD is non-perturbative! So the vacuum fluctuation could be sizable, so does strangeness! s quark belongs to the 2nd generation. So “ Lamb shift in QCD”!!! Would have been hugely suppressed in a perturbative theory. Tough to calculate!!!

  4. Contribution of s quark to the longitudinal momentum • Contribution to the nucleon mass • Contribution to the nucleon spin DIS N charm production: N scattering + hyperon mass splitting: likely 100% uncertainty • polarized inclusive DIS • elastic N scattering • polarized semi-inclusive DIS What Do We “Know” Already? All these indicate s quarks contribute sizably in nucleon structure, however with large uncertainties.

  5. Strange Vector Current Strange quark contributes to nucleon charge and magnetism? Define vector (EM) form factors: Distribution of nucleon’s charge and magnetization. EM quark current of the nucleon • Neglected heavier quarks • Charge symmetry Need one more constraint …

  6. Neutral-weak Current  Additional Constraint NC contains a vector and axial piece Charges in the unified electroweak theory in SM sin2qW = 0.2312 ± 0.00015 So define NC vector form factor: Kaplan and Manohar, 1988

  7. f/w N N s-quark form factors calculations at Q2=0

  8. e e N N Measuring the NC Form Factor: Parity Violation Elastic e-N scattering • NC amplitude suppressed • by ~10-4 • Difficult to see in cross-section measurement + PC PV However, if one measures the parity violation in the elastic scattering, one accesses the interference between EM and NC interactions  “amplify” the relative experimental sensitivity to NC interaction. Mckeown and Beck, 1989

  9. detector p’ R spin L e p e’ Measurement of Parity Violation First observation of parity violation in weak interaction; Madam Wu’s famous 1957 60Co beta decay experiment. 60Co 60Ni* + e- + e  detector 60Co B C. S. Wu In parity violating e-p scattering, the spin (helicity) of the electron is flipped back and forth.

  10. Parity Violating Asymmetry forward ep backward ep backward ed kinematic factors Assuming EM and axial form factors are known (with errors), each measurement yield GEs+GMs where

  11. Summary of PV Electron Scattering Experiments From D.H. Beck publishing, running publishing, running x2, published x2, running publishing, running

  12. Strange FF: Results at Q2=0.1 Combining world data (backward and forward) at Q2=0.1 GeV2 allows one to separate GEsand GMS 2=1 GMs= 0.550.28 GEs= -0.010.03 95% c.l.

  13. linac polarized source G0 A C B The Jefferson Laboratory

  14. The G0 Collaboration D.S.Armstrong1, J.Arvieux2, R.Asaturyan3, T.Averett1, S.L.Bailey1, G.Batigne4, D.H.Beck5, E.J.Beise6, J.Benesch7, L.Bimbot2, J.Birchall8, A.Biselli9, P.Bosted7, E.Boukobza2,7, H.Breuer6, R.Carlini7, R.Carr10, N.Chant6, Y.-C.Chao7, S.Chattopadhyay7, R.Clark9, S.Covrig10, A.Cowley6, D.Dale11, C.Davis12, W.Falk8, J.M.Finn1, T.Forest13, G.Franklin9, C.Furget4,D.Gaskell7, J.Grames7, K.A.Griffioen1, K.Grimm1,4,B.Guillon4, H.Guler2, L.Hannelius10, R.Hasty5, A. Hawthorne Allen14, T.Horn6, K.Johnston13, M.Jones7, P.Kammel5, R.Kazimi7, P.M.King6,5, A.Kolarkar11, E.Korkmaz15, W.Korsch11, S.Kox4, J.Kuhn9, J.Lachniet9, L.Lee8, J.Lenoble2, E.Liatard4, J.Liu6, B.Loupias2,7, A.Lung7, G.A.MacLachlan16, D.Marchand2, J.W.Martin10,17, K.W.McFarlane18, D.W.McKee16, R.D.McKeown10, F.Merchez4, H.Mkrtchyan3, B.Moffit1, M.Morlet2, I.Nakagawa11, K.Nakahara5, M.Nakos16, R.Neveling5, S.Niccolai2, S.Ong2, S.Page8, V.Papavassiliou16, S.F.Pate16, S.K.Phillips1, M.L.Pitt14, M.Poelker7, T.A.Porcelli15,8, G.Quéméner4, B.Quinn9, W.D.Ramsay8, A.W.Rauf8, J.-S.Real4, J.Roche7,1, P.Roos6, G.A.Rutledge8, J.Secrest1, N.Simicevic13, G.R.Smith7, D.T.Spayde5,19, S.Stepanyan3, M.Stutzman7, V.Sulkosky1, V.Tadevosyan3, R.Tieulent4, J.van de Wiele2, W.van Oers8, E.Voutier4, W.Vulcan7, G.Warren7, S.P.Wells13, S.E.Williamson5, S.A.Wood7, C.Yan7, J.Yun14 1College of William and Mary, 2Institut de Physique Nucléaire d'Orsay, 3Yerevan Physics Institute, 4Laboratoire de Physique Subatomique et de Cosmologie-Grenoble, 5University of Illinois, 6University of Maryland, 7Thomas Jefferson National Accelerator Facility, 8University of Manitoba, 9Carnegie Mellon University, 10California Institute of Technology, 11University of Kentucky, 12TRIUMF, 13Louisiana Tech University, 14Virginia Tech, 15University of Northern British Columbia, 16New Mexico State University, 17University of Winnipeg, 18Hampton University, 19Grinnell College

  15. Overview of the G0 experiment • Measure forward & backward asymmetries • 16 scintillator rings • recoil protons for forward measurement • electrons for backward measurements • elastic/inelastic for 1H, • quasi-elastic for 2H • Q2 = 0.12~1.0 for forward • Q2 = 0.3, 0.5, 0.8 for backward • Forward measurements complete (101 C electrons). 1013 protons per detector ring! Ebeam = 3.03 GeV, 0.33 - 0.93 GeV Ibeam = 40 uA, 80 uA Pbeam = 75%, 80% • = 52 – 760 , 104 – 1160  = 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

  16. G0 in Hall C superconducting magnet (SMS) Lumi monitors cryogenic supply beam monitoring girder scintillation detectors cryogenic target service module electron beamline

  17. Beam Properties Accelerator • 40uA, 32 ns time structure, much higher bunch charge. • Pbeam = 73.71.0%. • Need to minimize the helicity correlated beam properties to avoid false asymmetry. New Tiger laser system for G0 “IA” Pockels Cell (charge) and PZT mirrors (position) feedback system to minimize helicity correlated beam properties

  18. Spectrometer elastic protons detectors lead collimators beam target • Toroidal magnet, elastic protons dispersed in Q2 along focal surface • Acceptance 0.12<Q2<1.0 GeV for 3 GeV incident beam • 16 scintillator rings at the focal plane. 8 octants. • Azimuthally symmetric! • Detector 15 acceptance: 0.44-0.88 GeV2 • Detector 14: Q2 = 0.41, 1.0 GeV2 • Detector 16: “super-elastic”, crucial to measure the background

  19. 32 ns Ibeam +1 Beam Helicity -1 t Accelerator pulse structure ~500 ms “Macropulse” 1/30 s ON DAQ OFF t Timing in the Experiment Measurement timing Typical t.o.f. spectrum

  20. PMT Left Mean Timer Front PMT Right TDC / LTD Scalers: Histogramming Coinc PMT Left Mean Timer Back PMT Right Electronics • High rate counting experiment, coinc. rate ~1MHz per scintillator pair. Electronics deadtime well-understood. • Fast time encoding (ToF histogramming electronics. beam pick-off signal T=0) • DAQ rate, every helicity flip (30Hz)

  21. Analysis Overview Blinding Factor Raw Asymmetries, Ameas Beam and instrumental corrections: Deadtime Helicity-correlated beam properties Leakage beam Beam polarization Background correction Aphys Unblinding GEs+GMs Q2 nucleon form factors

  22. Electronics Deadtime Corrections To the first order • This shows up as • a decrease of normalized yield with Ibeam (almost like a target density reduction) • a correlation between Am and AQ . • The DT effect is largely corrected based on the model of the electronics • The residual AQ correlation is removed by the linear regression • The final residual Aphys dependence is corrected based on Aphys and fresidual (~0.050.05 ppm).

  23. 499MHz Leakage Beam Leakage demo, extreme case “G0” AQ = 1000 ppm “Leakage” AQ = -1000 ppm 0 16 32 ns • ~ 50 nA 499 MHz beam leaks into G0 beam (~ 40 uA) • Leakage current has large, varying asymmetry(A~ 600 ppm). BCM integrates charge, no sensitivity to the micro-structure! • Use “cut0” region in actual data to measure leakage current and asymmetry throughout run. Worked out nicely. Aleak = -0.710.14 ppm (global uncertainty!)

  24. Positive background asymmetry? Where do they come from ????

  25. Physics Origin of the Positive Background Asymmetries • Hyperons being produced in the scattering is highly polarized. • Y → N+π is a PV decay with very large asymmetry (~1) • However the decay-nucleon is highly supressed by the acceptance. GEANT simulation • Weak decay-particles rescatter inside the spectrometer that make into the detector. Low rate, large asymmetry!

  26. Results of the Hyperon Simulation Source explained; used measured data in the correction.

  27. Background Correction  Yield & Asymmetry “2-step” Fit constant quadratic 2/=31.1/40 2/=37.5/44 detector 8 • Extract bin-by-bin dilution factor fb(t) by fitting time-of-flight spectra (gaussian signal + 4nd order polynomial background) • Use results to perform asymmetry fit:Ae = const, Ab = quadratic

  28. Det. 15 Background Corrections • Elastic peak broadened (~6 ns) because of increased Q2 acceptance. • Smooth variation of the background yield and asymmetry over detector range 12-14, 16. So make linear interpretation over detector number to determine fb(t) and Ab(t).

  29. Detector 15 Asymmetry Compare interpolated background asymmetry and data Assume elastic asymmetry in each bin is a constant. Take interpolated fb and Ab, fit three Ae.

  30. Elastic Asymmetries • “No vector strange” asymmetry, ANVS, is A(GEs,GMs = 0) • Nucleon EM form factors: Kelly PRC 70 (2004) 068202 • Inner error bars: stat; outer: stat & pt-pt sys • Primary contributors to the global error band: • Leakage correction (low Q2) • background correction (high Q2)

  31. GEs+GMs, Q2 = 0.12-1.0 GeV2 • “Model” uncertainty is the EW rad. corr. unc. Dominated by the uncertainty of GAe. • 3 nucleon form factor fits; spread indicate uncertainties. • |Kelly-FW| heavily driven by the difference in GEn. Are the data consistent with zero? A 2 test based on the random and correlated errors: the non-vector-strangeness hypothesis is disfavored at 89%!

  32. World Data with G0 Q2=0.48 GeV2 Strange quark contributes to p at -10% level.

  33. “Complete” picture with G0 data For G0,  ~ 0.94Q2 Emerging picture: At low Q2 end, despite the small , the data are positive, consistent with a large and positive GMs. Data go toward zero around Q2~0.2, which suggests GEs might have a negative bump there. Data curve back up again for Q2>0.3 (note the growing ), indicating that GMs stays positive.

  34. Global Fits to World Data Toy model, minimal physics input à la Kelly dipole form • Kelly form ensures GEs(0)=0, GEs1/Q4 when Q2 large • Fixed b3 = 1, all other variables float • Fit all 24 data points: 18 G0, 3 HAPPEX, 2 A4, 1 SAMPLE

  35. Results of the fit Excellent fit 2 = 14.9 19 d.o.f. Correlation Coeff.

  36. GEs and GMs Separately • Compare GEs with GEn, and GMs with GMp • Recall the factor of -1/3 -1/3GEs(0.2)/GEn(0.2)~40% -1/3s/p = -18%

  37. Very Naïve Interpretation (Conclusion?) Interpret GEs and GMs results from the momentum space into spatial distribution. Naïve kaon cloud (s quark spatially outside) leads to a negative s. Disfavored by the data  “s quark skin”. Hannelius, Riska + Glozman, Nucl. Phys. A 665 (2000) 353 Recall the well-known charge distribution in the neutron: positive bump in GEn neutron has a positive core and negative “skin”. R. Jaffe, PLB 229 (1989) 275 -1/3GEs has a positive bump  s quark spatially outside on average! So the GEs and GMs results are both favor the picture that nucleon has an s quark “core” and s quark “skin” !

  38. Strange FF in the Near Future G0 backward: detect electrons atq = 108° Q2 = 0.3, 0.5, 0.8 GeV2 both LH2 and LD2 targets PV-A4 backward:q = 145° Q2 = 0.23, 0.47 GeV2 (underway) HAPPEX (H and He4) running now high precision at Q2 = 0.1 GeV2 high precision at Q2 = 0.6 GeV2 (proposed July 2005)

  39. Prospective G0 Data @ Q2 = 0.8, 0.23 GeV2 • Run in Spring 06 at Q2 = 0.79 GeV2 (H and D targets) • Possible run at Q2 = 0.23 GeV2 next (H alone?)

  40. Summary • The successful G0 forward angle experiment yield the first measurement of parity-violating asymmetries over broad Q2 range. PRL 95, 092001(2005) • Emerging picture: GMs and GEs are both likely nonzero • GMs positive at low Q2 and stays positive up to Q2=1.0 (GeV/c)2 • GEs might have a negative bump at Q2~0.2 (GeV/c)2 • s quark skin of the nucleon??? • Stay tuned for G0 backward results

  41. Target • 20 cm LH2, aluminum target cell • longitudinal flow, v ~ 8 m/s, P > 1000 W! • negligible density change < 0.5% • measured small boiling contribution: 260 ppm : 1200 ppm (stat. width)

  42. Formalism Including EW Rad. Corr. and At tree level, R’s are zeros. Where M.J. Mosolf et al, Phys Rep. 239, No. 1(1994) S.L Zhu et al, PRD 62,033008(2000) • Each asymmetry measurement can be cast into a linear combination of GEs and GMs, assuming everything else is known. • In forward angle, use theoretical value and uncertainty of GAe. Uncertainty dominated by the “anapole” term.

  43. Feedback Performance All parameters coverage to zero!

  44. Acceptance Large and continuous acceptance for protons.

  45. Helicity Correlated Beam Properties and Their Corrections So require • Small ΔPi • Small sensitivity to Pi • Azimuthal symmetry  large reduction of detector sensitivity to beam positions • Response of spectrometer to beam changes well understood • False asymmetries (and the uncertainty) due to helicity-correlated beam parameters very small (~-0.02 ppm)

  46. Measured Asymmetry upon Beam Spin Reversal

  47. Detector 1-14 Background Uncertainty • Allowed background yield varied within “lozenge”. Similar approach for asymmetry. • Separated point-to-point (pt-pt) uncertainties in background correction from global uncertainties. E.g. linear  quadratic model of Ab move Ae downward for detectors 1-14

  48. Det. 15 Asymmetry Compare interpolated background asymmetry and data Assume elastic asymmetry in each bin is a constant. Take interpolated fb and Ab, fit three Ae. In detector 15, the global uncertainties larger because bins are continuous.

  49. Different Nucleon EM FF Parametrizations

  50. Interpolate G0 Data Three overlapping Q2 with other experiments: Q2 = 0.1(HAPPEX, SAMPLE, A4),Q2 = 0.23 (A4),Q2=0.48 (HAPPEX) • Q2 = 0.1 extrapolate G0 using Ai/Q2i for first 3 Q2 points Q2 = {0.122, 0.128, 0.136} • Q2 = 0.23 (PVA4-I), 0.477 (HAPPEX-I) GeV2 Interpolate Ai/Q2i for Q2 = {0.210, 0.232, 0.262},{0.410, 0.511, 0.631} • Average the results of flat and linear interpolation. Use the ½ difference as an additional “model” uncertainty.

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