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Neutral and Charged Current at High–x and High-Q 2 at HERA

Neutral and Charged Current at High–x and High-Q 2 at HERA. Haiping Peng (On behalf of H1 and ZEUS Collaborations). QCD06 Montpellier, France. Outline. Introduction to HERA Deep inelastic scattering and structure functions F 2 , xF 3 measurement

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Neutral and Charged Current at High–x and High-Q 2 at HERA

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  1. Neutral and Charged Current at High–x and High-Q2 at HERA Haiping Peng (On behalf of H1 and ZEUS Collaborations) QCD06 Montpellier, France

  2. Outline • Introduction to HERA • Deep inelastic scattering and structure functions • F2, xF3 measurement • Polarised neutral and charged current • Summary peng@mail.desy.de 2006-07-04

  3. H1 and ZEUS at HERA Circumference～6.3km P • Proton –electron or positron collider; • Center of mass energy S = 319GeV; • Higher luminosity & polarised beam since 2003; • 2 collider experiments; • Nearly 4 detector coverage 27.5GeV 820/920GeV peng@mail.desy.de 2006-07-04

  4. Deep Inelastic Scattering • At fixed s: two independent kinematic variables: e.g. Q2 & x; • Q2 = “resolving power” of probe, High Q2 : resolve 1/1000th size of proton; • x = momentum fraction of proton carried by struck quark; • y = inelasticity of scattered lepton; Fraction of lepton’s energy transferred to the proton; • Related kinematic quantities; Q2 = Sxy peng@mail.desy.de 2006-07-04

  5. NC&CC Cross Sections NC Cross Section: CC Cross Section: Sensitive to u and d valence quarks at LO QCD peng@mail.desy.de 2006-07-04

  6. Kinematic Plane Coverage • Conventional QCD evolution only tells us Q2 dependence (DGLAP), x dependence must come from data; • HERA cover large region in x and Q2; • At low x : measure gluon PDFs and indirect determination of FL • At high x : measure xF3 and valence quark PDFs; • Large overlap with LHC region in x; x peng@mail.desy.de 2006-07-04

  7. Precision F2 Data HERAI • Sensitive to sum of quark and anti-quark; • Range in x : 0.00001 ～1; Range in Q2 : 1～30000GeV2; • Strong scaling violation, due to QCD gluon radiation, provide indirect constraint on gluon density; • Measured precision: 2 ～3%, still large error at highest Q2 and x; Higher Luminosity at HERAII • Data of HERA experiments in agreement and well described by QCD; peng@mail.desy.de 2006-07-04

  8. xF3 Determination HERAI • Difference between e+p and e-p  xF3 significant at high Q2 only; • Increases electron proton cross section and reduces positron proton cross section at large Q2; e- e+ L(e- p)  15pb-1 L(e+ p)  110pb-1 • Constrains the valence quark content; • Data is well described by QCD (CTEQ6D) and in agreement with global fit PDFs; • Uncertainty from e-p sample, higher luminosity is necessary; HERAII data(170pb-1) will greatly help; peng@mail.desy.de 2006-07-04

  9. HERA I e+p 65.1pb-1 NC at High x • Limitations in beam energies and measurement techniques: HERA x~0.65; • New method : Electron and Jet; Electron  Q2; One Jet  x; No Jet  highest bin xEdge<x<1; • Agree with the expectation of NLO using CTEQ6D PDFs; • A tendency to lie above the expectations at highest x bins; • Produce new constraint on PDFs at the higher values of x. peng@mail.desy.de 2006-07-04

  10. CC Cross Sections HERAI • e+p mostly sensitive to d(x,Q2); while e-p mostly sensitive to u(x,Q2); • e+p cross section is suppressed by factor (1-y)2; • At low Q2, the difference is less pronounce due to the sea quark contribution; • Constraint for the u and d valence quark; • Small cross section  Large statistical error; • Data agrees with NLO QCD global PDFs fit; peng@mail.desy.de 2006-07-04

  11. Polarised NC&CC Cross Sections • HERA upgrade : longitudinally polarized lepton beams ; higher luminosity; • SM Prediction: 1. The cross section dependent on the longitudinal polarization of the incoming lepton beam; 2. CC cross section : Linear function of polarization due to the absence of right-handed charged currents; 3. NC Cross section: decompose into unpolarised and polarised parts; peng@mail.desy.de 2006-07-04

  12. CC Cross Sections • Data: e-p : 2003-2004; e+p : 2005; HERA I; • Linear dependence is firmly established both for e+ and e- beam; • ZEUS and H1 measurements are in good agreement with SM; • Vanishing cross sections for left (right)-handed positrons (electrons); • Consistent with absence of right-handed weak currents; peng@mail.desy.de 2006-07-04

  13. NC Cross Sections e-p H1 e+p H1 • Rise of the ratio RH/LH for e+p; • drop of the ratio RH/LH for e-p;. • Indication of polarization effect on NC cross sections, although significance is moderate; • Well described by the predictions of SM for both H1 and ZEUS;. peng@mail.desy.de 2006-07-04

  14. xF3 at HERAII • Combine polarised data sets, and make small correction for residual polarization  a effective unpolarised date; • Combine HERAI data  xF3; • More precise xF3 measurement; • Data is well described by QCD (CTEQ6D) and in agreement with global fit PDFs; peng@mail.desy.de 2006-07-04

  15. Summary HERA measures proton structure function in a wide range of x and Q2, and plays important role in testing of SM. F2 is precisely measured, scaling violations are well described by pQCD; xF3 measurement at high Q2 is sensitive to valence quark content of proton, but statistically limited for HERAI, much improved at HERAII; CC cross section measurements at high Q2 constrain u and d density; HERAII polarised beam : Linear dependence of CC cross section on polarization is consistent with SM prediction, the absence of right-handed weak currents is confirmed. Effect of polarization in NC interactions is visible but significance is moderate; The coming HERAII polarised and higher luminosity (700pb-1, 2007) data will improve the measurement to a new level precision and enhance the potential in testing of SM. peng@mail.desy.de 2006-07-04

  16. Thanks peng@mail.desy.de 2006-07-04

  17. xG3 at HERAI peng@mail.desy.de 2006-07-04

  18. peng@mail.desy.de 2006-07-04

  19. CC Cross sections .VS. (1-y)2&x • e-p cross section is larger than e+p cross section • Different quark flavors probe by the w boson and the helicity structure of CC interaction • Agree well with SM expectations within errors • (1-y)2 is directly related to the scattering angle  in the electron–quark center-of-mass system; • Isotropic distribution and strong angular dependence on (1-y)2; • Isotropic distribution increase with decreasing x due to the increasing of sea quarks; • At large x, e+p sensitive to d quark; e-p sensitive to u quark; peng@mail.desy.de 2006-07-04

  20. NC at HERAII peng@mail.desy.de 2006-07-04

  21. xF3 at HERAII peng@mail.desy.de 2006-07-04

  22. CC Cross Sections Differential cross sections with different polarizations have the same shape, only the normalization is different! peng@mail.desy.de 2006-07-04

  23. CC & NC Cross sections HEARII HEARI  dominant • Spans more than two orders of magnitude in Q2, the cross section fall with Q2 by about six orders of magnitude for NC. while three orders for CC; • Agreement between H1, ZEUS and QCD fit in cross section; • NC cross section exceeds CC cross section at low Q2 due to the photon exchange in the NC process; • Unification of EM and weak interactions in DIS at high Q2; W boson Z interf. Diff u, d distribution. peng@mail.desy.de 2006-07-04

  24. peng@mail.desy.de 2006-07-04

  25. peng@mail.desy.de 2006-07-04

  26. NC at High x e-p 16.7 peng@mail.desy.de 2006-07-04

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