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Charged Current Cross Sections with polarised lepton beam at ZEUS

DIS2006, 20/April/2006-24/April/2006 at Congress Center EPOCHAL Tsukuba. Charged Current Cross Sections with polarised lepton beam at ZEUS. Hiroshi Kaji (KEK) on behalf of the ZEUS collaboration. Contents. Introduction Cross section measurement Single differential cross section

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Charged Current Cross Sections with polarised lepton beam at ZEUS

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  1. DIS2006, 20/April/2006-24/April/2006 at Congress Center EPOCHAL Tsukuba Charged Current Cross Sections with polarised lepton beam at ZEUS Hiroshi Kaji (KEK)on behalf of the ZEUS collaboration

  2. Charged Current Cross Sections with polarised lepton beam at ZEUS Contents • Introduction • Cross section measurement • Single differential cross section • Polarisation vs. total cross section • Limit on RHCC cross section • Double differential cross section • Summary

  3. Charged Current Cross Sections with polarised lepton beam at ZEUS Kinematic variables : virtuality of exchanged boson: Bjorken x: inelasiticity Introduction NC DIS process Deep inelastic scattering (DIS) can beclassified into two processes • Neutral Current (NC) processexchanging neutral gauge boson (g or Z) • Charged Current (CC) processexchanging charged gauge boson (W) -- A scattered neutrino is in the final state. -- Topic of this talk CC DIS process

  4. Charged Current Cross Sections with polarised lepton beam at ZEUS Unpolarised CC DIS cross section sensitive to density of d quark • e+p data • e–p data • ui (di) denotes densities of the u-type (d-type) quark and upper bar indicates anti-quark. Coupling Propagator Parton Distribution Function (PDF) sensitive to density of u quark

  5. Charged Current Cross Sections with polarised lepton beam at ZEUS Polarised CC cross section • Polarised electron beam at HERA. • Definition of polarisation, Pe, is asymmetry of No. of right-handed and left-handed electrons in the beam. • Since CC DIS is pure weak interaction, polarised CC cross section is as a function of Pe • If cross section is non zero at P=–1(+1) for e+p (e–p) data, • Evidence of right-handed component, sRHCC, which does not exist in the SM. Can be tested by HERA-II polarised data

  6. Charged Current Cross Sections with polarised lepton beam at ZEUS ZEUS CC analysis Since EPS2005-- e+p cross section has been published. (hep-ex 0602026, submitted to PLB) -- e–p analysis (new ZEUS preliminary) Data with integrated luminosity of 122 pb–1 were analyised. HERA-I (16.4 pb–1) Last summer (41.8 pb–1) e–p data dPe/Pe=2% (5%) for e+p (e–p) data

  7. Charged Current Cross Sections with polarised lepton beam at ZEUS Event characteristics Scattered neutrino escapes detection. CC DIS has large missing transverse momentum, PT,miss, which is interpreted as transverse momentum of scattered neutrino.

  8. Charged Current Cross Sections with polarised lepton beam at ZEUS Jacquet-Blondel method Kinematic reconstruction • Jacquet-Blondel method is used. • Kinematic variables are reconstructed with PT,miss and d of hadronic system. • where Ee is beam energy of electronand s is center-of-mass energy squared. All variables described here are measured by calorimeter. For hadron, s(E)/E=0.35/√E, in GeV

  9. Charged Current Cross Sections with polarised lepton beam at ZEUS Event selection Efficiency of vertex finding changes by a value of g0, which can be naively interpreted as scattering angle of quark. g0 We use different event selection in two g0 region. => See next slide.

  10. Charged Current Cross Sections with polarised lepton beam at ZEUS Event selection • Selection at g0>0.4 rad • PT,miss>12 GeV • PT,miss(–1ir)>10 GeV • NC rejection • PhP rejection • vertex cut • tracking cut • Selection at g0<0.4 rad • PT,miss>14 GeV • PT,miss(–1ir)>12 GeV • Selection based on forward CAL information • Kinematic range • Q2>200GeV2 • y<0.9 Tracking information is used for rejection of non-ep background at high-g0 region. Relatively higher threshold is used at low-g0 region. CAL shower shapes are used in stead of tracking information. This threshold is related to threshold of PT,miss.

  11. Charged Current Cross Sections with polarised lepton beam at ZEUS Event distribution : e–p data ZEUS preliminary • No. of events selected ~5000 events • Both Pe>0 and Pe<0 samples are plotted together. (122 pb–1) • Histograms are DJANGOH+ARIADNE with CTEQ5D PDF. • Contamination of background events from other processes <1% • Data were well described by MC. can be used for estimating • efficiency of selection • detector effects

  12. Charged Current Cross Sections with polarised lepton beam at ZEUS Single differential cross sections e+p CC cross section (hep-ex 0602026) e–p CC cross section (ZEUS preliminary) Both for e+p and e–p, measured cross sections from different polarisations have the same shape.Only their scales are different.Consistent with SM prediction =>

  13. Charged Current Cross Sections with polarised lepton beam at ZEUS Pe vs. Cross section ZEUS preliminary <Pe>=–0.27±0.01 <Pe>=+0.33±0.02 Cross sections clearly change with polarisation.Consistent with SM prediction.

  14. Charged Current Cross Sections with polarised lepton beam at ZEUS Pe vs. Cross section ZEUS preliminary Cross sections were meausured as a function of Pe. Linear relation between cross section and polarisation is clearly shown.

  15. Charged Current Cross Sections with polarised lepton beam at ZEUS Right-handed CC DIS cross section e+p result (hep-ex 0602026) Right-handed CC DIS cross section is extrapolated with new preliminary e–p and HERA-I result. ZEUS preliminary Uncertainty of polarisation is included in the systematic error. Rimit on RHCC, sRHCC<10.6 pb (95% C.L.), corresponds tomass of WR boson, MWR>180 GeV (95% C.L.) with an assumption that RHCC and LHCC have same coupling.

  16. Charged Current Cross Sections with polarised lepton beam at ZEUS Reduced cross sections ZEUS preliminary <Pe>=–0.268 -- Measurements were done with much larger statistics than before. -- The results will bring better understanding of PDFs.

  17. Charged Current Cross Sections with polarised lepton beam at ZEUS Reduced cross section ZEUS preliminary <Pe>=+0.329

  18. Charged Current Cross Sections with polarised lepton beam at ZEUS Summary • CC DIS was measured with HERA-II polarised data. • e+p result was published. (hep-ex 0602026) • e–p result has been updated with integrated luminosity of 122 pb–1. • Linear relation between cross section and polarisation is clearly shown. • e–p CC DIS cross section at P=+1 extrapolated to be consistent with SM • Reduced cross sections were measured with large luminosity of HERA-II. (ZEUS preliminary)

  19. Charged Current Cross Sections with polarised lepton beam at ZEUS Back up

  20. Charged Current Cross Sections with polarised lepton beam at ZEUS Event distribution : e+p data hep-ex 0602026 • Both P>0 and P<0 samples are plotted together. (23.8 pb–1) • No. of events selected • 469 events • Contamination of background events is suppressed well. • typically, <1% • 5% at low Q2 region

  21. Charged Current Cross Sections with polarised lepton beam at ZEUS ZEUS detector • Both Pe>0 and Pe<0 polarization samples were collected. ZEUS detector -- General detector for collider experiment. -- Mainly, calorimeter and tracking system were used this analysis.

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