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

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 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

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS Introduction NC DIS process Deep inelastic scattering (DIS) can be classified into two processes Neutral Current (NC) process exchanging neutral gauge boson (g or Z) Charged Current (CC) process exchanging charged gauge boson (W) -- A scattered neutrino is in the final state. -- Topic of this talk CC DIS process : virtuality of exchanged boson : Bjorken x : inelasiticity Kinematic variables

Unpolarised CC DIS cross section April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS Unpolarised CC DIS cross section e+p data e–p data ui (di) denotes densities of the u-type (d-type) quark and upper bar indicates anti-quark. sensitive to density of d quark Coupling Propagator Parton Distribution Function (PDF) sensitive to density of u quark

Polarised CC cross section April 21, 2006 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

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 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 Pe>0 Pe<0 e+p data L=12.3 pb–1 <Pe>=+0.32±0.01 L=11.5 pb–1 <Pe>=–0.41±0.01 e–p data L=42.7 pb–1 <Pe>=+0.33±0.02 L=78.8 pb–1 <Pe>=–0.27±0.01

Event characteristics April 21, 2006 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.

Kinematic reconstruction April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS Kinematic reconstruction Jacquet-Blondel method Jacquet-Blondel method is used. Kinematic variables are reconstructed with PT,miss and d of hadronic system. where Ee is beam energy of electron and 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

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 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.

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 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.

Event distribution : e–p data April 21, 2006 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

Single differential cross sections April 21, 2006 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 =>

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 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.

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 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.

Right-handed CC DIS cross section April 21, 2006 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 to mass of WR boson, MWR>180 GeV (95% C.L.) with an assumption that RHCC and LHCC have same coupling.

Reduced cross sections April 21, 2006 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.

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS Reduced cross section ZEUS preliminary <Pe>=+0.329

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 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)

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS Back up

Event distribution : e+p data April 21, 2006 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

Charged Current Cross Sections with polarised lepton beam at ZEUS April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS ZEUS detector ZEUS detector -- General detector for collider experiment. -- Mainly, calorimeter and tracking system were used this analysis. Both Pe>0 and Pe<0 polarization samples were collected.