1. Charged Current Cross Sections with polarised lepton beam at ZEUS
DIS2006, 20/April/ /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
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

3. 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

4. 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

5. 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

6. 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 EPS e+p cross section has been published. (hep-ex , submitted to PLB)
-- e–p analysis (new ZEUS preliminary) Data with integrated luminosity of 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

7. 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.

8. 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

9. 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.

10. 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.

11. 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

12. 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 )
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
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.

14. 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.

15. 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 )
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.

16. 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.

17. 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

18. 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 )
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
April 21, 2006
Charged Current Cross Sections with polarised lepton beam at ZEUS
Back up

20. 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
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
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.