Charged Current Cross Sections with polarised lepton beam at ZEUS

Hiroshi Kaji (KEK)on behalf of the ZEUS collaboration

DIS2006, 20/April/2006-24/April/2006at Congress Center EPOCHAL Tsukuba

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 2

Contents

• Introduction

• Cross section measurement– Single differential cross section– Polarisation vs. total cross section– Limit on RHCC cross section– Double differential cross section

• Summary

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 3

Introduction

CC DIS process

NC DIS process Deep inelastic scattering (DIS) can beclassified into two processes– Neutral Current (NC) process

exchanging neutral gauge boson ( or Z)

– Charged Current (CC) process exchanging charged gauge boson (W) -- A scattered neutrino is in the final state. -- Topic of this talk

kp

qpy

qp2

Qx

)'kk(qQ2

222

: virtuality of exchanged boson

: Bjorken x

: inelasiticity

Kinematic variables

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 4

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.

)x,Q(d)y1()x,Q(uQM

M

2

G

dxdQ

d 2i

22i

2

22W

2WF

2

peunpolCC

)x,Q(d)y1()x,Q(uQM

M

2

G

dxdQ

d 2i

22i

2

22W

2WF

2

peunpolCC

sensitive to density of d quark

Coupling Propagator Parton Distribution Function (PDF)

sensitive to density of u quark

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 5

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, RHCC, which does not exist in the SM.

)x,Q(2

P1)x,Q( 2pe

LHCCe2pe

polCC

Can be tested by HERA-II polarised data

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 6

ZEUS CC analysis

Pe>0 Pe<0

e+p data

L=12.3 pb–1

<Pe>=+0.32±0.01L=11.5 pb–1

<Pe>=–0.41±0.01

e–p data

L=42.7 pb–1

<Pe>=+0.33±0.02L=78.8 pb–1

<Pe>=–0.27±0.01

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)

Pe/Pe=2% (5%) for e+p (e–p) data

e–p data

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 7

Event characteristics

CC DIS has large missing transverse momentum, PT,miss, which is interpreted as transverse momentum of scattered neutrino.

Scattered neutrino escapes detection.

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 8

Kinematic reconstruction

• Jacquet-Blondel method is used.

• Kinematic variables are reconstructed with PT,miss and of hadronic system.

• where Ee is beam energy of electronand s is center-of-mass energy squared.

22

,

i

iY

i

iXmissT PPP

Jacquet-Blondel method

JB

JBJB

JB

missTJB

eJB

ys

Qx

y

PQ

Ey

2

2,2

1

2

i

iZ

i PE All variables described here are measured by calorimeter.

For hadron, (E)/E=0.35/√E, in GeV

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 9

Event selection

0

We use different event selection in two 0 region.

=> See next slide.

22

miss,T

22miss,T

0P

Pcos

Efficiency of vertex finding changes by a value of 0, which can be naively interpreted as scattering angle of quark.

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 10

Event selection• Selection at 0>0.4 rad

– PT,miss>12 GeV– PT,miss(–1ir)>10 GeV– NC rejection– PhP rejection– vertex cut– tracking cut

• Selection at 0<0.4 rad– PT,miss>14 GeV– PT,miss(–1ir)>12 GeV– Selection based on forward C

AL information• Kinematic range

– Q2>200GeV2

– y<0.9

Tracking information is used for rejection of non-ep background at high-0 region.

Relatively higher threshold is used at low-0 region.CAL shower shapes are used in stead of tracking information.This threshold is related to threshold of PT,miss.

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 11

Event distribution : e–p data

• 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 M

C.can be used for estimating– efficiency of selection– detector effects

ZEUS preliminary

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 12

Single differential cross sectionse+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 =>

)x,Q(2

P1)x,Q( 2

LHCCe2pe

pol

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 13

Pe vs. Cross section

pb.)sys(.)stat(3.17.46 5.13.1

peCC

pb.)sys(.)stat(3.14.87 6.25.2

peCC

<Pe>=–0.27±0.01

<Pe>=+0.33±0.02

ZEUS preliminary

Cross sections clearly change with polarisation.Consistent with SM prediction.

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 14

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.

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 15

Right-handed CC DIS cross section

pbsysstatPGeVQ epe .)(0.5.)(1.38.0)1,200( 22

pb.)sys(2.1.)stat(9.34.7peRHCC

ZEUS preliminary

Right-handed CC DIS cross section is extrapolated with new preliminary e–p and HERA-I result.

Uncertainty of polarisation is included in the systematic error.

e+p result (hep-ex 0602026)

Rimit on RHCC,RHCC<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.

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 16

Reduced cross sectionsZEUS preliminary

<Pe>=–0.268

-- Measurements were done with much larger statistics than before.

-- The results will bring better understanding of PDFs.

i

ii

i

W

W

F

dyux

dxdQ

d

M

QM

G

x

2

2

22

2

22

2

)1(

2~

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 17

Reduced cross section

i

ii

i

W

W

F

dyux

dxdQ

d

M

QM

G

x

2

2

22

2

22

2

)1(

2~

ZEUS preliminary

<Pe>=+0.329

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 18

Summary• CC DIS was measured with HERA-II polarised d

ata.• e+p result was published. (hep-ex 0602026)• e–p result has been updated with integrated lumi

nosity 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 lu

minosity of HERA-II.

pbsysstatPGeVQ epe .)(0.5.)(1.38.0)1,200( 22

(ZEUS preliminary)

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 19

Back up

April 21, 2006 Charged Current Cross Sections with polarised lepton beam at ZEUS 20

Event distribution : e+p data

• 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

hep-ex 0602026

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

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.