DIS 2011, Newport News, VA USA

Inderpal Singh

Panjab Univ./ MPI, Munich

On behalf of ZEUS collaboration

DIS cross-section can be described by :-Q²: Four momentum transfer ( probing power)x : Bjorken Scaling variable (momentumfraction of struck quark)y : inelasticity

Q2 = −q2 = −(k − k' )2,

y = q.k/p.k Q² = sxy

s = center of mass energy square

qp

Qx

.2

2

NC Deep inelastic scattering at HERA

2. Qxp

2DIS 2011

Motivation

The proton PDF’s are poorly determined at high-x.

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Relative difference of different valance u quark pdf’s

Is it possible to check this ?

Large x is relevant to understandPhysics at LHC

DGLAP evolution sensitive to values of x upto 1

Measurements from HERAupto x =1

HERA and LHC

DIS 2011 4

High x e±p

),(),(),(2)(

222

3

2

24

2

2

2

QxFyQxxFYQxFYxQdxdQ

pedNC

L

NCNCNC

Y 1 (1 y)2

At Born level (Lowest order in QED)

F2(x,Q2) x A f (Q

2)(q f (x,Q2) q f (x,Q

2))f

xF3(x,Q2) x B f (Q

2)(q f (x,Q2) q f (x,Q

2))f

contains parity violating part of cross-section which is negligible at low Q2

includes cross-section of both longitudinal and transversely polarized exchanged boson

Parton density function(PDF)probability density of finding parton q or q carrying the momentum fraction x at given Q2

Longitudinal structure function

NC cross sections and structure functions

5DIS 2011

)],(),(),([)(1

2)(~ 2

22

3

2

22

2

2

4

QxFY

yQxxF

Y

YQxF

dxdQ

ped

Y

xQpe

NC

L

NCNCNC

NC

Reduced cross section

HERA-I : 1992-2000HERA-II :2001-2007

In HERA-II the luminositycollected was at least 3 times of HERA-I

HERA luminosity

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ZEUS Prel. 2010

ZEUS Prel. 2011

Θjet<0.11, No jet foundGood Jet Electron

Jet defination: ET > 10. GeV , θjet>0.11

Event topology at high-x

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x : reconstructed from jet information, x<xedge

No jet information: x > xedge

FCALBCAL

RCAL

Q2 and x reconstruction

Electron + jets information used

Electron is well reconstructed for the Q2 range of this analysis

Define Q2 bins from Ee and e:

)cos1(2'2

eeeEEQ

In each Q2 bin, define x bins:If jets are not close to the beam pipe (θjet > 0.11) : • For one jet events Pt balance between jet and electron is used and

ET,jet is replaced by Ptel

If there is no well reconstructed jet count events as zero jet xedge < x < 1 integral of cross section

e

jetjetjetT

p

jetjetjetT

E

EE

Ex

2

)cos1(sin/12

)cos1(sin/

,

,

i

iTjetTEE

,, )(tan2 1 jetejet

jetT

iiiT

jet E

E

,

,

, ,

• For multijet events jet variables are calculated as:

DIS 2011 8

Calibration scheme

BCAL Electron

Calibration

BCAL + FCAL

jet energy scale

FCAL electron energy scale

Measured energy compared to expected energy to find the energy scale which is better than 1%

BCAL electron calibrationis key to whole detectorcalibration

Jet energy scale depends upon BCAL electronJet energy scale betterthan 2%

FCAL electron energy scale relies on jet energy scale

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Event propertiesElectron

Jet

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MC:ARIADNE with CTEQ5D

Event properties

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ET,jet > 10. GeV for Njet > 0

Double differential σ

σ integrated over x

Binning

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Results: NC(e±p) cross sections

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Comparison to theory: CTEQ6D

14DIS 2011

Comparison to theory: HERAPDF1.5

15DIS 2011

Comparison

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1. Latest HERA-II e+p data (142 pb-1)

2. New x-reconstruction method leading to better resolution

3. More x bins compared to HERA-l high x analysis.

4. Completes HERA-ll high-x analysis, cross sections for e-p were presented at DIS2010.

Expected outcome:

• F2 and xF3 can be calculated using e+p & e-p HERA-II data.

• PDF uncertainty at high x may reduce.

Thanks

Summary

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Back Up

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Q2 and x reconstruction

jetel TT pp

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)cos1(2'2

eeeEEQ

sy

Qx

2

y

p

Qjet

T

1

2

2

e

jetjet

p

jetjet

E

EE

Ex

2

)cos1(12

)cos1(

02

)(

E

PEy

jetz

)2

)cos1)(sin(1(2

)cos1)(sin(

e

jetjett

p

jetjett

E

pE

px

e

e

Reconstruction of x

Best resolution achieved

)1(

2

jbjb

t

ysy

px

jets

i

yi

xt

jetjetjets

ppp 222 )()(

e

ijetjet

jb E

E

yii

2

)]cos1([

)2

)cos1)(sin(1(2

)cos1)(sin(

e

jetjett

p

jetjett

E

pE

px

e

e

jet

jet

=

jet

Multi jet events

One jet events

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Projection of ZEUS detector onx and Q2 plane

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Jetelectron BCAL

BCAL

BCAL FCAL

FCAL

FCAL

RCAL

RCAL

RCAL

BCAL

FCAL

FCALFCAL

BCAL

BCAL

RCALRCAL

RCAL

At high x and high Q2 events the jets andthe electron can be found in BCAL and FCAL

Important to understand BCAL and FCAL electronand hadron energy scales