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“ Cross section measurements in DIS ”

K. PapageorgiouUniversity of the Aegean

On behalf of ZEUS and H1 Collaborations

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OUTLINE

Introduction

Cross sections

Power of data sets combination

FL measurement

Motivations & Method

Analysis strategy

H1 & ZEUS results

Summary

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HERA – The Worlds only electron-proton collider

Collides protons with

Ee : 27.5 GeV

Ep : 920 GeV

Centre of mass energy :

GeVs 320

Q2 corresponds to the spatial resolution (wavelength λ) of the probe

(set by s)

2/1~ Q252

max 10~ GeVQ

1000/ ~ 10~ 18min protonRm

e

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Approximately ~ 1 fb-1 of data collected by H1 & ZEUS

In operation since 1992

HERA Running ended 30 June 2007

HERA –I : Until year 2000

• Unpolarized e+ and e- beams

HERA – II : 2002 – Mar 2007

• High luminosity to allow more statistical sensitivity to high Q2

• Longitudinally polarized e+/e- beams to allow direct sensitivity to EW

Low energy Run : March 2007 – June 2007

• A special Run with low proton beam energy (460, 575 GeV) to measure directly FL structure function

HERA – I & II combined integrated Luminosity

~ 0.5 fb-1 per experiment

HERA Luminosity

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• A key tool to “look” inside the proton, allow testing of QCD

p(P) Proton remnant

γ, Ζ0Q2 = -q2=-(k-k΄)2

(xP)

}

Current jet

)(ke

)'(ke

NC

• DIS cross section can be described in terms of kinematic variables

Q2 : Virtuality of the intermediate boson

-- measure of the probing power

x : Bjorken scaling factor

-- fraction of proton’s momentum carried by struck quark

y : Inelasticity

-- energy fraction transferred from lepton in proton rest frame

s : Centre of mass energy of the ep system

qP

Qx

.2

2

kP

qPy

.

.

s x yQ 2

CC

DEEP INELASTIC SCATTERING KINEMATICS

p(P) Proton remnant

W±Q2 = -q2=-(k-k΄)2

(xP)

}

Current jet

)(ke

)'(kv

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The NC DIS cross section expressed in terms of structure functions

F2 -- dominant contribution

to the cross section

)q(q 22 xeF

qq

NC DIS cross section: F2

)],(),(),([2)( 2

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22

24

2

2

2

QxxFQxFY

yQxFY

xQ

a

dxdQ

pedL

F2 -- SF parameterize target structure, i.e. how far for point-like

If proton is point-like then,

YxQ

a

dxdQ

ped4

2

2

2 2)(

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NC DIS cross section: F2 (H1 & ZEUS combined data)

New HERA-I PDF fit predictions vs H1 & ZEUS combined data NC e+p: (see talk of LI, Gang )

Total uncertainties on the PDF fit predictions included but can barely be resolvedProton scan with high precision NLO fit (DGLAP) – good description of the data Systematic uncertainties are now smaller than statistical uncertainties

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CC e-p cross section (ZEUS)

Chiral structure of weak interactions is directly visible as a function of polarization Linear dependence with lepton polarization

Pe=1 for e- , Pe=-1 for e+

Consistent with SM prediction of No evidence of RH currents. Error band from PDF uncertainty

)0()1()(

epe

CCeepe

CC PPP

0RHCC

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CC e-p cross section (H1 & ZEUS)

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NC e+p cross section: (ZEUS)

Cross sections for NC DIS in e+p collisions with a longitudinal polarized positron beam have been measured using the ZEUS detector Results based on data corresponding to an L=113.3 pb-1 at sqrt(318) GeV are given for both positive and negative values of the longitudinal polarization of positron beam Single differential cross sections are shown in the kinematical region Q2>185GeV and y<0.95Measured cross section consistent with SM prediction

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NC e+p: (ZEUS)

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NC e+p: (H1)

Cross sections for NC DIS in e+p collisions with a longitudinal polarized positron beam have been measured using the H1 detector

Single differential cross section are shown for HERA I and II e+p data

Measured cross section is compared to the prediction of the SM

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xF3 -- is the parity-violating term arising from Z0 exchange

-- experimentally can be extracted by measuring the

-- negligible at small Q2

2

2

2

2 )()(

dxdQ

ped

dxdQ

ped

NC DIS cross section: xF3

)d(2u 3 vv3 x

xF Z

3xF

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FL MEASUREMENT : Motivation & Method

FL -- longitudinal SF, sizeable only at low x, observable at high y

-- FL=0 in QPM (spin ½ partons – Gallan-Gross),

-- propotional to gluon at higher orders, Direct sensitivity to gluon dynamics

)]()1(83

16[

42

2

1

32 zzg

z

xeF

z

dzx

aF

qq

x

sL

To separate F2 , FL w/o theory assumption , one needs to measure the cross section at fixed (x,Q2) and different values of y different beam energies (s)

)],(),([2)( 2

22

24

2

2

2

QxFY

yQxFY

xQ

a

dxdQ

pedL

~ :reduced cross section

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FL MEASUREMENT : Motivation & Method

Separate SF’s at given x ,Q2 using straight line fit and error compensation Intermediate energy for measurement control. At low y and fixed (x,Q2) cross sections need to coincide Renormalization used by H1 and by ZEUS

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DIS events: Identified by scattered electron (Electron Method)

High –y : Low energy electron Low –Q2:

)cos1(2 '2eeemeas EEQ

FL : Analysis strategy

High y / Low s (Ep=460 GeV) Low energy electron in the calorimeter; lot of hadronic activity

Electron finding is difficult at low Q2 large γp background, mostly γ’s and π’s faking an electron needs to be subtracted/controlled

Low y / High s (Ep=920 GeV) High energy well separated electron in the calorimeter; no background

)cos1(2

1'

ee

emeas E

Ey

H1 analysis covers both SpaCal and LAR

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Low-x: visible differences between data sets

High-x: cross sections are consistent

Low-x: the x sections turn over

Reduced cross section measurement (ZEUS)

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Reduced cross section measurement (H1)

H1 preliminary

Ep = 460 GeV

= 575 GeV

= 920 GeV

Combined LAr & SpaCal

ymax ~ 0.9

[@450 & 575 GeV]

xx

19 Results are consistent with FL from ZEUS JETS PDF’s

FL measurement (ZEUS)

20 Results are consistent with the prediction of H1 PDF

FL measurement (H1 published results)

21Results consistent with the prediction of H1 PDF fit (obtained using only the H1 high energy x-section data

Overview on all (medium & high Q2 ) FL data (H1)

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Results consistent with H1 fit and with expectations from global parton distribution fits at higher order pertubation theory

DGLAP describes low-x region at HERA

x- averaged FL in the full range (H1)

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Summary• First direct measurement of FL was performed in a new kinematical range

Q2 : (24-110) GeV2 [ZEUS] , Q2 : (12-800) GeV2 [H1]

and Bjorken x : 0.0002 – 0.05

• Data are in good agreement with higher order pQCD predictions

• First data have been published

• HERA experiments provide unique information on proton structure over wide

range of x

an important precision input for physics at LHC

• The combination of the data sets of ZEUS and H1 inclusive cross section is very successful and convincing

greatly improved precision compared to the measurements of each experiment separately

•Analysis is ongoing, thus can be expected to further constrain low-x

• A most remarkable success of HERA!

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Back-up slides

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Comparison of FL data with pQCD

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Comparison of FL data with Dipole Model

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NC e+p: (ZEUS)

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NC e+p: (ZEUS)

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NC e+/-p: (H1)