Studies of ProtonStructure at HERA

I. Tsurin

12th International QCD Conference

on behalf of ZEUS and H1

Montpellier July 4-9, 2005

• Neutral current (NC)• Charged current (CC)• Structure functions F2, xF3 , FL

• Parton distribution functions (PDF)• The strong coupling constant αS

bb2

cc2 F,F• New results on

ep-Collider HERA

H1

ZEUS

• Liquid argon calorimeter (H1) and compensated uranium calorimeter (ZEUS)• Tracking and vertex detectors• Silicon micro-strip detectors• Muon barrels and endcaps

Detectors instrumentation:

p-energy: 920 GeVe-energy: 27.6 GeV

GeV931s

HERA II:longitudinal e± beam polarisation

Deeply Inelastic Scattering

;)'(

k

kkPy

222 )k'(kqQ

yx

s

Q2

high x, low Q2 NC event

CC eventhigh Q2 NC event

H1 ZEUS

low Q2 NC event

e p

e p

NC and CC Cross-sections

ChargedCurrent

Neutral Current

HERA I

NC cross-section (ץ*, Z0):

CC cross-section (W±):

NCsNC σ

Qx

πα

Qx

)(x,Qσ ~24

2

2

22

CCW

WCC σMQ

M

π

G

Qx

)(x,Qσ ~2 22

22

2

22

][ 32

22),(~ xFYFyFYQx LNC

][ 32

22),(~ xWYWyWYQx LCC

Reduced crosssections:

Neutral Current

322 )(~),(~ xFyfFQxNC

Z exchange increases electron proton cross section and reduces positron proton cross section at large Q2

e-

e+

115)( pbpeL

1110)( pbpeL

QPM (FL=0):

22~ ZZ

)(),( 222 qqexQxF q

)(2),( 23 qqaexQxxF qq

Z

HERA I:

Z0 Contribution at High Q2

Measurements of xF3 provide a constrainton u, d valence quark densities at large x

xduxFx vvZ 2~3

)]()(2[3

13 DDUUxxF Z

sdDcuU ;

;;;

;;;

ssddddd

ccuuuuu

seaseav

seaseav

peNC

peNCxF

~3

Proton Structure at Low x

Steep rise towardslow x driven by:

gqsgqqqs PgPgPFQ

F

)(~log

222

- sea quark densities- gluon densities

δF2/ δ(lnQ2) at low x

reflects the gluon mo-

mentum distributions.

at low x gluon density dominates !

Structure Function FL

)()()(~ 22

22

2 QFY

yQFQ LNC

Decreasing of the NCcross section at low x(high y) is due to FL

xcQF )( 22

Extrapolation:

Charged Current

DxyxU

xDyUx

CC

CC2

2

)1(~~

)1(~~

at high x:

CC processes provide flavor information

vCCvCC xuxd ~~;~~

measurements of uV and dV densities

e-

e+

CC with Polarised e± Beams

)0()1()(

ppp peCC

peCC

Linear dependence of CC cross section on the longitudinal polarisation of the electron and positron beams

)0()1()(

ppp peCC

peCC

HERA II

Electron Beam

Positron Beam

Parton Distribution Functions and αS

Determination of PDFs by H1 and ZEUSusing NC and CC events measurements only

Strong coupling constant αS as measured from jet rates and from jets and NLO QCD fits to structure functions.

Contribution from Heavy Quarks

New measurement: a c-quarklifetime tagging using the silicondetectors.

Classical method: reconstruct D*

75 pb-1

22 /FFf qqqq

The contribution ofc and b quarks up to30% and 3% respect.

The first measurement of bbF2

Outlook

H1 and ZEUS collect data withhigh luminosity and e± polarizedbeams.

Physics of HERA II has begun.

More statistics will lead to higherprecision in structure analyses.

Acknowledgments

• H1 Collaboration

• ZEUS Collaboration

• DESY directorate