Parton distribution functions and jet cross sections at HERA

Parton distribution functionsand

jet cross sections at HERA

Enrico Tassi - Calabria University and INFN –

On behalf of the H1 and Zeus Collaborations

Introduction: proton PDFs @ HERA

Proton parton distribution functions (PDFs):• proton structure• predictions at hadron colliders → SM tests and Search Beyond SM

HERA main source of information on PDFs:• precise data over large kinematic region → low-x rise of F2 (sea, gluon)

Zeus and H1 DGLAP analyses of HERA-I(and fixed target) data:

→ H1 2000 PDFs → ZEUS-S PDFs Rise of F2 at low-x

(established with ~20 nb-1)Precise F2 determination( 1996-97 data samples)

HERA PDFs

pQCD describe F2 over 4 (3) order of mag. in Q2 (x) HERA PDFs: fair agreement

ZEUS: PRD67(2005)012007H1: EPJC30(2003)1

ZEUS-JETS PDFs

First fit using HERA incl and jets data- HERA-I inclusive cross sects- Inclusive jet cross sects in NC DIS- Dijet cross sects in photo-production

Data from one Experiment only:• Syst unc. well understood• No fixed target unc.• Valence quarks from high-Q2 NC

and CC cross sections

Jet data help in constraining

the gluon and αs:

• Reduction of gluon unc. by

~ factor 2 for 0.01 < x < 0.4

• Precise determination of αs

ZEUS: EPJC42(2005)1

Zeus-Pol PDFs

Since 2002 HERA has been runningwith long. polarized e± beam (P~30-40% )

ZEUS-prel-06-003

A new PDF fit has been performedincluding the new e-

L,R-p NC and CC cross sections

Improved determination of the PDFs at high-x: in particular xuv upedupe CCNC )( , )4()(

Q2=10 GeV2

Zeus-Pol PDFsZEUS-prel-06-003

Q2=10000 GeV2

New measurements

PDFs poorly known at large x:→New measurement of incl. cross sections at high-x

Jets have strong impact on the DGLAP analyses→New measurements of jet cross sections:• Jets in DIS (NC and CC)• Dijet in γ-p

High-x incl. cross sections

• Motivation: Poor knowledge of high-x PDFs (HERA x~0.65, BCDMS~ 0.75)• New technique which allows the meas. of incl. cross sections up to Bjorken x=1

ZEUS-pub-06-003

Electron-Jet Method:Use elec. and jet E and θ to reconst. Q2,x 1. Always use Ee and θe for Q2 2. For each Q2 bin define x-bins a) Jet far from beam pipe (low-x) → x from Ejet , θjet

b) Jet near the beam-pipe (high-x) → collect events in [xedge,1]

Re-analysis of HERA I high-Q2 e±p data(Q2 > 425 GeV2)

dxdQ

d2

21

2

2

edgex dxdQ

ddx

Measure:

and

High-x cross sections

Theory (with current PDF sets) underestimates the measurements at the highest values of x.

NC DIS: inclusive and dijet cross sections

Inclusive jets• Q2 > 125 GeV2

• EjetT,B > 8 GeV

• -2 < ηjetB < 1.5

Di-jet• Q2 > 125 GeV2

• Ejet1T,B > 12 GeV, Ejet2

T,B > 12 GeV• -2 < ηjet

B < 1.5

- Jets identified in the Breit frame using the kT cluster algorithm- 80 pb-1 of e±p data (1998-2000)

At LO:

Jet production in NC DIS:- testing ground of pQCD- constrains xg and αs

Precise measurement…(e.g. jet energy scale ~1% → Δσ ~ 5%)

ZEUS-pub-06-006

NC DIS: Inclusive jets

- The data are well described by NLO QCD predictions- Will be able to constrain further the gluon density in the proton- Much more data to come…

NC DIS: dijet cross sections

- The data are well described by NLO QCD predictions- Will be able to constrain further the gluon density in the proton- Much more data to come…

CC cross sections

- Jets identified in the LAB frame using the kT cluster algorithm- 125 pb-1 of polarized e-

L,R-p data (2004-2005)

Phase Space:• Q2 > 200 GeV2 and y < 0.9• Ejet

T > 11 GeV

Jet production in polarized CC DIS:- testing ground of pQCD and EW sector of the SM- sensitive to valence quarks

Larger data sample w.r.t. 20 pb-1 of e-p data at HERAI

ZEUS-prel-06-020

CC cross sections

- Cross sections compared to LL parton-shower MC models - Sensitive to different quark flavours

Photoproduction of Dijet at high-ET H1: EPJC41(2005)273

Data Selection:• 67 pb-1 of e+p data (1999-2000)• Q2 <1 and 0.1 < y < 0.9• Et,max > 25 GeV , Et,2nd > 15 GeV and -0.5 < ηjet < 2.75• 0.05 < xp < 0.7 and 0.1 < xγ < 1.0

xγ

Et,max

|cosθ*|

xp

Photoproduction of Dijet at High-ET

• Shape dominated by jet phase space• Large scale uncertainty

• Cut on dijet inv. mass • Reduced theoretical unc.

Photoproduction of Dijet at High-ET

•Good description by NLO pQCD calculations•Largest xp values reachable for forward-forward sample•Sensitivity to PDFs at large xp → data available for PDF Fits

Summary

•Mature DGLAP Analyses at HERA allow precise determination of PDFs

•A new set of incl. and jet observables has been presented which will help to determine PDFs with even higher precision

xF3 γZH1prelim-06-142 ZEUS-prel-06-022

See V. Chekelian talk