27.5 GeV e± 920 (820) GeV p

HEP2005 International Europhysics Conference on High Energy Physics

EPS05 (July 21st-27th 2005), Lisboa, Portugal

Claire Gwenlan, University of Oxfordon behalf of the H1 and ZEUS collaborations

The structure of the proton & NLO QCD fits

Outline introduction & motivation

HERA kinematics & ep physics

HERA NLO QCD fits

new “proton-structure sensitive”

measurements from HERA - NC cross sections at high-x

- dijets in p collisions at high-ET

summary & outlook

Introduction & Motivation proton structure described by Parton Density Functions (PDFs) needed to make predictions for any process involving protons

must be known as precisely as possible to maximise potential for

discovery at current and future colliders (Tevatron, LHC)

HERA is most important source of information on proton structure data are very precise and cover wide

kinematic region also in relevant x-region for LHC

precise extraction of PDFs now

possible within one experiment

presented here:

latest NLO QCD fit to HERA data

two new HERA measurements: 1) NC cross sections at high-x

2) dijet p cross sections

that will provide further constraints on

PDFs in future QCD analyses2

BUT still regions where PDFs not

well known high-x quarks, gluon

xp

p’ = xp + q

2 2 2

2

2

kinematics:

Q =-q =-(k-k') : Virtuality of exchanged boson

Qx = : Bjorken scaling variable

2p.q

p.qy= : inelasticity

p.k

Q = sxy; s = centre-of-mass energy squared

Overview of HERA kinematics

deep inelastic scattering (Q2 > 1 GeV2)

/Z0 exchange neutral current (NC)

W± exchange charged current (CC)

__________________________________________

photoproduction (Q2 < 1 GeV2)

“quasi-real” exchange

/Z0, W± q = k-k’

3

4

INCLUSIVE NC/CC DIS

HERA inclusive data: directly sensitive to quarks in proton

indirectly sensitive to gluon through

QCD radiation (scaling violation at low-x)

2

23

2L

2222

2

y)(11Y where

)Q(x,xFY)Q(x,Fy)Q(x,FY~dxdQ

p)(ed

Inclusive cross sections and structure functions

2 i i

23 i i

2L s

STRUCTURE FUNCTIONS:

F ~ x(q+q ) dominant contribution

xF ~ x(q -q ) contributes at high-Q

F ~ .xg(x,Q ) contributes at high-y

CC e-p (e+p) sensitive to u (d) valence flavour separation

HERA neutral current F2

5

JET PRODUCTION AT HERA

Jet cross section measurements

Scaling violations may give rise to

distinct jets in final state lowest

order contributions from:

QCD Compton (*q qg)

Boson Gluon Fusion (*g qqbar)

jets directly sensitive to gluon density

in proton through BGF process

also directly sensitive to s through

both BGF and QCDC breaks correlation between s and gluon

BGF QCDC

6

QCD analysis of HERA data

Global HERA OnlyValence

Predominantly fixed target data (-Fe & D/p)

High-Q2 NC/CC e cross sections

Sea Low-x from HERA NC DIS High-x from fixed target Flavour from fixed target

Low-x from HERA NC DIS High-x ? (need HERA-II) Flavour ? (assumptions needed)

Gluon Low-x from HERA dF2/dlnQ2 Mid-to-high-x from Tevatron jet data (some fits) High-x from momentum sum

Low-x from HERA dF2/dlnQ2 Mid-to-high-x from HERA jet data High-x from momentum sum

now precise PDFs can be determined using only HERA data free from heavy-target corrections, isospin symmetry assumptions, …

avoids complications associated with combining data from different experiments

where does the information come from in a QCD fit ?

7

The ZEUS-JETS QCD fit

_____________________________________________________________________________________Eur. Phys. J 050364, hep-ph/05030274 – available in LHAPDF version 4

good description of data: 2/points = 471/577

EPS05 abstract 324, ZEUS Collaboration

data included (ZEUS): - 94-00 NC/CC inclusive e+p & e-p - 96-97 inclusive jets in NC DIS - 96-97 dijets in p

kinematic coverage and cuts: - 6.3 ·10-5 < x < 0.65 - 2.7 < Q2 < 30000 GeV2 - W2 > 20 GeV2 (higher twist)

xf(x) = p1 xp2 (1-x)p3 (1+p4x) at

starting scale Q02 = 7 GeV2

f(x) = uv, dv, sea, g, =(dbar-ubar) 11 free parameters

evolve PDFs in Q2 using NLO DGLAP

heavy quarks: Thorne-Roberts variable flavour number scheme

correlated experimental uncertainties evaluated using Offset Method

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Valence quark, sea and gluon

distributions

8

valence quarks:

- high-x becoming competitive with global fits

sea-quarks & gluon: - low-x as good as global fits (information comes from HERA anyway)

- high-x improved by jet data

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comparison of gluon distribution

from fits with and without jets:

no significant change in shape:

no tension between jet and

inclusive data QCD factorisation

Impact of the jet data on the gluon

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HERA jet cross sections constrain

gluon in range x = 0.01 – 0.4

reduction in gluon uncertainties

by factor of ~2 in mid-x region

over the full range of Q2

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determination of s(MZ) from ZEUS-JETS:

s(MZ) = 0.1183±0.0028 (exp.)

±0.0008 (model) ±0.0050 (theory)

first extraction using only HERA data

scale uncertainty would improve in NNLO fit

in agreement with world average :

s(MZ) = 0.1182 ± 0.0027 (Bethke, 2004)

and with other extractions from HERA

addition of jet data also allows a precise extraction of s(MZ)

HERA-Ony without jet data

HERA-Only with jet data

ZEUS-JETS PDF

global fit

Determination of s(MZ)

ZEUS

NC cross sections at high-x from HERA

motivation

PDFs decrease quickly at high-x and

PDF uncertainties are large

need constraints from data at high-x

highest measured points in DIS are

at x = 0.75 (BCDMS)

- data at higher x exist but are in

resonance region and cannot be easily

interpreted in terms of PDFs

highest measured points from

HERA (H1/ZEUS) are at x = 0.65

new measurement from ZEUS new technique developed to measure differential NC cross sections up to Bjorken x = 1

EPS05 abstract 331, ZEUS Collaboration

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12

ELECTRON + JET METHOD:

b. if JET near beam-pipe high-x jet not well reconstructed ZERO jet collect events in bin with xedge < x < 1

measure integral cross section up to x=1:

2. in each Q2 bin, define x bins:

a. if JET far from beam-pipe low-x x from EJet, Jet good resolution in x

1. use ELECTRON information for Q2

electron well reconstructed for Q2 > 450 GeV2

good resolution in Q2 for all-x

)cos(1E2EQ e'ee

2

Method & data selection

edge

1 2 2

x(d /dxdQ )dx

12Information up to x = 1

kinematic coverage of data

DATA SELECTION 98-99 e-p (16.7pb-1) & 99-00 e+p (66.1pb-1)- high energy electron with strict fiducial cuts

- 0 or 1 jet with ETJet>10 GeV and Jet>0.12

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ZEUS

e-p NC cross section results

highest x point is integrated cross section up to x=1:edge

221

x

x1

)/dxdQ(ddx edge

14

e+p NC cross section results

edge

221

x

x1

)/dxdQ(ddx edge

highest x point is integrated cross section up to x=1:

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generally good description by NLO QCD (using CTEQ6D, ZEUS-S PDFs) new direct constraints on PDFs at high-x (and lower-x through sum rules) similar for e-p ratios

e+p NC cross section ratios

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Photoproduction of dijets with high-ET

new analysis from H1: 99-00 e+p data (66.6 pb-1) new high-precision measurement of high-ET dijets in photoproduction

AIM include in combined QCD fit with DIS data to extract PDFs and s

photoproduction (Q2 ~ 0)

perturbatively calculable if ET of jets

used as hard scale

at O(s), two processes contribute

jet,i

2jet,iOBS

p Ti=1p

1x = E e

2E for strong constraints on proton PDF, reduce dependence on photon structure H1: direct-enriched x

OBS > 0.8

jet,i2

jet,iOBS -T

i=1

1x = E e

2E

resolved direct

EPS05 abstract 680, H1 Collaboration

measurements of dijet photoproduction:

direct sensitivity to s and gluon in proton

resolved processes also sensitive to both

gluon and quark densities in photon

longitudinal momentum fractions xp and x participating in hard scatter:

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High-ET differential cross sections in pHigh-ET dijet cross sections:

longitudinally invariant kT algorithm in lab.

Q2 < 1 GeV2, 0.1 < y < 0.9

pT,max > 25 GeV, pT,2 > 15 GeV, 0.5 < Jet < 2.75

QCD models: PYTHIA6.1 (CTEQ5L p, GRV-LO PDFs) Frixione-Ridolfi NLO QCD (CTEQ6M p, GRV-HO ) - hadronisation corrs. (PYTHIA+HERWIG) - yellow band: scale uncert. - green band: total uncert. (incl. PDFs,

hadronisation)

cross sections differential in xp and x:

longitudinal momentum fractions:

0.05 < xp < 0.7 0.1 < x < 1.0

• low-xp (< 0.1) and high-xp (> 0.1) regions

roughly distinguish g and q scattering

low-x (< 0.8) and high-x (> 0.8) regions

roughly distinguish between resolved and

direct photon events

generally good description by NLO QCD

g enriched

q enriched

resolved enriched

direct enriched

18

Double differential cross sections

more detailed look in bins of

measurement divided into resolved-

(x < 0.8) and direct- (x > 0.8) enriched

generally good description by NLO QCD

(although data tends to lie below

prediction at high-xp when 1,2 > 1)

dominant uncertainties:

- experimental:

cal. e-scale, model (low-xp), stat. (high-xp)

- scale uncertainty smallest at high-xp

- PDFs better known at low- than high-xp

already constrained at low-x by

inclusive DIS data

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Double differential cross sections

more detailed look in bins of

similar conclusions for cross sections

in pT,max

direct-enhanced region (x > 0.8)

for cross sections in xp and pT,max

smaller scale uncertainties

reduced dependence on photon PDF

potentially strong constraints on

proton structure include in QCD fit

for PDFs and s

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Summary & OutlookHERA data now high precision and wide kinematic coverage new NLO

QCD combined fit to ZEUS inclusive DIS and jet data (ZEUS-JETS PDF)

simultaneous extraction of PDFs and s

rigorous inclusion of jet data for the first time in a QCD fit

significant reduction of gluon uncertainties at mid-to-high-x

precise determination of s from HERA data onlys(MZ) = 0.1183 ±0.0028 (exp.)±0.0008(model)±0.0050(theory)

new method developed by ZEUS to measure NC cross sections up to x = 1

first results in e+p (65 pb-1) and e-p (17 pb-1) data from HERA

new constraints on PDFs at high-x

under analysis within the framework of NLO QCD fitting

new high-ET dijet photoproduction cross sections measured by H1

67 pb-1 e+p data, reduced systematics compared to previous measurements

generally well described by NLO QCD

direct-enriched (x > 0.8) cross sections

smaller uncertainties and reduced dependence on structure

potentially strong constraints on gluon in proton

AIM: include data in combined NLO QCD analysis of inclusive DIS and jet data

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Extras …

22

In contrast with evaluation of structure functions from evolved PDFs, the calculation of jet cross sections at NLO requires much CPU time: O(10 hours) per PDF set unaffordable in a fit of the proton PDFs

METHOD

deconvolve PDFs and s from matrix elements in the NLO calculation:

construct “grids” containing matrix element part of cross section such

that calculations for jet cross sections can be performed sufficiently fast

(and accurately) for any PDF and any value of s O(1 second) per PDF set

Jet cross section calculations can be performed for ANY PDF set and

ANY value of s in a fast way and with an accuracy better than 0.5%

2k ji,a,n, ,Q

2

2

grid of w eights in the "(x= ,Q )" plane for a given bin of the

cross section (i), parton species (a) and order (n) typical si ( ,Q

G

) 100×10e 0z in

2k ji,a,n

2 n 2jet a k j s j

n=1,2 a=q,q,g j k, ,Q

(i)= f ( ,Q ). ( GQ ).

ZEUS-JETS: inclusion of jet data

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ZEUS-JETS fit: comparison with other PDFs

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agreement with other PDF fits within

uncertainties

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ZEUS-S global fit: no jet data included

ZEUS-JETS fit: jet data included

Only small increase in uncertainties when s freed

jet cross sections directly sensitive to s via *g qqbar (coupled to

gluon) and via *q qg (NOT coupled to gluon)

extraction of s NOT strongly correlated to gluon

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ZEUS-JETS fit: correlation between gluon and s(MZ)

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migration from low-x is very small zero-jet events really are high-x events!!!

xtrue distribution of the highest x bin

NC at high-x: check x migrations

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kinematic quantities electron quantities

MC gives good description of data

NC at high-x: Control plots

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One-jet events kinematics in zero-jet events

NC at high-x: Control plots (cont.)

MC gives good description of data

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NC at high-x: e-p NC cross section ratios

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dijets at high-ET: angular distributions - cos*

cos*

Jet

Jet

beamline

sensitive to dynamics of p

- low MJJ, sensitive to jet ET cuts

- cut at MJJ > 65 GeV reduces bias

distribution follows form of

QCD matrix elements:

shape described by NLO QCD

but data tends to lie below

predictions

gluon propagator ~ |(1-cos*)|-2

quark propagator ~ |(1-cos*)|-1