Inclusive jet photoproduction at HERA

Inclusive jet photoproduction at HERA

B.Andrieu (LPNHE, Paris)

On behalf of the collaboration

Outline:Introduction & motivationQCD calculations and Monte Carlo Detector & experimental facts Results Summary

DIS03, St-Petersburg, 25 Apr. 2003

B.Andrieu Inclusive jet photoproduction at HERA - 2

Jet photoproduction (parton level)

p

j(xp)

k

l

ee

e’

p p p

j(xp)

i(x)

k

l

e’

=direct (pointlike) + resolved (hadronlike

(VDM) + qq )

EEyW

EEγy

pep

e

4

/

σdμ,xfμ,xfQy,fdΩσ klijppj/pγγi/γ

ij

γ/ejetseep )()()( 222

max Σ

)( )()( 22// μ,xfμ,xf pppjγγi PDF of parton i (j ) in (p)

)( 2max/ Qy,f e photon flux in e

)( p factorisation scales in (p) ;

R renormalisation

scale

σd klij partonic cross section)()( 22 RSRem (direct)

)( 22 RS (resolved){

EeEeEx

p

llTkkTp 2

,,

yEeEeEx

e

llTkkT

2,,

{



Jet photoproduction (hadron level)

Higher orderQCD processes

LO hard process Soft processes

LO QCD partons jets of hadrons detector signals

Compare jets at the parton, hadron and detector level Jet algorithms must ensure

infrared and collinear safety minimal sensitivity to non-perturbative processes inclusive k (D=1) algorithm (Ellis&Soper, PRD48 (1993) 3160)

dij = min (ETi ,ET

j ) Rij /D ; ET weighted recombination scheme Cone (R=1) algorithm used for comparison with previous data



Motivation

High ET jets ( non-perturbative effects and scale uncertainty reduced)

Direct insight into parton dynamics Precise tests of perturbative QCD predictions Constrain photon and proton PDFs Search for new physics Low ET jets ( non-perturbative effects and scale uncertainty important)

Test phenomenological models of underlying event + fragmentation

Inclusive vs dijet + More statistics, extended kinematical range No direct reconstruction of x ,xp

+ No infrared sensitivity w.r.t. kinematical cuts as for dijet



QCD calculations and Monte Carlo

Most precise QCD calculations up to NLO (parton level) NLO QCD weighted parton Monte Carlo (Frixione, NPB507(1997)

295) Photon & proton PDFs: GRV & CTEQ5M

Other choice : photon AFG proton MRST99, CTEQ5HJ (enhanced gluon at high xp )

LO QCD Monte Carlo event generators to correct data and calculations to the hadron level: PYTHIA, PHOJET, HERWIG

QCD = 200 MeV Fragmentation: LUND String (PYTHIA, PHOJET) or Cluster

(HERWIG)

Underlying event:

Multiple Interactions (PYTHIA) pTmia =1.2 GeV

Dual Parton Model (PHOJET)Soft Underlying Event (HERWIG) 35 % resolved{



H1 detector at HERA

e

p

Central TrackingdpT /pT = 0.6 % . pT

LAr CalorimeterdE /E = 50 % / (E)(hadrons & jets)

{ 2% (high ET)

4% (low ET)Systematic uncertainty

Electron TaggerPhoton Detector

e p

dL/L = 1.5%

s = 300 GeV

Luminosity

{



Experimental facts (I)

Inclusive cross section as a function of ETjet and jet

count the number of jets in a given kinematical range jet measured in laboratory frame (cms ~ jet – 2)

High ET jets (ETjet > 21 GeV)

L = 24 pb-1, untagged (e+ undetected) data Q2 < 1 GeV2 , 95 < W p < 285 GeV (0.1 < y < 0.9)

Low ET jets (5 GeV< ETjet < 21 GeV)

L = 0.5 pb-1, tagged (e+ detected) data Q2 < 0.01 GeV2 , 164 < W p < 242 GeV (0.3 < y <

0.65)



Experimental facts (II)

Hadronisation corrections fragmentation (after parton showers) underlying event (after

fragmentation) reverse order consistent results

(1+hadr.) = (1+frag.) . (1+u.e.) frag.< 0 and when ET or u.e.> 0 and when ET or hadr.~ 30 (10) % for ET <10 (>20)

GeV Cone: hadr.~ 40 (20) % for ET <15 (>15) GeV

Data corrections bin migrations Important due tosteeply falling ET

spectrum selection efficiencies

Exclude regions of large migrations

high ET

<0 (photon region) low ET

>1.5 (proton region)

Hadron level cross sections obtained using Monte CarloHO QCD partons jets of hadrons detector signals



Experimental facts (III)

Systematic uncertainties: LAr hadronic energy scale 10-20 % (10 %) for low (high) ET Correction for detector effects < 10 % (8 %) for low (high) ET

(statistical 1/2 difference between Monte Carlo) Luminosity 1.5 % All other uncertainties (SPACAL energy scale, fraction of hadronic

energy flow carried by tracks, background subtraction, trigger efficiency) 1 %

Theoretical uncertainties: Hadronisation correction uncertainty 30 % (10 %) for low

(high) ET (statistical 1/2 difference between Monte Carlo) Renormalisation & factorisation scale (x2, /2) uncertainty <

10 %



ET & distribution (high ET)

LO too low at low ET and high Agreement with NLO very good, even w/o hadronisation corrections All predictions using different PDFs agree with the data



ET distribution (W p bins, high ET)

ET & fixed:

<x ,xp> 1/W p

LO prediction low ET & high W p

too low NLO prediction

high W p

very good agreement lowW p

reasonable agreement promising region toconstrain gluon at high xp



ET distribution: full range

LO prediction fails to reproduce shape

NLO prediction good agreement over

6 orders of magnitude! hadronisation corrections

needed

Fit Range: 5 < ET < 35 GeV

n = 7.5 0.3 (stat) +0.1– 0.5 (syst.)

compatible with similar fit on charged particle cross section

(EPJ C10 (1999) 363) n = 7.03 0.07 +-0.2 (syst.)

)/1( 0,n

TT EE



distribution (ET bins, high ET)

Good agreement with NLO even w/o hadronisation corrections

Precision of data equivalent to (or even better than) scale uncertainty

challenge for theory to reduce uncertainty

W p & fixed:

<x ,xp> ET



distribution (ET & W p bins, high ET)

Good agreement with NLO even w/o hadronisation corrections

Cross section maximum shifted towards lower values for higher W p (Lorentz boost) and lower ET

All PDFs consistent with data Precision of data equivalent to

(or better than) scale uncertainty

could be used to better constrain PDFs fits

fixed:<x ,xp> ET / W p



distribution (ET bins, low ET)

12 < ET < 21 GeV good agreement both NLO and hadronisation

corrections needed 5 < ET < 12 GeV data indicative of a trend

different from calculation challenge for Monte Carlo to

accurately estimate hadronisation corrections?

inadequacy of photon PDFs?

higher-order terms needed?



xT < 0.2 shape similar for

p and pp resolved photon ~ hadron

xT > 0.2 p harder than pp spectrum

enhanced quark density in the resolved photon w.r.t. a hadron

dominance of direct

point-like photon

Comparison with pp-Scaled cross section (independent of energy up

to scaling violations)

WEx

E

ExSpT

T

T

TT

2;dd

d2)(

23

-

-

Confirmation of the dual nature of the photon



Summary

New measurement of inclusive jet photoproduction cross section (L x 80 compared with previous one) using the k algorithm

Kinematical range extended to ET =75 GeV (~ xT = 0.5) Experimental uncertainties already competitive with (scale)

uncertainties Good agreement over 6 orders of magnitude in ET distribution NLO and hadronisation corrections needed, especially at low ET No discrimination of PDFs, but data helpful in global PDFs fits and

future measurement promising for the gluon at high xp Determination of hadronisation corrections challenging for theory

and phenomenology Comparison of scaled cross section with pp data confirms the

dual nature of the photon with a transition around xT = 0.2

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Inclusive jet photoproduction at HERA