Proton structure at high Q 2 at HERA

Proton structure at high Q 2 at HERA Yujin Ning Introduction Experiment setup High Q 2 PDF from HERA I New PDF from HERA II High x study in ZEUS Summary & outlook

Introduction

Deep inelastic scattering (DIS) an important tool to study the proton structure Deep: small scale Inelastic: proton is broken Lorentz-invariant variables: or Z 0: neutral current (NC) e p->e X W : charged current (CC) e-p-> X and e+p-> X Q 2=-q 2=-(k-k’)2 Virtuality: scale x=-q 2/2 p q Parton momentum fraction y=p q/p k Inelasticity: transferred energy fraction W 2=(p+q)2 Square of the invariant mass of hadronic final state s Square of center-of -mass

Neutral current cross sections and structure functions At Born level (Lowest order in QED) Longitudinal structure function valence + sea partons dominant valence quarks important at high Q 2 Parton density function(PDF) probability density finding parton q or q carry the momentum fraction x with Q

Charged current cross sections sensitive to d quark sensitive to u quark MW=80. 4 Ge. V Cross sections are suppressed CC good to disentangle flavor content of the proton (u or d by using e+/e-)

Experiment setup

HERA ep collider Ee=27. 6 Ge. V Ep=920(820 before year 98)Ge. V ep collisions x z Study the inner structure of proton and fundamental forces

ZEUS NC and CC events Scattered neutrino NC: CC: ü Well isolated electron with high transverse momentum ü Net transverse momentum is zero ü Large missing transverse momentum from the undetected neutrino

HERA I PDF

ZEUS NC and CC cross sections NC CC ZEUS-JETS (a new QCD fit using ZEUS inclusive cross section and jet production) describes data well

H 1 structure functions F 2 contribution from exchange x. F 3 FL from exchange Q 2<890 Ge. V 2 Good agreement with H 1 PDF 2000

ZEUS-JETS PDF § Only ZEUS data used in ZEUSJETS, well understood systematic uncertainty § High Q 2 NC/CC (94 -00), constrain at high x § DIS inclusive jet and dijet in photo production (PHP) (96 -97), jet cross section is sensitive to gluon density § First time HERA jet used in QCD fit ZEUS-JETS constrain the uncertainty on gluon density

HERA II PDF

HERA update HERA II Ee=27. 6 Ge. V Ep=920 Ge. V sqrt(s)=318 Ge. V e- 205 pb-1 • Longitudinally polarized lepton beam: sensitive to weak interaction • Increase specific luminosity: increase statistics at high Q 2 e- 62 pb-1 e+ 85 pb-1

ZEUS NC cross section and x. F 3 valence quark Unpolarized reduced cross section for e p More precise measurement of x. F 3 with 120 pb-1 e- data, 6 times more than HERA I

ZEUS CC e-p cross sections Higher statistics, good agreement

ZEUS-pol QCD fit with HERA II polarized lepton beam ZEUS-JET+polarized e-p All EM parameters fixed to SM value Center value does not change compared to ZEUS-JETS Uncertainties reduced, especially on u valence quark

High x study in ZEUS

Motivation for high x study • DGLAP does not predict xdependence • PDFs decrease very quickly at high x, hard to measure because of low statistics and large migration�� • Highest measured point x=0. 75, BCDMS, data is available at higher x but at low W, need huge correction. • HERA covers much larger phase space to high Q 2 and low x, but not to high x • ZEUS published x=0. 65 • The uncertainties on PDF grow with x, might be infinite at x=1 New reconstruction methods are needed to reach the highest x

ZEUS published method • Double angle method (DA) – All the energy deposits in CAL: scattered electron and all others->hadron – Electron polar angle – Spatial distribution hadronic deposits->hadronic polar angle • Insensitive to CAL overall energy scale • Sensitive to the accurate simulation of all effects in CAL – Hadronic energy lost in beam hole at high x, no accurate reconstruction on x – Highest published point, x=0. 65

New method • Electron + Jet • Electron is well reconstructed for Q 2>450 Ge. V 2, very high acceptance in whole x range • Define Q 2 bins from Ee and e: • In each Q 2 bin, define x bins: – If leading jet is NOT near the beam hole low x, jet is well reconstructed, x from Ejet and jet, good resolution – If jet IS near beam hole high x, jet is not well reconstructed no jet, count events without jet xedge<x<1 integral of cross section

Compare with ZEUS published In the ZUES published kinematic region, new method agrees well with published result New method measures cross section to x=1

Measured cross section low x x jet in FCAL high x jet lost in beam hole Agree well at measured low x region More information on high x

Cross section ratio to theory Theories describe data well at low x. Expectation is lower than data in the highest x bin, unmeasured before With new method, the cross section at high x was measured as precisely as other x range.

Summary • A lot of work done on PDF: ZEUS-JETS and H 1 PDF 2000. • A new method measure cross section to x=1. • The luminosity delivered by HERA II is more than 350 pb-1. • NC and CC DIS cross section for e+p and e-p are measured with HERA II and agree with the SM predictions. • New fit including HERA II polarized data, the uncertainty is reduced • HERA II will totally delivery 1 fb-1, more precise measurement.