Proton structure at high Q2 at HERA

Yujin Ning

IntroductionExperiment setupHigh Q2 PDF from HERA INew PDF from HERA IIHigh x study in ZEUSSummary & outlook

Introduction

Deep inelastic scattering (DIS)

Lorentz-invariant variables:Q2=-q2=-(k-k’)2 Virtuality: scalex=-q2/2pq Parton momentum

fractiony=pq/pk Inelasticity: transferred

energy fraction W2=(p+q)2 Square of the invariant

mass of hadronic final states Square of center-of -mass

or Z0: neutral current (NC)ep->eXW: charged current (CC)e-p->X and e+p->X

Deep: small scaleInelastic: proton is broken

an important tool to study the proton structure

Neutral current cross sections and structure

functions

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d2σ Born (e± p)

dxdQ2=

2πα 2

xQ4Y+F2

NC (x,Q2) mY−xF3NC (x,Q2) − y 2FL

NC (x,Q2)[ ]

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Y± ≡1± (1− y)2

At Born level (Lowest order in QED)

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F2(x,Q2) = x A f (Q2)(q f (x,Q2) + q f (x,Q2))f

∑

xF3(x,Q2) = x B f (Q2)(q f (x,Q2) − q f (x,Q2))f

∑ valence quarks important at high Q2

valence + sea partons dominant

Parton density function(PDF)probability density finding parton q or q carry the momentum fraction x with Q

Longitudinal structure function

Charged current cross sections

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d2σ CC (e+ p)

dxdQ2=

GF2

2πx(

MW2

MW2 + Q2

)2[u + c + (1− y)2(d + s)]

d2σ CC (e− p)

dxdQ2=

GF2

2πx(

MW2

MW2 + Q2

)2[u + c + (1− y)2(d + s)]

sensitive to d quark

sensitive to u quark

MW=80.4GeV Cross sections are suppressed

CC good to disentangle flavor content of the proton (u or d by using e+/e-)

Experiment setup

HERA ep colliderEe=27.6 GeVEp=920(820 before year 98)GeV

ep collisions

Study the inner structure of proton and fundamental forces

xz

NC: Well isolated electron with high transverse momentum

Net transverse momentum is zero

ZEUS NC and CC events

CC: Large missing transverse momentum from the undetected neutrino

Scattered neutrino

HERA I PDF

ZEUS NC and CC cross sections

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˜ σ NCe ± p =

xQ4

2πα 2

1

Y+

d2σ NC (e± p)

dxdQ2

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

CCNC

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˜ σ NCe ± p =

2πx

GF2

[MW

2 + Q2

MW2

]2 d2σ CC (e± p)

dxdQ2

H1 structure functionsF2 contribution from exchange

xF3

FL from exchangeQ2<890GeV2

Good agreement with H1 PDF 2000

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˜ σ e − p − ˜ σ e + p =Y−

Y+

2xF3

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F2 =˜ σ e + p

1+ ΔF 2 + ΔxF3+ ΔFL

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FL =Y+

y 2(F2 − ˜ σ e ± p )

ZEUS-JETS PDF

ZEUS-JETS constrain the uncertainty on gluon density

Only ZEUS data used in ZEUS-JETS, well understood systematic uncertainty High Q2 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

HERA II PDF

HERA update HERA IIEe=27.6 GeVEp=920 GeVsqrt(s)=318 GeV

• Longitudinally polarized lepton beam: sensitive to weak interaction• Increase specific luminosity: increase statistics at high Q2

e+ 85 pb-1

e- 205 pb-1

e- 62 pb-1

ZEUS NC cross section and xF3

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˜ σ e − p − ˜ σ e + p =Y−

Y+

2xF3

More precise measurement of xF3

with 120 pb-1 e- data, 6 times more than HERA IUnpolarized reduced cross section

for ep

xF3 valence quark

ZEUS CC e-p cross sections

Higher statistics, good agreement

ZEUS-pol QCD fit with HERA II polarized lepton beam

Uncertainties reduced, especially on u valence quark

ZEUS-JET+polarized e-pAll EM parameters fixed to SM value

Center value does not change compared to ZEUS-JETS

High x study in ZEUS

• DGLAP does not predict x-dependence

• 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 Q2 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

Motivation for high x study

• 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

ZEUS published method

• Electron + Jet• Electron is well reconstructed for

Q2>450GeV2, very high acceptance in whole x range

• Define Q2 bins from Ee and e:

• In each Q2 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

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Q2 = 2 × Ee × Ee' × (1+ cosθe )

New method

In the ZUES published kinematic region, new method agrees well with published result

New method measures cross section to x=1

Compare with ZEUS published

Measured cross section

low x

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d2σ

dxdQ2

jet in FCAL

jet lost in beam hole

x

Agree well at measured low x regionMore information on high x

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dxd2σ

dxdQ2xedge

1

∫1− xedge

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 H1

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 1fb-1, more precise measurement.