Neutral and Charged Current at High–x and High-Q 2 at HERA

Neutral and Charged Current at High–x and High-Q2 at HERA

Haiping Peng(On behalf of H1 and ZEUS Collaborations)

QCD06 Montpellier, France

QCD06 Montpellier [email protected] 2006-07-04

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Outline

Introduction to HERA

Deep inelastic scattering and structure functions

F2, xF3 measurement

Polarised neutral and charged current

Summary


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H1 and ZEUS at HERA

Proton –electron or positron collider; Center of mass energy S = 319GeV; Higher luminosity & polarised beam since 2003; 2 collider experiments; Nearly 4 detector coverage

e

Circumference ～ 6.3km

P

820/920GeV27.5GeV


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Deep Inelastic Scattering

At fixed s: two independent kinematic variables: e.g. Q2 & x;

Q2 = “resolving power” of probe, High Q2 : resolve 1/1000th size of proton;

x = momentum fraction of proton carried

by struck quark;

y = inelasticity of scattered lepton; Fraction of lepton’s energy transferred to the proton; Related kinematic quantities; Q2 = Sxy


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NC&CC Cross Sections

CC Cross Section:

NC Cross Section:

Sensitive to u and d valence quarks at LO QCD


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Kinematic Plane Coverage

Conventional QCD evolution only tells us Q2 dependence (DGLAP), x dependence must come from data;

HERA cover large region in x and Q2;

At low x : measure gluon PDFs and indirect determination of FL

At high x : measure xF3 and valence quark PDFs;

Large overlap with LHC region in x;

x


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Precision F2 Data

Sensitive to sum of quark and anti-quark;

Range in x : 0.00001 ～ 1; Range in Q2 : 1 ～ 30000GeV2;

Strong scaling violation, due to QCD gluon radiation, provide indirect constraint on gluon density;

Measured precision: 2 ～ 3%, still large error at highest Q2 and x;

Higher Luminosity at HERAII

Data of HERA experiments in agreement and well described by QCD;

HERAI


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xF3 Determination Difference between e+p and e-p xF3 significant at high Q2 only;

Increases electron proton cross section and reduces positron proton cross section at large Q2;

e-

e+

L(e- p) 15pb-1 L(e+ p) 110pb-1

Constrains the valence quark content;

Data is well described by QCD (CTEQ6D) and in agreement with global fit PDFs;

Uncertainty from e-p sample, higher luminosity is necessary; HERAII data(170pb-1) will greatly help;

HERAI


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NC at High x

Limitations in beam energies and measurement techniques: HERA x~0.65;

New method : Electron and Jet; Electron Q2; One Jet x; No Jet highest bin xEdge<x<1; Agree with the expectation of NLO using CTEQ6D P

DFs; A tendency to lie above the expectations at highes

t x bins; Produce new constraint on PDFs at the higher valu

es of x.

HERA I

e+p 65.1pb-1


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CC Cross Sections

e+p mostly sensitive to d(x,Q2); while e-p mostly sensitive to u(x,Q2);

e+p cross section is suppressed by factor (1-y)2;

At low Q2, the difference is less pronounce due to the sea quark contribution;

Constraint for the u and d valence quark;

Small cross section Large statistical error;

Data agrees with NLO QCD global PDFs fit;

HERAI


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HERA upgrade : longitudinally polarized lepton beams ; higher luminosity; SM Prediction: 1. The cross section dependent on the longitudinal polarization of the incoming lepton beam; 2. CC cross section : Linear function of polarization due to the absence of right-handed charged currents;

3. NC Cross section: decompose into unpolarised and polarised parts;

Polarised NC&CC Cross Sections


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CC Cross Sections

Data: e-p : 2003-2004; e+p : 2005; HERA I;

Linear dependence is firmly established both for e+ and e- beam;

ZEUS and H1 measurements are in good agreement with SM;

Vanishing cross sections for left (right)-handed positrons (electrons);

Consistent with absence of right-handed weak currents;


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NC Cross Sections

Rise of the ratio RH/LH for e+p;

drop of the ratio RH/LH for e-p;.

Indication of polarization effect on NC cross sections, although significance is moderate;

Well described by the predictions of SM for both H1 and ZEUS;.

H1 H1e+p e-p


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xF3 at HERAII

Combine polarised data sets, and make small correction for residual polarization a effective unpolarised date;

Combine HERAI data xF3; More precise xF3 measurement; Data is well described by QCD (CTEQ6D) and i

n agreement with global fit PDFs;


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Summary

HERA measures proton structure function in a wide range of x and Q2, and plays important role in testing of SM.

F2 is precisely measured, scaling violations are well described by pQCD;

xF3 measurement at high Q2 is sensitive to valence quark content of proton, but statistically limited for HERAI, much improved at HERAII;

CC cross section measurements at high Q2 constrain u and d density;

HERAII polarised beam : Linear dependence of CC cross section on polarization is consistent with SM prediction, the absence of right-handed weak currents is confirmed. Effect of polarization in NC interactions is visible but significance is moderate;

The coming HERAII polarised and higher luminosity (700pb-1, 2007) data will improve the measurement to a new level precision and enhance the potential in testing of SM.


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Thanks


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xG3 at HERAI


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CC Cross sections .VS. (1-y)2&x

(1-y)2 is directly related to the scattering angle in the electron–quark center-of-mass system;

Isotropic distribution and strong angular dependence on (1-y)2;

Isotropic distribution increase with decreasing x due to the increasing of sea quarks;

At large x, e+p sensitive to d quark; e-p sensitive to u quark;

e-p cross section is larger than e+p cross sectionDifferent quark flavors probe by the w boson and the helicity structure of CC interactionAgree well with SM expectations within errors


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NC at HERAII


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xF3 at HERAII


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CC Cross Sections

Differential cross sections with different polarizations have the same shape, only the normalization is different!


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CC & NC Cross sections

dominant

Z interf.

Diff u, d distribution.

W boson

Spans more than two orders of magnitude in Q2, the cross section fall with Q2 by about six orders of magnitude for NC. while three orders for CC;

Agreement between H1, ZEUS and QCD fit in cross section; NC cross section exceeds CC cross section at low Q2 due to the photon exchange in the

NC process; Unification of EM and weak interactions in DIS at high Q2;

HEARI HEARII


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NC at High x e-p 16.7

Documents

Neutral and Charged Current at High–x and High-Q 2 at HERA