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27.5 GeV e ± 920 (820) GeV p

The structure of the proton & NLO QCD fits. HEP2005 International Europhysics Conference on High Energy Physics EPS05 (July 21st-27th 2005), Lisboa, Portugal Claire Gwenlan, University of Oxford on behalf of the H1 and ZEUS collaborations. Outline  introduction & motivation

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27.5 GeV e ± 920 (820) GeV p

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  1. The structure of the proton & NLO QCD fits HEP2005 International Europhysics Conference on High Energy Physics EPS05 (July 21st-27th 2005), Lisboa, Portugal Claire Gwenlan, University of Oxford on behalf of the H1 and ZEUS collaborations 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 27.5 GeV e±920 (820) GeV p

  2. 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  BUT still regions where PDFs not well known  high-x quarks, gluon 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 analyses 2

  3. xp p’ = xp + q Overview of HERA kinematics q = k-k’ /Z0, W± deep inelastic scattering (Q2 > 1 GeV2)  /Z0 exchange neutral current (NC)  W± exchange charged current (CC) __________________________________________ photoproduction (Q2 < 1 GeV2)  “quasi-real”  exchange 3

  4. Inclusive cross sections and structure functions 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) HERA neutral current F2 CC e-p (e+p) sensitive to u (d) valence  flavour separation

  5. Jet cross section measurements JET PRODUCTION AT HERA • 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 where does the information come from in a QCD fit ? 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

  7. The ZEUS-JETS QCD fit EPS05 abstract 324, ZEUS Collaboration good description of data: 2/points = 471/577 •  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 _____________________________________________________________________________________Eur. Phys. J 050364, hep-ph/05030274 – available in LHAPDF version 4

  8. Valence quark, sea and gluon distributions  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 8

  9. Impact of the jet data on the gluon comparison of gluon distribution from fits with and without jets:  no significant change in shape: no tension between jet and inclusive data  QCD factorisation  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 9

  10. Determination of s(MZ) addition of jet data also allows a precise extraction of s(MZ)  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 ZEUS HERA-Only with jet data HERA-Ony without jet data global fit ZEUS-JETS PDF

  11. NC cross sections at high-x from HERA EPS05 abstract 331, ZEUS Collaboration 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) areat x = 0.65 new measurement from ZEUS  new technique developed to measure differential NC cross sections up to Bjorken x = 1 11

  12. Method & data selection • ELECTRON + JET METHOD: 1. use ELECTRON information for Q2  electron well reconstructed for Q2 > 450 GeV2  good resolution in Q2 for all-x 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 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: 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 Information up to x = 1 12

  13. e-p NC cross section results ZEUS  highest x point is integrated cross section up to x=1:

  14. e+p NC cross section results  highest x point is integrated cross section up to x=1:

  15. e+p NC cross section ratios • 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

  16. Photoproduction of dijets with high-ET EPS05 abstract 680, H1 Collaboration •  photoproduction (Q2 ~ 0) perturbatively calculable if ET of jets used as hard scale • at O(s), two processes contribute   measurements of dijet photoproduction:  direct sensitivity to s and gluon in proton  resolved processes also sensitive to both gluon and quark densities in photon resolved direct  longitudinal momentum fractions xp and x participating in hard scatter:  for strong constraints on proton PDF, reduce dependence on photon structure  H1: direct-enriched xOBS > 0.8 • 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

  17. High-ET differential cross sections in p High-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 q enriched 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) g enriched 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 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

  19. 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

  20. Summary & Outlook HERA 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 only s(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

  21. Extras …

  22. ZEUS-JETS: inclusion of jet data 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%

  23. ZEUS-JETS fit: comparison with other PDFs agreement with other PDF fits within uncertainties 23

  24. ZEUS-JETS fit: correlation between gluon and s(MZ) 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 ZEUS-S global fit: no jet data included ZEUS-JETS fit: jet data included Only small increase in uncertainties when s freed 24

  25. NC at high-x: check x migrations xtrue distribution of the highest x bin  migration from low-x is very small  zero-jet events really are high-x events!!!

  26. NC at high-x: Control plots kinematic quantities electron quantities  MC gives good description of data

  27. NC at high-x: Control plots (cont.) kinematics in zero-jet events One-jet events  MC gives good description of data

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

  29. dijets at high-ET: angular distributions - cos* Jet cos* beamline Jet  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

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