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Parton distribution functions and jet cross sections at HERA

Parton distribution functions and jet cross sections at HERA. Enrico Tassi - Calabria University and INFN – On behalf of the H1 and Zeus Collaborations. Rise of F 2 at low-x (established with ~20 nb -1 ). Precise F 2 determination ( 1996-97 data samples).

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Parton distribution functions and jet cross sections at HERA

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  1. Parton distribution functionsandjet cross sections at HERA Enrico Tassi - Calabria University and INFN – On behalf of the H1 and Zeus Collaborations

  2. Rise of F2 at low-x (established with ~20 nb-1) Precise F2 determination ( 1996-97 data samples) Introduction: proton PDFs @ HERA Proton parton distribution functions (PDFs): • proton structure • predictions at hadron colliders → SM tests and Search Beyond SM • HERA main source of information on PDFs: • precise data over large kinematic region • → low-x rise of F2 (sea, gluon) Zeus and H1 DGLAP analyses of HERA-I (and fixed target) data: → H1 2000 PDFs → ZEUS-S PDFs

  3. HERA PDFs H1: EPJC30(2003)1 ZEUS: PRD67(2005)012007 pQCD describe F2 over 4 (3) order of mag. in Q2 (x) HERA PDFs: fair agreement

  4. ZEUS-JETS PDFs ZEUS: EPJC42(2005)1 First fit using HERA incl and jets data - HERA-I inclusive cross sects - Inclusive jet cross sects in NC DIS - Dijet cross sects in photo-production Data from one Experiment only: • Syst unc. well understood • No fixed target unc. • Valence quarks from high-Q2 NC and CC cross sections Jet data help in constraining the gluon and αs: • Reduction of gluon unc. by ~ factor 2 for 0.01 < x < 0.4 • Precise determination of αs

  5. Zeus-Pol PDFs ZEUS-prel-06-003 Q2=10 GeV2 Since 2002 HERA has been running with long. polarized e± beam (P~30-40% ) A new PDF fit has been performed including the new e-L,R-p NC and CC cross sections Improved determination of the PDFs at high-x: in particular xuv

  6. Zeus-Pol PDFs ZEUS-prel-06-003 Q2=10000 GeV2

  7. New measurements PDFs poorly known at large x: →New measurement of incl. cross sections at high-x Jets have strong impact on the DGLAP analyses →New measurements of jet cross sections: • Jets in DIS (NC and CC) • Dijet in γ-p

  8. Measure: and High-x incl. cross sections ZEUS-pub-06-003 • Motivation: Poor knowledge of high-x PDFs (HERA x~0.65, BCDMS~ 0.75) • New technique which allows the meas. of incl. cross sections up to Bjorken x=1 Re-analysis of HERA I high-Q2 e±p data (Q2 > 425 GeV2) Electron-JetMethod: Use elec. and jet E and θ to reconst. Q2,x 1. Always use Ee and θe for Q2 2. For each Q2 bin define x-bins a) Jet far from beam pipe (low-x) → x from Ejet , θjet b) Jet near the beam-pipe (high-x) → collect events in [xedge,1]

  9. High-x cross sections Theory (with current PDF sets) underestimates the measurements at the highest values of x.

  10. NC DIS: inclusive and dijet cross sections ZEUS-pub-06-006 At LO: Jet production in NC DIS: - testing ground of pQCD - constrains xg and αs Precise measurement… (e.g. jet energy scale ~1% → Δσ ~ 5%) • Inclusive jets • Q2 > 125 GeV2 • EjetT,B > 8 GeV • -2 < ηjetB < 1.5 • Di-jet • Q2 > 125 GeV2 • Ejet1T,B > 12 GeV, Ejet2T,B > 12 GeV • -2 < ηjetB < 1.5 - Jets identified in the Breit frame using the kT cluster algorithm - 80 pb-1 of e±p data (1998-2000)

  11. NC DIS: Inclusive jets - The data are well described by NLO QCD predictions - Will be able to constrain further the gluon density in the proton - Much more data to come…

  12. NC DIS: dijet cross sections - The data are well described by NLO QCD predictions - Will be able to constrain further the gluon density in the proton - Much more data to come…

  13. CC cross sections ZEUS-prel-06-020 Jet production in polarized CC DIS: - testing ground of pQCD and EW sector of the SM - sensitive to valence quarks Larger data sample w.r.t. 20 pb-1 of e-p data at HERAI • - Jets identified in the LAB frame using the kT cluster algorithm • - 125 pb-1 of polarizede-L,R-p data (2004-2005) • Phase Space: • Q2 > 200 GeV2 and y < 0.9 • EjetT > 11 GeV

  14. CC cross sections - Cross sections compared to LL parton-shower MC models - Sensitive to different quark flavours

  15. Photoproduction of Dijet at high-ET H1: EPJC41(2005)273 xγ Et,max |cosθ*| xp • Data Selection: • 67 pb-1 of e+p data (1999-2000) • Q2 <1 and 0.1 < y < 0.9 • Et,max > 25 GeV , Et,2nd > 15 GeV and -0.5 < ηjet < 2.75 • 0.05 < xp < 0.7 and 0.1 < xγ< 1.0

  16. Photoproduction of Dijet at High-ET • Shape dominated • by jet phase space • Large scale uncertainty • Cut on dijet inv. mass • Reduced theoretical unc.

  17. Photoproduction of Dijet at High-ET • Good description by NLO pQCD calculations • Largest xp values reachable for forward-forward sample • Sensitivity to PDFs at large xp • → data available for PDF Fits

  18. Summary • Mature DGLAP Analyses at HERA allow precise determination • of PDFs • A new set of incl. and jet observables has been presented • which will help to determine PDFs with even higher precision

  19. xF3γZ H1prelim-06-142 ZEUS-prel-06-022 See V. Chekelian talk

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