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An NLO QCD Analysis of inclusive and jet data from ZEUS

Low-x Meeting Sinaia, 2005. An NLO QCD Analysis of inclusive and jet data from ZEUS. Enrico Tassi (Calabria University and INFN) on behalf of ZEUS. Based on: Zeus Coll., DESY-05-050 (accepted by EPJC). Proton PDFs and α s (M Z ).

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An NLO QCD Analysis of inclusive and jet data from ZEUS

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  1. Low-x Meeting Sinaia, 2005 An NLO QCD Analysisof inclusive and jet data from ZEUS Enrico Tassi (Calabria University and INFN) on behalf of ZEUS Based on: Zeus Coll., DESY-05-050 (accepted by EPJC)

  2. Proton PDFs and αs(MZ) A precise knowledge of the proton PDFs, is crucial for an understanding of the proton structure as well as necessary in any calculation of cross sections at hadron colliders (e.g. Standard Model tests and BSM searches @ LHC) QCD Factorisation: C. Glasman @DIS05 The strong coupling αsdetermines the strength of the interaction between coloured quanta and it is the fundamental parameter of QCD. Among the couplings of the Standard Model is, by far, the less well known…

  3. Motivation • Now, after the HERA I phase (1994-2000) of data-taking, the full set of e+p and • e-p inclusive Neutral Current (NC) and Charged Current (CC) cross sections are • available for QCD analysis: 96/97 e+p NC L=30 pb-1 2.7 < Q2 < 30000 GeV2 EPJ C21 443 (2001) “Nominal F2” 94-97 e+p CC L=33 pb-1 280.< Q2 < 17000 GeV2 EPJ C12 441 (2000) 98/99 e-p NC L=16 pb-1 200 < Q2 < 30000 GeV2 EPJ C28 175 (2003) 98/99 e-p CC L=16 pb-1 280 < Q2 < 17000 GeV2 PL B539 197 (2002) High-Q2/x data sets 99/00 e+p NC L=63 pb-1 200 < Q2 < 30000 GeV2 PR D70 052001 (2004) 99/00 e+p CC L=61 pb-1 280 < Q2 < 17000 GeV2 EPJ C32 16 (2003) • We decided to include in the analysis, in a rigorous way, our precise jet cross • sections (both in the DIS and γ-p regimes) • => with the full HERA I inclusive cross sections and the addition of the jet data we • can perform now a QCD analysis based on ZEUS data only • => improved determinations of the gluon density and αs • Lets look at the various observables included in the analysis and • discuss how they contribute to pdfs and αs….

  4. Inclusive NC/CC DIS Q2 = -(k-k’)2 xp x = momentum fraction of proton carried by quark (HERA: 10-6 ~ 1) Q2 = “resolving power” of probe Double differential cross sections:

  5. ZEUS F2 • Phase space: 2.7 < Q2 < 30 000 GeV2 6.3 10-5 < x < 0.65 • F2 dominates cross section • Precise determination of the low-x (x<10-2) sea(xS) and gluon (xg) densities • ZEUS high-x data still less precise than fixed target BCDMS: F2/F2~7% HERA: F2/F2~30%

  6. High-Q2 NC Reduced cross section: Z0 exchange gives a new “valence” structure function xF3 measurable from low- to high-x on a pure proton target,

  7. High-Q2 CC Reduced cross section: At LO: Used to constrain valence PDFs (instead of Fixed-target…) Still limited by statistics => need HERA II

  8. Inclusive jet production in NC DIS High-ET jet production in the Breit Frame: LO contributions: - boson-gluon fusion: - QCD-Compton: => Directly sensitive to αs and gluon/quark density in the proton

  9. Inclusive Jet Cross Sections in e+p NC DIS ZEUS coll., PL B547 164 (2002) • Phase space: • Q2 > 125 GeV2 • EBT,jet > 8 GeV and -2 < ηBjet < 1.8 • Jets identified with the kT cluster • algorithm in the Breit frame • Small Experimental uncertainties: • → jet energy scale (~1% for ET,jet>10 GeV) • => ± 5% on the cross sections • Small theoretical uncertainties: • - higher order terms ± 5% • - Hadronic corrections • (Chad <10 % and ΔChad ~ 1%)

  10. Dijet photoproduction Measure dijet production in γp collisions via ep scattering for Q2~0 At LO two processes contribute => Direct sensitivity to αs and gluon PDFs Direct process Resolved process However pQCD cross sections: depends ,via the resolved process component, on the photon PDFs. In order to suppress this dependence the QCD analysis was restricted to the direct-process-enriched region, xobs > 0.75, where is the fraction of the photon’s momentum taking part in the hard process.

  11. Dijet γp cross sections for x γobs > 0.75 ZEUS Coll., EPJ C23 615 (2002) • Phase space: • ETjet1,(2), > 14 (11) GeV and • -1 < ηjet1,2< 2.4 and xγobs > 0.75 • and • Q2<1 GeV2 and 134 <Wγp2< 277 GeV2 • Jets identified with the kT cluster • algorithm in the Lab frame • Small Experimental uncertainties: • → jet energy scale (~1% for ET,jet>10 GeV) • => ± 5% on the cross sections • Small theoretical uncertainties: • - higher order terms ± 10% • - Hadronic corrections • (Chad <10 % and ΔChad ~ 2-3%)

  12. Global vs ZEUS DGLAP fit Where does the information come from in a global fit compared to a fit including only ZEUS data ? • ANALYSES FROM ZEUS ONLY … •  Systematics well understood •  measurements from our own experiment •  No complications from heavy target Fe or D corrections

  13. DGLAP Analysis…details • Parameterised the PDFs u valence (xuv), d valence (xdv), total sea (xS), gluon (xg) and x=x(d-u) at the starting scale Q02= 7 GeV2 according to: xf(x) = p1xp2(1-x)p3 (1+p4x) • Constraints on the parameters {pi}: -momentum and number sum rules => p1(g) , p1(dv) , p1(uv) -no sensitive to the low-x behaviour of xuv and xdv => p2(uv)=p2(dv) -no sensitive to the flavour structure of the light sea => fix xΔ (Gottfried sum rule,Drell Yan data) - suppression by factor 2 of the of the strange sea (CCFR NuTeV dimuon data) • Evolve PDFs with Q2 using NLO DGLAP equations (MS scheme – QCDNUM evolution program) • Inclusive cross sections obtained convoluting the PDFs with coefficient functions (in the Roberts-Thorne Variable Flavour Number scheme). => 11 free parameters (+ αs(MZ) when free, otherwise αs(MZ) =0.118)

  14. Inclusion of Jet cross sections Computation of NLO jet cross sections extremely CPU intensive (~ O(10) hours ) => original programs cannot be used directly in the fit… …so we used this programs to compute αs- and PDFs-independent LO/NLO weights, σ, obtained by integrating the partonic cross sections over the 3(2)-dimensional bins of the (ξ,μR,μF) space. The NLO cross sections were then obtained by folding these weights with the PDFs and αs according to ~ This procedure reproduces the “exact” NLO predictions to better than 0.5%

  15. Data sets • A total of 577 data points • Kinematic region: • 2.7 < Q2 < 30 000 GeV2 • 6.3 10-5 < x < 0.65 • W2 > 20 GeV2 • Full account of the correlated • systematic uncertainties • A good description of the • measured cross sections is • obtained with: ZEUS-JETS and ZEUS-JETS-αs fits

  16. QCD fit results: Inclusive NC Very good description of the low- medium-Q2 e+p NC reduced cross sections

  17. QCD fit results: high-Q2 NC and CC NC: CC: Good description - still limited by statistics => HERAII

  18. QCD fit results: Jet cross sections Inclusive jet cross sections in e+p NC DIS Dijet cross sections in γp

  19. Valence distributions • At high-x not as well constrained • as when including fixed-target data • -…but becoming competitive + • being free from heavy-target • corrections, isospin-symmetry • assumptions etc. • To further improve here we need • precision high-Q2 e±p CC/NC data • from HERA II

  20. Glue & sea • Both are well determined at low-x (x<10-2) • -the sea distribution rises at low-x for all Q2 • the gluon density becomes valence-like • at low Q2 • The gluon uncertainty has been reduced • by the use of the jet data…

  21. Jet data & gluons Comparing the gluon distribution obtained from fits with and without jet data: - no significant change of shape: no tension between incl. and jet data - jet cross sections help in constraining the gluon density in the region: 0.01 < x < 0.4 - Sizeable reduction of the gluon unc: e.g. from 17% to 10% at x=0.06 and Q2=7 GeV2 →similar reduction by a factor two in the mid-x region over the full Q2 region

  22. PDFs uncertainties Checks: • Value of Q02 varied in the range: • 4< Q02 < 10 GeV2 • Parameterisation of the valence PDFs: • (1+p4x) → (1+p4x+p5√x) • -Tighter cuts on the jet cross sections • ET,jet>10 GeV (DIS) , ET,jet>17 GeV(γ-p) • -Hadronisation corrections • Photon PDFs • In general effects smaller than the experimental uncertainties

  23. Photon PDFs Including the resolved component (excluded in the ZEUS-JETS fit) Sensitivity to different PDFs (direct-enriched component)

  24. Comparing PDFs Compatible with ZEUS-S/MRST2001/CTEQ6.1M …and H1

  25. Determination of αs(MZ) - Simultaneous determination of PDFs and αs (free parameter) from ZEUS data alone - Inclusion of jet data greatly improves the determination of αs => - Exctracted value: αs(MZ) = 0.1183 ± 0.0007(uncor.) ± 0.0027(corr.) ± 0.0008(model) The theoretical uncertainty due to terms beyond NLO is : Δαs(th)= ±0.0050 (limited by theory => NNLO) In agreement with the world average αs(MZ) = 0.1182 ± 0.0027 (Bethke,2004)

  26. PDFs with αs free The extracted αs(MZ) value is very close to the fixed value (0.180) used in the ZEUS-JET fit => there are no significant changes in the central values of the PDFs parameters Uncertainties: - The valence and sea unc. are unaffected - The gluon unc. increases somewhat in the low-Q2 region

  27. ZEUS/HERA αs determinations C. Glasman @ DIS05

  28. Summary • ZEUS Inclusive and jet data used in a new NLO QCD analysis to determine the proton PDFs and αs (MZ) • Jet cross sections included “rigorously” in the DGLAP analysis • The input of jet-production data constrains the gluon and allows • an accurate determination of αs (MZ): • New PDFs available via HEPDATA-Durham web site αs(MZ) = 0.1183 ± 0.0007(uncor.) ± 0.0027(corr.) ± 0.0008(model) …more to come

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