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Tancredi.Carli@cern.ch

HERA as a QCD Precision Machine and the Implication for LHC. Tancredi.Carli@cern.ch. A personal collection: Recent results on PDF, the strong coupling and their uncertainties and their implications on LHC Note: HERA/LHC workshop with much more results !. p. ?. p.

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Tancredi.Carli@cern.ch

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  1. HERA as a QCD Precision Machine and the Implication for LHC Tancredi.Carli@cern.ch A personal collection: Recent results on PDF, the strong coupling and their uncertainties and their implications on LHC Note: HERA/LHC workshop with much more results !

  2. p ? p Proton gives access to highest energies: Mass reach up to~ 5 TeV ! LHC is a discovery machine, but S/B is small Production cross sectionand dynamics are largely controlled by QCD. Important discovery should stand of their own, Uncertainties should be determined from data, but need to develop tunable models and correct theories to do extrapolations LHC (Large Hadron Collider): •p-p collisions at √s = 14TeV What do we expect from a proton-proton collision at a centre-of-mass energy of 14 TeV? •bunch crossing every 25 ns low-luminosity: L ≈ 1033cm-2s-1 (L ≈ 10 fb-1/year) high-luminosity: L ≈ 1034cm-2s-1 (L ≈ 100 fb-1/year) -> the better our understanding of pp collisions, the better our sensitivity !

  3. Precision can reveal it ! …need excellent detector and theory ! …need to quantify the experimental and theoretical uncertainties ! …understanding of the proton structure and the dynamics of the constituents is important What if new physics is at the O(10 TeV) Scale ? Need to measure: Masses Couplings Cross-sections: Summer 2004 LHC ? ? Precision physics at hadron collider possible ? …learn form HERA !

  4. Jet physics: Single Inclusive Cross-section • - test of pQCD in an energy regime never probed! • - validate our understanding of pQCD at high momentum transfers Rather general String theory toy-model (hep-ph/0111298) Large momentum transfers and small-x ! from PDF from ren+fac scale reach in the first year At very small distances, particles disappear into curled extra-dimensions At the LHC the statistical uncertainties on the jet cross-section will be small. Main systematic errors ? Theory uncertainty ?

  5. LHC Parton Kinematics • LHC gives access: • to high momentum transfers • at relatively low-x • to high-x DIS: o) theoretically well defined o) experimentally clean HERA: o) measure strong coupling and parton densities o) verify/falsify DGLAP evolution o) develop techniques to constrain theory uncertainties from LHC data DGLAP evolution

  6. The Proton Structure Function Inclusive measurement: Scaling violations Gluon indirectly determined by scaling violations Fixed target: : 1-2% (low Q2, high-x) HERA: bulk: 2-3% -> 1% reachable with full HERA-I set 5% at high Q2 , large-x

  7. DIS: o) theoretically well defined o) experimentally clean contains spin dependence of eq scattering Gluon can be indirectly measured via inclusive DIS cross-section

  8. Factorisation scale NLO QCD Analysis Parton density Treatment of systematics and theoretical uncertainties subject to conventions

  9. Global NLO QCD Analysis Parton densities are from „global“ fits, i.e. from all available data: Recently (2002): PDF with uncertainties using phenomenological analysis (driven by HERA collabs) Quantifiable uncertainties on PDF and physical predictions Problem: complexity of global analysis as results from many experiments from variety of physical processes with diverse characteristics and errors often mutally not compatible and theoretical uncertainty can not be rigoursly quantified Tung (2004): „PDF-users must be well informed about nature of uncertainties !“

  10. Comparison MRST/CTEQ Example: Ratio MRST gluon is generally smaller quark smaller at low-x Largest difference at high-x where uncertainties are also largest

  11. Impact on Higgs Cross-section at LHC PDF/CTEQ Djouadi/Ferrag Ferrag/Djouadi 2004 Error bands have different size Uncertainty about 5%, Differences CTEQ/MRS similar to individual error bands….

  12. NLO QCD Analysis with HERA Data Only o) Clean way to treat experimental uncertainty and their correlations (one experiment !) o) No model uncertainties, e.g. on nuclear target mass corrections o) high-x valence quark information contrained by high Q2 neutral and charged current Fit is possible, but still large uncertainty at high-x ! ->include jet data !

  13. Strong Coupling from Jet Data Leading order: 5% jet cross section accuracy, based on 1% energy scale error Typical theory uncertainty: 0.05 (mainly from ren. scale dependence, Could be improved with NNLO) Can not compete with e.g. most recent LEP extraction (global e.w. fit) having uncertainty 0.03 but technique to extract strong coupling at hadron collider developed Note that LHC reaches scales up to 5 TeV ! …need to validate running of strong coupling over three orders of magnitude !

  14. NLO QCD Analysis of Jet Data H1 Collab., EPJ C19 (2001) 289 Problem: Solution: 1) Find observables where sensitivity to PDF cancels 2) Make a combined fit to PDF and strong coupling

  15. Strong Coupling from 3/2 jet ratio Data DESY 05-019 Idea: Uncertainty: PDF: 2% Had correction: 2% ren. Scale: 5% See also H1 PLB 515 (2001) 17

  16. Strong Coupling from HERA To be compared to an error of 0.03 from global e.w. fit to LEP data

  17. Large fraction of cross section is gluon induced • Jet cross section directly sensitive to strong coupling and gluon density Jet Cross Sections in DIS and Photoproduction Leading order: Quark-induced Gluon-induced Perturbatively calculable coefficientsFor incl. DIS: analytically knownFor jets: need computation via NLO MC program: - defined via jet algorithm - within detector acceptance calculation takes typically 1 day of CPU time -> needs special technique to be useful in fit (using look-up tables)

  18. NLO QCD Analysis with incl. DIS and Jet Data Jet give sensitivity to gluon at medium x~ 0.1 … and fix strong coupling in the fit ! Inclusive data give rather low strong coupling This technique can be extented to LHC ! constrain PDF uncertainties with LHC data In “commissioning” phase of data taking to prepare “discovery” phase Statistics limited, HERA-II improved sensitivity up to x ~0.3

  19. Impact on Inclusive Jet Cross-section at TeVatron no ‘tension’ of HERA jet data vs low x NC data (as in ‘conservative’ MRST analysis with Tevatron jets)

  20. Impact on W-Boson Cross-section at LHC Huge statistical samples & clean experimental channel. W and Z production ~105events containing W(pTW>400GeV) ~104events containingZ(pTZ >400 GeV) “Standard candles” at LHC: • Luminosity - detector calibration • - constrain quark and anti-quark densitiesin the proton. • Precision measurements MW etc. CTEQ 2005 To test the impact of low-x data: 20% intermediate cuts Strong cuts MRST state 20% change in W-cross section, if HERA low-x data are removed CTEQ does not see change in central value, but uncertainty increases !

  21. Impact on W-Boson Cross-section at LHC PDF uncertainties: 2 NNLO sets by MRST Mode=4 gives better description of Tevatron High Et jet data …after effort of 10 year a differential NNLO calculation is available ! NNLO NLO At NLO PDF uncertainties are absorbed In scale dependence At NNLO: PDF uncertainties are larger ! 1% difference visible/measurable at LHC ? Scale dependence at y=0: LO: 30% NLO: 6% NNLO: 0.6% No change in shape from NLO->NNLO

  22. W-mass Measurements Crucial test of SM window to new physics ? Very ambitious need to control electron energy for MZ -> MW to 0.02% Might be reduced and PDF dependence between Z and W correlated D0 D0 measured Insensitive to detector hadronic response Need correct model for PT,W Sensitive to detector hadronic response to multiple-interactions Insensitive to PT,W at Tevatron …but residual sensitivity at LHC -> need to understand non-perturbative region and soft gluon resummation -> missing higher orders of QCD corrections

  23. DIS Drell-Yan Impact of W-Boson Recoil Spectrum on W-mass For low PT,W: need to be resummed (Collin, Soper, Sterman formalism) Nadolsky et al., hep/ph/0410375 perturbative Non-perturbative kT of parton in proton resummation fixed order need to be matched can be estimated by looking at transverse energy flow in DIS data, but depends on x Non-perturbative input gives similar changes on electron pt spectrum than a shift of MW by 50 MeV Effect expected to be larger at LHC (low-x!)

  24. Recoil Spectrum on W-mass at LHC shift Broadening can affect at LHC: MW~100 MeV pT,el method ~ 10 MeV MT method Non-perturbative Rho(x) needed since DGLAP make a collinear approximation may be more appropriate framework is CCFM/KT factorisation using g(x,Q2, KT2) describe F2 abd had final state at HERA -> CASCADE, H. Jung -> LDC, L. Loennblad Need correct model to extrapolate Z->W !

  25. Recently new developments of MC generators: (>2002) 1) LO: automated tree level ME for up to 2-> 6 processes matched with PS (CKKW) SHERPA, MADEVENT, HERWIG++ 2) NLO: NLO-ME matched with PS (MC@NLO) SHERPA hep-ph/0409106 TU Dresden PS + intrinsic KT of 0.8 GeV

  26. Conclusions The proton gives us access to the highest possible energy, but is a rather complicated object HERA explores the proton structure and the interaction among its constituents from 1 fm to 1/1000 fm There are many interesting results on hard and soft interactions, perturbative and non-perturbative QCD This knowledge will be essential to improve the sensitivity of LHC to find new physics I have only presented a small collection of results … there is much more ! • In 2007 LHC will collide protons with an energy of 7 TeV ! • To explore full potential of new energy frontier need to prepare: • good detectors • good models and tools and theory calculations • program to calibrate detectors and to validate our understanding of SM processes

  27. The b-puzzle solved now ! ..after a lot of theoretical and experimental efforts, b-quark cross-section in pp collisions in agreement with theory, many reasons: most important was better understanding of b-quark fragmentation See M. Mangano, hep-ph/0411020 Total b-quark cross-section at Tevatron: x2 now Pre-HERA

  28. b-quark Production at HERA Historical Snapshoot (<2002) • Photoproduction (gP): Q2 < 1GeV2 • Hard scales: mb, pT Deep inelastic scattering (DIS): Q2 > 1GeV2 • Hard scales: mb, Q2, pT Now critical review of model assumptions

  29. b-quark Production in DIS DIS: DESY 05/004 Phase space region identified where NLO QCD describes the data ! Excess in forward region as in e.g. inclusive jet production p see also ZEUS, hep-ex/0405069

  30. LHC PYTHIA6.214 - tuned PHOJET1.12 Transverse < Nchg > x 3 x1.5 dNchg/dη at η=0 Pt (leading jet in GeV) LHC Tevatron (CDF data) √s (GeV) Predictions for LHC for Underlying Events Moraes, Buttar, Dawson (see also work of R. Field) After comprehensive study and tuning: Agreement with CDF data, but different predictions in region transverse to the leading jet ! MB can be easily measured at LHC UI more difficult Model tuning can, however, only be successful if model are more or less correct

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