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Parton densities for LHC

Parton densities for LHC. Introduction MRSW,CTEQ, HERA pdfs and data Some issues (uncertainties,u/d, s, HQ) Some LHC processes (Diffractive PDFs, LHCb not covered). Elisabetta Gallo (INFN Firenze) IFAE Bologna 28/3/2008. SM. 2XD. 4XD. 6XD. Cross sections at LHC.

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Parton densities for LHC

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  1. Parton densities for LHC • Introduction • MRSW,CTEQ, HERA pdfs and data • Some issues (uncertainties,u/d, s, HQ) • Some LHC processes • (Diffractive PDFs, LHCb not covered) Elisabetta Gallo (INFN Firenze) IFAE Bologna 28/3/2008

  2. SM 2XD 4XD 6XD Cross sections at LHC • Any search for new physics needs a precise understanding of the SM and therefore QCD • Parton densities, MCs, NLO-NNLO calculations, UE, are all important ingredients Example: search for Large Extra Dimensions in 2 jets. Main uncertainty is gluon density at high-x, of the same order as the difference between XDs IFAE 2008

  3. Cross sections at LHC Other processes, like Higgs or Z’ discovery potential not really affected by PDFs IFAE 2008

  4. A bit of history of PDFs Since then a lot of developments: First F2 from HERA 1993 • MRST/MSTW group • CTEQ group • HERA pdfs (H1 and ZEUS on own data) • unintegrated, small x • heavy quark effects • NNLO • hessian or offset method? IFAE 2008

  5. Factorization property in hard scattering i.e. at LHC Non-perturbative, parton densities, universal, from fit to exps. data Perturbative process, calculable in pQCD IFAE 2008

  6. How does a parton density look like? • Determine xu,xd,xS,xg from fits at a certain Q02 and then evolve in Q2with the DGLAP evolution equations • splitting functions calculated recently at NNLO IFAE 2008

  7. How does a parton density look like? • u,d-valence, dominate at high x • xS, sea, it is driven by the gluon, dominates at low x • gluon, steep rise at low x • evolution with Q2, example with a ZEUS QCD fit IFAE 2008

  8. Data for parton densities x1,2=(M/√s)exp(±yrap)~10-4-10-2 at LHC LHC Tevatron jets Fixed target HERA IFAE 2008

  9. Global fits • CTEQ and MRST groupsuse DIS+hard scattering data, H1 and ZEUS their data only • In principle 11 parton distributions (u,ubar,d,dbar,s,sbar,c,cbar,b,bbar,g) • Charm and beauty from BGF process (g→b bar, c cbar, with b=bbar, c=cbar) • s~sbar~0.2(ubar+dbar) at Q2=1 GeV2 • xf(x,Q20)=A(1-x)βxα(1+ε√x+γx) • 10-11 parameters to fit, momentum sum rule to fix normalizations. Recently up to 20 parameters in MSTW. IFAE 2008

  10. MRST/MSTW group (Martin, Stirling, Thorne et al., Durham UK) Roberts retired from project in 2006, G. Watt joined (MSTW). More recent ones MSTW2007, MSTW2008 • include NNLO. Uncertainties in NLO and NNLO are similar but SMALLER than individual uncertainties • Improved treatment of charm (General Mass Variable Number Scheme at NNLO) IFAE 2008

  11. MSTW2008 datasets IFAE 2008

  12. MSTW2008 Differences to MRST2001 sometimes larger than uncertainties IFAE 2008

  13. CTEQ (Wu-Ki Tung and collaborators in USA) • Global fit to HERA and colliders data • Latest is CTEQ6.6 (arXiv:0802.007) • Improves on CTEQ6.5 • Correlations for W,Z,ttbar production studied • s(x)≠sbar(x) • New charm scheme in CTEQ6.6/6.5 (GMVFNS) gives a different u,d at x~10-3 and Q~Mz W,Z correlated W,top anticorrelated IFAE 2008

  14. CTEQ6.6/5 vs CTEQ6.1 • Less c, more u,d →The different u,d densities cause a 8% difference in the W production x-section at LHC CTEQ6.6 CTEQ6.1 IFAE 2008

  15. H1, ZEUS • latest ZEUS-JETS, H1 PDF 2000 • Fit their own data (no nuclear effects, errors under control) • valence from NC/CC at high Q2 • gluon and sea from F2 data at low x • middle-x gluon from jets (latest ZEUS-JETS) IFAE 2008

  16. Comparisons IFAE 2008

  17. HERA F2 IFAE 2008

  18. Combined H1-ZEUS F2 Grows at low-x, precision ZEUS or H1 2-3%, <2% combined DGLAP works, can extract PDFs from fit Decrease at large x, 10% precision due to statistics Errors reduced in combination, each experiment ‘calibrates’ the other Fits on these combined data awaited IFAE 2008

  19. HERA charged current CC(e-p)~xuv CC(e+p)~(1-y)2dv Allows to separate the two flavors at high x and Q2 IFAE 2008

  20. HERA neutral current/jets gluon from F2 from F2+jets HERA I HERA II IFAE 2008

  21. Uncertainties • Hessian method (MRST,CTEQ,H1, allow data point to move according to correlated error, fit determines optimal setting of correlated errors) • Offset method (ZEUS, shift for each systematics and refit) • Experiments: distinguish between correlated and uncorrelated errors in tables of x-sections, • i.e. ZEUS, H1: - normalization, overall shift up-and-down - correlated, affect many points together, like the energy scale - uncorrelated, like efficiencies etc.. - statistical, only relevant at high x,Q2 IFAE 2008

  22. Uncertainties Experiments have learnt to provide correlated and uncorrelated systematics for every useful cross-section in order to make fits Ex. ZEUS 2002 gluon IFAE 2008

  23. Uncertainties • Note that band of uncertainties are smaller than differences of PDFs. • Blows up at high x, increases at low x CTEQ6.1M MRST04 IFAE 2008

  24. d/u ratio at large-x Ex. CTEQ6 fits • For x>0.4 NuTeV F2 data higher then CCFR, different magnetic field calibration and therefore muon momentum • tension in global fits between NuTeV (ν-Fe) data and other data, especially for d/u ratio IFAE 2008

  25. W-asymmetry at Tevatron Sensitive to the u,d, in practice measure the lepton asymmetry CDF,D0 II data have some influence especially on the d-valence density (compared to fixed-target) IFAE 2008

  26. Z production at Tevatron New precise data show some tension with MRSW and fits to W-asymmetry. Doing better at NNLO CTEQ NLO in good agreement Higher rapidities → smaller x, closer to LHC x-range IFAE 2008

  27. Tevatron jet data Constrain high-x gluon (Note MRSW also uses now HERA jet data to constrain middle-x gluon) CDFII prefers a smaller high-x gluon compared to Run I data in MRST IFAE 2008

  28. strange • Release assumption • Asymmetry not zero • Fit strange ‘directly’: in MSTW find reduced factor compared to usual r=½ of strange to u,d Determine strange from CCFR/NuTeV dimuon data IFAE 2008

  29. strange Asymmetry comes out compatible with zero. At Q2=10 GeV2 asymmetry is 0.0017 ± 0.0020 • Why bother? • Leaving strange free in these fits increases uncertainties in ubar, dbar from 1.5% to 2-2.5% • c+sbar→ H+ at LHC IFAE 2008

  30. Charm,beauty • For Q2~m2c the charm does not act as a parton, BGF process, massive scheme (FFNS) • For Q2>m2c the charm behaves has a massless parton (ZM-VFNS) • Variable number scheme in between (GM-VFNS) VFNS FFNS Different schemes, quite different predictions, data increasing precision IFAE 2008

  31. LHC “standard candles” W,Z Flavour decomposition in W,Z production and as determined in structure function data Note the high contribution bb→ Z IFAE 2008

  32. LHC “standard candles” W,Z • calculated at NNLO, accuracy at 1% • can be measured with high exp. accuracy • can be used as luminosity monitor, what about the pdf uncertainty? IFAE 2008

  33. LHC “standard candles” W ,Z Total cross sections known with an accuracy of <5% (driven by the sea-gluon) Note also small uncertainty of 2% claimed by MRST01, but difference to CTEQ6.1 (or CTEQ6.5) is bigger IFAE 2008

  34. LHC “standard candles” W,Z • correlations between W and Z cross-sections (not so at Tevatron). Ratio Z/W quite independent of PDFs, can be used as SM benchmark • CTEQ6.6 (GMVFN) a bit away from ZEUS-MSTW06 predictions • different HQ mass schemes in ZEUS make smaller difference, so CTEQ6.6 is away not only due to charm IFAE 2008

  35. Parton luminosities at LHC Uncertainties grow at high-x and rapidities IFAE 2008

  36. Extraction of PDFs from LHC • W asymmetry, Z rapidity → valence at smaller x~0.005 (gluon cancel in ratio in W-asymmetry) • Z+jets, γ+jet, prompt photons→ gluon • jet cross sections (jet energy scale has to be controlled) CMS IFAE 2008

  37. Conclusions • Precise prediction of SM essential for discovery of new signals • PDF uncertainties and effects to be considered; • - pdfs known only down to x~10-4 , where DGLAP works (no need for saturation or BFKL effects) • - uncertainties around 5% for W production • Heavy quark effects need to be watched out (CTEQ6.6 higher 8% W cross-section compared to CTEQ6.1) • LHAPDF accord, PDF4LHC workshop, DIS08 Workshop and HERA-LHC WS in May at CERN, fruitful discussion going on. IFAE 2008

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