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Hadron Colliders: a theorist’s perspective

Hadron Colliders: a theorist’s perspective. James Stirling IPPP, University of Durham. particle physics. the key questions. What is the origin of mass? Is it the Higgs boson? What is the origin of the matter-antimatter asymmetry in the universe? What are the properties of neutrinos?

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Hadron Colliders: a theorist’s perspective

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  1. Hadron Colliders: a theorist’s perspective James Stirling IPPP, University of Durham 1

  2. particle physics the key questions • What is the origin of mass? Is it the Higgs boson? • What is the origin of the matter-antimatter asymmetry in the universe? • What are the properties of neutrinos? • Is there unification of particles and forces including gravity? • What is the dark matter? CETA Inaugural Symposium

  3. bottom up top down Standard Model 6 quarks* (u,d,s,c,b,t) 6 leptons (e,,,e,,) gauge bosons (,W,Z,g) Higgs boson ~10-18m D=4 + supersymmetry? particle  sparticle dark matter? bottom up + string, brane theory? M-Theory? *quarks and gluons confined in hadrons: baryons (p,n), mesons () ~10-35m D=11? “Theory of Everything”?

  4. often rarely never! proton quark or gluon xP P hadron colliders – exploring the high-energy frontier dN/dEpart  Epart = (x1x2)Ecol  Ecol CETA Inaugural Symposium Epart

  5. past, present and future… Tevatron (1987→) Fermilab proton-antiproton collisions S = 1.8, 1.96 TeV - SppS (1981 → 1990) CERN proton-antiproton collisions S = 540, 630 GeV LHC (2007 → ) CERN proton-proton and heavy ion collisions S = 14 TeV CETA Inaugural Symposium

  6. c b W,Z t H, SUSY? 1970 1980 1990 2000 2010 2020 discovery … Higgs discovery at LHC SUSY discovery at LHC CETA Inaugural Symposium

  7. “Our intelligence experts say that Higgs bosons definitely exist – it’s only a matter of time before we find them.” CETA Inaugural Symposium

  8. precision… MW = cosw MZ  [ 1 + α F(mt,MH,SUSY,..)+ …] t + H W b precision W and top quark mass measurements at hadron colliders

  9. jet antiproton proton jet • deviation at large transverse momentum could signal New Physics, e.g. • new heavy resonance • new (contact) interaction • quark substructure • … precision and discovery? example: jet production CETA Inaugural Symposium

  10. electron q p proton X In the beginning…deep inelastic scattering and the parton model • variables • Q2 = –q2 (resolution) • x = Q2 /2p·q(inelasticity) • structure functions • d/dxdQ2 α2 Q-4 F2(x,Q2) • (Bjorken) scaling • F2(x,Q2) → F2(x) (SLAC, ~1970) CETA Inaugural Symposium

  11. q parton distributions e p parton model (Feynman) SLAC Scaling  free pointlike behaviour of proton ‘parton’ constituents F2(x) = x ei2qi(x) = 4/9 x u(x) + 1/9 x d(x) + … HERA

  12. + quark * antiquark - the Drell-Yan process  = M2/s “The full range of processes of the type A + B→ +- + X with incident p,, K, etc affords the interesting possibility of comparing their parton and antiparton structures” (Drell and Yan, 1970) (nowadays) … and to study the scattering of quarks and gluons, and how such scattering creates new particles CETA Inaugural Symposium

  13. jets! (1981) CETA Inaugural Symposium

  14. Log singularities from soft and collinear gluon emissions + * - factorisation • the factorisation of ‘hard scattering’ cross sections into products of parton distributions was experimentally confirmed and theoretically plausible • however, it was not at all obvious in QCD (i.e. with quark–gluon interactions included) • in QCD, for any hard, inclusive process, the soft, nonperturbative structure of the proton can be factored out & confined to universalmeasurable “parton distribution functions” (pdfs) fa(x,mF2).Collins. Soper, Sterman (1982-5)andevolution offa(x,mF2)in factorisation scale calculable (DGLAP, 1972-7) CETA Inaugural Symposium

  15. ^  • where X=W, Z, H, high-ET jets, SUSY sparticles, black hole, …, and Q is the ‘hard scale’ (e.g. = MX), usuallyF = R = Q, and  is known … • to some fixed order in pQCD and EWpt, e.g. • or in some leading logarithm approximation • (LL, NLL, …) to all orders via resummation the QCD factorization theorem for hard-scattering (short-distance) inclusive processes ^ CETA Inaugural Symposium

  16. F2(x,Q2) = q eq2 x q(x,Q2)etc xdependence of fi(x,Q2) determined by ‘global fit’ (MRST, CTEQ, …) to deep inelastic scattering (H1, ZEUS, …) and hadron collider data CETA Inaugural Symposium

  17. can we calculate everything?! Scattering processes at high energy hadron colliders can be classified as either HARD or SOFT Quantum Chromodynamics (QCD) is the underlying theory for all such processes, but the approach (and the level of understanding) is very different for the two cases For HARD processes, e.g. W or high-ET jet production, the rates and event properties can be predicted with some precision using perturbation theory For SOFT processes, e.g. the total cross section or diffractive processes, the rates and properties are dominated by non-perturbative QCD effects, which are much less well understood CETA Inaugural Symposium

  18. theoretical advances (mid 1980s → ) • numerical (‘spinor’) techniques for parton scattering amplitudes allow any leading-order (tree-level) amplitude to be calculated • e.g. qq→W+4gfor W+4jet background to top production (MADGRAPH, GRACE, …) • analytic techniques for real + virtual emission corrections to calculate exact higher order corrections in pQCD • e.g. recursive relations and master integrals multi-loop diagrams 1980s, 1990s: the NLO pQCD era CETA Inaugural Symposium

  19. not all NLO corrections are known! t b t b the more external coloured particles, the more difficult the NLO pQCD calculation Example: pp →ttbb + X bkgd. to ttH Nikitenko, Binn 2003 CETA Inaugural Symposium

  20. John Campbell, Collider Physics Workshop, KITP, January 2004 CETA Inaugural Symposium

  21. Glover NNLO: the perturbative frontier • The NNLO coefficient C is not yet known, the curves show guesses C=0 (solid), C=±B2/A (dashed) → the scale dependence and hence  σthis significantly reduced • Other advantages of NNLO: • better matching of partons hadrons • reduced power corrections • better description of final state kinematics (e.g. transverse momentum) Tevatron jet inclusive cross section at ET = 100 GeV CETA Inaugural Symposium

  22. soft, collinear jets at NNLO • 2 loop, 2 parton final state • | 1 loop |2, 2 parton final state • 1 loop, 3 parton final states • or 2 +1 final state • tree, 4 parton final states • or 3 + 1 parton final states • or 2 + 2 parton final state  rapid progress in last two years [many authors] • many 2→2 scattering processes with up to one off-shell leg now calculated at two loops • … to be combined with the tree-level 2→4, the one-loop 2→3 and the self-interference of the one-loop 2→2 to yield physical NNLO cross sections • this is still some way away but lots of ideas so expect progress soon! CETA Inaugural Symposium

  23. summary of NNLO calculations • p + p → jet + X *; in progress, see previous • p + p → γ + X; in principle, subset of the jet calculation but issues regarding photon fragmentation, isolation etc • p + p → QQbar + X; requires extension of above to non-zero fermion masses • p + p → (γ*, W, Z) + X *; van Neerven et al, Harlander and Kilgore corrected (2002) • p + p → (γ*, W, Z) + X differential rapidity distribution *; Anastasiou, Dixon, Melnikov (2003) • p + p → H + X; Harlander and Kilgore, Anastasiou and Melnikov(2002-3) Note: attention now turning to Electroweak corrections to hadron collider cross sections, since α ~ αS2 CETA Inaugural Symposium

  24. 4% total error (MRST 2002) what limits the precision of the predictions? • the order of the perturbative expansion • the uncertainty in the input parton distribution functions • example: σ(Z) @ LHC σpdf  ±3%, σpt  ± 2% →σtheory  ± 4% whereas for gg→H : σpdf << σpt  ± 10% CETA Inaugural Symposium

  25. highest precision cross sections D0 Run 2 error ~10% CDF Run 2 error ~ 6% In both cases dominated by the machine luminosity error! Nev =   L But note… theory error ~ 3% (MRST 2003) … so why not determine the Luminosity from the theoretical cross section? L = Nev  th CETA Inaugural Symposium

  26. another frontier: forward physics • ‘classical’ forward physics – σtot ,σel ,σSD,σDD, etc– a challenge for non-perturbative QCD models. Vast amount of low-energy data (ISR, Tevatron, …) to test and refine such models • output → deeper understanding of QCD, precision luminosity measurement (from optical theorem L ~ Ntot2/Nel) • ‘new’ forward physics – a potentially important tool for precision QCD and New Physics Studies at Tevatron and LHC p + p → p  X  p orp + p → M  X  M where  = rapidity gap = hadron-free zone, and X = χc, H, tt, SUSY particles, etc etc advantages? good MX resolution from Mmiss (~ 1 GeV?) (CMS-TOTEM) disadvantages? low event rate – the price to pay for gaps to survive the ‘hostile QCD environment’ CETA Inaugural Symposium

  27. ‘rapidity gap’ collision events Typical event Hard single diffraction Hard double pomeron Hard color singlet CETA Inaugural Symposium

  28. couples to gluons new selection rules • For example: Higgs at LHC (Khoze, Martin, Ryskin hep-ph/0210094) • MH = 120 GeV, L = 30 fb-1 , Mmiss = 1 GeV • Nsig = 11, Nbkgd = 4  3σ effect ?! Note:calibration possible via X = quarkonia or large ET jet pair Observation of p + p → p + χ0c (→J/ γ) + p by CDF? QCD challenge: to refine and test such models & elevate to precision predictions! CETA Inaugural Symposium

  29. future hadron colliders: energy vs luminosity? parton-parton luminosity: so that with  = MX2/s for MX > O(1 TeV), energy  3 is better than luminosity  10 (everything else assumed equal!) CETA Inaugural Symposium

  30. CETA Inaugural Symposium

  31. what next? • If the LHC discovers a light Higgs and perhaps TeV scale SUSY… • we will want to measure all the Higgs properties • and completely determine the SUSY spectrum  a Linear Collider ! Higgs branching ratios Chargino pair production near threshold (Blair 1999) CETA Inaugural Symposium Battaglia & Desch 2001

  32. New Physics? • The Standard Model is almost certainly an effective theory • New Physics could be • supersymmetry • extra dimensions • compositeness • strong electroweak symmetry breaking • something new?! CETA Inaugural Symposium

  33. + now G - then for example… Large Extra Dimension models have new resonances which could contribute to Drell-Yan  need to understand the SM contribution to high precision CETA Inaugural Symposium

  34. summary • hadron colliders are both discovery and precision machines! • there is a solid calculational framework for hard scattering processes (including New Physics) and much continuing theoretical activity to improve the precision... • the ~10% precision barrier has been broken for many processes • hard diffractive processes offer new challenges, both experimentally and theoretically; could also be good place to look for New Physics CETA Inaugural Symposium

  35. do experiments calculate predictions of theory analyse data to learn about theory construct more fundamental theories and finally … How we make progress: Theorists and experimentalists at Karlsruhe have made major contributions to this effort, and thanks to CETA this is sure to continue in the future. GOOD LUCK! CETA Inaugural Symposium

  36. extra slides CETA Inaugural Symposium

  37. pdf uncertainties encoded in parton-parton luminosity functions: with  = M2/s, so that for ab→X solid = LHC dashed = Tevatron Alekhin 2002 CETA Inaugural Symposium

  38. longer Q2 extrapolation smaller x CETA Inaugural Symposium

  39. Higgs cross section: dependence on pdfs Djouadi & Ferrag, hep-ph/0310209 CETA Inaugural Symposium

  40. uncertainty in gluon distribution (CTEQ) thenfg→σgg→X etc. CETA Inaugural Symposium

  41. g H t g HO corrections to Higgs cross section • the HO pQCD corrections to (gg→H) are large (more diagrams, more colour) • can improve NNLO precision slightly by resumming additional soft/collinear higher-order logarithms • example: σ(MH=120 GeV) @ LHC σpdf  ±3%, σptNNL0  ± 10%, σptNNLL  ± 8%, →σtheory  ± 9% Catani et al, hep-ph/0306211 CETA Inaugural Symposium

  42. Tevatron NNLO(S+V) NLO LO Kidonakis and Vogt, hep-ph/0308222 top quark production awaits full NNLO pQCD calculation; NNLO & NnLL “soft+virtual” approximations exist (Cacciari et al, Kidonakis et al), probably OK for Tevatron at ~ 10% level (> σpdf ) … but such approximations work less well at LHC energies CETA Inaugural Symposium

  43. HEPCODE: a comprehensive list of publicly available cross-section codes for high-energy collider processes, with links to source or contact person • Different code types, e.g.: • tree-level generic (e.g. MADEVENT) • NLO in QCD for specific processes (e.g. MCFM) • fixed-order/PS hybrids (e.g. MC@NLO) • parton shower (e.g. HERWIG) www.ippp.dur.ac.uk/HEPCODE/ CETA Inaugural Symposium

  44. Who? Alekhin, CTEQ, MRST, GKK, Botje, H1, ZEUS, GRV, BFP, … http://durpdg.dur.ac.uk/hepdata/pdf.html pdfs from global fits Formalism NLO DGLAP MSbar factorisation Q02 functional form @ Q02 sea quark (a)symmetry etc. fi (x,Q2) fi (x,Q2) αS(MZ ) Data DIS (SLAC, BCDMS, NMC, E665, CCFR, H1, ZEUS, … ) Drell-Yan (E605, E772, E866, …) High ET jets (CDF, D0) W rapidity asymmetry (CDF) N dimuon (CCFR, NuTeV) etc. CETA Inaugural Symposium

  45. Djouadi & Ferrag, hep-ph/0310209 CETA Inaugural Symposium

  46. the differences between pdf sets needs to be better understood! Djouadi & Ferrag, hep-ph/0310209 CETA Inaugural Symposium

  47. why do ‘best fit’ pdfs and errors differ? • different data sets in fit • different subselection of data • different treatment of exp. sys. errors • different choice of • tolerance to define  fi(CTEQ: Δχ2=100, Alekhin: Δχ2=1) • factorisation/renormalisation scheme/scale • Q02 • parametric form Axa(1-x)b[..] etc • αS • treatment of heavy flavours • theoretical assumptions about x→0,1 behaviour • theoretical assumptions about sea flavour symmetry • evolution and cross section codes (removable differences!) → see ongoing HERA-LHC Workshop PDF Working Group CETA Inaugural Symposium

  48. Kulesza Sterman Vogelsang qT (GeV) Bozzi Catani de Florian Grazzini resummation Z Work continues to refine the predictions for ‘Sudakov’ processes, e.g. for the Higgs or Z transverse momentum distribution, where resummation of large logarithms of the form n,m αSn log(M2/qT2)m is necessary at small qT, to be matched with fixed-order QCD at large qT CETA Inaugural Symposium

  49. comparison of resummed / fixed-order calculations for Higgs (MH = 125 GeV) qT distribution at LHC • Balazs et al, hep-ph/0403052 • differences due mainly to different NnLO and NnLL contributions included • Tevatron d(Z)/dqTprovides good test of calculations CETA Inaugural Symposium

  50. + interfacing NnLO and parton showers Benefits of both: NnLOcorrect overall rate, hard scattering kinematics, reduced scale, dependence, … PScomplete event picture, correct treatment of collinear logarithms to all orders, … e.g. MC@NLO (Frixione et al) CETA Inaugural Symposium

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