The Standard Model in the Light of the LHC

1. introduction and overview • HO corrections • PDFs • some comparisons with data • more speculative pQCD applications • summary The Standard Model in the Light of the LHC apologies for omitting many topics of interest! James Stirling Cambridge University 24thRencontres de Blois, May 2012

2. gauge sector flavour sector EWSB sector  mass sector The Standard Model Lagrangian … and beyond?

3. gauge sector flavour sector EWSB sector  mass sector The Standard Model Lagrangian … and beyond?

4. Status of the SM • With the exception of the Higgs sector, the SM has been tested and verified to very high precision • The high-energy colliders LEP1&2 and the Tevatron have played significant parts in this • The LHC will continue this effort ... in parallel with searching for BSM physics LEP/TEV EW WG

5. LEP/TEV EW WG Plots for Winter 2012

6. ... and the QCD coupling S. Bethke March 2012 αs(MZ)= 0.1185 ± 0.0007 Note: difficult for hadron colliders to be competitive!

8. What can the LHC add? • High collision energy  larger ‘Standard Candle’ cross sections and larger phase space for multi-parton, multi-gauge-boson production  study processes inaccessible at lower energy colliders, e.g. W + n jets, 3V, 4V, ... production (V=W,Z,)  check gauge couplings, pQCD dynamics • High collision energy + modest final-state-system mass MX studies of QCD at small x ~ MX/ s • Both of these provide challenges to theorists to match the precision of the calculations with that of the measurements!

9. ... and everything seems to work!

10. testing the SM EW gauge boson couplings arXiv:1111.5570 W W W W Z W W etc. Also V=, quartic couplings, etc

11. ... and everything seems to work!

12. tools for precision phenomenology at the LHC • The key theoretical tool is the QCD factorisation theorem: • precision SM tests require detailed knowledge of • perturbative corrections to the hard scattering cross sections (both EW and QCD) • the parton structure of the proton, as encoded in the parton distribution functions (PDFs) • accurate modeling of the ‘underlying event’, e.g. parton showers + tuned UE MCs, interfaced with LO or NLO hard scattering MEs • the precision we can ultimately achieve is highly process dependent – it can vary from O(few %) (super-inclusive quantities like tot(Z)) to O(100%) (multiparton production processes known only at LO in pQCD)

13. how precise in practice? ? NLO NLO NNLO Why are higher-order corrections so important for precision predictions? NLO NNLO

14. physical variable(s) process dependent coefficients depending on P general structure of a QCD perturbation series • choose a renormalisation scheme (e.g. MS) • calculate cross section to some order (e.g. NLO) • noted/d=0“to all orders”, but in practice d(N+n)/d= O((N+n)SN+n+1)  as many orders as possible! • can try to help convergence by using a “physical scale choice”, ~ P, e.g. = MZor = ETjet • what if there is a wide range of P’s in the process, e.g. W + n jets? – see later renormalisation scale

15. Top at Tevatron Bottom at LHC K. Ellis K. Ellis reason: new processes open up at NLO!

16. recent developments at NLO • traditional methods based on Feynman diagrams, then reduction to known (scalar box, triangle, bubble and tadpole) integrals • … and new methods based on unitarity and on-shell recursion: assemble loop-diagrams from individual tree-level diagrams • basic idea: Bern, Dixon, Kosower 1993 • cuts with respect to on-shell complex loop momenta: Cachazo, Britto, Feng 2004 • tensor reduction scheme: Ossola, Pittau, Papadopoulos 2006 • integrating the OPP procedure with unitarity: Ellis, Giele, Kunszt 2008 • D-dimensional unitarity: Giele, Kunszt, Melnikov 2008 • … • … and the appearance of automated programmes for one-loop, multi-leg amplitudes, either based on • traditional or numerical Feynman approaches (Golem, …) • unitarity/recursion (BlackHat, CutTools, Rocket, …)

17. some recent NLO results…* • pp  W+3j [Rocket: Ellis, Melnikov & Zanderighi] [unitarity] • pp  W+3j [BlackHat: Berger et al] [unitarity] • pp  tt bb [Bredenstein et al] [traditional] • pp  tt bb [HELAC-NLO: Bevilacqua et al] [unitarity] • pp  qq 4b [Golem: Binoth et al] [traditional] • pp  tt+2j [HELAC-NLO: Bevilacqua et al] [unitarity] • pp  Z,*+3j [BlackHat: Berger et al] [unitarity] • pp  W+4j [BlackHat: Berger et al] [unitarity] • … • with earlier results onV,H + 2 jets, VV,tt + 1 jet, VVV, ttH, ttZ, … • In contrast, for NNLO we still only have inclusive *,W,Z,H, WH (but with rapidity distributions and decays, although there is much progress on top,single jet, …) – for a recent review see M. Grazzini, indico.cern.ch/conferenceDisplay.py?confId=172986 *relevant for LHC

18. However... in complicated processes like W + n jets, there are often many ‘reasonable’ choices of scales: ‘blended’ scales like HT can seamlessly take account of different kinematical configurations: Berger et al., arXiv:0907.1984

19. the impact of NNLO: (Z) Anastasiou, Dixon, Melnikov, Petriello, 2004 • only scale variation uncertainty shown • central values calculated for a fixed set PDFs with a fixed value of S(MZ2)

20. the impact of NNLO: (Higgs) Harlander,Kilgore Anastasiou, Melnikov Ravindran, Smith, van Neerven … • the NNLO band is about 10%, or 15% if R and F varied independently

21. X x1P x2P proton proton SUSY, Higgs, W,Z, … DGLAP evolution * PDFs @ LHC • most SM and new physics processes sample PDFs in a region of x where they are already reasonably well known • current PDF uncertainties provide the benchmark for whether LHC can add new information • low-mass forward production (e.g. Drell-Yan in LHCb) can provide new information on small-x partons

22. the PDF industry • many groups now extracting PDFs from ‘global’ data analyses (MSTW, CTEQ, NNPDF, HERAPDF, AKBM, GJR, …) • broad agreement, but differences due to • choice of data sets (including cuts and corrections) • treatment of data errors • treatment of heavy quarks (s,c,b) • parameterisation at Q0 • theoretical assumptions (if any) about: • flavour symmetries • x→0,1 behaviour • … • definition of PDF uncertainties  see talk by Alberto Guffanti ... all now with NLO and NNLO sets new

23. parton luminosity* comparisons positivity constraint on input gluon Run 1 vs. Run 2 Tevatron jet data No Tevatron jet data or FT-DIS data in fit ZM-VFNS momentum sum rule *

24. convergence of pdfs! plots from Graeme Watt 2010 2011 ... although still some differences with ABKM, GJR, HERAPDF

25. Wl rapidity asymmetry • very sensitive to pdfs • complex interplay of uV, dV, Sea, V ± A decay • lots of 7 TeV data now! l± W

26. the MSTW08 valence quarks need to be retuned LHCb extends the reach to high rapidity

27. the LHC high pT jet data are now beginning to constrain the PDFs ...

28. but we need to be careful – the electroweak corrections become important at high pT + Moretti, Ross, Huston,Campanelli, Terron, in preparation V=W,Z Why? Non-cancellation of Sudakov double logarithms of the form [log2(pT/MW)]n

29. the top cross section is another good PDF discriminator ...  

30. LHCb – getting to very low x → detect forward, low pTDY muonsfrom

31.  see talk by Ronan McNulty

32. 5 finally, there are interesting SM processes where our theoretical understanding is much less developed...

33. It’s the Standard Model, Jim, but not as we know it....

34. DPS + SPS SPS single and double hard parton scattering e.g. X,Y = jj,bb,W,Z,J/,.. • folklore • studies of +3j production by CDF and D0 suggest eff~15 mb • use shape variables as a discriminator for DPS • however, simple factorisation hypothesis • now known to be invalid •  much recent theoretical activity, see • “Multi-Parton Interactions at the LHC”, P. Bartalini et al., arXiv:1111.0469 X,Y distinct: m=2 X,Y same: m=1

35. experimental measurements of DPS

36.     central exclusive production compare … • p + p  H + X • the rate (parton,pdfs, αS) • the kinematic distribtns. (d/dydpT) • the environment (jets, underlying event, backgrounds, …) with … • p + p  p + H + p • a real challenge for theory (pQCD + npQCD) and experiment (tagging forward protons, triggering, …) b b

37. gap survival central exclusive production – theory p + p → p  X  p • colliding protons interact via a colour singlet exchange and remain intact: can be triggered by adding proton detectors far down the beam-pipe or by using large rapidity gaps • a system of mass MXis produced at the collision point, and only its decay products are present in the central detector region. • the generic process pp → p + X + p is modeled perturbatively by the exchange of two t-channel gluons(‘Durham Model’ – Khoze Martin Ryskin) • the possibility of additional soft rescatterings filling the rapidity gaps is encoded in ‘eikonal’ and ‘enhanced’ survival factors X

38. CEP at LHC? p + p → p  X  p • in the limit that the outgoing protons scatter at zero angle, the centrally produced state X must have JZP = 0+ quantum numbers→ spin-parity filter/analyser • in certain regions of MSSM parameter space, couplings of Higgs to bb is enhanced, and CEP could be the discovery channel • or any exotic 0++state, which couples strongly to glue, is a real possibility: radions, gluinoballs, … • in the meantime, many ‘standard candle’ processes at RHIC, Tevatron, LHC: X= jj, , J/, c, b, , … • example: Durham/St Petersburg /Cambridge (Khoze, Martin, Ryskin, S, Harland-Lang,....) Manchester (Cox, Forshaw, Monk, Pilkington, Coughlin, ...) Helsinki (Orava, ...) Saclay (Royon, ...) Cracow (Szczurek, ...) … CDF(arXiv:0902.1271): KRYSTHAL (Khoze, Ryskin, S, Harland-Lang, arXiv:1005.0695 ):

39. Higgs production via CEP L. Harland-Lang, KRYSTHAL Collaboration

40. summary • relentless advance in improving phenomenology tools for precision hadron collider physics in recent years allows the Standard Model to be tested in many ways: Standard Candles, multiparton amplitudes etc. • the NLO revolution (but still ‘scale choice/variation’ issues), with NNLO the next frontier (but no “+jet” processes yet) • PDFs: convergence among groups and now precision tests at LHC • Monte Carlo: improved modelling, new tunes to LHC and increasing number of NLO processes included (e.g. MC@NLO, POWHEG, ... ) • … and don’t forget other more novel applications of pQCD (hard diffraction, multiple parton interactions, etc.) where more theoretical work and experimental data are needed

41. extra slides

42. recent global or quasi-global PDF fits ... all now with NLO and NNLO sets

43. probing heavy quark pdfs at LHC? take advantage of (a) qg dominates W,Z + jet production, (b) heavy quark suppression becomes weaker at high Q2, small x, (c) ability to tag c,b jets CMS: “W production in association with c jets” (CMS-PAS-EWK-11-013) sbar / s sbar + s Z differences at level of exptl. systematic error! Also: Z + c as a measure of charm pdf? S, Vryonidou, arXiv:1203.6781

44. Rc Rc • differences between the three sets easily understood by comparing the corresponding s,dpdfs.

45. Jet algorithms {phi}  {jk}  {partons} • Snowmass accord (1990) • simple to implement in experimental analyses as well as theory calculations • defined at any order in pQCD and yields finite results for rates at any order • yields a cross-section relatively insensitive to hadronisation • two main types • CONES: latest implementation SISCONE (Salam, Soyez, 2007) • SUCCESSIVE RECOMBINATION: Jade ... kT ... anti-kT (Cacciari, Salam, Soyez 2008) • anti-kT : hard stuff clusters with nearest neighbour, privilege collinear divergence over soft divergence; gives cone-like jets without using cones! Gavin Salam, “Towards Jetography” (2009)