Phenomenology tools for the LHC

Transcription

1 Phenomenology tools for the LHC James Stirling Cambridge University introduction and overview HO corrections apologies for omitting many PDFs LHC benchmarks topics of interest! MC tools more speculative pqcd applications summary PASCOS 2011 Cambridge

2 1 introduction and overview 2

3 phenomenology at hadron colliders Our goal is to make accurate predictions for event rates (cross sections and distributions) event shapes (content and structure) QCD is at the heart of everything electroweak effects are generally under control In many cases, perturbative QCD can be used to achieve high precision But in other contexts our understanding of the non-perturbative QCD effects is still quite primitive, and we have to resort to models 3

4 SM W & Z cross sections: comparison with theory 4

5 SM Higgs: Tevatron exclusion limits? cross section theory uncertainty 5

6 phenomenology tools ^ σ parton distribution functions event simulation (parton showers + tuned UE) MCs, interfaced with LO or NLO hard scattering MEs jet algorithms perturbation theory: LO, NLO, NNLO, supplemented by resummed N n LL improvements, EW corrections, all underpinned by the QCD factorization theorem for hardscattering (shortdistance) inclusive processes

7 2 higher-order perturbative QCD corrections 7

8 what if there is a wide range of P s in the process, e.g. W + n jets? see later general structure of a QCD perturbation series choose a renormalisation scheme (e.g. MSbar) calculate cross section to some order (e.g. NLO) physical variable(s) process dependent coefficients depending on P renormalisation scale note dσ/dµ=0 to all orders, but in practice dσ (N+n) /dµ= O((N+n)α S N+n+1 ) as many orders as possible! can try to help convergence by using a physical scale choice, µ ~ P, e.g. µ = M Z or µ = E T jet

9 how precise? LO for generic PS Monte Carlos, treelevel MEs NLO for NLO-MCs and many partonlevel signal and background processes in principle, less sensitivity to unphysical renormalisation and factorisation scales, µ R and µ F parton merging to give structure in jets more types of incoming partons more reliable pdfs better description of final state kinematics NNLO for a limited number of precision observables (W, Z, DY, H, )? NLO NLO NNLO NLO NNLO + E/W corrections, resummed HO terms etc 9

10 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, ) 10

11 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 on V,H + 2 jets, VV,tt + 1 jet, VVV, tth, ttz, In contrast, for NNLO we still only have inclusive γ*,w,z,h with rapidity distributions and decays (although much progress on top, single jet, ) *relevant for LHC 11

12 progress in W + jets NLO calculations* LO NLO W development of unitarity techniques *very important backgrounds to SUSY, etc. after Daniel Maître 12

13 calculation time: one-loop pure gluon amplitudes Giele and Zanderighi, 2008 loop tree 13

14 K. Ellis Top at Tevatron K. Ellis Bottom at LHC reason: new processes open up at NLO! 14

15 However... in complicated processes like W + n jets, there are often many reasonable choices of scales: blended scales like H T can seamlessly take account of different kinematical configurations: Berger et al., arxiv:

16 the impact of NNLO: W,Z Anastasiou, Dixon, Melnikov, Petriello, 2004 only scale variation uncertainty shown central values calculated for a fixed set pdfs with a fixed value of α S (M Z2 )

17 the impact of NNLO: Higgs Harlander,Kilgore Anastasiou, Melnikov Ravindran, Smith, van Neerven the NNLO band is about ±10%, or ±15% if µ R and µ F varied independently 17

18 SM Higgs: Tevatron exclusion limits? cross section theory uncertainty 18

19 3 parton distribution functions 19

20 parton distribution functions introduced by Feynman (1969) in the parton model, to explain Bjorken scaling in deep inelastic scattering (DIS) data; interpretation as probability distributions according to the QCD factorisation theorem for inclusive hard scattering processes, universal distributions containing long-distance structure of hadrons; related to parton model distributions at leading order, but with logarithmic scaling violations (DGLAP) determined from global fits to DIS and other data key ingredients for Tevatron and LHC phenomenology

21 MSTW = Martin, S, Thorne, Watt 21

22 LHC most SM and new physics sample pdfs in a region of x where they are already well known current pdf uncertainties provide the benchmark for whether LHC can add new information SUSY, Higgs, W,Z, DGLAP evolution low-mass forward production (e.g. b quarks, Drell-Yan) might provide new information on small-x partons γ* proton x 1 P X x 2 P proton 22

23 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) order of perturbation theory parameterisation at Q 0 theoretical assumptions (if any) about: flavour symmetries other x 0,1 behaviour definition of pdf uncertainties HERA-DIS FT-DIS Drell-Yan Tevatron jets Tevatron W,Z 23

24 recent global or quasi-global pdf fits pdfs authors arxiv ABKM CTEQ GJR HERAPDF MSTW NNPDF S. Alekhin, J. Blümlein, S. Klein, S. Moch, and others H.-L. Lai, M. Guzzi, J. Huston, Z. Li, P. Nadolsky, J. Pumplin, C.-P. Yuan, and others M. Glück, P. Jimenez-Delgado, E. Reya, and others H1 and ZEUS collaborations A.D. Martin, W.J. Stirling, R.S. Thorne, G. Watt R. Ball, L. Del Debbio, S. Forte, A. Guffanti, J. Latorre, J. Rojo, M. Ubiali, and others , , , , , , , , , , , , , , , , , , , , , , , , 24

25 MSTW08 CT10 NNPDF2.1 HERAPDF 1.0/1.5 ABKM09 HERA DIS GJR08 F-T DIS F-T DY TEV W,Z TEV jets GM-VFNS NNLO Note: each set comes with its own unique α S (M Z2 ) value (and uncertainty), correlated with the pdfs CT10, HERAPDF, NNPDF2.1 use recent combined HERA data NNPDF2.5(NNLO), CTEQ(NNLO) soon 25

26 4 LHC benchmark cross sections the following luminosity and cross section plots are from Graeme Watt: these and many more available at projects.hepforge.org/mstwpdf/pdf4lhc 26

27 parton luminosity functions a quick and easy way to assess the mass, collider energy and pdf dependence of production cross sections s a b X i.e. all the mass and energy dependence is contained in the X-independent parton luminosity function in [ ] useful combinations are and also useful for assessing the uncertainty on cross sections due to uncertainties in the pdfs (see later) 27

28 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 * 28

29 more restricted parametrisation Tevatron jet data not in fit 29

30 we note convergence of pdfs! plots from Graeme Watt although still some differences with other sets 30

31 plots from Graeme Watt 31

32 W lν rapidity asymmetry W l ± LHC 7 TeV MSTW2008 NLO A +- (y) very sensitive to pdfs complex interplay of u V, d V, Sea, V ± A decay 7 TeV data! W asymmetry lepton asymmetry, variable p Tlep (min) y lep or y W 32

33 33

34 SM Higgs and top cross sections differences from both pdfs AND α S! G. Watt,

35 5 MC Tools* *For a recent review, see Peter Richardson, Challenging the Standard Model IoP Half-Day meeting, indico.cern.ch/conferencedisplay.py?ovw=true&confid=

36 Monte Carlo Event Generators programs that simulates particle physics events with the same probability as they occur in nature widely used for signal and background estimates examples are PYTHIA, HERWIG, SHERPA,... the simulation comprises different phases: a) start by simulating a hard scattering process (LO, NLO) b) this is followed by the simulation of (soft and collinear) QCD radiation using a parton shower algorithm c) non-perturbative models are then used to simulate the hadronization of the quarks and gluons into the observed hadrons and the underlying event a) and b) well grounded theoretically, c) requires a model to be tuned to data: parameters relating to the final-state parton shower and hadronization are tuned to LEP data parameters relating to initial-state parton showers and multiple partonparton interactions are tuned to data (e.g. UA5, Tevatron) extrapolation to LHC?! 36

37 a Monte Carlo event Modelling of the soft underlying event Multiple perturbative scattering. Hard Perturbative scattering: Usually calculated at leading order in QCD, electroweak theory or some BSM model. Perturbative Decays calculated Initial and in Final QCD, State EW or parton showers resum the Finally the unstable hadrons are Non-perturbative some large BSM QCD modelling theory. logs. decayed. of the hadronization process. from Peter Richardson (HERWIG)

38 PYTHIA tunes to ATLAS 7 TeV MinBias data PYTHIA AMBT1 and HERWIG+JIMMY AUET1 tunes from ATLAS give a good description of ATLAS soft QCD physics without severely compromising Tevatron agreement. A. Buckley for ATLAS, Knoxville, November

39 interfacing N n LO and parton showers + Benefits of both: N n LO correct overall rate, hard scattering kinematics, reduced scale dep. PS complete event picture, correct treatment of collinear logs to all orders MC@NLO (Frixione, Webber, et al ): large range of processes available, integrated with Herwig FORTRAN and Herwig++ programs POWHEG (Nason): fewer processes, either standalone (Alioli, Nason, Oleari, Re) or integrated with Herwig++ (Hamilton, Richardson, Tully) or SHERPA (Hoeche, Krauss, Schonherr, Siegert)

40 (hadron collider) processes in from the 4.0 manual

41 HW++ vs. POWHEG vs. vs. MCFM Herwig Release Note (S. Gieseke et al) arxiv: [hep-ph] Tevatron Z 0 Z 0 LHC W + W - POWHEG vs. ATLAS jet data S. Alioli et al, arxiv: [hep-ph] 41

42 Jet algorithms {p h i } {j k } {partons} Snowmass accord (1990) a) simple to implement in experimental analyses as well as theory calculations b) defined at any order in pqcd and yields finite results for rates at any order c) yields a cross-section relatively insensitive to hadronisation two main types CONES: latest implementation SISCONE (Salam, Soyez, 2007) SUCCESSIVE RECOMBINATION: Jade... k T... anti-k T (Cacciari, Salam, Soyez 2008) anti-k T : hard stuff clusters with nearest neighbour, privilege collinear divergence over soft divergence; gives cone-like jets without using cones! Gavin Salam, Towards Jetography (2009) 42

43 Finally, a straightforward, robust, widely-accepted algorithm for jet studies at LHC that satisfies Snowmass accord...

44 6 finally, there are interesting processes where our theoretical understanding is much less developed... 44

45 central exclusive production µ µ b µ b µ compare p + p H + X the rate (σ parton, pdfs, α S) with the kinematic distribtns. (dσ/dydp T ) the environment (jets, underlying event, backgrounds, ) p + p p + H + p a real challenge for theory (pqcd + npqcd) and experiment (tagging forward protons, triggering, ) 45

46 central exclusive p + p p X p production theory 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 M X is 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) gap survival X the possibility of additional soft rescatterings filling the rapidity gaps is encoded in eikonal and enhanced survival factors

47 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 J Z P = 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, γγ, χ c, χ b, example: CDF(arXiv: ): X Durham/St Petersburg / Cambridge (Khoze, Martin, Ryskin, S, Harland-Lang,...) Manchester (Cox, Forshaw, Monk, Pilkington, Coughlin,...) KHRYSTHAL (Khoze, Ryskin, S, Harland-Lang, arxiv: ): Helsinki (Orava,...) Saclay (Royon,...) Cracow (Szczurek,...)

48 single and double hard parton scattering e.g. X,Y = jj,bb,w,z,j/ψ,.. folklore X,Y distinct: m=2 X,Y same: m=1 studies of γ+3j production by CDF and D0 suggest σ eff 15 mb use shape variables as a discriminator for DPS however, simple factorisation hypothesis now being called into question much recent theoretical activity, see DPS + SPS SPS MPI@LHC 2010: 2nd International Workshop on Multiple Partonic Interactions at the LHC, Glasgow, November 2010, 48

49 summary relentless advance in improving phenomenology tools for precision hadron collider physics in recent years the NLO revolution (but still scale choice/variation issues), with NNLO the next frontier (but no +jet processes yet) PDFs: convergence among groups and first precision tests at LHC Monte Carlo: improved modelling, new tunes to LHC and increasing number of NLO processes included (e.g. 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 49

50 extra slides

51 reference QCD and Collider Physics RK Ellis, WJ Stirling, BR Webber Cambridge University Press (1996)

52 MSTW08, ABKM09 and GJR08: α S (M Z2 ) values and uncertainty determined by global fit NNLO value about lower than NLO value, e.g. for MSTW08 pdfs and α S (M Z2 ) CTEQ, NNPDF, HERAPDF choose standard values and uncertainties world average (PDG 2009) note that the pdfs and α S are correlated! e.g. gluon α S anticorrelation at small x and quark α S anticorrelation at large x 52

53 CEP h MSSM bb at LHC 3σ statistical significance contours, M h max scenario Heinemeyer, Khoze, Ryskin, S, Tasevsky, Weiglein: arxiv: Trest