1. St. Andrews, Scotland J. Huston QCD Backgrounds to New Physics I J. Huston …thanks to Weiming Yao, John Campbell, Bruce Knuteson Nigel Glover for transparencies

2. St. Andrews, Scotland J. Huston Home again Houston Renfrewshire A large village located 6 miles (9.6 km) north-west of Paisley, the name derives from 'Hew's town', a 13th century village around the castle of Hugo de Kilpeter. In 1781 the castle was partly demolished and a New Town of 35 houses was erected. The Crosslee Mill (1793, demolished 1986) was used for cotton spinning but in 1916-17 a new factory was constructed for 'spinning cordite fuses'. The Mercat Cross has a sundial from the early 18th century and a shaft which may be 14th century; Houston House (17th century) and Barochan House (17th century and 1896) are other notable buildings. don’t worry about the extra o

3. St. Andrews, Scotland J. Huston Run 2 Monte Carlo Workshop Transparencies, video links to individual talks and links to programs can all be found at http://www- theory.fnal.gov/runiimc/ http://www- theory.fnal.gov/runiimc/ I’ll be referring to some of these programs in the course of my talk

4. St. Andrews, Scotland J. Huston Les Houches Two workshops on “Physics at TeV Colliders” have been held so far, in 1999 and 2001 (May 21- June 1) Working groups on QCD/SM, Higgs, Beyond Standard Model See web page: http://wwwlapp.in2p3.fr/conferences/ LesHouches/Houches2001/ especially for links to writeups from 1999 QCD writeup (hep-ph/0005114) is an excellent pedagogical review for many of the issues discussed here and at this SS

5. St. Andrews, Scotland J. Huston Other useful references (for pdf uncertainties) LHC Guide to Parton Distributions and Cross Sections, J. Huston; http://www.pa.msu.edu/~huston/lhc/lhc_pdfnote.ps The QCD and Standard Model Working Group: Summary Report from Les Houches; hep-ph/0005114 Parton Distributions Working Group, Tevatron Run 2 Workshop; hep- ph/0006300 A QCD Analysis of HERA and fixed target structure function data, M. Botje; hep-ph/9912439 Global fit to the charged leptons DIS data…, S. Alekhin; hep-ph/0011002 Walter Giele’s presentation to the QCD group on Jan. 12; http://www- cdf.fnal.gov/internal/physics/qcd/qcd99_internal_meetings.html Uncertainties of Predictions from Parton Distributions I: the Lagrange Multiplier Method, D. Stump, J. Huston et al.;hep-ph/0101051 Uncertainties of Predictions from Parton Distributions II: the Hessian Method, J. Pumplin, J. Huston et al.; hep-ph/0101032

6. St. Andrews, Scotland J. Huston Other references… …to the ability to calculate QCD backgrounds…to the need to do so

7. St. Andrews, Scotland J. Huston One person was more offended by the book than Carlo Rubbia “Stirling was a dapper Englishman, in his mid-thirties…” p. 228

8. St. Andrews, Scotland J. Huston Monojets in UA1 UA1 monojets (1983-1984)  Possible signature of new physics (SUSY, etc)  A number of backgrounds were identified, but each was noted as being too small to account for the observed signal  pp->Z + jets |_  pp->W + jets |_  |_ hadrons +  pp->W + jets |_ l +  pp->W + jets |_  |_ l + jet …but the sum was not  “The sum of many small things is a big thing.” G. Altarelli Can calculate from first principles or calibrate to observed cross sections for Z->e + e - and W->e Ellis, Kleiss, Stirling PL 167B, 1986.

9. St. Andrews, Scotland J. Huston Signatures of New Physics W’s, jets,  ’s, b quarks, E T …pretty much the same as signatures for SM physics How do we find new physics? By showing that it’s not old physics.  can be modifications to the rate of production  …or modification to the kinematics, e.g.angular distributions Crucial to understand the QCD dynamics of both backgrounds to any new physics and to the new physics itself Some backgrounds can be measured in situ  …but may still want to predict in advance, e.g. QCD backgrounds to H->  For some backgrounds, need to rely on theoretical calculations, e.g. ttbb backgrounds to ttH

10. St. Andrews, Scotland J. Huston Theoretical Predictions for New (Old) Physics There are a variety of programs available for comparison of data to theory and/or predictions.  Tree level  Leading log Monte Carlo  N n LO  Resummed Important to know strengths/weaknesses of each. In general, agree quite well…but before you appeal to new physics, check the ME. (for example using Comphep) Can have ME corrections to MC or MC corrections to ME. (in CDF->HERPRT) Perhaps biggest effort…include NLO ME corrections in Monte Carlo programs… correct normalizations. Correct shapes. N n LO needed for precision physics. Resummed description describes soft gluon effects (better than MC’s)…has correct normalization (but need HO to get it); resummed predictions include non-perturbative effects correctly…may have to be put in by hand in MC’s thresholdkTkT W,Z, Higgs dijet, direct  b space (ResBos) q t space Where possible, normalize to existing data. …in addition, worry about pdf, fragmentation uncertainties

11. St. Andrews, Scotland J. Huston W + Jet(s) at the Tevatron Good testing ground for parton showers, matrix elements, NLO Background for new physics  or old physics (top production) Reasonable agreement for the leading order comparisons using VECBOS (but large scale dependence) Good agreement with NLO (and smaller scale dependence) for W + >= 1 jet

12. St. Andrews, Scotland J. Huston W + jets For W + >=n jet production, typically use Herwig (Herprt) for additional gluon radiation and for hadronization Can also start off with n-1 jets and generate additional jets using Herwig

13. St. Andrews, Scotland J. Huston More Comparisons (VECBOS and HERWIG) Start with W + n jets from VECBOS Start with W + (n-1) jets from VECBOS

14. St. Andrews, Scotland J. Huston More Comparisons Start with W + n jets from VECBOS Start with W + (n-1) jets from VECBOS

15. St. Andrews, Scotland J. Huston When good Monte Carlos go bad Consider W + jet(s) sample Compare data (Run 0 CDF) to VECBOS+HERPRT (Herwig radiation+hadronization interface to VECBOS) normalized to W+X jets Starting with W + 1 jet rate in data, Herwig predicts 1 W+ >4 jet events; in data observe 10 …factor of ~2 every jet  very dependent on kinematic situation, though  jet E T cuts  center-of-mass energy  etc #events>1 jet >2 jets>3 jets>4 jet p T >10 GeV/c Data9202134210 VECBOS + HERPRT (Q= ) W + 1jet920178211 W + 2jet-----213436 W + 3jet----------42 10 VECBOS+HERPRT(Q=m W ) W + 1jet920176242 W + 2jet----213466 W + 3 jet---------427

16. St. Andrews, Scotland J. Huston Factors get worse at the LHC

17. St. Andrews, Scotland J. Huston Why? Some reasons given by the experts (Mangano, Yuan,Ilyin…)  Herwig (any Monte Carlo) only has collinear part of matrix element for gluon emission; underestimate for the wide angle emission that leads to widely separated jets  phase space: Herwig has ordering in virtualities for gluon emission while this is not present in exact matrix element calculations; more phase space for gluon emission in exact matrix element calculations  in case of exact matrix element, there are interferences among all of the different diagrams; these interferences become large when emissions take place at large angles (don’t know a priori whether interference is positive or negative)  unitarity of Herwig evolution: multijet events in Herwig will always be a fraction of the 2 jet rate, since multijet events all start from the 2-jet hard process  all “K-factors” from higher order are missed.

18. St. Andrews, Scotland J. Huston Tree Level Calculations Leading order matrix element calculations describe multi-body configurations better than parton showers Many programs exist for calculation of multi- body final states at tree-level  references at beginning of talk; see Run 2 MC workshop CompHep  includes SM Lagrangian and several other models, including MSSM  deals with matrix elements squared  calculates leading order 2->4-6 in the final state taking into account all of QCD and EW diagrams  color flow information; interface exits to Pythia  great user interface Grace  similar to CompHep Madgraph  SM + MSSM  deals with helicity amplitudes  “unlimited” external particles (12?)  color flow information  not much user interfacing yet Alpha + O’Mega  does not use Feynman diagrams  gg->10 g (5,348,843,500 diagrams)

19. St. Andrews, Scotland J. Huston An example using CompHep

20. St. Andrews, Scotland J. Huston Monte Carlo Interfaces To obtain full predictability for a theoretical calculation, would like to interface to a Monte Carlo program (Herwig, Pythia, Isajet)  parton showering (additional jets)  hadronization  detector simulation Some interfaces already exist  VECBOS->Herwig (HERPRT)  CompHep->Pythia A general interface accord was reached at the 2001 Les Houches workshop All of the matrix element programs mentioned will output 4-vector and color flow information in such a way as to be universally readable by all Monte Carlo programs CompHep, Grace, Madgraph, Alpha, etc, etc ->Herwig, Pythia, Isajet

21. St. Andrews, Scotland J. Huston Les Houches accord (draft)

22. St. Andrews, Scotland J. Huston Parton Showering Determination of the Higgs signal requires an understanding of the Higgs p T distribution at both LHC and Tevatron  for example, for gg->HX->  X, the shape of the signal p T distribution is harder than that of the  background; this can be used to advantage To reliably predict the Higgs p T distribution, especially for low to medium p T region, have to include effects of soft gluon radiation  can either use parton showering a la Herwig, Pythia, ISAJET or k T resummation a la ResBos  parton showering resums primarily the (universal) leading logs while an analytic k T resummation can resum all logs with Q 2 /p T 2 in their arguments; but expect predictions to be similar and Monte Carlos offer a more useful format Where possible it’s best to compare p T predictions to a similar data set to insure correctness of formalism; if data is not available, compare MC’s to a resummed calculation or at least to another Monte Carlo  all parton showers are not equal Note the large difference between PYTHIA versions 5.7 and 6.1. Which one is correct?

23. St. Andrews, Scotland J. Huston Change in PYTHIA Older version of PYTHIA has more events at moderate p T Two changes from 5.7 to 6.1  A cut has been placed on the combination of z and Q 2 values in a branching: u=Q 2 -s(1-z)<0 where s refers to the subsystem of hard scattering plus shower partons  corner of emissions that do not respect this requirement occurs when Q 2 value of space-like emitting parton is little changed and z value of branching is close to unity  necessary if matrix element corrections are to be made to process  net result is substantial reduction in amount of gluon radiation  In principle affects all processes; in practice only gg initial states  Parameter for minimum gluon energy emitted in space-like showers is modified by extra factor corresponding to 1/  factor for boost to hard subprocess frame  result is increase in gluon radiation The above are choices, not bugs; which version is more correct? ->Compare to ResBos S. Mrenna 80 GeV Higgs generated at the Tevatron with Pythia

24. St. Andrews, Scotland J. Huston Comparison of PYTHIA and ResBos for Higgs Production at LHC ResBos agrees much better with the more recent version of PYTHIA  Suppression of gluon radiation leading to a decrease in the average p T of the produced Higgs  Affects the ability of CMS to choose to the correct vertex to associate with the diphoton pair Note that PYTHIA does not describe the high p T end well unless Q max 2 is set to s (14 TeV)  Again, ResBos has the correct matrix element matching at high p T ; setting Q max 2 =s allows enough additional gluon radiation to mimic the matrix element

25. St. Andrews, Scotland J. Huston Comparisons with Herwig at the LHC HERWIG (v5.6) similar in shape in PYTHIA 6.1 (and perhaps even more similar in shape to ResBos) Is there something similar to the u- hat cut that regulates the HERWIG behavior?  Herwig treatment of color coherence? (study in progress)

26. St. Andrews, Scotland J. Huston The need for higher order…

27. St. Andrews, Scotland J. Huston What would we like? Bruce Knuteson’s wishlist from the Run 2 Monte Carlo workshop …all at NLO

28. St. Andrews, Scotland J. Huston What are we likely to get?

29. St. Andrews, Scotland J. Huston MCFM (Monte Carlo for Femtobarn Processes) J. Campbell and K.Ellis Goal is to provide a unified description of processes involving heavy quarks, leptons and missing energy at NLO accuracy There have so far been three main applications of this Monte Carlo, each associated with a different paper.  Calculation of the Wbb background to a WH signal at the Tevatron. R.K.Ellis, Sinisa Veseli, Phys. Rev. D60:011501 (1999), hep-ph/9810489.  Vector boson pair production at the Tevatron, including all spin correlations of the boson decay products. J.M.Campbell, R.K.Ellis, Phys. Rev. D60:113006 (1999), hep-ph/9905386.  Calculation of the Zbb and other backgrounds to a ZH signal at the Tevatron. J.M.Campbell, R.K.Ellis, FERMILAB-PUB-00-145- T, June 2000, hep-ph/0006304. The last of these references contains the most details of our method.

30. St. Andrews, Scotland J. Huston Higgs backgrounds using MCFM

31. St. Andrews, Scotland J. Huston Wbbar and Zbbar

32. St. Andrews, Scotland J. Huston Recent example of data vs Monte Carlo There is a discovery potential at the Tevatron during Run 2 for a relatively light Higgs (especially if Higgs mass is 115 GeV) …but small signal to background ratio makes understanding of backgrounds very important CDF and ATLAS recently went through similar exercises regarding this background  CDF using Run 1 data  ATLAS using Monte Carlo predictions

33. St. Andrews, Scotland J. Huston Data vs Monte Carlo

34. St. Andrews, Scotland J. Huston Sleuth strategy Consider recent major discoveries in hep  W,Z bosonsCERN 1983  top quarkFermilab 1995  tau neutrino Fermilab 2000  Higgs Boson? CERN 2000 In all cases, predictions were definite, aside from mass Plethora of models that appear daily on hep-ph Is it possible to perform a “generic” search? Transparencies from Bruce Knuteson talk at Moriond 2001

35. St. Andrews, Scotland J. Huston W2j We consider exclusive final states We assume the existence of standard object definitions These define e, μ, , , j, b, E T, W, and Z fi All events that contain the same numbers of each of these objects belong to the same final state Step 1: Exclusive final statesSleuth Bruce Knuteson Steps: 1) 1) eμE T Z4j eE T jj eE T 3j W3j ee  e  ZZ W  μμjj eμE T j  μμμ eee

36. St. Andrews, Scotland J. Huston Step 2: VariablesSleuthBruce Knuteson 2) Define variables What is it we’re looking for? The physics responsible for EWSB What do we know about it? Its natural scale is a few hundred GeV What characteristics will such events have? Final state objects with large transverse momentum What variables do we want to look at? p T ’ s

37. St. Andrews, Scotland J. Huston If the final state containsThen consider the variable 1 or more lepton 1 or more  /W/Z 1 or more jet missing E T (adjust slightly for idiosyncrasies of each experiment) Sleuth Bruce KnutesonStep 2: Variables

38. St. Andrews, Scotland J. Huston Input: 1 data file, estimated backgrounds transform variables into the unit box define regions about sets of data points  Voronoi diagrams define the “interestingness” of an arbitrary region  the probability that the background within that region fluctuates up to or beyond the observed number of events search the data to find the most interesting region, R determine P, the fraction of hypothetical similar experiments (hse’s) in which you would see something more interesting than R  Take account of the fact that we have looked in many different places For each final state... Output : R, P 3) Search for regions of excess (more data events than expected from background) within that variable space Step 3: Search for regions of excessSleuthBruce Knuteson

39. St. Andrews, Scotland J. Huston If the data contain no new physics, Sleuth will find  to be random in (0,1) If we find  small, we have something interesting If the data contain new physics, Sleuth will hopefully find  to be small If we find  large, is there no new physics in our data? or have we just missed it? How sensitive is Sleuth to new physics? Impossible to answer, in general (Sensitive to what new physics?) But we can provide an answer for specific cases SensitivitySleuth Bruce Knuteson

40. St. Andrews, Scotland J. Huston tt provides a reasonable sensitivity check [cf. DØ PRL (1997, 125 pb -1 )] in eμE T 2j:find  > 2  in  25% of an ensemble of mock experiments [cf. dedicated search: 2.75  (3 events with 0.2 expected)] in W 4j: find  > 3  in  25% of an ensemble of mock experiments [cf. dedicated search: 2.6  (19 events with 8.7 expected) w/o b-tag] [cf. dedicated search: 3.6  (11 events with 2.5 expected) w/ b-tag] Would we have “discovered” top with Sleuth? No. But results are nonetheless encouraging. Lessons:b-tagging, combination of channels important for top other sensitivity checks (WW, leptoquarks) give similarly sensible results SensitivitySleuth Bruce Knuteson

41. St. Andrews, Scotland J. Huston Results Results agree well with expectation No evidence of new physics is observed DØ data

42. St. Andrews, Scotland J. Huston Sleuth is a quasi-model-independent search strategy for new high p T physics  Defines final states and variables  Systematically searches for and quantifies regions of excess Sleuth allows an a posteriori analysis of interesting events Sleuth appears sensitive to new physics Sleuth finds no evidence of new physics in DØ data Sleuth finds no evidence of new physics in DØ data Sleuth has the potential for being a very useful tool  Looking forward to Run II hep-ex/0006011PRD hep-ex/0011067PRD hep-ex/0011071PRLConclusions

43. St. Andrews, Scotland J. Huston Fragmentation: Higgs->  and Backgrounds One of the most useful search modes for the discovery of the Higgs in the 100-150 GeV mass range at the LHC is in the two photon mode Higgs->  has very large backgrounds from QCD sources  Diphoton production   o and  o  o production; jets fragmenting into very high z  o ’s With excellent diphoton mass resolution, can try to resolve Higgs “bump” Still important to understand level of background

44. St. Andrews, Scotland J. Huston Diphoton Backgrounds in ATLAS Again, for a H->  search at the LHC, face irreducible backgrounds from QCD  and reducible backgrounds from  o and  o  o  in range from 70 to 170 GeV jet-jet cross section is estimated to be a factor of 2E6 times the  cross section and  -jet a factor of 8E2 larger  Need rejection factors of 2E7 and 8E3 respectively  PYTHIA results seem to indicate that reducible backgrounds are comfortably less than reducible ones  …but how to normalize PYTHIA predictions for very high z fragmentation of jets; fragmentation not known well at high z and certainly not for gluon jets

45. St. Andrews, Scotland J. Huston Different models predict different high z fragmentation Backgrounds to  production in Higgs mass region arise from fragmentation of jets to high z  o ’s  Pythia and Herwig predict very different rates for high z  all fragmentation is not equal Example of a background that can be measured in situ, but nice to be able to predict the environment beforehand DIPHOX (see Run 2 MC workshop) program can calculate ,  o, and  o  o cross sections to NLO  comparisons underway to Tevatron data gg->  and qqbar->  at NNLO may be available soon B. Webber, hep-ph/9912399

46. St. Andrews, Scotland J. Huston PDF Uncertainties What’s unknown about PDF’s  the gluon distribution  strange and anti-strange quarks  details in the {u,d} quark sector; up/down differences and ratios  heavy quark distributions  of quark distributions (q + qbar) is well- determined over wide range of x and Q 2  Quark distributions primarily determined from DIS and DY data sets which have large statistics and systematic errors in few percent range (±3% for 10 -4 <x<0.75)  Individual quark flavors, though may have uncertainties larger than that on the sum; important, for example, for W asymmetry information on dbar and ubar comes at small x from HERA and at medium x from fixed target DY production on H 2 and D 2 targets  Note dbar≠ubar strange quark sea determined from dimuon production in DIS (CCFR) d/u at large x comes from FT DY production on H 2 and D 2 and lepton asymmetry in W production

47. St. Andrews, Scotland J. Huston Example:Jets at the Tevatron Both experiments compare to NLO QCD calculations  D0: JETRAD, modified Snowmass clustering(R sep =1.3,  F =  R =E Tmax /2  CDF: EKS, Snowmass clustering (R sep =1.3 (2.0 in some previous comparisons),  F =  R =E Tjet /2 In Run 1a, CDF observed an excess in the jet cross section at high E T, outside the range of the theoretical uncertainties shown

48. St. Andrews, Scotland J. Huston Similar excess observed in Run 1B

49. St. Andrews, Scotland J. Huston Exotic explanations

50. St. Andrews, Scotland J. Huston Non-exotic explanations Modify the gluon distribution at high x

51. St. Andrews, Scotland J. Huston Tevatron Jets and the high x gluon Best fit to CDF and D0 central jet cross sections provided by CTEQ5HJ pdf’s

52. St. Andrews, Scotland J. Huston D0 jet cross section as function of rapidity E T (GeV)  d 2  dE T d  (fb/GeV) l 0.0    0.5 H 0.5    1.0 s 1.0    1.5 n 1.5    2.0 t 2.0    3.0 DØ Preliminary Run 1B Nominal cross sections & statistical errors only JETRAD  =E T max /2 CTEQ4HJ provides best description of data How reliable is NLO theory in this region? K-factors?

53. St. Andrews, Scotland J. Huston Chisquares for recent pdf’s For 90 data points, are the chisquares for CTEQ4M and MRSTgU “good”? Compared to CTEQ4HJ?

54. St. Andrews, Scotland J. Huston A new tool PDF uncertainties are important both for precision measurements (W/Z cross sections) as well as for studies of potential new physics (a la jet cross sections at high E T ) Most Monte Carlo/matrix element programs have “central” pdf’s built in, or can easily interface to PDFLIB Determining the pdf uncertainty for a particular cross section/distribution might require the use of many pdf’s  CTEQ Hessian pdf errors require using 33 pdf’s  GKK on the order of 100 Too clumsy to attempt to includes grids for calculation of all of these pdf’s with the MC programs ->Les Houches accord #2  Each pdf can be specified by a few lines of information, if MC programs can perform the evolution  Fast evolution routine will be included in new releases to construct grids for each pdf

55. St. Andrews, Scotland J. Huston Conclusions Great opportunity at Run 2 at the Tevatron for discovery of new physics; even better opportunity when the LHC turns on In order to be believeable, we must understand the QCD backgrounds to any new physics  Don’t rely totally on Monte Carlos and certainly not on one Monte Carlo alone  In the words of Ronald Reagan, “Trust but Verify”, if possible, theoretical predictions/formalisms with data  existing Run 1 data/Run 2 data  background” data to be taken at the LHC If no data, then verify with more complete theoretical treatments Many new tools/links between old tools are now being developed to make this job easier for experimenters Hopefully, we’ll find many more of the type of the event on the right to try them out on