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Backgrounds to New Physics Signals

Backgrounds to New Physics Signals. Peter Ratoff Lancaster University. 2003 CTEQ Summer School, Saint Feliou de Guixols, Catalonia. An experimental physicist’s perspective. Signatures of new physics Importance of backgrounds - lessons from history

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Backgrounds to New Physics Signals

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  1. Backgrounds to New Physics Signals Peter Ratoff Lancaster University 2003 CTEQ Summer School, Saint Feliou de Guixols, Catalonia

  2. An experimental physicist’s perspective ... • Signatures of new physics • Importance of backgrounds - lessons from history • Some of the basic background processes • Theoretical review - ME/MC - W+jets comparison • The “Qaero” program • Tevatron Run II results: • SM backgrounds processes • Examples of new physics searches Or, why am I here ? ... illustrated with recent examples from Tevatron Run II …and thanks to Joey Huston for use of some of his CTEQ 2002 Summer School slides!

  3. Signatures of New Physics • Ws, jets, gs, b quarks, missing ET • … pretty much the same as signatures for SM physics • How do we find new physics? By showing that its 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 and normalization 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gg • For some backgrounds, need to rely on theoretical calculations, e.g. ttbb backgrounds to ttH … look at some examples J.Huston, CTEQSS’02

  4. Importance of Backgrounds? A few lessons from history … • UA1 Monojets • CDF Run I inclusive jet cross-section • SM Higgs potential at the Tevatron

  5. #1 : 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 |_ nn • pp->W + jets |_ t + n |_ hadrons + n • pp->W + jets |_ l + n • pp->W + jets |_ t + n |_ l + n • …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->en • Ellis, Kleiss, Stirling PL 167B, 1986. jet J.Huston CTEQSS02

  6. #2 : CDF Inclusive Jet Cross-section Vital to understand QCD in order to perform precision/search physics T. Shears IOP/Durham ‘03 Run 1 inclusive jet cross section Consistent over 7 orders of magnitude deviation at high Et BUT

  7. Exotic explanations • Composite quarks - Eichten, Lane and Peskin (1983) • contact term added to LO QCD Lagrangian  increased cross-section for high Et jets • s stops running: conspiracy between new SUSY particles, colour sextet • new particle: (leptophobic) Z'

  8. SM explanation Run 2 - more high Et jets: Test QCD at high Et Discriminate between new physics and gluon PDF New bins for Run 2 Important gluon-gluon and gluon-quark contributions at high Et Gluon PDF @ high x not well known. T.Shears IOP/Durham ‘03

  9. #3 : SM Higgs searches at the Tevatron but WH/ZH production more accessible ... Gluon fusion Associated production WH or ZH

  10. SM Higgs decay branching ratios • For MH 135 GeV • H0  bb dominates … • but rate falling rapidly • QCD background • precludes gg H  bb • For MH  135 GeV • Gauge boson decays • dominate ( H0  WW )

  11. Tevatron: low mass Higgs searches For MH 135 GeV: use the same basic strategy as LEP … … study associated production of ZH and WH To the standard leptonic HZ channels add W  l  with H bb ... N.b. the qqbb channel is very difficult as the QCD backgrounds are severe • Low mass Higgs sensitivity depends on • the integrated luminosity collected • b-quark jet tagging performance • mass resolution of reconstructed bb jets • a good understanding of all backgrounds

  12. SM Backgrounds to light Higgs production WH (MH < 135 GeV)  lbb W+jets Wg*  Wbb lbb, W Z/*  lbb Wg*  Wjj  ljj, W Z/*  ljj (fake b jets) tt pairs tt  (W)Wbb  (l)lbb single top W  tb  Wbb  lbb qg  q’tb  q’Wbb  q’lbb Low mass Higgs search at the Tevatron ZH (MH < 135 GeV) llbb/bb W/Z+jets Zg*  Zbb  llbb/bb, Z Z/*  llbb/ bb Zg*  Zjj  lljj/jj, Z Z/*  lljj/ jj (fake b jets) Wg*  Wbb  (l)bb, W Z/*  (l)bb Wg*  Wjj (l)jj, W Z/*  (l)jj (fake b jets) tt pairs tt  WWbb  l()l()bb, (l)(l)bb single top (bb only) W  tb  Wbb  (l)bb qg  q’tb  q’Wbb  q’(l)bb QCD jets (bb only) gg  bb, gg  jj (fake b jets) •  Need to understand: • W/Z+jets • top (tt pairs, single top) • QCD jets

  13. Tevatron: high mass Higgs searches H VV (V=W,Z) MH > 135 GeV Trilepton final states ~ low bgnds but small rate Golden Modes: like-sign, like-flavour leptons Like sign dileptons + jets ~ many SM bgnds (VVV, Vtt, VVjj, tt, Vjjj) Dileptons + ET ~ large SM bgnds (VV, tt, , tW) Assoc. prod gg fusion

  14. SM Backgrounds to heavy Higgs production W/Z+H (MH > 135 GeV)  W/Z VV  l± l±jj VVV e.g. W+W-W+  l+() jj l+() ttV e.g. ttZ  WWbbZ  l±() jj bb l(l)  l± l±jj X VVjj e.g. WZjj  l±() l(l) jj  l± l±jj X tt pairs tt  WWbb  l ±()jjb(b)  l±()jj l(q)(b)  l± l±jj X Vjjj + fake j e e.g. Wjjj  l±() jj j  l±() jj “e”  l± l±jj High mass Higgs search at the Tevatron gg  H (MH > 135 GeV)  ll VV WW  ll, Z Z/*  ll, W Z/*  l l(l) tt pairs tt  WWbb  ll(bb)  pairs Z/*    l()l() •  Need to understand: • VVV, VV • VV+2 jets, V+3 jets • top (tt pairs, Vtt) •  pairs

  15. Tevatron: low mass Higgs searches Run II Higgs/SUSY Working Group, October 2000 Simplified generic detector, “unsophisticated” analysis small S/B  essential to understand all backgrounds! • Generic QCD jets bgnd in bb • (gg bb) cannot be reliably simulated • Study assumed = 50% of total bgnd • from the other sources (CDF Run I) • Must be determined from real data! ZH bb + llbb Mangano, Nason, Ridolfi Peterson frag.

  16. The basic background processes • Need to understand:- • W/Z boson production (+jets) • VV • V + 2 jets • VVV • VV+2 jets, V+3 jets • top production • tt pairs • single top • Vtt • Drell-Yan pairs (qq *  ee, ,) • QCD jets • ... • Can investigate these • background processes: • theoretically • experimentally • both (ideally)

  17. Theoretical Review • Tree level calculations • Monte-Carlo interfaces • Les Houches Accords • W + jets • Parton showering • Resummation of Large Logs • Higher orders

  18. Theoretical Predictions for New (Old) Physics There are a variety of programs available for comparison of data to theory and/or predictions. • Tree level Les Houches accord • Leading log Monte Carlo MC@NLO • NnLO • 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. NnLO 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 b space (ResBos) threshold kT W,Z, Higgs dijet, direct g qt space Where possible, normalize to existing data. J.Huston, CTEQSS’02

  19. Good testing ground for parton showers, matrix elements, NLO • Background for new physics or old physics (e.g. top production) • Reasonable agreement for the leading order comparisons using • VECBOS (but large scale dependence) W + jets at the Tevatron • Good agreement with NLO (and smaller • scale dependence) for W +  1 jet J.Huston, CTEQSS’02

  20. 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 J.Huston, CTEQSS’02

  21. ) More Comparisons (VECBOS and HERWIG) • Start with W + (n-1) jets from VECBOS • Start with W + n jets from VECBOS J.Huston, CTEQSS’02

  22. More Comparisons • Start with W + (n-1) jets from VECBOS • Start with W + n jets from VECBOS J.Huston, CTEQSS’02

  23. 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 • 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 QCD and EW diagrams • color flow information; interface exists 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) J.Huston, CTEQSS’02

  24. 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 (“Physics at TeV Colliders”) • 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 J.Huston, CTEQSS’02

  25. The Les Houches Accords 2001 • The Les Houches accords will be implemented in all ME/MC programs that experimentalists and theorists use • They will make it easy to generate the multi-parton final states crucial to much of the Run 2/HERA/LHC physics program and to compare the results from different programs • experimentalists/theorists can all share common MC data sets • They will make it possible to generate the pdf uncertainties for any cross sections • Accord #1 (MEMC): • PYTHIA, CompHEP, Wbbgen, • Madgraph, Herwig, Grace, AcerMC • Accord #2 (PDFs in ME/MC): • Interface is as easy to use as PDFLIB • (and easier to update) • First version has CTEQ6M, CTEQ6L, • all CTEQ6 error PDFs and MRST2001 • PDFs • Available in MCFM • See pdf.fnal.gov J.Huston, CTEQSS’02

  26. Parton Showering Higgs Pt case study • Determination of the Higgs signal requires an understanding of the Higgs pT distribution at both LHC and Tevatron • for example, for gg->HX->ggX, the shape of the signal pT distribution is harder than that of the gg background; this can be used to advantage • To reliably predict the Higgs pT distribution, especially for low to medium pT region, have to include effects of soft gluon radiation • can either use parton showering a la Herwig, Pythia, ISAJET or kT resummation a la ResBos • parton showering resums primarily the (universal) leading logs while an analytic kT resummation can resum all logs with Q2/pT2 in their arguments; but expect predictions to be similar and Monte Carlos offer a more useful format • Where possible it’s best to compare pT predictions to a similar data set to ensure 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? J.Huston, CTEQSS’02

  27. Changes in PYTHIA Higgs Pt case study • Older version of PYTHIA has more events at moderate pT • Two changes from 5.7 to 6.1 • A cut has been placed on the combination of z and Q2 values in a branching: u=Q2-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 Q2 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/g 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 J.Huston, CTEQSS’02

  28. Comparison of PYTHIA and ResBos for Higgs Production at LHC Higgs Pt case study • ResBos agrees much better with the more recent version of PYTHIA • Suppression of gluon radiation leading to a decrease in the average pT 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 pT end well unless Qmax2 is set to s (14 TeV) • Again, ResBos has the correct matrix element matching at high pT; setting Qmax2=s allows enough additional gluon radiation to mimic the matrix element J.Huston, CTEQSS’02

  29. Comparisons with Herwig at the LHC Higgs Pt case study • 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? J.Huston, CTEQSS’02

  30. Resummation of Large Logs • A1, B1 and (a bit of) A2 are effectively in Monte Carlos (especially Herwig) • A1,A2 and B1 for Higgs production are in current off-the-shelf version of ResBos • …as are C0 and C1 which control the NLO normalization • The B2 term has recently been calculated for ggH J.Huston, CTEQSS’02

  31. The need for higher order… John Campbell, FNAL

  32. What would we like? Bruce Knuteson’s wish-list from the Run 2 Monte Carlo workshop …all at NLO

  33. What are we likely to get ? NLO QCD Simulations • Single top production • Harris et al -fully differential final states • Harris, Laene, Phaf, Sullivan and Weinzierl (2000) • Diboson prod’n e.g. pp WW leptons • Baur et al - lepton correlation only partially included • Baur, Han and Ohnemus (1995, 1996) • Dixon et al - full correlations, anomalous couplings • Dixon, Kunszt and Signer (1999) • MCFM - full correlations, singly resonant contributions • Campbell and Ellis (1999) • Inclusive jets • JETRAD - 1 and 2 jets only • Giele, Glover and Kosower (1993) • Giele, Kilgore - 3 jet production • Giele and Kilgore (2000) • Drell-Yan + heavy flavours • MCFM - Wg* bb • Ellis and Veseli (1998) • MCFM - Zg* bb • Drell-Yan + jets • DYRAD - vector boson + 0 or 1 jets • Giele, Glover and Kosower (1993) • VECBOS - vector boson +  3 Z jets or 4 W jets • Berends, Kuijf, Tausk and Giele (1991) John Campbell, FNAL

  34. 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. MCFM Process List (included at NLO) ppbar  W/Z W+W W+ Z Z + Z W/Z + H W/Z + 1 jet W/Z + g*  bb Various leptonic and/or hadronic decays of the bosons are included as further sub-processes n.b. No NLO prediction for W/Z + 2 jets is available, but this is under construction in MCFM John Campbell, FNAL

  35. MCFM study for Tevatron Higgs sensitivity John Campbell, FNAL

  36. “Qaero (Sleuth)” Strategy • Consider recent major discoveries in hep • W,Z bosons CERN 1983 • top quark Fermilab 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

  37. Sleuth Bruce Knuteson Step 1: Exclusive final states We consider exclusive final states We assume the existence of standard object definitions These define e, μ, , , j, b, ET, W, and Z fi All events that contain the same numbers of each of these objects belong to the same final state W2j eETjj W3j eET3j ee eμET Z e W  μμμ μμjj eμETj Z4j eee

  38. probability to be SM Results DØ data Search for regions of excess (more data events than expected from background) within that variable space Results agree well with expectation No evidence of new physics is observed

  39. Tevatron Run IIW/Z, top, searches _ p-p collisions at s = 1.96 TeV

  40. CDF & D0 New tracking SVT displaced track trigger Particle ID (TOF) Muon, Calor coverage extended New tracking (+solenoid) SST displaced track trigger Preshower detectors Improved shielding + muon triggering

  41. Run 1 Run 2 Now Date 1992 – 1996 2001 - 2009 2003 Integrated Luminosity 110 pb-1 6.5 – 11 fb-1 ~190 pb-1 c.m. energy 1.8 TeV 1.96 TeV 1.96 TeV Luminosity 2 x 1031 cm-2 s-1 2 x 1032 cm-1 s-1 4.5 x 1031 cm-2 s-1 Bunch spacing 3.5 ms 396 – 132 ns 396 ns Tevatron operating parameters

  42. QCD Jet cross-section, shapes, multijet events Heavy flavour Lifetimes, cross-section, Bc, LB, Bs studies, CP violation, xs Electroweak W: mass, width, gauge couplings Top: mass, cross-section, branching ratios Searches Higgs, SUSY, extra dimensions, leptoquarks compositeness, etc. Physics in Run 2 # events in 1 fb-1 1014 1011 107 104

  43. Electroweak:W, Z Measurements in 2fb-1: m(W) measured to 40 MeV (sys. dominated - theory) G(W) measured to 30 MeV couplings measured to ~0.3 Run 2 benefits: s(W), s(Z)  12 % s(WW), s(ZZ)  13 - 22% W, Z essential calibration signals for high Et physics

  44. Selecting W’s & Z’s • Z Event selection • Two isolated high pT e’s • W Event selection • One isolated high pT central e, m or t • Large ET OR • One isolated high pT central m • A second isolated high pTtrack (minimum ionizing) We Z0m+m-

  45. W, Z results Wen Z mm Data/theory agree so far ..

  46. Z0teth • We have a clear Z0teth signal. • Further study of backgrounds is underway. • Our goal is to have a preliminary cross section measurement by summer. Not only interesting as an EWK measurement, it is important for Higgs and SUSY searches.

  47. W+jets production (1) • Selection • W( en) • Isolated e : pT > 20 GeV • |h| < 0.8 • Missing ET > 25 GeV • W(mn) • Isolated m : pT > 25 GeV • |h| < 1.5 • missing ET> 20 GeV • Jets • pT > 20 GeV • |h| < 2.5 • Compare PYTHIA MC with DATA • Normalized by area • Error includes stat. error and dominant syst. error from JES 1st leading jets W(en)+jets : Data : MC QCD BKG GeV 2nd leading jets : Data : MC QCD BKG GeV

  48. W+jets production (2) • Reconstructed di-jet mass and DR(= Df2 + Dh2 ) between jets • MC reproduces jet distributions well • First step towards study of W(leptons)H( bb) decay process W(en)+jets Di-jet Mass W(en)+jets DR between di-jets : Data : MC : Data : MC QCD BKG QCD BKG DRjj Mjj (GeV)

  49. W+jets production (3) • Di-jet mass and DRjj distribution for W(mn) + jets event DR between di-jets W(mn)+jets : Data :MC Di-jet Mass W(mn)+jets : Data :MC QCD BKG DRjj Mjj (GeV)

  50. Z+jets production (1) • Selections • 2 muons from Z(mm) • pT > 15 GeV • |h| < 2 • 2 electrons from Z( ee) • pT > 20 GeV • |h| < 2.3 • Jets • pT > 20 GeV • |h| < 2.5 2nd leading jets 1st leading jets • Compare PYTHIA MC • with DATA • Normalized by area • Error includes stat. error • and dominant syst. error • from JES Combined Z(ee)+jets and Z(mm)+jets

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