1. 1/33 Comparisons between Diphox/ResBos/Pythia for  bkg & signal at LHC/ATLAS Marc Escalier, LPNHE Paris, 13-14 octobre 2005 Eurogdr Supersymmetry workshop on SM Backgrounds http://lyoinfo.in2p3.fr/gdrsusy05/ Signal Background Reducible Irreducible significance

2. 2/33 very high energy accelerator : pp √s=14 TeV interaction rate: 40 MHz inelastic cross section: σ pp =70 mb luminosity: (2x)10 33 cm -2 s -1  10 34 cm -2 s -1 –per year:10 fb -1  100 fb -1 –1 year of LHC at 10 33 cm -2 s -1 ~10 years at prev. machines big detectors first collisions in Summer 2007 ATLAS and CMS at LHC Detectors optimised for Higgs boson and SUSY searches ATLAS: 25 m x 46 mCMS: 15 m x 21.5 m

3. 3/33 Constraints on Higgs mass High constraints Unitarity (W L W L diffusion) Triviality Low constraints vacuum stability Experimental constraints direct research at LEP: m H >114.4 GeV (95% CL=2  ) indirect research: global analysis of electroweak measurements (sensibles to m H ) m H =126 +73 -48 GeV, M H <280 GeV m H <700 GeV m H <750 GeV m H >139 GeV (m t =178,1 GeV) (  =10 16 GeV) m H >74 GeV (  =1 TeV) winter 2005 >0 uncertainty on m H The H   channel

4. 4/33 Signal To discuss what is background, we have to discuss what is signal significance if S>5 signal >5 x error on background P(bck fluctuates > 5  )=10 -7  discovery

5. 5/33 Opening of H  tt W/Z H associated production assoc. prod. ttH, bbH - - Dominant channel over the whole range of M H :gg  H VBF and assoc. prod. : distinct signatures  ↑S/B, ↓statistics VBF - M H (GeV) Higgs production at LHC

6. 6/33 2m W LEP 2m t M H <140 GeV: dominant decay mode bb and  overwhelmed by bck (inclusive)  (gg  H  bb)~20 pb;  (bb)~500 µb Accessible channels : ttH(H  bb), VBF (H   rare decay mode H   cleaner signature, inclusive production Higgs decay at LHC - 140<M H <180 GeV WW * and ZZ * channels have significant BR H  ZZ *  4l good mass reconstruction possible, but low stat. H  WW *  l l either inclusive or VBF production (better S/B) M H >180 GeV H  ZZ  4l gold-plated channel « easy » --

7. 7/33 Signal Direct production dominant Associated production WH, ZH, ttH

8. 8/33  (mb) production from Pythia gg  H LO Pythia NLO: K=  NLO /  LO =1.8 HiGlu (cross section only) ResBos (MC events) NNLO (Anastasiou et al.), prog FEHiP hep-ph/0501130, www.phys.hawaii.edu/~kirill/FEHiP VBF HiGlu  K=1.04 +BR corrections HDecay (NLO) Higgs production (eg: 120 GeV) K NLO =1,76 (HiGlu: 1,8) K NNLO =1.16 Tools available for signal LONLONNLO

9. 9/33 KSV=Kulesza, Sterman, Vogelsang, CTEQ6M RSVN=Ravindran, Smith, Van Neerven, CTEQ6M BCFG=Bozzi, Catani, De Florian, Grazzini, MRST2002 Uncertainty signal ~40 % Need data (bck) To better understand

10. 10/33 Parameters for analysis photons cuts: P T (1)>40 GeV, P T (2)>25 GeV |  |<2.4, exclude transition region at |  |=1.45  corrections clusters sizes (TDR): EMB:3x7:conv., 3x5 non-conv., EC :5x5 calibration constant term 0.6 % vertex correction: low lumi: vertex z : inner detector truth+smearing  =40 µm high lumi: vertex z : elmg clusters Resolution : 2 cm Study mass resolution of H  

11. 11/33 Resolution obtained

12. 12/33  +jet  jets quarks ~10>jets gluons Reducible background jet+jet 58 % jet gluon 4.7 % jet quark 20 millions > signal 37.3 % mixture Partons  hadrons (« jets »)

13. 13/33  /jet separation Hadronic leakages  (3x7/7x7)  (3x3/3x7) elmg calo 2nd layer Width  (3x5) 2 ème max.min-2 nd max Elmg calo 1 er layer HCAL  Some examples jets +1st layer: (width 40 strips, 3 strips, % E outside hearth shower)     Full simulation of detector Goal: efficiency 80 %, R=5000

14. 14/33 Efficiency for photons  ETET

15. 15/33 all jets: R~6680 quarks jets R~2880 gluons jets R~20650 Jets rejection ~3 P T >25GeV  0 =70 % remaining events

16. 16/33 Born(  QED 2 ) Bremsstrahlung (correction to Born) Box gg   (  s 2  QED 2 ) Irreducible background Diphox *NLO Born+brem w/o resum. *LO Box Eur. Phys. J. C 16,311-330 ResBos *NLO Born+brem w/ resum. *NLO Box w/ resum. Phys. Rev. D 57, 6934-6947,1998 [hep-ph/9712471] ~10 > signal Pythia distributions NLO computations NLO: cross-sections, scale dependence, final state P T M  (GeV)

17. 17/33 fragmentation

18. 18/33 Factorisation theorem Partonic cross-sectionlong distance physics (pdf) Resummation At NLO, divergent terms as log P T /Q P T <<Q  collinear and infrared divergences  isolation cone for photon (eg:R=0.4 E T <15 GeV)

19. 19/33 Background with ResBos P « Perturbative » contains all terms (divergent terms also) A « Asymptotic » contains divergent terms only W contains resummed « divergent terms » W+P-A, but resummation not valid at high Pt  smoothing fW+P(+F)-fA  fW+P(+F)-fA P T  Born construction Smoothing of background Frag=fragmentation P T (  ) ResBos not easy to use: need 3 distributions to have one physical distribution

20. 20/33 ResBos included Feynman diagrams

21. 21/33 No resummation in Diphox Phase space cut in P T Regularization of divergent terms Born+brem P T  (GeV)M  (GeV) better fragmentation in Diphox

22. 22/33 Box Diphox (LO) Bern, Dixon, Schmidt NLO Bern, Dixon, Schmidt LO ResBos NLO

23. 23/33 Events at LHC for 120 GeV and 30 fb -1 Diphox is scaled by 1.6 to take into accound NLO of box Gives an uncertainty of 5 %

24. 24/33 News from Pythia 6.3 « contains a completely new multiple interactions model, with new transverse- momentum-ordered showers for initial- and final-state radiation » To have backward compatibility: PYEVNT (old) and PYEVNW (new) http://www.thep.lu.se/~torbjorn/Pythia.html Hep-ph/0408302: transverse-momentum-ordered showers and interleaved multiple interactions Parton shower problem of Pythia

25. 25/33 Diphox suffers infrared divergence in the « q T » (P T  ) distribution ResBos resums effects of soft and/or collinear gluon emissions to all orders, predicts a smooth qT distribution. Comparison data/simulations

26. 26/33 w/ gg contribution wo/ gg contribution NLO Diphox Invariant mass M  at CDF, run II Pythia contrib x 2 207 pb -1 R. Blair, R. Culbertson, J. Huston, S. Kuhlmann, Y. Liu, X. Wu http://yliu.home.cern.ch/yliu/diphotonReBlessed_30Jan_2004/diphoton_reblessed.html

27. 27/33 R. Blair, R. Culbertson, J. Huston, S. Kuhlmann, Y. Liu, X. Wu Transverse momentum of pair q T =P T  ≠scales may give 10% effect ResBos < Diphox BUT Old version of ResBos (since NLO box included) Pythia contrib x 2 What about high PT ?  need more statistics

28. 28/33 R. Blair, R. Culbertson, J. Huston, S. Kuhlmann, Y. Liu, X. Wu   Pythia contrib x 2

29. 29/33 Details of values http://yliu.home.cern.ch/yliu/diphotonReBlessed_30Jan_2004/April25_2004/numbers.ps M  q T =P T   

30. 30/33 background is not so well constrainted: -resummation important to have a well description of high P T -difficult to have same parameters for the comparisons -need update comparisons background/data at Tevatron with *new version of ResBos, *more statistics (used 200 pb -1 so far…) *new version of Pythia (new parton shower) *take same theoretical assumptions (scales, pdfs, order of the pert. devel.)  need manpower

31. 31/33 Significance ↑50 % (NLO) 100 fb -1

32. 32/33 Likelihood ratio method qq et qg have fermions exchange in t channel   angles lower than those of Higgs boson (isotropic in  - Significance: 120 GeV, 30 fb -1 : 6.24  likelihood:9.24

33. 33/33 Conclusion First years of LHC are near : summer 2007 ? SM Higgs may be discovered in first years of operation with H   Need understanding of systematics Use data for detector performance understanding validation of pdf for cross-sections validation of MC tools More information M. Escalier, F. Derue, L. Fayard, M. Kado, B. Laforge, C. Reifen, G. Unal Search for a Standard Model Higgs boson in the ATLAS experiment on the H  gamma gamma channel, ATL-COM-PHYS-2005-054 M. Escalier, F. Derue, L. Fayard, M. Kado, B. Laforge, C. Reifen, G. Unal, Photon/jet separation with DC1 data, ATL-COM-PHYS-2005-048 M. Escalier, B. Laforge (dir.), Recherche expérimentale de la brisure spontanée de symétrie électrofaible dans le canal H \to \gamma \gamma et d'une solution au problème de hiérarchie dans ATLAS. Participation à la préparation de l'électronique du calorimètre électromagnétique, 2005 Paris : Paris 11, CERN-THESIS-2005-023

34. 34/33 Appendix

35. 35/33 Rumour on Diphox

36. 36/33 Correlations entre variables

37. 37/33 goal efficiency 80 % R=5000 Inner detector  /jet separation (II)  00 track veto Non converted  request : no tracks with P T >5 GeV in  R=0.2 Isolation request:  P T tracks in [0.1<  R<0.3] <4 GeV low lumi. <10 GeV high lumi.

38. 38/33 sym The Higgs mechanism solution : new field   Predict existence of a scalar boson non sym. v≠0 bosons get a dynamical mass : +interaction H-(W; Z) v  =v+   V(|  |)   other mechanism for fermions (µ 2 <0)   =M W /M Z cos  W =1

39. 39/33 Constraints on Higgs mass High constraints Unitarity (W L W L diffusion) Triviality Low constraints void stability Experimental constraints direct search at LEP: m H >114.4 GeV (95% CL=2  ) indirect search : Global analysis of electroweak measurements m H =126 +73 -48 GeV, M H <280 GeV m H <700 GeV m H <750 GeV m H >139 GeV (m t =178.1 GeV) (  =10 16 GeV) m H >74 GeV (  =1 TeV) hiver 2005 >0

40. 40/33 SM Discovery potential Almost all allowed mass range explored in 1 st year (10 fb -1 ) for ATLAS-CMS With 30 fb -1, more than 7  for the whole range (provided systematics on the background are under control)

41. 41/33 Higgs search in the H   channel Very rare decay (BR~10 -3 ), σ(pp  H 115 )xBr(H→  )=76 fb (NLO), S/B~0.05 NLO tools available for signal and bck. P T improve these results bck:irreducible  continuum, reducible:  -jet and jet-jet Keys: excellent energy and angular resolutions excellent  efficiency, jet rejection high granularity and response uniformity  LAr (ATLAS), PbWO 4 (CMS)   H width negligible, resolution dominated by detector  σ(m  )/m  ≈1% low masses (80-150 GeV)   pp H CERN/LHCC 96-40 ATLAS TDR 1 CERN/LHCC 96-41 ATLAS TDR 2 CERN/LHCC 97-33 CMS TDR 4 bck from sidebands

42. 42/33 Reducible background after rejection