1/33 Comparisons between Diphox/ResBos/Pythia for bkg & signal at LHC/ATLAS Marc Escalier, LPNHE...
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Transcript of 1/33 Comparisons between Diphox/ResBos/Pythia for bkg & signal at LHC/ATLAS Marc Escalier, LPNHE...
![Page 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.](https://reader033.fdocument.org/reader033/viewer/2022052603/56649e495503460f94b3c090/html5/thumbnails/1.jpg)
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Comparisons between Diphox/ResBos/Pythia
for bkg & signal at LHC/ATLAS
Marc Escalier, LPNHE Paris, 13-14 octobre 2005
Eurogdr Supersymmetry workshop on SM Backgroundshttp://lyoinfo.in2p3.fr/gdrsusy05/
• Signal• Background
• Reducible• Irreducible
• significance
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• very high energy accelerator : pp √s=14 TeV• interaction rate: 40 MHz• inelastic cross section: σpp=70 mb • luminosity: (2x)1033 cm-2s-11034 cm-2s-1
– per year: 10 fb-1 100 fb-1
– 1 year of LHC at 1033 cm-2s-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 searchesATLAS: 25 m x 46 m CMS: 15 m x 21.5 m
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Constraints on Higgs massHigh constraints• Unitarity (WLWL diffusion)• Triviality
Low constraints• vacuum stability
Experimental constraints• direct research at LEP:
mH>114.4 GeV (95% CL=2)• indirect research: global analysis of electroweak measurements (sensibles to mH) mH=126+73
-48 GeV, MH<280 GeV
mH<700 GeVmH<750 GeV
mH>139 GeV (mt=178,1 GeV) (=1016 GeV)mH>74 GeV (=1 TeV)
winter 2005
>0
uncertainty on mH
The H channel
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SignalTo discuss what is background, we have to discuss what is signal
/S BS N Nsignificance if S>5 signal >5 x error on background
P(bck fluctuates > 5 )=10-7
discovery
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Opening of Htt
W/Z H associated production assoc. prod. ttH, bbH
-
-
Dominant channel over the whole range of MH :ggHVBF and assoc. prod. : distinct signatures↑S/B, ↓statistics
VBF
-MH(GeV)
Higgs production at LHC
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2mW
LEP
2mt
• MH<140 GeV:dominant decay mode bb and overwhelmed by bck (inclusive)(ggHbb)~20 pb; (bb)~500 µb
Accessible channels :ttH(Hbb), VBF (Hrare decay mode Hcleaner signature, inclusive production
Higgs decay at LHC-
• 140<MH<180 GeVWW* and ZZ* channels have significant BRHZZ* 4l good mass reconstruction possible, but low stat. HWW*ll either inclusive or VBF production (better S/B)
• MH>180 GeVHZZ4l gold-plated channel « easy »
- -
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Signal
Direct production
dominant
Associated production WH, ZH, ttH
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(mb) production from PythiaggHLOPythiaNLO: K=NLO/LO=1.8• HiGlu (cross section only) • ResBos (MC events)NNLO (Anastasiou et al.), prog FEHiPhep-ph/0501130, www.phys.hawaii.edu/~kirill/FEHiP
VBF• HiGlu K=1.04+BR corrections HDecay (NLO)
Higgs production (eg: 120 GeV)
KNLO=1,76 (HiGlu: 1,8) KNNLO=1.16
Tools available for signal
LO NLO NNLO
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KSV=Kulesza, Sterman, Vogelsang, CTEQ6MRSVN=Ravindran, Smith, Van Neerven, CTEQ6MBCFG=Bozzi, Catani, De Florian, Grazzini, MRST2002
Uncertainty signal ~40 %
Need data (bck)To better understand
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Parameters for analysis• photons cuts: PT(1)>40 GeV, PT(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 detectortruth+smearing =40 µm• high lumi: vertex z : elmg clustersResolution : 2 cm
Study mass resolution of H
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Resolution obtained
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• +jet
jets quarks ~10>jets gluonsReducible background
• jet+jet
58 % jet gluon
4.7 % jet quark
20 millions > signal
37.3 % mixture
Partonshadrons (« jets »)
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/jet separation
Hadronic leakages (3x7/7x7) (3x3/3x7)
elmg ca
lo
2nd layer
Width (3x5) 2ème max. min-2nd max
Elmg ca
lo
1er la
yer
HCAL
Some examplesjets
+1st layer: (width 40 strips, 3 strips, % E outside hearth shower)
Full simulation of detector
Goal: efficiency 80 %, R=5000
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Efficiency for photons
ET
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• all jets: R~6680
• quarks jetsR~2880
• gluons jets R~20650
Jets rejection
~3
PT>25GeV
0=70 % remaining events
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• Born(QED2) • Bremsstrahlung
(correction to Born)
• Box gg (s2QED
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 computationsNLO: cross-sections, scale dependence, final state PT
M (GeV)
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fragmentation
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Factorisation theorem
Partonic cross-section long distance physics (pdf)
Resummation
At NLO, divergent terms as log PT/Q
PT<<Q collinear and infrared divergencesisolation cone for photon(eg:R=0.4 ET<15 GeV)
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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 Ptsmoothing fW+P(+F)-fA
fW+P(+F)-fA
PT Born construction
Smoothing of backgroundFrag=fragmentation
PT()
PT()
ResBos not easy to use: need 3 distributions to have one physical distribution
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ResBos included Feynman diagrams
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No resummation in DiphoxPhase space cut in PT
Regularization of divergent terms
Born+brem
PT (GeV)M (GeV)
better fragmentation in Diphox
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Box
Diphox (LO)
Bern, Dixon, Schmidt NLO
Bern, Dixon, Schmidt LO
ResBos NLO
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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 %
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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.htmlHep-ph/0408302: transverse-momentum-ordered showers and interleaved multiple interactions
Parton shower problem of Pythia
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Diphox suffers infrared divergence in the « qT » (PT) distribution
ResBos resums effects of soft and/or collinear gluon emissions to all orders, predicts a smooth qT distribution.
Comparison data/simulations
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w/ gg contributionwo/ 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. Wuhttp://yliu.home.cern.ch/yliu/diphotonReBlessed_30Jan_2004/diphoton_reblessed.html
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27/33R. Blair, R. Culbertson, J. Huston, S. Kuhlmann, Y. Liu, X. Wu
Transverse momentum of pair qT=PT
≠scalesmay give 10% effect
• ResBos < DiphoxBUTOld version of ResBos (since NLO box included)Pythia contrib x 2
• What about high PT ?need more statistics
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28/33R. Blair, R. Culbertson, J. Huston, S. Kuhlmann, Y. Liu, X. Wu
Pythia contrib x 2
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Details of values
http://yliu.home.cern.ch/yliu/diphotonReBlessed_30Jan_2004/April25_2004/numbers.ps
M
qT=PT
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• background is not so well constrainted:-resummation important to have a well description of high PT
-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
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Significance
↑50 % (NLO)100 fb-1
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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: 6.24likelihood:9.24
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Conclusion• First years of LHC are near : summer 2007 ?SM Higgs may be discovered in first years of operation with HNeed 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. UnalSearch 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
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Appendix
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Rumour on Diphox
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Correlations entre variables
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goal• efficiency 80 %• R=5000
Inner detector
/jet separation (II)
0
• track vetoNon converted request : no tracks with PT>5 GeV in R=0.2
• Isolationrequest: PT tracks in [0.1<R<0.3] <4 GeV low lumi.
<10 GeV high lumi.
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sym
The Higgs mechanismsolution : 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
(µ2<0)
=MW/MZcos W=1
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Constraints on Higgs massHigh constraints• Unitarity (WLWL diffusion)• Triviality
Low constraints• void stability
Experimental constraints• direct search at LEP:
mH>114.4 GeV (95% CL=2)• indirect search : Global analysis of electroweak measurements
mH=126+73-48 GeV, MH<280 GeV
mH<700 GeVmH<750 GeV
mH>139 GeV (mt=178.1 GeV) (=1016 GeV)mH>74 GeV (=1 TeV)
hiver 2005
>0
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SM Discovery potential
Almost all allowed mass range explored in 1st year (10 fb-1) for ATLAS-CMSWith 30 fb-1, more than 7 for the whole range (provided systematics on the background are under control)
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Higgs search in the H channel
Very rare decay (BR~10-3), σ(ppH115)xBr(H→)=76 fb (NLO), S/B~0.05
NLO tools available for signal and bck.PT improve these results
bck:irreducible continuum, reducible: -jet and jet-jetKeys: excellent energy and angular resolutions
excellent efficiency, jet rejectionhigh granularity and response uniformity
LAr (ATLAS), PbWO4 (CMS)H width negligible, resolution
dominated by detector σ(m)/m≈1%
low masses(80-150 GeV)
ppH
CERN/LHCC 96-40 ATLAS TDR 1CERN/LHCC 96-41 ATLAS TDR 2CERN/LHCC 97-33 CMS TDR 4
bck from sidebands
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Reducible background after rejection