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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

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

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