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Higgs to WW. Monte Carlo Tools and Cuts. Monte-Carlo tools. CMS. Monte Carlo generators. PYTHIA (LO): Event generator for a large number of physic processes: hard/soft interactions, parton distributions, initial/final state parton showers, multiple interactions, fragmentation and decay

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Higgs to ww

Higgs to WW

Monte Carlo Tools and Cuts

Monte carlo generators
Monte Carlo generators

  • PYTHIA (LO):

    Event generator for a large number of physic processes: hard/soft interactions, parton distributions, initial/final state parton showers, multiple interactions, fragmentation and decay

  • [email protected] (NLO):

    Fortran package. Monte Carlo event generator that provides Next-to-Leading-Order calculations of rates for QCD processes. It uses the Fortran HERWIG event generator

  • Alpgen:

    Hard multiparton processes. Produces matrix element (ME) level events to be passed to a parton shower (PS) / hadronization code (Pythia/HERWIG). The best Monte Carlo prediction of multi-jet final states

  • TopReX:

    Provides processes not implemented in PYTHIA, that can be later accessed from PYTHIA as external processes. Also stand alone event generator (partonic final states before showering)

  • MadGraph/MadEvent:

    Software that allows to generate amplitudes and events for any process (with up to 9 external particles) in any model

Rebeca Gonzalez Suarez (19/9/2010)

Background samples
Background Samples



POWHEG samples are also available in the official production of CMS, we tested them in V + jets processes

10 TeV

*POWHEG: The POWHEG method is a prescription for interfacing NLO calculations with parton shower generators, covers several processes, Higgs included (no negative weights)

Rebeca Gonzalez Suarez (19/9/2010)

Background cross sections
Background Cross-sections

  • For the considered backgrounds, the cross-sections are computed at NLO with MCFM

Rebeca Gonzalez Suarez (19/9/2010)

Higgs signal samples
Higgs Signal Samples

  • Higgs signal samples (to WW to 2 leptons) generated with PYTHIA

  • The differential Higgs boson transverse momentum distribution, PTH, is sensitive to higher order corrections

  • Differential reweighting technique has been applied: the leading order (LO) PYTHIA Higgs boson transverse momentum spectrum is reweighted to match the [email protected] differential distribution, as [email protected] incorporates next-to-leading order (NLO) matrix element calculations

  • The reweighting is done by means of PT dependant k-factors

Rebeca Gonzalez Suarez (19/9/2010)

Higgs cross sections
Higgs Cross-sections

  • The cross-sections have been calculated at different orders in perturbative QCD for the four main Higgs production mechanism at the LHC, using dedicated tools:

    • Associated production with Z/W:calculated at NLO using V2HV

    • Associated production with a top pair:at LO using HQQ

    • Vector-Boson fusion: at NLO using VV2H

    • Gluon-Fusion: (N)NLO using HIGLU and HggTotal

  • The Branching ratios of the SM Higgs are computed with the tool HDECAY


Rebeca Gonzalez Suarez (19/9/2010)

Two different styles

  • CMS and ATLAS havedifferentstrategiesforthe HWW analysisbasedonsequentialcuts:

    • CMS:

      • 0 jet bin

      • 3 final states (ee, eµ, µµ)

      • Mass-dependentapproach (differentcuts Vs mH)

    • ATLAS:

      • 0j, 1j, 2j

      • (one) CutmHdependant

Rebeca Gonzalez Suarez (19/9/2010)

Lepton selection

  • Initialstep of theanalysis

  • Withoutaccountingforthespecificqualitycuts, bothaskforopposite-signleptonpairswithsubstantial PTwithintheacceptance

    • CMS:

      • PT > 10, 20

      • η < 2.4

    • ATLAS:

      • PT > 15

      • η < 2.5

  • And eventswith a thirdlepton are vetoed

Rebeca Gonzalez Suarez (19/9/2010)

Things in common
Things in common

  • Jets: Same ET and η thresholds (details like JEC unknown in ATLAS, they just say “ET”)

    • Anti k-t calorimeter jets

    • Parameter size of 0.5 (CMS)

    • |η| < 3 / ET > 20 (RAW), 30 (corrected)

  • Use of:

    • MET

    • ΔΦll

    • Mll

    • PT of theleptons

  • But ATLAS uses more variables

Rebeca Gonzalez Suarez (19/9/2010)

Variables used
Variables Used

Rebeca Gonzalez Suarez (19/9/2010)

Cms cuts
CMS: Cuts


CutonMll > 12 included in the pre-selectionstep

Rebeca Gonzalez Suarez (19/9/2010)

Atlas cuts

Rebeca Gonzalez Suarez (19/9/2010)

0 jet bin comparison
0 jet bin, µµ Comparison

*W+jets, Z+jets no eventsremainafterallthecuts

Rebeca Gonzalez Suarez (19/9/2010)

Possible set of common cuts
Possible set of commoncuts

  • Selecteventswith 2 leptons :

    • |η| < 2.5

    • PT > 10, 20

  • Oppositecharge

  • Veto eventswith a thirdlepton

  • Jets: PT > 20 |η| < 3

→ Loosecuts

Eachexperiment, final state and jet binappliesthentheirownon top

Rebeca Gonzalez Suarez (19/9/2010)

Possible set of common cuts gen
Possible set of common cuts (GEN)

  • Events with 2 leptons (tau included in the simulation) :

    • |η| < 2.5

    • PT > 10/10 (5/5 or 0/0 if possible)

If the cuts are goint to be made at generation level, we should go for something looser, to not risk ‘problematic regions (low mass)

Rebeca Gonzalez Suarez (19/9/2010)