Higgs to WW

Higgs to WW

Monte Carlo Tools and Cuts

Monte-Carlo tools

CMS

Rebeca Gonzalez Suarez (19/9/2010) 3

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

• MC@NLO (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


Background SamplesSamples Generators

WW MC@NLO tt, W+jets, DY AlpgenWt TopReX + PYTHIAWZ, ZZ, QCD … PYTHIA

14TeV

Samples GeneratorsWW PYTHIA

ggW ggWW generator

tt, W+jets, DY, tW MadGraph

WZ, ZZ, QCD … PYTHIA

10 TeV

Samples GeneratorsWW PYTHIA

ggW ggWW generator

tt, W+jets, DY, tW MadGraph

WZ, ZZ, QCD … PYTHIA

7 TeV

POWHEG samples are also available in the official production of CMS, we

tested them in V + jets processes

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

http://virgilio.mib.infn.it/~nason/POWHEG/


Background Cross-sections

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


Higgs Signal Samples• Higgs signal samples (to WW to 2 leptons) generated with PYTHIA• The differential Higgs boson transverse momentum distribution, PT

H, 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 MC@NLO differential distribution, as MC@NLO incorporates next-to-leading order (NLO) matrix element calculations

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


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

http://people.web.psi.ch/spira/proglist.html

http://people.web.psi.ch/spira/proglist.html

Sequential Cut-based analysis

ATLAS – CMS


Two different styles

• CMS and ATLAS have different strategies for the HWW analysis based on sequential cuts:– CMS:• 0 jet bin• 3 final states (ee, eµ, µµ)• Mass-dependent approach (different cuts Vs mH)

– ATLAS:• 0j, 1j, 2j• (one) Cut mH dependant


Lepton selection• Initial step of the analysis• Without accounting for the specific quality cuts, both ask

for opposite-sign lepton pairs with substantial PT within the acceptance– CMS:

• PT > 10, 20• η < 2.4

– ATLAS:• PT > 15• η < 2.5

• And events with a third lepton are vetoed


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 the leptons

• But ATLAS uses more variables


Variables Used

CMS ATLAS

# of jets (CJV) # of jets

MET MET

ΔΦll ΔΦll

Mll Mll

PTmax ΔRll

PTmin Mll – MET

MT

PTll


CMS: Cuts

mH MET > MET < ΔΦll < Mll < PTmax > PTmax < PTmin >

120 44/35/35 -/80/60 69/100/90 50/60/40 20/20 48/56/65 10/15/20

130 44/35/35 -/85/60 69/100/90 50/60/40 20/20 48/56/65 10/15/20

140 44/40/30 -/80/75 69/80/70 50/70/50 20/26 48/66/50 10/19/25

150 48/40/45 -/85/105 57/80/70 50/65/45 28/27 50/66/50 25/25/30

160 48/40/45 -/85/105 57/80/70 50/65/45 28/26 50/66/50 25/25/25

165 48/40/45 -/-/105 57/80/70 50/65/45 28/29 50/66/50 25/25/25

170 48/45/45 -/-/105 57/80/65 50/75/50 28/29 50/78/55 25/25/25

180 55/45/55 -/-/105 57/80/65 50/80/60 36/29 70/78/65 25/25/20

190 55/45/65 -/-/90 57/80/60 50/80/80 36/29 70/82/70 25/25/10

200 55/45/65 -/-/90 57/100/65 50/80/95 36/29 70/82/80 25/25/15

µµ/ee/eµ

Cut on Mll > 12 included in the pre-selection step


ATLAS: Cuts# of jets 0, 1, 2ΔRll > 0.3Mll > 15 && |MZ – Mll| > 10 (ee and µµ)

MET > 30 (eµ) / 40 (ee and µµ)Mll – MET > 30Mll < 50ΔΦll < 1.3 rad (~74°)MT < MH (mass dependent)

# jets Extra-cuts0 PTll > 301 Anti b-tag && PTTOT < 30 && Zττ2 Anti b-tag && PTjetmax > 40 && |Etaj1j2| > 3.8 && Mj1j2 > 500 && PTTOT < 30 && Zττ


0 jet bin, µµ Comparison1fb-1 HWW160 WW tt WZ ZZ S/Sqrt(B)

CMS 10.78 3.83 0.52 0.15 0.08 5.04

ATLAS 19.75 19.18 2.87 0.77 0.49 4.09

1fb-1 HWW130 WW tt WZ ZZ S/Sqrt(B)

CMS 5.64 14.65 2.34 0.46 0.40 1.33

ATLAS 5.13 19.18 2.87 0.77 0.49 1.06

1fb-1 HWW190 WW tt WZ ZZ S/Sqrt(B)CMS 2.55 1.74 1.30 0.31 0.20 1.35

ATLAS 6.92 19.18 2.87 0.77 0.49 1.43

*W+jets, Z+jets no events remain after all the cuts


Possible set of common cuts• Select events with 2 leptons :

– |η| < 2.5– PT > 10, 20

• Opposite charge• Veto events with a third lepton• Jets: PT > 20 |η| < 3

Variable Common cuts

# of jets (CJV) -

MET > 30

ΔΦll < 1.8

Mll > 12

→ Loose cuts Each experiment, final state and jet bin applies then their own on top


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)

Variable Common cuts

# of jets (CJV) No cuts and no jet definition

MET > 20

Mll > 12

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

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Higgs to WW