Event Weights for Monte Carlo Samples

Introduction to Event Weights

When a Monte Carlo (MC) sample is generated the size of the sample is determined by requesting the generation of a certain number of events. The cross section of the sample is a fixed quantity dependent on the process generated. Since number of events, sample size, and luminosity are related according to σ = N_Events/L, the luminosity of a MC sample varies according to the number of events generated and the cross section of the process.

The amount of data collected in an experiment is expressed in terms of luminosity since this quantity can be determined from known properties of the colliding beams. In order to perform a simulation of the data at given luminosity, it is necessary to weight the MC sample to correspond to this luminosity. This allows, for example, to predict the number of top events that will be produced in an amount of data corresponding to that luminosity.

The weighting of the MC can be expressed as N = WN₀, where N is the number of weighted events, N₀ is the number of events in the original sample, and W is the weight. The weight must be a ratio of the desired luminsoity to the original luminosity of the sample, W = L/L₀. Expressing the original luminosity in terms of cross section and number of events,

W = σL/N₀.

All histograms are weighted by multiplying the quantity used to fill the histogram (N, pT, eta, etc) by the event weight. In the MSU analysis code, the actual weighting takes place in the method myTH1F::Fill().

NLO vs. LO

All the MC samples were genereated using a LO cross section prediction. For the single top (s, t, and Wt channels) ttbar processes, the cross sections are known to NLO. In this case the weight is scaled by the ratio of NLO to LO cross sections

W_NLO = W (σ_NLO / σ_LO ) = σ_NLOL/N₀

Branching Ratio

If the MC samples were genereated to produce a certain final state, for example ttbar -> dileptons, then the branching ratio, BR, for that final state must be included in the weight calculation;

W = BRσL/N₀

Sources of information

The information needed to calculate event weights were collected from the following places:

• DC3 Requests Page
• TopView Ntuple Availability Page
• W+Jets Alpgen Page
• CSC Single Top Note Page
• Top References Page

Weights for v15.5.1

We use weights from this website: https://twiki.cern.ch/twiki/bin/view/AtlasProtected/TopReferences10TeV. Note that the website recommendation is to multiply the cross-section by a factor that includes the k-factor as well as other factors.

Table of Weights

The following tables list the properties of the MC samples used in the Single Top analysis. The weights for the single top channels (s, t, and Wt) and ttbar channels were calculated using the NLO cross section.

For v13030, the column σ shows σ multiplied by the branching ratio.

Table for v13030

Run	Sample	Generator	Matrix Element	σ (pb)	BR	NEvents	Weight
5200	ttbar->no_had	Herwig+Jimmy	MC@NLO	833.0	0.555	36708.0	1.25944
5500	Wt	Pythia	acerMC	66.0	0.333	4000.0	0.54945
5501	s-channel	Phythia	acerMC	10.65	0.333	10000.0	0.03546
5502	t-channel	Phythia	acerMC	246.0	0.333	38950.0	0.21032
6410	W->enu+2Jets	Herwig+Jimmy	Alpgen	826.0	1.0	208000.0	0.39712
6411	W->enu+3Jets	Herwig+Jimmy	Alpgen	239.0	1.0	43150.0	0.55388
6412	W->enu+4Jets	Herwig+Jimmy	Alpgen	67.4	1.0	36000.0	0.18722
6413	W->enu+5Jets	Herwig+Jimmy	Alpgen	24.0	1.0	3000.0	0.80000
6414	W->munu+2Jets	Herwig+Jimmy	Alpgen	826.0	1.0	212300.0	0.38907
6415	W->munu+3Jets	Herwig+Jimmy	Alpgen	236.0	1.0	17100.0	1.38012
6416	W->munu+4Jets	Herwig+Jimmy	Alpgen	68.3	1.0	51950.0	0.13147
6417	W->munu+5Jets	Herwig+Jimmy	Alpgen	24.3	1.0	0.0	1.00000

In the 121403 MC version, run 1206, the W+Jets weights include branching ratio, alpgen matching efficiency and filtering efficiency.

Table for Version 121403, weights calculated for 100pb-1

Run	Sample	Generator	Version	Matrix_Ele	σLO(pb)	σNLO(pb)	BR	NEvents	Weight
6210	W+0Jets	Herwig+Jimmy	12000604	Alpgen	731.62	0.0	0.0	364950.0	0.20047
6211	W+1Jets	Herwig+Jimmy	12000604	Alpgen	6249.0	0.0	0.0	896949.0	0.69670
6212	W+2Jets	Herwig+Jimmy	12000604	Alpgen	2021.45	0.0	0.0	630834.0	0.32044
6213	W+3Jets	Herwig+Jimmy	12000604	Alpgen	818.0	0.0	0.0	133046.0	0.61482
52002	ttbar->dilep	Herwig+Jimmy	12000601	MC@NLO	830.63	833.0	0.111	79616.0	0.11614
5500	Wt	Pythia	12000601	acerMC	80.1	66.0	0.333	48350.0	0.04546
5501	s-channel	Phythia	12000601	acerMC	9.9	10.65	0.333	48300.0	0.00734
5502	t-channel	Phythia	12000601	acerMC	243.9	246.6	0.333	41428.68	0.19821
52001	ttbar->lep+jets	Herwig+Jimmy	12000601	MC@NLO	830.63	833.0	0.444	321720.0	0.11496
5200	ttbar->no_had	Herwig+Jimmy	12000601	MC@NLO	830.63	833.0	0.555	401336.0	0.11519

Table for Version 1214

Run	Sample	Generator	Version	Matrix_Ele	σLO(pb)	σNLO(pb)	NEvents	Lumi(pb-1)	Weight
005001	minBias	Phythia	11004103	-	-	80	-	5.00E+05	6.25E-06	5000	160000000.0
005200	ttbar->no_had	Herwig+Jimmy	11000505	MC@NLO	0.54	461	-	5.00E+05	1.08E+003	598700	0.925926
005204	ttbar->all_had	Herwig+Jimmy	11004206	MC@NLO	0.46	369	-	2.00E+05	5.42E+002	77600	1.845018
005205	ttbar->lep+jet	Pythia	11004206	acerMC	-	460	-	5.00E+05	-	434650	-
005223	W->ev(2parton_exe)	Herwig+Jimmy	11004206	ALPGEN	-	504	-	1.50E+04	8.24E+003	4000	0.121359
005224	W->ev(3parton_exe)	Herwig+Jimmy	11004206	ALPGEN	-	122	-	1.50E+04	6.58E+003	15200	0.151976
005225	W->ev(4parton_exe)	Herwig+Jimmy	11004206	ALPGEN	-	28.4	-	1.50E+04	16.3E+003	15200	0.061350
005226	W->ev(5patron_inc)	Hewwig+Jimmy	11004206	ALPGEN	-	6.1	-	1.50E+04	10.0E+003	7000	0.100000
005500	Wt	Pythia	11000505	acerMC	-	26.7	22.0	1.00E+04	3.75E+002	72600	2.197253
005501	s-channel	Phythia	11004209	acerMC	-	3.3	3.55	1.00E+04	3.03E+003	82950	0.355036
005502	t-channel	Phythia	11004207	acerMC	-	81.3	82.2	1.00E+04	1.23E+002	8400	8.220082
008203	W->mv(3parton_exe)	Herwig+Jimmy	11004206	ALPGEN	-	122	-	1.50E+04	7.91E+003	5000	0.126422
008204	W->mv(4parton_exe)	Herwig+Jimmy	11004206	ALPGEN	-	28.4	-	1.50E+04	16.2E+003	18050	0.061728

Table for Version 1213

File	Sample	Cross Section	Branching Ratio	Luminosity(pb-1)	MC Events	Weight
5200	t-tbar -> lepton + jets and dilepton	872.8	0.555	100	592300	0.08178356
5200	t-tbar -> dilepton	872.8	0.111	100	117516	0.08244052
5200	t-bar -> lepton + jets	872.8	0.444	100	474784	0.08162095
5500	Wt	62.1	1/3	100	48350	0.0428128
5501	s-channel (tb)	10.65	1/3	100	48050	0.0073881
5502	t-channel (tq)	246.6	1/3	100	43950	0.1870307

-- PatRyan - 19 Feb 2007, -- JennyHolzbauer - 01 Aug 2007-- JennyHolzbauer - 30 Jan 2008