28/01/10 1

Zvv bkg, how to get an estimate with first data ?

R. Brunelière

• Time schedule is tight. Goal : get an estimate by may/june if we get data from end february• Need to organize ourselves to make sure all topics are covered

28/01/10 2

Zvv signalZvv signal

• MC samples: • alpgen full simulation samples at 10 TeV are existing :

http://www-f9.ijs.si/atlpy/atlprod/prodtask/?q=10771&tatype__exact=G• alpgen samples at 7 TeV being generated at freiburg =>

then official mc request• alpgen matching uncertainties samples have been generated

at freiburg at 10 TeV• NLO MCFM production can be done for 1,2jets with new MCFM

version 5.7

• Objet and event selection cuts are defined by etmiss group

• Our data-driven methods should work for any set of cut, but need starting point for studies. Propose to concentrate :1. On 2-jets channel checking it works on the different

discriminating variables (,,MT2,MCT,HTmiss). Also because 2jets channel is only one where theory uncertainties can be fully studied and Wlv bkg is less sensitive to ttbar contribution.

2. Same exercise on 4jets channel

28/01/10 3

The different control samplesThe different control samples

List of possible control samples : • Zmumu• Zee• Wlv (l=mu,e)• Gamma (both prompt unconverted and converted

photons)

Any other ?

Difficult

y

CS

Statistics

28/01/10 4

Zll control sampleZll control sample

• Basic idea :

• 2 methods :• So-called “replace” method means N(Zll;data) applies same

cuts as N(Zvv;data) including Meff cut• So-called “MC” method means N(Zll;data) applies only

preselection cuts. N(Zvv;data) predicted assuming MC is predicting correct Meff shape

• If Meff cut is too high, only “MC” method is possible with first data but requires good understanding of theory uncertainties (in particular NLO/LO Meff shape)

);();(

);();( dataZllN

MCZllN

MCZNdataZN

28/01/10 5

ZZ control sample (1/2) control sample (1/2)

• MC samples: • alpgen full simulation samples at 10 TeV are existing :

http://www-f9.ijs.si/atlpy/atlprod/prodtask/?q=AlpgenJimmyZmumuNp&tatype__exact=K• alpgen samples at 7 TeV already generated and requested

by SM and top wg• alpgen matching uncertainties samples have been generated

at 10 TeV. Request for Atlfast1 being done now.• NLO MCFM production done by James Buchanan at 10 TeV.

Will probably redo later at 7 TeV.

• Objet and event selection cuts are defined by etmiss group, but can also start with Z/W+jets strawman selection for leptons : https://twiki.cern.ch/twiki/bin/view/AtlasProtected/WZJetsCOM09#Strawman_selection

• Trigger : mu10 + susy trigger to check trigger efficiency

28/01/10 6

ZZ control sample (2/2) control sample (2/2)

• Theory uncertainties on acceptance corrections: • NLO/LO using MCFM for 1,2 jets• Matching uncertainties using the different alpgen samples• Pdf using pdf reweighting tool from Yusuke• Turn on/off jimmy UE• Are FSR introducing any bias ?

• Detector uncertainties : JES, soft MET, muon id efficiency uncertainties

• Is it enough to take lepton id corrections from Z/W+jets wg ?

• How much is QCD background ? How to estimate it ?

• How do we compute “MET” ? Pt(1+2) or neutronification ?

• How much statistics+systematics uncertainties do we get after all preselection cuts ? After different Meff cut ?

• Suffers from statistics but benchmark channel for other control samples which can be cross-checked wrt Zmumu

28/01/10 7

Zee control sampleZee control sample

Same as Z, but electrons are also reconstructed as jets :

=>

• Bias “other” jets reconstructed like when R(jet,elec) < 0.6. How big is this effect ? Can we remove the two electron clusters and redo jets (and MET) ? Can we use existing tools : embedding, jet/met, redecay, simpler method ? On AODs ?

• Cannot directly check susy trigger. Need looking at trigger objects, remove objects associated to offline electrons, check trigger chain again

Not so easy to do. Can benefit again of work done in Z/W+jets wg (e.g. by Hugo

and Caterina)

28/01/10 8

Wlv control sample (1/2)Wlv control sample (1/2)

• Method :

• Good points:• Wlv has ~ same statistics as Zvv• W/Z ratio very stable and hopefully well known (up to 2

jets)• W/Z ratio is among first SM W/Z+jets paper => lot of work

being done there• Reversing method + using Zll can be used to predict Wlv

background

• Bad points:• For 3,4 jets, ttbar background very difficult to estimate

(except via using b-tagging)• For 3,4 jets, W/Z ratio no more predicted at NLO. Can we

still trust it ?

);();(

);();( dataWlN

MCWlN

MCZNdataZN

28/01/10 9

Wlv control sample (2/2)Wlv control sample (2/2)

• MC samples: • alpgen full simulation samples at 10 TeV are existing :

http://www-f9.ijs.si/atlpy/atlprod/prodtask/?q=AlpgenJimmyW&tatype__exact=K• alpgen samples at 7 TeV already generated and requested

by SM and top wg• alpgen matching uncertainties samples have been generated

at 10 TeV. Request for Atlfast1 being done now.• NLO MCFM production done by myself at 10 TeV. Soon

available on grid. Redo later at 7 TeV.

• Wev sample has same trouble as Zee. Easier to start with W

• Background rejection for 2jets ? How ?

• Who is already working on this topic ? Yusuke ?

28/01/10 10

Prompt photon control samplePrompt photon control sample

• Used by CMS for Zvv estimation. Statistics ~ Zvv. But tricky.

• MC samples: • alpgen full simulation samples at 10 TeV are existing :

http://www-f9.ijs.si/atlpy/atlprod/prodtask/?q=AlpgenJimmyGamNp&tatype__exact=K but too small !

• I have started generating alpgen samples at 7 TeV up to 5 jets. Already nearly done.

• In order to study theory uncertainty properly, photon fragmentation correction has to be included => use recent Sherpa : http://arxiv.org/abs/0912.3501

• Amir proposal concerning background rejection + obj selection efficiency : make use of converted/unconverted rate not affecting final distributions.

+jets/Zll+jets measurement and comparison with MC should be used as proof of feasibility

28/01/10 11

SummarySummary

Proposal : start with 2jets analysis as proof of feasibility for the various methods

Should quickly setup who is working on what to avoid duplicated effort

Examples few topic uncovered till now :• Theory systematics• Background rejection (specially for Wlv)• Prompt photon whole method• Trigger emulation

Concentrate myself to work on few topics :• MC samples production/study• Theory uncertainties (with MCFM/Alpgen/Sherpa)