W + jets: MET correction and measurement of the

W Transverse Mass

Autori: L. Brussich, I. Lazzizzera Padova, 7 ottobre 2008

Padova, 7 ottobre 2008Autori: L. Brussich, I. Lazzizzera

GOAL: Measurement of the Electroweak W Mass using the first CMS data in the channel W + jets

METHOD:the W Boson Mass MW will be extracted by fitting the transverse mass distribution as a function of MW.

FIRST-STEPS: - W transverse mass Mt(W) formula;- Using of GenMETCollection and CaloMETCollection in our

analysis how to correct these objects? …THE-NEXT-STEP:

event selection (eg: kinematic range) in order to analyse high-purity W samples using “suggestions” from the MC-distribution.

Padova, 7 ottobre 2008Autori: L. Brussich, I. Lazzizzera

W transverse mass Mt(W) formula:

The familiar form of the W transverse mass is:

Mt2 = 2 pt(l) pt(υ) [ 1 – cos(φ(l) – φ(υ)) ]

It is interesting to note that this is the extremum (w.r.t. the parallel

momentum of the neutrino) for the squared invariant mass M2 (W) as well.

Hence, from the cinematic point of view, we expect that distribution of M t2

has its maximum at the W invariant mass. But the dynamics may change the position of the upper bound: since this doesn’t happen, then the Mt

2 is a good discriminator for the invariant mass.

Padova, 7 ottobre 2008Autori: L. Brussich, I. Lazzizzera

GenMET Collection:

In CMSSW, MET is determined from the transverse vector sum over energy deposits in the calorimeters. Hence, it may also contain muons, since the muons deposit their energy above all in the Muon Chambers. The first “raw” correction we have done, was the vector subtraction of the transverse momenta of all muons from the MET.

Padova, 7 ottobre 2008Autori: L. Brussich, I. Lazzizzera

CaloMET Collection:

But the figures tell us that our correction gives a quite good agreement for GenMET (even if it could be improved!), while we can’t say the same for CaloMET.

As for the previous case, the same work was done for CaloMETCollection.

Padova, 7 ottobre 2008Autori: L. Brussich, I. Lazzizzera

MC vs Reco Muons:

The figure probes that CaloMET and GenMET Collection have different “ingredients” and hence they need different types of corrections (jets).

To understand if the difference between the Mt(W) distributions in the MC and Reco case is given by the use of HepgMuon vs RecoMuon, it is sufficient to observe the pt-muon distributions.

Padova, 7 ottobre 2008Autori: L. Brussich, I. Lazzizzera

Future improvement for muons:

To have high purity samples, same “quality” selections about muons will be done (cuts of pt over 100 GeV, isolation…)

Padova, 7 ottobre 2008Autori: L. Brussich, I. Lazzizzera

AIMS:

EW analysis in the channel W + jets with the first CMS data;

Work with MET, key element in the study of new physics (e.g. Dark Matter).