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Search for WR Bosons and Heavy Neutrinos of the Left-Right symmetric model in CMS

and first data in CMS

Mikhail Kirsanov (INR Moscow)

QUARKS-2010, Kolomna, 09 June 2010

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Analysis status.

CMS AN 2008/072 PAS EXO-08-006 (14 TeV) – approved

(Figures in this talk from this PAS) Previous studies at 14 TeV Last studies at 7 TeV CMS AN 2009/112 – being refereed

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Introduction

Heavy Neutrino: Predicted by many models, frequently

discussed: Left-Right Symmetric Model incorporates WR and Z’ and heavy right-handed

(usually Majorana) neutrino states Nl (l=e, μ, τ) that can be the partners of light neutrinos

light neutrino masses are generated via See-Saw mechanism

explains parity violation in weak interactions includes SM at ~1 TeV scale

in many SM extensions M(Nl) ~ 0.1- 1 TeV

Previous studies:S.N. Gninenko, M.M. Kirsanov, N.V. Krasnikov,

V.A. Matveev, CMS NOTE 2006/098 (2006)

Enhance Motivation to search for these

new particles at CMS!

SuperK'98 result: neutrinos are massive

But SM neutrino has no mass

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Model parameters

Masses: M(WR), M(Z’), M(Nl); l=e, μ, τ

Reactions: pp → Z’ → Nl + Nl + X pp → WR → l + Nl + X Nl → l + jet + jet

Signature: two high-Pt isolated leptons and two high-Pt jets

Current direct limits (by Tevatron experiments): M(WR) > 0.8 TeV M(Nl) > 0.3 TeV

Our reference points M(WR) = 1.5 TeV, M(Nl) = 600 GeV ; M(WR) = 1.2 TeV, M(Nl) = 500 GeV (LRRP1, LRRP2)• Simulation and reconstruction in CMSSW_3_1_5• Misalignment and miscalibration as expected for

int. luminosity of 0.1 fb-1

• Dielectron and dimuon events are studied

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Collision energies 14, 10, 7 TeV: cross section difference about factor 2

WR production cross sections at 14, 10, 7 TeV

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Leptons momenta: hardest lepton and second one

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Triggers and Datasets. The idea is to use single lepton triggers with as high threshold as possible

Electron channel:Signal efficiency >97%

Muon channel: Signal efficiency >97%. At 100 1/pb leptons are below 1 TeV: showers less important, so that triggers with isolation requirements can be used

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Lepton selection

Default leptons ID except isolation Pt cut 20 GeV Isolation cone radius 0.3 Isolation variable Econe/Pt Primary selection cut on isolation 0.2

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Lepton selection (2)

Isolation cuts: electrons 0.09, muons 0.05

These cuts remove QCD background at 10 and 7 TeV

Electrons selection efficiency for our signal (LRRP1) 74% (isolation 94%)

Muons selection efficiency 85% (isolation 95%)

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Jets

Cone 0.5, cut 40 GeV Default reconstruction Default energy corrections. E. g.

Gamma – jet calibration to be used for real data. Calibration accuracy 5 – 10 % at 100 1/pb

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Events selection: preliminary

At least 1 lepton and 1 jet (pT cut 20) At least 1 lepton with Pt > 42 GeV.

Probably will be used for the lumi > 100 1/pb. If we increase further this cut, the Z control sample drops rapidly

After this we should have about 3000 Z events for the efficiency control sample and events for the QCD fake sample

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Events selection: for signal (primary selection)

At least 2 leptons (of any flavour and sign) and 2 jets

At least 1 lepton with Pt > 42 GeV

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Primary selection efficiency, MW=2000

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Background reduction table, electrons

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Backgrouns, CSA09, muons

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Signal/background 100 1/pb, MW=1500 GeV, 14 TeV, electron channel

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Signal/background 100 1/pb, MW=1500 GeV, MN=600, electrons, 14 TeV

MW > 500 MW > 1000

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Fit

We use RooFit package for fitting Unbinned likelihood fit Model: (background) + (signal) P(MWc,MNc) = nsig*BW(MW,WW,MWc)* BW(r0,Wr,c) + nbg*exp(-k*MWc)*PBG(r)BW – Breit-Wigner functionPBG – BG histogram shapeMWc, MNc – inv. masses of WR and N candidatesr = MNc/MWc, r0 = MN/MWWe take the signal widths WW and Wr from the fitof the fully simulated signal

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Fit(3)

Free parameters of the fit: nsig, nbg, k Quasi-free parameters of the fit: WW, Wr (assume for the moment 2% for WW, Wr fixed) This means that about BG in the MWc

projection we only assume that it is a falling monotonous function of MWc

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Pseudo-experiments (toy MC)

Based on parameter from fit we do toy MC

1000 pseudo-experiments

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Significance BBS LLS /ln2

Electrons 7.53 Muons 5.5

LRRP2 (MWR=1200)

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BG control (2)Most important BG components:

tt events: electron – muon sample to control Z+jets events: sample with relaxed Mll cut

(80 GeV) to control: > 200 events with MW > 500 GeV. The shape can be different!

Events with fake leptons (W+jets, gamma+jets, QCD): Fake QCD sample and electron – muon sample with same sign to control

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BG control procedure

tt distribution shape: we compare 1D projections of the electron – muon sample with MC (6 events at 100 1/pb: rather big errors) and can use directly these corrections

Z+jets distribution shape: we compare the sample with relaxed Mll cut (80 GeV) with MC. The shape can be different, so probably we cannot use directly the corrections.

Events with fake leptons (W+jets, gamma+jets, QCD): check electron – muon sample with same sign and compare the number with all signs. If the factor is much different from 5, we introduce weight and eventually try to tighten the lepton selection cuts

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BG control procedure (2)

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BG control procedure (3)

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BG control procedure (4)

Events with fake leptons (W+jets, gamma+jets, QCD): Obtain fake QCD sample: lepton with a back-to-back jet and no no other loose lepton candidate in the event

Mostly events from QCD as shown by other PAG From this we obtain the probability of a jet to

produce a fake lepton p(ET) Take events with one lepton and jet(s), convert one

jet into lepton We obtain the shape of MWc distribution of BG with

fake leptons

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Efficiency from data

Needed to calculate sensitivity, limits etc., but also could change the shape of distributions

From the control Z sample Tag and probe or simply use

efficiency measured by POG Efficiency in presence of jets, in

particular isolation cuts efficiencies: additional checks

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Summary

WR and Heavy neutrino N of the minimal Left-Right symmetric model can be discovered at 100 1/pb for the masses of WR up to 1500 GeV and masses of N from 200 to 700 GeV in the electron channel

More narrow discovery region in the muon channel (up to ~1300 GeV)

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Summary (2). Discovery and limits plot electron channel, 14 TeV

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Towards the analysis of real data

The work presented here is the proof of concept and the estimation of the sensitivity. For the real data a few things are to be checked, a few methods are to be worked on in more details. For the next thesis

Check that QCD BG is small, if it is not small check its shape. Use fake sample (see above): check that it works

More studies of BG shape. Calculations predict something like (1/MWc)**a or (1-MWc/sqrt(s))**a * (1/MWc)**b. Are they different for tt, Zjets etc? Use FastSim

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Towards the analysis of real data (2)

Check that isolation cuts in data work as in MC. If not, work out ways of correcting the efficiencies

Various statistical checks and studies. What we see now is that the method of fit is more powerful than simple counting. But we must ensure that all possible real BG shapes are allowed by the fit. If we think that any funny BG shape is possible, even with bumps, then fit is useless

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Towards the analysis of real data (3)

Started to look at the data. 1.5 1/nb analysed. One Z candidate found (decay to electrons). It seems that nothing to do until we have a few tens of Z, just learn to read data

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CMS and LHC operation

By the end of May 18.7 1/nb delivered by LHC, 17 1/nb recorded: efficiency ~ 90%

Two last weekends luminosity was doubled both times and reached 2*1029

13 bunches now. Maximal possible number is 2808

Pictures below from D. Contardo, talk at IPRD10

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CMS and LHC operation(2)

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CMS and LHC operation(3)

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CMS and LHC operation(4)

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CMS and LHC operation(5)

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CMS and LHC operation