Search for Heavy Majorana Neutrino and WR Boson on CMS

Sergei Gninenko (INR), Anton Karneyeu (INP, LAPP), Mikhail Kirsanov (INR), Nikolai Krasnikov (INR), Ivan Mologin (INR), Alexander Toropin (INR),Viacheslav Duk (INR), Lyubov Menshikh (INR)

CERN12 March 2009

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Analysis status

• CMS AN 2008/072• PAS EXO-08-006• Analysis page:

https://twiki.cern.ch/twiki/bin/view/CMS/ExoticaWRHeavyNu

• Analysis approved on 22 of October• Presented at PANIC08 by M. Kirsanov (CMS CR

2009/018)• CMS discovery potential in terms of WR – heavy

neutrino masses at the LHC collision energies of 6 and 10 TeV have been studied on request of the CMS Exotica group (CMS AN 2009/007). The results were shown at the Les Houches LHC performance Workshop in February

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Introduction

Heavy Majorana Neutrino:• Predicted by many models, frequently discussed:

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

Majorana neutrino states Nl (l=e, μ, τ), which 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) > 0.7 TeV• M(Nl) > 0.3 TeV

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Samples

Signal:• LRRP1: M(WR) = 2 TeV, M(Nl) = 500 GeV

• LRRP2: M(WR) = 1.5 TeV, M(Nl) = 600 GeV

• LRRP3: M(WR) = 1.2 TeV, M(Nl) = 500 GeV

Background (CSA07):• ttbar, Z+jets, W+jets, γ+jets, QCD, WW+jets, WZ+jets

Conditions:• Simulation in CMSSW_1_4_12, reconstruction in

CMSSW_1_6_12• 100 pb-1 misalignment and miscalibration• Dielectron and dimuon events are studied

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Events selection

• At least 2 leptons and 2 jets

Leptons:

One lepton with pT > 20 GeVOne lepton with pT > 80 GeV

Isolation: in a cone 0.3

Total efficiency: 79% (e), 88% (μ)

Jets:

Jet energy correctionspT > 40 GeV

• Additional cuts:Mll > 200 GeVMWR > 600 GeV

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Backgrounds:

Main: ttbar + jets, Z + jets.Last column -- heavy flavours

PAS

electron channel:

muon channel:

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Signal/background100 pb-1, MWR = 1500 GeV, MN = 600 GeV

Electron channel Muon channel

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Significance calculation

• We use RooFit package for fitting• Unbinned likelihood fit• Model, idea: (signal) + (background)

BW – Breit-Wigner functionPBG – non-parametric (KEYS algorithm)

All parameters of the fit are free

),(

),,(),,(),( 2211

NW

NWNW

MMPBGnbkg

MMBWMMBWnsigMMP

R

RR

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Discovery potential (14 TeV)

Integrated luminosity 100 pb-1

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WR discovery potential

The dependence of the maximum MWR 5σ discovery reach in left-right symmetric models with 100 pb-1 integrated luminosity (CMS AN 2009/007).

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Summary

• CMS potential for discovery of heavy right-handed neutrino and WR boson is studied

• Good separation of the signal from SM background processes can be achieved due to resonance nature of signal

• For the integrated luminosity of 100 pb-1 particles can be observed with masses up to MWR = 2100 GeV and MN = 1200 GeV

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Plans

• Study e-mu channel• Process CSA08 backgrounds (thus moving to 10 TeV

collision energy) and new signal samples• Move to CMSSW_2_2_6• Start to use PAT• Improve significance and limits calculations:

- Estimate the effect of systematics on the limits- Breit-Wigner for signal in Nl projection too narrow?- Move to median values in the discovery plot

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Backup slides

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Triggers

• Trigger menu for luminosity L = 1032 cm-2 sec-1

• Electron channel:HLT1EMHighEt, HLT1EMVeryHighEtSignal efficiency 99%, rate 0.6 Hz

• Muon channel:HLT1MuonIsoSignal efficiency 93%, rate 18 Hz (very high, would like to select events with trigger candidate Pt cut 80 GeV: no efficiency drop)

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Backgrouns. Studied on CSA07 and private samples Main one tt + jets, then Z + jets

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Remarks about same sign background

Composition of background, 1500 GeV, wide W mass window:

all: 1.892

chowder: 0.7685

gumbo Gamma+jets: 0.675

Stew bbe e-enriched: 0.195

diboson WZ 0.111 (physical)

diboson WW 0.136

The measurements will not be based on the same sign signature because

we lose half of signal events and significance drops. This is only a

cross-check if we see a signal. So, just absolute data correction from the

same sign events in the Z peak could be sufficient.

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Signal/background: same sign leptons100 pb-1, MWR = 1500 GeV, MN = 600 GeV

Electron channel

Muon channel

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Signal/background100 pb-1, MWR = 1200 GeV, MN = 500 GeV

Electron channel

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BG control

• We should not be sensitive to the wrong prediction of the total number of BG events

• But still it would be good to find some signal-free region and look at it

• One of them – W masses from 500 to 800 GeV: signal excluded. Will be automatically taken into account in the fits.

• At the first stage can consider electron – muon channel as signal-free (assuming that the probability of strong mixing between heavy neutrinos is small)

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BG control (2)Most important BG components:

• tt events: electron – muon sample to controle-mu: 45 eventse-mu same-sign: 9 events

• 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): 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 (40 events: 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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Fit results(μμ channel, MWR = 1500 GeV/c2)

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Pseudo-experiments (toy MC)

• Based on parameter from fit• Only nsig and nbkg parameters are free• 2000 pseudo-experiments per Reference Point

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Significance distribution: BBS LLS /ln2