Outline Brief theoretical overview Search strategies Results Summary

Search for charged Higgs boson at CMS

Aruna Kumar Nayak

LIP, Lisbon

A Nayak, 09/08/2011 1DHEP Seminar, TIFR, Mumbai.

The Standard Model

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Standard Model is the most successful theory of particle physics

Most of the particles of SM have been discovered and their properties have been measured precisely

However, the Higgs boson (the fundamental ingredient of the model) is not yet discovered And there are also quite a few shortcomings.

Naturalness or fine-tunning problem The radiative correction to the Higgs mass squared diverges quadratically ( ~2, is cut of scale). In order to keep the Higgs mass in the range of electroweak symmetry breaking scale, an unnatural fine adjustment of parameters is needed.

Lack of Gauge Coupling UnificationElectromagnetic, weak and Strong interactions do not converge at certain energy scale

Dark matter problemSM particles only account for a small fraction of the matter observed in the universe

strong force

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Supersymmetry

SUSY postulates a new symmetry between bosons and fermions

For each SM fermionic spin-½ field there is bosonic spin-0 partner with exactly the same gauge quantum numbers. Similarly, each SM spin-1 field has a SUSY spin-½ partner.

Loops of particles and their supersymmetric partners have the ability to cancel the quadratic divergences in the Higgs field self-couplings hence solving the Naturalness problem.

SUSY foresees unification of couplings atlarge energy scales ~ 1015 GeV

Provides candidates for Dark Matter (LSP)

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MSSM Higgs Sector

The MSSM Higgs Sector

2 Higgs doublets, 5 physical bosons

3 neutral ( H and h with CP even; A with CP odd ) 2 charged (H±)

The observation of a charged Higgs boson will be an unambigous signature of new physics

Controlled by two parameters at tree level

tan(ratio between vacuum expectation values of the 2 Higgs fields) mA (mass of neutral CP-odd Higgs bosons)

Other parameters are important for radiative corrections

Here, we discuss only the search for a light charged Higgs boson in the decay of top quark.

H+ in Top quark decay

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CMS-PAS-HIG-11-008

Production and decay at tree level depends on MA and tan = v1/v2 (ratio of the v.e.vs)

Light charged Higgs (MH+ < Mtop) : Heavy charged Higgs (MH+ > Mtop) :

Search assumption : mH+ < mt, t→H+b, H+→,

BR(H+→) = 1 (high tan)

Three search channels included

1) Hadronic tau decay, hadronic W decay (+ jets) :

2) Hadronic tau decay, leptonic W decay ( + ) :

3) Leptonic tau decay, leptonic W decay (e + ) :

D0 Note 5715-CONF

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The Large Hadron Collider Superconducting magnets (NbTi) opperated at superfluid helium temp.(1.9K) and low vacuum (10-13 atm)

26.7 km pp (/heavy ion) collider located at swiss-french borders (CERN) close to Geneva reaching √s =7 TeV ( designed for 14 TeV )

Operating at instantaneous luminosity : L ~ 2 x10 33 cm-2s-1

Delivered integrated luminosity by 8th Aug : ~ 2.1 fb-1

Expected to go beyond 5 fb-1 in 2011

Analysis presented today with 2011 data Int. luminosity ~ 1 fb-1

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The Compact Muon Solenoid Detector

Hadronic Tau Identification

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• Tau decays To light leptons (e,μ) and 2 neutrinos with BR ~35% To hadrons(τh) and one neutrino with BR ~65 % Dominantly via π+/Κ+ ,ρ+ → π+π0 and a1 → π+π-π+(π+π0π0)

Particle Flow based Algorithm : ‐ Hadron + Strips (HPS) algorithm

• Decay mode Finding : Require there is a valid π,ρ,a1 combination (mass constraints on combination of particle candidates)• 3 possible isolation working points : Loose, medium, Tight depending on the cuts on particles in isolation cone

• Electron rejection : Low electron compatibility for leading charged hadron

• Muon rejection : Low muon compatibility for leading charged hadron

Tau Identification Performance

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Commissioned in data : Fake rates for jets in di-jet events, W + jets and muon enriched sample

Fake rates for electron/muons with tag & probe method using Z→ sample Efficiency measured (1) using Ζ → → μ + h with tag and probe, (2) Simultaneous fit for tau id efficiency and Z→ cross section and (3) comparison of measured Z→ cross section to measured Z→ee, cross section at CMS

CMS-PAS-TAU-11-001

b-Jet Tagging

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Track Counting b-Tagging Algorithm:

b-tag discriminator = 3-dimentional impact parameter significance = IP/IP

Two variants : High Efficiency : Two tracks with IP3D significance above a given threshold Discriminator : IP3D significance of 2nd track

High Purity : Three tracks with IP3D significance above a given threshold Discriminator : IP3D significance of 3rd track

Muon + Hadronic tau decay

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1.09 fb-1 of data used

Main backgrounds : , W+jets

Event Selection : Isolated single muon trigger (pT > 17 GeV/c) All physics objects are reconstructed using Particle Flow method with charged hadrons originating from pileup vertices removed.

One isolated muon pT >20 GeV/c

At least 2 jets pT>30 GeV/c (AK5, particle flow), || < 2.4

ET miss (particle flow) > 40 GeV

One tau pT >20 GeV/c

Opposite-Sign between muon and tau At least one b-jet , Track Counting High Efficiency Loose working point (At least two tracks with 3D impact parameter significance > 1.7)

bb

l

μW-

H+

t

tgg/q

g/q

-jet

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Muon Selection

Preselection : 1 muon + 3 jets(2 jets pT > 30 GeV/c + 1 jet pT > 20 GeV/c)

ID : Global muon prompt tight min tracker hits > 10, min muon hits > 1 2/ndof < 10 Global & tracker muon pT >20 GeV/c , || < 2.1

|d0| < 200 m

R (muon, jet) > 0.3

PF isolation cone 0.3 , rel.iso. < 0.2 Loose muon (electron) veto pT >10 GeV/c (15 GeV/c)

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Jet SelectionPreselection : 1 muon + 3 jets (2 jets pT > 30 GeV/c + 1 jet pT > 20 GeV/c) AK5 PFlow Jets pT > 30 GeV/c, || < 2.4

R (muon, jet) > 0.3

L1 FastJet + L2 (relative) +L3 (absolute) ,L2L3 Residual (data)

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MET and b-tagging

MET distribution for events with one isolated muon and at least three jets

B-tag jet multiplicity for events with one isolated muon+at least 3 jets +MET

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Tau Selections

Preselection : 1 muon + 3 jets + MET + ≥ 1 btag

pT > 20 GeV/c, || < 2.4

Hadron Plus Strips loose isolation

medium electron rejection, loose muon rejection

Not used in the selection

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Muon + Tau event yields

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Event yields at each selection step An excess of event yields expected in the presence of charged Higgs boson

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Data driven background estimation :

- Select jets in events with : 1 lepton + MET + 3 jets + ≥ 1 b-tagged jet

- Apply to every jet a“jettau probability (pt,eta,jet Width)”

Data driven -fake background

Main background from “fake” tau jets

Fake Rates as function of jet pt

W+≥1jet

QCD

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Jets in +ETmiss+3Jets events

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Jets that can fake taus in events with :

1 isolated muon pT>20 GeV/c

At least 2 jets with pT>30 GeV/c ( ≥ 1 b-tagged jet )

+ 1 jet with pT>20 GeV (If there is only one b-tagged jet, it is not counted. However if there are more than one b-tag jet in an event, all of them are counted)

MET > 40 GeV

Dominated by W+jets and tt~ -> l+jets events

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Uncertainties on -fake background -fake background fake rate applied in bins pT, eta, jet width

MVA method is used to apply tau fake probability to jets. k Nearest Neighbour (kNN) is trained with jets passing/failing the tau discriminators

closure test within uncertainty Contribution from real tau jets estimated from MC

The fake-tau background is estimated at “tau selection step”. In order to get the estimated fake-tau background at “OS selection step” an efficiency 0.66 +/- 0.04(estimated from MC) is applied.

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Uncertainties on -fake background Systematics Uncertainty on tau ID efficiency : 7%

JES,JER and MET uncertainty evaluated as function of jet pT and (includes unc. due to pileup, jet flavor b-jet, unclustered jet energy) Uncertainty on ttbar cross section : 20%

B-tagging/mis-tagging uncertainty : Data/MC scale factor for b-tagging efficiency and mis-tag rates used (CMS-PAS-BTV-11-001) to rescale MC and the corresponding uncertainty is propagated.

muon trigger & ID & Isolation: 1%

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Muon + Tau event yields

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Summary of event yields after final selection

Background measured from data

Data agrees well with the SM expectations within the uncertainty NO excess observed

Fully Hadronic final state

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Main backgrounds: QCD multi-jet, ttbar, W+jets General selection strategy is to suppress

QCD multi-jet background below ttbar and

other backgrounds

Event Selection : Trigger: Single tau + ET

miss trigger HLT_IsoPFTau35_Trk20_MET45 (340 pb-1), HLT_IsoPFTau35_Trk20_MET60 (740 pb-1) HLT MET > 60 GeV required offline to have uniform thresholds

Require one jet pT > 40 GeV/c, pT(leading particle)>20 GeV/c, || < 2.4

Take as -jet candidates the jets matching to the trigger jets HPS, tight isolation, againstElectronMedium, againstMuonTight, one prong ET

miss (particle flow) > 70 GeV At least 3 jets (AntiKt 0.5, particle flow), pT > 30 GeV/c, || < 2.4 At least one b-tagged jet, Track Counting High Efficiency Loose working point (At least two tracks with 3D impact parameter significance > 1.7)

1.08 fb-1 of data used

b

bq

qW-

H+

t

tgg/q

g/q

-jet

Tau and ETmiss Selection

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Distributions after id, excluding tau isolation

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Jet selection

Number of selected jets (pT > 30 GeV/c, |eta| < 2.4)

after id, lepton veto and MET cut, excluding tau isolation.

Number of selected b-jets after id, lepton veto, MET cut and jet number requirement, excluding tau isolation. B-tagging scale factors are applied.

Fully Hadronic Event Yields

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Event yields at each selection stepAn excess of event yields expected inthe presence of charged Higgs boson

Signal selection efficiency at each selection step

Background measurements

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QCD multi-jet background, measured from data Method based on factorisation of ET

miss + b-tagging from other selections

Apply same selections like signal analysis but in different order.

Factorize (ETmiss + b-tagging ) at a selection level where QCD contribution dominates.

(ETmiss + b-tagging ) estimated in bins of pT.

EWK and tt~ background (genuine taus within pT, η acceptance), measured from data

Based on tau embedding method

Select events with only one isolated lepton (pT > 40 Gev/c, || < 2.1) and 3 jets (pT > 30 GeV/c, || < 2.4). Replace the muon by a fully simulated and reconstructed tau with same momentum.

EWK and tt~ fake tau background (e/μ/jets mis-identified as taus, or genuine taus outside pT, η acceptance)

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Systematics

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Fully Hadronic Event Yields

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Summary of event yields after final selection

The major backgrounds are measured from data

Data agrees well with the SM expectations within the uncertainty NO excess observed

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Uncertainties on -fake background e final state

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0.98 fb-1 of data used

Deficit of total events expected in the presence of charged Higgs boson, because e/ from decay become soft

Tau decays leptonically Main background: Event selection: e- trigger one e pT>20 GeV/c ), one pT>20 GeV/c )

At least 2 jets (pT>30 GeV/c)

bb l

μ/eW-

H+

t

tgg/q

g/q

e/μl

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Uncertainties on -fake background Systematics

Low systematic uncertainty compared to other two channels

b-tagging is not used

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Uncertainties on -fake background Expected events vs BR

x = BR(tbH+)Ntt (in presence of H+) = NWH 2(1-x)x + NHH x2 + Ntt

SM (1-x)2

h channel e channel

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Statistical Interpretation

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A frequentist approach (CLs) is being used to obtain upper limit on BR(t-> bH+)

Define a likelihood ratio Q :

Run 105 toy expts to generate -2lnQ distributions for “background only” and “background + signal” hypothesis

Adjust “r” to get value of CLs = 0.05

“r” is signal strength modifier

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Uncertainties on -fake background Upper limit on BR (t → H+b)

fully hadronic muon+tau electron+muon

95 % CL upper limit on BR(t→H+b) using CLs method.

The signal is modelled as the excess (or deficit) of events yields in presence of H+

Nexcess (deficit) = NttSUSY – Ntt

SM = NWH 2(1-x)x + NHH x2 + NttSM ((1-x)2 – 1) , x = BR(t→H+b)

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Uncertainties on -fake background Results from Combination of three channels

combination of the fully hadronic, muon + tau and electron + muon channels

exclusion region in MSSM (mhmax )

MH+-tan parameter space

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Tevatron limit : 0.15 – 0.2

New World limit

Summary

The charged Higgs boson in the decay of Top quark is searched assuming

BR(H+→) = 1

Three search channels have been included : tau + jets, muon + tau and

electron + muon

Major backgrounds have been measured from data

No significant excess/deficit of events have been observed

Upper limit on BR(t→H+b) = 0.04 – 0.05 (new world limit) depending on H+

mass

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Thank You

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Previous limits on BR(tbH+)

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HIG-11-002

D0 BR Limit (multi-channel) CMS BR Limit ( single channel)

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QCD multi-jet background

Sample purity 60-90%

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EWK and tt~ tau background (genuine taus) data-driven

Control sample selection One muon, pT > 40 GeV/c, |η|<2.1

•Isolation by requiring no HPSTight-quality PFCandidates in 0.1 < ΔR < 0.4

Veto of isolated electrons and other muons, pT > 15 GeV/c At least 3 PF jets, pT > 30 GeV/c, |η| < 2.4

Tau embedding at PF levelSimulate and reconstruct tau with same momentum as muon Normalisation Tau trigger efficiency MET trigger efficiency with ”vector sum caloMET” > 60 GeV Muon trigger and ID efficiency with Tag and Probe

Result: 71 ± 5 (stat) ± 15 (syst)MC expectation: 78 ± 7 (stat)

Beforeisolation

Afterisolation

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EWK and tt~ bkg measurement

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