Search for charged Higgs boson at CMS
Aruna Kumar Nayak
LIP, Lisbon
Outline
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A Nayak, 09/08/2011
Brief theoretical overview
Search strategies
Results
Summary
DHEP Seminar, TIFR, Mumbai.
1
The Standard Model
 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 ( ~L2, L 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 Unification
Electromagnetic, weak and Strong interactions do not
converge at certain energy scale
strong force
 Dark matter problem
SM particles only account for a small fraction of the
matter observed in the universe
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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 at
large 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.
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H+ in Top quark decay
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+→tn,
BR(H+→tn) = 1 (high tan)
Three search channels included
D0 Note 5715-CONF
1) Hadronic tau decay, hadronic W decay (t + jets) :
2) Hadronic tau decay, leptonic W decay (m + t) :
3) Leptonic tau decay, leptonic W decay (e + m) :
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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
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Hadronic Tau Identification
• 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
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Tau Identification Performance
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→ll sample
Efficiency measured (1) using Ζ → tt → μ + th with tag and probe,
(2) Simultaneous fit for tau id efficiency and Z→tt cross section
and (3) comparison of measured Z→tt cross section to measured Z→ee,mm cross
section at CMS
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CMS-PAS-TAU-11-001
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b-Jet Tagging
Track Counting b-Tagging Algorithm:
b-tag discriminator =
3-dimentional impact parameter
significance = IP/sIP
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
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Muon + Hadronic tau decay
1.09 fb-1 of data used
Main backgrounds :
, W+jets
g/q
g
 Event Selection :
Isolated single muon trigger (pT > 17 GeV/c)
t
t-jet
t
H+
b
b
nt
nt
t
nl
g/q
W
All physics objects are reconstructed using Particle Flow method with charged μ
hadrons originating from pileup vertices removed.
One isolated muon pT >20 GeV/c
At least 2 jets pT>30 GeV/c (AK5, particle flow), |h| < 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)
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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
c2/ndof < 10
Global & tracker muon
 p >20 GeV/c , |h| < 2.1
T


|d0| < 200 mm

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 Selection
Preselection : 1 muon + 3 jets
(2 jets pT > 30 GeV/c + 1 jet pT > 20 GeV/c)
 AK5 PFlow Jets
 p > 30 GeV/c, |h| < 2.4
T

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
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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, |h| < 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
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 t-fake background
Fake Rates as function of jet pt

Main background from “fake” tau jets
W+≥1jet
Data driven background estimation :
- Select jets in events with :
1 lepton + MET + 3 jets
+ ≥ 1 b-tagged jet
- Apply to every jet a
“jettau probability (pt,eta,jet Width)”
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QCD
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Jets in l+ETmiss+3Jets events
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
t-fake background
t-fakeon
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 t-fake background
Systematics

Uncertainty on tau ID efficiency : 7%
JES,JER and MET uncertainty evaluated as function of jet pT and h (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
Summary of event yields
after final selection
Background measured from data
Data agrees well with the SM expectations within the uncertainty
NO excess observed
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Fully Hadronic final state
1.08 fb-1 of data used

Main backgrounds:
QCD multi-jet, ttbar, W+jets

General selection strategy is to suppress
QCD multi-jet background below ttbar and
g/q
g
other backgrounds
 Event Selection :
g/q
Trigger: Single tau + ETmiss trigger

HLT_IsoPFTau35_Trk20_MET45 (340
pb-1),
t-jet
t
t
H+
nt
b
b
t
q
W-
HLT_IsoPFTau35_Trk20_MET60 (740
nt
pb-1)
q
HLT MET > 60 GeV required offline to have uniform thresholds

Require one t jet pT > 40 GeV/c, pT(leading particle)>20 GeV/c, |h| < 2.4
Take as t-jet candidates the jets matching to the trigger t jets
HPS, tight isolation, againstElectronMedium, againstMuonTight, one prong
ETmiss (particle flow) > 70 GeV
 At least 3 jets (AntiKt 0.5, particle flow), p > 30 GeV/c, |h| < 2.4
T
 At least one b-tagged jet, Track Counting High Efficiency Loose working point

(At least two tracks with 3D impact parameter significance > 1.7)
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Tau and ET
miss
Selection
Distributions after t  id, excluding tau isolation
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Jet selection
Number of selected jets (pT > 30
GeV/c, |eta| < 2.4)
after t  id, lepton veto and MET cut,
excluding tau isolation.
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Number of selected b-jets after t  id, lepton
veto, MET cut and jet number requirement,
excluding tau isolation. B-tagging scale factors
are applied.
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Fully Hadronic Event Yields
Event yields at each selection step
An excess of event yields expected in
the presence of charged Higgs boson
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Signal selection efficiency at each
selection step
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Background measurements
 QCD multi-jet background, measured from data
Method based on factorisation of ETmiss + b-tagging from other selections
Apply same selections like signal analysis but in different order.
Factorize e(ETmiss + b-tagging ) at a selection level where QCD contribution dominates.
e (ETmiss + b-tagging ) estimated in bins of t 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, |h| < 2.1) and 3 jets (pT > 30 GeV/c, |h| <
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)
Expected to be small, estimated from simulation
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Systematics
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Fully Hadronic Event Yields
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
t-fake
em on
final
statebackground
g/q
g/q
g
t
t
H+
t
b
b
W-
0.98 fb-1 of data used
e/μ
nl
nt nt
nl
μ/e
Tau decays leptonically
Main background:
 Event selection:
e-m trigger
one e (pT>20 GeV/c ), one m (pT>20 GeV/c )
At least 2 jets (pT>30 GeV/c)
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Deficit of total events expected in the
presence of charged Higgs boson,
because e/m from t decay become soft
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Uncertainties
on t-fake background
Systematics
Low systematic uncertainty compared to other two channels
b-tagging is not used
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Expected
BR
Uncertainties
onevents
t-fakevs
background
mth channel
em channel
x = BR(tbH+)
Ntt (in presence of H+) = NWH 2(1-x)x + NHH x2 + NttSM (1-x)2
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Statistical Interpretation
A frequentist approach (CLs) is being used to obtain upper limit on BR(t-> bH+)
Define a likelihood ratio Q :
“r” is signal strength modifier
Run 105 toy expts to generate -2lnQ distributions for
“background only” and “background + signal” hypothesis
Adjust “r” to get value of CLs = 0.05
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Uncertainties
Upper limitonont-fake
BR (t background
→ 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)
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= NttSUSY – NttSM = NWH 2(1-x)x + NHH x2 + NttSM ((1-x)2 – 1) , x = BR(t→H+b)
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Results from
Combination
Uncertainties
on t-fake
background
of three channels
combination of the fully hadronic, muon + tau and electron + muon channels
New World limit
Tevatron limit : 0.15 – 0.2
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exclusion region in MSSM (mhmax )
MH+-tan parameter space
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Summary
 The charged Higgs boson in the decay of Top quark is searched assuming
BR(H+→tn) = 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
D0 BR Limit (multi-channel)
+
BR(tbH )
CMS BR Limit ( single channel)
HIG-11-002
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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 level
Before
isolation
Simulate and reconstruct tau with same momentum as muon

Normalisation

Tau trigger efficiency

MET trigger efficiency with
After
isolation
”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)
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EWK and tt~ bkg measurement
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