Search for Charged Higgs Bosons in ATLASDaniel Pelikan on behalf of the ATLAS Collaboration
Uppsala University
Motivation
In several theories beyond the Standard Model (SM), including the MinimalSupersymmetric Model (MSSM), two doublets of scalar fields, and thereby five physicalstates, are predicted in the Higgs sector. Two of these are charged (H+ and H−) and,among the three neutral Higgs bosons, two are CP-even states, h0 and H0, and one isCP-odd, A0. In the MSSM, a light H+ decays primarily to cs̄, bb̄W + and τ+ν, dependingon tan β and mH+. For tan β > 3, B(H+ → τν) exceeds 90%.The aim of the analyses presented here is to search for the charged Higgs boson in tt̄decays with a hadronically or leptonically decaying τ lepton in the final state. 1.03 fb−1 ofATLAS data have been analysed.
Single- and Di-Lepton Final States
Single-Lepton
Event selection:I 1 trigger-matched leptonI at least 4 jetsI 2 b jetsI Emiss
T
The assignment of the b jetsis done by minimising:
χ2 =(Mjjb −Mtop)2
σ2top+
(Mjj −MW )2
σ2W
*θcos1 0.5 0 0.5 1
Eve
nts
/ 0
.2
0
200
400
600
800
1000
1200
1400
*θcos1 0.5 0 0.5 1
Eve
nts
/ 0
.2
0
200
400
600
800
1000
1200
1400
ATLAS Preliminary
= 130 GeV+H
m
) = 10%+
bH→Br(t
1Ldt = 1.03 fb∫
)+
(with Htt
)
W+
(WttSingle top
Z + jets
W + jets
Diboson
QCD
Data 2011
Figure 1: Single lepton cos θ∗ldistribution.
[GeV]HTm
20 40 60 80 100 120 140 160 180
Eve
nts
/ 1
0 G
eV
0
20
40
60
80
100
120
[GeV]HTm
20 40 60 80 100 120 140 160 180
Eve
nts
/ 1
0 G
eV
0
20
40
60
80
100
120
ATLAS Preliminary
= 130 GeV+H
m
) = 10%+
bH→Br(t
1Ldt = 1.03 fb∫
)+
(with Htt
)
W+
(WttSingle top
Z + jets
W + jets
Diboson
QCD
Data 2011
Figure 2: mHT distribution for
events with cos θ∗l < −0.6and mW
T < 60 GeV.
I cos θ∗l =2m2
bl
m2top−m2
W− 1 ' 4 pb·pl
m2top−m2
W− 1
with pb and pl being the 4-momenta of the b quark and charged lepton l (electron ormuon) and mbl being their invariant mass.
I Novel transverse mass: (mHT )2 =
(√m2
top + (~plT + ~pbT + ~pmissT )2 − pbT
)2−(~plT + ~pmiss
T
)2
Table 1: Expected and observed number of events with∫Ldt = 1.03 fb−1, a fitted value of
165.1 pb is used for σbbWW .
Di-Lepton
Event selection:I 2 oppositely charged
leptons with at least onetrigger-match
I at least 2 jetsI 2 b jets
I EmissT cut
side+
*, Hθcos1 0.5 0 0.5 1
Eve
nts
/ 0
.2
0
100
200
300
400
500
600
side+
*, Hθcos1 0.5 0 0.5 1
Eve
nts
/ 0
.2
0
100
200
300
400
500
600
ATLAS Preliminary
= 130 GeV+H
m
) = 10%+
bH→Br(t
1Ldt = 1.03 fb∫
)+
(with Htt
)
W+
(WttSingle top
Z + jets
Diboson
QCD & W
Data 2011
Figure 3: cos θ∗l distributionon the “H+ side”.
[GeV]HT2m
40 60 80 100 120 140 160 180 200
Eve
nts
/ 1
0 G
eV
0
50
100
150
200
250
[GeV]HT2m
40 60 80 100 120 140 160 180 200
Eve
nts
/ 1
0 G
eV
0
50
100
150
200
250
ATLAS Preliminary
= 130 GeV+H
m
) = 10%+
bH→Br(t
1Ldt = 1.03 fb∫
)+
(with Htt
)
W+
(WttSingle top
Z + jets
Diboson
QCD & W
Data 2011
Figure 4: MHT2 distribution for
events with cos θ∗l < −0.6.
mHT2 is computed by a numerical maximisation of: mH
T2 = max{constraints}
[mHT (~pH+
T )]
with the definition: (mHT (~pH+
T ))2 =(√
m2top + (~pH+
T + ~p bT )2 − pbT
)2
−(~pH+
T
)2.
I Assign the leptons and the b jets:. check cos θ∗l for the two pairs and remove unphysical pairings (cos θ∗l > 1). if both pairings are correct: take the smallest sum of ∆R(l , b)pair 1 + ∆R(l , b)pair 2
. assign the smallest cos θ∗l to the “H+ side” and the other to the “W side”
Table 2: Expected and observed number of events with∫Ldt = 1.03 fb−1, a fitted value of
150.4 pb is used for σbbWW .
Limits on the Branching Ratio of t → bH+
Assuming B(H+ → τν) = 1, upper limits are extracted on the branching ratio B(t → bH+),as a function of the charged Higgs boson mass, using a profile likelihood statistical analysis.
[GeV]+H
m
90 100 110 120 130 140 150 160
+ b
H→
t 9
5%
C.L
. u
pp
er
bo
un
d o
n B
r
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Observed CLsExpected
σ 1±σ 2±
ATLAS Preliminary = 7 TeVsData 2011
1Ldt = 1.03 fb∫
Figure 7: Expected and observedcombined upper limits with 95% C.L. as
a function of mH+.
[GeV]+H
m
90 100 110 120 130 140
βta
n
0
10
20
30
40
50
60
Expected Limit
σ 1±Expected
σ 2±Expected
Observed Limit
σ 1±Observed,
theor. uncertainties
1Ldt = 1.03 fb∫Data 2011
maxhm
ATLAS Preliminary
Figure 8: Combined limits for H+
production in the mH+-tan β plane(mmax
H scenario).
The τ+jets Final State
Multi-jet background estimation and jet-to-τmisidentification probability
The main sources of background in charged Higgs
boson searches are coming from processes as:I tt̄ productionI multi-jet eventsI single top quark
I W+jets
All are determined with data-driven methods.
Jet→ τ misidentification probability is measured fromγ-jet events for jets with 1 or 3 associated tracks as afunction of pT and η.
[GeV]τT
p
20 30 40 50 60 70 80 90 100
Jet m
isid
entification p
robabili
ty
310
210
110
1
10
1 trackτ
3 tracksτ
1Data 2011 L dt = 1.03 fb∫
ATLAS Preliminary
Figure 9: Jet → τ misidentificationprobability.
The multi-jet event estimate is determined by a fit to the EmissT distribution in data after all
selection cuts using two shapes (one for the multi-jet model, and one for all otherbackground processes, dominated by tt̄ and W+jets).
I To study this shape in a data-driven way, acontrol region is defined where the τ identificationand b-tagging requirements are inverted.
I The multi-jet event fraction estimated after all
selection cuts is (23± 10)%.
[GeV]missTE
0 50 100 150 200 250 300
Eve
nts
/ 3
0 G
eV
0
5
10
15
20
25
30
35
Multijet
Other MC
Data 2011
1 L dt = 1.03 fb∫
PreliminaryATLAS
Figure 10: Multi-jet QCD estimation.
The embedding methodTo estimate the background from true τ jets an embedding method is used, which consists incollecting a control sample of tt̄, single-top, and W+jets events with a muon in data, andreplacing the detector signature of this muon with that of a simulated τ lepton.
[GeV]miss
TE
0 50 100 150 200 250
Arb
itra
ry u
nits
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Embedded data 2011
MC
1 L dt=1.03 fb∫
ATLAS Preliminary
Figure 11: Validation of the embeddingmethod.
I The hybrid event is reconstructed and used toestimate the background to the H+ selection.
I With the embedding method it is possible toinclude the underlying event and pile-up, missingenergy, b quark jets and light-quark jets fromdata.
Results and limits
The transverse mass mT distribution of collision data,and the estimates from data-driven methods arecompared.mT is defined as:
mT =√
2pτTEmissT (1− cos ∆φ).
The expected and observed 95% C.L. exclusion limitfor charged Higgs boson production from top quarkdecays as a function of mH+ is determined.
[GeV]Tm
0 50 100 150 200 250 300
Events
/ 2
0 G
eV
0
5
10
15
20
25Data 2011
misidτ→e
misidτ→Jet
τTrue
Multi−jets
(130), B=0.1+
H
+background+
H
1 L dt=1.03 fb∫
ATLAS Preliminary
Figure 12: mT distribution after eventselection.
Table 3: Expected and observed number of events with∫Ldt = 1.03 fb−1.
[GeV]+H
m
90 100 110 120 130 140 150 160
)τν
→+
H B
(×
) b
+H
→ t95
% C
L o
n B
(
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
0.22
Expected Limit
σ 1±Expected
σ 2±Expected
Observed Limit
D0 Observed
1 L dt= 1.03 fb∫Data 2011
ATLAS Preliminary
Figure 13: Expected and observed limits with95% C.L. as a function of mH+.
[GeV]+H
m
90 100 110 120 130 140 150 160
βta
n
0
10
20
30
40
50
60Expected Limit
σ 1±Expected
σ 2±Expected
Observed Limit
σ 1 ±Observed,
theor. uncertainties
1 L dt= 1.03 fb∫Data 2011
maxhm
ATLAS Preliminary
Figure 14: Limits for H+ production in themH+-tan β plane (mmax
H scenario).
[1] Search for a charged Higgs boson H+ → τlep + ν in tt̄ events with one or two leptons, using1.03 fb−1 of pp collision data recorded at
√s = 7 TeV with the ATLAS detector,
ATLAS-CONF-2011-151[2] Search for Charged Higgs Bosons in the τ+jets Final State in tt̄ Decays with 1.03 fb−1 of ppCollision Data Recorded at
√s = 7 TeV with the ATLAS Experiment, ATLAS-CONF-2011-138
Hadron Collider Physics Symposium, Nov 14 - 18, 2011 Paris, France http://atlas.ch/
