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/