SEARCH FOR BSM HIGGS BOSONS IN THE µ

A TL-PHYS-PROC-2012-255 23 November 2012

SEARCH FOR BSM HIGGS BOSONS IN THE µ

µ

DECAY CHANNEL

1

WITH THE ATLAS EXPERIMENT

2

SEBASTIAN STERN

3

†On behalf of the ATLAS collaboration.

4

Max-Planck-Institut f¨ ur Physik, Munich, Germany

5

Two searches for Higgs bosons in theories beyond the Standard Model (BSM) with the ATLAS experiment at the LHC are presented. The search for the neutral MSSM Higgs bosons decaying directly to two muons,

, was performed using

collisions recorded with the ATLAS detector in 2011 at a centre-of-mass energy of

= 7 TeV and corresponding to an integrated luminosity of 4.8 fb

. The search for the light CP-odd Higgs boson of the NMSSM,

, was performed on a data set with integrated luminosity of 35 pb

, recorded in 2010 at

s

= 7 TeV. In both searches, no significant excess of events was found in the data with respect to the Standard Model expectation. Exclusion limits at the 95 % confidence level were evaluated.

6

1 Introduction

7

Supersymmetry provides attractive solutions of several problems of the Standard Model of parti-

8

cle physics, like for instance the fine-tuning and hierarchy problems and the unification of gauge

9

couplings. Compared to the Standard Model Higgs mechanism [1–4], supersymmetric models

10

have an expanded Higgs sector with two separate complex scalar doublets which are needed

11

to generate couplings individually to up- and down-type fermions. Extending the Standard

12

Model with minimal modifications gives rise to the Minimal Supersymmetric Standard Model

13

(MSSM) [5, 6] which provides solutions for the problems mentioned above. In order to ensure

14

that the MSSM predictions are compatible with the results of the SM-like Higgs boson searches

15

at LEP [7], the µ-term must be fine-tuned in an unnatural way. In the Next-to-Minimal Super-

16

symmetric Standard Model (NMSSM) an additional complex scalar singlet is introduced which

17

solves this µ-term problem through the spontaneous symmetry breaking.

18

For large parts of the allowed parameter spaces in the MSSM and NMSSM couplings between

19

Higgs bosons and down-type fermions are enhanced leading to an increased branching fraction

20

of the µ

µ

decay. The direct decay into µ

µ

is experimentally interesting because of the

21

excellent muon identification and µ

µ

mass resolution of the ATLAS detector.

22

Two searches for MSSM and NMSSM Higgs bosons decaying to µ

µ

performed with the

23

ATLAS detector [8] at the LHC are presented. The search for the neutral MSSM Higgs bosons [9],

24

h/A/H → µ

µ

, was performed using pp collision with an integrated luminosity of 4.8 fb

25

recorded in 2011 with √

s = 7 TeV. The search for the light CP-odd Higgs boson of the NMSSM

26

[10], a

→ µ

µ

, was performed with 35 pb

of pp collisions recorded in 2010.

27

2 Higgs bosons in the MSSM and NMSSM

28

2.1 MSSM Higgs bosons

29

The Higgs sector in the MSSM comprises two scalar doublets which couple separately to the up-

30

and down-type fermions. This results in five physical Higgs bosons, two of which are electrically

31

charged (H

), two are neutral and CP-even (h, H) and one is neutral and CP-odd (A). At tree

32

level, their properties are determined by two free parameters, typically chosen to be the mass of

33

the CP-odd Higgs boson, m

, and the ratio of the vacuum expectation values of the two scalar

34

doublets, tan β. Especially for large values of tan β the couplings to the down-type fermions are

35

enhanced which increases the branching fraction for the neutral MSSM h/A/H → µ

µ

decay.

36

The analysis presented in Section 3 was performed within the m

benchmark scenario [11]

37

where the parameters of the model are chosen such that the mass of the light CP-even boson,

38

h, is maximised for given m

and tan β.

39

Two mechanisms for neutral MSSM Higgs boson production are dominant at the LHC.

40

First, b-quark associated production which can produce accompanying b jets in the final state

41

and second, the gluon fusion production process.

42

2.2 NMSSM Higgs bosons

43

Compared to the MSSM, the Higgs sector of the NMSSM is extended by a complex scalar

44

singlet leading to in total seven Higgs bosons. Two Higgs bosons are electrically charged (H

)

45

while three are neutral and CP-even (h

, h

, h

) and two are neutral and CP-odd (a

, a

). Of

46

particular experimental interest is the so-called Ideal Higgs Scenario where the mass of the

47

lightest CP-odd Higgs boson is below the threshold to produce B hadron pairs, m

< 2m

. In

48

this scenario decays to b ¯ b are suppressed and thus branching fractions of decays to τ

τ

and

49

µ

µ

are enhanced.

50

For the analysis presented in Section 4 the gluon fusion production of a

with direct decay

51

into µ

µ

pairs was considered.

52

3 The Search for the MSSM h/A/H → µ

µ

decay

53

The observable in the search for the h/A/H → µ

µ

decay is the µ

µ

invariant mass. The

54

signal signature is characterized by a pair of isolated muon tracks with high transverse mo-

55

menta and opposite electric charges. In addition, signal events have small missing transverse

56

momentum.

57

Standard Model background processes are in particular Z/γ

production in association with

58

jets and non-resonant production of t ¯ t, b ¯ b, W W and single top quarks. A large fraction of the

59

Z/γ

background can be rejected by requiring that the event contains at least one identified

60

b jet. The analysis is therefore performed separately in two independent event categories with

61

b-tagged and b-vetoed µ

µ

events. A hypothetical Higgs signal would rise up as a narrow

62

resonance on the high-mass tail of the Z boson invariant mass distribution superimposed on a

63

continuous mass distribution from the non-resonant backgrounds.

64

The invariant mass distributions for the signal and the total background are both parametrized

65

with analytic functions. The signal model is given by the sum of three Breit-Wigner functions

66

convolved with Gaussian distributions corresponding to the h, A and H bosons. Separate

67

parametrization is used for the two main production modes. Masses, widths and cross sec-

68

tions of the three Higgs bosons are determined by the m

scenario in dependence of m

and

69

tan β. The background model is constructed from a combination of Breit-Wigner and hyperbolic

70

functions convolved with a Gaussian distribution to account for the finite µ

µ

mass resolution.

71

The background estimate is obtained from sideband fits to the µ

µ

invariant mass distribution

72

where the signal window changes for each studied point in the m

− tan β plane.

73

No significant excess of data above the expected Standard Model background is observed.

74

Exclusion limits at the 95 % confidence level are set using the modified frequentist approach,

75

CL

[12]. The exclusion limits are shown in Figure 1(a) for the h/A/H production in the

76

MSSM m

scenario as a function of m

and tan β. In addition, limits are evaluated for the

77

production cross section times branching fraction of a generic scalar boson, φ, produced in either

78

gluon fusion or in association with b quarks, shown in Figure 1(b) as a function of its mass, m

.

79

[GeV]

mA

100 150 200 250 300

βtan

10 20 30 40 50 60

Preliminary ATLAS

channels µ µ

Ldt = 4.7 - 4.8 fb-1

∫

= 7 TeV s

>0 µ

max, mh

Observed CLs Expected CLs

σ

± 1 σ

± 2 LEP

[GeV]

mφ

100 150 200 250 300

) [pb]µµ→φ BR(×φσ95% CL upper limit on

10-2

10-1

1

Preliminary ATLAS

Ldt = 4.8 fb-1

∫

= 7 TeV, s channels

µ

µ Observed bb→φ CLs

φ

→ Expected bb

φ CLs

→ Observed gg

φ

→ Expected gg

φ

→ σ bb

± 1 φ

→ σ bb

± 2

Figure 1: Exclusion limits at the 95 % confidence level from

final states on (left) the MSSM

production as a function of

and tan

in the

scenario and (right) on the production cross section times

branching fraction of a generic scalar boson produced in gluon fusion or in association with

quarks [9].

4 The Search for the NMSSM a

→ µ

µ

decay

80

The search for the light CP-odd Higgs boson of the NMSSM, a

→ µ

µ

, looks for a narrow

81

resonance in the µ

µ

invariant mass distribution in the mass ranges 6 − 9 GeV and 11 − 12 GeV

82

around the Υ resonances. The mass region 9 − 11 GeV is not considered because of uncertainties

83

in the expected rate of the Υ production.

84

Signal candidate events are required to contain two muons with transverse momenta above

85

p

= 4 GeV. Oppositely charged dimuon pairs with invariant mass 4.5 < m

< 14 GeV and

86

a well reconstructed common vertex are selected. To increase the sensitivity to a hypothetical

87

a

→ µ

µ

signal events with muon pairs not originating from a single particle decay are

88

rejected by means of the multi-variate likelihood ratio method. The likelihood ratio, R, is

89

constructed from the goodness of the µ

µ

vertex fit and muon isolation variables. Probability

90

density functions used in R are derived from data using the sidebands in the µ

µ

invariant

91

mass distribution and from simulated CP-odd Higgs bosons with masses between 6 and 12 GeV.

92

As in the MSSM analysis the signal and background models are parametrized with analytic

93

functions. For the signal a double Gaussian distribution is chosen and the background is modeled

94

by a fourth-order polynomial describing the continuous background plus three double Gaussian

95

distributions accounting for the Υ resonances.

96

No statistically significant excess of data compared to the background expectation is found.

97

Exclusion limits on the production cross section times branching fraction of the process gg →

98

a

→ µ

µ

are evaluated at the 95 % confidence level using the PCL [13] approach. The

99

result is shown in Figure 2(a). Fluctuations in the observed limit, in particular for the excess

100

[GeV]

µ

mµ

7 8 9 10 11

[pb]µµ → 1 a→gg σ

0 200 400 600 800 1000 1200 1400

1600 ATLAS Preliminary = 7 TeV s

L dt = 39 pb-1

∫

Observed limit Expected limit

Figure 2: Exclusion limits at the 95 % confidence level on the gluon fusion production times branching fraction of the light CP-odd NMSSM Higgs boson decaying directly to

as a function of the

invariant mass [10].

at m

= 7 GeV, are consistent with statistical fluctuations taking into account the look-

101

elsewhere effect [14] which results in trial factors of 70 to 90. Within the Ideal Higgs scenario

102

of the NMSSM the obtained limit constrains regions with high tan β and small CP-odd Higgs

103

boson mixing angle, cos θ

≈ 1.

104

5 Summary

105

Searches for neutral MSSM Higgs bosons and the light CP-odd Higgs boson of the NMSSM

106

in direct µ

µ

decays at the ATLAS experiment were presented. The search for the neutral

107

MSSM Higgs bosons, h/A/H, used 4.8 fb

of pp collisions recorded in 2011 at a centre-of-mass

108

energy of √

s = 7 TeV. The search for the light CP-odd NMSSM Higgs boson was performed

109

on pp collisions recorded in 2010 at √ s = 7 TeV, corresponding to an integrated luminosity of

110

35 pb

. No significant excess of data over the Standard Model backgrounds have been observed

111

in either search. For the MSSM search exclusion limits at the 95 % confidence level have been

112

set on the MSSM parameter tan β as a function of the CP-odd Higgs boson mass, m

, and

113

on the production cross section times branching fraction for a generic boson produced in either

114

gluon fusion or association with b quarks. For the NMSSM search exclusion limits were set on

115

the production cross section times branching fraction of the process gg → a

→ µ

µ

. Detailed

116

discussions of the analyses can be found in the references to the two searches [9, 10].

117

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118

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