Higgs Decays to Unstable Neutrinos: Collider Constraints from Inclusive Like-Sign Dilepton Searches

Abstract

We study the pair production of fourth generation neutrinos from the decay of
an on-shell Higgs produced by gluon fusion. In a fourth generation scenario,
the Higgs branching fraction into fourth generation neutrinos may be quite
large. In the case that the unstable heavy neutrinos are a mixed Majorana and
Dirac state, neutrinos pair-produced from Higgs decay will yield a substantial
number of like-sign dilepton events. In this article we use inclusive like-sign
dilepton searches from hadron colliders to constrain the theoretical parameter
space of fourth generation leptons.

Full text

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arXiv:1107.2123v1 [hep-ph] 11 Jul 2011
UCI-TR-2011-13
July, 2011
Higgs Decays to Unstable Neutrinos:
Collider Constraints from Inclusive Like-Sign Dilepton Searches
Linda M. Carpenter1and Daniel Whiteson1
1Department of Physics and Astronomy
University of California, Irvine, CA 92697
We study the pair production of fourth generation neutrinos from the decay of an on-shell Higgs
produced by gluon fusion. In a fourth generation scenario, the Higgs branching fraction into fourth
generation neutrinos may be quite large. In the case that the unstable heavy neutrinos are a mixed
Majorana and Dirac state, neutrinos pair-produced from Higgs decay will yield a substantial number
of like-sign dilepton events. In this article we use inclusive like-sign dilepton searches from hadron
colliders to constrain the theoretical parameter space of fourth generation leptons.
PACS numbers:
I.INTRODUCTION
A fourth generation is among the simplest possibilities for new physics at the weak scale. If a fourth generation
exists, recent work has shown that bounds on unstable fourth generation leptons may be well under 100 GeV, making
the leptons the lightest new states. Heavy unstable fourth generation neutrinos would decay through the process
N1 → ℓW. Fourth generation neutrinos may be light, LEP placed mass bounds under 100 GeV [1] [2]. Most
generally, fourth generation lepton sectors may have neutrinos with mixed Dirac and Majorana masses and, in this
case, mass bounds on the lightest neutrinos may be 62.1, 79.9 or 81.8 GeV, depending on whether the final state
lepton is τ, µ, or e [3].
If a fourth generation exists, Higgs physics may provide a powerful handle for constraining these models. The
dominant Higgs production mode at both Tevatron and LHC is gluon fusion, gg → h. This process proceeds through
a heavy-quark loop and is substantially enhanced by the presence of fourth generation quarks. The enhancement
is largely independent of quark mass and gives an increase in Higgs production of approximately a factor of eight
compared to production in the Standard Model, see for example [4] [5].
The presence of fourth generation particles also greatly effects the Higgs branching fractions. For SM Higgs masses
up to 160 GeV, the Higgs decay width is dominated by decay to bottom quarks; however, this proceeds through a
Yukawa coupling that is not extremely large. If other heavier states exist in the theory, they may easily dominate
and require us to look for the Higgs in non-standard channels. For example, many recent ’hidden Higgs’ scenarios
yield highly altered Higgs branching fractions and a variety of standard Higgs final states, for example see [6]-[10].
A fourth-generation lepton sector also offers new channels for Higgs decays. Fourth generation neutrinos with both
Dirac and Majorana masses will have significant couplings to the Higgs depending on the Dirac mass component. In
fact, as long as the Dirac mass parameter is larger than the bottom mass, the Higgs decay width to neutrinos will
dominate that of bottoms for any sufficiently heavy Higgs mass. In the window of Higgs masses between roughly 120
and 160 GeV, the dominant branching fraction of the Higgs may be to heavy neutrinos. For larger values of the Higgs
mass, where the decay channel into on-shell electro-weak gauge bosons is open, the branching fraction into neutrinos
is still significant, remaining above 10 percent for Higgs masses up to 200 GeV and above one percent up to 500 GeV.
In principle fourth generation neutrinos may be stable or unstable, leading to very different Higgs decay signatures.
Stable Majorana neutrinos may be quite light, under 50 GeV [11] [12] [13] [14], and the possibility of light stable
neutrinos as a Higgs decay channel has been recently explored [15] [16]. Unstable Majorana neutrinos may decay into
a standard model charged lepton and a W boson, N1→ Wℓ. If this decay is flavor-democratic, or is dominated by
τ decays, the heavy neutrino may be as light as 61.2 GeV. Since a Majorana neutrino may decay into a SM lepton

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of either sign, any process which pair-produces these fourth generation neutrinos produces many like-sign dileptons
[17] [18] . As like-sign dileptons are a clear, low-background signature at hadron colliders, the possibility of Higgs
decays in this channel is quite interesting. Previous works have proposed searches for heavy neutrino pairs produced
from a Z and which decay into like-sign dileptons. In this case, the signal of like-sign dileptons plus jets become an
interesting new channel in which to look for the Higgs. If the Higgs production as well as the branching fraction to
fourth generation neutrinos are large enough, a simple inclusive like-sign dileptons search may be used to constrain
large parts of fourth generation parameter space - this possibility is the focus of this paper. We will use published
inclusive same-sign dilepton analyses from LHC and the Tevatron to place constraints on various fourth generation
lepton scenarios.
This paper is organized as follows: Section 2 reviews fourth generation neutrino masses and couplings, Section
3 discusses Higgs production and branching fractions in fourth generations scenarios, Section 4 analyses inclusive
like-sign dilepton searches at hadron colliders, and Section 5 concludes.
II.REVIEW OF FOURTH GENERATION NEUTRINO MASSES
In the most general case, fourth generation neutrinos may have both a Dirac and a Majorana mass. Note that here
we use the notation of [19], where the Lagrangian is
L = mDL4NR+ MNN2+mD
v
HLiNR
The mass matrix is given by,
Lm= −1
2(Qc
RNc
R)
?
0mD
Mmd
??QR
NR
?
+ h.c.(1)
where ψc= −iγ2ψ∗. There are then two Majorana neutrinos with different mass eigenvalues:
M1= −(M/2) +
?
?
m2
D+ M2/4
M2= (M/2) +
m2
D+ M2/4
The mass eigenstates can be expressed in terms of the left-right eigenstates
N1= cosθQc
N2= −isinθQc
L+ sinθNR+ cosθQL+ sinθNc
L+ icosθNR+ isinθQL− icosθNc
R
R
where we have defined the mixing angle
tanθ = m1/mD
The Higgs couples to the neutrino mass eigenstates with coupling proportional to powers of the neutrino mixing
angle. The Higgs coupling to the lightest neutrino pair is given by
mD
vh
HN1N1sin(2θ).
The neutrino mixing angle varies between π/4 for pure Dirac-type neutrinos and π/2 for Majorana-type. We see that
in the limit where the neutrinos are pure Majorana, mD approaches zero, the mixing angle θ approaches π/2, and
the Higgs decouples from the neutrinos as required, while for pure Dirac states the coupling is maximal.
III.HIGGS PRODUCTION AND BRANCHING FRACTIONS
At hadron colliders, the largest Higgs production rate comes from gluon fusion. The Higgs-gluon coupling is
generated by loops of the heavy quarks. The gg → h production cross section is given by

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200300400500
10
100
50
20
200
30
15
150
70
mH (GeV)
σ (pb)
FIG. 1: Dependence of gg → h production on Higgs mass, including effects of fourth generation quarks, at LHC with√s = 7
TeV.
100500200300150
1.000
0.500
0.100
0.050
0.010
0.005
mH (GeV)
b.f.
WW
ZZ
NN
gg
bb
_
tt
_
FIG. 2: Dependence of Higgs branching fractions on mass for the benchmark point mn1= 63GeV , mn2= 300GeV .
σ(p¯ p → h) =
1
16Γ(h → gg)16π2
smh
?1
m2
h/s
dx
xg(x)g(m2
h
sx),
where Γ(h → gg) is the Higgs to gluon decay width
Γ(h → gg) =αsGFm3
h
16√2π3Σi(τi(1 + (1 − τi)f(τi)))
with
τi=
4m2
m2
fi
h
,f(τi) = (sin−1?
1/τi)2
where the index i runs over heavy quark flavors. Due to their large Yukawa couplings, the additional of fourth
generation quarks enhances the h → gg decay width substantially; the resulting decay width is largely independent
of the new heavy quark mass, and larger than the Standard Model prediction by about a factor of 8. The gg → h
production cross-section in a fourth generation scenario for LHC is shown in Figure 1; note that the Higgs production
remains above a picobarn for Higgs masses up to 500 GeV.
For Higgs masses below twice the W or Z masses, the most important standard model Higgs branching fractions
are b quarks and gluons. This changes significantly with the addition of a fourth generation. The Higgs decay width
into a Dirac particle is given by
ΓD=mhy2
D
8π
(1 −4m2
d
m2
h
)3/2

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The branching ratio is proportional to the square of the Yukawa coupling. We see that the bottom, which normally
dominates the Higgs decay for low masses, does not have a particularly large Yukawa coupling. A fourth generation
neutrino with substantial Dirac component may easily overtake the bottom decay mode, see Figure 2.
The ratio of h → N1N1to h → bb is
Γ(h → n1n1)
Γ(h → bb)
=m2
n1sin(2θ)
m2
b
(1 −4m2
(1 −
n1
mh2)3/2
4m2
b
mh2)3/2
Notice, however, that the coupling is proportional to the mixing angle. Only when the neutrino is in the deep
Majorana limit where θ → π/2 does the Higgs branching fraction to neutrinos drop substantially. Figure 3 shows
the effect of mixing angle on Higgs branching fraction. These are contour plots over the N1,N2mass plane for two
benchmark Higgs mass values. The branching fraction drops as one increases N2mass and enters the Majorana region.
020406080100
0
500
1000
1500
2000
2500
3000
.6
.1
.2
.3
.4
.5
mN (GeV)
2
mN (GeV)
1
050100150200250
0
500
1000
1500
2000
2500
3000
mN (GeV)
1
mN (GeV)
2
.06
.05
.04
.03
.02
.01
FIG. 3: Higgs branching fraction h → N1N1 Higgs mass values of 150 GeV (left) and 400 GeV (right).
In the Standard Model, the next largest branching fraction comes from Higgs decaying to gluons. The decay width
of Higgs to gluons is given above. With the addition of a fourth generation, the branching fraction of Higgs to gluons
is substantially larger than it is in the Standard Model and overtakes that of h → bb for low masses. However, in the
region just above twice the mass of the lightest fourth generatio neutrino, h → N1N1is the dominant decay mode, see
Figure 2. Once the Higgs becomes large enough to decay into a pair of on-shell electroweak gauge bosons, this mode
dominates the Higgs branching fraction. However, the decay width into fourth generation neutrinos remains larger
than all other channels. The Higgs branching fraction to neutrinos remains appreciable, above 10 percent, even for
Higgs masses out to 200 GeV and above a percent up to Higgs masses of 500 GeV.
In the heavy Higgs region the most sensitive search channel is the h → ZZ mode. However, to suppress the multi-
jet background at least one leptonic decaying Z is required, which significantly reduces the cross-section due to the
small Z → ℓℓ branching ratio. If the Higgs decay to fourth-generation neutrinos is very distinctive, it may remain an
important channel for high mass Higgs searches.
IV.COLLIDER BOUNDS
A.LHC
We consider Higgs production from gluon fusion and decay to a pair of light fourth generation neutrinos, where the
lightest neutrino is unstable and decays via the process N1→ Wℓ, giving
gg → h → N1N1→ WWℓℓ → ℓℓ + qq′+ qq′
The N1→ Wℓ decay rate is determined by the values of a four-generation lepton mixing matrix. In the simple case
that we consider here, we allow only a single decay rate to be non-zero, so that the fourth generation neutrino decays
entirely to a single flavor of lepton.

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200250300350
mh
400450500
100
120
140
160
180
mN
acc >.5
.1< acc <.2
acc <.1
.3< acc <.4
FIG. 4: Plot of ATLAS fiducial region acceptance (defined in text) in the mh, N1 mass-plane.
Because the N1state is Majorana, it may decay to a final state lepton of either sign with equal probability: thus
the decay results in like-sign dileptons half of the time. Therefore, there is a significant rate of Higgs production with
decay into states with high pT like-sign dileptons, a low-background signature.
ATLAS has completed an inclusive like-sign dilepton search which looks for like sign muons of high invariant mass
in 34 pb−1of data. Even with a small data set, ATLAS has set limits on the cross-section of like-sign dimuon events
at < 170 pb in a simple fiducial region. We calculate the predicted cross-section in the ATLAS fiducial region for
fourth-generation Higgs decays to to constrain fourth generation parameter space with the inclusive like sign dilepton
data. Higgs events were generated using MADGRAPH [20] decayed with BRIDGE [21] and showered with PYTHIA
[22]. The fiducial region requires two like-sign muons with
• pT>20 GeV and ηµ< 2.5
• isolation cone of R > 0.4 between muons and quarks or gluons
• di-muon invariant mass of > 110 GeV
The acceptance for h → n1n1→ ℓℓWW in this fiducial region is shown in Figure 4. Notice that the acceptance
becomes large when the Higgs mass is large, due to the high invariant mass cut on the like-sign leptons. The acceptance
also falls as the N1mass becomes lighter and the final state lepton becomes soft.
Figure 5 shows the expected exclusion at 95 percent confidence level in the Higgs, N1 mass plane from ATLAS
data for the fixed value mN2= 300 GeV. Figure 5 gives an exclusion in the N1,N2mass plane shown for different
values of the Higgs mass. For large mixing between neutrinos, light neutrino masses between 100 and 200 GeV can be
excluded. Notice that regions of parameter space are ruled out as long as the Higgs branching fraction to neutrinos
remains large: Again, when N2increases, the neutrino becomes Majorana-like and decouples from the Higgs, thus
exclusions can no longer be made.
B. Tevatron
The CDF experiment has recently completed an inclusive like-sign dilepton analysis using 6.1 fb−1of data [23].
Though Tevatron’s gg → h production cross section is smaller than LHC’s, it is still possible to use the inclusive
like-sign dilepton search to constrain fourth generation parameter space for Higgs masses under 200 GeV. First of
all, in a fourth generation scenario, Higgs production from gluon fusion is substantially enhanced. In addition, in
the regime of lighter Higgs masses, the lightest Majorana neutrinos occupy a significant or even dominant portion of
the Higgs branching fraction, and we thus expect many like-sign dilepton events from Higgs production. In addition,
Tevatron’s inclusive like-sign dilepton search did not require the large invariant mass cut, increasing the sensitivity to
the fourth generation Higgs decays where the neutrino is light.
We consider again the process gg → h → N1N1→ WWℓℓ → ℓℓ+qq′+qq′. CDF’s search considered both electrons
and muons in the final state. Note that the weakest bound in fourth generation lepton scenario occurs when the fourth
generation neutrino decay, N1→ Wℓ, is flavor democratic: then the lightest allowed neutrino mass is 61.2 GeV. We
expect that we will be able to improve the bound using the inclusive like-sign dilepton search only in the flavor