Light charged Higgs bosons to AW/HW via top decay
Received: May
Light charged Higgs bosons to AW=H W
Felix Kling 0
Adarsh Pyarelal 0
Shufang Su 0
at the LHC. 0
0 Department of Physics, University of Arizona
While current ATLAS and CMS measurements exclude a light charged Higgs the possibility of a light charged Higgs produced in top decay via single top or top pair production, which is the most prominent production channel for a light charged Higgs We consider the subsequent decay H
decay; Supersymmetry Phenomenology; Hadronic Colliders

< 160 GeV) for most of the parameter region in the context of the MSSM
scenarios, these bounds are signi cantly weakened in the Type II 2HDM once the exotic decay
channel into a lighter neutral Higgs, H
! AW=HW , is open. In this study, we examine
! AW=HW , which can reach a
sizable branching fraction at low tan
once it is kinematically permitted. With a detailed
collider analysis, we obtain exclusion and discovery bounds for the 14 TeV LHC
assuming the existence of a 70 GeV neutral scalar. Assuming BR(H
and BR(A=H !
and 0.03% for single top and top pair production respectively, with an integrated
luminosity of 300 fb 1
. The discovery reaches are about 3 times higher. In the context of
the Type II 2HDM, discovery is possible at both large tan
> 17 for 155 GeV < mH
165 GeV, and small tan
< 6 over the entire mass range. Exclusion is possible in the
! AW=HW ) = 100%
versus mH
The exotic decay channel H
plane except for charged Higgs masses close to the top threshold.
1 Introduction 2 3 4
Theoretical motivation
Current limits
Collider analysis
Single top production
Top pair production
Introduction
5 Implication for the type II 2HDM 1 2 5
In July 2012, both the ATLAS and the CMS collaborations announced the discovery of
a new resonance with a mass of 126 GeV, which is consistent with the predictions of the
Standard Model (SM) Higgs boson [1, 2]. The data obtained in the following years allowed
measurements of its mass and couplings and a determination of its CP properties and
spin [3{5]. Nevertheless, there are many reasons, both from theoretical considerations and
experimental observations, to expect physics beyond the SM, such as the hierarchy problem,
neutrino masses and dark matter. There have been numerous attempts to build new physics
models which can explain these puzzles. Some well known examples are the Minimal
Supersymmetric Standard Model (MSSM) [6{8], the Next to Minimal Supersymmetric
Standard Model (NMSSM) [9, 10] and the Two Higgs Doublet Models (2HDM) [11{14].
Many of these new physics models involve an extended Higgs sector with an interesting
phenomenology that might be testable at the LHC. In addition to the SMlike Higgs boson
in these models, the low energy spectrum includes other CPeven Higgses1 H, CPodd
Higgses A, and a pair of charged Higgses H . The discovery of one or more of these new
particles would be a clear indication of an extended Higgs sector as the source of
electroweak symmetry breaking (EWSB). A number of searches have been performed at the
LEP, Tevatron and the LHC, mainly focusing on decays of Higgses into SM particles [15{
22]. However, exotic decay channels, in which a heavy Higgs decays into either two lighter
Higgses, or a Higgs plus an SM gauge boson, open up and can even dominate if
kinematically allowed, reducing the reach of the conventional search channels. Some of these
channels have already been studied both in a theoretical [23{31] and experimental [32{34]
1Note that we use h0 and H0 to refer to the lighter or the heavier CPeven Higgs for models with two
CPeven Higgs bosons. When there is no need to specify, we use H to refer to the CPeven Higgses.
setting. Soon, more of those exotic Higgs decay channels will be accessible at the LHC. It
is therefore timely to study the LHC reach of those channels more carefully.
In the current study we examine the detectability of a light charged Higgs boson,
< mt. The dominant production mode for such a light charged Higgs at the
LHC is via top decay, given the large top production rate at the LHC. BR(t ! H b)
can be enhanced at both large and small tan , due to the enhanced top and bottom
Yukawa couplings. Current search strategies assume that the charged Higgs decays either
leptonically (H
) or hadronically (H
! cs). The null search results at both the
ATLAS and CMS exclude a light charged Higgs below a mass of about 160 GeV for most of
such that the H
channel is kinematically open, the branching fractions
into the conventional nal states
and cs are suppressed and the exclusion bounds can
be signi cantly weakened. Due to experimental challenges at low energies, such a light
neutral Higgs has not been fully excluded yet. A relatively large region of mH
. 20 is still allowed, while no limits exist for mH
The exotic decay channel of H
! AW=HW , on the other hand, o ers an additional
opportunity for the detection of a light charged Higgs, closing the loophole of the current
light charged Higgs searches. While there are strong constraints on the mass of the light
charged Higgs from
avor [35{37] and precision [38{42] observables, those limits are
typically model dependent and could be relaxed when there are contributions from the other
sectors of the model [43]. A direct search for a light charged Higgs, on the other hand,
provides a modelindependent and complementary reach. It is thus timely and worthwhile
to fully explore the discovery or exclusion potential of the light charged Higgs at the LHC.
In this paper we study the exotic decay of a charged Higgs H
! AW=HW with A=H
. We focus on the light charged Higgs produced via top decay, considering
both the single top and top pair production channels. The exclusion bounds and discovery
reach will be explored and interpreted in the context of the Type II 2HDM. A collider
analysis considering the same decay channel of a heavy charged Higgs produced in H tb
associate production has been performed in [26].
We will proceed as follows. In section 2, we brie y introduce the Type II 2HDM and
present scenarios that permit a large branching fraction for the process H
In section 3, we summarize the current experimental constraints on a light charged Higgs.
In section 4, we present the details of our collider analysis. We investigate the single top
and top pair production channels in section 4.1 and 4.2, respectively, and present the model
independent 95% C.L. exclusion and 5
discovery limits for both processes at the 14 TeV
LHC with various luminosities in section 4.3. In section 5, we discuss the implications of
our analysis for the Type II 2HDM and translate our results into reaches in parameter
! AW=HW .
space. We conclude in section 6.
Theoretical motivation
i =
(vi + i0 + iGi)=p2
where v1 and v2 are the vacuum expectation values of the neutral components which
symmetry imposed on the Lagrangian, we are left with six free parameters, which can be
chosen as four Higgs masses (mh0 , mH0 , mA, mH ), a mixing angle
between the two
CPeven Higgses, and the ratio of the two vacuum expectation values (tan
= v2=v1).
In the case where a soft breaking of the Z2 symmetry is allowed, there is an additional
parameter, m212. In the Type II 2HDM, 1 couples to the leptons and down type quarks,
while 2 couples to the up type quarks. Details of the Type II 2HDM can be found in the
review paper [11].
The Higgs mass eigenstates contain a pair of CPeven Higgses (h0; H0), one CPodd
Higgs A and a pair of charged Higgses H , which can be written as:
gW tb = p
pmt=mb
becomes close to the top mass.
the corresponding couplings being
gH cs =
+ mc cot )
If there is an additional light neutral Higgs boson h0 or A, additional decay channels into
as the mixing angles [45]:
If the charged Higgs is light, the top quark can either decay into W b or into H b. The
rst decay is controlled by the SM gauge coupling
with g being the SM SU(2)L coupling, while the second decay depends on tan
Type II 2HDM or MSSM:
gH tb =
This coupling is enhanced for both small and large tan . In gure 1, we present contours
of the branching fraction BR(t ! H b) in the mH
plane, calculated using the
2HDMC [44]. We can see that the decay branching fraction BR(t ! H b) can reach
values of 5% and above for both large and small tan , but reaches a minimum at tan
8. The branching fraction decreases rapidly when the charged Higgs mass
Conventionally, a light charged Higgs is assumed to either decay into
or cs, with
with p being the incoming momentum for the corresponding particle.
BRHt ® H±bL
Figure 1. Branching fractions of BR(t ! H b) in the mH
CPeven neutral Higgs H0 to be the observed 126 GeV SMlike Higgs. In this case j cos(
! h0W channel for a light charged Higgs is open only if we demand the heavy
1 is preferred by experiments and the H h0W
coupling is unsuppressed. The
coupling is independent of sin(
) and always unsuppressed. There is no H
H0W channel since it is kinematically forbidden given mH
< mt and mH0
In the generic 2HDM, there are no mass relations between the charged scalars, the
scalar and pseudoscalar states. Therefore both the decays H
can be accessible or even dominant in certain regions of the parameter space. It was shown
in ref. [37] that in the Type II 2HDM with Z2 symmetry, imposing all experimental and
theoretical constraints still leaves large regions in the parameter space that permit such
exotic decays with unsuppressed decay branching fractions.
In the left panel of gure 2, we show the contours of the branching fraction BR(H
AW ) in the mH
plane assuming mA = 70 GeV, h
0 being the SMlike Higgs
and mH0 decoupled. This branching fraction dominates for values of tan
less than 10
to 30 for charged Higgs masses in the range between 155 GeV and 170 GeV. For large
values of tan , the
channel dominates, as shown in the right panel of
gure 2 for
decay is kinematically suppressed. Similar results can be obtained for H
mh0 = 70 GeV, sin(
0 and decoupled mA.
The MSSM Higgs mass spectrum is more restricted. At tree level, the mass matrix
depends on mA and tan
only, and the charged Higgs mass is related to mA by m2H
m2A + m2W . Large loop corrections are needed to increase the mass splitting to permit the
decay of H
! AW . In the nondecoupling region of MSSM with H0 being the SMlike
Higgs, the light CPeven Higgs h0 can be light: mh0 < mH
mW . The branching fractions
can reach values up to 10% [46] in some regions of parameter space. In the NMSSM the
Higgs sector is enlarged by an additional singlet. The authors of [47, 48] have shown that
decays of H
plane. The right panel shows the branching fractions of H
! AW ) in the Type II 2HDM in
! AW (red),
and cs (blue) as a function of tan
for a 160 GeV H . Both plots assume the existence of a 70 GeV
CPodd scalar A, h0 being the SMlike Higgs and H0 decoupled.
Current limits
Searches for a light charged Higgs boson with mass mH
< mt have been performed by
both ATLAS and CMS [16, 17] with 19.7 fb 1 integrated luminosity at 8 TeV and 4.6
fb 1 integrated luminosity at 7 TeV. The production mechanism considered is top pair
production in which one top quark decays into bH
while the other decays into bW .
These studies focus on the H
decay channel, which is dominant in most parts of
the parameter space in the absence of decays into lighter Higgses. Assuming a branching
for the top quark branching fraction BR(t ! H b) varying between 1.2% to 0.16% for
charged Higgs masses between 80 GeV and 160 GeV. This result can be translated into
bounds on the MSSM parameter space. The obtained exclusion limits for the MSSM mhmax
scenario can be seen in the right panel of gure 3 (the region to the left of the red line). Only
charged Higgs masses in the small region 155 GeV < mH
are still allowed. The ATLAS results [16] are similar.
< 160 GeV around tan
= 8
A search with the H
nal states has been performed by ATLAS [18] using
4.7 fb 1 integrated luminosity at 7 TeV and by CMS [19] using 19.7 fb 1 integrated
luminosity at 8 TeV. Assuming BR(H
! cs) = 100%, the ATLAS results imply an upper
bound for BR(t ! bH ) around 5% to 1% for charged Higgs masses between 90 GeV and
150 GeV while the CMS searches impose an upper bound of BR(t ! bH ) around 2% to
7% for a charged Higgs mass between 90 and 160 GeV.
These limits get weaker once we assume realistic branching fractions smaller than
100%. The left panel of
gure 3 shows how the CMS limits on the branching fraction
BR(t ! H b) can change signi cantly in the presence of an additional light neutral Higgs.
Relaxed CMS Exclusion
25 Type II 2HDM with mA=70 GeV
146 148 150 152 154 156 158 160
146 148 150 152 154 156 158
) (black line) [17], as well as the weakened limits in the Type II 2HDM in the
presence of a light neutral Higgs for tan
= 1 (red), tan
= 7 (blue) and tan
= 50 (green). Right
panel: the excluded region in mH
plane assuming a 100% BR(H
) (yellow and cyan
regions) and the weakened limits with a light neutral Higgs (cyan region). Here we have assumed
the light neutral Higgs to be a 70 GeV CPodd scalar A.
The black curve shows the CMS limits presented in [17] assuming a 100% BR(H
The modi ed limits assuming the presence of a 70 GeV CPodd neutral Higgs are shown
= 1 (red), tan
= 7 (blue) and tan
= 50 (green). We can see that for large
tan , the limits stay almost unchanged since H
is the dominating decay channel,
but for smaller values of tan
these limits are weakened signi cantly.
The right panel of gure 3 shows how the CMS limits in the mH
in the presence of an additional light Higgs. The yellow shaded region (plus the cyan region)
assumes a 100% BR(H
) while the cyan region assumes the Type II 2HDM branching
fractions in the presence of a 70 GeV CPodd neutral Higgs. For tan
< 15, the surviving
region in mH
is much more relaxed, extending down to about 150 GeV. Therefore, the
presence of exotic decay modes substantially weakens the current and future limits based
on searches for the conventional H
; cs decay modes.
A light charged Higgs could have a large impact on precision and avor observables [49].
For example, in the 2HDM, the bounds on b ! s restrict the charged Higgs to be heavier
than 300 GeV. A detailed analysis of precision and
avor bounds in the 2HDM can be
found in refs. [35{37]. Flavor constraints on the Higgs sector are, however, typically
modeldependent, and could be alleviated when there are contributions from other new particles
in the model [43]. Since our focus in this work is on collider searches for a light charged
Higgs and their implications for the Type II 2HDM, we consider the scenario of a light
charged Higgs: mH
< mt, as long as it satis es the direct collider Higgs search bounds.
constrained by the A=H !
to the di culties in the identi cation of the relatively soft taus and the overwhelming
SM backgrounds for soft leptons and jets. Furthermore, LEP limits [22] based on V H
associated production do not apply for the CPodd A or the nonSM like CPeven Higgs.
LEP limits based on AH pair production can also be avoided as long as mA + mH >
208 GeV. Therefore, in our analyses below, we choose the daughter (neutral) Higgs mass
to be 70 GeV.2
There have been theoretical studies on other light charged Higgs production and decay
channels. The authors of [50, 51] analyzed the possibility of using the single top production
mode to observe a light charged Higgs boson decaying into a
nal state. The
detectability of a charged Higgs decay into a
nal state or a
nal state via AW with a light
charged Higgs produced via top decay in top pair production has been investigated in [52]
The H tb associated production with H
! AW=HW
has been analyzed in detail
in ref. [26], which focuses on heavy charged Higgs bosons (mH
> mt). Given the same
Higgs coming from top decay with top pair production. Furthermore, we analyze single
signal due to its unique kinematic features.
Collider analysis
In our analysis we study the exotic decay H
! AW=HW of light charged Higgs bosons
< mt) produced via top decay. We consider two production mechanisms: tchannel
single top production3 (tj) and top pair production (tt) [54].
The light neutral Higgs boson can either be the CPeven H or the CPodd A. In the
analysis that follows, we use the decay H
as an illustration. Since we do not
use angular correlations of the charged Higgs decay, the bounds obtained for H
apply to H
fermionic decay A !
The neutral Higgs boson (A) itself will decay further. In this analysis we look at the
for single top production and both the
and the hadronic bb
modes for top pair production. While the bb mode would have the advantage of a large
branching fraction BR(A ! bb), the
case has smaller SM backgrounds and therefore
leads to a cleaner signal. We study both leptonic and hadronic
decays and consider three
cases: had had, lep had and lep lep. The lep had case is particularly promising since we
can utilize the same sign dilepton signal with the leptons from the decays of the W and
We use Madgraph 5/MadEvent v1.5.11 [55, 56] to generate our signal and background
events. These events are passed to Pythia v2.1.21 [57] to simulate initial and
radiation, showering and hadronization. The events are further passed through Delphes
2The mass of 70 GeV is also chosen to be above the hSM ! AA threshold to avoid signi cant deviations
of the 126 GeV SMlike Higgs branching fractions from current measurements.
3We only consider the dominant tchannel single top mode since the schannel mode su ers from a very
small production rate and the tW mode has a nal state similar to that of the top pair production case.
The cuts that we have imposed are:
1. Identi cation cuts. (4.1) (4.2) (4.3)
Case A ( had had). One lepton ` = e or , two
tagged jets, zero or one b tagged
jet and at least one untagged jet:
n` = 1; n = 2; nb = 0; 1; nj
We require the tagged jets to have charges of opposite signs.
Case B ( lep had). Two leptons, one
tagged jet, zero or one b tagged jet and at
least one untagged jet:
n` = 2; n = 1; nb = 0; 1; nj
We require that both leptons have the same sign, which is opposite to the sign of the
Case C ( lep lep). Three leptons, no
tagged jet, zero or one b tagged jet and at
least one untagged jet:
n` = 3; n = 0; nb = 0; 1; nj
3.07 [58] with the Snowmass combined LHC detector card [59] to simulate detector
effects. The discovery reach and exclusion bounds have been determined using the program
RooStats [60, 61] and thetaauto [62].
In this section, we will present model independent limits on the
BR for both 95%
C.L. exclusion and 5
discovery for both single top and top pair production with possible
bbW W /bbbbW W . We consider the parent particle mass mH in
the range 150
Single top production
For single top production, we consider the channel
pp ! tj ! H bj ! AW bj !
The dominant SM backgrounds are W
production, which we generate with up to two
additional jets (including b jets); and top pair production with both fully and semileptonic
decay chains, which we generate with up to one additional jet. We also take into account
the SM backgrounds tj
and ttll with l = (e; ; ).
The following selection cuts for the identi cation of leptons, b jets and jets are used:
j `;b; j < 2:5; j j j < 5; pT;`1;j;b > 20 GeV and pT;`2 > 10 GeV.
2. Neutrino reconstruction. We reconstruct the momentum of the neutrino using the
missing transverse momentum and the momentum of the hardest lepton as described
in [63], assuming that the missing energy is solely from W
the neutrino reconstruction is relatively poor since there is additional missing energy
from the leptonic
−1
(left panel) and the transverse momentum of the tj
system pT;tj (right panel) for the signal (red, solid) and the dominant SM backgrounds: tt (blue,
dotted) and W
(green, dotted). The imposed cuts are indicated by the vertical dashed lines.
The histograms shown are for case A with mH
= 160 GeV and mA = 70 GeV.
3. Neutral Higgs candidate A. The jets (case A), the jet and the softer lepton (case B) or the two softer leptons (case C) are combined to form the neutral Higgs
candidate. In cases B and C the mass reconstruction is relatively poor due to missing
energy from the neutrino associated with the leptonic
4. Charged Higgs candidate H . The neutral Higgs candidate, the reconstructed
neutrino and the hardest lepton are combined to form the charged Higgs candidate.
5. Mass cuts. We place upper limits on the masses of the charged and neutral Higgs
candidates, optimized for each mass combination. For mH
= 160 GeV and mA =
70 GeV, we impose
< 48 GeV and m
W < 148 GeV.
to peak around cos
analysis we require
6. Angular correlation. A unique kinematical signature of single top production is
the distribution of the angle
, which is the angle between the top momentum in the
tj system's rest frame and the tj system's momentum in the lab frame, as suggested
in [64]. The di erential distribution for cos
is shown in the left panel of gure 4
for signal (red, solid), tt (blue, dotted) and W
(green, dotted). The signal tends
1 while the background is at for W
and tt.4 In our
4As shown in [64], the cos
distribution for tt background would peak around cos
= 1 if the top
quark could be reliably identi ed. However, in this paper we approximate the top quark momentum by the
momentum of the charged Higgs candidate, which results in a at distribution of cos
for the tt system.
A: Identi cation [eq. (4.2)]
Mass cuts [eq. (4.6)]
B: Identi cation [eq. (4.3)]
Mass cuts [eq. (4.6)]
and pT;tj [eq. (4.7), (4.8)]
and pT;tj [eq. (4.7), (4.8)]
C: Identi cation [eq. (4.4 )]
Mass cuts [eq. (4.6)]
and pT;tj [eq. (4.7), (4.8)]
1.39 0.002
S=pB
= 160 GeV and mA = 70 GeV at the 14 TeV LHC.
We have chosen a nominal value for
BR(pp ! tj ! H jb !
W bj) of 100 fb to illustrate the cut e ciencies for the signal process.
7. Top and recoil jet system momentum. In single top production, we expect that
the transverse momentum of the top quark and recoil jet should balance each other,
as shown in the right plot of gure 4 by the red solid curve. We impose the cut for
the transverse momentum of the tj system:
pT;tj < 30 GeV:
This further suppresses the top pair background in the presence of additional jets
coming from the second top.
In Table 1, we show the signal and major background cross sections with cuts for
a signal benchmark point of mH
The rst row shows the total cross section before cuts, calculated using MadGraph. The
following rows show the cross sections after applying the identi cation cuts, mass cuts and
the additional cuts on cos
chosen a nominal value for
and pT;tj for all three cases as discussed above. We have
BR(pp ! H bj !
W bj) of 100 fb.5
We can see that the dominant background contributions after particle identi cation
are tt for cases A and C, and W
for case B. The reach is slightly better in case B in which
the same sign dilepton signature can reduce the tt background su ciently. Nevertheless,
soft leptons from underlying events or bdecay can mimic the same sign dilepton signal.
The obtained results are sensitive to the
tagging e ciency as well as the misidenti cation
rate. In our analyses, we have used a
tagging e ciency of tag = 60% and a mistagging
increase the signi cance of this channel.
5For the Type II 2HDM the top branching fraction into a charged Higgs for mH
= 160 GeV is typically
between 0.1% and 1% (see
assuming the branching fractions BR(H
! AW ) = 100% and BR(A !
) = 8:6% leads to the stated
BR of around 21
tt [fb] ttll [fb]
had had: Identi cation
lep had: Identi cation
lep lep: Identi cation
column of S=p
bbW W ) of 1000 fb to illustrate the cut e ciencies for the signal process. The last
the identi cation cuts and m
Top pair production
We now turn to the top pair production channel
pp ! tt ! H tb ! AbbW W !
A detailed collider study with the same nal states has been performed in [26] with a focus
on high charged Higgs masses. The same strategy has been adopted for the light charged
Higgs case and we refer to ref. [26] for details of the analysis.
To analyze this channel, we consider decay modes of the neutral Higgs into had had,
had lep, lep lep and bb. For the two W bosons, we require one to decay leptonically and
the other to decay hadronically to reduce backgrounds.
The dominant SM background for the
channel is semi and fully leptonic tt pair
. We ignored the subdominant backgrounds from single vector boson production,
W W , ZZ, single top production, as well as multijet QCD background. Those backgrounds
are either small or can be su ciently suppressed by the cuts imposed. Similar backgrounds
are considered for the bb process.
In Table 2, we show the signal and major background cross sections of the
with cuts for a signal benchmark point of mH
= 160 GeV and mA = 70 GeV at the 14 TeV
LHC, similar to Table 1. We have chosen a nominal value for
BR(pp ! tt ! H tb !
bbW W ) of 1000 fb to illustrate the cut e ciencies for the signal process.
After the cuts, the dominant background contributions are tt ( had had, lep lep) as well
as ttll ( had lep) while the backgrounds including vector bosons do not contribute much. We
nd that the case in which one
decays leptonically and the other
decays hadronically
gives the best reach. This is because the same sign dilepton signature can reduce the tt
background su ciently.
Figure 5 displays the 95% C.L. exclusion (green curve) and 5 discovery (red curve) limits
at the 14 TeV LHC for both the single top (left) and top pair (right) channel . The
dotdashed, solid and dashed line show the results for three luminosities: 100 fb 1, 300 fb 1 and
1000 fb 1, respectively. In these plots we have combined all three cases of
decays. While in
the single top channel, all three cases contribute roughly the same to the overall signi cance,
the highest sensitivity in the top pair production channel comes from the lep had case.
Due to the small number of events in both channels, the statistical error dominates over
the assumed 10% systematic error in the background cross sections. Therefore, higher
luminosities lead to better reaches. Assuming 300 fb 1 integrated luminosity, the 95%
C.L. limits on
BR are about 35 and 55 fb for the single top and top pair production
processes respectively. The discovery reaches are about 3 times higher.
Assuming a 100% branching fraction BR(H
! AW ) and BR(A !
) = 8:6%,6
we can reinterpret
BR limits as limits on the branching fraction BR(t ! H b) as
indicated by the vertical axis on the right. While the cross section limits are better in the
single top channel, the corresponding limits on the branching fraction BR(t ! H b) are
weaker due to the smaller single top production cross section. The 95% C.L. exclusion
limit on BR(t ! H b) is about 0.2% for the single top process and 0.03% for the top pair
production process, respectively.
A study of the A ! bb decay using the top pair production channel leads to worse
results due to the signi cantly higher SM backgrounds. For the 14 TeV LHC with 300 fb 1
the exclusion limit on
BR is about 7 pb for a charged Higgs with mass mH
= 160 GeV,
typical ratio of BR(A=H ! bb) : Br(A=H !
in the bb case is much worse than that in the
3mb2=m2, we conclude that the reach
We reiterate here that the exclusion and discovery limits on
BR are completely
model independent. Whether or not discovery/exclusion is actually feasible in this channel
should be answered within the context of a particular model, in which the theoretically
predicted cross sections and branching fractions can be compared with the exclusion or
discovery limits. We will do this in section 5 using the Type II 2HDM as a speci c example.
Implication for the type II 2HDM
The results in the previous section on BR(t ! bH ) can be applied to any beyond the SM
scenarios containing a light charged Higgs boson with the H
! AW=HW channel being
kinematically accessible. To give a speci c example of the implication of this channel, we
will now apply the exclusion and discovery limits in the context of the Type II 2HDM.
The 2HDM allows us to interpret the observed Higgs signal either as the lighter
CPeven Higgs (h0126) or the heavier CPeven Higgs (H0126). The authors of ref. [37]
and including all the experimental and theoretical constraints. In the h0126 case, we are
restricted to either a SMlike region at sin(
) =
1 with tan
< 4 or an extended
6Assuming bb and
cschannel is enhanced.
are the dominant decay modes of a light A, BR(A !
) = 8:6% in the Type II
2HDM or MSSM for medium to large tan . This branching fraction decreases for small tan
H b) (right vertical axis) assuming BR(H
! AW ) = 100% and BR(A !
BR and BR(t !
) = 8:6% for
production channels. The dotdashed, solid and dashed lines correspond to an integrated luminosity
of 100, 300 and 1000 fb 1 respectively. Here, we have assumed a 10% systematic error on the
fmH ; mA; mh0 ; mH0 g GeV
BP1: f160; 70; 126; 700g
BP2: f160; 700; 70; 126g
Favored Region sin( sin( )
of the Type II 2HDM. The checkmarks indicate kinematically allowed channels. Also shown are
the typical favored region of sin(
) for each case (see ref. [37]).
region with 0:6 < sin(
) < 0:9 and 1:5 < tan
< 4 with relatively unconstrained
masses. In the H0126 case, an SMlike region, around sin(
) = 0 and tan
an extended region with
) < 0:05 and tan
up to 30 or higher, survive all
We can interpret the results of the previous section in two ways: the light neutral
Higgs in the charged Higgs decay could either be the light CPeven Higgs h0 or the
CPodd Higgs A. The decay mode H
! H0W is not possible given that mH0
The decay H
! AW is possible in both the h0126 and H0126 case and the partial
decay width is independent of sin(
). The decay branching fraction, however,
depends on whether H
! h0W is open or not. For simplicity, we choose a benchmark
kinematically accessible. The decay width H
! h0W depends on sin(
) and is only
sizable in the H0126 case. We illustrate this case with a second benchmark point BP2:
We list the benchmark points in Table 3.
In the left panel of
gure 6, we show the branching faction BR(H
BP1, which is independent of sin(
) and decreases with increasing tan
due to the
8160,70,126,700<
8160,700,70,126<
for BP1 and BP2, respectively.
! AW (left panel) and H
! h0W (right panel)
enhancement of the
mode. The branching fraction can reach values of 90% or larger for
< 4 and stays the dominating channel until tan
= 12.
The right panel of gure 6 shows the branching fraction, BR(H
It reaches maximal values around sin(
compared to BP1 due to the suppressed H h0W coupling.
) = 0 and decreases for larger j sin(
In gure 7, we display the 95% exclusion (yellow regions enclosed by the solid lines as
well as the cyan regions) and 5 discovery reach (cyan regions enclosed by the dashed lines)
for BP1 (left panel) and BP2 (right panel) at the 14 TeV LHC with 300 fb 1 integrated
luminosity. The red lines refer to the limits based on top pair production, and the blue
lines refer to the limits based on single top production.
For the benchmark point BP1 with H
! AW , the exclusion reach based on top pair
production covers the entire parameter space, while discovery is possible for small tan
and large tan
> 18, independent of sin(
). Intermediate values of tan
have a reduced
branching fraction BR(t ! H b) (see gure 1) and therefore the total
BR is suppressed.
At high tan , BR(t ! H b) is enhanced su ciently to overcome the reduced branching
! AW ). The search based on single top production is only e ective in the
region, with an exclusion reach of tan
< 4 and a discovery reach of tan
The right panel of gure 7 shows the reach for BP2. The exclusion region for top pair
production covers the entire parameter space except for j sin(
)j > 0:85 and tan
Discovery is possible for large tan
> 18 with j sin(
)j < 0:5 and for small tan
The reach for single top production is limited to the small tan
gure 8, we show the reach in the mH
plane for H
! AW with mA =
70 GeV with both h0 and H0 outside the kinematic reach. These limits also apply for
! h0W with mh0 = 70 GeV and sin(
95% exclusion (yellow regions enclosed by the solid lines as well as the cyan regions) and
5 discovery limits (cyan regions enclosed by the dashed lines) for an integrated luminosity
) = 0 with a decoupled A. We display the
discovery reach (cyan regions enclosed by the dashed lines) obtained by the tjchannel
(blue) and ttchannel (red) in the tan
) plane for BP1 (left panel) and BP2 (right
panel), with an integrated luminosity of 300 fb 1 at the 14 TeV LHC.
of 300 fb 1 at the 14 TeV LHC. Superimposed are the current CMS limits (black hatched
region) [17] which exclude the large tan
region at mH
The best reach is obtained by the top pair channel, as indicated by regions enclosed by
the red lines. The model can be excluded up to 167 GeV for all tan
and up to 170 GeV
< 4 or tan
> 29. Discovery is possible for both low tan
<6 in the entire region
of 150 GeV < mH
< 170 GeV and high tan
> 17 with 155 GeV < mH
< 165 GeV. The
reach is weakened for intermediate tan
due to the reduced branching fraction t ! H b.
The single top channel (blue lines) only provides sensitivity in the low tan
permits exclusion (discovery) for tan
We conclude this section with the following observations:
and intermediate values of tan . The reach in the H
mode is signi cantly
weakened in the presence of the H
! AW=h0W modes, in particular for small to
intermediate tan , leaving the possibility of a light charged Higgs that has escaped
detection so far.
Both the H
H0126 case permit exclusion and discovery in large regions of the parameter space.
! AW channel for the h0126 case and the H
! h0W channel in the
The reach in the exotic channels H
the conventional search channel H
! AW=h0W is complementary to the reach in
, especially for small to intermediate values
While the top pair production channel covers a large region of parameter space, the
single top channel permits discovery/exclusion in the low tan
discovery (cyan regions bounded by the dashed lines) imposed by the tjchannel (blue) and
ttchannel (red) in the mH
The same limits apply for mh0 = 70 GeV and sin(
) = 0 if A is decoupled. The black hatched
region indicates the region excluded by the CMS search based on H
After the discovery of the rst fundamental scalar by both the ATLAS and CMS
collaboration, it is now time to carefully measure its properties to determine the nature of this
particle. Current measurements still permit the possibility that the discovered signal is
not the SM Higgs particle, but just one scalar particle contained in a larger Higgs sector,
as predicted by many extensions of the SM. While most of the current searches for the
nonSM Higgs bosons focus on conventional search channels, increasing attention is being
paid to exotic Higgs decay channels [23{34] into a pair of lighter Higgses or a Higgs plus
nal states that can become dominant once kinematically allowed.
In this paper we consider the possibility of a light charged Higgs mH
< mt produced
via top decay t ! H b. Due to the large single top and top pair production cross section
at the LHC, the charged Higgs can be produced copiously. Assuming that a light charged
Higgs predominantly decays into
, both ATLAS and CMS exclude a light charged Higgs
for most regions of the MSSM and the Type II 2HDM parameter spaces. The branching
) can be signi cantly reduced once the exotic decay channel into
fraction BR(H
a light Higgs, H
search get weakened, in particular for small and intermediate tan , leaving the possibility
of a light charged Higgs open. This loophole, however, can be closed when we consider the
alternative charged Higgs decay channel: H
! AW=HW .
In this paper we analyze the possibility of discovering a light charged Higgs via the
While the top pair channel bene ts from a large production cross section, the
single top channel permits a cleaner signal due to its unique kinematic features. Assuming
the existence of a light neutral Higgs of mass 70 GeV, the model independent 95% C.L.
exclusion limits on
BR based on
channel are about 35 fb for the single top channel
and 55 fb for the top pair channel. The discovery reaches are about three times higher.
Assuming BR(H
! AW=HW ) = 100% and BR(A=H
limits on BR(t ! H+b) are about 0.2% and 0.03% for single top and top pair production,
respectively. A signi cantly worse reach is obtained in the bb channel.
) = 8:6%, the exclusion
We discuss the implications of the obtained exclusion and discovery bounds in the
context of the Type II 2HDM, focusing on two scenarios: the decay H
a light A in the h0126 case and the decay H
! h0W in the H0126 case. The top
pair channel provides the best reach and permits discovery for both large tan
around mH
= 160 GeV and small tan
< 6 over the entire mass range, while exclusion
is possible in the entire tan
versus mH
plane except for charged Higgs masses close
to the top threshold. The single top channel is sensitive in the low tan
region and
permits discovery for tan
< 3. In particular, the low tan
region is not constrained by
channel, making the H
Higgs searches.
! AW=h0W a complementary channel for charged
While most of the recent searches for additional Higgs bosons have focused on
conventional decay channels, searches using exotic decay channels have just started [23{34].
Studying all of the possibilities for the nonSM Higgs decays will allow us to explore the
full potential of the LHC and future colliders in understanding the nature of electroweak
symmetry breaking.
Acknowledgments
We thank B. Coleppa for his participation at the beginning of this project. We would also
like to thank Peter Loch and Matt Leone for helpful discussions. This work was supported
by the Department of Energy under Grant DEFG0213ER41976.
Open Access.
This article is distributed under the terms of the Creative Commons
Attribution License (CCBY 4.0), which permits any use, distribution and reproduction in
any medium, provided the original author(s) and source are credited.
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