The top quark data in the lepton plus $\tau$ channel offers a viable probe
for the charged Higgs boson signal. We analyse the recent Tevatron collider
data in this channel to obtain a significant limit on the $H^\pm$ mass in the
large $\tan\beta$ region.Comment: 8 pages, LaTeX file; 2 figures included (PS files

A.C. Bawa

A.D. Martin

B.K. Bullock

D. Garcia

D. P. Roy

D.P. Roy

E. Berger

F. Abe

J.F. Gunion

M. Drees

Monoranjan Guchait

O. Helene

P.H. Chankowski

R. M. Barnett

R.M. Godbole

S. Raychaudhuri

V. Barger

English

arXiv

The top quark data in the lepton plus &#964; channel offers a viable probe for the charged Higgs boson signal. We analyze the recent Tevatron collider data in this channel to obtain a significant limit on the H&#177; mass in the large tan&#946; region

Guchait, Monoranjan

Roy, D. P.

Publications of the IAS Fellows

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TIFR/TH/96-58
hep-ph/yymmnn
Constraints on the Charged Higgs Sector from the
Tevatron Collider Data on Top Quark Decay
Monoranjan Guchait1 and D.P. Roy2
Theoretical Physics Group
Tata Institute of Fundamental Research
Homi Bhabha Road, Mumbai 400 005, India
Abstract
The top quark data in the lepton plus τ channel offers a viable probe for the charged
Higgs boson signal. We analyse the recent Tevatron collider data in this channel to obtain
a significant limit on the H± mass in the large tanβ region.
Pacs Nos: 11.30.Pb,13.35.Dx,14.65.Ha
1e-mail:guchait@@theory.tifr.res.in
2e-mail:dproy@@theory.tifr.res.in
1
The discovery of the top quark signal at the Tevatron collider [1,2] has generated a good
deal of current interest in the search of new particles in top quark decay. The large mass
of top offers the possibility of carrying on this search to a hitherto unexplored mass range
for these particles. In particular the top quark decay is known to provide by far the best
discovery limit for one such new particle, i.e. the charged Higgs boson [3] of the minimal
supersymmetric standard model (MSSM). The signature of the charged Higgs boson in top
quark decay is based on its preferential coupling to the τ channel in contrast to the universal
W boson coupling. Thus a departure from the universality prediction can be used to separate
the charged Higgs boson signal from the W background in
t→ bH(bW )→ bτν. (1)
In particular, the top quark decay into the τ lepton channel provides a promising signature
for charged Higgs boson in the region
tan β >∼mt/mb, (2)
where tanβ denotes the ratio of the vacuum expectation values of the two Higgs doublets in
MSSM.
In this note we shall analyse the recent CDF data on tt¯ decay events in the ℓτ channel,
where ℓ denotes e and µ [2,4,5]. This channel has the advantage of a low background. As we
shall see below, the number of tt¯ events in this dilepton channel relative to the ℓ + multijet
channel gives a significant lower bound on the H± mass in the large tan β region (2).
In the diagonal KM matrix approximation, the charged Higgs boson couplings to the
fermions are given by
L = g√
2mW
H+ {cot βmuiu¯idiL + tan βmdiu¯idiR + tan βmℓiν¯iℓiR}+ h.c. (3)
where the subscript i denotes quark and lepton generation. The leading log QCD correction
is taken into account by substituting the quark mass parameters by their running masses
evaluated at the H± mass scale [6]. The resulting decay widths are
Γt→bW =
g2
64πm2Wmt
λ
1
2
(
1,
m2b
m2t
,
m2W
m2t
)
[
m2W (m
2
t +m
2
b) + (m
2
t −m2b)2 − 2m4W
]
(4)
Γt→bH =
g2
64πm2Wmt
λ
1
2
(
1,
m2b
m2t
,
m2H
m2t
)
[
(m2t cot
2 β +m2b tan
2 β)(m2t +m
2
b −m2H)− 4m2tm2b
]
(5)
ΓH→τν =
g2mH
32πm2W
m2τ tan
2 β (6)
2
ΓH→cs¯ =
3g2mH
32πm2W
(m2c cot
2 β +m2s tan
2 β). (7)
They clearly show a large branching fraction for t→ bH decay at tanβ<∼1 and tanβ>∼mt/mb,
while the branching fraction for H → τν decay is ≃ 1 at tan β ≫ 1. Thus one expects a
large charged Higgs boson signal in top quark decay into the τ channel (1) in the large tanβ
region (2).
In the present analysis, we shall concentrate in the tanβ ≫ 1 region, for which the charged
Higgs boson decays dominantly into τν. The basic process of interest is tt¯ production via
quark-antiquark (or gluon-gluon) fusion, followed by their decays into charged Higgs or W
boson channels, i.e.
qq¯ → tt¯→ bb¯(W+W−,W±H∓, H+H−). (8)
The most important tt¯ signal is observed in the ℓ + multijet channel [1,2]. It comes from the
W+W− final state with a branching fraction of 24/81. The corresponding branching fraction
from this final state into the ℓτ channel is only 4/81. However there would be an additional
contribution to the ℓτ channel from the W±H∓ final state with a large branching fraction
of 4/9, which is to be weighted of course by the ratio Γt→bH/Γt→bW . Thus a comparison of
the number of tt¯ events in the two channels leads to an upper limit on this ratio, which can
be translated into lower limit on H± mass for a given tanβ.
For a quantitative estimate of the above limit, we have to consider the various kinematic
cuts and detection efficiencies [2,5]. Our analysis is based on a parton level Monte Carlo
simulation of tt¯ production using the quark and gluon structure functions of [7]. This is
followed by the decays
t
W+−→ bℓν, t¯ W−−→ b¯qq¯′ (9)
and vice versa for the ℓ + multijet channel. The quark jets are merged according to the CDF
jet cone algorithm of ∆R = (|∆η|2+ |∆φ|2)1/2 = 0.7. The resulting final state is required to
satisfy the CDF cuts [2]
pℓT > 20 GeV, |ηℓ| < 1, E/T > 20 GeV and njet ≥ 3 (with EjT > 15 GeV, |ηj| < 2). (10)
We estimate the efficiency factor for these kinematic and topological cuts to be 52% for
mt = 175 GeV. This has to be supplemented by the following CDF efficiency factors [2,5,8];
ǫℓtr = .93, ǫ
ℓ
id = .87, ǫ
ℓ
iso = .9, ǫb = .4, ǫaz = .85, (11)
corresponding to lepton triggering, identification and isolation-cut along with those due to
SVX b-tagging and azimuthal gaps in the detector. The combined efficiency factor is 12.8%,
in reasonable agreement with the CDF estimate of 11.8% [2] including hadronisation and a
more exact detector simulation.
The measured t¯t cross-section from this channel, including SLT b-tagging, is [2]
σt = 7.5± 1.5 pb. (12)
This is 40-50% higher than the NLO QCD prediction for mt = 175 GeV [9]. We shall use
this cross-section for normalisation. Thus our results will be independent of any theoretical
3
model for the t¯t cross-section. It will only depend on the preferencial H± coupling to the τ
channel vis-a-vis the universal W± boson coupling. It should be noted here that the above
cross-section corresponds to the WW contribution to the t¯t cross-section, represented by the
1st term of (8), since any contribution from the other terms would have very small detection
efficiency for this channel.
The ℓτ channel of our interest corresponds to the decays
t
W+−→ bℓν, t¯ W
−(H−)−→ b¯τν (13)
and vice versa, where the τ lepton coming from W (H) decay has a definite polarization
Pτ = −1(+1). It is identified in its hadronic decay mode as a thin jet containing 1 or
3 charged prongs [4,5]. This accounts for a τ branching fraction of about 64% [10]. The
dominant contributions come from
τ → πν(12%), ρν(25%), a1ν(15%), (14)
adding up to a little over 80% of the hadronic τ decay. We shall combine these three decay
modes and scale up their sum by 20% to simulate hadronic τ -decay. The decay distributions
are simply given by [11]
dΓπ
Γπd cos θ
=
1
2
(1 + Pτ cos θ), (15)
dΓv
Γvd cos θ
=
1
2
(
1 +
m2τ − 2m2v
m2τ + 2m
2
v
Pτ cos θ
)
, v = ρ, a1 (16)
where θ is the direction of the decay hadron in τ rest frame relative to the τ line of flight.
It is related to the fraction x of τ momentum carried by the hadron,
cos θ =
2x− 1−m2π, v/m2τ
1−m2π, v/m2τ
. (17)
This decay hadron momentum is refered to below as pτ .
The resulting final state is required to satisfy the CDF kinematic cuts [4,5],
pℓT > 20 GeV, |ηℓ| < 1, pτT > 15 GeV, |ητ | < 1.2. (18)
The corresponding efficiency factors are shown in the first column of Table 1 for the WW
and WH contributions with different charged Higgs boson masses. It includes the hadronic
branching fraction of τ along with a factor of 0.8 due to azimuthal gaps in the detector,
resulting in 15% (5%) loss to ℓ(τ) detection efficiency [8]. The opposite polarizations of τ
coming from W and H decays results in a somewhat larger efficiency factor for the latter,
which increases further with increasingH mass. The second column shows the CDF efficiency
factors for the lepton trigger, isolation and idendification as well as the τ identification. These
are expected to be essentially process independent. The last column shows the efficiency
factors for the topological and missing-ET cuts [4,5]
njet ≥ 2 (with EjT > 10 GeV, |ηj| < 2), (19)
4
H = pℓT + p
τ
T + E/T +
∑
j
EjT > 180 GeV, (20)
σE/T = E/T /
√
pℓT + p
τ
T +
∑
j
EjT > 3 GeV
1/2. (21)
For the WW contribution, the efficiency factors for the kinematic and topological cuts from
the CDF simulation [5] are shown in parantheses for comparison. For both cases they are
15-20% below our MC estimates, which indicate an overall error of ∼ 30% in our MC result.
The product of the efficiency factors in the three columns of Table 1 gives the overall
acceptance factor for the ℓτ channel. This is to be multiplied by the branching fraction
of 4/81 for the WW contribution and 4/9 times Γt→bH/Γt→bW for the WH . The resulting
factor gives the corresponding ℓτ cross-section as a fraction of the σt of (12).
Fig. 1 shows the predicted cross-section in the ℓτ channel against tanβ for several charged
Higgs boson masses. The scale on the right shows the corresponding number of events for
the accumulated CDF luminosity of 110 fb−1. The prediction includes the WW contribution
of 14 fb, i.e. 1.5 events. The corresponding number from the CDF simulation is 1.2 events
[5,8]. It may be noted that the dominant contribution comes from WW for tanβ = 5− 10,
where t→ bH width has a pronounced dip. However the WW is overwhelmed by the WH
contribution, when kinematically allowed, for tanβ >∼ mt/mb. The preliminary CDF data
shows 4 events in this channel against a background of 2 ± 0.4 [2,4,5]. The corresponding
95% CL limit of 7.7 events [10,12] is indicated in the figure. This implies a H± mass limit
of 100 GeV for tan β ≥ 40, going up to 120 GeV for tanβ ≥ 50. One may scale down the
predicted cross-section by 30% to account for the difference between the acceptance factors
of CDF simulation and ours. This would correspond to an upward shift of the above tanβ
limit by about 10 units for a given H± mass. Nonetheless it would still represent a very
significant constraint on the charged Higgs boson mass in the large tan β region.
The mass limits of Fig. 1 for different values of tanβ are converted into a 95% exclusion
contour in the mH − tan β plane in Fig. 2. As mentioned above, a 30% reduction in the
signal cross-section would correspond to a rightward shift of this contour by roughly 10 units
in tan β. The scale on the right shows the corresponding pseudo-scalar Higgs mass mA from
the MSSM mass relation, m2H± −m2A = m2W , at the tree level [13]. The radiative correction
to this mass relation is known to be no more than a few GeV. One sees from this figure
that a relatively light pseudo-scalar mass (mA ≤ 60 GeV) is disallowed for tanβ ≥ 40. It
is precisely in this region of mA and tan β that one expects to get a significant radiative
correction to Rb (Γ
b
z/Γ
had
z ) from the Higgs sector of MSSM [14]. Thus the so called large
tan β solution to the (so called!) Rb anomaly seems to be strongly disfavoured by the above
CDF data.
Recently the CDF collaboration has obtained a limit on the charged Higgs mass in the
large tanβ region [15] on the basis of their analogous data in the inclusive τ channel. Thus it
is instructive to compare the relative merits of the two channels for probing the charged Higgs
signal. The inclusive τ channel corresponds to a larger branching fraction than the ℓτ channel
analysed here. However it is compensated by much stronger experimental cuts, required
to control the background. Consequently the final signal cross-section in the inclusive τ
channel is similar to that in the ℓτ channel. This can be seen by comparing the predicted
cross-sections in the two channels in the region of tan β = 5− 10. The reason we get a much
5
larger signal cross-section in the large tanβ region compared to [15] and hence a stronger
mass limit is due to the different normalisation procedure followed in the two cases. We
use the tt¯ cross-section in the WW decay mode as measured via the lepton plus ≥ 3 jets
channel for our normalisation, while the QCD prediction for the inclusive tt¯ cross-section is
used for normalisation in [15]. The former method is evidently more powerful in the large
tan β region and should be used in the analysis of the inclusive τ channel as well.
It should be noted here that even with stronger cuts the number of estimated background
events in the inclusive τ channel are 5 times larger than in the ℓτ channel, for equal lumi-
nosity [5,15]. Thus the ℓτ channel will be clearly more advantageous at Tevatron upgrade,
which promises a 20 times higher luminosity along with a 2 times larger tt¯ cross-section.
In particular the ℓτ channel with b-tagging seems to be practically free from non-top back-
ground [4,5]. The main background in this case is from the tt¯ decay via the WW mode.
This can be suppressed relative to the H± signal by exploiting the opposite polarizations
of τ lepton in the two cases [16]. Thus the ℓτ channel with b-tagging is best suited for the
charged Higgs boson search at the Tevatron upgrade as well as the LHC.
In summary, the tt¯ data in the ℓτ channel is well suited to probe for a charged Higgs
boson signal because of the small background. On the basis of the recent CDF data in this
channel we can already get a significant limit on the H± mass in the large tan β region. With
a much higher luminosity expected at the Tevatron upgrade the probe can be extended over
a significantly wider range of H± mass and tan β.
It is a pleasure to thank Prof. M. Hohlmann of the CDF collaboration and Prof. N.K.
Mondal of DO/ for many helpful discussions.
6
References
1. CDF Collaboration: F. Abe et. al., Phys. Rev. Lett. 74, 2626 (1995);
DO/ Collaboration: S. Abachi et. al., Phys. Rev. Lett. 74, 2632 (1995).
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A.C. Bawa, C.S. Kim and A.D. Martin, Z. Phys. C47, 75 (1990);
R.M. Godbole and D.P. Roy, Phys. Rev. D43, 3640 (1991);
R.M. Barnett, R. Cruz, J.F. Gunion and B. Hubbard, Phys. Rev. D47, 1048 (1993).
4. CDF Collaboration: S. Leone, XI Topical Workshop on p¯p Collider Physics, Padova
(1996), Fermilab-Conf. 96/195-E.
5. CDF Collaboration: M. Hohlmann, Lake Louise Winter School (1996).
6. M. Drees and D.P. Roy, Phys. Lett. B269, 155 (1991);
D.P. Roy, ibid B283, 403 (1992).
7. A.D. Martin, R.G. Roberts and W.J. Stirling, Phy. Lett. B306, 145 (1993) and B309,
492 (1993).
8. M. Hohlmann, Private Communication.
9. E. Berger and H. Contoparagos, Phys. Lett. B361, 115 (1995);
S. Catani et. al., hep-ph/9602208 (1996).
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P.H. Chankowski and S. Pokarski, Nucl. Phys. B (to be published).
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16. S. Raychaudhuri and D.P. Roy, Phys. Rev. D52, 1556 (1995) and D53, 4902 (1996).
7
Table 1: The efficiency factors for the ℓτ channel corresponding to the indicated kine-
matical and topological cuts. For the WW process, the corresponding efficiencies from
the CDF simulation are shown in parenthesis. The middle column shows the triggering,
isolation and identification efficiencies from the CDF simulation.
Eff. pℓ,τT & geom. ǫ
ℓ
tr, ǫ
ℓ
iso, jets, HT & E/T
Process ǫℓid, ǫ
τ
id
WW .16 (.13) .93 × .9 .64 (.54)
WH(80) .19 × .87 × .5 .61
WH(100) .21 = .36 .62
WH(120) .22 .64
WH(140) .22 .65
Figure Captions
Fig.1 : The predicted cross-section (No. of events) shown against tanβ for different H±
masses. The 95% C.L limit corresponding to 7.7 events is shown as a dashed line.
Fig.2 : The 95% C.L exclusion contour in the H± mass and tanβ plane. The corresponding
pseudoscalar mass mA is indicated on the right.
8
0100
200
300
400
500
10 20 30 40 50 60 70 80 90 100
0
11
22
33
44
55
Cr
os
s 
Se
ct
io
n(i
n f
b)
N
um
be
r o
f e
ve
nt
s
tanβ
95% C.L
60
80
100
120
140
Fig.1
60
80
100
120
140
160
10 20 30 40 50 60 70 80 90 100
Ch
ar
ge
d 
Hi
gg
s 
m
as
s
Ps
eu
do
sc
al
ar
 H
ig
gs
 m
as
s
tanβ
Excluded region
at 95% C.L
Fig.2
60
90
115


Constraints on the charged Higgs sector from the Tevatron collider data on top quark decay

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Constraints on the Charged Higgs Sector from the Tevatron Collider Data
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Constraints on the Charged Higgs Sector from the Tevatron Collider Data on Top Quark Decay

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