Multi-photon signatures at the Fermilab Tevatron

Abstract

Fermiophobic Higgs bosons $h_f$ exhibiting large branching ratios to two photons can arise in models with two or more scalar doublets and/or triplets. In such models the conventional production mechanisms at hadron colliders may be rendered ineffective due to severe mixing angle suppression. In this scenario, double fermiophobic Higgs production may proceed via complementary mechanisms leading to events with up to 4 photons. We perform a simulation of the detection prospects of $h_f$ in the multi-photon (>3) channel at the Fermilab Tevatron and show that a sizeable region of the ($m_{H^\pm},m_{h_f}$) parameter space can be probed during Run II.

Full text

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arXiv:hep-ph/0512077v2 31 Oct 2006
KEK-TH-1056
hep-ph/0512077
December 2005
Multi-photon signatures at the Fermilab Tevatron
A.G. Akeroyda, A. Alvesb, Marco A. D´ ıazc, O.´Ebolib
a: Theory Group, KEK, 1-1 Oho, Tsukuba, Japan 305-0801
b: Instituto de F´ ısica, Universidade de S˜ ao Paulo, S˜ ao Paulo, Brazil
c: Departamento de F´ ısica, Universidad Cat´ olica de Chile,
Avenida Vicu˜ na Mackenna 4860, Santiago, Chile
Abstract
Fermiophobic Higgs bosons (hf) exhibiting large branching ratios to two photons
can arise in models with two or more scalar doublets and/or triplets.
models the conventional production mechanisms at hadron colliders, which rely
on the hfV V coupling (V = W,Z), may be rendered ineffective due to severe
mixing angle suppression. In this scenario, double hfproduction may proceed via
the complementary mechanism qq′→ H±hfwith subsequent decay H±→ hfW∗,
leading to events with up to 4 photons. We perform a simulation of the detection
prospects of hf in the multi-photon (> 3) channel at the Fermilab Tevatron and
show that a sizeable region of the (mH±,mhf) parameter space can be probed during
Run II.
In such
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1Introduction
Neutral Higgs bosons with very suppressed couplings to fermions – “fermiophobic Higgs
bosons” (hf) [1]– may arise in specific versions of the Two Higgs Doublet Model (2HDM)
[2],[3] or in models with Higgs triplets [4]. Such a hf would decay dominantly to two
photons, hf → γγ, for mhf< 95 GeV or to two massive gauge bosons, hf → V V(∗),
(V = W±,Z) for mhf> 95 GeV [5, 6]. The large branching ratio (BR) for hf → γγ
would provide a very clear experimental signature, and observation of such a particle would
strongly constrain the possible choices of the underlying Higgs sector [5, 6, 7, 8, 9, 10, 11].
Experimental searches for hf at LEP and the Fermilab Tevatron have been negative
so far. Mass limits have been set in a benchmark model which assumes that the coupling
hfV V has the same strength as the Standard Model (SM) Higgs coupling V V φ0, and that
all fermion BRs are exactly zero. Lower bounds of the order mhf>
obtained by the LEP collaborations OPAL[12], DELPHI[13], ALEPH[14], and L3[15],
utilizing the channel e+e−→ hfZ, hf → γγ. At the Tevatron Run I, the limits on
mhffrom the DØ and CDF collaborations are respectively 78.5 GeV [16] and 82 GeV
[17] at 95% C.L., using the mechanism qq′→ V∗→ hfV , hf → γγ, with the dominant
contribution coming from V = W±. For an integrated luminosity of 2 fb−1, Run II
will extend the coverage of mhfin the benchmark model slightly beyond that of LEP
[18],[20]. In addition, Run II will be sensitive to the region 110GeV < mhf< 160GeV
and BR(hf→ γγ) > 4% which could not be probed at LEP. A preliminary search in the
inclusive 2γ + X channel has been performed with 190pb−1of Run II data [21].
However, the hfV V coupling in a specific model could be suppressed relative to the
φ0V V coupling by a mixing angle, leading to a weakening of the above mass limits. If this
suppression were quite severe (hfV V/φ0V V < 0.1) a very light hf (mhf<< 100 GeV)
would have eluded the searches at LEP and the Tevatron Run I in production mechanisms
which rely upon the hfV V coupling. Therefore it is of interest to consider other production
mechanisms for hfwhich may allow observable rates if the hfV V coupling is suppressed.
Since the couplings hfV V and hfV H (where H is another Higgs boson in the model) are
complementary, two LEP collaborations, i.e. OPAL [12] and DELPHI [13], also searched
for fermiophobic Higgs bosons in the channel e+e−→ A0hf, and ruled out the region
mA+ mhf< 160 GeV. However, a very light mhf< 50 GeV is still possible if mA is
sufficiently heavy.
An alternative production mechanism which also depends on the complementary
hfV H coupling is the process qq′→ H±hf [22], [23]. Such a mechanism is exclusive
to a hadron collider, and can offer promising rates at the Tevatron Run II provided that
H±is not too far above its present mass bound mH± > 90 GeV. This alternative ex-
perimental signature depends on the decays of H±. In fermiophobic models the decay
H±→ hfW(∗)can have a larger BR than the conventional decays H±→ tb,τν [24],[25],
which leads to double hfproduction.
In this paper we analyze the inclusive production of multi-photon (3γ’s or 4γ’s) final
states at the Tevatron RUN II via the mechanism:
∼100 GeV have been
p¯ p → hfH±→ hfhfW±→ γγγ(γ) + X .
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In the 2HDM the multi-photon signature arises in the parameter space mhf<
mH±<
∼200 GeV, and tanβ > 1. In this region, BR(hf → γγ) ∼ 1 and BR(H±→
hfW∗±) ∼ 1, leading to a 4γ + leptons or jets signature. The multi-photon signature has
the added virtue of being extremely clean concerning the background contamination, in
contrast to the conventional searches for single hfproduction in the channels γγ +V and
γγ + X. In the present work we show that the multi-photon signal can be observed in
a large fraction of the mhf⊗ mH± plane at the Tevatron RUN II. In fact, at 3σ level of
statistical significance, the RUN II will be able to exclude Higgs masses up to mH±<
GeV for very light mhf, or mhf<
∼100 GeV for mH± ≈ 100 GeV.
Our work is organized as follows. In Section 2 we give a brief introduction to fermio-
phobic Higgs bosons, exhibiting the main decay channels of hf and H±. The possible
fermiophobic Higgs production mechanisms and respective signatures are described in
Section 3. We present our analyses in Section 4 and Section 5 contains our conclusions.
∼90 GeV,
∼240
2 Fermiophobic Higgs bosons
In this section we briefly review the properties of hf. For a detailed introduction we refer
the reader to [6], [9], [10], [11]. Fermiophobia can arise in i) 2HDM (Model I) and ii) Higgs
triplets models. In (i) the imposition of a discrete symmetry together with a vanishing
mixing angle ensures exact fermiophobia at tree-level. In (ii), gauge invariance forbids
any coupling of hf to quarks while lepton couplings are strongly constrained by neu-
trino oscillation data and lepton flavour violation experiments, resulting in approximate
fermiophobia at tree-level.
2.1 2HDM (Model I)
If Φ1and Φ2are two Higgs SU(2) doublets with hypercharge Y = 1, the most general
SU(2) × U(1) gauge invariant scalar potential is [26]:
V= m2
?
+1
Φ†
Φ†
11Φ†
1Φ1+ m2
22Φ†
?2+ λ3
1Φ2
2Φ2−
?
?
m2
12Φ†
1Φ2+ h.c.
?
?
+1
2λ1
?
Φ†
1Φ1
?2
?
2λ2
?
?
2Φ2
1Φ1
??
1Φ1
Φ†
2Φ2
?
+ λ4
Φ†
1Φ2
??
1Φ2+ h.c.
Φ†
2Φ1
(1)
+
1
2λ5
?
Φ†
?2+λ6
?
Φ†
?
+ λ7
?
Φ†
2Φ2
??
Φ†
?
.
If the discrete symmetry Φ1→ −Φ1is imposed the couplings λ6= λ7= 0. However, the
term proportional to m2
and still ensure that Higgs-mediated tree-level flavour changing neutral currents are absent
[3]. Note that the above 2HDM potential contains one more free parameter than those
studied in Refs. [10],[11].
The potential in eq. (1) breaks SU(2)L× U(1)Y down to U(1)emwhen the two Higgs
doublets acquire vacuum expectation values
1
√2
v1
12can remain as a soft violation of the above discrete symmetry
?Φ1? =
?0
?
,?Φ2? =
1
√2
?0
v2
?
(2)
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which must satisfy the experimental constraint m2
The minimization conditions that define the vacuum expectation values in terms of the
parameters of the potential (λ6= λ7= 0) are
Z=1
2(g2+ g′2)(v2
1+ v2
2) ≈ (91 GeV)2.
t1 = m2
t2 = m2
11v1− m2
22v2− m2
22can be solved in favour of m2
12v2+1
12v1+1
2λ1v3
2λ2v3
1+1
2+1
2(λ3+ λ4+ λ5)v1v2
2(λ3+ λ4+ λ5)v2
2= 0
1v2 = 0(3)
from which m2
The neutral CP-odd Higgs mass matrix is, after using the minimization conditions,
11and m2
Zand tanβ ≡ v2/v1.
M2
A=
?m2
12tβ− λ5v2s2
−m2
β
−m2
m2
12+ λ5v2sβcβ
12/tβ− λ5v2c2
12+ λ5v2sβcβ
β
?
(4)
and is diagonalized by a rotation in an angle β. We define sβ= sinβ, cβ= cosβ, and
tβ = tanβ.M2
while its second eigenvalue is the mass of the physical CP-odd Higgs boson A,
Ahas a zero eigenvalue corresponding to the neutral Goldstone boson
m2
A=m2
12
sβcβ
− λ5v2
(5)
with v2= v2
1+ v2
2. The charged Higgs mass matrix is given by
M2
H± =
?m2
12tβ−1
−m2
2(λ4+ λ5)v2s2
2(λ4+ λ5)v2sβcβ
β
−m2
m2
12+1
12/tβ−1
2(λ4+ λ5)v2sβcβ
2(λ4+ λ5)v2c2
12+1
β
?
(6)
which also is diagonalized by a rotation in an angle β. It has a zero eigenvalue corre-
sponding to the charged Goldstone boson, and the charged Higgs mass is
m2
H± = m2
A+1
2(λ5− λ4)v2.(7)
Here we see that the charged and the CP-odd Higgs masses are independent parameters,
as opposed to supersymmetry, where the mass squared difference is equal to m2
level.
The neutral CP-even Higgs mass matrix is given by
Wat tree
M2
H0 =
?
m2
As2
β+ λ1v2c2
β
−m2
Asβcβ+ (λ3+ λ4)v2sβcβ
m2
−m2
Asβcβ+ (λ3+ λ4)v2sβcβ
Ac2
β+ λ2v2s2
β
?
(8)
and the two eigenvalues are the masses of the neutral CP-even Higgs bosons h and H. It
is diagonalized by an angle α defined by
sin2α =
[−m2
β+ λ2v2s2
A+ (λ3+ λ4)v2]s2β
?2+ [m2
??
m2
Ac2β− λ1v2c2
βA− (λ3+ λ4)v2]2s2
2β
.(9)
Fermiophobia is caused by imposing the mentioned discrete symmetry Φ1 → −Φ1
which forbids Φ1 coupling to the fermions. This model is usually called “Type I” [2].
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However, fermiophobia is erased due to the mixing in the CP–even neutral Higgs mass
matrix, which is diagonalized by the mixing angle α, when both CP–even eigenstates h0
and H0acquire a coupling to the fermions.
The fermionic couplings of the lightest CP–even Higgs h0take the form h0ff ∼
cosα/sinβ, where f is any fermion. Small values of cosα would strongly suppress the
fermionic couplings, and in the limit cosα → 0 the coupling h0ff would vanish at tree–
level, giving rise to fermiophobia. This is achieved if
m2
A= (λ3+ λ4)v2. (10)
Despite this extra constraint, the parameters mA, mH±, and tanβ are still independent
parameters in this model.
However, at the one-loop level, Higgs boson couplings to fermions receive contributions
from loops involving vector bosons and other Higgs bosons,
hf
f
f
∼
1
16π2(gmW)(g2
8)mfC0(m2
h,0,0;0,m2
W,m2
W)
where we have naively estimated the contribution of the loop with the Passarino–Veltman
function C0. To get an order of magnitude of the correction we approximate C0∼ 1/m2
expected in the limit of large Higgs mass, and compare this correction with the tree-level
vertex in the SM gφ0ff∼ gmf/2mW. We find
∆ghff
gφ0ff
64π2
h,
∼
g2
?mW
mh
?2
. (11)
This estimation is also applicable if mh<
correction. Nevertheless, we note that the proper renormalization of the φ0f¯f vertex
involves a counterterm that has to be taken into account. It is conventional to define an
extreme hf in which all BRs to fermions are set to zero. This gives rise to benchmark
BRs which are used in the current searches to set limits on mhf.
∼mW replacing mhby mW. This is a very small
2.2Higgs Triplet Models
Fermiophobia (or partial fermiophobia) can arise for scalar fields in isospin I = 1 triplet
representations. Gauge invariance forbids any couplings of the triplet fields (χ) to quarks.
For hypercharge Y = 2 triplets, the neutral Higgs field χ0can couple to leptons (νν) via
the following Yukawa type interaction [27]:1
hijψT
iLCiτ2∆ψjL+ h.c .(12)
1Note that there is no such interaction for Y = 0 triplets, which are rendered fermiophobic as a
consequence of gauge invariance
5