600
M. Toharia
the end of Run II of the LHC, both ATLAS and CMS report that the main production
channel of the Higgs (gluon fusion) is consistent with the SM expectation.
As mentioned in the abstract, both ATLAS and CMS experiments have
announced the observation of tth channel with a combined observed significance of
6.3 sigma and 5.2 sigma respectively [1, 2]. In both results the expected significance
is slightly lower (5.1 sigma for ATLAS and 4.2 for CMS) in both cases caused by
mild excesses in the tt → H → γ γ channel (very mild excesses with measured
cross sections of σ (t ¯
tH ) × B(H → γ γ ) = 1.59
+0.43
−0.39 fb for ATLAS [3] and
σ (t ¯
tH ) × B(H → γ γ ) = 1.56
+0.34
−0.32 fb for CMS [4], to be compared with the SM
expected cross section of 1.13
+0.08
−0.11 fb). Of course, measurements in the tth channel
are hindered by poor statistics, and moreover, not all decay channels give slightly
enhanced signal strengths. Nevertheless, should these tantalizing signals survive
more precise measurements at higher luminosity, they would provide the much
awaited signals for new physics.
One possibility to explain a possible tth enhancement is the modification of
the Higgs coupling to the top quark. A simple explanation would be to invoke the
presence of vector-like quarks which will mix with the SM top quark and thus can
modify their couplings to the Higgs, and in particular it could enhance them (see for
example [5] for an early proposal or the more recent [6] and references therein). In
this framework, the rates for loop-induced processes (specially gluon fusion Higgs
production) could remain SM-like by compensating effects between SM fermions
and the new vector-like fermions in the loops [6]. The masses of these vector-like
quarks must be of order 1–2 TeV in order to modify significantly the top Yukawa
coupling; such masses may still be safe from the current LHC lower limits on their
masses, m VLQ 1000 GeV [7].
2 Top and Bottom Partners: A Doublet and Two Singlets
The simple scenario that we consider contains the usual SM gauge groups and
matter fields, with the addition of a vector-like quark SU (2) doublet and two
vector-like quarks SU (2) singlets, one with up-type gauge charge and another with
down-type gauge charge. We denote q 0
L ≡
t 0
L
b 0
L
as the SM third generation
doublet, and t 0
R as the SM right handed top. Using similar notation we define
Q L,R ≡
Q t
L,R
Q b
L,R
as the new vector-like quark doublet, T R,L as the new vector-like
up-type quark singlet, and B L,R as the new vector-like down-type singlet. We can
write down the Yukawa couplings between the SM top quark and the new vector-like
quarks, which will lead to 5 × 5 fermion mass matrices. The mass and interaction
Lagrangian in the top sector can be then written as
M. Toharia
the end of Run II of the LHC, both ATLAS and CMS report that the main production
channel of the Higgs (gluon fusion) is consistent with the SM expectation.
As mentioned in the abstract, both ATLAS and CMS experiments have
announced the observation of tth channel with a combined observed significance of
6.3 sigma and 5.2 sigma respectively [1, 2]. In both results the expected significance
is slightly lower (5.1 sigma for ATLAS and 4.2 for CMS) in both cases caused by
mild excesses in the tt → H → γ γ channel (very mild excesses with measured
cross sections of σ (t ¯
tH ) × B(H → γ γ ) = 1.59
+0.43
−0.39 fb for ATLAS [3] and
σ (t ¯
tH ) × B(H → γ γ ) = 1.56
+0.34
−0.32 fb for CMS [4], to be compared with the SM
expected cross section of 1.13
+0.08
−0.11 fb). Of course, measurements in the tth channel
are hindered by poor statistics, and moreover, not all decay channels give slightly
enhanced signal strengths. Nevertheless, should these tantalizing signals survive
more precise measurements at higher luminosity, they would provide the much
awaited signals for new physics.
One possibility to explain a possible tth enhancement is the modification of
the Higgs coupling to the top quark. A simple explanation would be to invoke the
presence of vector-like quarks which will mix with the SM top quark and thus can
modify their couplings to the Higgs, and in particular it could enhance them (see for
example [5] for an early proposal or the more recent [6] and references therein). In
this framework, the rates for loop-induced processes (specially gluon fusion Higgs
production) could remain SM-like by compensating effects between SM fermions
and the new vector-like fermions in the loops [6]. The masses of these vector-like
quarks must be of order 1–2 TeV in order to modify significantly the top Yukawa
coupling; such masses may still be safe from the current LHC lower limits on their
masses, m VLQ 1000 GeV [7].
2 Top and Bottom Partners: A Doublet and Two Singlets
The simple scenario that we consider contains the usual SM gauge groups and
matter fields, with the addition of a vector-like quark SU (2) doublet and two
vector-like quarks SU (2) singlets, one with up-type gauge charge and another with
down-type gauge charge. We denote q 0
L ≡
t 0
L
b 0
L
as the SM third generation
doublet, and t 0
R as the SM right handed top. Using similar notation we define
Q L,R ≡
Q t
L,R
Q b
L,R
as the new vector-like quark doublet, T R,L as the new vector-like
up-type quark singlet, and B L,R as the new vector-like down-type singlet. We can
write down the Yukawa couplings between the SM top quark and the new vector-like
quarks, which will lead to 5 × 5 fermion mass matrices. The mass and interaction
Lagrangian in the top sector can be then written as