174
5 Direct Searches for New Physics
determined by the parameters κ L and κ R , with the corresponding projection operators
P L and P R . Similarly to (5.3), the Lagrangian describing the weak interaction of the
b
∗ is given by [978]
L =
g W
√
2
W
+
μ tγ
μ
(g L P L + g R P R ) b
∗
+ h.c.,
(5.5)
where g L and g R determine the left- and right-handed b
∗ couplings to W t. When
both interactions are considered, g L/R = 0 and κ L/R = 0, the b
∗ can be produced
through the strong interaction of (5.4) or the weak interaction of (5.5), where the
latter results in an identical signature as single VLQ production. However, production
through the weak interaction is largely suppressed because of the necessity of a top
quark in the initial state, which requires a g → tt splitting. The resulting production
of a b
∗ in association with a t and a forward jet has a cross section more than an order
of magnitude smaller than the cross section of bg → b
∗ , such that a singly produced
b
∗ is rarely produced with a forward jet. Besides this, the signature of b
∗
→ W t is
similar to single B production with B → W t. Single VLQ searches targeting the
W t channel can be reinterpreted in b
∗ models, but the requirement of a forward jet,
which is often used to define the signal region in single VLQ searches, reduces the
sensitivity to a singly produced b
∗ . Vice versa, b
∗ searches not requiring a forward
jet do not achieve the optimal sensitivity for single B production.
The first analysis searching for b
∗
→ W t has been performed by ATLAS using
4.7 fb
−1 of 7 TeV data [979]. The analysis considers fully resolved b
∗ decays and
selects events with either two leptons and exactly one small-R jet, or with one lepton
and three small-R jets, one of which has to be b-tagged. The signal signature is
similar to tt production, where one b is not reconstructed or produced outside the
detector acceptance, such that this constitutes the largest background. The second
largest background is the irreducible SM single top production in the W t channel in
the dilepton selection and W +jets production in the +jets selection. The b
∗ mass
can be reconstructed in the +jets final state and serves as sensitive observable to
discriminate between signal and background. In the dilepton final state S T is used,
which is defined as the scalar p T -sum of the leptons, p
miss
T
and the small-R jet.
For vector-like couplings, i.e. identical left- and right-handed b
∗ couplings κ L =
κ R = g L = g R = 1, b
∗ masses below 1030 GeV can be excluded at 95% CL with
this search. This search has been improved for the analysis of 8 TeV data, where
also jet substructure information has been introduced in the +jets channel [905].
Large-R jets with a trimmed mass larger than 50 GeV are used to reconstruct either
the W or t. No further requirements are imposed on large-R jets in order to be
as inclusive as possible. The signal region is divided into a b
∗ and a B category,
depending on the presence of a forward jet. Because of this categorisation, the analysis
achieves sensitivity to both heavy bottom partners, produced through the strong and
weak force. The main backgrounds in this search are estimated from simulation. The
normalisation and modelling uncertainties are constrained with the help of control
regions, which are obtained by requiring no b-tagged small-R jets for the W +jets
control region and at least two b-tagged small-R jets for the tt control region. The
5 Direct Searches for New Physics
determined by the parameters κ L and κ R , with the corresponding projection operators
P L and P R . Similarly to (5.3), the Lagrangian describing the weak interaction of the
b
∗ is given by [978]
L =
g W
√
2
W
+
μ tγ
μ
(g L P L + g R P R ) b
∗
+ h.c.,
(5.5)
where g L and g R determine the left- and right-handed b
∗ couplings to W t. When
both interactions are considered, g L/R = 0 and κ L/R = 0, the b
∗ can be produced
through the strong interaction of (5.4) or the weak interaction of (5.5), where the
latter results in an identical signature as single VLQ production. However, production
through the weak interaction is largely suppressed because of the necessity of a top
quark in the initial state, which requires a g → tt splitting. The resulting production
of a b
∗ in association with a t and a forward jet has a cross section more than an order
of magnitude smaller than the cross section of bg → b
∗ , such that a singly produced
b
∗ is rarely produced with a forward jet. Besides this, the signature of b
∗
→ W t is
similar to single B production with B → W t. Single VLQ searches targeting the
W t channel can be reinterpreted in b
∗ models, but the requirement of a forward jet,
which is often used to define the signal region in single VLQ searches, reduces the
sensitivity to a singly produced b
∗ . Vice versa, b
∗ searches not requiring a forward
jet do not achieve the optimal sensitivity for single B production.
The first analysis searching for b
∗
→ W t has been performed by ATLAS using
4.7 fb
−1 of 7 TeV data [979]. The analysis considers fully resolved b
∗ decays and
selects events with either two leptons and exactly one small-R jet, or with one lepton
and three small-R jets, one of which has to be b-tagged. The signal signature is
similar to tt production, where one b is not reconstructed or produced outside the
detector acceptance, such that this constitutes the largest background. The second
largest background is the irreducible SM single top production in the W t channel in
the dilepton selection and W +jets production in the +jets selection. The b
∗ mass
can be reconstructed in the +jets final state and serves as sensitive observable to
discriminate between signal and background. In the dilepton final state S T is used,
which is defined as the scalar p T -sum of the leptons, p
miss
T
and the small-R jet.
For vector-like couplings, i.e. identical left- and right-handed b
∗ couplings κ L =
κ R = g L = g R = 1, b
∗ masses below 1030 GeV can be excluded at 95% CL with
this search. This search has been improved for the analysis of 8 TeV data, where
also jet substructure information has been introduced in the +jets channel [905].
Large-R jets with a trimmed mass larger than 50 GeV are used to reconstruct either
the W or t. No further requirements are imposed on large-R jets in order to be
as inclusive as possible. The signal region is divided into a b
∗ and a B category,
depending on the presence of a forward jet. Because of this categorisation, the analysis
achieves sensitivity to both heavy bottom partners, produced through the strong and
weak force. The main backgrounds in this search are estimated from simulation. The
normalisation and modelling uncertainties are constrained with the help of control
regions, which are obtained by requiring no b-tagged small-R jets for the W +jets
control region and at least two b-tagged small-R jets for the tt control region. The