5.8 Leptoquarks
205
on an additional parameter κ, which is a dimensionless coupling and takes a value
of κ = 1 in the Yang-Mills case and κ = 0 in the minimal coupling case [1259]. In
experimental searches for LQ pair production, the acceptance is unchanged when
considering vector instead of scalar LQs, such that the upper cross sections limits
apply to both cases. The mass exclusion is stronger for vector LQs due to the larger
production cross section.
Collider searches for the pair production of LQs with decays to third-generation
quarks have been performed in the decay channels LQ → bτ , and LQ → bν at
√
s = 8 TeV [1168, 1260–1265] and at
√
s = 13 TeV [1266, 1267]. The first search
targeting top-quark couplings in the decay channel LQ → tτ , has been performed
by CMS at 8 TeV [1268]. The search is performed in τ h +jets final states. One of
the difficulties in this search is the estimation of tt and W +jets backgrounds from
misidentified τ h at high p T , where dedicated measurements of the τ h misidentification
rate have been essential. With 35.9 fb
−1 of 13 TeV data [1269], this search has been
improved by adding the τ h τ h final state, made possible by the larger data set and
higher
√
s. Additionally, a background estimation from data has been developed,
making measurements of the τ h misidentification rate unnecessary. The background
from misidentified τ h candidates is obtained from control regions with inverted τ h
isolation criteria. The measurement uses the reconstructed top quark p T as sensitive
observable, which is predicted to have a very different shape for LQ → tτ production
than from SM processes. In contrast to p T of the τ h candidate, this observable is not
biased from the inversion of the τ h isolation. The measured distributions of top
quark p T in two signal regions are shown in Fig. 5.30, where a different background
composition is observed for events with opposite-sign and same-sign lepton charges.
This search excludes scalar LQ masses below 900 GeV for B(LQ → tτ ) = 1. Very
recently, ATLAS has performed an analysis in search for LQ → tτ using 139 fb
−1
of 13 TeV data [1270]. The search considers τ h , τ h τ h , τ h , τ h τ h and τ h final
states, where the categories with additional light leptons could be included thanks
to the larger dataset compared to the CMS search at 13 TeV. The search defines
seven signal categories, defined by the lepton charges, and measures the distribution
in the effective LQ mass in each category. The sensitivity is dominated by the τ h
category, but the τ h and τ h categories bring a significant improvement. The
search excludes scalar LQs below 1430 GeV for B(LQ → tτ ) = 1.
Constraints on LQ decays to neutrinos can be obtained from a reinterpretation
of SUSY searches for direct squark production, in the limit of a massless neutralino. Since squarks are scalar particles and the neutralino is a spin-1/2 fermion,
the same phenomenological signature is obtained for ˜
q → q ˜
χ
0
1 as expected from
LQ → qν [1228]. This has been exploited for the first time by CMS in a reinterpretation of a SUSY search for the direct production of third-generation squarks [1170],
which results in excluded scalar LQ masses below 1100 and 1020 GeV for B(LQ →
bν) = 1 and B(LQ → tν) = 1, respectively [1271]. ATLAS has combined a search
for LQ → bτ using 36.1 fb
−1 of 13 TeV data [1272], with the reinterpretation of
four SUSY searches [528, 1273–1275], to cover all possible final states with thirdgeneration couplings, LQ → (t/b)(τ/ν). Masses of scalar LQs below 800 GeV are
excluded for any combination of branching fractions. The highest mass limit of
205
on an additional parameter κ, which is a dimensionless coupling and takes a value
of κ = 1 in the Yang-Mills case and κ = 0 in the minimal coupling case [1259]. In
experimental searches for LQ pair production, the acceptance is unchanged when
considering vector instead of scalar LQs, such that the upper cross sections limits
apply to both cases. The mass exclusion is stronger for vector LQs due to the larger
production cross section.
Collider searches for the pair production of LQs with decays to third-generation
quarks have been performed in the decay channels LQ → bτ , and LQ → bν at
√
s = 8 TeV [1168, 1260–1265] and at
√
s = 13 TeV [1266, 1267]. The first search
targeting top-quark couplings in the decay channel LQ → tτ , has been performed
by CMS at 8 TeV [1268]. The search is performed in τ h +jets final states. One of
the difficulties in this search is the estimation of tt and W +jets backgrounds from
misidentified τ h at high p T , where dedicated measurements of the τ h misidentification
rate have been essential. With 35.9 fb
−1 of 13 TeV data [1269], this search has been
improved by adding the τ h τ h final state, made possible by the larger data set and
higher
√
s. Additionally, a background estimation from data has been developed,
making measurements of the τ h misidentification rate unnecessary. The background
from misidentified τ h candidates is obtained from control regions with inverted τ h
isolation criteria. The measurement uses the reconstructed top quark p T as sensitive
observable, which is predicted to have a very different shape for LQ → tτ production
than from SM processes. In contrast to p T of the τ h candidate, this observable is not
biased from the inversion of the τ h isolation. The measured distributions of top
quark p T in two signal regions are shown in Fig. 5.30, where a different background
composition is observed for events with opposite-sign and same-sign lepton charges.
This search excludes scalar LQ masses below 900 GeV for B(LQ → tτ ) = 1. Very
recently, ATLAS has performed an analysis in search for LQ → tτ using 139 fb
−1
of 13 TeV data [1270]. The search considers τ h , τ h τ h , τ h , τ h τ h and τ h final
states, where the categories with additional light leptons could be included thanks
to the larger dataset compared to the CMS search at 13 TeV. The search defines
seven signal categories, defined by the lepton charges, and measures the distribution
in the effective LQ mass in each category. The sensitivity is dominated by the τ h
category, but the τ h and τ h categories bring a significant improvement. The
search excludes scalar LQs below 1430 GeV for B(LQ → tτ ) = 1.
Constraints on LQ decays to neutrinos can be obtained from a reinterpretation
of SUSY searches for direct squark production, in the limit of a massless neutralino. Since squarks are scalar particles and the neutralino is a spin-1/2 fermion,
the same phenomenological signature is obtained for ˜
q → q ˜
χ
0
1 as expected from
LQ → qν [1228]. This has been exploited for the first time by CMS in a reinterpretation of a SUSY search for the direct production of third-generation squarks [1170],
which results in excluded scalar LQ masses below 1100 and 1020 GeV for B(LQ →
bν) = 1 and B(LQ → tν) = 1, respectively [1271]. ATLAS has combined a search
for LQ → bτ using 36.1 fb
−1 of 13 TeV data [1272], with the reinterpretation of
four SUSY searches [528, 1273–1275], to cover all possible final states with thirdgeneration couplings, LQ → (t/b)(τ/ν). Masses of scalar LQs below 800 GeV are
excluded for any combination of branching fractions. The highest mass limit of