204
5 Direct Searches for New Physics
5.8 Leptoquarks
Leptoquarks (LQs)
8 are hypothetical particles that can decay to SM quarks and leptons. They are triplets with respect to the strong interaction, have fractional electric
charge, and can be either scalar (spin 0) or vector (spin 1) particles. Many extensions
to the SM, among them grand unification [1213–1215], technicolour [1216, 1217],
and compositeness models [1218, 1219], predict the existence of these particles.
The effective Buchmüller–Rückl–Wyler model [1220] incorporates the assumption
that LQ interactions with SM fermions are renormalisable and gauge invariant, leading to restrictions on the allowed quantum numbers of LQs [1221]. Depending on
its quantum numbers and the coupling structure, a given LQ can decay to any one
of a number of different combinations of SM fermions. The couplings of LQs to
leptons and quarks of different generations introduce flavour changing neutral currents that may be observable in precision measurements [1222]. Therefore, most
searches at the Tevatron [1223], HERA [1224] and the LHC have focussed on LQs
with couplings to quarks and leptons of the same generation. While simultaneous
couplings to the first and second generations are tightly constrained by experimental
data [1225], the bounds are weaker for couplings to the second and third generation,
thus allowing the existence of LQs with non-diagonal couplings in the generation
matrix [1226–1228]. Leptoquarks have received considerable theoretical and experimental attention recently, because of significant deviations from the predictions of
the SM in measurements of decays of B mesons. In particular, deviations have been
seen in the values of the ratio R D (∗) , defined as the ratio of the B → D
(∗)
τ ν branching
fraction to the B → D
(∗)
μν branching fraction. These deviations from the SM were
first reported by BaBar [1229, 1230] and Belle [1231–1234] and have been confirmed by LHCb [1235, 1236] with a combined significance of about three standard
deviations [1237, 1238]. The ratios of the branching fractions of B → K
(∗)
μμ to
B → K
(∗) ee, R K and R K (∗) , as measured by LHCb [1239–1243], show departures
from lepton universality by 2.6 and 2.4 standard deviations, respectively. The measurement of the muon anomalous magnetic moment a μ , one of the most precisely
measured quantities in particle physics [1244], also deviates from the SM prediction
by 3.5 standard deviations [1245]. These anomalies are among the most significant
deviations from the SM observed so far. The existence of LQs with masses at the
TeV scale and large couplings to third-generation quarks [1226, 1227, 1246–1256]
has been proposed as a possible explanation for one, two, or all of these deviations.
At the LHC, pair production of LQs is possible via gluon-gluon fusion or quarkantiquark annihilation, allowing direct searches to be performed. Single LQ production via quark-gluon scattering is subdominant for LQs coupled to heavy quarks, as it
requires a heavy quark in the initial state. The pair production cross section depends
on the mass of the scalar LQ and is known at NLO precision [1257]. The pair production cross section for vector LQs has been calculated at LO [1258] and is much
larger than the scalar LQ cross section. The cross section for vector LQs depends
8 The text in the first paragraph of this subsection has been taken from [1212] and has been written
by the author. It has been adjusted to fit this book.
5 Direct Searches for New Physics
5.8 Leptoquarks
Leptoquarks (LQs)
8 are hypothetical particles that can decay to SM quarks and leptons. They are triplets with respect to the strong interaction, have fractional electric
charge, and can be either scalar (spin 0) or vector (spin 1) particles. Many extensions
to the SM, among them grand unification [1213–1215], technicolour [1216, 1217],
and compositeness models [1218, 1219], predict the existence of these particles.
The effective Buchmüller–Rückl–Wyler model [1220] incorporates the assumption
that LQ interactions with SM fermions are renormalisable and gauge invariant, leading to restrictions on the allowed quantum numbers of LQs [1221]. Depending on
its quantum numbers and the coupling structure, a given LQ can decay to any one
of a number of different combinations of SM fermions. The couplings of LQs to
leptons and quarks of different generations introduce flavour changing neutral currents that may be observable in precision measurements [1222]. Therefore, most
searches at the Tevatron [1223], HERA [1224] and the LHC have focussed on LQs
with couplings to quarks and leptons of the same generation. While simultaneous
couplings to the first and second generations are tightly constrained by experimental
data [1225], the bounds are weaker for couplings to the second and third generation,
thus allowing the existence of LQs with non-diagonal couplings in the generation
matrix [1226–1228]. Leptoquarks have received considerable theoretical and experimental attention recently, because of significant deviations from the predictions of
the SM in measurements of decays of B mesons. In particular, deviations have been
seen in the values of the ratio R D (∗) , defined as the ratio of the B → D
(∗)
τ ν branching
fraction to the B → D
(∗)
μν branching fraction. These deviations from the SM were
first reported by BaBar [1229, 1230] and Belle [1231–1234] and have been confirmed by LHCb [1235, 1236] with a combined significance of about three standard
deviations [1237, 1238]. The ratios of the branching fractions of B → K
(∗)
μμ to
B → K
(∗) ee, R K and R K (∗) , as measured by LHCb [1239–1243], show departures
from lepton universality by 2.6 and 2.4 standard deviations, respectively. The measurement of the muon anomalous magnetic moment a μ , one of the most precisely
measured quantities in particle physics [1244], also deviates from the SM prediction
by 3.5 standard deviations [1245]. These anomalies are among the most significant
deviations from the SM observed so far. The existence of LQs with masses at the
TeV scale and large couplings to third-generation quarks [1226, 1227, 1246–1256]
has been proposed as a possible explanation for one, two, or all of these deviations.
At the LHC, pair production of LQs is possible via gluon-gluon fusion or quarkantiquark annihilation, allowing direct searches to be performed. Single LQ production via quark-gluon scattering is subdominant for LQs coupled to heavy quarks, as it
requires a heavy quark in the initial state. The pair production cross section depends
on the mass of the scalar LQ and is known at NLO precision [1257]. The pair production cross section for vector LQs has been calculated at LO [1258] and is much
larger than the scalar LQ cross section. The cross section for vector LQs depends
8 The text in the first paragraph of this subsection has been taken from [1212] and has been written
by the author. It has been adjusted to fit this book.