136 research outputs found
Colored scalars as flavor messengers in grand unified theories
We critically review the proton decay due to the scalar leptoquark exchanges within SU(5) and flipped SU(5) frameworks to address the issue of the model dependence of the relevant tree level operators. We quantify if, and when,
it is necessary to have the leptoquark mass close to a grand unification scale. We summarize novel results regarding a possibility to have a collider accessible leptoquark without a rapid proton decay. The relevant state could be observed indirectly through its influence on physical processes such as the forward-backward asymmetry
in t¯t production due to an antisymmetric set of couplings to a pair of up-quarks. The same leptoquark could affect the muon anomalous magnetic moment through the interaction of a lepton-quark nature. We accordingly investigate whether both sets of couplings can be simultaneously sizable without any conflict with matter stability
Unification versus proton decay in SU(5)
We investigate unification constraints in the simplest renormalizable
non-supersymmetric SU(5) framework. We show that in the scenario where the
Higgs sector is composed of the 5, 24, and 45 dimensional representations the
proton could be practically stable. We accordingly demonstrate that of all the
SU(5) scenarios only the non-renormalizable one with the 5, 24, and 15
dimensional Higgs multiplets can be verify if low-energy supersymmetry is not
realized in Nature.Comment: 10 pages, minor correction
Leptoquark toolbox for precision collider studies
We implement scalar and vector leptoquark (LQ) models in the universal
FeynRules output (UFO) format assuming the Standard Model fermion content and
conservation of baryon and lepton numbers. Scalar LQ implementations include
next-to-leading order (NLO) QCD corrections. We report the NLO QCD inclusive
cross sections in proton-proton collisions at 13 TeV, 14 TeV, and 27 TeV for
all on-shell LQ production processes. These comprise (i) LQ pair production () and (ii) single LQ + lepton production ()
for all initial quark flavours (, , , , and ). Vector LQ
implementation includes adjustable non-minimal QCD coupling. We discuss several
aspects of LQ searches at a hadron collider, emphasising the implications of
gauge invariance, electroweak and flavour constraints, on the possible
signatures. Finally, we outline the high- search strategy for LQs recently
proposed in the literature to resolve experimental anomalies in -meson
decays. In this context, we stress the importance of complementarity of the
three LQ related processes, namely, , ,
and .Comment: 18 pages, 5 figures, 3 tables. Version published in JHE
Novel Leptoquark Pair Production at LHC
We introduce a novel mechanism for the leptoquark pair production at LHC that
is of a -channel topology and is quark-quark initiated. This mechanism
operates under fairly general conditions. One of them is that the two
leptoquarks in question couple to the same lepton and the other one is that the
fermion numbers of these two leptoquarks differ by two. The strength of the
proposed mechanism provides an alternative way to the conventional processes to
efficiently constrain the parameter space of the two leptoquark scenarios at
LHC whenever the aforementioned conditions are met. We accordingly present one
case study to outline the physics potential of this novel production mechanism.Comment: 14 pages, 5 figure
A comparative study of the and leptoquark effects in the light quark regime
We study the phenomenology of two leptoquarks, the Standard Model
singlets and , with regard to the latest experimental data from the
low-energy flavor physics measurements, LHC, and the IceCube neutrino
experiment. We consider a scenario when scalar (vector) leptoquark
() couples exclusively to the down quark and the neutrinos (charged
leptons) of all flavors, where the leptoquark in question couples to the SM
lepton doublets. The couplings of () to the up-type quarks and the
charged leptons (neutrinos) are in turn uniquely determined via
symmetry. We find that the most important constraints on the leptoquark
parameter space originate from flavor physics measurements, followed by the LHC
search limits that take over the flavor physics ones in the large LQ mass
regime. We furthermore show that () marginally improves (spoils) the
fit of the current IceCube data with respect to the SM case within the region
of parameter space that is otherwise consistent with various low-energy flavor
physics measurements and the latest LHC input. Our study offers an up-to-date
analysis for these two leptoquarks in view of the latest experimental data.Comment: 15 pages, 9 figures, modified version with the inclusion of the
latest IceCube dat
Light colored scalars from grand unification and the forward-backward asymmetry in top quark pair production
The experimental results on the t bar t production cross section at the
Tevatron are well described by the QCD contributions within the standard model,
while the recent measurement of the forward-backward asymmetry cannot be
accounted for within this framework. We consider light colored scalars
appearing in a particular SU(5) GUT model within the 45-dimensional Higgs
representation. A virtue of the model is that it connects the presence of a
light colored SU(2) singlet (Delta_6) and a color octet weak doublet (Delta_1)
with bounds on the proton lifetime, which constrain the parameter space of both
scalars. We find that both the t bar t production cross section and the
forward-backward asymmetry can be accommodated simultaneously within this
model. The experimental results prefer a region for the mass of Delta_6 around
400 GeV, while Delta_1 is then constrained to have a mass around the TeV scale
as well. We analyze possible experimental signatures and find that Delta_6
associated top production could be probed in the t bar t + jets final states at
Tevatron and the LHC.Comment: 12 pages, 13 figures, version as publishe
Limits on scalar leptoquark interactions and consequences for GUTs
A colored weak singlet scalar state with hypercharge 4/3 is one of the
possible candidates for the explanation of the unexpectedly large
forward-backward asymmetry in t tbar production as measured by the CDF and D0
experiments. We investigate the role of this state in a plethora of flavor
changing neutral current processes and precision observables of down-quarks and
charged leptons. Our analysis includes tree- and loop-level mediated
observables in the K and B systems, the charged lepton sector, as well as the Z
to b bbar decay width. We perform a global fit of the relevant scalar
couplings. This approach can explain the (g-2)_mu anomaly while tensions among
the CP violating observables in the quark sector, most notably the nonstandard
CP phase (and width difference) in the Bs system cannot be fully relaxed. The
results are interpreted in a class of grand unified models which allow for a
light colored scalar with a mass below 1TeV. We find that the renormalizable
SU(5) scenario is not compatible with our global fit, while in the SO(10) case
the viability requires the presence of both the 126- and 120-dimensional
representations.Comment: 26 pages, 7 figures; version as publishe
Light Colored Scalar as Messenger of Up-Quark Flavor Dynamics in Grand Unified Theories
The measured forward-backward asymmetry in the t tbar production at the
Tevatron might be explained by the additional exchange of a colored weak
singlet scalar. Such state appears in some of the grand unified theories and
its interactions with the up-quarks are purely antisymmetric in flavor space.
We systematically investigate the resulting impact on charm and top quark
physics. The constraints on the relevant Yukawa couplings come from the
experimentally measured observables related to D0--D0bar oscillations, as well
as di-jet and single top production measurements at the Tevatron. After fully
constraining the relevant Yukawa couplings, we predict possible signatures of
this model in rare top quark decays. In a class of grand unified models we
demonstrate how the obtained information enables to constrain the Yukawa
couplings of the up-quarks at very high energy scale.Comment: 13 pages, 11 figures, version as published in PR
Fully Testable Axion Dark Matter within a Minimal GUT
We present a minimal Grand Unified Theory model, based on gauge
symmetry and a global Peccei-Quinn symmetry, that predicts the existence
of an ultralight axion dark matter within a narrow mass range of
neV. This mass window is determined through an interplay
between gauge coupling unification constraints, partial proton decay lifetime
limits, and the need to reproduce the experimentally observed fermion mass
spectrum. The entire parameter space of the proposed model will be probed
through a synergy between several low-energy experiments that look for proton
decay (Hyper-Kamiokande), axion dark matter through axion-photon coupling
(ABRACADABRA and DMRadio-GUT) and nucleon electric dipole moments (CASPEr
Electric).Comment: 28 pages + references, 9 figures, version accepted in PR
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