31 research outputs found
Single Leptoquark Solutions to the -physics Anomalies
We examine various scenarios in which the Standard Model is extended by a
light leptoquark state to explain one or both -physics anomalies. Combining
low-energy constraints and direct searches at the LHC, we confirm that the only
single leptoquark model that can explain both anomalies at the same time is a
vector leptoquark, known as . Focusing on , we highlight the
complementarity between LHC and low--energy constraints, and argue that
improving the experimental bound on by two orders
of magnitude could compromise its viability as a solution to the -physics
anomalies.Comment: 6 pages, 1 figure, 1 table; contribution to the 2019 EW session of
the 54th Rencontres de Moriond; based on arXiv:1808.0817
Dark Matter Phenomenology of SM and Enlarged Higgs Sectors Extended with Vector Like Leptons
We will investigate the scenario in which the Standard Model (SM) Higgs
sector and its 2-doublet extension (called the Two Higgs Doublet Model or 2HDM)
are the "portal" for the interactions between the Standard Model and a
fermionic Dark Matter (DM) candidate. The latter is the lightest stable neutral
particle of a family of vector-like leptons (VLLs). We will provide an
extensive overview of this scenario combining the constraints purely coming
from DM phenomenology with more general constraints like Electro-weak Precision
Tests (EWPT) as well as with collider searches. In the case that the new
fermionic sector interacts with the SM Higgs sector, constraints from DM
phenomenology force the new states to lie above the TeV scale. This requirement
is relaxed in the case of 2HDM. Nevertheless, strong constraints coming from
Electroweak Precision Tests (EWPT) and the Renormalization Group Equations
(RGEs) limit the impact of VLFs on collider phenomenology.Comment: 32 pages, 12 figure
Vector-like top/bottom quark partners and Higgs physics at the LHC
Vector-like quarks (VLQs) that are partners of the heavy top and bottom
quarks are predicted in many extensions of the Standard Model (SM). We explore
the possibility that these states could explain not only the longstanding
anomaly in the forward-backward asymmetry in -quark production at LEP,
, but also the more recent deviation of the cross
section for the associated Higgs production with top quark pairs at the LHC,
. Introducing three illustrative models for VLQs with
different representations under the SM gauge group, we show that the two
anomalies can be resolved while satisfying all other theoretical and
experimental constraints. In this case, the three different models predict VLQ
states in the TeV mass range that can be soon probed at the LHC. In a
second step, we discuss the sensitivity on the VLQ masses and couplings that
could be obtained by means of a percent level accuracy in the measurement of
ratios of partial Higgs decay widths, in particular and . We show that top and bottom VL partners with
masses up to TeV and exotic VLQs with masses in the TeV range can
be probed at the high-luminosity LHC.Comment: 27 pages, 6 figures; v2: added reference
Scenarii for interpretations of the LHC diphoton excess: two Higgs doublets and vector-like quarks and leptons
An evidence for a diphoton resonance at a mass of 750 GeV has been observed
in the data collected at the LHC run at a center of mass energy of 13 TeV. We
explore several interpretations of this signal in terms of Higgs-like
resonances in a two-Higgs doublet model and its supersymmetric incarnation, in
which the heavier CP-even and CP-odd states present in the model are produced
in gluon fusion and decay into two photons through top quark loops. We show
that one cannot accommodate the observed signal in the minimal versions of
these models and that an additional particle content is necessary. We then
consider the possibility that vector-like quarks or leptons strongly enhance
the heavy Higgs couplings to photons and eventually gluons, without altering
those of the already observed 125 GeV state.Comment: 13 pages, 2 figures. v3: Discussion in section 3 extended and
inconsistency in the pseudoscalar coupling corrected; conclusions unchange
Diboson resonances within a custodially protected warped extra-dimensional scenario
We propose an interpretation of the diboson excess recently observed by the
ATLAS and CMS collaborations in terms of Kaluza-Klein excitations of
electroweak gauge bosons stemming from a realization of a warped
extra-dimensional model that is protected by a custodial symmetry. Besides
accounting for the LHC diboson data, this scenario also leads to an explanation
of the anomalies that have been observed in the measurements of the
forward-backward asymmetries for bottom quarks at LEP and top quarks at the
Tevatron.Comment: 10 pages, 2 figure
Integrating out new fermions at one loop
We present the fermionic universal one-loop effective action obtained by integrating out heavy vector-like fermions at one loop using functional techniques. Even though previous approaches are able to handle integrating out heavy fermions with non-chiral interactions, i.e. vanishing γ5 interaction terms, the computations proceed in a tedious manner that obscures a physical interpretation. We show how directly tackling the fermionic functional determinant not only allows for a much simpler and transparent computation, but is also able to account for chiral interaction terms in a simple, algorithmic way. Finally, we apply the obtained results to integrate out at one loop the vector-like fermions appearing in a toy model and in a fermionic model that exhibits strong cosmological phase transitions
Integrating Out New Fermions at One Loop
We present the fermionic universal one--loop effective action obtained by
integrating out heavy vector--like fermions at one loop using functional
techniques. Even though previous approaches are able to handle integrating out
heavy fermions with non--chiral interactions, i.e. vanishing
interaction terms, the computations proceed in a tedious manner that obscures a
physical interpretation. We show how directly tackling the fermionic functional
determinant not only allows for a much simpler and transparent computation, but
is also able to account for chiral interaction terms in a simple, algorithmic
way. Finally, we apply the obtained results to integrate out at one loop the
vector--like fermions appearing in a toy model and in a fermionic model that
exhibits strong cosmological phase transitions.Comment: 36 pages, no figure
Lepton Flavor Violation and Dilepton Tails at the LHC
Starting from a general effective Lagrangian for lepton flavor violation
(LFV) in quark-lepton transitions, we derive constraints on the effective
coefficients from the high-mass tails of the dilepton processes (with ). The current (projected) limits derived in this paper
from LHC data with () can be applied
to generic new physics scenarios, including the ones with scalar, vector and
tensor effective operators. For purely left-handed operators, we explicitly
compare these LHC constraints with the ones derived from flavor-physics
observables, illustrating the complementarity of these different probes. While
flavor physics is typically more constraining for quark-flavor violating
operators, we find that LHC provides the most stringent limits on several
flavor-conserving ones. Furthermore, we show that dilepton tails offer the best
probes for charm-quark transitions at current luminosities and that they
provide competitive limits for tauonic transitions at the
high-luminosity LHC phase. As a by-product, we also provide general numerical
expressions for several low-energy LFV processes, such as the semi-leptonic
decays , and .Comment: 14 pages, 2 figures, 5 tables, published versio