73 research outputs found
Effective theory of a doubly charged singlet scalar: complementarity of neutrino physics and the LHC
We consider a rather minimal extension of the Standard Model involving just
one extra particle, namely a single singlet scalar and its
antiparticle . We propose a model independent effective operator, which
yields an effective coupling of to pairs of same sign weak gauge
bosons, . We also allow tree-level couplings of
to pairs of same sign right-handed charged leptons of the
same or different flavour. We calculate explicitly the resulting two-loop
diagrams in the effective theory responsible for neutrino mass and mixing. We
propose sets of benchmark points for various masses and couplings
which can yield successful neutrino masses and mixing, consistent with limits
on charged lepton flavour violation (LFV) and neutrinoless double beta decay.
We discuss the prospects for discovery at the LHC, for these
benchmark points, including single and pair production and decay into same sign
leptons plus jets and missing energy. The model represents a minimal example of
the complementarity between neutrino physics (including LFV) and the LHC,
involving just one new particle, the .Comment: 57 pages, 14 figures, 10 tables, version accepted for publication in
JHE
Extra Quarks Decaying to Dark Matter Beyond the Narrow Width Approximation
We explore the effects induced by a finite width in processes of pair
production of a heavy top-quark partner and its subsequent decay into a bosonic
Dark Matter (DM) candidate -- either scalar or vector -- and a SM up-type quark
at the Large Hadron Collider (LHC). We discuss the configurations of masses,
widths and couplings where this phenomenology can be important in a simple
model with just one such objects. Finally, we emphasise the correct definition
of signal and background to be adopted as well as stress the importance of new
dedicated experimental searches.Comment: 5 pages, 6 figures, proceeding LHCp 201
Production of extra quarks decaying to Dark Matter beyond the Narrow Width Approximation at the LHC
This paper explores the effects of finite width in processes of pair
production of an extra heavy quark with charge 2/3 (top partner) and its
subsequent decay into a bosonic Dark Matter (DM) candidate -- either scalar or
vector -- and SM up-type quarks at the Large Hadron Collider (LHC). This
dynamics has been ignored so far in standard experimental searches of heavy
quarks decaying to DM and we assess herein the regions of validity of current
approaches, based on the assumption that the extra quarks have a narrow width.
Further, we discuss the configurations of masses, widths and couplings where
the latter breaks down.Comment: 20 pages, 16 figures. Version accepted by PR
Large width effects in processes of production of extra quarks decaying to Dark Matter at the LHC
This paper explores the effects of finite width in processes of pair
production of a heavy eXtra Quark (XQ) with charge 2/3 and its subsequent decay
into a Dark Matter (DM) candidate -- either scalar or vector -- and Standard
Model (SM) up-type quarks at the Large Hadron Collider (LHC). This dynamics has
been ignored so far in standard experimental searches of heavy quarks decaying
to DM and we assess herein the regions of validity of current approaches, based
on the assumption that the XQ has a narrow width. Further, we discuss the
configurations of masses, widths and couplings where the latter breaks down.Comment: 6 pages, 7 figures, proceeding DIS 2017, title changed to avoid
overlap with corresponding paper. arXiv admin note: text overlap with
arXiv:1706.0400
LHC signatures of vector-like quarks
This work provides an overview on the current status of phenomenology and
searches for heavy vector-like quarks, which are predicted in many models of
new physics beyond the Standard Model. Searches at Tevatron and at the LHC,
here listed and shortly described, have not found any evidence for new heavy
fermionic states (either chiral or vector-like), and have therefore posed
strong bounds on their masses: depending on specific assumptions on the
interactions and on the observed final state, vector-like quarks with masses up
to roughly 400-600 GeV have been excluded by all experiments. In order to be as
simple and model-independent as possible, the chosen framework for the
phenomenological analysis is an effective model with the addition of a
vector-like quark representation (singlet, doublet or triplet under SU(2))
which couples through Yukawa interactions with all SM families. The relevance
of different observables for the determination of bounds on mixing parameters
is then discussed and a complete overview of possible two-body final states for
every vector-like quark is provided, including their subsequent decay into SM
particles. A list and short description of phenomenological analyses present in
literature is also provided for reference purposes.Comment: review, 21 pages, 8 figures, 4 tables, typos corrected, references
added, updated LHC bounds. Version accepted for publication in AHE
LHC Missing-Transverse-Energy Constraints on Models with Universal Extra Dimensions
We consider the performance of the ATLAS and CMS searches for events with
missing transverse energy, which were originally motivated by supersymmetry, in
constraining extensions of the Standard Model based on extra dimensions, in
which the mass differences between recurrences at the same level are
generically smaller than the mass hierarchies in typical supersymmetric models.
We consider first a toy model with pair-production of a single vector-like
quark U1 decaying into a spin-zero stable particle A1 and jet, exploring the
sensitivity of the CMS alphaT and ATLAS meff analysis to U1 mass and the U1-A1
mass difference. For this purpose we use versions of the Delphes generic
detector simulation with CMS and ATLAS cards, which have been shown to
reproduce the published results of CMS and ATLAS searches for supersymmetry. We
then explore the sensitivity of these searches to a specific model with two
universal extra dimensions, whose signal is dominated by the pair production of
quark recurrences, including searches with leptons. We find that the LHC
searches have greater sensitivity to this more realistic model, due partly to
the contributions of signatures with leptons, and partly to events with large
missing transverse energy generated by the decays of higher-level Kaluza-Klein
recurrences. We find that the CMS alphaT analysis with ~5/fb of data at 7 TeV
excludes a recurrence scale of 600 GeV at a confidence level above 99%,
increasing to 99.9% when combined with the CMS single-lepton search, whereas a
recurrence scale of 700 GeV is disfavoured at the 72% confidence level.Comment: 29 pages, 11 figures, 5 tables, references adde
Higgsphobic and fermiophobic Z' as a single dark matter candidate
A spin-1 Z' particle as a single dark matter candidate is investigated by
assuming that it does not directly couple to the Higgs boson and standard model
fermions and does not mix with the photon and Z boson. The remaining dominant
vertices are quartic Z'Z'ZZ and Z'Z'W+W-, which can induce effective
Z'Z'q\bar{q} couplings through standard-model gauge-boson loops. We discuss
constraints from the cosmological thermal relic density, and direct and
indirect-detection experiments, and find that a dark Z' can only exist above
the W boson mass threshold, and the effective quartic coupling of Z'Z'VV is
bounded in the region of 10^{-3}~10^{-2}.Comment: 18pages, 14 figure
Heavy Vector-like Top Partners at the LHC and flavour constraints
We consider the phenomenology at the Large Hadron Collider of new heavy
vector-like quarks which couple mainly to the third generation quarks via
Yukawa interactions, with special emphasis on non-standard doublet
representations which are less constrained from present data. We also discuss
in detail the flavour limits at tree level and loop level and implications of a
generalised CKM mixing matrix to these cases.Comment: 45 pages, 20 figures, 8 tables. Updated limits in the B-physics part,
typos corrected, minor modifications in the LHC par
A comprehensive exploration of t-channel simplified models of dark matter
We analyse six classes of t-channel dark matter simplified models in which
the Standard Model field content is extended by a coloured mediator and a dark
matter state. The two new states are enforced to be odd under a new parity,
while all Standard Model fields are taken even so that dark matter stability is
guaranteed. We study several possibilities for the spin of the new particles
and the self-conjugate property of the dark matter, and we focus on model
configurations in which the dark matter couples to the right-handed up quark
for simplicity. We investigate how the parameter spaces of the six models can
be constrained by current and future cosmological, astrophysical and collider
searches, and we highlight the strong complementary between those probes. Our
results demonstrate that scenarios featuring a complex (non self-conjugate)
dark matter field are excluded by cosmology and astrophysics alone, the only
possibility to avoid these bounds being to invoke very weak couplings and
mechanisms such as conversion-driven freeze-out. For models with self-conjugate
dark matter, mediator and dark matter masses are pushed deep into the TeV
regime, with the lower limits on the mediator mass reaching 3 to 4 TeV and
those on the dark matter mass 1 to 2 TeV. Those bounds are much stronger than
those obtained in previous studies of t-channel dark matter models, due in
particular to overlooked contributions to LHC signal modelling that we
incorporate in this work for the first time.Comment: 19 pages, 6 figures, happy holidays
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