We analyze a modular invariant model of lepton masses, with neutrino masses
originating either from the Weinberg operator or from the seesaw. The
constraint provided by modular invariance is so strong that neutrino mass
ratios, lepton mixing angles and Dirac/Majorana phases do not depend on any
Lagrangian parameter. They only depend on the vacuum of the theory,
parametrized in terms of a complex modulus and a real field. Thus eight
measurable quantities are described by the three vacuum parameters, whose
optimization provides an excellent fit to data for the Weinberg operator and a
good fit for the seesaw case. Neutrino masses from the Weinberg operator
(seesaw) have inverted (normal) ordering. Several sources of potential
corrections, such as higher dimensional operators, renormalization group
evolution and supersymmetry breaking effects, are carefully discussed and shown
not to affect the predictions under reasonable conditions.
A consistent model for vector mediators to dark matter needs to be
anomaly-free and include a scalar mode from mass generation. For the leading
U(1) extensions we review the structure and constraints, including kinetic
mixing at loop level. The thermal relic density suggests that the vector and
scalar masses are similar. For the LHC we combine a $Z'$ shape analysis with
mono-jets. For the latter, we find that a shape analysis offers significant
improvement over existing cut-and-count approaches. Direct detection limits
strongly constrain the kinetic mixing angle and we propose a $\ell^+\ell^- E_T$
search strategy based on the scalar mediator.
Mono-X searches are standard dark matter search strategies at the LHC. First,
we show how in the case of initial state radiation they essentially collapse to
mono-jet searches. Second, we systematically study mono-X signatures from
decays of heavier dark matter states. Direct detection constraints strongly
limit our MSSM expectations, but largely vanish for mono-Z and mono-Higgs
signals once we include light NMSSM mediators. Finally, the decay topology
motivates mono-W-pair and mono-Higgs-pair searches, strengthening and
complementing their mono-X counterparts.
I gently introduce the diagrammatic birdtrack notation, first for vector
algebra and then for permutations. After moving on to general tensors I review
some recent results on Hermitian Young operators, gluon projectors, and
multiplet bases for SU(N) colour space.
We introduce a new and highly efficient tagger for hadronically decaying top
quarks, based on a deep neural network working with Lorentz vectors and the
Minkowski metric. With its novel machine learning setup and architecture it
allows us to identify boosted top quarks not only from calorimeter towers, but
also including tracking information. We show how the performance of our tagger
compares with QCD-inspired and image-recognition approaches and find that it
significantly increases the performance for strongly boosted top quarks.
Precision phenomenology at the LHC requires accounting for both higher-order
QCD and electroweak corrections as well as for photon-initiated subprocesses.
Building upon the recent NNPDF3.1 fit, in this work the photon content of the
proton is determined within a global analysis supplemented by the LUXqed
constraint relating the photon PDF to lepton-proton scattering structure
functions: NNPDF3.1luxQED. The uncertainties on the resulting photon PDF are at
the level of a few percent, with photons carrying up to 0.5% of the proton's
momentum. We study the phenomenological implications of NNPDF3.1luxQED at the
LHC for Drell-Yan, vector boson pair, top quark pair, and Higgs plus vector
boson production. We find that photon-initiated contributions can be
significant for many processes, leading to corrections of up to 20%. Our
results represent a state-of-the-art determination of the partonic structure of
the proton including its photon component.
Dark matter (DM) coupled to light mediators has been invoked to resolve the
putative discrepancies between collisionless cold DM and galactic structure
observations. However, $\gamma$-ray searches and the CMB strongly constrain
such scenarios. To ease the tension, we consider asymmetric DM. We show that,
contrary to the common lore, detectable annihilations occur even for large
asymmetries, and derive bounds from the CMB, $\gamma$-ray, neutrino and
antiproton searches. We then identify the viable space for self-interacting DM.
Direct detection does not exclude this scenario, but provides a way to test it.
Searches for invisible Higgs decays in weak boson fusion are a well-known
laboratory for jets and QCD studies. We present a series of results on tagging
jets and central jet activity. First, precision analyses of the central jet
activity require full control of single top production in some analyses.
Second, the rate dependence on the size of the tagging jets is not limited to
weak boson fusion. For the first time, we show how subjet information on the
tagging jets and on the additional jet activity can be used to extract the
Higgs signal. The additional observables relieve some of the pressure on other,
critical observables. Finally, we compare the performance of weak boson fusion
and associated Higgs production.
It has been suggested that a colour-entanglement effect exists in the
Drell-Yan cross section for the 'double T-odd' contributions at low transverse
momentum $Q_T$, rendering the colour structure different from that predicted by
the usual factorisation formula . These T-odd contributions can come from
the Boer-Mulders or Sivers transverse momentum dependent distribution
functions. The different colour structure should be visible already at the
lowest possible order that gives a contribution to the double Boer-Mulders
(dBM) or double Sivers (dS) effect, that is at the level of two gluon
exchanges. To discriminate between the different predictions, we compute the
leading-power contribution to the low-$Q_T$ dBM cross section at the two-gluon
exchange order in the context of a spectator model. The computation is
performed using a method of regions analysis with Collins subtraction terms
implemented. The results conform with the predictions of the factorisation
formula. In the cancellation of the colour entanglement, diagrams containing
the three-gluon vertex are essential. Furthermore, the Glauber region turns out
to play an important role - in fact, it is possible to assign the full
contribution to the dBM cross section at the given order to the region in which
the two gluons have Glauber scaling. A similar disentanglement of colour is
found for the dS effect.
In the presence of approximate global symmetries that forbid relevant
interactions, strongly coupled light Dark Matter (DM) can appear weakly coupled
at small energy and generate a sizable relic abundance. Fundamental principles
like unitarity restrict these symmetries to a small class, where the leading
interactions are captured by effective operators up to dimension-8. Chiral
symmetry, spontaneously broken global symmetries and non-linearly realized
supersymmetry are examples of this. Their DM candidates (composite fermions,
pseudo Nambu-Goldstone Bosons and Goldstini) are interesting targets for LHC