Large Higgs-electron Yukawa coupling in 2HDM

The present upper bound on $\kappa_e$, the ratio between the electron Yukawa coupling and its Standard Model value, is of ${\cal O}(600)$. We ask what would be the implications in case that $\kappa_e$ is close to this upper bound. The simplest extension that allows for such enhancement is that of two Higgs doublet models (2HDM) without natural flavor conservation. In this framework, we find the following consequences: (i) Under certain conditions, measuring $\kappa_e$ and $\kappa_V$ would be enough to predict values of Yukawa couplings for other fermions and for the $H$ and $A$ scalars. (ii) In the case that the scalar potential has a softly broken $Z_2$ symmetry, the second Higgs doublet must be light, but if there is hard breaking of the symmetry, the second Higgs doublet can be much heavier than the electroweak scale and still allow the electron Yukawa coupling to be very different from its SM value. (iii) CP must not be violated at a level higher than ${\cal O}(0.01/\kappa_e)$ in both the scalar potential and the Yukawa sector. (iv) LHC searches for $e^+e^-$ resonances constrain this scenario in a significant way. Finally, we study the implications for models where one of the scalar doublets couples only to the first generation, or only to the third generation.Comment: 14 pages, 2 figure

GeV-scale dark matter: production at the Main Injector

Assuming that dark matter particles interact with quarks via a GeV-scale mediator, we study dark matter production in fixed target collisions. The ensuing signal in a neutrino near detector consists of neutral-current events with an energy distribution peaked at higher values than the neutrino background. We find that for a $Z'$ boson of mass around a few GeV that decays to dark matter particles, the dark matter beam produced by the Main Injector at Fermilab allows the exploration of a range of values for the gauge coupling that currently satisfy all experimental constraints. The NO$\nu$A detector is well positioned for probing the presence of a dark matter beam, while future LBNF near-detectors would provide more sensitive probes.Comment: 25 pages, 9 figure

Relaxion and light (pseudo)scalars at the HL-LHC and lepton colliders

We study the potential of future lepton colliders, running at the Z-pole and above, and the High-Luminosity LHC to search for the relaxion and other light scalars $\phi$. We investigate the interplay of direct searches and precision observables for both CP-even and -odd couplings. In particular, precision measurements of exotic Z-decays, Higgs couplings, the exotic Higgs decay into a relaxion pair and associated $Z\phi$ and $\gamma \phi$ production are promising channels to yield strong bounds.Comment: 27 pages + references, 5 figures, 2 table

Fermion Hierarchy from Sfermion Anarchy

We present a framework to generate the hierarchical flavor structure of Standard Model quarks and leptons from loops of superpartners. The simplest model consists of the minimal supersymmetric standard model with tree level Yukawa couplings for the third generation only and anarchic squark and slepton mass matrices. Agreement with constraints from low energy flavor observables, in particular Kaon mixing, is obtained for supersymmetric particles with masses at the PeV scale or above. In our framework both the second and the first generation fermion masses are generated at 1-loop. Despite this, a novel mechanism generates a hierarchy among the first and second generations without imposing a symmetry or small parameters. A second-to-first generation mass ratio of order 100 is typical. The minimal supersymmetric standard model thus includes all the necessary ingredients to realize a fermion spectrum that is qualitatively similar to observation, with hierarchical masses and mixing. The minimal framework produces only a few quantitative discrepancies with observation, most notably the muon mass is too low. We discuss simple modifications which resolve this and also investigate the compatibility of our model with gauge and Yukawa coupling Unification.Comment: 42 pages, 11 figure

Dark matter beams at LBNF

High-intensity neutrino beam facilities may produce a beam of light dark matter when protons strike the target. Searches for such a dark matter beam using its scattering in a nearby detector must overcome the large neutrino background. We characterize the spatial and energy distributions of the dark matter and neutrino beams, focusing on their differences to enhance the sensitivity to dark matter. We find that a dark matter beam produced by a $Z'$ boson in the GeV mass range is both broader and more energetic than the neutrino beam. The reach for dark matter is maximized for a detector sensitive to hard neutral-current scatterings, placed at a sizable angle off the neutrino beam axis. In the case of the Long-Baseline Neutrino Facility (LBNF), a detector placed at roughly 6 degrees off axis and at a distance of about 200 m from the target would be sensitive to $Z'$ couplings as low as 0.05. This search can proceed symbiotically with neutrino measurements. We also show that the MiniBooNE and MicroBooNE detectors, which are on Fermilab's Booster beamline, happen to be at an optimal angle from the NuMI beam and could perform searches with existing data. This illustrates potential synergies between LBNF and the short-baseline neutrino program if the detectors are positioned appropriately.Comment: 26 pages, 7 figure

Current and future perspectives of positronium and muonium spectroscopy as dark sectors probe

Positronium and Muonium are purely leptonic atoms and hence free of an internal sub-structure. This qualifies them as potentially well suited systems to probe the existence of physics beyond the Standard Model. We hence carry out a comprehensive study of the sensitivity of current Positronium and Muonium precision spectroscopy to several new physics scenarios. By taking properly into account existing experimental and astrophysical probes, we define clear experimental targets to probe new physics via precise spectroscopy. For Positronium we find that, in order for the spectroscopy bounds to reach a sensitivity comparable to the electron gyromagnetic factor, an improvement of roughly five orders of magnitude from state-of-the-art precision is required, which would be a challenge based on current technology. More promising is instead the potential reach of Muonium spectroscopy: in the next few years experiments like Mu-MASS at PSI will probe new regions of the parameter space testing the existence of medium/short range (MeV and above) spin-dependent and spin-independent dark forces between electrons and muons.Comment: 8 pages, 6 figures. Version changed to match journal version. Figures update

Phenomenology of relaxion-Higgs mixing

We show that the relaxion generically stops its rolling at a point that breaks CP leading to relaxion-Higgs mixing. This opens the door to a variety of observational probes since the possible relaxion mass spans a broad range from sub-eV to the GeV scale. We derive constraints from current experiments (fifth force, astrophysical and cosmological probes, beam dump, flavour, LEP and LHC) and present projections from future experiments such as NA62, SHiP and PIXIE. We find that a large region of the parameter space is already under the experimental scrutiny. All the experimental constraints we derive are equally applicable for general Higgs portal models. In addition, we show that simple multiaxion (clockwork) UV completions suffer from a mild fine tuning problem, which increases with the number of sites. These results favour a cut-off scale lower than the existing theoretical bounds.Comment: 46 pages, 6 figures, v3: typos fixed, references added, version matches the version published in JHE

Event generation for beam dump experiments

A wealth of new physics models which are motivated by questions such as the nature of dark matter, the origin of the neutrino masses and the baryon asymmetry in the universe, predict the existence of hidden sectors featuring new particles. Among the possibilities are heavy neutral leptons, vectors and scalars, that feebly interact with the Standard Model (SM) sector and are typically light and long lived. Such new states could be produced in high-intensity facilities, the so-called beam dump experiments, either directly in the hard interaction or as a decay product of heavier mesons. They could then decay back to the SM or to hidden sector particles, giving rise to peculiar decay or interaction signatures in a far-placed detector. Simulating such kind of events presents a challenge, as not only short-distance new physics (hard production, hadron decays, and interaction with the detector) and usual SM phenomena need to be described but also the travel has to be accounted for as determined by the geometry of the detector. In this work, we describe a new plugin to the {\sc MadGraph5\_aMC@NLO} platform, which allows the complete simulation of new physics processes relevant for beam dump experiments, including the various mechanisms for the production of hidden particles, namely their decays or scattering off SM particles, as well as their far detection, keeping into account spatial correlations and the geometry of the experiment.Comment: LaTeX, 42 pages, 14 figure