Federico Mescia

We perform a comprehensive study of the Direct Detection phenomenology of singlet Dark Matter t-channel portal models. For that purpose, we present a complete one-loop matching onto a Heavy Dark-Matter Effective Field Theory, leading to a complete computation of the loop induced Direct Detection cross-section for both scalar and fermionic Dark Matter candidates. The results are compared with current and future bounds from Direct Detection experiments, as well as with the requirement of the correct Dark Matter relic density.

There are two tensions related to the Cabibbo angle of the CKM matrix. First, the determinations of V us from K μ2, K ℓ3, and τ decays disagree at the 3σ level. Second, using the average of these results in combination with β decays (including super-allowed β decays and neutron decay), a deficit in first-row CKM unitarity with a significance of again about 3σ is found. These discrepancies, known as the Cabibbo Angle anomaly, can in principle be solved by modifications of W boson couplings to quarks. However, due to SU (2) L invariance, Z couplings to quarks are also modified and flavour changing neutral currents can occur. In order to consistently assess the agreement of a new physics hypothesis with data, we perform a combined analysis for all dimension-six Standard Model Effective Field Theory operators that generate modified W couplings to first and second generation quarks. We then study models with …

We study the impact of renormalization group effects on QCD axion phenomenology. Focusing on the Dine-Fischler-Srednicki-Zhitnitsky model, we argue that the relevance of running effects for the axion couplings crucially depends on the scale where the heavier Higgs scalars are integrated out. We study the impact of these effects on astrophysical and cosmological bounds as well as on the sensitivity of helioscopes experiments such as IAXO and XENONnT, showing that they can be sizable even in the most conservative case in which the two Higgs doublets remain as light as the TeV scale. We provide simple analytical expressions that accurately fit the numerical solutions of the renormalization group equations as a function of the mass scale of the heavy scalars.

Loss of unitarity in an effective field theory is often cured by the appearance of dynamical resonances, revealing the presence of new degrees of freedom. These resonances may manifest themselves when suitable unitarization techniques are implemented in the effective theory, which in the scalar-isoscalar channel require making use of the coupled-channel formalism. Conversely, experimental detection of a resonance may provide interesting information on the couplings and constants of the relevant effective theory. By applying the systematic procedure developed in previous works, we will attempt to accommodate a possible scalar resonance with mass around 650 GeV for which there is preliminary evidence at the LHC in the vector-boson fusion channel. The results are interesting; the resonance can be accommodated within the experimentally allowed range of next-to-leading order coefficients in the Higgs …

In this work, we explore in detail the presence of scalar resonances in the W W fusion process in the context of the LHC experiments working in the theoretical framework provided by Higgs effective field theories (HEFTs). While the phenomenology of vector resonances is reasonably understood in the framework of Weinberg sum-rules and unitarization studies, scalar resonances are a lot less constrained and, more importantly, do depend on HEFT low-energy effective couplings different from the ones of vector resonances that are difficult to constrain experimentally. More specifically, unitarization techniques combined with the requirement of causality allows us to set nontrivial bounds on Higgs self-interactions. This is due to the need for considering coupled channels in the scalar case along the unitarization process. As a byproduct, we can gain some relevant information on the Higgs sector from W W→ W W elastic …

Loss of unitarity in an effective field theory is often cured by the appearance of dynamical resonances, revealing the presence of new degrees of freedom. These resonances may manifest themselves when suitable unitarization techniques are implemented in the effective theory, which in the scalar-isoscalar channel require making use of the coupled-channel formalism. Conversely, experimental detection of a resonance may provide interesting information on the couplings and constants of the relevant effective theory. By applying the systematical procedure developed in previous works, we will attempt to accommodate a possible scalar resonance with mass around GeV for which there is preliminary evidence at the LHC in the vector boson fusion channel. The results are interesting: the resonance can be accommodated within the experimentally allowed range of next-to-leading order coefficients in the HEFT but in a rather non-trivial manner. Interestingly, its width and production cross section turn out to agree with the tentative experimental results.

It has been recently pointed out that in certain axion models it is possible to suppress simultaneously both the axion couplings to nucleons and electrons, realizing the so-called astrophobic axion scenarios, wherein the tight bounds from SN1987A and from stellar evolution of red giants and white dwarfs are greatly relaxed. So far, however, the conditions for realizing astrophobia have only been set out in tree-level analyses. Here we study whether these conditions can still be consistently implemented once renormalization group effects are included in the running of axion couplings. We find that axion astrophobia keeps holding, albeit within fairly different parameter space regions, and we provide analytical insights into this result. Given that astrophobic axion models generally feature flavor-violating axion couplings, we also assess the impact of renormalization group effects on axion-mediated flavor-violating …

Axion production from astrophysical bodies is a topic in continuous development, because of theoretical progress in the estimate of stellar emission rates and, especially, because of improved stellar observations. We carry out a comprehensive analysis of the most informative astrophysics data, revisiting the bounds on axion couplings to photons, nucleons and electrons, and reassessing the significance of various hints of anomalous stellar energy losses. We confront the performance of various theoretical constructions in accounting for these hints, while complying with the observational limits on axion couplings. We identify the most favorable models, and the regions in the mass/couplings parameter space which are preferred by the global fit. Finally, we scrutinize the discovery potential for such models at upcoming helioscopes, namely IAXO and its scaled versions.

The rare flavor-changing top quark decay t→ c Z is a clear sign of new physics and experimentally very interesting due to the huge number of top quarks produced at the LHC. However, there are few (viable) models which can generate a sizable branching ratio for t→ c Z—in fact vectorlike quarks seem to be the only realistic option. In this paper, we investigate all three representations (under the Standard Model gauge group) of vectorlike quarks (U, Q 1 and Q 7) that can generate a sizable branching ratio for t→ c Z without violating bounds from B physics. Importantly, these are exactly the three vectorlike quarks which can lead to a sizable positive shift in the prediction for W mass, via the couplings to the top quark also needed for a sizable Br (t→ c Z). Calculating and using the one-loop matching of vectorlike quarks on the Standard Model effective field theory, we find that Br (t→ c Z) can be of the order of 10− 6, 10 …

The rare flavour changing top quark decay is experimentally very interesting due to the huge number of top quarks produced at the LHC. Furthermore, in the Standard Model the corresponding rate is suppressed, such that any observation of this process in the foreseeable future would prove the presence of new physics. However, there are few (viable) models which can generate a sizable branching ratio for --in fact vector-like quarks seem to be the only realistic option. In this Letter, we investigate all three representations (under the Standard Model gauge group) of vector-like quarks (, and ) that can generate a sizable branching ratio for without violating other bounds. Nonetheless, these vector-like quarks still give rise to loop-effects in physics and electroweak precision observables which we calculate by matching the model on the Standard Model Effective Field Theory (at one-loop). Taking into account the resulting constraints …

Muon colliders provide a unique route to deliver high energy collisions that enable discovery searches and precision measurements to extend our understanding of the fundamental laws of physics. The muon collider design aims to deliver physics reach at the highest energies with costs, power consumption and on a time scale that may prove favorable relative to other proposed facilities. In this context, a new international collaboration has formed to further extend the design concepts and performance studies of such a machine. This effort is focused on delivering the elements of a 10 TeV center of mass (CM) energy design to explore the physics energy frontier. The path to such a machine may pass through lower energy options. Currently a 3 TeV CM stage is considered. Other energy stages could also be explored, eg an s-channel Higgs Factory operating at 125 GeV CM. We describe the status of the R&D; and design effort towards such a machine and lay out a plan to bring these concepts to maturity as a tool for the high energy physics community.

This article describes BabyIAXO, an intermediate experimental stage of the International Axion Observatory (IAXO), proposed to be sited at DESY. IAXO is a large-scale axion helioscope that will look for axions and axion-like particles (ALPs), produced in the Sun, with unprecedented sensitivity. BabyIAXO is conceived to test all IAXO subsystems (magnet, optics and detectors) at a relevant scale for the final system and thus serve as prototype for IAXO, but at the same time as a fully-fledged helioscope with relevant physics reach itself, and with potential for discovery. The BabyIAXO magnet will feature two 10 m long, 70 cm diameter bores, and will host two detection lines (optics and detector) of dimensions similar to the final ones foreseen for IAXO. BabyIAXO will detect or reject solar axions or ALPs with axion-photon couplings down to g aγ∼ 1. 5× 10− 11 GeV− 1, and masses up to m a∼ 0. 25 eV. BabyIAXO will …

Motivated by the result of the Muon g-2 experiment and the long-standing anomalies in semileptonic meson decays, we systematically build a class of minimal models that can address both experimental results thanks to the contributions of a set of new fields that include a thermal Dark Matter candidate.

We study the constraints imposed by perturbative unitarity on the new physics interpretation of the muon g− 2 anomaly. Within a Standard Model effective field theory approach, we find that scattering amplitudes sourced by effective operators saturate perturbative unitarity at about 1 PeV. This corresponds to the highest energy scale that needs to be probed in order to resolve the new physics origin of the muon g− 2 anomaly. On the other hand, simplified models (eg, scalar-fermion Yukawa theories) in which renormalizable couplings are pushed to the boundary of perturbativity still imply new on-shell states below 200 TeV. We finally suggest that the highest new physics scale responsible for the anomalous effect can be reached in nonrenormalizable models at the PeV scale.

In light of the recent result of the muon g− 2 experiment and the update on the test of lepton flavor universality R K published by the LHCb Collaboration, we systematically study for the first time a set of models with minimal field content that can simultaneously give (i) a thermal dark matter candidate;(ii) large loop contributions to b→ s ℓ ℓ processes able to address R K and the other B anomalies;(iii) a natural solution to the muon g− 2 discrepancy through chirally enhanced contributions. Moreover, this type of model with heavy particles and chiral enhancement can evade the strong limits from direct searches but can be tested at present and future colliders and direct-detection searches.

We study renormalizable models with minimal field content that can provide a viable dark matter candidate through the standard freeze-out paradigm and, simultaneously, accommodate the observed anomalies in semileptonic B-meson decays at one loop. Following the hypothesis of minimality, this outcome can be achieved by extending the particle spectrum of the Standard Model either with one vectorlike fermion and two scalars or two vectorlike fermions and one scalar. The dark matter annihilations are mediated by t-channel exchange of other new particles contributing to the B anomalies, thus resulting in a correlation between flavor observables and dark matter abundance. Again based on minimality, we assume the new states to couple only with left-handed muons and second and third generation quarks. Besides an ad hoc symmetry needed to stabilize the dark matter, the interactions of the new states are …

The strongest upper bounds on the axion mass come from astrophysical observations like the neutrino burst duration of SN1987A, which depends on the axion couplings to nucleons, or the white-dwarf cooling rates and red-giant evolution, which involve the axion-electron coupling. It has been recently argued that in variants of Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) models with generation-dependent Peccei-Quinn charges an approximate axion-nucleon decoupling can occur, strongly relaxing the SN1987A bound. However, as in standard DFSZ models, the axion remains in general coupled to electrons, unless an ad hoc cancellation is engineered. Here we show that axion-electron decoupling can be implemented without extra tunings in DFSZ-like models with three Higgs doublets. Remarkably, the numerical value of the quark mass ratio m u/m d∼ 1/2 is crucial to open up this possibility.

The flavour neutrino puzzle is often addressed by considering neutrino mass matrices m with a certain number of vanishing entries (m ij= 0 for some values of the indices), since a reduction in the number of free parameters increases the predictive power. Symmetries that can enforce textures zero can also enforce a more general type of conditions f (m ij)= 0 with f some function of the matrix elements m ij. In this case m can have all entries non-vanishing with no reduction in its predictive power. We classify all generation-dependent U (1) symmetries which, in the presence of two leptonic Higgs doublets, can reduce the number of independent high-energy parameters of type-I seesaw to the minimum number compatible with non-vanishing neutrino mixings and CP violation. These symmetries are broken above the scale where the effective operator is generated and can thus remain covert, in the sense that no explicit …

Axions are a natural consequence of the Peccei-Quinn mechanism, the most compelling solution to the strong-CP problem. Similar axion-like particles (ALPs) also appear in a number of possible extensions of the Standard Model, notably in string theories. Both axions and ALPs are very well motivated candidates for Dark Matter, and in addition, they would be copiously produced at the sun's core. A relevant effort during the last decade has been the CAST experiment at CERN, the most sensitive axion helioscope to-date. The International Axion Observatory (IAXO) is a large-scale 4th generation helioscope. As its primary physics goal, IAXO will look for solar axions or ALPs with a signal to background ratio of about 5 orders of magnitude higher than CAST. Recently the IAXO collaboration has proposed and intermediate experimental stage, BabyIAXO, conceived to test all IAXO subsystems (magnet, optics, detectors and sun-tracking systems) at a relevant scale for the final system and thus serve as pathfinder for IAXO but at the same time as a fully-fledged helioscope with record and relevant physics reach in itself with potential for discovery. BabyIAXO was endorsed by the Physics Review committee of DESY last May 2019. Here we will review the status and prospects of BabyIAXO and its potential to probe the most physics motivated regions of the axion & ALPs parameter space.

We investigate new-physics contributions to [Formula omitted] transitions in the context of an effective field theory extension of the Standard Model, including operator mixing at one loop. We identify the few scenarios where a single Wilson coefficient,[Formula omitted], induces a substantial shift in the lepton flavour universality ratios [Formula omitted] and [Formula omitted] at one loop, while evading Z-pole precision tests, collider bounds, and other flavour constraints. Novel fits to the present data are achieved by a left-handed current operator with quark-flavour indices (2, 2) or (3, 3). Interestingly, the running of the Standard Model Yukawa matrices gives the dominant effect for these scenarios. We match the favoured effective-theory scenarios to minimal, single-mediator models, which are subject to additional stringent constraints. Notably, we recognise three viable instances of a leptoquark with one coupling to …