arXiv subjects

Daniele Barducci

Publications and source records attributed to Daniele Barducci.

At least 37 records · Page 2Linked to original sources

arXiv2021

Perturbative unitarity constraints on generic Yukawa interactions

We study perturbative unitarity constraints on generic Yukawa interactions where the involved fields have arbitrary quantum numbers under an $\prod_i SU(N_i) \otimes U(1)$ group. We derive compact expressions for the bounds on the Yukawa couplings for the cases where the fields transform under the trivial, fundamental or adjoint representation of the various $SU(N)$ factors. We apply our results to specific models formulated to explain the anomalous measurements of $(g-2)_μ$ and of the charged- and neutral-current decays of the $B$ mesons. We show that, while these models can generally still explain the observed experimental values, the required Yukawa couplings are pushed at the edge of the perturbative regime.

hep-ph↗

arXiv2021

Gravitational tests of electroweak relaxation

We consider a scenario in which the electroweak scale is stabilized via the relaxion mechanism during inflation, focussing on the case in which the back-reaction potential is generated by the confinement of new strongly interacting vector-like fermions. If the reheating temperature is sufficiently high to cause the deconfinement of the new strong interactions, the back-reaction barrier then disappears and the Universe undergoes a second relaxation phase. This phase stops when the temperature drops sufficiently for the back-reaction to form again. We identify the regions of parameter space in which the second relaxation phase does not spoil the successful stabilization of the electroweak scale. In addition, the generation of the back-reaction potential that ends the second relaxation phase can be associated to a strong first order phase transition. We then study when such transition can generate a gravitational wave signal in the range of detectability of future interferometer experiments.

astro-ph.CO↗

arXiv2021

The see-saw portal at future Higgs Factories

We consider an extension of the Standard Model with two right-handed singlet fermions with mass at the electroweak scale that induce neutrino masses, plus a generic new physics sector at a higher scale $Λ$. We focus on the effective operators of lowest dimension $d=5$, which induce new production and decay modes for the singlet fermions. We assess the sensitivity of future Higgs Factories, such as FCC-ee, CLIC-380, ILC and CEPC, to the coefficients of these operators for various center of mass energies. We show that future lepton colliders can test the cut-off of the theory up to $Λ\simeq 500 - 1000\;$TeV, surpassing the reach of future indirect measurements of the Higgs and $Z$ boson widths. We also comment on the possibility of determining the underlying model flavor structure should a New Physics signal be observed, and on the impact of higher dimensional $d=6$ operators on the experimental signatures.

hep-ph↗

arXiv2020

Gravitational traces of broken gauge symmetries

We investigate first order phase transitions arising from hidden sectors which are in thermal equilibrium with the Standard Model bath in the Early Universe. Focusing on two simplified scenarios, an higgsed U(1) and a two scalar singlet model, we show the impact of friction effects acting on the bubble walls on the gravitational wave spectra and on the consequences for present and future interferometer experiments. We further comment on the possibility of disentangling the properties of the underlying theory featuring the first order phase transition should a stochastic gravitational wave signal be discovered.

hep-ph↗

arXiv2020

Reinterpretation of LHC Results for New Physics: Status and Recommendations after Run 2

We report on the status of efforts to improve the reinterpretation of searches and measurements at the LHC in terms of models for new physics, in the context of the LHC Reinterpretation Forum. We detail current experimental offerings in direct searches for new particles, measurements, technical implementations and Open Data, and provide a set of recommendations for further improving the presentation of LHC results in order to better enable reinterpretation in the future. We also provide a brief description of existing software reinterpretation frameworks and recent global analyses of new physics that make use of the current data.

hep-ph↗

arXiv2020

Minimal flavor violation in the see-saw portal

We consider an extension of the Standard Model with two singlet leptons, with masses in the electroweak range, that induce neutrino masses via the see-saw mechanism, plus a generic new physics sector at a higher scale, $Λ$. We apply the minimal flavor violation (MFV) principle to the corresponding Effective Field Theory ($ν$SMEFT) valid at energy scales $E \ll Λ$. We identify the irreducible sources of lepton flavor and lepton number violation at the renormalizable level, and apply the MFV ansätz to derive the scaling of the Wilson coefficients of the $ν$SMEFT operators up to dimension six. We highlight the most important phenomenological consequences of this hypothesis in the rates for exotic Higgs decays, the decay length of the heavy neutrinos, and their production modes at present and future colliders. We also comment on possible astrophysical implications.

hep-ph↗

arXiv2018

Combined explanations of B-physics anomalies: the sterile neutrino solution

In this paper we provide a combined explanation of charged- and neutral-current $B$-physics anomalies assuming the presence of a light sterile neutrino $N_R$ which contributes to the $B \to D^{(*)} τν$ processes. We focus in particular on two simplified models, where the mediator of the flavour anomalies is either a vector leptoquark $U_1^μ\sim ({\bf 3}, {\bf 1}, 2/3)$ or a scalar leptoquark $S_1 \sim ({\bf \bar 3}, {\bf 1}, 1/3)$. We find that $U_1^μ$ can successfully reproduce the required deviations from the Standard Model while being at the same time compatible with all other flavour and precision observables. The scalar leptoquark instead induces a tension between $B_s$ mixing and the neutral-current anomalies. For both states we present the limits and future projections from direct searches at the LHC finding that, while at present both models are perfectly allowed, all the parameter space will be tested with more luminosity. Finally, we study in detail the cosmological constraints on the sterile neutrino $N_R$ and the conditions under which it can be a candidate for dark matter.

hep-ph↗

arXiv2018

Quark flavour-violating Higgs decays at the ILC

Flavour-violating Higgs interactions are suppressed in the Standard Model such that their observation would be a clear sign of new physics. We investigate the prospects for detecting quark flavour-violating Higgs decays in the clean ILC environment. Concentrating on the decay to a bottom and a light quark $j$, we identify the dominant Standard Model background channels as coming from hadronic Standard Model Higgs decays with mis-identified jets. Therefore, good flavour tagging capabilities are essential to keep the background rate under control. Through a simple cut-based analysis, we find that the most promising search channel is the two-jet plus missing energy signature $e^+e^-\to bj+E_{T}^\text{miss}$. At 500 GeV, the expected $95\,$% CL upper limit on $\mathcal{B}(h\to bj)$ is of order $10^{-3}$. Correspondingly, a $5\,σ$ discovery is expected to be possible for branching ratios as low as a few $10^{-3}$.

hep-ph↗

arXiv2018

In search of a UV completion of the Standard Model - 378.000 models that don't work

Asymptotically safe extensions of the Standard Model have been searched for by adding vector-like fermions charged under the Standard Model gauge group and having Yukawa-like interactions with new scalar fields. Here we study the corresponding renormalization group beta functions to next and next-to-next to leading order in the perturbative expansion, varying the number of extra fermions and the representations they carry. We test the fixed points of the beta functions against various criteria of perturbativity to single out those that are potentially viable. We show that all the candidate ultraviolet fixed points are unphysical for these models: either they are unstable under radiative corrections, or they cannot be matched to the Standard Model at low energies.

hep-ph↗

arXiv2018

An almost elementary Higgs: Theory and Practice

We study models that interpolate between an elementary and a composite Higgs boson. Such models, arising in theories with new vector-like fermions with electro-weak quantum numbers and charged under a confining gauge interaction, are entirely compatible with current data, with only weak bounds from flavor, CP-violation and precision tests. After classifying the models from the point of view of symmetries, we study their collider phenomenology at LHC. In the most relevant scenarios, bounds from present searches exclude heavy scalar isospin triplets and quintuplets up to $\sim 200\;$GeV and we show how dedicated searches of simple signals such as $pp\to 3 γW$ could improve the reach by at least a factor of 2 with present data, reaching $O(1\;{\rm TeV})$ with higher integrated luminosities. States that mix with the SM Higgs can be tested in a variety of final states, such as $2b2γ$ searches relevant for double Higgs production.

hep-ph↗

arXiv2017

Status and discovery prospects for light pseudoscalars in the NMSSM

While most BSM searches at the LHC focus on heavy new states, the NMSSM contains the possibility of new light states that have escaped detection due to their singlet nature. Here we focus on light pseudoscalars, investigating the parameter space impact of recent LHC searches for such light states stemming from the decay of the 125 GeV Higgs boson. It is shown that, though direct searches can not yet compete with the requirement of the 125 GeV scalar having SM-like couplings, the searches are touching the allowed parameter space and should make a phenomenological impact in the near future.

hep-ph↗

arXiv2017

Cornering pseudoscalar-mediated dark matter with the LHC and cosmology

Models in which dark matter particles communicate with the visible sector through a pseudoscalar mediator are well-motivated both from a theoretical and from a phenomenological standpoint. With direct detection bounds being typically subleading in such scenarios, the main constraints stem either from collider searches for dark matter, or from indirect detection experiments. However, LHC searches for the mediator particles themselves can not only compete with -- or even supersede -- the reach of direct collider dark matter probes, but they can also test scenarios in which traditional monojet searches become irrelevant, especially when the mediator cannot decay on-shell into dark matter particles or its decay is suppressed. In this work we perform a detailed analysis of a pseudoscalar-mediated dark matter simplified model, taking into account a large set of collider constraints and concentrating on the parameter space regions favoured by cosmological and astrophysical data. We find that mediator masses above 100-200~GeV are essentially excluded by LHC searches in the case of large couplings to the top quark, while forthcoming collider and astrophysical measurements will further constrain the available parameter space.

hep-ph↗

arXiv2016

Implications of a High-Mass Diphoton Resonance for Heavy Quark Searches

Heavy vector-like quarks coupled to a scalar $S$ will induce a coupling of this scalar to gluons and possibly (if electrically charged) photons. The decay of the heavy quark into $Sq$, with $q$ being a Standard Model quark, provides, if kinematically allowed, new channels for heavy quark searches. Inspired by naturalness considerations, we consider the case of a vector-like partner of the top quark. For illustration, we show that a singlet partner can be searched for at the 13$\,$TeV LHC through its decay into a scalar resonance in the $2γ+\ell + X$ final states, especially if the diphoton branching ratio of the scalar $S$ is further enhanced by the contribution of non coloured particles. We then show that conventional heavy quark searches are also sensitive to this new decay mode, when $S$ decays hadronically, by slightly tightening the current selection cuts. Finally, we comment about the possibility of disentangling, by scrutinising appropriate kinematic distributions, heavy quark decays to $St$ from other standard decay modes.

hep-ph↗

arXiv2016

Review of LHC experimental results on low mass bosons in multi Higgs models

A number of searches at the LHC looking for low mass ($2m_μ - 62\ \mathrm{GeV}$) bosons in $\sqrt{s} = 8\ \mathrm{TeV}$ data have recently been published. We summarise the most pertinent ones, and look at how their limits affect a variety of supersymmetric and non-supersymmetric models which can give rise to such light bosons: the 2HDM (Types I and II), the NMSSM, and the nMSSM.

hep-ph↗

arXiv2016

Monojet searches for momentum-dependent dark matter interactions

We consider minimal dark matter scenarios featuring momentum-dependent couplings of the dark sector to the Standard Model. We derive constraints from existing LHC searches in the monojet channel, estimate the future LHC sensitivity for an integrated luminosity of 300 fb$^{-1}$, and compare with models exhibiting conventional momentum-independent interactions with the dark sector. In addition to being well motivated by (composite) pseudo-Goldstone dark matter scenarios, momentum-dependent couplings are interesting as they weaken direct detection constraints. For a specific dark matter mass, the LHC turns out to be sensitive to smaller signal cross-sections in the momentum-dependent case, by virtue of the harder jet transverse-momentum distribution.

hep-ph↗

arXiv2016

Drell-Yan production of multi Z'-bosons at the LHC within Non-Universal ED and 4D Composite Higgs Models

The Drell-Yan di-lepton production at hadron colliders is by far the preferred channel to search for new heavy spin-1 particles. Traditionally, such searches have exploited the Narrow Width Approximation (NWA) for the signal, thereby neglecting the effect of the interference between the additional Z'-bosons and the Standard Model Z and γ. Recently, it has been established that both finite width and interference effects can be dealt with in experimental searches while still retaining the model independent approach ensured by the NWA. This assessment has been made for the case of popular single Z'-boson models currently probed at the CERN Large Hadron Collider (LHC). In this paper, we test the scope of the CERN machine in relation to the above issues for some benchmark multi Z'-boson models. In particular, we consider Non-Universal Extra Dimensional (NUED) scenarios and the 4-Dimensional Composite Higgs Model (4DCHM), both predicting a multi-Z' peaking structure. We conclude that in a variety of cases, specifically those in which the leptonic decays modes of one or more of the heavy neutral gauge bosons are suppressed and/or significant interference effects exist between these or with the background, especially present when their decay widths are significant, traditional search approaches based on the assumption of rather narrow and isolated objects might require suitable modifications to extract the underlying dynamics.

hep-ph↗

arXiv2016

One jet to rule them all: monojet constraints and invisible decays of a 750 GeV diphoton resonance

The ATLAS and CMS collaborations recently reported a mild excess in the diphoton final state pointing to a resonance with a mass of around 750 GeV and a potentially large width. We consider the possibility of a scalar resonance being produced via gluon fusion and decaying to electroweak gauge bosons, jets and pairs of invisible particles, stable at collider scales. We compute limits from monojet searches on such a resonance and test their compatibility with the requirement for a large width. We also study whether the stable particle can be a a dark matter candidate and investigate the corresponding relic density constraints along with the collider limits. We show that monojet searches rule out a large part of the available parameter space and point out scenarios where a broad diphoton resonance can be reconciled with monojet constraints.

hep-ph↗

arXiv2016

Bounding wide composite vector resonances at the LHC

In composite Higgs models (CHMs), electroweak precision data generically push colourless composite vector resonances to a regime where they dominantly decay into pairs of light top partners. This greatly attenuates their traces in canonical collider searches, tailored for narrow resonances promptly decaying into Standard Model final states. By reinterpreting the CMS same-sign dilepton (SS2$\ell$) analysis at the Large Hadron Collider (LHC), originally designed to search for top partners with electric charge $5/3$, we demonstrate its significant coverage over this kinematical regime. We also show the reach of the 13 TeV run of the LHC, with various integrated luminosity options, for a possible upgrade of the SS2$\ell$ search. The top sector of CHMs is found to be more fine-tuned in the presence of colourless composite resonances in the few TeV range.

hep-ph↗