arXiv subjects

Luca Di Luzio

Publications and source records attributed to Luca Di Luzio.

At least 91 records · Page 5Linked to original sources

arXiv2018

$B_s$-$\bar B_s$ mixing interplay with $B$ anomalies

After reviewing the theoretical uncertainties entering the Standard Model determination of the mass difference of the neutral $B_s$-$\bar B_s$ meson system, $ΔM_s^{\rm SM}$, we discuss the implications of its updated value for new physics models addressing the experimental anomalies in semi-leptonic $B$ decays. Using the most recent FLAG average of lattice results for the non-perturbative matrix elements and the CKM-fitter determination of $V_{cb}$ points to a $1.8 \, σ$ discrepancy in $ΔM_s^{\rm SM} > ΔM_s^{\rm exp}$. Extending the analysis in Ref. [1] we show that the latter tension cannot be easily accommodated within single mediator models, whenever the same mediator is also responsible for the $b \to s \ell \ell$ anomalies.

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arXiv2018

Axion properties in GUTs

We summarize recent studies of realistic nonsupersymmetric Grand Unified Theories (GUTs) extended with a global $U(1)_{\rm PQ}$ symmetry, so as to accommodate the axion solution to the strong CP problem. Aside from solving the CP problem and unifying the gauge structure of the SM, these models can also yield realistic spectra and mixings, including neutrino masses, and allowing for a consistent cosmological history that accounts for inflation, dark matter and baryogenesis. In our studies of $SO(10)$ and $SU(5)$ theories, we determined the mass and couplings of the axion in terms of the relevant threshold scales, and assessed how the former are constrained from the requirements of gauge coupling unification, proton decay searches and collider bounds. The axion mass ends up being rather constrained for GUT scale axions, particularly in the case of $SU(5)$, and could be probed by upcoming dark matter experiments, such as ABRACADABRA and CASPEr-Electric.

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arXiv2018

Maximal Flavour Violation: a Cabibbo mechanism for leptoquarks

We propose a mechanism that allows for sizeable flavour violation in quark-lepton currents, while suppressing flavour changing neutral currents in quark-quark and lepton-lepton sectors. The mechanism is applied to the recently proposed 4321 renormalizable model, which can accommodate the current experimental anomalies in $B$-meson decays, both in charged and neutral currents, while remaining consistent with all other indirect flavour and electroweak precision measurements and direct searches at high-$p_T$. To support this claim, we present an exhaustive phenomenological survey of this fully calculable UV complete model and highlight the rich complementarity between indirect and direct searches.

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arXiv2018

Axion mass prediction from minimal grand unification

We propose a minimal realization of the Peccei Quinn mechanism in a realistic SU(5) model, where the axion mass is directly connected to the grand-unification scale. By taking into account constraints from proton decay, collider searches and gauge coupling unification, we predict the axion mass: $m_a \in [4.8, 6.6]$ neV. The upper bound can be relaxed up to $m_a < 330$ neV, at the cost of tuning the flavour structure of the proton decay operators. The predicted mass window will be complementarily probed by the axion dark matter experiments ABRACADABRA and CASPER-Electric, which could provide an indirect evidence for the scale of grand unification before the observation of proton decay.

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arXiv2018

Astrophobic Axions

We propose a class of axion models with generation dependent Peccei-Quinn charges for the known fermions that allow to suppress the axion couplings to nucleons and electrons. Astrophysical limits are thus relaxed, allowing for axion masses up to ${\cal O}(0.1)$ eV. The axion-photon coupling remains instead sizeable, so that next generation helioscopes will be able to probe this scenario. Astrophobia unavoidably implies flavor violating axion couplings, so that experimental limits on flavour-violating processes can provide complementary probes. The astrophobic axion can be a viable dark matter candidate in the heavy mass window, and can also account for anomalous energy loss in stars.

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arXiv2018

One constraint to kill them all?

Many new physics models that explain the intriguing anomalies in the $b$-quark flavour sector are severely constrained by $B_s$-mixing, for which the Standard Model prediction and experiment agreed well until recently. The most recent FLAG average of lattice results for the non-perturbative matrix elements points, however, in the direction of a small discrepancy in this observable. Using up-to-date inputs from standard sources such as PDG, FLAG and one of the two leading CKM fitting groups to determine $ΔM_s^{\rm SM}$, we find a severe reduction of the allowed parameter space of $Z'$ and leptoquark models explaining the $B$-anomalies. Remarkably, in the former case the upper bound on the $Z'$ mass approaches dangerously close to the energy scales already probed by the LHC. We finally identify some model building directions in order to alleviate the tension with $B_s$-mixing.

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arXiv2017

Gauge leptoquark as the origin of B-physics anomalies

The vector leptoquark representation, $U_μ= (3,1,2/3)$, was recently identified as an exceptional single mediator model to address experimental hints on lepton flavour universality violation in semileptonic $B$-meson decays, both in neutral ($b \to s μμ$) and charged ($b \to c τν$) current processes. Nonetheless, it is well-known that massive vectors crave an ultraviolet (UV) completion. We present the first full-fledged UV complete and calculable gauge model which incorporates this scenario while remaining in agreement with all other indirect flavour and electroweak precision measurements, as well as, direct searches at high-$p_T$. The model is based on a new non-abelian gauge group spontaneously broken at the TeV scale, and a specific flavour structure suppressing flavour violation in $ΔF = 2$ processes while inducing sizeable semileptonic transitions.

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arXiv2017

Maxi-sizing the trilinear Higgs self-coupling: how large could it be?

In order to answer the question on how much the trilinear Higgs self-coupling could deviate from its Standard Model value in weakly coupled models, we study both theoretical and phenomenological constraints. As a first step, we discuss this question by modifying the Standard Model using effective operators. Considering constraints from vacuum stability and perturbativity, we show that only the latter can be reliably assessed in a model-independent way. We then focus on UV models which receive constraints from Higgs coupling measurements, electroweak precision tests, vacuum stability and perturbativity. We find that the interplay of current measurements with perturbativity already exclude self-coupling modifications above a factor of few with respect to the Standard Model value.

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arXiv2017

What is the scale of new physics behind the $B$-flavour anomalies?

Motivated by the recent hints of lepton flavour non-universality in $B$-meson semi-leptonic decays, we study the constraints of perturbative unitarity on the new physics interpretation of the anomalies in $b \to c \ell \bar ν$ and $b \to s \ell \bar \ell$ transitions. Within an effective field theory approach we find that $2 \to 2$ fermion scattering amplitudes saturate the unitarity bound below 9 TeV and 80 TeV, respectively for $b \to c \ell \bar ν$ and $b \to s \ell \bar \ell$ transitions. Stronger bounds, up to few TeV, are obtained when the leading effective operators are oriented in the direction of the third generation, as suggested by flavour models. We finally address unitarity constraints on simplified models explaining the anomalies and show that the new physics interpretation is ruled out in a class of perturbative realizations.

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arXiv2017

Window for preferred axion models

We discuss phenomenological criteria for defining "axion windows", namely regions in the parameter space of the axion-photon coupling where realistic models live. Currently, the boundaries of this region depend on somewhat arbitrary criteria, and it would be highly desirable to specify them in terms of precise phenomenological requirements. We first focus on hadronic axion models within post-inflationary scenarios, in which the initial abundance of the new vectorlike quarks $Q$ is thermal. We classify their representations $R_Q$ by requiring that $i)$ the $Q$ are sufficiently short lived to avoid issues with long-lived strongly interacting relics, $ii)$ the theory remains weakly coupled up to the Planck scale. The more general case of multiple $R_Q$ is also studied, and the absolute upper and lower bounds on the axion-photon coupling as a function of the axion mass is identified. Pre-inflationary scenarios in which the axion decay constant remains bounded as $f_a\leq 5\cdot 10^{11}\,$GeV allow for axion-photon couplings only about 20% larger. Realistic Dine-Fischler-Srednicki-Zhitnitsky type of axion models also remain encompassed within the hadronic axion window. Some mechanisms that can allow to enhance the axion-photon coupling to values sizeably above the preferred window are discussed.

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arXiv2017

Accidental Peccei-Quinn symmetry protected to arbitrary order

A $SU(N)_L\times SU(N)_R$ gauge theory for a scalar multiplet $Y$ transforming in the bi-fundamental representation $(N,\bar N)$ preserves, for $N>4$, an accidental $U(1)$ symmetry firstly broken at operator dimension $N$. Two configurations are possible for the vacuum expectation value of $Y$, which correspond to the (maximal) little groups $\mathcal{H}_s=SU(N)_{L+R}$ and $\mathcal{H}_h=SU(N-1)_L\times SU(N-1)_R\times U(1)_{L+R}$. In the first case the accidental $U(1)$ gets also broken, yielding a pseudo Nambu-Goldstone boson with mass suppression controlled by $N$, while in the second case a global $U(1)$ remains unbroken. The strong CP problem is solved by coupling $Y$ to new fermions carrying color. The first case allows for a Peccei-Quinn solution with $U(1)_{PQ}$ protected up to order $N$ by the gauge symmetry. In the second case $U(1)$ can get broken by condensates of the new strong dynamics, resulting in a composite axion. By coupling $Y$ to fermions carrying only weak isospin, models for axion-like particles can be constructed.

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arXiv2017

Redefining the Axion Window

A major goal of axion searches is to reach inside the parameter space region of realistic axion models. Currently, the boundaries of this region depend on somewhat arbitrary criteria, and it would be desirable to specify them in terms of precise phenomenological requirements. We consider hadronic axion models and classify the representations $R_Q$ of the new heavy quarks $Q$. By requiring that $i)$ the $Q$ are sufficiently short lived to avoid issues with long lived strongly interacting relics, $ii)$ no Landau poles are induced below the Planck scale, fifteen cases are selected, which define a phenomenologically preferred axion window bounded by a maximum (minimum) value of the axion-photon coupling about twice (four times) larger than commonly assumed. Allowing for more than one $R_Q$, larger couplings, as well as complete axion-photon decoupling, become possible.

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arXiv2016

Massive vectors and loop observables: the $g-2$ case

We discuss the use of massive vectors for the interpretation of some recent experimental anomalies, with special attention to the muon $g-2$. We restrict our discussion to the case where the massive vector is embedded into a spontaneously broken gauge symmetry, so that the predictions are not affected by the choice of an arbitrary energy cut-off. Extended gauge symmetries, however, typically impose strong constraints on the mass of the new vector boson and for the muon $g-2$ they basically rule out, barring the case of abelian gauge extensions, the explanation of the discrepancy in terms of a single vector extension of the standard model. We finally comment on the use of massive vectors for $B$-meson decay and di-photon anomalies.

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arXiv2015

Neutrino-axion-dilaton interconnection

We show that a recently proposed framework that provides a simple connection between Majorana neutrinos and an invisible axion in minimal scalar extensions of the standard electroweak model can be naturally embedded in a classically scale-invariant setup. The explicit breaking of the scale invariance à la Coleman-Weinberg generates the Peccei-Quinn and electroweak scales. The spontaneous breaking of the chiral $U(1)_{PQ}$ triggers the generation of neutrino masses via Type-II seesaw and, at the same time, provides a dynamical solution to the strong CP problem as well as the axion as a dark matter candidate. The electroweak and neutrino mass scales are obtained via a technically natural ultraweak limit of the singlet scalar interactions. Accordingly, a realistic and perturbatively stable scalar spectrum, possibly in the reach of the LHC, is naturally obtained. A very light pseudodilaton characterizes such a setting. The vacuum stability of the extended setup is discussed.

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arXiv2015

Stability of the electroweak ground state in the Standard Model and its extensions

We review the formalism by which the tunnelling probability of an unstable ground state can be computed in quantum field theory, with special reference to the Standard Model of electroweak interactions. We describe in some detail the approximations implicitly adopted in such calculation. Particular attention is devoted to the role of scale invariance, and to the different implications of scale-invariance violations due to quantum effects and possible new degrees of freedom. We show that new interactions characterized by a new energy scale, close to the Planck mass, do not invalidate the main conclusions about the stability of the Standard Model ground state derived in absence of such terms.

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arXiv2015

Accidentally safe extensions of the Standard Model

We discuss a class of weak-scale extensions of the Standard Model which is completely invisible to low-energy indirect probes. The typical signature of this scenario is the existence of new charged and/or colored states which are stable on the scale of high-energy particle detectors.

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arXiv2015

R-parity violation in SU(5)

We show that judiciously chosen R-parity violating terms in the minimal renormalizable supersymmetric SU(5) are able to correct all the phenomenologically wrong mass relations between down quarks and charged leptons. The model can accommodate neutrino masses as well. One of the most striking consequences is a large mixing between the electron and the Higgsino. We show that this can still be in accord with data in some regions of the parameter space and possibly falsified in future experiments.

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arXiv2015

Accidental matter at the LHC

We classify weak-scale extensions of the Standard Model which automatically preserve its accidental and approximate symmetry structure at the renormalizable level and which are hence invisible to low-energy indirect probes. By requiring the consistency of the effective field theory up to scales of 10^15 GeV and after applying cosmological constraints, we arrive at a finite set of possibilities that we analyze in detail. One of the most striking signatures of this framework is the presence of new charged and/or colored states which can be efficiently produced in high-energy particle colliders and which are stable on the scale of detectors.

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