arXiv subjects

Luca Di Luzio

Publications and source records attributed to Luca Di Luzio.

At least 73 records · Page 4Linked to original sources

arXiv2021

The Anomalous Case of Axion EFTs and Massive Chiral Gauge Fields

We study axion effective field theories (EFTs), with a focus on axion couplings to massive chiral gauge fields. We investigate the EFT interactions that participate in processes with an axion and two gauge bosons, and we show that, when massive chiral gauge fields are present, such interactions do not entirely originate from the usual anomalous EFT terms. We illustrate this both at the EFT level and by matching to UV-complete theories. In order to assess the consistency of the Peccei--Quinn (PQ) anomaly matching, it is useful to introduce an auxiliary, non-dynamical gauge field associated to the PQ symmetry. When applied to the case of the Standard Model (SM) electroweak sector, our results imply that anomaly-based sum rules between EFT interactions are violated when chiral matter is integrated out, which constitutes a smoking gun of the latter. As an illustration, we study a UV-complete chiral extension of the SM, containing an axion arising from an extended Higgs sector and heavy fermionic matter that obtains most of its mass by coupling to the Higgs doublets. We assess the viability of such a SM extension through electroweak precision tests, bounds on Higgs rates and direct searches for heavy charged matter. At energies below the mass of the new chiral fermions, the model matches onto an EFT where the electroweak gauge symmetry is non-linearly realised.

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arXiv2021

What is the scale of new physics behind the muon $g-2$?

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 (SMEFT) 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 (e.g.~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 non-renormalizable models at the PeV scale.

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arXiv2021

Breakdown of chiral perturbation theory for the axion hot dark matter bound

We show that the commonly adopted hot dark matter (HDM) bound on the axion mass $m_a \lesssim$ 1 eV is not reliable, since it is obtained by extrapolating the chiral expansion in a region where the effective field theory breaks down. This is explicitly shown via the calculation of the axion-pion thermalization rate at the next-to-leading order in chiral perturbation theory. We finally advocate a strategy for a sound extraction of the axion HDM bound via lattice QCD techniques.

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arXiv2021

Dark Matter from self-dual gauge/Higgs dynamics

We show that a new gauge group with one new scalar leads to automatically stable Dark Matter candidates. We consider theories where the Higgs phase is dual to the confined phase: it is known that SU(2) gauge theories with a scalar doublet (like the Standard Model) obey this non-trivial feature. We provide a general criterion, showing that this self-duality holds for SU(N), SO(N), Sp(N) and G_2 gauge dynamics with a scalar field in the fundamental representation. The resulting Dark Matter phenomenology has non-trivial features that are characteristic of the group, and that we discuss case by case. Just to mention a few, SU(N) has an accidental conserved dark baryon number, SO(2N+1) leads to stable glue-balls thanks to a special parity, G_2 leads to a Dark Matter system analogous to neutral kaons. The cosmological Dark Matter abundance is often reproduced for masses around 100 TeV: all constraints are satisfied and lighter dark glue-balls can affect Higgs physics. These theories acquire additional interest and predictivity assuming that both the dark and weak scales are dynamically generated.

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arXiv2021

Selective enhancement of the QCD axion couplings

We present a mechanism wherein the QCD axion coupling to nucleons, photons, or electrons, can be enhanced selectively without increasing the axion mass. We focus in particular on the axion-nucleon couplings, that are generally considered to be largely model-independent, and we show how nucleophilic axion models can be constructed. We discuss the implications of a nucleophilic axion for astrophysics, cosmology and laboratory searches. We present a model with enhanced axion couplings to nucleons and photons that can provide an excellent fit to the anomalous emission of hard X-rays recently observed from a group of nearby neutron stars, and we argue that such a scenario can be thoroughly tested in forthcoming axion-search experiments.

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arXiv2021

Axion-mediated forces, CP violation and left-right interactions

We compute the CP-violating (CPV) scalar axion coupling to nucleons in the framework of baryon chiral perturbation theory and we apply the results to the case of left-right symmetry. The correlated constraints with other CPV observables show that the predicted axion nucleon coupling is within the reach of present axion-mediated force experiments for $M_{W_R}$ up to $1000$ TeV.

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arXiv2021

Axion quality from the (anti)symmetric of SU(N)

We propose two models where a U(1) Peccei-Quinn global symmetry arises accidentally and is respected up to high-dimensional operators, so that the axion solution to the strong CP problem is successful even in the presence of Planck-suppressed operators. One model is SU$(N)$ gauge interactions with fermions in the fundamental and a scalar in the symmetric. The axion arises from spontaneous symmetry breaking to SO$(N)$, that confines at a lower energy scale. Axion quality in the model needs $N \gtrsim 10$. SO bound states and possibly monopoles provide extra Dark Matter candidates beyond the axion. In the second model the scalar is in the anti-symmetric: SU$(N)$ broken to Sp$(N)$ needs even $N \gtrsim 20$. The cosmological DM abundance, consisting of axions and/or super-heavy relics, can be reproduced if the PQ symmetry is broken before inflation (Boltzmann-suppressed production of super-heavy relics) or after (super-heavy relics in thermal equilibrium get partially diluted by dark glue-ball decays).

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arXiv2020

Accidental SO(10) axion from gauged flavour

An accidental U(1) Peccei-Quinn (PQ) symmetry automatically arises in a class of SO(10) unified theories upon gauging the SU(3)$_{f}$ flavour group. The PQ symmetry is protected by the $\mathbb{Z}_4 \times \mathbb{Z}_3$ center of $\text{SO(10)} \times \text{SU(3)}_f$ up to effective operators of canonical dimension six. However, high-scale contributions to the axion potential posing a PQ quality problem arise only at $d=9$. In the pre-inflationary PQ breaking scenario the axion mass window is predicted to be $m_a \in [7 \times 10^{-8}, 10^{-3}]$ eV, where the lower end is bounded by the seesaw scale and the upper end by iso-curvature fluctuations. A high-quality axion, that is immune to the PQ quality problem, is obtained for $m_a \gtrsim 0.02$ eV. We finally offer a general perspective on the PQ quality problem in grand unified theories.

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arXiv2020

Solar axions cannot explain the XENON1T excess

We argue that the interpretation in terms of solar axions of the recent XENON1T excess is not tenable when confronted with astrophysical observations of stellar evolution. We discuss the reasons why the emission of a flux of solar axions sufficiently intense to explain the anomalous data would radically alter the distribution of certain type of stars in the color-magnitude diagram in first place, and would also clash with a certain number of other astrophysical observables. Quantitatively, the significance of the discrepancy ranges from $3.3σ$ for the rate of period change of pulsating White Dwarfs, and exceedes $19σ$ for the $R$-parameter and for $M_{I,{\rm TRGB}}$.

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arXiv2020

Pati-Salam Axion

I discuss the implementation of the Peccei-Quinn mechanism in a minimal realization of the Pati-Salam partial unification scheme. The axion mass is shown to be related to the Pati-Salam breaking scale and it is predicted via a two-loop renormalization group analysis to be in the window $m_a \in [10^{-11}, \, 3 \times 10^{-7}]$ eV, as a function of a sliding Left-Right symmetry breaking scale. This parameter space will be fully covered by the late phases of the axion Dark Matter experiments ABRACADABRA and CASPEr-Electric. A Left-Right symmetry breaking scenario as low as 20 TeV is obtained for a Pati-Salam breaking of the order of the reduced Planck mass.

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arXiv2020

Covert symmetries in the neutrino mass matrix

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 evidence of the symmetry can be read off the neutrino mass matrix, and different symmetries can give rise to the same low-energy structure. We find that only two cases are viable: one yields a structure with two zero-textures already considered in the literature, the other has no zero-textures and has never been considered before. It predicts normal ordering, a lightest neutrino mass $ \sim 10$ meV, a Dirac phase $δ\sim \frac{3π}{2}$ and definite values for the Majorana phases.

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arXiv2019

Flavour Violating Axions

I review the physics case for flavour violating axions. In particular, I argue that relaxing the assumption of the universality of the Peccei-Quinn current opens up new pathways, including: the relaxation of the Supernova bound on the axion mass, a possible connection with the Standard Model flavour puzzle and the experimental opportunity of discovering the axion via flavoured axion searches.

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arXiv2019

$ΔM_s$ theory precision confronts flavour anomalies

Based on recent HQET sum rule and lattice calculations we present updated Standard Model predictions for the mass differences of neutral $B$ mesons: $ΔM_s^{\rm SM} = \left(18.4^{+0.7}_{-1.2} \right) \mbox{ps}^{-1}$ and $ΔM_d^{\rm SM} = \left(0.533^{+0.022}_{-0.036} \right) \mbox{ps}^{-1}$ and study their impact on new physics models that address the present hints of anomalous data in $b \to s \ell \ell$ transitions. We also examine future prospects of further reducing the theory uncertainties and discuss the implications of a 2025 scenario with $ΔM_s^\text{SM 2025} = \left(18.4 \pm 0.5\right) \mbox{ps}^{-1}$. In particular, the latter yields upper bounds $M_{Z'}\lesssim 9~\text{TeV}$ and $M_{S_3}\lesssim 30~\text{TeV}$ for the minimal $Z'$ and $S_3$ lepto-quark explanations of the $b \to s \ell \ell$ anomalies, respectively.

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arXiv2019

Axion-electron decoupling in nucleophobic axion models

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 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 $m_u/m_d\sim 1/2$ is crucial to open up this possibility.

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arXiv2019

Coset Cosmology

We show that the potential of Nambu-Goldstone bosons can have two or more local minima e.g. at antipodal positions in the vacuum manifold. This happens in many models of composite Higgs and of composite Dark Matter. Trigonometric potentials lead to unusual features, such as symmetry non-restoration at high temperature. In some models, such as the minimal $\rm SO(5)/SO(4)$ composite Higgs with fermions in the fundamental representation, the two minima are degenerate giving cosmological domain-wall problems. Otherwise, an unusual cosmology arises, that can lead to supermassive primordial black holes; to vacuum or thermal decays; to a high-temperature phase of broken $\mathrm{SU}(2)_L$, possibly interesting for baryogenesis.

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arXiv2019

Implications of perturbative unitarity for scalar di-boson resonance searches at LHC

We study the constraints implied by partial wave unitarity on new physics in the form of spin-zero di-boson resonances at LHC. We derive the scale where the effective description in terms of the SM supplemented by a single resonance is expected to break down depending on the resonance mass and signal cross-section. Likewise, we use unitarity arguments in order to set perturbativity bounds on renormalizable UV completions of the effective description. We finally discuss under which conditions scalar di-boson resonance signals can be accommodated within weakly-coupled models.

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arXiv2019

$U(1)$ flavour symmetries as Peccei-Quinn symmetries

We investigate to what extent a generic, generation-dependent $U(1)$ symmetry acting on the quark Yukawa operators can reduce the number of free parameters by forcing some entries in the Yukawa matrices to vanish. The maximal reduction compatible with CP violation yields nine real parameters and one phase, which matches the number of physical observables, implying that such models have no free parameters. We derive a set of results: (i) the only possible structures have the form $M_4 \oplus M_5$, where the subscripts indicate the number of real parameters in the Yukawa matrices, (ii) there are only two inequivalent Yukawa structures, each one giving rise to six different models depending on quark flavour assignments, (iii) the $U(1)$ symmetries that generate these textures all have a QCD anomaly, and hence are Peccei-Quinn symmetries, reinforcing the idea of a possible connection between the quark flavour puzzle and the axion solution to the strong CP problem, (iv) in some cases the contributions to the QCD anomaly of two generations cancels out, and this opens the possibility that the axion coupling to nucleons could be strongly suppressed. Flavour-violating axion couplings to quarks are completely fixed, up to the axion decay constant, providing a non-trivial complementarity between low-energy flavour-violating processes and standard axion searches.

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arXiv2018

Probing new electroweak states via precision measurements at the LHC and future colliders

Several new physics scenarios, motivated e.g. by dark matter, feature new electroweakly charged states where the lightest particle in the multiplet is stable and neutral. In such cases direct searches at LHC are notoriously difficult, while electroweak precision tests both at hadron and lepton colliders offer the possibility to indirectly probe those states. In this work, we assess the sensitivity of the high-luminosity phase of the LHC on new electroweak multiplets via the modification of neutral and charged Drell-Yan processes, and compare the reach of future hadron and lepton colliders presently under consideration.

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