arXiv subjects

Alessandro Strumia

Publications and source records attributed to Alessandro Strumia.

At least 37 records · Page 2Linked to original sources

arXiv2025

New physics in spin entanglement

We propose a theory that preserves spin-summed scattering and decay rates at tree level while affecting particle spins. This is achieved by breaking the Lorentz group in a non-local way that tries avoiding stringent constraints, for example leaving unbroken the maximal sub-group SIM(2). As a phenomenological application, this new physics can alter the spins of top-antitop pairs (and consequently their entanglement) produced in $pp$ collisions without impacting their rates. Some observables affected by loops involving top quarks with modified entanglement receive corrections.

hep-ph↗

arXiv2024

Cosmological collider non-Gaussianity from multiple scalars and $R^2$ gravity

Cosmological collider signals of primordial non-Gaussianity arise at tree level when an extra scalar has Hubble mass during inflation. We critically review the formalism finding that a large class of inflationary theories, based on Planck-scale physics, predict a scalar bi-spectrum around the gravitational floor level. This mild signal arises for example in $R^2$ gravity, in the regime where its gravitational scalar has Hubble-scale mass. Signals much above the gravitational floor arise in theories where scalars undergo multiple turns during inflation, thanks to sub-Planckian physics.

astro-ph.CO↗

arXiv2024

Is nature natural?

I summarize the apparent relevance of anthropic selection in understanding unnatural features of the Standard Model, and the consequent implications for theory. This topic, pioneered by Weinberg, could make him the last giant of physics.

hep-ph↗

arXiv2024

Solving the strong CP problem without axions

We formulate general conditions under which the strong CP problem is solved by spontaneous CP violation. Quark-mass matrix elements are polynomials in the CP-breaking order parameters, engineered such that their determinant is a real constant. This scheme permits only a limited number of textures. These conditions can be realized in supersymmetric theories with CP as an anomaly-free local flavour symmetry, suggesting a unified solution to the strong CP problem and the flavour puzzle. Our solution can be implemented using either modular invariance or a local U(1) symmetry. We present modular-invariant realizations where matter fields are assigned small modular weights $\pm2$ ($\pm1$), utilising higher levels $N=2$ ($N=3$). Heavy quarks are in general not required, but their presence allows for models where colored particles fill non-singlet representations of the flavour group.

hep-ph↗

arXiv2024

Traversing a kinetic pole during inflation: primordial black holes and gravitational waves

We consider an inflationary kinetic function with an integrable pole that is traversed during inflation. This scenario leads to enhanced spectra of primordial scalar inhomogeneities with detectable signals: formation of primordial black holes (that could explain Dark Matter) and scalar-induced gravitational waves (that could reproduce the recent Pulsar Timing Array observation, or predict signals in future detectors such as LISA or ET). Spectral signatures depend on whether the inflaton mass dimension at the pole is above or below 2. Values mildly below 2 allow a big power spectrum enhancement with a mild tuning. Finally, we discuss the possibility that a kinetic pole can arise as anomalous dimension of the inflaton due to quantum effects of Planckian particles that become light at some specific inflaton field value.

hep-ph↗

arXiv2024

Dark Matter from freeze-in and its inhomogeneities

We consider generic freeze-in processes for generation of Dark Matter, together with the consequent re-thermalization of the Standard Model fluid. We find that Dark Matter inherits the Standard Model adiabatic inhomogeneities on the cosmological scales probed by current observations, that were super-horizon during freeze-in. Thereby, freeze-in satisfies the bounds on iso-curvature perturbations.

hep-ph↗

arXiv2023

A boosted muon collider

A muon collider could produce the heavier Standard Model particles with a boost, for example in resonant processes such as $μ^-μ^+\to h$ or $μ^-μ^+\to Z$. We propose machine configurations that produce the boost (asymmetric beam energies, tilted beams) and estimate how much the luminosity is reduced or perhaps enhanced. The feasibility of the proposed configurations, as well as an estimation of the beam-induced backgrounds and beam energy spread, needs to be evaluated in order to derive more solid conclusions on the physics potential of such boosted collider configurations. If achievable, the boost can provide new interesting observational opportunities. For example it can significantly enhance the sensitivity to long-lived new particles decaying in a far-away detector, such as dark higgses or sterile neutrinos produced in $h$ or $Z$ decays.

hep-ph↗

arXiv2023

Probing the Dark Matter density with gravitational waves from super-massive binary black holes

Supermassive black hole binaries source gravitational waves measured by Pulsar Timing Arrays. The frequency spectrum of this stochastic background is predicted more precisely than its amplitude. We argue that Dark Matter friction can suppress the spectrum around nHz frequencies, where it is measured, allowing to derive robust and significant bounds on the Dark Matter density, which, in turn, controls indirect detection signals from galactic centers. A precise spectrum of gravitational waves would translate in a tomography of the DM density profile, potentially probing DM particle-physics effects that induce a characteristic DM density profile, such as DM annihilations or de Broglie wavelength.

astro-ph.CO↗

arXiv2023

Modular invariance and the QCD angle

String compactifications on an orbi-folded torus with complex structure give rise to chiral fermions, spontaneously broken CP, modular invariance. We show that this allows simple effective theories of flavour and CP where: i) the QCD angle vanishes; ii) the CKM phase is large; iii) quark and lepton masses and mixings can be reproduced up to order one coefficients. We implement such general paradigm in supersymmetry or supergravity, with modular forms or functions, with or without heavy colored states.

hep-ph↗

arXiv2023

The collider landscape: which collider for establishing the SM instability?

Capabilities of future colliders are usually discussed assuming specific hypothetical new physics. We consider the opposite possibility: that no new physics is accessible, and we want to learn if the unnatural Standard Model is part of a vast landscape. We argue that a main step in this direction would be establishing the possible instability scale of the Higgs potential. This primarily needs reducing the uncertainty on the strong coupling and on the top quark mass. We show that the top quark mass can be measured well enough via a $t \bar t$ threshold scan with low $10^{33}\,{\rm cm}^{-2}{\rm sec}^{-1}$ luminosity, that seems achievable at a `small' $e^+ e^-$ collider in the LEP tunnel, or at the first low-energy stage of a muon collider.

hep-ph↗

arXiv2023

Black holes don't source fast Higgs vacuum decay

We argue that the rate of Standard Model vacuum or thermal decay seeded by primordial black holes is negligible (because non-perturbatively suppressed by the small quartic Higgs coupling) and independent of the non-minimal coupling of the Higgs to gravity.

hep-ph↗

arXiv2022

Pole inflation from non-minimal coupling to gravity

Transforming canonical scalars to the Einstein frame can give a multi-field generalization of pole inflation (namely, a scalar with a divergent kinetic term) at vanishing field-dependent Planck mass. However, to obtain an attractor, the scalar potential must obey certain non-generic conditions. These are automatically satisfied in Quantum Field Theories with dimension-less couplings. The resulting models of pole inflation have special inflationary predictions determined by the full RG running of couplings. Acceptable predictions for the tensor/scalar ratio arise for perturbative but moderately large couplings, so we explore the possible QFT runnings: to confinement, to an IR fixed point, and to a UV fixed point.

hep-th↗

arXiv2022

Interpreting electroweak precision data including the $W$-mass CDF anomaly

We perform a global fit of electroweak data, finding that the anomaly in the $W$ mass claimed by the CDF collaboration can be reproduced as a universal new-physics correction to the $T$ parameter or $| H^\dagger D_μH|^2$ operator. Contributions at tree-level from multi-TeV new physics can fit the anomaly compatibly with collider bounds: we explore which scalar vacuum expectation values (such as a triplet with zero hypercharge), $Z'$ vectors (such as a $Z'$ coupled to the Higgs only), little-Higgs models or higher-dimensional geometries provide good global fits. On the other hand, new physics that contributes at loop-level must be around the weak scale to fit the anomaly. Thereby it generically conflicts with collider bounds, that can be bypassed assuming special kinematics like quasi-degenerate particles that decay into Dark Matter (such as an inert Higgs doublet or appropriate supersymmetric particles).

hep-ph↗

arXiv2022

Dark Matter abundance via thermal decays and leptoquark mediators

We explore a new mechanism for reproducing the Dark Matter (DM) abundance: scatterings of one DM particle on light Standard Model particles. Strong bounds on its decays can be satisfied if DM undergoes freeze-in and has a mass around or below the pion mass. This happens, for example, in theories with a right-handed neutrino interacting with charged fermions through a leptoquark exchange. These leptoquarks can be linked to the ones motivated by the B-physics anomalies if assumptions about the flavour structure are made. DM signals are unusual, with interesting possibilities for direct and indirect detection. Achieving thermal freeze-out instead requires models with more than one DM flavour, and couplings parametrically smaller than what needed by the usual pair annihilations.

hep-ph↗

arXiv2022

Higgstory repeats itself

We consider a scalar potential with two minima, one of which is arbitrarily deep, such as could be the case for the Higgs potential in the Standard Model. A recent calculation within the thin-wall approximation [arXiv:2205.10240] concludes that regions in which the scalar field takes values beyond the top of the potential barrier are forced by gravity to collapse, while they remain hidden behind a black hole horizon. We show that the thin-wall approximation is not applicable to this problem. We clarify the issue through numerical and analytical solutions to the field equations of the gravity-scalar system. We find that regions around the deeper minimum expand, and would thereby engulf the Universe in post-inflationary cosmology. We also show that black holes with Higgs hair are unstable. Even though the physics of the true vacuum is different, our final conclusion replicates the earlier `Higgstory' paper [arXiv:1505.04825].

hep-ph↗

arXiv2022

Dark Matter-Induced Multi-Phase Dynamical Symmetry Breaking

We consider the classically scale invariant Higgs-dilaton model of dynamical symmetry breaking extended with an extra scalar field that plays the role of dark matter. The Higgs boson is light near a critical boundary between different symmetry breaking phases, where quantum corrections beyond the usual Gildener-Weinberg approximation become relevant. This implies a tighter connection between dark matter and Higgs phenomenology. The model has only three free parameters, yet it allows for the observed relic abundance of dark matter while respecting all constraints. The direct detection cross section mediated by the Higgs boson is determined by the dark matter mass alone and is testable at future experiments.

hep-ph↗

arXiv2022

Dark Matter as dark dwarfs and other macroscopic objects: multiverse relics?

First-order phase transitions can leave relic pockets of false vacua and their particles, that manifest as macroscopic Dark Matter. We compute one predictive model: a gauge theory with a dark quark relic heavier than the confinement scale. During the first-order phase transition to confinement, dark quarks remain in the false vacuum and get compressed, forming Fermi balls that can undergo gravitational collapse to stable dark dwarfs (bound states analogous to white dwarfs) near the Chandrasekhar limit, or to primordial black holes.

hep-ph↗

arXiv2022

Dark Matter interpretation of the neutron decay anomaly

We add to the Standard Model a new fermion $χ$ with minimal baryon number 1/3. Neutron decay $n \to χχχ$ into non-relativistic $χ$ can account for the neutron decay anomaly, compatibly with bounds from neutron stars. $χ$ can be Dark Matter, and its cosmological abundance can be generated by freeze-in dominated at $T \sim m_n$. The associated processes $n \to χχχγ$, hydrogen decay ${\rm H}\to χχχν(γ)$ and DM-induced neutron disappearance $\barχn \to χχ(γ)$ have rates below experimental bounds and can be of interest for future experiments.

hep-ph↗