arXiv subjects

Gonzalo Alonso-Álvarez

Publications and source records attributed to Gonzalo Alonso-Álvarez.

At least 19 recordsLinked to original sources

arXiv2026

Probing light axion-like particles in vector boson fusion at CMS with data parking and scouting

Axion-like particles (ALPs) with masses between 10 MeV and 10 GeV and moderately small couplings sit in an experimental blind spot. They decay too fast for intensity-frontier experiments and are too feebly coupled and too light for conventional collider searches. The LHC produces them at substantial rates through vector boson fusion (VBF), the dominant production mode for electromagnetically coupled ALPs, but two obstacles keep this signal out of reach. The events are soft, so trigger thresholds discard most of them, and the Lorentz boost merges the two decay photons into a single calorimeter deposit. We show that the Compact Muon Solenoid (CMS) experiment can overcome both by combining its data-acquisition strategies with the tracker-based reconstruction of merged photon pairs. Data parking on the VBF jet topology in Run 3 (312 fb$^{-1}$) and trigger-level data scouting at the High-Luminosity LHC (HL-LHC, 3 ab$^{-1}$) remove the need for a threshold on the photons. Photons that convert to electron-positron pairs in the silicon tracker resolve the merged pair, since the tracker measures their direction far more finely than the calorimeter granularity. Reconstructing the ALP decay vertex from the conversion tracks removes the prompt backgrounds but not the long-lived $K_L\toγγ$. In Run 3, where the full event is recorded, the merged diphoton invariant mass confines the $K_L$ to a narrow window around its own mass. In the scouting stream, where only trigger-level information survives, the diphoton $p_T$, the conversion tracks, and the hadronic activity in the event take over this role. We project that the parked Run 3 data, already recorded, reach a region of the $(m_a, g_{aγγ})$ plane that no measurement has probed. Level-1 trigger data scouting at the HL-LHC extends that reach by roughly an order of magnitude in both coupling and mass.

hep-ph↗

arXiv2026

Wideband Search for Axionlike Dark Matter Using Octupolar Nuclei in a Crystal

Most of the matter in the Universe is in the form of dark matter, which has evaded detection so far. Ultralight axionlike particles (ALPs) are a class of dark matter candidates that produce measurable signatures in the form of oscillating violations of discrete symmetries in nuclei. We report results from a search for an oscillating parity-odd time-reversal-odd nuclear Schiff moment of $^{153}$Eu ions in a crystal, which leads to constraints on ALP-gluon coupling strength across a wide band spanning eight decades in ALP mass.

physics.atom-ph↗

arXiv2025

Dark photon distortions of NO$ν$A and T2K neutrino oscillations

Dark photons coupling to $L_μ-L_τ$ lepton number difference are a highly studied light dark matter candidate, with potential to be discovered through their impact on terrestrial neutrino oscillation experiments. We re-examine this in the light of claimed tensions between the NO$ν$A and T2K long baseline experiments, also taking into account data from the MINOS experiment. We obtain leading limits on the $L_μ-L_τ$ gauge coupling $g'$ versus dark photon mass $m_{A'}$, and find no statistically significant alleviation of the tension from inclusion of the new physics effect.

hep-ph↗

arXiv2024

Dark Astronomy with Dark Matter Detectors

We present a novel way of probing non-gravitational dark matter interactions: dark astronomy, which leverages the dark radiation emitted by dissipative dark sectors. If the mediator of the dark matter self interactions is a dark photon with a small mass that kinetically mixes with the visible photon, the dark radiation flux becomes accessible to underground experiments. We argue that the emission may be dominantly longitudinally polarized, thereby enhancing the sensitivity of direct detection experiments such as XENON and SENSEI to this signal. We introduce a new detection mechanism based on resonant dark-photon-to-photon conversion at the surface of conducting materials, which offers unique directional sensitivity to dark radiation. This mechanism facilitates the development of experiments that combine dark matter detection techniques with methods of traditional astronomy, opening the possibility to map dark radiation sources within our galaxy.

astro-ph.CO↗

arXiv2024

The first limit on invisible decays of $B_s$ mesons comes from LEP

Motivated by the recent evidence for $B^+\to K^+\barν ν$ decays at Belle II, we point out that fully invisible $B_d$ and $B_s$ meson decays are strongly constrained by LEP. A reinterpretation of an old inclusive ALEPH search for $b$-hadron decays with large missing energy allows us to place the limits $\mathrm{Br}(B_d \rightarrow \mathrm{invisible}) < 1.4\times10^{-4}$ and $\mathrm{Br}(B_s \rightarrow \mathrm{invisible}) < 5.6\times10^{-4}$, both at $90\%$ CL. The $B_d$ limit is only a factor of 6 looser than the world-leading one provided by the BaBar collaboration, while the $B_s$ one is the first limit in the literature on this decay mode. These results are relevant in the context of new light states coupled to quarks and exemplify the power of a future Tera-$Z$ factory at FCC-ee to look for $B$ meson decays containing missing energy.

hep-ph↗

arXiv2024

Self-interacting dark matter solves the final parsec problem of supermassive black hole mergers

Evidence for a stochastic gravitational wave (GW) background, plausibly originating from the merger of supermassive black holes (SMBHs), is accumulating with observations from pulsar timing arrays. An outstanding question is how inspiraling SMBHs get past the "final parsec" of separation, where they have a tendency to stall before GW emission alone can make the binary coalesce. We argue that dynamical friction from the dark matter (DM) spike surrounding the black holes is sufficient to resolve this puzzle, if the DM has a self-interaction cross section of order $\,$cm$^2$/g. The same effect leads to a softening of the GW spectrum at low frequencies as suggested by the current data. For collisionless cold DM, the friction deposits so much energy that the spike is disrupted and cannot bridge the final parsec, while for self-interacting DM, the isothermal core of the halo can act as a reservoir for the energy liberated from the SMBH orbits. A realistic velocity dependence, such as generated by the exchange of a massive mediator like a dark photon, is favored to give a good fit to the GW spectrum while providing a large enough core. A similar velocity dependence has been advocated for solving the small-scale structure problems of cold DM.

astro-ph.CO↗

arXiv2024

Baryogenesis through Asymmetric Reheating in the Mirror Twin Higgs

We present the $νϕ$MTH, a Mirror Twin Higgs (MTH) model realizing asymmetric reheating, baryogenesis and twin-baryogenesis through the out-of-equilibrium decay of a right-handed neutrino without any hard $\mathbb{Z}_2$ breaking. The MTH is the simplest Neutral Naturalness solution to the little hierarchy problem and predicts the existence of a twin dark sector related to the Standard Model (SM) by a $\mathbb{Z}_2$ symmetry that is only softly broken by a higher twin Higgs vacuum expectation value. The asymmetric reheating cools the twin sector compared to the visible one, thus evading cosmological bounds on $ΔN_{\mathrm{eff}}$. The addition of (twin-)colored scalars allows for the generation of the visible baryon asymmetry and, by the virtue of the $\mathbb{Z}_2$ symmetry, also results in the generation of a twin baryon asymmetry. We identify a unique scenario with top-philic couplings for the new scalars that can satisfy all cosmological, proton decay and LHC constraints; yield the observed SM baryon asymmetry; and generate a wide range of possible twin baryon DM fractions, from negligible to unity. The viable regime of the theory contains several hints as to the possible structure of the Twin Higgs UV completion. Our results motivate the search for the rich cosmological and astrophysical signatures of twin baryons, and atomic dark matter more generally, at cosmological, galactic and stellar scales.

hep-ph↗

arXiv2023

The Flavor of QCD Axion Dark Matter

We argue that demanding a consistent cosmological history, including the absence of domain walls and strongly interacting relics at the Peccei-Quinn scale, singles out two concrete realizations of hadronic QCD axions as viable dark matter models. These realizations generally feature flavor-violating axion couplings to Standard Model quarks that are unsuppressed at low energies. As a consequence, experiments looking for flavor-violating hadronic processes involving the axion can be sensitive probes of QCD axion dark matter models. In particular, we show that the NA62 and KOTO experiments could detect the $K\rightarrowπ+ a$ decay for axions consistent with the observed dark matter abundance via the post-inflationary misalignment mechanism.

hep-ph↗

arXiv2023

Nonabelian Kinetic Mixing in a Confining Phase

Dark matter from a hidden sector with SU($N$) gauge symmetry can have a nonabelian kinetic mixing portal with the standard model. The dark photon becomes massive in the confining phase without the need for spontaneous symmetry breaking. Depending on the particle content of the dark sector, there can be two or more composite vectors that get kinetic mixing through a heavy mediator particle $X$. This provides a model of composite dark photons giving a portal for direct detection of dark baryons. Avoiding exotic charged relics requires additional couplings allowing $X$ to decay to dark quarks and standard model fields, leading to further portals between the dark matter and the standard model. We comprehensively study the constraints on such models from colliders, rare decays, direct detection, and big bang nucleosynthesis.

hep-ph↗

arXiv2023

Tracking axion-like particles at the LHC

Highly boosted axion-like particles decaying into photon pairs are notoriously hard to detect at the LHC. The collimated decay photons cannot be individually reconstructed using only electromagnetic calorimeter information, making the signal less distinguishable from background. In this note we propose a search strategy to address this issue, exploiting the fact that a fraction of the decay photons convert into electron-positron pairs inside the tracking detector. The resulting tracks can be resolved with high resolution, allowing to separate the two collimated photons and resolve a displaced decay vertex. To demonstrate the effectiveness of this approach, we apply it to ALPs in the challenging MeV-GeV range produced via vector boson fusion. We find that such a search could give sensitivity to untested parameter space.

hep-ph↗

arXiv2023

Distortion of neutrino oscillations by dark photon dark matter

A weakly coupled and light dark photon coupling to lepton charges $L_μ-L_τ$ is an intriguing dark matter candidate whose coherent oscillations alter the dispersion relations of leptons. We study how this effect modifies the dynamics of neutrino flavor conversions, focusing on long baseline and solar oscillations. We analyze data from the T2K, SNO, and Super-Kamiokande experiments in order to obtain world-leading limits on the dark photon gauge coupling for masses below $\sim 10^{-11}\,\mathrm{eV}$. Degeneracies between shifts in the neutrino mass-squared differences and mixing angles and the new physics effect significantly relax the current constrains on the neutrino vacuum oscillation parameters.

hep-ph↗

arXiv2022

The Strange Physics of Dark Baryons

Dark sector particles at the GeV scale carrying baryon number provide an attractive framework for understanding the origin of dark matter and the matter-antimatter asymmetry of the universe. We demonstrate that dark decays of hadronic states containing strange quarks -- hyperons -- offer excellent prospects for discovering such dark baryons. Building up on novel calculations of the matrix elements relevant for hyperon dark decays, and in view of various collider, flavor, and astrophysical constraints, we determine the expected rates at hyperon factories like BESIII and LHCb. We also highlight the interesting theoretical connections of hyperon dark decays to the neutron lifetime anomaly and Mesogenesis.

hep-ph↗

arXiv2022

Sterile neutrino production at small mixing in the early universe

Sterile neutrinos can be produced in the early universe via interactions with their active counterparts. For small active-sterile mixing angles, thermal equilibrium with the standard model plasma is not reached and sterile neutrinos are only produced via flavor oscillations. We study in detail this regime, taking into account matter potentials and decoherence effects caused by elastic scatterings with the plasma. We find that resonant oscillations occurring at temperatures $T\lesssim 10\,\mathrm{GeV}$ lead to a significant enhancement of the sterile neutrino production rate. Taking this into account, we improve constraints on the active-sterile mixing from Big Bang nucleosynthesis and the cosmic microwave background, excluding mixing angles down to $θ_s\sim 10^{-10}-10^{-16}$ for sterile neutrino masses in the $10\,\mathrm{MeV}$ to $10\,\mathrm{GeV}$ range. We observe that if sterile neutrinos predominantly decay into metastable hidden sector particles, this process provides a novel dark matter production mechanism, consistent with the sterile neutrino origin of light neutrino masses via the seesaw mechanism.

hep-ph↗

arXiv2022

Unleashing the full power of LHCb to probe Stealth New Physics

In this paper, we describe the potential of the LHCb experiment to detect Stealth physics. This refers to dynamics beyond the Standard Model that would elude searches that focus on energetic objects or precision measurements of known processes. Stealth signatures include long-lived particles and light resonances that are produced very rarely or together with overwhelming backgrounds. We will discuss why LHCb is equipped to discover this kind of physics at the Large Hadron Collider and provide examples of well-motivated theoretical models that can be probed with great detail at the experiment.

hep-ph↗

arXiv2022

Sterile neutrino dark matter catalyzed by a very light dark photon

Sterile neutrinos ($ν_s$) that mix with active neutrinos ($ν_a$) are interesting dark matter candidates with a rich cosmological and astrophysical phenomenology. In their simplest incarnation, their production is severely constrained by a combination of structure formation observations and X-ray searches. We show that if active neutrinos couple to an oscillating condensate of a very light $L_μ-L_τ$ gauge field, resonant $ν_a$-$ν_s$ oscillations can occur in the early universe, consistent with $ν_s$ constituting all of the dark matter, while respecting X-ray constraints on $ν_s\toν_aγ$ decays. Interesting deviations from standard solar and atmospheric neutrino oscillations can persist to the present.

hep-ph↗

arXiv2021

Gauging lepton flavor SU(3) for the muon $g-2$

Gauging a specific difference of lepton numbers such as $L_μ-L_τ$ is a popular model-building option, which gives rise to economical explanations for the muon anomalous magnetic moment. However, this choice of gauge group seems rather arbitrary, and additional physics is required to reproduce the observed neutrino masses and mixings. We address these shortcomings by embedding $L_μ-L_τ$ in the vectorial SU(3) gauge symmetry of lepton flavor. The vacuum expectation values (VEVs) of scalar fields in the fundamental, six-dimensional and adjoint representations allow for phenomenologically viable lepton and gauge boson masses. The octet scalar gives rise to charged lepton masses, and together with the triplet scalar generates masses for all the leptophilic gauge bosons except for the $L_μ-L_τ$ one. The latter gets its smaller mass from the sextet VEVs, which also generate the neutrino masses, and are determined up to an overall scaling by the observed masses and mixings. The model predicts three heavy neutral leptons at the GeV-TeV scale as well as vectorlike charged lepton partners; it requires the mass of the lightest active neutrino to exceed $10^{-4}\,$eV, and it naturally provides a resolution of the Cabibbo angle anomaly.

hep-ph↗

arXiv2021

Collider Signals of Baryogenesis and Dark Matter from $B$ Mesons: A Roadmap to Discovery

Low-scale baryogenesis could be discovered at $B$-factories and the LHC. In the $B$-Mesogenesis paradigm [G. Elor, M. Escudero, and A. E. Nelson, PRD 99, 035031 (2019), arXiv:1810.00880], the CP violating oscillations and subsequent decays of $B$ mesons in the early Universe simultaneously explain the origin of the baryonic and the dark matter of the Universe. This mechanism for baryo- and dark matter genesis from $B$ mesons gives rise to distinctive signals at collider experiments, which we scrutinize in this paper. We study CP violating observables in the $B^0_q-\bar{B}_q^0$ system, discuss current and expected sensitivities for the exotic decays of $B$ mesons into a visible baryon and missing energy, and explore the implications of direct searches for a TeV-scale colored scalar at the LHC and in meson-mixing observables. Remarkably, we conclude that a combination of measurements at BaBar, Belle, Belle II, LHCb, ATLAS and CMS can fully test $B$-Mesogenesis.

hep-ph↗

arXiv2021

Leading Logs in QCD Axion Effective Field Theory

The axion is much lighter than all other degrees of freedom introduced by the Peccei-Quinn mechanism to solve the strong CP problem. It is therefore natural to use an effective field theory (EFT) to describe its interactions. Loop processes calculated in the EFT may, however, explicitly depend on the ultraviolet cutoff. In general the UV cutoff is not uniquely defined, but the dimensionful couplings suggest to identify it with the Peccei-Quinn symmetry-breaking scale. An example are $K \rightarrow π+ a$ decays that will soon be tested to improved precision in NA62 and KOTO and whose amplitude is dominated by the term logarithmically dependent on the cutoff. In this paper, we critically examine the adequacy of using such a naive EFT approach to study loop processes by comparing EFT calculations with ones performed in complete QCD axion models. In DFSZ models, for example, the cutoff is found to be set by additional Higgs degrees of freedom and to therefore be much closer to the electroweak scale than to the Peccei-Quinn scale. In fact, there are non-trivial requirements on axion models where the cutoff scale of loop processes is close to the Peccei-Quinn scale, such that the naive EFT result is reproduced. This suggests that the existence of a suitable UV embedding may impose restrictions on axion EFTs. We provide an explicit construction of a model with suitable fermion couplings and find promising prospects for NA62 and IAXO.

hep-ph↗