Light Axion-like Particles Research Articles

Exploring axionlike particle couplings through single top tW -channel and top pair production at the LHC

In this paper, we explore the interactions between light axionlike particles (ALPs) and top quarks, gluons, and W-bosons. Utilizing high-energy probes at the LHC, we probe these couplings in the context of single top quark production associated with a W-boson and an ALP. We analyze the latest LHC Run II data on tt¯ spin correlation measurements to establish constraints on these couplings. Additionally, we compare these constraints with those derived from loop-induced couplings and indirect searches involving B-meson decays. Furthermore, we derive two-dimensional limits on the ALP couplings with top quarks and gluons. Published by the American Physical Society 2024

Interpretation of excess in H→Zγ using a light axionlike particle

We interpret the recent excess in a rare decay of the Higgs boson, H→Zγ, using a light axionlike particle (ALP) in the mass range 0.05–0.1 GeV. The dominant decay of such a light ALP is into a pair of collimated photons, whose decay is required to happen before reaching the ECAL detector, such that it mimics a single photon in the detector. It can explain the excess with a coupling CaZHeff/Λ∼4×10−5 GeV−1, while the decay of the ALP before reaching the ECAL requires the diphoton coupling Cγγeff/Λ≥0.35 TeV−1(0.1 GeV/ma)2. A potential test would be the rare decay of the Z boson Z→aH*→a(bb¯) at the Tera-Z option of the future FCC and CEPC. However, it has a branching ratio of only O(10−12), and thus barely testable. The production cross section for pp→Z*→aH via the same coupling CaZHeff/Λ at the LHC is too small for detection. Published by the American Physical Society 2024

Muon g−2 , long-range muon spin force, and neutrino oscillations

Recent studies have proposed using a geocentric muon spin force to account for the (g−2)μ anomaly, with the long-range force mediator being a light axionlike particle. The mediator exhibits a CP-violating scalar coupling to nucleons and a normal derivative coupling to muons. Due to the weak symmetry, this axion inevitably couples to neutrinos, providing potential impact on neutrino oscillations. By utilizing neutrino data from BOREXINO, IceCube DeepCore, Super-Kamiokande, and SNO, we have identified that both atmospheric and solar neutrino data can impose stringent constraints on the long-range muon spin force model and the (g−2)μ parameter space. With optimized data analysis techniques and the potential from future experiments, such as JUNO, Hyper-Kamiokande, SNO+, and IceCube PINGU, there exists a promising opportunity to achieve even greater sensitivities. Indeed, neutrino oscillations offer a robust and distinctive cross-check for the model, offering stringent constraints on the (g−2)μ parameter space. Published by the American Physical Society 2024

Flavor-violating ALPs, electron g−2 , and the Electron-Ion Collider

We revisit the possibility that light axionlike particles (ALPs) with lepton-flavor-violating couplings could give significant contributions to the electron’s anomalous magnetic moment ge−2. Unlike flavor diagonal lepton-ALP couplings, which are exclusively axial, lepton-flavor-violating couplings can have arbitrary chirality. Focusing on the e−τ ALP coupling, we find that the size of the contribution to ge−2 depends strongly on the chirality of the coupling. A significant part of the parameter space for which such a coupling can explain experimental anomalies in ge−2 can be probed at the Electron-Ion Collider, which is uniquely sensitive to the chirality of the coupling using the polarization of the electron beam. Published by the American Physical Society 2024

On the coupling of axion-like particles to the top quark

In this paper we explore the coupling of a light axion-like particle (ALP) to top quarks. We use high-energy LHC probes, and examine both the direct probe to this coupling in associated production of a top-pair with an ALP, and the indirect probe through loop-induced gluon fusion to an ALP leading to top pairs. Using the latest LHC Run II data, we provide the best limit on this coupling. We also compare these limits with those obtained from loop-induced couplings in diboson final states, finding that the tt¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ t\\overline{t} $$\\end{document}+MET channel is the best current handle on this coupling.

Determining the spin of light primordial black holes with Hawking radiation

We propose a method to determine the mass and spin of primordial black holes (PBHs) in the mass range 5 × 107–1012 kg (Hawking temperatures ~ 10 MeV–200 GeV), based on measuring the energy of specific features in the photon Hawking emission spectrum, including both primary and secondary components. This is motivated by scenarios where PBHs in this mass range spin up as they evaporate, namely the string axiverse, where dimensionless spin parameters overset{sim }{a} ~ 0.1–0.5 can be achieved through the Hawking emission of hundreds or even thousands of light axion-like particles. Measuring the present PBH mass-spin distribution may thus be an important probe of physics beyond the Standard Model. Since the proposed method relies on the energy of the photons emitted by a given PBH, rather than on the associated flux, it is independent of the PBH-Earth distance and, as a byproduct, can also be used to infer the latter.

Axion couplings in grand unified theories

We show that the couplings of axions to gauge bosons are highly restricted in Grand Unified Theories where the standard model is embedded in a simple 4D gauge group. The topological nature of these couplings allows them to be matched from the UV to the IR, and the ratio of the anomaly with photons and gluons for any axion is fixed by unification. This implies that there is a single axion, the QCD axion, with an anomalous coupling to photons. Other light axion-like particles can couple to photons by mixing through the QCD axion portal and lie to the right of the QCD line in the mass-coupling plane. Axions which break the unification relation between gluon and photon couplings are necessarily charged under the GUT gauge group and become heavy from perturbative mass contributions. A discovery of an axion to the left of the QCD line can rule out simple Grand Unified models. Axion searches are therefore tabletop and astrophysical probes of Grand Unification.

Flavor physics with generalized ALP-quark couplings

A light axionlike particle or an axionlike particle (ALP) not just gives rise to interesting and spectacular signals of new physics as final states in meson decays, it necessarily leaves tell-tale signatures in processes that involve standard model (SM) fields only (i.e., SM processes). These effects result in the violation of the Gell-Mann-Okubo mass relation, modified form factors, altered integrated and differential rates for various SM transitions etc. This suggests that in the presence of a low lying state, such as an ALP, extraction of masses, mixing angles, and form factors in an entirely data-driven way from meson-physics observables is a highly nontrivial exercise. However, once done correctly, these same observables may, in turn, provide important (indirect) bounds on ALP physics, which remain robust even in the limits where new physics effects conspire to weaken the bounds from direct searches. Starting with a generalized ALP-quark Lagrangian (where restrictions due to parity are removed) we demonstrate this approach by focusing on ${K}_{{\ensuremath{\ell}}_{3}}^{+}$ decays, where we derive (indirect) bounds on ALP physics using $\mathrm{NA}48/2$ data and lattice results. We also find sum rules that not just show deviations in the presence of an ALP but also give hints towards the specific nature of the ALP physics itself.

Searching for dark-matter waves with PPTA and QUIJOTE pulsar polarimetry

The polarization of photons emitted by astrophysical sources might be altered as they travel through adark matter medium composed of ultra light axion-like particles (ALPs). In particular, the coherent oscillations of the ALP background in the galactic halo induce a periodic change on thepolarization of the electromagnetic radiation emitted by local sources such as pulsars. Building up on previous works, we develop a new, more robust, analysis based on the generalised Lomb-Scargle periodogram to search for this periodic signal in the emission of the Crab supernova remnant observed by the QUIJOTE MFI instrument and 20 Galactic pulsars from the Parkes Pulsar Timing Array (PPTA) project. We also carefully take into account the stochastic nature of the axion field, an effect often overlooked in previous works.This refined analysis leads to the strongest limits on the axion-photon couplingfor a wide range of dark matter masses spanning 10-23 eV ≲ ma ≲ 10-19 eV. Finally, we survey possible optimal targets and the potential sensitivity to axionic dark-matter in this mass range that could be achieved using pulsar polarimetry in the future.

How Do Magnetic Field Models Affect Astrophysical Limits on Light Axion-like Particles? An X-Ray Case Study with NGC 1275

Axion-like particles (ALPs) are a well-motivated extension to the standard model of particle physics, and X-ray observations of cluster-hosted AGN currently place the most stringent constraints on the ALP coupling to electromagnetism, g a γ , for very light ALPs (m a ≲ 10−11 eV). We revisit limits obtained by Reynolds et al. using Chandra X-ray grating spectroscopy of NGC 1275, the central AGN in the Perseus cluster, examining the impact of the X-ray spectral model and magnetic field model. We also present a new publicly available code, ALPro, which we use to solve the ALP propagation problem. We discuss evidence for turbulent magnetic fields in Perseus and show that it can be important to resolve the magnetic field structure on scales below the coherence length. We reanalyze the NGC 1275 X-ray spectra using an improved data reduction and baseline spectral model. We find the limits are insensitive to whether a partially covering absorber is used in the fits. At low m a (m a ≲ 10−13 eV), we find marginally weaker limits on g a γ (by 0.1–0.3 dex) with different magnetic field models, compared to Model B from Reynolds et al. (2020). A Gaussian random field (GRF) model designed to mimic ∼50 kpc scale coherent structures also results in only slightly weaker limits. We conclude that the existing Model B limits are robust assuming that β pl ≈ 100, and are insensitive to whether cell-based or GRF methods are used. However, astrophysical uncertainties regarding the strength and structure of cluster magnetic fields persist, motivating high-sensitivity RM observations and tighter constraints on the radial profile of β pl.

Axion-like particle oscillations

String theory compactifications may generate many light axion-like particles (ALPs) with weak couplings to electromagnetism. In general, a large number of ALPs may exist, with a linear combination having a potentially observable coupling to electromagnetism. The basis in which only one ALP couples to electromagnetism is in general misaligned with the mass basis. This leads to mixing between the `electromagnetic' ALP and a number of `hidden' ALPs that do not interact directly with the photon. The process is analagous to neutrino oscillations. I will discuss the phenomenological consequences of this mixing for astrophysical ALP signals, in particular showing that it may significantly reduce the predicted signal in experiments such as the CERN Axion Solar Telescope.

Photon-jet events as a probe of axionlike particles at the LHC

Axionlike particles (ALPs) are predicted by many extensions of the Standard Model. When ALP mass lies in the range of MeV--GeV, the cosmology and astrophysics will be largely irrelevant. In this work, we investigate such light ALPs through the ALP-strahlung process $pp\ensuremath{\rightarrow}Va(\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})$ at the 14 TeV LHC with an integrated luminosity of $3000\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ (high-luminosity LHC). With the photon-jet algorithm, we demonstrate that our approach can probe the mass range of ALPs, which has been inaccessible in previous LHC experiments. The obtained result can surpass the existing limits on ALP-photon coupling in the ALP mass range from 0.3 to 10 GeV.

Searching for axionlike particles from core-collapse supernovae with Fermi LAT’s low-energy technique

Light axion-like particles (ALPs) are expected to be abundantly produced in core-collapse supernovae (CCSNe), resulting in a $\sim$10-second long burst of ALPs. These particles subsequently undergo conversion into gamma-rays in external magnetic fields to produce a long gamma-ray burst (GRB) with a characteristic spectrum peaking in the 30--100-MeV energy range. At the same time, CCSNe are invoked as progenitors of {\it ordinary} long GRBs, rendering it relevant to conduct a comprehensive search for ALP spectral signatures using the observations of long GRB with the \textit{Fermi} Large Area Telescope (LAT). We perform a data-driven sensitivity analysis to determine CCSN distances for which a detection of an ALP signal is possible with the LAT's low-energy (LLE) technique which, in contrast to the standard LAT analysis, allows for a a larger effective area for energies down to 30~MeV. Assuming an ALP mass $m_a \lesssim 10^{-10}$~eV and ALP-photon coupling $g_{a\gamma} = 5.3\times 10^{-12}$ GeV$^{-1}$, values considered and deduced in ALP searches from SN1987A, we find that the distance limit ranges from $\sim\!0.5$ to $\sim\!10$~Mpc, depending on the sky location and the CCSN progenitor mass. Furthermore, we select a candidate sample of twenty-four GRBs and carry out a model comparison analysis in which we consider different GRB spectral models with and without an ALP signal component. We find that the inclusion of an ALP contribution does not result in any statistically significant improvement of the fits to the data. We discuss the statistical method used in our analysis and the underlying physical assumptions, the feasibility of setting upper limits on the ALP-photon coupling, and give an outlook on future telescopes in the context of ALP searches.

Probing charged lepton flavor violation with axion-like particles at Belle II

We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), X, at the B-factory experiment Belle II. We focus on production of the ALP in the tau decays τ → Xl with l = e, μ, followed by its decay via X → l−l+. The ALP can be either promptly decaying or long-lived. We perform Monte-Carlo simulations, recasting a prompt search at Belle for lepton-flavor-violating τ decays, and propose a displaced-vertex (DV) search. For both types of searches, we derive the Belle II sensitivity reaches in both the product of branching fractions and the ALP coupling constants, as functions of the ALP mass and lifetime. The results show that the DV search exceeds the sensitivity reach of the prompt search to the relevant branching fractions by up to about a factor of 40 in the long decay length regime.

PASSAT at future neutrino experiments: Hybrid beam-dump-helioscope facilities to probe light axionlike particles

There are broadly three channels to probe axionlike particles (ALPs) produced in the laboratory: through their subsequent decay to Standard Model (SM) particles, their scattering with SM particles, or their subsequent conversion to photons. Decay and scattering are the most commonly explored channels in beam-dump type experiments, while conversion has typically been utilized by light-shining-through-wall (LSW) experiments. A new class of experiments, dubbed PASSAT (particle accelerator helioscopes for slim axionlike-particle detection), has been proposed to make use of the ALP-to-photon conversion in a novel way: ALPs, after being produced in a beam-dump setup, turn into photons in a magnetic field placed near the source. It has been shown that such hybrid beam-dump-helioscope experiments can probe regions of parameter space that have not been investigated by other laboratory-based experiments, hence providing complementary information; in particular, they probe a fundamentally different region than decay or LSW experiments. We propose the implementation of PASSAT in future neutrino experiments, taking a DUNE-like experiment as an example. We demonstrate that the magnetic field in the planned DUNE multipurpose detector is already capable of probing the ALP-photon coupling down to ${g}_{a\ensuremath{\gamma}\ensuremath{\gamma}}\ensuremath{\sim}\mathrm{few}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$ for ALP masses ${m}_{a}\ensuremath{\lesssim}10\text{ }\text{ }\mathrm{eV}$. The implementation of a CAST or BabyIAXO-like magnet would improve the sensitivity down to ${g}_{a\ensuremath{\gamma}\ensuremath{\gamma}}\ensuremath{\sim}{10}^{\ensuremath{-}6}\text{ }\text{ }{\mathrm{GeV}}^{\ensuremath{-}1}$.

Parametrics of electromagnetic searches for axion dark matter

Light axion-like particles occur in many theories of beyond-Standard-Model physics, and may make up some or all of the universe's dark matter. One of the ways they can couple to the Standard Model is through the electromagnetic $F_{\mu\nu} \tilde F^{\mu\nu}$ portal, and there is a broad experimental program, covering many decades in mass range, aiming to search for axion dark matter via this coupling. In this paper, we derive limits on the absorbed power, and coupling sensitivity, for a broad class of such searches. We find that standard techniques, such as resonant cavities and dielectric haloscopes, can achieve O(1)-optimal axion-mass-averaged signal powers, for given volume and magnetic field. For low-mass (frequency $\ll$ GHz) axions, experiments using static background magnetic fields generally have suppressed sensitivity - we discuss the physics of this limitation, and propose experimental methods to avoid it, such as microwave up-conversion experiments. We also comment on the detection of other forms of dark matter, including dark photons, as well as the detection of relativistic hidden sector particles.

New collider searches for axionlike particles coupling to gluons

Axion-like particles (ALPs) are pseudo Nambu-Goldstone bosons associated with spontaneously broken global symmetries emerging in many extensions of the Standard Model. Assuming the most general effective Lagrangian up to dimension-5 operators for an ALP interacting with the SM fields, we investigate for the first time the sensitivity of the LHC13 to the ALP production in association with a di-jet. This study is focused on light ALPs which appear as invisible particles at the detector. Performing a realistic detector simulation and deploying a multivariate technique to best discriminate the signal from backgrounds, we set expected upper bounds on the ALP coupling to gluons. A comprehensive set of background processes is considered, and it is shown that this process provides significant sensitivity to the ALP-gluon coupling and the resulting bound is more stringent than those already obtained at the LHC. We also present prospects for the HE-LHC27 and FCC-hh100 and show that these future colliders are able to improve the limits from the LHC by roughly one order of magnitude.

Astrophysical Limits on Very Light Axion-like Particles from Chandra Grating Spectroscopy of NGC 1275

Abstract Axions/axion-like particles (ALPs) are a well-motivated extension of the Standard Model and are generic within String Theory. The X-ray transparency of the intracluster medium (ICM) in galaxy clusters is a powerful probe of light ALPs (with mass ); as X-ray photons from an embedded or background source propagate through the magnetized ICM, they may undergo energy-dependent quantum mechanical conversion into ALPs (and vice versa), imprinting distortions on the X-ray spectrum. We present Chandra data for the active galactic nucleus NGC 1275 at the center of the Perseus cluster. Employing a 490 ks High Energy Transmission Gratings exposure, we obtain a high-quality 1–9 keV spectrum free from photon pileup and ICM contamination. Apart from iron-band features, the spectrum is described by a power-law continuum, with any spectral distortions at the <3% level. We compute photon survival probabilities as a function of ALP mass m a and ALP-photon coupling constant for an ensemble of ICM magnetic field models, and then use the NGC 1275 spectrum to constrain the -plane. Marginalizing over magnetic field realizations, the 99.7% credible region limits the ALP-photon coupling to (depending upon magnetic field model) for masses . These are the most stringent limit to date on for these light ALPs, and have already reached the sensitivity limits of next-generation helioscopes and light-shining-through-wall experiments. We highlight the potential of these studies with the next-generation X-ray observatories Athena and Lynx, but note the critical importance of advances in relative calibration of these future X-ray spectrometers.

Axions in a highly protected gauge symmetry model

We study QCD axion or cosmological axion-like particles (ALPs) in a model inspired by the recent interest in 4-dimensional clockwork models, with the global symmetry being accidentally enforced by a gauge abelian quiver with scalar bifundamental fields. For the QCD axion, we analyze the connection between the degree of protection of the axion mass against gravitational corrections, the explanation of the hierarchy f_a ll M_P and the number of colored fermions needed to generate anomalous couplings to gluons, all linked together by the underlying gauge symmetries. Based on that model and on the comparison with earlier models in the literature, we derive certain general conclusions on QCD axion models that use accidental global symmetries. For the ALPs, assuming that their mass is solely given by gravitational corrections, we identify the parameter space where the decay constant and the mass are consistent with the DM abundance, and we show that this clockwork-inspired model is a particularly economical model for a very light ALP DM candidate.

Constraints on axion-like particles and nucleon pairing in dense matter from the hot neutron star in HESS J1731-347

If the thermal evolution of the hot young neutron star in the supernova remnant HESS J1731-347 is driven by neutrino emission, it provides a stringent constraint on the coupling of light (mass $\ll 10$ keV) axion-like particles to neutrons. Using Markov-Chain Monte Carlo we find that for the values of axion-neutron coupling $g_{ann}^2 > 7.7 \times 10^{-20}$ (90% c.l.) the axion cooling from the bremsstrahlung reaction $n+n\rightarrow n+n +a$ is too rapid to account for the high observed surface temperature. This implies that the Pecci-Quinn scale or axion decay constant $f_a > 6.7 \times 10^7$ GeV for KSVZ axions and $f_a > 1.7 \times 10^9$ GeV for DFSZ axions. The high temperature of this neutron star also allows us to tighten constraints on the size of the nucleon pairing gaps.