Testing the heterotic string with the axion-photon coupling

We minimally extend the Standard Model field content by adding new vectorlike fermions at the TeV scale to allow gauge coupling unification at a realistic scale. We embed the model into an SU(5) grand unified theory that is asymptotically safe and features an interacting fixed point for the gauge coupling. There are no Landau poles of the U(1) gauge and Higgs couplings in this new setting. Gauge, Yukawa, and Higgs couplings are retraced from the fixed point and matched at the grand unification scale to those of the Standard Model rescaled up to the same energy. All couplings, their fixed point values, and critical exponents always remain in the perturbative regime.

String unification, universal one-loop corrections and strongly coupled heterotic string theory

Opportunities for searches for axionlike particles (ALPs) coupling to photons in e+e− collisions at the Future Circular Collider (FCC-ee) and International Linear Collider (ILC) are investigated. We perform a study of the photon-fusion production of ALPs decaying into two photons, e+e−→γγe+a(γγ)e−, over the light-by-light continuum background, for the planned FCC-ee and ILC center-of-mass energies and integrated luminosities. An analysis of the feasibility measurements is presented using parametrized simulations for two types of detectors. Upper limits at 95% confidence level (C.L.) on the cross section for ALP production, σ(γγ→a→γγ), and on the ALP-photon coupling are obtained over the ma≈0.1–1000 GeV ALP mass range, and compared to current and future collider searches. Production cross sections down to σ(γγ→a→γγ)≈1 fb (1 ab) will be probed at ma≈1 (300) GeV, corresponding to constraints on the axion-photon coupling as low as gaγγ≈2×10−3 TeV−1. Published by the American Physical Society 2024

Axionlike particles (ALPs), hypothetical particles beyond the Standard Model (SM), are considered as promising dark matter candidates. ALPs can convert into photons and vice versa in a magnetic field via the Primakoff effect, potentially generating detectable oscillation in γ-ray spectra. This study analyzes 16.5 years of data from the Fermi Large Area Telescope (Fermi-LAT) on NGC 1275, the brightest galaxy in the Perseus cluster, to constrain the ALP parameter space. Our results improve the previous 95% exclusion limits of the photon-ALP coupling gaγ by a factor of 2 in the ALP mass range of 4×10−10 eV≲ma≲5×10−9 eV. Moreover, we investigate the projected sensitivity of the future Very Large Area γ-ray Space Telescope (VLAST) on searching for ALPs. We find that (i) the expected sensitivity on the photon-ALP coupling gaγ can be stronger than that from the upcoming International Axion Observatory in the ALP mass range of 2×10−11 eV≲ma≲1×10−7 eV, with the best sensitivity of gaγ∼7×10−13 GeV−1 at ma∼2×10−10 eV; (ii) VLAST can extend the sensitivity of the ALP masses below 5×10−12 eV, where the photon-ALP coupling gaγ≳1.5×10−11 GeV−1 will be excluded; (iii) the entire parameter space of ALP accounting for TeV transparency can be fully tested. These results demonstrate that VLAST will offer an excellent opportunity for ALPs searches.

Gauge coupling unification and the success of TeV-scale weakly interacting dark matter are usually taken as evidence of low energy supersymmetry (SUSY). However, if we assume that the tuning of the higgs can be explained in some unnatural way, from environmental considerations for example, SUSY is no longer a necessary component of any Beyond the Standard Model theory. In this paper we study the minimal model with a dark matter candidate and gauge coupling unification. This consists of the SM plus fermions with the quantum numbers of SUSY higgsinos, and a singlet. It predicts thermal dark matter with a mass that can range from 100 GeV to around 2 TeV and generically gives rise to an electric dipole moment that is just beyond current experimental limits, with a large portion of its allowed parameter space accessible to next generation EDM and direct detection experiments. We study precision unification in this model by embedding it in a 5-D orbifold GUT where certain large threshold corrections are calculable, achieving gauge coupling and b-tau unification, and predicting a rate of proton decay just beyond current limits.

This article describes BabyIAXO, an intermediate experimental stage of the International Axion Observatory (IAXO), proposed to be sited at DESY. IAXO is a large-scale axion helioscope that will look for axions and axion-like particles (ALPs), produced in the Sun, with unprecedented sensitivity. BabyIAXO is conceived to test all IAXO subsystems (magnet, optics and detectors) at a relevant scale for the final system and thus serve as prototype for IAXO, but at the same time as a fully-fledged helioscope with relevant physics reach itself, and with potential for discovery. The BabyIAXO magnet will feature two 10 m long, 70 cm diameter bores, and will host two detection lines (optics and detector) of dimensions similar to the final ones foreseen for IAXO. BabyIAXO will detect or reject solar axions or ALPs with axion-photon couplings down to gaγ ∼ 1.5 × 10−11 GeV−1, and masses up to ma ∼ 0.25 eV. BabyIAXO will offer additional opportunities for axion research in view of IAXO, like the development of precision x-ray detectors to identify particular spectral features in the solar axion spectrum, and the implementation of radiofrequency-cavity-based axion dark matter setups.

Axionlike particles (ALPs) with lepton flavor-violating (LFV) interactions are predicted within a wide range of flavored ALP models. The proposed μTRISTAN high-energy e−μ+ and μ+μ+ collider will provide a good opportunity to explore flavor physics in the charged lepton sector. In this work, based on a model-independent effective Lagrangian describing the ALP leptonic interactions, we investigate the potential of μTRISTAN to probe ALP LFV couplings. We analyze the testability of selected ALP production channels with potential sensitivity at μTRISTAN, considering different beams and collision energies, including e−μ+→aγ, e−μ+→e−τ+a, μ+μ+→μ+τ+a, and e−μ+→τ−μ+a. The produced ALP a is either long-lived or can promptly decay to flavor-violating or flavor-conserving charged lepton final states. In particular, combining the above LFV ALP production modes with a suitable LFV decay mode, one can identify signatures that are virtually free of Standard Model background. We show the resulting sensitivity of μTRISTAN to LFV ALP couplings and compare it with multiple low-energy leptonic constraints and the future improvements thereof. We find that μTRISTAN can be generally complementary to searches for low-energy LFV processes and measurements of the leptonic magnetic dipole moments and has the capability to explore unconstrained parameter space for ALP masses in the O(1)–O(100) GeV range. In the light ALP regime, however, the parameter space that μTRISTAN is sensitive to has been already excluded by low-energy searches for LFV decays. Published by the American Physical Society 2024

FASER (the Forward Search Experiment) is a compact detector located about 480 m downstream of the ATLAS interaction point at CERN’s Large Hadron Collider (LHC). It is designed to explore new Beyond the Standard Model (BSM) physics by searching for light, weakly interacting, and long-lived particles (LLPs) produced in the far-forward region. This unique setup—shielded by approximately 100 m of rock and concrete—enables FASER to perform highly sensitive searches for exotic states such as dark photons (A′) and axion-like particles (ALPs), potentially mediating interactions between the visible and dark sectors. During Run 3 (2022–2024), FASER has recorded 190 $\mathrm{fb^{⁻¹}}$ of data with over 97\% efficiency, and A dark photon and ALP searches are performed. We will present these physics results, probing previously unexplored mass and coupling ranges. FASER is now planning an upgrade of its preshower sub-detector to improve diphoton resolution and background discrimination, aiming for enhanced sensitivity in upcoming ALP and other new physics searches. With continued data collection through Run 3 and a substantially increased data set during the High-Luminosity LHC era (Run 4), FASER will further extend its discovery potential for long-lived particles and other novel BSM signatures. In this talk, we will present the status of the experiment, including detector design, detector performance, and physics results of new particle searches from Run 3 data.

Top quark mass prediction in superstring derived standard-like models

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.

Dipole moments of black holes and string states

Duality, compactification, and [formula omitted] effects in the heterotic string theory

We consider the constraints from Supernova 1987A on particles with small couplings to the Standard Model. We discuss a model with a fermion coupled to a dark photon, with various mass relations in the dark sector; millicharged particles; dark-sector fermions with inelastic transitions; the hadronic QCD axion; and an axion-like particle that couples to Standard Model fermions with couplings proportional to their mass. In the fermion cases, we develop a new diagnostic for assessing when such a particle is trapped at large mixing angles. Our bounds for a fermion coupled to a dark photon constrain small couplings and masses ≲ 200 MeV, and do not decouple for low fermion masses. They exclude parameter space that is otherwise unconstrained by existing accelerator-based and direct-detection searches. In addition, our bounds are complementary to proposed laboratory searches for sub-GeV dark matter, and do not constrain several benchmark-model targets in parameter space for which the dark matter obtains the correct relic abundance from interactions with the Standard Model. For a millicharged particle, we exclude charges between 10−9–few×10−6 in units of the electron charge, also for masses ≲ 200 MeV; this excludes parameter space to higher millicharges and masses than previous bounds. For the QCD axion and an axion-like particle, we apply several updated nuclear physics calculations and include the energy dependence of the optical depth to accurately account for energy loss at large couplings. These corrections allow us to rule out a hadronic axion of mass between 0.1 and a few hundred eV, or equivalently to put a bound on the scale of Peccei-Quinn symmetry breaking between a few×104 and 108 GeV, closing the hadronic axion window. For an axion-like particle, our bounds disfavor decay constants between a few×105 GeV up to a few×108 GeV, for a mass ≲ 200 MeV. In all cases, our bounds differ from previous work by more than an order of magnitude across the entire parameter space. We also provide estimated systematic errors due to the uncertainties of the progenitor.

We investigate the potential of using the signature of mono-Higgs plus large missing energies to constrain on two new ph ysics models, namely the model of an axion-like particle (ALP) and the model of sterile neutrinos. We focus on the Higgs-ALP interactions starting at dimension-six and the Higgs-sterile neutrino interactions starting at dimension-five, via the processes pp → haa for ALP production and pp → hNN for sterile neutrinos at the LHC and High Luminosity LHC (HL-LHC), followed by the Higgs decay h→bb¯. We establish bounds on the ALP-Higgs coupling CaHΛ2 and sterile neutrino-Higgs coupling λ3M∗, respectively, for ALP and sterile-neutrino mass ranging from 1 to 60 GeV, using the recent ATLAS data on mono-Higgs plus missing energies at the LHC (s = 13 TeV and L = 139 fb−1). The most stringent constraint occurs in the missing transverse energy MET range 200 < MET ≤ 350 GeV. We also estimate the sensitivities that we can achieve at the HL-LHC (s = 14 TeV and L = 3000 fb−1). We obtain improved sensitivities across various missing energy regions. The ALP model exhibits better sensitivities, particularly at lower mass range, compared to the sterile neutrino model, which shows weaker sensitivities across similar mass and energy ranges. Our results underscore the potential of the mono-Higgs signature as a robust probe for physics beyond the Standard Model.

A bstract We study axion-like particles (ALPs) whose dominant interactions are with gluons and third-generation quarks, and whose couplings to light Standard Model (SM) particles arise at one loop. These loop-induced effects lead to ALP decays and production channels that can be probed at the LHC, even when tree-level couplings are absent. Using an effective field theory (EFT) description that includes momentum-dependent corrections from radiative effects, we reinterpret a wide range of LHC measurements via the Contur framework to derive model-independent constraints on the ALP parameter space. We show that LHC data place meaningful bounds in the plane of effective couplings $$ {c}_t^0/{f}_a $$ c t 0 / f a and $$ {c}_{\overset{\sim }{G}}^0/{f}_a $$ c G ~ 0 / f a , and that these limits are sensitive to the UV origin of the ALP-top and ALP-gluon couplings. We discuss representative scenarios where either $$ {c}_t^0 $$ c t 0 or $$ {c}_{\overset{\sim }{G}}^0 $$ c G ~ 0 vanishes at the matching scale, and highlight the role of EFT running and mixing in generating observable signals. We also assess the domain of validity of the EFT approach by comparing the typical momentum transfer $$ \sqrt{\hat{s}} $$ s ̂ in sensitive regions to the underlying scale f a . Our results demonstrate the power of loop-aware EFT reinterpretation of SM measurements in probing otherwise elusive ALP scenarios. The framework presented here can be readily extended to include couplings to other fermions and to accommodate ALP decay or long-lived signatures.