The ForwArd Search ExpeRiment (FASER) is an approved experiment dedicated to searching for light, extremely weakly interacting particles at the LHC. Such particles may be produced in the LHC’s high-energy collisions and travel long distances through concrete and rock without interacting. They may then decay to visible particles in FASER, which is placed 480 m downstream of the ATLAS interaction point. In this work we briefly describe the FASER detector layout and the status of potential backgrounds. We then present the sensitivity reach for FASER for a large number of long-lived particle models, updating previous results to a uniform set of detector assumptions, and analyzing new models. In particular, we consider all of the renormalizable portal interactions, leading to dark photons, dark Higgs bosons, and heavy neutral leptons; light B-L and Li-Lj gauge bosons; axionlike particles that are coupled dominantly to photons, fermions, and gluons through nonrenormalizable operators; and pseudoscalars with Yukawa-like couplings. We find that FASER and its follow-up, FASER 2, have a full physics program, with discovery sensitivity in all of these models and potentially far-reaching implications for particle physics and cosmology.

FASER’s physics reach for long-lived particles

The Physics Beyond Colliders initiative is an exploratory study aimed at exploiting the full scientific potential of the CERN's accelerator complex and scientific infrastructures through projects complementary to the LHC and other possible future colliders. These projects will target fundamental physics questions in modern particle physics. This document presents the status of the proposals presented in the framework of the Beyond Standard Model physics working group, and explore their physics reach and the impact that CERN could have in the next 10-20 years on the international landscape.

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Physics Beyond Colliders at CERN: Beyond the Standard Model Working Group Report

Dark Matter through the Higgs portal

Lower bounds on the mass of a light scalar (Higgs) or pseudoscalar (axion) particle are found in three ways: (1) by requiring that their effect on primordial nucleosynthesis not yield a deuterium abundance outside present experimental limits, (2) by requiring that the photons from their decay thermalize and not distort the microwave background, and (3) by requiring that their emission from helium-burning stars (red giants) not disrupt stellar evolution. The best bound is from (3); it requires the axion or Higgs-particle mass to be greater than about 0.2 MeV.

Astrophysical bounds on the masses of axions and Higgs particles

Axion models with generation-dependent Peccei-Quinn charges can lead to flavor-changing neutral currents, thus motivating QCD axion searches at precision flavor experiments. We rigorously derive limits on the most general effective flavor-violating couplings from current measurements and assess their discovery potential. For two-body decays, we use available experimental data to derive limits on $q\ensuremath{\rightarrow}{q}^{\ensuremath{'}}a$ decay rates for all flavor transitions. Axion contributions to neutral-meson mixing are calculated in a systematic way using chiral perturbation theory and operator product expansion. We also discuss in detail baryonic decays and three-body meson decays, which can lead to the best search strategies for some of the couplings. For instance, a strong limit on the $\mathrm{\ensuremath{\Lambda}}\ensuremath{\rightarrow}na$ transition can be derived from the supernova SN 1987A. In the near future, dedicated searches for $q\ensuremath{\rightarrow}{q}^{\ensuremath{'}}a$ decays at ongoing experiments could potentially test Peccei-Quinn breaking scales up to ${10}^{12}\text{ }\text{ }\mathrm{GeV}$ at NA62 or KOTO and up to ${10}^{9}\text{ }\text{ }\mathrm{GeV}$ at Belle II or BES III.

Quark Flavor Phenomenology of the QCD Axion

The low-energy electronic recoil spectrum in XENON1T provides an intriguing hint for potential new physics. At the same time, observations of horizontal branch stars favor the existence of a small amount of extra cooling compared to the one expected from the Standard Model particle content. In this note, we argue that a hidden photon with a mass of $\sim 2.5$ keV and a kinetic mixing of $\sim 10^{-15}$ allows for a good fit to both of these excesses. In this scenario, the signal detected in XENON1T is due to the absorption of hidden photon dark matter particles, whereas the anomalous cooling of horizontal branch stars arises from resonant production of hidden photons in the stellar interior.

Hidden photon dark matter in the light of XENON1T and stellar cooling

Model-independent bounds on light pseudoscalars from rare B-meson decays

We investigate direct detection signatures of dark matter particles interacting with quarks via a light spin-0 mediator with general CP phases. Since tree-level scattering may be strongly suppressed in the non-relativistic limit, loop contributions play an important role and can lead to observable signals in near-future experiments. We study the phenomenology of different mediator masses and CP phases with an emphasis on scenarios with maximal CP violation and Higgs portal models. Intriguingly, the sum of the rates obtained at tree- and loop-level can give a characteristic recoil spectrum not obtainable from a single type of interaction. We furthermore develop a novel method for decomposing the two-loop contribution to effective interactions between dark matter and gluons into two separate one-loop diagrams, which in our case substantially simplifies the calculation of the important top-quark contribution.

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Loop-induced direct detection signatures from CP-violating scalar mediators

Studies of axion-like particles (ALPs) commonly focus on a single type of interaction, for example couplings only to photons. Most ALP models however predict correlations between different couplings, which change the phenomenology in important ways. For example, an MeV-scale ALP coupled to Standard Model gauge bosons at high energies will in general interact with photons, W ± and Z bosons as well as mesons and nucleons at low energies. We study the implications of such scenarios and point out that astrophysical constraints, in particular from SN1987A, may be substantially relaxed, open- ing up new regions of parameter space that may be explored with laboratory experiments such as NA62.

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On the interplay between astrophysical and laboratory probes of MeV-scale axion-like particles

The low-energy electronic recoil spectrum in XENON1T provides an intriguing hint for potential new physics At the same time, observations of horizontal branch stars favor the existence of a small amount of extra cooling compared to the one expected from the Standard Model particle content In this note, we argue that a hidden photon with a mass of $\sim 25$ keV and a kinetic mixing of $\sim 10^{-15}$ allows for a good fit to both of these excesses In this scenario, the signal detected in XENON1T is due to the absorption of hidden photon dark matter particles, whereas the anomalous cooling of horizontal branch stars arises from resonant production of hidden photons in the stellar interior

Fatih Ertas

Papers

Hidden photon dark matter in the light of XENON1T and stellar cooling

Model-independent bounds on light pseudoscalars from rare B-meson decays

Loop-induced direct detection signatures from CP-violating scalar mediators

On the interplay between astrophysical and laboratory probes of MeV-scale axion-like particles

Hidden Photon Dark Matter in the Light of XENON1T and Stellar Cooling