Axion-like particles (ALPs), which are gauge-singlets under the Standard Model (SM), appear in many well-motivated extensions of the SM. Describing the interactions of ALPs with SM fields by means of an effective Lagrangian, we discuss ALP decays into SM particles at one-loop order, including for the first time a calculation of the a → πππ decay rates for ALP masses below a few GeV. We argue that, if the ALP couples to at least some SM particles with couplings of order (0.01 − 1) TeV−1, its mass must be above 1 MeV. Taking into account the possibility of a macroscopic ALP decay length, we show that large regions of so far unconstrained parameter space can be explored by searches for the exotic, on-shell Higgs and Z decays h → Za, h → aa and Z → γa in Run-2 of the LHC with an integrated luminosity of 300 fb−1. This includes the parameter space in which ALPs can explain the anomalous magnetic moment of the muon. Considering subsequent ALP decays into photons and charged leptons, we show that the LHC provides unprecedented sensitivity to the ALP-photon and ALP-lepton couplings in the mass region above a few MeV, even if the relevant ALP couplings are loop suppressed and the a → γγ and a → l+l− branching ratios are significantly less than 1. We also discuss constraints on the ALP parameter space from electroweak precision tests.

/pdf/collider-probes-of-axion-like-particles-1je1gfzke8.pdf

Collider probes of axion-like 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 the 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.

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

Abstract Particle physics today faces the challenge of explaining the mystery of dark matter, the origin of matter over anti-matter in the Universe, the origin of the neutrino masses, the apparent fine-tuning of the electro-weak scale, and many other aspects of fundamental physics. Perhaps the most striking frontier to emerge in the search for answers involves new physics at mass scales comparable to familiar matter, below the GeV-scale, or even radically below, down to sub-eV scales, and with very feeble interaction strength. New theoretical ideas to address dark matter and other fundamental questions predict such feebly interacting particles (FIPs) at these scales, and indeed, existing data provide numerous hints for such possibility. A vibrant experimental program to discover such physics is under way, guided by a systematic theoretical approach firmly grounded on the underlying principles of the Standard Model. This document represents the report of the FIPs 2022 workshop, held at CERN between the 17 and 21 October 2022 and aims to give an overview of these efforts, their motivations, and the decadal goals that animate the community involved in the search for FIPs. 

Feebly-interacting particles: FIPs 2022 Workshop Report

The sensitivities to the anomalous quartic gauge boson coupling γγγZ are estimated via γZ production with intact protons in the forward region at the LHC. Proton tagging proves to be a powerful tool to suppress the background, which allows consideration of the hadronic decays of the Z boson in addition to the leptonic ones. We discuss the discovery potential for an integrated luminosity of 300 fb−1 and 3000 fb−1. The sensitivity we obtain at 300 fb−1 goes beyond the one expected from LHC bounds on the Z → γγγ decay by about three orders of magnitude. The γZ channel provides important discriminatory information with respect to the exclusive γγ channel, as many particles beyond the Standard Model (such as a radion or Kaluza Klein gravitons) predict a signal in the latter but not the former.

/pdf/probing-the-anomalous-gggz-coupling-at-the-lhc-with-proton-3vctjee1tv.pdf

Probing the anomalous γγγZ coupling at the LHC with proton tagging

We propose the use of FASER as a light-shining-through-walls experiment to search for axions and axion-like particles (ALPs). LHC collisions generate a high intensity and high energy photon flux in the forward direction which can oscillate into ALPs in the magnetic fields that are used to confine the beam. These ALPs then pass through about 100~m of rock before reaching the magnetic fields of FASER, where they can convert back into photons and be detected by an electromagnetic calorimeter. In the next years, FASER and its successor FASER2 at the Forward Physics Facility will be able to explore regions of the ALPs parameter space inaccessible by other laboratory-based experiments.

http://link.aps.org/pdf/10.1103/PhysRevD.106.055036

ALP searches at the LHC: FASER as a light-shining-through-walls experiment

The total and the differential cross sections for the reaction e+e ~ ~,~(~,) have been measured with the DELPHI detector at LEP using an integrated luminosity of 36.9 pb -~. The results agree with the QED predictions and consequently there is no evidence for non-standard channels with the same experimental signature. The lower limits obtained on the QED cutoff parameters are A+ > 143 GeV and A > 120 GeV, and the lower bound on the mass of an excited electron with an effective coupling constant hr = 1 is 132 GeV/c 2. Upper limits on the branching ratios for the decays Z ° ~ ~O', Z° ~ 7r°~ ', Z° ~ rff and Z ° ~ ~O')' have been determined to be 5.5 x 10 -5, 5.5 × 10 -5, 8.0 x 10 -5, and 1.7 x 10 s respectively. All the limits are at the 95% confidence level.

/pdf/measurement-of-the-e-e-cross-section-at-lep-energies-3atkieqnh9.pdf

Measurement of the e +e- ) cross section at LEP energies

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Measurement of the e +e- ) cross section at LEP energies