Feebly-interacting particles: FIPs 2020 workshop report

With the establishment and maturation of the experimental programs searching for new physics with sizeable couplings at the LHC, there is an increasing interest in the broader particle and astrophysics community for exploring the physics of light and feebly-interacting particles as a paradigm complementary to a New Physics sector at the TeV scale and beyond. FIPs 2020 has been the first workshop fully dedicated to the physics of feebly-interacting particles and was held virtually from 31 August to 4 September 2020. The workshop has gathered together experts from collider, beam dump, fixed target experiments, as well as from astrophysics, axions/ALPs searches, current/future neutrino experiments, and dark matter direct detection communities to discuss progress in experimental searches and underlying theory models for FIPs physics, and to enhance the cross-fertilisation across different fields. FIPs 2020 has been complemented by the topical workshop “Physics Beyond Colliders meets theory”, held at CERN from 7 June to 9 June 2020. This document presents the summary of the talks presented at the workshops and the outcome of the subsequent discussions held immediately after. It aims to provide a clear picture of this blooming field and proposes a few recommendations for the next round of experimental results.

Muon g − 2 from millicharged hidden confining sector

We provide a novel explanation to the muon g − 2 excess with new physics contributions at the two-loop level. In this scenario, light millicharged particles are introduced to modify the photon vacuum polarization that contributes to muon g − 2 at one additional loop. The muon g − 2 excess can be explained with the millicharged particle mass mχ around 10 MeV and the product of the multiplicity factor and millicharge squared of Nχε2 ∼ 10−3. The minimal model faces severe constraints from direct searches at fixed-target experiments and astrophysical observables. However, if the millicharged particles are also charged under a hidden confining gauge group SU(Nχ) with a confinement scale of MeV, hidden-sector hadrons are unstable and can decay into neutrinos, which makes this scenario consistent with existing constraints. This explanation can be well tested at low-energy lepton colliders such as BESIII and Belle II as well as other proposed fixed-target experiments.

Confirming $$U(1)_{L_\mu -L_{\tau }}$$ as a solution for $$(g-2)_\mu $$ with neutrinos

The recent measurement of the muon anomalous magnetic moment by the Fermilab E989 experiment, when combined with the previous result at BNL, has confirmed the tension with the SM prediction at $$4.2\,\sigma $$ 4.2 σ CL, strengthening the motivation for new physics in the leptonic sector. Among the different particle physics models that could account for such an excess, a gauged $$U(1)_{L_\mu -L_{\tau }}$$ U ( 1 ) L μ - L τ stands out for its simplicity. In this article, we explore how the combination of data from different future probes can help identify the nature of the new physics behind the muon anomalous magnetic moment. In particular, we contrast $$U(1)_{L_\mu -L_{\tau }}$$ U ( 1 ) L μ - L τ with an effective $$U(1)_{L_\mu }$$ U ( 1 ) L μ -type model. We first show that muon fixed target experiments (such as NA64$$\mu $$ μ ) will be able to measure the coupling of the hidden photon to the muon sector in the region compatible with $$(g-2)_\mu $$ ( g - 2 ) μ , and will have some sensitivity to the hidden photon’s mass. We then study how experiments looking for coherent elastic neutrino-nucleus scattering (CE$$\nu $$ ν NS) at spallation sources will provide crucial additional information on the kinetic mixing of the hidden photon. When combined with NA64$$\mu $$ μ results, the exclusion limits (or reconstructed regions) of future CE$$\nu $$ ν NS detectors will also allow for a better measurement of the mediator mass. Finally, the observation of nuclear recoils from solar neutrinos in dark matter direct detection experiments will provide unique information about the coupling of the hidden photon to the tau sector. The signal expected for $$U(1)_{L_\mu -L_{\tau }}$$ U ( 1 ) L μ - L τ is larger than for $$U(1)_{L_\mu }$$ U ( 1 ) L μ with the same kinetic mixing, and future multi-ton liquid xenon proposals (such as DARWIN) have the potential to confirm the former over the latter. We determine the necessary exposure and energy threshold for a potential $$5\,\sigma $$ 5 σ discovery of a $$U(1)_{L_\mu -L_{\tau }}$$ U ( 1 ) L μ - L τ boson, and we conclude that the future DARWIN observatory will be able to carry out this measurement if the experimental threshold is lowered to $$1\,{\mathrm {keV}}_{\mathrm {nr}} $$ 1 keV nr .

Portal Effective Theories. A framework for the model independent description of light hidden sector interactions

We present a framework for the construction of portal effective theory (PETs) that couple effective field theories of the Standard Model (SM) to light hidden messenger fields. Using this framework we construct electroweak and strong scale PETs that couple the SM to messengers carrying spin zero, one half, or one. The electroweak scale PETs encompass all portal operators up to dimension five, while the strong scale PETs additionally contain all portal operators of dimension six and seven that contribute at leading order to quark-flavour violating transitions. Using the strong scale PETs, we define a set of portal currents that couple hidden sectors to QCD, and construct portal chiral perturbation theory (χPTs) that relate these currents to the light pseudoscalar mesons. We estimate the coefficients of the portal χPT Lagrangian that are not fixed by SM observations using non-perturbative matching techniques and give a complete list of the resulting one- and two-meson portal interactions. From those, we compute transition amplitudes for three golden channels that are used in hidden sector searches at fixed target experiments: i) charged kaon decay into a charged pion and a spin zero messenger, ii) charged kaon decay into a charged lepton and a spin one half messenger, and iii) neutral pion decay into a photon and a spin one messenger. Finally, we compare these amplitudes to specific expressions for models featuring light scalar particles, axion-like particles, heavy neutral leptons, and dark photons.

DUNE Prospect for Leptophobic Dark Matter

Highly energetic proton/electron beam fixed-target experiments extend an opportunity to probe the sub-GeV dark matter and associated interactions. In this work, we have explored the sensitivity of DUNE (Deep Underground Neutrino Experiment) for sub-GeV leptophobic dark matter, i.e., this dark matter barely couples with the leptons. Baryon number gauge theory can predict the existence of leptophobic cold dark matter particle candidates. In our work, the dark matter candidate is considered to be scalar whose mass is defined by the symmetry breaking of new baryonic gauge group U 1 B . In this scenario, a light scalar dark matter couples with the standard model candidates via vector boson mediator V B which belongs to the baryonic gauge group U 1 B . This leptophobic dark matter dominantly couples to the quarks. Under this scenario, new parameter space for α B is explored by DUNE for leptophobic dark matter candidates. This new parameter space allowed α B to get a lower value than the present exiting constraint value of α B , i.e., 1 0 − 6 .

Layouts for fixed-target experiments and dipole moment measurements of short-lived baryons using bent crystals at the LHC

Several studies are on-going at CERN in the framework of the Physics Beyond Collider study group, with main aim of broadening the physics research spectrum using the available accelerator complex and infrastructure. The possibility to design a layout that allows fixed-target experiments in the primary vacuum of the CERN Large Hadron Collider (LHC), without the need of a dedicated extraction line, is part of these studies. The principle of the layouts presented in this paper is to deflect beam halo protons on a fixed-target placed in the LHC primary vacuum, by means of the channeling process in bent crystals. Moreover, the presence of a second bent crystal adjacent to the target opens a unique opportunity for the first direct measurement of electric and magnetic dipole moments of short-lived baryons. Two possible layouts are reported, together with a thorough evaluation on their expected performance and impact on LHC operations.