FASER’s physics reach for long-lived particles

Abstract

FASER, the ForwArd Search ExpeRiment, 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 (HNLs); light B−L and L i −L j gauge bosons; axion-like particles (ALPs) that are coupled dominantly to photons, fermions, and gluons through non-renormalizable 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.

Searches for decays of new particles in the DUNE Multi-Purpose near Detector

“…Overall, we find that the DUNE MPD will be able to extend sensitivity to a dark photon in the sub-GeV mass regime for kinetic mixing in the ε 2 ≈ 10 −16 − 10 −14 range. This complements other future experiments, such as FASER [73,74], SHiP [66], and SeaQuest [67], which will be sensitive to larger values of ε 2 , as the detectors associated with these proposals are significantly closer to their production targets. If dark photons exist and are produced in this type of environment, then those produced at CERN (for detection at FASER/SHiP) would be much more boosted than those produced at DUNE.…”

Direct detection and complementary constraints for sub-GeV dark matter

“…As discussed in section 4.2, the particle physics constraints hence need to be adapted correspondingly, and we thus only keep those limits shown in figure 3 that are still relevant in this situation (and add that from BaBar [103] for the case of m S = 1 GeV). 13 For invisible decays, furthermore, there is also no upper boundary to the area excluded by energy loss arguments in supernovae (as in figure 3). This implies that for 2m χ < m S 0.2 GeV, unlike the situation in figure 5 for visible decays, the combination of SN bounds and SIDM constraints indeed does combine to a very competitive bound on σ SI (though, as discussed in section 5.4, SN limits should be interpreted with care).…”

“…At the intensity frontier, in particular, there is a plethora of both ongoing and planned activities that aim to explore new physics in the sub-GeV range. Prominent examples for the latter include, but are not limited to, planned upgrades to current experiments such as NA62 [9] and NA64 [10], the recently approved LHC add-on FASER [11][12][13][14][15] as well as dedicated new experiments like LDMX [16] and SHiP [17][18][19] that are planned to be run at the new Beam Dump Facility at CERN. Finally there are proposals for LHC based intensity frontier experiments such CODEX-b [20] and MATHUSLA [21][22][23][24].…”

Baryogenesis from a dark first-order phase transition

“…LHCb [65,66], Belle-II [60,67], AWAKE [68] (10 16 electrons of 50 GeV), HPS [69], SeaQuest [70], LDMX [71] (HL-LDMX with E beam = 16 GeV), FASER [72] (LHC Run 3 with 150 fb −1 ), NA62 [73], and SHiP [74]. Additionally, the NA64 bounds should improve soon [75].…”

“…The constraints specific to our models, namely that m γ ≤ m π 0 /2 and that the dark photon decays before SM neutrinos decouple around T ∼ 3 MeV, are in light blue and red, respectively. Also shown in rainbow colors are projections from future experiments [60,[65][66][67][68][69][70][71][72][73][74]. Left: Viable parameter space for the dark neutron dark matter case with the predicted m n = 1.33 GeV and assuming m π ∼ 0.5m n .…”