Feebly Interacting Particles (FIPs) exist within both the Standard Model (SM) and in theories that extend beyond it. The FASER experiment [1] is the first of two experiments approved for data taking in Run 3 aiming to study these elusive particles. Located 480 meters from the ATLAS interaction point, FASER uses an emulsion-based setup to closely examine SM FIPs originating from there: specifically neutrinos produced at the highest-ever machine-made energies. Additionally, thanks to its spectrometer, the experiment aims to detect Beyond the Standard Model (BSM) FIPs created at the ATLAS interaction point that could travel long distances before decaying, within the FASER volume. 

The FASER experiment started data collection at the beginning of Run 3 in 2022 and has since successfully detected collider neutrinos, marking a significant milestone in the exploration of Standard Model FIPs at TeV-scale energies [2]. It has also placed constraints on new physics models predicting FIPs, such as ‘dark photons’, achieving sensitivity in regions of parameter space inaccessible since the 1990s and excluding previously viable models motivated by dark matter [3]. These accomplishments were made possible by using the electronic components of the FASER detector and analysing final states involving charged particles. The FASER tracker was crucial in mitigating backgrounds that could obscure the desired signals.

This year, the experiment reached important milestones by conducting its first measurements and searches independently of its tracker. Using its emulsion detector, it performed neutrino measurements  [4]. It also produced preliminary results regarding FIPs decaying into photons [5], enhancing the search for the theoretically well-motivated axion-like particles (ALPs). 

Neutrino measurements with the FASER emulsion detector. 

The FASER emulsion detector, FASERν, consists of 730 alternating layers of tungsten plates and emulsion films, with a total mass of 1.1 tonnes. To maintain low hit occupancy, the emulsion films are replaced frequently, approximately every 25 fb⁻¹ in 2022 and 2023 data-taking conditions. The analysed films recorded 9.5 fb⁻¹ of data in 2022. Processing the films is lengthy and the reason the analysis performed so far has only used 14% of the full emulsion volume, which corresponds to a target mass of about 130 kg. Despite the challenging operational aspects, FASERν has exceptional performance characteristics, notably a spatial resolution of 300 nm.

High-energy neutrinos from decays of hadrons originating from LHC pp collisions at the ATLAS IP interact with the tungsten nuclei in the emulsion detector, leading to both charged-current interactions (ν_ℓ + N → ℓ + X) and neutral-current interactions (ν_ℓ + N → ν_ℓ +X), where ℓ represents any lepton flavour, N is the target nucleus, and X is the set of hadrons produced in the interaction. A neutrino interaction in the FASER emulsion detector is illustrated in figure 1.

Event reconstruction in the emulsion detector involves tracking individual particle paths, reconstructing interaction vertices, and identifying electrons and muons. Events are selected based on the presence of a reconstructed vertex associated with either an electron or a muon, thereby focusing on charged-current neutrino interaction events. These leptons must be highly energetic, boosted in the forward direction, and well-separated from other particles at the vertex.

Similar event characteristics can arise from neutral hadrons interacting within the detector, resulting in signatures that mimic those of electrons or muons. Unlike neutrinos, these neutral hadrons are much less energetic and exhibit less forward boosting. The background from neutral hadrons is estimated using simulations and is validated in data; it is quantified at 0.025 for electron-neutrino selections and 0.22 for muon-neutrino selections, with uncertainties ranging from 30 to 50% in both cases.

The expected number of neutrino signal events meeting the selection criteria ranges from 1.1 to 3.3 for electron-neutrinos and from 6.5 to 12.4 for muon-neutrinos, where the range covers the uncertainties, dominated by the neutrino flux originating from the interaction point. We observed four electron neutrino events and eight muon-neutrino events, achieving statistical significance exceeding 5σ for both neutrino types.

Figure 1: Sketch of a neutrino interaction in the emulsion detector of the FASER experiment. It illustrates a muon-neutrino charged-current interaction. Only emulsion detector signals are used in the analysis presented in the recent FASER publication [4]. The rest of the FASER detector components are also shown in a simplified way. 

These results mark the first direct observation of electron neutrinos produced at a particle collider and the first direct detection of neutrino interaction vertices at the LHC. They also provide measurements of the neutrino interaction cross-section per nucleon across a previously unexplored energy range for both electron- and muon-neutrinos, as shown in figure 2. The primary source of uncertainty in these measurements is statistical in nature. The second most significant source of uncertainty arises from the neutrino flux. These findings illustrate the ability of the FASER emulsion-based detector to investigate flavour-specific neutrino interactions at TeV energies. Further measurements will be instrumental in enhancing our understanding of neutrino production at these high energies. The FASER collaboration is currently analysing approximately forty times more FASERν data than has been collected to date.

Figure 2: First measurement of the interaction cross-section per nucleon for electron-neutrinos (top) and muon-neutrinos (bottom).

First FASER searches for ALPs.

ALPs represent a fascinating class of pseudoscalar particles that includes axions, which are well-motivated by attempts to resolve the strong CP problem. These particles also serve as dark matter candidates, emerging from several extensions of the SM. In the FASER experiment, they could be produced in flavour-changing neutral current (FCNC) decays of heavy-flavoured hadrons, with their decays into pairs of photons being a critical search focus. A representative Feynman diagram is shown in figure 3. FASER recently conducted its first search for ALPs using data from LHC proton-proton collisions collected in 2022 and 2023. The collisions occurred at a centre-of-mass energy of 13.6 TeV, corresponding to an integrated luminosity of 57.7 fb⁻¹.

Figure 3: An example of a Feynman diagram in which an ALP is produced in the FCNC decay of a b or s-flavoured hadron and decays to two photons.  

Figure 4: Interpretation of the signal region yield as ALP exclusion limits with the assumption of 0.42 ± 0.38 neutrino background events. Individual limits from other experiments are also shown. 

Detecting photons in the FASER experiment is an enormous challenge. FASER is equipped with a preshower detector comprising two scintillator layers, each preceded by a 3 mm-thick layer of tungsten, and a calorimeter made up of four spare LHCb ECAL modules, each with a total depth of 25 radiation lengths (see figure 1). The signal requirements include charge deposits in the preshower consistent with an electromagnetic shower and significant energy deposition in the calorimeter, while ensuring no signal in the veto scintillators. 

With the current detector design, resolving the two photons is not feasible. Therefore, stringent selection criteria are applied to minimize various background components, primarily from neutrino interactions in the detector. This background is meticulously estimated using simulations, which are validated in control regions to match data within the given uncertainties. It is estimated that 0.42±0.38 neutrino events are expected after applying signal selections, with about 85% of this uncertainty originating from the neutrino flux. Other potential backgrounds, such as neutral hadrons entering the detector, muons bypassing the veto scintillators, and non-collision backgrounds, are considered negligible based on simulations and data-driven methods. 

One data event was observed in the signal region, consistent with the background expectation. For the studied model of ALPs coupled to weak gauge bosons, FASER demonstrates sensitivity to uncharted parameter spaces, mapping out new frontiers in ALP mass and coupling as illustrated in figure 4.

An exciting detector upgrade is underway to enhance FASER’s capability to search for ALPs and other particles in final states involving photons. The current preshower detector will be replaced by a high-granularity, multi-layered monolithic pixel preshower detector, ready for data collection in 2025 [6]. This upgraded detector will continue to operate through Run 4, during which the FASER experiment is already approved to proceed with data taking. The prospects for tantalizing results are better than ever before! 

References

[1] FASER Collaboration, “The FASER Detector,” arXiv:2207.11427 [physics.ins-det].

[2] FASER Collaboration, “First Direct Observation of Collider Neutrinos with FASER at the LHC,” Phys. Rev. Lett. 131 (Jul, 2023) 031801. 

[3] FASER Collaboration, “Search for dark photons with the FASER detector at the LHC,” Physics Letters B 848 (2024) 138378.

[4] FASER Collaboration, “First Measurement of the ν_e and ν_µ Interaction Cross Sections at the LHC with FASER’s Emulsion Detector,” arXiv:2403.12520 [hep-ex].

[5] FASER Collaboration, “Search for Axion-like Particles in Photonic Final States with the FASER Detector at the LHC,” tech. rep., CERN, Geneva, 2024. https://cds.cern.ch/record/2892328.

[6] L. Paolozzi, A new High-Granularity pre-shower detector for FASER, EP Newsletter, June 2022, https://ep-news.web.cern.ch/content/new-high-granularity-pre-shower-detector-faser.