ALPs at FASER: The LHC as a Photon Beam Dump

TL;DR

This work investigates a novel ALP search channel at FASER in which the LHC acts as a high-energy photon beam dump. Forward photons from the IP collide with the TAXN absorber to produce ALPs through the Primakoff process, and the ALPs travel to FASER and decay to two photons, providing a distinctive di-photon signature. The authors find sensitivity to $m_a \sim 30-400~\text{MeV}$ and $g_{a\gamma\gamma} \sim 10^{-6}-10^{-3}~\text{GeV}^{-1}$, with potentially up to $\mathcal{O}(10^5)$ signal events at the HL-LHC, dependent on detector granularity and depth. This photon-beam-dump channel offers a complementary, high-sensitivity probe of ALPs in a regime challenging for other experiments, and places stringent requirements on calorimeter spatial resolution to resolve the two-photon final state.

Abstract

The goal of FASER, ForwArd Search ExpeRiment at the LHC, is to discover light, weakly-interacting particles with a small and inexpensive detector placed in the far-forward region of ATLAS or CMS. A promising location in an unused service tunnel 480 m downstream of the ATLAS interaction point (IP) has been identified. Previous studies have found that FASER has significant discovery potential for new particles produced at the IP, including dark photons, dark Higgs bosons, and heavy neutral leptons. In this study, we explore a qualitatively different, `beam dump' capability of FASER, in which the new particles are produced not at the IP, but through collisions in detector elements further downstream. In particular, we consider the discovery prospects for axion-like particles (ALPs) that couple to the standard model through the $a γγ$ interaction. TeV-scale photons produced at the IP collide with the TAN neutral particle absorber 130 m downstream, producing ALPs through the Primakoff process, and the ALPs then decay to two photons in FASER. We show that FASER can discover ALPs with masses $m_a \sim 30 - 400~\text{MeV}$ and couplings $g_{aγγ} \sim 10^{-6} - 10^{-3}~\text{GeV}^{-1}$, and we discuss the ALP signal characteristics and detector requirements.

Figures (9)

• Figure 1: The dominant ALP production and decay processes considered in this study. Upper panel: FASER is located $480~\text{m}$ downstream from the IP along the beam collision axis (dotted line) after the main LHC tunnel curves away. Lower left panel: High-energy photons are produced at the IP with small angles $\theta_{\gamma}$ relative to the beamline. These photons then collide with the neutral particle absorber (TAN or TAXN), producing ALPs $a$ at similarly small angles $\theta_a$ relative to the beamline. Note the extreme difference in horizontal and vertical scales. Lower right panel: The ALPs then travel $\sim 350~\text{m}$ further downstream and decay through $a \to \gamma \gamma$ to two highly collinear, high-energy photons in FASER, which is located in the side tunnel TI18 close to the UJ18 hall.
• Figure 2: ALP production in the Primakoff process (left) and light meson decay (right).
• Figure 3: Distributions of photons (left), ALPs produced in the TAXN (center), and ALPs decaying at FASER's position within the range $(L_{\text{min}}, L_{\text{max}})$ (right) in the $(\theta, p)$ plane, where $\theta$ is the particle's angle with respect to the beam axis, and $p$ is the particle's momentum. We assume the HL-LHC integrated luminosity $3~\text{ab}^{-1}$ and a TAXN radius of $R_{\text{TAXN}} = 12.5~\text{cm}$, which implies that ALPs produced at the TAXN typically have $\theta_a < R_{\text{TAXN}} / L_{\text{TAXN}} \approx 1~\text{mrad}$.
• Figure 4: Left: For the ALP production mechanisms indicated, the number of ALP decays in FASER in the $(m_a, g_{a\gamma\gamma})$ parameter space, given an integrated luminosity of $3~\text{ab}^{-1}$. Right: Reach for FASER ($N=3$ signal events) for integrated luminosities $300~\text{fb}^{-1}$ and $3~\text{ab}^{-1}$. For comparison we also show the projected reach of other proposed ALP searches. The projected reach at Belle-II assumes the full expected integrated luminosity of $50~\text{ab}^{-1}$Dolan:2017osp. The reach for NA62 assumes $\sim 3.9 \times 10^{17}$ protons on target (POT) while running in a beam dump mode that is being considered for LHC Run 3 Dobrich:2015jyk. The SeaQuest reach assumes $\sim 1.44 \times 10^{18}$ POT, which could be obtained in two years of parasitic data taking and requires additionally the installation of a calorimeter Berlin:2018pwi. The reach for proposed beam dump experiment SHiP assumes $\sim 2 \times 10^{20}$ POT collected in 5 years of operation Dobrich:2015jyk.
• Figure 5: Left: Number of ALP decays as a function of radius $R$ for various choices of $(m_a, g_{a\gamma\gamma})$. Right: Reach for FASER for $3~\text{ab}^{-1}$ in ALP parameter space for different values of the radius $R$. The stars corresponds to the benchmarks in the left panel. These results show that the ALP signal is highly collimated within distances $\sim {\cal O}(10)~\text{cm}$ of the line of sight, and even a small detector with radius $R \sim 1~\text{cm}$ may probe currently unconstrained regions of ALP parameter space.