Axionlike particles at FASER: The LHC as a photon beam dump

The goal of ForwArd Search ExpeRiment (FASER) 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 axionlike particles (ALPs) that couple to the standard model through the $a\gamma \gamma$ interaction. TeV-scale photons produced at the IP collide with the TAXN 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 $m_a\sim 30–400\mathrm{MeV}$ and couplings $g_{a\gamma \gamma }\sim {10}^{-6}-{10}^{-3}{\mathrm{GeV}}^{-1}$, and we discuss the ALP signal characteristics and detector requirements.

Figure 1

The dominant ALP production and decay processes considered in this study. Upper panel: FASER is located 480 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 beam line. These photons then collide with the neutral particle absorber (TAN or TAXN), producing ALPs $a$ at similarly small angles ${\theta }_a$ relative to the beam line. Note the extreme difference in horizontal and vertical scales. Lower right panel: The ALPs then travel $\sim 350\mathrm{m}$ 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 $(L_{\min },L_{\max })$ (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 $3{\mathrm{ab}}^{-1}$ and a TAXN radius of $R_{\mathrm{TAXN}}=12.5\mathrm{cm}$, which implies that ALPs produced at the TAXN typically have ${\theta }_a<R_{\mathrm{TAXN}}/L_{\mathrm{TAXN}}\approx 1\mathrm{mrad}$.

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 $3{\mathrm{ab}}^{-1}$. Right: Reach for FASER ($N=3$ signal events) for integrated luminosities $300{\mathrm{fb}}^{-1}$ and $3{\mathrm{ab}}^{-1}$. 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 $50{\mathrm{ab}}^{-1}$ [51]. 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 [29]. 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 [18]. The reach for proposed beam dump experiment SHiP assumes $\sim 2\times {10}^{20}$ POT collected in 5 years of operation [29].

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 $3{\mathrm{ab}}^{-1}$ 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 $\sim \mathcal{O}(10)\mathrm{cm}$ of the line of sight, and even a small detector with radius $R\sim 1\mathrm{cm}$ may probe currently unconstrained regions of ALP parameter space.

Figure 6

Left: Efficiency $\epsilon$ to detect the diphoton signal as a function of ALP energy for different choices of ALP mass $m_a$ and detector resolution $\delta$, the minimal separation of two photons that can be distinguished as two photons in the EM calorimeter. The detector depth is set to $\mathrm{\Delta }=10\mathrm{m}$; to an excellent approximation, $\epsilon$ depends on the ratio $\delta /\mathrm{\Delta }$. Right: The reach in ALP parameter space for the diphoton signal for various values of $\delta$.

Figure 7

Momentum transfer $\sqrt{t}$ (left), form factor $F(t)$ (center), and the Primakoff differential cross section of Eq. (7) (right) for different combinations of ALP mass $m_a$ and photon energy $E_{\gamma }$. These results are for scattering off iron with $Z_{\mathrm{nuc}}=26$ and $A_{\mathrm{nuc}}=56$.

Figure 8

Left: The ratio between the Primakoff and pair-production cross sections, ${\sigma }_{\text{Prim}}/{\sigma }_{\mathrm{conv}}$, as a function of the photon energy $E_{\gamma }$, for different values of $m_a$ and $g_{a\gamma \gamma }=1{\mathrm{GeV}}^{-1}$. Right: The branching ratios $B({\pi }^0,\eta \to a\gamma \gamma )$ as functions of $m_a$ for $g_{a\gamma \gamma }=1{\mathrm{GeV}}^{-1}$.

Figure 9

Left: Geometric setup for ALP production and detection in FASER. Right: Intersection of the cone for ALP production with fixed ${\theta }_{a\gamma }$ with the plane at $z=L_{\max }$ containing FASER.