FASER: ForwArd Search ExpeRiment at the LHC

TL;DR

FASER proposes a compact forward detector placed downstream of the LHC IPs to search for light, weakly-coupled new particles produced in the very forward region, where the enormous forward event rate and collimation increase discovery potential. The study uses dark photons as a concrete benchmark, detailing production via meson decays and proton bremsstrahlung, plus a qualitative (and cautiously conservative) note on direct production, and evaluates two on-axis locations (far at ~400 m and near at ~150 m behind the TAN) with background considerations. It provides detector- and geometry-driven signal expectations, including a decay-in-volume probability and track-separation requirements, and shows that a small detector (order 0.01–1 m^3 depending on location) can yield substantial signal yields (potentially 1–10^5 events) in viable regions of parameter space $m_{A'} ext{(MeV–GeV)}$ and $oldsymbol{\e}$. Backgrounds are analyzed and found manageable with robust timing, vertexing, and particle-ID; the far location offers a near-background-free environment, while the near location promises higher signal acceptance despite larger beam-induced backgrounds. Overall, FASER presents a cost-effective, complementary approach to long-lived particle searches at ATLAS/CMS, with potential reach comparable to SHiP in the high-$oldsymbol{\e}$ regime and the flexibility to explore a range of forward-produced new physics scenarios.

Abstract

New physics has traditionally been expected in the high-$p_T$ region at high-energy collider experiments. If new particles are light and weakly-coupled, however, this focus may be completely misguided: light particles are typically highly concentrated within a few mrad of the beam line, allowing sensitive searches with small detectors, and even extremely weakly-coupled particles may be produced in large numbers there. We propose a new experiment, ForwArd Search ExpeRiment, or FASER, which would be placed downstream of the ATLAS or CMS interaction point (IP) in the very forward region and operated concurrently there. Two representative on-axis locations are studied: a far location, $400~\text{m}$ from the IP and just off the beam tunnel, and a near location, just $150~\text{m}$ from the IP and right behind the TAN neutral particle absorber. For each location, we examine leading neutrino- and beam-induced backgrounds. As a concrete example of light, weakly-coupled particles, we consider dark photons produced through light meson decay and proton bremsstrahlung. We find that even a relatively small and inexpensive cylindrical detector, with a radius of $\sim 10~\text{cm}$ and length of $5-10~\text{m}$, depending on the location, can discover dark photons in a large and unprobed region of parameter space with dark photon mass $m_{A'} \sim 10~\text{MeV} - 1~\text{GeV}$ and kinetic mixing parameter $ε\sim 10^{-7} - 10^{-3}$. FASER will clearly also be sensitive to many other forms of new physics. We conclude with a discussion of topics for further study that will be essential for understanding FASER's feasibility, optimizing its design, and realizing its discovery potential.

Figures (12)

• Figure 1: Schematic drawings of the LHC ring and the current very forward infrastructure downstream from the ATLAS and CMS interaction points, along with the representative far and near on-axis detector locations for FASER. Note the extreme difference in the transverse and longitudinal scales in the lower figure. Details of the geometry and sample tracks have been taken from Refs. Mokhov:2003haMokhov:2006ndFornieri:2015. See the text for details.
• Figure 2: Particle multiplicities in $13~\text{TeV}$$pp$ collisions at the LHC. Left: $\pi^0$ and $\eta$ multiplicities from EPOS-LHC Pierog:2013ria (circles), QGSJET-II-04 Ostapchenko:2010vb (squares), and SIBYLL 2.3 Ahn:2009wxRiehn:2015oba (triangles). Right: $\pi^0$, $\pi^{\pm}$, $\eta$, $\omega$, $\rho$, and $p$ multiplicities from EPOS-LHC Pierog:2013ria.
• Figure 3: Distribution of $\pi^0$ (top) and $\eta$ (bottom) mesons in the $(\theta, p)$ plane, where $\theta$ and $p$ are the meson's angle with respect to the beam axis and momentum, respectively. The different panels show results from the simulation codes EPOS-LHC Pierog:2013ria (left), QGSJET-II-04 Ostapchenko:2010vb (center) and SIBYLL 2.3 Ahn:2009wxRiehn:2015oba (right). The total number of mesons is the number produced in one hemisphere ($0 < \cos \theta \le 1$) in 13 TeV $pp$ collisions at the LHC with an integrated luminosity of $300~\text{fb}^{-1}$. The bin thickness is $1/5$ of a decade along each axis. The dashed line corresponds to $p_T = p \sin \theta = \Lambda_{\text{QCD}} \simeq 250~\text{MeV}$.
• Figure 4: Distribution in the $(\theta, p)$ plane, where $\theta$ and $p$ are the angle with respect to the beam axis and momentum, respectively, for dark photons produced by $\pi^0$ decays (left), $\eta$ decays (center), and proton bremsstrahlung (right), for $A'$ parameters $(m_{A'}, \epsilon) = (20~\text{MeV}, 10^{-4})$ (top) and $(100~\text{MeV}, 10^{-5})$ (bottom). The right-hand axis indicates the dark photon's decay length; see Eq. (\ref{['eq:ap_decay_length']}). The total number of dark photons is the number produced in one hemisphere ($0 < \cos \theta \le 1$) in $13~\text{TeV}$$pp$ collisions at the LHC with an integrated luminosity of $300~\text{fb}^{-1}$. The bin thickness is $1/5$ of a decade along each axis. The black dashed, dotted, and dash-dotted lines correspond to $p_{T,A'} = \Lambda_{\text{QCD}} \simeq 250~\text{MeV}$, $m_{A'}^2$, and $10~\text{GeV}$, respectively.
• Figure 5: Distribution in the $(\theta, p)$ plane, where $\theta$ and $p$ are the angle with respect to the beam axis and momentum, respectively, for dark photons that decay in the interval $(L_{\text{min}}, L_{\text{max}}) = (390~\text{m}, 400~\text{m})$ (the far detector location) and are produced by $\pi^0$ decays (left), $\eta$ decays (center), and proton bremsstrahlung (right) for $A'$ parameters $(m_{A'}, \epsilon) = (20~\text{MeV}, 10^{-4})$ (top) and $(100~\text{MeV}, 10^{-5})$ (bottom). The total number of $A'$s is the number produced in one hemisphere ($0 < \cos \theta \le 1$) in 13 TeV $pp$ collisions at the LHC with an integrated luminosity of $300~\text{fb}^{-1}$. The bin thickness is $1/5$ of a decade along each axis. The dashed and dashed-dotted lines correspond to $p_{T,A'} = \Lambda_{\text{QCD}} \simeq 250~\text{MeV}$ and $10~\text{GeV}$, respectively. In each plot the right $y$-axis indicates the dark photon's characteristic decay length $\bar{d}$ (see Eq. (\ref{['eq:ap_decay_length']})). The angular coverage of the detector is indicated via vertical gray dashed lines.