The FASER experiment at the Large Hadron Collider

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This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Large Hadron Collider (LHC) at CERN as the world's most powerful particle smasher. When two protons collide at nearly the speed of light, they create a chaotic explosion of new particles flying out in all directions. Most of these particles are caught by giant detectors like ATLAS or CMS, which sit right at the collision point.

But there's a problem: some particles are "shy." They are very light, interact very weakly with matter, or are neutrinos (ghostly particles that rarely bump into anything). These particles don't stop at the main detectors; they fly straight through the rock and concrete, heading in a very specific direction: straight ahead, along the beamline.

Enter FASER (ForwArd Search ExpeRiment). Think of FASER as a long-distance sniper's nest or a hidden listening post located 480 meters (about 5 football fields) away from the collision point, buried deep inside an old, unused tunnel. It is perfectly aligned to catch the "shy" particles that the main detectors miss.

Here is a breakdown of what FASER does, how it works, and what it has found, explained simply:

1. The Two Main Missions

FASER has two distinct jobs, like a detective looking for two different types of clues:

Mission A: The "Dark Sector" Hunt (New Particles)
Physicists suspect there is a "dark sector" of the universe filled with invisible particles (like Dark Matter) that we can't see. FASER is looking for a "messenger" particle (like a Dark Photon) that might be produced in the collision, fly 480 meters through solid rock without stopping, and then suddenly decay (disappear) into visible particles (like an electron and a positron) right inside FASER's detector.
- Analogy: Imagine throwing a ball through a thick forest. If the ball is invisible, you won't see it. But if the ball suddenly bursts into a shower of glitter right in front of your face, you know something invisible passed through the trees and exploded. FASER is waiting for that glitter.
Mission B: The Ghost Catcher (Neutrinos)
Neutrinos are the ultimate ghosts. They can pass through light-years of lead without hitting anything. The LHC produces trillions of them, but they fly in a tight beam straight ahead. FASER is the first experiment in history to catch and study these high-energy neutrinos from a particle collider.
- Analogy: If the main detectors are like a net trying to catch fish in a pond, FASER is a specialized trap set in the river downstream where the fish are swimming in a straight line.

2. How the Machine Works

FASER is surprisingly small and simple compared to the massive detectors at the collision point. It's about the size of a small room (7 meters long) and fits in a narrow tunnel.

The Shield (Veto System): Before the particles enter the main area, they pass through "scintillator" walls. These are like motion sensors. If a regular, boring particle (like a muon) tries to sneak in, the sensors scream "STOP!" and the computer ignores it. FASER only cares about particles that didn't set off the alarm.
The Magnet: Inside, there are three giant magnets. These act like a funnel, bending the path of charged particles. This helps scientists figure out what the particle is and how heavy it is.
The Camera (Tracker & Calorimeter):
- The Tracker is like a high-speed camera that takes pictures of the particle's path to see where it's going.
- The Calorimeter is a heavy block of lead and plastic that stops particles and measures how much energy they have.
The "Emulsion" Detector (FASERν): This is the coolest part. To catch the ghostly neutrinos, FASER uses a stack of 730 tungsten plates sandwiched with special photographic film (emulsion). When a neutrino finally hits a tungsten atom, it creates a tiny track. The film is then developed (like old-school photography) and scanned by robots to find these microscopic tracks. It's like finding a single needle in a haystack by looking at every single straw under a microscope.

3. What Has FASER Found?

Since starting operations in 2022, FASER has been incredibly successful:

First Sightings of Collider Neutrinos: In 2023, FASER announced the first-ever observation of neutrinos created by a particle collider. Before this, we only knew about neutrinos from the sun or nuclear reactors. FASER proved we can make and study them in a lab.
Measuring the Unmeasurable: FASER has measured how neutrinos interact with matter at energies never seen before (in the TeV range). This helps us understand the fundamental rules of the universe.
Hunting for New Physics: FASER has looked for "Dark Photons" and "Axion-like particles." So far, they haven't found them yet, but they have ruled out many places where scientists thought they might be hiding. It's like searching a dark room for a cat; even if you don't find the cat, you know exactly where it isn't.

4. Why Does This Matter?

Understanding the Universe: Neutrinos are everywhere, but we don't fully understand them. FASER gives us a new lab to study them at extreme energies.
Dark Matter: If the "Dark Sector" exists, FASER is one of the best places to find it. Finding a dark photon would be a Nobel Prize-winning discovery that changes our understanding of physics.
Cosmic Rays: The data FASER collects helps scientists understand what happens when cosmic rays (particles from space) hit the Earth's atmosphere, which is crucial for understanding the universe's most energetic events.

5. The Future

FASER is just getting started. The LHC is running until 2026, and FASER will keep collecting data. They are already planning upgrades for the future, including a massive new facility called the Forward Physics Facility (FPF) that will be built during the next long shutdown. This new facility will be like a "super-FASER," capable of catching millions of neutrinos and searching for even more exotic particles.

In a nutshell: FASER is a clever, low-cost experiment hiding in a tunnel, waiting for the universe's shyest particles to fly past the main detectors and reveal their secrets. It's a testament to the idea that sometimes, to find the biggest answers, you have to look in the most unexpected places.

1. Problem Statement

The Large Hadron Collider (LHC) produces a vast number of particles in proton-proton collisions, but standard detectors (ATLAS, CMS, LHCb) are optimized for central rapidity. A significant "forward" region (very small angles relative to the beamline) remains largely unexplored. This region is theoretically predicted to be a rich source of:

Light, Weakly-Interacting New Particles (BSM): Such as Dark Photons ( $A'$ ), Axion-Like Particles (ALPs), and Heavy Neutral Leptons. These particles are often produced in the forward direction via the decay of light mesons (e.g., $\pi^0$ ) and can travel hundreds of meters before decaying.
High-Energy Neutrinos: The LHC produces a collimated flux of high-energy neutrinos (TeV scale) from the decay of forward hadrons. Prior to FASER, no collider-based experiment had detected or studied neutrinos of all flavors ( $\nu_e, \nu_\mu, \nu_\tau$ ) at these energies, nor measured their interaction cross-sections in this regime.

The challenge is to detect these rare signals in a location 480 meters downstream from the interaction point (IP1), shielded by 100 meters of rock, while rejecting an immense background of high-energy muons and residual hadrons.

2. Methodology and Detector Design

The FASER experiment is located in the TI12 tunnel, aligned with the beam collision axis (Line of Sight - LOS). The detector design is a hybrid system comprising an electronic detector and a passive emulsion detector.

A. Electronic Detector Components

Veto System: Four scintillator stations (upstream and downstream) designed to veto incoming charged particles (primarily muons) with an efficiency $>99.999\%$ .
Decay Volume: A 1.5-meter long region surrounded by a permanent dipole magnet (0.57 T) where long-lived BSM particles are expected to decay.
Tracking Spectrometer: Three stations of silicon microstrip detectors (using spare ATLAS SCT modules) to reconstruct charged particle trajectories, momentum, and charge.
Electromagnetic Calorimeter: Four modules from the LHCb experiment (lead/scintillator sandwich) to measure energy deposits of electrons and photons.
Trigger & DAQ: A custom FPGA-based system capable of handling trigger rates up to ~2 kHz with minimal dead time.
Upgrades (2024-2025):
- Calorimeter: Split readout into Low-Energy (LE) and High-Energy (HE) channels to extend dynamic range up to 3 TeV.
- Preshower: Replaced with a high-granularity silicon/tungsten system to resolve closely spaced photon pairs (crucial for ALP searches).

B. FASER $\nu$ (Emulsion Detector)

Structure: A 1.1-tonne target consisting of 730 tungsten plates interleaved with nuclear emulsion films.
Function: Provides sub-micron spatial resolution to identify neutrino interaction vertices and distinguish lepton flavors ( $\nu_e, \nu_\mu, \nu_\tau$ ) and neutral current interactions.
Operation: Due to high track occupancy, the detector is periodically replaced (every ~10–30 fb $^{-1}$ ) during LHC technical stops. Films are developed and scanned using high-speed automated microscopes (HTS).

3. Key Contributions

The paper provides a comprehensive status report of FASER up to early 2026, covering:

First Observation of Collider Neutrinos: The first detection of neutrinos produced at a particle collider, including the separation of neutrino and antineutrino interactions.
BSM Searches: The first experimental constraints on Dark Photons and ALPs in the forward region, probing parameter spaces motivated by dark matter relic density.
Cross-Section Measurements: The first measurements of neutrino-nucleon charged-current (CC) cross-sections in the TeV energy regime.
Forward Hadron Production: Using neutrino data to constrain models of forward pion and kaon production, which are critical for cosmic-ray air-shower simulations.
Operational Milestones: Documentation of the detector's successful operation through Run 3 (2022–2025), collecting over 311 fb $^{-1}$ of data.

4. Key Results

A. Searches for New Particles

Dark Photons ( $A'$ ):
- Using 177 fb $^{-1}$ of data (2022–2024), FASER observed zero signal events in the $A' \to e^+e^-$ channel.
- Result: Excluded Dark Photon couplings ( $\epsilon$ ) in the range $4 \times 10^{-6}$ to $2 \times 10^{-4}$ for masses between 10 MeV and 200 MeV. This covers a region of parameter space relevant for dark matter models.
Axion-Like Particles (ALPs):
- Searching for ALPs decaying to two photons ( $a \to \gamma\gamma$ ).
- Result: Observed 1 event in the signal region (consistent with background). Set exclusion limits on ALP couplings to $W$ bosons and photons, probing masses up to several hundred MeV.

B. Neutrino Physics

First Observation: In 2023, FASER reported the first observation of collider neutrinos (150 events) with a statistical significance exceeding 5 $\sigma$ .
Flavor Identification:
- $\nu_\mu$ : Measured the cross-section for $\nu_\mu$ and $\bar{\nu}_\mu$ separately using the spectrometer's charge identification.
- $\nu_e$ : In March 2026, the first observation of collider $\nu_e$ interactions using the electronic detector was reported (65 events, 5.5 $\sigma$ significance).
- $\nu_\tau$ : FASER $\nu$ emulsion data has identified $\nu_\tau$ candidates, though statistics are currently limited.
Cross-Section Measurements:
- Measured the CC cross-section for $\nu_\mu$ and $\nu_e$ in the TeV range. The results are consistent with Standard Model predictions (Bodek-Yang model) but provide the first data points in this energy regime.
- The measured fluxes suggest that current forward hadron production models (e.g., DPMJET, EPOS-LHC) have significant uncertainties, particularly regarding charm production.

C. Detector Performance

Efficiency: The detector recorded 97% of the delivered luminosity.
Background Rejection: The veto system achieved an inefficiency of $<10^{-5}$ per scintillator, rendering the experiment effectively background-free for BSM searches.
Stability: The tracking spectrometer maintained >99.7% operational channels with stable noise and gain over four years of operation.

5. Significance

New Physics Reach: FASER has established a unique sensitivity to light, weakly-coupled particles that are inaccessible to central detectors due to their long lifetimes and forward production kinematics. The exclusion limits on Dark Photons and ALPs significantly constrain theoretical models for dark matter.
Neutrino Physics: FASER has opened a new field of "Collider Neutrino Physics." It is the first experiment to measure neutrino interactions at TeV energies, providing crucial data for understanding neutrino cross-sections at energies previously only accessible via astrophysical sources (IceCube) or lower-energy beam dumps.
Hadronic Physics: The neutrino flux measurements serve as a direct probe of forward hadron production, helping to tune models used in ultra-high-energy cosmic ray physics.
Future Outlook: The success of FASER has paved the way for the Forward Physics Facility (FPF), a proposed dedicated facility for the High-Luminosity LHC (HL-LHC) era, which aims to house multiple experiments to further explore the forward region with significantly higher luminosity.

In summary, the paper demonstrates that FASER has successfully transitioned from a concept to a fully operational, high-performance experiment, delivering the first-ever observations of collider neutrinos and setting world-leading limits on light dark sector particles.