Latest Results from the FASER Experiment

✨

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) as the world's most powerful particle smasher. It smashes protons together at nearly the speed of light, creating a chaotic explosion of new particles. Usually, scientists look at the debris flying out in all directions. But the FASER experiment is like a detective who decided to stand 480 meters (about 1,500 feet) away, hiding behind a massive wall of rock and steel.

Why stand so far away? Because only the "ghosts" of the particle world—neutrinos and muons—can pass through that thick wall of rock. Everything else gets stopped or deflected. This makes FASER a unique "forward-looking" telescope that sees things no other detector can.

Here is a breakdown of their latest "detective work" from 2026, explained simply:

1. The Ghost Hunt: Searching for Dark Photons

The Goal: Scientists are looking for a mysterious particle called a "Dark Photon." Think of it as a secret cousin to the regular light particle (photon) that we can't see, but it might exist in a hidden dimension.
The Strategy: FASER is looking for these dark photons to decay (break apart) into pairs of electrons and positrons hundreds of meters away from the crash site.
The New Trick: In the past, they only looked for two tracks left by these particles. But sometimes, the particles are moving so fast (like a speeding bullet) that the two tracks merge into one, or the explosion is so messy it looks like one big blob.

The Analogy: Imagine trying to spot a car in a fog. Previously, you only looked for cars with two headlights on. Now, FASER said, "Wait, if the fog is thick, maybe we'll only see one headlight, or maybe the car is just a blur. Let's look for those too!"
The Result: They didn't find any dark photons, but by looking harder and smarter, they set the strictest rules in the world on where these particles could be hiding. They effectively told the universe: "If you are a dark photon, you can't be this heavy or this strong."

2. The Ghostly Photographers: FASERν (The Emulsion Detector)

The Tool: This part of FASER is like a giant, ultra-sensitive camera made of 730 layers of tungsten plates and photographic film (emulsion). It's heavy (1.1 tons) and acts like a 3D film reel that captures the exact moment a neutrino hits a nucleus.
The New Photos:

Measuring the "Size" of Neutrinos: They measured how often neutrinos hit atoms (cross-section) with much better precision than ever before. It's like finally getting a clear ruler to measure a ghost.
The "Charm" Hunt: They started looking for "charm" particles (a specific type of heavy quark) being created when neutrinos hit matter. This is the first time anyone has tried to spot this specific interaction at these high energies. It's like looking for a specific, rare type of fish in the deepest, darkest part of the ocean. They have a few suspects, but they are still "unblinding" the data (taking off the blindfold) to confirm.

3. The Electronic Detector: Catching the "Electron Neutrino"

The Achievement: While the emulsion detector is great at taking high-res photos, the electronic detector is like a high-speed video camera.
The Big News: For the first time, FASER saw electron neutrinos (a specific type of ghost) using this electronic camera.

The Evidence: They saw a "glow" (an electromagnetic shower) in their calorimeter (a device that measures energy) that matched exactly what an electron neutrino would look like.
The Confidence: They are 99.9999% sure (5.5 sigma) that this wasn't a mistake or background noise. It's like finally hearing a whisper in a hurricane and being certain it was a human voice, not just the wind.

4. The Double-Check: Measuring Muon Neutrinos in 3D

The Breakthrough: They didn't just count how many muon neutrinos hit the detector; they mapped them out in two dimensions at the same time: Energy (how hard they hit) and Rapidity (how fast they were moving forward).

The Analogy: Imagine a baseball pitcher throwing balls. Before, we just counted how many balls hit the catcher's mitt. Now, FASER is mapping exactly where on the mitt they hit and how fast they were thrown. This helps physicists understand the "physics of the throw" (how protons behave when smashed together) much better.

What's Next? (The Future)

The team isn't stopping. They are:

Connecting the dots: Trying to match the high-res photos from the emulsion detector with the speed data from the electronic detector to get a perfect 4D picture of every neutrino.
New Detectors: They installed two new "side-view" detectors in early 2026. Think of these as new security cameras placed at different angles to catch particles that the main camera might miss. These will help them study the "glue" (gluons) that holds atoms together at levels never seen before.

The Bottom Line

FASER is proving that standing far away from the chaos of the particle collider is actually a superpower. By using a mix of old-school film photography and modern high-speed electronics, they are:

Setting the world's best limits on invisible "dark" particles.
Taking the most precise measurements of neutrino interactions ever recorded.
Opening a new window into the universe that was previously too dark to see.

They are essentially turning the "back of the room" of the LHC into the most exciting front row seat in physics.

Based on the provided paper, here is a detailed technical summary of the latest results from the FASER experiment.

1. Problem and Motivation

The FASER (ForwArd Search ExpeRiment) detector is located 480 meters downstream of the ATLAS interaction point at the LHC, directly on the proton-proton collision axis. This unique "far-forward" location allows it to detect particles that traverse $\sim$ 100 meters of rock shielding, which absorbs or deflects most Standard Model particles. Consequently, only neutrinos and muons reach the detector.

The experiment addresses two primary physics goals:

Beyond-the-Standard-Model (BSM) Searches: Specifically, the search for light, long-lived particles like dark photons ( $A'$ ) that decay into $e^+e^-$ pairs hundreds of meters from the collision point.
High-Energy Neutrino Physics: Studying neutrino interactions in the previously unexplored TeV energy regime, measuring cross-sections, and observing neutrino flavors ( $\nu_e, \nu_\mu$ ) and potential charm production.

2. Methodology and Experimental Setup

The results presented utilize data from LHC Run 3 ( $\sqrt{s} = 13.6$ TeV), collected between 2022 and 2024. The analysis relies on two distinct detector subsystems:

FASER $\nu$ (Emulsion Detector): A passive stack of 730 interleaved layers of 1.1 mm tungsten plates and nuclear emulsion films (total mass 1.1 tonnes, with an updated 681 kg tungsten target for specific analyses). It provides sub-micrometer spatial resolution to identify interaction vertices and classify neutrino flavors.
Electronic Detector: Located downstream of FASER $\nu$ $ν$ , this active system includes:
- Scintillator veto systems (front and timing).
- An interface tracker (IFT).
- A 1.5 m decay volume within a 0.57 T dipole magnet.
- A tracking spectrometer (using ATLAS SCT silicon strips) and additional magnets for momentum and charge-sign measurement.
- An Electromagnetic (EM) calorimeter (four LHCb ECAL modules) with a newly installed dual-readout system.

Data Sets Used:

Dark Photon Search: 177 fb $^{-1}$ of electronic detector data.
Neutrino Cross Sections & Charm Search: 9.5 fb $^{-1}$ of 2022 emulsion data.
$\nu_e$ Observation: 176.8 fb $^{-1}$ of electronic detector data (2022–2024).
$\nu_\mu$ Double-Differential Measurement: 186 fb $^{-1}$ of electronic detector data.

3. Key Contributions and Methodological Improvements

A. Dark Photon Search ( $A'$ )

Strategy Upgrade: Introduced an improved analysis strategy using the full 177 fb $^{-1}$ dataset.
Signal Regions: Defined two orthogonal regions to recover efficiency:
1. "One Track Signal Region": Relaxes the strict two-track requirement to at least one good fiducial track. This captures events where tracks are unresolvable due to high boost or where EM showers create extra tracks.
2. "Segment Signal Region": Uses reconstructed track segments within individual tracking stations to detect decays occurring inside the spectrometer, extending the effective fiducial volume by 2.5 m.
Impact: These changes improved sensitivity by more than a factor of two compared to previous analyses.

B. Neutrino Physics (Emulsion Detector)

Advanced Reconstruction: For the first time, muon neutrino energy was reconstructed using a regression Boosted Decision Tree (BDT). This algorithm combines muon momentum (measured via multiple Coulomb scattering) with hadronic activity at the vertex.
Charm Search: Performed the first search for charm hadron production ( $D^0, D^\pm, D_s^\pm, \Lambda_c^\pm$ ) in high-energy neutrino charged-current (CC) interactions. This utilized dedicated tools to reconstruct secondary displaced vertices and a multivariate analysis to discriminate against hadronic backgrounds.

C. Neutrino Physics (Electronic Detector)

$\nu_e$ Observation: Identified $\nu_e$ CC interactions via large, contained EM showers in the calorimeter with no upstream activity. Backgrounds (primarily $\nu_\mu$ interactions) were estimated using a data-driven approach extrapolated via a migration matrix.
Double-Differential Measurement: Performed the first measurement of $\nu_\mu$ interactions as simultaneous functions of energy ( $E$ ) and rapidity ( $y$ ). The ratio $q_\mu/p_\mu$ at the detector level served as a proxy for neutrino rapidity.

4. Key Results

Dark Photon Limits:
- Observation: Zero events observed in both signal regions.
- Background: Expected background is $0.077^{+0.028}_{-0.042}$ events.
- Exclusion: Set world-leading exclusion limits at 90% CL for dark photon couplings $\epsilon \sim 10^{-5}–10^{-4}$ and masses $m_{A'} \sim 10–150$ MeV.
Neutrino Cross Sections (Emulsion):
- Measured $\nu_\mu + \bar{\nu}_\mu$ cross sections in two energy bins and $\nu_e + \bar{\nu}_e$ in a single bin.
- Selected 33 $\nu_\mu$ CC and 7 $\nu_e$ CC candidate events.
- Results are consistent with Standard Model predictions and represent the most precise measurements at TeV energies to date.
Charm Production Search:
- Applied to 40 neutrino interaction candidates.
- Partial unblinding performed; full results pending.
First Observation of $\nu_e$ (Electronic Detector):
- Observed an excess of $65 \pm 12$ events above the background-only expectation.
- This corresponds to a significance of 5.5 $\sigma$ , constituting the first observation of electron neutrinos in the FASER electronic detector.
- The observed signal is consistent with the expected $\nu_e$ yield of $42 \pm 27$ .
$\nu_\mu$ Double-Differential Measurement:
- Observed $766.8 \pm 29.6$ interaction events after background subtraction.
- Unfolded data into a 100 mm-radius fiducial volume, providing interaction rates as functions of both energy and rapidity.
- These results provide new constraints on forward QCD processes, charm production, and neutrino flux modeling relevant to atmospheric neutrinos.

5. Significance and Future Outlook

BSM Physics: The FASER experiment has established the most stringent limits to date on dark photons in the 10–150 MeV mass range, significantly narrowing the parameter space for light BSM particles.
Neutrino Physics: The experiment has successfully transitioned from discovery to precision measurement in the TeV regime. The first observation of $\nu_e$ and the double-differential $\nu_\mu$ measurement validate the detector's capabilities and provide critical data for understanding high-energy neutrino interactions and forward QCD.
Future Developments:
- Emulsion-Spectrometer Matching: Ongoing work to match tracks between the emulsion and electronic detectors will enable charge-sign-tagged cross-section measurements.
- New Detectors: Two off-axis detectors (AHCAL and FASERCal) were installed in January 2026 to probe gluon PDFs at extremely low $x$ ( $10^{-6}–10^{-7}$ ) and serve as demonstrators for the HL-LHC Run 4 program.

In conclusion, the FASER experiment has delivered a comprehensive suite of results that demonstrate its unique capability to explore the far-forward regime of the LHC, setting world records in dark photon searches and achieving milestone observations in high-energy neutrino physics.