First Measurement of the Muon Neutrino Interaction Cross Section and Flux as a Function of Energy at the LHC with FASER

Using 13.6 TeV proton-proton collision data, the FASER experiment reports the first differential measurement of the muon neutrino interaction cross section and flux in the TeV energy range, confirming Standard Model predictions and distinguishing contributions from pion and kaon decays.

The Gist

The Big Picture: Catching Ghosts at a Particle Smasher

Imagine the Large Hadron Collider (LHC) at CERN 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. Most of these particles are heavy, slow, or interact strongly with matter, so they get stopped by the thick concrete walls of the collider tunnel.

But there is one type of particle that is a master of stealth: the neutrino. Neutrinos are like cosmic ghosts. They have almost no mass and rarely interact with anything. They can pass through light-years of lead without stopping. Because they are so elusive, the main detectors at the LHC (which are huge, like cathedrals) miss them entirely because the neutrinos just fly right through the walls and out the front door.

The FASER Experiment is like setting up a tiny, high-tech "ghost trap" right in the path of these escaping neutrinos. Located 480 meters down a tunnel from the collision point, FASER is the first detector to successfully catch and count these high-energy neutrinos coming directly from the LHC.

What They Did: The "Ghost Hunt"

In this specific study, the FASER team looked at data collected in 2022 and 2023. They were hunting for muon neutrinos (a specific "flavor" of neutrino) and their antimatter twins.

1. The Trap: The detector is built like a sandwich. It has layers of heavy tungsten (a very dense metal) alternating with special films. When a neutrino finally decides to interact with a tungsten atom, it creates a "spark" of new particles, including a muon (a heavy cousin of the electron).
2. The Filter: The detector is surrounded by sensors that act like a bouncer at a club. If a regular particle (like a stray proton or a cosmic ray) tries to enter, the sensors kick it out. But because neutrinos are ghosts, they slip past the bouncer, hit the tungsten, and create a muon inside the detector.
3. The Count: The team found 338 confirmed neutrino interactions. They carefully subtracted the "noise" (background events that looked like neutrinos but weren't) to get this clean number.

The Two Big Questions They Answered

The paper focuses on two main measurements, which they approached like a detective solving a mystery from two different angles:

1. How "sticky" are neutrinos? (The Cross Section)
Imagine neutrinos are like tiny, invisible darts, and the tungsten atoms are targets. The "cross section" is a measure of how likely a dart is to hit a target.

• The Challenge: We knew how sticky neutrinos were at low energies (from old experiments) and at incredibly high energies (from space), but we had a huge gap in the middle (the TeV range).
• The Result: FASER filled that gap. They measured exactly how often these high-energy neutrinos hit the tungsten. The result matched the Standard Model (our current best theory of physics) perfectly. It's like checking a map and finding the terrain is exactly where the map said it would be.

2. How many ghosts are there? (The Flux)
Imagine standing in a rainstorm. You can measure how hard the rain is hitting your umbrella (the cross section) to figure out how many raindrops are falling (the flux).

• The Result: Using the known "stickiness" of neutrinos, they calculated how many neutrinos were flying through their detector. They found that the number of neutrinos matched the predictions from their computer simulations.

The "Recipe" of the Neutrinos

One of the most interesting findings was figuring out where these neutrinos came from. In the particle smasher, neutrinos are born when heavier particles decay (fall apart). The two main "parents" are pions and kaons (types of subatomic particles).

• The Analogy: Think of pions and kaons as two different types of factories. One factory (pions) makes neutrinos that tend to be a bit slower. The other factory (kaons) makes faster, more energetic neutrinos.
• The Discovery: By analyzing the energy of the neutrinos they caught, the team realized there were more neutrinos coming from the "Pion Factory" than they expected.
• Why it matters: This helps solve a long-standing puzzle in astrophysics called the "Muon Puzzle." Scientists have been confused about why cosmic rays hitting Earth's atmosphere seem to produce more muons than our models predict. This new data suggests that our models of how particles behave at high speeds might need a slight tweak, specifically regarding how often strange particles (like kaons) are made compared to pions.

The Bottom Line

This paper is a milestone because it is the first time scientists have measured the behavior of neutrinos in this specific, high-energy range (between 360 GeV and 6.3 TeV) using a collider.

• They caught the ghosts: They identified hundreds of neutrino interactions.
• They checked the map: The results agree with the Standard Model of physics.
• They found a clue: They discovered that neutrinos from pion decays are more common than previously thought, which might help explain why cosmic rays behave the way they do in the universe.

In short, FASER has opened a new window into the universe, proving that we can study these "ghost" particles right here on Earth using the world's biggest particle accelerator.

Technical Summary

Technical Summary: First Measurement of the Muon Neutrino Interaction Cross Section and Flux as a Function of Energy at the LHC with FASER

Problem and Motivation
The Large Hadron Collider (LHC) generates an intense forward beam of neutrinos originating from the decay of pions, kaons, and charmed hadrons produced in proton-proton ($pp$) collisions. While these neutrinos offer a unique opportunity to study neutrino properties, quantum chromodynamics (QCD), and phenomena beyond the Standard Model, they have historically remained unmeasured because they fall outside the instrumented acceptance of typical LHC experiments. Previous measurements of neutrino interaction cross sections were limited to fixed-target experiments at energies below 360 GeV or to astroparticle data at energies above 10 TeV, leaving a significant gap in the TeV range. The FASER collaboration previously achieved the first direct observation of collider neutrinos and the first measurement of cross sections at TeV energies using an emulsion detector. This paper addresses the need for a differential measurement of the charged current (CC) interaction cross section and the flux of muon neutrinos ($\nu_\mu$) and anti-neutrinos ($\bar{\nu}_\mu$) as a function of energy using the active electronic components of the FASER detector.

Methodology
The analysis utilizes $pp$ collision data collected during 2022 and 2023 at a center-of-mass energy of 13.6 TeV, corresponding to an integrated luminosity of $(65.6 \pm 1.4) \text{ fb}^{-1}$. The FASER detector is located approximately 480 m downstream of the ATLAS interaction point (IP1), shielded by 100 m of rock and concrete to absorb background particles while allowing neutrinos to pass.

• Event Selection: The analysis targets CC $\nu_\mu$ interactions within a fiducial volume defined as a cylinder with a 200 mm diameter around the spectrometer axis. Events are selected if they trigger downstream scintillators, originate from a colliding bunch crossing, and exhibit no significant charge in the upstream VetoNu scintillator (indicating no incoming charged particles). A key discriminant is the "reduced charge" in the VetoNu scintillators, calculated by integrating charge over a time window expected for a background muon; neutrino interactions typically yield significantly lower reduced charges due to the time-of-flight difference between particles produced in the target.
• Track Reconstruction: Selected events require at least one track passing through all three tracking spectrometer stations with a momentum greater than 100 GeV. Tracks must satisfy strict radial and angular cuts to reject background particles entering at large radii or angles.
• Background Estimation: The primary background consists of high-momentum muons from upstream that evade the VetoNu scintillators. This is estimated using a sideband of events with momenta below 100 GeV and extrapolated to the signal region. Contributions from neutral hadrons, cosmic rays, and beam backgrounds are found to be negligible. Non-signal neutrino interactions (e.g., $\nu_e$ CC, $\nu_\tau$ CC, NC) are estimated via simulation and subtracted.
• Unfolding and Analysis: The reconstructed muon momentum ($p_\mu$) is scaled by a factor of 0.8 to estimate the incident neutrino energy. The data is unfolded from six bins of the ratio $q/p'_\mu$ (where $q$ is the muon charge and $p'_\mu$ is the scaled momentum) to six bins of neutrino energy ($E_\nu$) using a response matrix derived from simulation (GENIE 3.04.0 for interactions, GEANT4 for detector response). A binned extended maximum likelihood fit is employed to extract the number of neutrino interactions.

Key Contributions and Results
The paper reports the first differential measurement of the $\nu_\mu$ and $\bar{\nu}_\mu$ interaction cross section and flux in the TeV energy range.

• Event Yield: After background subtraction, the analysis identifies $338.1 \pm 21.0$ CC $\nu_\mu$ and $\bar{\nu}_\mu$ interaction events in the fiducial volume. This is consistent with the expected value of $298.4 \pm 42.6$.
• Cross Section and Flux: Using the likelihood fit, the collaboration determines $1242.7 \pm 137.1$ CC interactions inside the fiducial volume.
  • Cross Section: The energy-dependent neutrino-nucleon cross section is measured in six energy bins spanning from approximately 100 GeV to the TeV range. The results are consistent with Standard Model predictions (specifically the Bodek-Yang model and more recent models) and bridge the gap between fixed-target and IceCube astroparticle measurements.
  • Flux: Conversely, assuming theoretical cross sections, the differential flux of muon neutrinos and anti-neutrinos is measured.
• Production Mechanisms: By performing a template fit on the unfolded neutrino energy spectrum, the contributions from pion and kaon decays are separated. The analysis observes a larger pion-to-kaon ratio than predicted by Standard Model expectations (yielding a $p$-value of 5.9%), which disfavors explanations for the "cosmic muon puzzle" that rely on enhanced forward strangeness production.

Significance
This work marks a milestone in collider neutrino physics by providing the first differential measurement of the muon neutrino interaction cross section and flux in the TeV energy range. The results validate the Standard Model predictions in a previously unexplored energy regime and demonstrate the capability of forward detectors at the LHC to perform precision neutrino physics. The measurement of the pion-to-kaon ratio provides new constraints on hadron production models, which are critical for understanding cosmic ray air showers. The paper concludes that these results, combined with previous emulsion-based measurements, represent the dawn of collider neutrino physics, offering broad implications for neutrino properties, QCD, and astroparticle physics. The full experimental covariance matrix and results are made available via HEPData.