Measurement of the muon neutrino charged-current cross section with SND@LHC

Using proton-proton collision data from LHC Run 3, the SND@LHC experiment reports the first measurement of the muon neutrino charged-current cross section on tungsten, observing 31 candidate events against a small expected background to determine a cross section of $(37^{+24}_{-12})\times 10^{-35}~\text{cm}^2$ at a median energy of 228 GeV.

The Gist

Imagine the Large Hadron Collider (LHC) at CERN as a massive, high-speed train station where two trains of protons crash into each other. Usually, scientists look at the debris from these crashes to study new particles. But sometimes, this crash creates a special, invisible passenger: a neutrino.

Neutrinos are like ghosts. They have almost no mass and don't interact with anything. They can pass through the entire Earth without stopping. Because they are so elusive, catching them is incredibly difficult.

This paper describes how the SND@LHC experiment successfully caught a specific type of ghostly passenger: the muon neutrino. Here is the story of how they did it, explained simply.

1. The Setup: A "Ghost Trap" 480 Meters Away

The scientists built a special detector called SND@LHC. They didn't put it right next to the crash site (where it would be destroyed by the explosion). Instead, they placed it 480 meters away in a tunnel, directly in the path of the "forward" spray of particles.

Think of the collision point as a cannon firing a massive cloud of particles. Most particles hit the walls of the tunnel and stop. But the neutrinos, being ghosts, fly straight through the walls and keep going. The detector is like a net placed far down the track, waiting to catch the few neutrinos that make it all the way there.

2. The Detector: A Hybrid "Sandwich"

The detector is a bit like a high-tech sandwich with different layers:

• The Veto (The Bouncer): At the front, there are sensors that act like a bouncer. If a regular particle (like a charged muon) tries to enter from the side, the bouncer shouts "Stop!" and tags it. We only want the neutrinos that sneak in without being tagged.
• The Target (The Tungsten Wall): Inside, there are heavy blocks of tungsten (a very dense metal). This is the "trap." When a neutrino finally decides to interact, it smashes into the tungsten.
• The Tracker (The Camera): Behind the tungsten, there are layers of fiber-optic sensors that take pictures of the crash.
• The Calorimeter (The Energy Meter): Finally, there are layers of iron and sensors that measure how much energy was released in the crash.

3. The Hunt: Finding the Needle in the Haystack

The problem is that the "haystack" is huge. Every second, billions of particles fly through the detector. The neutrinos are the "needles."

To find them, the scientists used a computer program to filter out the noise. They looked for a very specific pattern:

1. No Bouncer Tag: The particle must have entered without hitting the side sensors (meaning it was a neutral ghost).
2. The Big Smash: It must hit the tungsten and create a shower of other particles (a "hadronic shower").
3. The Outgoing Ghost: Crucially, a muon neutrino interaction creates a muon (a heavier cousin of the electron) that flies out the back. The detector needs to see this muon leaving the scene.

4. The Results: 31 Ghosts Caught

The scientists analyzed data from 2022 and 2023.

• The Total: They found 31 candidate events that looked exactly like neutrino interactions.
• The Noise: They calculated that about 5 of these might have been false alarms (like a regular particle sneaking past the bouncer or a glitch).
• The Real Deal: After subtracting the noise, they were left with about 26 real neutrino interactions. This matched their theoretical predictions almost perfectly.

5. Measuring the Energy: The "Calorimetric" Breakthrough

One of the coolest parts of this paper is that they didn't just count the ghosts; they weighed them.
Using special test data from particle beams (like a "practice run" with known particles), they calibrated their "Energy Meter" (the calorimeter).

• They measured how much energy the neutrinos deposited when they hit the tungsten.
• They found energies ranging from a few GeV up to 390 GeV (gigaelectronvolts).
• This is the first time scientists have measured the energy of neutrinos created in a particle collider this way. It's like finally being able to weigh a ghost instead of just knowing it was there.

6. The Conclusion: A Perfect Match

The paper concludes that the number of neutrinos they caught and the energy they measured match the predictions of the Standard Model of physics (the rulebook for how particles behave).

• They calculated the "cross-section" (a fancy word for the probability of the neutrino hitting the tungsten).
• Their measurement was 37 (with some uncertainty), while the theory predicted 34.
• This is a great match, confirming that our understanding of neutrinos at these incredibly high energies is correct.

Summary

In simple terms, the SND@LHC team built a specialized "ghost trap" 480 meters away from a massive particle crash. They successfully caught 31 muon neutrinos, filtered out the background noise, and for the first time, measured exactly how much energy these invisible particles carried. It's a major step forward in understanding the "ghostly" side of the universe.

Technical Summary

Technical Summary: Measurement of the Muon Neutrino Charged-Current Cross Section with SND@LHC

Problem and Motivation
The Large Hadron Collider (LHC) serves as a source of very high-energy neutrinos, produced in the forward direction from the decay of high-energy hadrons generated in proton–proton ($pp$) collisions. While previous experiments have established the "dawn of neutrino physics at colliders," precise measurements of neutrino interactions at these energies remain challenging. This paper addresses the need for an updated measurement of the muon neutrino ($\nu_\mu$) charged-current (CC) interaction cross section on tungsten. It aims to improve upon previous results by utilizing a larger dataset, expanding the fiducial volume to increase acceptance, and introducing a calibrated measurement of hadronic energies, which had not been previously performed for collider neutrino interactions.

Methodology
The analysis utilizes data collected by the SND@LHC (Scattering and Neutrino Detector at the LHC) experiment during LHC Run 3 (2022 and 2023) at a center-of-mass energy of $\sqrt{s} = 13.6$ TeV. The dataset corresponds to an integrated luminosity of $68.6 \text{ fb}^{-1}$.

• Detector: The analysis relies exclusively on the electronic subsystems of the hybrid SND@LHC detector, located 480 m downstream from the ATLAS interaction point (IP1). The system includes a veto system, a target consisting of tungsten blocks interleaved with emulsion bricks, a scintillating-fibre (SciFi) tracker, and a muon system interleaved with iron absorbers acting as a hadron calorimeter.
• Event Selection: A cut-based procedure reduces the data volume by a factor of $5.5 \times 10^8$. Key selection criteria include:
  • Fiducial Volume: Interactions must occur in the target walls (specifically walls 2–5, an expansion from previous analyses) and within defined SciFi and muon system fiducial areas.
  • Background Rejection: Events must have no hits in the upstream veto system or the first SciFi station (to reject side-entering and penetrating muons).
  • Signal Identification: Candidates must exhibit a hadron shower (defined by $>35$ SciFi hits and total upstream charge $Q_{US} > 600$ units) and a reconstructed muon track in the downstream (DS) system consistent with the interaction vertex.
• Background Estimation: Backgrounds are dominated by side-entering and penetrating muons. These are estimated using data-driven "ABCD" methods and survival probability calculations based on known muon fluxes and detector inefficiencies. Neutral hadron and non-muon neutrino backgrounds are estimated via simulation.
• Hadronic Energy Measurement: A calorimetric measurement of hadronic energy is performed using a linear combination of SciFi and upstream calorimeter signals ($E_{tot} = k \cdot Q_{SciFi} + \alpha \cdot Q_{US}$). Calibration constants are derived from dedicated test-beam campaigns (2023 and 2024) using pion and electron beams.
• Simulation: Signal events are generated using DPMJET-III for $pp$ collisions, FLUKA for neutrino propagation, and Genie 3.2.0 for neutrino interactions. Systematic uncertainties are evaluated by varying selection criteria and comparing data with Monte Carlo (MC) simulations, including test-beam data.

Key Contributions

1. Expanded Dataset and Acceptance: The analysis doubles the integrated luminosity compared to the previous SND@LHC publication and increases the fiducial volume acceptance for $\nu_\mu$ CC interactions from 8% to 19%.
2. Calorimetric Energy Measurement: This work presents the first calorimetric measurement of hadronic energies in collider neutrino interactions, reconstructing energies up to 0.4 TeV with a resolution ranging from 22% (at 100 GeV) to 12% (at 300 GeV).
3. Cross Section Determination: The paper provides a measurement of the combined $\nu_\mu$ and $\bar{\nu}_\mu$ CC cross section on tungsten, averaged over the detector's energy spectrum.

Results

• Event Yields: 31 $\nu_\mu$ CC candidate events were selected. The expected background is $5.0 \pm 1.1$ events. The signal expectation, based on the EPOS-LHC generator for light hadrons and POWHEG for charm hadrons, is $24^{+10}_{-9}$ events.
• Signal Strength: The measured signal strength is $\hat{\mu} = 1.09^{+0.72}_{-0.37}$, indicating consistency between the observation and theoretical predictions.
• Cross Section: The combined CC cross section on tungsten is determined to be:
  $$\sigma(\nu_\mu + \bar{\nu}_\mu) = (37^{+24}_{-12}) \times 10^{-35} \text{ cm}^2$$
  This measurement is taken at a median neutrino energy of 228 GeV (average energy 343 GeV).
• Systematics: The total systematic uncertainty on the signal efficiency is $[-34.9, +8.6]\%$, dominated by the negative side by mismodelling of the QUS distribution in MC and the positive side by uncertainties in the SciFi hit distribution.

Significance and Claims
The paper claims that the measured cross section is consistent with the Genie prediction. The observation of 31 events against a background of 5.0 and a signal expectation of 24 confirms the capability of the SND@LHC electronic detector system to identify and measure high-energy neutrino interactions.

The authors state that this work demonstrates the calorimetric capability of the detector, providing the first measurement of hadronic energy spectra in collider neutrino interactions. They note that these results serve as a precursor for higher-precision studies planned for the high-luminosity Run 4 of the LHC, following a planned upgrade to the detector's silicon microstrip system. The paper modestly frames these results as an updated measurement that validates the detector's performance and the theoretical models used for neutrino flux and interaction predictions at the LHC.