A SHIFT of Perspective: Observing Neutrinos at CMS and… — Plain-Language Explanation

The Big Idea: A "Ghost" Hunter in a Giant Machine

Imagine the Large Hadron Collider (LHC) at CERN as a massive, high-speed train track where tiny particles (protons) race around at near the speed of light. Usually, scientists crash these trains together head-on to see what explodes.

This paper proposes a new, slightly different way to use the LHC. Instead of just looking at the head-on crashes, they suggest setting up a "gas trap" (a fixed target) about 100 meters down the track from the main crash site.

The Analogy:
Think of the main collision point as a busy highway intersection. The "gas trap" is like a small, invisible net placed on the side of the road, 100 meters away. When the proton beam passes this net, it smashes into the gas molecules inside. This creates a spray of new particles, much like a car hitting a puddle and spraying water everywhere.

Most of this spray flies forward, like water from a hose. Among these particles are neutrinos.

What are Neutrinos?

Neutrinos are like invisible ghosts. They have almost no mass and no electric charge. They can pass through entire planets without stopping. Because they are so hard to catch, we usually need massive, specialized detectors to find them.

The Paper's Claim:
The authors suggest that if we use this "gas trap" setup, the LHC's main detectors (CMS and ATLAS)—which are huge, multi-story buildings sitting further down the track—will act as giant ghost catchers.

They calculate that even if we only use 1% of the LHC's scheduled time for this experiment, these main detectors could catch thousands of neutrino interactions.

Muons (a type of heavy electron): About 10,000 interactions.
Electrons: About 1,000 interactions.
Energy: These ghosts would be carrying energy ranging from a light bulb's worth (20 GeV) to a lightning bolt's worth (1 TeV).

Why is this Special? (The "New View")

Usually, the LHC detectors look at what happens right in the middle of the crash. They miss the particles that fly off at very sharp, forward angles.

The Analogy:
Imagine a fireworks display. The main cameras are set up to film the explosion in the center. But this new setup allows the cameras to film the sparks flying off at a sharp angle, which no one has ever been able to see clearly before in this specific energy range.

This setup lets scientists look at a "blind spot" in the universe:

The Angle: It sees particles flying at angles (pseudorapidity) that current detectors can't see.
The Source: It helps us understand how particles (pions and kaons) are made and decay before they hit the detector.
The Comparison: It fills a gap between the low-energy neutrinos we see from the sun or atmosphere and the super-high-energy ones from deep space.

How Will They Catch the Ghosts?

The detectors (CMS and ATLAS) are like giant, layered sandwiches.

The Layers: They have layers of metal and sensors.
The Interaction: When a neutrino (the ghost) finally hits a nucleus inside the detector's metal layers, it creates a tiny explosion of energy (a shower of particles).
The Signal: This explosion leaves a trace. The scientists can tell the difference between a muon-neutrino and an electron-neutrino based on the shape of the explosion and the type of particle that flies out.

The Challenges (The "Noise")

The paper admits this won't be easy.

The Background Noise: When the gas trap is hit, it also creates regular particles (like muons) that travel alongside the neutrinos. It's like trying to hear a whisper (the neutrino) while a loud band (the other particles) is playing nearby.
The Solution: The scientists think they can filter this out. The neutrinos will hit the detector at a slightly different angle or time than the loud background noise. They also plan to use the outer layers of the detector to spot the "loud" particles and ignore them, focusing only on the "whispers" that made it through.
The Confusion: Sometimes, a neutral particle can mimic an electron. The paper notes this is a problem they will need to solve with better computer simulations later.

What Will They Learn?

If this works, it's a historic first: detecting neutrinos inside a general-purpose particle collider detector.

This isn't just about finding ghosts; it's about understanding the "recipe" of the universe.

Atmospheric Neutrinos: Experiments that look for neutrinos coming from the Earth's atmosphere (like IceCube or DUNE) need to know exactly how these particles are made. This experiment provides a controlled "lab" to test those recipes.
New Materials: Because the detectors are made of different metals (brass, copper, steel, tungsten), scientists can see how neutrinos interact with different materials, which helps improve our understanding of physics.

Summary

The paper proposes turning a side-section of the LHC into a neutrino factory. By shooting protons into a gas trap, they can create a beam of neutrinos that flies straight into the main detectors. Even with a small amount of time, they expect to catch thousands of these elusive particles, opening a new window to study how matter behaves at the very edge of our current knowledge.

Technical Summary: Observing Neutrinos at CMS and ATLAS via the SHIFT@LHC Concept

Problem and Motivation
While the LHC has traditionally been viewed as a source of high-energy particle collisions for discovering new physics, it also naturally produces an intense, forward-collimated flux of neutrinos. Recent experiments like FASER and SND@LHC have successfully observed these neutrinos, opening a program for studying hadron production, neutrino cross-sections, and new physics. However, existing forward detectors are limited to specific pseudorapidity ranges and energy scales. The SHIFT@LHC proposal originally envisioned installing a gaseous fixed-target 100 meters downstream from the main interaction points (IPs) to search for long-lived particles. This paper explores a distinct physics opportunity enabled by the same setup: utilizing the gaseous fixed-target collisions as a source of neutrinos to be detected directly within the general-purpose LHC detectors, CMS and ATLAS. The authors aim to demonstrate that this configuration can generate a measurable flux of neutrinos with energies from 20 GeV to 1 TeV, filling a kinematic gap complementary to existing forward detectors and dedicated neutrino beam experiments.

Methodology
The study employs a multi-stage simulation chain to estimate neutrino production and interaction rates:

Production Simulation: Proton-gas collisions are simulated using PYTHIA8 with a 6.8 TeV proton beam impinging on a stationary proton target. The fixed-target is modeled at approximately 160 meters upstream of the CMS (IP5) or ATLAS (IP1) interaction points. Events are generated in bins of hard-process transverse momentum ( $\hat{p}_T$ ).
Propagation and Decay: The propagation of hadrons and muons through the LHC tunnel environment is analyzed. The authors consider three scenarios for neutrino origin: (a) decay before reaching the surrounding rock, (b) decay within the rock, and (c) muon decay within the rock. A GEANT4 simulation of standard rock determines that hadrons are stopped within 1 meter, effectively filtering out neutrinos from hadrons decaying deep within the rock. Muon decays within the decay volume are found to be negligible due to the muon lifetime.
Interaction Simulation: Neutrino propagation and interactions within the CMS and ATLAS detectors are simulated using GENIE. The study focuses on charged-current (CC) interactions above 20 GeV, utilizing the G18_02a tune for energies up to 1 TeV and GHE19_00b for higher energies.
Detection Criteria: The analysis targets interactions within the innermost calorimetric systems (electromagnetic and hadronic calorimeters). Selection criteria require the outgoing charged lepton and hadronic shower to carry energies above 3 GeV and 10 GeV, respectively. The outer muon systems are used to tag accompanying muons to isolate neutrino interactions. The authors assume that advances in reconstruction algorithms and the high granularity of the detectors will allow for the reconstruction of displaced jet+lepton vertices.

Key Results
Assuming 1% of the LHC Run-4 integrated luminosity (approximately $7.15 \text{ fb}^{-1}$ or $4 \times 10^{25}$ protons-on-target) is dedicated to this study, the authors estimate the following interaction rates:

Total Yield: Approximately $O(10^4)$ muon-neutrino ( $\nu_\mu$ ) interactions and $O(10^3)$ electron-neutrino ( $\nu_e$ ) interactions.
Flavor Distribution: The majority of events are $\nu_\mu$ CC interactions, with a peak energy distribution between 20–100 GeV. The dominant source of these neutrinos is kaon decay, while pion decays become relevant below 15 GeV.
Detector Location: In CMS, over 70% of interactions occur in the hadronic calorimeters (brass absorbers), with a non-negligible fraction in the electromagnetic calorimeters (PbWO $_4$ ). In ATLAS, the forward calorimeter (tungsten/copper) sees the highest rate, followed by the hadronic end-cap and barrel.
Pseudorapidity Coverage: The setup provides access to hadron production in the pseudorapidity range $5 < \eta < 8$ , a region largely inaccessible to standard collider-mode analyses and very-forward detectors like FASER.
Target Sensitivity: Varying the target position by $\pm 30$ meters from the nominal 160 m location results in a $\sim 30\%$ change in interaction rates, driven by the available decay length for parent hadrons.

Challenges and Limitations
The paper acknowledges several significant challenges:

Backgrounds: A primary background is the flux of muons produced in the same hadronic decays as the neutrinos. Simulations suggest that while accompanying muons are common, their multiplicity and energy (average max 38 GeV) are unlikely to overwhelm the calorimeter response. However, a realistic assessment of pileup from concurrent $pp$ collisions is required.
$\nu_e$ Contamination: Neutral-current (NC) interactions can mimic $\nu_e$ CC signatures if they produce a hard $\pi^0$ that initiates an electromagnetic shower. The authors estimate that the surviving NC background in the CMS hadronic end-cap could be comparable to the $\nu_e$ signal, necessitating detailed detector-level simulations to assess discrimination capabilities.
Reconstruction: The analysis assumes optimistic reconstruction efficiencies. The requirement for full containment of hadronic showers and reliable lepton reconstruction to determine neutrino energy may restrict the effective fiducial mass and accessible pseudorapidity range.

Significance and Claims
The authors claim that if realized, this configuration would mark the first detection of neutrinos in a general-purpose LHC detector. The significance of this work lies in:

Complementarity: It extends the physics reach of the SHIFT concept to neutrino measurements, probing a forward pseudorapidity regime ( $5 < \eta < 8$ ) and an energy range (20 GeV – 1 TeV) that complements FASER, SND@LHC, and past fixed-target experiments (e.g., NOMAD, NuTeV).
Hadron Production Constraints: The high statistics of CC events could provide direct experimental constraints on forward hadronic production (pion and kaon contributions), addressing uncertainties relevant to atmospheric neutrino experiments like KM3NeT-ORCA, IceCube-DeepCore/Upgrade, DUNE, and Hyper-Kamiokande.
Cross-Section Measurements: The diverse nuclear compositions of the calorimeter materials (brass, PbWO $_4$ , tungsten, copper, steel) allow for ratio measurements of neutrino cross-sections on various nuclei, including antineutrinos and electron-neutrinos, for which data is currently scarce.
Feasibility Demonstration: The study serves as a proof-of-concept that neutrino measurements are feasible within the complex environment of ATLAS and CMS, provided that reconstruction algorithms can handle displaced vertices and background suppression.

The paper concludes that while detailed simulations of hadron propagation, pileup effects, and detector performance are needed to refine these estimates, the proposed setup offers a unique and valuable opportunity to study neutrino physics within the LHC infrastructure.

A SHIFT of Perspective: Observing Neutrinos at CMS and ATLAS