New physics searches at NA62

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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 universe as a giant, bustling cosmic highway. On this highway, tiny particles called Kaons and Pions are speeding along, trying to get from point A to point B. Usually, they follow the strict traffic laws written in the "Standard Model" (the rulebook of physics we already know). But sometimes, they might take a secret shortcut through a hidden tunnel—a portal to a "Dark Sector" of physics that we haven't discovered yet.

The NA62 experiment at CERN is like a high-tech traffic camera and detective team stationed on this highway. Their job is to watch these particles decay (break apart) and look for anything suspicious that doesn't fit the rulebook.

Here is a breakdown of what this paper is about, using some everyday analogies:

1. The Main Job: Catching the "Ghost" Decay

The primary goal of NA62 is to study a very rare event: a Kaon turning into a Pion and... nothing else visible.

The Analogy: Imagine a magician (the Kaon) on stage. He throws a ball (the Pion) into the air. According to the laws of physics, he should also throw a second, invisible ball (a neutrino) that we can't see.
The Mystery: If the magician throws the ball and nothing else happens, but the math says he should have thrown two invisible balls, we know we are seeing the standard magic trick. But if the math says he should have thrown one invisible ball, and we see a different pattern, it might mean he actually threw a secret, invisible object (a new particle) that we don't know about.
The Result: The team watched millions of these "magic tricks" between 2016 and 2022. They found that the Kaons are behaving exactly as the Standard Model predicts (within a tiny margin of error). It's like checking a lock and finding it fits the key perfectly. This is good news for the rulebook, but it also means they didn't find the "secret tunnel" yet.

2. The "Missing Mass" Detective Work

How do they know if something invisible was thrown? They use a concept called Missing Mass.

The Analogy: Think of a scale. You put a heavy box (the Kaon) on one side. It breaks open, and a lighter box (the Pion) flies out. You weigh the Pion. If the math says the Pion should weigh 10 pounds, but the original box weighed 100 pounds, there is a "missing" 90 pounds.
The Twist: Usually, that missing weight is the invisible neutrinos. But the NA62 team looked closely at that "missing weight" to see if it was a specific amount that would indicate a new, heavy particle (like a Dark Photon or a Heavy Neutral Lepton) instead of just invisible neutrinos.
The Search: They scanned the data like a metal detector on a beach, looking for a specific "ping" that would indicate a new particle hiding in the missing mass. They found nothing suspicious, which allowed them to set strict limits on how heavy or how common these new particles could be.

3. The "Heavy Neutral Lepton" Hunt

The paper also discusses a search for Heavy Neutral Leptons (HNLs) using Pions instead of Kaons.

The Analogy: Imagine a Pion is a delivery truck. Usually, it drops off a package (an electron) and a tiny, invisible drone (a neutrino). The team is looking for a scenario where the truck drops off the package and a giant, heavy boulder (the HNL) instead of the tiny drone.
The Challenge: If that boulder is too heavy, the truck might not be able to carry it, or it might decay immediately. The team looked for a specific "signature" in the data where the math didn't add up, suggesting a heavy boulder was involved.
The Result: They didn't find the boulder. However, by not finding it, they were able to say: "If these heavy boulders exist, they must be extremely rare (less than 1 in a billion chance) or very light." This narrows down the search area for future scientists.

4. Why Does This Matter?

You might ask, "If they didn't find new particles, why is this paper important?"

Ruling Out the "Wild Guesses": In science, knowing what isn't there is just as important as knowing what is. By proving that these new particles aren't hiding in the places they looked, they are closing doors on many theories about the "Dark Sector."
The "Dark Portal" Models: The paper tested four different "portals" (ways new particles could interact with our world). They put up "Do Not Enter" signs on all four, telling future physicists: "If you want to find new physics, you have to look somewhere else or build a more powerful detector."
The Future: The experiment is still running! They plan to collect three times more data by 2026. It's like upgrading from a pair of binoculars to a giant telescope. If there is a secret tunnel, they are determined to find it with the next batch of data.

Summary

The NA62 team acted like cosmic detectives, watching billions of particle collisions. They confirmed that the "Standard Model" (our current rulebook) is still holding up very well. While they didn't find the "Dark Particles" they were hunting for, they successfully mapped out the "No-Go Zones," telling the rest of the physics world exactly where not to look, and setting the stage for even more precise searches in the future.

1. Problem and Motivation

The NA62 experiment at CERN aims to probe the Standard Model (SM) with high precision and search for physics beyond the Standard Model (BSM). The primary motivations are:

Rare Kaon Decays: The decay $K^+ \to \pi^+ \nu \bar{\nu}$ is a flavor-changing neutral current (FCNC) process highly suppressed in the SM (Branching Ratio $\mathcal{B} \sim 8 \times 10^{-11}$ ) due to the GIM mechanism and CKM suppression. Its clean theoretical prediction makes it a sensitive probe for new physics.
Hidden Sector Searches: The experiment searches for deviations in the $K^+ \to \pi^+ \nu \bar{\nu}$ spectrum that could indicate the production of a new, invisible particle $X$ (e.g., dark photons, scalars, or Axion-Like Particles) in the decay $K^+ \to \pi^+ X$ .
Heavy Neutral Leptons (HNLs): The experiment searches for HNLs ( $N$ ) produced in pion decays ( $\pi^+ \to e^+ N$ ), which would manifest as a peak in the missing mass spectrum of electronic pion decays.

2. Methodology and Experimental Setup

The analysis utilizes data collected during Run 1 (2016–2018) and Run 2 (2021–2024).

Beam and Target: A 400 GeV proton beam from the SPS hits a secondary target, producing a secondary beam of 75 GeV particles (70% $\pi^+$ , 23% $p$ , 6% $K^+$ ).
Detector Components:
- KTAG: Tags incoming kaons.
- GTK: Measures kaon momentum.
- STRAW Spectrometer: Measures outgoing pion momentum.
- LKr (Liquid Krypton Calorimeter): Identifies pions and rejects backgrounds.
- RICH (Ring Imaging Cherenkov): Provides particle identification (PID) and timing.
- MUV1,2,3 & LAV: Muon vetos and large-angle vetos to reject background activity.
Analysis Strategy for $K^+ \to \pi^+ \nu \bar{\nu}$ :
- The analysis identifies events with a single pion originating from a kaon decay within the fiducial volume.
- Kinematic Selection: The squared missing mass ( $m_{miss}^2$ ) is calculated. Signal regions are defined to avoid the dominant backgrounds: $K \to 3\pi$ , $K \to \pi^+ \pi^0$ , and $K \to \mu \nu$ .
- Background Rejection: PID cuts reduce muonic decay backgrounds by a factor of $O(10^8)$ . The total background rejection factor is $O(10^{-4})$ from kinematic cuts alone.
- Sensitivity: The single event sensitivity is calculated as $\mathcal{B}_{SES} = (8.48 \pm 0.29) \times 10^{-12}$ .
Analysis Strategy for HNL Search ( $\pi^+ \to e^+ N$ ):
- Uses the same trigger line as the kaon analysis.
- Selects events with a single positron in the final state.
- Assumes HNL lifetimes $> 50$ ns; decays of the HNL into SM particles are neglected.
- Searches for a peak in the missing mass spectrum $m_{miss}^2(\pi) = (P_\pi - P_e)^2$ .

3. Key Contributions and Results

A. Measurement of $K^+ \to \pi^+ \nu \bar{\nu}$

Data Sample: 2016–2022 data.
Observation: 51 signal events were observed with an expected background of $18^{+3}_{-2}$ .
Significance: The background-only hypothesis is rejected with $> 5\sigma$ significance.
Branching Ratio: The measured branching ratio is:
$\mathcal{B}(K^+ \to \pi^+ \nu \bar{\nu}) = \left(13.0^{+3.0}_{-2.7} \text{ (stat)} +^{1.3}_{-1.3} \text{ (syst)}\right) \times 10^{-11}$
Comparison: This result is consistent with the Standard Model prediction within 1.7 $\sigma$ .

B. Search for $K^+ \to \pi^+ X$ (New Physics)

Method: The missing mass spectrum was scanned for peaks corresponding to a new particle $X$ with mass $m_X$ .
Results:
- No significant excess was found.
- Upper Limits: Upper limits on the branching ratio $\mathcal{B}(K^+ \to \pi^+ X)$ were established at the $10^{-11}$ level.
- Model Constraints: These limits were interpreted to constrain four "portal" models:
  1. Vector (Dark Photon)
  2. Scalar
  3. ALP with fermion coupling
  4. ALP with gluon coupling
- The results provide competitive exclusion limits across a wide mass range for these scenarios.

C. Search for Heavy Neutral Leptons ( $\pi^+ \to e^+ N$ )

Data Sample: 2017–2024 data.
Method: A peak search was performed in the missing mass spectrum of electronic pion decays.
Results:
- No evidence for HNLs was found.
- Upper Limits: Upper limits on the extended neutrino mixing matrix element $|U_{e4}|^2$ were established at the $10^{-8}$ level.
- Mass Range: These limits apply to HNL masses between 95 and 126 MeV/c $^2$ .

4. Significance and Outlook

Standard Model Validation: The measurement of $K^+ \to \pi^+ \nu \bar{\nu}$ confirms the SM prediction with high precision, validating the theoretical framework of FCNC suppression.
New Physics Constraints: The NA62 collaboration has set the world's most stringent constraints (or among the most competitive) for dark sector particles (dark photons, scalars, ALPs) and Heavy Neutral Leptons in the specific mass and coupling ranges probed.
Future Prospects: Data taking continues into 2026. The total dataset is expected to be 3 times larger than the 2016–2022 sample, which will further reduce statistical uncertainties and improve sensitivity to rare decays and new physics signals.

In summary, this paper presents a comprehensive analysis of rare kaon and pion decays, successfully measuring the SM benchmark $K^+ \to \pi^+ \nu \bar{\nu}$ and simultaneously setting rigorous limits on a broad spectrum of BSM scenarios, including dark sectors and heavy neutral leptons.