Recent Results from NA62 in Kaon and Dump Mode

This paper reports the NA62 experiment's latest results, including a Standard Model-compatible measurement of the ultra-rare decay $K^+\to\pi^+\nu\bar\nu$ and the setting of new upper limits on heavy neutral lepton couplings based on a null search for new-physics particles in beam-dump mode data.

The Gist

Imagine the NA62 experiment as a high-tech, ultra-sensitive "particle detective agency" located at CERN in Switzerland. Their job is to watch tiny particles called kaons (a type of subatomic particle) as they zoom through a long, empty tunnel and see how they behave.

This paper reports on two different "cases" the detectives solved using data collected between 2016 and 2024.

Case 1: The "Ghostly" Disappearance (Kaon Mode)

In their standard mode, the experiment acts like a high-speed camera trying to catch a very rare event: a kaon turning into a pion (a lighter particle) and then vanishing into thin air, leaving behind only invisible particles called neutrinos.

• The Challenge: This is like trying to spot a single specific grain of sand falling from a beach, while millions of other grains are falling around it. Most kaons decay in predictable, noisy ways. The team needed to filter out the "noise" to find the "signal."
• The Method: They built a massive vacuum tunnel (117 meters long) to ensure the particles don't bump into air molecules. They used a series of "guards" (detectors) to check every particle's ID card. If a particle didn't match the strict rules of the "ghostly disappearance," it was thrown out.
• The Result: They caught this rare event more times than ever before. The number of times they saw it matched the predictions of the "Standard Model" (the rulebook of physics) almost perfectly.
• The Takeaway: The universe is behaving exactly as the rulebook says it should. This result is so precise that it rules out some wild new theories that tried to predict different outcomes, pushing the limits of our knowledge up to scales of 100,000 trillion meters.

Case 2: The "Dump Mode" Hunt for Hidden Monsters

The experiment has a second setting, called "Beam-Dump Mode." Imagine instead of letting the particles fly freely, you slam the proton beam into a giant wall (a dump) to stop it.

• The Goal: When protons smash into this wall, they might create heavy, invisible particles that don't exist in the standard rulebook. These are hypothetical "Heavy Neutral Leptons" (HNLs)—think of them as heavy, ghostly cousins of the neutrino that might explain why the universe has so much matter.
• The Strategy: The team looked for these heavy ghosts as they traveled through the detector and decayed (broke apart) into a mix of charged particles (like pions or electrons).
• The Filter: They set up a "safe zone" (a specific volume in the tunnel) where these ghosts should appear. They used smart computer algorithms to ignore background noise, like stray muons (another type of particle) that usually cause false alarms.
• The Result: They looked very hard at data collected over 31 days of running. They found zero ghosts. Not a single one.
• The Takeaway: While they didn't find the new particles, finding nothing is still a huge success. It allows them to draw a "No Trespassing" sign on a map of particle physics. They can now say with 90% confidence that these heavy ghosts do not exist in a specific weight range (between 150 and 2000 MeV) or with a specific strength of interaction.

Summary

In short, the NA62 team did two things:

1. Confirmed the Rulebook: They watched a rare particle decay and found it matches the existing laws of physics perfectly.
2. Ruled Out the Unknown: They looked for new, heavy particles in a "dump mode" and found none, effectively narrowing the search area for future physicists.

They didn't find new physics this time, but they successfully closed the door on several possibilities, telling us exactly where not to look next.

Technical Summary

1. Problem and Motivation

The NA62 experiment at CERN's SPS addresses two primary frontiers in particle physics:

1. Precision Flavor Physics: The ultra-rare decay $K^+ \to \pi^+ \nu \bar{\nu}$ is a "golden mode" for testing the Standard Model (SM). It is theoretically clean and highly sensitive to New Physics (NP) contributions, such as Supersymmetry or extra dimensions, which could alter the branching ratio (BR) significantly.
2. Search for Long-Lived Particles (LLPs): Many Beyond the Standard Model (BSM) scenarios, particularly those involving Heavy Neutral Leptons (HNLs) or sterile neutrinos, predict the existence of particles with masses between 150 MeV and 2 GeV that decay into visible final states ($h^\pm \ell^\mp$). These particles are often long-lived and require a "beam-dump" configuration to be produced and detected effectively, as they cannot be kinematically accessed in standard kaon decays.

2. Methodology

The paper reports on two distinct operational modes of the NA62 experiment:

A. Kaon Mode (Standard Operation)

• Beam Setup: A 400 GeV/c proton beam strikes a Beryllium target, producing a secondary unseparated hadron beam. A momentum of 75 GeV/c is selected, containing approximately 6% $K^+$.
• Detection Strategy:
  • Identification: Kaons are identified by the KTAG (differential Cherenkov counter) and their momentum measured by the GTK (silicon pixel detector).
  • Decay Volume: A 75 m fiducial volume (FV) inside a 117 m vacuum vessel.
  • Reconstruction: The analysis reconstructs the missing mass squared ($m^2_{miss} = (P_K - p_\pi)^2$) by matching the incoming kaon 4-momentum to a downstream pion.
  • Background Rejection:
    • Muon Rejection: Combined Ring Imaging Cherenkov (RICH) and calorimeter (LKr) PID achieves $\mathcal{O}(10^7)$ rejection.
    • $\pi^0$ Rejection: Veto systems (SAV, LAV, LKr) reject events with extra photon activity, achieving $\mathcal{O}(10^8)$ rejection.
    • Upstream Background: Improved algorithms were used to characterize and reject "upstream" backgrounds (decays occurring before the FV).
• Data Set: Analysis covers data collected from 2016 to 2024, with a specific focus on the 2023–2024 dataset, which doubled the normalization statistics (measured via $K^+ \to \pi^+ \pi^0$ events).

B. Beam-Dump Mode

• Configuration: The Be target is lifted, and the TAX collimators are closed to act as a beam dump. The 400 GeV protons interact with the dump, producing particles with a center-of-mass energy of $\sqrt{s} \approx 27.3$ GeV.
• Target Physics: Searches for long-lived particles decaying into $h^\pm \ell^\mp$ (where $h^\pm \in \{\pi^\pm, \pi^\pm\pi^0, \pi^\pm 2\pi^0, K^\pm\}$ and $\ell \in \{e, \mu\}$).
• Signal Selection:
  • Requires exactly one charged hadronic track and one oppositely charged leptonic track.
  • Vertexing: Tracks must form a high-quality vertex within a specific subset of the FV to suppress backgrounds from muons interacting with detector material.
  • Kinematic Cuts: A Signal Region (SR) is defined based on the distance of closest approach (CDATAX) and longitudinal coordinate (ZTAX) relative to the proton beam line. This exploits the closed kinematics of HNL decays to distinguish them from upstream hadron decays.
  • Veto: Events with activity in LAV, ANTI0, or CHANTI are rejected.
• Data Set: 31 days of operation between 2021 and 2024, corresponding to $(6.3 \pm 1.3) \times 10^{17}$ protons on target (PoT).

3. Key Contributions

• Updated $K^+ \to \pi^+ \nu \bar{\nu}$ Measurement: The paper presents the most precise measurement to date, utilizing the full 2016–2024 dataset. It introduces significant improvements in trigger, tracking, and PID algorithms, and a refined evaluation of the dominant upstream background.
• First Beam-Dump Results for NA62: This is the first report of NA62 results in beam-dump mode, establishing a new search channel for heavy neutral leptons in the 150–2000 MeV mass range.
• Model-Independent Search: The HNL search is presented in a model-independent way but is interpreted using the Alpinist tool for specific HNL benchmark scenarios (Majorana HNLs with Yukawa couplings).

4. Results

A. $K^+ \to \pi^+ \nu \bar{\nu}$ Branching Ratio

• 2023–2024 Result: The new dataset yields a branching ratio of:
  $$BR_{2023-2024} = (7.2^{+2.3}_{-2.1}) \times 10^{-11}$$
  This is compatible with previous NA62 results and the SM expectation.
• Combined Result (2016–2024): The statistically combined result is:
  $$BR_{2016-2024} = (9.6^{+1.9}_{-1.8}) \times 10^{-11}$$
• Significance: The result is consistent with the Standard Model prediction (ranging from $7.86 \times 10^{-11}$ to $8.60 \times 10^{-11}$) within $1\sigma$.
• Constraints: The measurement constrains BSM scenarios up to energy scales of 100 TeV.

B. Heavy Neutral Lepton (HNL) Search

• Observation: Zero events were observed across all considered signal channels in the beam-dump mode.
• Exclusion Limits:
  • Upper limits were set on the mixing parameter $U^2$ (coupling suppression) as a function of HNL mass ($m_N$).
  • Excluded Region: HNLs with masses between 0.4 GeV and 1.0 GeV and coupling suppressions of $U^2 \sim 10^{-6}$ are excluded at the 90% Confidence Level (CL).
  • Scenarios: Limits were derived for four specific coupling scenarios:
    1. Electrophilic ($U^2_\mu = U^2_\tau = 0$)
    2. Muonphilic ($U^2_e = U^2_\tau = 0$)
    3. Normal Hierarchy ($U^2_\mu = U^2_\tau, U^2_e = 0$)
    4. Inverted Hierarchy ($U^2_e = U^2_\mu = U^2_\tau$)
• Background Suppression: The analysis successfully demonstrated that signal events are evenly distributed in the fiducial volume, while background events cluster near dense beamline objects (collimators, LAV stations) or the start of the FV, validating the kinematic separation strategy.

5. Significance

• Standard Model Validation: The updated $K^+ \to \pi^+ \nu \bar{\nu}$ measurement reinforces the validity of the CKM unitarity and the SM description of flavor-changing neutral currents. The agreement within $1\sigma$ tightens constraints on models predicting large deviations in this channel.
• New Physics Reach: The beam-dump results significantly extend the sensitivity to HNLs in the mass range where previous experiments (like CHARM, PS191, and NuTeV) had limited reach or different systematic limitations. By excluding $U^2 \sim 10^{-6}$ for masses up to 1 GeV, NA62 probes parameter space relevant for neutrino mass generation mechanisms (e.g., the $\nu$MSM).
• Experimental Technique: The successful transition to beam-dump mode demonstrates the versatility of the NA62 detector, proving its capability to search for a wide variety of long-lived particles beyond its primary kaon physics goals. The use of kinematic separation (CDATAX/ZTAX) provides a robust method for background rejection in high-intensity fixed-target environments.