arxiv: 2604.08954 · v1 · submitted 2026-04-10 · ✦ hep-ex

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New physics searches at NA62

Elizabeth Long (for the NA62 collaboration)

Authors on Pith no claims yet

Pith reviewed 2026-05-10 17:15 UTC · model grok-4.3

classification ✦ hep-ex

keywords NA62kaon decaysnew physicsdark photonheavy neutral leptonsportal modelsbranching ratio limitsALP

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The pith

NA62 sets upper limits at the 10^{-11} level on K+ to pi+ X branching ratios to constrain new physics in portal models.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The NA62 experiment uses data from kaon and pion decays collected from 2016 through 2024 to search for rare processes that could signal new particles. No evidence is observed for either K+ to pi+ X or pi+ to e+ N decays. Upper limits are placed on the branching ratio at the 10^{-11} level and on the heavy neutral lepton mixing parameter at the 10^{-8} level. These bounds restrict couplings in dark photon, scalar, ALP, and heavy neutral lepton scenarios across all four portal models. A reader cares because the limits shrink the allowed parameter space for dark sector physics that might otherwise remain hidden at current energies.

Core claim

Searches for the decays K+→π+X and π+→e+N in NA62 data from 2016-2022 and 2017-2024, respectively, find no signals. Upper limits on the K+→π+X branching ratio are established at the 10^{-11} level, providing constraints on dark photon, scalar and ALP couplings. From the search for heavy neutral lepton production in π+→e+N decays, upper limits of |U_e4|^2 are established at the 10^{-8} level over the mass range 95-126 MeV/c².

What carries the argument

Kinematic reconstruction of missing mass or invariant mass distributions in selected kaon and pion decay events to search for narrow peaks or missing energy signatures from new particles.

If this is right

Dark photon couplings to standard model particles are bounded in the relevant mass ranges.
Scalar particle and ALP couplings receive direct limits from the branching ratio constraints.
Heavy neutral lepton mixing matrix elements are restricted at the 10^{-8} level for masses between 95 and 126 MeV/c².
The results apply uniformly across all four portal model scenarios for new physics.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

Ongoing NA62 data collection could push these branching ratio limits lower in future updates.
The limits can be combined with results from other experiments to further restrict portal model parameter spaces.
Null results imply that any new physics in these channels, if present, must occur at higher masses or weaker couplings than currently probed.

Load-bearing premise

The new-physics interpretations assume signals would appear in the four portal scenarios with no additional backgrounds or detector effects beyond those modeled in the analysis.

What would settle it

An excess of events above expected background in the missing-mass signal region for K+→π+X at a specific mass value would indicate a new particle and invalidate the null-result limits.

Figures

Figures reproduced from arXiv: 2604.08954 by Elizabeth Long (for the NA62 collaboration).

**Figure 2.** Figure 2: shows the kinematic distribution of kaon decays from a control data sample, before applying particle identification (PID) cuts and photon rejection. Choosing the signal regions to avoid the three main kaon decays (K → 3π, K → π +π 0 and K → µν) results in background rejection of O(10−4 ). Additional PID cuts reduce background from the muonic decay by a further factor of O(108 ), giving a single event sensi… view at source ↗

**Figure 3.** Figure 3: Squared missing mass (m2 miss as a function of pion momentum for events passing signal selection. The signal regions are highlighted in red. 0 5 10 15 20 25 30 35 40 BNL NA62: 2016-2018 NA62: 2021-2022 NA62: 2016-2022 SM [JHEP 09 (2022) 148] SM [EPJC 82 (2022) 615] [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

**Figure 5.** Figure 5: Missing mass spectrum of [PITH_FULL_IMAGE:figures/full_fig_p006_5.png] view at source ↗

**Figure 7.** Figure 7: Exclusion limits from NA62 searches for K+ → π +Xinv assuming X is, respectively, a vector, a scalar, an ALP with fermion coupling, an ALP with gluon coupling. 5 Conclusions and Outlook The NA62 collaboration has measured the branching ratio of K+ → π +νν, showing agreement with the Standard Model to within 1.7σ. The K+ → π +X search has allowed the collaboration to place competitive limits on all 4 dark s… view at source ↗

**Figure 8.** Figure 8: Squared missing mass distribution of electronic pion decays [PITH_FULL_IMAGE:figures/full_fig_p008_8.png] view at source ↗

read the original abstract

The NA62 experiment at CERN has collected a large sample of $K^+$ and $\pi^+$ decays in flight during Run 1 in 2016--2018 and the ongoing Run 2 which started in 2021. Searches for the decays $K^+\rightarrow\pi^+X$ and $\pi^+\rightarrow e^+N$ are presented using NA62 data collected in 2016--2022 and 2017--2024, respectively. Results are interpreted to constrain a range of new physics scenarios covering all four portal model scenarios. Upper limits on the $K^+\rightarrow\pi^+X$ branching ratio are established at the $10^{-11}$ level, providing constraints on dark photon, scalar and ALP couplings. From the search for heavy neutral lepton production in $\pi^+\rightarrow e^+N$ decays of beam pions, upper limits of the extended neutrino mixing matrix element $|U_{e4}|^2$ are established at the $10^{-8}$ level over the heavy neutral lepton mass range 95--126~MeV/$c^2$.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

NA62's new limits from expanded 2016-2024 data tighten constraints on low-mass portal models at the 10^{-11} and 10^{-8} levels, but the results rest on standard efficiency and background modeling that needs direct verification.

read the letter

The main takeaway is that NA62 has used its Run 1 and Run 2 data to push upper limits on K+ to pi+ X down to 10^{-11} and on |U_e4|^2 to 10^{-8} in the 95-126 MeV range, which does give better coverage for dark photon, scalar, ALP, and heavy neutral lepton scenarios at low masses than some other experiments provide right now. This is a straightforward extension of prior NA62 work rather than a new method or first observation. The paper applies the same decay-in-flight techniques to a larger sample and reports the limits clearly in the context of the four portal models. That part is useful incremental progress for the rare-decay community. The analysis is data-driven with no fitted parameters or circular claims, which keeps the central result clean. The non-observation translates directly into the quoted bounds once efficiencies are applied. The soft spot is exactly where the stress-test note points: the limits only hold if signal efficiencies for each model and background estimates in the missing-mass or kinematic windows are accurate. Any mismatch in resolution, particle ID, or unmodeled pile-up would loosen the constraints, and the abstract gives no numbers on those checks. If the full paper shows sideband validations and simulation cross-checks that look solid, the results are fine; without that, the numbers stay plausible but not yet confirmed at the quoted precision. This paper is for people who follow NA62 results or build models in the light new-physics sector. A reader who needs the latest experimental bounds on these portals will find the updated numbers worth checking. It deserves peer review because it reports fresh data from a running experiment that can actually be used in model scans.

Referee Report

1 major / 0 minor

Summary. The manuscript reports searches for the rare decays K⁺ → π⁺X and π⁺ → e⁺N using NA62 data collected in 2016–2022 and 2017–2024, respectively. Non-observation of signals is interpreted in the four portal models to set upper limits on the K⁺ → π⁺X branching ratio at the 10^{-11} level (constraining dark-photon, scalar, and ALP couplings) and on the heavy-neutral-lepton mixing parameter |U_{e4}|^2 at the 10^{-8} level for masses 95–126 MeV/c².

Significance. If the efficiency and background modeling are robust, the results provide competitive, data-driven constraints on beyond-Standard-Model scenarios that are directly relevant to current portal-model phenomenology. The use of a large in-flight decay sample from both Run 1 and Run 2 strengthens the experimental reach relative to earlier searches.

major comments (1)

[Analysis and Results sections] The central limits on BR(K⁺ → π⁺X) and |U_{e4}|^2 are obtained only after folding in signal efficiencies and background estimates that are model-dependent. The manuscript must demonstrate, with quantitative validation (e.g., control-sample studies or sideband fits), that missing-mass resolution, particle identification, and pile-up effects are correctly modeled for each portal scenario; any mismatch directly scales the quoted limits.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the positive assessment of its significance. We address the single major comment below.

read point-by-point responses

Referee: [Analysis and Results sections] The central limits on BR(K⁺ → π⁺X) and |U_{e4}|^2 are obtained only after folding in signal efficiencies and background estimates that are model-dependent. The manuscript must demonstrate, with quantitative validation (e.g., control-sample studies or sideband fits), that missing-mass resolution, particle identification, and pile-up effects are correctly modeled for each portal scenario; any mismatch directly scales the quoted limits.

Authors: We agree that explicit quantitative validation of the modeling is required to support the robustness of the limits. In the revised manuscript we have added dedicated subsections in the Analysis section that present control-sample studies for particle identification and sideband fits for background modeling. These studies demonstrate agreement between data and simulation for missing-mass resolution and pile-up effects over the relevant mass ranges. The validations are performed using data-driven methods that are independent of the specific portal-model assumptions; model-dependent signal efficiencies are applied only after the underlying detector response has been validated. Additional figures and tables have been included to quantify the level of agreement. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental upper limits are data-driven with no self-referential derivation.

full rationale

The paper reports direct experimental searches for K+→π+X and π+→e+N decays using NA62 data, setting upper limits on branching ratios and |U_e4|^2 from non-observation. These limits follow from counting observed events against modeled backgrounds and signal efficiencies, without any mathematical derivation, fitted parameters renamed as predictions, or load-bearing self-citations to uniqueness theorems. The central results are falsifiable against external data and do not reduce to inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

No free parameters or invented entities are introduced; the paper relies on standard assumptions of the Standard Model plus portal extensions.

axioms (2)

domain assumption Standard Model decay kinematics and detector response are correctly modeled.
Invoked to extract branching-ratio limits from observed events.
domain assumption New physics appears only through the four portal scenarios with no additional interactions.
Used to translate limits into coupling constraints.

pith-pipeline@v0.9.0 · 5483 in / 1269 out tokens · 39683 ms · 2026-05-10T17:15:03.572176+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

5 extracted references · 5 canonical work pages

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