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Study of neutral pion decays at the NA62 experiment at CERN

Periodic Reporting for period 1 - PIZERO_NA62 (Study of neutral pion decays at the NA62 experiment at CERN)

Reporting period: 2019-06-01 to 2021-05-31

The Standard Model (SM) of particle physics represents our current understanding of physical laws at subnuclear scales. The SM very successfully describes the interactions of elementary particles, but arguably, it is not the final theory of particle physics. For example, it does not provide any dark matter candidate with properties compatible with astrophysical observations; it also does not explain non-zero neutrino masses. The SM is not entirely satisfactory from the theoretical point of view either; it lacks a description of gravity, and it does not explain the stabilization of the electroweak scale with respect to higher energy scales (the hierarchy problem of the SM). Therefore the primary goal of contemporary particle physics is the search for physics beyond the SM.

Direct searches at experiments such as ATLAS and CMS at CERN are trying to observe new heavy states produced in the collisions of high-energy protons. No signal of direct production of new particles has been observed in these experiments so far. Other experiments are trying to find deviations from the SM by performing high precision measurements at lower energies. The presence of new particles or interactions could be revealed indirectly through virtual quantum corrections. One of the most important experiments at the high precision frontier that look for new physics is the NA62 experiment representing the kaon physics research program at CERN.

The PIZERO_NA62 project has studied neutral pion decays at the NA62 experiment at CERN using the data sample collected by the experiment in the years 2016--2018. The neutral pion is the lightest meson that plays a crucial role in the study of the low-energy properties of the strong nuclear force. The project's main objective is a new measurement of the branching fraction of the very rare decay of neutral pion into an electron-positron pair. This observable represents a direct probe of the Standard Model of particle physics.
In the first part of the project implementation, Monte-Carlo (MC) samples of neutral pion decays were studied, and basic selection criteria for signal decay modes were developed. The selection procedures were then optimized to boost the signal acceptance by employing a flexible particle identification algorithm that relies on the specific kinematics of the studied pion decays.

The MC samples were compared to the data collected by the NA62 experiment. It was found that the simulation of the NA62 Straw spectrometer was not satisfactory and did not reproduce the features observed in the data. Therefore, a significant update of the MC simulation of the Straw detector has been carried. The update improved the agreement between the simulation and the experimental data.

The trigger chain used to collect studied neutral pion decays comprises both a hardware di-electron trigger and a software trigger identifying multi-track events. A minimum bias trigger, collected in parallel to the physics triggers, was used to evaluate the time-dependent efficiencies of various signal trigger components.

The central part of the analysis focused on the background processes that can contribute to the signal samples. Apart from the expected processes originating from misidentification of other pion and charged kaon decays, an additional important background process was identified. This process involves a photon conversion into an electron-positron pair in the Straw detector material. A dedicated biasing of the MC simulation was implemented to study this process in detail and provide precise background estimates for the final measurement.

By studying the data set collected by the NA62 experiment, it was estimated that the sample size of pion decays into an electron-positron pair is comparable to the previous best measurement by the KTeV experiment. The data analysis is in the state of finalization, and its results will be published soon.
The new measurement of the neutral pion decay rate into an electron-positron pair aims to resolve the existing tension between the SM theoretical prediction and the latest experimental result by the experiment KTeV. Theoretical modeling of neutral pion decays is of great importance since it enters theoretical computations of the muon's anomalous magnetic moment, which exhibits a significant disagreement between the theoretical predictions and the latest experimental results by the Muon g-2 experiment at Fermilab.

The improvements and studies performed in this project also contributed to other publications by the NA62 collaboration.

The preparation of the final publication is in progress, and the results are expected to be published soon.
photo of the NA62 experiment
diagram of the studied decay of a neutral pion into an electron and a positron
logo of the NA62 experiment