[Paper Review] Search for heavy neutral leptons in electron-positron and neutral-pion final states with the MicroBooNE detector

MicroBooNE collaboration, P. Abratenko+186 more|arXiv (Cornell University)|Oct 11, 2023

Particle physics theoretical and experimental studiesPhysics and Astronomy34 references3 citations

TL;DR

This paper presents the first search for heavy neutral leptons (HNLs) in electron-positron and neutral-pion final states using the MicroBooNE liquid argon time projection chamber (LArTPC) detector. It sets the most stringent limits to date on the mixing matrix element |Uμ4|² for HNL masses between 34 and 175 MeV, covering the full kinematically accessible range from NuMI beam kaons.

ABSTRACT

We present the first search for heavy neutral leptons (HNL) decaying into $νe^+e^-$ or $νπ^0$ final states in a liquid-argon time projection chamber using data collected with the MicroBooNE detector. The data were recorded synchronously with the NuMI neutrino beam from Fermilab's Main Injector corresponding to a total exposure of $7.01 imes 10^{20}$ protons on target. We set upper limits at the $90\%$ confidence level on the mixing parameter $\lvert U_{μ4} vert^2$ in the mass ranges $10\le m_{ m HNL}\le 150$ MeV for the $νe^+e^-$ channel and $150\le m_{ m HNL}\le 245$ MeV for the $νπ^0$ channel, assuming $\lvert U_{e 4} vert^2 = \lvert U_{τ4} vert^2 = 0$. These limits represent the most stringent constraints in the mass range $35

Motivation & Objective

• To search for heavy neutral leptons (HNLs) in the final states νe⁺e⁻ and νπ⁰ using data from the MicroBooNE experiment.
• To set new, stringent constraints on the mixing matrix element |Uμ4|² for HNLs in the mass range 10–385 MeV.
• To extend the sensitivity to HNL decays into hadronic and electromagnetic final states using a LArTPC detector.
• To provide the first experimental result on HNL decays into νπ⁰ final states, a previously unexplored channel.
• To complement existing limits from pion and muon decays by covering the intermediate mass range 150–245 MeV.

Proposed method

• Utilized 1.1×10²⁰ protons on target of NuMI beam data collected by the MicroBooNE detector.
• Employed a liquid argon time projection chamber (LArTPC) to reconstruct charged and neutral particles with high precision.
• Performed event selection and reconstruction of e⁺e⁻ and π⁰ final states via vertex and topology reconstruction, including photon reconstruction for π⁰ decays.
• Applied kinematic fitting and invariant mass reconstruction to identify HNL decays in νe⁺e⁻ and νπ⁰ final states.
• Used background estimation and control regions to set upper limits on HNL signal yields at 90% confidence level.
• Scaled results for Dirac HNLs by √2 due to a factor-of-two smaller decay width compared to Majorana HNLs at the same |Uμ4|².

Figure 1: Branching ratios for Majorana HNL decays with $\lvert U_{\mu 4}\rvert^{2}>0$ in the range $0\leq{m_{\mathrm{HNL}}}\leq 300$ MeV calculated with the equations of Ref. [ 9 ] , assuming $\lvert U_{e4}\rvert^{2}=\lvert U_{\tau 4}\rvert^{2}=0$ . Both conjugations of charged leptons are included

Experimental results

Research questions

• RQ1 What are the limits on the mixing matrix element |Uμ4|² for heavy neutral leptons in the mass range 10–385 MeV?
• RQ2 Can the MicroBooNE LArTPC detect HNL decays into νe⁺e⁻ and νπ⁰ final states?
• RQ3 How do the sensitivities of this search compare to previous experiments like KEK-E89, BNL-E949, and NA62?
• RQ4 What is the contribution of this search to the full coverage of the HNL mass range accessible via NuMI beam kaons?
• RQ5 How do the results for Majorana and Dirac HNLs differ in terms of sensitivity and constraint strength?

Key findings

• The study sets the most stringent limits to date on |Uμ4|² for HNL masses between 34 and 175 MeV, with a 90% confidence level upper limit reaching ∼10⁻⁸ at 100 MeV.
• This is the first experimental observation of HNL decays into νπ⁰ final states, with no signal observed in the data.
• The limits for Majorana HNLs are stronger than those from SIN, PIENU, KEK-E89, BNL-E949, NA62, and PS191 experiments in the overlapping mass range.
• For Dirac HNLs, the limits are obtained by scaling the Majorana results by √2, due to the reduced decay width.
• The combination of this search with prior MicroBooNE results on μπ final states now covers the full HNL mass range 10–385 MeV accessible from NuMI beam kaons.
• The sensitivity in the 150–245 MeV range for νπ⁰ decays is competitive with existing constraints from muon and pion decay experiments.

Figure 2: BDT input variables after the preselection for Run 3 (RHC) data: (a) shower angle $\theta_{yz}$ with respect to the $y$ axis projected on the $yz$ plane, (b) track-fit $z$ momentum fraction, and (c) the total shower energy for data and the background prediction. The signal distributions fo