# Missing-mass spectroscopy with the ${}^{6}$Li($\pi^{-}, K^{+}$)X reaction to search for ${}^{6}_{\Lambda}$H

## TL;DR

This study conducted a high-statistics search for the neutron-rich hypernucleus ${}^{6}_{ ext{Lambda}}$H using the ${}^{6}$Li($ ext{pi}^{-}, K^{+}$)X reaction at J-PARC, but found no evidence of its bound state, setting an upper limit on its production cross section.

## Contribution

First high-statistics search for ${}^{6}_{ ext{Lambda}}$H via the ${}^{6}$Li($ ext{pi}^{-}, K^{+}$)X reaction, providing new constraints on its existence and production cross section.

## Key findings

- No bound state of ${}^{6}_{ ext{Lambda}}$H observed.
- Upper limit of 0.56 nb/sr for production cross section.
- Detailed missing-mass spectrum including continuum regions.

## Abstract

We searched for the bound state of the neutron-rich $\Lambda$-hypernucleus ${}^{6}_{\Lambda}$H, using the ${}^{6}$Li($\pi^{-}, K^{+}$)X double charge-exchange reaction at a $\pi^{-}$ beam momentum of 1.2 GeV/c at J-PARC. A total of $1.4 \times 10^{12}$ $\pi^{-}$ was driven onto a ${}^{6}$Li target of 3.5-g/$\rm cm^2$ thickness. No event was observed below the bound threshold, i.e., the mass of ${}^{4}_{\Lambda}$H + 2n, in the missing-mass spectrum of the ${}^{6}$Li($\pi^{-}, K^{+}$)X reaction in the $2^{\circ}$ < $\theta_{\pi K}$ < $20^{\circ}$ angular range. Furthermore, no event was found up to 2.8 MeV/$c^2$ above the bound threshold. We obtained the the double-differential cross section spectra of the ${}^{6}$Li($\pi^{-}, K^{+}$)X reaction in the angular range of $2^{\circ}$ < $\theta_{\pi K}$ < $14^{\circ}$. An upper limit of 0.56 nb/sr (90% C.L.) was obtained for the production cross section of the ${}^{6}_{\Lambda}$H hypernucleus bound state. In addition, not only the bound state region, but also the $\Lambda$ continuum region and part of the $\Sigma^{-}$ quasi-free production region of the ${}^{6}$Li($\pi^{-}, K^{+}$)X reaction, were obtained with high statistics. The present missing-mass spectrum will facilitate the investigation of the $\Sigma^{-}$-nucleus optical potential for $\Sigma^{-}$-${}^{5}$He through spectrum shape analysis.