Search for the deeply bound $K^-pp$ state from the semi-inclusive forward-neutron spectrum in the in-flight $K^-$ reaction on helium-3

TL;DR

This experiment searched for the $K^-pp$ bound state in in-flight $K^-$ reactions on helium-3 but found no significant signal, setting upper limits on its production cross section depending on assumed widths.

Contribution

First semi-inclusive search for $K^-pp$ in in-flight $K^-$ reactions on helium-3, providing new upper limits on production cross sections.

Findings

No significant peak observed in neutron missing-mass spectrum.

Upper limits on production cross section range from 30 to 270 μb/sr.

Results constrain the existence or production rate of the $K^-pp$ state.

Abstract

An experiment to search for the $K^-pp$ bound state was performed via the in-flight $^3$He($K^-,n)$ reaction using 5.3 $\times$ $10^9$ kaons at 1 GeV/$c$ at the J-PARC hadron experimental facility. In the semi-inclusive neutron missing-mass spectrum at $\theta_{n}^{lab}=0^\circ$, no significant peak was observed in the region corresponding to $K^-pp$ binding energy larger than 80 MeV, where a bump structure has been reported in the $\Lambda p$ final state in different reactions. Assuming the state to be isotropically decaying into $\Lambda p$, mass-dependent upper limits on the production cross section were determined to be 30--180, 70--250, and 100--270 $\mu$b/sr, for the natural widths of 20, 60, and 100 MeV, respectively, at 95\% confidence level.