  A precise measurement of the hypertriton lifetime is presented. In this
letter, the mesonic decay modes $\mathrm{{^3_\Lambda}H \rightarrow ^3He +
\pi^-}$ and $\mathrm{{^3_\Lambda}H \rightarrow d + p + \pi^-}$ are used to
reconstruct the hypertriton from Au+Au collision data collected by the STAR
collaboration at RHIC. A minimum $\chi^2$ estimation is used to determine the
lifetime of $\tau = 142^{+24}_{-21}\,{\rm (stat.)} {\pm} 31\,{\rm (syst.)}$ ps.
This lifetime is about 50\% shorter than the lifetime $\tau = 263\pm2$ ps of a
free $\Lambda$, indicating strong hyperon-nucleon interaction in the
hypernucleus system. The branching ratios of the mesonic decay channels are
also determined to satisfy B.R.$_{(^3{\rm He}+\pi^-)}/$(B.R.$_{(^3{\rm
He}+\pi^-)}+$B.R.$_{(d+p+\pi^-)})$ = $0.32\rm{\pm}0.05\,{\rm (stat.)}\pm
0.08\,{\rm (syst.)}$. Our ratio result favors the assignment
$J(\mathrm{^{3}_{\Lambda}H})$ = $\frac{1}{2}$ over
$J(\mathrm{^{3}_{\Lambda}H})$ = $\frac{3}{2}$. These measurements will help to
constrain models of hyperon-baryon interactions.
