6533b852fe1ef96bd12aabbb

RESEARCH PRODUCT

Hindered M1 radiative decay ofϒ(2S)from lattice NRQCD

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We present a calculation of the hindered M1 $\mathrm{\ensuremath{\Upsilon}}(2S)\ensuremath{\rightarrow}{\ensuremath{\eta}}_{b}(1S)\ensuremath{\gamma}$ decay rate using lattice nonrelativistic quantum chromodynamics. The calculation includes spin-dependent relativistic corrections to the NRQCD action through $\mathcal{O}({v}^{6})$ in the quark's relative velocity, relativistic corrections to the leading order current which mediates the transition through the quark's magnetic moment, radiative corrections to the leading spin-magnetic coupling and for the first time a full error budget. We also use gluon field ensembles at multiple lattice spacing values, all of which include $u$, $d$, $s$ and $c$ quark vacuum polarization. Our result for the branching fraction is $\mathcal{B}(\mathrm{\ensuremath{\Upsilon}}(2S)\ensuremath{\rightarrow}{\ensuremath{\eta}}_{b}(1S)\ensuremath{\gamma})=5.4(1.8)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, which agrees with the current experimental value.