Observation of Polarization in Bottomonium Production at√s=38.8GeV

We present a measurement of the polarization observed for bottomonium states produced in $p-\mathrm{Cu}$ collisions at $\sqrt{s}=38.8\mathrm{GeV}$. The angular distribution of the decay dimuons of the $\Upsilon \left(1S\right)$ state shows no polarization at small values of the fractional longitudinal momentum $x_F$ and transverse momentum $p_T$ but significant positive transverse production polarization for either $p_T>1.8\mathrm{GeV}/c$ or for $x_F>0.35\mathrm{}$. The $\Upsilon (2S+3S)$ (unresolved) states show a large transverse production polarization at all values of $x_F$ and $p_T$ measured. These observations challenge NRQCD calculations of the polarization expected in the hadronic production of bottomonium states.