Transverse Spin Dependent Azimuthal Correlations of Charged Pion Pairs in $p^{\uparrow}p$ Collisions at $\sqrt s = 200$ GeV at STAR

At leading twist, the transversity distribution function, $h^{q}_{1}(x)$, where $x$ is the longitudinal momentum fraction of the proton carried by quark $q$, encodes the transverse spin structure of the nucleon. Extraction of it is difficult because of its chiral-odd nature. In polarized proton-proton ($p^\uparrow p$) collisions, it can be coupled with a spin-dependent fragmentation function. This coupling leads to experimentally measurable oppositely charged hadron-pair (di-hadron) azimuthal correlations, $A_{UT}$, between the spin of the fragmenting quark and the final state di-hadron, which directly probes $h_{1}^{q}(x)$. The STAR experiment at RHIC has previously measured non-zero $A_{UT}$ for $\pi^+\pi^-$ pairs using $p^\uparrow p$ collisions at $\sqrt{s} = 200$ GeV from 2006, corresponding to an integrated luminosity, $L$, of $1.8\ \mathrm{pb^{-1}}$. In 2015, STAR collected $L\sim 48\ \mathrm{pb^{-1}}$ of $p^{\uparrow}p$ data at $\sqrt{s}=200$ GeV. This dataset provides highest precision $A_{UT}$ measurements at $\sqrt{s}=200$ GeV to date, which covers quark momentum fractions $0.1\textless x\textless 0.4$, sensitive to valence quark $h_{1}^{q}(x)$. We will present preliminary results on $A_{UT}$ for $\pi^+\pi^-$pairs based on this dataset.