Measurements of Transverse Spin Dependent $\pi^+\pi^-$ Azimuthal Correlation Asymmetry and Unpolarized $\pi^+\pi^-$ Cross Section in $pp$ Collisions at $\sqrt s = 200$ GeV at STAR

The transversity distribution function, $h_1^{q}(x)$, where $x$ is the longitudinal momentum fraction of the proton carried by quark $q$, encodes the proton's transverse spin structure at the leading twist. Extraction of it is difficult because of its chiral-odd nature. However, it can be coupled with a spin-dependent interference fragmentation function (FF), $H_1^{\sphericalangle, q}$, in polarized proton-proton ($p^\uparrow p$) collisions. The coupling of $h_1^{q}(x)$ and $H_1^{\sphericalangle, q}$ produces an experimentally measurable azimuthal correlation asymmetry, $A_{UT}$, between the spin of the fragmenting quark and the final state di-hadron. A model-independent extraction of transversity from these measurements relies on the knowledge of di-hadron FFs, namely the unpolarized di-hadron FFs. Extraction of these FFs requires measurements of the unpolarized di-hadron cross section in $pp$ collisions, which are desperately needed. We will present preliminary results on $A_{UT}$ for $\pi^+\pi^-$ pairs with $p^\uparrow p$ data at $\sqrt{s} = 200$ GeV taken in 2015, as well as an update on the unpolarized $\pi^+\pi^-$ cross-section measurement with the $pp$ data at $\sqrt{s} = 200$ GeV taken in 2012, at the STAR experiment.