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, $h_1^{q}(x)$, encapsulates the transverse spin structure of the proton at the leading twist, where $x$ represents the longitudinal momentum fraction carried by the quark $q$. Extracting $h_1^{q}(x)$ poses a formidable challenge due to its chiral-odd nature. Measurements of final-state dihadron pairs in transversely polarized proton-proton ($p^uparrow p$) collisions directly probe the collinear quark transversity through its coupling with a chiral-odd interference fragmentation function (IFF), $H_1^{sphericalangle, q}$. This coupling leads to an experimentally measurable azimuthal correlation asymmetry, $A_{UT}$.

Therefore, precise knowledge of the IFF and unpolarized dihadron fragmentation functions are necessary to achieve a model-independent extraction of $h_1^q(x)$ from these measurements. Obtaining these fragmentation functions requires measuring the unpolarized dihadron cross section in $pp$ collisions, which is crucial for constraining the gluon fragmentation function. In this presentation, we will share preliminary results on $A_{UT}$ using $p^uparrow p$ data collected in 2015, and the unpolarized cross section using $pp$ data collected in 2012 for $pi^+pi^-$ pairs at $sqrt{s} = 200$ GeV by the STAR experiment.