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 leading twist. Difficulties arise when extracting $h_1^q(x)$ due to its chiral-odd nature. However, it can be coupled with a spin-dependent interference fragmentation function (FF), $H_1^{\sphericalangle, h_1h_2}$, in a dihadron ($h_1h_2$) production channel in polarized proton-proton ($p^\uparrow p$) collisions. The coupling of $h_1^{q}(x)$ and $H_1^{\sphericalangle, h_1h_2}$ produces an experimentally measurable azimuthal correlation asymmetry, $A_{UT}$, between the spin of the fragmenting quark and the final state dihadron. A model-independent extraction of transversity from these measurements relies on the knowledge of dihadron FFs, namely the unpolarized dihadron FFs, $D_1^{h_1h_2/q(g)}$ for quarks, \emph{q} (gluons, \emph{g}). Extraction of these FFs requires measurements of the unpolarized dihadron cross-section in $pp$ collisions, which are urgently needed. In $pp$ collisions, the unpolarized cross-section measurement provides access to the $D_1^{h_1h_2}$ for both quarks and gluons. This thesis outlines the measurements of the \dipion azimuthal correlation asymmetry in the forward ($\eta > 0$) and backward ($\eta < 0$) pseudorapidity regions with respect to the polarized beam using the RHIC Run 2015 polarized $pp$ data and the measurement of the unpolarized \dipion cross-section in the invariant mass bins in the mid-pseudorapidity ($|\eta|<1$) region using the RHIC Run 2012 $pp$ data at $\sqrt{s}=200$ GeV. These data sets were collected at the STAR experiment. The STAR Time Projection Chamber (TPC), Barrel Electromagnetic Calorimeter (BEMC), and Time-of-Flight Detector (TOF) were used in conjunction to measure outgoing particle energy, tracking, and identification. / Physics