18 Chapter 2: Motivation Figure 2.3: HBT parameters [109, 110] compared to various hydrodynamics cal- culations [111–113]; none describe the data well. Figure adapted from [77]. hadronization is in the low- p$_{T}$ sector, and there is no reason to believe that experimental measurements in, say, p + p collisions can provide independent information on the process. Similarly the low momenta involved makes it non- perturbative; there is little theoretical insight, either, although recombination [117, 118] and quark coalescence [119, 120] are two proposed mechanisms for hadronization. More sophisticated 3 + 1 D hydrodynamics treatments that in- clude hadronic rescattering (a so-called hadronic ‘afterburner’) have had good success in reproducing both single particle spectra and azimuthal anisotropy