Abstract
We survey the current phenomenological status of semi-inclusive deep inelastic scattering at moderate hard scales and in the limit of very large transverse momentum. As the transverse momentum becomes comparable to or larger than the overall hard scale, the differential cross sections should be calculable with fixed order pQCD methods, while small transverse momentum (TMD factorization) approximations should eventually break down. We find large disagreement between HERMES and COMPASS data and fixed order calculations done with modern parton densities, even in regions of kinematics where such calculations should be expected to be very accurate. Possible interpretations are suggested.
Highlights
Transverse momentum spectra are of theoretical interest for many reasons, and processes with an electromagnetic hard scale like Drell-Yan scattering (DY) and semiinclusive deeply inelastic scattering (SIDIS) are ideal clean probes of the underlying hadronic correlation functions
We will focus on SIDIS, lðlÞ þ ProtonðPÞ → l0ðl0Þ þ HadronðPHÞ þ X; wherein a single identified hadron with momentum PH is observed in the final state
We have argued that there is tension between existing fixed order perturbative QCD (pQCD) calculations and at least two sets of large transverse momentum measurements where those calculations should be reasonably accurate and that this disagreement is too large to be attributable to qT being too small
Summary
Transverse momentum spectra are of theoretical interest for many reasons, and processes with an electromagnetic hard scale like Drell-Yan scattering (DY) and semiinclusive deeply inelastic scattering (SIDIS) are ideal clean probes of the underlying hadronic correlation functions. Most especially, identifying properties of transverse momentum dependence that are truly intrinsic to specific hadrons requires that they be disentangled from those that are generated in processspecific hard collisions This can be delicate, especially at the smaller values of Q typical of SIDIS experiments, because the three regions enumerated above begin to be squeezed into an increasingly small range of qT Understanding the transition to the qT ∼ Q region is important for clarifying the general nature of transverse momentum dependence in processes like SIDIS, especially for more moderate values of Q where the transition between regions is not obvious These are highly constrained calculations since the only input objects that involve prior fitting—the PDFs and FFs—are those taken from collinear factorization.
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