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Primordial $k_{\perp}$

It is customary to assign a primordial transverse momentum to the shower initiator, to take into account the motion of quarks inside the original hadron, basically as required by the uncertainty principle. A number of the order of $\langle k_{\perp}\rangle \approx m_{\mathrm{p}}/3 \approx 300$ MeV could therefore be expected. However, in hadronic collisions much higher numbers than that are often required to describe data, typically of the order of 1 GeV at fixed-target energies[EMC87] and 2 GeV at collider energies [Miu99,Bál01], if a Gaussian parameterization is used. Thus, an interpretation as a purely nonperturbative motion inside a hadron is difficult to maintain.

Instead a likely culprit is the initial-state shower algorithm. This is set up to cover the region of hard emissions, but may miss out on some of the softer activity, which inherently borders on nonperturbative physics. By default, the shower does not evolve down to scales below $Q_0 = 1$ GeV. Any shortfall in shower activity around or below this cutoff then has to be compensated by the primordial $k_{\perp}$ source, which thereby largely loses its original meaning. One specific reason for such a shortfall is that the current initial-state shower algorithm does not include non-ordered emissions in $Q^2$, as is predicted to occur especially at small $x$ and $Q^2$ within the BFKL/CCFM framework [Lip76,Cia87].


next up previous contents
Next: Remnant Kinematics Up: Beam Remnants Previous: Lepton Beams   Contents
Stephen_Mrenna 2012-10-24