Primordial Partonic kt

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This is one of the "golden measurements" planned for eRHIC, with emphasis on gluonic kT especially in eA, since a big change in gluonic kT from ep to eA could be evidence of parton saturation (CGC) effects in the nucleus.

Another interesting point is that various model codes, including PYTHIA, have increased primordial kT for higher energy processes. The DEFAULT rms kT in PYTHIA 6.4 is 2 GeV! The model authors point out that most hadron-hadron observables and even some DIS observables don't really do a good job at distinguishing between pT coming from QCD and that coming from primordial kT so this may very well just be a funny way to parameterize higher order QCD effects that haven't been handled ideally. But such a tune will have observable consequences at eRHIC where we should be able to scan a W region corresponding to a range from 0.5 to 2 GeV in expected "effective kT".

Using ep and eA collisions we should have the sensitivity to make the distinction between primordial kT and QCD effects and to measure the difference between ep and eA for all partons as well as for tagged gluonic events.

White Gold Measurement: Di-hadron correlations in eA, ep, dA etc.

This is the measurement which has been discussed in the various white papers and which is the focus of much theoretical and simulation work. For a recent summary see Liang Zhang's talk at the RIKEN/BNL Forward Physics Workshop (July/August 2012). It has a lot of theoretical interest behind it and has the virtue that pp and pA (or dA) measurements can be made now and we don't have to wait for the more incisive ep and eA measurements.

Gamma + jet measurements

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Seagull Plots and Pt-Compensation Plots

Seagull plots refer to <p²T> vs. xF. Larger primordial partonic kT leads to a symmetric increase at large |xF|, while both soft and hard QCD contribute primarily forward and fragmentation effects contribute independent of xF. Presumably Intra-nuclear cascading in eA will also contribute modestly at negative xF.

Pt-compensation plots consider a trigger particle and define that particle's pT orientation as +x. It then plots <px> vs. y* (rapidity in hadronic cm frame along virtual photon axis). Regions of y* where the pT is being compensated then have a negative value of <px>. EMC showed that if you trigger very forward or very backward, then large primordial kT would have shown up in a negative-going peak in the opposite hemisphere, while QCD effects were more spread out. INC effects should be local and won't correlate positive and negative hemispheres.

These plots have a long history and the EMC versions were used to tune the kT in LEPTO. See for instance the classic paper: PUT CITATION & LINK HERE. Taken together, the EMC results supported a modest contribution from kT and from QCD at s=(20GeV)². Perhaps more importantly, at least for eRHIC purposes, they showed that these methods were very sensitive to the difference between primordial kT and radiative QCD effects.

Mark Baker put together some Notes on 7/5/2012 summarizing these ideas.

AQs: Asked Questions (with answers)

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Open Questions

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Cahn Effect

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