Hadronization of pions and kaons from nuclei using DIS
by Kenneth Hicks (for the CLAS Collaboration)

The CLAS experiment called eg2 was run with a variety of nuclear 
targets using a 5.5 GeV electron beam from the 
continuous electron beam accelerator facility (CEBAF) at Jefferson Lab.
The goal of this experiment is to measure observables related to 
the propagation of a quark (struck by the virtual photon from 
deep inelastic scattering--DIS) through cold nuclear matter. These 
results could be contrasted with quark propagation through hot 
QCD matter, sometimes called the quark-gluon plasma, formed in 
relativistic heavy ion collisions (RHIC). Quarks from RHIC are 
tagged by back-to-back ``jets" of high energy hadrons formed by 
hard quark-quark collisions.  In contrast with RHIC results, where 
the quark propagation is severely damped by passage through hot 
QCD matter, the preliminary DIS results suggest that the struck quark 
propagates relatively freely (compared with fully formed mesons) 
through cold nuclear matter.  

The measured quantities of the DIS measurements are the squared 
four-momentum transfer ($Q^2$) and energy transfer ($\nu$) from 
the scattered electron and the energy fraction ($z = E_h/\nu$) 
and transverse momentum ($p_T$) of the leading hadron.  By comparing 
the $z$ and $p_T$ distributions of pions and kaons from a deuterium 
target with that from various nuclear targets, the quark propagation 
and formation into a hadron can be inferred.  Theoretical predictions 
in a given model provide guidance for interpretation of the results. 
Ratios of the number of hadrons detected, normalized to the number 
of DIS events in a given kinematic bin of $Q^2$ and $\nu$, for a 
nuclear target divided by the same quantities for a deuterium target 
are one quantitative measure of the effects of propagation through 
cold nuclear matter.  The subtraction of the average value of $p_T^2$ 
seen for a nuclear target minus that for a deuterium target is 
another measure which, according to theoretical models, is sensitive 
to gluonic radiation by the quark before it forms into a hadron. 
By comparing these two quantitative measures with theoretical 
calculations over a variety of kinematics, then models of hadronization 
can be tested.  The HERMES experiment has already published the 
nuclear attenuation ratios for several types of hadrons at DIS 
kinematics in the range $8<\nu<23$ GeV.  The CLAS data are for a 
lower range, with $2.6<\nu<4.3$ GeV corresponding to shorter 
formation times.  At HERMES kinematics, calculations suggest that 
the quark propagates through the full nucleus, followed by hadron 
formation outside of the nuclear radius.  At CLAS, we expect to 
see hadron formation inside the radius of larger nuclei.

Preliminary results for positive pions produced at CLAS for the 
eg2 experiment indicate a plateau for the difference $\Delta p_T^2$ 
as the nuclear size increases (targets C to Pb).  This suggests 
that, at these kinematics, the average hadron formation length 
is shorter than the radius of Pb.  In contrast, nuclear attenuation 
ratios cannot be explained by theoretical models unless some 
finite propagation of a "pre-hadron" (with a smaller cross section 
than fully-formed hadrons) is assumed.  Attenuations ratios for 
neutral kaons appear to be smaller than those for $\pi^+$ particles, 
but results are still preliminary.

In conclusion, the CLAS data provide a new kinematic window for 
hadronization at the expected length scale near to the radius of 
heavy nuclei.  Statistical uncertainties are small due to an 
experimental luminosity of about 100 more than for the HERMES data. 
Both $\pi^+$ and $K_s^0$ nuclear attenuation ratios and $\Delta p_T^2$ 
are expected to be submitted for publication before the end of 2008.

