Exploring a direct measurement of quark energy loss using semi-inclusive deep inelastic scattering
C. I. Peña 1*, W. Brooks 1, H. Hakobyan 1, M. Arratia 1
1 Departamento de Fisica, Universidad Técnica Federico Santa María
Quantum Chromodynamics (QCD), the theory for explaining the interactions of quarks and gluons has been successfully developed and tested over the last two decades in the high energy region where the strong coupling constant is sufficiently small such that perturbation theory can be employed. Further successes indirectly attributable to QCD have been acquired through the use of effective fields theories in the low-energy, non-perturbative regime. An increasing trend within the field of nuclear physics has been to attempt to relate observables in hadronic and nuclear systems with the QCD theory, which is supposed to underlie their behavior. In spite of the successfully application of QCD over a wide range of phenomena, the space-time characteristics of the hadronization process is still poorly understood, due to the very few constraints provided by experimental data.
The aim of these studies is to explore a direct measurement of the quark energy loss using semi-inclusive deep inelastic scattering (DIS) in nuclear matter. We use the data acquired by the EG2 experiment at Jefferson Laboratory in USA. The EG2 experiment studied high-energy electrons scattering off a series of nuclei such as D, C, Fe, Pb. Outgoing hadrons in the final state were detected and analyzed with CLAS detector which is capable of detecting multiple particles with precise momentum resolution and good particle identification.
We analyze the data in DIS kinematics where the virtual photon interacts with a single valance quark inside a nucleon. Afterwards, the struck quark traverses the nuclear medium, while propagating the quark undergoes multiple scattering and radiates gluons, this process will subsequently cause a visible energy loss and transverse momentum broadening (PT-broadening) measurable for the final state hadron. Looking at the outgoing positive pion energy distribution for different nuclei compared with deuterium and relying on the fact that in most of our events the struck quark traverses a path below certain critical length (Lc) where according to BDMPS the parton energy loss rate is independent of the parton energy we expect to find a simple energy shift in the energy distribution of pions created using each of the nuclei compared to ones using deuterium. The procedure is developed and tested for pions with energy between 1.5-2.5 GeV which is half of the available pion energy spectrum using explicit Feynman x cuts and implicit Zh cuts. The results are compared with PT-broadening results obtained in a complete different way and both seem to be in agreement according to the pQCD relation in which -dE/dx is proportional to p2T (see figure 1).
Finally, this method has space for further improvements which will reduce errors. Despite of the fact that these results should be compared with simulation that includes nuclear cascade models in order to rule out hadronic interaction effects. The results obtained are promising and hopefully are a real contribution for a better understanding of space-time characteristics of the hadronization process.
Quark Energy Loss: blue data points correspond to quark energy loss data. Red line correspond to a fit to the data points.