Predictions from a Simple Hadron Rescattering Model for pp Collisions at the LHC

With studies of heavy ion and pp physics already under way at the LHC, it is necessary to consider how hadron rescattering will effect the observed results from experiments such as ALICE, ATLAS and CMS. Through the use of a simple, relativistic kinematics based hadron rescattering model, this dissertation shows that the hadron rescattering phase can obscure some signals for radial flow in pp collisions at LHC energies.

This dissertation presents an in depth description of the hardware based alignment monitoring system developed for the ALICE Inner Tracking System. It details the development of the ITSAMS, which uses geometric optics and a CMOS array to measure micron scale motion between two points. By monitoring three strategic points on the ITS in relation to the TPC endplate, the ITSAMS can determine translational shifts between the two detectors to a resolution of 9.4 µm in the transverse plane and 78 um along the longitudinal axis. The ITSAMS can measure rotational shifts to 10 urad or better about all three axes.

After a brief discussion of the ALICE experiment and the theory and practice of two-particle intensity interferometry, this dissertation details a simple hadron rescattering computer model developed by Dr. T. J. Humanic. The process of porting the model to the C++ computer language is presented here, along with the improvements made. The model has been updated with a new space-time distribution scheme that is more appropriate for pp collision studies.

The model is then compared with final-state PYTHIA generated Monte-Carlo data. It is shown that the hadron rescattering model accurately reproduces pseudorapidity distributions for pp collisions at sqrt(s) = 0.9, 7, 10, and 14 TeV. Moreover, except for a slight overprediction of kaons and a slight underprediction of protons, the rescattering model accurately reproduces PYTHIA p_T spectra.

This dissertation then endeavours compare results to the HBT radii present in the ALICE collaboration's analysis of several hundred thousand pp collisions. It is shown that after tuning the model, there is very good agreement between the rescattering model and ALICE data in terms of multiplicity dependence of 1D HBT radii. There is also considerable agreement between the two on the k_T dependence of 1D HBT radii, although the model does tend to slightly overpredict radius sizes at larger k_T values.

Finally, this dissertation makes predictions for 1D and 3D identical pion, kaon, and proton correlation functions from pp collisions using the rescattering model for sqrt(s) = 0.9, 7, 10, and 14 TeV. These correlation functions are binned by multiplicity and k_T, both separately and together, and by m_T. Fits to these correlation functions indicate that any k_T signal of radial flow in pp collisions would be largely obscured by rescattering in the post hadronization phase. However, they also indicate that m_T scaling would still provide a clear indication for radial flow without obfuscation from hadron rescattering.