Nicholas Hunt-Smith - Honours Project 2019

Reformulating Tension in the Lund String Model of Hadronisation

The work I performed in my Honours reserach project centred on the field of non-perturbative quantum chromodynamics (QCD) within particle physics, specifically the Lund string fragmentation model of hadronization.

Hadronization is an essential stage of high-energy particle collisions that describes how quarks and gluons come to produce composite particles that can be measured by a detector. The Lund model is one of the most ubiquitous models of hadronization, able to describe a wide range of experimental results through its implementation in the state-of-the-art PYTHIA Monte Carlo event generator.

However, the Lund model is currently unable to account for the apparent existence of collective effects in pp collisions at the LHC, such as the presence of strangeness enhancements and flow-like effects. This is due to an underlying assumption that the fragmentation of a string stretched between any given qqbar pair does not depend on other nearby strings.

Theoretical attempts to modify the Lund model have primarily focused on differentiating the case of multiple strings in pp collisions from the case of a single string such as in hadronic Z decays, but given this newfound reevaluation of the Lund model it is worth considering if there is a more fundamental aspect of single string dynamics that is not being captured.

In particular, it was recently demonstrated that different regions of a string are necessarily entangled, resulting in a significant entropy with an associated temperature that depends inversely on proper time as T ∝ 1/τ. We interpreted this concept within the Lund model by introducing a modified string tension that depends inversely on proper time, and reduces to a constant at late times in accordance with lattice QCD observations. Any such modifications to the case of a single string are subject to the constraints of archival e+e- results from LEP, so we compared the predictions of our model to these measurements via PYTHIA. The bulk of the work I did during my project consisted of modifying the normal C++ PYTHIA code to correctly implement this time-dependent string tension, before analysing various parameterisations of the model in order to develop meaningful results. We found that our model remained consistent with the existing experimental data, while introducing interesting differences for certain quantities that have not yet been analysed in the context of e+e- collisions. Specifically, there were clear variations in average transverse momentum (pT) and strange hadron yields as a function of charged multiplicity compared to baseline PYTHIA. Following my Honours year I continued to work on this research, culminating in a paper that has been accepted for publication with the European Physical Journal C.

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