PhD defence by Darius Keijdener 'Azimuthal Differences of Quenched Jets'

On September 1st 2023, Darius Keijdener has successfully defended his thesis. Darius Keijdener studied Theoretical Physics at Utrecht University. He accomplished his PhD at the Institute for Gravitational and Subatomic Physics (GRASP), ALICE Group. The defense has taken place in the Academiegebouw in Utrecht.

Summary PhD thesis

Azimuthal Differences of Quenched Jets

The study of high-energy partons in heavy ion collisions and proton collisions can provide us with an insight into the strong interaction. These products of hard collisions fragment and hadronize in a parton shower, so-called jets. In heavy ion collision the parton shower is modified in a process called quenching, wherein the parton and its products are interacting with a Quark Gluon Plasma (QGP). In this process the parton shower transfers energy to the QGP, on average ‘losing’ energy, changing the geometric shape and energy profile. Intuitively one would expect the quenching effects to scale with the path length in the medium. A possible observable which could measure this path length dependence uses the naturally occurring azimuthal differences in non-central heavy ion collisions. The elliptic flow observed in these events is linked to the second order event plane. Whether jets are in the direction of this event plane or perpendicular to it influences possibly the amount of quenching.

In this work the difference between jets was studied depending on their angle to the event plane for three different equally large bins. This was done with the help of the study of correlations between pairs of hadrons. When the angles between them are small there is a contribution that can be attributed to jets, even though at low energies a sizeable background is present as well. This is subtracted with a model that connects the combinatorial background in the three plane-bins. Special attention was paid to the roll of the correlations between event planes. This model was tested in a Toy Monte Carlo model where a bootstrap was used to compute the average value and the errors. This procedure is necessary to establish the correct uncertainties, and is able to reproduce the input parameters reliably. 

Then the same background model was used in an analysis of the 2011 ALICE data of Lead-Lead collisions. Here several signs are present that the plane dependent trigger background fit (PDF) model does not perform as well as expected on basis of the Toy Monte Carlo Model, especially at lower transverse momenta of the associated particle (pT ,a). Several cross checks have been performed to see whether this difference could be explained within the confines of the PDF model by changing the event plane correlation or resolution parameters. But results for the near-side jet peak are not consistent with the more reliable method of large ∆η (LDE) subtraction, except for pT ,a > 3 GeV/c on the near-side and pT ,a > 4 GeV/c on the away side. Since an analysis of the away-side is not possible with the LDE model, and the PDF model is not suitable for the high momentum range where statistics is low, a model based on a Fourier analysis of the near-side range is introduced. This reproduces different results as well in the low momentum region. For all these models the Iplane parameters are introduced, a fraction of the associated particle yield from a trigger that has a specific angle to the event plane through a associated particle yield from a trigger without selection. A near-identical analysis is performed in a AMPT Monte Carlo model, where the same inconsistencies are present for the lower pT ,a.

For both the AMPT Monte Carlo and the ALICE data we can conclude that the LDE results are mostly in between the Fourier and the PDF results. The differences between these methods are significant with respect to the statistical uncertainty. At high pT ,a no difference could be found within the current experimental and theoretical constraints. The expected effect of the path length dependence in the AMPT model is also small, and not distinguishable within current uncertainties. At lower pT ,a the differences between background models are too large to draw conclusion. In order to test this at these energies some extension of the background model is necessary, and several of these possible improvements are discussed.

PhD supervisors: prof. dr. R.J.M. Snellings and dr. M. van Leeuwen