Rapidity Correlation of K/π to ϕ in γSCSM and Pythia

Abstract

Pythia with Rope Hadronization, and the Canonical-Statistical Model with strangeness undersaturation (γSCSM), have shown remarkably similar results when simulating strange hadrons yields relative to pion yields as a function of event-multiplicity. As the underlying assumptions in these models are very different, a novel observable is warranted. The aim of this thesis is to show that K0S/(π+ + π−) in ϕ-triggered events is an observable which distinguishes between Pythia and γSCSM. For this purpose K0S/(π+ + π−) in three rapidity windows w.r.t ϕ was studied in proton-proton collisions, namely, |∆y| < 0.1, |∆y| < 0.5 and |∆y| < 1 as a function of event multiplicity. With these considerations, stark differences were observed between the two models. The Lund String Model implemented with Pythia conserves strangeness on the level of string breaks, which results in larger K0S yields relative to π+ + π− in regions closer to ϕ. Contrary to this, the statistical nature of γSCSM does not predict any increase of K0/π in ϕ-triggered events when compared to non-triggered events. Moreover, Pythia events were generated for multiple values of the vector-meson mixing angle (θV ). This allowed for a study of ϕ/π based on the probability of ss¯, uu¯ or d¯d projecting onto ϕ, and how that affects K0S/(π+ +π−) in the different rapidity windows. Here it was observed that larger deviations from ideal-mixing(θV = 35.3◦) resulted in larger ϕ/π, whilst only minimal decreases in K0S/(π+ + π−) were noticed.

Popular Abstract

Physicists like to study the fundamental building blocks of our universe by colliding protons at very high speeds. At the moment of impact, the particles inside the protons (called quarks and gluons) interact and form new quarks. Some of these quarks are less likely to be created, compared to other types of quarks, due to their heavier mass and a variety of other reasons. An important quark for this study is the strange quark, which is much heavier compared to other quarks called the up and down quarks. These quarks are impossible to observe, since they are glued together by the gluons. As such, what is observed are the particles (hadrons) containing the multiple quarks inside. A particular type of hadron, consisting of at-least one strange quark is referred to as a strange hadron. The production of a strange hadron is suppressed relative another type of hadron containing only light-quarks (up and/or down quarks). However, a team of researchers working with the ALICE detector at the Large-Hadron Collider have observed that when a large number of hadrons are produced the suppression decreases. This phenomenon is called strangeness-enhancement.

Strangeness enhancement has typically been considered as a signature for the state of matter called Quark-Gluon Plasma (QGP), where quarks and gluons are not confined within the hadrons. However, models that do not assume a QGP, have been able to simulate strangeness enhancement. Here we explore The Lund String Model which does not assume QGP, and compare it to a QGP model called the Canonical-Statistical Model with a strangeness undersaturation factor (γSCSM).

In the Lund String model, the gluons holding together a pair of quarks are treated as a classical string with a string tension κ = 1 GeV/fm. We call this a ”colour field”. It’s very similar to a rubber band, where the ends of the band represent the quarks, and the rest of the band represents the the colour field. Like a rubber band, if you pull hard enough, it splits into two—forming new quark pairs. There are many different types of quarks that can be formed in this way, but it is more probable to produce lighter quarks, such as up and down quarks, compared to heavier quarks, such as strange quarks. This is due to the energy stored in the colour
field, which is proportional to the string tension κ. However, if multiple strings are allowed to interact with each other, the overall string tension increases. As a results, the probability of producing strange quarks—relative to up and down quarks—also increases.

In the γSCSM, the process of hadron production is explained by a thermal ”fireball” (QGP) emitting hadrons. This fireball has multiple properties that determine which hadron will be produced. Here, strange hadrons are suppressed due to their heavier masses and by the factor γS. This factor changes depending on how populated a collision event is, where for larger populations, it decreases—leading to more strange hadrons being produce.

We have two different models predicting the same thing. Therefore, we need a new observable to distinguish between them. A potential observable may be a correlation between two types of strange hadrons, kaons and phi-mesons. This is because the Lund-String model predicts that the presence of a phi-meson increases the yield of kaons close to it, since both of them contain strange quarks. Whilst in γSCSM, everything is statistically produced, so the presence of a phi-meson will not affect the kaon yield.

• author: Doroci, Leutrim
• supervisor: Christian Bierlich
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20251
• year: 2025
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• language: English
• id: 9206212
• date added to LUP: 2025-06-30 13:37:28
• date last changed: 2025-06-30 13:38:24