Probing the Structure of the f0(980) via K +K − Coalescence in Proton–Proton Collisions at √ s = 5.02 TeV

Abstract

The internal structure of the scalar meson f0 (980) remains an open question, with competing interpretations as a conventional qq ̄ state, a tetraquark, or a loosely bound K +K − hadronic molecule. This thesis investigates the molecular hypothesis through a Monte Carlo coalescence calculation applied to proton–proton collisions at √ s = 5.02 TeV. Kaon phase-space distributions are generated with PYTHIA 8 using the Monash 2013 tune, validated against ALICE measurements, and a pT-dependent correction is applied to isolate discrepancies arising from the coalescence model itself. The formation probability is evaluated from the Wigner-function overlap of a Gaussian molecular wavefunction, with additional scans of sharp momentum and spatial cutoffs used to assess parameter sensitivity. The model successfully reproduces the overall shape and magnitude of the measured f0 (980) spectrum at low pT, supporting the presence of a significant K +K − molecular component in this region. At higher pT, the predicted yield increasingly underestimates the data, a deficit that persists across all molecular sizes considered and follows naturally from the molecular picture. The results suggest a mixed internal structure, with the high-pT excess pointing toward an additional contribution from a more compact configuration such as a qq ̄ state or tetraquark.

Popular Abstract

On a rainy day, droplets of water cling to a window, slowly merging into larger drops before slipping downward. It’s simple, almost hypnotic, tiny pieces coming together, growing, and suddenly becoming something new. We rarely stop to think about it. But this quiet, familiar process mirrors something happening in one of the most violent environments ever created.

Inside particle accelerators like the Large Hadron Collider (LHC), protons are smashed to- gether at enormous energies. For a fleeting instant, the collision produces a chaotic spray of particles, a microscopic fireball where matter exists in an almost unrecognizable state. Everything is fast, dense, and unpredictable. And yet, as this system cools, order begins to emerge. New particles form out of the chaos.

How does that happen? One possibility is surprisingly intuitive. It may follow a principle not so different from raindrops merging on glass.

This idea is known as coalescence, this is where smaller particles combine to form a larger, composite object. Rather than being created directly, some particles may emerge when oth- ers drift close enough moving in just the right way to effectively “stick together”.

My work focuses on one particularly puzzling particle: the f0(980). Discovered decades ago, it has resisted a clear explanation ever since. Most particles can be neatly described as combinations of fundamental building blocks called quarks. But the f0(980) does not fit into this picture. Its properties hint that it might not be a simple object at all, but something more complex.

One interesting possibility is that it forms from a pair of particles called kaons, themselves short-lived combinations of quarks loosely bound together. In this picture, the f0(980) would 1 resemble a kind of molecular structure, much like atoms binding together to form molecules. But instead of atoms, these structures are held together by the strong force, the powerful interaction that tightly binds the smallest building blocks of matter.

To explore this idea, I recreate these collisions using detailed computer simulations, attempt- ing to capture their chaotic aftermath particle by particle. Within this complexity, I track the production of kaons the potential building blocks of the f0(980) and study how they move and interact.

The key question sounds simple, but hides a deeper challenge: when kaons are produced close together, how often do they actually combine to form an f0(980)? And under what conditions does this happen? By modeling this coalescence process and comparing the results with experimental data, I can begin to test whether this picture reflects reality or whether something else is happening.

What makes this powerful is the direct link between idea and observation. If the simulations reproduce what experiments see, it strengthens the case that the f0(980) is not a conven- tional particle, but a composite structure emerging from the dynamics of the collision. If they do not, then the mystery deepens and our understanding must evolve.

At a deeper level, this question connects to one of the central challenges of modern physics: how simple fundamental particles give rise to the complex structures we observe. The theory describing these interactions, known as Quantum Chromodynamics. Our best framework for understanding how quarks and the strong force behave is notoriously difficult to solve in such situations. Each new insight is a small step toward making sense of this complexity.

In the end, the same principle appears across vastly different scales from raindrops on a window to particles emerging in high-energy collisions. Small things come together, and something new emerges. By uncovering how this happens at the smallest scales imaginable, we are not just studying particles we are learning how complexity itself can arise from simplicity.

• author: Patel, Vishvam Kaushikkumar
• supervisor: Peter Christiansen; Sushanta Tripathy
• organization: Department of Physics; Particle and nuclear physics
• course: FYSK04 20261
• year: 2026
• type: M2 - Bachelor Degree
• subject: Physics and Astronomy
• language: English
• id: 9240730
• date added to LUP: 2026-06-18 08:58:24
• date last changed: 2026-06-18 08:58:24