Charmed Meson Spectra and Radiative Transitions in a Relativistic Quark Model.

The fundamental constituents of matter, quarks and gluons, combine to form hadrons, composite particles including the more familiar protons and neutrons. Understanding the internal structure of these hadrons, particularly those containing charm and strange quarks, provides crucial insights into the strong force, one of the four fundamental forces of nature. Researchers at the Centre for High Energy Physics, University of the Punjab, namely Saba Noor, Faisal Akram, and Bilal Masud, investigate the properties of excited charmed and charmed-strange mesons, particles composed of a charm or charm-strange quark and its antiquark. Their work, detailed in a study titled ‘Study of excited and mesons in a relativized quark model’, employs a modified relativistic quark model to calculate the masses and wavefunctions of these mesons, incorporating both spin and mixing effects. The team utilises a differential evolution technique to refine the model’s parameters against existing experimental data and further explores the spectrum of hybrid mesons, formed by including a gluon within the quark-antiquark system, examining the impact of this gluonic excitation on their properties and predicting radiative transitions which may aid in their future detection.

Meson and hadron physics currently experiences a period of notable advancement, driven by a combination of theoretical modelling and experimental investigation. Researchers actively explore the properties and spectra of hadrons, composite particles made of quarks and gluons, employing relativistic quark models and analysing data from experiments such as LHCb and BaBar to refine understanding of these particles. This research increasingly focuses on exotic hadrons, states that deviate from the conventional meson (two quarks) and baryon (three quarks) classification, including tetraquarks (four quarks) and pentaquarks (five quarks), prompting a re-evaluation of the strong force interactions governing their existence.

Theoretical physicists construct modified relativistic quark models to predict the masses and wavefunctions of charmed and charmed-strange mesons, incorporating spin mixing – a phenomenon where particles exist in a superposition of different spin states – and utilising optimisation algorithms, specifically differential evolution, to fit model parameters to existing experimental data. These calculations allow for the investigation of excited states and gluonic excitations, where gluons, the force carriers of the strong force, contribute to the particle’s quantum state, significantly influencing the spectrum of resultant hybrid mesons. The inclusion of gluonic excitation effects provides a more nuanced understanding of hadron structure and interactions.

Experimental collaborations, particularly those at LHCb, located at the Large Hadron Collider, and BaBar, a now-decommissioned experiment at SLAC National Accelerator Laboratory, contribute significantly to this field by observing and measuring the properties of hadrons, providing crucial data for validating theoretical models and identifying new resonance states. Analysis of hadron decays, including precise measurements of decay rates and branching fractions – the proportion of times a particle decays into a specific set of products – offers valuable insights into the internal structure and interactions of these particles.

Researchers currently determine radiative transitions, where a hadron emits a photon, of both conventional and hybrid open charm mesons, comparing calculated results with experimental data and other theoretical works to validate models and identify areas for further refinement. This comparative analysis serves as a critical test of theoretical predictions, ensuring accuracy and consistency with observed phenomena.

Current research focuses heavily on hadrons containing bottom and charm quarks, owing to their relatively strong binding and enhanced detectability, allowing for more precise measurements and facilitating the identification of resonant states. The presence of heavier quarks increases the mass of the hadron, making it easier to detect amidst the background noise of particle collisions.

The application of optimisation algorithms, such as Particle Swarm Optimisation, to data analysis and model fitting proves essential for extracting meaningful results from complex experimental datasets, enhancing the accuracy and reliability of our findings. These algorithms efficiently search for the best-fit parameters within a model, given the available data. Furthermore, the use of relativistic quark models, incorporating spin mixing, allows for a more accurate description of hadron properties, providing a deeper understanding of their internal structure.

The ongoing interplay between theoretical modelling and experimental observation promises to further illuminate the complex landscape of hadron physics and deepen our understanding of the strong force, one of the four fundamental forces of nature.