Beautiful and charming: physicists discover a new tetraquark

A new study reveals the existence of a tetraquark made up of beauty and charm quarks, expanding our knowledge of subatomic particle physics and strong force interactions.

Scientists from the Institute of Mathematical Science (IMSc) and the Tata Institute of Fundamental Research (TIFR) are exploring the complex domain of subatomic particles and recently published a new finding in the journal Physical Assessment Letters. Their research illuminates a new horizon in Quantum Chromodynamics (QCD), shedding light on exotic subatomic particles and pushing the boundaries of our understanding of the strong force.

Schematic representation of the predicted Tetraquark, Tbc, made of four quarks: beauty quarks, charm quarks and two light anti-quarks. Credit: Nilmani Mathur

The enigmatic strong force and the world of Hadrons

At the heart of this exploration lies the enigmatic fundamental strong force, which generates almost all the mass of all visible matter in the universe. A handful of fundamental particles known as quarks, which form intriguing interactions by exchanging gluons, create all the composite subatomic particles that ultimately make up all the visible matter of our universe.

Central to this understanding is the theory of Quantum Chromodynamics (QCD), which governs the dynamics of strong interactions. QCD enables the formation of color-neutral combinations of quarks into subatomic particles, commonly referred to as hadrons.

Traditionally, hadrons are classified into two main categories: mesons, such as pions, consisting of one quark and one antiquark, and baryons, such as protons, consisting of three quarks. Outside these categories, however, lie exotic hadrons, including those with four, five or six quarks, and even particles with gluons, such as glueballs.

However, until relatively recently, the existence of these exotic hadrons remained largely unknown territory for particle physicists. Over the past fifteen years, a stream of experimental discoveries has illuminated this previously obscure domain, revealing a rich spectra of exotic hadrons that defy conventional views of the strong force and challenge our understanding of subatomic particles.

Schematic representation of the formation of Tbc by interactions of a soil and a charm meson. Credit: A. Radhakrishnan and V. Raj

Discovering tetraquarks: a new frontier in particle physics

Among these exotic hadrons are tetraquarks, which are composed of four quarks (more precisely, two quarks and two anti-quarks). They could exist in very compact forms or as loosely bound molecules of two mesons or something else: their precise structures remain a mystery. They are also observed in the most common exotic species and it is expected that many more will be discovered in the future. Theoretical studies can help discover these by predicting their quark contents and possible energy ranges.

In this recent work, Prof. Nilmani Mathur and a postdoctoral researcher, Dr. Archana Radhakrishnan, from the Department of Theoretical Physics, TIFR, and Dr. M. Padmanath from IMSc have predicted the existence of a new tetraquark. This new subatomic particle is composed of a beauty and a charm quark, along with two light anti-quarks, and belongs to a family of tetraquarks called T_bc: the beautiful-charming tetraquarks.

They used the computing facilities of the Indian Lattice Gauge Theory Initiative (ILGTI) to perform this calculation. The formation of this specific tetraquark was investigated using the interactions between a bottom and a charm meson. Using variational techniques over different lattice distances and valence light quark masses, this study investigated the energy eigenvalues ​​of the interacting meson systems within finite volumes, and concluded about the existence of this tetraquark. Like the predicted particle, there may be other tetraquarks with the same quark content, but different spin and parity.

This prediction comes at a coincidental time, coinciding with the recent discovery of a tetraquark (T_cc) with two charm quarks and two light antiquarks. Consequently, there is a distinct possibility that the newly predicted particle or a related variant could be discovered using similar experimental methodologies, given that the energy range and brightness required for their production and detection are becoming increasingly accessible.

Moreover, the binding energy of the predicted particle exceeds that of all discovered tetraquarks and the binding becomes weaker as the mass of the light quark increases, which points to the complicated dynamics of strong interactions between different quark mass regimes and also clarifies the intriguing features of strong force . in hadron formation, especially those with heavy quarks.

This also brings additional motivation to search for heavier exotic subatomic particles in next-generation experiments, which can be used in deciphering the strong force and unlocking its full potential.

Reference: “Bound Isoscalar Axial Vector 𝑏𝑐𝑢¯𝑑¯ Tetraquark 𝑇𝑏𝑐 from Lattice QCD using Two-Meson and Diquark-Antidiquark Variational Basis” by M. Padmanath, Archana Radhakrishnan and Nilmani Mathur, May 14, 2024, Physical Assessment Letters.
DOI: 10.1103/PhysRevLett.132.201902