An international research team including Korean scientists has, for the first time, measured the precise properties of a particle composed of four quarks, known as a 'tetraquark'.

Quarks are the smallest fundamental particles that make up protons and neutrons. Given that multi-quark states like tetraquarks are expected to be common in the universe, this research is anticipated to provide clues to understanding the beginning and end of the universe.

The CMS international research team at the Large Hadron Collider (LHC) of the European Organization for Nuclear Research (CERN) published their findings on the characteristics of the 'all-charm tetraquark', composed of four heavy quarks (c-quarks), in 'Nature' on the 3rd (local time). The CMS international research team analyzes data from the Compact Muon Solenoid (CMS), one of the four detectors at the LHC. Korea is participating in the production and research of the CMS detector.

The traditional quark model was explained only by 'baryons', composed of three quarks, represented by protons and neutrons, and particles composed of two quarks. Since the 2000s, the discovery of tetraquarks, whose identity was previously unknown, and pentaquarks, composed of five quarks, has challenged the traditional quark model. Particles composed of quarks in numbers other than two or three are referred to as 'exotic hadrons'.

Professor Yoon Jin-hee of the Department of Physics at Inha University stated, "In recent particle physics, a major research topic is determining whether exotic hadrons are tightly bound states of four or five quarks, or loosely bound states of two mesons (particles composed of one quark and one antiquark) or a meson and a baryon."

More than 99% of the matter that constitutes the universe is composed of quarks and gluons, the particles that transmit the force that binds quarks together. Especially since quarks and gluons might have existed in unique bound states like exotic hadrons immediately after the Big Bang, understanding the binding state of exotic hadrons is crucial for uncovering the origins of the early universe.

In this study, the CMS international research team analyzed particles in tetraquark states X(6900), X(6600), and X(7100) in the data to determine the quark binding characteristics of tetraquarks. They observed the process where these particles decay into two 'J/psi' particles, each of which further decays into a muon and its antiparticle, the 'antimuon'.

The J/psi particle is composed of a 'c-quark' and an 'anti c-quark'. Muons are fundamental particles of the universe that carry charge like electrons but have greater mass. Through analysis of the angles at which muons decay, the research team sequentially ruled out various possibilities regarding the quark binding characteristics of tetraquarks.

As a result, the research team discovered that the analyzed tetraquark is likely to have a spin of 2 and be symmetric with respect to parity and charge conjugation. Spin refers to the way a particle rotates, parity refers to the property of being symmetric when flipped left to right, and charge conjugation refers to the physical state of being symmetric when the charge of a particle is reversed to become an antiparticle.

The research team concluded that the tetraquark analyzed, with its characteristics, is consistent with the observation that it is a state where four c-quarks are strongly bound together. Professor Yoon evaluated, "This provides a clue to understanding how the early universe, starting from the Big Bang, generated particles and matter, and how the universe will evolve in the future."

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