To date, the Large Hadron Collider (LHC) at CERN has discovered 80 particles. The most famous is the Higgs boson, a crucial ingredient in the fundamental laws of the Universe. The rest are particles called hadrons made up of quarks, which allow physicists to investigate the intriguing properties of the strong nuclear force.
Of the hadrons discovered so far, most are familiar sets of two or three quarks (so-called mesons and baryons, respectively). But one of the LHC's most striking discoveries is the confirmation of exotic hadrons composed of four or five quarks.
The exact nature of these exotic hadrons is far from established. Some models describe them as tightly bound tetraquarks or pentaquarks, others as loosely bound pairs of standard hadrons, and still others as both simultaneously.
In a paper published today in the journal Nature, the CMS collaboration has taken an important step in disentangling the true nature of exotic hadrons by reporting the first measurement of the quantum properties of a family of three "all-charm" tetraquarks.
Quarks come in six types: up, down, charm, strange, top and bottom. The tetraquarks and pentaquarks discovered so far, at the LHC and other colliders, usually contain a charm quark and its antimatter counterpart (a charm antiquark), with the remaining two or three quarks being up, down or strange quarks or their antiquarks. But in recent years other types have been found.
Finding and studying different kinds of tetraquarks and pentaquarks helps physicists to better understand how the strong force binds quarks into not only these exotic particles but also conventional hadrons such as the more familiar protons and neutrons, which make up the nuclei of atoms and thus all the matter around us.
The family of tetraquarks investigated by CMS is particularly promising because it comprises three tetraquarks composed entirely of heavy charm quarks, more precisely two charm quarks and two charm antiquarks. Such all-charm tetraquarks provide an extreme, yet more theoretically tractable, playground for understanding the strong force than their lighter equivalents.
The three particles are known as X(6600), X(6900) and X(7100), where the numbers within parentheses indicate their approximate mass in million electron volts. The second most massive particle of the three was first reported by the LHCb collaboration in 2020 and, since then, also by the ATLAS and CMS collaborations independently. CMS has also reported the least and most massive particles of the trio.
To determine the quantum properties of these heavyweights of the tetraquark world, the CMS team analysed data collected by the CMS detector from 2016 to 2018, during the second run of the LHC. By studying the decays of each X particle into two J/psi particles (comprising a charm quark-charm antiquark pair), which in turn each decay into two muons, the researchers measured three quantum properties: spin (a form of intrinsic angular momentum), parity symmetry (how the particle and its decay products behave under a mirror reflection), and charge conjugation symmetry (how the system transforms when every particle is replaced by its antiparticle).
For all the three tetraquarks, the parity and charge conjugation symmetries were found to be 1 and the spin was consistent with 2. These numbers put limits on the possible internal structure of these tetraquarks, indicating that they are more likely to be made of objects of tightly bound quarks.
"While our results do not definitively determine the internal structure of these exotic hadrons, they favour the hypothesis of tightly bound tetraquarks," says Andrei Gritsan, one of the lead contributors to the analysis. "With additional data from the ongoing LHC Run 3 and the upcoming High-Luminosity LHC, we will deepen our understanding of these exotic hadrons and other particles governed by nature's strongest force."
