One of the detectors of the Large Hadron Collider has discovered a new particle, consisting of four charmed quarks. Physicists believe that this is the first representative of an undescribed class of particles.
The (lower-right) two-dimensional distribution of the di-J / ψ candidates and its projections onto (lower left) M (1) μμ and (upper) M (2) μμ. Four components are present because each projection consists of the signal and background J / ψJ / ψ candidates. The labels J / ψ1, 2 and bkg1,2 represent, respectively, the contributions of the signal and background to the distribution M (1), (2) μμ.
The LHCb (Large Hadron Collider beauty experiment) collaboration has found a new type of four-quark particle that has never been seen before. The discovery was presented at a recent seminar at CERN, and is also described in an article on the arXiv preprint site. This discovery will help scientists understand quarks - fundamental particles of the Standard Model of the Universe.
They usually come together in groups of two (quark - antiquark) or three to form protons and neutrons. Larger particles are considered exotic, but scientists have long assumed that they may be composed of four or five quarks (the so-called tetraquarks and pentaquarks). In recent years, experiments at the Large Hadron Collider (LHC) have confirmed the existence of such hadrons. They are ideal for studying the strong nuclear force, one of the four fundamental forces in the universe that binds protons, neutrons and atomic nuclei together.
"The particle we just discovered is the first to be composed of heavy quarks of the same type: two charmed quarks and antiquarks," says LHCb spokesman Giovanni Passaleva. "So far, LHCb and other experiments have only recorded tetraquarks with a maximum of two heavy quarks, and none of them had more than two quarks of the same type."
To search for new tetraquarks T cccc, the LHCb team calculated their possible mass and studied the data obtained at the detector during the periods of the first and second launches of the LHC in 2009-2013 and 2015-2018. She found two energy surges in the 6900 and 6400-6600 megaelectronvolts range. In an attempt to describe the results obtained, scientists found more than five standard deviations in the range of 6200-7400 megaelectronvolts. This is enough to announce the discovery of a new particle. Moreover, such jumps correspond to the mass T cccc. "This particle is unique - an exotic hadron containing four quarks instead of two or three in ordinary particles of matter, and the first containing heavy quarks," the scientists say.
It is not yet clear whether the new particle is a "true tetraquark", that is, a system of tightly bound four quarks, or whether it consists of two ordinary pairs. In any case, the new particle will help theorists test models of quantum chromodynamics, which describes the strong interaction of particles. The authors intend to continue research during the third launch of the LHCb, in March 2021.