Scientists suspect that X (3872) is either a compact tetraquark or an entirely new kind of molecule made from not atoms but two loosely bound mesons - subatomic particles that themselves are made from two quarks. Only recently have physicists begun to see signs of exotic "tetraquarks" - particles made from a rare combination of four quarks. "For years we had thought that for some reason, nature had chosen to produce particles made only from two or three quarks," Lee says. The basic building blocks of matter are the neutron and the proton, each of which are made from three tightly bound quarks. The study's co-authors are members of the CMS Collaboration, an international team of scientists that operates and collects data from the Compact Muon Solenoid, one of the LHC's particle detectors. In the next few years we want to use the quark-gluon plasma to probe the X particle's internal structure, which could change our view of what kind of material the universe should produce." "This is just the start of the story," says lead author Yen-Jie Lee, the Class of 1958 Career Development Associate Professor of Physics at MIT. The results, published in Physical Review Letters, mark the first time researchers have detected X particles in quark-gluon plasma - an environment that they hope will illuminate the particles' as-yet unknown structure. Amid this ultradense, high-energy particle soup, the researchers were able to tease out about 100 X particles, of a type known as X (3872), named for the particle's estimated mass. The team used machine-learning techniques to sift through more than 13 billion heavy ion collisions, each of which produced tens of thousands of charged particles. Now physicists at MIT's Laboratory for Nuclear Science and elsewhere have found evidence of X particles in the quark-gluon plasma produced in the Large Hadron Collider (LHC) at CERN, the European Organization for Nuclear Research, based near Geneva, Switzerland. Today, X particles are extremely rare, though physicists have theorized that they may be created in particle accelerators through quark coalescence, where high-energy collisions can generate similar flashes of quark-gluon plasma. In the chaos before cooling, a fraction of these quarks and gluons collided randomly to form short-lived "X" particles, so named for their mysterious, unknown structures.
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