The ALICE, CMS and LHCb collaborations present new measurements that show how particles containing charm quarks can serve as “messengers” of hadrons and the quark–gluon plasma, carrying information about these forms of matter. This media update is part of a series related to the 2020 Large Hadron Collider Physics (LHCP2020) conference, taking place 25–30 May 2020.
Dr. Leontsinis, a senior physicist at University of Zurich who was part of the internal review committee that approved the CMS analysis, noted that "the X(3872), also known as χc1(3872) is a very interesting particle that opened the window to exotic, quarkonium-like spectroscopy. Although discovered by Belle in 2003, its nature still remains unclear. Many theoretical interpretations exist, categorizing it as either a tetra-quark candidate, a molecule particle, or a mixture of those with a conventional charmonium state. The X(3872) was never previously observed in the decays of B mesons with strange quarks in them, and this new CMS measurement provides an additional understanding of X(3872) properties."
The main results are:
·The LHCb team obtained the most precise yet measurements of two properties of a particle known as Xc1(3872), a charmed hadron. The particle was discovered in 2003 and it has remained unclear whether it is a two-quark hadron, a more exotic hadron such as a tetraquark – a system of four quarks tightly bound together – or a pair of two-quark particles weakly bound in a molecule-like structure. Pinning down the nature of this charmed hardon could extend physicists’ understanding of how quarks bind into hadrons.
“Our results are consistent with Xc1(3872) being a pair of two-quark particles loosely bound together, but it does not fully rule out the tetraquark hypothesis or other possibilities,” says LHCb spokesperson Giovanni Passaleva.
·The CMS collaboration observed for the first time the transformation, or “decay”, of another particle, called B0s, into the same Xc1(3872) particle. The researchers compared this decay with the previously observed decay of the B+ meson, which had led to the first detection of the Xc1(3872) in 2003. Both types of decay link the behaviour of this charmed hadron to the up and strange quarks.
“Measured differences in the decay rates are intriguing and could provide further insight into the nature of the Xc1(3872), which has not yet been fully established,” says CMS spokesperson Roberto Carlin.
·The ALICE collaboration measured the so-called elliptic flow of hadrons containing charm quarks, in heavy-ion collisions. The hadrons are created during collisions that also create a quark–gluon plasma. Hadrons containing heavy quarks, like the charm quark, are excellent “messengers” of the quark–gluon plasma, meaning they carry important information about it.
“The pattern observed by ALICE indicates that the heavy charm quarks are dragged by the quark–gluon plasma’s expansion,” says ALICE spokesperson Luciano Musa.
For more information:
In the past the EPFL group worked actively on the measurements of the X(3872) properties with Belle data, see e.g. . Later, whithin LHCb, the EPFL team also had an activity on these exotic states, in particular the Z(4430)+, see e.g.