Researchers Observe New Charmonium Particle with High Spin Value for the First Time

The LHCb collaboration has identified a new particle, the mass and other properties of which position it precisely in the charmonium family. This family includes the well-known J/ψ particle—the first particle consisting of a “charm quark” to be discovered and won a Nobel prize in physics for its finders.

Forthcoming research on the characteristics of this new charmonium state and its relatives will enable physicists to gain better insights into the strong force binding the quarks together, which are one of the smallest known particles.

Charmonium particles are two-quark particles (known as mesons) formed of a charm quark and a charm antiquark, the antimatter counterpart of the charm quark. Of six known quark types, charm quarks are the third most massive. Quite similar to atoms, it is possible to observe mesons in higher energy excited states, where the constituent quarks of the mesons move around each other in various configurations. As a result of these different arrangements, a series of particles are formed with different masses and quantum characteristics such as spin, which can be regarded as the rotation of a system around its axis.

Through the observation of such excited states and evaluating their properties, models of quantum chromodynamics (QCD) can be tested. The QCD theory describes the way in which quarks are locked together into composite particles. Furthermore, in-depth insights into the complete collection of these states will be helpful in identifying the exotic states with more than three quarks, like tetraquarks, that are also proposed by QCD but have been discovered only recently. By taking all of the excited states into consideration, physicists can be self-assured that any remaining quarks are exotic.

The new charmonium particle was trapped by the LHCb collaboration, one of the four main experiments at the Large Hadron Collider, by investigating the decays of charmonium states generated in proton–proton collisions into D mesons pairs, with the help of data recorded between 2011 and 2018; D mesons are the lightest particles that consist of charm quarks.

The mass range of the D-meson pairs was evaluated by the collaboration, which then added up the number of times each mass value was recorded within the measured range. Then, in this mass distribution, they looked for an excess of events (or bump) and observed a new, narrow peak at a mass is in accordance with a previously unseen charmonium state known as the ψ₃(1D). The spin value of the particle is 3, rendering this the first-ever observation of a spin-3 charmonium state. The narrow width of the peak and the fact it has been found after such a long time could be explained by the high spin value.

Source: https://home.cern/