New Charmonium Particle Discovered

Physicists from the Large Hadron Collider beauty (LHCb) Collaboration at CERN have discovered a previously unknown particle that consists of a charm quark and its antimatter counterpart, the charm antiquark. The mass and other properties of the new particle, ψ3(1D), place it squarely in the charmonium family that includes the better-known J/ψ meson, which was the first particle containing a charm quark to be discovered.

One of the first heavy-ion collisions with stable beams recorded by LHCb. Image credit: CERN.

One of the first heavy-ion collisions with stable beams recorded by LHCb. Image credit: CERN.

Charmonium particles are two-quark particles (called mesons) composed of a charm quark and a charm antiquark.

Just like atoms, mesons can be observed in excited states of higher energy, in which the mesons’ constituent quarks move around each other in different configurations.

These different arrangements give rise to a gamut of particles with different masses and quantum properties such as spin, which can be thought of as the rotation of a system around its axis.

Observing such excited states and measuring their properties provides a way of testing models of quantum chromodynamics (QCD), the theory that describes how quarks are stuck together into composite particles.

What’s more, knowledge of the full collection of these states helps identify exotic states with more than three quarks, such as tetraquarks, that are also predicted by QCD but have only recently been discovered. If all of the excited states are accounted for, scientists can be more confident that any remaining ones are exotic.

To catch the new charmonium particle, physicists from the LHCb Collaboration studied the decays of charmonium states produced in proton-proton collisions into pairs of D mesons (the lightest particles containing charm quarks), using data recorded between 2011 and 2018.

The researchers measured the range of masses of the D-meson pairs and then added up how many times they recorded each mass value within the measured range.

They then looked for an excess of events, or bump, in this mass distribution, and found a new, narrow peak at a mass that corresponds to a previously unobserved charmonium state, ψ3(1D).

The particle has a spin value of 3, making this the first observation of a spin-3 charmonium state.

The high spin value could account for the peak’s narrow width and the fact it has taken so long to find.

“The observed mass (3842.72 MeV/c2) and narrow natural width (2.79 MeV) suggest the interpretation of the new state as the previously unobserved ψ3(1D) charmonium state,” the physicists said.

“This represents the first spin-3 charmonium state observed.”

The scientists announced the discovery February 26 at the International Workshop ‘e+e- Collisions from Phi to Psi 2019.’

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Vanya Belyaev et al (LHCb Collaboration). Quarkonium studies at LHCb. PHIPSI19

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