Physicists from the Compact Muon Solenoid (CMS) Collaboration at CERN’s Large Hadron Collider (LHC) have observed three J/ψ particles emerging from a single collision between two protons.
A proton-proton collision event with six muons (red lines) produced in the decays of three J/ψ particles. Image credit: CMS / CERN.
The J/ψ meson (J/psi meson or psion) is a flavor-neutral meson about three times more massive than a proton.
This particle consists of a charm and an anti-charm pair held together tightly via the strong force.
It was discovered in two experiments unexpectedly in 1974, in the so-called ‘November Revolution’ of particle physics. This, in turn, led to the discovery of the charm quark, a heavier cousin of the up quark.
It fetched the pioneers of the experiments — Burton Richter and Samuel Ting — the Physics Nobel prize in 1976, and, importantly, opened a new window in the field of high-energy physics.
ATLAS, CMS and LHCb experiments have previously seen one or two J/ψ particles coming out of a single particle collision, but never before have they seen the simultaneous production of three J/ψ particles — until the new CMS analysis.
The rarity of this process is reflected in the number of events found, which is a meager 5, out of almost 100 billion proton-proton interactions that occurred during the Run-2 operation of the LHC spanning 2015-2018.
The new result, published on the CERN CDS information server, has a statistical significance of more than five standard deviations — the threshold used to claim the observation of a particle or process in particle physics.
“This result provides a novel independent approach to study the evolution of the transverse density of quarks and gluons inside the proton by exploiting, for the first time, the production of three J/ψ particles,” said CMS physicist Dr. Stefanos Leontsinis and his colleagues.
“In the next 20 years, the LHC experiments expect to accumulate data samples of proton-proton collisions that are 20 times larger than those analyzed by our team.”
“Future studies will thus allow us to measure more precisely the triple production of the J/ψ particle — and other heavier particles — and thereby improve greatly our understanding of the proton structure at the highest energy scales ever probed.”
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CMS Collaboration. 2021. Observation of triple J/ψ meson production in proton-proton collisions at s√= 13 TeV. CMS-PAS-BPH-21-004
