In a first for particle physics, the CMS collaboration has observed three
J/ψ particles emerging from a single collision between two protons.
It’s a triple treat. By sifting through data from particle collisions at the
Large Hadron Collider (LHC), the CMS collaboration has seen not one, not two
but three J/ψ particles emerging from a single collision between two
protons. In addition to being a first for particle physics, the observation
opens a new window into how quarks and gluons are distributed inside the
proton.
The J/ψ particle is a special particle. It was the first particle containing
a charm quark to be discovered, winning Burton Richter and Samuel Ting a
Nobel prize in physics and helping to establish the quark model of composite
particles called hadrons.
Experiments including ATLAS, CMS, and LHCb at the LHC 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 trick? Analysing the vast amount of high-energy proton–proton collisions
collected by the CMS detector during the second run of the LHC, and looking
for the transformation of the J/ψ particles into pairs of muons, the heavier
cousins of the electrons.
From this analysis, the CMS team identified five instances of single
proton–proton collision events in which three J/ψ particles were produced
simultaneously. The result 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.
These three-J/ψ events are very rare. To get an idea, one-J/ψ events and
two-J/ψ events are about 3.7 million and 1800 times more common,
respectively. “But they are well worth investigating,” says CMS physicist
Stefanos Leontsinis, “A larger sample of three-J/ψ events, which the LHC
should be able to collect in the future, should allow us to improve our
understanding of the internal structure of protons at small scales.”
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