CERN’s LHCb experiment has announced the discovery of the Xicc++ particle CERN

There’s a new particle in town, and it’s a double-charmingly heavy beast. Researchers working on the LHCb experiment at CERN’s Large Hadron Collider have announced the discovery of the esoterically named Xi cc ++ particle. The find could help us probe our understanding of one of the four fundamental forces that govern the universe.

The Xi cc ++ is a baryon, the family of particles that make up most ordinary matter and includes the likes of protons and neutrons. All baryons are made up of three quarks, a type of fundamental particle that comes in six different flavours. Theory suggests you can mix up these six in a variety of combinations to produce a whole host of baryons, but many of these combinations have yet to be observed in the real world.

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“Obviously you want to discover all of the states so the picture is complete,” says LHCb member Abolhassan Jawahery at the University of Maryland. “It doesn’t represent new physics, but it does help fill the picture that we have of the quark model. It may resolve some outstanding puzzles.”


This latest find is exciting particle physicists because it is the first confirmed to contain two heavier quarks known as charm quarks – the third constituent is an up quark. That means unlike in other baryons, where the three quarks rotate around each other equally, the two charm quarks are thought to sit at the centre of the Xi cc ++ with the lighter up quark orbiting around them.

A firm discovery

All this extra mass means Xi cc ++ weighs in at around 3621 megaelectronvolts, four times heavier than the proton. That’s in line with theorists’ expectations, says LHCb member Sheldon Stone at Syracuse University, New York, unlike previous results.

In 2002, the SELEX experiment at US particle accelerator Fermilab found evidence for a similar particle, but with a different mass that puzzled theorists. “It had a very strange mass that looked suspicious,” says Stone. “It would have done chaos to our model of how things are put together. So this is going to be very comforting to the theorists.”

The result announced today, at the European Physical Society Conference on High Energy Physics in Venice, Italy, has a statistical significance of over 7 sigma – a measure of how confident the researchers are in the find. Traditionally, physicists treat anything over 5 sigma as a firm discovery. The SELEX result only reached 4.8 sigma.

Physicists hope studying the new particle will help them test quantum chromodynamics, the theory of the strong force, which is responsible for holding quarks together in baryons. As the Xi cc ++ is the first baryon to contain two heavy quarks, it will push the theory in a new direction. “It really tests the theory if you put two of them together with a light quark,” says Stone. “It’s a nice result.”