The discovery of the particles, known as Xi_b'- and Xi_b*-, were announced by CERN, the European Organization for Nuclear Research today and published online on the physics preprint server Arxiv.They have been submitted to the scientific journal Physical Review Letters.
"Nature was kind and gave us two particles for the price of one," said one of the CERN collaborators, Matthew Charles, of the CNRS's LPNHE laboratory at Paris VI University, in a statement.
The new particles, just the third and fourth ever discovered at the Large Hadron Collider, were found using an experiment called LHCb that is run by an international collaboration of scientists.
The new particles are baryons – a type of particle each made up of three elemental subatomic particles called quarks. The protons and neutrons that make up atoms are also baryons, but the new particles are about six times more massive than a proton.
That's because they contain a very heavy kind of quark called a b quark – also known as a beauty or bottom quark. The two other quarks in the particles are the d or down quark – a very light type of quark that is also found in protons and neutrons – and a middleweight strange quark.
The existence of the two new baryons had been predicted in 2009 by Canadian particle physicists Randy Lewis of York University and Richard Woloshyn of the TRIUMF, Canada's national particle physics lab in Vancouver.
Lewis said he saw the paper when it was first published online last night around 8 p.m.
"I saw the title [and] I thought, 'Oh, I predicted those — I wonder how it turned out?" he recalled. "I looked up their numbers and I said, 'Yeah, that looks a lot like what I predicted — great!"
Lewis and Woloshyn had predicted the composition and mass of the new baryons using a computer calculation based on a theory called lattice quantum chromodynamics, which describes the mathematical rules for how quarks behave.
Knowing where to look
The predictions give scientists at particle accelerators like the Large Hadron Collider an idea of where to look for undiscovered particles and, if they find something, what it might be.
Lewis said he's quite sure the LHCb scientists "were hoping that they would discover these particles in this particular mass range," as physicists have great confidence that the theory behind the calculations are correct.
On the other hand, Lewis said, he's never quite sure that his calculations are correct until the discovery of particles that match his predictions: "One always worries, 'Have I made a mistake?'"
Wendy Taylor, a particle physicist at York University who has studied b quarks in the past, said such discoveries are always neat.
"We don't find new particles every day."
She added that the Large Hadron Collider, with its ultra high energy collisions, provides the opportunity to find rarer particles with more unusual properties than particles discovered in the past, such as the two new heavy baryons.
That's due to the phenomenon described by Albert Einstein's famous equation E = mc^2.
"You can only have mass 'm' if you have enough energy 'E,'" Taylor said. "Higher energy collisions allow you to produce, hopefully, some of these higher mass particles."
The b quark is the second-heaviest of the six types or "flavours" of quarks, and the heaviest that can combine with other quarks to form baryons. (The very heaviest quark, the top quark, can't form baryons).
One interesting thing about the two new particles is that they are made up of exactly the same three quarks, whose masses add up the same number – but CERN researchers detected that the two new particles don't have the same mass.
That's because each quark has a quantum mechanical property called "spin" – a physics phenomenon that only applies to very small particles. When the spins of two quarks are aligned, they add energy that manifests as additional mass. That makes the the Xi_b*- baryon a little heavier than the Xi_b'- baryon.
Getting the details right
"I'm happy," Lewis said, noting that such details are difficult to both calculate and detect. "These are little details of physics that we're getting right."
The new particles are the third and fourth ever discovered at the Large Hadron Collider. The first, found by the CMS experiment in 2012, was a related particle called Xi_b*0, which also contains a b quark and a strange quark, but contains an up quark instead of a down quark. The second was the famous Higgs boson.
The LHC is currently shut down so it can be equipped with more intense beams that will operate at higher energies when it restarts in the spring of 2015.
"I'm quite confident that they will be discovering many more particles at CERN," Lewis said, "and I think that will be really valuable for us to solidify our understanding."