Two never-before-seen particles have just been detected at CERN’s Large Hadron Collider, the world’s largest particle accelerator, by the international LHCb collaboration. Known as Xi_b'- and Xi_b*-, the new particles belong to the baryon family.
Baryons are made from three fundamental, subatomic particles called quarks, bound together by a strong force. The more familiar protons and neutrons are also baryons -- these combine with electrons to make up everything on the periodic table. “The building blocks of all known things, including cars, planets, stars and people, are quarks and electrons, which are tied together by strong, electromagnetic forces,” Steven Blusk of Syracuse University explains in a news release.
And the quarks in these newly discovered baryons aren’t even the same type: Each of the new particles contains one beauty (b), one strange (s), and one down (d) quark.
The new particles are more than six times as massive as a proton, thanks to their heavyweight b quarks and their angular momentum -- a particular attribute of quarks known as “spin.” In the Xi_b'- state, the spins of the two lighter quarks point in the opposite direction to the b quark, and in the Xi_b*- state, the spins are aligned. This difference in configuration makes Xi_b*- a little heavier.
“Nature was kind and gave us two particles for the price of one," says Matthew Charles of the CNRS's LPNHE laboratory at Paris VI University in a CERN statement. "The Xi_b'- is very close in mass to the sum of its decay products: If it had been just a little lighter, we wouldn't have seen it at all using the decay signature that we were looking for.” But thanks to the sensitivity and precision of the LHCb detector, Blusk adds, “we’ve been able to separate a clean, strong signal from the background.”
In addition to the masses of the new particles, the team also studied their relative production rates, their widths (a measurement of how unstable they are), and a few other details of their decays. The new baryons are very short-lived, CBC explains, lasting only a thousandth of a billionth of a second before breaking up into five smaller pieces.
The existence of these particles were previously predicted in 2009, but no one has ever seen them until now. Right after the findings were released online this week, "I saw the title [and] I thought, 'Oh, I predicted those -- I wonder how it turned out?" Randy Lewis of York University tells CBC. "I looked up their numbers and I said, 'Yeah, that looks a lot like what I predicted -- great!" Lewis and colleagues predicted the mass and composition of the new particles based on mathematical rules for how quarks behave.
The findings are available through arXiv and will be published in Physical Review Letters.