Particle physicists working at the Large Hadron Collider have detected two new subatomic particles that were predicted to exist but never seen. The discovery of the two new baryon particles stands to deepen our understanding of the universe.
The two new baryons (known as Xi_b'- and Xi_b*-) are made from three quarks bound together by a strong force. Quarks are elementary particles that are believed to be the basic building blocks of protons, neutrons, and a variety of other, heavier particles. Baryons are subatomic particles with masses equal to or greater than that of protons.
The mass difference spectrum; the results show strong evidence of the existence of the two particles. Credit: LHCb Collaboration.
The new particles, which were predicted by the quark model, will expand our knowledge of how things work at atomic scales, and beyond the Standard Model's attempt to explain how the building blocks of matter contribute to the constitution of the universe.
The new baryons are actually quite heavy. They're more than six times as massive as the protons that CERN scientists have been deliberately smashing into each other in the 17-mile (27-km) tunnel. The heavier weight of the two particles is due to their "spins" in opposite directions. But unlike the well-known protons that the LHC accelerates, the new particles contain quarks that are quite different. As noted by CERN:
[The] new Xib particles both contain one beauty (b), one strange (s), and one down (d) quark. Thanks to the heavyweight b quarks, they are more than six times as massive as the proton. But the particles are more than just the sum of their parts: their mass also depends on how they are configured. Each of the quarks has an attribute called "spin". In the Xi_b'- state, the spins of the two lighter quarks point in opposite directions, whereas in the Xi_b*- state they are aligned. This difference makes the Xi_b*- a little heavier.
"This is a very exciting result," said Syracuse University's Steven Blusk in response to the discovery. "Thanks to LHCb's excellent hadron identification, which is unique among the LHC experiments, we were able to separate a very clean and strong signal from the background."
The find also demonstrates the incredible sensitivity and precision of the LHCb detector, say the researchers. The results of the experiment now appears at Physical Review Letters.
"Nature was kind and gave us two particles for the price of one," noted CERN team member Matthew Charles in a 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."
The data for the discovery was collected by CERN scientist from 2011 to 2012.
UPDATE: 5:46 PM ET
Some commentators are not happy with CERN's choice of words, arguing that "subatomic particles" can easily be misconstrued by the public to mean "elementary particles", which is clearly not the case.
Indeed, it may have been more accurate for CERN to say that previously undiscovered — but predicted — bound states of subatomic particles were identified.