LHCb experiment finds new particles [View all]

[DreamGypsy]

...from Symmetry, November 19, 2014: A new LHCb result adds two new composite particles to the quark model

Today the LHCb experiment at CERN’s Large Hadron Collider announced the discovery of two new particles, each consisting of three quarks.

The particles, known as the Xi_b'- and Xi_b*-, were predicted to exist by the quark model but had never been observed. The LHCb collaboration submitted a paper reporting the finding to the journal Physical Review Letters.

Similar to the protons that the LHC accelerates and collides, these two new particles are baryons and made from three quarks bound together by the strong force.

But unlike protons—which are made of two up quarks and one down quark—the new Xi_b particles both contain one beauty quark, one strange quark and one down quark. Because the b quarks are so heavy, these particles are more than six times as massive as the proton.

“We had good reason to believe that we would be able to see at least one of these two predicted particles,” says Steven Blusk, an LHCb researcher and associate professor of physics at Syracuse University. “We were lucky enough to see two. It’s always very exciting to discover something new and unexpected.”

The research paper describing the discovery is available here: Observation of two new Ξ−b baryon resonances

Two structures are observed close to the kinematic threshold in the Ξ0bπ− mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb−1 recorded by the LHCb experiment. In the quark model, two baryonic resonances with quark content bds are expected in this mass region: the spin-parity JP=12+ and JP=32+ states, denoted Ξ′−b and Ξ∗−b. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be
m(Ξ′−b)−m(Ξ0b)−m(π− =3.653±0.018±0.006 MeV/c2,
m(Ξ∗−b)−m(Ξ0b)−m(π− =23.96±0.12±0.06 MeV/c2,
Γ Ξ∗−b)=1.65±0.31±0.10 MeV, where the first and second uncertainties are statistical and systematic, respectively. The width of the lighter state is consistent with zero, and we place an upper limit of Γ Ξ′−b)<0.08 MeV at 95% confidence level. Relative production rates of these states are also reported.

Congratulations to each of the 702 CERN collaborators who contributed to this discovery.