New Baryons Discovered

The periodic table of baryons has now been supplemented with several heavyweight members. Like elements 116 and 118, recently added to the chemical periodic table (see PNU 797), the new members of the baryonic periodic table are unstable and ephemeral, but their observed existence serves to expand our understanding of matter in the universe. The new baryons, the heaviest yet with masses around 5.8 gigaelectronvolt, were sifted from trillions of proton-antiproton collisions conducted at an energy of 2 teraelectronvolt at the Fermi National Accelerator Laboratory.

According to the toolbox of the standard model, all matter is assembled from a family of six leptons or a family of six quarks. Among the leptons, only the electron is of account in ordinary atoms, and among the quarks only the up (u) and down (d) quarks help to fill out protons and neutrons. Thus the proton is really a u-u-d quark troika while the neutron's lineup is d-d-u. But one can imagine other baryons (particles made of three quarks) made of different quark combinations, or with different spin values (the proton and neutron both have a nominal spin value of 1/2). Although they can be made artificially in particle collisions, baryons containing the other quarks -- strange (s), charm (c), bottom (b), or top (t) -- are unstable and quickly decay. Still, to understand the strong force that governs nuclear matter, physicists strive to create and measure all those other candidate baryons. (For a picture of the baryon hierarchy see Physics News Graphics.)

Up to now there was only one well established bottom-quark-bearing baryon, the so called Lambda_b. The first evidence for its existence was reported by CERN and Fermilab in late 1990s based on a handful of events. Now the CDF collaboration at Fermilab is claiming discovery of two baryon types, each on the basis of about 100 events. Actually there are four new so-called Sigma_b baryons: two positively charged baryons with a u-u-b combination (one with spin 1/2, one with spin 3/2), the first of which constitutes a sort of bottom-proton; and two negatively charged baryons with a d-d-b combination (one each with a spin of 1/2 or 3/2). In all cases, the Sigma decays almost immediately into a Lambda_b particle (with a u-d-b set of quarks) plus a pion. In the detector the Lambda typically flies about 100 microns before decaying into Lambda_c (a Lambda baryon with a c quark instead of a b), which quickly decays into an ordinary proton.

Is there sufficient data in this case to claim a "discovery" of these particles? The new results were announced at a recent talk at Fermilab by Petar Maksimovic, of Johns Hopkins University. Jacobo Konigsberg, of the University of Florida, the co-spokesperson for the CDF group says that the statistical odds against the Sigma_b particles being real are at the level of a few parts in 10¹⁹.

Fermilab press release
Image at Physics News Graphics