American Institute of Physics
SEARCH AIP
home contact us sitemap
Physics News Update
Number 798, October 25, 2006 by Phil Schewe, Ben Stein, and Davide Castelvecchi

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 Lambdab. 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 Sigmab 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 Lambdab 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 Lambdac (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 Sigmab particles being real are at the level of a few parts in 1019.

Fermilab press release
Image at Physics News Graphics

Fermion Superfluidity in an Optical Lattice

In this summer of 2006, while Europe and North America have been buffeted by record high temperatures, Wolfgang Ketterle's lab in Cambridge, Massachusetts, continues to explore matter at record low temperatures. In three new papers -- one each in Nature, Science, and Physical Review Letters Ketterle and his MIT colleagues report on several new forms of quantum behavior in a research area at the crossroads between atomic and condensed matter physics. The samples used are dilute atomic gases (two of them with fermion atoms and one with boson atoms), but the properties studied -- things like conductivity and fluid flow -- are more typical of liquids and solids.

Here are the three new results.

1. First direct observation of phase separation between a fluid and a superfluid.

The MIT group had previously obtained visual proof, in the form of vortex images, that lithium-6 atoms had paired up and condensed into a superfluid (see PNU 734). As fermions (particles whose net spin has a half-integral value), lithium-6 atoms obey the Pauli exclusion principle, which forbids fermi atoms from joining a common quantum state -- like the one enjoyed when bosonic atoms (whose net spin is an integer) form a Bose-Einstein condensate, or BEC.

On the other hand, lithium-6 atoms can be manipulated with external magnetic fields to interact in a variety of ways. Paired up, they can, like bosons, proceed to form a condensed, superfluid state. In later work, the MIT physicists were able to contrive a lithium-6 superfluid in which there was an imbalance in the population of atoms with opposite spin orientation. This allowed the atomic gas to exist partly as a superfluid and partly as a normal fluid. In new work, this separation of the fluid and superfluid phases has been imaged

Ketterle says he believes this is the first time a quantum-condensed material (e.g., a superfluid or superconductor) has been imaged right along with normal phase. In this case the superfluid phase is seen to lie within a normal-phase cocoon.

Shin et al., Physical Review Letters, 21 July 2006
See images on the MIT Web site
Also see Nature, 6 July 2006

2. First observation of Mott insulator shells.

A Mott insulator (named for Neville Mott) is a sort of frustrated conductor; even though in the material there ought to be places in a lattice for extra charges to move into, strong interactions among electrons depress conductivity, making the material into an insulator, even when it should be a conductor (see PNU 645).

In the MIT work, the moving particles are not electrons but neutral atoms (rubidium atoms in a Bose-Einstein condensate), and the underlying lattice is not a matrix of atoms but an optical lattice -- a kind of artificial diffuse "solid," where laser beams trap one or more atoms at the interstices of a criss-crossing light field.

By careful tuning of external magnetic fields, a layered Russian-doll structure is achieved: Mott insulator layers, one inside another, are separated by superfluid layers. This structure was deduced by the careful application of spectroscopy technology used in atomic clocks (locking a microwave transmitter onto the receptive absorbing ability of supercooled atoms). Ketterle says that Mott/BEC vapors might, in their turn, help to make atomic clocks more precise.

Immanuel Bloch's group in Mainz might also be publishing new results in this area.

Campbell et al., Science, 4 August 2006

3. First fermion superfluidity observed in an optical lattice.

This represents the first time the paired fermion particles constituting a quantum fluid were nominally lodged within a crystal-like configuration of forces. This is a big step toward one of the big goals of research with ultracold fermi atoms, namely the ability to create an artificial crystalline superfluid or superconductor where the interaction parameters can be tuned at will. In this case the evidence for the quantum coherence of the atoms in residence within the optical lattice is indirect and consists of an interference pattern emerging when the atoms are released from pairs, a development controlled the an external magnet.

In a subject as fast moving as the study of trapped ultracold atoms, there are plenty of other related results. For example, a Harvard-George Mason-NIST group (including Charles Clark, NIST, charles.clark@nist.gov) has also obtained some insights on Mott insulators in quantum gases: see Rey et al., Physical Review A, June 2006, and Rey et al., 2006 March Meeting of the American Physical Society.

Randy Hulet and his group at Rice University reported the direct observation of phase separation in an article in Science in January 2006.  They too are about to have some new results on imbalanced spin populations.

Physical Review Letters, upcoming article
(See preprint on the Rice University Web site)
Journal of Low Temperature Physics, upcoming article
(See preprint)
Chin et al., Nature, 26 October 2006

Back to Physics News Update