Finding the Ωb– baryon
The Ω– baryon played an important role in the evolution of particle theory. Its much heralded discovery in 1964 at precisely the mass (1.67 GeV) predicted by symmetry arguments about charge and strangeness led promptly to the quark model of the strongly interacting particles. The quark model described the Ω– as a bound state of three strange (s) quarks. The relatively straightforward “naïve” quark model has long since been incorporated into quantum chromodynamics, a much more complete theory from which, however, precise predictions are notoriously hard to extract. But QCD does predict that the Ω– should have a heavy-quark analogue, called Ωb– , with a mass of about 6.0 GeV—more than six times that of the proton. In the Ωb– , one of the three s quarks is replaced by the much heavier bottom (b) quark. Now, having combed through 1014 proton–antiproton collisions accumulated at Fermilab’s Tevatron collider over the past four years, the collaboration that runs the collider’s D0 detector complex has reported finding 18 events in which the expected decay of an Ωb– to an Ω– plus a charmonium meson is clearly discerned. The discovery was difficult not only because so very few collisions produce an Ωb– , but also because the newly discovered baryon is so short-lived that it moves only about a millimeter before decaying. The 6.1-GeV mass extracted from the observed events is reassuringly close to that predicted by the number-crunching lattice-gauge calculations to which QCD theorists have to resort. (V. Abasov et al., D0 collaboration, http://arxiv.org/abs/0808.4142.) — Bertram Schwarzschild
Comments
The discovery that the Omega b - particle has a measured mass of 6.1 GeV mass, a value very close to the 6.0 GeV mass predicted by lattice QCD is a great triumph of our computational methods involving QCD physics.
However, when the LHC resumes operation after its repair, as well as further research; at the Relativistic Heavy Ion Collider, the proposed Rare Isotope Accelerator, and the proposed 1 TeV to 1.5 TeV Linac progresses, we may discover another boundary condition or transition zone in the behavior of sub-atomic physics relative to the current limits of the QCD physics that is analogous to the discovery of quarks relative to the previous simplified nuclear models involving the previous notion that protons and neutrons are non-composite particles.
However, even if further levels of sub-structure are detected within the make up of quarks, the methods of lattice QCD I believe will still be a very useful model for predicting the behavior of quarks which can lead to the prediction and discovery of additional composite particles. One simply has to take the example of the physics of the conservation of baryon number and other models of the nucleus of the atoms as these models existed in the early to the middle of the 20th century, and the predictions that such models made as they were applied to the field of nuclear energy, and nuclear physics, to see the value of well established courser grained models.
Another simple and obvious example is the existence of college chemistry texts that are full of predictive models that are utilized throughout industry and research while neglecting the existence of the up-quark and down-quark based compositions of the proton and neutron.
While making no attempt to brag, my father had received a PhD in nuclear engineering from MIT at the top of his class during the mid 1960s and would often state to me that he had no idea what quarks were during his studies there. Yet he was able to write a masterpiece thesis in certain aspects of the dynamics of nuclear reactors and worked as a Naval Officer and then as a D.O.E./ D.O.D. civil service employee as a nuclear engineer quite successfully without resorting to knowledge of quarks.
Posted by: James M. Essig | September 30, 2008 9:37 PM