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A new heavy cousin of the proton was found hiding in a pile
of data at the Fermi National Accelerator Laboratory in
The new particle, long predicted to exist, is made of a bottom quark — the second-heaviest of all quarks — and two, much lighter strange quarks essentially orbiting around it, says Fermilab physicist Dmitri Denisov. The laboratory announced the discovery on September 3 and submitted a paper for publication to Physical Review Letters.
The particle, known as omega-b-minus (Ωb-), is one of many possible combinations of quarks predicted by the standard model of particle physics, the accepted foundation of the subject. The 1964 discovery of a particle made of three strange quarks was the landmark that established the mathematical basis for what would later be the theory of quarks, says Michael Peskin, a theoretical physicist at the Stanford Linear Accelerator Center in Menlo Park, Calif. “This much later discovery is just another feather in the cap of this excellent theory,” he says.
Researchers working at Fermilab’s DZero detector, which smashes together protons and antiprotons circling almost at light speed inside the Tevatron accelerator, took about one year to sift through data gathered between 2002 and 2006.
The data described the debris coming from the proton-antiproton collisions, in which the particles’ huge energy — equivalent by E=mc2 to roughly 1,000 times their normal mass — turns in part into matter, creating hundreds of new quarks and other particles.
The physicists were looking for telltale signs of the rare collision events that led some of the quarks to combine into the new particle. “We needed 100 trillion events to select events which can be interpreted as an omega-b-minus,” and only a handful fit the bill, says Denisov, DZero’s co-spokesperson. In each case, the researchers did not observe the new particle itself, because it decayed almost immediately. Instead, the DZero detector picked up a signature combination of five particles leftover by omega-b-minus decay. The experiment also produced a roughly equal number of antimatter versions of the new particle, as predicted by theory, Denisov says, for a total of 18 particles.
The physicists were able to calculate the new particle’s mass, which at 6.2 billion electron volts is about six times that of a proton — very close to theorists’ predictions.
A similar effort last year led physicists at DZero and also at CDF, the Tevatron’s other detector, to discover a particle called the cascade baryon (SN: 7/7/07, p. 13). The omega-b-minus is the 13th particle to be discovered out of 20 predicted proton “cousins” — particles made of three quarks, as is the proton, and having magnetic properties similar to a proton’s.
Fermilab physicists are now searching for signs of one more such particle. Some of the remaining ones probably lie outside the Tevatron’s reach, Denisov says, but could be found at the Large Hadron Collider, the new, more powerful particle accelerator that’s starting up this fall deep below the Swiss-French border (SN 7/19/08, p. 16). The LHC is also expected to find new elementary particles (rather than combinations of other particles), some of which, if found to exist, may require extending or rethinking the standard model.
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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.
Note that the proposed existence of 20 baryons, each made of 3 quarks, and with similar magnetic properties to the proton, as well as the existence of particles referred to as pentaquarks, etc., seems to be leading us to the discovery of a particle zoo, a slang term given in the 20th century for the proliferation of the discovery of numerous particles in the 1950s and 1960s most of which turned out to be composite particles composed of quarks. This begs the question as to whether quarks are composite themselves, and by corollary, whether or not there are additional nuclear or any sub-nuclear forces.
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