Lonely top quarks

Dugan O'Neil from Simon Fraser University presented the results at a seminar Friday, December 8.

At the Wine and Cheese seminar Friday, December 8, the DZero collaboration announced that it has found, for the first time ever, evidence for top quarks produced singly, rather than in pairs. Ten years ago, the top quark was discovered at the Tevatron, but it was not alone. Each top quark observed was accompanied by its anti-matter partner, produced by the strong nuclear force in a relationship that has been carefully studied ever since. However, theorists have long pointed out that a top quark could also be found on its own, a lonesome particle without its mate.

At roughly the same mass as a gold atom, the top quark is a heavyweight in the world of elementary particles. However, searching for single-top production is frustratingly similar to panning for gold: only once in every twenty billion proton-antiproton collisions is a top quark expected to be produced by itself. Making matters worse, the signature of such occurrences is easily mimicked by other "background" processes that occur at much higher rates. To stand a chance of seeing the single-top signal, physicists at the DZero experiment had to develop sophisticated selection procedures. The first stage selected approximately 1,400 candidate events. Of these candidates, only about 60 single-top events were expected, and experimenters exploited every bit of information to unambiguously establish their presence. DZero used three different techniques that combine many discriminating variables in ways that allows physicists to recognize the true single-top events--much like a mother's ability to distinguish identical twins.

With a high degree of agreement among the three measurement techniques, DZero physicists have measured the single-top production cross section at 4.9 +/- 1.4 pb, in good agreement with the theory prediction. They estimate the chance of this value to be the result of a background fluctuation at only one in 2,800. Because of this high degree of certainty, the DZero Collaboration has, for the first time, established evidence for single-top production. This analysis also allows the first direct measurement of |V_tb| at >0.68, consistent with the presence of only three families of quarks.

These lonely top quarks will find themselves in good company as they join a rich history of high-energy physics at Fermilab.

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You may find this aspect of the Nobel Prize ceremony amusing.

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Every once in a while you share safety tips.
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From The Scotsman, December 11, 2006:

'Ray gun' cancer cure nears speed of light

Medical scientists will soon be able to offer cancer patients a radical new treatment using hugely accelerated ion particles to target tumors precisely without the dangerous side-effects of current procedures.

The carbon-ion therapy accelerates ions to up to 73 percent of the speed of light in a synchrotron--a machine similar to the particle accelerator at the CERN laboratory in Switzerland--before beams are fired into patients' cancerous cells.

Dr Juergen Debus, the chief radiologist at the Heidelberg Ion Beam Therapy Centre (HIT) in Germany, where the treatment will first be available, claimed the process turned the carbon ions into "miniature precision-guided missiles" that can destroy cancer cells with pin-point accuracy.