Fermilab collider experiments discover rare single top quark
March 9, 2009 | 9:13 am
Scientists at the Department of Energy’s Fermilab have observed a new subatomic process. In particle collisions produced by the Tevatron collider, currently the world’s most powerful operating particle accelerator, two teams of scientists found single top quarks. The discovery has significance for the ongoing search for the Higgs particle.
Previously, top quarks had only been observed when produced by the strong nuclear force. That interaction leads to the production of pairs of top quarks. The production of single top quarks, which involves the weak nuclear force and happens almost as often as the strong force production, is harder to identify experimentally. Now, scientists working on the CDF and DZero collider experiments at Fermilab achieved this feat, almost 14 years to the day of the top quark discovery at Fermilab in 1995.
A single top quark candidate event recorded by the DZero collaboration. The top quark decayed into a bottom quark, a muon, and a neutrino.
Searching for single-top production makes finding a needle in a haystack look easy. Only one in every 20 billion proton-antiproton collisions produces a single top quark. Even worse, the signal of these rare occurrences is easily mimicked by other “background” processes that occur at much higher rates.
“Observation of the single top quark production is an important milestone for the Tevatron program,” says Dennis Kovar, associate director of the DOE Office of Science for High Energy Physics. ”The highly sensitive and successful analysis is an important step in the search for the Higgs.”
Discovering the single top quark production presents challenges similar to the Higgs boson search in the need to extract an extremely small signal from a very large background. Advanced analysis techniques pioneered for the single top discovery are now in use for the Higgs boson search at Fermilab. In addition, the single top and the Higgs signals have backgrounds in common, and collisions that produce single top quarks can mimic collisions events that create a Higgs particle.
To make the single-top discovery, physicists of the CDF and DZero collaborations spent years combing independently through the results of proton-antiproton collisions recorded by their experiments, respectively. Each team identified several thousand collision events that looked the way experimenters expect single top events to appear. Sophisticated statistical analysis and detailed background modeling showed that a few hundred collision events produced the real thing. On March 4, the two teams submitted their independent results to Physical Review Letters (CDF paper, DZero paper).
The CDF detector at Fermilab weighs about 6000 tons. It records the particles emerging from high-energy proton-antiproton collisions produced by the Tevatron.
A couple of years ago, the two collaborations had reported preliminary results on the search for the single top. Since then, experimenters have more than doubled the amount of data analyzed and sharpened selection and analysis techniques, making the discovery possible. For each experiment, the probability that background events have faked the signal is now only one in nearly four million, allowing both collaborations to claim a bona fide discovery that paves the way to more discoveries.
“I am thrilled that CDF and DZero achieved this goal,” says Fermilab Director Pier Oddone. “The two collaborations have been searching for this rare process for the last fifteen years, starting before the discovery of the top quark in 1995. Investigating these subatomic processes in more detail may open a window onto physics phenomena beyond the Standard Model.”
The Standard Model of particles and their interactions is one of the most successful theoretical frameworks in physics, yet scientists know that it only explains a fraction of all matter and energy in the universe. Dark matter and dark energy, which the Standard Model does not explain, make up about 95 percent of the universe.
Kurt Riesselmann
Posted in Uncategorized |
13 Comments »
March 9th, 2009 at 12:45 pm
I’m confused – how can quarks be produced singly? I thought this was impossible due to the nature of the interquark force (asymptotic freedom etc)
March 9th, 2009 at 12:57 pm
Dear Cormac,
The single top quark is produced simultaneously with an anti-bottom quark. There is no single quark produced, just a single top quark. Before, scientists always had seen a top quark and an anti-top quark produced at the same time. Here is a diagram that shows how a single top quark and anti-bottom quark can come from a collision at the Tevatron:
http://www.fnal.gov/pub/presspass/images/feynman_tb_tqb_prl2009-A-mr.jpg
March 10th, 2009 at 6:02 am
Oops, I’m with ya now. Thansk for the reference. Ecxcellent blog by the way!
March 10th, 2009 at 12:46 pm
Is your decay mislabeled? The neutrino shows an arc and the muon does not, neutrino is neutral and shouldn’t bend in the detectors field.
Jim
March 11th, 2009 at 10:32 am
Jim, I think you’re right that the yellow segment is mislabeled. In other end-views of DZero I’ve seen related to top quark detection, it’s been labeled “Missing transverse energy”. If I’m not mistaken, the neutrino would move directly opposite to the muon, and is represented by the (faint) straight track directly opposite the muon.
March 11th, 2009 at 12:50 pm
Would the actual neutrino track be shown on such a diagram at all? I mean, D0 isn’t directly sensitive to neutrinos, is it? I would guess that (a) the muon had sufficiently high momentum that it didn’t curve noticeably; (b) the yellow bar is the “missing transverse energy” carried off by the neutrino; (c) the curving track which ends at the yellow bar is actually a bit of random debris, not a neutrino (notice that it seems to have deposited a bit of energy of its own); (d) the “straight track directly opposite the muon” is actually part of a faint red triangle highlighting the tracks comprising one of the b jets (compare the image here, for example).
March 11th, 2009 at 1:09 pm
[...] this month. Less than a week ago, the DZero collaboration submitted a paper on the discovery of single top quark production at the Tevatron collider. In the last year, the collaboration has published 46 scientific papers [...]
March 11th, 2009 at 4:31 pm
Jim and Howard: Neutrinos do not leave tracks in particle detectors. They are neutral particles, always travel in a straight line, and they very rarely interact with matter at all. (On the rare occasion that a neutrino hits an atom, then the charged particles emerging from that collision leave tracks. That’s how neutrino detectors work.) In the graphic above, the bent particle track that leads to the neutrino label is a coincidence. It is a different, charged particle with very little energy. (For comparison: the muon, a charged particle identified in the graphic, has high energy: it travels in almost a straight line, and the green block indicates its energy.) Neutrinos are idenfied by missing energy and momentum. The yellow block indicates that there is a lot of missing energy in this particular direction. How does the DZero collaboration know there is missing energy: it’s like seeing billard balls collide, and one ball is invisible. But knowing exactly the tracks and velocities of all other balls, you can identify the track and energy of the invisible ball.
March 11th, 2009 at 6:53 pm
Thanks for the explanation.
Jim
March 12th, 2009 at 3:32 pm
[...] Just look at the discovery that the CDF and DZero collaborations announced less than a week ago: single top quark production. An amazing feat in its own right, the discovery also showed that the Fermilab Higgs hunters may [...]
March 24th, 2009 at 10:25 am
[...] recently announced the first observation of the single top quark, a top quark produced without an anti-top quark. Fermilab physicists discovered the top quark in [...]
October 31st, 2009 at 6:27 pm
There is of course the possibility that this isn’t important or even interesting at all ! The way I understand it, this was just a confirmation of something they expected all along. Though I suppose it is comforting that the standard model is once again confirmed, I do not find this particularly exciting.
November 1st, 2009 at 7:54 am
One aspect of science is that you don’t trust a theory blindly. If scientists did, people would never have questioned Newton’s explanation of gravity. So you need experiments to test all predictions of a theory, and you just don’t know when one of these tests will show a surprising result instead of confirming the expected. The top quark is very different from the other quarks. It is much, much heavier; it doesn’t live long enough to form composite particles; and theorists have developed models that suggest that the top quark could play a major role in understanding the origin of mass. To find out whether that is true, physicists need to measure and understand the various types of interactions that the top quark exhibits. This particular measurement was particularly hard — it took 14 years longer to observe the electroweak interaction of the top quark than to find its strong interaction — and is an important test of the Standard Model.