2011. május 3., kedd

Interesting effect at the Tevatron hints at new physics

March 18, 2011 | 9:00 am
Fermilab's Wilson Hall in the shape of a t, the symbol for the top quark.
Fermilab's Wilson Hall in the shape of a t, the symbol for the top quark.
Scientists at the Large Hadron Collider may be on the verge of discovering a new particle, according to mounting evidence from experiments at Fermilab’s Tevatron.
Judging by its behavior, it’s not the Higgs.
Scientists are finding signs of new physics through the study of a particle Fermilab physicists discovered at the Tevatron, the top quark.
When top quarks and their anti-particles, anti-top quarks, are created in particle collisions at the Tevatron, detectors note the direction in which they fly. Theory predicts that the particles will favor one direction slightly over the other, traveling that way about 5 percent of the time more.

However, in studies by the DZero collaboration and the CDF collaboration, the particles seemed to be picky 15 percent of the time. Top quarks went forward and anti-top quarks went backward. This month, the CDF collaboration announced results with an even larger asymmetry.
They also recently released a study in which top quarks and their partners showed this unexpected behavior almost half of the time in collisions above a certain energy.
“It’s really challenging for us to construct a convincing theory to explain this,” said theorist Susanne Westhoff, who presented on the subject at the Rencontres de Moriond conference on Wednesday. “All of the proposed explanations involve a new particle.”
Scientists think the cause of the unexpected asymmetry could be the interference of an undiscovered particle, one just heavy enough to go undetected by the Tevatron. If that’s true, experiments at the recently restarted Large Hadron Collider may be just weeks from collecting enough data to find it.
The Higgs particle would not have this kind of effect, said physicist Fabrizio Margaroli, CDF top quark group leader, who presented the experimental results at the conference.
In particle colliders, energy converts to mass to create heavy particles that haven’t been around in abundance in nature since moments after the big bang. The more energy a collider has, the more massive particles it can create. Scientists search for particles with certain masses by studying collisions with the corresponding energy.
If a new particle is affecting top and anti-top quark production, scientists should see greater effects in collisions closer to the energy at which the new particle is created. CDF physicists saw just that. In collisions above 450 GeV, top and anti-top quarks favored one direction over the other 48 percent of the time.
The Tevatron experiments did not have quite enough confidence in the measurements considering collisions at any energy to call what they saw evidence of a deviation from the expected. However, the study of collisions above 450 GeV is statistically significant enough to make the cut.
“At this point, things become really interesting,” Margaroli said. “People have a reason to be curious.”
The CDF and DZero experiments expect to announce updated results by this summer. If their findings are similar, the combined evidence from the two experiments could tell scientists with 99.9999 percent certainty that what they’re seeing is no fluke; new physics are most likely afoot.
Kathryn Grim 


Research at the Cosmic Frontier
Ninety-five percent of the universe consists of unknown dark matter and dark energy. Fermilab conducts some of the world’s most advanced cosmic-frontier experiments to discover their nature and plans to remain a leader in the next generation of world-class projects. In 2015, the Dark Energy Survey will be in the middle of its initial 5-year run, and Fermilab scientists are planning upgrades to extend the operation of the Dark Energy Camera for an additional five years. The CDMS and COUPP collaborations are developing plans for larger-scale underground experiments, much more sensitive to dark-matter particles than any currently operating experiment. Fermilab scientists will work on the LSST collaboration to build a wide-field optical survey telescope to observe more than half the sky every four nights. The LSST identified as a top priority in the 2010 decadal study of the National Research Council, will explore dark energy, supernovae and time-variable phenomena.
The Dark Energy Camera will take pictures of 300 million galaxies over five years starting in late 2011.


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