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 October 6, 2009

UR Physicists Determine Mass Difference between Top and Antitop Quarks

A team of physicists in the Department has performed the first direct measurement of the mass difference between a quark and its antimatter partner (antiquark). Quarks are the fundamental particles that make up protons and neutrons, the stuff that regular matter is made of. Before antimatter was discovered, Paul Dirac introduced it as a requirement of the laws of quantum mechanics and Einstein's relativity. Scientists have been producing and studying antimatter such as antiprotons, antineutrons and antielectrons (or positrons) for many decades. All antimatter particles are supposed to have the same masses and lifetimes as their matter partners, but with opposite electric charges. Indeed, the masses and lifetimes of antimatter particles have been measured and found to be identical to their normal matter counterparts. Even more complicated objects such as pions, kaons, light nuclei, and hydrogen atoms appear to agree with this matter and antimatter "symmetry."

Studying isolated quarks is challenging because they combine with other quarks to form heavier and more complicated particles very rapidly -- between 10-22 seconds -- before they leave the regions of their creation.

Nonetheless, the heaviest of the quarks, the top quark, behaves differently: it decays so fast -- in around 10-24 seconds -- that it has no time to form any compounds. And that is the quark that the Rochester scientists have used to check the mass difference between a quark and its antiquark.

Analyzing the data collected in the DZERO experiment of the Tevatron proton-antiproton collider at Fermilab near Chicago, Illinois, the group was able to measure the mass of the top and its antitop quark partner produced in billions of collisions, out of which only around 160 collisions were identified as containing top-antitop pairs.

UR physicists discover the difference in mass between top and antitop quarks Image: Professor Regina Demina, Professor Tom Ferbel, graduate student Carlos Garcia, visiting scientist Ivan Nikolaev, and senior researcher Michael Wang, who together with other DZERO collaborators, were able to determine for the first time the mass difference between the top and antitop quarks. The measured difference is 3.8 ± 3.7 GeV, or a difference of (2.2 ± 2.2)%, since the top quark mass is close to 172 GeV. Hence, within the limits of experimental error, the masses appear to be identical.

While this result was expected, it is a crucial measurement, because the very heavy top quark was a natural place to hope to see something unexpected.

This result has been published in Phys. Rev. Lett. 103, 132001 (2009) and was a Research Highlight in Nature 461, 572 (1 October 2009).

Top vs antitop mass in electrons and muonsImage: The measured top mass versus antitop mass in the two channels studied: electrons (and positrons, left) and muons (and antimuons, right). The contours represent the number of standard deviations (sd) away from the central value of the measurement. (From Physics Today Physics Update June 2009).

--Submitted by Assistant Professor of Physics Aran Garcia-Bellido