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Fermilab | Batavia, IL | USA

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Using a better scale for matter and antimatter

A previous Result of the Week studied the difference between the masses of the top quark and antiquark. This week, DZero returns to the question, this time with a more precise scale.

This article appeared in Fermilab Today April 14.

In July of 2009, the DZero collaboration using the Tevatron at Fermilab reported on a measurement of the difference between the mass of top quarks and antitop quarks. This is a very interesting topic. One of the most fundamental tenets of the Standard Model is that the laws of physics apply equally well to matter and antimatter. This is called Charge, Parity and Time (CPT) invariance. Translated into English, CPT invariance means that if we reverse the left-right directions, go backwards in time instead of forward, and swap positive charges to negative ones, that it is impossible to tell the difference between the values. One crucial consequence is that matter and the corresponding antimatter particles must have identically the same mass. In fact, this tenet is even more fundamental than the Standard Model and physicists think that any theory that describes our universe must incorporate this necessary principle.

Of course a fundamental tenet is a challenge to physicists. Finding that this tenet is false would result in a total shakeup of our understanding. While measuring the mass differences between matter and antimatter is straightforward to do for leptons, it is very difficult to do for quarks. Quarks typically interact with their surroundings in the fractions of a second after they are created and before they decay. This makes it difficult to get a good measurement of the quarks’ masses.

Top quarks have the unique property that they decay very rapidly, indeed so rapidly that they have decayed before they undergo any subsequent interactions. Thus we know the mass of the top quarks with more precision than any other type of quark. This provides an interesting opportunity to test CPT invariance, especially since we suspect that any new and unobserved physics is more likely to occur at very high masses. Given that the top quark is the highest mass particle ever discovered, measuring the differences between the masses of top quarks and antiquarks is an ideal way to study this question.

Using nearly four times more data than the previous result, DZero physicists’ new measurement is twice as precise as the earlier one. The bottom line is that there is no indication of a mass difference, which is a marvelous confirmation of CPT invariance.

– Don Lincoln

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