Is the bottom quark a spectator or a team player?

Does the bottom quark behave differently depending on which other quarks it teams up with to form a composite particle?

A bottom quark doesn’t last long before it decays into a charm quark or an up quark. Does its company during that brief moment affect the length of its life? Quarks are never seen on their own. Only their groupings, by twos into mesons or by threes into baryons, are observable. If the bottom quark were just a spectator in its own composite particle then all mesons and baryons containing bottom quarks would have the same lifetime before they decayed, due to the bottom quark inside of them decaying. But the complex web of strong interactions between the quarks may play a role in the lifetime of these composite particles.

A recent DZero analysis focused on comparing the observed lifetimes when the bottom quark teamed up with different sets of partner quarks. In one case, it was teamed up with an up quark and a down quark into a Λb baryon. In the other case, the bottom quark partnered with an anti-down quark into a B0 meson. The analyzers focused on events where each of these composite particles decayed by emitting charmonium, a charm-anticharm meson. Such Λb baryon and B0 meson decays leave a similar signature in the DZero detector. The analyzers looked for events with two pairs of two tracks with each pair emanating from positions that may be centimeters apart within the detector. Measuring the lifetimes of these composite particles in such similar events allows many uncertainties to cancel out when taking the ratio of their lifetimes.

If the bottom quark were simply a spectator, the lifetimes of these two composite particles would be equal. DZero measures the Λb baryon’s lifetime to be about 86 percent that of the B0 meson, which agrees well with the latest theoretical predictions about the changing behavior of the bottom quark when acting as a team player.

—Mike Cooke 

These physicists made major contributions to this analysis.

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