Quantum Mechanical Interference in Charmed Meson Decay

(Click here for a higher-resolution image.)

Professor Mats Selen and his group study the decay of charmed mesons. The colorful picture shown above, known as a Dalitz plot, is interesting not only because of its aesthetic appeal, but also because it represents some fascinating physics. A very basic knowledge of the concepts of energy and momentum conservation, which are discussed in most high-school physics courses and which are also the underpinnings of all high-energy particle physics research, are all that is necessary to understand the underlying idea of this picture.

Most particle physics experiments use high-energy accelerators to produce collisions in which variety of new particles are created (hence the name). These particles do not last long in the laboratory, and in a very short time (typically 10^–12 seconds) they will decay (explode) into lighter particles, whose energy and momentum we can measure with our experimental detectors.

If a heavy particle decays into two lighter ones, the energy and momentum of these two "daughter" particles can be easily predicted just by knowing that the total energy and momentum before and after the decay must be the same. Reasoning backwards, we can say that measuring both the energy and momentum of the two daughter particles will allow us deduce what the energy and momentum (hence the mass) of the heavy "parent" particle must have been.

If a parent (P) decays instead into three daughters (d1, d2, d3), the situation is slightly more complicated, and thus much more interesting. Luckily, we can think of such a "three-daughter" decay as a sequence of two "two-daughter" decays, the physics of each described in the simple way outlined in the previous paragraph. In other words, P ® d₁ + d₂ + d₃ can be written P ® d₁ + Y, followed by Y ® d₂ + d₃. However, we could just as well write it as P ® X + d₃, followed by X ® d₁ + d₂ or for that matter as P ® Z + d₂, followed by Z ® d₁ + d₃. Furthermore, since each of the above three decay paths are equally valid, quantum mechanics tells us that we need to consider the possibility that all three will happen at the same time, leading to a fascinating phenomenon known as "interference".

In the above picture we are studying the decay of a large collection of parent D⁰ mesons ("P" in the above paragraph) to three daughters K^–, p⁺ and p⁰ (d₁, d₂ and d₃ above). Shown on the horizontal axis is the square of the calculated mass of X, and on the vertical axis is the square of the calculated mass of Y. The color represents the probability of finding X and Y at the indicated masses; red indicates a higher likelihood and blue a lower one. The interesting pattern of colors in the Dalitz plot tells us about the properties (such as mass and angular momentum) of any X-, Y- and Z-type contributing resonances, as well as details of the interference between them.

Click here to learn more about the High Energy Physics Group at the University of Illinois.

© Copyright 2000-02 by the Department of Physics at the University of Illinois at Urbana-Champaign.

If you have questions about this page, please e-mail webmaster.

Links to external sites are provided as a convenience to our users.
The Department of Physics does not control or endorse the content of external sites.

cme