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Science Today: D Meson Mixing—Might it be New Physics?

One way to hunt for new physics beyond the Standard Model is to observe processes that are expected to be very rare. If the rates for rare processes are found to be higher, or lower, than what is predicted by the Standard Model, this gives a clear signal for the existence of new physics. A particularly important class of rare processes is known as meson mixing, where neutral mesons spontaneously transform into their own antiparticles. This transition occurs because of a property of quantum mechanics that allows virtual particles to pop in and out of existence.

We have already learned much from the rates of meson mixing processes. In the 1960's, the very slow rate of K meson mixing was a major problem for the theory of weak interactions. Glashow, Iliopoulos, and Maiani solved this problem by predicting the existence of the charm quark, which was later discovered at SLAC and Brookhaven. In the late 1980's, B mixing was discovered at DESY with a rate that seemed amazingly large. This was the first evidence for the very large value of the top quark mass. Now the Standard Model seems to explain both rates well, and theorists use these processes to obtain constraints on physics beyond the Standard Model, or perhaps even find evidence for new effects.

Last March, BaBar and Belle announced the discovery of mixing between the neutral D meson and its antiparticle. The D meson is a bound state of a charm quark with an anti-up quark. This discovery marked the first time that a rare process involving a charm quark transition had been observed! Experiments had been searching for decades for this process. Since rare D meson transitions had never before been detected, no theoretical model, not even the Standard Model, had been tested with processes of this type.

I found this very exciting and literally lay awake all night after the discovery was announced, thinking of the countless calculations one could do. I knew I was in for a very busy time ahead. The result of that sleepless night (and all the hard work that followed) is an 86 page paper co-authored with Eugene Golowich at University of Massachusetts, Sandip Pakvasa at University of Hawaii and Alexey Petrov at Wayne State University. We examined the rates for D meson mixing in 22 different models, including the Standard Model, and models containing new fermions, new gauge bosons, new scalar bosons, new symmetries, and even new dimensions of space.

Surprisingly, the prediction of the Standard Model is difficult to pin down. The dominant contributions involve light-meson physics rather than quark physics. Many powerful techniques have been developed to compute properties of mesons involving heavy quarks, such as the b quark, but these methods do not work well for the lighter D mesons. For most of our techniques, the D meson is either too light or too heavy to accurately compute the Standard Model prediction. We conclude that it is possible for the Standard Model to account for the observed rate of D meson mixing, but the calculations are too uncertain to know this with confidence. More work is definitely needed here.

New Physics models can induce D mixing transitions through processes that have simple pictures at the quark level, so accurate predictions can be made. Of course, the New Physics models involve particles that are yet to be discovered, and which have unknown interaction strengths. In some cases, we found that a model will not generate D meson mixing at the observed level for any values of its parameters. More often, however, the new models give too large a rate for D mixing, and the experimental measurement places severe constraints on the new particles and interactions. In some scenarios, the constraint is strong enough to tell us that this type of new physics cannot be observed at the LHC.

D meson mixing thus provides a new and powerful probe of physics beyond the Standard Model. Future refinements in the experimental measurements and in the Standard Model theory, and sharp comparisons to LHC observations, will increase its power. The mixing rate of the D meson, like the similar rates of the K and B mesons, has much to teach us, and we are just beginning to learn.

—JoAnne Hewett, SLAC Today, June 14, 2007