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Lifetime difference in Bs mixing July 13, 2007

Posted by apetrov in Particle Physics, Physics, Science.
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We recently finished another paper that was started a long time ago — in fact this one was actually started in 2004 (I even gave a talk about the results at ICHEP conference in 2006)! I guess this year is about finishing up old projects… But this is an exciting event since one of my co-authors is my second grad student Andriy, for whom it is the first paper (the first paper of the first student, Mohammad, is this one). So what did we do there?

Well, the paper deals with an exciting subject, oscillations of neutral heavy mesons. The system under consideration, B_s, is a state that is built out of heavy b-antiquark and  light s-quark. The thing is, this system can oscillate into its antiparticle, anti-B_s, which is by itself is a cool fact. The point of serious study of this system is that this transition can only happen in the Standard Model (SM) via quantum effects (a so-called one-loop amplitude), so it is sensitive to quantum effects of New Physics (NP) particles. How is it possible to test if there is New Physics?

According to CPT-theorem, mass of B_s is the same as mass of anti-B_s. However, since there is interaction that connects the two, they are no longer the mass eigenstates — this is effect is similar to appearance of the fine structure of levels in atomic physics when extra interactions split degenerate energy levels. The point is that when you actually find the mass eigenstates that diagonalize the Hamiltonian in the presence of that interaction connecting B_s and anti-B_s, you’d find that there are two new states, B_L and B_H (where L and H stand for “heavy” and “light”) and you can ask yourself what the differences between the masses and the lifetime s of those states are. Those quantities (aka Delta M and Delta Gamma) can be computed and measured. And the best thing is that they can be affected by New Physics!

So our paper addresses two things. First, we compute 1/m_b^2 corrections to the lifetime difference (or Delta Gamma)  in the Standard Model. Not very exciting, but this is needed for precise SM predictions of Delta Gamma — after all, if you want ot find NP, you’d better know what to expect in the SM. Second, we show that NP can directly affect Delta Gamma at an appreciable rate and even parameterize all possible effects of any possible NP model on Delta Gamma. This is exciting, as it turns out, there are examples of NP where Delta Gamma can even dominate the Standard Model result, which we provide!

And that’s what makes it interesting!

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