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Search for the Decay Bs ® m+ m- j in Proton-Antiproton Collisions


The CDF Collaboration


June 2001



The goal of particle physics research is to understand what are the elementary particles that exist in nature and what are the rules that describe their behavior. Particle physicists are always searching for new, undiscovered particles or new ways in which known particles can interact. Because lighter particles tend to be produced more copiously than more massive particles, new physical processes are most likely to be discovered at the highest masses that can be produced by today’s particle accelerators. Thus, physicists often seek to produce new particles directly at the world’s highest energy accelerators, such as the Fermilab Tevatron. As accelerators are improved to reach higher energies and intensities, particles that are more massive and processes that are more rare can be observed.


When a hypothetical new particle is too massive to have been observed directly at even the highest-energy accelerators, physicists can still search for its effects in the decays of lighter particles. For example, while the charm quark was first observed directly in 1974, its existence had been postulated years earlier, based on measurements of the decays of neutral kaons (which are bound states of quarks that are much lighter than the charm quark).


The Bs meson is a bound state of a strange quark and a bottom antiquark. One well-known Bs decay mode is Bs ® J/y j, where J/y is a bound state of a charm quark and a charm antiquark, and j is a bound state of a strange quark and a strange antiquark. The J/y can then decay to a pair of muons (m+,m-), and the j to a pair of kaons (K+, K-). A Bs decay mode that is expected to be much more rare is Bs ® m+ m- j, where the muons do not come from the decay of a J/y. In the Standard Model of particle physics, the latter Bs decay is expected to occur about 50 times less often than the former Bs decay. However, the existence of some new particles, such as charged Higgs bosons, could enhance the rate of the latter decay. Thus, by searching for rare decays of the well-known Bs meson, which is light enough to be produced copiously at the Tevatron, we are indirectly searching for new particles of much higher mass.


In our search, which uses data recorded between 1992 and 1995 by the Collider Detector at Fermilab (CDF), we did not observe any statistically significant Bs ® m+ m- j signal. (We observe two candidate events, where we would expect on average to observe one event from other, unrelated “background” processes. The observation of two events is statistically consistent with the prediction of one background event.) Since we observe no signal, we cannot measure the rate at which Bs ® m+ m- j occurs, but we can rule out the possibility that it occurs at a very high rate. A very high rate of Bs ® m+ m- j decays would be inconsistent with our observation of only two candidate events. The result of our statistical analysis is that the decay Bs ® m+ m- j occurs at most 1.2 times at often as the reference decay (Bs ® J/y j, followed by J/y ® m+ m-). The result presented in this paper is the first published search for this decay of the Bs.


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