Excitement about B mesons

Distribution of B**+ candidates is shown with black squares, and a smooth curve in blue shows the result of a fit. The mass is measured as a mass difference Q to improve the mass resolution.

In the last century, physicists discovered that the proton-electron system of a hydrogen atom has discrete energy levels that depend on the relative position, angular momentum and spin orientation of the two constituents. This meant that electromagnetic force that bound them would permit the electron to live only at particular energies relative to the proton. The discovery was essential in the development of quantum mechanics.

Just as the hydrogen atom served as a probe of the electromagnetic force, measurements of the energy levels of B mesons provide information about the strong force. B mesons contain a heavy anti-bottom quark and a lighter up, down, strange or charm quark. The quark-antiquark pair is bound by the strong interaction.

Physicists at CDF have made an exciting new measurement of B mesons. In this analysis by CDF using the full Tevatron Run II data set, scientists studied excited B meson states that decay into a B meson ground state and a pion or kaon. The states that can be described by an orbital angular momentum L=1 — B1 and B2*, collectively referred to as B** states — are seen as three signal peaks in a mass difference plot, where two overlap in the case of B mesons with an up or down quark (see above figure). Scientists have observed the B** states with an up quark for the first time.

We measure several properties of the three B** states, such as mass, width and production and decay rate, and compare them with theoretical predictions. In general there is very good agreement between our measurements and those of DZero and the LHCb experiment at CERN.

In addition to the expected B** signals, another small peak appears for the first time in the mass difference distribution at a Q value of about 0.55 GeV (see above figure). The peak is seen in both the B**0 and B**+ distributions. The combined result has a statistical significance of 4.4 standard deviations, corresponding to a probability of about 10-5 that this is merely a background fluctuation. This is also just below the 5-sigma level required to claim a discovery. The CDF collaboration members have measured the properties of this new excited B meson state, called B(5970), and these measurements will serve as input to the interpretation by the theoretical community.

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edited by Andy Beretvas

These CDF physicists contributed to this data analysis. From left: Michael Feindt, Martin Heck, Thomas Kuhr and Manuel Kambeitz (not shown) all from University of Karlsruhe.