Plain English Summary of
Measurement of semileptonic branching fractions of B mesons to narrow D** states"

B mesons, which are bound states of the b-quark and a lighter quark, play a special role in the elementary particle physics due to their heaviness and long lifetime. The B mesons were subject of extensive studies during recent years with thousands of papers published throughout the world. In this study we investigated properties of B mesons decaying to the excited D mesons (also known as D**) accompanied by a charged and neutral leptons :  a muon and neutrino. This paper explores a universal theoretical description of B meson decays: Heavy Quark Effective Theory (HQET), which exploits the heaviness of the B meson . We measured a parameter predicted by the theory : the ratio of probabilities to decay to one of two possible narrow states.  

Physicists at Fermilab have a long history of studying B mesons at the Tevatron collider. These mesons are abundantly produced in the proton-antiproton collisions and experiments use complicated trigger algorithms and electronics to select those events out of billions of other collisions. At the analysis phase B mesons are reconstructed combining a muon and several other charged particles. Muons are very similar in properties to electrons but they are about 200 times heavier.  Experimentalists like muons because they do not interact much with matter and reach the outmost part of the detector penetrating through thick slabs of iron. This allows for easy identification of the muons. The other particles in the same event have to form at least one excited D meson.

The D mesons are composed of a "charm" quark and a light ("up" or "down") anti-quark. Similarly to the hydrogen atom the quarks inside the D meson can have relative orbital momentum or to be "excited".  Following the same analogy the "excitement" is removed by a decay to a state with lower potential energy (or in this case to a state with lower mass) emitting an extra particle (typicaly a charged or neutral pion) . As always in quantum mechanics the orbital momentum is quantized and also obeys selection rules, which allow only certain decays. In our study we concentrate on allowed chain decays of the type shown below:
The D mesons can be reconstructed as peaks in the invariant mass distributions as shown in the figures below. To select  the D*+ meson we look at the mass difference between the kaon-pion mass and kaon-pion-pion mass, see the plot below.

Four species of the D** mesons different in quantum numbers are predicted by the theory and two out of four are expected to be "narrow". The term "narrow" in this case applies to the width of their invariant mass distribution (~10 MeV/c^2) and means that they appear as clearly visible resonances in the invariant mass of products of their decay : D* and  pion, see the plot below. The peak in the plot can be interpreted as two merged narrow resonances, which are also shown separately in the same plot. The obtained statistics is 10 times larger than seen previously in other experiments and this allowed us to accurately measure the rates of how often the B mesons decay to those two D** resonances and compare them to each other.
  This result was submitted for publication in Phys.Rev.Letters in July 2005.

The full text of the submitted publication can be found at the link below.

Please address your further inquires to the primary authors: Gennadi Borissov, Sergey Burdin, Andrei Nomerotski.