Calculating hadron widths in quantum chromodynamics
|Research Area||Plasma & Particle Physics|
|Principal Investigator(s)||Dr. Laurent Lellouch|
We have good reasons to believe that quantum chromodynamics (QCD) describes the interactions between quarks and gluons at low energies, where these elementary constituents combine into hadrons in a highly nonlinear fashion. A further confirmation of this fact was provided by our collaboration’s calculation of light hadron masses with fully controlled systematic uncertainties (Science 322, 1224, Nov. 2008). Our results are in complete agreement with the experimentally measured masses to within a few percent. However, there are important effects which such a calculation only tests indirectly: the creation and annihilation of quarkantiquark pairs, which is allowed in accordance with Heisenberg’s uncertainty principle. These effects have been particularly challenging to account for in quantitative investigations of nonperturbative QCD and have been the focus of most of the activity in the field in the last few years. Here we propose to test these effects directly by studying the emblematic decay of the rho meson into two pions, using lattice QCD. We will perform calculations in the full dynamical 2+1 flavor theory, with a strange quark at its physical mass and degenerate up and down quarks taken all the way down to their physical masses. This will obviate the need for difficult extrapolations of results obtained from simulations carried out with heavier quarks. Such calculations are only now becoming feasible, thanks to improvements in algorithms and to the advent of petascale computers. Our studies will provide a reliable determination of the rho meson decay width in QCD which is free of extraneous model assumptions and will mark an important first step in the calculation of hadron widths with fully controlled uncertainties. Moreover, the techniques and codes developped for these studies will be applicable to other important problems, such as the direct violation of CP symmetry in the weak decays of kaons.