High Precision Experiments to Study Light Neutral Meson Decay Rates and the Proton Charge Radius Using Electromagnetic Probes

The award includes three projects at Jefferson Lab (JLab): (a) developing a new experiment to measure the proton charge radius with a sub-percent precision; (b) extracting the lifetime of the pi-zero meson with an unprecedented 1.4% precision from the PrimEx-II data set; and (c) developing and preparing the approved experiment to measure the lifetime of the eta meson in the newly developing Hall D. As one of the most fundamental quantities, accurate knowledge of the proton size was always important in physics. Very recently, a new result from a muonic-hydrogen experiment, with its unprecedented less than 0.1% precision, stated that the proton size is up to 7 standard deviations smaller than it was previously known to be. This experimental fact triggered the well-known 'proton radius crisis' in physics. To address this crisis, a collaboration led by the PI of this award developed a uniquely designed electron-scattering experiment that was approved by PAC38 at JLab with the major condition of developing a new windowless gas hydrogen target. Precision measurements of the light pseudo-scalar meson lifetimes continue to be an important part of this award. The PrimEx collaboration performed the first measurement (PrimEx-I) of the pi-zero meson lifetime with the best precision up to date (2.8% in total). The results have been published and included in the Particle Data Group average. To reach the projected unprecedented 1.4% precision, our collaboration upgraded the experimental setup and performed the second (PrimEx-II) experiment in the fall of 2010. Under this award, the analysis process will be completed, and the results published. Under the leadership of the PI, a award to measure the eta meson lifetime with the best up to now precision (3%) was developed and approved to run in Hall D at JLab. We will prepare this experiment to run for the next few years.

Precise knowledge of the proton size is critically important for understanding of the structure of hadronic matter. In addition, it is very important for atomic physics, in particular, the spectroscopy of atomic hydrogen. The currently developed experimental situation with the proton size limits the determination of the Rydberg constant, one of the basic and the most precisely known constant in physics up to now. The proposed experiment, with its independent novel approach and projected precision, has a direct potential to either significantly shift the current value of the Rydberg constant (if it confirms recent muonic-hydrogen result) or question the sufficiency of the basic theory calculations in muonic-hydrogen system. In the latter case, it may also indicate new physics beyond the Standard Model. The lifetimes of the light pseudo-scalar mesons carry direct information about the symmetries, and in particular, their partial breaking effects in the theory of interaction in physics. Therefore, the precise knowledge of these important quantities will serve as benchmark tests to our understanding of Nature. In particular, the anticipated high precision measurement of the eta lifetime will significantly improve the ratio of light quark masses in a direct and most model independent way in physics. The quark masses and their ratios are input into the contemporary particle physics from the experiment. This project will provide advanced and highest scientific training for African-American students in a field where they are currently under-represented.