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STAR focus: Measurement of interaction between antiprotons

The STAR Collaboration has for the first time directly measured the interaction between antiprotons, as presented in a recent publication Nature 527, 345–348(2015).

The interaction between two antinucleons is the basic interaction that binds the antinucleons into antinuclei, and this has not been directly measured previously. By applying a technique similar to Hanbury Brown and Twiss intensity interferometry, the force between two antiprotons at short range is found to be attractive. In addition, two key parameters that characterize the corresponding strong interaction are reported, namely, the scattering length and the effective range. The measured parameters are consistent within errors with the corresponding values for proton–proton interactions. The results provide direct information on the interaction between two antiprotons, one of the simplest systems of antinucleons, and so are fundamental to understanding the structure of more complex antinuclei and their properties. Of equal importance, one aspect of the current measurement is a test of matter–antimatter symmetry, more formally known as CPT—a fundamental symmetry of physical laws under the simultaneous transformations of charge conjugation (C), parity transformation (P) and time reversal (T).

Left figure: Correlation functions and their ratio. Correlation functions for proton–proton pairs (top) and antiproton–antiproton pairs (middle). The ratio of the former to the latter is shown in the bottom panel. Errors are statistical only. Cinclusive(k*), are plotted as solid lines, and the term 1 + xpp[Cpp(k*; Rpp) − 1] is shown as dashed lines.

Lower right figure: d0 versus f0 for (anti)nucleon-(anti)nucleon interactions.

The singlet s-wave scattering length (f0) and the effective range (d0) for the antiproton–antiproton interaction (red star) is plotted together with the s-wave scattering parameters for other nucleon–nucleon interactions.

Posted Nov. 5, 2015

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