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Measurement of interaction between antiprotons

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One of the primary goals of nuclear physics is to understand the force between nucleons, which is a necessary step for understanding the structure of nuclei and how nuclei interact with each other. Rutherford discovered the atomic nucleus in 1911, and the large body of knowledge about the nuclear force that has since been acquired was derived from studies made on nucleons or nuclei. Although antinuclei up to antihelium-4 have been discovered1 and their masses measured, little is known directly about the nuclear force between antinucleons. Here, we study antiproton pair correlations among data collected by the STAR experiment2 at the Relativistic Heavy Ion Collider (RHIC)3, where gold ions are collided with a centre-of-mass energy of 200 gigaelectronvolts per nucleon pair. Antiprotons are abundantly produced in such collisions, thus making it feasible to study details of the antiproton–antiproton interaction. By applying a technique similar to Hanbury Brown and Twiss intensity interferometry4, we show that the force between two antiprotons is attractive. In addition, we report two key parameters that characterize the corresponding strong interaction: the scattering length and the effective range of the interaction. Our measured parameters are consistent within errors with the corresponding values for proton–proton interactions. Our 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.

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Figure 1: A schematic of the two-particle correlation process in a heavy-ion collision.
Figure 2: m2 versus nσz for negatively charged particles.
Figure 3: Correlation functions and their ratio.
Figure 4: d0 versus f0 for (anti)nucleon-(anti)nucleon interactions.

Change history

  • 18 November 2015

    In the Methods, the equation for the equal-time reduced Bethe-Salpeter amplitude (which can be approximated by the outer solution of the scattering problem) was corrected.


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We thank the RHIC Operations Group and RCF at BNL, the NERSC Center at LBNL, the KISTI Center in Korea, and the Open Science Grid consortium for providing resources and support. This work was supported in part by the Office of Nuclear Physics within the US DOE Office of Science, the US NSF, the Ministry of Education and Science of the Russian Federation, NSFC, the MoST of China (973 Programme No. 2014CB845400), CAS, MoST and MoE of China, the Korean Research Foundation, GA and MSMT of the Czech Republic, FIAS of Germany, DAE, DST and UGC of India, the National Science Centre of Poland, National Research Foundation, the Ministry of Science, Education and Sports of the Republic of Croatia, and RosAtom of Russia.

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The STAR Collaboration. Measurement of interaction between antiprotons. Nature 527, 345–348 (2015).

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