Probing symmetry between u and d quarks with WASA

Up and down quarks are the basic constituents of protons and neutrons which, together with electrons, build all visible matter in the universe. Both nucleons are affected by the residual strong force binding them into atomic nuclei almost identically. Almost, because the up and down quarks differ slightly in mass. Quark masses, or rather quark mass differences, have important impact on our very existence. In a world with equal masses of the up and down quarks, the proton-neutron mass difference would be exclusively based on electromagnetic effects, resulting in the proton being heavier than the neutron. In such a world the proton, instead of the neutron, would have a finite lifetime, and stable hydrogen atoms will not exist. Individual light-quark masses, however, are not directly accessible by experiments. Instead, we can probe experimentally net effects of quark-mass differences or quark-mass ratios in hadronic reactions.

The approximate symmetry between up and down quarks is called isospin symmetry. My Master and PhD projects concentrated on investigation of broken charge symmetry – a special case of isospin symmetry describing the interchange of up and down quarks and, thus, a rotation by $180^{\circ}$ around the $I_2$ axis in isospin space. Charge symmetry breaking observables, like, e.g., cross section of the $dd \rightarrow {}^{4}\text{He}\pi^{0}$ reaction, are by far less sensitive to the electromagnetic effects, and therefore allow easier access to quark-mass effects.

Using the WASA experiment at the Cooler Synchrotron of the Institute for Nuclear Physics at the Research Center Jülich we measured the $dd \rightarrow {}^{4}\text{He}\pi^{0}$ reaction at an excess energy of 60 MeV. We determined for the first time the differential cross section analyzing the data from a dedicated ten-week long beamtime. The result has shown the importance of the contribution of higher partial waves and provide an important input to a theoretical analysis based on Chiral Perturbation Theory which should allow for deep insight into the dynamics of the nucleon-nucleon interaction and the role of quark masses in hadron dynamics.

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Publications

. Charge symmetry breaking in $dd \rightarrow {}^4\text{He} \pi^0$ with WASA-at-COSY. In: Phys. Lett. B739, 2014.

Project DOI