Precision measurement of the mass difference between light nuclei and anti-nuclei

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2.76 TeV
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Heavy ions are collided at very high energies at the CERN Large Hadron Collider (LHC) to study matter at extremely high temperatures and densities. Under these conditions, a phase called the quark-gluon plasma is created, a state which supposedly prevailed in the early Universe. Such collisions are also a rich source of matter and anti-matter particles, and thus are suitable for an experimental investigation of their properties such as mass and electric charge. The measurement of the mass differences for systems bound by the strong force has reached a very high precision with protons and anti-protons. The extension of such measurement from (anti-)baryons to (anti-)nuclei allows one to probe any difference in the interactions between nucleons and anti-nucleons encoded in the (anti-)nuclei masses. This force is a remnant of the underlying strong interaction among quarks and gluons and can be described by effective theories, but not yet directly derived from Quantum Chromodynamics (QCD). Here we report a measurement of the difference $\Delta \mu = \Delta (m / |z|)$ between the ratios of the mass and charge of deuterons (d) and anti-deuterons ($\rm{\bar{d}}$), and $\rm{^3 He}$ and $\rm{^3 \overline{He}}$ nuclei carried out with the ALICE (A Large Ion Collider Experiment) detector in Pb-Pb collisions at a center-of-mass energy per nucleon pair of $\sqrt{s_{\rm NN}}=~2.76~\rm{TeV}$. Our direct measurement of the mass-over-charge differences yields $\bf{{\it \Delta \mu}_{d\bar{d}}/{\it \mu}_{d} = [0.9 \pm 0.5 (stat.) \pm 1.4 (syst.)] \times 10^{-4}}$ and $\bf{{\it \Delta \mu}_{^{3}He ^{3}\overline{He}}/{\it \mu}_{^{3}He} = [-1.2 \pm 0.9 (stat.) \pm 1.0 (syst.)] \times  10^{-3}}$. Moreover, combining these results with existing measurements of the masses of the (anti-)nucleons, the relative binding energy differences are $\bf{{\it \Delta \epsilon}_{d\bar{d}}/{\it \epsilon}_{d} = -0.04 \pm 0.05(stat.) \pm 0.12(syst.)}$ and $\bf{{\it \Delta \epsilon}_{^{3}He ^{3}\overline{He}}/{\it \epsilon}_{^{3}He} = 0.24 \pm 0.16(stat.) \pm 0.18(syst.)}$. These results confirm, to an unprecedented precision in the sector of light nuclei, the invariance of a fundamental symmetry of nature, CPT, which  implies that all physics laws should be the same under the simultaneous exchange of particles with anti-particles (charge conjugation C), the reflection of spacial coordinates (parity transformation P) and the time inversion (T).

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