  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 cannot yet be directly derived from quantum chromodynamics. Here we report
a measurement of the difference between the ratios of the mass and charge of
deuterons and anti-deuterons, and $^{3}{\rm He}$ and $^3\overline{\rm He}$
nuclei carried out with the ALICE (A Large Ion Collider Experiment) detector in
Pb-Pb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV. Our
direct measurement of the mass-over-charge differences confirm CPT invariance
to an unprecedented precision in the sector of light nuclei. This fundamental
symmetry of nature, which exchanges particles with anti-particles, implies that
all physics laws are the same under the simultaneous reversal of charge(s)
(charge conjugation C), reflection of spatial coordinates (parity
transformation P) and time inversion (T).
