Physicists from the ALICE experiment at CERN’s Large Hadron Collider have measured the difference between mass-to-charge ratios for antideuterons and deuterons (a proton, or hydrogen nucleus, with an additional neutron), as well as for antihelium-3 nuclei and helium-3 nuclei (two protons plus a neutron).
The ALICE detector. Image credit: Mona Schweizer / ALICE / CERN.
The new result, reported online in the journal Nature Physics, confirms the fundamental symmetry known as CPT in these light nuclei.
This symmetry of nature implies that all of the laws of physics are the same under the simultaneous reversal of charges (charge conjugation C), reflection of spatial coordinates (parity transformation P) and time inversion (T).
The result, which comes exactly fifty years after the discovery of the antideuteron at CERN and in the United States, improves on existing measurements by a factor of 10-100.
The ALICE (A Large Ion Collider Experiment) experiment records high-energy collisions of lead ions at the Large Hadron Collider, enabling it to study matter at extremely high temperatures and densities. The lead-ion collisions provide a copious source of particles and antiparticles, and nuclei and the corresponding antinuclei are produced at nearly equal rates.
This allows ALICE to make a detailed comparison of the properties of the nuclei and antinuclei that are most abundantly produced.
The experiment makes precise measurements of the curvature of particle tracks in the detector’s magnetic field and of the particles’ time of flight, and uses this information to determine the mass-to-charge ratios for the nuclei and antinuclei.
Deuteron – antideuteron (top) and helium-3 – antihelium-3 (bottom) mass-over-charge ratio difference measurements as a function of the particle rigidity. Image credit: J. Adam et al.
“The high precision of our time-of-flight detector, which determines the arrival time of particles and antiparticles with a resolution of 80 picoseconds, associated with the energy-loss measurement provided by our time-projection chamber, allows us to measure a clear signal for deuterons/antideuterons and helium-3/antihelium-3 over a wide range of momentum,” said Dr Paolo Giubellino, spokesperson for the ALICE collaboration.
The measured differences in the mass-to-charge ratios are compatible with zero within the estimated uncertainties, in agreement with expectations for CPT symmetry.
These measurements, as well as those that compare the protons with antiprotons, may further constrain theories that go beyond the existing Standard Model of particles and the forces through which they interact.
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J. Adam et al. Precision measurement of the mass difference between light nuclei and anti-nuclei. Nature Physics, published online August 17, 2015; doi: 10.1038/nphys3432
