Abstract:
The foundation of our modern understanding of physics is built on our understanding the symmetries of nature. Of these symmetries, the universal symmetry of CPT is one of the most fundamental, underpinning both Quantum Field Theory and the General Theory of Relativity. CPT symmetry states that all systems are invariant under a simultaneous reversal of charge, parity and time direction. As a test of CPT symmetry, we measured the mass difference of ³H, ³He, and ⁴He with their anti-matter counterparts. The mass of nuclei is determined not just by the masses of the constituent nucleons, but also by the binding energy holding them together resulting from the strong interaction. Using Ru+Ru and Zr+Zr collision data at 200 GeV, colloquially known as the isobar dataset from STAR, we were able to identify and measure the mass difference of these products produced by the collisions. The mass difference of Triton, Helium 3, and Alpha with their antimatter counterparts is -0.64±0.58(stat.)±1.7(sys.), -0.98±0.61(stat.)±1.6(sys.), and -0.83±21.3(stat.)±34.4(sys.) MeV/c² Respectively. These are the first published results of the measurement of mass difference between triton and anti-triton and alpha and anti-alpha. All results are consistent with zero, as predicted by CPT symmetry. 

Figure 1: <dE/dx> plots of all tracks with a TOF mass² measurement within 20% of expected mass value. The black lines correspond to fit lines for the expected <dE/dx> measurement for the respective nuclei. For all nuclei there is a clear grouping of tracks around the expected <dE/dx> measurement curve, indicating the presence of these nuclei in the debris of the collision. Curves for (anti) Helium are fitted to correct for 0.88 sigma offset.

Figure 2: ∆m²/m² for identified protons (|nσ_<dE/dx>| <3.0) as a function of Transverse Momentum. The  P_T dependence is due to calibration error of the TPC caused by spatial charge distortions [Nucl.Instrum.Meth. A566 (2006) 22-25]. This mass measurement difference was used to correct the momentum measurements for the light nuclei. Protons in particular were chosen over other light hadrons due to their clear signal at high P_T.

Figure 3: mass distributions of respective Nuclei. Red data points are the positively charged nuclei. Black Data are the negatively charged nuclei. The Negatively charged mass distribution data was rescaled and shifted along the x axis to minimize its shift to minimize the x² between it and the positively charged mass distribution data. This method of comparison removes the need from background estimation when fitting the distribution.

Figure 4: measurements of ∆m/m of respective particles. For Clarity there is a y axis offset for the Neutron (+0.1 GeV/c² offset), and both ³He and Triton measurements(±0.1 GeV/c² offset). Brackets represent systematic errors, bars represent statistical errors. The value and error of the mass difference for ⁴He is scaled down by a factor of 0.1.

Conclusion:

In this work we have measured the mass differences between Triton, Helium 3, and Alpha with their antimatter counterparts to be -0.64±0.58(stat.)±1.7(sys.), -0.98±0.61(stat.)±1.6(sys.), and -0.83±21.3(stat.)±34.4(sys.) MeV/c² Respectively. These values are consistent with zero, as predicted by CPT symmetry. This is the first time the mass of anti-alpha has been measured after its first observation in 2011, and is the most massive antimatter state measured. The mass difference measurement of Helium 3 is over a 2 fold increase in precision over the previous result. 

Target Journal: Physical Review Letters