A charming result

These plots show the effective lifetime asymmetries as function of decay time for D →K+K (top) and D → π+π (bottom) samples. Results of the fits not allowing for (dotted red line) and allowing for (solid blue line) CP violation are overlaid.

Physicists gave funny names to the heavy quark cousins of those that make up ordinary matter: charm, strange, bottom, top. The Standard Model predicts that the laws governing the decays of strange, charm and bottom quarks differ if particles are replaced with antiparticles and observed in a mirror. This difference, CP violation in particle physics lingo, has been established for strange and bottom quarks. But for charm quarks the differences are so tiny that no one has observed them so far. Observing differences larger than predictions could provide much sought-after indications of new phenomena.

A team of CDF scientists searched for these tiny differences by analyzing millions of decays of particles decaying into pairs of charged kaons and pions, sifting through roughly a thousand trillion proton-antiproton collisions from the full CDF Run II data set. They studied CP violation by looking at whether the difference between the numbers of charm and anticharm decays occurring in each chunk of decay time varies with decay time itself.

The results have a tiny uncertainty (two parts per thousand) but do not show any evidence for CP violation, as shown in the upper figure. The small residual decay asymmetry, which is constant in decay time, is due to the asymmetric layout of the detector. The combined result of charm decays into a pair of kaons and a pair of pions is the CP asymmetry parameter AΓ , which is equal to -0.12 ± 0.12 percent. The results are consistent with the current best determinations. Combined with them, they will improve the exclusion constraints on the presence of new phenomena in nature.

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These physicists are the primary analysts for this result. Top row, from left: Angelo Di Canto (CERN, formerly Fermilab/INFN Pisa), Sabato Leo (University of Illinois) and Paolo Maestro (University of Siena). Second row, from left: Kevin Pitts (University of Illinois) and Diego Tonelli (CERN, formerly Fermilab).