Another charm revolution?

Mass distributions of the charm (left) and anticharm (right) decays used in the measurement. The different heights of the narrow peaks indicate instrumental and physics asymmetries.

Among quarks, the charm has played probably the biggest role in the development of the Standard Model. When its existence was postulated in 1970 to explain the absence of particle decays that should have otherwise been observed, not many physicists believed charm could be real. But it is, as it was spectacularly shown by two experiments, at Brookhaven and SLAC, four years later. The discovery of the charm quark is known in physicists’ lingo as the “November revolution” because it triggered a chain of experimental and theoretical advancements that produced the Standard Model as we know it today.

The Standard Model predicts that laws governing the decays of charm quarks differ if these particles are replaced by their antiparticles and observed in a mirror. This difference, called CP violation, is so tiny that no one has observed it to date.

A team of CDF scientists searched for the tiny differences by analyzing hundreds of thousands of charm decays into pairs of charged kaons and pions by sifting through thousands of billions proton-antiproton collisions from the Tevatron.

Because the detector is made of matter and not antimatter, it can generate instrumental differences that mimic or mask the physics differences. The CDF scientists devised a special measurement technique in which any instrumental effects cancel out from the results.

The results of our measurement of CP violating asymmetry parameter is: Δ ACP = -0.0062 ± 0.0021 (stat) ± 0.0010 (syst). This is a 2.7 sigma deviation from the expected result of there being no asymmetry. This measurement is in good agreement with the earlier result from the LHCb experiment at CERN. Calculations by theorists are unable to consistently explain an effect of this size. This measurement puts much pressure on the need for revising and improving the prediction techniques to make a clear distinction between the contributions expected from Standard Model physics and those from possible new interaction forces. Ideas for new precision measurements are likely to turn out as well. Our hope is that this is the beginning of a second revolution in charm physics.

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These CDF physicists contributed to this data analysis. From left: Angelo Di Canto (INFN Pisa/Heidelberg University), Diego Tonelli (Fermilab/CERN), Luciano Ristori (INFN Pisa, Fermilab and CDF co-spokesperson), Giovanni Punzi (Pisa University), Michael J. Morello (INFN Pisa).