A better understanding of the Standard Model

Mass distribution of the suppressed B-→D0K- decay for both           positive and negative charges. A large asymmetry is predicted for the signal shown in red.

Mass distribution of the suppressed B→D0K decay for both positive and negative charges. A large asymmetry is predicted for the signal shown in red.

Up until 1964, physicists believed that the laws of physics governing our universe were identical by interchanging particles with their antiparticles (C symmetry or charge reversal invariance) and by observing the particles through a mirror (P symmetry or mirror symmetry). This was not the case.

Physicists discovered that a type of meson called a kaon violates this charge-parity (CP) symmetry and that other heavier cousins, called B mesons, show even larger violations.

CDF looked at CP violation in B meson decays. The decay B→D0K, is a decay channel for which large CP asymmetries are expected. This means that the behavior of positive and negative B mesons differ greatly in this channel.

In principle, the asymmetry in itself is easy to measure; one simply has to look for a difference in the number of positively charged decays and the number of negatively charged decays. However, this decay is rare and easily confused with other similar decays, so what is easy to describe is complicated in practice.

Once the asymmetry is accurately measured, scientists can determine the value of angle gamma, the least understood parameter of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. The CKM matrix tells us how quarks interact with each other in the Standard Model. If physicists observe deviations from what the CKM matrix predicts, that would indicate the presence of new particles or interactions. The angle gamma’s value is a critically important ingredient in this game.

Once a B meson is produced, the B→D0K decay occurs one out of 10 million decays. Its reconstruction requires a lot of care, to not confuse the B decay from the aforementioned background processes that could mimic it. Because of its rarity, only a handful of these suppressed decays have been identified and measured by dedicated experiments at B factories, such as BaBar and Belle.

In principle, larger samples are accessible at hadron colliders, but the backgrounds are even more severe and have previously prevented this kind of analysis. A CDF team has now been able to reconstruct this decay for the first time in hadron collisions. Using an integrated luminosity of 7 inverse femtobarns, CDF physicists obtained a signal for B→D0K with significance above 3 σ, and measured an asymmetry of -0.82±0.44±0.09. The result contributes to the world knowledge of the gamma angle.

Learn more

—Edited by Andy Beretvas and
Diego Tonelli

These CDF physicist contributed to this data analysis. From the left: Paola Squillacioti, Paola Garosi (both from INFN Pisa and Siena University, Italy) and Giovanni Punzi (Pisa University and CDF co-spokesperson).

These CDF physicist contributed to this data analysis. From the left: Paola Squillacioti, Paola Garosi (both from INFN Pisa and Siena University, Italy) and Giovanni Punzi (Pisa University and CDF co-spokesperson).