The LHCb collaboration has just presented at the Rencontres de Moriond EW and in a special CERN Seminar the first observation of CP violation in charm particle decays. Quarks can be split into two sectors: those with the same electrical charge as the up quark (up-type quarks, charge +2/3), and those with the same as the down quark (down-type quarks, charge -1/3). Differences in the properties of matter and antimatter, arising from the so-called phenomenon of CP violation, had been observed in the past using the decays of K and B mesons, i.e. of particles that contain strange or beauty quarks, which are both down-type quarks. By contrast, despite decades of experimental searches, CP violation in the decays of charmed particles, i.e. containing the charm quark, which is an up-type quark, escaped detection so far. The result announced today constitutes the first observation of CP violation in decays of a charmed particle.

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The final result, which uses essentially the full data sample collected by LHCb so far, is given by the quantity ΔACP=(-0.154±0.029)%, whose difference from zero quantifies the amount of CP violation observed. In particular, this turns out to differ from zero by 5.3σ (5.3 standard deviations), thus surpassing the threshold of 5σ adopted by particle physicists to assert without reservation that a discovery is made. This represents the first observation of CP violation in charm particle decays, opening up a new field in particle physics: the study of CP-violating effects in the sector of up-type quarks, and searches for new physics effects using charm CP asymmetry measurements.

CERN: Study sheds light on one of physics' biggest mysteries – why there's more matter than antimatter

Why do we exist? This is arguably the most profound question there is and one that may seem completely outside the scope of particle physics. But our new experiment at CERN’s Large Hadron Collider has taken us a step closer to figuring it out.

To understand why, let’s go back in time some 13.8 billion years to the Big Bang. This event produced equal amounts of the matter you are made of and something called antimatter. It is believed that every particle has an antimatter companion that is virtually identical to itself, but with the opposite charge. When a particle and its antiparticle meet, they annihilate each other – disappearing in a burst of light.

Why the universe we see today is made entirely out of matter is one of the greatest mysteries of modern physics. Had there ever been an equal amount of antimatter, everything in the universe would have been annihilated. Our research has unveiled a new source of this asymmetry between matter and antimatter.

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