The LHCb collaboration at CERN has observed a phenomenon known as CP (charge and parity) violation in the decays of the D0 meson. The term CP refers to the transformation that swaps a particle with the mirror image of its antiparticle. The weak interactions of the Standard Model of particle physics are known to induce a difference in the behaviour of some particles and of their CP counterparts, an asymmetry known as CP violation. The finding, has been presented in a dedicated CERN seminar.
The D0 meson is made of a charm quark and an up antiquark. So far, CP violation has only been observed in particles containing a strange or a bottom quark. These observations have confirmed the pattern of CP violation described in the Standard Model by the so-called Cabibbo-Kobayashi-Maskawa (CKM) mixing matrix, which characterises how quarks of different types transform into each other via weak interactions. The deep origin of the CKM matrix, and the quest for additional sources and manifestations of CP violation, are among the big open questions of particle physics. The discovery of CP violation in the D0 meson is the first evidence of this asymmetry for the charm quark, adding new elements to the exploration of these questions.
To observe this CP asymmetry, the LHCb researchers used the full dataset delivered by the Large Hadron Collider (LHC) to the LHCb experiment between 2011 and 2018 to look for decays of the D0 meson and its antiparticle, the anti-D0, into either kaons or pions. “Looking for these two decay products in our unprecedented sample of D0 particles gave us the required sensitivity to measure the tiny amount of CP violation expected for such decays. Measuring the extent of the violation then boiled down to counting the D0and anti-D0 decays and taking the difference,” explained Giovanni Passaleva, spokesperson for the LHCb collaboration.
The result has a statistical significance of 5.3 standard deviations, exceeding the threshold of 5 standard deviations used by particle physicists to claim a discovery. This measurement will stimulate renewed theoretical work to assess its impact on the CKM description of CP violation built into the Standard Model, and will open the window to the search for possible new sources of CP violation using charmed particles.
The INFN Milano Unit participates in the LHCb collaboration with a group coordinated by Nicola Neri, which is responsible for the design and construction of the Upstream Tracker, a silicon tracker for the upgrade of the experiment currently underway.
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