The Big Bang is thought to have created equal amounts of matter and antimatter, yet the Universe today is made almost entirely of matter, so something must have happened to create this imbalance. The weak force of the Standard Model of particle physics is known to induce a behavioural difference between matter and antimatter – known as CP symmetry violation – in decays of particles containing quarks, one of the building blocks of matter. But these differences, or asymmetries, are hard to measure and insufficient to explain the matter–antimatter imbalance in the present-day Universe, prompting physicists to both measure precisely the known differences and to look for new ones. At a seminar held at CERN today, the LHCb collaboration reported how it has measured, more precisely than ever before, two key parameters that determine such matter–antimatter asymmetries. In 1964, James Cronin and Val Fitch discovered CP symmetry violation through their pioneering experiment at Brookhaven National Laboratory in the US, using decays of particles containing strange quarks. This finding challenged the long-held belief in this symmetry of nature and earned Cronin and Fitch the Nobel Prize in Physics in 1980. In 2001, the BaBar experiment in the US and the Belle experiment in Japan confirmed the existence of CP violation in decays of beauty mesons, particles with a beauty quark, solidifying our understanding of the nature of this phenomenon. This achievement ignited intense research efforts to further understand the mechanisms behind CP violation. In 2008, Makoto Kobayashi and Toshihide Maskawa received the Nobel Prize in Physics for their theoretical framework that elegantly explained the observed CP violation phenomena. It its latest studies, using the full dataset recorded by the LHCb detector during the second run of the Large Hadron Collider (LHC), the LHCb collaboration set out to measure with high precision two parameters that determine the amount of CP violation in decays of beauty mesons. One parameter determines the amount of CP violation in decays of neutral beauty mesons, which are made up of a bottom antiquark and a down quark. This is the same parameter as that measured by the BaBar and Belle experiments in 2001. The other parameter determines the amount of CP violation in decays of strange beauty mesons, which consist of a bottom antiquark and a strange quark. Specifically, these parameters determine the extent of time-dependent CP violation. This type of CP violation stems from the intriguing quantum interference that occurs when a particle and its antiparticle undergo decay. The particle has the ability to spontaneously transform into its antiparticle and vice versa. As this oscillation takes place, the decays of the particle and antiparticle interfere with each other, leading to a distinctive pattern of CP violation that changes over time. In other words, the amount of CP violation observed depends on the time the particle lives before decaying. This fascinating phenomenon provides physicists with key insights into the fundamental nature of particles and their symmetries. For both parameters, the new LHCb results, which are more precise than any equivalent result from a single experiment, are in line with the values predicted by the Standard Model. “These measurements are interpreted within our fundamental theory of particle physics, the Standard Model, improving the precision with which we can determine the difference between the behaviour of matter and antimatter,” explains LHCb spokesperson Chris Parkes. “Through more precise measurements, large improvements have been made in our knowledge. These are key parameters that aid our search for unknown effects from beyond our current theory.” Future data, from the third run of the LHC and the collider’s planned upgrade, the High-Luminosity LHC, will further tighten the precision on these matter–antimatter asymmetry parameters and perhaps point to new physics phenomena that could help shed light on what is one of the Universe’s best-kept secrets.  Find out more on LHCb's website: precise measurement of the CP-violating phase φs and precise measurement of the unitarity triangle angle β  

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Today the international LHCb collaboration at the Large Hadron Collider (LHC) presented new measurements of rare particle transformations, or decays, that provide one of the highest-precision tests yet of a key property of the Standard Model of particle physics, known as lepton flavour universality. Previous studies of these decays had hinted at intriguing tensions with the theoretical predictions, potentially due to the effects of new particles or forces. The results of the improved and wider-reaching analysis based on the full LHC dataset collected by the experiment during Run 1 and Run 2, which were presented at a seminar at CERN held this morning, are in line with the Standard Model expectation. A central mystery of particle physics is why the 12 elementary quarks and leptons are arranged in pairs across three generations that are identical in all but mass, with ordinary matter comprising particles from the first, lightest generation. Lepton flavour universality states that the fundamental forces are blind to the generation to which a lepton belongs. In recent years, however, an accumulation of results from LHCb and experiments in Japan and the US have suggested that this might not be the case, generating cautious excitement among physicists that a more fundamental theory – perhaps one that sheds light on the Standard Model’s mysterious flavour structure – might reveal itself at the LHC. Interest in the “flavour anomalies” peaked in March 2021, when LHCb presented new results comparing the rates at which certain B mesons, composite particles that contain beauty quarks, decay into muons and electrons. According to the theory, decays involving muons and electrons should occur at the same rate, once differences in the leptons’ masses are accounted for. But the LHCb results hinted that B mesons decay into muons at a lower rate than predicted, as indicated by the results’ statistical significance of 3.1 standard deviations from the Standard Model prediction. The new LHCb analysis, which has been ongoing for the past five years, is more comprehensive. It considers two different B-meson decay modes simultaneously for the first time and provides better control of the background processes that can mimic the decays of B-mesons to electrons. In addition, the two decay modes are measured in two different mass regions, thus yielding four independent comparisons of the decays. The results, which supersede previous comparisons, are in excellent agreement with the principle of lepton flavour universality. “Measurements of the ratios of rare B-meson decays to electrons and muons have generated much interest in recent years because they are theoretically ‘clean’ and show consistency with a pattern of anomalies seen in other flavour processes,” explains LHCb spokesperson Chris Parkes of the University of Manchester and CERN. “The results shown today are the product of a comprehensive study of the two main modes using our full data sample and applying new, more robust techniques. These results are compatible with the expectation of our theory.”  New datasets will allow LHCb – one of the four large experiments at the LHC at CERN – to investigate lepton flavour universality further, in addition to conducting a wider research programme that includes studies of new hadrons, including the search for exotic tetraquarks and pentaquarks and investigation of the differences between matter and antimatter. An upgraded version of the experiment now in operation for LHC Run 3 will collect larger datasets that will allow even higher-precision tests of rare particle decays. “Earlier LHCb indications of anomalies concerning lepton flavour universality triggered excitement,” says theoretical physicist Michelangelo Mangano of CERN. “That such anomalies could potentially have been real shows just how much remains unknown, since theoretical interpretations exposed a myriad of unanticipated possible phenomena. The latest LHCb findings take nothing away from our mission to push the boundary of our knowledge further, and the search for anomalies, guided by experimental hints, goes on!” Read more on the LHCb website and in the CERN Courier.

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Protons can hold an elementary particle heavier than themselves.

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Brian Stelter working hard at CNN. In June TGP’s Cristina Laila reported that Brian Stelter’s days were numbered at CNN. We were right! CNN canceled the Sunday media affairs program Reliable Sources this week. Brian Stelter is out. NPR reported: CNN is canceling its Sunday media affairs show Reliable Sources, and host Brian Stelter is…

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