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  

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.

Protons can hold an elementary particle heavier than themselves.

Today, the LHCb experiment at CERN is presenting a new discovery at the European Physical Society Conference on High Energy Physics (EPS-HEP). The new particle discovered by LHCb, labelled as Tcc+, is a tetraquark – an exotic hadron containing two quarks and two antiquarks. It is the longest-lived exotic matter particle ever discovered, and the first to contain two heavy quarks and two light antiquarks. Quarks are the fundamental building blocks from which matter is constructed. They combine to form hadrons, namely baryons, such as the proton and the neutron, which consist of three quarks, and mesons, which are formed as quark-antiquark pairs. In recent years a number of so-called exotic hadrons – particles with four or five quarks, instead of the conventional two or three - have been found. Today’s discovery is of a particularly unique exotic hadron, an exotic exotic hadron if you like. The new particle contains two charm quarks and an up and a down antiquark. Several tetraquarks have been discovered in recent years (including one with two charm quarks and two charm antiquarks), but this is the first one that contains two charm quarks, without charm antiquarks to balance them. Physicists call this “open charm” (in this case, “double open charm”). Particles containing a charm quark and a charm antiquark have “hidden charm” – the charm quantum number for the whole particle adds up to zero, just like a positive and a negative electrical charge would do. Here the charm quantum number adds up to two, so it has twice the charm! The quark content of Tcc+ has other interesting features besides being open charm. It is the first particle to be found that belongs to a class of tetraquarks with two heavy quarks and two light antiquarks. Such particles decay by transforming into a pair of mesons, each formed by one of the heavy quarks and one of the light antiquarks. According to some theoretical predictions, the mass of tetraquarks of this type should be very close to the sum of masses of the two mesons. Such proximity in mass makes the decay “difficult”, resulting in a longer lifetime of the particle, and indeed Tcc+ is the longest-lived exotic hadron found to date. The discovery paves the way for a search for heavier particles of the same type, with one or two charm quarks replaced by bottom quarks. The particle with two bottom quarks is especially interesting: according to calculations, its mass should be smaller than the sum of the masses of any pair of B mesons. This would make the decay not only unlikely, but actually forbidden: the particle would not be able to decay via the strong interaction and would have to do so via the weak interaction instead, which would make its lifetime several orders of magnitude longer than any previously observed exotic hadron. The new Tcc+ tetraquark is an enticing target for further study. The particles that it decays into are all comparatively easy to detect and, in combination with the small amount of the available energy in the decay, this leads to an excellent precision on its mass and allows the study of the quantum numbers of this fascinating particle. This, in turn, can provide a stringent test for existing theoretical models and could even potentially allow previously unreachable effects to be probed. Read more on the LHCb website and in the CERN Courier.