Yesterday at the International Conference on High Energy Physics in Valencia, Spain, CERN’s LHC experiments presented their most recent results concerning the properties of the Higgs boson and searches for new physics and new physical processes.
Among these new results were fresh measurements of the mass and width of the Higgs boson and a wealth of new searches for supersymmetry by the ATLAS and CMS collaborations, as well as studies of an unusually-behaved hadron by LHCb. Many of the presentations underlined the essential element of repeatability in science, as both ATLAS and CMS presented results backing up earlier results from the other experiment.
Over the past year, CMS and ATLAS have both greatly improved their analysis techniques and calibrations of their detectors. This has enabled both experiments to improve substantially on their previous measurements of the properties of the Higgs boson. All of these measurements are aligning with the Standard Model’s predictions.
Today ATLAS presented their first measurements of Higgs production in a process with energy far above the mass of the particle itself. This probes how strongly these so-called virtual Higgs bosons interact with other particles in a different way to measurements with real Higgs bosons. The study can also be recast into a constraint on the width (lifetime) of the Higgs particle, and suggests that the width is consistent with, and not more than a few times larger than the value expected in the Standard Model. A similar analysis was presented previously by CMS.
The experiments have also improved their ability to track the different types of Higgs boson decays. CMS was able to verify with 5-sigma significance that the Higgs boson decays to two photons at the rate predicted by the Standard Model, backing up a similar measurement previously reported by ATLAS. The collaboration also presented its final mass measurement of the Higgs boson based on this decay pattern.
A better understanding of the Higgs bosons’ decay patterns has also allowed ATLAS and CMS to combine results from the different decay channels. Both experiments presented their most up-to-date mass measurements of the Higgs boson by combining measurements from its decays to two photons and with measurements from its decays to four leptons through two Z bosons.
All of these new mass measurements are centred around 125 GeV and have significantly reduced experimental uncertainties. These new mass measurements are still dominated by statistical uncertainty, however, which will only be improved once the experiments are able to collect more data when the LHC restarts in 2015.
The two experiments presented an impressive number of new analyses in the search for supersymmetric particles, testing new scenarios and revisiting old ones with all the data collected in the LHC’s first run. Searching for new particles is like looking for a missing person over a very large territory. Each experimental analysis is like the report from one search party. Combining these new results with previous ones allows scientists to zoom in on the most likely places that supersymmetric particles might be found. The studies showed today made two points clear: some theoretical models are now highly disfavoured while others models predict possible discoveries after the LHC resumes operation at higher energy in 2015.
CMS and ATLAS also presented first evidence for the production of a top-antitop quark pair in association with a W or Z boson. This is a very rare process, about one thousand times rarer than top-antitop production alone, and it is of special interest because of the heavy particles involved in the process. Studying this process is a milestone on the way to observing the even rarer production of a Higgs boson radiated off a top quark that was produced as top-antitop pair.
The LHCb experiment presented new results about the decay patterns of a short-lived hadron. The experiment found that a beauty-anti-beauty meson is actually produced through the radioactive decay of one of its excited states about 50% of the time. Because the rate of production during the proton-proton collision does not match the rate of production during the lead-lead collisions, this new result could be evidence for the formation of a quark-gluon-plasma during heavy ion collisions.
For more information, visit the experiments’ websites: