Explore new ways to see the Higgs Boson.
The ATLAS and CMS collaborations presented their latest results on new signatures to detect the Higgs boson at CERN’s Large Hadron Collider.
This media update is part of a series related to the 2020 Large Hadron Collider Physics conference, which took place May 25–30, 2020. Originally planned to take place in Paris, the conference was held entirely in line due to the COVID-19 pandemic.
The ATLAS and CMS collaborations presented their latest results on new signatures to detect the Higgs boson at CERN’s Large Hadron Collider. These include searches for rare transformations of the Higgs boson into a Z boson, which is the carrier of one of the fundamental forces of nature, and a second particle. Observing and studying transformations that are predicted to be rare helps advance our understanding of particle physics and could also point the way to new physics if observations differ from predictions. The results also included searches for signs of Higgs transformations in “invisible” particles, which could illuminate possible dark matter particles. The analyzes involved nearly 140 inverse femtobarns of data, or about 10 million million proton and proton collisions, recorded between 2015 and 2018.
ATLAS and CMS detectors can never see a Higgs boson directly: an ephemeral particle transforms (or “decays”) into lighter particles almost immediately after being produced in proton and proton collisions, and the lighter particles allow the difference between them. signatures on detectors. However, other standard model signatures can produce similar signatures. Therefore, scientists must first identify the individual pieces that match this signature and then accumulate enough statistical evidence to confirm that the collisions had produced Higgs bosons.
When it was discovered in 2012, the Higgs boson was observed mainly in transformations in pairs of Z bosons and pairs of photons. These so-called “decay channels” have relatively clean signatures that make them more easily detectable, and have been observed at the LHC. Other transformations are expected to occur very rarely, or to have a less clear signature, and are therefore difficult to detect.
At LHCP, ATLAS presented the latest results of its searches for such a rare process, in which a Higgs boson transforms into a Z boson and a photon. The Z thus produced, being unstable, is transformed into pairs of leptons, either electrons or muons, leaving a signature of two leptons and one photon in the detector. Given the low probability of observing a Higgs to Z transformation with the volume of data analyzed, ATLAS was able to rule out the possibility that more than 0.55% of the Higgs bosons produced at the LHC were transformed into Z. “ With this analysis, “says Karl Jakobs, spokesperson for the ATLAS collaboration,” we can show that our experimental sensitivity for this signature has now come close to the prediction of the standard model. “ The best extracted value for H-Z signal intensity, defined as the ratio of observed standard model signal performance to predicted standard model signal performance, is 2,0^+ 1.0_0,9.
CMS presented the results of the first search for Higgs transformations that also involve a Z boson, but accompanied by an é (rho) or la (phi) meson. The Z boson transforms once more into pairs of leptons, while the second particle transforms into pairs of pions in the case of o and into pairs of kaons (KK) in the case of... “These transformations are extremely rare “says Roberto Carlin, spokesperson for the CMS collaboration,” and it is not expected to be observed at the LHC unless TB physics is involved. “ The analyzed data allowed CMS to rule out that more than 1.9% of Higgs bosons could transform into Z and more than 0.6% could transform into Z. Although these limits are much greater than the predictions of the Standard Model, they demonstrate the ability of detectors to make forays into the pursuit of physics beyond the Standard Model.
The so-called “dark sector” includes hypothetical particles that could form dark matter, the mysterious element that represents more than five times the mass of ordinary matter in the universe. Scientists believe that the Higgs boson could contain clues about the nature of dark matter particles, as some extensions of the Standard Model propose that a Higgs boson could transform into dark matter particles. These particles would not interact with ATLAS and CMS detectors, which means that they remain “invisible” to them. This would allow them to escape direct detection and manifest as “missing energy” in the collision event. At LHCP, ATLAS presented its latest upper limit, 13%, on the probability that a Higgs boson could transform into invisible particles known as weakly interacting massive particles, or WIMPs, while CMS presented the results of a new search for Higgs transformations in four leptons. through at least one intermediate “dark photon”, which also has limits on the probability of such a transformation occurring at the LHC.
The Higgs boson remains invaluable in helping scientists test the Standard Model of particle physics and search for physics that may lie beyond. These are just some of the many results regarding the Higgs boson that were presented at the LHCP. You can read more about them on the ATLAS and CMS websites.
Technical note:
When data volumes are not high enough to claim a definitive observation of a particular process, physicists can predict the limits they hope to place on the process. In the case of Higgs transformations, these limits are based on the product of two terms: the rate at which a Higgs boson is produced in proton and proton collisions (production cross-section) and the rate at which it will undergo a particular transformation to lighter particles (branching fraction).
ATLAS expected to place an upper limit of 1.7 times the Standard Model expectation for the process involving Higgs transformations into a Z boson and a photon (H-Z) if such a transformation were not present; the collaboration was able to place an upper limit of 3.6 times this value, approaching the sensitivity to the predictions of the standard model. The CMS searches were a much rarer process, predicted by the Standard Model to occur only once in every million Higgs transformations, and the collaboration was able to set upper limits of approximately 1000 times the Standard Model expectations for the H-Z and H-Z processes.
