Evidence for Higgs boson contributions to the production of Z boson pairs at high energies

The Higgs boson, the fundamental subatomic particle associated with the Higgs field, was first discovered in 2012 as part of the ATLAS and CMS experiments, which both analyze data collected at CERN’s Large Hadron Collider (LHC) , the most powerful particle accelerator that exists. . Since the discovery of the Higgs boson, research teams around the world have been trying to better understand the properties and characteristics of this unique particle.

The CMS Collaboration, the large group of researchers involved in the CMS experiment, recently obtained an updated measurement of the width of the Higgs boson, while collecting the first evidence of its out-of-shell contributions to the production of Z boson pairs. Their findings, published in Natural Physicsare consistent with the predictions of the standard model.

“The quantum-theoretical description of fundamental particles is probabilistic in nature, and if you consider all the different states of a collection of particles, their probabilities must always add up to 1, whether you look at that collection now or later,” Ulascan said. Sarica, researcher for the CMS collaboration, told Phys.org. “When analyzed mathematically, this simple statement imposes restrictions, the so-called unitarity limits, on the probabilities of high-energy particle interactions.”

Since the 1970s, physicists have predicted that when pairs of Z or W heavy vector bosons are produced, the typical high-energy restrictions would be violated, unless a Higgs boson contributes to the production of these pairs. Over the past ten years, theoretical physics calculations have shown that the occurrence of these high-energy Higgs boson contributions should be measurable using existing data collected by the LHC.

“Further research has shown that the total decay width of the Higgs boson, which is inversely proportional to its lifetime and predicted in the Standard Model to be particularly small (4.1 mega-electron volts wide, or 1 .6×10^-22 seconds of lifetime) can be determined using these high-energy events with an accuracy at least a hundred times better than other techniques limited by detector resolution (1000 mega-electron volts in width measurements total and 1.9 × 10^-13 seconds in lifetime measurements),” Sarica explained.

“For these reasons, our paper had two objectives: to search for the presence of Higgs boson contributions to the production of high-energy heavy dibosons, and to measure the total Higgs boson decay width as precisely as possible via these contributions.”

As part of their recent study, the CMS collaboration analyzed some of the data collected between 2015 and 2018, as part of the second LHC data collection campaign. They specifically focused on events characterized by the production of pairs of Z bosons, which then decayed into four charged leptons (i.e. electrons or muons) or into two charged leptons and two neutrinos.

Past experimental analyzes suggest that these two unique patterns are the most sensitive to the production of high-energy heavy boson pairs. By analyzing events that fit these patterns, the team therefore hoped to gather clearer and more reliable results.

“We observed the first evidence of Higgs boson contributions in the production of high-energy Z boson pairs with a statistical significance of more than 3 standard deviations,” said Li Yuan, another member of the CMS collaboration, at Phys.org. “The result strongly supports the spontaneous electroweak symmetry breaking mechanism, which preserves unitarity in the production of heavy dibosons at high energies.”

In addition to gathering evidence for the Higgs boson’s contributions to ZZ production, the CMS collaboration was able to significantly improve existing measurements of the width or total lifetime of the Higgs boson decay. The measurement they collected was considered unattainable 10 years ago, given the small width of the particle (i.e. 4.1 mega-electron-volts as predicted by the Standard Model of Physics particles).

“Our result for this measurement is 3.2 mega-electron-volts with an upper error of 2.4 mega-electron-volts and a lower error of 1.7 mega-electron-volts,” Yuan said. “This result is consistent with Standard Model expectations so far, but there is still room for future measurement with even greater precision to deviate from the prediction.”

Recent work from the CMS collaboration offers new insights into the properties of the Higgs boson, while highlighting its contribution to the production of Z boson pairs. In their next studies, the researchers plan to continue their exploration of this fascinating subatomic particle using new data collected at the LHC and advanced analysis techniques.

“While our results reached statistical significance above the threshold of 3 standard deviations, generally taken as evidence in the particle physics community, more data are needed to be able to reach the threshold of 5 standard deviations in order to to claim a discovery,” Sarica said.

The third LHC data collection campaign started this year and is expected to continue until the end of 2025. Sarica, Yuan and the rest of the CMS collaboration have already started preparations that will allow them to measure the width of the boson from Higgs with precision using new data collected in this third round of data collection.

“Additionally, our CMS analysis does not yet include the analysis of high-energy events with four charged leptons from the 2018 data, and preparations are underway for its inclusion in an update,” Sarica added. .

“The recent preliminary results from the ATLAS collaboration, presented on November 9 at the Higgs 2022 conference, also provide independent confirmation of the evidence found by CMS. So once their results are submitted for peer review, we hope that the two collaborations will be able to discuss how the two analyzes can be combined to provide the best measurements of the contributions of the high-energy Higgs boson and its full width.”

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