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

Left: two-parameter probability scans of the output signal strength parameters gg and EW outside the shell, 𝜇off-shellF and 𝜇off-shellV, respectively. The dashed and dashed lines surround 68% (−2Δln𝐿 = 2.30) and 95% (−2Δln𝐿 = 5.99) CL regions. The intersection indicates the lower bound, and the blue diamonds represent the SM projections. The integrated brightness only reaches 138 fb−1 as 4ℓ events on the hull are not included in this scan procedure. Right: observed (solid) and predicted (dashed) one-parameter probability scans over H. Scans are shown to collect 4ℓ on-shell data with 4ℓ out-of-shell (purple) or 2ℓ2ν out-of-shell data (green) alone, or with both datasets ( black). The horizontal lines indicate 68% (−2Δln𝐿 = 1.0) and 95% (−2Δln𝐿 = 3.84) CL regions. The integrated luminosity reaches 140 fb−1 as 4ℓ events on the shell are included in making these scans. Excluding the no outer shell hypothesis corresponds to 3.6 s.d. in both panels. Credit: CMS Collaboration.

The Higgs boson, the fundamental subatomic particle associated with the Higgs field, was first discovered in 2012 as part of ATLAS and CMS experiments, both of which analyze data collected at CERN’s Large Hadron Collider (LHC), the most powerful particle accelerator in existence. . 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, a large group of researchers involved in the CMS experiment, recently obtained an updated measurement of the width of the Higgs boson, while also gathering the first evidence of its extra-chance contributions to the production of Z-boson pairs. Their findings, published in nature physicsagrees with the predictions of the standard model.

“The quantum theoretical description of fundamental particles is probabilistic in nature, and if you look at all the different states of a group of particles, their probabilities should always add up to 1 regardless of whether you look at that group now or later,” Ulascan told Sarika, a researcher at CMS Collaboration, Phys.org website. “When analyzed mathematically, this simple statement imposes constraints, so-called unit limits, on the probabilities of particle interactions at high energies.”

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

Further investigations have shown that the total decay width of the Higgs boson, which is inversely proportional to its lifetime and would be expected in the Standard Model to be very small (4.1 MeV in width, or 1.6 x 10).-22 seconds in a lifetime) using these high-energy events with a resolution at least a hundred times better than other techniques limited by detector resolution (1000 MeV in total width measurements, and 1.9 x 10-13 seconds in lifetime measurements), Sarika explained.

“For these reasons, our paper had two goals: to search for the existence of contributions of the Higgs boson to the production of heavy binary bosons at high energies, and to measure as accurately as possible the width of the total decay of the Higgs boson from these contributions.”

As part of their latest study, the CMS collaboration analyzed some data collected between 2015 and 2018, as part of the Large Hadron Collider’s second data collection run. They focused in particular on events characterized by the production of pairs of Z bosons, which subsequently decay into either four charged leptons (i.e., electrons or muons) or charged leptons and two neutrinos.

Previous experimental analyzes indicate that these two unique models are most sensitive to producing heavy pairs of bosons at higher energies. By analyzing events that matched these patterns, the team hoped to get more clear and reliable results.

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

In addition to gathering evidence of Higgs boson contributions to ZZ production, the CMS collaboration was able to significantly improve existing measurements of displaying Higgs boson decay or lifetime. The measurement they collected was thought out of reach 10 years ago, given the particle’s narrow width (that’s 4.1 MeV according to predictions from the Standard Model of particle physics).

“Our result for this measurement is 3.2 MeV, with a higher error of 2.4 MeV and a lower error of 1.7 MeV,” Yuan said. “This result is consistent with Standard Model predictions so far, but there is still scope because future measurement with greater precision can deviate from the prediction.”

Recent work by the CMS collaboration provides new insight into the properties of the Higgs boson, while also highlighting its contribution to the production of Z boson pairs. In their next studies, the researchers plan to continue their exploration of these fascinating subatomic particles using new data collected at the LHC and advanced analysis techniques.

“While our results reached statistical significance beyond the 3 standard deviations threshold, usually taken as evidence in the particle physics community, more data is needed to reach the 5 standard deviations threshold in order to claim a discovery,” Sarika said.

The LHC’s third data collection began this year and is expected to continue through the end of 2025. Sarika, Yuan, and the rest of the CMS collaboration have already begun preparations that will allow them to measure the width of the Higgs boson with more precision. Accuracy using new data collected as part of this third round of data collection.

“In addition, our CMS analysis does not yet include analysis of high-energy events with four charged leptons from the 2018 data, and preparations are underway to include them in the update,” Sarika added.

“Recent preliminary results from the ATLAS Collaboration, presented November 9 during the Higgs 2022 conference, also provide independent confirmation of the evidence CMS is discovering, so once its results have been subject to peer review, we hope the two collaborations can discuss how the two analyzes can be combined to provide the best measures of the contributions of The Higgs boson at high energy and its total width.”

more information:
The CMS Collaboration, Measuring the Width of the Higgs Boson and Evidence for its Extra-Coincidence Contributions to ZZ Production, nature physics (2022). DOI: 10.1038/s41567-022-01682-0

Conference: indico.cern.ch/event/1086716/

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