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

Left: Two-parameter probability scan of the off-shell gg and EW production signal strength parameters, 𝜇off-shellF and 𝜇off-shellV, respectively. The dotted and dashed outlines enclose the 68% (−2Δln𝐿=2.30) and 95% (−2Δln𝐿=5.99) CL regions. The cross marks the minimum and the blue diamond marks the SM expectation. The integrated brightness only reaches 138 fb−1, as on-shell 4ℓ events are not included in running this scan. Right: observed (solid) and expected (dotted line) single-parameter probability scans over ΓH. Scans are shown for the combination of 4ℓ on-shell data with 4ℓ off-shell (magenta) or 2ℓ2ν off-shell data (green) alone, or with both datasets (black). The horizontal lines indicate the 68% (−2Δln𝐿=1.0) and 95% (−2Δln𝐿=3.84) CL regions. The integrated brightness reaches up to 140 fb−1 as on-shell 4ℓ events are included in running these scans. The exclusion of the no-off-shell hypothesis corresponds to 3.6 sd in both panels. Credit: The CMS Collaboration.

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, 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 sought 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 also gathering the first evidence of its off-shell contributions to the production of Z boson pairs. Their findings, published in nature physicsare consistent with standard model predictions.

“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 this collection now or sometime later,” Ulascan Sarica, researcher for the CMS Collaboration, told Phys.org. “When analyzed mathematically, this simple statement imposes constraints, called unitarity limits, on the likelihood of particle interactions at high energies.”

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

“Other studies have shown that the total decay width of the Higgs boson, which is inversely related to its lifetime and predicted in the Standard Model to be extremely small (4.1 mega-electron volts wide, or 1.6 × 10-22 seconds in lifetime) can be determined using these high-energy events with an accuracy at least a hundred times better than other techniques limited by the resolution of the detector (1000 mega-electron volts in overall width measurements and 1.9 × 10-13 seconds in lifetime measurements),’ explained Sarica.

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

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

Previous experimental analyzes suggest that these two unique patterns are most sensitive to the production of heavy pairs of bosons at high energies. By analyzing events that matched these patterns, the team therefore hoped to collect clearer and more reliable results.

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

In addition to accumulating evidence of Higgs boson contributions to ZZ production, the CMS collaboration was able to significantly improve existing measurements of the total Higgs boson decay width or lifetime. The measurement they collected was considered unachievable 10 years ago, given the particle’s narrow width (i.e., 4.1 mega-electron volts according to predictions from the Standard Model of particle physics).

“Our result for this measurement is 3.2 megaelectronvolts with an upper error of 2.4 megaelectronvolts and a lower error of 1.7 megaelectronvolts,” Yuan said. “This result is in line with the Standard Model’s expectation so far, but there is still scope that a future measurement could deviate from the prediction with even greater precision.”

The CMS collaboration’s recent work 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 this fascinating subatomic particle using new data collected at the LHC and advanced analysis techniques.

“While our results have reached statistical significance above the 3 standard deviation threshold, typically considered evidence in the particle physics community, more data is needed to reach the 5 standard deviation threshold to claim a discovery,” said Sarica.

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

“In addition, 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 inclusion in an update,” added Sarica.

“Recent preliminary results from the ATLAS collaboration, presented Nov. 9 at the Higgs 2022 conference, also provide independent confirmation of the evidence CMS finds, so once their results are peer-reviewed, we hope the two collaborations can discuss how the two analyzes can be combined to provide the best measurements of Higgs boson contributions at high energy and its total width.”

More information:
The CMS collaboration, measurement of the Higgs boson width and evidence of its off-shell contributions to ZZ production, nature physics (2022). DOI: 10.1038/s41567-022-01682-0

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

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Quote: Evidence of contributions from Higgs bosons to production of Z boson pairs at high energies (2022, Nov. 25) Retrieved Nov. 27, 2022 from https://phys.org/news/2022-11-evidence-higgs-boson- contributions-production .html

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