Evidence of contributions of the Higgs boson to the production of pairs of high-energy Z bosons

Left: Two-parameter likelihood scan of off-shell production signal intensity parameters gg and EW, 𝜇off-shellF and 𝜇off-shellV, respectively. The dotted and dotted contours 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 built-in brightness only reaches up to 138 fb-1 since 4ℓ on-shell events are not included in the run of this scan. Right: Observed (solid) and expected (dashed) one-parameter probability scans on ΓH. Scans are shown for the combination of 4ℓ on-shell data with 4ℓ off-shell data (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 built-in brightness reaches up to 140 fb-1 as 4ℓ on-shell events are included in the execution of these scans. The exclusion of the no off-shell hypothesis is consistent with 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 largest existing powerful particle accelerator. Since the discovery of the Higgs boson, research teams from 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, also gathering the first evidence of its off-shell contributions to Z boson pair production. Their results , 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 look at all the different states of a set of particles, their probabilities must always add up to 1, regardless of whether you look at this set now or at some time next,” Ulascan Sarica, researcher for the CMS Collaboration, told Phys.org. ‘When analyzed mathematically, this simple statement imposes restrictions, so-called unit limits, on the probabilities of high-energy particle interactions.’

Since the 1970s, physicists have predicted that when Z or W heavy vector boson pairs are produced, typical high-energy restrictions would be violated, unless a Higgs boson contributes to the production of these pairs. Over the past decade, 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.

“Other investigations have shown that the total decay amplitude of the Higgs boson, which is inversely proportional to its lifetime and predicted in the standard model, is remarkably small (4.1 mega-electron volts wide, or 1.6×10-22 seconds in life) can be determined using these high-energy events with at least a hundred times better accuracy than other techniques limited by detector resolution (1000 mega-electron volts in full width measurements and 1.9×10-13 seconds in waist measurements),” Sarica explained.

“For these reasons, our paper had two objectives: to look for the presence of Higgs boson contributions to the production of heavy dibosons at high energies, and to measure the total width of the Higgs boson decay as precisely as possible through 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. They particularly focused on events known from the production of pairs of Z bosons, which subsequently decayed into four charged leptons (i.e. electrons or muons) or into two charged leptons and two neutrinos.

Previous experimental analyzes suggest that these two unique models are the most sensitive to the production of heavy pairs of high-energy bosons. By analyzing events that matched these patterns, therefore, the team hoped to glean 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,” Li Yuan, another member of the CMS collaboration, told 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 contributions of the Higgs boson to ZZ production, the CMS collaboration was able to significantly improve existing measurements of the total decay amplitude or lifetime of the Higgs boson. The measurement they collected was believed to be unattainable 10 years ago, given the particle’s narrow width (i.e., 4.1 mega-electron volts as predicted by the Standard Model of particle physics).

“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 expectations from the standard model thus far, but there is still room for a future measurement with even greater precision to deviate from the prediction.”

Recent work by the CMS collaboration offers new insights into the properties of the Higgs boson, 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.

“Although our results reached statistical significance beyond the 3 standard deviation threshold, which is generally regarded as evidence in the particle physics community, more data is needed for us to reach the 5 standard deviation threshold in order to claim a discovery.” Sarah said.

The third round of LHC data collection 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 Higgs boson with dimensions still greater. accuracy using the new data collected as part of this third round of data collection.

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

“The recent preliminary results of the ATLAS collaboration, presented on November 9 during the Higgs 2022 conference, also provide independent confirmation of the evidence found by CMS, so once their findings are peer reviewed, we hope the two collaborations can discuss how two analyzes can be combined to provide the best measurements of the contributions of the high-energy Higgs boson and its overall width.”

More information:
The CMS Collaboration, Measuring the amplitude of the Higgs boson and proving its off-shell contributions to ZZ production, Natural Physics (2022). DOI: 10.1038/s41567-022-01682-0

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

© 2022 Science X Network

Citation: Evidence for contributions of the Higgs boson to the production of high energy Z boson pairs (2022 Nov 25) retrieved Nov 25 2022 from https://phys.org/news/2022-11-evidence-higgs-boson- contributions-production .html

This document is subject to copyright. Except in all propriety for the purpose of private study or research, no part may be reproduced without written permission. The content is provided for informational purposes only.

Leave a Comment

%d bloggers like this: