The Higgs boson plays a special role in the Standard Model: It is through the Higgs mechanism that the elementary particles acquire their masses, and their values are determined by the strength of their interaction, also referred to as “coupling,” with the Higgs boson. The Higgs boson also interacts with itself, and the strength of this self-coupling has profound implications on the mechanism of the electroweak phase transition the universe underwent shortly after the Big Bang and on the ultimate fate of the universe itself. This self-coupling strength can be measured at the LHC by measuring the rate of double Higgs boson production. Precise measurements of this parameter are among the top priorities of particle physics and the LHC physics program.
A new search for Higgs boson pairs (HH) with the CMS experiment has pushed us closer than ever to a measurement of the Higgs boson self-coupling. CMS physicists searched for two so-called boosted Higgs bosons that have momentum so large that the decay products of each Higgs boson merge within a single jet, i.e., into a single conical spray of particles.
Figure 1: A candidate event in which two Higgs bosons produced at large transverse momenta decay into collimated bottom quark-antiquark pairs, represented by the orange cones. The event signature is consistent with gluon fusion production of a Higgs boson pair. Image: CMS collaboration
A candidate event captured by the CMS detector is shown in Fig. 1. New results from the search were recently published by the CMS collaboration, which show this to be the most sensitive single channel at CMS in its search for the double Higgs boson process.
The data are found to agree with the background-only hypothesis, as shown in Fig. 2, and an observed (expected) upper limit at 95% confidence level is set at 9.9 (5.1) times the Standard Model expectation. Moreover, the search makes a novel measurement of the vector-boson fusion production mode of a pair of Higgs bosons, and obtain for the first time ever, at more than 99.99% confidence level, a non-zero value of the coupling strength between a pair of Higgs bosons and a pair of electroweak vector bosons.
Figure 2: The distributions for data, background and signal contributions for the sub-leading jet mass. The lower panel shows the ratio of the data and the total prediction, with its uncertainty represented by the shaded band. Image: CMS collaboration
A key feature of the new result is the use of state-of-the-art machine learning methods, which are quickly becoming critical tools in particle physics data analysis. The current search uses graph neural networks trained on kinematic and topological data of highly granular particle constituents of jets to optimally distinguish a boosted jet produced by a Higgs boson from similar jets produced by background processes. This novel Higgs boson jet-tagging approach has resulted in an overall improvement of the boosted Higgs boson pair search sensitivity by more than a factor of 10 over the previous best result, thereby making this channel the most sensitive one at CMS.
Figure 3: These physicists comprise the LPC team that contributed to the search for Higgs boson pairs at CMS. Image: CMS collaboration
As the focus turns to the High-Luminosity LHC (HL-LHC), the boosted Higgs pair channel may play a key role in the future observation of the HH process. The significance for observing HH production at the end of HL-LHC has been projected to be close to 4 standard deviations. With the inclusion of this highly sensitive boosted HH channel, in combination with all other HH searches at the LHC, it is possible that the golden 5-standard-deviation threshold for observation of HH would be achieved at the HL-LHC.
Artur Apresyan is a scientist at Fermilab, and Si Xie is a joint associate scientist at Fermilab and Caltech research assistant professor.
CMS communications are coordinated by Fermilab distinguished scientist Pushpa Bhat.