In a recent result, the CMS experiment has combined a comprehensive set of searches for the production of not one but two Higgs bosons – the result is a significant step towards observation of this elusive process, and constitutes a legacy of the Run 2 data taking era at the LHC.
Have you ever wondered if Higgs bosons can interact with themselves? Probing this interaction – dubbed Higgs self-coupling – can give us clues about the formation of structure in the early universe immediately after the Big Bang, and tell us how stable the vacuum of our universe really is!
Since the CMS and ATLAS experiments announced the discovery of the Higgs boson in 2012, they have been measuring its mass and interaction with other particles with ever increasing precision. So far, all the measurements appear to be consistent with the Standard Model (SM) of particle physics. The CMS experiment is now doubling down its efforts to experimentally observe the interaction of the Higgs boson with itself, an expected property that has been elusive so far.
The Higgs self-interaction can be studied directly by investigating the production of pairs of Higgs bosons (HH) at the LHC. In proton-proton collisions, the HH pairs can be produced in two primary ways. The first is the so called "gluon-gluon fusion" where gluons, constituents of the protons, interact via a top quark loop (Fig. 1); this mechanism allows us to study the interaction between one intermediate-state Higgs boson and two final-state Higgs bosons. In the second, two vector bosons are radiated by quarks, also constituents of the protons, and the vector bosons fuse with each other, creating one or two Higgs bosons (Fig. 2); this allows us to study the interaction between two Higgs bosons and two vector bosons.
Figure 1: Producing Higgs boson pairs by fusing gluons.
Figure 2: Producing Higgs boson pairs by fusing vector bosons.
The recent CMS study was done using 138 fb-1 of data collected in proton-proton collisions at 13 TeV. Analysis was performed combining multiple HH decay channels, wherein a plethora of final states resulting from Higgs bosons decaying to bottom quarks, W bosons, tau leptons, and photons were considered. Analysing all the data simultaneously – and utilising sophisticated analysis techniques, including boosted decision trees and deep neural networks – allows us to extract more information and achieve better results than ever before.
Despite all the efforts, so far, Higgs boson pair production remains unobserved, The study establishes upper bounds at 95% confidence level on the Higgs boson pair production rate (Fig. 3). The measured bounds are currently at 3.5 times the SM expectation for the total HH production, and 79 times the SM expectation for HH production by fusing vector bosons.
Figure 3: 95% confidence level bounds on rates of HH production (black markers). The inner (green) and the outer (yellow) bands indicate the 68% and 95% quantiles, respectively, under a background-only hypothesis.
Along with measuring limits on the SM Higgs self-couplings, the recent CMS result also probed scenarios beyond the SM. This was done by seeking, and placing bounds on, anomalous values of the couplings, including new ones in the framework of the “Higgs Effective Field Theory”.
With the Run 3 data taking era of the LHC in progress, the CMS experiment has already doubled the amount of collected data, and we are in the process of analysing it. One of the most interesting prospects for Higgs boson self-couplings is the upcoming High-Luminosity LHC (HL-LHC), whose operations are scheduled to start in 2030. In this new phase, the accelerator will deliver the highest instantaneous luminosity ever reached at a collider, resulting in a remarkable integrated luminosity of 3000 fb-1 being collected over the anticipated decade of data taking. Considering projections of luminosity and systematic uncertainties, we have estimated that we might start to see the first evidence for Higgs boson pair production (significance of three standard deviations) with about half of the HL-LHC data. Thrilling prospects await us …
Read more about these results:
-
CMS Physics Analysis Summary (HIG-20-011): " Combination of searches for nonresonant Higgs boson pair production in proton-proton collisions at 13 TeV "
-
@CMSExperiment on social media: LinkedIn - Facebook - Twitter - Instagram
- Do you like these briefings and want to get an email notification when there is a new one? Subscribe here