ATLAS produces the first measurement of Higgs production in association with top quarks

The ATLAS and CMS experiments at the Large Hadron Collider at CERN have independently observed the production of Higgs bosons in association with two top quarks. The results presented this week at the LHCP conference in Bologna allow for the first time a direct determination of the couplings between top quarks and the Higgs boson. Dr. Stefan Gadatsch from the ATLAS group of Prof. Tobias Golling here at DPNC was one the leaders of the analysis and an editor of the paper.

In the Standard Model, all fundamental particles obtain their mass through interactions with the Higgs field. ATLAS tests this theory using high-precision measurements of the interactions between the Higgs boson and other particles. While the ATLAS and CMS experiments at CERN had previously measured the Higgs boson decaying to pairs of W or Z bosons, photons or tau leptons, its coupling to quarks had not been observed yet and only some first evidence had been observed.

With the results presented this week, the ATLAS Collaboration has observed the so-called "ttH" production of the Higgs boson together with a top-quark pair. This is a very rare process with only about 1% of all Higgs bosons being produced this way. This result establishes a direct measurement of the "top Yukawa coupling", an important parameter of the Standard Model that defines the interaction between the top quark and the Higgs boson. As the top quark is the heaviest particle in the Standard Model, this measurement is one of the most sensitive tests of the Higgs mechanism.

With a combination of all available data taken from 2015 until 2017, ATLAS has observed the production of the Higgs boson in association with a top-quark pair with a significance of 6.3 standard deviations over the background-only hypothesis. This is far above the required 5 standard deviations usually required by the community. The measured ttH production cross section is consistent with the Standard Model prediction. This supports the theoretical Standard Model mechanism that gives mass to the top quark obtains through its interaction with the Higgs field.