The CMS collaboration at the Large Hadron Collider published thefirst observation of ttH production in Physical Review Letters (PRL) on June 4^th, 2018. This observation gives direct proof that the Higgs particle couples to the top quark, which is a key element in understanding the origin of fermion mass.

After the discovery of the Higgs boson at the ATLAS and CMS experiments in 2012, two new interactions were predicted: the Yukawa coupling between the Higgs and fermions, and the Higgs self-interaction. However, there were no direct observations in any experiments of these two interactions, until this first report of a direct observation of the Higgs interacting with one of the fermions: the top quark. This observation of ttH indicates the coupling between the Higgs and fermions does exist.

The CMS group from the Institute of High Energy Physics was involved in the effort to hunt for ttH production, and made a significant contribution to ttH observation in the most sensitive Higgs decay channels, the WW, bb, di-photon and ZZ channels.

To extract ttH signal from overwhelming backgrounds, they developed a dedicated method to tag jets from higgs boson decays, and introduced new variables to distinguish whether a lepton is isolated from jets or not. These methods were two of the key elements in the success of the ttHWW analysis. This analysis makes ttHWW turned out to be the most important channel contributing to the observation of ttH.

The IHEP CMS group gave the pre-approval talk within the CMS collaboration on behalf of the ttHWW analysis team. They also made a significant contribution to ttHbb analysis by introducing the b-jetness method.

However, the observed production rate of ttH is slightly higher than the prediction from the Standard Model of particle physics, within 2 sigma of uncertainty, leaving room for new physics. Only data taken up to 2016 was used in this observation, and further analysis with more data in future is expected to probe the details of the Higgs-top quark Yukawa coupling through ttH production.

For more information please see:

CMS press release: http://cms.cern/news/tth-announcement

CERN Courier: http://cerncourier.com/cws/article/cern/71524

CERN press release: https://press.cern/press-releases/2018/06/higgs-boson-reveals-its-affinity-top-quark

APS Viewpoint: https://physics.aps.org/articles/v11/56

Best fit value of the ttH signal strength modifier u_ttH compared to SM expectation (Image by CMS Collaboration)