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Direct coupling of the Higgs boson to the top quark observed

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An observation made by the Compact Muon Solenoid (CMS) experiment at CERN, published in Physical Review Letters today, connects for the first time the two heaviest elementary particles of the Standard Model.

Particle physicists, led by Professor Peter Hobson, in the Department of Electronic and Computer Engineering at Brunel University London have contributed to the CMS experiment since Brunel joined in 1995.

On 4 July 2012, two of the experiments at the CERN’s Large Hadron Collider (LHC), ATLAS and CMS, reported independently the discovery of the Higgs boson. The announcement created headlines worldwide: the discovery confirmed the existence of the last missing elementary particle of the Standard Model, half a century after the Higgs boson was predicted theoretically. At the same time the discovery marked also the beginning of an experimental programme aimed to determine the properties of the newly discovered particle. Reporting today in Physical Review Letters, the CMS collaboration announces a milestone in that programme.

In the Standard Model, the Higgs boson can couple to fermions, with a coupling strength proportional to the fermion mass. While associated decay processes have been observed, the decay into top quarks, the heaviest known fermion, is kinematically impossible. Therefore, alternative routes to directly probing the coupling of the Higgs boson to the top quark are needed. One is through the production of a Higgs boson and a top quark–antiquark pair (shown in the figure below). This is the production mechanism that has now been observed for the first time, and in doing so, the CMS collaboration accomplished one of the primary objectives of the Higgs physics programme.

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Two routes to observing the coupling of the Higgs boson to the top quark are the production of a Higgs boson in the fusion of
(a) a top quark–antiquark pair or (b) through radiation from a top quark

“That milestone has been passed considerably earlier than expected,” says ETH professor and CMS Deputy Spokesperson Günther Dissertori. “This is due to the availability of excellent experimental data, but also to a good part thanks to the use of sophisticated analysis methods, ensuring that the required statistical precision could be reached.”

With the observation of the coupling between the two heaviest elementary particles of the Standard Model, the LHC physics programme to characterise and more fully understand the Higgs boson has taken an important step. While the strength of the measured coupling is consistent with the Standard Model expectation, the precision of the measurement will improve in the coming years, and we wait with excitement to see what whether the Higgs will reveal the presence of physics beyond the Standard Model.

Further information can be found in a press release issued by CERN today, on the occasion of the opening of the LHCP2018 conference in Bologna (Italy), where also the ATLAS collaboration is presenting their latest results for the first time.

‘Observation of ttH Production’, by A M Sirunyan et al. (CMS Collaboration), is published today in Physical Review Letters.

Main image: The CMS solenoid magnet, the central element of the CMS apparatus, during the 2013 technical stop. Photo by CERN/CMS, Michael Hoch

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