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One step beyond understanding the Higgs boson and the quark top

​An observation made by the CMS experiment at CERN, published in Physical Review Letters this week, connects for the first time the two heaviest elementary particles of the Standard Model. This makes the CMS collaboration achieve one of the primary objectives of its Physics program.

The particle physics group at IFCA participates in the understanding of the Higgs boson, specifically in the disintegration channel to two WW bosons. It has also participated in precision measurements of the top quark.

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 (see the figure). 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.


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 top quark–antiquark pair (left) or through radiation from a top quark (right).

That milestone has been passed considerably earlier than expected, says 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.

The present achievement is a case in point. 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 still leaves room for contributions from new physics. In the coming years, much more data will be collected and the precision will be improved, in order to see if the Higgs reveals the presence of physics beyond the Standard Model.

The particle physics group at IFCA, Santander, has been working for more than 20 years on the CMS detector both in data analysis and in the construction of the detector. The analyzes in which it actively participates are related to the top quark, the Higgs boson and electroweak bosons (W / Z), as well as the search for dark matter and supersymmetry. They contributed to the alignment system of the muon chambers in the detector part and are currently working on the R & D of 3D silicon pixel technology to be included in the CMS trace detector.

Further information can be found in the press release issued by CERN, 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.

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