Heavy Weigths World Match
The challenge was raised for a long while: who would first observe the interaction of the Higgs boson with the top quark, the two heaviest elementary particles known so far? (The Higgs mass exceeds that of a bunch of 130 protons, the top even 180).
Indirect evidence has been around for a while, but only on April 8th the CMS collaboration could claim the unambiguous observation of a sufficient number of events where the Higgs was produced coupled to a top-antitop pair out of billions of proton proton collisions at the Large Hadron Collider at CERN.
The result was soon after sent for publication to Physical Review Letters, and published on June 4th. The article was highlighted as PRL Editor\’s Suggestion and was accompanied by a comment article.
This result, confirmed just these days by an independent measurement by another LHC experiment, ATLAS, was highlighted on Monday 4th by professor Roberto Carlin, the elected spokeperson of the CMS collaboration, full professor of General Physics in the University of Padova, during the VI International Conference on LHC Physics, an assembly of more than 400 LHC Physicists who meet these days in Bologna to discuss LHC results. The CMS members from Padova University and our division constitute one of the largest groups of the CMS collaboration.
The Higgs boson was discovered in July 2012 simultaneously by ATLAS and CMS. It is the keystone of the electroweak theory of fundamental interactions. Each particle in the theory gains its mass through the interaction with the Higgs, so that a simple relation can be stated between the particle mass and its coupling to the boson. Checking this relation, by measuring independently masses and couplings, provides a strong test of the theory.
While the top and the Higgs masses are precisely known, the measurement of their coupling is very difficult, because the process is very rare (it happens once in hundred Higgs events, and Higgs production happens once in a billion proton proton collisions at the LHC); in addition, top and Higgs are observed in very complicated topologies. A lot of different decay modes have been independently inspected by several research groups, and only at the end the individual results have been merged to allow the discovery. The measurement confirms the theory prediction with about 25% accuracy.
\”The result we achieved was expected only in the far future, for the so called second phase of LHC operations. We face a transition from discovery to precision measurement, the challenge is to reach few percent precision in most of Higgs couplings to the other particles. In the next decades we expect to collect about forty times the events analyzed so far, with a much improved detector. We aim to observe the coupling of the Higgs boson with particles as light as the muons, and the Higgs self-coupling . Besides clarifying beyond any doubt the way mass is provided to elementary particles, this will throw light upon possible extensions of the electroweak theory and will drive the future steps in accelerator Physics\”, says Prof. Carlin.
