A novel method to measure the lifetime of the Brout-Englert-Higgs boson

A significant progress regarding the properties of the Brout-Englert-Higgs (H) boson has just been presented on thursday 21 march 2014 at the Moriond conference, as a « hot topic ». A novel measurement method of the H boson lifetime, 100 times more precise than the most precise previous measurement, has been used. A team of researchers from the IIHE-Université Libre de Bruxelles, the Université Catholique de Louvain-la-Neuve and CERN has contributed to this result in a significant way.

The H boson discovered in july 2012 by the ATLAS and CMS experiments at the Large Hadron Collider of CERN is special : in the standard model of particle physics, it interacts with all elementary particles proportionally to their mass. The data collected at CERN seem to confirm this behaviour for the most massive particles : interactions with the W and Z bosons, as well as with the massive b-quark and tau-lepton, have been observed directly. Interactions with the top quark have been observed indirectly, since it is through this interaction that the H boson can decay to a photon pair, one of the decay modes by which the H boson was discovered.

The CMS collaboration has just presented a constraint on the width of the resonance peak of the H boson, a property that is inversely proportional to H boson lifetime. The standard model predicts a width of 4.15 megaelectronvolts (MeV) for the H boson, which corresponds to a lifetime of 1.59 x 10-22 second. Should the observed width differ from the prediction, the H boson would either interact with known particles differently than what the standard model predicts, or it would decay into yet-unknown particles.

The novel method consists of interpreting the data with the help of recent theoretical calculations that establish a relation between the H boson width and the contribution of the H boson to very rare processes – the production of Z boson pairs with high invariant mass, above the H boson mass. The contribution searched for is tiny and the method requires all the expertise from the researchers. In principle, the method allows the exclusion (at 95% confidence level) of width values 8.5 larger than the standard model prediction, compared to 800 times larger for the previous best method. When applied to the data, an upper limit 4.2 larger than the predicted width is obtained, marginally consistent with the expected limit.

The forthcoming high-energy data taking of the LHC, expected to start in april 2015, should provide even better sensitivity. In the meantime, the researchers are trying to understand why the method provides twice better limits than expected.

More information

The CMS experiment at the LHC collider at CERN is developed by a collaboration of 193 institutes, among which 6 Belgian universities, the Universiteit Antwerpen, the Universiteit Gent, the Université Catholique de Louvain, the Université Libre de Bruxelles, the Université de Mons and the Vrije Universiteit Brussel.

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The Interuniversity Institute for High Energies, IIHE (ULB-VUB), was created in 1972 at the initiative of the academic authorities of both the Université Libre de Bruxelles and Vrije Universiteit Brussel.
Its main topic of research is the physics of elementary particles. The present research programme is based on the extensive use of the high energy particle accelerators and experimental facilities at CERN (Switzerland) and DESY (Germany) as well as on non-accelerator experiments at the South Pole. The main goal of this experiments is the study of the strong, electromagnetic and weak interactions of the most elementary building blocks of matter. All these experiments are performed in the framework of large international collaborations and have led to important R&D activities and/or applications concerning particle detectors and computing and networking systems.
Research at the IIHE is mainly funded by Belgian national and regional agencies, in particular the Fonds National de la Recherche Scientifique (FNRS) en het Fonds voor Wetenschappelijk Onderzoek (FWO) and by both universities through their Research Councils.
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