Physics Department News

Researchers from many institutions worldwide, including Warwick Univesity, have analysed data from the ATLAS experiment at the Large Hadron Collider and have found evidence for the decay of Higgs Bosons to pairs of tau leptons.

See the details here in today’s seminar slides.

The Higgs boson discovery was announced by the ATLAS and CMS experiments in July 2012. This confirmed the existence of the Higgs boson as predicted by Higgs, Brout & Englert and others in 1964. It is the particle needed in the theory of Particle Physics, the Standard Model, in order to give mass to all fundamental particles. Following its discovery in 2012, Higgs and Englert were awarded the Nobel Prize in October 2013 for their theoretical prediction.

Professor Peter Higgs said of this latest measurement of the Higgs decaying to tau leptons “it is very pleasing to know that further measurements are confirming the role of the Higgs boson in the Standard Model. I congratulate once again the ATLAS collaboration on their heroic achievements”.

So far the Higgs boson has been shown to decay to W, Z and gamma particles, all of which are in the group of particles knows as bosons , whereas the tau is a fundamentally different class of particle known as a fermion.

The bosons are the particles which we think of as “force carriers”, mediating the weak force by the exchange of W or Z bosons; the electromagnetic force by the exchange of gamma particles (photons) and the strong force by exchange of gluons. In fact the Higgs boson gives a large mass to the W and Z bosons and for this reason the weak force has a very short range and took a long time to discover. (Think of trying to throw a canon ball a long distance.)

The fermions are the particles which make up everyday material: the nucleus is made of protons and neutrons which are in turn made up of quarks which are fermions, and the electrons which circle the nucleus forming the atom are also fermions. The Higgs particle was proposed to give mass to the bosons and it was later realised, theoretically, that it could do the same job for fermions. Our work observing the decay of the Higgs boson to tau pairs heralds a stronger affirmation of the Standard Model Higgs theory. Further, it proves experimentally that the Higgs boson is responsible not only for the mass of the W and Z bosons, but also for the fermions, which make up the material of our everyday lives including our own bodies! The evidence is strong - it is at the level of 4.1 standard deviations. Usually 3 standard deviations is considered to define “evidence for” a phenomenon, and 5 standard deviations constitutes discovery.

This result is exciting because it tells us three facts about the Higgs boson. First, the tau is a fermion, so now we know the Higgs interacts with fermions. Previously this could be inferred indirectly via the main production mechanism, gluon fusion, which is supposed to involve the conversion of the gluons into a pair of top quarks before those create the Higgs boson. . Here we have a direct measurement. Also, the Tevatron and CMS had some evidence of this being the case. Second, it definitely decays to leptons. The most everyday example of a lepton is the electron, which is present in atoms which constitute the known universe. A tau is exactly like an electron but about 3500 times heavier. Third, the tau is a "down-type" fermion whereas the indirect evidence we had before was a coupling of the Higgs boson to the top quark, an "up-type" fermion.

For more information, contact Sinead Farrington (s.m.farrington@warwick.ac.uk), leader of the Warwick ATLAS group and co-convenor of the Higgs to tau tau group at ATLAS.

Further details can be found here.