A Lay-Scientist's Guide to the Higgs Boson
Dr Wahid Bhimji is a researcher at the University of Edinburgh, working on the search for the Higgs boson with the ATLAS experiment. He is UK Higgs analysis convenor for the ATLAS Experiment.
The Standard Model of particle physics is a remarkably successful theory: with its predictions confirmed by experiment to exceptional precision. A striking example of this comes from electroweak unification, under which the weak nuclear force, which is responsible for radioactive beta decay, is shown to be fundamentally the same force as electromagnetism. This underlying electroweak symmetry was confirmed by the observation of the heavy particles that mediate the force (the so-called W+, W- and Z particles) and measurement of the weak mixing angle that relates their masses.
These forces do not appear the same in everyday interactions, however, because the symmetry is broken leading to the W and Z particles being massive unlike the carrier of electromagnetism, the photon. Breaking this symmetry for a theory that is also both relativistic and gauge-invariant (one in which the same results are obtained no matter how certain properties are measured) would not be possible without a mechanism such as that postulated by Peter Higgs of Edinburgh University and others. In the Higgs mechanism the carriers of the weak force, and other fundamental particles, acquire mass through interacting with a Higgs field that fills the vacuum. This mechanism also has a clear experimental signature as it predicts the existence of an additional fundamental particle, the Higgs Boson. This is the only particle predicted by the Standard Model that has not been observed experimentally.
The Standard Model and the Higgs mechanism predict certain properties of the Higgs particle such as that it has a spin of zero (hence the term boson), that it has a mass, and that it will decay to certain other fundamental particles. However the value of the mass itself is a free parameter of the theory. The mass of the Higgs Boson affects what energy and type of particles would need to be used to produce it in particle colliders, as well as what particles it will decay into – and so how it can subsequently be observed in experimental particle detectors. The Large Hadron Collider (LHC) and the ATLAS detector were designed, in part, to be able to produce and detect the Higgs Boson if it had a mass between those values already excluded by previous experiments and those required by theory. Even so, the Higgs Boson would only be produced in a small fraction of the 100s of millions of collisions occurring every second at the LHC and the observations also need to be separated from other processes that create the same particles in the detector.
The ATLAS collaboration involves 1000s of physicists, including around 20 from the University of Edinburgh, who operate the particle detector and analyse the data produced. By convention in particle physics, it is only when the probability of misidentification is less than around one in 3 million that an observation is claimed. This threshold was achieved in 2012 leading to the announcement by ATLAS of the “Observation of a New Particle in the Search for the Standard Model Higgs Boson”. It remains however to establish that the properties of this new particle are indeed exactly those required for it to be a Higgs Boson and so that this particle can provide its essential role in completing the Standard Model.
References:
Electroweak unification:
S. L. Glashow, Nucl. Phys. 22, 579 (1961).
S. Weinberg, Phys. Rev. Lett. 19, 1264 (1967).
A. Salam, in Elementary Particle Theory, p. 367 (Almqvist and Wiksell, Stockholm, 1968).
G. ’t Hooft, M. Veltman, Nucl. Phys. B44, 189 (1972).
Higgs Mechanism:
F. Englert, R. Brout, Phys. Rev. Lett. 13, 321 (1964).
P. W. Higgs, Phys. Lett. 12, 132 (1964).
P. W. Higgs, Phys. Rev. Lett. 13, 508 (1964).
G. S. Guralnik, C. R. Hagen, T. W. B. Kibble, Phys. Rev. Lett. 13, 585 (1964).
Experimental Observation:
ATLAS Collaboration, Phys.Lett. B716 (2012) 1-29
CMS Collaboration, Phys.Lett. B716 (2012) 30-61