The latest measurements of the mass of the W boson, one of two elementary particles that mediate the weak nuclear force, are a powerful reminder of the profound beauty in the standard model of particle physics.
The most precise measurement so far of the mass of the W boson is reported by researchers in the CDF Collaboration (Aaltonen et al.1) at the Tevatron collider, Fermilab, near Chicago, in Physical Review Letters. The W boson is the particle that carries the weak nuclear force, which is responsible for radioactive β-decay. The particle decays in less than one billion-trillionth of a second, and many of the things that it decays to are hard to detect. The measurement of its mass is an experimental tour de force, a consistency test for the standard model of particle physics, and a pointer that will aid the discovery of new phenomena at the Large Hadron Collider (LHC) at CERN, Europe's particle-physics laboratory near Geneva, Switzerland.
The 'standard model' is the rather prosaic name given to the collection of quantum field theories that describes the Universe at the shortest distances and highest energies. You will often hear particle physicists hoping for physics 'beyond the standard model' because an anomalous experimental result might help to resolve some of the issues left unexplained by the model. These problems — including why the Universe contains more matter than antimatter, and where gravity fits in — are discussed so often that it is easy to forget how accurate and economical the standard model is.
In the model, the fundamental forces are carried by particles known as gauge bosons. The photon, for example, is the gauge boson that carries the electromagnetic force, whereas the W and Z bosons mediate the weak force. Bosons are particles that have an integer quantum of intrinsic angular momentum, otherwise known as spin. 'Gauge' means that the particles are generated by mathematical symmetries.
Symmetry is a central feature in the standard model. For the model to work in its most symmetric form, the W and Z bosons should, like the photon, have zero mass. This is not the case. Yet without symmetry, the standard model loses all predictive power — a finding2 that earned Gerardus 't Hooft and Martinus Veltman the Nobel Prize in Physics in 1999.
The way out of this impasse could be supplied by the discovery of the Higgs boson. Its existence would indicate that symmetry remains in the standard model at high energies, even though it is manifestly broken in the low-energy Universe in which we live.
The non-zero mass of the W boson is intimately connected with the Higgs boson, with the origins of mass in general and with our understanding of physics in terms of quantum field theories. It is a quantity well worth measuring precisely — just as the Tevatron experimenters1 have done.
This was a hugely challenging analysis. The Tevatron (Fig. 1), which recently ceased operation, was a high-energy collider that stored protons and antiprotons, accelerated them to high energies — almost 1 teraelectronvolt (1012 eV) — and forced them into head-on collisions. The energy and frequency of the collisions were sufficient to produce large numbers of W bosons. W bosons decay rapidly, and their decay products could generally be detected in the Collider Detector at Fermilab (CDF), or in the rival detector D0, which is also located at the Tevatron. In the CDF analysis, the researchers used 1,094,834 W-boson decays to measure the W-boson mass.
The W boson can decay in many different ways, but those decays that produce an electron or a muon — a short-lived particle similar to the electron — are the most useful for measuring the W boson's mass because electrons and muons can be reliably detected. However, an electrically neutral particle called a neutrino that is hard to detect is also produced in these decay events. This is problematic because the neutrino's momentum is needed to determine the W boson's mass; however, this momentum can be deduced only indirectly from an analysis of all the other particles produced in the decay event.
The CDF is a cylinder constructed such that proton and antiproton beams enter at either end and collide in the centre. Although the neutrino cannot be detected, its presence — and the component of its momentum transverse to the beam — can be deduced by applying the law of conservation of momentum to all the other particles produced in the collision. In addition to the electron and the muon, this includes composite particles known as hadrons, which are generated when elementary particles called quarks and gluons are scattered from the colliding particles and then combine.
Detailed analyses of all of these components led the CDF Collaboration to obtain a value for the W boson's mass of 80,387 MeV with an error of 19 MeV, a precision of about two parts in 10,000. This value is consistent with that obtained from an experiment performed with the D0 detector, which found3 a mass of 80,367 MeV with an error of 26 MeV.
The W boson and the top quark, the heaviest of all known elementary particles, contribute to many particle-production and scattering processes that have been accurately measured in particle-physics experiments. In these processes, the particles enter quantum loops as virtual particles with fleeting existence but measurable effect — at least, if the measurement is precise enough. If it exists, the Higgs boson must also appear in these loops. By combining these measurements with their value of the W boson's mass, the authors were able to conduct a precise test of the symmetry structure of the standard model.
Knowledge of the W boson's mass has imposed limits on the range of possible mass values for the Higgs boson. This range has been further curtailed4,5 by data from the direct searches for the Higgs at the LHC. And yet there is still a region of overlap. If the hints seen at the LHC do turn out to be the Higgs, then the particle's mass is consistent with that inferred from standard-model calculations, using the W boson's mass, of an array of particle-physics processes. This consistency is built into the quantum loops and symmetries of the standard model. A theorist, or even a mathematician, might call this a highly non-trivial consistency test of the theory. I call it beautiful.
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Abazov, V. M. et al. Phys. Rev. Lett. 108, 151804 (2012).
Aad, G. et al. Phys. Lett. B 710, 49–66 (2012).
Chatrchyan, S. et al. Phys. Lett. B 710, 26–48 (2012).