Introducing the Higgs Bosom
On July 4, 2012, the two major experimental collaborations operating at CERN’s Large Hadronic Collider (LHC), the CMS and ATLAS experiments, independently announced that they had each confirmed the discovery of a previously unknown boson with a mass-energy of about 125 GeV, whose behavior so far has been “consistent with” that of a Higgs boson. They cautioned that additional data and analysis were needed before positively identifying the new particle as definitively being the Higgs boson. The physics community in general, not being so cautious, is now convinced that the long sought after Higgs boson has been discovered. This is not a “breakthrough” or unexpected discovery, because it has been long anticipated and was the motivation for constructing the LHC in the first place, but it is very significant. It represents a confirmation of the standard model of particle physics. It is a major step toward understanding the universe in which we live. The CERN/LHC announcement has already stimulated many discussions of the Higgs boson in the popular press, but in this column we want to delve a bit deeper into the meaning and significance of this discovery. We will do this using the familiar Q&A format.
Q: What is a boson? The fundamental particles of physics, depending on their intrinsic angular momentum (or spin), come in two distinct “personality types”: the individualistic fermions and the group-oriented bosons. Fermions, named for Italian physicist Enrico Fermi, have half-integer spins (normally 1/2 of an Ã unit), obey Fermi-Dirac statistics, and behave as described by the Pauli exclusion principle, with at most one fermion allowed in any given quantum state. Bosons, named for Indian theorist Satyendra Nath Bose, have integer spins (normally 0, 1, or 2 Ã units), and obey Bose-Einstein statistics. Bosons tend to congregate together in the same quantum state, and under the right circumstances many identical bosons can pile up in the same quantum state to produce Bose-Einstein condensation.
In the standard model of particle physics, the mediating particles that serve as the carriers of the forces are always bosons. The photon (mass=0, spin=1) is the boson that mediates the electromagnetic interaction. The Z0 and W± bosons (masses=91.2 GeV and 80.4 GeV, spin=1) mediate the weak interaction. The gluon (mass=0, spin=1) is the boson that mediates the strong interaction. Also hypothesized is the graviton (mass=0, spin=2) that, at the quantum level, should mediate the gravitational interaction. As we will discuss below, the Higgs boson (mass=125 GeV, spin=0) is the boson that is expected to mediate interactions with the Higgs field, the field that gives mass to some other bosons and all fermions.
Q: What is the Higgs field? The Higgs field was suggested by Peter Higgs in 1964, as a way of explaining why some fundamental particles have mass and others do not. The idea is that there exists a scalar field (i.e., a field with no preferred direction) that is an intrinsic characteristic of all space, even completely empty space. The fundamental particles have mass depending on how strongly they couple to and interact with the Higgs field. Photons and gluons do not interact with it at all, and thus have zero mass. Quarks and leptons interact with it to varying degrees, accounting for their non-zero masses.
In principle, the electromagnetic and weak forces should act in the same way and be indistinguishable. The reason the weak force is so different from the electromagnetic force is that the mediating particles of the weak interaction, the Z0 and W± bosons, interact strongly with the Higgs field, while the photon does not. This difference in mediating masses “breaks the symmetry” between the forces, leading to very different behaviors in the two forces. Since the mass of the mediating particle determines the “range” or distance over which the interaction acts, the weak interaction, mediated by massive particles, is a very short range force while the electromagnetic interaction, mediated by massless photons, has infinite range.
The Higgs field is often characterized as a sort of “molasses” that impedes the progress of massive particles interacting with the Higgs field as they move through it. This is not really a very good analogy, however, because moving through molasses involves loss of energy, while moving through the Higgs field does not. It is better to think of a moving massive particle as surrounded by a cloud of virtual particles (see below) that the moving particle must carry along as “baggage” as it moves through the Higgs field. Massless particles like the photon carry no such baggage and move at the speed of light.
Q: What is a Higgs boson? If the Higgs field is a still pond of water, the Higgs boson is the ripples in the pond. The presence of a force field implies the existence of a particle that is a minimal disturbance in that field. Examples are photons (electromagnetic field) and gravity waves (gravitational field). The Higgs boson is a wave disturbance in the Higgs field, and is the lowest possible energy state of such a disturbance. The Higgs boson is also the mediating particle of the Higgs field, so that the massive particles that are strongly coupled to the Higgs field will be surrounded by a cloud of virtual Higgs bosons that give the particles their mass.
The mass-energy of the Higgs boson was expected to be roughly one hundred times larger than that of a proton (mass energy=0.938 GeV), but the standard model provides no definitive prediction of what the Higgs mass should be. The LHC experiments have reported the mass-energy of their candidate Higgs to be about 125 GeV, consistent with this expectation.
Q: Why do some fundamental particles couple to the Higgs field while others do not? Our present understanding of particle physics is what we call the standard model. It does not provide an answer to this question. In analogy with electric forces, there must be some “Higgs charge” that fundamental particles possess in varying degrees. It is not quantized, like spin and electric charge, and it can be very large, giving the top quark a mass-energy of 171.2 GeV, or very small, giving the electron neutrino a mass-energy of around 0.000000001 GeV. The Higgs charge can also be zero, as it is for the photon, gluon, and graviton. What sets the coupling strength of a particle to the Higgs field, and why that strength can vary over such a large dynamic range, are questions that the present standard model does not answer. We need a better theory.
It should also be noted that composite particles like the proton and neutron do not owe most of their mass to the Higgs mechanism. A proton is a system of two up quarks (mass=0.0024 GeV) and one down quark (mass=0.0048 GeV) bound together by the strong interaction. The quarks themselves contribute only a small fraction of the total mass of the proton. The majority of the proton’s mass-energy is attributable to the action of Heisenberg’s uncertainty principle. The strong interaction confines the quarks in a proton to a rather small ‘box,’ making their positions very well localized. The uncertainty principle requires that such localization in position be accompanied by a rather large uncertainty in momentum, meaning that on the average the bound quarks have a large momentum, and thus a large kinetic energy. The kinetic energy of the three quarks, as they rattle around in their proton box, accounts for most of the mass-energy of the proton (0.938 GeV)
Q: How do the CERN experimenters know that the particle they have observed is a Higgs boson and not some random unrelated particle? The Higgs boson is very unstable, and when produced it exists for a very short time before it decays into other, less massive particles. Because of the way that the Higgs field couples to other particles and because of its zero spin, there are some decay modes that are favored and others that are “forbidden” or disfavored. In searching for the Higgs boson, analysis of p+p collisions at the LHC has focused on favored decay channels, for example the decay into a pair of Z0 bosons, and looked for “mass bumps” in those channels that would indicate a Higgs boson of a certain mass-energy. Correlated studies must show that the candidate particle must not decay into forbidden channels when produced. The evidence so far strongly suggests a 125 GeV mass-bump appearing in the favored channels and the absence of such a bump in forbidden channels, but more data needs to be analyzed to make the identification of the new particle as a Higgs boson unambiguous.
Q: Is there only one kind of Higgs boson? The standard model does not require that there be any more than one kind of Higgs boson, but some of the extensions of the standard model suggest that there may be two or even more Higgs-type particles in nature. The more massive ones, if they exist, are yet to be discovered. In particular, one extension called “supersymmetry” would predict a second Higgs-type particle. The experiments at the LHC are focused on this question, and as more data is collected and the analysis becomes more selective and statistically significant, some indications of additional Higgs-type particles may emerge. Theoretical extensions of the standard model will be supported or falsified depending on the outcome of this work. At this writing (late July 2012) there are unconfirmed rumors from the direction of the LHC that a second Higgs may be showing up at a mass-energy around 140 GeV.
Q: Does the discovery of the Higgs boson explain everything about the origins of mass? Unfortunately, the answer is no. As mentioned above, mass and energy are related, and the mass of the proton arises mainly from kinetic energy rather than from the Higgs-derived mass of its constituent quarks. Further, even “massless” particles like the photon exhibit the properties of mass like deflection by gravity because they have energy. The term “massless” means only that they have intrinsic rest mass of zero. And while interactions with the Higgs field may explain the mass of fundamental particles, it does not completely explain the origins of inertia, the tendency of mass to resist acceleration. Some deeper explanation, perhaps Mach’s Principle, is required to account for the origins of inertial forces and explain how a mass “knows” that acceleration is changing its reference frame.
Q: What is the significance of the discovery of the Higgs for everyday life and the real world in which we live? In one important sense, if the Higgs field did not exist, we would not be here to worry about it. The Higgs mechanism gives mass to the electrons that orbit atoms. The massive electrons organize themselves into shells that account for chemical valences and lead to the chemical properties that make life possible. If one could throw a switch that would make the Higgs field vanish from space, all particles would become massless and could travel only at the speed of light, all solid matter would evaporate in a cloud of lightspeed particles, and all life would abruptly end.
In another sense, the Higgs discovery has everyday importance because today we understand something new about how the universe works that we did not understand, or at least were not sure of, yesterday. When the large accelerator at Fermilab that became the Tevatron was first being constructed in the early 1970s, there was reportedly a hearing in the U. S. Senate at which a senator asked Robert R. Wilson, then the director of Fermilab, if his new accelerator could be used to defend the United States. “No, Senator,” Wilson is supposed to have answered, “but building machines like this to learn how the universe works is one of the things that makes the United States worth defending.”
AV Columns Online: Electronic reprints of over 160 “The Alternate View” columns by John G. Cramer, previously published in Analog, are available online at: http://www.npl. washington.edu/av.
Discovery of the Higgs Boson:
“CERN experiments observe particle consistent with long-sought Higgs boson”, CERN Press release, July 4, 2012, http://press.web.cern. ch/press/PressReleases/Releases2012/PR17.12E.html.
“Physicists Find Elusive Particle Seen as Key to Universe”, Dennis Overbye, New York Times, July 4, 2012.
Copyright © 2012 John G. Cramer