Particle physicists have made many remarkable discoveries over the past few decades, but one fundamental question remains: why do particles have mass? The answer is predicted to lie in the Higgs boson, the only particle predicted by the Standard Model that has yet to be observed.
Last week however, rumors began to circulate that the Higgs boson might have finally been detected at the Large Hadron Collider. And Tuesday morning, an official announcement from CERN suggested that the 40-year search might be coming to a close … but the data do not suggest a definitive discovery yet.
The Standard Model of Particle Physics
For those of us who aren’t particle physicists, let’s begin with the basics. The Standard Model of particle physics describes all known subatomic particles and the forces that mediate their interactions. First formalized in the 1970s, it includes 12 elementary particles with half-integer spin (an odd multiple of ½), known as fermions, and 4 elementary particles with integer spin, known as bosons, which mediate their interactions. The 12 fermions consist of six quarks (up, down, charm, strange, top, and bottom) that constitute protons and neutrons (amongst many other particles), plus six leptons (the electron, muon, and tau, and one neutrino associated with each).
The Standard Model describes three forces, or types of particle interactions: the electromagnetic, weak, and strong nuclear forces. Gravity however, isn’t yet incorporated into the model, as it will require an elusive theory of quantum gravity. Each of these interactions is represented by the exchange of a particular boson, which “mediates” the force. The electromagnetic force is mediated by the exchange of a photon, the weak interaction by the massive intermediate bosons W± and Z0, and the strong nuclear interaction by a massless boson known as a gluon.
The Higgs Boson
The Standard Model also predicts the existence of the Higgs boson, a hypothetical massive elementary particle thought to hold the key to why particles have mass. It is theorized to be a “scalar boson,” signifying that it has spin 0, and therefore no intrinsic angular momentum — only mass. The Higgs boson is a quantum of the theoretical Higgs field, just as the photon is a quantum of the electromagnetic field.
The Higgs field is thought to permeate the entire Universe — even empty space. Hence it has a “vacuum expectation,” which describes how the field behaves in the absence of matter. Every massive particle couples to this field to a greater or lesser degree, giving them the property of “mass.” This so-called Higgs mechanism also predicts the ratio between the W boson and Z boson masses, as well as how they couple to each other and to quarks and leptons. If it is confirmed to exist, it will solve the largest remaining question in the Standard Model.
The LHC is searching for the Higgs boson by colliding protons at extremely high energies (up to 7 TeV); these protons then rapidly decay (in quadrillionths of a second or less) through a series of other unstable particles, the end results of which are tracked and measured to reconstruct the entire particle cascade. Which particles are formed and destroyed within these super-energetic fireballs depends on a number of factors, including pure chance. One possibility is that at some point in a decay series a Higgs particle is created.
There are two main channels by which the Higgs boson may be theoretically produced: two gluons may convert to two top/anti-top quark pairs that then combine to make a neutral Higgs boson, or two quarks may each emit a W or Z boson that combine to make a neutral Higgs boson. See the above Feynman diagrams. The Higgs boson will then decay quickly in a similar fashion to how it was made (i.e. into two top/anti-top quark pairs), and the LHC experiments will pick up the debris of that decay and reconstruct the event.
ATLAS and CMS Experiments
Two experiments searching for the Higgs boson are currently in progress at the Large Hadron Collider (LHC), the largest particle accelerator ever built. The project is run by the European Organization for Nuclear Research, CERN. The LHC is comprised of a 26 km underground ring with the capacity to accelerate particles to 99.99991% of the speed of light! Two fast proton beams circle clockwise and counterclockwise inside the tunnel and are allowed to cross one another only inside a detector when the results of the energetic collision can be analyzed.
The LHC feeds particles to seven experiments, each of which studies the collisions with a different scientific goal in mind and uses a wide range of technologies to do so. Both the ATLAS and CMS experiments are searching for the Higgs boson by analyzing the broad spectrum of particles and phenomena produced in high-energy collisions.
ATLAS (A Toroidal LHC AppartuS) is 44 meters long and 25 meters in diameter. It is the largest detector ever built and weighs 7000 tonnes. The detector consists of four major components: the inner detector, which measures the momenta of charged particles, the calorimeter, which measures the particle energies, the muon spectrometer, which identifies and measures the momenta of muons, and the magnet system, which bends the charged particles for the final momenta measurements.
CMS (Compact Muon Solenoid) is smaller than ATLAS but outweighs it by nearly a factor of two. It consists of layers of material that exploit the different properties of particles in order to measure the energy and momentum of each one. It consists primarily of a solenoid magnet with an overall length of 13m and a diameter of 7m; within the solenoid are the tracker and the calorimeter. By measuring the path of a particle in the magnetic field produced by the solenoid one can determine the particle’s momentum (a particle with a high momentum will traverse a less curved path).
On 13 December 2011, both experiments presented new results in the search for the elusive Higgs boson. The press release may be found here. The conclusion? Both experiments have made significant progress in the search, but not quite enough to make any conclusive statement on the existence or non-existence of the Higgs boson.
Previous experiments have narrowed the energy (i.e. mass) range of the Higgs boson to be between 120 GeV and 140 GeV (One GeV, or giga-electronvolt, is one billion times the energy gained by a single electron in a one-volt electron field). The Heisenberg uncertainty principle dictates that since the particle lives such a short amount of time, the uncertainty in its energy will be large. So the Higgs boson will have a range of energies, but physicists believe it should be approximately 126 times heavier than a proton and 250,000 times heavier than an electron.
The ATLAS experiment has now constrained the mass range of the Higgs boson to 116-130 GeV, while the CMS experiment has constrained it to 115-127 GeV. Excitingly, the recent press release indicates that we have now seen hints of the Higgs boson in this region. However, both experiments are complicated by the fact that we can’t view the Higgs boson directly, as it is too short-lived and can decay in many different ways. Discovery thus relies on observing the particles it decays into rather than the particle itself.
Particle physicists must first calculate the expected background, or the total signal from decays due to currently known particles (i.e. in this case, without including the presence of a Higgs Boson). The next step is to look for an excess signal above this background, the presence of which will indicate a new particle. There are multiple ways for the Higgs boson to decay, and both ATLAS and CMS have analyzed several decay channels.
ATLAS detected a slight excess above the expected background energy at 126 GeV. The significance of this result is 2.5 sigma or a confidence level of 98%. This is scientific parlance for a 98% chance that the observed energy peak is due to the presence of a new particle and not simply due to statistical fluctuations. ATLAS has only obtained data on two of the many decay channels, so their next step is to look at other channels. If the Higgs boson exists at a certain mass, one would expect to find an excess at the same energy and of the same magnitude in each decay channel.
CMS found similar results, noting an energy excess at less than 127 GeV. Its significance is smaller: 1.9 sigma, or a confidence level of 94%. Separately, the statistical significances of these results are too small to claim a discovery; particle physicists require a confidence level of 99.9999% (5 sigma) before they declare that a new particle has been found. At this certainty level the chance that the result is erroneous and was obtained simply due to random statistical fluctuations is only one in a million. The fact that both experiments independently found excesses at similar energies indicates that they may be on the right track to finding the Higgs boson. A definitive answer on the existence or non-existence of this elusive particle will require much more data, and is not likely until late in 2012 according to the recent press release.
While the discovery of the Higgs boson would answer the most fundamental outstanding question in the Standard Model of particle physics — why particles have mass — there is still much to be learned about both matter and energy. The Standard Model, even if completely confirmed, is not the final theory capable of explaining everything. It is able to describe only 4% of the matter/energy content of the Universe, the rest being composed of dark matter and dark energy, of which we know very little. But a Standard Model Higgs boson could point to theories that go beyond the Standard Model itself. Once we confirm its existence we will study its behavior, where subtleties may indicate new physics. As an example, if the Higgs boson has an energy of less than 120 GeV, we expect a supersymmetric model to hold. Thus not only will the Higgs boson prove a long-awaited theoretical prediction, but further observational evidence could pave the way for new and exciting physics.