The Higgs

FAS Astronomers Blog, Volume 30, Number 9.

It has been ten years since July 4, 2012. Yes, ten years since that massive discovery. With this discovery a weight was lifted off all of physics. (Puns intended). It was the discovery of the Higgs.

The underlying structure of the universe is described by the standard model of particle physics. For many years, this model was incomplete. It was missing something called the Higgs boson.

The Higgs boson is the manifestation of the Higgs field, which is something other particles interact with through the Higgs mechanism to achieve mass. The more a particle interacts with the Higgs field, the more mass it has. It is thought that particles initially achieved mass during something called “spontaneous symmetry breaking” when the four fundamental forces we see today broke away from a unified super force shortly after the big bang at the very beginning of the universe.

The general idea is that the Higgs acts like thick fluid. Some particles, such as the photon, are small and fast enough to move through the fluid without any interaction, so they are massless and continue on at the speed of light. Other particles, such as the weak force W^+/- and Z⁰ bosons are heavy and slow. They interact with the fluid, and this gives them mass. Ian Sample describes how the Higgs field works in the beginning of this video.

The theories underlying the process through which particles achieved mass were first proposed in the early 1960s. Several physicists including Yoichiro Nambu, Jeffery Goldstone, and Phillip Anderson developed some of the early ideas. Three groups of physicists followed. (Robert Brout and Francois Englert), Peter Higgs, and (Gerald Guralnik, Carl Richard Hagen, and Tom Kibble) published three pivotal papers on the subject in 1964 issues of Physical Review Letters.

Peter Higgs published an initial paper in late 1964. He then sent a second paper to the same journal, but it was rejected. He revised the paper by adding a short section, which identified a particle associated with the process, and sent it to the Physical Review Letters. Little did he know at the time that this small change would introduce the particle that would eventually bear his name and become one of the most sought-after objects in physics.

Later during the 1970s, Sheldon Glashow, Steven Weinberg, and Abdus Salam develop a theory unifying electromagnetism and the weak nuclear force. Martinus J. G. Veltman and Gerard ‘t Hooft solidified the mathematical foundation for this theory. All included features of the Higgs. These scientists received Nobel prizes for their work; Glashow, Weinberg, and Salam in 1979, and Veltman and ‘t Hooft in 1999. 

In 1993, Leon Lederman with Dick Teresi published a book, The God Particle: If the Universe is the answer, what is the Question?, possibly assigning more significance to the Higgs than it deserved. Near the end of chapter 1, Lederman says that “The Godd**n Particle” might have been a better title, but the publisher demurred. In the preface to the book’s 2006 edition, he remarks that “The title ended up offending two groups: 1) those who believe in God, and 2) those who do not. We were warmly received by those in the middle.” In the end, this reference caught the eye of journalists all over the world, and it made its way into many popular science articles. So, the Higgs boson became the “God Particle.”

Peter Higgs has always been uncomfortable with his name being attached to all this. Ian Sample (Massive) and Frank Close (The Infinity Puzzle), report that Higgs suggested it be called the ABEGHHK’tH mechanism after Anderson, Brout, Englert, Guralnik, Hagen, Higgs, Kibble and ‘t Hooft. Ben Lee first referred to it as the Higgs in 1972. Over time the name Higgs has stuck with us.

In a quirk of particle physics, it takes a huge particle collider to uncover the smallest constituents of nature. For many years, the Higgs boson remained elusive. Existing colliders were simply not large enough to find a massive particle such as the Higgs.

Eventually, CERN build the Large Hadron Collider (LHC) with enough energy to produce a Higgs boson. Prior to its startup, there were speculations that the LHC would create microscopic black holes, which would destroy the Earth. In September 2008, the LHC was turned on for the first time … and it soon suffered an explosion. But don’t worry, no black holes were created, and the Earth is still here.

By November 2009, it was up and running again and the search for the Higgs began. CERN crept slowly toward the Higgs discovery with a preliminary announcement in December 2011. They were close and in a press conference on July 4, 2012, the ATLAS and CMS experiment teams presented their results. Then CERN’s Director General Rolf Heuer stepped forward and said, “I think we have it.”

More data was collected and published by the end of 2012. The press continued to be involved – so much so that the Higgs boson was nominated for Time Magazine’s particle of the year. Finally, in March 2013, the folks at CERN were ready. They confirmed that they had, in fact, discovered the Higgs boson.

After the discovery, two of the six authors of the 1964 papers, Peter Higgs, and Francois Englert, were awarded the 2013 Nobel Prize in Physics. Englert’s coauthor Robert Brout had passed away in 2011 and was no longer eligible. Guralnik, Hagen, and Kibble were bypassed presumably because they published last, and the award is only given to at most three individuals.

As an aside, it is the Higgs that gives mass to other particles (almost). The original focus was on the particles associated with the weak nuclear and electromagnetic interactions. The Higgs does impart mass to quarks, which interact via the strong nuclear force (gluons) and are the components of protons and neutrons. However, most of the mass attributed to nucleons (protons and neutrons) comes from the energy associated with the binding of quarks inside the nucleons. I’ve come across a couple of explanations for this. 

• The strong force exhibits something called asymptotic freedom, which means it gets stronger as the distance between quarks increases. As such, the closer quarks are to each other, the more freedom they have to move about and the more energy they have (from increased mass as they approach a fraction of the speed of light). Therefore, protons and neutrons derive most of their mass from this energy (and corresponding mass) and not from the rest mass of the quarks themselves. (adapted from Marcus Chown).
• Nucleons contain three “valence quarks. There are also virtual quark and anti-quark pairs popping in and out of existence inside the nucleons. It is the mass/energy from these virtual particles that contribute most of the mass rather than from the mass of the “valence” quarks. (adapted from Sean Carroll).

The Higgs (and the Standard Model) is associated with the stuff we can see and touch (aka baryonic matter). This is only around 5% of what is out there. Ninety five percent of the universe is composed of dark matter and dark energy, of which we know very little.