The Heart of Mass

The term “God particle” tends to rankle physicists. The flippant reference to the recently discovered particle believed to be the Higgs boson was coined by Leon Lederman, the former director of the Department of Energy’s Fermilab and Nobel Prize winning physicist. But, says Ken Krane, nuclear scientist and emeritus professor of physics at Oregon State University, had it not been for the name, the discovery might not have generated such headlines in July. It was good, he adds, to see physics in the news.

It’s no exaggeration to call the discovery momentous. In July, two teams working at the world’s largest atom smasher (the Large Hadron Collider at the European Center for Nuclear Research, known as CERN, near Geneva, Switzerland) announced independently that they had strong experimental evidence for the existence of the Higgs. In an interview shortly after the announcement, Krane explained what scientists found, what it means for their science and why it matters to the rest of us.

Krane chaired the Oregon State Department of Physics from 1984 to 1998 and has written or edited nearly 20 books and monographs, as well as dozens of research articles. The American Association of Physics Teachers recognized his exceptional teaching by awarding him its Millikan Medal in 2004.

Terra: What did the physicists at CERN find?

Krane: Every time we bang particles together like we do at CERN, we create dozens of new particles. These are not particles in the sense that we ordinarily think of them. They live for a definite but very brief time and then decay into a multitude of other particles. We don’t see the original particles that were produced, but what we see is all the decay products that must live long enough to make it into the detector where they generate signals that we can record. If you add up all those signals, you can get to the mass of the particle that was created. So you need to be fairly sure you’ve captured everything. And then you’ve got to be fairly sure of what the background (random fluctuations of energy) is in that region. And that’s what they’ve done.

The result is a particle with a mass that is nothing special in terms of particles we normally deal with. It has about half the mass of a uranium atom. Not insignificant, but it’s not a huge massive particle in a realm that’s not been explored before.

Scientists talk about “a five-sigma result.” That’s five standard deviations (five times the average distance between all data and the mean), and that’s pretty secure, strong confirmation that this is a real observation, not just some random event. In my experiments in a very different area, if I were to see a peak on a background that is five standard deviations above the background, that’s a peak (evidence of a particle), not a fluctuation of the background. By analogy, in a random distribution of men with an average height of 6 feet with a standard deviation of 2 inches, someone who is 6’10” would certainly stand out.

Terra: What does it mean for physics?

Krane: We know an elephant is more massive than a mouse, and we understand why. The mass of a body is basically the total mass of all the atoms of which the body is composed, and there are more atoms in an elephant’s body than in a mouse’s body. At a more fundamental level, we understand why a single atom of a heavy element such as lead is more massive than a single atom of the lightest element, hydrogen. The mass of an atom is essentially the total mass of all its constituent protons and neutrons, and the lead atom has about 200 protons and neutrons while the hydrogen atom has only one. So a common way of understanding the mass of an object is on the basis of the masses of its constituent parts.

Particles have their particular masses because of how they interact with the Higgs field.
— Ken Krane

Unfortunately this type of logical reasoning breaks down at the most fundamental level. The electrons in an atom are members of a family of three particles that are “elementary” in the sense that they have no internal structure and can’t be taken apart into still smaller entities. The electron is the lightest member of this family. The other two members are 200 and 3500 times as massive as the electron. Why do these three particles have different masses? Why these particular values? And where does their mass come from if it can’t be accounted for in terms of constituents? This is the question that particle physicists have been trying to answer by searching for the Higgs particle.

In 1964, British physicist Peter Higgs proposed that the universe is filled with a force field now known as the Higgs field and that particles get their masses by interacting with this field. Each different particle has a different strength of interaction with this field, which results in the different masses for the particles. According to this explanation, particles would otherwise be massless, but they get their apparent masses by interacting with this gooey field, much as an object that flies effortlessly through air is more sluggish in traveling through water.

In theoretical physics, each type of force (gravity, electromagnetic, nuclear, etc.) can be accounted for through a force carrier, which is a type of particle called a boson. The force carrier for the Higgs field is known as the Higgs boson, and its observation would amount to a verification of Higgs’ hypothesis and the first step for scientists to be able to study the origin of mass. Now instead of throwing up our hands and saying, “Particles have their particular masses just because they do,” we can now say, “Particles have their particular masses because of how they interact with the Higgs field.” The latter explanation gives scientists a basis for a deeper understanding of the way the universe works, and it enables new experiments to study this interaction and achieve a better understanding of how mass originates.

From a scientific point of view, the concept of mass is not terribly well understood, nor is the standard of mass well defined in terms of measurable quantities. A century ago, we replaced other unit standards in physics, the standard second and standard meter, with very precise atomic standards. Originally the meter was based on two scratches in a bar kept in a vault in Paris. Everybody else could take their bar and compare it to the standard bar. That’s how standards of weights and measures were done.

Now, we’ve replaced the standard for length with an atomic wavelength and the standard of the second with an atomic frequency, and if you look at the dozens and dozens of basic standards in physics, they’ve almost all been replaced with very precisely determined atomic standards, except for mass. The standard of mass is this kilogram sitting in a glass bell-jar in a vault in Paris. We don’t yet have a way of going from that standard mass to an atomic mass.

Terra: What did it take for physics to arrive at this point?

Krane: The Large Hadron Collider (the “hadron” family includes particles such as protons and neutrons that interact through the so-called “strong” force) is a circular racetrack 17 miles long buried in a tunnel more than 500 feet below the border between France and Switzerland. It was designed to smash together beams of protons traveling in opposite directions around the circle at speeds in excess of 99.999999% of the speed of light. The accelerator, which began operating in 2008, was designed to optimize the search for the Higgs particle. It was built at a cost of approximately $9 billion by an international consortium of nations, because such a large project is beyond the science resources of any one nation. The European Center for Nuclear Research (CERN), which operates the accelerator, includes several thousand scientists and engineers on its staff. Many more thousands of scientific visitors travel to CERN to participate in experiments.

There are two detector systems at the LHC. Each reported results supporting the existence of the Higgs particle. The construction and operation of these detector systems involved in excess of 5,000 scientists from more than 30 countries and 200 universities and scientific institutes.

Terra: Why does it matter for society?

Krane: Science is a human endeavor, first of all. It is an indication of our desire, manifest for three millennia since the ancient Greeks first speculated about the existence of atoms, to achieve an understanding of the fundamental nature of matter. Our instincts as human beings push us toward a deeper understanding of nature. Just like small children, scientists are always asking “why?” and seeking answers.

The discovery of the Higgs field probably offers no cure for any disease. Nor will it solve the energy crisis or contribute to reversing global warming. But it does advance our understanding of the way the universe works at the most fundamental level.

In the 19th century, little was known about atoms or the ways that atoms combined to form chemical compounds. The development of the Periodic Table of the Elements showed that many thousands of chemical compounds could be understood on the basis of fewer than 100 basic building blocks, the chemical elements. And that understanding in turn led to successful theories of atomic structure and to the development of new chemical compounds and electronic devices that are now essential to our lives.

The situation was similar for particle physicists in the 20th century. Out of a complicated array of thousands of “elementary” particles came an underlying order called the Standard Model, which consisted of 6 “light” particles called leptons, 6 “heavy” particles called quarks, 4 field particles called bosons that are the carriers of the different forces by which the particles interact (electromagnetism, strong and weak nuclear forces), and the Higgs boson. All of these components of the Standard Model have been observed in laboratories except the Higgs boson. It is the last remaining “element” of the Standard Model that is needed to complete our understanding of the fundamental nature of matter.

Even though the outcome of the research may have no immediate practical applications, the research leads to technological developments that benefit society. Research in high-energy particle physics has produced advances in the technology of particle accelerators, which are used for medical diagnosis and treatment. The need for better imaging of the results of particle physics experiments led to the development of devices that are now widely used in digital cameras. Complex particle physics experiments require advanced computing hardware and software. Computing techniques developed for these experiments are now being used to map the human genome and to search for new molecular structures that could be used to develop new types of medicines. The need to share large data files among international teams of particle physicists led to the development of the World Wide Web. In support of accelerators such as the European facility where the Higgs was observed, industries that build components, including superconducting magnets, computer systems and particle detectors, develop more efficient manufacturing and testing techniques that result in improved consumer goods.

Completing the Standard Model isn’t the end of particle physics. It’s not like people won’t be interested in it any longer and the graduate students will be going to other projects. There are still lots of interesting things to be found out there.

There are some other wrinkles that have been thrown into the Standard Model recently. The model was originally based on particles called neutrinos (three of the “light” particles in the Standard Model) having zero mass. That’s not true any longer. In the last 10 or 20 years, we’ve learned that neutrinos have a very, very tiny mass, but definitely not zero. The basics of the Standard Model don’t include neutrinos with mass. So already we have to fix it up because we don’t understand neutrino mass (and it’s not yet clear if the Higgs mechanism could be applied to give mass to the neutrinos).

Another mystery concerns the nature of “dark matter” which comprises about 25% of the universe (in addition to 5% ordinary matter and 70% “dark energy,” which is even less understood). Dark matter has mass and is affected by gravity, but it’s not composed of anything like ordinary matter, so it’s not there in the Standard Model. If you put things together in the Standard Model, you get the protons and neutrons of ordinary matter, and dark matter isn’t composed of protons and neutrons.

Another puzzle is why the Higgs mass has the particular value that it does. The Higgs particle can explain the masses of the other particles, but calculations of the mass of the Higgs particle come up with values many orders of magnitude bigger than the actual value that it seems to have, based on this discovery. So again, the models have to be fixed up to bring the Higgs mass down to this actual value.

Terra: This discovery is one of many that have changed our understanding of the subatomic world in the past several decades. How has your view of physics changed over the course of your career?

Krane: The quark model was introduced in the 1960s when I was a graduate student. Previous to that, there was a list of hundreds and hundreds of particles. There was no way of categorizing them or understanding them. Now we can talk about the substructure and interactions that produce these particles. You can understand the way these particles form groups and families, like the way you can talk in chemistry about compounds of halogens because they have certain characteristics and certain valences, and they form compounds in certain ways.

The other thing that I get excited about in the time since I was a graduate student is the coming together of astrophysics and particle physics, which were very separate realms. Astrophysics has become a precise experimental science in the last couple of decades, where we can now measure things to two or three significant figures. We can pin down the age of the universe within 1 percent and other parameters as well, such as the temperature of the universe, in the same way.

I mentioned that there are three members of the electron family. Each of these is paired with a member of the neutrino family, and there are also three pairs of quarks. Why do they match up and why only three? Should we build a big accelerator to find out if there’s a fourth generation of electron-like particles? The answer to that is “no.” We know that because the evolution of the early universe would have been measurably different if there had been four members of the electron family and four pairs of quarks. The early temperature would have been different. The early composition would have been different.

And the present composition, determined by its early evolution, would be different. Today we can study such characteristics of the universe as the relative amounts of helium and hydrogen or the relative amounts of deuterium (“heavy hydrogen”) and ordinary hydrogen, from which it can be concluded that there almost certainly cannot be another generation of electron-like particles or neutrinos. So there is no point in building an accelerator to search for such particles.

There are now numerous university research programs known as “particle astrophysics.” The two separate fields, one dealing with the very small and the other with the very large, have been joined into a new research specialty. Somehow, the theory of elementary particles is at some point going to have to work in the neutrino masses, which is an astrophysics discovery, and they’re also somehow going to have to include dark matter, which is another.

Someday in some accelerator we’ll be able to smash things together and create dark matter, whatever it is. The particle physicists will have to go to work to do those experiments and interpret those experiments and put the results into the framework of the Standard Model. It’s remarkable to see how these two fields have come together and have common goals in understanding the universe.

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Learn more about Oregon State research in nanoelectronics, photovoltaic and magnetic materials, biophysics, and computational and theoretical physics.

Scientists confirmed and updated their findings in papers published on August 1, 2012.

Read the July 4 announcement from CERN about the particle believed to be the Higgs boson.

Watch a video produced by CERN to explain how physicists came up with the Standard Model.