On July 4, two experiments analyzing data from the Large Hadron Collider at the European Center for Nuclear Research (CERN) at the border of Switzerland and France announced that they had discovered a new particle, which looks similar to the long-sought Higgs boson. One of these two experiments is ATLAS, a collaboration of 163 institutions and nearly 3,000 physicists. UC Berkeley and Lawrence Berkeley National Laboratory are among these 163 institutions and have been members since 1994. Currently, about 40 scientists (10 of whom are graduate students) from Berkeley work on ATLAS and contributed to this discovery.
The discovery was made using the largest scientific instrument ever built: the Large Hadron Collider, or LHC. At this collider, protons get accelerated in a circular tunnel 100 yards below the Earth’s surface to 99.999999 percent of the speed of light. The tunnel has a circumference of 16.5 miles, and the protons take one full turn 11,254 times each second. Eventually, they collide with protons traveling in the opposite direction at points in the tunnel where the experiments are located. The LHC is the largest of a long series of accelerators, the first of which was invented by Ernest O. Lawrence in 1931 in Berkeley and had a diameter of only five inches. Lawrence received the 1939 Nobel Prize for this invention, and our local national laboratory on the hill above campus is named after him.
When the protons collide, they interact with each other, and some process occurs. This can be the production of a Higgs boson, but much more often it is just the production of other particles, e.g. two quarks or a photon and a quark, etc. Since the first collisions in 2009, 800 trillion interactions have taken place, and only about 200 were identified to be due to this new Higgs-like particle. Finding those 200 events relies on a sophisticated system of detector technologies that are used to identify different particle species and to measure their energies and angles.
This discovery made big news all over the world, and one might wonder why scientists are so excited about this. There are in fact several reasons for this excitement.
First of all, the known interactions of the elementary particles fail to account for one of their most crucial properties: mass. Without mass, nature would be very different, as complex structures such as atoms, materials and stars would be unable to form. It was the quest for the origin of mass that was the biggest single motivation for building the LHC. The first theory for producing mass, the Higgs mechanism, was invented nearly 50 years ago in 1964. The idea was that empty space was not really empty but was filled with a field that affected the propagation of particles, giving them mass. It is different from the more familiar electric, magnetic and gravitational fields. These fields result from symmetries and are only generated by sources. But there are some similarities. Oscillations of fields lead to quantum particles: the photon for electric and magnetic fields and the Higgs boson for the Higgs field.
Second, the Higgs boson is different, unlike any other known fundamental particle. All other particles are either matter particles or force carriers, and the Higgs boson is neither of those. For instance, the electrons and quarks are constituents of the atoms, while the photon is the force carrier of the electromagnetic force. The Higgs is not part of the building blocks of the atom, and it also does not mediate a conventional force. The matter and force particles interact in a very simple and beautiful way that is dictated by symmetry and has only one parameter for each force. The Higgs, however, has a plethora of interactions with many parameters. Furthermore, it destroys some of the original symmetries, leading to the observed diversity of particle masses and to the complexity of the structures we see in nature.
Third, if this particle really is a Higgs boson, and it certainly looks like one so far, we are now confronted with the pressing question of the instability of the Higgs field. In the mid-1970s, a very puzzling aspect of the Higgs was discovered; the Higgs field that pervades all space has a tendency to grow in strength, increasing particle masses almost without limit. In other words, the Higgs seems to do its job too well! In some sense, discovering a Higgs boson is a huge relief, as the entire theory of how fundamental particles interact with each other needed a mass-generation mechanism. But in another sense it leads to a huge puzzle: Why aren’t the particle masses much larger?
The program of the LHC has just started. In the coming decade, the number of proton collisions at the LHC will increase by about two orders of magnitude. The particle just discovered will be subjected to precision tests to confirm its Higgs nature and quite possibly to see deviations from the simplest Higgs theory. The LHC will also probe theories that were invented to solve the instability puzzle. The leading idea is called supersymmetry, which extends the symmetries of space and time. Over more than 35 years, Berkeley theorists played a leading role in inventing and developing these theories. Supersymmetry predicts lots of new particles, which are being eagerly sought at the LHC but have not shown up yet. Is this because they decay in unexpected ways to give signals in the detector that are hard to distinguish from the many other processes that occur? Or are the superparticles heavier than expected, requiring an upgrade in the proton beam energy that is planned to come online in about two years? Another possibility, actively pursued at Berkeley, is the multiverse. Maybe there are many universes, with most having very strong Higgs fields but no observers to make measurements.
Where will the Higgs lead — more symmetries, more universes … or something else entirely?
Lawrence Hall is a professor and theoretical physicist at UC Berkeley. Beate Heinemann is a professor and experimental physicist at UC Berkeley and Lawrence Berkeley National Laboratory as well as a member of the ATLAS experiment.