Knocking on Heaven's Door - Lisa Randall [60]
THE STANDARD MODEL AT THE LHC
The Standard Model of particle physics tells us how to make predictions about the light particles we’re made of. It also describes other heavier particles with similar interactions. These heavy particles interact with light and nuclei through the same forces the particles that constitute our bodies and our solar system experience.
Physicists know about the electron, and heavier similarly charged particles called the muon and the tau. We know that these particles—called leptons—are paired with neutral particles (particles with no charge that don’t directly experience electromagnetic interactions) called neutrinos, which interact only via the prosaically named weak force. The weak force is responsible for radioactive beta decay of neutrons into protons (and beta decay of nuclei in general) and to some of the nuclear processes that occur in the Sun. All Standard Model matter experiences the weak force.
We also know about quarks, which are found inside protons and neutrons. Quarks experience both the weak and electromagnetic forces, as well as the strong nuclear force, which holds light quarks together inside protons and neutrons. The strong force poses calculational challenges, but we understand its basic structure.
The quarks and leptons, together with the strong, weak, and electromagnetic forces, form the essence of the Standard Model. (See Figure 23 for a summary of the particle physics Standard Model.) With these ingredients, physicists have been able to successfully predict the results of all particle physics experiments to date. We understand the Standard Model’s particles and how its forces act very well.
[ FIGURE 23 ] The elements of the Standard Model of particle physics, which describe matter’s most basic known elements and their interactions. Up-and down-type quarks experience the strong, weak, and electromagnetic forces. Charged leptons experience the weak and electromagnetic forces, while neutrinos experience only the weak force. Gluons, weak gauge bosons, and the photon communicate these forces. The Higgs boson is yet to be found.
However, some big puzzles remain.
Chief among these challenges is how gravity fits in. That’s a big question that the LHC has some chance to explore but is far from guaranteed to resolve. The LHC’s energy—though high from the perspective of what has been previously achieved here on Earth and from the requirement of what it will take to address some of the big puzzles that come next on this list—is much too low to definitively answer the questions relating to quantum gravity. To do so, we would need to study the infinitesimally tiny lengths where both quantum mechanical and gravitational effects can emerge—and that is far beyond the reach of the LHC. If we’re lucky, and gravity plays a big role in addressing the particle problems that we’ll soon consider related to mass, then we will be in a much better position to answer this question and the LHC might reveal important information about gravity and space itself. Otherwise, experimental tests of any quantum theory of gravity—including string theory—are most likely a long way off.
However, gravity’s relation to the other forces isn’t the only major question left unanswered at this point. Another critical gap in our understanding—one that the LHC is definitively poised to resolve—is the way in which the masses of the fundamental particles arise. That probably sounds like a pretty strange question (unless of course you read my first book) since we tend to think of the mass of something as a given—an intrinsic inalienable property of the particle.
And in some sense that is correct. Mass is one of the properties—along with charge and interactions—that define a particle. Particles always carry nonzero energy, but mass is an intrinsic property that can take many possible values including zero. One of Einstein’s major insights was to recognize that the value of a particle’s mass tells how much energy it has when it’s at rest. But particles don’t always have a nonvanishing value for their masses.