A recent CMS study cast a wide net and put limits on three theories of physics beyond the Standard Model: dark matter, extra dimensions and unparticles. There is a common misconception that the LHC was built only to search for the Higgs boson. It is intended to answer many different questions about subatomic particles and the nature of our universe, so the collision data are reused by thousands of scientists, each studying their own favorite questions. Usually, a single analysis…
The former presence of a cat on the patio can be inferred from where the rain didn’t land. Similarly, sterile neutrinos may be inferred from their effects on normal neutrinos, which themselves are barely visible. What does it mean for something to be invisible? If it does not reflect light with the right wavelengths, it is not visible to humans, though it might be detected by a specialized instrument. Neutral particles, such as the neutrons in an atom, do not…
The author has been waiting a long time to use this joke. If you listen in on particle physics conversations, you’ll hear a lot of alphabet soup, such as “b to s gamma,” “Z to tau tau” and “q q-bar to X.” Reactions among particles provide a view to the underlying physics: You can learn how particles are related by how willing they are to collide and how often they decay a particular way. I’ve long held a secret hope…
In Eve’s Diary, a short story by Mark Twain, Eve writes, “This majestic new world is marvelously near to being perfect, notwithstanding the shortness of the time, but there are too many stars in some places and not enough in others.” If you can get a good view of the sky, far from city lights, you’ll see that stars are grouped in clumps: random but not uniformly random. Part of this is due to gravity. Stars that are close to…
The Higgs field was recently found to give mass to leptons and quarks (matter particles) as well as the bosons (force particles). Lines in the diagram above indicate interactions between particles: The red lines are new. Nearly 50 years before its discovery, the Higgs field was proposed as a way to explain why particles have mass. The Standard Model would be internally inconsistent if particles could have mass on their own (that is, as an intrinsic property like charge), but…
The trajectories of protons in the LHC are controlled by magnetic fields. An upward-pointing magnetic field (B) applies a force (F) to the right on protons flowing through the beam pipe (into the plane of the picture), and this steers them around the (imperceptible) curve of the ring. Image courtesy of Jim Pivarski Manipulating small objects, such as the cogs of an old-fashioned watch, is difficult with bulky fingers, so special tools are needed to fit them together into a…
Supersymmetric particles produced in proton collisions might have any of a variety of decay patterns, but the total energy of all decaying particles corresponds to the mass of the first particle in the cascade. Supersymmetry is hard to kill. It is more general than most theories of physics beyond the Standard Model: It is the basic idea that particles and forces are fundamentally the same thing but appear different because something creates an effective distinction between them, similar to the…
In particle collisions like the one shown above, it is common for debris to be grouped into clumps known as jets rather than uniformly distributed in a circle. This event has about 10 jets, but most have only two or three. Image courtesy of ATLAS In these articles, I often get stuck when I need to describe jets. Jets are complicated yet ubiquitous in particle physics — they’re hard to avoid and hard to explain. In this article, I intend…
This plot shows how well the Higgs half-life (λ) is known: Less than 20 yoctoseconds is ruled out with 95 percent confidence, but greater than 20 yoctoseconds is still possible. The standard prediction is that the half-life is 100 yoctoseconds. It is almost two years since physicists from the CMS and ATLAS collaborations announced the discovery of a Higgs-like boson. Today, the evidence has strengthened to the point that they no longer qualify it as “Higgs-like.” The signal is now…
A jet of water sprayed through water loses energy and changes shape, as illustrated by this Jacuzzi jet. CMS scientists studied a similar phenomenon in an exotic liquid of quarks and gluons. Despite the complexity of particle colliders and the instrumentation needed to analyze their results, the ultimate aim of most particle physics experiments is to understand something simple. At a fundamental level, most natural phenomena turn out to be simple in profound ways. By contrast, our macroscopic world is…