## Jim Pivarski

#### Is the universe getting bigger or am I getting smaller?

It is a well-established fact that the universe is expanding. It grows without center, like an inflating raisin cake, but an infinite raisin cake filling all of space in all directions. The raisins are the galaxies. A problem I’ve had with this explanation is that if everything were to double in size — galaxies, houses, you and me, rulers — then we’d never notice. I might be a towering giant, but if the room is equally huge, I wouldn’t know….

#### Looking in Schrödinger’s box

CMS scientists study the possible ways a B meson decays into a kaon and two muons.

#### Keeping cool

Liquid nitrogen plumbing often develops a layer of ice, even in the summer. Photo: tibchris How do you cool off in the summer? A cold drink? A dip in the pool? Air conditioning? I guarantee that it involves making something else hotter: The second law of thermodynamics requires heat to flow from high temperatures to low temperatures unless additional energy is added to the system. Cooling off with water heats up the water, and air conditioners expel more heat outside…

#### The wonderful thing about triggers

Somehow, you have to get those marvelous tracks to film. Imagine you’re a particle physicist in 1932. You have a cloud chamber that can show you the tracks of particles, and you have a camera to capture those tracks for later analysis. How do you set up an apparatus to take pictures whenever tracks appear? At first, you might just try to be quick with your finger, but since the tracks disappear in a quarter of a second, you’d end…

#### The strong/weak force

Click to enlarge. The symmetries of the strong force (left) and the weak force (right) might be unified in a suite of particles that transform as a color-octet weak-triplet. (Strong force charges are called colors, merely because they are built from three fundamental values, like the primary colors). When describing a result from the CMS experiment, I usually try to relate it to everyday experience because it reminds me that the esoteric phenomena observed at the LHC are as much…

#### Magnets for measurements

Magnet systems in modern particle physics experiments are used to analyze particle charge and momentum, but the field is strong enough and covers enough volume to give a whale an MRI exam. Broadly speaking, a modern particle physics detector has three main pieces: (1) tracking, which charts the course of charged particles by letting them pass through thin sensors, (2) calorimetry, which measures the energy of charged or neutral particles by making them splat into a wall and (3) a…

#### Two Z’s or three?

A coupling is a connection in the paths of particles viewed in a diagram in which one axis represents the flow of time. A triple coupling could be one particle decaying into two or two particles colliding and forming one. In the Standard Model, W–W-photon is allowed, but Z–Z-photon is not. Most everyday phenomena, from magnetism to the chemical processes of life, are due to exchanges of photons. The only exceptions are gravity and radioactive decay (if you consider that…

#### Fractured protons

Photograph of out-of-focus Christmas tree lights (large circles) that instead focuses on dust on the lens of the camera (small dark spots with rings). The rings around the dust are caused by diffraction. Photo: Jon Rista In a bright light, you can sometimes see specks in your field of vision that resemble the picture above. If you try to turn your eyes to look at them, they move because these objects are sitting on your eyeball — blinking jostles them….

#### Observe neutral particles with this one weird trick

A shower produces dozens of particles that could be observed individually (inset figure) or collectively in a calorimeter (bottom). The previous Physics in a Nutshell introduced tracking, a technique that allows physicists to see the trajectories of individual particles. The biggest limitation of tracking is that only charged particles ionize the medium that forms clouds, bubbles, discharges or digital signals. Neutral particles are invisible to any form of tracking. Calorimetry, which now complements tracking in most particle physics experiments, takes…

#### Rule of three

The three-fold symmetry of electrons, muons and taus may be broken by Higgs decays. (Design adapted from a neolithic spiral and the flag of Sicily.) In Rendezvous with Rama, Arthur C. Clarke imagined an artifact built by aliens who have three arms with three fingers each, so everything about it has a three-fold symmetry. One could argue that our fondness for bilateral symmetries (in the design of cars, planes, cathedrals, etc.) comes from the ubiquity of this shape in life…