This shows a particle identified in a photograph of a bubble chamber (left) and a computer reconstruction of signals from a silicon tracker (right). Much of the complexity of particle physics experiments can be boiled down to two basic types of detectors: trackers and calorimeters. They each have strengths and weaknesses, and most modern experiments use both. This and the next Physics in a Nutshell are about trackers and calorimeters, to kick off a series about detectors in general. The…
Despite the difference in scale, medical PET scanners and the CMS experiment both observe the remnants of particle collisions and use them to reconstruct the source. Antimatter is becoming increasingly common in medical technology. One technique, called positron emission tomography (PET scan), follows the flow of a positron-emitting fluid through the body. Wherever an antimatter positron encounters a normal-matter electron, the pair annihilates and produces particles whose trajectories trace back to the point of annihilation, allowing a computer to reconstruct…
A photograph from the day that Sweden switched from driving on the left to driving on the right. A physicist might call this a change in chirality, since one driving pattern was replaced by its mirror image. Photo courtesy of rarehistoricalphotos.com One morning in 1967, all cars in Sweden had to stop, move over to the right side of the road, wait 10 minutes and then resume driving. From that day forward, Sweden has been a drive-on-the-right country, like its…
Distribution of mass calculations for pairs of jets: Black points are real data and colored peaks show what two sample models would look like (W’ and excited quarks). See the paper for detailed plots. Four months ago, I wrote about a CMS result that had implications for three distinct theories of new physics. I spoke too soon — a new result constrains seven. In this new measurement, CMS scientists identified proton collisions that produce two jets and, assuming that these…
This spiderweb was overtaken by a sudden frost. The last few days have been cold. Really cold. It prompted me to make some comparisons: cold as outer space, cold as laser beams, cold as an electron — but most of these are nonsensical. The concept of temperature only makes sense in settings with large numbers of particles and a diffuse distribution of energy, such as our everyday world. A single particle doesn’t have a temperature, and the temperature of a…
A branch that is one-dimensional to a chameleon is two-dimensional to an ant, and particles or waves that travel along the short dimension can loop around and even resonate. Perhaps the most surprising thing about the LHC is that it has the potential to discover new dimensions. This is strange because dimensions are mutually perpendicular directions, like length, width and height — a new dimension would be a direction that is perpendicular to all three. Not only is it hard…
Bandwidth, or the spreading of a radio station onto multiple, neighboring frequencies, is related to uncertainty in quantum mechanics. When I first started teaching, I was stumped by a student who asked me if quantum mechanics affected anything in daily life. I said that the universe is fundamentally quantum mechanical and therefore it affects everything, but this didn’t satisfy him. Since then, I’ve been noticing examples everywhere. One surprising example is the effect of Heisenberg’s uncertainty principle on Wi-Fi communication…
Muons (red) are 18 times lighter than tau leptons (blue), so we expect Higgs decays to muon pairs to be about 300 times less common than Higgs decays to tau pairs. Since the discovery of the Higgs boson two years ago, about 80 analyses have helped to pin down its properties. Today, we know that it does not spin, that it is mirror-symmetric, and that it decays into pairs of W bosons, pairs of Z bosons, pairs of tau leptons,…
Fractals like the one above have a property known as scale invariance. Some exotic forms of matter may also be scale-invariant. The first property of matter that was known to be quantized was not a surprising one like spin — it was mass. That is, mass only comes in multiples of a specific value: The mass of five electrons is 5 times 511 keV. A collection of electrons cannot have 4.9 or 5.1 times this number — it must be…
Most particles produced by proton collisions originate in the point where the beams cross. Those that do not are due to intermediate particles that travel some distance before they decay. The main concern for most searches for rare phenomena is to control the backgrounds. Backgrounds are observations that resemble the one of interest, yet aren’t. For instance, fool’s gold is a background for gold prospectors. The main reason that the Higgs boson was hard to find is that most Higgs…