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Coffeeshop Physics
by Jim Pivarski

The Higgs boson may decay into a Z boson and a photon through an intermediate pair of charged particles.

Branching Fractions off the Menu

20 Sep 2013

This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).

A particle’s branching fractions — that is, the probability that it will decay into one set of particles rather than another — are a good way to see if physicists really understand what’s happening at microscopic scales. Many things can affect a particle’s decision to decay into, say, electrons rather than photons. If the physicists’ predictions match the observed probability of decay, then the underlying mechanisms may be well understood, especially if it is a tight balance between opposing forces. If not, then there might be a new intermediate particle involved or some other new phenomena to be discovered. read more »

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Dark Energy, Chunky or Smooth?

13 Sep 2013

One of the primary questions about dark energy is whether it is the same everywhere ("smooth") or varies in density from place to place ("chunky").

This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).

The discovery of dark energy was the most surprising scientific breakthrough in my lifetime. Many physicists consider it the most baffling of nature’s mysteries, and still little is known about it.

To say that it caused the textbooks to be rewritten is literally true. When I first studied cosmology, every textbook had a section on how the fate of the universe depends on the amount of matter it contains. As the universe expands, the gravitational force of all matter pulls against this expansion, slowing it down. If there is enough matter, the expansion can be reversed, pulling everything together into a big crunch. If there is too little, the universe will merely slow down as it flies apart. Dark energy is the discovery that the universe is not slowing down at all, but speeding up. read more »

Data Mining for b-quarks

23 Aug 2013

Tracks from a b-quark (yellow) and an ordinary quark or gluon (purple), overlaid on a photo of the CMS tracker, in approximately the position where these particles were observed.

This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).

Of the six known types of quarks, only two can be distinguished in a typical particle physics experiment. The top quark, once produced, has a dramatic signature involving cascades of decays from heavy particles into lighter ones. The bottom (b) quark also decays into lighter particles, but these are hidden in a spray of additional particles that form along with it, called a jet. A jet is essentially random: random particles moving in nearly random directions. The lighter quarks—charm, strange, up, and down—produce only jets when they decay.

In practice, this means that it’s almost impossible to distinguish an up-quark from a down-quark. Fortunately, most of the questions that scientists want to address do not rely on telling the difference.

The b-quark, however, is interesting for a variety of reasons: It can be part of a signal for new phenomena; it is part of the top-quark decay chain; and it probes fundamental symmetries in the laws of nature. Finding a way to distinguish b-jets from all other jets would help many scientists at once. read more »

The Shape of Things that Were

16 Aug 2013

This shows the shape of the early universe as seen from outside of space and time. One spatial dimension is shown—the circumference of the bowl—and time is represented by the direction away from the bottom of the bowl. Inflation, nucleosynthesis, the cosmic microwave background and the first stars are not drawn to scale.

This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).

In our culture, the phrase "big bang theory" is often used to mean the idea that the universe was created in one explosive moment (or it’s a TV sitcom or a Styx album). For cosmologists, however, "big bang" means the early expansion of the universe, which might or might not have begun in an instant. The late stages of this process are better understood than the beginning: It ended with a sky full of stars, but at the beginning, even the laws of physics are unknown. Is a point of infinitesimal size and infinite density even possible? No one knows. read more »

R-Parity Violation

26 Jul 2013

Not only would the discovery of supersymmetry double the number of known particles, but it would introduce a new type of charge: Standard Model particles have positive R-parity and supersymmetric particles have negative R-parity.

This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).

Many of the CMS results presented in this column involve supersymmetry, the idea that there is a deep relationship between matter and forces. If nature is supersymmetric, then for each type of matter particle, there would be a corresponding supersymmetric force, and for each of the four forces, there would be a corresponding particle of supersymmetric matter. No evidence of supersymmetry has yet been found, despite the fervent searches, so you might be wondering why scientists are still looking for it. There are two reasons: (1) It would greatly increase our understanding of how all known particles unify, and (2) there are so many different ways that supersymmetry might manifest itself that the searches performed so far are not exhaustive. read more »

How Real is Relativity?

19 Jul 2013

Rotating a picture frame mixes horizontal and vertical in much the same way that relativity mixes space and time.

This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).

Special relativity is a well-established fact of nature. Although we rarely encounter relativistic effects in everyday life, they are routine in the world of subatomic particles and in the cosmos. Objects traveling close to the speed of light become spatially compressed and experience time at a slower rate. For example, lead nuclei in a stationary brick are roughly spherical, but when these same nuclei accelerate and collide in the LHC, they flatten into pancakes that collide face-on. Particles resulting from the collision, such as kaons, take a longer time to decay than stationary kaons because their internal clocks run slower. These are all measurable effects that have been observed in colliders for decades.

But how real is this stretching of time and squashing of length? Perspective makes faraway objects look small and nearby objects look large, but we do not say that they really are smaller when they’re farther away. This is because the same object can look small to a faraway person and large to a nearby person at the same time. We usually don’t call an effect real unless it is consistent among observers. read more »

Weird Tracks

12 Jul 2013

Clockwise from left: CMS event display, bubble chamber photograph, cloud chamber photograph.

This is a reprint of an article from Fermilab Today (which are limited to 300–500 words).

If you’ve ever seen computer displays like the one above, or old bubble chamber photographs, or even tinkered with a homemade cloud chamber, then you’ve seen particle tracks. Tracking is an important tool for particle physics experiments because tracks show you the comings and goings of individual particles. When coupled with a magnetic field, they also tell you the momentum of each particle, since slow particles curve in the field while fast ones fly straight through. Irène Joliot-Curie, daughter of Marie Curie and an early adopter of tracking for her radioactivity research in the 1930s, called it "the most beautiful phenomenon in the world, apart from childbirth." read more »