Friday, March 29, 2013

Have a safe day!

Friday, March 29

3:30 p.m.

4 p.m.
Joint Experimental-Theoretical Physics Seminar - One West
Speaker: Steve Sekula, Southern Methodist University
Title: Latest ATLAS Higgs Results

Monday, April 1


3:30 p.m.

4 p.m.
All Experimenters' Meeting - Curia II
Special Topics: MICE Status/Progress; Liquid-Argon Purity Demonstrator Progress

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Wilson Hall Cafe

Friday, March 29

- Breakfast: blueberry-stuffed French toast
- Vegetarian chili
- Ye olde fish and chips
- Southern fried chicken
- Smart cuisine: seafood linguine
- Eggplant parmesan panini
- Assorted pizza by the slice
- Breakfast-for-lunch omelet bar

Wilson Hall Cafe Menu
Chez Leon

Friday, March 29

Wednesday, April 3
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Physics in a Nutshell

Electromagnetism, the simplest force

Much like the game of go, the basic rules of the electromagnetic force are simple, yet they play out in complex ways.

Electromagnetism is the training ground for modern physics, both in its historical development and in classrooms today. It is the simplest of the four forces, compared to the nuclear weak force, nuclear strong force and gravitation, but it gives rise to intricately rich patterns. All of the complex phenomena of everyday life, except for gravity and radioactivity, are due to the workings of electromagnetism. It makes chemical bonds, forming the basis of life, gives structure to solid and liquid matter, and makes lightning and aurora borealis twist across the sky.

And yet the fundamental rules of electromagnetism are startlingly simple: Like charges repel and opposite charges attract. If fundamental forces were board games, electromagnetism would be go, in which the rules can be learned in a few minutes but for which the strategy takes a lifetime to master. The other forces are more like chess, with more complicated fundamental interactions. The only problem with this analogy is that electromagnetism took physicists two centuries to learn and has never been mastered.

Electromagnetism was the first of the forces to be understood in a quantum context, in the late 1940s. The key insight was understanding that like charges repel because one emits a virtual photon and the other catches it, as described in the last Nutshell. Analogous to people in rowboats tossing a sack of flour back and forth, the momentum of the exchanged photons is transferred to the charges, pushing them apart.

So how do opposite charges attract? How can one boatman throw a sack of flour to another and have the boats drift toward each other? As it turns out, the quantum nature of the photon is essential. Photons can carry momentum without moving in a straight line between the throw and the catch: They pop from one location to another randomly. When two opposite charges attract, it is as though the sack of flour pops into existence behind the second boatman, pushing him toward the first. Think about that the next time two socks electrically cling in the laundry.

You may have heard of photons as "particles of light." That's true, too. Photons can be found in a virtual state, in which they act as described above, or a real one. Real photons are massless quanta of light, radio, gamma rays and any other frequency of electromagnetic radiation. Virtual photons are sometimes massive, sometimes here, sometimes there, always flitting back and forth between charges to bring them together or push them apart. Moreover, "virtualness" or "realness" is a matter of degree—visible light only approximates a massless train of waves.

The one aspect of electromagnetism that makes it seem simple (in comparison) is the fact that photons themselves do not attract or repel each other; they only affect charged particles. This feature separates electromagnetism from all the other forces. Strong force gluons are a sticky mess of gluons attracting gluons by emitting gluons, weak force dynamics are complicated by self-couplings, and gravity can attract itself in the form of a black hole. But the surprising thing is that the two nuclear forces look a lot like electromagnetism—they follow the same paradigm of virtual particles being tossed back and forth. They differ only in the details of how the pieces interact as they move across the board.

Jim Pivarski

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The attraction between opposite charges is a bit like two boaters tossing boomerangs, rather than sacks of flour. The photon appears behind the second charged particle, pushing it toward the first. Image courtesy of Dan Claes
Photo of the Day

Bright lights, big city

Chicago's city lights are visible from Wilson Hall. Photo: Marty Murphy, AD
In the News

Synopsis: A year-long search for dark matter

From Physics, March 28, 2013

Clues to the nature of dark matter could come from evidence that high-energy neutrinos are produced in the Sun. The neutrinos, according to certain dark matter theories, would result from particles called WIMPs (weakly interacting massive particles) becoming trapped by the Sun's gravitational field and annihilating with each other. Now, the collaboration running the world's largest neutrino telescope, the IceCube experiment at the South Pole, reports in Physical Review Letters its most comprehensive search to date for the predicted neutrinos.

Read more
Frontier Science Result: MiniBooNE

Stealthier than a neutrino

MiniBooNE observes excesses of 78.4 ±20.0 (stat) ±20.3 (syst) and 162.0 ±28.1 (stat) ±38.7 (syst) candidate electron neutrino events in antineutrino (top) and neutrino (bottom) modes, respectively. Here they are given as a function of reconstructed neutrino energy.

The search for sterile neutrinos has reached a new milestone. After collecting data for the past decade in both neutrino and antineutrino modes, the MiniBooNE experiment reports in a paper accepted for publication in Physical Review Letters an excess of events that suggests there may be additional neutrinos to the known three. MiniBooNE observed a combined excess of these events with 3.8 sigma significance.

It took 25 years to observe the electron neutrino after it was predicted to exist, so it is not surprising that it could take even longer to observe the proposed sterile partners of neutrinos. A sterile neutrino, unlike the neutrinos of the Standard Model, would not interact through the weak force. The existence of such neutrinos would be a sign of physics beyond the Standard Model.

The excess that MiniBooNE sees is consistent with neutrino oscillations at the approximately 1 eV2 mass scale, which implies these additional neutrinos are sterile. It is also consistent with neutrino oscillation evidence from the Los Alamos National Laboratory LSND experiment, which hinted at the existence of these as yet unobserved particles. In the antineutrino mode, the model with one sterile neutrino fits MiniBooNE data reasonably well, with 66 percent probability. In the neutrino mode, on the other hand, agreement between data and the fitted model is low—only 6 percent probability. However, expanded models with two or more sterile neutrinos improve these so-called best-fit probabilities, as well as agreement with LSND data, by allowing different oscillation patterns for neutrinos and antineutrinos (CP violation).

Future experiments at Fermilab and CERN will be able to test the LSND and MiniBooNE results and determine whether short-baseline oscillations and sterile neutrinos exist. These experiments include MINOS+, MicroBooNE, ICARUS-NESSIE at CERN and nuSTORM. MINOS+ will start taking data later this year with a more intense neutrino beam and will have better sensitivity for muon neutrino disappearance, which is a hallmark of all sterile-neutrino models. MicroBooNE will begin taking data in 2014 and will test whether the excesses observed by MiniBooNE are due to outgoing electrons from neutrino events, as expected by sterile-neutrino models, or to outgoing photons, which could point to other, unexplored phenomena. ICARUS-NESSIE plans to search for sterile neutrinos beginning in 2016. Finally, nuSTORM is a future muon storage ring experiment in which the neutrinos that arise from muon decay are well understood. By building two detectors at different distances from the storage ring, nuSTORM will be able to make a definitive test of short-baseline oscillations and sterile neutrinos.

If sterile neutrinos are proven to exist, then they will have a big impact on particle physics, nuclear physics, astrophysics, and cosmology.

—Zarko Pavlovic

The oscillation allowed regions as a function of Δm2 and sin22θ for antineutrino mode (left) and neutrino mode (right). Also shown are the allowed regions from the LSND experiment, which took data with antineutrinos, as well as limits from the KARMEN and ICARUS experiments.

Road D closed - today

Martial arts class - begins April 1

Nominations open for 2013 Tollestrup Postdoc Award - through April 1

Hiring managers: submit summer personnel requisitions by April 12

C2ST: Chemical Innovations: Molecular Modeling - April 3

Garden Club spring meeting - April 3

The World According to Higgs - Chris Quigg - April 12

Fermilab Arts Series: Barynya: Music & Dance of Russia - April 20

Writing for Results: E-mail and More - May 3

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