Friday, Aug. 1, 2014

Have a safe day!

Friday, Aug. 1

3:30 p.m.

4 p.m.
Joint Experimental-Theoretical Physics Seminar - One West
Speaker: Aaron Higuera, University of Rochester, Guanajuato University
Title: Coherent Charged-Pion Production at MINERvA

Monday, Aug. 4


3:30 p.m.

4 p.m.
All Experimenters' Meeting - Curia II

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Wilson Hall Cafe

Friday, Aug. 1

- Breakfast: blueberry-stuffed French toast
- Breakfast: chorizo and egg burrito
- Cajun chicken sandwich
- Smart cuisine: white fish florentine
- Kielbasa and kraut
- Roast beef and cheddar panino
- Cilantro lime chicken bowl
- Clam chowder
- Texas-style chili
- Assorted pizza by the slice

Wilson Hall Cafe menu
Chez Leon

Friday, Aug. 1

Wednesday, Aug. 6
- Lemongrass shrimp over rice vermicelli and vegetables
- Jasmine chai rice pudding

Chez Leon menu
Call x3524 to make your reservation.


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Physics in a Nutshell

Baryon acoustic oscillations

Distribution of galaxies observed by SDSS: Each dot is a galaxy (image source). More distant, older galaxies are on the top of the image, with closer, more recent galaxies on the bottom, showing the development of structure over time, from smooth to textured.

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 one another gravitationally attract and tend to form tight clusters with gaps between the clusters. The same is true of whole galaxies — nearby galaxies tend to amalgamate into superclusters.

The other part of the explanation has to do with the distribution of matter in the early universe. The early universe was nearly uniform, but not perfectly so. Tiny quantum fluctuations, stretched to cosmic proportions by the expansion of space, provided the initial seeds that helped matter start coalescing into galaxies, stars, planets and us.

This phenomenon is known to astronomers as baryon acoustic oscillations (BAO). Baryons are particles of ordinary matter (as opposed to dark matter) — "baryon" is a general term for particles such as protons and neutrons, which provide most of the mass of normal atoms. The "acoustic oscillations" part refers to the fact that fluctuations in the early universe were actually sound waves. Before the universe was cool enough to transition from a glowing plasma into a transparent gas (the first 380,000 years), the light from the Big Bang exerted pressure on charged particles in the plasma, and the waves of high- and low-pressure were analogous to sound in air.

When matter became transparent, the light passed through it unimpeded, and this light is today visible as cosmic microwave background (CMB). Matter, on the other hand, was suddenly released from outward pressure and became subject only to gravity. The CMB is effectively a snapshot of the lumpiness of the universe when it was 380,000 years old, and the BAO is the same distribution amplified by gravity over the last 13.8 billion years.

Although the CMB and BAO have been separate for most of the history of the universe, the imprint of CMB-scale fluctuations has been discovered in the distribution of galaxies today. The characteristic wavelength between crests of galactic superclusters and troughs of intergalactic voids is about 500 million light-years, which corresponds to 73 octaves below middle C. This is exactly what would be expected from propagating the crests and troughs of the CMB forward by 13.8 billion years.

Jim Pivarski

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Photo of the Day

Farewell, Feynman van

The Feynman van, which was on display in front of Wilson Hall from April to June, finally departs Fermilab for California. Photo: Lauren Biron
In the News

Dark matter search enters round 2

From Scientific American, July 28, 2014

Dark matter scientists are doubling down on efforts to catch the elusive particles thought to constitute most of the matter in the universe. These theorized particles make themselves felt through gravity: They appear to tug on the normal matter throughout the universe but they otherwise can't be seen or touched. Experiments aiming to observe the rare occasions when dark matter particles interact with normal atoms have been operating for decades without success and have already ruled out many of the most basic explanations for dark matter. Rather than give up the search, however, three of the largest experiments recently won approval to make big upgrades, potentially allowing them to reach the sensitivities needed to finally pin down these cagey missing particles.

Read more

Frontier Science Result: MINERvA

Pion on the break shot

This MINERvA event display shows a coherent pion production candidate interaction. The neutrino enters the detector from the left and interacts with a nucleus, producing a muon and a pion. The colors indicate the amount of energy deposited at that point.

Para una versión en español, haga clic aquí. Para a versão em português, clique aqui.

In February, the MINERvA experiment at Fermilab reported its findings of what happens when a neutrino produces a pion (a particle made of a quark and an antiquark) by interacting with a proton or neutron inside the nucleus. In today's wine and cheese seminar, MINERvA will release its measurement of what happens when a neutrino or antineutrino produces a pion outside a nucleus by interacting with the nucleus as a whole but leaving the nucleus intact. Neutrino physicists refer to this reaction as coherent pion production.

A neutrino interaction with a nucleus is like the break shot at the beginning of a billiards game where the cue ball is shot into a tightly packed group of target balls to break up the group. If coherent pion production were to happen in billiards, the target balls would remain tightly packed after being struck and an additional ball (the pion) would emerge from the collision.

Coherent pion production can be a background to neutrino oscillation experiments that measure how neutrinos change from one type of neutrino to another as they travel through space. Predictions for coherent pion production disagree in how much background the reaction should produce in oscillation experiments. In addition, recent experiments that looked for coherent pion production at neutrino energies important to oscillation experiments came up empty — until now, that is.

MINERvA has measured coherent pion production on carbon atoms where the interaction changes the neutrino (or antineutrino) into a muon (a heavier cousin of the electron). MINERvA searches for coherent pion production using its defining characteristic — that the interaction does not breakup the nucleus.

MINERvA can see whether or not breakup of the nucleus occurs in two ways. First, it can detect the particles ejected from the nucleus when it is broken up and can require that only a muon and a pion are detected at the interaction point. Second, MINERvA can measure the momentum transferred to the nucleus by measuring the muon and pion momentum and can require it be consistent with not breaking the nucleus apart.

These two signatures together greatly reduce the background and allow MINERvA to measure, for the first time, the details of coherent pion production to understand how it produces background for oscillation experiments.

Aaron Mislivec, University of Rochester

These plots show the interaction rate of neutrinos (left) and antineutrinos (right) with nuclei in MINERvA as a function of the measured momentum transferred to the nucleus t. The data exhibit a definite excess above the predicted background rate at low t, consistent with coherent pion production. The signal prediction is from the model of coherent pion production currently used by neutrino oscillation experiments.
Aaron Mislivec of the University of Rochester (left) and Aaron Higuera of University of Guanajuato and University of Rochester worked on the antineutrino and neutrino analyses, respectively. Aaron Higuera will give a talk on both results at today's wine and cheese seminar.
In the News

New correction to speed of light could explain SN1987 neutrino burst

From Physics World, July 28, 2014

The effect of gravity on virtual electron–positron pairs as they propagate through space could lead to a violation of Einstein's equivalence principle, according to calculations by James Franson at the University of Maryland, Baltimore County. While the effect would be too tiny to be measured directly using current experimental techniques, it could explain a puzzling anomaly observed during the famous SN1987 supernova of 1987.

Read more


Today's New Announcements

C++ FNAL Software School - Aug. 4-8

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Meet with Pace on train station shuttles - today

Fermilab prairie plant survey - Aug. 9

Deadline for the UChicago tuition remission program - Aug. 18

Call for applications: URA Visiting Scholars Program - apply by Aug. 25

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