Thursday, April 3, 2014

Have a safe day!

Thursday, April 3

11 a.m.
Academic Lecture Series - One West
Speaker: Wolfgang Altmannshofer, Perimeter Institute
Title: EDMs and Higgs and Flavor Physics

2:30 p.m.
Theoretical Physics Seminar - Curia II
Speaker: Yanou Cui, University of Maryland
Title: Baryogenesis from WIMPs

3:30 p.m.

Friday, April 4

3:30 p.m.

4 p.m.
Joint Experimental-Theoretical Physics Seminar - One West
Speaker: Oleg Brandt, University of Heidelberg
Title: Top Quark Mass Measurements at DZero: How Precise Does It Get?

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Wilson Hall Cafe

Thursday, April 3

- Breakfast: Canadian bacon, egg and cheese Texas toast
- Breakfast: Corned beef hash and eggs
- Carolina pulled pork sandwich
- Smart cuisine: barbecue chicken breast
- Honey baked ham
- Buffalo chicken tender wrap
- Grilled- or crispy-chicken Caesar salad
- White chicken chili
- Chef's choice soup
- Assorted pizza by the slice

Wilson Hall Cafe menu

Chez Leon

Friday, April 4

Wednesday, April 9
- Bayou catfish with Creole sauce
- Island rice
- Sauteed green beans
- Chocolate pecan pie with Bourbon cream

Chez Leon menu
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From symmetry

The oldest light in the universe

The Cosmic Microwave Background, leftover light from the big bang, carries a wealth of information about the universe — for those who can read it. Photo: Reidar Hahn

Fifty years ago, two radio astronomers from Bell Labs discovered a faint, ever-present hum in their telescope that they couldn't identify. After ruling out radio broadcasts, radar signals, a too-warm receiver and even droppings from pigeons nesting inside the scope, they realized they'd found a soft cosmic static that originated from beyond our galaxy. Indeed, it seemed to fill all of space.

Fast-forward five decades, and the static has a well-known name: the cosmic microwave background, or CMB. Far from a featureless hum, these faint, cold photons, barely energetic enough to boost a thermometer above absolute zero, have been identified as the afterglow of the big bang.

This light — the oldest ever observed — offers a baby picture of the very early universe. How early? The most recent result, announced on Saint Patrick's Day 2014 by the researchers of the BICEP2 experiment, used extremely faint signals imprinted on CMB photons to reach back to the first trillionth of a trillionth of a trillionth of a second after the big bang — almost more of a cosmic sonogram than a baby picture. This image offered the first direct evidence for the era of cosmic inflation, when space itself ballooned outward in a turbocharged period of expansion.

CMB photons have more to tell us. Combined with theoretical models of cosmic growth and evolution, ongoing studies will expand this view of the very early universe while also looking forward in time. The goal is to create an entire album chronicling the growth of the universe from the very moments of its birth to today.

Further studies promise clear insight into which of the many different models of inflation shaped our universe, and can also help us understand dark matter, dark energy and the mass of the neutrino — if researchers can read the CMB in enough detail.

That's not easy, though, because the afterglow has faded. During its epic 13-billion-year-plus journey, light that originally blazed through the universe has stretched with space itself, its waves growing billions of times longer and cooler and quieter.

Relic radiation
The Standard Model of Cosmology says that about 13.8 billion years ago, the universe was born from an unimaginably hot, dense state. Before a single second had ticked away, cosmic inflation increased the volume of the universe by an amount that varies according to the particular model, but always features a 10 followed by about 30 to 80 zeroes.

When inflation hit the brakes, leftover energy from that expansion created many of the particles we see around us today: gluons, quarks, photons, electrons and their bigger brethren, muons and taus, and neutrinos. Primordial photons scattered off free-floating electrons, bouncing around inside the gas cloud that was the universe. Hundreds of thousands of years later, the cosmic cloud of particles cooled enough that single protons and helium nuclei could capture the electrons they needed to form neutral hydrogen and helium. This rounded up the free electrons, clearing the fog and releasing the photons. The universe began to shine.

These photons are the cosmic microwave background. Although now weak, they are everywhere; CMB photons bathe the Earth — and every other star, planet, black hole and hunk of rock in the universe — in their cold light.

Read more

Lori Ann White

Photo of the Day

Bubble chamber at SiDet

This gorgeous shot of the bubble chamber was taken on a fall night. Photo: Steve Krave, TD
Video of the Day

Science students at Fermilab

Fermilab helps teachers meet the Next Generation Science Standards. Elementary and middle school teachers learn how to provide their students with authentic science experiences. High school physics teachers partner with particle physicists at Fermilab and 50 other QuarkNet Centers. The students take measurements with authentic data and develop relationships with physicists, who become role models for students and teachers alike. View the video, which is part of the new National Science Teachers Association TV initiative. Video: Fermilab
In the News

Where gravitational waves are found: behind the scenes at BICEP2

From Popular Science, March 28, 2014

The astronomical instrument BICEP2 was deployed at the South Pole in 2009 to look for evidence that would support the theory of inflation, which tries to explain how the universe looked a trillionth of a second after the Big Bang.

Why the South Pole? Because there the sky is the cleanest and the clearest — no man-made light or radio pollution and minimal water vapour in the atmosphere. These would absorb the signals that the instrument was developed to record. The regions of sky targeted for observation are known to be particularly clear of contaminating microwave emissions from the galaxy and are always above the horizon.

On March 17, headlines around the world hailed the BICEP2 as having made biggest scientific discovery of the year. So what was it like to work on this historic project?

Read more

In the News

Art and science collide in the discovery of the Higgs boson

From Smithsonian, March 31, 2014

What do ancient cave paintings, Renaissance frescoes and Cubist sculptures have to do with the discovery of the Higgs boson?

Quite a lot, says theoretical physicist Savas Dimopoulos. "Why do we do science? Why do we do art? It is the things that are not directly necessary for survival that make us human."

Read more

Frontier Science Result: DZero

Top quarks as forward as expected

The Standard Model predicts that the forward-backward asymmetry of top-antitop events changes depending on the mass of the top-antitop system. For alternative models, this dependence can be quite different. A recent DZero paper on the lepton-plus-jets channel finds good agreement between the data (the points in the above plot) and various calculations of Standard Model predictions (the horizontal lines).

Disponible en español

Of the hundreds of publications released by the Tevatron experiments in recent years, few have provoked as much interest, from both theorists and experimentalists, as the measurements of the directional asymmetry in the production of top-antitop quark pairs. This parameter expresses the tendency for produced top quarks to follow the initial proton direction, and top antiquarks the antiproton direction, and is sensitive to many possible theories beyond the Standard Model. Previous measurements from both the DZero and CDF collaborations found asymmetries appreciably larger than were expected from Standard Model processes, giving a tantalizing hint that there may be some new physics at play and prompting a vast body of theoretical work predicting different models to explain this behavior.

Significantly, this forward-backward asymmetry parameter is unique to the Tevatron, because it is defined only for matter-antimatter collisions. A similar quantity can be measured at the LHC experiments, but the sensitivity is hampered by the different production mechanism of top-antitop quarks. As such, further improved measurements from the Tevatron are very important to help resolve the anomaly. This week the DZero collaboration released a much anticipated new publication on this subject, in which the full data set is analyzed for the first time using the lepton-plus-jets channel.

The precision of previous measurements was constrained by the limited size of the signal sample, and so a major focus of this latest analysis was to increase the signal efficiency. In the lepton-plus-jets channel, the top-antitop quarks decay to a single charged lepton (a muon or electron), four quark jets and an undetected neutrino. By accepting events in which one jet remains unreconstructed in the detector, the detection efficiency for signal events is almost doubled. The cost of this change is an increase in the background contamination; this is mitigated by analyzing the data in separate subsamples of differing signal purity.

Two experimental challenges merit a brief explanation here. First, it is nontrivial to determine the original direction of the top quark because the undetected neutrino carries away an unknown momentum. For events with a missing jet, there is an additional loss of information. To get around this, other properties of the event are used to statistically evaluate the most likely direction of the initial top (and antitop) quark. Using this method, the direction of the top quark is correctly assigned for more than 75 percent of cases.

A second challenge lies in the fact that the major background process (W boson plus jets) itself has a large directional asymmetry, which needs to be accurately determined and corrected for in the final measurement.

With all of the improvements, the new analysis had the sensitivity required to reach the milestone 5-sigma discovery criterion, if the asymmetry had remained at the value reported in previous Tevatron papers. However, the final measured forward-backward asymmetry is in fact consistent with the Standard Model prediction, with a value of 10.6 ± 3.0 percent.

The asymmetry is also determined in terms of the mass of the top-antitop system, which is especially useful for distinguishing between various new physics models. Again, the observed dependence versus mass is in good agreement with the expectations.

While much of the community might have been cheering for an observation of new physics in this measurement, it is important to remember that when we ask Nature the profound questions, we should listen to the answers. In this case, once again, the Standard Model has come out on top!

Mark Williams

These DZero members, all from the University of Rochester, made significant contributions to this publication, as well as a similar paper exploring the lepton asymmetry of top-antitop quarks.
The University of DZero has been active since 2010 and, in this time, has provided almost 50 lectures targeted to the interests of the particle physics community. Topics range from technical tutorials on experimental and theoretical aspects of particle physics to presentations of careers outside of academia, often given by former DZero collaborators. The University Deans are Jenny Holzbauer (University of Mississippi) and Hang Yin (Fermilab), who arrange and coordinate talks and, perhaps most importantly, ensure that pizza is available for attendees. For more information, please visit the DZero Web page.

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English country dancing with live music at Kuhn Barn - April 6

LabVIEW seminars scheduled on April 10

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Edward Tufte artist reception - April 16

MySQL relational database management course - April 22-23

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