Friday, March 1, 2013

Have a safe day!

Friday, March 1

3:30 p.m.

4 p.m.
Joint Experimental-Theoretical Physics Seminar - One West
Speaker: Tingjun Yang, Fermilab
Title: New Photon Results from CDF

Monday, March 4

2:30 p.m.
Particle Astrophysics Seminar - One West
Speaker: Meng Su, Harvard University
Title: Recent Evidence for Gamma-ray Line Emission from Fermi-LAT: WIMP or Artifact?

3:30 p.m.

4 p.m.
All Experimenters' Meeting - Curia II
Topics: Proton Improvement Plan; Portfolio Review Summary; Experiment Operational Readiness Review Summary

Click here for NALCAL,
a weekly calendar with links to additional information.

Ongoing and upcoming conferences at Fermilab


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Secon Level 3

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Wilson Hall Cafe

Friday, March 1

- 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 1

Wednesday, March 6
- Grilled lemongrass beef and noodle salad
- Almond cake

Chez Leon Menu
Call x3524 to make your reservation.


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Physics in a Nutshell

Cross section

A beam of particles is like a shower of arrows— the probability of any one hitting the target depends on their cross-sectional area and the space between them.

Sometimes, everyday words are co-opted by scientists and used as technical terms. One of these is the word "berry." Talking to a botanist friend of mine, I learned that tomatoes are berries, but strawberries are not—the scientific meaning of a berry has more to do with the reproductive structures of the plant than the way it tastes. The term "cross section" is a berry of particle physics—its technical meaning is very different from the common usage.

In everyday speech, "cross section" refers to a slice of an object. A particle physicist might use the word this way, but more often it is used to mean the probability that two particles will collide and react a certain way. For instance, when CMS scientists measure the proton-proton to top-antitop cross section, they are counting how many top-antitop pairs were created when a given number of protons were fired at each other.

How did particle physicists come to use "cross section" in such a strange way? It's a long story. In the early days of particle physics, particles were thought to be tiny indestructible balls. When marbles or billiard balls are rolled at each other, the probability that they will collide is proportional to the size of the balls, unless they are precisely aimed. Subatomic particles are so small that aiming individual particles at each other is out of the question—the best anyone can do is to shoot a lot of them in the same general area. The collision probability for a cloud of projectiles is simply the ratio of area covered by them to the total area of the cloud. When Xerxes darkened the sky with arrows at the Battle of Thermopylae, the probability of getting hit by an arrow was very high.

Early collision experiments were intended to measure the size of particles from their collision rate. Rutherford's experiment, which collided alpha particles and gold nuclei in 1911, revealed that nuclei are much smaller than previously supposed. But soon, disparities arose: Neutrons are more likely to collide with certain nuclei when they are moving slowly than when they are fast. It is as though the neutrons change the area of their cross-section mid-flight. Particles like neutrons are actually quantum clouds that pass through each other or interact with an energy-dependent probability—the likelihood of collision has little to do with a solid, cross-sectional area. Even though hard spheres is the wrong mental image, the term cross section stuck, and it's common for a physicist to say, "this cross section depends on energy" when it would be nonsensical to imagine the size of the particle actually changing.

But why use "cross section" when alternatives like "probability" and "reaction rate" exist? Cross section is independent of the intensity and focus of the particle beams, so cross section numbers measured at one accelerator can be directly compared with numbers measured at another, regardless of how powerful the accelerators are. Arrows are arrows, no matter how many of them are fired into the sky.

Jim Pivarski

Want a phrase defined? Have a question? E-mail

Photo of the Day

Snow E Coyote

Fermilab user Salavador Carillo of Universidad Iberoamericana took this photo of a coyote ankle-deep in snow near the corner of Batavia and Eola roads.
In Brief seeks nominations for women in STEM to profile

DOE is accepting nominations for women in STEM to be featured on this month. Nominations should be submitted by March 8. Image courtesy of AnneMarie Ashburn, DOE Office of Economic Impact and Diversity

In honor of Women's History Month, the Energy Department's Office of Economic Impact and Diversity will profile 50 Women in STEM (science, technology, engineering and mathematics) in an online feature to be published next month on Nominations should be submitted by March 8 and can be for any woman who works in STEM throughout the DOE: at a national lab, field site or even headquarters.

The profiles will feature a photo of each woman, a paragraph biography and answers to the following questions:

  1. What inspired you to work in STEM?
  2. What excites you about your work at the Energy Department?
  3. How can our country engage more women, girls and other underrepresented groups in STEM?
  4. Do you have tips you'd recommend for someone looking to enter your field of work?
  5. When you have free time, what are your hobbies?

Please e-mail your nominations to AnneMarie Ashburn. The DOE's Office of Economic Impact and Diversity will contact nominees, conduct 20-minute phone interviews and request a jpeg photo from those being considered for the profiles. The office plans to publish the collection of profiles by March 24 and will encourage cross-publication by national labs throughout the DOE, as well as the Office of Personnel Management and the White House Council of Women and Girls.

In the News

Astronomers measure how fast a supermassive black hole spins

From Forbes, Feb. 27, 2013

Astronomers using observations from NASA's NuStar space telescope and the ESA's XMM-Newton have finally solved an astronomical mystery. By doing so, they've finally been able to find a definitive measurement for how fast a supermassive black hole spins.

Read more
Frontier Science Result:
Theoretical Astrophysics

Gamma-ray bubbles and dark matter

Scientists have seen evidence of gamma-ray bubbles extending tens of thousands of light-years north and south of the Galactic Center. A new study finds that some of this gamma-ray emission may be the result of annihilating dark-matter particles. Image: NASA

Although it's been two and a half years since a group of Harvard astrophysicists discovered a pair of bright "bubbles" in data from the Fermi Gamma-Ray Space Telescope, the origin of these gamma-rays is still not well understood. Last summer, Harvard's Tracy Slater and I began to think about ways that we could test different scenarios for how the Fermi bubbles may have formed. We found a very different gamma-ray signal, with a possible connection to dark matter.

The Fermi bubbles extend tens of thousands of light years north and south of the Galactic Center—the center of the Milky Way—and are likely the consequence of a very active period in the recent history of the galaxy, maybe having to do with the rate of star formation in the inner galaxy or with an eruption from a supermassive black hole. Early in our investigation of the bubbles, we noticed that their spectrum varies a lot with galactic latitude. At high latitudes—far from the Galactic Plane—the spectrum looks much like we would expect and can be easily explained by cosmic-ray electrons interacting with radiation and the galactic magnetic field. Within ten thousand light-years or so of the plane, however, the spectrum looks very different, exhibiting a sharp and bright feature, peaking at a few GeV. No realistic spectrum of cosmic rays could account for this strange signal.

So if not from cosmic rays, where does this extra GeV emission come from? One possibility is that it comes from a large number of rapidly spinning pulsars—too faint to be detected individually, but perhaps collectively bright enough to make up this gamma-ray signal. This interpretation is a bit strained, however, by the degree of isotropy that is exhibited by the gamma-ray background. If enough bright pulsars were present far away from the Galactic Plane to make up the signal from the bubbles, then one would expect the gamma-ray background to be less isotropic than is observed. Those pulsars should lead to a clumpy gamma-ray background instead of the very smooth distribution that Fermi sees.

There is another, more exciting, interpretation. If the dark matter is made up of particles that can annihilate with each other, then we should expect those annihilations to produce a sharply peaked spectrum of gamma-rays, very much like the observed from the low-latitude regions of the Fermi bubbles. The angular distribution of the observed gamma-rays is also in good agreement with what we expect from dark matter. Furthermore, the gamma-rays from the inner few degrees around the Galactic Center exhibit the same bump and overall distribution, just as predicted from dark matter annihilations.

Are we finally seeing evidence of dark matter particles? According to the old adage, "If it looks like a duck, swims like a duck and quacks like a duck, then it is probably a duck." The gamma-ray signal we have discovered certainly looks, swims and quacks like dark matter, but more work remains to be done before we can rule out some very dark matter-like imitators.

Read our new paper.

—Dan Hooper

In Brief

Nominations sought for Employee Advisory Group

If you have insights or suggestions that could help improve Fermilab policies and programs, the Employee Advisory Group needs you.

Nominations are now being accepted for new members to serve on the EAG, which provides Fermilab's senior management with recommendations from an employee perspective. The committee meets approximately once a month and is regularly joined by several members of the senior management team. Fermilab benefits from the EAG's formal recommendations that come after the EAG studies a topic, as well as the extensive discussions of complex issues. The discussions are guided by a facilitator from outside the laboratory.

Nominations are encouraged from all job categories, all divisions, sections and centers, and from new and long-time employees. Members serve three-year terms, attend monthly meetings and are expected to communicate with their fellow employees about issues under consideration. EAG members spend approximately three to five hours per month (including monthly meetings) on committee-related work.

Nominations are due by March 20. Employees are welcome to nominate their colleagues or to self-nominate.

Nomination forms are available online or in the Office of Communication on the atrium level of Wilson Hall. More information about the EAG is available on the EAG website.


Today's New Announcements

Fermilab Management Practices courses now available for registration

URA Thesis Award competition applications accepted until today

International Folk Dance 25th Anniversary party - March 2

Welsh country dancing for St. David's Day - March 3

Deadline for UChicago Tuition Remission Program - March 7

Fermilab Chamber Series: Arianna String Quartet - March 10

Fermilab Arts Series: The Believers (documentary) - March 15

Fermilab Arts & Lecture Series: ScrapArtsMusic - March 23

DOEGrids certificates to be decommissioned - March 23

Nominations open for 2013 Tollestrup Award - through April 1

2013 FRA scholarship applications accepted until April 1

Interpersonal Communication Skills course offered in May

Scottish country dancing meets Tuesday evenings in Kuhn Barn

Indoor soccer

Employee discounts

Find new classified ads on Fermilab Today.