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In this issue:
Symmetry Explains It in 60 Seconds: Particle Event
Science Today: Lensless Nanoscale Imaging
SLAC at the LHC: The ATLAS Trigger

SLAC Today

Thursday - September 4, 2008

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Symmetry Explains It in 60 Seconds: Particle Event

A particle event is a particle collision or interaction that is observed by some type of particle detector. Collected by the hundreds, thousands, or millions, particle events are the raw material that scientists use to explore the subatomic world.

To capture these precious events, particle physicists build "cameras" that record signals such as the tracks of particles emerging from a collision. The interesting features of particle events often occur on submillimeter scales, and the cameras act as extremely powerful microscopes.

The cameras have a wide variety of shapes and sizes. At particle laboratories around the world, detectors as large as houses take snapshots of the bursts of matter and energy that emerge when particles ram into each other. In other places, large arrays of detectors—sometimes covering thousands of square kilometers—record cosmic-ray showers created when protons originating from outer space smash into air molecules high above the ground.

Each experiment archives its particle events and assigns a unique number to each event. Physicists refer to these event numbers when discussing unusual events that might hint at particles or phenomena never seen before. See the original article in symmetry magazine.

(Daily Column - Science Today)

Lensless Nanoscale Imaging

Specialized diffraction image setup. (Photo courtesy of SSRL Headlines.)

A team of researchers working at the Stanford Synchrotron Research Laboratory Beamline 13-3 have devised an imaging technique that combines methods from traditional X-ray crystallography and X-ray holography, circumventing one of the major technical hurdles associated with capturing detailed images of non-crystal, or "nonperiodic" materials. The results were published in the August 15 edition of Physical Review Letters.

X-ray diffraction has been widely used to determine the structure of large molecules like proteins, but samples must first be grown into a crystal form, with molecules arranged in a periodic pattern. The regular ordering of the molecules in the sample makes it much easier to recover information about the phase of diffracted X-rays, which then enables researchers to recreate the structure of the molecules within the sample.

The desire to image all kinds of natural and artificial nanostructures or materials that exhibit nanoscale ordering has led to the development of X-ray imaging techniques that do not rely on any form of sample periodicity. One such method is real space X-ray microscopy, using special X-ray lenses. Other "lensless" methods use coherent X-ray scattering, wherein the reciprocal space "speckle" pattern needs to be inverted into real space to determine structure. This step is impeded by a well-known "phase problem." The SSRL researchers found a new way to solve the phase problem by capturing two scattering patterns of a sample—in this case, microscopic polystyrene beads on a specially prepared thin film—using coherent X-rays of two different energies. A "resonant" beam was used to obtain a diffraction pattern that highlighted the carbon atoms in the sample, while a "non-resonant" beam captured a baseline diffraction pattern as a reference. The researchers then combined the two images to recover the phase information and recreate a two-dimensional image of the sample. The ability to capture detailed images of non-periodic structures holds great promise for imaging all kinds of nanostructures, which in most cases are not periodic.

For more details, see the full scientific highlight.

SLAC at the LHC:
the ATLAS Trigger

SLAC postdoctoral researcher Ignacio Aracena in the ATLAS control room. (Click for larger image.)

When the Large Hadron Collider proton beams smash head-on inside the ATLAS detector, the great majority of the results will involve physics we already know. And that's a good thing. The LHC proton beams will cross about 40 million times every second, and recording each collision requires so many gigabytes of data that researchers can save only about 200 collisions per second. To whittle down this huge amount of data without losing the good bits, ATLAS uses what's called a trigger system. Its job is to quickly determine which collisions show new, unexplored physics and which do not. SLAC researchers are working on several aspects of the trigger, including a system that ensures its quick decisions are based on the best information possible, and another that helps the trigger's many computers work independently.

"The trigger code must deliver the optimal answer and do it very fast," said Su Dong, who co-leads the SLAC ATLAS team with SLAC physicist Charlie Young. "We're working to come up with various ways to help make this possible."

Part of the eventually 72-rack system hosting the ATLAS high level trigger computing nodes. (Click for larger image.)

In order to ignore as many "boring" events as possible while not accidentally discarding the interesting ones, the trigger system processes the data in several steps. It first makes a very quick and cursory comparison between each detected event and expected signatures of new, interesting events. Using custom-built fast electronics, this "Level 1" step takes about 3 microseconds to reject events that clearly don't match any of the signatures. This reduces the 40 million collisions recorded each second to about 100,000 potentially interesting collisions per second. Then, with fewer collisions to consider, the Level 2 trigger can spend a little more time—about 40 milliseconds—analyzing each event. Using a farm of up to 500 computers, it conducts a second round of analysis, searching for subtle characteristics that may indicate new or particularly interesting physics. Finally, a farm of up to 1600 computers carefully analyzes the most promising events and permanently records approximately 200 of them per second for later study.   Read more...

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