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In this issue:
From the Theory Group: Anticipating the First Steps Beyond the Standard Model
SLAC Colloquium Monday: First Bio-Imaging Results from LCLS

SLAC Today

Thursday - June 3, 2010

From the Theory Group: Anticipating the First Steps Beyond the Standard Model

(Image - the Large Hadron Collider)
 (Image: Sandbox Studios.)

The particle physics community has just entered a critical new era with the commissioning of the Large Hadron Collider. The LHC has broken through the energy frontier and the first results are beginning to come out. Ultimately, the LHC will let scientists probe the laws of physics in new ways and may potentially reveal mysterious new phenomena, indicating that our current understanding of nature is incomplete. Within the next year, the LHC could discover the first signatures of new particles, new symmetries or even extra dimensions of space-time. The data collected could solve outstanding puzzles in physics or reveal new mysteries.

Physicist's knowledge of elementary particles is encapsulated in the Standard Model of particle physics, which currently describes almost everything we've seen. Yet there is compelling evidence that the Standard Model cannot be the complete description of nature. For example, despite all of its successes, the Standard Model describes only 20 percent of the mass of the Universe. Eighty percent of the mass is known as "dark matter," which we have never directly observed and know next to nothing about.

Another mystery of the Standard Model is that it typically predicts that the masses of all particles are incredibly heavy. To get the observed particle masses, the parameters of the theory have to be carefully balanced—precise to one part in 1032—in order to add up many large contributions and get a small answer. The probability that this occurs randomly is like the chances of winning the California lottery four times in a row! We've seen such absurd tunings before in theories, and they almost always indicate that some physical mechanism is causing the delicate balance of parameters. The LHC is designed to discover the cause of this fine tuning of the Standard Model parameters. Of course, we don't know what mechanism solves this fine tuning problem, but numerous theories have been developed over the past 30 years.  Read more...

SLAC Colloquium Monday:
First Bio-Imaging Results from LCLS

Monday, June 7 at 4:15 p.m. researcher Henry Chapman from the DESY physics lab in Hamburg, Germany, will discuss the first bio-imaging results from the Linac Coherent Light Source.

The ultrafast pulses from X-ray free-electron lasers are of high enough intensity and sufficiently short duration that individual single-shot diffraction patterns can be obtained from a sample before significant damage occurs. This "diffraction before destruction" method may enable the determination of structures of proteins that cannot be grown into large enough crystals or are too radiation sensitive for high-resolution crystallography.

In order to address the many challenges of attempting molecular diffraction, Chapman and colleagues have carried out experiments in coherent diffraction from membrane protein nanocrystals at the LCLS. The periodicity of these objects provides high-angle scattering signals in order to determine the effects of pulse duration and fluence on the high-resolution structure of single objects. The crystals are filtered to sizes less than 2 micron, and are delivered to the pulsed X-ray beam in a liquid jet. Snapshot diffraction patterns were recorded from individual crystals as small as 300 nm. It was possible to record millions of diffraction patterns, and the team is assembling these data into 3-D patterns for structure determination.

This new form of protein nanocrystallography may open a new avenue for high-throughput membrane protein crystallography. These experiments were carried out as part of a large international collaboration between CFEL, ASU, the PULSE Institute at SLAC, LCLS, University of Uppsala, Max Planck Institute for Medical Research and others, using the CAMP apparatus which was designed and built by the Max Planck Advanced Study Group at CFEL. 

The colloquium will take place in Panofsky Auditorium. The event is free and open to the public.

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