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In this issue:
SULI Mentors Needed
Parking Limited in Lot C Tomorrow
Science Today: Twin Higgs Models
The Slippery Substance of Water

SLAC Today

Thursday - March 16, 2006

(Photo - SULI Mentors)
Phil Marshall's SULI students Marissa Cevallos and Lowry Kirkby with Jonathan Dorfan at the 2005 SULI luncheon.

SULI Mentors Needed

The Summer Undergraduate Laboratory Internship (SULI) program is looking for SLAC mentors to share the excitement of their research with undergraduates interested in science and engineering careers. SULI is a DOE science education program that takes place at many of the national labs each summer.

"Students often come from situations where they don't normally get to do research, so they're keen," said KIPAC's Phil Marshall. "As a consequence, you do get useful results out of them." Marshall recently won a DOE mentor award for his work with three SULI students over the past year. One of the students published work in the DOE Journal of Undergraduate Research and another gave a poster presentation at a major conference.

"Working with a student on a fun exploratory project is a very nice way to spend the summer," said Marshall, adding that having a student tidy up a project that should have been finished long ago was also a good use of the student's time.  Read more...

(Daily Column - Science Today)

Twin Higgs Models

The high energy community is practically holding its breath in anticipation of the Large Hadron Collider (LHC). The LHC will probe the TeV scale and hopefully discover the mechanism responsible for electroweak (EW) symmetry breaking. In the standard model this would amount to discovering and measuring the properties of the Higgs boson.

In addition to discovering the Higgs, the LHC is expected to probe, and hopefully discover, other new physics, beyond the standard model, that lies at the TeV scale, such as supersymmetry, extra spatial dimensions, etc.

But why do we expect new physics at the TeV scale? And if such new physics is there, would the LHC discover it?

One of the prime motivations for new particles at a TeV is the fact that the EW scale is much smaller compared to the Plank scale or the Grand Unification (GUT) scale. This large hierarchy between scales is unstable if one only assumes the standard model.

The heart of the problem is that the SM top quark and gauge bosons contribute loop diagrams, making large contributions to the Higgs mass. These contributions are set by the high scale in the theory beyond which the standard model is not valid, e.g. the Planck or GUT scales. Barring a remarkably unnatural cancellation, this drives the weak scale to this high scale. Models that solve this problem invoke new particles that also contribute to the Higgs mass through loops. Due to the symmetry of the theory, the new physics contribution to the Higgs mass cancels the SM contribution leaving a naturally small EW scale. The most popular theory that solves this problem is suprsymmetry, where the new particles are the superpartners of the SM fields—the stop, the guaginos, etc.

Models with a natural EW scale will thus have new particles at the TeV scale. Because such new particles must be related to the top quark, all of the models that have been constructed thus far involve new particles charged under QCD. This was always good news for the LHC—the cross section for the production of new colored particles at the LHC is very high.

Last spring, collaborators Zackaria Chacko and Hock-Seng Goh from U. of Arizona and I proposed a new class of models to stabilize the weak scale. In these models, dubbed "the Twin Higgs," all of the new particles that are added beyond the standard model are not charged under the standard model. They are related to the standard model by a discrete mirror, or twin, symmetry that ensures the cancellation of quadratically divergent loops. If such a mechanism is realized in nature, it will be extremely challenging to expose this new physics at the LHC.

Parking Limited in Lot C Tomorrow

Beginning early tomorrow morning, half of parking lot C will be cordoned off for guests of the Kavli Building Dedication.  Security guards will be on hand to help SLAC staff and users find parking along the Loop Road.  The lot will fully reopen after 5:00 p.m. on Friday evening.

The Slippery Substance of Water

(Photo - Water)

Images by Diana Rogers

Liquid water is essential, beautiful, powerful, and completely strange.

The substance whose properties we take for granted actually acts like no other liquid on the planet. It expands when it freezes, holds heat exceptionally well (allowing Europe to have a moderate climate instead of a Siberian one), and has a high surface tension (allowing plants to pull water up from their roots). Fish and other lake life survive the winter because water's maximum density is several degrees above the freezing point; cold water sinks to the bottom, but water close to freezing resides at the top, ultimately forming a protective layer of ice. Water can put out fires, even though its individual components—hydrogen and oxygen—incite flames. And liquid water is an ideal solvent for the biological molecules and systems that help us circulate oxygen in our bodies, reproduce, and exist.

"Many of these unique properties are the basis of our existence," says Anders Nilsson, a chemical physicist at SLAC's Stanford Synchrotron Radiation Laboratory (SSRL).

Everyone knows the chemical formula for water, but as strange as it sounds, scientists have only recently begun to discover what liquid H2O really looks like and how it really behaves. Using new methods at synchrotron radiation laboratories, researchers have come to some startling, and sometimes controversial, conclusions.

"The impact of understanding the structure of water correctly is important to every part of biology and many parts of chemistry. If we don't even understand pure, simple liquid water, how are you going to understand how life works," says Uwe Bergmann, an SSRL scientist who collaborates with Nilsson.

Scientists have found creatures that don't need oxygen, but so far liquid water appears indispensable to life. Even exobiologists searching for life on other planets look for signs of liquid water.

Liquid water is a slippery substance to pin down in the lab.
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