Monday, 7 January 2013

seminar in kathmandu


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Saturday, 21 July 2012

Stars draw atoms closer together

Magnetism may be the secret to a strong marriage between atoms in the atmospheres of stars. Computer simulations show that a previously unknown type of powerful chemical bond should be induced by the stars’ ferocious magnetic fields. If the effect can be harnessed in the lab, ‘magnetized matter’ could be exploited for quantum computing.
Chemists identify two classes of strong molecular bonds: ionic bonds, in which electrons from one atom hop over to another, and covalent bonds, in which electrons are shared between atoms. But Trygve Helgaker, a quantum chemist at the University of Oslo, and his colleagues accidentally discovered a third bonding mechanism when they simulated how atoms should behave under magnetic fields of about 105 tesla — 10,000 times the biggest fields that can be generated on Earth. Their results are published in Science today1.
White-dwarf stars have huge magnetic fields that could force molecular bonds into powerful new modes.
NASA/ESA/H. Bond (STScI)/M. Barstow (Univ. Leicester)
The team first examined how the lowest energy state, or ground state, of a two-atom hydrogen molecule was distorted by the magnetic field. The dumb-bell-shaped molecule oriented itself parallel to the direction of the field and the bond became shorter and more stable, says Helgaker. When one of the electrons was boosted to an energy level that would normally break the bond, the molecule simply flipped so that it was perpendicular to the field and stayed together.
“We always teach students that when an electron is excited like this, the molecule falls apart,” says Helgaker. “But here we see a new type of bond keeps the atoms hanging together.” The team also reports that a similar effect should occur between helium atoms, which normally don't bond at all.
The atoms are held together by the way their electrons dance around the magnetic-field lines, explains Helgaker. “The way electrons move relative to the field, and their kinetic energy, can become as important for chemical bonding as the electrostatic attraction between the electrons and the nuclei,” he says. Depending on their geometry, molecules will turn to allow electrons to rotate around the direction of the magnetic field.

Star field

If the new states remain bound at very high temperatures, they could well exist in the atmospheres of some white dwarfs and neutron stars, where the magnetic fields are similar to those simulated by the team. But it will be difficult to spot them, says Dong Lai, an astrophysicist at Cornell University in Ithaca, New York. The team will need to extend its model to see whether the unusual bonding states would modify the spectra of light coming from the stars in a way that can be detected, he says. The simulation of the states “is an important step, but several more are needed to see how relevant this is in astrophysics”.
Closer to home, it is virtually impossible to generate such high magnetic fields, because they are accompanied by drastic changes in the chemistry of everything affected by them. The bond length between atoms can shrink by around 25% under such high fields, says Helgaker. “The experimental apparatus would cease to be an apparatus in these extreme conditions!”
Nevertheless, the findings boost hopes that ‘magnetized matter’ in the lab could have properties that may be exploited.
In 2009, physicists created a weakly bound state called a Rydberg molecule2, which some people have suggested could be used to carry information in a quantum computer. Rydberg molecules are highly sensitive to magnetic effects, says Chris Greene, an atomic physicist at the University of Colorado Boulder, who was one of the first people to posit the molecules' existence3. “That means we could use magnetic fields as a knob to tightly control the strength of the binding, to manipulate them to store and erase quantum memory as needed.”
For full story : http://www.nature.com/news/stars-draw-atoms-closer-together-1.11045
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Monday, 16 July 2012

Particle Physics and Cosmology in Auckland

As I mentioned in my last post, I’m now in Auckland. Richard Easther, a repatriated Kiwi who came here from Yale last year to head up the physics department, has organized a workshop on “The LHC, Particle Physics and the Cosmos“, at which I gave a talk this morning.
This is a very different affair to ICHEP. In Melbourne there were 800 or so participants, filling a gigantic conference hall for the plenary talks, whereas there are something like 30 44 participants at this workshop, roughly split between New Zealand academics (faculty, postdocs and students), and those of us from abroad. ICHEP was a terrific conference, but more usually I strongly prefer these small, intimate workshops to huge meetings. They tend to be more focused and I typically seem to leave having learned more from the talks.
This meeting kicked off with a public lecture on Thursday evening, at which Mark Kruse from Duke University gave a skilled account of “Why do we care about the Large Hadron Collider”.
There were something like 400 people at his talk, and the thing that struck me was the quality of the questions that people asked at the end. There was even a question that was essentially about triggers, and the risk that one might miss important physics due to them. As you’ll have seen discussed before, the sheer volume of data produced by each collision at the LHC, combined with the frequency of these collisions means that it is just impossible to save each individual event. Instead, a decision has to be made extremely rapidly whether to save a given event, understanding that doing this means that many other events will then be missed. This decision is based on the expectations we have of the kind of signals that we expect the new physics to exhibit. Of course, a consequence is that there exist possible signals of new physics that will evade these triggers. This is a subtle question and one that I’m surprised to hear asked in a public lecture.
Yesterday the research talks began. The topics have spanned quite a number of topics, including talks from people on ATLAS and CMS on their Higgs, and other results. There have been talks on dark matter, neutrinos, variance in the Hubble flow in cosmology, and a number of other topics, including one on the Phenomenological Minimal Supersymmetric Standard Model from Tom Rizzo from SLAC. I particularly enjoyed a talk from Pat Scott, who is a postdoc at McGill, about cosmology with ultracompact minihalos of dark matter. These potentially provide a way to probe the extent to which the statistics of structure formation deviates from that expected from gaussian primordial seeds. As such, it seems that it may provide another way to look at non-gaussianity beyond that we usually think of in the microwave background, and about which we hope to see interesting results from the Planck mission.
This morning Tom Appelquist (Yale) and Jay Wacker (SLAC/Stanford) gave interesting theory talks, and our own JoAnne spoke about the physics that may be probed through a program of physics at the intensity frontier. This afternoon Michele Redi from CERN gave an interesting talk on the implications of a light Higgs for composite models. It is one thing to find the object that breaks the electroweak symmetry, but another to pin down whether it is a fundamental or composite particle. Compositeness is attractive in some ways, since it may provide a way to tackle the hierarchy problem, but finding the Higgs at the rather light mass announced last week presents particular challenges to models in which the Higgs is composite, and leads to some specific predictions. Michele is interested in models in which the Higgs is a pseudo-Goldstone boson and showed that in many such models, naturalness, coupled with a 125 GeV Higgs implies that there should also be new fermions in the model that are quite light, and may be within the reach of the LHC.
Well I’m off to have tea and then chair a parallel session in which there will be a lot of theory talks, about which I may report soon.
source http://blogs.discovermagazine.com/cosmicvariance/2012/07/13/particle-physics-and-cosmology-in-auckland/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+CosmicVarianceBlog+%28Cosmic+Variance%29
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