Posts Tagged ‘physics’

Custodial Symmetry for Z coupling

Sunday, July 16th, 2006

Kaustubh Agashe

Kaustubh Agashe

Agashe, Contino, Da Rold, and Pomarol recently realized that assigning top and bottom quarks to different representations of  SU(2)_L \times SU(2)_R can protect the coupling of the Z to the left-handed bottom quark from non-standard model corrections. Typically extra dimensional models of electroweak symmetry breaking with low mass scales for the Kaluza-Klein modes have corrections of the order of 20-40% while measurements from LEP and SLAC require less than a 0.25% deviation from the standard model prediction. The left-handed top and bottom quarks in their models are in the (2,2) representation rather than the (2,1) so there is a heavy top and an exotic quark to look for.

Negative S from AdS

Monday, June 12th, 2006

Veronica Sanz, photo by Johannes Hirn

Veronica Sanz, photo by Johannes Hirn

Hirn and Sanz have examined models of electroweak symmetry breaking in anti-de Sitter space via boundary conditions. They include additional bulk breaking of electroweak symmetry and find that they can change the spectrum of vector and axial vector resonances so as to make the S parameter negative and thus compatible with precision electroweak tests.


Linear Confinement in AdS

Monday, March 6th, 2006

the rho tragectory from Karch et. al.

the rho tragectory from Karch et. al.

A new paper by Karch, Katz, Son, and Stephanov attempts to modify the 5D AdS background that is used to model QCD in order to obtain the behaviour of a string state with large spin or excitation number.  They find that this can occur in a non-trivial dilaton background.


Runaway String “Vacua”

Wednesday, February 1st, 2006

Ken Intriligator

Ken Intriligator

A new paper by Intriligator and Seiberg points out that some recent string theory models of dynamical supersymmetry breaking do not actually break supersymmetry but have runaway “vacua”.  The vacua from flux compactifications and moduli stabilization that are under perturbative control all seem to break supersymmetry near the Planck scale, so it would be interesteing to know if there are any known string vacua that are rigorously stabilized and contain the MSSM with weak scale supersymmetry.

Warped Passages

Sunday, January 22nd, 2006

Lisa Randall

Lisa Randall

Lisa Randall gave a HEFTI public lecture, based on her recent popular book“Warped Passages” at UC Davis on Jan 10.  While students got in free, the general public paid $7, and the 400 seat ballroom sold out, with about 100 people turned away. Many people were surprised that so many people would pay to hear about cutting edge physics. It seems to be further evidence that the public thinks physics is actually cool, provided that someone will bother to try to explain it in an accessable way.

Chiral Phase Transition

Monday, December 12th, 2005
shapeimage

QCD phase boundary from Braun and Gies

A new paper by Jens Braun and Holger Gies gives a nice picture of the chiral phase transition that links the finite temperature phase transition with the chiral phase transition that appears as the number of quark flavors is varied at zero temperature.  The idea is that in the chiral symmetric phase there is an infrared fixed point for the gauge coupling.  As the temperature or number of flavors in lowered, the fixed point coupling moves to stronger coupling.  Eventually the fixed point coupling is large enough that four quark operators become relevant (i.e have scaling dimensions less than four) and produce chiral symmetry breaking and destroy the infrared fixed point.  It would be nice if lattice gauge theorists who do Monte Carlo simulations would try to verify this picture.


LHC Olympics

Thursday, December 1st, 2005

olympic rings

olympic rings

The Large Hadron Collider (LHC) is planned to start running in 2007.  In order to help theorists to prepare themselves, the LHC Olympics are  currently underway. This is a “competition” to analyze simulated data from different models of new physics.  The simulated collisions have been put through a Pretty Good Simulator (PGS) to simulate the results from a LHC detector such as ATLAS or CMS. (By the way, PGS was written by John Conway of UC Davis.)  The idea is to get particle theorists thinking about what real data from the LHC will look like.

New Axion Analysis

Monday, November 28th, 2005

photon-axion mixing

A cartoon of photons leaving a supernova, partially converting to axions, and the axions passing through the Earth.

There is a new analysis by Yong-Seon Song and Wayne Hu of photon-axion oscillations using data from Type IA supernovae, the Wilkinson Microwave Anisotropy Probe, and baryon oscillations in the density power spectrum seen by Sloan Digital Sky Survey.  They find that while photon-axion oscillations cannot explain supernova dimming in the absence of dark energy, photon-axion oscillations with dark energy are preferred by the data over a cosmological constant. For more background on photon-axion oscillations click here.