Particle Astrophysics

Particle physicists have put enormous efforts into the study of the strong, electromagnetic, and weak interactions. From these studies, the Standard Model of the three forces has emerged, incorporating the ideas of gauge theory and unification. Unification of the electromagnetic and weak forces into the electroweak force, at a mass scale of 100 GeV seems on solid ground at this time. Grand Unified Theories, which attempt to unify the electroweak and the strong interaction at much higher energy scales, have not had as much predictive power. Limits on the decay lifetime of the proton are in conflict with the simplest of these theories. The fourth force, gravity, is not currently integrated into the Standard Model. In particular, there is not yet a standard quantum theory of gravity.

Large amounts of experimental data collected by particle physicists starting in the late 1940s has been at the heart of our increasing understanding of the Standard Model. It thus seems unlikely that gravity can be incorporated into the Standard Model without more experimental information than is currently available. Probing gravity in sectors where little experimental information exists, such as relativistic strong field gravity and the related gravitational waves, is an important avenue of fundamental particle physics research, difficult though the experiments may be.

SLAC is addressing these issues with its particle astrophysics studies program, one aspect of which is x-ray and gamma-ray astrophysics. Work in this area involves analyzing archival x-ray and gamma-ray data, and collaborating on the Unconventional Stellar Aspect (USA) satellite experiment. For the longer term, there is an effort on a Gamma-ray Large Area Space Telescope (GLAST) that is planned as a follow-on space mission to the Energetic Gamma-ray Experiment Telescope (EGRET) currently operating on the Compton Gamma-ray Observatory (CGRO). In addition to x-ray and gamma-ray particle astrophysics, faculty are also interested in measuring gravitational waves and are exploring various ideas to make the first direct measurement. Data are now available at SLAC from NASA’s Rossi X-ray Timing Explorer (RXTE) satellite. These measurements are focused on the timing and energy of x-rays emitted by neutron stars and black hole binary systems in our galaxy and nearby galaxies, and will be used to study the strong gravitational fields of these compact stellar objects. The particle astrophysics program also has access to gamma-ray archival data from EGRET, and the Burst and Transient Source Experiment (BATSE), which is currently operating on CGRO as well.

The U. S. Air Force will launch the USA experiment into low-earth orbit on the ARGOS (P91-1) satellite in 1998 or 1999. Data from USA will yield unprecedented high-resolution timing information, correlated x-ray energy measurements, and about a factor of 10 or more longer x-ray exposures than previously obtained from about 40 selected astronomical sources. SLAC is a major collaborator on USA, and has built part of the hardware for the experiment; we have helped design and program some of the flight software, and are participating in launch preparations and science planning.

SLAC physicists are also using the RXTE x-ray data mentioned previously to help develop the data-analysis software for USA, as well as getting physics results from this data. The GLAST Program began at Stanford in early 1992, and has progressed considerably since that time. SLAC physicists are currently working in the context of a growing international GLAST collaboration of astrophysicists/astronomers and particle physicists. The GLAST mission is in the strategic plan for NASA proposed for a new mission start in 2002, and the GLAST instrument design and proposed physics program have also been given high marks in DOE reviews.
 

The GLAST research and development to date demonstrates the feasibility of a new design for a high-energy gamma-ray telescope (Gamma-ray energies from 20 MeV to 1 TeV). The instrument development is proceeding toward the goal of a major advance in high-energy astrophysics instrumentation. GLAST will have a much improved angular and energy resolution and active area, leading to about two orders of magnitude higher sensitivity over a much broader energy range than achieved by EGRET. To achieve these goals, GLAST will use modern particle physics technology. This includes silicon strip detectors for tracking (using silicon strip technology closely related to that currently being developed for the superconducting
Large Hadron Collider detectors at CERN), CsI detectors with PIN diode readout for calorimetry, a solid-state data acquisition system, and a high-energy collider-experiment type triggering system employing powerful on-board computing.

There are many opportunities for graduate students interested in pursuing a thesis in particle astrophysics at SLAC. Work would usually involve a mix of instrument development and construction, with analysis of x-ray and gamma-ray data. Thesis topics would typically be related to particle physics issues, rather than purely astronomical issues.
 
 

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