Office: 303H Osmond Laboratory: (814) 865-2015
Lab: 14, 212 Osmond Laboratory: (814) 865-2013
Mailing address: Department of physics, 104 Davey Laboratory PMB241, The Pennsylvania State University, University Park, PA 16802
Fax: (814) 863-3297

Teaching

Physics 457 Laboratory, Spring 1998, Fall 1998, Fall 2001, Fall 2002.
Physics 237 Introduction to Quantum Physics, Spring 1999, Fall 1999.
Physics 211 General Physics: Mechanics, Spring 2000, Fall 2000, Fall 2003.
Physics 559 Graduate Laboratory, Spring 2001, Spring 2002, Spring 2003.
Physics 590 Current Research (Graduate Seminar), Spring 2001, Spring 2002, Spring 2003.

Particle Astrophysics

I am interested in experimental high-energy particle astrophysics, which is the study of the universe at the point where the mind-boggingly vast meets the infinitesimally tiny. I have been studying high-energy cosmic rays, particles that rain down on Earth from the depths of space. Among these are particles of antimatter, which can be used to search for candidates for the elusive dark matter that appears to pervade the universe but is detectable, at present, only through its gravitational influence. Or else they can point to new and interesting mechanisms for particle production and acceleration within the Galaxy. At the very highest energies, an entirely different and essentially unexplored regime opens up, with particles most likely originating from outside our own Galaxy, and carrying more energy than ever achieved artificially in the laboratory by particle accelerators.

I am involved in the NASA-supported High-Energy Antimatter Telescope (HEAT) program, a series of high-altitude balloon-borne experiments to study antimatter in the primary cosmic radiation. We have studied high-energy positrons and antiprotons using two different instruments. We use large and complex detectors with redundant particle identification techniques to unambiguously select out the particles of interest from a large background of ordinary cosmic-ray protons and nuclei. We fly such instruments to the very edges of the atmosphere, essentially into space, by means of enormous helium-filled balloons.

I also participate in the Monopole, Astrophysics, and Cosmic Ray Observatory (MACRO) project, an experiment buried deep under a mountain in Italy. The experiment has shut down at the end of 2000. MACRO was a very large instrument, 10 meters-high by 12 meters-wide by 72 meters-long, using multiple particle tracking and identification techniques to study those particles capable of traveling through more than 1000 meters of rock. Among the many physics topics covered by this multi-purpose detector is the search for magnetic monopoles, hypothetical particles that would carry magnetic (instead of electric) charge, and that are predicted by some theories but have never been observed experimentally. When cosmic rays strike the Earth's atmosphere after traveling through the Galaxy for many millions of years, they generate a cascade of atmospheric secondary particles, known as an air shower. MACRO studies the very penetrating muon components of air showers, and this information in turn can be used to infer the mass of the primary cosmic ray that initiated the air shower. By looking at muons traveling upwards, MACRO detected neutrinos that have interacted in the rock underneath the detector. Such neutrinos could arise from dark matter particle annihilation at the center of the Earth or the Sun. Finally, MACRO searched for bursts of neutrino events arising from supernova explosions in our Galaxy.

I am also involved with the Pierre Auger Observatory project. This is an ambitious plan to construct two huge arrays of detectors, one in the northern hemisphere, and one in the southern. These arrays will study the highest-energy particles in the Universe, and open up a new window on the physical world. Although these particles are very rare (and therefore require very large arrays of sensitive detectors), they are known to exist from previous measurements done by smaller detectors. At present it is not known how such tremendous energies can ever be attained, and the Auger observatory will provide information crucial to the resolution of the puzzle of their existence. The southern detector site is currently under construction in Western Argentina.

I am involved in the Cosmic Rays Energetics And Mass (CREAM) project. This is a NASA-sponsored effort to build an advanced balloon payload to measure the mass composition of high-energy cosmic rays up to the astrophysical "knee" (a spectral feature at an energy of a few times 10^15 eV). Direct measurements at such high energies have been difficult because of very small particle fluxes. To remedy this, CREAM will achieve long exposures by flying on ultra-long duration balloons, or possibly by multiple long flights in Antarctica.

I am a participant in NASA-sponsored studies in progress for a next generation of space-borne high-energy cosmic-ray detectors (the ACCESS project), which would be operated on the International Space Station. I am also interested in developing payloads for the NASA program of long-duration balloon flights currently under development, which will increase the total exposure possible for the study of rare particle phenomena.

Our Penn State Particle Astrophysics group homepage with more technical information and links is here.

Some Recent Research Articles

Click here for a more complete list of refereed papers, some retrievable in postscript format.

Click here for a long list of unrefereed papers (mostly conference proceedings).

M. Ambrosio et al. (MACRO Collaboration). 2001. Neutrino Astronomy with the MACRO Detector. Astrophys. J. 546:1038.

S. Coutu et al. (HEAT Collaboration). 2000. Energy Spectra, Altitude Profiles and Charge Ratios of Atmospheric Muons. Phys. Rev. D 62:032001.

M. Ambrosio et al. (MACRO Collaboration). 2000. Low Energy Atmospheric Muon Neutrinos in MACRO. Phys. Lett. B 478:5.

M. Ambrosio et al. (MACRO Collaboration). 2000. Nuclearite Search with the MACRO Detector at Gran Sasso. Eur. Phys. J. C 13:453.

M. Ambrosio et al. (MACRO Collaboration). 2000. A Search for Lightly Ionizing Particles with the MACRO Detector. Phys. Rev. D 62:052003.

S. Coutu et al. (HEAT Collaboration). 1999. Cosmic-Ray Positrons: Are There Primary Sources? Astropart. Phys. 11:429.

M. Ambrosio et al. (MACRO Collaboration). 1999. Measurement of the Energy Spectrum of Underground Muons at Gran Sasso with a Transition Radiation Detector. Astropart. Phys. 10:11.

M. Ambrosio et al. (MACRO Collaboration). 1999. Limits on Dark Matter WIMPs Using Upward Going Muons in the MACRO Detector. Phys. Rev. D. 60:082002.

M. Ambrosio et al. (MACRO Collaboration). 1999. Observation of the Shadowing of Cosmic Rays by the Moon Using a Deep Underground Detector. Phys. Rev. D 59:012003.

M. Ambrosio et al. (MACRO Collaboration). 1999. Measurement of the Atmospheric Neutrino-Induced Upgoing Muon Flux Using MACRO. Phys. Lett. B 434:451.

M. Ambrosio et al. (MACRO Collaboration). 1998. The Observation of Upgoing Charged Particles Produced by High Energy Muons in Underground Detectors. Astropart. Phys. 9:105.

M. Ambrosio et al. (MACRO Collaboration). 1998. Real Time Supernova Neutrino Burst Detection with MACRO. Astropart. Phys. 8:123.

S. Barwick et al. (HEAT Collaboration). 1998. The Energy Spectra and Relative Abundances of Electrons and Positrons in the Galactic Cosmic Radiation. Ap. J. 498:779.

S. Barwick et al. (HEAT Collaboration). 1997. Cosmic Ray Reentrant Electron Albedo: High-Energy Antimatter Telescope Balloon Measurements from Fort Sumner, New Mexico. J. Geophys. Res. 103:4817.

S. Barwick et al. (HEAT Collaboration). 1997. The High-Energy Antimatter Telescope (HEAT): an Instrument for the Study of Cosmic-Ray Positrons. Nucl. Inst. and Meth. 400:34.

S. Barwick et al. (HEAT Collaboration). 1997. Measurements of the Cosmic-Ray Positron Fraction from 1 to 50 GeV. Ap. J. 482:L191.

M. Ambrosio et al. (MACRO Collaboration). 1997. The Performance of MACRO Liquid Scintillator in the Search for Magnetic Monopoles with 0.001 < beta < 1. Astropart. Phys. 6:113.

M. Ambrosio et al. (MACRO Collaboration). 1995. Performance of the MACRO Streamer Tube System in the Search for Magnetic Monopoles. Astropart. Phys. 4:33.

S. Barwick et al. (HEAT Collaboration). 1995. Cosmic Ray Positrons at High Energies: a New Measurement. Phys. Rev. Lett. 75:390.

S.P. Ahlen et al. (MACRO Collaboration). 1995. Atmospheric Neutrino Flux Measurement Using Upgoing Muons. Phys. Lett. B. 357:481.

S.P. Ahlen et al. (MACRO Collaboration). 1995. Vertical Muon Intensity Measured with MACRO at the Gran Sasso Laboratory. Phys. Rev. D. 52:3793.

Click here for a PowerPoint Colloquium on Cosmic Antimatter.