The Physics of Ultrahigh Energy Cosmic Rays. Example Poster Presentation Physics 5110 Spring 2009 Reminder: Posters are due Wed April 29 in class.

Transcription

1 The Physics of Ultrahigh Energy Cosmic Rays Example Poster Presentation Physics 5110 Spring 2009 Reminder: Posters are due Wed April 29 in class. 1

2 Introduction to Cosmic Rays References: rdable_cosmic_ray_detector.htm Cosmic Rays are subatomic particles and radiation of extraterrestrial origin. First discovered in 1912 by German scientist Victor Hess, measuring radiation levels aboard a balloon at up to 17,500 feet (without oxygen!) Hess found increased radiation levels at higher altitudes: named them Cosmic Radiation 2

3 The Cosmic Ray Energy Spectrum Cosmic Rays have been observed with energies at up to ~10 20 ev: (1 ev = energy drop of one electron through a 1 V battery/cell e.g. a 40 W light bulb uses about ev in one hour) The flux (rate of particles per unit area per solid angle) follows a power law ~E -3 (very rapidly falling) Reference: Introduction To Ultrahigh Energy Cosmic Ray Physics by Pierre Sokolsky (Dean, COS), Westview Press (1988) 3

4 Mystery of Ultra-High Energy Cosmic Rays 1 Cosmic Rays with energy > ev are referred to as Ultra-High Energy (UHE) Cosmic Rays 1. What are they? We think they are mostly protons. In the ev range: ~50% protons, ~25% alpha particles ~13% C/N/O nuclei, <1% electrons, <0.1% gamma rays. Reference: Introduction To Ultrahigh Energy Cosmic Ray Physics by Pierre Sokolsky 4

5 Mystery of Ultra-High Energy Cosmic Rays 2 2. How are they accelerated? We re not sure Some plausible theories based on ideas of Enrico Fermi: Reference: High Energy Astrophysics (Vol 1 and 2) by Malcolm.S. Longair, Cambridge University Press (1992 and 1998) 5

6 Mystery of Ultra-High Energy Cosmic Rays 3 3. Where do they come from? Suspected sources are large, energetic structures where strong shocks are found e.g. supernovae. problems: ery/sn1006.html (a) difficult to explain energies >10 18 ev. (b) UHE cosmic rays do not point back to known supernovae 6

7 Extra-Galactic UHECR Sources? Other Possible Sources of UHE Cosmic Rays: Very Energetic objects in the nearby Universe outside of our galaxy: Active galaxies (with jets extending to ~100 Ly) Colliding Galaxies? Decay/annihilation of some unknown super-heavy particles or cosmological relics from the creation of the Universe. Reference: High Energy Astrophysics (Vol 1 and 2) by Malcolm.S. Longair, Cambridge University Press (1992 and 1998) 7

8 Detection of Ultra-High Energy Cosmic Rays At < ev: flux is large enough to allow DIRECT measurement on balloons, satellites, shuttle missions. At > ev, we expect a flux < /m 2 Sr s: A 1 m 2, 2π Sr. detector sees < 1 event/50 yrs. Direct measurement is impractical!!! Reference: Introduction To Ultrahigh Energy Cosmic Ray Physics by Pierre Sokolsky 8

9 One Possible Solution: Extensive Air Showers (EAS) Use the Earth s atmosphere as part of our detector system!!! Pierre Auger discovered air showers a) The primary cosmic ray particle collides with air nucleus leading to b) a cascade of secondary particles, which in turn c) make more collisions producing a shower of a billion or more particles. Reference: Cosmic Rays at the Energy Frontier by James W. Cronin, Thomas K. Gaisser and Simon P. Swordy, Scientific American, Jan

10 The Fluorescence Technique The particle shower leaves a faint glow in its trail: like a 100 W, ultra-violet light- bulb moving at the speed of light. This flash of light lasts only a few millionths of a second. Original Illustration by Tamara Young, Dept. of Physics, University of Utah This faint glow can be seen by extremely fast, sensitive electronic cameras on clear, moonless nights. This technique was successfully used by University of Utah in 1976 Reference: Introduction To Ultrahigh Energy Cosmic Ray Physics by Pierre Sokolsky 10

11 : 1993: The Fly s Eye Experiment sites separated by 3.4 km, pixels (PMTs) per mirror Each pixel/pmt covers a 5 degree cone in the sky. 12 years of operation: The highest energy particle ever measured was seen in 1991 by the original Fly s Eye Experiment at University of Utah: E = 3.2x10 20 ev (~50 joules or ~12 calories) Reference: Detection of a Cosmic Ray with Measured Energy Well Beyond the Expected Spectral Cutoff Due to Cosmic Microwave Radiation, D. Bird et al. (Fly s Eye) Astrophys. J (1995) 11

12 Violation of the Greisen-Zatsepin Zatsepin-Kuzmin (GZK) 6x10 19 ev. Problem: We should not see cosmic rays above 6x10 19 ev. At above this energy, it becomes possible to interact with photons in the Cosmic Microwave Background (CMB). Cosmic rays above this GZK cut-off can travel at most ~ million light-years before losing energy in such interactions. 150 million light-years takes us only to the boundaries of our Local Super-cluster of galaxies (the visible horizon is ~20 billion light-years away). BUT: the arrival direction does not point to any known object: Mystery Deepens Possibly new Physics at work! Refrences: (1) Greisen, K., Phys. Rev. Lett., : p (2) Zatsepin, G.T. and V.A. K'uzmin, Pis'ma Zh. Eksp. Teor. Fiz., : p. 114; [JETP Lett. 4, 78 (1966)]. 12