Physics 5110 Spring 2009 1
The Physics of Ultrahigh Energy The Physics of Ultrahigh Energy Cosmic RaysCosmic Rays
Example Poster PresentationPhysics 5110Spring 2009
Reminder: Posters are due Wed April 29 in class.
Physics 5110 Spring 2009 2
Introduction to Cosmic RaysIntroduction to Cosmic Rays• Cosmic Rays are
subatomic particles and radiation of extra-terrestrial 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
References:
http://www.phy.bris.ac.uk/groups/particle/PUS/Affordable_Cosmic_Ray_Detector.htm
http://www.fnal.gov/pub/inquiring/timeline/04.html
Physics 5110 Spring 2009 3
The Cosmic Ray Energy SpectrumThe Cosmic Ray Energy Spectrum• Cosmic Rays have been
observed with energies at up to ~1020 eV:(1 eV = energy drop of one electron through a 1 V battery/cell… e.g. a 40 W light bulb uses about 1024 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) http://lpnhe-auger.in2p3.fr/slides/spectrum.gif
Physics 5110 Spring 2009 4
Mystery of UltraMystery of Ultra--High Energy Cosmic Rays 1High Energy Cosmic Rays 1Cosmic Rays with energy > 1018 eV are referred to
as “Ultra-High Energy (UHE) Cosmic Rays”
1. What are they?We think they are mostly protons.In the 1012 - 1015 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
Physics 5110 Spring 2009 5
Mystery of UltraMystery of Ultra--High Energy Cosmic Rays 2High 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)
Physics 5110 Spring 2009 6
Mystery of UltraMystery of Ultra--High Energy Cosmic Rays 3High 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:
(a) difficult to explain energies >1018 eV.
(b) UHE cosmic rays do not point back to known supernovae
http://heasarc.gsfc.nasa.gov/docs/asca/gallery/sn1006.html
Physics 5110 Spring 2009 7
ExtraExtra--Galactic UHECR Sources?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)
Physics 5110 Spring 2009 8
Detection of UltraDetection of Ultra--High Energy Cosmic RaysHigh Energy Cosmic Rays• At < 1014 eV: flux is large
enough to allow DIRECTmeasurement on balloons, satellites, shuttle missions.
• At > 1017 eV, we expect a flux < 10-10/m2 Sr s:
A 1 m2, 2π Sr. detectorsees < 1 event/50 yrs.
Direct measurement is impractical!!!
Reference: Introduction To Ultrahigh Energy Cosmic Ray Physics by Pierre Sokolsky
Physics 5110 Spring 2009 9
One Possible SolutionOne Possible Solution: : Extensive Air Showers (EAS)Extensive Air Showers (EAS)Use the Earth’s atmosphere as part of our detector system!!!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 turnc) 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 1997.
Physics 5110 Spring 2009 10
The Fluorescence TechniqueThe 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
Reference: Introduction To Ultrahigh Energy Cosmic Ray Physics by Pierre Sokolsky
• 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
Physics 5110 Spring 2009 11
19801980--1993: The Fly’s 1993: The Fly’s Eye ExperimentEye Experiment
• 101 mirrors @ 2 sites separated by 3.4 km, 12-14 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 = 3.2x1020 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. 441 144 (1995)
Physics 5110 Spring 2009 12
Violation of the GreisenViolation of the Greisen--ZatsepinZatsepin--Kuzmin (GZK) Kuzmin (GZK) CutCut--off @ off @ 6x106x101919 eV.eV.
Problem: We should not see cosmic rays above 6x1019 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 ~100-150 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 DeepensPossibly new Physics at work!
Refrences: (1) Greisen, K., Phys. Rev. Lett., 1966. 16: p. 748. (2) Zatsepin, G.T. and V.A. K'uzmin, Pis'ma Zh. Eksp. Teor. Fiz., 1966. 4: p. 114; [JETP Lett. 4, 78 (1966)].
