Cosmic Rays M. Swartz 1
History Cosmic rays were discovered in 1912 by Victor Hess: he discovered that a charged electroscope discharged more rapidly as he flew higher in a balloon hypothesized they were electromagnetic (wrong) won 1936 Nobel Prize They actually consist of charged primaries interacting with the atmosphere to produce secondary particles 2
What are the Primaries? 88% are protons (ionized Hydrogen atoms) 10% are alpha particles (ionized Helium atoms) 1% ionized nuclei of heavier atoms - common heavier elements (C,O,Mg,Si,Fe) are found in same relative abundances as in solar system - rare lighter elements (Li,Be,B) are overabundant: probably produced from collisions of heavy primaries with interstellar gas 1% electrons (unknown why e- are not accelerated like p s are) 3
Where do they come from? Galactic Cosmic Rays - confined by Galactic magnetic field to radius of galaxy: E<10 18 ev [LHC energy = 3.5x10 12 ev] - very old: ~10x10 6 years [last dinosaurs 65x10 6 years] - particles probably accelerated by shock waves from exploding supernovae (1/50 years) B expanding plasma magnetic shock wave particle 4
Where do they come from? Solar Cosmic Rays - accelerated by solar flares and coronal shock waves - typical energies: (1-10)x10 6 ev, max E~10 9 ev Extra-Galactic Cosmic rays - very rare, E>10 18 ev - acceleration mechanism unknown - still don t point to sources unless they are nearby (see GZK Cutoff). 5
Primary Energy Spectrum abundance falls as E -a with increasing energy (a~1.6) largest observed energies in excess of 10 20 ev! - single proton with energy larger than the kinetic energy of a baseball traveling at 100 MPH! 6
Secondary Cosmic Rays p mass: 938 MeV p 6 mass: 140 MeV carrier of intranucleon force, decays: p 6 Lm 6 n m 6 mass: 106 MeV heavy electron decays: m 6 Le 6 nn p8 mass: 135 MeV decays: p8lgg 7
Cosmic Microwaves In 1965, Penzias and Wilson discovered that the universe was filled with a uniform background of microwaves: cooled remnant of big bang black body spectrum corresponding to 2.7 deg K Penzias and Wilson share 1978 Nobel Prize Spatial fluctuations are now used to study the cosmology of the universe (see dark energy and dark matter) 8
In 1966, Greisen and Zatsepin, Kuzmin predicted that very high energy cosmic rays could collide with the remnant microwaves to produce D + particles (excited p s), p cosmic ray! microwave p #$ p # +! the neutral pion decays: p8lgg the charged pion decays: p 6 Lm 6 n the neutron decays: nlpe - n always produces a final state proton with significantly less energy than the initial energy should produce high energy neutrinos! What is the minimum energy of the primary proton for this to happen? " + p 9! " + n
the microwave photons have very little energy, the square of invariant mass of the photon and the proton must be equal to the square of the delta mass m 2 D=(E p + E g ) 2 (p p E g ) 2 =Ep 2 + Eg 2 + 2E p E g p 2 p Eg 2 + 2p p E g m 2 p + 4E p E g solving for E p E g 2.82k B T = 6.6 10 4 ev E p = m2 D m2 p 4E g = 2.40 10 20 ev This suggests that very high energy cosmic rays with E p >2x10 20 ev cannot propagate very far (GZK cutoff). 10
the correct calculation makes use of the whole microwave power spectrum and the total (D + dominated) cross section for gp scattering 11
to yield a calculation of energy loss length as a function of primary cosmic ray energy: primary protons with energies E p <2x10 20 ev can propagate more than 1000 Mpc primary protons with energies E p >2x10 20 ev can propagate less than 15 Mpc 12
1 Mpc is a typical distance to a nearby galaxy 50 Mpc is the size of our local supercluster of galaxies no known nearby sources of E p >2x10 20 ev cosmic rays Distance Scales 13
several experiments do see cosmic rays with energies E p >2x10 20 ev: they should come from cosmological distances (much more than 50 Mpc) even if that were true, there still isn t a known acceleration mechanism cannot be ionized nuclei - nuclei would break-up from interactions with microwave photons at much lower energies than 10 20 ev 14
Cosmic Ray Experiments Very high energy cosmic ray primaries (E p >10 19 ev) produce huge showers in the atmosphere that spread out over several kilometers by the time that they reach the ground 10 11 particles at sea level - mostly e s and g s the ionized atmospheric N 2 fluoresces (emits blue light) can be detected by either charged secondaries or fluorescence 15
Surface detector arrays measure: energy from total number of hit detectors zenith angle from time of arrival of signals surface detectors L! D Dt = L/c = D cosq/c Could do this with Cosmic Ray e-lab if there is a dense enough grid of school detectors? D 16
Florescence detectors image the glowing air molecules left by the passing shower: image is recorded on the focal plane of a PMT camera stereo reconstruction of the image allows accurate measurement of the primary cosmic ray direction 17
Pierre Auger Observatory The world s largest search for the very highest energy cosmic rays is now underway in Argentina (plans for 2nd site in Utah). Consists: Surface detectors for charged secondaries - tanks of water viewed by PMTs fluorescence detectors to make redundant measurements 18
Auger Southern Obs 1600 surface dets 4 stations: 6 fluor dets 3000 km2 area Surface Detector Station Fluorescence Detecto fluorescence detector surface detector 19
Western Argentina near the Andes: low population density 20
surface detectors are big tanks of water viewed by PMTs cerenkov light detectors solar powered connected via cell phone tech. Pierre Auger cartoon 21
surface detectors are big tanks of water viewed by PMTs cerenkov light detectors solar powered connected via cell phone tech. Pierre Auger cartoon 21
Example Surface Array Event Θ~ 48º, ~ 70 EeV (7x10 19 ev) Lateral density distribution Flash ADC traces Flash ADC traces 22
Example Hybrid Event Θ~ 30º, ~ 8 EeV 23
A Tri-ocular Event! ~20EeV 24
A Tri-ocular Event! ~20EeV 24
A Tri-ocular Event! ~20EeV 24
A Tri-ocular Event! ~20EeV 24
A Tri-ocular Event! ~20EeV 24
A Tri-ocular Event! ~20EeV 24
A Tri-ocular Event! ~20EeV 24
Ice Cube A search for very high energy neutrinos at the south pole has been constructed recently: neutrinos aren t deflected by magnetic fields should point to astrophysical sources 1x1x1 km array of PMTs embedded in the antarctic ice ice is relatively clear and transmits Cerenkov light 25
Holes are drilled into the surface layer (firn) and then with jets of hot water (expensive at the South Pole). Photosensitive detectors are lowered into the holes. 26
The phototube depths vary between 1450m and 2450m: deep ice has good optical properties 4800 digital optical modules 27
Interacting neutrinos produce energetic leptons: n m NLm6 X n e NLe6 X n t NLt6 X tlmn m n t [18%] tlen e n t [17%] tlh 6 (ng)n t [48%] The charged leptons then produce rings of Cerenkov light that can be detected by the PMTs. It is possible to distinguish between these cases. 28
The experiment can see several toplogies: downward-going muons or muon bundles (top) background from atmospheric CRs electrons from n e interactions (middle) taus from n t interactions and their decays make double bang topologies (bottom) upward-going muons from n m interactions So far: only limits have been published 29