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Particle Accelerators and Detectors


Contents

Cosmic Rays
Accelerators
Detectors
An Example - The LHC (Large Hadron Collider)
Wakefield Acceleration in Plasma
Future Particle Colliders

Cosmic Rays

Cosmic rays are energetic particles come from outer space traveling near the speed of light. Before the development of particle accelerators, cosmic rays provided physicists with their only sources of high-energy particles to study. Although low-energy cosmic rays are emitted from the Sun, the origin of the highest energy cosmic rays is one of the outstanding puzzles in astrophysics. These particles (mostly protons, but including heavier atomic nuclei and gamma rays) have energies of up to 108 TeV. The sources of high-energy cosmic rays could be supernovae, blackholes, or defects in space-time (Figure 01a). The UHECRs (ultra-high-energy cosmic rays) events are rare, but they should not be here at all; because within a few hundred million years at most, they should be slowed down by successive collisions with the omnipresent photons of the cosmic microwave background. They could be produced nearby, e.g., they could be originated within some 150 million lightyears of the Milky Way such as from the active galaxy M87,
Cosmic Rays Cosmic Ray Spectrum which is about 60 million lightyears away. However, this kind of source seems to be unlikely since UHECRs are isotropic, and not affected by magnetic fields. The yellow line in Figure 01b is the observed cosmic ray energy spectrum. The three other lines show how suspected sources possibly contribute to the overall signature of cosmic rays. It is not unknown what causes the kinks at the "knee" and "ankle". Normally, cosmic rays bombard the Earth at a rate of about 1 particle/km2-sec. At ultra-high energies,

Figure 01a Cosmic Rays, Origins of [view large image]

Figure 01b Spectrum [view large image]

above 1019 ev, the rate falls to less than 1 particle/km2-year.


UHECR New results from the Pierre Auger Observatory in 2007 indicate that UHECRs have sky directions statistically consistent with the positions of nearby active galactic nuclei (AGN). These galactic centers are known to emit great amounts of light and are likely powered by large black holes. It also concludes that the UHECRs are protons. Figure 02b is an artist's view of a cosmic ray striking the Earth's atmosphere and creating a shower of secondary particles detectable on the surface. The small inserts are the images of the AGN Centaurus A and the detectors of the Pierre Auger Observatory.

Figure 02b UHECR
[view large image]

High Energy Electrons A report in November 2008 reveals that there is an excess of cosmic ray electrons at energies of 300 - 800 Gev. The finding is consistent with other measurements as indicated in Figure 02c, where the new measurement is in red filled circles. It is compared with previous observations in various symbols. The solid curve is calculated with a power-law spectral index of E-3.2 (the new measurement is scaled with an index of -3.0). The dashed curve is the solar modulated electron spectrum. The source of the bump can be ascribed either from nearby astrophysical objects (such as pulsar or micro-quasar) or from the annihilation of dark matter particles (such as a KK particle with a mass of about 620 Gev).

Figure 02c High Energy Electrons [view large image]

Another possibility is via the annihilation of the supersymmetric neutralinos, which is its own anti-particle (Figure 02d) although it doesn't quite fit the observational data.

Neutralino Dark Matter Experiments Figure 02e shows experiments over the years trying to obtain evidence for dark matter. It reveals a disparate results not fitting to any particular candidate. A theory introducing a new force between the WIMPs has been proposed to resolve the contradiction between theory and observation. It invokes a heavy force carrying boson that weighs about the same as

Figure 02d Neutralino
[view large image]

Figure 02e Dark Matter Experiments

a proton. It claims to reconcile the many facets of the observations:

  1. The new dark force boosts the annihilation rate of slow-moving WIMPs giving a surplus of electrons and positrons. Yet it has no effect on much faster particles that filled the early universe leaving plenty of WIMPs around today.
  2. The absence of surplus anti-protons is explained by the heavy mass of the mediating boson, which can decay into electrons and positrons only.
  3. The 511 Kev photons observed by INTEGRAL can be identified to the annihilation of the electron-positron pair.
  4. The scenario favours the DAMA detectors, which use material with heavier nuclei (e.g., iodine). While much lighter germanium and silicon is inside CDMS, which detects no signal of any WIMP.
NASA's Fermi satellite, launched in 2008, could confirm with a high degree of accuracy the excess of electrons over a wide range of energy. If WIMPs really are 600 times as massive as protons, there should be an abrupt drop in the number of electrons above a certain energy threshold. It should also be able to see gamma rays that are produced when WIMPs annihilate. The energies of these rays should differentiate the KK particles from neutralinos. Fermi will even be able to pinpoints the gamma ray sources in the sky. If it detects a big nearby clump, then neutralinos will be back in favours as the claims against them assume that dark matter is evenly spread throughout the galactic halo. Of course, the LHC will render a big helping hand if such force carrier is detected there.

An April 2009 report by the Fermi team indicates that there is no bump in the 300 - 800 Gev range of the cosmic electrons. In July 2009, the same Fermi team reports detection of a spike in cosmic gamma ray at 100 Gev.

Supernova Shockwave Two 2013 reports suggest strongly that the UHECRs were originated from the shock wave associated with supernova explosion. Gamma-ray data on 2 supernova remnants, called IC443 and W44, from NASA's Fermi space telescope found abundance of gamma-ray photons in the range of energies expected from pion decay. The pion is produced by collision of high energy proton with matter. In the other study, the spectrum from supernova remnant 1006 reveals the presence of high energy protons in the shock wave front.

Figure 02f Supernova Shockwave

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