Observations and Experiments

Back to Standard Model

Table of Contents

  1. Detecting Particles
  2. Discovery of Electron, Proton, and Neutron
  3. Cosmic Rays
  4. Pierre Auger Observatory (pea-ERR OH-GCHAY)
  5. Particle Accelerators and Detectors
  6. Large Hadron Collider (LHC)
  7. LHC and its Particle Detectors: ALICE, ATLAS, LHCb, CMS
  8. Higgs Boson
  9. Predicted Probabilities of the Ways the Higgs Decays
  10. Two Photons Observed at the CMS Detector
    1. CMS
    2. Video
  11. Peter Higgs

Detecting Particles

  • Quantum Fields are detected only by their particles, which in turn are detected only by their tracks and traces, in some cases only by the tracks and traces of their debris.
  • Particles are detected by
    • Laboratory Experiments
    • Observation of Cosmic Rays
    • Particle Accelerators
  • Some particles were discovered by happenstance:
    • Electron, Proton, Neutron, Muon
  • Others were predicted:
    • Neutrino, Positron, Mesons, Photon, W and Z Bosons, Higgs Boson

Discovery of Electron, Proton, and Neutron

  • Electron 1897
    • In 1897 the English physicist J.J. Thomson identified cathode rays as streams of electrons. The electron was the first subatomic particle to be discovered.
  • Proton 1898
    • While studying streams of ionized gas, Wilhelm Wien, in 1898, identified a positive particle equal in mass to the hydrogen atom.
  • Neutron 1932
    • In 1932 James Chadwick observed that beryllium, when exposed to bombardment by alpha particles, released an unknown radiation that in turn ejected protons from the nuclei of various substances. 

Cosmic Rays

  • Cosmic Rays are streams of high-energy particles entering Earth’s atmosphere from outer space, traveling near the speed of light. Most originate within the Milky Way, including the Sun.  The rest come from distant galaxies.
  • The particles are
    • Electrons
    • Protons
    • Nuclei of atoms from helium (2 protons) to lead (82 protons)
  • Cosmic ray particles break up into “secondaries” when they strike atmospheric nuclei.
  • Their tracks are observed by detectors on the ground, high-altitude balloons, satellites, rockets, and under the ground.
  • By analyzing cosmic-ray particle interactions, Carl David Anderson discovered the positron in 1932 and the muon in 1936. Cecil Frank Powell discovered the pion in 1947.

Pierre Auger Observatory (pea-ERR OH-GCHAY)

The Pierre Auger Observatory, located on a vast, high plain in western Argentina, is the world’s largest cosmic ray observatory.  The Observatory, operating since 2008, consists of 1660 water Cherenkov particle detector stations spread over 3000 km2, overlooked by 24 air fluorescence telescopes.

Each dot corresponds to one of the 1660 surface detector stations.
 A surface detector station
  • The Auger Observatory uses two independent methods to detect cosmic rays.
    • Charged particles passing through water at speeds greater than light produce Cherenkov radiation, detected by the 1,660 water surface detector tanks.
    • The  fluorescence detectors detect the ultraviolet light emitted by charged particles interacting with atmospheric nitrogen, 

Particle Accelerators and Detectors

A Particle Accelerator (Collider) is a machine that uses electromagnetic fields to accelerate charged particles (electrons, protons, ions) to almost the speed of light, smashing them together to create a shower of particles in Particle Detectors. The particles’ tracks are analyzed for evidence of new particles.

Large Hadron Collider (LHC)

  • The Large Hadron Collider (LHC) at CERN is the most powerful particle accelerator in the world, consisting of a circular tunnel 17 miles in circumference. (CERN developed WWW)
  • The heart of the LHC is a ring, a few centimeters in diameter, running through the tunnel.
  • The ring is evacuated to a higher degree than deep space and is cooled to within two degrees of absolute zero.
  • Inside the ring, two beams of billions of protons or heavy ions, circulating in opposite directions, are accelerated to speeds within one-millionth of a percent of the speed of light. Collisions occur a billion times per second.
  • The collisions are detected at one of four particle detectors: ALICE, ATLAS, LHCb, and CMS

LHC and its Particle Detectors: ALICE, ATLAS, LHCb, CMS

Higgs Boson

  • Peter Higgs postulated the Higgs Field and Higgs Boson in 1964 to account for the masses of the other elementary particles.
  • But Higgs Boson’s predicted lifespan was so short (10-22 seconds) that it leaves no tracks in particle detectors. It would have to be detected indirectly, by its decay products.
  • The Higgs was predicted to decay in a variety of exotic ways. 
  • Luckily, it was also predicted to decay into two photons 0.2% of the time. Not only can photons be directly observed, but their energy and momentum can be measured very precisely, giving an accurate reconstruction of the mass of the decaying particle.

Predicted Probabilities of the Ways the Higgs Decays

  • The Standard Model predicts that 0.2% of the time the Higgs Boson decays into two photons, which are easily detected.
  • The decay is by way of a loop of virtual top quarks

Two Photons Observed at the CMS Detector


The Compact Muon Solenoid (CMS) is a general-purpose detector at the Large Hadron Collider (LHC).


After being accelerated in the LHC, two protons collide at the CMS detector, resulting in two high energy photons (dashed orange lines and towers)


Peter Higgs