Cosmic rays and how a Bristol Physicist won the Nobel Prize Dr Helen Heath

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100 years of Cosmic Rays

NASA TV

What are cosmic rays Primary Cosmic rays 85% protons 12% helium nuclei (alpha particles) 3% heavier nuclei 2% electrons

Particle Physics Primer

f Baryons Mesons

Particle Energy(MeV) v year Mass of lightest “new particles” Natural Radioactivty

Cecil Frank Powell Nobel Prize 1950 “for his development of the photographic method of studying nuclear processes and his discoveries regarding mesons made with this method.

The Start of Big Physics? “These developments in international collaboration formed an essential background to the setting up of large international laboratories such as CERN” D.H.Perkins 40 years of Particle Physics Conference Bristol July 1987

Sub-atomic particles Nucleus Protons Neutrons Electrons First antimatter particle – positron Photons

Yukawa Proposed a field theory of nuclear forces. Field theory requires field quanta. We can estimate the mass of such a quanta from the Uncertainty Principle  E  t ≈ h (h=6.63x Js)  t ≈ /c ≈ 0.3x s  E ≈ 2.2x J  E = mc 2  m=0.24 x kg=0.14m p

Mesons If Yukawa’s theory was correct there must be a field particle The hunt was on for a meson Today we use meson to mean a combination of a quark and an antiquark Originally it was a particle with a mass between the proton and electron Mesos – Intermediate

(Modern Aside) Mass is unexplained Proposed Higgs Mechanism Requires Higgs Field..and therefore the Higgs boson

Experimental techniques Cloud Chambers Bubble Chambers Emulsions Solid detectors

Cloud Chamber The discovery of the positron Carl D. Anderson – Physical Review 1933

Bubble Chamber ©CERN photo

Cecil Powell Cecil Powell started an autobiography which can be found at He succeeded academically winning a scholarship to Judd School and then moving on to Sidney Sussex Cambridge From an early age he was an enthusiastic, if not always, talented investigator.

After Cambridge Nearly became a teacher Research student at Cambridge Supervised by Wilson Recruited by Tyndall to Bristol in 1928

The emulsion technique Cloud chambers and bubble chambers need to be photographed Emulsions are continuously active Emulsions are also rather portable Important for early observatories & balloons

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Photographic Emulsion Grains of silver bromide suspended in gelatine. Light causes changes to the silver bromide. Developer changes the affected grains to silver Fixer removes the remaining silver bromide

Powell’s research group

The pion In 1936 C. Anderson and S Neddermeyer observed negatively charged particles with mass intermediate between that of the proton and the electron Initially called mesotrons Renamed the mu-meson in 1947 Now mesons are a subset of hadrons The mu meson is a lepton

Measuring energies Energy loss in the Emulsion is approximately continuous. The range of a particle depends on its energy

Double meson events Seen in events from the Jungfrau Joch The second meson has a range of 600  m

Interpretation In Double meson events one meson decays to another  -  -  Since the pion has stopped all the kinetic energy of the decay products comes from the change in mass Pion mass =139.6 MeV/c 2 Muon mass = MeV/c 2

After the Nobel Prize Powell continued to work with emulsions Left is the first example of a Kaon decay to three pions Work switched to balloons rather than mountains.

Modern Cosmic Ray Work LHC Energies

Open Question Origin of very energetic cosmic rays GZK –Cutoff (Griesen- Zatsepin- Kuzmin) Cronin, J. W., 1999, Cosmic Rays: the most energetic particles in the universe, RvMA 71, 165–172

Large arrays – Pierre Auger Pampa Amarilla plain Argentina 1604 detectors 3000 km²

Large arrays – Pierre Auger

Sparse Array – Large area HiSPARC project High School Project on Astrophysics Research with Cosmics >100 detector across The Netherlands

HiSPARC – school detector Students build their own detector And place it on top of the roof of the school