Particle Physics Lesson 6 The Particle Zoo

What are the charges? γ (photon) p (proton) n (neutron) ν (neutrino) e- (electron) e+ (positron) μ- (muon) μ+ (anti-muon)

Learning Objectives Define the electron volt. Describe how scientists look for new particles. State some quantities which are conserved during decays. Determine whether a decay occurs based on the conservation rules.

The Electron Volt Defined as:- An electron volt is the kinetic energy gained by an electron when it is accelerated through a potential difference of 1V. Energy = Charge × Potential Difference = 1.60×10−19 C × 1 V = 1.60×10−19 J = 1 eV Note that 1 V = 1 JC −1

Worked Example The ionisation energy of a hydrogen atom is 13.6 eV, what is this energy in Joules? EI = 13.6 eV 1 eV = 1.6 × 10-19 J EI = 13.6 × 1.6 × 10-19 = 2.18 × 10-18 J (to 3 s.f.)

Worked Example 2 A photon has an energy of 5.43 × 10-15 J. What is its energy in eV? E = 5.43 × 10-15 J 1 eV = 1.6 × 10-19 J E = 5.43 × 10-15 J/ 1.6 × 10-19 J = 33.9 keV ( to 3 s.f.)

Specification Hadrons: baryons (proton, neutron) and antibaryons (antiproton and antineutron) and mesons (pion, kaon). Hadrons are subject to the strong nuclear force. The proton being the only stable baryon into which other baryons eventually decay; in particular, the decay of the neutron should be known. Leptons: electron, muon, neutrino (electron and muon types). Leptons being subject to the weak interaction. Baryon numbers for the hadrons.  Lepton numbers for the leptons will be given in the data booklet.

Cosmic Rays

Cosmic Rays High energy protons or small nuclei that travel through space from the stars. They are moving at very high speeds so gain more mass. When they enter the atmosphere they collide with gas atoms. This results in the creation of new particles and antiparticles with very short lifetimes and photons. A type of natural particle accelerator.

Cosmic Rays In the 1930’s – 1940’s Physicists discovered some exotic partciles by studying cosmic rays with weather balloons.

Reminder - Discovery of the Positron Cosmic ray experiment. Charged particles leave an ion trail as they pass through a super-cooled gas. Causes vapour to condense to leave vapour trails – like jet plane. Magnetic field causes charged particles to bend. The particle bent the wrong way in a magnetic field.

The Problem with Cosmic Rays Physicists wanted to study Cosmic rays further. However cosmic rays posed further problems:- The detectors needed to explore cosmic rays further were getting too big to fit on weather balloons. You never know where to look to see really high energy collisions. It is purely based on chance. To overcome these problems Physicists invented the Particle Accelerator – a way to make cosmic rays on Earth.

Video In Search of Giants (4 of 15) The Existence of Quarks In Search of Giants (13 of 15) Particle Accelerators and the Higgs Particle

Accelerators A TV tube accelerates electrons through a potential difference of about 5,000 V. Stanford linear accelerator in California accelerates electrons through a p.d. of 50 GV. The Large Hadron Collider is designed to accelerate charged particles to energies of over 7000 GV.

Also discovered… The muon (μ-) – a negatively charge particle with a rest mass over 200 times that of the electron The pion (pi meson) – can be positively (π+) or negatively charged (π-) or have no charge (π0), rest mass greater than muon but less than proton. The kaon (K meson) – which can also be positive (K+), negative (K-) or neutral (K0), rest mass greater than pion but less than proton.

New Particle Summary Name Symbol Mass (MeV/c2) Lifetime (s) Antiparticle Muon μ- 106 2.2 × 10-6 μ+ Pion π0 135 0.8 × 10-16 Itself π+ 140 2.6 × 10-8 π- Kaon K+ 494 1.2 × 10-8 K- K0 498 8.9 × 10-11

β− decay Decay of neutrons, which are slightly more energetic than a proton.  Isolated protons are stable; isolated neutrons last about 10 minutes.

Decay Modes K mesons decay into pi mesons, muons and antineutrinos, and antimuons and neutrinos. Charged pi mesons decay into muons and antineutrinos, or antimuons and neutrinos. Chargeless pion decays into high energy photons.

Some other decay modes… Muons and antimuons decay into electrons and antineutrinos, or positrons and neutrinos. Note:- The decays always obey the conservation rules for energy, momentum and charge.

Something strange going on… Pions and kaons are both produced through the strong interaction. However, kaons last much longer than Pions and decay through the weak interaction. This led to kaons being referred to as strange particles  we’ll talk about this later…

The Conservation Rules Translates as “what goes in must equal what comes out.” Energy Momentum Charge  we can use this to decide if a decay is allowed. NOT Mass

Worked Example 1 Is the following decay allowed? Q:  The charge on both sides adds up to 0. So this decay is allowed.

Worked Example 2 Is the following decay allowed? Q:  The charge on left hand side adds up to 0 but adds up to -1 on the right hand side. So this decay is not allowed.

Summary Scientists look for new particles using a particle accelerator. Energy, momentum and charge are conserved in allowed decays. Conservation of charge can be used to determine whether or not a decay occurs.