Introduction to Experimental Particle Physics

Introduction to Experimental Particle Physics Heavily indebted to 1. Steve Lloyd – Queen Mary’s College, London – 2004 2. Robert S. Orr – University of Toronto – 2007 3. Z. Vilakazi – University of Cape Town -2006 For use of ideas/material from their lectures Among the many books I’ve consulted the following were the ones I found most instructive •Introduction to High Energy Physics – D.H. Perkins •Particle Physics – B.R.Martin & G.Shaw •The Fundamental Particles and Their Interactions – William B. Rollnick Gian Gopal

Particle Physics – A short History

Introduction to Experimental Particle Physics Total of 12 Lectures over the next 4 weeks to Provide an understanding of the field of Particle Physics including • Its history • Techniques deployed – mainly experimental but with the necessary mathematics & theory • Experimental phenomena, discoveries and their significance Gian Gopal


Introduction to Experimental Particle Physics Outline of the Course: 1. The History & Fundamentals 2. Special Relativity & Units 3. Accelerators 4. Detectors 5. Attributes & QPM 6. Forces – Electromagnetic 7. Weak Force 8. CP Violation 9. Quantum Chromodynamics 10.ElectroWeak Unification 11.Neutrino Physics 12.Summary of SM & Brief Look Beyond Gian Gopal


The Fundamentals Particle Physics studies the deepest elementary constituents of matter and the forces between them. Making use of the two most basic theoretical constructs of Physics • Special Relativity • Quantum Mechanics to Probe the Structure of Matter Large objects with our Eyes

Receiving Light reflected/emitted by the objects – limited by the wavelength of the visible range of the e.m. spectrum Go deeper – see smaller structures within matter - Need probes (Waves) of much smaller wavelength (de Broglie’s Particle-Wave Duality) de Broglie’s relationship betweenMomentum (p) & Wavelength (λ): λ = h/p So for p = 1 Gev/c , λ = 1.2 fm with h= 197MeV fm Gian Gopal


Different Levels of Matter as we know it:

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The History: Up to 1930 known particles: Electron, photon and neutrino (postulated to explain the missing energy in β-decay) , proton and neutron (inside Nucleus) The 1st Anti-particle – the positron – was discovered by Carl Anderson B

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X


The same year (1932) James Chadwick observes a free Neutron In 1933 Fermi sets out his Theory of Beta Decay – n → p e- ¯υe 1935 Yukawa proposes his Meson Hypothesis – Nuclear force due to exchange of particles with mass (Mesons). Anderson & Seth Nedermeyer discover the Muon (assumed to be Yukawa’s proposed Meson – but) was far too penetrating in matter to be the required exchange particle between nucleons AND it decayed to an e± at rest rather than being absorbed by the nucleus as a meson would be. 1946 – Cecil Powell discovers the Charged Pion observing the decay

π+ → µ+ υµ followed by µ+ → e+ υµ υe

Neutral Pion seen in 1950 decaying to 2 Photons. Gian Gopal


1947 – Rochester & Butler see ‘Strange’ long-lived in the Cloud Chamber K+ → µ+ υµ

Neutral K0

A New quantum Number ‘Strangeness’ Chamberlain & Segre observe the antiproton in 1955 –

p p → 8 Pions in the final state – NO NUCLEON

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1960/1970s saw observation of a whole zoo of long and short lived sub-atomic particles

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A Real Mess that needed theoretical input to sort it out

Gellman and Salam independently proposed an underlying structure – the 8-fold way based on quaintly named quarks –u, d, s - to explain the zoo of strongly interacting particles – theory that led to the prediction of a new particle

The Ω- made up of 3 strange quarks Gian Gopal


D.H. Perkins Gian Gopal


Theoretical Highlights 1950 sees advent of the Quantum Theory of Electromagnetism -QED – describing the interaction of charged particles via photon exchange. The Architects:

Richard Feynman Gian Gopal

Julian Schwinger

Sin-itiro Tomonaga Particle Physics – A short History

Gellman’s Quarks were followed in the 1970s with the Theory of Quantum Chromodynamics – postulates a) New quantum numbers – 3 Colours – red, blue & green – quarks carry colour b) Strong Interactions occur via exchange of gluons Progress in understanding the Weak Interaction as a manifestation of QED at high Energies - ElectroWeak Interaction – exchange particles W+, Wand Z0 as the Carriers of the Weak Force and the γ the carrier of the electromagnetic Force

Sidney Glashow Gian Gopal

Abdus Salam

Steven Weinberg Particle Physics – A short History

Back to Experimental Discoveries A new fourth Quark Charm (c) postulated by Glashow, Iliapolous & Maiani in (the GIM mechanism) in 1970

Discovered simultaneously at SLAC (Richter)& Brookhaven (Sam Ting) in 1974

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SLAC e+ e- → Ψ hadrons

Brookhaven p + Be → J (→ e+ + e- )+ X

dimuons

e+ e-

M=3096MeV, Γ=91 KeV – a narrow cc bound state Gian Gopal

D.H. Perkins


A new third charged Lepton – Tau – was discovered a year later (1975) at SPEAR (SLAC Positron Electron Collider) – Martin Perl e+ + e- → τ+ + τ- with τ+ → µ+ + υµ + ¯υτ and τ- → e- + υ¯ e + υτ Event appears as e+ + e- →µ+ + ee+

µ+ τ+ τ-

-

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e-

e Particle Physics – A short History

Now 3 charged leptons, 2 neutrinos, 4 - (u,d) (c,s) – quarks ! Must be another neutrino to go with the Tau And for aesthetic symmetry another pair of quarks with the charged leptons and their neutral partners 1978 sees the discovery of the b-quark (b for beauty/bottom) at Fermilab p ¯p → e+ e- + ….. M=9.46GeV, Γ=53KeV again a narrow bb bound state. Open B-hadrons need 10.58 GeV Gian Gopal


Still missing the neutrino to go with the Tau lepton to complete symmetry since Leptons come in pairs Direct Observation ντ+p→τ++… Not Yet ? Count the number of massless (low mass < 45 GeV) neutrinos One of the 1st Results to come from CERN LEP Collider

Measure the missing width of the Z0 i.e. to all invisible final states (pairs of neutrinos)

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The b-quark must have a partner since quarks come in pairs – a 17 year wait – 1995 Fermilab discovers the t-quark (Truth/Top)

pp→WbW’b’ – 4-Jet Event Gian Gopal

Vilakazi 2006


The symmetry of matter is complete 3 pairs of quarks and 3 pairs of leptons What Remains – Vector Bosons of Weak & Strong Interactions 1979 – The Gluon at PETRA e+ eCollider in Hamburg in 3-Jet events

1983 – W Z at CERN at SPS p p Collider at CERN Gian Gopal


Various expts (1998 onwards) Search for neutrino oscillation indicate neutrinos NOT massless LEP II (105 GeV e+ on 105 GeV e- ) indicates existence of the HIGGS Boson

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Basic Building Blocks of Matter (at least ones we know of, so far !)

1) Quarks -------> Hadrons -------> Nuclei 3 Generations of quark-pairs – each quark with JP = ½+ - each with a unique new Quantum Number

Up-Down

Charge = 2/3 u

Charge = -1/3 d

I=½

M = 1.5 – 4.0 MeV

M = 4.0 – 8.0 MeV

Charm-Strange

c

s

I=0

M = 1.15 – 1.35 GeV

C=1

M = 80 – 130 MeV S = -1

Top-Bottom

t

I=0

b

M = 174 – 178 GeV T=1

M = 4.1 – 4.9 GeV B = -1

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Protons & Neutrons uds – light quarks s-quarks with u/d produce strange particles cbt – Heavy quarks -


Basic Building Blocks of Matter (Cont.) 2)

Leptons

Again 3 generations of Lepton-pairs – each Lepton with J = ½

Charged

Neutral (Neutrino)

e (Electron)

νe

M = 0.5 MeV,

Le = 1

µ (Muon) M = 105.7 MeV, Lµ = 1

τ (Tau) M = 1.78 GeV,

Lτ = 1

M < 3 eV,

Le = -1

νµ M < 0.19 MeV, Lµ = -1

ντ M < 18.2 MeV, Lτ = -1

Each pair has a unique Lepton Quantum Number which is conserved within the Standard Model with Zero Mass Neutrinos.

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