Introduction to particle physics Hal

Particle physics want to answer  What are the “elementary” constituents of Matter?  What are the forces that control their behaviour at the most basic level?

What is elementary means ?  The word “elementary” is used in the sense that such particles have no known structure, they are “pointlike. ”  “elementary” depends on the spatial resolution of the probe used to investigate possible structure.

Units in particle physics

History of Constituents of Matter

Дмитрий Иванович Менделеев

History of Constituents of Matter Electrons were first discovered as the constituents of cathode rays. In 1897 British physicist J. J. Thompson showed the rays were composed of a previously unknown negatively charged particle, which was named electron.

History of Constituents of Matter radioactive source  particles target (very thin Gold foil) fluorescent screen Detector (human eye)

Proton discovery  particle Atom: spherical distribution of electric charges impact parameter b For Thomson’s atomic model the electric charge “seen” by the  – particle is zero, independent of impact parameter  no significant scattering at large angles is expected An atom consists of a positively charged nucleus surrounded by a cloud of electrons

History of Constituents of Matter James Chadwick's 1932 discovery of the neutron Neutron

Neutron discovery: observation and measurement of nuclear recoils in an “expansion chamber” filled with Nitrogen at atmospheric pressure incident neutron (not visible) scattered neutron (not visible) recoil nucleus (visible by ionization) Neutron discovery

Plate containing free hydrogen (paraffin wax) Incident neutron direction proton tracks ejected from paraffin wax Recoiling Nitrogen nuclei Assume that incident neutral radiation consists of particles of mass m moving with velocities v < V max Determine max. velocity of recoil protons (U p ) and Nitrogen nuclei (U N ) from max. observed range U p = V max  m m + m p U N = V max  m m + m N From non-relativistic energy-momentum conservation m p : proton mass; m N : Nitrogen nucleus mass U p m + m N U N m + m p = From measured ratio U p  U N and known values of m p, m N determine neutron mass: m  m n  m p Present mass values : m p = .  MeV/c  ; m n = .  MeV/c 

Antimatter Dirac equation two solutions P.A.M. Dirac For each solution of Dirac’s equation with electron energy E  there is another solution with E  What is the physical meaning of these “negative energy” solutions ? Motion of an electron in an electromagnetic field: presence of a term describing (for slow electrons) the potential energy of a magnetic dipole moment in a magnetic field  existence of an intrinsic electron magnetic dipole moment opposite to spin electron spin electron magnetic dipole moment  e 1928

Antimatter discovery  mm thick Pb plate  MeV positron  MeV positron Cloud chamber photograph by C.D. Anderson of the first positron ever identified. The positron must have come from below since the upper track is bent more strongly in the magnetic field indicating a lower energy Carl D. Anderson Cosmic-ray “shower” containing several e + e – pairs 1932

Neutrinos A puzzle in  – decay: the continuous electron energy spectrum First measurement by Chadwick (  ) Radium E:  Bi  (a radioactive isotope produced in the decay chain of 238 U) (Two body decay) electron total energy E = [M(A, Z) – M(A, Z+  )]c  (E. Fermi, )   decay: n  p + e  +

First neutrino detection (Reines, Cowan 1953) + p  e + + n detect  MeV  -rays from e + e –   (t =  ) E  =  MeV neutron “thermalization” followed by capture in Cd nuclei  emission of delayed  -rays (average delay ~   s) H  O + CdCl  I, II, III: Liquid scintillator  m Event rate at the Savannah River nuclear power plant:  events  hour (after subracting event rate measured with reactor OFF ) in agreement with expectations

History of Constituents of Matter Late  ’s – early  ’s: discovery of many particles at the high energy proton accelerators (Berkeley Bevatron, BNL AGS, CERN PS), all with very short mean life times (  –  –  –  s,collectively named “hadrons”) ARE HADRONS ELEMENTARY PARTICLES?

History of Constituents of Matter 1964 (Gell-Mann, Zweig): Hadron classification into “families”; observation that all hadrons could be built from three spin ½ “building blocks.” (named “quarks” by Gell-Mann) The three quarks are u(+2/3),d(-1/3),s(-1/3). And three antiquarks ( u, d, s ) with opposite electric charge.

Mesons: quark – antiquark pairs Examples of non-strange mesons: Examples of strange mesons: Baryons: three quarks bound together Antibaryons: three antiquarks bound together Examples of non-strange baryons: Examples of strangeness –  baryons: Examples of strangeness –  baryons:

What are the “elementary” constituents of Matter? 3 x 6 = 18 quarks + 6 leptons = 24 fermions (constituents of matter) + 24 antiparticles 48 elementary particles consistent with point-like dimensions within the resolving power of present instrumentation ( ~ cm)

What are the forces that control their behaviour at the most basic level? Gravitational interaction (all particles) Totally negligible in particle physics Electromagnetic interaction (all charged particles) Infinite interaction radius

No static fields of forces In Relativistic Quantum Mechanics static fields of forces DO NOT EXIST ; the interaction between two particles is “transmitted” by intermediate particles acting as “interaction carriers” Example: electron – proton scattering (an effect of the electromagnetic interaction) is described as a two-step process : . incident electron  scattered electron + photon . photon + incident proton  scattered proton The photon (  ) is the carrier of the electromagnetic interaction incident electron ( E e, p ) scattered electron ( E e, p’ ) incident proton ( E p, – p ) scattered proton ( E p, – p’ )   “ Mass” of the intermediate photon: Q   E   – p   c  = –  p  c  (  – cos  ) The photon is in a VIRTUAL state because for real photons E   – p   c  =   (the mass of real photons is ZERO ) – virtual photons can only travel over  very short distances thanks to the “Uncertainty Principle” Energy – momentum conservation: E  =  p  = p – p ’ (  p  =  p ’  )

Weak interaction The weak interaction is the only force affecting neutrinos (except for gravitation). Its most familiar effect is beta decay. The weak interaction is unique in a number of respects: 1.It is the only interaction capable of changing flavour. 2.It is the only interaction which violates parity symmetry P (because it almost exclusively acts on left-handed particles). 3.It is mediated by massive gauge bosons.

Strong interaction In particle physics, the strong interaction, or strong force, or color force, holds quarks and gluons together to form protons, neutrons, baryons and mesons. The interaction radius   –  cm. The theory about strong force is quantum chromodynamics (QCD). Each quark exists in three states of a new quantum number named “colour” Particles with colour interact strongly through the exchange of spin 1 particles named “gluons”, in analogy with electrically charged particles interacting electromagnetically through the exchange of spin 1 photons.

Strong interaction Free quarks, gluons have never been observed experimentally; only indirect evidence from the study of hadrons – WHY? CONFINEMENT: coloured particles are confined within colourless hadrons because of the behaviour of the colour forces at large distances The attractive force between coloured particles increases with distance  increase of potential energy  production of quark – antiquark pairs which neutralize colour  formation of colourless hadrons (hadronization)

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Conclusion

1937: Theory of nuclear forces (H. Yukawa) Existence of a new light particle (“meson”) as the carrier of nuclear forces Relation between interaction radius and meson mass m: mc 2   MeV for R int    cm Yukawa’s meson initially identified with the muon – in this case  – stopping in matter should be immediately absorbed by nuclei  nuclear breakup (not true for stopping  + because of Coulomb repulsion -  + never come close enough to nuclei, while  – form “muonic” atoms) Experiment of Conversi, Pancini, Piccioni (Rome, 1945): study of  – stopping in matter using  – magnetic selection in the cosmic rays In light material (Z   ) the  – decays mainly to electron (just as  + ) In heavier material, the  – disappears partly by decaying to electron, and partly by nuclear capture (process later understood as  – + p  n + ). However, the rate of nuclear captures is consistent with the weak interaction. the muon is not Yukawa’s meson Hideki Yukawa

 : Discovery of the  - meson (the “real” Yukawa particle) Observation of the  +   +  e + decay chain in nuclear emulsion exposed to cosmic rays at high altitudes Four events showing the decay of a  + coming to rest in nuclear emulsion Nuclear emulsion: a detector sensitive to ionization with ~   m space resolution (AgBr microcrystals suspended in gelatin) In all events the muon has a fixed kinetic energy ( .  MeV, corresponding to a range of ~   m in nuclear emulsion)  two-body decay m  = .  MeV  c   spin =  Dominant decay mode:  +   + +  and  –   – +  Mean life at rest:    =  x    s =  ns  – at rest undergoes nuclear capture, as expected for the Yukawa particle A neutral  – meson (  °) also exists: m (  °) = .  MeV  c   Decay:  °   + , mean life = .  x   s  – mesons are the most copiously produced particles in proton – proton and proton – nucleus collisions at high energies

CONCLUSIONS The elementary particles today: 3 x 6 = 18 quarks + 6 leptons = 24 fermions (constituents of matter) + 24 antiparticles 48 elementary particles consistent with point-like dimensions within the resolving power of present instrumentation ( ~ cm) 12 force carriers ( , W ±, Z, 8 gluons) + the Higgs spin  particle (NOT YET DISCOVERED) responsible for generating the masses of all particles

Dr. Vitaly KudryavtsevExperimental Astroparticle PhysicsLectures 1-2, slide 35 Cosmic rays underground  At the surface, muons contribute more than a half to the total cosmic ray flux.  The energy spectrum of muons: dN / dE  E -3.7 (at E>1 TeV).  Only muons and neutrinos can penetrate to large depths underground.  The background from cosmic-ray muons for underground experiments will be considered later in the course.  Atmospheric neutrinos are hard to detect due to small interaction cross-section. Nevertheless their flux has been measured and the deficit of muon neutrinos has been observed pointing to the neutrino oscillations - this will be considered in detail later.

Dr. Vitaly KudryavtsevExperimental Astroparticle PhysicsLectures 1-2, slide 36 Cosmic rays underground  Review of Particle Physics: pdg.lbl.gov (Cosmic Rays).  Muon flux as a function of depth underground: x (m w. e.) = depth (m)   (g/cm 3 )  Neutrino-induced muons dominate at x > 15 km w. e.

Dr. Vitaly KudryavtsevExperimental Astroparticle PhysicsLectures 1-2, slide 37 Energy loss of muons

Thomson (1897): Discovers electron

Neutron discovery: observation and measurement of nuclear recoils in an “expansion chamber” filled with Nitrogen at atmospheric pressure incident neutron (not visible) scattered neutron (not visible) recoil nucleus (visible by ionization) An old gaseous detector based on an expanding vapour; ionization acts as seed for the formation of liquid drops. Tracks can be photographed as strings of droplets Neutron discovery

Example: Rutherford’s scattering radioactive source  particles target (very thin Gold foil) fluorescent screen Detector (human eye)  c ~ resolving power of Rutherford’s experiment  particle mass