Introduction to Particle Physics [email protected]
From atoms to quarks An elementary historical review of concepts, discoveries and achievements
Recommended reading: D.H. Perkins, Introduction to High Energy Physics F.E. Close, The cosmic onion
Mainly based on talks at CERN Lecture series The “elementary particles” in the 19th century: The Atoms of the 92 Elements
-24 1. Hydrogen Mass MH 1.7 10 g N A 2. Helium n 3. Lithium 4 3 increasing mass A ...... V R ...... 3
92. Uranium Mass 238 MH Estimate of a typical atomic radius 23 -1 NA 6 10 mol (Avogadro constant) 푵 Number of atoms /cm3: 풏 = 푨 흆 A: molar mass 푨 : density ퟒ Atomic volume: 푽 = 흅푹ퟑ Packing fraction: f 0.52 - 0.74 ퟑ
1/ 3 3 f -3 R Example: Iron (A = 55.8 g; = 7.87 g cm ) 4n R = (1.1 - 1.3) 10 - 8 cm 1894 - 1897: Discovery of the electron Study of “cathode rays”: electric current in tubes at very low gas pressure (“glow discharge”)
Measurement of the electron mass: me MH/1836 “Could anything at first sight seem more impractical than a body which is so small that its mass is an insignificant fraction of the mass of an atom of hydrogen?” (J.J. Thomson) J.J. Thomson Atoms are not elementary
Thomson’s atomic model: . Electrically charged sphere . Radius ~ 10-8 cm . Positive electric charge . Electrons with negative electric charge embedded in the sphere 1896: Discovery of natural radioactivity (Henri Becquerel)
1909 - 1913: Rutherford’s scattering experiments fluorescent screen Discovery of the atomic nucleus a - particles radioactive source
Ernest Rutherford target Detector (very thin Gold foil) (human eye)
a - particles : nuclei of Helium atoms spontaneously emitted by heavy radioactive isotopes. Typical velocity of a - particle in radioactive decay 0.05 c (c : speed of light) Expectations for a - atom scattering a - atom scattering at low energies is dominated by Coulomb interaction a - particle
Atom: spherical distribution of electric charges impact parameter b
a - particles with impact parameter = b “see” only electric charge within sphere of radius = b (Gauss theorem for forces proportional to r-2 )
For Thomson’s atomic model the electric charge “seen” by the a - particle is zero, independent of impact parameter no significant scattering at large angles is expected Rutherford’s observation
Significant scattering of a - particles at large angles, consistent with scattering expected for a sphere of radius few 10-13 cm and electric charge = Ze, with Z = 79 (atomic number of gold) and e = |charge of the electron| An atom consists of a positively charged nucleus surrounded by a cloud of electrons Nuclear radius 10-13 cm 10-5 atomic radius Mass of the nucleus mass of the atom (to a fraction of 0.1% ) Two questions
. Why did Rutherford need a - particles to discover the atomic nucleus? . Why do we need huge accelerators to study particle physics today?
Answer to both questions from basic principles of Quantum Mechanics
Observation of very small objects using visible light
opaque screen with small circular aperture
point-like photographic light source plate l = 0.4 mm (blue light)
focusing lenses Diffraction of light
Aperture diameter: D = 20 mm Focal length: 20 cm Observation of light diffraction, interpreted as evidence that light consists of waves since (mm) y the end of the 17th century Angular aperture of the first circle (before focusing): a = 1.22 l / D x (mm)
Opaque disk, diam. 10 mm in the centre Presence of opaque disk is detectable Opaque disk of variable diameter diameter = 4 mm diameter = 2 mm diameter = 1 mm
no opaque disk The presence of the opaque disk in the centre is detectable if its diameter is larger than the wavelength l of the light The Resolving Power of the observation depends on the wavelength l Visible light: not enough resolution to see objects smaller than 0.2 - 0.3 mm Opaque screen with two circular apertures
aperture diameter: 10 mm distance between centres: 15 mm Image obtained by shutting one aperture y(mm) alternatively for 50% of the exposure time
x (mm)
Image obtained with both y(mm) apertures open simultaneously
x (mm) Photoelectric Effect : evidence that light consists of particles
glass tube under vacuum
Current measurement Observation of a threshold effect as a function of the frequency of the light impinging onto the electrode at negative voltage (cathode):
Frequency n < n0 : electric current = zero, independent of luminous flux;
Frequency n > n0 : current > 0, proportional to luminous flux Interpretation (A. Einstein) . Light consists of particles (“photons”) . Photon energy proportional to frequency: E = h n (Planck constant h = 6.626 10 -34 J s)
. Threshold energy E0 = hn0: the energy needed to extract an electron from an atom (depends on the cathode material) Albert Einstein Dual nature of light
Repeat the experiment with two circular Apertures using a very weak light source aperture diameter: 10 mm Luminous flux = 1 photon /second distance between centres: 15 mm (detectable using modern, commercially available photomultiplier tubes) Need very long exposure time
Question: which aperture will photons choose? y(mm)
Answer: diffraction pattern corresponds to both apertures simultaneously open, independent of luminous flux x (mm) Photons have both particle and wave properties simultaneously It is impossible to know which aperture the photon traversed The photon can be described as a coherent superposition of two states 1924: De Broglie’s principle Not only light, but also matter particles possess both the properties of waves and particles 풉 Relation between wavelength and momentum: 흀 = 풑 h: Planck constant p = m v : particle momentum Louis de Broglie Hypothesis soon confirmed by the observation of diffraction pattern in the scattering of electrons from crystals, confirming the wave behaviour of electrons (Davisson and Germer, 1927)
Wavelength of the a - particles used by Rutherford in the discovery of the atomic nucleus:
-34 h 6.62610 J s -15 -13 l -27 7 -1 6.710 m 6.710 cm ma v (6.610 kg)(1.510 m s ) ~ resolving power a-particle 0.05 c of Rutherford’s mass experiment Typical tools to study objects of very small dimensions
풉 Resolving 흀 = 풑 power
Optical microscopes Visible light ~ 10-4 cm
Electron microscopes Low energy electrons ~ 10- 7 cm
Radioactive sources a-particles ~ 10-12 cm
Accelerators High energy electrons, protons ~ 10-16 cm Units in particle physics
Energy 1 electron-Volt (eV): the energy of a particle with electric charge = |e|, initially at rest, after acceleration by a difference of electrostatic potential = 1 Volt (e 1.60 10 -19 C)
1 eV = 1.60 10 -19 J
Multiples: 1 keV = 103 eV ; 1 MeV = 106 eV 1 GeV = 109 eV; 1 TeV = 1012 eV Energy of a proton in the LHC : 6.5 TeV = 1.04 10 -6 J (the same energy of a body of mass = 1 mg moving at speed = 1.43 m /s) Energy and momentum for relativistic particles (velocity v comparable to c ) Speed of light in vacuum c = 2.99792 108 m / s 2 2 m0c m : relativistic mass Total energy: E mc 1- (v/c)2 m0 : rest mass 1 Expansion in powers of (v/c): E m c2 m v2 ... 0 2 0
energy “classical” associated kinetic with rest mass energy
m v Momentum: p mv 0 1- (v/ c)2 pc v E c 2 2 2 2 2 E – p c = (m0c ) “relativistic invariant” (same value in all reference frames)
Special case: the photon (v = c in vacuum) E / p = n l = c (in vacuum) E = h n E2 – p2c2 = 0 l = h / p photon rest mass mg = 0 Momentum units: eV/c (or MeV/c, GeV/c, ...) Mass units: eV/c2 (or MeV/c2, GeV/c2, ...)
Numerical example: electron with v = 0. 99 c 2 Rest mass: me = 0.511 MeV/c 1 g 7.089 (often called “Lorentz factor”) 1- (v/ c)2
2 Total energy: E = g me c 7.089 0.511 MeV 3.622 MeV
Momentum: p = g me v = (v / c) (E / c) = 0.99 3.62 = 3.58 MeV/c First (wrong) ideas about nuclear structure : before 1932 • Mass values of light nuclei multiples of proton mass Observations (to few %) (proton nucleus of the hydrogen atom) • decay: spontaneous emission of electrons by some radioactive nuclei Hypothesis : • The atomic nucleus is a system of protons and electrons strongly bound together • Nucleus of the atom with atomic number Z and mass number A : a bound system of A protons and (A – Z) electrons • Total electric charge of the nucleus = [A – (A – Z)]e = Z e Problem with this model: the “Nitrogen anomaly” • Spin of the Nitrogen nucleus = 1 • Spin: intrinsic angular momentum of a particle (or system of particles) • In Quantum Mechanics only integer or half-integer multiples of ℏ = 풉Τퟐ흅 are possible: – integer values for orbital angular momentum (e.g., for the motion of atomic electrons around the nucleus) – both integer and half-integer values for spin
Electron, proton spin = ½ħ (measured) Nitrogen nucleus (A = 14, Z = 7): 14 protons + 7 electrons = 21 spin ½ particles Total Spin Must Have Half-Integer Value, But Measured Spin = 1 Discovery of the Neutron : James Chadwick, 1932 Neutron : a particle with mass proton mass but with zero electric charge Solution to the nuclear structure problem: Nucleus with atomic number Z and mass number A: a bound system of Z protons and (A – Z) neutrons
Nitrogen anomaly: no problem if neutron spin = ½ħ Nitrogen nucleus (A = 14, Z = 7): 7 protons, 7 neutrons = 14 spin ½ particles total spin has integer value Neutron source in Chadwick’s experiments: a 210Po radioactive source (5 MeV a - particles ) mixed with Beryllium powder emission of electrically neutral radiation capable of traversing several centimetres of Pb:
4 9 12 1 He2 + Be4 C6 + n0 Basic principles of particle detection Passage of charged particles through matter Interaction with atomic electrons ionization (neutral atom ion+ + free electron) excitation of atomic energy levels (de-excitation photon emission) Ionization + excitation of atomic energy levels energy loss Mean energy loss rate –dE /dx • proportional to (electric charge)2 of incident particle • for a given material, function only of incident particle velocity • typical value at minimum : -dE /dx = 1 - 2 MeV /(g cm-2) Note: traversed thickness (dx) is given in g /cm2 to be independent of material density (for variable density materials, such as gases) - multiply dE /dx by density (g/cm3) to obtain dE /dx in MeV/cm Residual range
Residual range of a charged particle with initial energy E0 losing energy only by ionization and atomic excitation:
R Mc2 M: particle rest mass 1 R dx dE MF(v) v: initial velocity dE / dx 2 2 0 E0 E0 Mc / 1- (v/c) the measurement of R for a particle of known rest mass M is a measurement of the initial velocity Passage of neutral particles through matter: no interaction with atomic electrons detection possible only in case of collisions producing charged particles Neutron discovery: observation and measurement of nuclear recoils in an “expansion chamber” filled with Nitrogen at atmospheric pressure
scattered neutron (not visible) An old gaseous detector based on an expanding vapour; ionization acts as incident seed for the formation of liquid drops. neutron recoil nucleus (not visible) Tracks can be photographed as strings (visible by ionization) of droplets Wilson Cloud chamber : 1910 Ions act as nuclei for the condensation of supersaturated water vapour Wilson’s original cloud chamber, 1911 Combined with the invention of fast photography, one could record particle tracks in the cloud chamber Discovaries via imaging
Alphas, Philipp 1926 4 MeV ~ 5cm in air
X-ray, Wilson 1912
Not possible to study interaction of particles, Position resolution : 500mm, …. 22 Incident neutron Neutron mass direction Plate containing free hydrogen (paraffin wax)
Recoiling Nitrogen nuclei proton tracks ejected from paraffin wax Assume that incident neutral radiation consists of particles of mass m moving with velocities v < Vmx
Determine maximum velocity of recoil protons (Up) and Nitrogen nuclei (UN) from maximum observed range From non-relativistic energy- 2m 2m U = V momentum Conservation mp: proton Up = Vmx N m + m mx m + mp N mass; mN: Nitrogen nucleus mass U m + m From measured ratio U / U and known values of m , m p = N p N p N UN m + mp determine neutron mass: m mn mp 2 2 Present mass values : mp = 938.272 MeV/c ; mn = 939.565 MeV/c Pauli’s exclusion principle In Quantum Mechanics the electron orbits around the nucleus are “quantized”: only some specific orbits (characterized by integer quantum numbers) are possible. Example: allowed orbit radii and energies for the Hydrogen atom 2 2 40 n -10 2 Rn 2 0.5310 n [m] m = memp/(me + mp) me n = 1, 2, ...... me 4 13.6 En - 2 2 2 - 2 [eV] 2(40 ) n n In atoms with Z > 2 only two electrons are found in the innermost orbit - WHY? Answer (Pauli, 1925): two electrons (spin = ½) can never be in the same physical state Hydrogen (Z = 1) Helium (Z = 2) Lithium (Z = 3) .....
Lowest energy Wolfgang Pauli state Pauli’s exclusion principle applies to all particles with half-integer spin (collectively named Fermions) Antimatter Predicted “theoretically” by P.A.M. Dirac (1928) Dirac’s equation : a relativistic wave equation for the electron Two surprising results:
. Motion of an electron in an electromagnetic field: P.A.M. Dirac 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
e -5 me 5.7910 [eV/T] 2me electron magnetic dipole moment me . For each solution of Dirac’s equation with electron energy E > 0 there is another solution with E < 0 What is the physical meaning of these “negative energy” solutions ? Generic solutions of Dirac’s equation: complex wave functions Y( r , t) In the presence of an electromagnetic field, for each negative-energy solution the complex conjugate wave function Y* is a positive-energy solution of Dirac’s equation for an electron with opposite electric charge (+e) Dirac’s assumptions: . nearly all electron negative-energy states are occupied and are not observable. . electron transitions from a positive-energy to an occupied negative-energy state are forbidden by Pauli’s exclusion principle. . electron transitions from a positive-energy state to an empty negative-energy state are allowed electron disappearance. To conserve electric charge, a positive electron (positron) must disappear e+e– annihilation. . electron transitions from a negative-energy state to an empty positive-energy state are also allowed electron appearance. To conserve electric charge, a positron must appear creation of an e+e– pair. empty electron negative-energy states describe positive energy states of the positron Dirac’s perfect vacuum: a region where all positive-energy states are empty and all negative-energy states are full.
Positron magnetic dipole moment = me but oriented parallel to positron spin Experimental confirmation of antimatter
Detector: a Wilson cloud - chamber (visual detector based on a gas volume containing vapour close to saturation) in a magnetic field, exposed to cosmic rays
Measure particle momentum and sign of electric charge from magnetic curvature Lorentz force f ev B projection of the particle trajectory in a plane perpendicular to 푩 is a circle 10 p [GeV/c] Circle radius for electric charge |e|: R [m] 3B [T] p : momentum component perpendicular to magnetic field direction –e NOTE : impossible to distinguish between positively and negatively charged particles going in opposite directions +e need an independent determination of the particle direction of motion First experimental observation 23 MeV positron of a positron : C.D. Andersion, 1932 15000 Gauss 6 mm thick Pb plate
direction of high-energy photon 63 MeV positron
Production of an electron-positron pair Cosmic-ray “shower” by a high-energy photon containing several e+ e– pairs in a Pb plate Neutrinos (not neutrons, a completely different particle, properties) A puzzle in - decay: the continuous electron energy spectrum First measurement by Chadwick (1914) 210 Radium E: Bi83 (a radioactive isotope produced
in the decay chain of 238U)
/dE dN
E
If - decay is (A, Z) (A, Z+1) + e–, then the emitted electron is mono- energetic: electron total energy E = [M(A, Z) – M(A, Z+1)]c2 (neglecting the kinetic energy of the recoil nucleus ½p2/M(A,Z+1) << E) Several solutions to the puzzle proposed before the 1930’s (all wrong), including violation of energy conservation in - decay December 1930: public letter sent by W. Pauli to a physics meeting in Tübingen
Zürich, Dec. 4, 1930 Dear Radioactive Ladies and Gentlemen, ...because of the “wrong” statistics of the N and 6Li nuclei and the continuous -spectrum, I have hit upon a desperate remedy to save the law of conservation of energy. Namely, the possibility that there could exist in the nuclei electrically neutral particles, that I wish to call neutrons, which have spin ½ and obey the exclusion principle ..... The mass of the neutrons should be of the same order of magnitude as the electron mass and in any event not larger than 0.01 proton masses. The continuous -spectrum would then become understandable by the assumption that in -decay a neutron is emitted in addition to the electron such that the sum of the energies of the neutron and electron is constant...... For the moment, however, I do not dare to publish anything on this idea ...... So, dear Radioactives, examine and judge it. Unfortunately I cannot appear in Tübingen personally, since I am indispensable here in Zürich because of a ball on the night of 6/7 December. .... W. Pauli
. Pauli’s neutron is a light particle not the neutron that will be discovered by Chadwick one year later . As everybody else at that time, Pauli believed that if radioactive nuclei emit particles, these particles must exist in the nuclei before emission Theory of -decay : E. Fermi, 1932-33 - − 14 14 - decay: 풏 → 풑 + 풆 + 흂ഥ (e.g., C6 N7 e 휈ҧ) + 14 14 + decay: 풑 → 풏 + 풆 + 흂 (e.g., O8 N7 + e + n) 흂 : the particle proposed by Pauli (named “neutrino” by Fermi) 흂ഥ : its antiparticle (antineutrino) Fermi’s theory: a point interaction among four spin ½ particles, using p the mathematical formalism of creation and annihilation n - operators invented by Jordan particles emitted in - decay need not exist before emission – n they are “created” at the instant of decay Prediction of - decay rates as a function of only one parameter: Fermi coupling constant GF (determined from experiments) Energy spectrum dependence on neutrino mass m (from Fermi’s original article, published in German on Zeitschrift für Physik, following rejection of the English version by Nature) Measurable distortions for m > 0 near the end-point
(E0 : maximum allowed electron energy) Fermi theory of beta decay (1933) Neutrino detection Prediction of Fermi’s theory : 흂ഥ + 풑 → 풆+ + 풏 (Inverse beta decay)
흂ഥ - p interaction probability in thickness dx of hydrogen-rich material (e.g., H2O)
Incident 흂ഥ Target: Flux F [흂ഥ cm–2 s–1 ] surface S, thickness dx (uniform over surface S) containing n protons cm-3
dx 흂ഥ p interaction rate = F S n s dx interactions per second s : 흂ഥ- proton cross-section (effective proton area, as seen by the incident 흂ഥ) 흂ഥ p interaction probability = n s dx = dx / l Interaction mean free path: l = 1 / n s Interaction probability for finite target thickness T = 1 – exp(–T / l) s(흂ഥ p) 10–4 3 cm2 for 3 MeV 흂ഥ l 150 light-years of water !
–18 Interaction probability T / l very small (~10 per metre H2O) need very intense sources for antineutrino detection Nuclear reactors : very intense antineutrino sources 235 Average fission: n + U92 (A1, Z) + (A2, 92 – Z) + 2.5 free neutrons + 200 MeV
nuclei with large neutron excess a chain of decays with very short lifetimes: (until a stable or long lifetime (A, Z) − (A, Z + 1) − (A, Z + 2) − .... 풆 + 흂ഥ풆 풆 + 흂ഥ풆 풆 + 흂ഥ풆 nucleus is reached) On average, 6 흂ഥ풆 per fission 6P n production rate t 1.87 1011 P n/s 200 MeV1.610-13 t
Pt: reactor thermal power [W] conversion factor MeV Joule
9 For a typical reactor: Pt = 3 10 W 20 5.6 10 흂ഥ풆 / s (isotropic) Continuous 흂ഥ풆 energy spectrum - average energy ~3 MeV
ퟏΤퟐ ퟐ ퟐ −ퟒퟒ ퟐ 흈 푬흂ഥ풆 = ퟗ. ퟏퟑ ± ퟎ. ퟏퟏ 푬흂ഥ풆 − ퟏ. ퟐퟗퟑ × 푬흂ഥ풆 − ퟏ. ퟐퟗퟑ − ퟎ. ퟓퟏퟏ × ퟏퟎ 풄풎 First neutrino detection : (Reines, Cowan 1953) + 흂ഥ풆 + 퐩 → 풆 + 풏 • Detect 0.5 MeV g-rays from 풆+풆− → 휸휸 (t=0) • Neutron “thermalisation” followed by capture in Cd nuclei emission of delayed g-rays (average delay ~30ms)
Reines Cowan
2 m H2O + CdCl 2 Eg = 0.511 MeV
0.3 & 0.35 MeV I, II, III: 2.5ms delayed 5.8 & 3.3 MeV Liquid scintillator Event rate at the Savannah River nuclear power plant: 3.0 0.2 events / hour (after subracting event rate measured with reactor OFF ) in agreement with expectations Cosmic Rays . Discovered by V.F. Hess in the 1910’s by the observation of the increase of radioactivity with altitude during a balloon flight . In early 30’s sudden high energy particles were observed during the study of radioactive materials .Until the late 1940’s, the only existing source of high-energy particles Composition of cosmic rays at sea level - two main components . Electromagnetic “showers”, consisting of many e and g-rays, mainly originating from: g + nucleus e+e– + nucleus (pair production); e + nucleus e + g + nucleus (“bremsstrahlung”) The typical mean free path for these processes (“radiation
length”, X0 ) depends on Z. For Pb (Z = 82) X0 = 0.56 cm Thickness of the atmosphere 27 X0 . Muons ( m ) capable of traversing as much as 1 m of Pb without interacting (except −dE/dx loss); tracks observed in cloud chambers in the 1930’s. Determination of the mass by simultaneous measurement of momentum p = mv(1 – v2/c2)-½ (track curvature in magnetic Cloud chamber image of an field) and velocity v (ionization) electromagnetic shower. 2 mm = 105.66 MeV/c 207 me 1.27 cm thick Pb plates Discovery of muon, Phys Rev 52(1937) 1003
New Evidence for the Existence of Particle of Mass Intermediate Between the Proton and Electron
Proton (density of H- = 9.6 104 gauss- ionisation. 2.6 times cm, even though more than thick penetrated the lead track) H- = 2 106 block & -ve charge gauss-cm and +ve M ~130 m e High energy penetrating track 37 Cosmic ray vs collider
for Ebeam>>mp
ECM 2mp Ebeam
E-2.7 Expect only 1000 2 Galactic events/year/km with S>14 TeV, Extra-Galactic whereas in LHC, 109 -3.1 event/sec LHC (pp) E
Fermi acceleration Ankle 2 (1st and 2nd order) (1 particle /km yr)
32 orders of magnitude of orders 32 near shock-fronts in
supernova explosion
LHC
Tevatron RHIC
12 orders of magnitude 38 Atmospheric cascade • Low fluxes, Ground based observation • Atmosphere acts as a Radiator amplifier
TeV PeV & Above ------70KM ------ Cherenkov light pool Charged particle Night sky background
One source at a time All sources in the sky Duty cycle ~ 5% 100%
Area ~ 104 m2 104 m2
Pointing telescope ~ 1o Direction of each event ~1o History of accelerators
q(v B) mv2 / r
PT 0.3Br Electron in Magnetic field
• The picture shows and electron with 16.9 MeV initial energy. It spirals about 36 times in the magnetic field. • At the end of the visible track the energy has decreased to 12.4 MeV. From the visible path length (1030cm) the energy loss by ionization is calculated to be 2.8MeV. • The observed energy loss (4.5MeV) must therefore be cause in part by Bremsstrahlung. • The curvature indeed shows sudden changes as can most clearly be seen at about the seventeenth circle.
Fast electron in a magnetic field at the Bevatron, 1940
41 Particle interactions (as known until the mid 1960’s) In order of increasing strength: . Gravitational interaction (all particles) : Totally negligible in particle physics Example: static force between electron and proton at distance D m m 1 e2 Gravitational: e p Electrostatic: f fG GN 2 E 2 D 40 D – 4 0 Ratio fG / fE 4.4 10 . Weak interaction (all particles except photons) Responsible for b decay and for slow nuclear fusion reactions in the star core Example: in the core of the Sun (T = 15.6 106 ºK) 4p 4He + 2e+ + 2n Solar neutrino emission rate ~ 1.84 1038 neutrinos / s Flux of solar neutrinos on Earth ~ 6.4 1010 neutrinos cm-2 s –1
Very small interaction radius Rint (maximum distance at which two particles interact) (Rint = 0 in the original formulation of Fermi’s theory) . Electromagnetic interaction (all charged particles) Responsible for chemical reactions, light emission from atoms, etc. Infinite interaction radius (example: the interaction between electrons in transmitting and receiving antennas) Nature of interactions Gravitational --solar system/galaxy Electromagnetic --photon
Weak --radioactivity Strong --binding of nucleus . Strong interaction ( neutron, proton, .... NOT THE ELECTRON ! ) Responsible for keeping protons and neutrons together in the atomic nucleus –13 Independent of electric charge & Interaction radius Rint 10 cm 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 : 1) incident electron scattered electron + photon 2) photon + incident proton scattered proton The photon ( g ) is the carrier of the electromagnetic interaction scattered electron In the electron - proton ( E , p’ ) centre-of-mass system incident electron e ( Ee , p ) q g Energy – momentum conservation: incident proton ( E , – p ) Eg = 0 p scattered proton pg = p – p ’ ( | p | = | p ’| ) ( Ep , – p’ )
2 2 2 2 2 2 “Mass” of the intermediate photon: Q Eg – pg c = – 2 p c ( 1 – cos q ) 2 2 2 The photon is in a VIRTUAL state because for real photons Eg – pg c = 0 (the mass of real photons is ZERO ) - virtual photons can only travel over very short distances thanks to the “Uncertainty Principle” The Uncertainty Principle
Classical Mechanics Position and momentum of a particle can be measured independently and simultaneously with arbitrary precision
Quantum Mechanics Werner Heisenberg Measurement perturbs the particle state position and momentum measurements are correlated:
(also for y and z components) xpx
Numerical example: -13 px 100 MeV/c x 1.9710 cm Theory of nuclear forces : (1937) Hideki Yukawa Existence of a new light particle (“meson”) as the carrier of nuclear forces Relation between interaction radius and meson mass m: mc2 200 MeV Rint for R 10 -13 cm mc int
Yukawa’s meson initially identified with the muon - in this case m– stopping in matter should be immediately absorbed by nuclei nuclear breakup (not true for stopping m+ because of Coulomb repulsion - m+ never come close enough to nuclei, while m– form “muonic” atoms) Experiment of Conversi, Pancini, Piccioni (Rome, 1945): study of m– stopping in matter using m– magnetic selection in the cosmic rays In light material (Z 10) the m– decays mainly to electron (just as m+) In heavier material, the m– disappears partly by decaying to electron, and partly by nuclear capture (process later understood as m– + p n + n). However, the rate of nuclear captures is consistent with the weak interaction. the muon is not Yukawa’s meson 1947: Discovery of the - meson (the “real” Yukawa particle) Observation of the + m+ 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 ~1 mm space resolution (AgBr microcrystals suspended in gelatin) In all events the muon has a fixed kinetic energy (4.1 MeV, corresponding to a range of ~ 600 mm in nuclear emulsion) two-body decay 2 m = 139.57 MeV/c ; spin = 0 Dominant decay mode: 흅+ → 흁+ + 흂 (and 흅− → 흁− + 흂ഥ) -8 Mean life at rest: t = 2.6 10 s = 26 ns – at rest undergoes nuclear capture, as expected for the Yukawa particle Cecil Frank Powell A neutral - meson (°) also exists: m (°) = 134. 98 MeV /c2 Decay: ° g + g , mean life = 8.4 10-17 s - mesons are the most copiously produced particles in proton - proton and proton - nucleus collisions at high energies Conserved Quantum Numbers Why is the free proton stable? Possible proton decay modes (allowed by all known conservation laws: energy - momentum, electric charge, angular momentum): p ° + e+ p ° + m+ p + + n . . . . . No proton decay ever observed - the proton is STABLE 26 34 Limit on the proton mean life: tp > 1.6 10 (1.6 10 ) years Invent a new quantum number : “Baryonic Number” B B = 1 for proton, neutron B = -1 for antiproton, antineutron B = 0 for e , m , neutrinos, mesons, photons Require conservation of baryonic number in all particle processes:
B B i f i f ( i : initial state particle ; f : final state particle) Strangeness Late 1940’s: discovery of a variety of heavier mesons (K - mesons) and baryons (“hyperons”) - studied in detail in the 1950’s at the new high-energy proton synchrotrons (the 3 GeV “cosmotron” at the Brookhaven National Lab and the 6 GeV Bevatron at Berkeley) Mass values : Mesons (spin = 0): m(K ±) = 493.68 MeV/c2 ; m(K°) = 497.67 MeV/c2 Hyperons (spin = ½): m(L) = 1115.7 MeV/c2 ; m(S ±) = 1189.4 MeV/c2 m(X°) = 1314.8 MeV/c2 ; m(X – ) = 1321.3 MeV/c2 Properties . Abundant production in proton - nucleus , - nucleus collisions . Production cross-section typical of strong interactions (s > 10-27 cm2) . Production in pairs (example: – + p K° + L ; K– + p X – + K+ ) . Decaying to lighter particles with mean life values 10–8 - 10–10 s (as expected for a weak decay)
Examples of decay modes : K± ± ° ; K± ± +– ; K± ± ° ° ; K° +– ; K° ° ° ; . . . L p – ; L n ° ; S+ p ° ; S+ n + ; S+ n – ; . . . X – L – ; X° L ° Invention of a new, additive quantum number “Strangeness” (S) (Gell-Mann, Nakano, Nishijima, 1953)
S S . conserved in strong interaction processes: i f i f S - S 1 . not conserved in weak decays: i f f S = +1: K+, K° ; S = –1: L, S±, S° ; S = –2 : X°, X– ; S = 0 : all other particles (and opposite strangeness –S for the corresponding antiparticles)
° e+ e - g (a rare decay) Example of a K- stopping in liquid hydrogen: K - + p L + ° L is (strangeness conserving) – produced in followed by the decay A and decays in B L p + - (strangeness violation) p K- Nobel Prizes • 1948 : Patrick Maynard Stuart Blackett : "for his development of the Wilson cloud chamber method, and his discoveries therewith in the fields of nuclear physics and cosmic radiation."
• 1960 : Donald Arthur Glaser : “for the invention of the bubble chamber”
• 1968 : Luis Walter Alvarez : “for his decisive contributions to elementary particle physics, in particular the discovery of a large number of resonance states, made possible through his development of the technique of using hydrogen bubble chamber and data analysis” Discovery of antiproton (Phys Rev 100 (1955) 947) Nobel Prize in 1959 • Dirac equation in 1928 and the discovery of Positron in 1932 predicted also an antiproton. • But, to generate antiproton (conservation of baryon number, 1.19 GeV/c antiproton in spectrometer), need an accelerator, which can give about 6.5 GeV proton. • 1955 : Segre & Chamberlin used bevatron of 6.5GeV proton at Lawrence Berkeley National Laboratory (LBNL) : Used momentum and velocity (scintillator and Cherenkov) detector. One in 44000 Emilio Gino Segrè particles produced in the interaction is antiproton
Owen Chamberlain 52 Antiproton discovery (1955) Threshold energy for antiproton ( 풑ഥ) production in proton - proton collisions Baryon number conservation simultaneous production of 풑ഥ and p (or 풑ഥ and n) Example: p p p p p p Threshold energy ~ 6 GeV . build a beam line for 1.19 GeV/c momentum . select negatively charged particles (mostly – ) . reject fast – by Čerenkov effect: light emission in transparent medium if particle velocity v > c / n (n: refraction index) - antiprotons have v < c / n no Čerenkov light
. measure time of flight between counters S1 and S2 (12 m path): 40 ns for – , 51 ns for antiprotons For fixed momentum, time of flight gives particle velocity, hence particle mass
“Bevatron”: 6 GeV 풑 = 휸 풎 풗 proton synchrotron 풑 in Berkeley 풎 = ퟏ − 풗ퟐ/풄ퟐ 풗 Example of antiproton annihilation at rest in a liquid hydrogen bubble chamber Muon decay : 흁± → 풆± + 흂 + 흂ഥ m Decay electron momentum distribution
Muon spin = ½
Cosmic ray muon stopping in a cloud chamber and decaying to an electron
-6 Muon lifetime at rest: tm = 2.197 10 s 2.197 ms Muon decay mean free path in flight: decayed electron track vt m pt m p p : muon momentum ldecay t m c 2 1-(v / c) mm mm c tm c 0.66 km muons can reach the Earth surface after a path 10 km because the decay mean free path is stretched by the relativistic time expansion Another neutrino A puzzle of the late 1950’s: the absence of m e g decays Experimental limit: < 1 in 106 of 흁+ → 풆+흂 흂ഥ decays A possible solution: existence of a new, conserved “muonic” quantum number distinguishing muons from electrons To allow 흁+ → 풆+흂 흂ഥ decays, 흂ഥ must have “muonic” quantum number but not n in m+ decay the 흂ഥ is not the antiparticle of n + + two distinct neutrinos (ne , nm) in the decay 흁 → 풆 흂풆 흂ഥ흁 + − − Consequence for - meson decays: m nm ; 흅 → 흁 흂ഥ흁 to conserve the “muonic” quantum number
High energy proton accelerators: intense sources of -mesons 흂흁, 흂ഥ흁
Experimental method 흂흁, 흂ഥ흁
decay region proton beam Shielding Neutrino target to stop all other particles, detector including m from decay
– – – If nm ne , nm interactions produce m and not e (e.g., nm + n m + p) Discovery of nm at the Brookhaven AGS : 1962 AGN : Alternating Gradient Synchrotron
A 30 GeV proton synchrotron running at 17 GeV for the neutrino experiment
Neutrino energy spectrum known from , K production and m , K m decay kinematics 13. 5 m iron shielding (enough to stop 17 GeV muons) Spark chamber each with 9 Al plates (112 112 2.5 cm) mass 1 Ton
Muon - electron separation Muon: long track Electron: short, multi-spark event from electromagnetic shower Neutrino detector Lederman Schwartz Steinberger
64 “events” from a 300 hour run:
. 34 single track events, consistent with m track . 2 events consistent with electron shower (from small, calculable ne contamination in beam)
Clear demonstration that nm ne
Three typical single-track events in the BNL neutrino experiment Atmospheric neutrino : Historical context Detection of atmospheric neutrinos • Markov (1960) suggests Cherenkov light in deep lake or ocean to detect atmospheric n interactions for neutrino physics • Greisen (1960) suggests water Cherenkov detector in deep mine as a neutrino telescope for extraterrestrial neutrinos • First reported events in deep mines with electronic detectors, 1965: KGF detector (Menon et al.), CWI detector (Reines et al.); Two methods for calculating atmospheric neutrinos: • From muons to parent pions infer neutrinos (Markov & Zheleznykh, 1961; Perkins) • From primaries to , K and m to neutrinos (Cowsik, 1965 and most later calculations) • Essential features known since 1961: Markov & Zheleznykh, Zatsepin & Kuz’min • Monte Carlo calculations follow second method n backgroundbackground for forp-pdecay- Stability of matter: search for proton decay, 1980’s decay • IMB & Kamioka -- water Cherenkov detectors nm • KGF, NUSEX, Frejus, Soudan -- iron tracking calorimeters ne • Principal background is interactions of atmospheric neutrinos e • Need to calculate flux of atmospheric neutrinos 0.1 1 10 KGF at 1951 : Beginning of neutrino physics in India First report on Atmospheric neutrinos
Atmospheric neutrino detector at Kolar Gold Field -1965 Physics Letters 18, (1965) 196, dated 15th Aug 1965
PRL 15,PRL (1965), 15, (1965), 429, 429, dated dated 30th 30th Aug. Aug. 19651965 KGF neutrino and proton decay experiments Collaboration: INDIA , UK and Japan Proton Decay Experiment
140 ton (at 2.3 km) Absorber : Iron ½” Detector : 10cm 10cm 4(6)m Proportional counter 8.41years Prof. NK Mondal’s thesis
5.53years 340 ton (at 2km) 60 horizontal array
V = 6m 6m 6m Revival of underground neutrino experiment India-based neutrino Observatory (INO) at BodiHills
50Kton ICAL neutrino Detector The “STATIC” Quark Model Late 1950’s - early 1960’s: discovery of many strongly interacting particles at the high energy proton accelerators (Berkeley Bevatron, BNL AGS, CERN PS), all with very short mean life times (10–20 - 10–23 s, typical of strong decays) catalog of > 100 strongly interacting particles (collectively named “hadrons”)
Are Hadrons Elementary Particles ? 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): u d s Electric charge 2/3 -1/3 -1/3 ( units |e| ) Baryonic number 1/3 1/3 1/3 Strangeness 0 0 -1
and three antiquarks ( 풖ഥ, 풅ഥ, 풔ത ) with opposite electric charge, opposite baryonic number and also opposite strangeness Mesons: quark - antiquark pairs Examples of non-strange mesons: ud ; - ud ; 0 (dd - uu)/ 2 Examples of strange mesons: K - su ; K 0 sd ; K su ; K 0 sd
Baryons: three quarks bound together Antibaryons: three antiquarks bound together Examples of non-strange baryons: proton uud ; neutron udd Examples of strangeness –1 baryons: S suu ; S0 sud ; S- sdd
Examples of strangeness –2 baryons: X0 ssu ; X- ssd Prediction and discovery of the W- particle ퟑ The “decuplet” of spin baryons A success of the static quark model ퟐ Strangeness Mass (MeV/c 2 ) 0 N*++ N*+ N*° N*– 1232 () uuu uud udd ddd
–1 S*+ S*° S*– 1384 suu sud sdd
–2 X*° X*– 1533 ssu ssd
–3 W– 1672 (predicted) sss W- : the bound state of three s-quarks with the lowest mass with total angular momentum = 3/ 2 Pauli’s exclusion principle requires that the three quarks cannot be identical Three colour charge of quarks (Red, Green, Blue) : not real colour, but certain combinations of quarks could be “neutral”, in the sense that three ordinary colours can yield white, a neutral colour. The first W– event (observed in the 2 m liquid hydrogen bubble chamber at BNL using a 5 GeV/c K- beam from the 30 GeV AGS)
Chain of events in the picture: K– + p W – + K+ + K° (strangeness conserving)
W – X ° + – (S = 1 weak decay)
X ° ° + L (S = 1 weak decay)
L – + p (S = 1 weak decay)
° g + g (electromagnetic decay) with both g - rays converting to an e+e – in liquid hydrogen (very lucky event, because the mean free path for g e+e – in liquid hydrogen is ~10 m)
W– mass measured from this event = 1686 ± 12 MeV/c2 PDG : 1672.45 0.29 MeV/c2 History of particle/physics discoveries
Today > 200 particles listed in PDG But only 27 have ct > 1μm and only 13 have ct > 500μm
Neutral current
DIS
Already 20 particles Discovered X,Y,Z (cc+) …images and logic discoveries Higgs 68 Discovery of neutral current, the most important discovery in HEP in last century, but no Nobel prize “DYNAMIC” Evidence for Quarks Electron - proton scattering using a 20 GeV electron beam from the Stanford two - mile Linear Accelerator (1968 - 69). The modern version of Rutherford’s original experiment: resolving power wavelength associated with 20 GeV electron 10-15 cm Three magnetic spectrometers to detect the scattered electron: . 20 GeV spectrometer (to study elastic scattering e– + p e– + p) . 8 GeV spectrometer (to study inelastic scattering e– + p e– + hadrons) . 1.6 GeV spectrometer (to study extremely inelastic collisions) The Stanford two-mile electron linear accelerator (SLAC)
Jerome I. Friedman
Henry W. Kendall
Richard E. Taylor Electron elastic scattering from a point-like charge |e| at high energies: differential cross-section in the collision centre- of-mass (Mott’s formula) 2 2 2 ds a (c) cos (q/ 2) e2 1 s a dW 8E 2 sin 4 (q/ 2) M c 137
Scattering from an extended charge distribution: multiply sM by a “form factor”:
ds 2 F( Q )sM F(|Q2|) dW |Q| = ħ / D : mass of the exchanged virtual photon D: linear size of target region contributing to scattering Increasing |Q| decreasing target electric charge F (|Q2| ) = 1 for a point-like particle |Q2| (GeV2) the proton is not a point-like particle Inelastic electron - proton collisions
scattered electron incident electron ( Ee’ , p’ ) ( Ee , p ) q g F(|Q2|) incident proton ( Ep , – p ) Hadrons (mesons, baryons)
2 2 Total hadronic energy : W 2 E - p c2 i i i i For deeply inelastic collisions, the cross-section depends only weakly on |Q2| , suggesting a collision with a Point-Like object |Q2| (GeV2) Interpretation of deep inelastic e - p collisions Deep inelastic electron - proton collisions are elastic collisions with point-like, electrically charged, spin ½ constituents of the proton carrying a fraction x of the incident proton momentum
Each constituent type is described by its electric charge ei (units of | e | ) and by its x distribution (dNi /dx) (“structure function”) If these constituents are the u and d quarks, then deep inelastic e - p collisions provide information on a particular combination of structure functions:
dN 2 dNu 2 dNd eu ed dx e-p dx dx
Comparison with 흂흁 - p and 흂ഥ흁 - p deep inelastic collisions at high energies under the assumption that these collisions are also elastic scatterings on quarks – – 흂흁 + p m + hadrons : 흂흁 + d m + u (depends on dNd / dx ) + + 흂ഥ흁 + p m + hadrons : 흂ഥ흁 + u m + d (depends on dNu / dx ) (Neutrino interactions do not depend on electric charge) All experimental results on deep inelastic e - p , 흂흁 - p, 흂ഥ흁 - p collisions are 2 2 consistent with eu = 4 / 9 and ed = 1 / 9 the proton constituents are the quarks Knowledge about constituent of matter
~10 KeV 10 MeV 100 MeV GeV
Thomson Rutherford Chadwick SLAC 1897 1909 1932 1968 Probe smaller distance with higher energy ➔ Newer structure Electromagnetic interactions : Maxwell’s equation
D = 4 Inverse square law
B = 0 No magnetic charges
c E = B /t Electromagnetic induction
c H = - D /t Ampere’s circuital law + 4J James Clerk Maxwell + charge conservation (1831 - 1879)
Unified Theory of Electricity and Magnetism
mn n c m F = 4 J mFnl + nFlm + lFmn = 0
The road to unification Simplest gauge-invariant construction Fermi’s Theory of Beta Decay (1932) + Prediction of W
1 1 - 0 n1p e n e
had IVB Hypothesis Jm (1958)
Enrico Fermi Julian Schwinger (1901 - 1954) m Jl e p (1918 - 1994) current-current interaction
Intermediate Vector Boson (bounded interaction rate) e = g sinqW
2 Sin qW = 0.2312 Another massive intermediate vector boson (1961)
J helec a d IVB theory is very similar to m QED it can be written as a gauge theory g Sheldon Glashow (1932 - ) QED J m intermediate vector boson lmuon e p
e e- W W -
Also need a neutral intermediate vector boson to have finite production rate Unification of fundamental forces
Observation in higher energy More unification The Mass Problem
Maxwell’s equations (in the Lorenz gauge) lead to the (inhomogeneous) wave equation
Invariant under because CONVERSELY gauge invariance demands
in the Lorenz gauge ( ) But if the photon has a mass...... gauge invariance is lost... Generating W, Z boson masses
Generate masses for the W and For every broken continuous symmetry Z bosons by using the hidden in a quantum field theory, there exists a symmetry mechanism massless scalar (spin-0) boson
Yoichiro Nambu Jeffrey Goldstone (1921 - ) (1933 - )
Conflict between broken symmetry and Goldstone Boson 80 HIDDEN SYMMETRY
Such things happen all the time! We call them phase transitions...
Space is isotropic
Ferromagnet
Space points ↑ Symmetry is not really lost... it reappears on an ensemble...
Hidden symmetry
What if there is only one system... the Universe...?
Spontaneous symmetry-breaking
There could be domains of different phase in the Universe, like bubbles in water...
Prediction of new scalar particle
The Gauge field ‘ate’ the Goldsone boson, thereby acquiring both a mass and a third polarization state (particular choice of gauge)
Physical consequences of this model?
Prediction of a massive scalar (‘Higgs particle’) 1961-67: Formulation of a Unified Electroweak Theory
Glashow Salam Weinberg • 4 intermediate spin 1 interaction carriers (“bosons”): – The photon (g) : responsible for all electromagnetic processes – Three weak, heavy bosons W+ , W– , Z : W responsible for processes with electric charge transfer = ±1 (Charged Current processes) − − − − – 풏 → 풑 풆 흂ഥ풆 ∶ 퐧 → 풑 + 푾 followed by 푾 → 풆 흂ഥ풆 + + + + + + – 흁 → 풆 흂풆 흂ഥ흁 ∶ 흁 → 흂ഥ흁 + 푾 followed by 푾 → 풆 흂풆 – Z responsible for weak processes with no electric charge transfer • (Neutral Current processes) • Processes never observed before • Require neutrino beams to search for these processes, to remove the much larger electromagnetic effects expected with charged particle beams Gargamelle Bubble chamber : Discovery of neutral current in 1973
l=4.8m d=2m Wt=1000ton 18 ton liquid
freon (CF3Br)
MIP : ~300 bubble/cm, increase with temp Bubble diameter ~30 μm
• A liquefied gas like H2, D2, Ne, freon etc is kept in a pressure vessel below but close to its boiling point • After the passage of ionising radiation, the volume of the chamber is expanded by the fast movement of a piston during about 1μs-1ms is such a was that the boiling temp is exceeded. • Along the ionising tracks gas bubbles are formed in the liquid, due to the heat developed by the recombination of ions. 86 Discovery of Neutral Currents (1973)
GARGAMELLE was the largest bubble chamber ever built Discovery of Neutral Currents (1973)
- + n m e e - e - e- Z e - e + e+ e- e e- Different types of Particle Collider
Begins with fix target (one beam), for Ebeam>>mpro , E CM 2mpro Ebeam Proton-Proton Collider (e.g. LHC) Electron-Positron Collider (e.g. LEP) Discovery machine Mainly for precision study d d - + u u e e u u Electrons are elementary particles, so Eproton1 = Ed1 + Eu1 + Eu2 + Egluons1 E = E + E + E + E proton2 d2 u3 u4 gluons2 Ecollision = Ee- + Ee+ = 2 Ebeam Collision could be between quarks or gluons, so e.g., in LEP, Ecollision ~ 91 GeV 0 < Ecollision < (Eproton1 + Eproton2) = mZ i.e., with a single beam energy you can i.e., can tune beam energy so that “search” for particles of unknown mass! you always produce a desired particle!
Limited by the strength of bending magnet Limited by synchrotron radiation loss Combination of these two, electron+proton are also there, e.g. HERA Energy Frontier (Goal was Higgs, Now SUSY,… …)
(Goal was top..) rising total pp cross section
ECM 2mt arget Ebeam vs 2Ebeam
Initially increase of energy by ~10 fold in every ~12 year LHC Accelerator Layout (not to 1939 → 2009 scale) Energy ~107 times 5 1989-2001 Size is also ~10 2008- 6551 45 GeV 8KM
2.2KM 450 GeV
1976 1.4 GeV 1720 Power converters > 9000 magnetic elements 50 MeV 7568 Quench detection systems 200M 25 GeV 1088 Beam position monitors 1959 ~4000 Beam loss monitors
150 tones helium, ~90 tones atISR 1.9 K : 1971-1985 300 MJ stored beam energy in 2016 1.2 GJ magnetic energy per sector at 6.5 TeV 1. In the LHC, each RF cavity is tuned to oscillate at 400 MHz 2. 8.33T bending magnets (1232 dipoles). Detectors Concept of stable particles
± ± ± ± 풑, 풑ഥ, 풌 , 흅 , 풆 , 휸, 풏, 풏ഥ, 풌푳풐풏품, 흁 , 흂, 흂ഥ
Where do we put magnet ?
• Design detector which can detect all these stable particles • No single detector can identity and measure energies/momenta of all particles • Layers of subdetectors to detect different particle in different detector layer • Each layer identifies and measures (remeasures) the energy of particles unmeasured by the previous layer
Acceptance : Close to 4 (efficiency), also to detect (흂, 흂ഥ) Building block of any HEP detector F q v B
•Bending of charge particle in presence of magnetic field provides the information of momentum of charge particle Calorimeter is mainly used for the measurement of energy of neutral particle
Design detector to detect these stable particles :
± ± ± ± e , γ, p, 풑ഥ, K , π , KL, n, 풏ഥ, μ , 흂, 흂ഥ Charm (4th quark) discovery
2
94 Charm discovery
s(s) ~ 1 in 104
Beam size 3 6 mm2 Beam size 1 0.1 50 mm3
Particle data book (2020)
MJ/ = 3096.900 0.006 MeV J/ = 92.9 2.8 keV 95 The Nobel Prize in Physics 1976
• Prize motivation: "for their pioneering work in the discovery of a heavy elementary particle of a new kind."
Samuel C C Ting Burton Richter 97 Physics with e+e– Colliders Two beams circulating in opposite directions in the same magnetic ring and colliding head-on e+ e–
푬, 풑 푬, −풑 A two-step process: e+ + e– virtual photon f + 풇ത f : any electrically charged elementary spin ½ particle (m , quark) (excluding e+e– elastic scattering) 2 2 2 2 2 Virtual photon energy - momentum : Eg = 2E , pg = 0 Q = Eg – pg c = 4E 2 2 2 2a c 2 Cross - section for 풆+풆− → 풇 풇ത : s e (3- ) 3Q2 f a = e2/(ħc) 1/137 formula precisely e : electric charge of particle f (units |e |) f verified for e+e– m+m– = v/c of outgoing particle f Assumption: e+e– quark ( q ) + antiquark ( 풒ഥ ) hadrons 2 at energies E >> mqc (for q = u , d , s) 1: s (ee- hadrons) 4 1 1 2 R e 2 e 2 e 2 s (ee- m m - ) u d s 9 9 9 3 Experimental results from the Stanford e+e– collider SPEAR (1974 - 75):
R
Q = 2E (GeV) • For Q < 3. 6 GeV R 2. If each quark exists in three different states, R 2 is consistent with 3 ( 2 / 3). This would solve the W– problem. • Between 3 and 4.5 GeV, the peaks and structures are due to the production of quark-antiquark bound states and resonances of a fourth quark (“charm”, c) of electric charge +2/3 • Above 4.6 GeV R 4.3. Expect R 2 (from u, d, s) + 3 (4 / 9) = 3.3 from the addition of the c quark alone. So the data suggest pair production of an additional elementary spin ½ particle with electric charge = 1 (later identified as the t - lepton (no strong interaction) with mass 1777 MeV/c2 ). Discovery of the Tau in the MARK-I experiment at SPEAR: 1974-76 One of the first large solid-angle, general Stanford Positron Electron purpose detector build for colliding beam Asymmetric Rings
System
Lead
\
Upgrade for better electron and muon identification Discovery of t-lepton, PRL 35(1975) 1491 Final state : an electron - muon pair + missing energy e e- t t m nn Martin L. Perl enn Ee+ + Ee- = s=4.8 GeV
s = 2E (GeV) Evidence for production of pairs of heavy leptons t± : - 0 1.6 GeV • Peak is clearly visible, while used background (non resonance component) subtracted distributions • Not only one, two peaks with sass = 9.44 0.03 & 10.17 0.05 GeV 102 The Modern Theory of Strong Interactions • The interactions between quarks based on “Colour Symmetry” Quantum ChromoDynamics (QCD) formulated in the early 1970’s – 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 • A major difference with the electromagnetic interaction – Electric charge: positive or negative – Photons have no electric charge and there is no direct photon-photon interaction – Colour: three varieties • Mathematical consequence of colour symmetry: the existence of eight gluons with eight variety of colours, with direct gluon - gluon interaction – The observed hadrons (baryons, mesons ) are colourless combinations of coloured quarks and gluons – The strong interactions between baryons, mesons is an “apparent” interaction between colourless objects, in analogy with the apparent electromagnetic interaction between electrically neutral atoms 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) • At high energies (e.g., in 풆+ + 풆− → 풒 + 풒ഥ ) expect the hadrons to be produced along the initial direction of the q 풒ഥ pair production of hadronic “jets” • Confinement, Hadronisation : properties deduced from observation. So far, the properties of colour forces at large distance have no precise mathematical formulation in QCD. Discovery of gluon • Started with only one 3-jet events, .. all four experiments observed it through various event shape variables (Thrust , Sphericity, acoplanarity, ..) • 1979, EPS @Geneva and then LP@FNAL – Mark-J : PRL 43 (1979) 830) – PLUTO : Phys. Lett B86 (1979) 418 – JADE : Phys. Lett B91 (1980) 142 – TASSO : Phys. Lett B86 (1979) 243 OPAL 105 Evidence for a spin-1 gluon in three jet events : TASSO ퟐ풔풊풏휽풊 풙풊 = 풔풊풏휽ퟏ + 풔풊풏휽ퟐ + 풔풊풏휽ퟑ 풙 − 풙 풔풊풏휽 − 풔풊풏휽 풄풐풔휽෩ = ퟐ ퟑ = ퟐ ퟑ 풙ퟏ 풔풊풏휽ퟏ ퟏ 풅흈 ퟐ휶 풙ퟐ+풙ퟐ Vector : = 풔 ퟏ ퟐ + cycl. perm. of 1,2,3 흈ퟎ 풅풙ퟏ풅풙ퟐ ퟑ흅 (ퟏ−풙ퟏ)(ퟏ−풙ퟐ) ퟏ 풅흈 휶 풙ퟐ Scalar : = 풔 ퟑ + cycl. perm. of 1,2,3 흈ퟎ 풅풙ퟏ풅풙ퟐ ퟑ흅 (ퟏ−풙ퟏ)(ퟏ−풙ퟐ) 3-jet in JADE • The three-jet data at s =30 GeV are consistent with the QCD model of vector gluons and exclude a scalar gluon with a confidence of about 10-4 Measured rates of Neutral Current events 1975: Proposal to transform the new 450 GeV CERN proton synchrotron (SPS) into a proton-antiproton collider (C. Rubbia) p 풑ഥ Van der Meer C. Rubbia Beam energy = 315 GeV total energy in the centre-of-mass = 630 GeV Beam energy necessary to achieve the same collision energy on a proton at rest (E m c2 )2 - p2c2 (630 GeV)2 p Ecosmic = 210 TeV Production of W and Z by quark - antiquark annihilation: u d W u d W - u u Z d d Z Estimate of the W and Z masses (not very accurately, because of the small number of events): 2 2 MW 70 - 90 GeV/c ; MZ 80 - 100 GeV/c too high to be produced at any accelerator in operation in the 1970’s UA1 and UA2 experiments (1981 - 1990) Search for W± e± + n (UA1, UA2) ; W± m± + n (UA1) Z e+e– (UA1, UA2) ; Z m+ m– (UA1) UA2 : non-magnetic, calorimetric UA1 : magnetic volume with trackers, detector with inner tracker surrounded by “hermetic” calorimeter and muon detectors One of the first W e + n events in UA1 48 GeV electron identified by surrounding calorimeters Discovery of W-boson (Phys. Lett. B122(1983), 103) UA1 Paper did not talk much about statistics, but probability of background fluctuation to observe these six event, should be less than 10-7 UA2 : PLB122, 476 110 Discovery of Z-boson (Phys.Lett. B126(1983), 398) • Four event was sufficient to establish a new resonance First Zμμ events in UA1 111 UA2 final results Events containing two high-energy electrons: Distributions of the “invariant mass” Mee 2 2 2 2 2 (Meec ) (E1 E2 ) -( p1 p2 ) c + – (for Z e e Mee = MZ) Events containing a single electron with large transverse momentum (momentum component perpendicular to the beam axis) and large missing transverse momentum (apparent violation of momentum conservation due to the escaping neutrino from W en decay) mT (“transverse mass”): invariant mass of the electron - neutrino pair calculated from the transverse components only MW is determined from a fit to the mT 2 distribution: MW = 80.35 ± 0.37 GeV/c Proton-proton collisions at the Large Hadron Collider Beam size ~ 5.5 cm 15 μm 15 μm 6.51012 eV Beam Energy 3564 (2808/2556) Bunches/Beam 11245 Turns/sec 1.11011 Protons/Bunch 2.11034 cm-2 s-1 Luminosity 24.96ns Proton 6.5 TeV Proton colliding beams << Hz n - 2.710-5 Hz e e q 6 + µ - Bunch Crossing 28.7410 Hz 1 - ~ µ q q Z 9 ~ Proton Collisions 1.410 Hz g p H p p p ~ q Parton Collisions Z m + µ - q ~ 0 m 2 (New) Particle Production µ - ~ (W/Z/Higgs, SUSY, ....) 113 1 0 34 -36 -2 -2 2 N (HZZ*mmmm) = L.s.1.2…1.2…= 2.1 10 (44 10 ) (2.62 10 ) (3.36 10 ) Detector of Today ~77 million electronic channels readout in every 25ns Higgs discovery (CMS, ATLAS 2012) at LHC (ATLAS) Phys. Lett. B716 (2012),1 (CMS) Phys. Lett. B716 (2012),30 115 Nobel Prize in 2013 “For the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider." François Englert Peter W. Higgs High Energy Phycics and the society: some applications • Synchrotron light source producing high energy X-rays Probe superconducting materials • Materials research Beams of photons, neutrons and muons: study materials at the atomic level. • Protein modeling Synchrotron light to solve the 3D structure • Controlling power plant gas emission electron beams control emission of sulphur , nitrogen oxides. • Super conducting accelerators in hospitals short-life radio-nuclides for diagnosis • Hadron therapy Proton and ion beams for the treatment of deep seated tumours. • Positron Emission Tomography (PET) Radioisotopes used in PET-CT scanning • Ion implantation for electronics Build fast transistors and chips for digital electronics • RPC/gaseous detector Used to detect halls inside pyramid, indication of volcanic eruption • Electronic trigger system Fast electronics for communication There is no price of fundamental research • A new born baby : What is the gain of the society ? • Why apple falls to ground ? • Mysterious light passed though most of the object @Strasbourg : X - ray • Stimulated emission of light @Burkley : LASER • Nuclear Magnetic Resonance @ Cornel : MRI Scan • Neutron interaction with heavy atom @Kaiser Wilhelm : Nuclear power • Connecting few Personal Computers @Geneva : Internet • A mold that developed on a staphylococcus culture plate@London : Penicillin - a discovery changed the course of medicine Difference between neutron and neutrino Neutron Neutrino • Mass : 939.5654 MeV • Mass < 2 eV • Discovered in 1932 • Predicted in 1930, but first detection in 1956 • Not a fundamental particle, a • Fundamental particles (there are composite object, made of three quarks three types of neutrino, ne, nm & nt) • Effectively participated in all kind of • Participated only in the weak interactions (interactions of quarks) interaction • Main sources are nuclear reaction, • Main source is the Sun nuclear fission • Most of them absorbed in 10cm of • Require 1019cm of water for the same water absorption (passed through > light years of water without interaction) • Negligible in air • Second abundant particles in the universe (after photon) Sources of neutrinos Atmospheric - cosmic ray interaction in upper atmosphere produce and m whose decay generates roughly two nm for every ne, En 4 -2 -1 GeV, neutrino flux F(ne ) 10 m sec 14 -2 Sun - En 0.1-15 MeV, F(ne ) 6 10 m -1 sec 4 4H He 2e 2n e g Man made sources such as nuclear power 13 -2 -1 reactors - E 0.1-5 MeV, F(ne ) 10 m sec GWth-1 at 1 km and accelerators – E < 200 9 -2 -1 GeV, F(ne ) 10 m sec at 1 km Geo (earth’s crust & interior) – from beta decays of 40K, U, Th chains (contributing 40% heat production in earth) E 0.1-2 MeV, 6 -2 -1 F(ne ) 610 m sec Supernova – exploding stars in cosmos - 99% energy released as neutrinos (Nn over ~10 sec 60 53 10 ) : Eb~10 ergs, En~10-30 MeV -4 Big bang cosmic background - En~ 4×10 eV 330 cm-3 (like 2.7K microwave bkgd) Bethe’s Theory of Stellar Evolution (1934-39) 26 Lsun ~ 4 10 W 2×1038 n/sec 2Lsun 1 10 -1 -2 CNO cycle contributes only Fn 2 710 sec cm 25MeV 4 (1AU) about 1% of the solar luminosity121 4 1 1H + 2 e− → 4 2He + 2 e+ + 2 e− + 2 ν e + 3 γ + 24.7 MeV → 4 2He + 2 ν e + 3 γ + 26.7 MeV Observe neutrino flux is much smaller than predicted 푬흂 > ퟎ. ퟐퟑퟑ 푴풆푽 푬흂 ≥ ퟒ − ퟕ. ퟔ 푴풆푽 (Cherenkov+noise threhold) 푬흂 > ퟎ. ퟖퟏퟒ 푴풆푽 100 ton GaCl3-HCL • Something wrong with target (30.3 ton Ga) in 100,000 gallon C Cl 54m3 tank 50 Kton our understanding of 2 4 water the Sun? • Neutrino oscillation? 123 Atmospheric Neutrinos No second peak due to poor energy resolution Reduction of nμ flux, while detected at large distance. Oscillation dependents on length, L Neutrino decay or de coherence models are not sinusoidal (also not so appealing theoretically) Confirmation of oscillation : SNO PRL, 89 (2002) 011301 - - Elastic Scattering n x e n x e • Charged Current :n - e n e d p p e -1.44MeV • Neutral Current n +n +n : e m t n x d n x p n - 2.22MeV • 7.6s difference, but (90.08.00)% of total 12 nm,t are coming from the Sun (Oscillation) ! 5 Neutrino oscillation & INO • For neutrinos weak Eigen states may be different from mass eigen states. n e n1 cosq n 2 sinq n m -n1 sinq n 2 cosq • In a weak decay one produces a definite weak eigen state -iE1t -iE2t • Then at a later time t, n (t) n1e cosq n 2e sinq ퟏ. ퟐퟕ횫퐦ퟐ퐋 푷 흂 → 흂 ; 푳 = 풔풊풏ퟐퟐ휽 풔풊풏ퟐ 풆 풇 퐄 n e UUUeee123 n1 Angles (mixing) are very n UUU n much different than in m mmm123 2 quark sector nt UUUtt123 t n 3 Pontecorvo-Maki-Nakagawa-Sakata Cabibbo-Kobayashi-Maskawa Physics Nobel Prize 2002 & 2015 2002 : “For pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos." Raymond Davis Jr. Masatoshi Koshiba 2015 : “For the discovery of neutrino oscillations, which shows that neutrinos have mass." Takaaki Kajita Arthur B. McDonald 127 Conclusion The elementary particles today: 3 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 ( ~ 10−16 cm) 12 force carriers (g, W, Z, 8 gluons) Matter Matter Particles + the Higgs spin 0 (fermions) particle (Discovered in 2012) responsible for generating the bosons masses of all vector of Interactions by exchange by particles to Scalar boson Scalar Provides mass mass Provides other particles other Free parameters in the SM • Charged lepton mass 3 • Neutrino section (Still not • Quark masses +6 understood properly) • Quark mixing angles (CKM) +3 – Neutrino mixing angle • Quark CP violation (CKM phase) +1 (PMNS) +3 • U(1) gauge coupling +1 – Neutrino CP violation (PMNS • SU(2) gauge coupling +1 phase) +1 • SU(3) gauge coupling +1 • Neutrino masses +3 • Strong CP phase, q3 +1 • Higgs vacuum expectation value +1 • Total : 19 + 7 = 26! • Higgs boson mass +1 • Total 19