Par$cles and Interac$ons

Transcription

1 Par$cles and Interac$ons Prof. Marina Cobal Fisica Sperimentale Nucleare e Subnucleare 2016/2017

2 What is the world made of? In the ancient *me: 4 elements 19 century atoms Beginning 20 th century electrons, protons, neutrons Today quarks and leptons

3 All things around us are made of atoms Human Hair ~ 50 µm = m = m Magritte Atom ~ m = m

4 The atom in the 20 th century... Atoms reacts through chemical reactions More than 100 atoms known (H, He, Fe ) The internal structure is not well known

5 Table of elements Atoms are grouped in families which present similar proper*es The elements table is done This symmetry suggests a structure with simpler cons*tuents.

6 Atomic Model With the experiments now we are able to break up atoms Light par*cles (electrons) with nega*ve charge around a posi*ve and heavy nucleus Prac*cally, the atom is empty!

7 Rutherford: atoms are not elementary par*cles! 1911 Rutherford found a nucleus in the atom by firing alpha particles at gold and observing them bounce back Precursor of modern scattering experiments at accelerator

8 Atoms Atoms are all similarly made of: - protons and neutrons in the nucleus - electrons orbiting around proton The electron was the first elementary particle to be discovered (JJ Thomson 1897) Protons, neutrons are made up of quarks electron neutron

9 Nucleus The nucleus is small and dense. For a while it was thought to be pointlike. However, there were so many different nuclea as many atoms Simplifica*on: all nuclea are made of neutrons and protons!

10 New Par$cles Collisions of eleyrons and nuclea in the cosmic rays and in the par*cle accelerators at the beginning of the 30ies, brought to the discovery of many other par*cles. Some of them were predicted, other were discovered as surprises, being completely unexpected. At the beginning it was thought that all these par*cles were fundamentals.

11 At the beginning just a few And many others

12 Introducing the quarks..

13 "Young man, if I could remember the names of these par8cles, I would have been a botanist! E.Fermi to his student L. Lederman (both Nobel laureates) The Par*cle Physicist s Bible: Par*cle Data Book hyps://pdg.lbl.gov Most particles are not stable and can decay to lighter particles..

14 Quarks Today we know that also protons and neutrons are not fundamental units. They are made of smaller par*cles called quarks For the moment looks like quarks are pointlike

15 Sub-atomic dimensions

16 The Modern Atom A cloud of electrons moving constantly around the nucleus Protons and neutrons moving in the nucleus Quarks moving in protons and neutrons

17 What is fundamental? Physicists have found hundreds of new par*cles. Today we know that most of them are not fundamental A theory has been developed that seems to explain quite well what we do observe in nature: the theory is called Standard Model This model includes 6 quarks, 6 leptons and 13 par*cles which carry the force in between quarks and leptons.

18 Is the whole Universe made only of quarks and electrons? No! There are also neutrinos! ν Electron, proton and neutrons are rarities! For each of them in the Universe there is 1 billion neutrinos Neutrinos are the most abundant matter-particles in the Universe! Within each cm 3 of space: ~300 neutrinos from Big Bang ν ν ν ν ν νννννννν νν ν ν νν ν ννννννν ν νν νν νν ν ν ν ν ν νν ν ν νν ν νννννν ν νν νν νν ννννννν νν νν ν ννν ννννννν νν νν ν ννν ννννννν 1 cm 1 cm Neutrinos are everywhere! in the outer space, on Earth, in our bodies..

19 Neutrinos get under your skin! Every cm 2 of Earth surface is crossed every second by more than 10 billion (10 10 ) neutrinos produced in the Sun Within your body at any instant: roughly 30 million neutrinos from the Big Bang neutrinos per second from Sun are zipping through you ν No worries! Neutrinos do not harm us. Our bodies are transparent to neutrinos

20 Quarks and Leptons Structureless building blocks down to a spa*al extension of m Well defined spin and charge Leptons have well defined mass as well Low Mass MaYer Cons*tuents under ordinary condi*ons (low energy/t) Cons*tuents of unstable par*cle (produced at high energy, in astrophysical systems). They decay to lower mass par*cles High Mass 20

21 Families The 6 quarks and leptons are organized in families The 3 families have analogies Quarks have charge +2/3 e -1/3. Leptons have charge -1 and 0.

22 Fermions Elementary par*cles: fermions 1 st generation 2 nd generation Why 3 families? Are there more? 3 rd generation Quarks Massa (MeV) 2/3-1/3 Charge (e) 2/3-1/3 Leptons

23 What is the world made of? Real world is not done by single quarks Quarks exist only in groups, to form the so-called hadrons (protons and neutrons are hadrons) Example: a proton is made of two quarks of up type and one quark of type down. The mayer around, and even each of us, is made of quarks up and down and of electrons.

24 A reduc*onist example: the Deuterium Atom p = ( u, u, d) n = ( u, d, d) m m

25 And the leptons? There are 6 leptons: 3 charged and 3 neutral. They look like pointlike par*cle without an internal structure. Electrons are the most common and are found in the ordinary mayer. Muons (µ) and taus (τ) are heavier and charged as the electrons. Neutrinos (ν) have no charge and they have an extremely small mass.

26 Wave-par$cle duality of Nature Central concept of quantum mechanics: all particles present wave-like properties De Broglie showed that moving particles have an equivalent wavelength λ Not only light has a dual nature λ λ 1 p So high momentum gives us short wavelengths so we can make out small details Electron Microscope Image Gold atoms (0.2 nm apart) Example: electron microscope Copyright FEI

27 Energy and distance

28 Probing the proton

29 Quarks detected within protons Freeway miles long accelerator End Station A experimental area Stanford (SLAC), California, late 1960s Fire electrons at proton: big deflections seen!

30 Quark masses Mass (GeV) E= mc 2 1 proton mass ~ 1GeV (10-27 Kg) Quarks GeV Top (discovered 1995) Up Down Strange Charm Bottom Top The mass grows larger in each successive family

31 Quantum Mechanics Atoms and par*cles behavior is described by the Quantum Mechanics Some proper*es, like energy for example, can only assume some discrete values, they don t belong to a con*nuum Par*cle proper*es are described by these values (quantum numbers). Some examples: Electric charge Colour charge Flavour Spin

32 Pauli principle We can use the par*cle quan*s*c proper*es to classify them Some par*cles, called fermions, obey to the Pauli principle. While some others the bosons do not.

33 The Spin/Sta$s$cs Theorem Half-integer Spin Particles 1 2 3!,!,... Fermions 2 Fermi-Dirac Statistics (W. Pauli, 1940) Bose-Einstein Statistics Integer Spin Par*cles 0,!,2!,... Bosons Consequences of the Spin/Sta*s*cs Theorem: formal: wave func*ons, field operators commuta*on rules experimental: nuclear and atomic structure, Bose-Einstein condensates 33

34 Fermions and Bosons

35 The Wave Func*on must have the correct symmetry under interchange of iden*cal par*cles. If 1, 2 are iden*cal par*cles : 2 1, x2) ψ ( x2, 1) ψ ( x = x 2 (probability must be conserved upon excange of iden*cal par*cle) ψ +ψ (1 2) Iden*cal Bosons ψ ψ (1 2) (symmetric) Iden*cal Fermions (an*symm.) A consequence of the Spin/Sta*s*cs Theorem: for two iden*cal Fermions 1,2 in the same quantum state x: ψ(x 1, x 2 )=ψ(x 2, x 1 )= ψ(x 1, x 2 ) ψ(x 1, x 2 ) = 0 Because identical Spin/Statistics Theorem Pauli Exclusion Principle!

36 Par$cles/An$par$cles: the birth of Par$cle Physics 1928: Dirac Equa*on, merging Special Rela*vity and Quantum Mech. A rela*vis*c invariant Equa*on for spin ½ par*cles. E.g. the electron µ ( i m) ψ = γ µ E>0, s=+1/2 E>0, s=-1/2 E<0, s= +1/2 E<0, s= -1/2 0 Rest frame Upon solutions: reinterpreta*on 4 independent states: of nega*ve-energy states as an*par*cles of the electron: Electron, s=+1/2 Electron, s=-1/2 Positron, s=1/2 Positron, s=-1/2 The positron, a par*cle iden*cal to the electron e - but with a posi*ve charge: e +. The first predic*on of the rela*vis*c quantum theory.

37 An$-maXer For every fundamental par*cle of mayer there is an an*-par*cle with same mass and proper*es but opposite charge Matter Anti-Matter 0 ν e u +2/3 ν e 0 u -2/3 Bar on top to indicate anti-particle e - -1 d -1/3 e /3 d positron Correspondent anti-particles exist for all three families Anti-matter can be produced using accelerators

38 An$par$cles

39 MaXer-an$maXer pair crea$on Electron-positron pair created out of photons hitting the bubble-chamber liquid Example of conversion of photon energy into matter and anti-matter Matter and anti-matter spiral in opposite directions in the magnetic field due to the opposite charge Energy and momentum is conserved

40 4 Forces There are 4 fundamental interac*ons in Nature All forces can be brought back to these interac*ons The gravity is ayrac*ve, all other forces can also be repulsive The interac*ons are also responsible of the nucleus decays

41 How do par$cles interact? Objects can interact without being in contact How do magnets ayract or repulse? How does the Sun ayract the Earth? A force is something which is propaga*ng in between objects

42 The concept of Force In Classical Physics : Instantaneous action at a distance Field (Faraday, Maxwell) In Quantum Physics : Exchange of Quanta Inverse square law F = k r 2

43 Constituents properties: Quarks: Electric charge Color Effective mass Spin (1/2) Leptons: Electric charge Mass Spin (1/2) All Contituents (Quarks, Leptons) are Fermions. 3 families Costituents of Matter Force Carriers 43

44 Forces in Nature Force Intensity Carriers Happens in Strong Nuclear ~ 1 Gluons (massless) Atomic nuclea Elettromagnetic ~ 10-3 Photons (massless) Atomic levels Weak Nuclear ~10-5 W +,W -,Z 0 (heavy) Beta radioactive decay Gravitation ~10-38 Gravitons (?) Heavy bodies

45 Classical/Quantum Force Let us consider two par*cles at a distance r.. r If a source par*cle emits a quantum that reaches the other par*cle, the change in momentum will be: r Δp! And since: c Δt r Δp! Δt r c r F = Δp Δt k r We have: 2 45

46 Range of interac$ons

47 Forces and distances R > 10 6 m (gravita*onal force) R ~ m (forza elettromagne*ca) R ~ m (forza forte)

48 Electromagne$sm The electromagne*c forces are such that opposite charges ayract and equal charges repulse The force carrier is the photon (γ) The photon is massless and move at the speed of light

49 Electromagne$c force The repulsive force that two approaching electrons feel e - e - Photon is the particle associated to the electromagnetic force smallest bundle of force Photon

50 Photon exchange Feynman Diagram e- e- γ e- e-

51 Electromagne$c residual force Normally the atoms are neutral having the same number of protons and neutrons The charged part of an atom can ayract the charged part of another atom The atoms can link themselves to form molecules

52 Why nuclea do not explode? A heavy nucleus contains many protons, all with posi*ve charges These protons repulse each others Why the nucleus does not explode?

53 Strong force In aggiunta alla carica eleyrica, i quarks portano anche un nuovo *po di carica, deya carica di colore La forza tra le par*celle che hanno carica di colore e deya forza forte

54 The gluon The strong force keeps the quark together, to form the hadrons. The force carriers are the gluons: there are 8 different gluons The strong force acts only on short distances

55 Colour and an$-colour There are 3 colour charges and 3 an*-colour charges Note that these colours are not at all related with the standard colour and with the visible light. It is just a way to describe the physics (quantum numbers)

56 Quarks and colour All quark flavours come in 3 versions, called colours u u u up d d d down +2/3-1/3 Quarks combine together to form colourless particles - Baryons (three quarks: red+ green + blue = white) proton p Strong forces glue quarks together in bound states - Mesons (quark-antiquark pair) such as red+anti-red u-ubar state pion π u u

57 Building more par$cles B mesons (bq) c c J/ψ b u B - b d u b B + d b b b Y B 0 B 0 Many more mesons and baryons

58 Coloured quarks and gluons Every quark has one of the three colour charges and every an*quark has one of the three an*colour charges Barions and mesons are neutral in colour

59 Quarks confinement The colour force increase with the distance Partciles with colour charge cannot exist as isolated par*cles Quarks are confined with other quarks to form hadrons Hadrons are neutral in colour

60 Quarks emit gluons When a quark emits or absorbs a gluon, the colour of the quark changes, in order to save the colour charge A red quark emits a red/an*-blue gluon, and becomes blu

61 Strong force: gluons Gluons interact with quarks Gluons interact with other gluons

62 Quark confinement There are no free quarks, quarks and antiquarks are confined in colourless doublet (mesons) or triplets (baryons) by the exchange of gluons Deca y Z 0 Gluon hold quarks together as they move further apart until the gluon connection snaps, and other quark-antiquark pairs are created out of the energy released The new quarks bound to the old quarks and form new mesons

63 Residual strong force The strong force between the quarks of a proton and the quarks of another proton is strong enough to win on the repulsive electromagne*c force

64 Weak force Weak interac*ons are responsible for the decays of heavy quarks and leptons Example: the neutron decays in a proton + electrons + neutrino This explains why all mayer is composed by the lighter leptons and quarks

65 Weak force: beta decay n p Antineutrin o Electron

66 Neutron β-decay At quark level: d u e - ν e u d d n 15 min u u p d e - ν e A (free) neutron decays after 15 min Long life time (15min is an eternity in particle physics!) weak

67 Electroweak force In the SM the electromagne*c and weak forces have been unified in a single electro-weak force At very short distances (~10-18 meters), the weak and electromagne*c interac*ons have the same intensi*es The carriers are photons, W and Zs.

68 Weak force: W -,W +,Z 0 β-decay n peν e W - Electric charge conserved at each vertex

69 Muon decay Example of a par*cle decay Here the produced par*cles are not pieces of the ini*al par*cle, but are completely new ones.

70 How does a par$cle decay? When a par*cle decay, it is transformed in a lighter par*cle and in a par*cle which is one of the carriers of the weak force (the W boson) A par*cle decays if its total mass is higher than the sum of the masses of the decay products and if there is a force which mediates the decay

71 Missing mass In most of the decays, the par*cles and the nucleus which are leo have a total mass smaller than that of the ini*al par*cle or of the origina*ng nucleus The missing mass it is transformed in kine*c energy of the decay products

72 The unstable nucleus We saw that strong forces keep the nucleus together, against the repulsive force between protons. However, not all nuclea are stable Some of them decay The nucleus can split in smaller nuclea This happens for example in a nuclear reactor

73 Virtual Par$cles Par*cles decay via par*cles which are force-carriers In some cases, a par*cle may decay via a force-carrier that is more massive than the ini*al par*cle The force-carrier par*cle is immediately transformed into lower-mass par*cles The short-lived massive par*cle appears to violate the law of energy conserva*on

74 Annihila$on Annihila*ons are not decays but, in the same way, they take place thanks to virtual par*cles The annihila*on of light quarks at high energies can bring to the produc*on of heavy quarks in the laboratory This bubble chamber shows an an*-proton which hits against a proton, annihilates and produces 8 pions. One of the pion then decays in a muon and a neutrino (neutrino does not leave any trace)

75 Electron-Positron annihila$on

76 And the gravity? Gravity is very weak Is important at macroscopic distances The force carriers (gravitons) is predicted by theory but not yet observed

77 Mysteries and failures of SM The SM is a theory of the Universe It gives a good descrip*on of the phenomena which we observe experimentally Under many respects is not complete: why there are 3 genera*ons? What is the dark mayer? As Enstein has extended the laws of mechanics of Newton with the Rela*vity theory, we now have to go beyond the SM We have to do it to explain masses, gravity etc..

78 Three families There are 3 families of fundamental par*cles Why only 3? And why we just see one of them in the real world?

79 What can we say about mass? The SM cannot explain why a given par*cle is characterized by its mass. Physicists invented a new field, called Higgs field, which interacts with all the other par*cles to give their masses The Higgs boson has been discovered with the LHC experiment in 2013 aoer ~ 70 years of search!.

80 Beyond the Standard Model:Unifica*on of forces ELECTRO- MAGNETIC GRAVITY UNIFIED FORCE? STRONG WEAK Looking for a simple elegant unified theory

81 Theory of Grand Unifica$on It is believed that a GUT will unify the strong, weak, and electromagne*c forces. These 3 forces will then be visible as different manifesta*ons at low energy of a unique force The 3 forces will unify at a very high energy

82 Supersymmetry Some physicists, in the ayempt to unify gravity with the other fundamental forces, have suggested that every fundamental par*cle should have a shadow par*cle. It is more than 20 years that we are looking for these supersymmetric par*cles!

83 Today, physics has a theory for quantum mechanic, for rela*vity and for gravity, but these 3 theories are not unified.. If we would live in a world with more than 3 spa*al dimensions, maybe this problem could be overpassed. The string theory suggests that in a world where there are the 3 standard dimensions and some addi*onal (but smaller) dimensions, the par*cles would be like strings.

84 Extra Dimensions The String theory requires more than just 3 dimensions These extra-dimensions can be so small that we cannot see them Experiments now look for evidence of these extra-dimensions

85 Dark MaXer It seems that the Universe is made by a different mayer than our Sun and stars The dark mayer ayracts normal mayer, but it has not been iden*fied yet

86