Particle Physics: Status and Perspectives Part 1: Particles

Particle Physics: Status and Perspectives Part 1: Particles
Manfred Jeitler SS 2018

Overview (1) what are elementary particles?
the first particles to be discovered historical overview a few formulas relativistic kinematics quantum mechanics and the Dirac equation common units in elementary particle physics the Standard Model detectors accelerators Often, elementary particle physics is also called “High Energy Physics” because most particles become visible only at high energies, or can only be produced via high energies. Apart from the “physics”, I will also present the basics of the techniques of Elementary Particles. For more in-depth information, there are specialized courses: detectors Manfred Krammer (summer semester)

Overview (2) completing the Standard Model the Higgs boson
the second generation (charm and J/ψ) the third generation (beauty (bottom) and Υ (“upsilon”), top) gauge bosons of electroweak interactions: the W and Z bosons testing at the Precision Frontier: the magnetic moment of the leptons the Higgs boson fundamental symmetries and their violation parity violation CP-violation T-violation

Overview (3) neutrinos and neutrino oscillations
particle physics and cosmology, open questions the Energy Frontier and the Precision Frontier Supersymmetry dark matter gravitational waves slides and formulas at

Literature A few useful books are:
Donald Perkins, Introduction to High Energy Physics Otto Nachtmann, Elementary Particle Physics Robert Klauber, Student Friendly Quantum Field Theory You will find many other good books in your library On recent experiments, much useful information can be found on the internet (Wikipedia, home pages of the various experiments etc.)

What are (elementary) particles?
The ancient Greek philosophized already about elementary particles (ἄτομος – that which cannot be separated into parts, from τέμνω to cut; even today, this idea is valid in a certain sense: atoms cannot be split up by chemical means). Their reasoning was not based on experiment but only on philosophical speculation. (Can one always continue dividing things into smaller parts? Or do we arrive at something that cannot be divided any more? Both concepts seem problematic!) But what is really elementary? Which particles do YOU consider elementary? Where are we now for undividable (elementary) objects?

e- the electron J.J.Thomson 1897
the modern, scientific investigation into elementary particles starts with the discovery of the electron as a particle by Thomson to date, the electron appears as an elementary particle; its size is too small to be measured (with present-day instruments) and is below one millionth of the size of an atom Thomson’s classic experiment showed that the specific charge of the electron was surprisingly large, or conversely, that the electrons mass was very small compared to molecular masses known from chemistry the cathode-ray tube TV set used to be the most wide-spread kind of particle accelerator! J.J. (Joseph John) Thomson: discovered cathode tube rays in 1897 Nobel prize in 1906 „the electron is a particle“ George Page Thomson (son of J.J.): Nobel prize in 1937: the electron behaves like a wave Bohr’s model of the atom, 1913: electrons do not behave like classical particles! --> later development: quantum mechanics name: ἤλεκτρον white gold, amber; from ἠλέκτορ shining sun, shining 1897

J.J. Thomson’s “plum-pudding model” of the atom
... the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification, ... After the carrier of the negative charge had been found, it became obvious that one had to look for the carrier of the positive charge - but it was not obvious that this had to be a “particle” model: electrons and hypothetical positive particles are distributed homogeneously in matter (plum-pudding model)

e- p the proton 1914 Rutherford 1897 1900-1924
classic experiment (Geiger-Marsden experiment, based on Rutherford’s ideas, 1909): alpha-particles (heavy, therefore not influenced by electrons) are shot at a gold foil; surprising result: matter consists largely of empty space, the positive charge and the main part of the matter are concentrated in an extremely small region  nucleus, Rutherford’s model of the atom the positive particle in the nucleus of the hydrogen atom is called “proton” (πρῶτος first) 1914 1897

classic experiment (Geiger-Marsden experiment, based on Rutherford’s ideas, 1909)

Compton’s “plum-pudding model” was replaced by Rutherford’s model of the atom: matter consists largely of empty space.

e- p g the photon 1900-1924 Planck Einstein Compton 1897
the photon, the particle of light, was “rediscovered” (φῶς light) an early theory of light had been Newton’s corpuscular theory this was later discarded in favor of the wave theory of light because only this allowed to explain the effects of interference and refraction in 1900, Planck introduced (much against his will) the idea of quantization of light energy to explain the radiation of hot bodies Einstein generalized this idea and postulated the light quantum called photon, which allowed to explain the photoelectric effect (Nobel prize) In 1924, Compton managed to show that photons colliding with electrons actually behave like particles (“billiards balls”; Nobel prize 1927) quantum theory predicts that waves have properties of particles, and vice versa (duality) Compton 1897

e- g n the neutron p 1932 Chadwick 1914 1897 1900-1924
atomic masses known from chemistry show that in the nucleus there must be, apart from protons, other, neutral particles (so, helium has two electrons and therefore two protons in the nucleus, but four times the mass of hydrogen, which has one proton) this neutral particle was observed by Chadwick by shooting α-particles at beryllium (1932) which emitted a neutral particle - the neutron - and transformed into carbon (Nobel prize 1935) Which other interactions play a role in this context? Which force is it that keeps protons and neutrons together? --> A new fundamental type of interaction: the “strong interaction” we have now got all the particles that constitute ordinary matter! But for some reason, there are many more ... 1932 1897 1914

the positron (anti-matter) p n
g e+ the positron (anti-matter) p n Anderson another phenomenon (nowadays well-know to the man - or the woman - in the street from science-fiction stories) which was first predicted by theoreticians: antimatter when trying to obtain a wave equation of the electron, Dirac found a second solution for this equation, which described a “mirror image” of the electron with positive charge he first interpreted this as “holes” in a sea filled with electrons this “anti-particle” was called the positron and was experimentally found by Anderson in 1932 (cosmic radiation in cloud chamber; then confirmed by shooting gamma rays at thorium carbide, thus creating electron-positron pairs; Nobel prize in 1936) by and by, antiparticles were found also for other particles; today we believe that for all particles there are anti-particles (in some cases, they are identical to the original particle, like in the case of the photon) however, we do not see much antimatter in the universe - why? (Will come back to this later: CP-violation) still, we can now produce anti-particles and store them for days Dirac 1932 1897 1914 1947 1937

Cosmic radiation: first seen in 1912 by Viktor Hess (Austria; Nobel prize in 1936)
during balloon flights he found out that the natural radiation background increases with height; this was rather unexpected because before people had believed it to be caused by terrestrial sources the measurement was done with an electrometer

Low-energy cosmic particles are absorbed in the atmosphere and can only be observed by satellites or high-flying (stratospheric) balloons. Particles of higher energies give rise to atmospheric showers; Cherenkov light and (if the cosmic particle’s energy is high enough) secondary particles can pass the whole atmosphere and reach the ground.

e- g µ the muon p n e+ 1937 Who ordered this ? Hess Anderson,
Neddermeyer e- g the muon p n Who ordered this ? e+ once electron, proton and neutron had been found, all particles were known which seemed to constitute matter it therefore came as a surprise when in cosmic radiation a new particle was observed which seemed to be similar to an electron but was 200 times heavier; this particle was baptized the “muon” (μ) this new particle did not seem to fit into the picture and the question arose: “what is this good for?” atmospheric muons yield a nice illustration of the effect of time dilation predicted by the special theory of relativity: muons come from the decay of pions created in the high layers of the atmosphere by high-energy cosmic particles; the muons’ life time is 2.2 μs; during this time span, even light cannot go further than about 600 meters; this shows that the muon’s “inner clock” is slowed down due to relativistic effects when it travels approximately at the speed of light originally, the muon was thought to be the particle that mediates strong interactions; when this mistake was found out, the particle that really plays this role was called “pi-meson” or “pion” (π); in the older literature, muons were therefore sometimes referred to as “μ-mesons” 1937 1897 1914 1932

muon lifetime muon lifetime ~ 2.2 μs
speed of muons: almost speed of light speed of light = ? path travelled by muons = ?

relativistic kinematics
elementary particles travel mostly at speeds close to speed of light because their masses are small compared to typical energies  (almost) always use relativistic kinematics in particle physics, “special relativity” is sufficient most of the time remember a few basic formulae !

a bit of maths Special Relativity Quantum Mechanics Dirac Equation

relativistic kinematics
1/γ 1

units: energy and mass + - e-
10-4 eV: 3 K cosmic background radiation (~ 0.25 meV) 10-2 eV: room temperature (~ 30 meV) eV: ionisation energy for light atoms (13.6 eV in hydrogen) 103 eV (keV): X-rays in heavy atoms 106 eV (MeV): mass of electron me = 511 keV/c2 109 eV (GeV): mass of proton (~1GeV/c2) ~ 100 GeV/c2: mass of W, Z ~ 200 GeV/c2: mass of top 1012 eV (TeV): range of present-day man-made accelerators 1020 eV: highest energies seen for cosmic particles 1028 eV (1019 GeV/c2): ~ Planck mass + - e- 1V the electron-volt (eV) an electron volt is the energy obtained by an electron when accelerated over a potential difference of 1 volt in elementary particle physics, usual units are GeV (giga electron volt, 109 eV) and MeV (mega electron volt, 106 eV) due to the equivalence of mass and energy in special relativity (‘‘E=mc²‘‘) mass can be measured in units of GeV/c² momentum is measured in units of GeV/c ( E = pc + mc² ! ) the velocity is often measured in units of the speed of light, so c is set to unity (c2 = 1) and mass is sloppily expressed in GeV like “Mach” for planes - only c cannot be exceeded 1 GeV/c2 is about the mass of the proton 1 MeV/c2 is about the mass of two electrons (me = 511 keV) how much do you weigh (in GeV/c2) ? (Avogadro constant!) conversion to macroscopic units: 1 GeV ~ 1.6 * J so, energy scale is smaller by factor of 1012 (if we produce 100 W on a bicycle ergometer) but mass scale is smaller by factor of ~1028, in other words, typically energy per mass is much larger for elementary particles than for macroscopic bodies  “High Energy Physics”

units: speed and distance
velocity: speed of light ~ 3 * 108 m/s ~ 30 cm/ns approximately, all speeds are equal to the speed of light in high-energy particle physics ! all particles are “relativistic” distance: fm (femtometer) 1 fm = m sometimes also called “Fermi” how fast do you drive (in units of c) ? how late can you come to class and still make it plausible that it’s just due to time dilation on the subway? h has the dimension of an action: energy * time = energy * length / speed

relations and constants
waves λ × ν = c ω = 2π ν quantum mechanics h Planck constant (“Planck’sches Wirkungsquantum”) h = h / 2π hν = hω = E numerical survival kit c = h = 1 as long as you need no “usual” units; and then, use: c ~ 3 × 108 m/s speed of light hc ~ 200 MeV × fm ??? protons / kg (~ GeV / kg) Avogadro’s number e = ??? As (coulomb) 1 eV ~ ??? K Boltzmann’s constant 1 eV ~ ??? J 1 Tesla = ??? gauss more accurate value of Boltzmann’s constant: 1 eV = Kelvin

relations and constants
waves λ × ν = c ω = 2π ν quantum mechanics h Planck constant (“Planck’sches Wirkungsquantum”) h = h / 2π hν = hω = E numerical survival kit c = h = 1 as long as you need no “usual” units; and then, use: c ~ 3 × 108 m/s speed of light hc ~ 200 MeV × fm ~ 6 × protons / kg (~ GeV / kg) Avogadro’s number e ~ 1.6 × 10−19 As (coulomb) 1 eV ~ 104 K Boltzmann’s constant 1 eV ~ 10-19 J 1 Tesla = gauss more accurate value of Boltzmann’s constant: 1 eV = Kelvin 1 eV ~ 1.6 * J 1 GeV ~ 1.6 * J

“natural” units c = h = 1  length ~ time ~ 1/energy
c ~ length/time speed of light hc ~ energy × length  length ~ time ~ 1/energy 1 GeV−1 ~ 10−16 m (=0.1 fm) ~ 10−24 s V = -G m1m2 / r gravitational attraction  G ~ m-2 G = MPlanck-2 particles with this mass would at ~proton-size distance have gravitational energy of ~proton mass MPlanck ~ 1019 GeV LPlanck = 1/MPlanck ~ m tPlanck = 1/MPlanck ~ s in daily life: “I live ten minutes away from the university”; 1 km = 15 min more accurately (from hc ~ 200 MeV * fm): 1 GeV−1 = 0.2 fm= 2/3 10−24 s MPlanck = 1.2 * 1019 GeV LPlanck = 1/MPlanck = 1.6*10-31 m tPlanck = 1/MPlanck = 5.4*10-44 s

gravitation is weak! Vgrav = - G m1m2 / r gravitational potential
= - MPlanck-2 m1m2 / r ~ m1m2 / r Velec = (1 / (4πε0) ) q1e q2e / r electrostatic potential = (e2 / (4πε0 hc) ) q1q2 / r = α q1q2 / r α = fine structure constant ~ (1/137) q1q2 / r ~ q1q2 / r  Vgrav / Velec ~ / = 10-36 more accurately (from hc ~ 200 MeV * fm): 1 GeV−1 = 0.2 fm= 2/3 10−24 s MPlanck = 1.2 * 1019 GeV LPlanck = 1/MPlanck = 1.6*10-31 m tPlanck = 1/MPlanck = 5.4*10-44 s

e- g p the pion p n µ e+ 1947 Yukawa Powell 1937 1914 1932 1897
the pion (π) was originally predicted theoretically by Yukawa as the particle mediating the strong interaction according to his theory the strong interaction was postulated as the force that keeps protons (and neutrons) together in the nucleus Yukawa calculated the pion’s mass fairly accurately a particle with this mass was found by Powell in 1947 (he got the Nobel prize in 1950; Yukawa had received the Nobel prize in 1949) at first, the muon found in cosmic radiation had been wrongly identified as this particle general mechanism in elementary particle physics: interactions between particles are mediated by other particles 1947 1914 1937 1897 1932

Force carriers gauge bosons
L.J. Curtis gauge bosons Interaction between particles due to exchange of other (“virtual”) particles

e- g n the neutrino p n µ p e+ 1932 Pauli Reines 1947 1914 1937 1897
in 1930, Wolfgang Pauli postulated a new particle, the neutrino (ν) to explain β-decay: it had been puzzling why the electron‘s energy in β-decay varied strongly in a two-body decay, it should be always the same (due to energy and momentum conservation) an extreme way out was to question energy conservation Pauli‘s solution was that in β-decay a third particle is created which interacts so weakly with other particles and matter in general that it is (almost) invisible 1932 1947 1897 1914 1937

only much later did very challenging and difficult experiments allow to observed neutrinos directly (1959, by Cowan, Reines et al.; Nobel prize for Reines in 1995) this was possible only by using a very strong source of neutrinos, namely a nuclear reactor neutron capture about 5 microsec later even today, neutrino detection is characterized by very low efficiency: few events big and expensive detectors strong sources (nuclear reactors, accelerators, the sun) we are constantly being bombarded by a large number of neutrinos from the sun (~ 7*10^10 ~10^11 /cm2 /sec) and from the earth’s atmosphere (cosmic rays creating pions, with π -> μ + νμ and subsequent μ -> νμ + e + νe) , most of which do not interact at all on earth and pass it without being seen

e- g „strange“ particles p n µ p e+ n 1947-... L Rochester, Butler,
K „strange“ particles p S n p e+ n „strange“ particles were observed which did not fit into the prevailing theoretical picture of the time a “particle zoo” appeared - a cornucopia of elementary particles, which did not seem to be “needed” to explain our world at the time, these particles had only been seen in cosmic radiation and seemed unnecessary for “terrestrial physics” Rochester and Butler, 1947: cosmic radiation in cloud chamber: Λ --> p π ns KS --> π+ π ns what seemed particularly strange was the fact that these particles are often created in collisions but decay slowly production via strong interactions, decay via weak interactions 1897 1914 1932 1947 1937

Too many particles! Wie “zu viele Noten” im Mozart-Film “Amadeus” (Ausspruch von Joseph II.) Das sind nur die Mesonen – nur ein Teil aller subatomaren Teilchen!

E=1eV the particle zoo life time (s) mass (GeV/c2) e- p n m KL D Kc KS
100000 e- p n m KL D Kc KS p0 h t B f J/y 1s 2s 3s 4s w r D* Sc S0 W- n 1s 1 ms E=1eV 1 µs m KL pc Kc Sc 1 ns W- KS B D t 10-15s p0 this map of the ”particle zoo“ shows two important properties of particles: life time (in seconds, on a logarithmic scale!) versus mass (in GeV/c2) for comparison: 1 ns is the clock rate of a computer 1 ns is the time needed by light to fly 30 cm (Lemo cable!) a year has x 107 seconds (~ π x 107 seconds :)! ) we live for ~ 3 x 109 seconds (3 x 109 heart beats) a “moment” (shortest time we can distinguish) is ~ 3 x 10-2 seconds so we live for ~1011 moments lifetime range of particles: 1027 size of proton: ~ 1 fm = m light travels this distance in length/speed = 1 fm / c = m/(3x108 m/s) = 3 x seconds why are there several “islands” in this map? h S0 1s 2s 3s J/y 10-20s D* w f 4s W±, Zo r 10-25s mass (GeV/c2)

e- g p n µ p e+ n „I have heard it said that the finder
In his Nobel prize speech in 1955, Willis Lamb expressed nicely the general attitude at the time: e- g p „I have heard it said that the finder of a new elementary particle used to be rewarded by a Nobel Prize, but that now such a discovery ought to be punished by a $10,000 fine.“ n Lamb p e+ n at the beginning of the 1960ies, about 200 particles had been identified how could one classify all these particles? analogy with chemistry (periodic system of the elements): a large number of elements was reduced to four basic constituents (proton, neutron, electron, and the photon to mediate interactions) „holes“ in the periodic system allowed to predict elements (and their properties) that had not been found previously by the same token, the theories of elementary particles allowed to predict new particles based on the principles that had been found L K 1947 S 1897 1914 1932 1937

The “particle zoo” of the subatomic world
Is there something analogous to the Periodic Table of the elements?

Is there something missing?

The periodic table today

g Wechselwirkungen Teilchen ne nm nt u c t g e m t d s b d u d u d u q
„Leptonen“ „Quarks“ Ladung stark ne nm nt u c t strong g +2/3 electromagnetic g -1 e m t d s b -1/3 d u d u d u weak W, Z q q gravitation ? schwach +1/3 +1 Proton Neutron

g Wechselwirkungen Anti-Teilchen ne nm nt u c t g e m t d s b d u ?
Ladung stark ne nm nt u c t strong g -2/3 electromagnetic g +1 e m t d s b +1/3 d u weak W, Z gravitation ? schwach +1 Pion (p)

D++ u d u s L0 s u K- p0 d c d D+ b  u s S+

g the Standard Model fermions (spin ½) interactions ? ne nm nt u c t m
strong weak gravitation ? W, Z electromagnetic g force carriers = bosons (spin 1) leptons quarks charge ne nm nt +2/3 u c t m -1 e t -1/3 d s b d u d u the Standard Model describes all known particles leptons are elementary particles that can be observed as such quarks are elementary particles that are observed only in combinations of three quarks (“hadrons”) or of a quark and an anti-quark (“mesons”); differently from leptons, they bear the so-called “color charge” both leptons and quarks are “fermions”, i.e. particles with half-integer spin which obey the “Fermi-Dirac statistics”: no two identical particles can occupy the same quantum state (“Pauli exclusion principle”) both leptons and quarks exist in 3 “generations”; only the first-generation particles are stable (except for neutrinos) interactions (4 types) are mediated by particles: “gauge bosons” (bosons are particles with integer spin and obey the “Bose-Einstein statistics”: identical particles tend to flock together in the same location and quantum state) +1 proton neutron baryons

g anti-particles -2/3 +1 +1/3 interactions d u s c b t e ne m t nm nt
leptons quarks interactions charge d u s c b t strong e ne m t nm nt -2/3 ne nm nt u c t strong g g +1 +1/3 e m t d s b electromagnetic weak W, Z gravitation ? for all particles there is an anti-particle with same mass, lifetime, spin etc but opposite electromagnetic charge, opposite “baryon number”, “strangeness” etc, and “complementary” “color charge” (red<->antired(=cyan), blue<->antiblue(=yellow), green<->antigreen(magenta)) by combining a quark with an anti-quark of complementary color charge one arrives at a color-neutral (“white”) state called “meson” while a state consisting of two quarks (or two anti-quarks) would be “colored” and not allowed in the model weak force carriers = bosons (spin 1)

the 4 fundamental interactions
Gravitation Strong Interaction Electromagnetism Weak Interaction Why 4 different interactions? How can we separate them? Gravitation affects all bodies with mass and is always positive (= attractive). However, it is so weak (coupling constant of that for electromagnetic interactions) that in elementary particle physics it can be neglected most of the time (except, maybe, in formation of mini black holes?). Electromagnetism is much stronger than gravitation (in pulling up a needle, a little magnet is stronger than whole earth below!) and has infinite range. It is most of the time not visible in everyday life because positive and negative charges cancel. “Strong interactions” have a very limited range but are by a factor of ~100 stronger than electromagnetism. They were first postulated and found as the force that keeps the nucleons in the nucleus together. Weak interactions are much weaker than strong interactions and have limited range. They become visible in cases where the stronger interactions cannot act for some reason (“violation of conservation laws”). The range of interactions is correlated with the mass of the “gauge boson” that mediates it: photons and (hypothetical) gravitons are massless and the range of their interactions is infinite. The range of “weak interactions” is very short because of the high mass of the mediating bosons (“vector bosons”), W and Z. When assuming that these “virtual” particles travel approximately at the speed of light, the range can be estimated from Heisenberg’s uncertainty relation: Δt * ΔE < h > range ~ h / m This is not a “proof” but experiment shows that this relation is a good approximation!

can now understand reason for “islands” in mass/lifetime plot: states decaying by specific interactions not all interactions can mediate all transitions: certain transitions are “forbidden” for certain interactions general rule: the weaker the interaction, the more transitions can it mediate (otherwise we would not see it!)

lifetime and width remember: hc ~ 200 MeV × fm
due to the uncertainty principle, the lifetime of a state (= unstable particle) and the accuracy, with which its mass (= rest energy) is reproduced at subsequent measurements, are correlated: Δt × ΔE ~ h lifetime can be measured directly for fairly long-lived states ( > s) width can be measured directly for short-lived states (becomes immeasurably small for long-lived states) both properties can always be converted into each other: τ = h / Γ Γ = h / τ remember: hc ~ MeV × fm c = 3 ~ 1023 fm/s  h ~ 2/3  MeV × s

cross section

cross section defined via scattering probability W = n × σ
n ... number of scatterers in beam σ ... cross section of individual scatterer naive picture: each scatterer has a certain “area” and is completely opaque absorption cross section can also be used for elastic scattering ... into certain solid angle dΩ: dσ/dΩ ... or particle transformation differential cross section for a certain reaction unit: “barn”: (10 fm)2 = 100 fm2 = m2 = cm2 The etymology of the unit “barn” is clearly whimsical and jocular—the unit is said to be "as big as a barn" compared to the typical cross sections for nuclear reactions. During wartime research on the atomic bomb, American physicists who were deflecting neutrons off uranium nuclei, (similar to Rutherford scattering) described the uranium nucleus as “big as a barn.” Physicists working on the project adopted the name barn for a unit equal to square centimeters, about the size of a uranium nucleus. Initially they hoped the American slang name would obscure any reference to the study of nuclear structure; eventually, the word became a standard unit in particle physics.

cross section

cross sections at LHC Large Hadron Collider

fundamental interactions
Strong electro-magnetic Weak gravity gauge boson gluon photon W, Z graviton mass ~ 100 GeV range 1 fm  10-3 fm source “color charge” electric charge “weak charge” coupling ~ 1 α ~ 1/137 10-5 10-38 typical σ fm2 10-3 fm2 10-14 fm2 - typical lifetime (s) 10-23 10-20 10-8

Feynman diagrams Very similar Feynman diagrams can depict seemingly different physical processes. Particles flying “forward in time”, i.e. behaving as we would expect them to behave, react similarly to anti-particles moving “backward in time”. (We will revisit these concepts later on.) In the left diagram, two electrons approach each other and scatter elastically from each other by exchanging a virtual photon. In the center, an electron and a positron annihilate with each other, produce a virtual photon, and this transforms again into an electron-positron pair. In the right graph, all electrons transformed into positrons. Two positrons scatter by exchanging a virtual photon. Why are these photons “virtual”? Could they be “real” photons? What is a “virtual” photon?

electron scattering (Bhabha scattering)
Very similar Feynman diagrams can depict seemingly different physical processes. Particles flying “forward in time”, i.e. behaving as we would expect them to behave, react similarly to anti-particles moving “backward in time”. (We will revisit these concepts later on.) In the left diagram, two electrons approach each other and scatter elastically from each other by exchanging a virtual photon. In the center, an electron and a positron annihilate with each other, produce a virtual photon, and this transforms again into an electron-positron pair. In the right graph, all electrons transformed into positrons. Two positrons scatter by exchanging a virtual photon. Why are these photons “virtual”? Could they be “real” photons? What is a “virtual” photon?

Feynman diagrams for electromagnetic interactions
Feynman diagrams are a symbolic representation of the propagation of particles (“real” and “virtual”) in time no more than 3 lines meet in any point (“vertex”, “vertices”) (different from Fermi’s point-like interaction between 4 particles!) Feynman diagrams can be read in any direction particles going forward in time correspond to anti-particles going backward in time usually anti-particles are depicted with arrows “going the wrong way”

Feynman diagrams for Weak interactions
Feynman diagrams describe really “elementary” particles (leptons, quarks, gauge bosons) baryons and mesons are not “elementary”: apart from the quark undergoing a reactions, there are “spectator quarks”

The total neutrino-nucleon scattering cross section scales linearly with energy, just as one would expect it for scattering on point-like constituent particles (“partons”).

The cross section for the formation of hadrons in electron-positron collisions behaves like the one for the formation of a muon pair, i.e. of two point-like (according to present-day knowledge) particles.

experimental setup for measuring
deep-inelastic electron-proton scattering (from Robert Hofstadter’s Nobel prize lecture, 1961) If we can use electrons to "see" protons inside the nucleus, can we also use them to see inside protons? The direct evidence for the existence of quarks inside the proton is provided by deep inelastic scattering. The idea is to accelerate electrons to very high energies, then allow them to interact with a stationary proton, and investigate what happens. But why is this called deep inelastic scattering? At high energies, the wavelengths associated with the electrons are much smaller than the size of a proton. Hence the electrons can probe distances that are small compared with the proton - that is, DEEP within the proton. However, the high energies tend to disrupt the proton, so that it produces several new particles (hadrons). This means the scattering is INELASTIC because the target has been changed in the process. Deep inelastic scattering may be viewed in two ways: as inelastic scattering off a proton because it has constituents inside, or as elastic scattering from one of the constituents inside (ignoring the whole proton and other constituents). We are able to say that the constituents appear to be point-like and so can be considered to be fundamental particles. The “deep-inelastic” scattering of electrons on nucleons allowed to study their structure. Due to their high energies, the electrons can penetrate into the nucleon, the nucleon appears to become “transparent” for the electrons.

color charge Apart from their electric charge, quarks also have “color charge”. The particles which convey this interaction and keep the quarks together are called gluons. color anticolor RED CYAN BLUE YELLOW GREEN MAGENTA In particle physics, color charge is a property of quarks and gluons which are related to their strong interactions in the context of quantum chromodynamics (QCD). This has analogies with the notion of electric charge of particles, but because of the mathematical complications of QCD, there are many technical differences. The "color" of quarks and gluons has nothing to do with visual perception of color; rather, it is a whimsical name for a property which has almost no manifestation at distances above the size of an atomic nucleus. The term "color" itself is simply derived from the fact that the property it describes has three aspects (analogous to the three primary colors), as opposed to the single "aspect" of electromagnetic charge. The color force is much stronger than the electromagnetic force, so particles of like charge can stay together in nucleons and nuclei. Seen from outside, the color of a system must be “white“; this also guarantees integer values of electric charge. Only quarks have color charge, leptons do not feel color.

Free quarks have never been observed, they always appear in bound states (quark confinement).
2 types of bound states are observed: 3 quarks of three different colors: baryons 2 quarks of a color and its anticolor: mesons d u mesons q q baryons q Confinement: a feature of the strong interaction is, that the force does not decrease but increase with distance (not like gravitation or electromagnetism, but rather like a rubber band or spring). So, quarks cannot be separated because when they are far enough from each other, there will be enough energy to create the mass of a quark-antiquark pair, and mesons or hadron will be created (“hadronisation”)

Feynman diagrams for Strong interactions

(Aleph experiment, LEP Collider, CERN, Geneva, Switzerland)
3-jet event (Aleph experiment, LEP Collider, CERN, Geneva, Switzerland) A jet is a narrow cone of hadrons and other particles produced by the hadronization of a quark or gluon in a particle physics or heavy ion experiment. Because of QCD confinement, particles carrying a color charge, such as quarks, cannot exist in free form. Therefore they fragment into hadrons before they can be directly detected, becoming jets. These jets must be measured in a particle detector and studied in order to determine the properties of the original quark.

... ... p+ K-  L0 D+ d u d b u s c b d u d u u u u s u d D++ mesons
baryons u s u d proton neutron D++ L0 the “building blocks” of the Standard Model allow constructing a large number of bound states which correspond to the particles found by experiment ordinary matter consists only of the up and down quarks and the electron. Baryons are the family of subatomic particles with a baryon number of 1. Each baryon has a corresponding antiparticle (anti-baryon) where quarks are replaced by their corresponding antiquarks and their corresponding antiquarks replaced by quarks. Amongst the baryons are the protons and neutrons, which make up atomic nuclei, but many other unstable baryons exist as well. The term "baryon" is derived from the Greek βαρύς (barys), meaning "heavy," because at the time of their naming it was believed that baryons were characterized by having greater mass than other particles. A meson is a strongly interacting boson—that is, a hadron with integer spin. In the Standard Model, mesons are composite (non-elementary) particles composed of an even number of quarks and antiquarks. All known mesons are believed to consist of a quark-antiquark pair—the so-called valence quarks—plus a "sea" of virtual quark-antiquark pairs and virtual gluons. Searches for exotic mesons that have different constituents are ongoing. The valence quarks may exist in a superposition of flavor states; for example, the neutral pion is neither an up-antiup pair nor a down-antidown pair, but an equal superposition of both. Pseudoscalar mesons (spin 0), where the quark and antiquark have opposite spin, have the lowest rest energy. Next lowest in rest energy are vector mesons (spin 1), where the quark and antiquark have parallel spin. Both come in higher-energy versions where the spin is augmented by orbital angular momentum. All mesons are unstable. nucleus atom He nucleus (a-particle) matter

Robert Hofstadter (Nobel prize lecture, 1961)
Robert Hofstadter (February 5, 1915 – November 17, 1990) was the winner of the 1961 Nobel Prize in Physics "for his pioneering studies of electron scattering in atomic nuclei and for his consequent discoveries concerning the structure of nucleons." Robert Hofstadter (Nobel prize lecture, 1961)

e µ ne p nm nm p e- e- e+ e+ p decays & scattering K K p
What do we observe? decays & scattering e ne p decay nm nm  26 ns  2200 ns K p e- e- K scattering e+ e+ p particles may undergo decays, or may interact with each other by scattering (when two particles approach each other closely) scattering may be “elastic“ (particles are only deviated from their trajectory) or “inealastic”: particles may transform into other particles, new particles may be created out of the collision energy (which will mostly be unstable): this is like a “mini big bang” - particles that “died out” long ago can be “revived” scattering may appear in cosmic radiation or in accelerator experiments accelerators create high-energy particles in a confined space, where they can be easily investigated p

g Astro Accelerator the Standard Model fermions (spin ½) interactions
strong weak gravitation ? W, Z electromagnetic g leptons quarks charge ne nm nt +2/3 u c t Astro Accelerator m -1 e t -1/3 d s b the Standard Model describes all known particles leptons are elementary particles that can be observed as such quarks are elementary particles that are observed only in combinations of three quarks (“hadrons”) or of a quark and an anti-quark (“mesons”); differently from leptons, they bear the so-called “color charge” both leptons and quarks are “fermions”, i.e. particles with half-integer spin which obey the “Fermi-Dirac statistics”: no two identical particles can occupy the same quantum state (“Pauli exclusion principle”) both leptons and quarks exist in 3 “generations”; only the first-generation particles are stable (except for neutrinos) interactions (4 types) are mediated by particles: “gauge bosons” (bosons are particles with integer spin and obey the “Bose-Einstein statistics”: identical particles tend to flock together in the same location and quantum state)

Particle Physics: Status and Perspectives Part 1: Particles

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Particle Physics: Status and Perspectives Part 1: Particles

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Presentation on theme: "Particle Physics: Status and Perspectives Part 1: Particles"— Presentation transcript:

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