From Nucleus to Quarks

Transcription

1 From Nucleus to Quarks

2 Rutherford, having discovered the nucleus, next observed the first laboratory-initiated nuclear reaction in α N 7 1 p O 8 In terms of nuclei this can be described as:- He + N H + O Rutherford coined the term proton for the positivelycharged nucleus of hydrogen. He regarded it as a fundamental building block of matter. The subscripts in the top reaction refer to the electric charge of each nucleus. Charge is conserved. The superscripts refer to the relative mass of each nucleus. Mass is conserved. Rutherford speculated that the differences between mass and charge could be supplied by an uncharged (and as yet undiscovered) particle which he named the neutron.

3 Following Rutherford s work with α-particles, many other researchers around the world started to try to initiate nuclear reactions. Irene Curie (Marie Curie s daughter) and her husband, Frederic Joliot, showed that radiation from Beryllium bombarded by α-particles could penetrate lead foil and ejected 5.7 MeV protons from paraffin.

4 In 1932, James Chadwick, who was a former student of Rutherford, correctly deduced that the new radiation was a beam of particles with the mass of the proton, but with no electric charge. He realized that it was the neutron. Their presence in the nucleus accounts for the discrepancies between mass and charge. The neutron also gives rise to isotopes, nuclei of an element with the same charge but with different masses.

5 The fundamental particles (so far) Electron: charge -1, doesn t feel strong force Proton: charge +1, feels strong force Neutron: charge 0, feels strong force Positron: (the anti-electron) Same mass and opposite charge as the electron. Predicted in 1928 by Dirac based on relativistic generalization of the Schrodinger equation. Discovered in Cosmic Rays in (All particles have antiparticles. The anti-proton was discovered in 1956)

6 In 1932, Carl D. Anderson discovered the positron in a cosmic ray shower using a cloud chamber in a magnetic field. This verification of Dirac s theory earned Dirac the Nobel prize in 1933 and earned Anderson the prize in This firmly established the existence of antiparticles. 4

7 Cosmic Rays Cosmic rays are very energetic particles, mostly protons, that come from interstellar space. They collide with particles in the earth s atmosphere, producing showers of very high energy particles. Their energies can be as high as ev, about a billion times the highest energy human-built accelerator.

8 The fundamental particles (so far) (continued) Neutrino: charge 0, doesn t feel strong force. Predicted by Pauli in 1930, in order to conserve momentum in nuclear b decay. Discovered in Photon: charge 0, associated with the electromagnetic force The photon carries or mediates the EM force by being exchanged virtually between charged particles. This is represented in Feynman diagrams: t e - g e - e - e -

9 The Prediction of the Pion In 1935, based on an analogy with the photon as the mediator of the EM force, Yukawa argued that there also should be a particle that mediates the strong force. The mass of Yukawa s particle (the π-meson or pion) can be estimated by the uncertainty principle: Range of nuclear force is Δx ~ 1 fm A pion travels this distance in Δt ~ Δx /c ==> 3.3E-24 sec Uncertainty in energy necessary for pion to exist for this time is: ΔE ~ ħ/2xδt = 6.58E-16/2x3.3E-24 = ~100 MeV ==> Mp = ~100 MeV/c 2 Hideki Yukawa

10 (note: m p > m p > m e ) t p p n p n Only 2 years after Yukawa s prediction, a new particle was discovered in cosmic rays with just the right mass. But it was not Yukawa s particle!!! (more on this later.)

11 Yukawa s pion finally discovered in in cosmic rays. It comes in three varieties: Charged pions p ±, with charge ±1 and mass 140 MeV/c 2. They are antiparticles of each other. They live with a mean lifetime of 2.6x10-8 seconds before decaying to lighter particles. The neutral pion p 0, with charge 0 and mass 135 MeV/c 2. It is its own antiparticle. It lives about 8.4x10-17 seconds before decaying into two photons. p p 0 n n p ± p p n p n

12 More Particles Muon discovered. Its mass was 106 MeV/c 2. (just right for Yukawa s particle ) But subsequent experiments showed that it did not interact strongly, passing easily through dense matter. (not right for Yukawa s particle) In many ways the muon (charge ±1) behaves like a heavy electron. Who ordered that? - I.I. Rabi

13 Many other new particles found in cosmic rays: K-meson (Kaon) and the L-Baryon (heavier than the proton). These had some Strange properties, such as unexpectedly long life-times. In 1950 s more discoveries: S-Baryons and h-mesons, and many more! The particle zoo is getting crowded! Some organization is needed.

14 However, before discussing the classification of elementary particles, I want to introduce you to some important developments that had been made in two fields since the days of Rutherford, Geiger and Marsden. Accelerators: Most nuclear reactions are initiated by particles (protons or α-particles) that had sufficient energy to overcome the Coulomb repulsion of the target nucleus. Rutherford had been dependent on naturally-occurring radioactive decays for his nuclear probes. But these typically have low rates and insufficient energy to probe deeply into the nucleus. More intense and energetic probes became available using electric field to accelerate the charged projectiles. Detectors: Those long hours waiting to dark-adapt before counting infrequent scintillations from the Zinc Sulfide screen convinced Marsden and, particularly Geiger, that there had to be a better way! Elementary particles are, of course, extremely small submicroscopic. Their presence and their properties can only be inferred from their interactions in matter. Charged particles passing through matter produce ionization or excitation of the atoms in the detector.

15 Accelerators The first nuclear reaction initiated by an accelerator was performed by John Cockcroft and Ernest Walton (again in Rutherford s group at Cambridge). The Cockcroft- Walton accelerator was a complicated arrangement of transformers, diode rectifiers, and capacitors. The energy obtained was about 500 kev. A rival electrostatic accelerator of the 1930 s was the van de Graaff generator, which made use of a continuous moving belt carrying charge to the center of a metal dome. Such an accelerator could accelerate particles to over 1 MeV.

16 A major breakthrough in accelerator design was the invention of the cyclotron by Ernest Lawrence. A magnetic field was used to bend a charged particle into a circular orbit. A relatively low voltage could be used to accelerate the particle which then traveled in a circle so that the same voltage could be used again (and again). As the energy and the velocity of the particle increased, the radius of the circle increased but the period of the orbit remained the same. Cyclotrons enable particles to be accelerated up to energies as high as about 10 MeV.

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18 The energy upper limit of cyclotrons is set by relativity. When the velocity of the particle starts to approach the velocity of light, the time to complete a full revolution increases and so the frequency of the accelerating voltage has to drop to compensate. In this case, the accelerator is called a synchrocyclotron. The MSU cyclotron is actually a synchrocyclotron with a maximum energy of 1200 MeV (or 1.2 GeV). An alternative approach that permits a considerable increase in energy is the synchrotron invented in 1945 by McMillan and Veksler. As the energy of the particle is increased, it is constrained into an orbit of constant radius by an ever-increasing magnetic field in a ring of magnets. The accelerators at Fermilab and at CERN are synchrotrons.

19 Until the 1960 s nuclear collisions consisted of accelerated particles striking stationary fixed targets. This is not as efficient as when both particles in the collision are accelerated in opposite directions. This process occurs in particle colliders where the beams of particles rotate in opposite directions in an accelerator.

20 Detectors Charged particles passing through matter produce ionization or excitation of the atoms in the detector. Ionization means that the particle knocks atomic electrons from the material of the detector. Thus there is a trail of negative electric charge that appears along the track of the high energy particle. Excitation means that the atom gains energy from the passage of the high energy particle. Often it releases this energy as a flash of visible light. Scintillation counters: usually a plastic sheet which gives a flash of light when a charged particle traverses it. The light is made to fall onto a photomultiplier tube, which uses the photoelectric effect to turn the light into an amplified electrical signal so that the signal can be used or recorded, for example in a computer. In a recent variation, the scintillating material can be embedded in very thin light fibers that permit the light to be brought out from the detector and away from high radiation environments or troublesome magnetic fields.

21 The Geiger counter: this usually consists of a chamber containing a gas at low pressure and a thin, insulated wire charged to a high positive voltage. The high energy particle ionizes electrons in the gas and the negative charge is attracted to the positively charged wire. In turn this charge gains energy as it moves towards the wire and becomes capable of ionizing even more charge. The passage of a single charged particle produces an avalanche of charge which can be recorded or made audible as a click. The Cloud chamber was invented by C.T.R. Wilson. It consists of a gas under pressure maintained by a piston. If the piston is suddenly withdrawn, the gas expands and condensation takes place. Condensation droplets are formed preferentially along the tracks of the ionizing particles. Discovery of the positron C.T.R. Wilson Nobel Prize 1927

22 The Bubble Chamber was invented by Donald Glaser (at the University of Michigan) in It is based on a similar principle to the cloud chamber except that in this case the pressure on a liquid is suddenly reduced so that it starts to boil. Again, the preferred sites for the bubbles are along the tracks of the ionizing particles. The most common liquid used is hydrogen which boils at -411 o F. Bubble chambers are usually placed in a large magnetic field so that particles will be bent giving information on both the charge of the particle and its momentum. Donald Glaser Nobel Prize 1960

23 The Spark Chamber consists of a gas volume sandwiched between metal plates that are alternately charged. The voltage between the plates is adjusted so that sparking is about to occur. If a charged particle passes through the gas, the ionization causes a spark at the location of the track. Successive plates will enable the trajectory of the particle to be seen and photographed. A variant is to replace some of the metal plates with a wire grid. Sparks take place on individual wires that can be recorded directly into the computer via fast electronics the wire chamber. A more recent variation, the drift chamber, measures the time taken for the ionization charge to drift to the nearest wire. With modern electronics, this measurement can yield a very precise determination of the particle s trajectory.

24 Forces Gravity: Important in everyday lives and in astronomical phenomena, but negligible for elementary particles. Electromagnetic: Electricity and Magnetism unified into a single fundamental interaction by Maxwell. The force carrier is the photon, which can extend over long range. Strong: Holds protons and neutrons inside nuclei. Very strong, but short range. Pion can be considered to carry the force, but a more fundamental description will come later. Weak: A very short range force, which is responsible for b-decay of nuclei, and the decay of many other elementary particles.

25 Classification of Particles There are three broad categores: Leptons: Particles such as electrons, muons, and neutrinos, which do not feel the strong force. Leptons always have spin 1/2 h. Hadrons: Particles which do participate in strong interactions. (any spin) Gauge particles (Gauge Bosons): The particles responsible for carrying the forces. The only one we have met so far is the photon.

26 Q uick uiz What type of particle is the neutron? (A) (B) (C) (D) (E) Lepton Hadron Gauge boson Heavy neutrino Anti-proton

27 Q uick uiz What type of particle is the neutron? (A) (B) (C) (D) (E) Lepton Hadron Gauge boson Heavy neutrino Anti-proton

28 Leptons There are believed to be six leptons (along with their associated anti-leptons). They come in three generations (pairings of a charged lepton and a neutrino). Generation Particle Charge Mass e MeV/c 2 n e 0 ~ 0 m MeV/c 2 n m 0 ~ 0 t MeV/c 2 n t 0 ~ 0

29 The t (Tau) lepton was discovered by Martin Perl and collaborators at the Stanford Linear Accelerator (SLAC) in Heavier charged leptons decay to the lighter ones. For example: m - Æ e - + n m + n e (The t can also have hadrons in its decay.) In the last couple years it has been verified that neutrinos do have a mass (although very small). This was seen indirectly through oscillations from one type of neutrino to another. These oscillations can only occur if the neutrinos have nonzero mass.

30 Hadrons Hadrons feel the strong force. They can be further subdivided into Baryons and Mesons. Mesons are hadrons with integral spin (mostly 0 or 1, but sometimes 2 or higher). Most have masses between that of the electron and proton. The pion (p), Kaon (K), and eta meson (h) are examples. Baryons are hadrons with 1/2 integral spin (mostly 1/2, but sometimes 3/2 or higher). The lightest baryons are the nucleons (proton and neutron).

31 Force Particles Each of the four basic forces is mediated by the exchange of a force particle. Force Electromagnetic Particle photon (g) Strong nuclear pion (p) * Weak nuclear Gravity Intermediate Boson (W ±, Z) Graviton * The modern, more fundamental formulation of the strong force has the gluon (g) as the carrier, as we shall see.

32 W particles were first proposed by Fermi. e - n e p t W - n W ± and Z particles discovered at CERN. Gravitons not yet observed directly.

33 Q uick uiz What type of particle is the electron? (A) (B) (C) (D) (E) Lepton Meson Baryon Gauge boson Anti-proton

34 Q uick uiz What type of particle is the electron? (A) (B) (C) (D) (E) Lepton Meson Baryon Gauge boson Anti-proton

35 Q uick uiz What type of particle is the neutron? (A) (B) (C) (D) (E) Lepton Meson Baryon Gauge boson Anti-proton

36 Q uick uiz What type of particle is the neutron? (A) (B) (C) (D) (E) Lepton Meson Baryon Gauge boson Anti-proton

37 Conservation Laws Certain quantities are always conserved. In addition to energy, momentum, and electric charge, they are: Baryon number: The generalization of conservation of nucleons (each with baryon number 1). Anti-Baryons have baryon number -1. Mesons, leptons and gauge particles have baryon number 0. e - + p Æ e - + p + n + n Baryon # = (-1) Lepton number: The number of leptons of each generation is conserved. For example, e - (electron) and n e have electron number 1, e + (positron) and n e have electron number -1.

38 Example, Muon Decay m - Æ e - + n m + n e muon # 1 = electron # 0 = (-1)

39 Strangeness: The K-mesons and L- baryons had strange properties. They were almost always produced in pairs, and their lifetimes were exceptionally long. These properties could be explained by a new quantum number, Strangeness. Strangeness is conserved by the EM and strong force, but not by the weak force. These particles are produced strongly, in strange - antistrange pairs. But they decay weakly with long lifetimes. By plotting the strangeness vs. EM charge, many regularities were observed.

40 Mesons Strangeness K 0 p - p 0 h p + K - K 0 K EM Charge Strangeness n p S - S 0 L 0 S Baryons EM Charge

41 Quarks Early 1960 s - Murray Gell-Mann (and others) introduced the idea that the hadrons were built out of more fundamental objects, which he called quarks. Quarks have - spin 1/2 and - charges +2/3 and -1/3. The protons and neutrons are made from up (+2/3) and down (-1/3) quarks. A third strange quark (-1/3) accounts for strangeness. (Of course, there are also antiquarks, with opposite charges.)

42 Much later, three new (and heavier) quarks were discovered: Charm (+2/3) was discovered in 1974 (by Ting and Richter). Bottom (-1/3) was discovered in 1977 (by Lederman). Top (+2/3) was discovered in 1995 (by D0 and CDF collaborations at Fermilab). Just like the leptons, the quarks pair up into 3 generations.

43 Quarks Generation Particle Charge Mass up (u) +2/3 ~3 MeV/c 2 down (d) -1/3 ~7 MeV/c 2 charm (c) +2/3 ~1.3 GeV/c 2 strange (s) -1/3 ~100 MeV/c 2 top (t) +2/3 ~174 GeV/c 2 bottom (b) -1/3 ~4.3 GeV/c 2 The EM and strong forces cannot change the flavor of the quark, The weak force can change the sign of the quark, and can even change the generation (but with a suppression factor).

44 Quark Structure of Hadrons (ds) (us) (du) (ud) (su) (sd) (uu,dd,ss) Meson = quark + anti-quark (udd) (uud) (dds) (uus) (dss) (uss) (uds) Baryon = 3 quarks

45 Mesons Strangeness K 0 p - p 0 h p + K - K 0 K EM Charge Strangeness n p S - S 0 L 0 S Baryons EM Charge

46 Evidence for Quarks Quarks were originally suggested as a mathematical invention to describe the properties of the hadrons. But later, evidence from scattering experiments showed that the quarks have a physical meaning as constituents of the hadrons.

47 Q uick uiz What is the charge of a Λ c (a particle consisting of an up, a strange and a charm quark (u, s, c)? (A) +2 (B) +4/3 (C) +1 (D) 0 (E) -1

48 Q uick uiz What is the charge of a Λ c (a particle consisting of an up, a strange and a charm quark (u, s, c)? (A) +2 (B) +4/3 (C) +1 (D) 0 (E) /3-1/3 + 2/3 = +1

49 1950 s - Hofstadter at SLAC scattered electrons off protons, and found the proton to be a smooth, featureless sphere of about meters m a group (led by Friedman, Kendall, and Taylor) at SLAC did the same, but now with much higher energies of 20 GeV. They found that at these high energies the electron appeared to scatter off point-like objects within the proton ===> The quarks! m m

50 Problems with the Quark Model 1. Quarks have not been directly observed. 2. The quark hypothesis seems to conflict with the Pauli exclusion principle. Let s look at problem 2 first.

51 COLOR There exists a baryon, W -, whose spin is 3/2, whose charge is -1, and whose strangeness is -3. The quark model then says that the state is W - s s s This is forbidden by the Pauli exclusion principle! The resolution is a new quantum number called color (having nothing to do with the colors that we see). Each quark must be red, green, or blue and each anti-quark must be anti-red, anti-green, or anti-blue.

52 Furthermore, all hadrons must be formed out of color-less combinations of quark and/or anti-quarks. Thus, baryons are made out of 1 red quark, 1 green quark, and 1 blue quark: p u u d n u d d Mesons are made out of colored quark - anticolored antiquark combination: p - d u or d u but not d u Problem 2 solved.

53 Quantum Chromodynamics The addition of the color quantum number suggested to theorists a new explanation of the strong force! Quantum Chromodynamics (QCD) is a generalization of Quantum Electrodynamics (QED). The colors play the role of the charge. The force carriers (analogous to the photon) are called gluons, and they carry coloranticolor charges. There are 8 gluons: (RG, GB, BR, GR, RB, BG, RR, BB, GG) Only 2 combinations are gluons

54 Force between quarks and also between gluons: u g(gb) d u d Due to the gluon self-coupling, the force of attraction between quarks increases as the separation between the quarks increases. It would take an infinite amount of energy to separate two quarks. This concept is called confinement. Quarks and gluons must combine into colorless objects. It is impossible to see a free colored quark.

55 Energetically favorable to create quark antiquark pair on which gluon flux can terminate Energy is lowered by shortening the flux tube lengths

56 The Weak Force Responsible for b-decay. n Æ p + e - + n e p e - n e In 1934, Fermi suggested that this occured through the exchange of a charged gauge particle (W): n p e - n e or in terms of quarks: n W - p u d u e - n e W - n u d d The weakness of the force is due to the fact that the W is very heavy (80 GeV).

57 Electroweak Unification Early 1960 s Sheldon Glashow showed that the EM force could be unified with the weak force. n e e - W + W - e - n e e - W - e - Æ n e W + n e

58 g, Z 0 g, Z 0 The W ± and Z 0 were predicted to be very heavy (M W± = 80 GeV, M Z = 91 GeV) and were discovered at CERN in 1982.

59 A problem: The photon (γ) is massless The W and Z are massive The symmetry between them must be broken. A mechanism for this was proposed by Steven Weinberg and Abdus Salam. Glashow, Weinberg, and Salam won the Nobel Prize in The simplest model for symmetry breaking predicts a single neutral particle, called the Higgs Boson. It is presently being searched for at Fermilab and at CERN.

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62 Grand Unified Theories Grand Unified Theories (GUTS) are an attempt to extend ElectroWeak unification to include QCD (strong force), and eventually gravity. GUTS usually predict new very massive particles, which can lead to Baryonnumber-violating processes, such as Proton decay. This decay should be very rare and, as yet, has not been observed.

63 SU(5) Unification RR GR BR RG GG BG RB GB BB g, Z 0 W - W + g, Z 0 New ultra-heavy Gauge particles, which can lead to proton decay.

64 The unification of the other forces with gravity is very difficult. (Gravity and quantum mechanics are difficult to reconcile. Einstein spent the last 35 years of his life devoted to this goal, without success). Current ideas include: String Theory: All types of particles are just different excitations of one type of (very tiny) string. Supersymmetry: All particles come in (integer spin)-(1/2 integer) spin partners. electron Extra-dimensions: More than 4 space-time dimensions. e - selectron

65 Unanswered Questions 1. Why 3 generations? 2. Why the masses of the particles? 3. How is electroweak symmetry broken? Is there a single Higgs particle or something else? 4. Do the forces unify and how? 5. Are Strings, Supersymmetry, extradimensions real? 6. Why is the universe essentially all matter, but no anti-matter? 7. Are there other connections to the early universe, shortly after the big bang?