Particle Physics. Tommy Ohlsson. Theoretical Particle Physics, Department of Physics, KTH Royal Institute of Technology, Stockholm, Sweden

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1 Particle Physics Tommy Ohlsson Theoretical Particle Physics, Department of Physics, KTH Royal Institute of Technology, Stockholm, Sweden International Baccalaureate T. Ohlsson (KTH) Particle Physics 1/ 35

2 Outline This lecture is organized as follows: Scales What are elementary particles? Timeline of discovery of elementary particles The Standard Model of particle physics Fermions Bosons Symmetries and conservation laws Hadrons What are they? Nature and range of interactions Feynman diagrams Confinement and asymptotic freedom LHC Large Hadron Collider The Higgs particle and the Higgs mechanism Open questions in particle physics T. Ohlsson (KTH) Particle Physics 2/ 35

3 Scales Scales in Nature: T. Ohlsson (KTH) Particle Physics 3/ 35

4 Scales II Scales in physics: T. Ohlsson (KTH) Particle Physics 4/ 35

5 What are elementary particles? T. Ohlsson (KTH) Particle Physics 5/ 35

6 What are elementary particles? Definition of an elementary particle A particle is called elementary if it is not made out of any smaller component particles. T. Ohlsson (KTH) Particle Physics 5/ 35

7 What are elementary particles? Definition of an elementary particle A particle is called elementary if it is not made out of any smaller component particles. Advanced definition of an elementary particle An elementary particle is an irreducible representation of the Poincáre group. T. Ohlsson (KTH) Particle Physics 5/ 35

8 What are elementary particles? Definition of an elementary particle A particle is called elementary if it is not made out of any smaller component particles. Advanced definition of an elementary particle An elementary particle is an irreducible representation of the Poincáre group. There are two classes of elementary particles: fermions (half-integer spin particles) matter particles leptons quarks bosons (integer spin particles) gauge bosons exchange particles scalar boson T. Ohlsson (KTH) Particle Physics 5/ 35

9 Timeline of discovery of elementary particles Year Particle Discovered by T. Ohlsson (KTH) Particle Physics 6/ 35

10 Timeline of discovery of elementary particles Year Particle Discovered by 1897 electron J.J. Thomson 1932 positron (or antielectron) C.D. Anderson T. Ohlsson (KTH) Particle Physics 6/ 35

11 Timeline of discovery of elementary particles Year Particle Discovered by 1897 electron J.J. Thomson 1932 positron (or antielectron) C.D. Anderson 1937 muon S. Neddermeyer, C.D. Anderson, J.C. Street, and E.C. Stevenson 1956 electron antineutrino F. Reines and C.L. Cowan 1962 muon neutrino L. Lederman, M. Schwartz, J. Steinberger et al light quarks SLAC 1974 charm B. Richter et al. and S. Ting et al. T. Ohlsson (KTH) Particle Physics 6/ 35

12 Timeline of discovery of elementary particles Year Particle Discovered by 1897 electron J.J. Thomson 1932 positron (or antielectron) C.D. Anderson 1937 muon S. Neddermeyer, C.D. Anderson, J.C. Street, and E.C. Stevenson 1956 electron antineutrino F. Reines and C.L. Cowan 1962 muon neutrino L. Lederman, M. Schwartz, J. Steinberger et al light quarks SLAC 1974 charm B. Richter et al. and S. Ting et al tau M.L. Perl et al bottom Fermilab 1978 gluon PLUTO Collaboration (DESY) 1983 W and Z bosons C. Rubbia, S. van der Meer, and UA1 Collaboration (CERN) 1995 top CDF and D Collaborations (Fermilab) 2000 tau neutrino DONUT Collaboration (Fermilab) T. Ohlsson (KTH) Particle Physics 6/ 35

13 Timeline of discovery of elementary particles Year Particle Discovered by 1897 electron J.J. Thomson 1932 positron (or antielectron) C.D. Anderson 1937 muon S. Neddermeyer, C.D. Anderson, J.C. Street, and E.C. Stevenson 1956 electron antineutrino F. Reines and C.L. Cowan 1962 muon neutrino L. Lederman, M. Schwartz, J. Steinberger et al light quarks SLAC 1974 charm B. Richter et al. and S. Ting et al tau M.L. Perl et al bottom Fermilab 1978 gluon PLUTO Collaboration (DESY) 1983 W and Z bosons C. Rubbia, S. van der Meer, and UA1 Collaboration (CERN) 1995 top CDF and D Collaborations (Fermilab) 2000 tau neutrino DONUT Collaboration (Fermilab) 2012 Higgs ATLAS and CMS Collaborations (CERN) T. Ohlsson (KTH) Particle Physics 6/ 35

14 The Standard Model (SM) of particle physics T. Ohlsson (KTH) Particle Physics 7/ 35

15 Fermions The fermions ( matter particles ) are divided into: leptons quarks T. Ohlsson (KTH) Particle Physics 8/ 35

16 Fermions The fermions ( matter particles ) are divided into: leptons quarks Some basic facts about leptons There are six types of leptons: e, ν e; µ, ν µ; τ, ν τ. For the masses of e, µ, and τ, it holds that m e m µ m τ. There is solid evidence that all six leptons exist and conclusive evidence that neutrinos have small but non-zero mass. All leptons interact with the weak interaction and the charged leptons (i.e. e, µ, and τ ) also interact with the electromagnetic interaction. T. Ohlsson (KTH) Particle Physics 8/ 35

17 Fermions The fermions ( matter particles ) are divided into: leptons quarks Some basic facts about leptons There are six types of leptons: e, ν e; µ, ν µ; τ, ν τ. For the masses of e, µ, and τ, it holds that m e m µ m τ. There is solid evidence that all six leptons exist and conclusive evidence that neutrinos have small but non-zero mass. All leptons interact with the weak interaction and the charged leptons (i.e. e, µ, and τ ) also interact with the electromagnetic interaction. Some basic facts about quarks There are six types of quarks: u, d; c, s; t, b. For the electric charges, it holds that Q u = Q c = Q t = 2 3 e and Q d = Q s = Q b = 1 3 e. There is solid evidence that all six quarks exist. Quarks interact with the strong, weak, and electromagnetic interactions. T. Ohlsson (KTH) Particle Physics 8/ 35

18 Bosons The bosons are divided into: gauge bosons ( exchange particles ) scalar boson (the Higgs boson) We will come back to this particle later! T. Ohlsson (KTH) Particle Physics 9/ 35

19 Bosons The bosons are divided into: gauge bosons ( exchange particles ) scalar boson (the Higgs boson) We will come back to this particle later! Definition an exchange particle In particle physics, an interaction between two particles is interpreted as the exchange of another particle the exchange particle between them. T. Ohlsson (KTH) Particle Physics 9/ 35

20 Bosons II Example of an exchange particle (the photon): In case of the electromagnetic interaction, the exhange particle is a photon. One electron emits a photon and then another electron (or actually, a positron) absorbs it. The photon carries momentum, so the two electrons change their momenta, and thus experience forces. T. Ohlsson (KTH) Particle Physics 10/ 35

21 Bosons II Example of an exchange particle (the photon): In case of the electromagnetic interaction, the exhange particle is a photon. One electron emits a photon and then another electron (or actually, a positron) absorbs it. The photon carries momentum, so the two electrons change their momenta, and thus experience forces. T. Ohlsson (KTH) Particle Physics 10/ 35

22 Bosons III The exhange particles of the SM are: Electromagnetic interaction: photon (γ) Weak interaction: W and Z bosons (W ±, Z 0 ) Strong interaction: eight gluons (g) T. Ohlsson (KTH) Particle Physics 11/ 35

23 Bosons III The exhange particles of the SM are: Electromagnetic interaction: photon (γ) Weak interaction: W and Z bosons (W ±, Z 0 ) Strong interaction: eight gluons (g) Some basic facts about the photon The photon has zero rest mass and always propagates at the speed of light in vacuum. The photon is a stable particle and its own antiparticle and has no electric charge and spin 1. The photon has two independent polarization states two transversal polarizations. T. Ohlsson (KTH) Particle Physics 11/ 35

24 Bosons III The exhange particles of the SM are: Electromagnetic interaction: photon (γ) Weak interaction: W and Z bosons (W ±, Z 0 ) Strong interaction: eight gluons (g) Some basic facts about the photon The photon has zero rest mass and always propagates at the speed of light in vacuum. The photon is a stable particle and its own antiparticle and has no electric charge and spin 1. The photon has two independent polarization states two transversal polarizations. Some basic facts about the W and Z bosons The W and Z masses are m W = ( ± 0.015) GeV/c 2 and m Z = ( ± ) GeV/c 2, respectively. We will come back to why these particles have mass later! The W bosons have either positive or negative electric charge and are each other s antiparticles. The Z boson has no electric charge and is its own antiparticle. All three particles have spin 1 and three independent polarization states. T. Ohlsson (KTH) Particle Physics 11/ 35

25 Bosons IV Some basic facts about the eight gluons All eight gluons have zero rest mass. Note that this is a theoretical values, since no free gluons have been observed! Experimental limit: m g < 1.3 MeV/c 2. The gluons have no electric charge but eight different color charges. Like the photon, all eight gluons have spin 1 and two independent polarization states. T. Ohlsson (KTH) Particle Physics 12/ 35

26 Bosons IV Some basic facts about the eight gluons All eight gluons have zero rest mass. Note that this is a theoretical values, since no free gluons have been observed! Experimental limit: m g < 1.3 MeV/c 2. The gluons have no electric charge but eight different color charges. Like the photon, all eight gluons have spin 1 and two independent polarization states. Strong interaction There are eight independent gluons. Intuitively: Quarks carry three types of color and antiquarks carry three types of anticolor. Gluons carry both color and anticolor. This gives nine possible combinations for gluons, which are: red-antired, red-antigreen, red-antiblue green-antired, green-antigreen, green-antiblue blue-antired, blue-antigreen, blue-antiblue These are not the actual color states of observed gluons, but rather effective states. To correctly describe gluons, it is necessary to consider the mathematics of color in more detail eight gluons. T. Ohlsson (KTH) Particle Physics 12/ 35

27 Symmetries and conservation laws Some keywords Symmetry or invariance: In physics, a symmetry is a physical or mathematical property that is preserved under some applied transformation. Conservation law: A symmetry property of a physical system is intimately related to the conservation law characterizing that system. Conserved quantity: The number quantizing the conservation law a quantum number. T. Ohlsson (KTH) Particle Physics 13/ 35

28 Symmetries and conservation laws Some keywords Symmetry or invariance: In physics, a symmetry is a physical or mathematical property that is preserved under some applied transformation. Conservation law: A symmetry property of a physical system is intimately related to the conservation law characterizing that system. Conserved quantity: The number quantizing the conservation law a quantum number. Examples in physics Symmetry translation in time translation in space rotation in space Conserved quantity energy momentum angular momentum T. Ohlsson (KTH) Particle Physics 13/ 35

29 Symmetries and conservation laws II Example of conserved quantities in particle physics: electric charge Q baryon number B strangeness S Violated in weak interactions! electron lepton number L e muon lepton number L µ tau lepton number L τ lepton number L = L e +L µ +L τ T. Ohlsson (KTH) Particle Physics 14/ 35

30 Symmetries and conservation laws II Example of conserved quantities in particle physics: electric charge Q baryon number B strangeness S Violated in weak interactions! electron lepton number L e muon lepton number L µ tau lepton number L τ lepton number L = L e +L µ +L τ Conservation laws in particle physics In all particle physics reactions, the above-mentioned quantities are conserved, i.e. they have the same values before and after the reactions. T. Ohlsson (KTH) Particle Physics 14/ 35

31 Symmetries and conservation laws III Quantum numbers for particles particle Q B S Le Lµ Lτ L u 2 3 e ū 3 2e d 1 3 e d 1e s 1 3 e s 1e c 2e c 3 2e b 3 1e b 1 e t 2e t 2 3 e e e νe νe µ µ νµ νµ τ τ ντ ντ T. Ohlsson (KTH) Particle Physics 15/ 35

32 Symmetries and conservation laws IV 1. Consider the decays 0 p + π and Λ 0 p + π, which are both possible. T. Ohlsson (KTH) Particle Physics 16/ 35

33 Symmetries and conservation laws IV 1. Consider the decays 0 p + π and Λ 0 p + π, which are both possible. 0 p + π Λ 0 p + π T. Ohlsson (KTH) Particle Physics 16/ 35

34 Symmetries and conservation laws IV 1. Consider the decays 0 p + π and Λ 0 p + π, which are both possible. 0 p + π Λ 0 p + π Q 0 = = B 1 = = S 0 = OK! T. Ohlsson (KTH) Particle Physics 16/ 35

35 Symmetries and conservation laws IV 1. Consider the decays 0 p + π and Λ 0 p + π, which are both possible. 0 p + π Λ 0 p + π Q 0 = = B 1 = = S 0 = OK! For Λ 0 p + π, strangeness is violated The weak interaction is responsible for this decay. T. Ohlsson (KTH) Particle Physics 16/ 35

36 Symmetries and conservation laws IV 1. Consider the decays 0 p + π and Λ 0 p + π, which are both possible. 0 p + π Λ 0 p + π Q 0 = = B 1 = = S 0 = OK! For Λ 0 p + π, strangeness is violated The weak interaction is responsible for this decay. 2. The reactions p + p π 0 + π 0 + n and K 0 π + + π + e do not occur. Why? T. Ohlsson (KTH) Particle Physics 16/ 35

37 Symmetries and conservation laws IV 1. Consider the decays 0 p + π and Λ 0 p + π, which are both possible. 0 p + π Λ 0 p + π Q 0 = = B 1 = = S 0 = OK! For Λ 0 p + π, strangeness is violated The weak interaction is responsible for this decay. 2. The reactions p + p π 0 + π 0 + n and K 0 π + + π + e do not occur. Why? p + p π 0 + π 0 + n K 0 π + + π + e T. Ohlsson (KTH) Particle Physics 16/ 35

38 Symmetries and conservation laws IV 1. Consider the decays 0 p + π and Λ 0 p + π, which are both possible. 0 p + π Λ 0 p + π Q 0 = = B 1 = = S 0 = OK! For Λ 0 p + π, strangeness is violated The weak interaction is responsible for this decay. 2. The reactions p + p π 0 + π 0 + n and K 0 π + + π + e do not occur. Why? p + p π 0 + π 0 + n K 0 π + + π + e Q = B T. Ohlsson (KTH) Particle Physics 16/ 35

39 Symmetries and conservation laws IV 1. Consider the decays 0 p + π and Λ 0 p + π, which are both possible. 0 p + π Λ 0 p + π Q 0 = = B 1 = = S 0 = OK! For Λ 0 p + π, strangeness is violated The weak interaction is responsible for this decay. 2. The reactions p + p π 0 + π 0 + n and K 0 π + + π + e do not occur. Why? p + p π 0 + π 0 + n K 0 π + + π + e Q = B For p + p π 0 + π 0 + n, baryon number is not conserved. For K 0 π + + π + e, charge is not conserved. T. Ohlsson (KTH) Particle Physics 16/ 35

40 Symmetries and conservation laws IV 1. Consider the decays 0 p + π and Λ 0 p + π, which are both possible. 0 p + π Λ 0 p + π Q 0 = = B 1 = = S 0 = OK! For Λ 0 p + π, strangeness is violated The weak interaction is responsible for this decay. 2. The reactions p + p π 0 + π 0 + n and K 0 π + + π + e do not occur. Why? p + p π 0 + π 0 + n K 0 π + + π + e Q = B For p + p π 0 + π 0 + n, baryon number is not conserved. For K 0 π + + π + e, charge is not conserved. 3. Which of the following reactions are possible? Σ K +π 0 +π 0 Σ + p +π 0 µ e +γ +γ γ e +e + n p +e +γ e +p n+ν µ T. Ohlsson (KTH) Particle Physics 16/ 35

41 Hadrons What are they? T. Ohlsson (KTH) Particle Physics 17/ 35

42 Hadrons What are they? Definition of a hadron A hadron is a composite particle built up by quarks. It is not an elementary particle. T. Ohlsson (KTH) Particle Physics 17/ 35

43 Hadrons What are they? Definition of a hadron A hadron is a composite particle built up by quarks. It is not an elementary particle. Definitions of a baryon and a meson Quarks can combine in just two ways to form hadrons: When three quarks combine, they form a baryon. When one quark and one antiquark combine, they form a meson. T. Ohlsson (KTH) Particle Physics 17/ 35

44 Hadrons What are they? Definition of a hadron A hadron is a composite particle built up by quarks. It is not an elementary particle. Definitions of a baryon and a meson Quarks can combine in just two ways to form hadrons: When three quarks combine, they form a baryon. When one quark and one antiquark combine, they form a meson. Example: The proton is a baryon made out of two u and one d, whereas the neutron is another baryon made out of two d and one u. Thus, the electric charges of the proton (uud) and the neutron (ddu) are Q p = 2 e + 2 e +( 1 e) = e and Q n = ( 1 e) + ( 1 e) + 2 e = 0, respectively. Of course, these two values are the measured values. T. Ohlsson (KTH) Particle Physics 17/ 35

45 Hadrons What are they? II Since baryons are made from three quarks, all baryons have baryon number 1, whereas all antibaryons have baryon number 1. All mesons have baryon number 0. Other particles not made from quarks also have baryon number 0. T. Ohlsson (KTH) Particle Physics 18/ 35

46 Hadrons What are they? II Since baryons are made from three quarks, all baryons have baryon number 1, whereas all antibaryons have baryon number 1. All mesons have baryon number 0. Other particles not made from quarks also have baryon number 0. Pentaquarks Recently, in 2015 at the LHCb experiment, a new type of hadrons has been confirmed. This type is called pentaquarks and they consist of five quarks, or actually, four quarks and one antiquark. However, the binding mechanism for pentaquarks is not yet clear. Either five quarks combine or they are more loosely bound and consist of a baryon and a meson interacting relatively weakly with each other, i.e. a meson-baryon molecule. T. Ohlsson (KTH) Particle Physics 18/ 35

47 Hadrons What are they? II Since baryons are made from three quarks, all baryons have baryon number 1, whereas all antibaryons have baryon number 1. All mesons have baryon number 0. Other particles not made from quarks also have baryon number 0. Pentaquarks Recently, in 2015 at the LHCb experiment, a new type of hadrons has been confirmed. This type is called pentaquarks and they consist of five quarks, or actually, four quarks and one antiquark. However, the binding mechanism for pentaquarks is not yet clear. Either five quarks combine or they are more loosely bound and consist of a baryon and a meson interacting relatively weakly with each other, i.e. a meson-baryon molecule. Glueballs A glueball is a hypothetical composite particle, which consists solely of gluons. The prediction that glueballs exist is one of the most important predictions of the SM that has not yet been confirmed experimentally. However, there are several experimental candidates for glueballs. T. Ohlsson (KTH) Particle Physics 18/ 35

48 Nature and range of interactions There are four fundamental interactions in Nature: electromagnetic (EM) weak strong gravitational T. Ohlsson (KTH) Particle Physics 19/ 35

49 Nature and range of interactions There are four fundamental interactions in Nature: electromagnetic (EM) weak strong gravitational Not included in the SM! T. Ohlsson (KTH) Particle Physics 19/ 35

50 Nature and range of interactions There are four fundamental interactions in Nature: electromagnetic (EM) weak strong gravitational Not included in the SM! The four fundamental interactions Interaction Exchange particle Acts on Relative strength Long dist. behavior Range electromagnetic γ charge r 2 weak W and Z flavor r e mr m strong g color 1 r m gravitational graviton (?) mass r 2 T. Ohlsson (KTH) Particle Physics 19/ 35

51 Nature and range of interactions II The three fundamental interactions in the SM Interaction Exchange particle Acts on Experiencing particles electromagnetic γ charge all electrically charged particles weak W and Z flavor leptons, quarks, W, and Z strong g color quarks and g T. Ohlsson (KTH) Particle Physics 20/ 35

52 Nature and range of interactions II The three fundamental interactions in the SM Interaction Exchange particle Acts on Experiencing particles electromagnetic γ charge all electrically charged particles weak W and Z flavor leptons, quarks, W, and Z strong g color quarks and g T. Ohlsson (KTH) Particle Physics 20/ 35

53 Feynman diagrams Feynman diagrams In 1948, R.P. Feynman introduced pictorial representations of particle interactions, which are today called Feynman diagrams. These diagrams clearly express the idea that particle interactions involve exchange particles. T. Ohlsson (KTH) Particle Physics 21/ 35

54 Feynman diagrams Feynman diagrams In 1948, R.P. Feynman introduced pictorial representations of particle interactions, which are today called Feynman diagrams. These diagrams clearly express the idea that particle interactions involve exchange particles. T. Ohlsson (KTH) Particle Physics 21/ 35

55 Feynman diagrams II Example of a Feynman diagram: Beta decay of a neutron n p+e +ν e or actually d u+w where W e +ν e. T. Ohlsson (KTH) Particle Physics 22/ 35

56 Feynman diagrams II Example of a Feynman diagram: Beta decay of a neutron n p+e +ν e or actually d u+w where W e +ν e. T. Ohlsson (KTH) Particle Physics 22/ 35

57 Confinement and asymptotic freedom The theory for the strong interaction is called quantum chromodynamics (QCD). T. Ohlsson (KTH) Particle Physics 23/ 35

58 Confinement and asymptotic freedom The theory for the strong interaction is called quantum chromodynamics (QCD). Confinement In QCD, at low energies (or at long distances), confinement is the phenomenon that colored (or color charged) particles cannot be isolated. Therefore, colored particles cannot be directly observed. Quarks and gluons must combine to form hadrons. T. Ohlsson (KTH) Particle Physics 23/ 35

59 Confinement and asymptotic freedom The theory for the strong interaction is called quantum chromodynamics (QCD). Confinement In QCD, at low energies (or at long distances), confinement is the phenomenon that colored (or color charged) particles cannot be isolated. Therefore, colored particles cannot be directly observed. Quarks and gluons must combine to form hadrons. However, at extremely high energies (or at extremely short distances), the situation is totally different. T. Ohlsson (KTH) Particle Physics 23/ 35

60 Confinement and asymptotic freedom The theory for the strong interaction is called quantum chromodynamics (QCD). Confinement In QCD, at low energies (or at long distances), confinement is the phenomenon that colored (or color charged) particles cannot be isolated. Therefore, colored particles cannot be directly observed. Quarks and gluons must combine to form hadrons. However, at extremely high energies (or at extremely short distances), the situation is totally different. Asymptotic freedom In QCD, asymptotic freedom is a property that causes the strong interaction between particles to become asymptotically weaker as the energy scale increases and the corresponding length scale decreases. T. Ohlsson (KTH) Particle Physics 23/ 35

61 Confinement and asymptotic freedom II The asymptotic freedom of QCD was discovered in 1973 by D. Gross, D. Politzer, and F. Wilczek. T. Ohlsson (KTH) Particle Physics 24/ 35

62 Confinement and asymptotic freedom II The asymptotic freedom of QCD was discovered in 1973 by D. Gross, D. Politzer, and F. Wilczek. Nobel Prize in Physics 2004 Press release (see nobelprize.org) David Gross, David Politzer and Frank Wilczek have made an important theoretical discovery concerning the strong force, or the colour force as it is also called. [...] What this year s Laureates discovered was something that, at first sight, seemed completely contradictory. The interpretation of their mathematical result was that the closer the quarks are to each other, the weaker is the colour charge. When the quarks are really close to each other, the force is so weak that they behave almost as free particles. This phenomenon is called asymptotic freedom. The converse is true when the quarks move apart: the force becomes stronger when the distance increases. This property may be compared to a rubber band. The more the band is stretched, the stronger the force. T. Ohlsson (KTH) Particle Physics 24/ 35

63 Confinement and asymptotic freedom III Reference: Quantum Chromodynamics by S. Bethke, G. Dissertor, and G.P. Salam in C. Patrignani et al. (Particle Data Group), Chin. Phys. C 40, (2016). T. Ohlsson (KTH) Particle Physics 25/ 35

64 Confinement and asymptotic freedom IV In fact, no physical quantities are constant with respect to energy. They all run and so do the couplings constants. T. Ohlsson (KTH) Particle Physics 26/ 35

65 Confinement and asymptotic freedom IV In fact, no physical quantities are constant with respect to energy. They all run and so do the couplings constants. Running coupling constants in the SM (left) and with the introduction of supersymmetry (right): In the SM, the coupling constants for the three fundamental interactions do not meet at one point, but with supersymmetry, they do meet at one point. This is known as unification. T. Ohlsson (KTH) Particle Physics 26/ 35

66 LHC Large Hadron Collider Some facts about the LHC World s largest and most powerful particle collider: Circumference: 26, 659 m 27 km Reaction: p +p a+b+..., where a,b,... are final particles. Center-of-mass energy: s 13 TeV It was built between 1998 and It is in operation since November 20, Number of collisions per second: 1 billion T. Ohlsson (KTH) Particle Physics 27/ 35

67 LHC Large Hadron Collider II The purposes of the LHC are: to discover the Higgs particle, DONE! to search for supersymmetry, to search for so-called extra dimensions, and many other things. T. Ohlsson (KTH) Particle Physics 28/ 35

68 LHC Large Hadron Collider II The purposes of the LHC are: to discover the Higgs particle, DONE! to search for supersymmetry, to search for so-called extra dimensions, and many other things. The main experiments at the LHC are: ATLAS CMS LHCb ALICE T. Ohlsson (KTH) Particle Physics 28/ 35

69 LHC Large Hadron Collider II The purposes of the LHC are: to discover the Higgs particle, DONE! to search for supersymmetry, to search for so-called extra dimensions, and many other things. The main experiments at the LHC are: ATLAS CMS LHCb ALICE The Higgs particle was discovered by the ATLAS and CMS experiments. T. Ohlsson (KTH) Particle Physics 28/ 35

70 LHC Large Hadron Collider III A section of LHC ATLAS CMS T. Ohlsson (KTH) Particle Physics 29/ 35

71 The Higgs particle In fact, the symmetry of the SM forbids γ, W ±, and Z 0 to have mass. T. Ohlsson (KTH) Particle Physics 30/ 35

72 The Higgs particle In fact, the symmetry of the SM forbids γ, W ±, and Z 0 to have mass. Facts: γ has no mass. OK! However, W ± and Z 0 have mass... T. Ohlsson (KTH) Particle Physics 30/ 35

73 The Higgs particle In fact, the symmetry of the SM forbids γ, W ±, and Z 0 to have mass. Facts: γ has no mass. OK! However, W ± and Z 0 have mass... Mathematical solution: The electroweak symmetry in the SM that forbids the electroweak gauge bosons to have mass is (spontaneously) broken and in this process the Higgs boson receives a non-zero mass. This process is the so-called Brout Englert Higgs mechanism (or the Higgs mechanism for short) and it generates the masses of the SM particles including the W ± and Z 0. T. Ohlsson (KTH) Particle Physics 30/ 35

74 The Higgs particle In fact, the symmetry of the SM forbids γ, W ±, and Z 0 to have mass. Facts: γ has no mass. OK! However, W ± and Z 0 have mass... Mathematical solution: The electroweak symmetry in the SM that forbids the electroweak gauge bosons to have mass is (spontaneously) broken and in this process the Higgs boson receives a non-zero mass. This process is the so-called Brout Englert Higgs mechanism (or the Higgs mechanism for short) and it generates the masses of the SM particles including the W ± and Z 0. Conclusion: The Higgs particle H is responsible for the mass of the particles of the SM, especially for the masses of W ± and Z 0. T. Ohlsson (KTH) Particle Physics 30/ 35

75 The Higgs particle In fact, the symmetry of the SM forbids γ, W ±, and Z 0 to have mass. Facts: γ has no mass. OK! However, W ± and Z 0 have mass... Mathematical solution: The electroweak symmetry in the SM that forbids the electroweak gauge bosons to have mass is (spontaneously) broken and in this process the Higgs boson receives a non-zero mass. This process is the so-called Brout Englert Higgs mechanism (or the Higgs mechanism for short) and it generates the masses of the SM particles including the W ± and Z 0. Conclusion: The Higgs particle H is responsible for the mass of the particles of the SM, especially for the masses of W ± and Z 0. Some basic facts about the Higgs particle The mass of the H is m H = (125.09±0.24) GeV/c 2. The H has no electric charge and spin 0 it is a so-called scalar boson. T. Ohlsson (KTH) Particle Physics 30/ 35

76 The Higgs particle II Event from ATLAS Event from CMS H µ + µ+ + µ + µ+ H γ+γ cds.cern.ch/record/ cds.cern.ch/record/ T. Ohlsson (KTH) Particle Physics 31 / 35

77 The Higgs particle III Advanced: The Higgs field The Higgs field a mathematical function is assumed to exist that fills all of space and has no external source. The Higgs boson is an elementary excitation of this field, i.e. the corresponding elementary particle to the Higgs field. T. Ohlsson (KTH) Particle Physics 32/ 35

78 The Higgs particle III Advanced: The Higgs field The Higgs field a mathematical function is assumed to exist that fills all of space and has no external source. The Higgs boson is an elementary excitation of this field, i.e. the corresponding elementary particle to the Higgs field. The source of the Higgs field is the Higgs field itself. In other words, the Higgs bosons in the condensate attract each other. The resulting potential energy V(φ) = µ 2 φ 2 +λ φ 4 has its minimum at a µ non-zero value of the field (φ min = 2 ): 2λ T. Ohlsson (KTH) Particle Physics 32/ 35

79 The Higgs particle III Advanced: The Higgs field The Higgs field a mathematical function is assumed to exist that fills all of space and has no external source. The Higgs boson is an elementary excitation of this field, i.e. the corresponding elementary particle to the Higgs field. The source of the Higgs field is the Higgs field itself. In other words, the Higgs bosons in the condensate attract each other. The resulting potential energy V(φ) = µ 2 φ 2 +λ φ 4 has its minimum at a µ non-zero value of the field (φ min = 2 ): 2λ T. Ohlsson (KTH) Particle Physics 32/ 35

80 The Higgs particle IV Generation of mass All elementary particles are massless and therefore propagate with the speed of light. However, most of them bounce off the Higgs bosons in vacuum, and hence effectively propagate with finite velocities. Their kinetic energies are transformed into the rest energies, i.e. the masses of the elementary particles. T. Ohlsson (KTH) Particle Physics 33/ 35

81 The Higgs particle IV Generation of mass All elementary particles are massless and therefore propagate with the speed of light. However, most of them bounce off the Higgs bosons in vacuum, and hence effectively propagate with finite velocities. Their kinetic energies are transformed into the rest energies, i.e. the masses of the elementary particles. Some particles (including the Higgs boson itself) interact more frequently with the Higgs field than others they become more massive. T. Ohlsson (KTH) Particle Physics 33/ 35

82 The Higgs particle IV Generation of mass All elementary particles are massless and therefore propagate with the speed of light. However, most of them bounce off the Higgs bosons in vacuum, and hence effectively propagate with finite velocities. Their kinetic energies are transformed into the rest energies, i.e. the masses of the elementary particles. Some particles (including the Higgs boson itself) interact more frequently with the Higgs field than others they become more massive. Note! Photons and gluons do not interact at all with the Higgs field they remain massless. T. Ohlsson (KTH) Particle Physics 33/ 35

83 Open questions in particle physics Some open fundamental questions in particle physics are: Is there unification of the three fundamental interactions in particle physics? Is there unification of all four fundamental interactions in Nature? Is there only one type of Higgs particle? Are neutrinos their own antiparticles? Is there supersymmetry in Nature? Why are there three generations of leptons and quarks? Is the proton fundamentally stable? What combinations of quarks are possible? Do glueballs exist? What is the dark matter? Why is it far more matter than antimatter in the observable Universe? T. Ohlsson (KTH) Particle Physics 34/ 35

84 Questions Thank you for your attention! Questions? T. Ohlsson (KTH) Particle Physics 35/ 35

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