Particle physics today. Giulia Zanderighi (CERN & University of Oxford)

Transcription

1 Particle physics today Giulia Zanderighi (CERN & University of Oxford)

2 Particle Physics Particle Physics is fundamental research, as opposed to many applied sciences (medicine, biology, chemistry, nano-science, material science...) The ultimate aim is to understand the fundamental laws of nature. In a nutshell the main objectives today are to understand the origin, structure, composition of the universe the constituents of matter the fundamental forces between them

3 Particle Physics Since Particle Physics aims at pushing the boundaries of our knowledge, it faces big technological challenges, e.g. the development of accelerators with highest energies, operating at high magnetic field, low temperatures treatment of huge amount of data (fast processing of information, storage) detectors with highest tracking/calorimetry/timing information... Often this leads to great spin-offs, e.g. world wide wed (www) radiotherapy, diagnose new computational methods and tools...

4 Particle Physics Hence, particle physics lead, and will continue to lead to important technological advances. Still, the pleasure and challenge of exploring the unknown maintains a key role in this field Furthermore, even if it is not an applied science the by far most relevant scientific and technological progress resulted from fundamental breakthroughs in particle physics (e.g. electromagnetisms, relativity, quantum theory...) Do encourage young talented students to work on fundamental sciences!

5 Particle Physics Particle physics went through an incredible journey in the 20 s century Since these developments are covered in many books, I will instead focus only on those discoveries that have a close connection to today s activities Some references L. Lederman The God Particle (1993) M. Veltman Facts and Mysteries in Elementary Particle Physics (2003) F. Close Particle Physics: A Very Short Introduction (2004) F. Close The New Cosmic Onion: Quarks and the Nature of the Universe (2006) G.Giudice A Zeptospace Odyssey, A Journal into the physics of the LHC (2010)

6 All that remains to do in physics is to fill in the sixth decimal place Albert Michelson 1894

7 All that remains to do in physics is to fill in the sixth decimal place Albert Michelson 1894

8

9 CERN and Particle Physics At CERN s Large Hadron Collider (LHC), the approach is to accelerate particles to highest possible energies, make them collide and produce new particles High energy is important as heavy particles need high energy: E = m c 2 This is even more so at a proton collider since the elementary particle entering the hard scattering is a quark or gluon in the proton that carries only a fraction of the proton s energy

10 The Large Hadron Collider CMS Lake of Geneva LHCb ALICE ATLAS

11 The LHC in a nutshell The machine: 2 beams of protons, circulating in two magnetized rings of 27 km, steered by 1200 dipoles, 9 Tesla, operating at 1.5 K Run 1: pp or HI collisions at ECM = 7/8 TeV, ongoing Run2: 13TeV 25 fb -1 of data collected in Run 1, i.e trillions of pp collisions Four main experiments: ATLAS & CMS: Higgs studies, New Physics searches LHCb: flavour physics, CP violation, heavy-quark physics ALICE: heavy ion collisions, study of quark-gluon plasma

12 Collision recorded by ATLAS

13 Collision recorded by ATLAS

14 Aim By analyzing carefully the collisions the hope is to discover the direct production of new particles find that some particles are produced at a different rate than what we expect Collisions look very complicated. Is it very difficult? yes! But such deviations, if confirmed, would be the sign of New Physics (new particles, new forces), i.e. of Physics Beyond our Standard Model of particles physics Let s start discussing the Standard Model then

15 Matter content in the Standard Model

16 Force carriers in the Standard Model Electromagnetic electric charge massless photon coupling e U(1) Weak force weak charge massive W ± /Z coupling gw SU(2) Strong (QCD) colour charge 8 massless gluons coupling gs SU(3) 0 ev/c GeV/c GeV/c 2 0 ev/c γ ±1 W Z 0 1 g

17 The Higgs Boson Higgs boson responsible for mass generation only scalar (spin=0) elementary particle so far GeV/c H More on the Higgs boson later

18 Feynman diagrams Interaction through exchange of gauge bosons. Feynman diagrams still used a lot today Example: e + e - Z e + e - space gauge boson time

19 Feynman diagrams Interaction through exchange of gauge bosons. Feynman diagrams still used a lot today Example: e + e - Z e + e - space gauge boson time Precision calculations through higher order corrections in the coupling loop corrections (virtual particles) real emissions

20 The Standard Model (SM) Lagrangian: encodes all possible ways in which SM particles interact with each other

21 The Standard Model (SM) Lagrangian: encodes all possible ways in which SM particles interact with each other

22 Running couplings Abelian case (quantum electrodynamics): Higher energies smaller distances, less screening from vacuum polarization coupling increases with energy (intuitive)

23 Running couplings Non-abelian case (electroweak and strong force): Include extra contributions with field-strength self-interaction Overall effect: coupling decreases with energy, i.e. the closer the more free particles are. The more you try to pull particles apart, the stronger the interaction

24 Running couplings weaker stronger At collider energies interactions are perturbative, i.e. the coupling constant is smaller than one and a description though a power expansion is possible

25 Coupling unification? weaker stronger Improved unification of gauge couplings is one of the strong argument used in support of supersymmetry (we will come back to this later)

26 QCD: confinement Strong interaction becomes stronger at low energies (larger distances). What does it mean? When you try to pull apart two quarks, you pull out of the vacuum a pair of quark + anti-quark, so that you build two colour singlet states that can move apart This simple picture is used today to model the hadronization of quarks into colourless meson (quark-antiquark) and baryons (3quarks)

27 If quarks are confined, how do we know that they exist?

28 QCD matter sector u up d down s strange 1 st 2 nd quark generation 1/3 + 2/3 electric charge p e q e q X Feynman diagram describing DIS of an electron on a proton The light quark's existence was validated by the MIT/SLAC's deep inelastic scattering (DIS) experiments in 1968: strange was a necessary component of Gell-Mann and Zweig's three-quark model, it also provided an explanation for the kaon and pion mesons discovered in cosmic rays in 1947

29 QCD matter sector u up d down c charm s strange 1/3 + 2/3 electric charge K d c s s d c K Feynman diagram describing the mixing of a kaon into its anti-particle. The black boxes indicate weak effective fourfermion interactions 1 st 2 nd quark generation G 2 F sin 2 m 2 c c MW 2 In 1970 Glashow, Iliopoulos, and Maiani (GIM mechanism) presented strong theoretical arguments for the existence of the as-yet undiscovered charm quark, based on the absence of flavor-changing neutral currents [S. L. Glashow, J. Iliopoulos and L. Maiani, Phys. Rev. D 2 (1970) 2]

30 QCD matter sector u up d down c charm s strange 1/3 + 2/3 electric charge 1 st 2 nd quark generation Computer reconstruction of a ψ decay in the Mark I detector at SLAC, making a near-perfect image of the Greek letter ψ Charm quarks were observed almost simultaneously in November 1974 at SLAC and at BNL as charm anti-charm bound states (charmonium). The two groups had assigned the discovered meson two different symbols, J and ψ. Thus, it became formally known as the J/ψ meson (Nobel Prize 1976)

31 QCD matter sector u up d down c charm s strange b bottom 1/3 + 2/3 electric charge! " ' 1 st 2 nd quark generation 3 rd Unitarity triangle measuring the amount of CP violation in the standard model The bottom quark was theorized in 1973 by Kobayashi and Maskawa in order to accommodate the phenomenon of CP violation, which requires the existence of at least three generations of quarks in Nature (Nobel Prize 2008) [M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49 (1973) 652]

32 QCD matter sector u up d down c charm s strange b bottom 1/3 + 2/3 electric charge The bump at 9.5 GeV that lead to the discovery of the bottom quark at FNAL in st 2 nd quark generation 3 rd In 1977, physicists working at the fixed target experiment E288 at FNAL discovered the Υ (Upsilon) meson. This discovery was eventually understood as being the bound state of the bottom and its anti-quark (bottomonium)

33 QCD matter sector u up d down c charm s strange t top b bottom 1/3 + 2/3 electric charge B d b W t t W + Diagram involving the virtual exchange of top quarks that induces a mass difference in the B meson system b d B 1 st 2 nd quark generation 3 rd M B G 2 F m B f 2 B V td 2 m 2 t The measurement of the oscillations of B mesons into its own antiparticles in 1987 by ARGUS led to the conclusion that the top-quark mass has to be larger than 50 GeV. This was a big surprise at that time, because in 1987 the top quark was generally believed to be much lighter

34 QCD matter sector u up d down c charm s strange 1 st 2 nd quark generation t top b bottom 3 rd 1/3 + 2/3 electric charge t, b Z, W Z, W t t b W Z t b Diagrams that feature a quadratic dependence on the top-quark mass It was also realized that certain precision measurements of the electroweak vector-boson masses and couplings are very sensitive to the value of the top-quark mass. By 1994 the precision of these indirect measurements led to a prediction of the top-quark mass between 145 GeV and 185 GeV

35 QCD matter sector jet µ + u up d down c charm s strange 1 st 2 nd quark generation t top b bottom 3 rd 1/3 + 2/3 electric charge Top anti-top production in proton anti-proton collision at the Tevatron jet t p t W b b t b W + p jet µ jet The top quark was finally discovered in 1995 by CDF and D0 at FNAL. The mass of the top quark is today quite well known, mt = (173.0 ± 0.6 ± 0.8) GeV

36 QCD matter sector u up d down c charm s strange t top b bottom 1/3 + 2/3 electric charge mq/mtop up-type quarks down-type quarks c s t b 1 st 2 nd quark generation 3 rd u d proton charge 2/3 mass= charge -1/3 mass = up few MeV down few MeV charm 1.6 GeV strange 100 MeV top 172 GeV bottom 5 GeV

37 QCD matter sector u up d down c charm s strange t top b bottom 1/3 + 2/3 electric charge mq/mtop up-type quarks down-type quarks c s t b 1 st 2 nd quark generation 3 rd u d 10 6 The masses of the six different quark flavors range from around 2 MeV for the up quark to around 173 GeV for the top. Why these masses are split by almost six orders of magnitude is one of the big mysteries of particle physics

38

39 Z decay width Let s consider an example: the production and decay of a Z boson, e.g. at an e+e- collider (LEP) one has From the position of the resonance in the invariant mass distribution of the decay products one can extract the Z boson mass What about the width of the distribution?

40 Z decay width The width of the distribution is sensitive to the total number of possible decay products. For the Z boson we find production and decay processes obey a number of conservation rules [why is there no top quark?]

41 Z decay width The width of the distribution is sensitive to the total number of possible decay products. For the Z boson we find the measurement of the Z-boson width is compatible with 3 families for quarks and leptons and with 3 colors for each quark

42 Extrapolation to today similarly with what happened in the past, we might see the presence of new particles only indirectly first through the way they affect measurable quantities when they are exchanged as virtual states (this happens if the collider energy is not enough to produce real New Physics particles) when a real new particle is produced, it decays similarly to the Z- boson. The particle might manifests itself as a bump in the invariant mass combinations of particles in the final state from properties of the decay products one can infer properties of the decaying particle (because of conservation rules) by studying the width of the mass distributions one can learn also about invisible decay modes This is what is done today at the LHC