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1 - Chapter 1 - The Standard Model of Particle Physics Section 1: Introduction and Overview Section 2: Theory recap about the SM Section 3: Measurements at the Z pole Section 4: Hadronic collision: a closer look Physique des Particules - CS10 Arnaud Duperrin - P3TMA 1

e. verify) current theoretical models (ex: Standard Model) this course (CS10) - discover new particles and check

2 Section 1: Introduction and overview There are two complementary approaches in physics: - Theory: - provides models explaining current known particles - and unify phenomena without apparent direct relationship - Experiment: - check (i.e. verify) current theoretical models (ex: Standard Model) this course (CS10) - discover new particles and check consistency with new models (like supersymmetry for instance) (CS11) Physique des Particules - CS10 Arnaud Duperrin - P3TMA 2

3 1.1 History of Particle Physics 1.2 Experiments in Particle Physics Section 1: Introduction and Overview Physique des Particules - CS10 Arnaud Duperrin - P3TMA 3

1.1 History of Particle Physics The aim of particle physics is to describe the elementary constituents of matter and the interactions between them.

Röntgen in 1895, ( Nobel Prize 1901) Radioactivity by H.

4 1.1 History of Particle Physics The aim of particle physics is to describe the elementary constituents of matter and the interactions between them. This field of physics entered its modern phase at the end of the nineteenth century with a series of exciting discoveries: X-rays by W.C. Röntgen in 1895, ( Nobel Prize 1901) Radioactivity by H. Becquerel in 1896, ( Nobel Prize 1903) Image of Becquerel's photographic plate which has been fogged by exposure to radiation from a uranium salt. The shadow of a metal Maltese Cross placed between the plate and the uranium salt is clearly visible Henri Becquerel ( ) Prix Nobel 1903 Physique des Particules - CS10 Arnaud Duperrin - P3TMA 4

The electron as the first particle (still considered elementary today!) discovered by J.

Prior to his work, it was believed that atoms were the fundamental building blocks of matter

thermal junction and comparing this with the magnetic deflection of the rays.

5 The electron as the first particle (still considered elementary today!) discovered by J.J. Thomson in Prior to his work, it was believed that atoms were the fundamental building blocks of matter J.-J. Thomson ( ) Nobel Prize 1906 Thomson discovered this through his explorations on the properties of cathode rays. He estimated the mass of cathode rays by measuring the heat generated when the rays hit a thermal junction and comparing this with the magnetic deflection of the rays. His experiments suggested not only that cathode rays were over 1000 times lighter than the hydrogen atom, but also that their mass was the same whatever type of atom they came from. cathode rays Physique des Particules - CS10 Arnaud Duperrin - P3TMA 5

6 Radioactive decays, discovered in uranium, invalidated the common belief in unchangeable chemical elements or atoms. indeed the phenomenon of radioactivity involves the three interactions still studied today in particle physics (and reminded in this first chapter): the strong, the electromagnetic and the weak interaction. Physique des Particules - CS10 Arnaud Duperrin - P3TMA 6

γ-rays: the atomic nucleus - as a composite object of charged and neutral particles -may undergo transitions, for example from an excited state to the ground state:

7 γ-rays: the atomic nucleus - as a composite object of charged and neutral particles -may undergo transitions, for example from an excited state to the ground state: radioactive decays under emission of γ-rays: identified as photons, this is a manifestation of the electromagnetic interaction Physique des Particules - CS10 Arnaud Duperrin - P3TMA 7

interaction the change of particle identity appears at a more

8 β-rays: radioactive decays under emission of electrons: change neutrons into protons This is a manifestation of the charged weak interaction the change of particle identity appears at a more elementary level (nucleons). Physique des Particules - CS10 Arnaud Duperrin - P3TMA 8

Over the last 100 years, experimental results on the weak interaction have shown many surprises.

Pauli in 1930 to ensure energy-momentum and angular momentum conservation in β decays (see previous

in 1957, (wait for chapter 4 to know more about this) 3. discovery of CP violation by J.W. Cronin, V.

9 Over the last 100 years, experimental results on the weak interaction have shown many surprises. Among these drastic changes in physics: 1. postulation of neutrinos by W. Pauli in 1930 to ensure energy-momentum and angular momentum conservation in β decays (see previous slide), 2. discovery of Parity violation in charged weak decays by C.S. Wu et al. in 1957, (wait for chapter 4 to know more about this) 3. discovery of CP violation by J.W. Cronin, V.L. Fitch et al. in 1964 (chapter 4) Wolfgang Pauli ( ) Nobel Prize 1945 Chien-Shiung Wu Physique des Particules - CS10 Arnaud Duperrin - P3TMA 9

10 4. discovery of neutral weak interactions by F.J. Hasert et al. in 1973 (in GARGAMELLE neutrino experiment) These weak interactions (within the framework of gauge theories): exchange a very massive spin-1 gauge bosons: the charged W boson or the neutral Z boson. weak = short range W and Z vector bosons were discovered by C. Rubbia et al. in Physique des Particules - CS10 Arnaud Duperrin - P3TMA 10

11 α-rays: radioactive decays of heavy nuclei under emission of Helium nuclei. Helium nuclei is composed of neutral and charged nucleons, called neutrons and protons. nucleons are bound together by the strong interaction Physique des Particules - CS10 Arnaud Duperrin - P3TMA 11

In order to describe the three phenomenologically vastly different strong, weak and electromagnetic interactions in a common framework the

particle physics is a renormalisable quantum field theory of the electroweak and strong interactions.

12 In order to describe the three phenomenologically vastly different strong, weak and electromagnetic interactions in a common framework the following concepts are used: relativity, quantum theory, local gauge symmetry, and spontaneous symmetry breaking The Standard Model (SM) of particle physics is a renormalisable quantum field theory of the electroweak and strong interactions. This theory is able to explain (or at least accommodate) all experimental results in particle physics obtained so far! Physique des Particules - CS10 Arnaud Duperrin - P3TMA 12

13 The electroweak theory part of the SM was developed by S.L. Glashow, S.Weinberg and A.Salam from 1961 to 1968, S. Glashow, A. Salam, S. Weinberg Nobel Prize 1979 Provides a description of the weak and electromagnetic interactions. Leaving aside, however, the problem of mass generation. Physique des Particules - CS10 Arnaud Duperrin - P3TMA 13

The problem of mass generation in gauge theories was solved in 1964 by P.W.

The mechanism was first proposed in 1962 by Philip Warren Anderson.

Brout and Francois Englert; Peter Higgs; and Gerald Guralnik, C. R.

14 The problem of mass generation in gauge theories was solved in 1964 by P.W. Higgs et al. It is named the Higgs mechanism. The mechanism was first proposed in 1962 by Philip Warren Anderson. The relativistic model was developed in 1964 by three independent groups: Robert Brout and Francois Englert; Peter Higgs; and Gerald Guralnik, C. R. Hagen, and Tom Kibble. Nobel prize 2013! 2010 J.J. Sakurai Prize Kibble, Guralnik, Hagen, Englert, and Brout Physique des Particules - CS10 Arnaud Duperrin - P3TMA 14

thooft in 1971. G. thooft came recently to visit us!

15 Renormalisability of the Standard Model for the strong and electroweak interactions was proven by G. thooft in G. thooft came recently to visit us! Physique des Particules - CS10 Arnaud Duperrin - P3TMA 15

What Renormalization means: renormalization is any of a collection of techniques used to treat infinities arising in calculated quantities.

16 What Renormalization means: renormalization is any of a collection of techniques used to treat infinities arising in calculated quantities. o The idea was to realize that the quantities initially appearing in the theory's formulae (in the Lagrangian for instance) representing such things as the electron's electric charge or mass o do not actually correspond to the physical constants measured in the laboratory. o As written, they are what we call bare quantities that do not take into account the contribution of virtual-particle loop effects. diagram contributing to electron-electron scattering in QED. The loop has an ultraviolet divergence. Physique des Particules - CS10 Arnaud Duperrin - P3TMA 16

The strong interaction theory (or quantum chromodynamics, QCD) was

17 The strong interaction theory (or quantum chromodynamics, QCD) was developed in 1973 by H. Fritzsch, M. Gell-Mann, H. Leytwyler, D.J. Gross, F. Wilczek and many others. The concept of confinement of quarks and gluons inside hadrons was suggested by S.Weinberg in The asymptotic freedom property was discovered by D.J. Gross, F. Wilczek and H.D. Politzer in Physique des Particules - CS10 Arnaud Duperrin - P3TMA 17

1.2 Experiments in Particle Physics Until the middle of the 20 th century, particle physics experiments were mainly based on the study of radioactive materials and cosmic rays.

18 1.2 Experiments in Particle Physics Until the middle of the 20 th century, particle physics experiments were mainly based on the study of radioactive materials and cosmic rays. Cosmic rays are immensely high-energy radiation, mainly originating outside the Solar System. (mostly protons and hadrons) Physique des Particules - CS10 Arnaud Duperrin - P3TMA 18

s = Fixed-target experiments: Collider experiments: p b =0 m a E a,p a m b s = m a2 + m b 2 + 2 E a E

19 With the progress of particle accelerators, the experimental emphasis shifted towards fixed-target and colliding-beam experiments, allowing to perform experiments under controlled beam conditions. s = Fixed-target experiments: Collider experiments: p b =0 m a E a,p a m b s = m a2 + m b E a E b (beam energy>> particle masses) m E, P E, -P m = 2 E Physique des Particules - CS10 Arnaud Duperrin - P3TMA 19

20 Colliders = more energy for the collision Comparison: Beam of 7 TeV proton on a fix proton target: E cm = 0.1 TeV ( ) 2 beams of 7 TeV colliding: E cm = 14 TeV (LHC) ( = 2 E ) To reach the same energy with a fix target, on should have a beam of: E(beam)= TeV!!! Cosmic ray flux versus particle energy Only way to have more energies Cosmic rays (Cosmic rays have more energies but initial state conditions are not under control) Physique des Particules - CS10 Arnaud Duperrin - P3TMA 20

21 Conditions to have colliding-beam: particles must be stable (i.e. not to decay while circulating) the beam must be made of charged particles (i.e. to be accelerated) Leaving as possibilities: electrons, protons, and their antiparticles. Simon Van der Meer invented the technique of stochastic cooling of particle beams: Nobel Prize1984 his technique was used to accumulate intense beams of antiprotons Physique des Particules - CS10 Arnaud Duperrin - P3TMA 21

22 1.2.1 Proton and antiproton colliders Why on circular orbits, protons and antiprotons can be accelerated to much higher energy than electrons and positrons? Because their masses are higher and therefore the energy loss due to synchrotron radiation is thus much reduced. higher center-of-mass energies in collisions are more easily obtained at proton-proton (or proton-antiproton) colliders (like at LHC) allowing the production and thus discovery of new particles with high masses. σ (proton - antiproton) 1 million x σ (e + e - ) Physique des Particules - CS10 Arnaud Duperrin - P3TMA 22

The SPS accelerator at CERN in Geneva was switched on in 1976 at CERN.

6 TeV: The SPS experiments UA1 and UA2 discovered in 1983 the heavy intermediate vector

23 The SPS accelerator at CERN in Geneva was switched on in 1976 at CERN. In 1983, it provided proton-antiproton collisions at center-of-mass energies of up to 0.6 TeV: The SPS experiments UA1 and UA2 discovered in 1983 the heavy intermediate vector bosons: the charged W boson and the neutral Z boson, with masses around 80 GeV and 90 GeV, respectively ( Nobel prize to C. Rubbia) By the end of data taking at the SPS in 1990, a few hundred W and Z bosons were accumulated. A W eν event as seen in the UA1 tracker, Physique des Particules - CS10 Arnaud Duperrin - P3TMA 23

The TEVATRON proton-antiproton collider at Fermilab near Chicago USA

8 TeV which is a factor of 3 larger than that of the SPS.

collected more than 100 pb 1 of luminosity each, yielding several

slide) The second phase of the Tevatron (Run II) started in 2001 and

24 The TEVATRON proton-antiproton collider at Fermilab near Chicago USA (in its first phase, Run I) operated at a center-of-mass energy of 1.8 TeV which is a factor of 3 larger than that of the SPS. By the end of run I in 1996, the TEVATRON experiments CDF and DØ have collected more than 100 pb 1 of luminosity each, yielding several W and Z bosons, and the discovery of the top quark (see next slide) The second phase of the Tevatron (Run II) started in 2001 and finished in 2011 with 10 fb -1 Physique des Particules - CS10 Arnaud Duperrin - P3TMA 24

The most important discovery at the TEVATRON: the 6 th and heaviest quark the top quark, by CDF and DØ experiments in 1995.

25 The most important discovery at the TEVATRON: the 6 th and heaviest quark the top quark, by CDF and DØ experiments in The top-quark mass is about 175 GeV it is so large that it couldn t be pairproduced at e + e colliders (like at the LEP at CERN in 1990 s). more about the top quark in chapter 3 Physique des Particules - CS10 Arnaud Duperrin - P3TMA 25

26 Another very important result from the Tevatron is the excess with a significance of 2.9σ seen in 2012 in the combination of CDF and DØ's H bb channels interpreted as coming from a Higgs boson with a mass in the region of 115 to 135 GeV. (remember that as of today, the LHC has not yet demonstrated the discovery of the Higgs in the bb channel ) H bb qq WH νbb qq ZH + bb qq ZH ννbb use associated production modes to get better S/B more about the Higgs boson in CS11 class Physique des Particules - CS10 Arnaud Duperrin - P3TMA 26

27 Summer 2012 Tevatron Combination Observed Limit (i.e. what data tell you) SM Prediction Significant excess, 2-3 sigma for GeV Expected Limit (assuming no Higgs) Physique des Particules - CS10 Arnaud Duperrin - P3TMA 27 27

A quick look at several Standard Model processes which are nowadays considered as backgrounds for searches Ex: SM processes involving jets (many at hadron colliders!

28 A quick look at several Standard Model processes which are nowadays considered as backgrounds for searches Ex: SM processes involving jets (many at hadron colliders! illustrated here with Tevatron): - Instrumental backgrounds QCD multijet (i.e. faking lepton ) - Derived from ( sidebands ) data Physique des Particules - CS10 Arnaud Duperrin - P3TMA 28

29 W/Z+jets backgrounds - Physics backgrounds: W/Z+jets with real / misidentified heavy flavour Physique des Particules - CS10 Arnaud Duperrin - P3TMA 29

30 Dibosons backgrounds - Dibosons Physique des Particules - CS10 Arnaud Duperrin - P3TMA 30

31 Top and single top backgrounds - ttbar and Single Top Physique des Particules - CS10 Arnaud Duperrin - P3TMA 31

32 The big thing now is the LHC: CERN was founded in 1954 as one of the first European joint ventures and has now 21 member states. The ATLAS and CMS experiments Physique des Particules - CS10 Arnaud Duperrin - P3TMA 32

17 th December 2012, the CERN completed the first LHC proton run (Run 1) after 3 years of run of the world s most powerful

33 A beam in the LHC is not a continuous string of particles it is divided into hundreds of bunches, each a few tens of centimeters long. Each bunch contains more than a hundred billion protons. 17 th December 2012, the CERN completed the first LHC proton run (Run 1) after 3 years of run of the world s most powerful particle accelerator. The collision energy was increased from 7 TeV in 2011, to 8 TeV in 2012, and on 3 rd June 2015 it reached 13 TeV. Physique des Particules - CS10 Arnaud Duperrin - P3TMA 33

34 Of the 5 billion collisions collected in 2011 and collisions produced results compatible with the Higgs particle whose discovery was announced in July 4, Followed by a long maintenance stop between 2013 and the beginning of At the end of year meeting in 2015 at CERN, something intriguing was reported in the presentation Physique des Particules - CS10 Arnaud Duperrin - P3TMA 34

35 @CERN on December 15 th 2015:? Physique des Particules - CS10 Arnaud Duperrin - P3TMA 35

36 After the short winter shutdown, running resumed in march 2016 at ~13 TeV Summer 2016.Finally not confirmed. Physique des Particules - CS10 Arnaud Duperrin - P3TMA 36

1.2.2 Electron and positron colliders In contrast, electron-positron colliders offer a much cleaner experimental environment due to the point-like nature of the colliding beam particles.

37 1.2.2 Electron and positron colliders In contrast, electron-positron colliders offer a much cleaner experimental environment due to the point-like nature of the colliding beam particles. Particles, interactions and theories are tested with high experimental precision. E 2 E Circular (with radius R): o Limited by synchrotron radiation: E E 4 /(m 4 R) each cyle (i.e. about 4 GeV at e + e - LEP2) o hadronic colliders favored compared to electron colliders: (m e /m p ) 4 ~ o Next generation of e + e - collider should be linear? Since a few years, the design of a μ + μ collider is also actively studied Hadron or electron circular collider also actively investigated (TLEP projects etc.). Too early to elaborate on this (see for details). Physique des Particules - CS10 Arnaud Duperrin - P3TMA 37

e + e colliders are particularly well suited to study properties of the Z boson via e + e Z production: In 1989, both the e + e colliders SLC at SLAC (Stanford) and LEP at CERN started to operate at

38 e + e colliders are particularly well suited to study properties of the Z boson via e + e Z production: In 1989, both the e + e colliders SLC at SLAC (Stanford) and LEP at CERN started to operate at center of-mass energies close to 91 GeV. This energy corresponds to the mass of the neutral Z boson. Z SLC is a linear accelerator At the SLC, electron and positron beams are accelerated in a single pass through a linear structure. At the end of the two-mile linear accelerator, they are bent into two arcs and then brought to a head-on collision at the center of a very large particle detector. Stanford, USA linear, e + e -, 3.2 km long, CME = 100 GeV data taking ended in June 1998 Physique des Particules - CS10 Arnaud Duperrin - P3TMA 38

The four LEP@CERN experiments ALEPH, DELPHI, L3 and OPAL collected until 1995, more than 16 million Z-boson decays (this period is called LEP I) At SLC@SLAC, until 1998, more than half a million Z

39 The four experiments ALEPH, DELPHI, L3 and OPAL collected until 1995, more than 16 million Z-boson decays (this period is called LEP I) At until 1998, more than half a million Z bosons have been produced with longitudinally polarized electron beams. This large amount of Z data combined with the clean experimental situation leads to high-precision measurements of the properties of the Z boson, such as: its mass, total and partial decay widths, and the neutral-current coupling constants of fermions. => see section 3 of this chapter measurement at the Z pole. Genève circular, e + e -, 27 km long, CME < 206 GeV data taking ended in 2001 Physique des Particules - CS10 Arnaud Duperrin - P3TMA 39

40 e + e colliders are also well suited to study properties of the W boson: From 1996 until the year 2000 the LEP collider operates in its second phase, called LEP II, where the center-of-mass energy is more than doubled to a range from 160 GeV up to 200 GeV. These center-of-mass energies allow the pair-production of on-shell W ± bosons, e + e W + W. Nearly W-pair events in total are recorded per experiment, making it possible to study the W boson precisely and in particular to measure its mass and its gauge couplings (the study continued at the hadron collider Tevatron and now at LHC). see chapter 3 Physique des Particules - CS10 Arnaud Duperrin - P3TMA 40

electron-proton colliders : HERA @DESY study proton structure using a beam of

(Hadron Elektron Ring Anlage) : 6.34 km in circumference.

Beam of accelerated electrons to energies of 27.

41 electron-proton colliders : study proton structure using a beam of electrons H1 ZEUS PETRA circular accelerator followed by the larger HERA ring (Hadron Elektron Ring Anlage) : 6.34 km in circumference. Collide protons with either electrons or positrons. Beam of accelerated electrons to energies of 27.5 GeV The other beam is made of protons accelerated to energies of 920 GeV in the opposite direction. Closed down on June 30 in Physique des Particules - CS10 Arnaud Duperrin - P3TMA 41

Recap: accelerators in past 15 years LEP (CERN) e +

Comfort the SM SLC, and then PEP-II (Stanford) e + e

1 GeV e + shutdown in 2008 Babar experiment (see

HERA ep X 920 GeV p / 28 GeV e shutdown in 2007

p-pbar X s ~ 2 TeV shutdown in 2011 Tests of the SM

but evidence for Higgs bb in 2012 LHC (CERN) pp X s

Rediscover the SM Higgs boson discovered in 2012,

42 Recap: accelerators in past 15 years LEP (CERN) e + e - X s~ 200 GeV shutdown in 2001 Precision physics Comfort the SM SLC, and then PEP-II (Stanford) e + e - X s~ 91 GeV (1998), and 9.1 GeV e - / 3.1 GeV e + shutdown in 2008 Babar experiment (see Chapter 4) Study of million of B mesons at PEP-II. HERA ep X 920 GeV p / 28 GeV e shutdown in 2007 Proton structure Comfort the SM Tevatron (FERMILAB) p-pbar X s ~ 2 TeV shutdown in 2011 Tests of the SM Comfort the SM (top quark discovery) No New Physics but evidence for Higgs bb in 2012 LHC (CERN) pp X s ~ 7 (puis 14) TeV started in November 2010 Rediscover the SM Higgs boson discovered in 2012, and search for New Physics on going Physique des Particules - CS10 Arnaud Duperrin - P3TMA 42

43 Futures' accelerators projects HL-LHC Physique des Particules - CS10 Arnaud Duperrin - P3TMA 43

Weak Interactions. The Theory of GLASHOW, SALAM and WEINBERG

Weak Interactions. The Theory of GLASHOW, SALAM and WEINBERG Weak Interactions The Theory of GLASHOW, SALAM and WEINBERG ~ 1959-1968 (Nobel 1979) Theory of the unified weak and electromagnetic interaction, transmitted by exchange of intermediate vector bosons mass