Teilchenphysik 2 — W/Z/Higgs an Collidern

Sommersemester 2019

Matthias Schroder¨ und Roger Wolf | Vorlesung 3

INSTITUTFUR¨ EXPERIMENTELLE TEILCHENPHYSIK (ETP)

KIT – Die Forschungsuniversitat¨ in der Helmholtz-Gemeinschaft www.kit.edu 2. The Electroweak Sector of the Standard Model

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 2/24 2. Electroweak Sector of the Standard Model

2.1 Gauge theory ◦ Global and local phase transformations ◦ Example: QED ◦ Abelian and non-Abelian gauge theories 2.2 The electroweak sector of the Standard Model – I ◦ Properties of the weak interaction, weak isospin ◦ Formulation of the Standard Model (without masses) 2.3 Discovery of W and Z bosons ◦ History towards discovery ◦ Experimental methods 2.4 The Higgs mechanism ◦ Problem of massive gauge bosons and massive fermions ◦ Idea of the Higgs mechanism: examples of spontaneous symmetry breaking 2.5 The electroweak sector of the Standard Model – II ◦ The Standard Model Higgs mechanism ◦ Yukawa couplings and fermion masses ◦ The Higgs boson

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 3/24 2. Electroweak Sector of the Standard Model

2.1 Gauge theory ◦ Global and local phase transformations ◦ Example: QED ◦ Abelian and non-Abelian gauge theories 2.2 The electroweak sector of the Standard Model – I ◦ Properties of the weak interaction, weak isospin ◦ Formulation of the Standard Model (without masses) 2.3 Discovery of W and Z bosons ◦ History towards discovery ◦ Experimental methods 2.4 The Higgs mechanism ◦ Problem of massive gauge bosons and massive fermions ◦ Idea of the Higgs mechanism: examples of spontaneous symmetry breaking 2.5 The electroweak sector of the Standard Model – II ◦ The Standard Model Higgs mechanism ◦ Yukawa couplings and fermion masses ◦ The Higgs boson

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 4/24 2.3 Discovery of the W and Z bosons

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 5/24 Weak Interactions: Divergences

e� scattering as contact interaction Weak Interaction: Divergencies Divergences in theories of physics �e �e → predictive◦ Divergence power limited 1: contact interaction in Fermi theory of weak interaction 2 ◦ Coupling constant GF dimensional quantity [GF] = energy− 2 Divergence #1:◦ contactPredicted scattering interactions cross-section in grows as σ ∝ Ecms = s Weak Interactions:Fermi Divergences theory◦ Solution:of weakexchange interactions of massive vector bosonse– e–

Coupling constant GF dimensional quantity e� scatteringeν scattering as as contact contact interaction interaction ee�ν scatteringscattering withwith vector-boson vector boson exchange exchange [GF] = energy–2 Divergences in theories of physics �e �e �e �e → predictive power limited Predicted scattering cross section grows with square of center-of-mass energy: σ ~ s W– Divergence #1: contact interactionsIncrease in with s exceeds Froissart bound e– e– σ ~ ln(s/s0)2 → divergent for s → ∞ Fermi theory of weak interactions e– e– e– e–

Coupling constant GF dimensional quantity Solution (O. Klein,e� scattering 1938) with: vector boson exchange –2 [GF] = energy Z Replace contact interaction�e by exchange�e G = 1.16639(2) · 10 5 GeV 2 Predicted scattering cross section grows with F − − of massive vector bosons→ m ≈ O 1 O 100 GeV V ( √G ) = ( ) square of center-of-mass energy: σ ~ s F �e �e W– 316 Particle Physics I (4022031) – Lecture #8 Winter Semester 2017/2018 Increase with s exceeds Froissart bound Matthias Schroder¨ –e W/Z/Higgs– an Collidern (Sommersemestere– 2019) Vorlesung 3 6/24 σ ~ ln(s/s0)2 → divergent for s → ∞ e– e–

Solution (O. Klein, 1938): Replace contact interaction by exchange Z of massive vector bosons �e �e

316 Particle Physics I (4022031) – Lecture #8 Winter Semester 2017/2018 Weak Interactions: Divergences Weak Interaction: Divergencies Divergence #2: WW pair production in V–A theory ◦ Divergence 2: WW pair production in V − A theory Solution: require◦ Solution: direct require interaction direct interaction among among three three gauge gauge bosons (trilinearbosons (trilinear couplings,couplings, or or tripletriple gauge gauge couplings, couplings, TGC) TGC) → indirect hint→ indirectto existence hint to existence of Z of bosonZ boson Neutrino exchange Photon and Z boson exchange + + + + e e+ W e W+ e+ W W+ + + e e+ W W+

νe νe γ γ Z Z e− e− W− W− e− e− W− e− W− e− W− W−

+ + + + DivergenceFigure 1.4:Figure #3: Feynman 1.4: longitudinal Feynman diagrams diagrams (CC03) (CC03) for W the for processboson the process e e− scattering e We− WW− atW the− at Born the (W Born level.LW level.L → WLWL) → → Solution: additional contributions to scattering amplitude from exchange of scalar particles → indirect hint to existence of Higgs boson + + Z Z Quartice+ ande trilinear gaugeZ Z boson couplingse+ e Higgs-boson exchange Matthias+ Schroder¨ – W/Z/Higgs an+ Collidern (Sommersemester 2019) Vorlesung 3 7/24 Wlong Wlonge e e e − − − e Z − e H e Z e Z H Z – – Wlong Wlong + Figure 1.5: Feynman diagrams (NC02) for the process+ e e− ZZ at the Born level. Figure 1.5: Feynman diagrams (NC02) for the process e e− ZZ→ at the Born level. → 317 Particle Physics I (4022031) – Lecture #8 Winter Semester 2017/2018

e+ W+ e+ W+ e+ W+ e+ W+

γ Z γ/Z γ γ/Z Z γ/Z γ/Z e− W− e− W− e− W− e− W− + Figure 1.6: Feynman diagrams for the process e e− WWγ and WWZ at the Born level + → Figure 1.6:involving Feynman quartic diagrams electroweak-gauge-boson for the process e vertices.e− WWγ and WWZ at the Born level → involving quartic electroweak-gauge-boson vertices.

+ − + + + − e νe e e e νe + W − + /Z + + W − e ν e γ e e ν e W+ W− e /Z W γ/Z W γ γ/Z + W − W − W − − W − γ/Z e e e ν e ν γ/Z W e W e − − − − e e e νe e νe Figure 1.7: Vector-boson fusion diagrams for the single W/Z/γ process at the Born level. Figure 1.7: Vector-boson fusion diagrams for the single W/Z/γ process at the Born level.

24 24 Weak Interactions: Divergences

Divergence #2: WW pair production in V–A theory Solution: require direct interaction among three gauge bosons (trilinear couplings, or triple gauge couplings, TGC) → indirect hint to existence of Z boson Neutrino exchange Photon and Z boson exchange + + + + e e+ W e W+ e+ W W+ + + e e+ W W+

νe νe γ γ Z Z e− e− W− W− Weak Interaction:e− Divergenciese− W− e− W− e− W− W−

+ + + + DivergenceFigure◦ Divergence 1.4:Figure #3: Feynman 1.4: longitudinal Feynman diagrams3: longitudinal diagrams (CC03) (CC03) for WW the boson for processboson the process scattering e e− scattering e We− W(WW− atLWW the−Lat Born→ the (W Born level.LLWW level.L) L → WLWL) → → Solution:◦ additionalSolution: additional contributions contributions to to scattering scattering amplitude amplitude from exchangefrom exchange of of scalar particles → indirect hint to existence of Higgs boson scalar particles → indirect hint to existence of Higgs boson + + Z Z Quartice+ ande trilinear gaugeZ Z boson couplingse+ e Higgs-boson exchange + + Wlong Wlonge e e e − − − e Z − e H e Z e Z H Z – – Wlong Wlong + Figure 1.5: Feynman diagrams (NC02) for the process+ e e− ZZ at the Born level. Figure 1.5: Feynman diagrams (NC02) for the process e e− ZZ→ at the Born level. → 317 Particle Physics I (4022031) – Lecture #8 Winter Semester 2017/2018

e+ W+ e+ W+ e+ W+ e+ W+

γ Z γ/Z γ γ/Z Z γ/Z γ/Z e− W− e− W− − − − − Matthias Schroder¨ e – W/Z/Higgs an Collidern (SommersemesterW 2019) e WVorlesung 3 8/24 + Figure 1.6: Feynman diagrams for the process e e− WWγ and WWZ at the Born level + → Figure 1.6:involving Feynman quartic diagrams electroweak-gauge-boson for the process e vertices.e− WWγ and WWZ at the Born level → involving quartic electroweak-gauge-boson vertices.

+ − + + + − e νe e e e νe + W − + /Z + + W − e ν e γ e e ν e W+ W− e /Z W γ/Z W γ γ/Z + W − W − W − − W − γ/Z e e e ν e ν γ/Z W e W e − − − − e e e νe e νe Figure 1.7: Vector-boson fusion diagrams for the single W/Z/γ process at the Born level. Figure 1.7: Vector-boson fusion diagrams for the single W/Z/γ process at the Born level.

24 24 Search for Neutral Currents

◦ Strategy: interactions of neutrinos ◦ No electromagnetic processes ◦ Charged-particle scattering: huge background of electromagnetic processes → impossible ◦ CERN neutrino beam (since 1970) + + ◦ Protons from CERN PS on target: muon neutrinos, e. g. π → µ νµ ◦ Detection: bubble chamber experiment Gargamelle

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 9/24 The Gargamelle Bubble Chamber

ν beam

cds.cern.ch

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 10/24 Discovery of Neutral Currents

◦ Gargamelle: neutrino beam hits heavy liquid (freon) ◦ Neutrinos interact with electrons and atomic nuclei

◦ Neutral-current interaction: scattered electron → electromagnetic shower (bremsstrahlung and e+e− production) ◦ Charged-current interaction: muon in final state → long track

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 11/24 Neutrino–ElectronNeutrino-Electron Neutral NC Currents

(–) (–) νµ µ− νµ νµ

W Z 121 Phys. Lett. (1973) B46 hs et 4 17)121 (1973) B46 Lett. Phys. e− νe e− e−

Neutrino Interaction Beam electromagnetic shower

348 Matthias Schroder¨ – W/Z/Higgs an Collidern (SommersemesterParticle Physics 2019) I (4022031) – Lecture #9 Vorlesung 3Winter Semester 12/24 2017/2018 W and Z Boson Production

◦ Production process: lepton production in hadronic interactions (“Drell–Yan process”) ◦ Requirement to produce real W and Z bosons at accelerators: Ecms of initial-state fermions at expected boson mass (≈ 100 GeV) ¯ f l− q¯′ ν

Z W+

f l+ q l+ √ p ◦ Fixed-target setup: s = 2mpE → E = 5 TeV → unrealistic √ ◦ Electron-positron collider: s = E1 + E2 → 50 GeV beam energy → technically feasible only from the 1990s, only Z production ◦ Proton-antiproton collider: W/Z production by annihilation of valence quark in proton and valence antiquark in antiproton

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 13/24 SppS ◦ pp collider reach

◦ Momentum fractions of colliding valence (anti)quarks x1 ≈ x2 ≈ 0.2 √ √ ! √ → estimated Ecms: ˆs ≈ x1x2s = 100 GeV → s = 500 GeV ◦ SPS (Super Proton Synchrotron) at CERN ◦ 6.9 km circumference ◦ 400 GeV protons UA2 experiment Idea (C. Rubbia, 1976): upgrade to a pp collider SppS SppS

◦ Ecms = 540 GeV initially later upgrade to 630 GeV ◦ UA1 and UA2 experiments data taking from 1981 UA1 experiment

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 14/24 SppS

◦ Antiprotons ◦ p beam on target: approximately 1 p in 109 final-state particles ◦ Very large emittance of p beam: reduction without violating Liouville’s theorem? ◦ Solution (S. van der Meer, 1968): stochastic cooling1 with pick-up and kicker ◦ Further challenges ◦ p and p share same beam pipe ◦ Hermetic 4π detectors for the cerncourier.com first time at hadron colliders

1Details e. g. Nucl. Instrum. Meth. A532 (2004) 11 Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 15/24 Discovery of the W Boson (1983)

◦ Process pp → W → lν (l = e, µ)

q¯′ ν 103 (1983) B122 Lett. Phys.

W+

q l+

◦ Analysis strategy: charged lepton+ E/ T ◦ Charged lepton: clean detector signature ◦ Neutrino: missing transverse momentum ◦ 2-body decay W → lν: lepton and neutrino

(E/ T) back-to-back in W-boson rest frame ◦ QCD jet production background: no

preferred relative directions of lepton and E/ T

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 16/24 Discovery of the W Boson (1983)

◦ Process pp → W → lν (l = e, µ)

q¯′ ν hs et 12(93 103 (1983) B122 Lett. Phys. W+

q l+

◦ Analysis strategy: charged lepton+ E/ T ◦ Charged lepton: clean detector signature ◦ Neutrino: missing transverse momentum ◦ 2-body decay W → lν: lepton and neutrino

(E/ T) back-to-back in W-boson rest frame ◦ QCD jet production background: no

preferred relative directions of lepton and E/ T

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 17/24 Discovery of the Z Boson (1983)

◦ Process pp → Z → l+l− (l = e, µ)

¯f l− 398 (1983) B126 Lett. Phys.

Z

f l+ ◦ Analysis strategy: charged leptons ◦ Invariant mass of lepton pair ◦ Depends on lepton momenta and opening angle (lepton masses neglected)

2 mZ ≈ 2 · |~pl+ | · |~pl− | · (1 − cos φl+l− )

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 18/24 Discovery of the Z Boson Candidate event with 2 electrons: before cuts

Phys. Lett. B126 (1983) 398

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 19/24 Discovery of the Z Boson Candidate event with 2 electrons: after cuts

Phys. Lett. B126 (1983) 398

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 20/24 Nobel Prize 1984 Nobel Prize 1984

C. Rubbia S. van der Meer

“for their decisive contributions to the large project, which led to the discovery of the field particles W and Z, communicators of weak interaction”

www.nobelprize.org[nobelprize.org]

355 Particle Physics I (4022031) – Lecture #9 Winter Semester 2017/2018

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 21/24 Summary

◦ Central prediction of the electroweak theory confirmed ◦ New neutral current interactions observed in neutrino-electron and neutrino-nucleus scattering (Gargamelle, CERN 1973) ◦ Mediator particles W boson and Z boson produced in pp collisions (UA1 and UA2 at SppS, CERN 1983)

◦ W signature: lepton+E/ T, back-to-back ◦ Z signature: same-flavour (SF) opposite charge-sign (OS) lepton pair with large invariant mass ◦ pp collider SppS ◦ Stochastic cooling: p-beam emittance reduced ◦ Experiments (UA1, UA2) with electronic readout

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 22/24 Summary

10 5 Z

4 − 10 e+e →hadrons hs et 2 20)257 (2006) 427 Rept. Phys. Cross-section (pb)

10 3

2 CESR + - 10 DORIS PEP W W PETRA TRISTAN KEKB SLC PEP-II 10 LEP I LEP II 0 20 40 60 80 100 120 140 160 180 200 220 Centre-of-mass energy (GeV)

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 23/24 Programme

Date Room Type Topic

Wed Apr 24. Kl. HS B LE 01 1. Organisation and introduction: particle physics at colliders + W/Z/H history Tue Apr 30. 30.23 11/12 — no class Wed May 01. Kl. HS B — no class Tue May 07. 30.23 11/12 LE 02 2.1 Gauge theory & 2.2 The electroweak sector of the SM I Wed May 08. Kl. HS B LE 03, EX 01 2.3 Discovery of the W and Z bosons & EX gauge theories Tue May 14. 30.23 11/12 LE 04 2.4 The Higgs mechanism Wed May 15. Kl. HS B EX 02 Exercise “SM Higgs mechanism” Tue May 21. 30.23 11/12 — no class Wed May 22. Kl. HS B LE 05 2.5 The electroweak sector of the SM II (Higgs mechanism + Yukawa couplings) Tue May 28. 30.23 11/12 SP 01 Specialisation of 2.4 and 2.5 Wed May 29. Kl. HS B LE 06 3.1 From theory to observables & 3.2 Reconstruction + analysis of exp. data Tue Jun 04. 30.23 11/12 EX 03 Exercise “Trigger efficiency measurement” Wed Jun 05. Kl. HS B LE 07 3.3 Measurements in particle physics (part 1) Tue Jun 11. 30.23 11/12 EX 04 Exercise on statistical methods Wed Jun 12. Kl. HS B LE 08 3.3 Measurements in particle physics (part 2) Tue Jun 18. 30.23 11/12 SP 02 Specialisation “Limit setting” Wed Jun 19. Kl. HS B SP 03 Specialisation “Unfolding” Tue Jun 25. 30.23 11/12 LE 09 4.1 Determination of SM parameters Wed Jun 26. Kl. HS B LE 10 4.2 Measurement and role of W/Z bosons at the LHC Tue Jul 02. 30.23 11/12 EX 05 Paper seminar “Z pole measurements” Wed Jul 03. Kl. HS B LE 11 4.3 Processes with several W/Z bosons Tue Jul 09. 30.23 11/12 EX 06 Paper seminar Higgs Wed Jul 10. Kl. HS B LE 12 5.1 Discovery and first measurements of the Higgs boson Tue Jul 16. 30.23 11/12 EX 07 Exercise “Machine learning in physics analysis” Wed Jul 17. Kl. HS B LE 13 5.2 Measurement of couplings and kinematic properties Tue Jul 23. 30.23 11/12 EX 08 Presentations: results of ML challenge Wed Jul 24. Kl. HS B LE 14 5.3 Search for Higgs physics beyond the SM & 5.4 Future Higgs physics

Matthias Schroder¨ – W/Z/Higgs an Collidern (Sommersemester 2019) Vorlesung 3 24/24