Particle Physics II

1. Particle Physics II 1st Handout • Experimental electroweak physics: W &Z • Resonances • Measuring the number of neutrinos from the width of the Z0 • W mass & branching ratio Chris Parkes Room 455, chris.parkes@cern.ch

2. Course Content • Experimental Electroweak: W & Z • Number of generations from Z resonance • Triple boson vertices • Top Quark & Higgs • Direct & indirect searches • Heavy Flavour Physics • Flavour changing interactions • CP Violation • matter anti-matter asymmetry • Beyond the Standard Model • Neutrinos • Supersymmetry • Astro-particle physics Text books: 1.Alessandro Bettini, Introduction to Elementary Particle Physics, Cambridge University Press (2008) 2. B.R. Martin and G. Shaw, Particle Physics (2nd edition), Wiley (2001) 3. Donald H. Perkins, Introduction to High Energy Physics (4th edition), Cambridge University Press (2000).

3. Resonances and particles • Many particles are too short-lived to be observed directly • Observe from decay particles • e.g. ppX+other stuff, Xµ+µ- Energy uncertainty ΔE is known as the particles width . Uncertainty principle relates width and mean lifetime τof particle Three resonances =2ΔE FWHM Exponential decay process for particles

4. Particle Width/lifetime Shorter lifetime, larger width • Depends on: • Strength of interaction • Strong G~10-100MeV t~10-22-10-24s • EM G~10-100keV t~10-16-10-20s • Weak G<0.01eV t~10-8-10-13s • Number of different states the particle can decay into • Can see long-lived particle through their decay distance • Can see short-lived particles through their width STRONG EMAG WEAK Flight distance

5. Breit-Wigner Energy spins • The total width G is related to the strength of the interaction • The particle can usually decay to • A number of different final states • Each individual decay mode has a width, Gi, such that sum of “partial widths”=total width. • Branching Ratio • Fraction of decays to that state • BR=Gi/G • Total width from width of invariant mass distribution • Mass from centre of distribution • Partial width of initial and final states from height of distribution • Shape is same for all decays

6. Z0 production • e+e- has two diagrams g and Z0 exchange • Photon exchange dominates at low c.m. energies • Z0 exchange results in a resonance at Mz • At High cm energy, both photon and Z exchange– EW unification • Calculation of relative strength of xsection

7. Much Ado about Nothing:Getting Nn from GZ Final states X in Z-decays  Counting states Ge=Gm=Gt Gl Gne=Gnm=Gnt  Gn Gu=Gc  Gu Gd=Gs=Gb  Gd GZ=Ghad+3Gl+NnGn  How to get Nn Total width from shape in any decay channel Partial widths from all seen decay channels What isn’t seen is the neutrinos Ghad

8. Number of neutrinos • Mass – peak of distribution • MZ=91.1876±0.0021GeV (0.02% error!) • Total width - width GZ=2.4952±0.0023GeV (0.1%) • Partial Widths – number events Ghad=1.741±0.006 Gl=0.0838±0.0003 • Neutrino width from theory • determine number • Nn=2.97±0.07 LEP

9. What does it mean? • Proves there are only three generations of particles • 3 neutrino families  3 generations • Satisfies anomaly condition • What is the catch? • Assumed neutrinos massless in theory width • Neutrinos now know to have mass (more later) but very small • Mass hierarchy • We have summed Z X+X- If there is an extra neutrino X With mass > Z mass/2 Z could not decay to it Z0

10. How is Z mass / lineshape reconstructed ? • Energy of electron&positron beams in accelerator adjusted • Cross-section measured at different energies • Breit-wigner fitted to cross-sections Peak value gives mass

11. Aside: Measuring the LEP beam energy TRAINS The accuracy with which the LEP beam energy is known controls the accuracy of MZ Return current for train –TGV- changed magnet currents Tidal effects change the shape of the ring TIDE TIDE

12. W Discovery • UA1, UA2 1983 • Proton anti-proton collider • We-+ν • Lots of soft tracks + electron ‘transverse’ mass E2-p2, taking only components In transverse plane

13. LEP e+e-W+W- • Can be used to make precision tests of electroweak theory Recall: 3 diagrams contribute First Event: Me et al. !

14. Demonstration of triple boson vertex • All three diagrams contribute and interference • Total cross-section agrees with theory, requires • Triple boson vertex

15. W boson: mass measurement Why: • Test of SM, recall • Allows measure top mass, higgs mass in SM • See later How: • Measure momentum of all charged particles • Highly relativistic E≈p • Energy of neutrals in calorimeters • Less well measured than p • Cluster particles into jets • Obtain quark momenta • ‘Missing’ momenta • Gives neutrino • Pair jets/leptons • Form W • Reconstruct mass from • E2-p2=m2 • Use known constraints to improve • Total energy from beams • Total momentum=0

16. W boson: branching fractions • Count possible decays of W- • Leptonic: • Hadronic: [Hadronic is approximation – discuss CKM matrix later] • but 3 colours • 3 leptonic+2 hadronic *3 colours =9 states • Approx 1/9 B.R per state • 6/9= 67% hadronic • Width per state =225 MeV • Total width W=9*225MeV~2 GeV Not as too heavy Q)What is lifetime ? Q)What fraction e+e-W+W- are fully hadronic/leptonic/mixed decays? Which can be used for mass measurement ?