Standard Model of Particle Physics SS 2012

Transcription

1 Lecture: Standard Model of Particle Physics Heidelberg SS 2012 W- and Z-Bosons 1

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3 Contents Discovery of real W- and Z-bosons Intermezzo: QCD at Hadron Colliders LEP + Detectors W- and Z- Physics at LEP W- and Z-Physics at Hadron Colliders (Tevatron+LHC) 3

4 Prediction of W and Z masses SM predictions: e = g sin θw = g ' cos θw Measurement of Weinberg angle: g 0.6 sin 2 θw 0.25 Low energy limits of W-propagator G F / 2= g2 /8 M 2W M W 80 GeV Relation from vector-boson mass matrix M 2W g2 2 = = cos θw MZ g +g ' 4 M Z 90 GeV

5 W,Z Physics at Hadron Colliders l q W, Z _ l' _ q' 5

6 Intermezzo QCD QCD Lagrangian (physical fields) vector coupling Covariant derivative: SU(3) group generators Gluon field: non-abelian coupling SU(3) structure constants 6 self coupling

7 SU(3) Group Representation 1 r= 0 0 color states 0 g= b= generators (N*N-1) t 1= i 0 t 2= i ( ) ( ) ( t 3= ) ( ( t 4= i t 5= i t 7 = 0 0 i 0 i t 8= ( ( ) ( t 6= ) ) ) )

8 Quantum Chromodynamics anti-blue g6 green Meson q q q g5 q q g3 g8 g2 q anti-red g4 q Hadron q q q red g1 g7 q anti-green blue 8

9 Running of alphas ~ Λ QCD Distance Energy Asymptotic Freedom 9 Confinement

10 Asymptotic Freedom Oh Brother, where art thou? (2000) 10

11 Confinement The force between two quarks is N!!! binding energy distance consequence: free quarks or gluons are not observable 11

12 Three-Jet Event at PETRA Reaction: _ + e e qqg Hard gluon emission calculable in pqcd event topology Soft gluon emissions parton showers (non-pqcd) high particle multiplicities collinear emissions lead to jet structure Hadronisation long distance scale formation of hadrons from quarks and gluons 12

13 Parton Showers energy Parton Shower in Deep Inelastic Scattering 13

14 Luminosity-Function At Hadron Colliders: how to get from the proton to the parton? 1 Lq q = z q z / z 2 q z 2 dz 2 /with z2 s =z s q q z=z 1 z 2 s = total cms energy ^s = cms energy of hard parton interaction Parton density function q = q(x,μ2) In Lepton-Nucleon Scattering parton splitting (factorisation) scale μ = Q2 Question: Which scale determines parton splitting in hadron-colliders? Answer: factorisation scale typically: μ F = s Input from lepton-nucleon scattering needed! 14

15 Parton Dynamics The x-dependence of q(x,μ) can not be calculated from first principles! Parton densities have to be measured by experiments Evolution of parton densities in Q2 is described by DGLAP equations (splitting functions) 15

16 W,Z Production in Hadron Collisions Reaction: _ q q W (Z) X Collider energy: s1/2 ~ 500 GeV Boson masses MW,Z ~ 100 GeV MW,Z ~ ^s = x1 x2 s parton momentum fractions: x1, x2 ~ 0.2 valence-quark region _ p p W (Z) X need anti-protons! PDFLIB

17 W,Z Cross Section Reasonable cross section of 0.1 nb at s1/2/mw ~2 Typically: x1, x2 ~ 0.4 need high luminosity! 17

18 Proton Parton Densities multiply by 20! SPS energy 18

19 Super Proton (Antiproton) Synchrotron 270 GeV protons 270 GeV anti-protons

20 Cooling of Anti-protons electron cooling of anti-protons High luminosities are obtained for small beam emittances! Antiprotons are hot after production! Stochastic cooling of anti-protons Simon van de Meer 20

21 UA1 Experiment modern high energy collider experiment able to run at high collision rates (fast electronics) 21

22 UA2 experiment 22

23 Candidate Z ee 23

24 Z-candidate Event Signature U.Uwer 24

25 W-candidates U.Uwer 25

26 W-candidates exploit momentum conservation! U.Uwer 26

27 Kinematic Reconstruction of W-bosons U.Uwer 27

28 (MW / 2)2 U.Uwer 28

29 Final Result 2 ρ= MW 2 2 M Z cos θw Rho parameter consistent with 1 confirmation of the SM Nobel Prize for Physics 1984: C.Rubbia and S van de Meer 29

30 Large Electron Positron Collider e+e- collider s1/2 = GeV 30

31 Hadron Production in e e + - MZ = ± GeV 31

32 WW Pair Production at LEP U.Uwer 32

33 U.Uwer 33

34 W-Pair Production at LEP2 34

35 W leptonic branching fractions 35

36 Tevatron at Fermilab Proton Antiproton Collider at s1/2 = 2 TeV 36

37 Missing Transverse Momentum Jacobian peak at D0: ptv W ev electron neutrino ptv = ptmiss ptmiss 37

38 Latest Results W-mass Method: normalise W-mass measurement to Z-mass measurement and take input (precise Z-mass) from LEP } Tevatron Run 1 LEP2 March 2012 } Tevatron Run 2 World Average W-mass measurement important for Top and Higgs Mass predictions 38

39 LHC 2011: s1/2 = 7 TeV proton-proton collisions! 2012: s1/2 = 8 TeV >2014: s1/2 = 14 TeV 39

40 ATLAS Detector 40

41 LHC Kinematic Plane W,Z production dominated by sea quarks low x-region very well constrained by HERA W,Z region 41 W,Z production can be used to measure proton-pdfs and LHC luminosity!

42 Proton Parton Densities multiply by 20! LHC energy SPS energy 42

43 Quark Flavors in Z Production q qbar Z yz = pseudorapidity of Z-boson: 43 y = ln tan θ/2

44 Z ee candidate at ATLAS 44

45 Z-Peak at ATLAS 2011 data LHC is a Vector-Boson factory! 45

46 W-Production at LHC Charge Asymmetric! Handle to disentangle d and u valence quarks 46

47 W-boson Production at LHC 47

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49 Lepton Universality Check at LHC 49

50 Summary W, Z boson discovered in 1983 W, Z masses consistent with SM predictions Ratio of W and Z mass consistent with Weinberg angle measured in Neutral Currents Lepton universality tested in W, Z Decays W+ W- pair production cross section measured. Confirmation of triple gauge couplings (WWZ) W and Z mass relevant for Higgs mass predictions 50

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