Standard Model of Particle Physics - Max Planck … · Schöning/Rodejohann 1 Standard Model of Particle Physics SS 2012 Lecture: Standard Model of Particle Physics Heidelberg SS
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Schöning/Rodejohann 1 Standard Model of Particle Physics SS 2012
Lecture:
Standard Model of Particle Physics
Heidelberg SS 2012
W- and Z-Bosons
Schöning/Rodejohann 2 Standard Model of Particle Physics SS 2012
Schöning/Rodejohann 3 Standard Model of Particle Physics SS 2012
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)
Schöning/Rodejohann 4 Standard Model of Particle Physics SS 2012
Prediction of W and Z masses
e = g sinθW = g ' cosθW
SM predictions:
GF /√2= g2/8 MW
2
Low energy limits of W-propagator
Measurement of Weinberg angle:
sin2θW ≈ 0.25 → g ≈ 0.6
→ MW ≈ 80GeV
Relation from vector-boson mass matrix
MW2
M Z2
=g2
g2+g '2= cos2
θW→ M Z ≈ 90GeV
Schöning/Rodejohann 5 Standard Model of Particle Physics SS 2012
W,Z Physics at Hadron Colliders
q
q'_
W, Z
l
l'_
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Intermezzo QCDQCD Lagrangian (physical fields)
Covariant derivative:
Gluon field: non-abelian coupling
self coupling
SU(3) group generators
SU(3) structure constants
vector coupling
Schöning/Rodejohann 7 Standard Model of Particle Physics SS 2012
SU(3) Group Representation
8 generators (N*N-1)
r=100 g= 010 b= 001
t1=(0 1 01 0 00 0 0) t2=(
0 −i 0i 0 00 0 0) t3=(
1 0 00 −1 00 0 0 )
t4=(0 0 10 0 01 0 0 ) t5=(
0 0 −i0 0 0i 0 0 ) t6=(
0 0 00 0 10 1 0 )
t7=(0 0 00 0 −i0 i 0 ) t8=
1
√3 (1 0 00 1 00 0 −2 )
color states
Schöning/Rodejohann 8 Standard Model of Particle Physics SS 2012
Quantum Chromodynamics
qqq
Meson
Hadron
q
q q
q
q
qg7
g6 g5
g4
g2g1g3 g8
green
blue
red
anti-greenanti-red
anti-blue
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Running of alphaS
Distance
Energy
„Confinement“„Asymptotic Freedom“
~ Λ QCD
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„Asymptotic Freedom“
„Oh Brother, where art thou?“ (2000)
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ConfinementThe force between two quarks is 50000 N !!!
consequence: free quarks or gluons are not observable
distance
binding energy
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Three-Jet Event at PETRA
e+ e- → q q g_Reaction:
Hard gluon emissioncalculable in pQCDevent topology
Soft gluon emissionsparton showers (non-pQCD)high particle multiplicitiescollinear emissions leadto “jet” structure
Hadronisationlong distance scaleformation of hadronsfrom quarks and gluons
Schöning/Rodejohann 13 Standard Model of Particle Physics SS 2012
Parton Showers
Parton Shower in
Deep Inelastic Scattering
energy
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Luminosity-Function
q q
Lq q =∫z
1q z / z2q z2dz2 / z2 z=z1 z2with
ss=z
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: μF= s
At Hadron Colliders: how to get from the proton to the parton?
Input from lepton-nucleon scattering needed!
s = total cms energy
s = cms energy ofhard parton interaction
^
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Parton Dynamics● The x-dependence of q(x,μ) can not be calculated from first principles!
Parton densities have to bemeasured by experiments
Evolution of parton densities in Q2 is described by DGLAPequations (splitting functions)
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W,Z Production in Hadron Collisions
MW,Z
~ 100 GeV
s1/2 ~ 500 GeV
MW,Z
~ s = x
1 x
2 s ^
x1 , x
2 ~ 0.2
→ valence-quark region
Collider energy:
Boson masses
parton momentum fractions:
PDFLIB 2000
Reaction:
p p → W (Z) X_
q q → W (Z) X_
need anti-protons!
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W,Z Cross Section
x1 , x
2 ~ 0.4
Reasonable cross section of 0.1 nb at
s1/2/MW
~2
Typically:
need high luminosity!
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Proton Parton Densities
SPS energy
multiply by 20!
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Super Proton (Antiproton) Synchrotron
270 GeV protons
270 GeV anti-protons
1981-1984
Schöning/Rodejohann 20 Standard Model of Particle Physics SS 2012
Cooling of Anti-protonselectron cooling of anti-protons
Stochastic cooling of anti-protons
High luminosities are obtained for small beam emittances ! Antiprotons are hot after production!
Simon van de Meer
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UA1 Experiment
“modern” high energycollider experiment ableto run at high collision rates(fast electronics)
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UA2 experiment
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Candidate Z → ee
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Z-candidate Event Signature
U.Uwer
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W-candidates
U.Uwer
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W-candidates
U.Uwer
exploit momentum conservation!
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Kinematic Reconstruction of W-bosons
U.Uwer
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(MW
/ 2)2
U.Uwer
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Final Result
Rho parameter consistent with 1 → confirmation of the SM
ρ=MW
2
M Z2 cos2
θW
Nobel Prize for Physics 1984: C.Rubbia and S van de Meer
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Large Electron Positron Collider
e+e- collider
s1/2 = 90-200 GeV
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Hadron Production in e+ e-
MZ = 91.1876 ± 0.0021 GeV
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WW Pair Production at LEP
U.Uwer
Schöning/Rodejohann 33 Standard Model of Particle Physics SS 2012U.Uwer
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W-Pair Production at LEP2
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W leptonic branching fractions
Schöning/Rodejohann 36 Standard Model of Particle Physics SS 2012
Tevatron at Fermilab
Proton – Antiproton Collider at s1/2 = 2 TeV
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Missing Transverse Momentum
electron
neutrino
pT
v = pT
miss
pT
v
pT
miss
W → e v
Jacobian peak at D0:
Schöning/Rodejohann 38 Standard Model of Particle Physics SS 2012
Latest Results W-mass
Tevatron Run 1
Tevatron Run 2
LEP2
}
}
World Average
W-mass measurement important for Top and Higgs Mass predictions
March 2012
Method: normalise W-mass measurement to Z-mass measurement and take input (precise Z-mass) from LEP
Schöning/Rodejohann 39 Standard Model of Particle Physics SS 2012
LHC2011: s1/2 = 7 TeV
2012: s1/2 = 8 TeV
>2014: s1/2 = 14 TeV
proton-proton collisions!
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ATLAS Detector
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LHC Kinematic Plane
W,Z region
W,Z productiondominated bysea quarks
low x-region verywell constrainedby HERA
W,Z productioncan be usedto measureproton-PDFs andLHC luminosity!
Schöning/Rodejohann 42 Standard Model of Particle Physics SS 2012
Proton Parton Densities
SPS energy
multiply by 20!
LHC energy
Schöning/Rodejohann 43 Standard Model of Particle Physics SS 2012
Quark Flavors in Z Production
q qbar → Z
yz = pseudorapidity of Z-boson: y =−ln tanθ/2
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Z → ee candidate at ATLAS
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Z-Peak at ATLAS
2011 data
LHC is a Vector-Boson factory!
Schöning/Rodejohann 46 Standard Model of Particle Physics SS 2012
W-Production at LHC
Charge Asymmetric! Handle to disentangle d and u valence quarks
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W-boson Production at LHC
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Lepton Universality Check at LHC
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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
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