Electroweak Physics Lecture 3

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Electroweak PhysicsLecture 3
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Status so far

• 6 parameters out of 18

Today 7 more!
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The Grand Reckoning

• Correlations of all yesterdays Z peak
  observables from all 4 LEP experiments

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Physics Menu for Today

• The continuing legacy of LEP SLC
• t-polarisation
• Quark final states
• Introduction to hadron colliders

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Key Quantities from last Lecture
(curly) A measures Vf, Af and sin2?W

• Amount of polarisation

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t-polarisation measurement

• Parity-violation (V-A) results in longitudinal
  polarised fermions in ee-?Z?ff
• At LEP t is the only fermion whose polarisation
  can be measured
• Thats because taus can decay in the detector
• We can look at the decay modes to determine the
  polarisation

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Polarisation, Helicity Chirality

• Polarisation measures helicity states.
• Theory tells us about the chirality states
• Chirality ?L½(1-?5)? ?R½(1?5)?
  
• Helicity projection of spin on momentum sp
• In the relativistic limit
• left-handed chirality is same as -ve helicity
• right-handed chirality is same as ve helicity

s ve helicity of t- (-ve for t)
s- -ve helicity of t- (ve for t)
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Polarisation Distribution

• Couplings of Z to chirality states
• The polarisation of a t- produced in ee-?Z?tt-
  depends on cos?
• Ae At are nearly uncorrelated. Insensitive to
  Atfb.
• Another measure of V-A structure, sin2?W

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t?p?t Decays

• Use momentum of p as handle on t polarisation
• Helicity of ? is in same direction as t-helicity
• (True in limit of massless particles)
• ? Effects resulting momentum distribution of p
• In t-?p-? decay
• If P(t-)1 momentum of p- is higher than for
  P(t)-1

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Fit to Obtain Helicity

• Event-by-event measurement of polarisation not
  possible.
• Use statistical fit
• Sum of
• t with ve helicity
• t with -ve helicity
• In Lab Frame

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Other Modes

• t??? followed by ??pp
• t?a1? followed by a1?ppp
• t?µ?? and t?e??

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Final t-polarisation Results

• Extracted values for Ae At

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Measuring Quark Final States

• Up-type and down-type quarks couple differently
  to Z boson
• We can try to identify the type of quark produced
  by looking at the properties of the jets
• Some separation can be made

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b-tagging

• b-quarks have a higher mass and longer lifetime
  than the other quarks.
• Identify b-quark jets by b-tagging
• Can look for
• vertices away from the interaction point, due to
  long lifetime
• high-pt e or µ in the jet due to b?cl? decays
  (20)
• charmed hadrons from b?cl? decays
• 
  (61)

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Rc and Rb Results

• Was historically difficult to tag b and c quarks
  at LEP
• Values of Rc and Rb were wrong initially
• Now agree v. well with EWK prediction

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Rc and Rb Results
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Forward Backward Asymmetry

• sF cross section in the forward hemisphere
• Forward defined by b-quark (not bbar quark) at
  cos?gt0
• At tree level, angular distribution of quark is
• Measures Z couplings to quarks, sin2?W

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Thrust

• Thrust measures the distribution of jets in a
  event.
• The unit vector n is where T is maximized is
  known at the thrust axis
• The range of T is ½ltTlt1
• T½ for an isotropic event
• T1 for an event with 2 back-to-back jets

gt
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Charge of the Quark

• Need to separate b quarks from bbar quarks
• Longitudinal momentum w.r.t. thrust axis
• ? tunable parameter 0.3 ?1

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AFB with quarks

• ?q is probability to estimate quark charge
  correctly

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Quark Asymmetries Measurements

• Corrections for QCD effects

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Quark Asymmetry Results

• At vsMZ

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At SLC

• Electron polarisation allows the measurement of
  ALRfb for quarks
• Differential Cross section w.r.t cos?, including
  electron polarisation

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Quark Asymmetry Results
Oh, isnt the Standard Model great
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Status with the Z Pole Measurements
t polarisation asymmetry
b and c quark final states

• 13 parameters out of 18
• not bad for 5 experiments in 6 years

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LEP SLD Before and After

• Truly established the EWK theory as the correct
  description of fermion interactions at vs lt 100
  GeV

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Next Topic Physics at Hadron Colliders

• More physics from LEPI/SLC and LEPII still to
  come
• But lets change gear a little here and talk
  about physics at hadron colliders
• 3 hadron colliders
• SppS collider at CERN
• TeVATRON at Fermilab
• LHC at CERN
• W, Z and top physics

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Super Proton Antiproton Synchrotron

• Ran from 1981 to 1984
• Proton anti-proton collider
• CM energy 400 GeV
• 6km in circumference
• Two experiments
• UA1 and UA2

Physics Highlight discovery of W and Z bosons!
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W Bosons Discovered at CERN 82

• W- bosons were discovered at CERNs SppS
  collider in 1982 by the UA1 and UA2 detectors
• pp ? W anything
• W ? e ? or ? ?
• event topology isolated charged lepton (with
  high-pT) plus large amount of missing transverse
  energy (due to the neutrino)
• very little background contamination in these
  event samples they are spectacularly clean
  signatures

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Z Bosons Discovered at CERN 83
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Nobel Prize for Physics 1984

• Given to Carlo Rubbia and Simon van der Meer
• For their decisive contributions to large
  projects, which led to the discovery of the field
  particles W and Z, communicators of the weak
  interaction.

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TeVatron

• At Fermilab, 40km west of Chicago
• Proton anti-proton collider
• 1987 to 2009
• Run 1 from 1987 to 1995 vs1.8 TeV
• Run 2 from 2000 to 2009 vs1.96 TeV
• Two experiments CDF and DØ

Physics Highlight discovery of the top quark
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Discovery of the Top Quark

• 1995 On Friday, February 24, 1995, at precisely
  11 a.m. Central Standard Time, collaborators from
  DOE Fermilabs CDF and DZero experiments
  simultaneously pushed the buttons on their
  computers submitting to Physical Review Letters
  their papers announcing the discovery of the top
  quark, the last remaining quark of the Standard
  Model.

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Large Hadron Collider

• 2007-2020??
• At CERN in LEP tunnel
• Proton-proton collisions
• CM energy 7 TeV
• 4 experiments
• ATLAS, CMS
• LHCb, ALICE

Physics Highlight ???
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Discovery of W, Z top

• The weak vector bosons and the top quark were all
  discovered at Hadron Colliders
• why? because these new particles were too heavy
  to produce at existing ee- machines
• hadron colliders can operate at much higher
  energies (less synchrotron radiation)
• The quark and gluon content of the proton means
  that QCD is a very important feature of hadron
  collider physics

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Next Lecture

• Relating the production of W, Z and top to the EW
  Lagrangian
• Measuring W, Z top properties at Tevatron