9 April 2009 Nijmegen Colloquium 1 Precision electroweak measurements at the LEP e + e – collider Jan Timmermans / NIKHEF Amsterdam
9 April 2009 Nijmegen Colloquium 1
Precision electroweak measurements at the LEP e+e– collider
Jan Timmermans / NIKHEF Amsterdam
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• First study B. Richter 1976: C~43 km, E=2*100 GeV, L=1032 cm-2s-1, 8 ip’s
•Les Houches study 1979:
study of Z prod. and decay
study of WW production
Higgs search
searches for new leptons, quarks
3 and 4 jet structures, scaling
violations
• 1983: C=27 km; start construction
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Particles in the Standard Model• Fermions (matter particles)
b
t
s
c
d
u
ee
,,
,,
leptons
quarks
• Gauge bosons
, Z, W± (electroweak interaction)
gluons g (strong interaction)
Higgs boson H (resulting from EW symmetry breaking)
E.g. decays: Z +- , Z bb , W+ e+e , W+ cs
WZW mm cos
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Some events at LEP1
e+ e– 2-fermions
Z + – Z e+ e–
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Z + –
Z q q
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3 jets: Z q q g (2 quarks and gluon)
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And some events at LEP2e+ e– 4-fermions
The first WW event!
1996: s = 161 GeV
e+ e– W+W– q1q2q3q4
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e+ e– ZZ +– qqe+ e– Z** +– e+e–
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events/experiment:
4.5 M Z events
10000 WW events 600 ZZ events 250 single-W events
LEP1 (1989-1995): ~200 pb-1/exp LEP2 (1996-2000): ~700 pb-1/exp
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Z line shape and asymmetriese+
e- f
f
Z
e+
e- f
f
int22222
20
/)()(
ZZZ
Zffff msms
ss
220 12
Z
ffee
Zff m
pole cross-section:
)( 22AfVfffgg partial width:
invqqeeZ total width:
vector and axial-vector couplings
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Forward-backward asymmetry:
cos)cos1(8
3
cos
1 2FBAd
d
At the Z pole:2222
,0 22
4
3
4
3
AfVf
AfVf
AeVe
AeVefe
fFB gg
gg
gg
ggAAA
• Measure lepton partial width ll and F/B asymmetry AFB gives the couplings gVl and gAl (for l=e, , )
• Effective weak mixing angle:
Effective couplings contain radiative corrections, which depend on the top quark mass and Higgs boson mass
)1(4
1sin 2
Al
Vllepteff g
g
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Some examples of radiative corrections:
e+
e- b
b
/Z
t
tW
e+
e- f
fZ
H
e+
e- f
f
t
t
Higgs loop
top quark loop
W loop
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Z line shape
Moriond 1990:
mZ = 91171 ± 12 ± 32 (incl. ELEP) MeV
Z = 2538 ± 26 ± 28 MeV
N = 3.04 ± 0.12
first evidence 3 generations
Now:
mZ = 91187.5 ± 2.1 MeV
Z = 2495.2 ± 2.3 MeV
N = 2.9841 ± 0.0083
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Beam energy precision 0.2 MeV from resonant depolarisation
But corrections needed due to:
TIDES LEVEL LAKE TGV
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Axial and Vector couplings for leptons
Measured from partial widths ll,, forward-backward asymmetries AFB
0,l and τ polarisation
gVf / gAf = 1 – 4 | Qf | sin2eff f
Couplings and lepton-universality established at
1 per-mille (gAl) and few % (gVl)
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Vertex Detectors:
Precise -lepton & b-quark studies
Here DELPHI LEP2 version
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Rb = b / had and top quark mass
Summer 1992:
b = 3739 MeV I3b = -1/2
(370 MeV; for I3b =0: 24 MeV)
Top quark must exist
Rb is also sensitive to top quark mass,
but much less than sin2eff !lept
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Top mass: predicted by LEP
Prediction mtop from EW fit
EPS93 Marseille:
mtop = 166 –19 -22 GeV
ICHEP94 Glasgow:
mtop = 178 -11 -19 GeV
and CDF saw excess due to top at
mtop = 174 -10 -23 GeV
EPS95 Brussels: both CDF and D0 observed top at predicted mass.
+17 +19
+11 +18
+10 +13
Great success for the Standard Model
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Two most precise sin2eff values from
SLD ALR and LEP A0,b differ by 2.9
lept
FB
SM: 0.1036
A0,b (and also A0,c ) prefer a high
value for the Higgs boson massFBFB
A puzzle left ….. : comparison sin2efflept
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lepton: a puzzle solved
lifetime related to lifetime, mass, mass and leptonic branching ratio
Up to 1992 a 2-2.3 discrepancy between the coupling constants g and g
In 1992 a new, more precise mass by BES
Now - -e universality at few per-mille, thanks to high-precision Si vertex detectors
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only exchange
No ZWW vertex
WW cross section
Clear evidence for the SU(2) x U(1)Gauge structure
Exp. Errors 1-3% / energy point
CC03 diagrams:
Theory predictions include full O(em) corrections; theory error 0.5%
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Single W WW
DLO
Quartic Gauge Couplings
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W mass measurement
Mw21 – –––– –––– 1 – r Mw
2
MZ2
GF 2WW
t
b
W
WW
H
Mw ln MH
Mw Mt2
W mass defined by relativistic Breit-Wigner
lineshape of the W propagator with
s-dependent width
Mtop MW
Direct 1.3 GeV 25 MeV
Indirect ~10 GeV 32 MeV
• precision MW measurement 0.04%
• gives handle on Higgs mass
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W mass• W masses reconstructed directly from the decay products
•Constrained fits improve mass resolution from 8 GeV to 3 GeV
before 4C fit
after 4C fit
qqqq
qqqqe
W mass: LEP (prel.) combined results•Single measurements are FINAL
•Working on combination (systematics)
Apart from NuTeV result (from rates of CC and NC (anti-) scattering)
very good agreement between direct and indirect measurements
*
*2008 Tevatron average; now D0 alone: 80.401 ± 0.044 GeV
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We searched…..
and searched…..
and searched…..
and searched…..
and searched…..
No Higgses, 4th-generation, sleptons, squarks, charginos, neutralinos....
Large part of the MSSM parameter space excluded
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SM Higgs search
Higgs strahlung: dominant WW fusion: small(can go beyond ‘kinematical limit’)
Branching ratios (mH=115 GeV):
• H bb (74%)
• H (8%)
• H WW (8%)
Final states:
• bbqq (4-jet channel)
• bb (missing energy channel)
• bbee, bb (leptonic channel)
• bb, bb (tau channel)
Selections: cut based or NN
• select multi-hadronic events
• look for b-tagged jets
• lepton identification
• apply constraint fits + sometimes W/Z mass constraints
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Mass distributions:
• But not only mass information useful!
• Combine all information in a discriminating variable (event likelihood or NN output)
• 2-dim inputs to statistical analysis:
- invariant mass of bb jet combination
- discriminating variable (containing b-tags, kinematics, jet-properties)
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CLs+b1-CLb
Likelihood test: sig+bkg bkg
)(
)()(
i
iiHi bL
bsLmQ
i
iQQ )ln(2)ln(2
•1-CLb measures incompatibility with bkg
•CLs+b a measure of compatiblity with s+b
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•Small excess around 116 GeV but less than 2 (mainly coming from ALEPH candidates and from four-jet events)
•Excess was 2.9 on 3/11/2000
• CLs = CLs+b / CLb
• when CLs < 0.05 the hypothesis is rejected at 95% CL
MH > 114.4 GeV (115.3 expected)
FINAL 2003
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Global electroweak fitResults of different fits:
• using only Z pole data
• using all data
%9.910/0.16/
111
173
2
19060
1310
probndof
GeVm
GeVm
H
t
%)19(13/2.17/
90
3.12.173
2
3627
probndof
GeVm
GeVm
H
t
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•Very good consistency between direct and indirect measurement of mt and mW
• Both prefer low Higgs boson mass
mH < 163 GeV at 95% CL
(Tevatron excludes 160-170 GeV)
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Conclusions LEP was great!
Full gauge structure of the SM has been measured, and many measurements in good agreement with the SM prediction
Largest discrepancy between AbFB (LEP) and ALR (SLD) at ~3
but much less w.r.t. average in terms of sin2eff !
SM Higgs boson not yet found: MH > 114.4 GeV
(and MH outside 160-170 GeV window from Tevatron)
A task for the Tevatron and Large Hadron Collider experiments
lept
total output: ~ 300 journal papers/experiment
PhD theses in NL: ~50 (L3 + DELPHI) , ~half of which RUN
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Backup slides
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We searched…..
and searched…..
and searched…..
and searched…..
and searched…..But sometimes……
No Higgses, 4th-generation, sleptons, squarks, charginos, neutralinos....
Large part of the MSSM parameter space excluded
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•1995: 130-136 GeV
•4-jet events
•Sum of di-jet masses with smallest M
•16 events (8.3 exptd)
•Prob. accumulation in 6.3 GeV bin: 0.01%
ALEPH
Why it is good to have more than one expt.
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QCD: test of gluon selfcouplingFrom fit to angular distributions in 4-jet events.
Rel. strength of couplings qqg, ggg, gqq depends on gauge group through Casimir factors CF , CA , TF
TF /
CF
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QCD: running of s
Results from fits to different event shape variables.
Combined fit at all energies:
s(MZ) = 0.1201 0.0003
0.0009
0.0009
0.0047
(stat)
(exp.syst)
(theor.hadr.)
(theor.ev.shapes)