1. Direct and Indirect Probes of EWSB at LEP and SLC
Outline:
Patrick Janot, CERN
First Lecture: Second Lecture:
Higgs EP
(and SLC too!)
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

2. Indirect Probes of EWSB at LEP and SLC
Common Goals of LEP and SLC:
Check the internal consistency of
the Standard Model of Electroweak
interactions (Z & W studies);
Test with precision the predictions
of Electroweak Symmetry Breaking
(mW vs mZ);
Predict heavy particle masses and
new physics scale (mtop, mH, ?)
First Lecture Outline:
1. Brief History & Overview
2. Brief Theory Reminder:
Why is Precision needed?
3. List of “Electroweak” Observables
4. A few Precision Measurements
Z Lineshape & Beam Energy (LEP)
Left-Right Asymmetry
and Beam Polarization (SLC)
Heavy Flavour Rates (LEP&SLC)
W mass (LEP)
5. The top mass prediction
6. The Higgs boson mass prediction
7. Standard Model consistency W Z H
mW=mZ cosW
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

3. Probes of Electroweak Symmetry Breaking at LEP and SLC
A Little Bit of History
1967: Electroweak unification, with W, Z
and H (Glashow, Weinberg, Salam);
1973: Discovery of neutral currents in
e scattering (Gargamelle, CERN)
1983: W and Z discovery (UA1, UA2);
LEP and SLC construction start;
First Z detected in the world:
UA2 -
qq  Z  e+e-
1974: Complete formulation of the standard
model with SU(2)WU(1)Y (Illiopoulos)
1981: The CERN SpS becomes a pp collider;
LEP and SLC approved before W/Z
discovery;
1989: First collisions in LEP and SLC;
Precision tests of the SM (mtop);
1995: Discovery of the top (FNAL);
Precision tests of the SM (mH);
2000: First hints of the Higgs boson? -
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

4. LEP and SLD Overview (I)
COMPLEMENTARY to LEP
Conventional collider e+e- ring;
Energy upgradeable;
Energy measurable;
Four detectors (A,L,D,O);
Large luminosity;
“Linear” e+e- Collider (with arcs);
Electron polarized;
Small beam transverse dimensions
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

5. LEP and SLD Overview (II)
Total Luminosity: 1000 pb-1
550,000 Z decays
SLD Run
20 Million Z’s
Precision: 0.1%
LEP Run
40,000 W+W-
Average e- Polarization: 73%
Small transverse beam sizes;
Small beam pipe;
Energy: 88  GeV
A few Higgses?
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

6. LEP: Luminosity, Energy, Precision Why is Precision Needed?
Electroweak Observables (i.e., related to W and Z) sensitive to vacuum polarization effects:
L Couplings (v+a) 
R Couplings (v-a)
0.1% Precision needed!
with
Determine  and sin2W from LEP/SLD data;
Predict mtop and mW;
Compare with direct measurements;
Predict mH;
Compare with direct measurements.
eff
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

7. Precision Electroweak Observables (I)
LEP1
LEP2
Heavy Flavour
Rates
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

8. Dependence on mtop, mH of mW and Rb
(mW/mZ)2  (mW /mZ)2 (1 + Dr)
220
200
180
160
140 mW
mtop (GeV/c2) Rb
Rb  Rb (1 + dVb) Dr
Combined
mH (GeV/c2)
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

9. Precision Electroweak Observables (II)
of the
Detectors
LEP
eff
1-4|Qi|sin2qW
sin2qW
eff
e.g.
Precise
Energy
Zm+m-
High Luminosity to
5.10-4
+ b-Tagging
t-selection mW
...
...
Polarized Beam
(SLC only)
Consistency
Checks!
tpnt
eff
sin2W = 1 – mW2/mZ2(1+)
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

10. Probes of Electroweak Symmetry Breaking at LEP and SLC
The Z Lineshape at LEP
At tree-level:
 (1+)2
Measure s and s
Correct for QED and QCD
-30% for s0
+200 MeV for mZ
+4% for Gqq
Fit for the Z parameters
(mass, total width, peak cross
section and partial widths)
s =
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

11. Measurement of the LEP Beam Energy (I)
Approximation: LEP is a circular ring
immersed in a uniform magnetic field:
1) The electrons get transversally polarized
(i.e., their spin tends to align with B), but
Bdipole
Process very sensitive to imperfections
( slow, typically hours, and limited
to o(10%) polarization)
Process very sensitive to beam-beam
interactions ( one beam, no polarization
in collisions)  p e- R
LEP
(L = 2pR = 27km)
E  p = e B R = (e/2p) B L
In real life:
B non-uniform, ring not circular
To be
measured
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

12. Measurement of the LEP Beam Energy (II)
2) The spin precesses around B with
a frequency proportional to B.
The number of revolutions
for each LEP turn is thus
proportional to B L (in fact,
to  B dl, and then to Ebeam)
3) Measure ns : B 1 2
Bx: oscillating field with
frequency n, in one point. 3
-Bx Bx
Vary n until Polarization = 0
Peak-2
Peak
Peak+2
1993
Precision
 210-6
DEbeam  100 keV !
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

13. Measurement of the LEP Beam Energy (III)
A dispersion of 10 MeV is observed ( 100 keV)
in the same machine conditions. Correlation with
the moon found on 1992, Nov 11th:
At midnight, the electrons see
less magnetic field, E is smaller;
At noon, they see more magnetic
field, and E is larger.
LEP at midnight
Longer by 300 mm
LEP at noon
Shorter by 300 mm
 S
 U
 N
Prediction and data
fit perfectly …
However, the electron orbit length
is fixed by the RF frequency:
L = c  t
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

14. Measurement of the LEP Beam Energy (IV)
Bdipole
RF frequency: Hz
(1 Hz)
Electron orbit fixed by RF
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

15. Probes of Electroweak Symmetry Breaking at LEP and SLC
QUAD
QUAD
Dipole
Dipole  
Beam Orbit
Monitors (BOM)  
QUAD
QUAD
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

16. Measurement of the LEP Beam Energy (V)
Other 10 MeV-ish effects understood even later:
Effect of the rain: water pressure in the
mountains change LEP circumference;
(controlled with the BOM’s)
Effect of the TGV: currents induced on the
LEP beam pipe change the magnetic field
(controlled by 16 NMR probes)
Understood after one-day strike
Understood after three rainy months
Now: DEbeam  2 MeV
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

17. Z Lineshape: Final State Identification (I)
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

18. Z Lineshape: Final State Identification (II)-
e) Z->nn -
Z  nn :
Not detectable.
Z  t+t- : Two low multiplicity jets + missing
energy carried by the decay neutrinos
Z  qq : Two jets, large particle multiplicity.
Z  e+e-, m+m- : Two charged particles (e or .) -
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

19. Z Lineshape: Final State Identification (III)
Leptonic decays: Low multiplicity,
with (t) or without (e, m) missing energy
600,000
gg Collisions:
Low multiplicity,
Low mass
16 million
Selections with
High Efficiency;
High Purity;
600,000
Count events : Easy?
Hadronic decays:
High multiplicity
High mass
Systematic Uncertainty  0.1%
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

20. Z Lineshape: Results (I)
s varied from 88 to 94 GeV;
Points are from data in both
coordinates;
Lines are from a standard model
fit to the Z parameters;
MEASURE
10-5
Dominant (and common) error:
LEP Beam Energy Measurement
MEASURE
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

21. Z Lineshape: Results (II)
Three Weeks
after start
Z Lineshape: Results (II)
Nn = 2
13 October 1989
DELPHI L3
OPAL
Nn = 3
Nn = 4 L3
L3: 2538 hadronic Z’s
Nn = 3.42  0.48
ALEPH: 3112 hadronic Z’s
ALEPH
Nn = 3.27  0.30
OPAL: 4350 hadronic Z’s
OPAL
Nn =  0.008
Nn = 3.1  0.4
DELPHI: 1066 Hadronic Z’s
MarkII, Aug. 1989,
with 106 Z decays:
Nn = 3.8  1.4.
13-Oct-1989:
Nn = 3.16  0.20
DELPHI
Nn = 2.4  0.4
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

22. Z Lineshape: Results (III)
(1998)
10-3
shad/sl
500 MeV
in 1989
With this measurement alone:
mtop  165  25 GeV/c2
GZ  (1 + )2
(+small sensitivity to mH)
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

23. Heavy Flavour Rates: Identification (I)
b- and c-hadrons decay weakly towards c- and s-hadrons, with a
macroscopic lifetime (1.6 ps for b’s), corresponding to few mm’s at LEP
3d-vertexing determines
secondary and tertiary
vertices.
High resolution is crucial.
Impact parameters of
reconstructed tracks
allow b quarks to be
tagged with very good
purity.
Mass of secondary vertex
tracks is a very powerful
discriminator of flavour
(b, c, and light quarks):
mb5 GeV/c2, and
mc1.5 GeV/c2.
 10 cm
 1 cm
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

24. Heavy Flavour Rates: Identification (II)
Vertex detectors (Si m-strips, CCD’s, pixels):
At LEP: inner radius 6 cm, good resolution;
At SLC: inner radius 2.3 cm, superior resolution.
SLD can do both b- and c-tagging with good purity.
Data
Vertex Mass for Events
With a Secondary Vertex
In SLD
b (MC)
c (MC)
uds (MC)
Vertex Mass (GeV/c2)
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

25. Heavy Flavour Rates: Results
Use double-tag method to reduce uncertainties
from the simulation, e.g., in bb events:
Take ec, euds, and rb (all small) from simulation.
Solve for eb and Rb ! -
Rb = Gbb / Ghad
3 10-3
With this measurement alone:
mtop  150  25 GeV/c2
(dependence on mH, as, … cancel in the ratio)
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

26. Prediction of mtop from EW Measurements
A top mass of 177 GeV/c2 was predicted by LEP & SLC with a precision of
10 GeV/c2 in March 1994.
/ SLD
One month later, FNAL
announced the first 3
evidence of the top.
/ SLD
/ SLD
In 2001:
/ SLD
(actually 2.9s)
Perfect consistency between prediction
and direct measurement. Allows a global
fit of the SM (with mH) to be performed.
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

27. Asymmetries: Measurement of ALR at SLD (I)
(e- beam polarization)
ALR ( Ae ~ 14% if Pe = 100%) is 10 times more sensitive to sin2W
than AFB ( AeAl ~ 1-2%);
ALR is independent of the final state (Z  hadrons, +-, +-);
ALR is independent of the detector acceptance;
Most of the theoretical corrections, uncertainties (QED, QCD, …)
cancel in the ALR ratio.
lept
Statistical
&
Systematic
uncertainties compete with LEP asymmetry measurements
(unpolarized)
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

28. Asymmetries: Measurement of ALR at SLD (II)
Condition # 1: Have a longitudinally 100% polarized electron beam.
Get longitudinally polarized e- by illuminating a GaAs photo cathode with
circularly polarized Lasers (frequency: 2  60 = 120 Hz)
In principle, Pe  100% can be reached. In practice, 80% was achieved.
Change the sign of the polarization
on a random basis to ensure that
equal amount of data are taken with
both signs, and that the luminosity
is not tied to any periodic effects
in SLC.
Transport, accelerate and collide the
polarized electrons, with enough care
to keep the same polarization at the
interaction point. SLC was designed
to do so from the beginning (unlike
LEP).
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

29. Asymmetries: Measurement of ALR at SLD (III)
Condition # 2: Measure the e- polarization Pe with 0.5% accuracy.
Collide 45.6 GeV long. polarized e- with
2.33 eV (532 nm) circularly polarized
photons every 7th bunch (17 Hz);
Detect Compton back-scattered e- as
a function of their energy after a bend
magnet;
s(E) = s0  [1 + A(E) Pe Pg] s0
A(E)
s0 and A(E) are
theoretically well
known (pure QED
process)
Cerenkov Detectors
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

30. Asymmetries: Measurement of ALR at SLD (IV)
Reverse on a random basis the sign of the photon polarization Pg
(close to 100%, optically measured with filters);
Count the number of e- detected in each of the Cerenkov channels
(about a hundred electrons per channel at each beam crossing)
Deduce the e- beam polarization from the asymmetry
Uncertainty
dP/P
Laser Polarization
0.1%
Analyzing Power
0.4%
Linearity
0.2%
Electronic Noise
TOTAL
0.5%
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

31. Asymmetries: Measurement of ALR at SLD (V)
Cross-checks:
Count Compton-scattered gammas at the kin.
threshold with two additional calorimeters.
Agree with the main measurement to 0.4%.
Measure the positron polarization to be 0.0%
with an accuracy better than 0.1%.
Condition # 3: Count events in SLD
Complete data set:
e.g., in :
NL = 183,355; NR = 148,259
Pe = 72.92%
 Ae =  (stat.)  (syst.)
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

32. Asymmetries: Results for sin2W
eff
Asymmetries: Results for sin2W
Leptons:
0.23100.0002
???
Quarks:
0.23230.0003
Still statistically acceptable, but
a couple of add’l years at LEP
and SLD would have helped…
5 10-4
ALL: 
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

33. Asymmetries: Results for al and vl
Axial and vector couplings (al, vl) from ALR (SLC) and Z  l+l- (LEP)
Before LEP&SLC
After LEP&SLC
Errors 10
 200
Precision on sin2W: , adequate to become sensitive to mH.
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

34. Asymmetries: Prediction of mW
Predict mW in the SM:
mW2 = mZ2(1+ )cos2W
eff
Direct
Measurements*
Precision
Measurements
mH dependence in the SM
through quantum corrections
(see later)
* Coming now for mW at LEP;
See Y.-K. Kim lecture for mtop
and mW at the Tevatron.
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

35. W mass at LEP 2: Production and Decay
s  2mW
45.6%
43.8%
10.8% - - - -
W+W-  q1q2q3q4:
Four well separated jets.
W+W-  q1q2ln:
Two hadronic jets,
One lepton, missing energy.
W+W-  l1n1l2n2:
Two leptons, missing energy -
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

36. W mass at LEP 2: Threshold cross section
mW (thresh.) = (80.40  0.22) GeV/c2
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

37. W mass at LEP 2: Direct measurement (I)
0 unknowns,
5C fit
5 Constraints:
Fitted mass (GeV)
Initial State Radiation;
(change the effective beam energy)
Detector Resolution;
(change the jet directions)
Other four-fermion diagrams
(create non-Breit-Wigner bkgds)
3 unknowns,
2C fit
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

38. W mass at LEP 2: Direct measurement (II)
 Mostly rely on simulated mass distributions to determine mW
10,000 W pairs per experiment
LEP Combination:
mW(qqqq) =  GeV/c2
mW(qql) =  GeV/c2
Good compatibility between the two
final state: Dm = 9  46 MeV/c2
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

39. W mass at LEP 2: Result and Uncertainties
Good consistency between experiments;
Good consistency with hadron colliders
(see Y.-K. Kim lecture);
Good consistency with Z data (LEP/SLD).
mW(LEP) =   0.030
(stat.) (syst.)
Limited by systematic uncertainties
on final state interaction (Bose-Einstein
Correlation and Colour Reconnection).
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

40. W mass at LEP 2: Final State Interactions
Interactions between W hadronic decay products may
cause a shift between the mass from fully hadronic and
semileptonic events:
Colour Reconnection: QCD interaction between
quarks from different W’s;
Bose-Einstein Correlations between identical
hadrons (pions, kaons), well established in single
W or Z decays. q q
SMALL q
SMALL q
Use the data to constrain them ! + =
Without FSI
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

41. Global Fit of the Standard Model to mH (I)
Knowing mtop, most electroweak observables have a sensitivity to mH through Dr
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

42. Global Fit of the Standard Model to mH (II)
Global fit of mH and mtop:
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

43. Global Fit of the Standard Model to mH (III)
Internal Consistency of the Standard Model?
Pull distribution = Normal Gaussian?
Mean: 0.22  0.28
Sigma: 1.1  0.4
Largest discrepancy (-2.9s) well inside
statistical expectation;
2 probability = 8%. Just fine.
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC

44. Global Fit of the Standard Model to mH (IV)
What Next?
1st Lecture Conclusions
LEP and SLD allowed the internal
consistency of the standard model
to be tested with great precision;
LEP and SLD checked the predictions
of the Electroweak Symmetry Breaking
mechanism (e.g., mW = mZcosqW);
LEP and SLD allowed the mass of the
top quark to be predicted several years
before it was discovered in 1995 at the
Fermi National Laboratory;
LEP and SLD measurements led to the
prediction of a relatively small Higgs
boson mass (around 100 GeV/c2);
Future Machines (Tevatron, LHC, LC, mC)
will be important for EW Physics.
Now: dmtop =  5.1 GeV, dmW =  34 MeV
With dmtop =  2 GeV
With dmW =  15 MeV
With dmtop =  2 GeV and dmW =  15 MeV
7th Hellenic School, Corfu
Aug. 31-Sep. 13, 2001
Probes of Electroweak Symmetry Breaking at LEP and SLC