Post on 03-Mar-2021
From LEP to LHC
1. Physics of the Z boson (I)2. Physics of the Z boson (II)3. Physics of the W boson4. Physics of the top quark5. Tests of the Standard Model6. Search for the Higgs boson (I)7. Search for the Higgs boson (II)
Salvatore MeleCERN & INFN
(Salvatore.Mele@cern.ch)
From LEP to LHC
Salvatore MeleCERN & INFN
(Salvatore.Mele@cern.ch)
1. Physics of the Z boson (I)• LEP (Statistics interlude)
2. Physics of the Z boson (II)• LEP (Statistics interlude)
3. Physics of the W boson• LEP, Tevatron, LHC (Statistics interlude)
4. Physics of the top quark• Tevatron, LHC
5. Tests of the Standard Model• LEP, Tevatron (Statistics interlude)
6. Search for the Higgs boson (I)• LEP (Statistics interlude)
7. Search for the Higgs boson (II)• Tevatron, LHC
Salvatore Mele | From LEP to LHC | Troisième cycle 3
Physics of the Z boson
•Z boson: what to measure?•The LEP accelerator and detectors•The Z “lineshape”•Energy and luminosity measurements•The tau polarisation•Measurements with heavy quarks•The SLD accelerator and SLC detector•Measurements with polarized electrons•A problem?•Extraction of the Z-boson couplings
#1
#2
Salvatore Mele | From LEP to LHC | Troisième cycle 4
Main bibliographic reference
The ALEPH, DELPHI, L3, OPAL, SLD Collaborations,the LEP Electroweak Working Group,the SLD Electroweak and Heavy Flavour Groups
[~2500 authors]
Physics Reports 427 (2006) 257arXiv:hep-ex/0509008
Salvatore Mele | From LEP to LHC | Troisième cycle 5
Z boson: what do we want to measure?Fundamental properties: mass, width, couplings
Fundamental Standard Model parameters (#5)
Two parts:1. How to measure mZ and ΓZ2. How to measure gAf and gVf
•mZ the Z boson mass•ΓZ the Z boson width•gAf (f=e,µ,τ,...,c,b)•gVf (f=e,µ,τ,...,c,b)
Salvatore Mele | From LEP to LHC | Troisième cycle 6
How to measure mZ and ΓZ ?1. Produce Z bosons UA1 PLB 129 (1983) 273
UA1 PLB 134 (1984) 469
Salvatore Mele | From LEP to LHC | Troisième cycle 7
What’s on the market ?
2. Produce LOTS of Z bosons
PDG J. Phys. G 33 (2006) 1
Salvatore Mele | From LEP to LHC | Troisième cycle 8
The e+e- -> ff process: producing Z bosons_
Z bosons decay into quarks (70%) leptons (9%) neutrinos(21%)
Salvatore Mele | From LEP to LHC | Troisième cycle 9
Useful definition in Z-boson physics
As above, with“leptonic universality”
A0FB
l
Leptonic forward-backwardasymmetries
A0FB
e, A0FBµ, A0
FB τ
As above, with“leptonic universality”R0
l= Γhad/Γll
Leptonic widths (expressed asfraction of hadronic)
R0e= Γhad/Γee
R0µ= Γhad/Γµµ
R0τ= Γhad/Γ ττ
ee->Z->qq cross sectionσ0had
Z-boson widthΓZ
Z-boson massmZ
Salvatore Mele | From LEP to LHC | Troisième cycle 10
Forward-backward asymmetry
l+
e+e-
l-
backwardforward
Salvatore Mele | From LEP to LHC | Troisième cycle 11
How to measure mZ and ΓZ ?
Information on mZ and ΓZ contained inσ0
had, R0l= Γhad/Γll, A0
FBl
CERN Report 89-08
Salvatore Mele | From LEP to LHC | Troisième cycle 12
Can’t you just take the invariant mass?
Salvatore Mele | From LEP to LHC | Troisième cycle 13
Requirements
• Produce Z bosons• Count Z bosons decaying into quarks
σ0had ∝ (NHad-NBack)/εSelL
• Count Z bosons decaying into leptonsR0
l ∝ Nhad/Nll
• Study angular distributions of decayproductsA0
FBl ∝ (NF-NB)/(NF+NB)
Salvatore Mele | From LEP to LHC | Troisième cycle 14
Producing Z bosons
Salvatore Mele | From LEP to LHC | Troisième cycle 15
Salvatore Mele | From LEP to LHC | Troisième cycle 16
Salvatore Mele | From LEP to LHC | Troisième cycle 17
Ferm
ion-
pair
sel
ecti
on
+21% νν
qq, 70%
µµ, 3% ττ, 3%
ee, 3%
Salvatore Mele | From LEP to LHC | Troisième cycle 18
Event selection
Salvatore Mele | From LEP to LHC | Troisième cycle 19
Event selection
Salvatore Mele | From LEP to LHC | Troisième cycle 20
The LEP data samplex103
Combination of four experiments halves statistical uncertainties.
Effects of uncorrelated systematic uncertainties are reduced by the combining experiments.
Salvatore Mele | From LEP to LHC | Troisième cycle 21
Off-peak data. Lumi/precision tradeoff
20/pb off peak
20/pb off peak
} 7/pb off peak
Salvatore Mele | From LEP to LHC | Troisième cycle 22
Life is not as easy as you would like it to be...
Salvatore Mele | From LEP to LHC | Troisième cycle 23
The “pseudo-observables”
σhad & σ0had
Salvatore Mele | From LEP to LHC | Troisième cycle 24
The “pseudo-observables”
AFBl & A0
FBl
Salvatore Mele | From LEP to LHC | Troisième cycle 25
Z “line shape”, from 15 million Z-> hadrons
Salvatore Mele | From LEP to LHC | Troisième cycle 26
Details of the hadron cross-section measurement
Salvatore Mele | From LEP to LHC | Troisième cycle 27
Measuring AFB
!
d"
dcos#$1+ cos
2# +8
3AFBcos#
Salvatore Mele | From LEP to LHC | Troisième cycle 28
Measurement strategy• Count Z bosons decaying into quarks to get σhad=(NHad-NBack)/εSelL• Correct σhad to σ0
had
• Count Z bosons decaying into leptons• Correct Nhad/Nll to R0
l (for each flavour)• Study angular distributions of decay leptons• Correct AFB
l = (NF-NB)/(NF+NB) to A0FB
l (for eachflavour)
• Fit σ0had, R0
l and A0FB
l to get mZ and ΓZ (9- and 5-parameter fit)
• (Combine measurements from the four experiments)
Salvatore Mele | From LEP to LHC | Troisième cycle 29
Results of the 9-parameter fit
Salvatore Mele | From LEP to LHC | Troisième cycle 30
Results of the 5-parameter fit
Salvatore Mele | From LEP to LHC | Troisième cycle 31
Statistical interludeCombine XA±δA and XB±δB
1. δA and δB are uncorrelated2. δA and δB are partially correlated
1.trivial answer: weighted average
!
< x >=
xA
A
2+xB
B
2
1
A
2+1
B
2
!
"x
=1
1
"A
2+1
"B
2
!
2=(x
A# < x >)
2
A
2+(x
B# < x >)
2
B
2
2 measurements- 1 parameter to determine= 1 d.o.f.
Salvatore Mele | From LEP to LHC | Troisième cycle 32
2. partially correlated uncertainties• Split the uncertainties in correlated and
uncorrelated parts
!
xA
± "A
U
± "A
C
!
xB
± "B
U
± "B
C
• Build error matrix
!
V ="A
U2
0
0 "B
U2
#
$
% %
&
'
( ( +
"A
C2
"A
C"B
C
"A
C"B
C "B
C2
#
$
% %
&
'
( ( =
"A
2 "A
C"B
C
"A
C"B
C "B
2
#
$
% %
&
'
( (
!
" =#A
C#B
C
#A
2#B
2• Calculate correlation coefficient
• Get the average, the uncertainty and the χ2
!
< x >=
Vij"1xi
i ,1
#
Vij"1
i ,1
#
!
"x
= Vij#1
i ,1
$
!
"2
= (xi# < x >)Vij
#1
i,1
$ (x j# < x >)
• Rephrase: the <x> is the one minimising the χ2
Combining experimentsXm=( ; ;... )
Measured DELPHIMeasured ALEPH
X=( , x4)Parameters of the fit
Correlation matrix of the common uncertainties
Correlation matrices of •LEP energy uncertainty ·t-channel correction•Luminosity uncertainty ·theoretical uncertainties
The solution minimises:
Measured L3Measured OPAL
Salvatore Mele | From LEP to LHC | Troisième cycle 34
A matrix at the timeMatrix of the correlation matrices
Salvatore Mele | From LEP to LHC | Troisième cycle 35
A matrix at the time
0.061% diagonal +small correlationbetween
Salvatore Mele | From LEP to LHC | Troisième cycle 36
Results of the 9-parameter fit
Results of the 5-parameter fit
Salvatore Mele | From LEP to LHC | Troisième cycle 37
Results of the 5-parameter fit
Salvatore Mele | From LEP to LHC | Troisième cycle 38
Measure of mZ to 23 p.p.m.
Control of systematic uncertainties is essential:•Theoretical uncertainties•Luminosity uncertainties•Energy calibration
The LEP luminosity
Need accurate measurement, beam parameters notenough to estimate L with the precision needed to
measure the electroweak parameters
!
L =N
b +Nb "frevkb#
$ x$ y
100µm 10µm
3x1012 11 kHz 4 0.01-1
=2 x 1031 cm-2s-1
How to measure the LEP luminosity
Cross sections are calculated knowing counts,efficiency, background and luminosity
Luminosity can be calculated kowing counts,efficiency, background and cross section!
Need to go to the per-mille. Need process with:•Extremely low background•Extremely well known cross section•Very high statistics
Bhabha scattering
Need to go to the per-mille. Need process with:•Extremely low background•Extremely well known cross section•Very high statistics
t-channel dominated at small angleExperimental uncertainty <0.07-0.10%/expExp. unc. mostly uncorrelated -> 0.05% totTheoretical uncertainty 0.11%
Salvatore Mele | From LEP to LHC | Troisième cycle 42
L3Luminosity monitors
•Small-angle calorimeters,possibly with tracker in front•1.4-1.8°<θ<3.1-3.3°•Mechanical precision/positionmeasurement crucial•Select events, count events,control efficiency andbackground
~2.5m
Luminosity measurement at LEP
Measurement of the beam energyDirect inference from the measurement
of the magnetic field
!
< Ebeam
>= e
2"Bdlorbit#
Flux loop and NMR allow a precision of 3 x 10-4
Need 1 order of magnitude better!
Resonant depolarizationCERN Report 89-08
Solokov-Ternoveffect
S S S S
~10%
BY
BX
•In electron storage rings, the bending in the XZ plane and the associateradiation give vertical polarisation in the transverse (Y) direction
•Electron spins precess with
•Vertical bending field does not induce polarisation. An additionalhorizontal field does
•The precession carries information on Ebeam:
!
dr S
dt=
r " #
r S
!
r " = #
e
ae$(1+ a
e$)
r B
!
ae" =
ae
mec2Ebeam
Resonant depolarizationVariation of Bx can bring S in the horizontal plane, wherepolarisation disappears
Use a single coil in the ring, oscillate Bx and look for aresonance with the turning beams
v = fdepolarisation/frevolution gives Ebeam through
!
r " = #
e
ae$(1+
ae
mec2
Ebeam)r B
Salvatore Mele | From LEP to LHC | Troisième cycle 46
Other parts of the saga• Resonant depolarisation precision of
200KeV!• Polarisation disrupted by beam-beam effect• Measurement performed with single-beam
special runs• Need extrapolations, including other info as
direct measurements of magnetic field.• Final precision 1MeV (2 x 10-5)
Salvatore Mele | From LEP to LHC | Troisième cycle 47
The energy is not the samefor all the experiments
(the 9- and 5- parameters fits need to know this!)
Salvatore Mele | From LEP to LHC | Troisième cycle 48
Attraction of the moon (Earth crust tides)•Beam movement of 13µm changes Ebeam of 1MeV!•Earth tides change diameter of the ring•Quadrupoles move off the orbit (or viceversa...)
10MeV effect daily, continuosly corrected for
Salvatore Mele | From LEP to LHC | Troisième cycle 49
Level of Geneva lake
10MeV effect weekly
Salvatore Mele | From LEP to LHC | Troisième cycle 50
Effects of the TGV
NMR measures the field, needed in the estrapolation for Ebeam
Salvatore Mele | From LEP to LHC | Troisième cycle 51
Three flavours and a Standard Model
Salvatore Mele | From LEP to LHC | Troisième cycle 52
Number of “light” neutrino species
τ mass correction -0.23%