From LEP to LHC - CUSO...7.Search for the Higgs boson (II) Salvatore Mele CERN & INFN...

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

([email protected])

From LEP to LHC

Salvatore MeleCERN & INFN

([email protected])

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


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


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)


How to measure mZ and ΓZ ?1. Produce Z bosons UA1 PLB 129 (1983) 273

UA1 PLB 134 (1984) 469


What’s on the market ?

2. Produce LOTS of Z bosons

PDG J. Phys. G 33 (2006) 1


The e+e- -> ff process: producing Z bosons_

Z bosons decay into quarks (70%) leptons (9%) neutrinos(21%)


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


Forward-backward asymmetry

l+

e+e-

l-

backwardforward


How to measure mZ and ΓZ ?

Information on mZ and ΓZ contained inσ0

had, R0l= Γhad/Γll, A0

FBl

CERN Report 89-08


Can’t you just take the invariant mass?


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)


Producing Z bosons


Ferm

ion-

pair

sel

ecti

on

+21% νν

qq, 70%

µµ, 3% ττ, 3%

ee, 3%


Event selection


The LEP data samplex103

Combination of four experiments halves statistical uncertainties.

Effects of uncorrelated systematic uncertainties are reduced by the combining experiments.


Off-peak data. Lumi/precision tradeoff

20/pb off peak

20/pb off peak

} 7/pb off peak


Life is not as easy as you would like it to be...


The “pseudo-observables”

σhad & σ0had


The “pseudo-observables”

AFBl & A0

FBl


Z “line shape”, from 15 million Z-> hadrons


Details of the hadron cross-section measurement


Measuring AFB

!

d"

dcos#$1+ cos

2# +8

3AFBcos#


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)


Results of the 9-parameter fit


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.


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


A matrix at the timeMatrix of the correlation matrices


A matrix at the time

0.061% diagonal +small correlationbetween


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%


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


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)


The energy is not the samefor all the experiments

(the 9- and 5- parameters fits need to know this!)


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


Level of Geneva lake

10MeV effect weekly


Effects of the TGV

NMR measures the field, needed in the estrapolation for Ebeam


Three flavours and a Standard Model


Number of “light” neutrino species

τ mass correction -0.23%

From LEP to LHC - CUSO...7.Search for the Higgs boson (II) Salvatore Mele CERN & INFN...

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Transcript of From LEP to LHC - CUSO...7.Search for the Higgs boson (II) Salvatore Mele CERN & INFN...