what we have learned from lep and slc?
DESCRIPTION
Precision tests of electroweak interactions-. What we have learned from LEP and SLC?. Krzysztof Doroba, Warsaw University & DELPHI Collaboration. XXVIII Mazurian Lakes Conference on Physics, Aug 31 – Sep 7 2003. Outline of the talk:. - PowerPoint PPT PresentationTRANSCRIPT
What we have learned from LEP and SLC?Krzysztof Doroba, Warsaw University & DELPHI Collaboration
XXVIII Mazurian Lakes Conference on Physics, Aug 31 – Sep 7 2003
Precision tests of electroweak interactions-
Outline of the talk:
•Strategy of the Standard Model tests•Radiative corrections•LEP/SLC and detectors•Z0 line shape•Z0 decays to heavy quarks•Asymmetries at the Z0 pole•Direct W mass measurement•Direct Higgs search• Global fit•Conclusions from the tests
Strategy of the test.
WMinimal Standard Model (MSM) describes electroweak interactions of quarks (q), leptons (l) and Higgs boson(s) (h) by exchange of GeVMZ
GeVMW
m
Z
W
92
81
0
0
first step: build LEP1 (SLC) collider at GeVs 90
(with possible electron beampolarization at SLAC)
second step: increase the energy to GeVs 160 (LEP only)
•Study W and Z production•Check model internal consistency•Look for Higgs boson(s) and supersymetric particles
and
Input parameters of Minimal Standard Model (MSM)
-electromagnetic fine structure constant
FG -Fermi constant- determines charged current strength
ZM - Z0 boson mass, measured at LEP with high precision
above parameters are sufficient to perform MSM calculations on thetree level. However due to high precision of the LEP/SLC measure-ments tree level is not sufficient and radiative corrections are required.This brings into the game more parameters:
fm - fermion masses (mt)
Hm - Higgs boson mass
2Zs M - strong coupling constant at 22
ZMq (for quarks in final state)
Radiative corrections
Pure QED corrections factorize from electroweak part
+ ..........
Electroweak part:
Vacuum polarization
Vertex correction
QED:
This leads to improved Born approximation; the improved amplitude for the process has same form as Born amplitude for this processbut with effective coupling constants:
_0 ffZee
55
sgsgsgsg AfVfAeVe
The electroweak corrections dependence is:• quadratic on top quark mass• logarithmic on Higgs boson mass
For electroweak corrections two loop level is achieved today for most of the processes.
Numerical calculations are performed using the programmesTOPAZ0 and ZFITTER.
LEP and detectors
Large Electron Positon collider• 27 km circumference• peak luminosity L=2.*1031cm-2s-1 (design value 1.6*1031)• maximum energy 208 GeV• beam energy known with precision of about 2 MeV (at Z0 peak)
To operate LEP special „LEP standard model” took into account• earth tides generated by moon and sun• rainfalls in Jura• Lake Geneva water level• leakage currents from trains
Four experiments have been operating at LEP (ALEPH, DELPHI,L3and OPAL). At Z0 peak ADLO collected about 17 M events.
LEP I running at Z0 peak
_0 qqZee
quark and antiquark fragment into two separete jets
LEP II running at GeVs 205
__
qqqqWWee
four jets in the final state
SLAC Linear Collider
SLC, the first linear e+e- collider ever• operated with good luminosity and polarization from 1992 till 1998• had worse then LEP beam energy resolution• run only at Z0 peak (600 k events)But...• its electron beam was longitudinally polarized• its beam spot was much smaller (1.5μm*.7μm vs. 150μm*5μm)
The designs of LEP and SLC detectors are quite similar.
Slac Linear Detector (SLD) had better vertex reconstructiom(CCD vs. micro-strip)
for example, due to• lower repetition rate• smaller beam spot
But,
s
mff
f
sssHdss2
_
4
_
'','
""""2
2
2
2_
_
Z
Ms
Ms
ss
Z
ZZ
Zpeak
ff
calculated from SM, not fittedX-section formula at Z0 peak:
H(s,s’)-radiative function
43
1
112
1
1222
0__
fZ
fe
ZQEDff
peak
ff QM
Fit performed to the hadron data:
MZ, ΓZ, σ0had, Rl
and to the lepton data:
Γe, Γμ, Γτ, or (lepton universality) Γlept
lept
hadlR
_
qqee
_
llee
BF
BFFB NN
NNA
ADLO results (with lepton universality)
0010.00171.0
025.0767.20
037.0540.41
0023.04952.2
0021.01875.91
,0
0
0
lFB
l
had
Z
Z
A
R
nb
GeV
GeVM
Values of Mz,Гz,Гμ,Гτ,Гe,Rl,... extracted with use of SM elementsObservables Pseudo-observables
N
sl
hadl R
0
00
leff2sin
fQCDQED
f Z
fAf
Z
fVf
Z
fZ m
mg
m
mg
m
m
1*
41
21
41
2
22
2
22
2
2
0
12
2 3
0Zf
C
mGN
SM expresion for Z
The number of light neutrino families
3
3
llept
Z
lept
inv
invlepthadZ
R
312
0
lZhad
l
SMlept
RM
RN
depends strongly on 0had
invN
N
0083.09841.2 N
Predicted cross-section fortwo, three and four (massless)neutrino species with SM couplings
Z0 decays to heavy quarks (charm and beauty)
• two (or more) jets are formed in_
0 qqZee process,following the quark fragmentation into hadrons.
• jet (initial quark) direction is established from thrust axis.
• in the final state we observe hadrons, not quarks. How to select Z0 decays into particular flavour ?.....,,
__
ccbb
Flavour tagging:
• heavy flavours tagged by leptons (high p,pT), lifetime, secondary vertex mass,....Works well for b and c quarks. thanks to vertex detectors:
b hadron on average travels 3 mm,position of the secondary vertex is measured with accuracy of 300 μm.
Different methods use different tags combinations to establish flavour of the initial (heavy) quark .
cb
cb GeVmGeVm
5.15 secondary vertex mass and/or high p, pT allows
to distinguish between b anc c hadrons.
For tagged sample one has to know:• purity (up to 96%)• efficiency (up to 26%) usually requires very good
Monte-Carlo program
Most precise – double tag method 1996
Pseudo-observables:
had
bbR
had
ccR
Most recent values: 00066.021638.0 bR
0030.01720.0 cREPS Aachen 2003
Asymmetries at Z0 pole
Z0 couplings to right-handed and left-handed fermions are different.
for_
0 ffZee even for unpolarized e beams Z0 is polarized along beam direction (LEP)
forward (F) – e- beam direction.R (L) means right (left) handed fermions in final state
For polarized electron beam (SLC):
tot
RBLFLBRFFBpol
tot
BF
tot
LBRBLFRFFB
tot
LR
LBLFRBRF
LBLFRBRFpol
A
A
A
,,,,
,,,,
,,,,
,,,,
rl
rBlBrFlFLRFB
tot
rlLR
PA
PA
,,,,1
1r(l) means right (left) handed electron beam polarization.<P> - mean beam polarization
At the Z0 pole:
fLRFB
eLR
feFB
eFBpol
fpol
AA
AA
AAA
AA
AA
4
3
4
34
3
0
0
0
0,
0
asymmetry parameter forfermion f
fA
2
1
2
Af
Vf
Af
Vf
f
gg
gg
A
When the couplings conform to the SM structure:
fefff
Af
Vf Qg
g2sin41
Studies of asymmetry parameters provide very sensitive measurementof the f
eff2sin ,particulary good for leptonf
Particulary cute- ALR at SLAC
precise, direct measurement of Ae with hadron events eeff2sin
Another precise measurements: cFB
bFB AA ,0,0 ,
LEPSLC
021.0898.0 bA
020.0925.0 bA
combined
013.0903.0 bA vs. 935.0
Standard Model
EPS, Aachen 2003
LEP and SLAC measurementsof Ab are consistent. But the combined Ab value seems to disagree with SM prediction.
LEP Ab (and Ac ) result can be expresed in terms of
lepteff2sin
Direct W mass and width measurement.
From CDF and D0 experiments at 1 Tevproton antiproton collider at Fermilab:
059.0454.80 WM
From direct measurements at LEP 2:
• GeVMW 22.040.80
• study of decay channels:
lqqWW_
or__
qqqqWW
important corrections coming from:• Bose-Einstein correlations• color reconnection
LEP 2 result: GeVGeVM WW 091.0150.2042.0412.80
cross section for processat the treshold (161 GeV)
Very good agreement between electron and hadroncolliders!
Combined result: GeVGeVM WW 069.0139.2034.0426.80
But
NuTeV experiment measures from the ratio of the
neutral to charged current interactions in and_
beams:
W2sin
0016.02277.01sin 2
22
Z
WW M
M
Using MZ from LEP I GeVMW 084.0136.80
This indirect measurement differs more then 3σ from direct one !
Standard Model Higgs Search
The production (and decay) of Higgs particle is predicted in the SMas a function of its (unknown) mass.
For mH=115 GeV
%74)(_
bbHBR
Background:WW,ZZ,2f
main production channel
ZH decay channels
_
__
__
__
,
,
,
,
bbZH
llZbbH
ZbbH
qqZbbH
b-tagging plays essential role in Higgs search!
At LEP I serches in fully hadronic channels excluded by background
LEP I serches in other channels - negative
At LEP II main sources of background in Higgs search:
_00 ,, qqWWZZ
Selection of Higgs candidate events: on the Monte-Carlo basis • topology• btag
HB
HBSi mL
mLQ
Does the data sample contains signal and background or onlybackground ?
• for each candidate i introduce the likelihoods ratio:• Qi is estimated from topology combined with mass information.• MC determines expected Qi distributions• the global likelihood:
cQQi
i ln2ln2
s and s+b equally likely for-2ln(Q)=0
ADLO result by M.Duehrssen, EPS, Aachen 2003
Conclusion from furtherstatistical analysis:mH<114.4 GeVis excluded @ 95% CL
green and yellow bands indicate 1σ and 2σ limitsof backround only hypotesis.
The Global Fit
Fit of the five Standard Model parameters to all available electroweak results.
GeVmmmMMM H
HtZZsZhad 1022)5( log,,,,
Some fit results already presented:
Wt Mvsm .
The purpose of the fit• check internal consistency of the Standard Model• constrain the Higgs mass
5076999035.13710
1
0)5(
with
ssss
hadtl
Runing coupling constant shad5 -from dispersion integral
and low energy e+e- data.
If in the global fit replace 6 parameters )(sin,,,,, 2,0,0,0 hadfb
leffll
cFB
bFB
lFB QSLDAPAAAA
with lepteff2sin
then for global fit fod ..10/152 -probability=13%
value fitted to the above parameters
3 σ from Standard Model prediction !
-very precise measurement at low <Q2>~20 GeV2
Removing NW 2sin from fit changes χ2 probability (to 28%) but
does not influence SM parameters values much.
Global EW fit with average
lepteff2sin
lepteff2sin
and without
NW 2sin
.%70..94.62 probfod
OK. for global fit but
NW 2sin
NW 2sin problem remains...
Conclusions from the tests
• precision (above tree level) predictions of the Standard Model have been compared with experimental results from LEP and SLC.• Standard Model looks fine after that comparison. SM is a well established (effective) theory. • no need for New Physics.• where is (if at all) the Higgs boson(s)?• further measurements of MW, mt, (mH? .....) will make tests more stringent and perhaps will show the road to New Physics. Tools: Tevatron (Run II) + ......... Large Hadron Collider (2007) Next Linear Collider
• this talk is ,by all means, not exhaustive. Supersymmetry, Grand Unification, Multi doublet Higgs Models, MSSM, TGC,... were left behind.