Download - Standard Model Results from LEP
Standard Model Results from LEP
Pat WardUniversity of Cambridge
� Introduction
� Electroweak Physics
� QCD and Two-photon
Results
10-1
1
10
10 2
10 3
10 4
80 100 120 140 160 180 200√s / GeV
Cro
ss-s
ecti
on /
pb e+e−→hadronse+e−→µ+µ−
e+e−→γγ (cosθ<0.93)
e+e−→ZZe+e−→WW (prel.)e+e−→WWγ (Eγ> 2.5 GeV)
e+e−→qqγγ (Eγ> 5 GeV)
13/02/2004 K. Sachs
Pat Ward September 2004 1
Introduction
LEP1 1989–1995 � � � � 4.5M Z events / expt
LEP2 1996–2000 161 � � � 209 GeV 10k WW events/expt
� LEP was shut down at the end of 2000, so why are we still giving LEP talks?
� Many physics results still being produced
� In the last 2 years, the 4 LEP experiments (ALEPH, DELPHI, L3 and OPAL)
have submitted for publication � 100 papers
About 25% of these were on LEP1 physics
� The 4 expts have submitted � 120 papers to ICHEP04
Majority on Standard Model physics
Mostly final results, but some new preliminary results too
� This talk will concentrate on results which have been finalized, or are new,
in the last year (but include some older important results too)
Pat Ward September 2004 2
Introduction
LEP Standard Model physics covers a wide range of topics; no time in this talk
for, e.g.,
� Precise measurement of the � lifetime (DELPHI)
� � = 290.9 � 1.4 � 1.0 fs (c.f. PDG2004: 290.6 � 1.1 fs)
� � branching ratios and strange spectral functions (DELPHI, OPAL)
� ��� ��
, �� �� and hard-photon production in two-photon collisions
(DELPHI, L3, OPAL)
� Excited b-hadrons, �� �� �� oscillations, B spectral moments (DELPHI)
Will give a brief overview of current results; for more details consult the
experiments’ web pages: http://aleph.web.cern.ch/aleph/
http://delphiwww.cern.ch/
http://l3.web.cern.ch/l3/
http://opal.web.cern.ch/Opal/PPwelcome.html
Pat Ward September 2004 3
LEP1 Electroweak Physics� Z lineshape measurements final
since summer 2000
� � = 91.1875 � 0.0021 GeV
� � = 2.4952 � 0.0023 GeV
�� � = 41.540 � 0.037 nb
� � = 20.767 � 0.025
� � � ��� = 0.01714 � 0.00095
� New measurement of � ��
(DELPHI)
� � new LEP heavy-flavour com-
bination
-0.05
0
0.05
0.1
0.15
89 90 91 92 93 94√s [GeV]
AF
B(√
s)
LEP
AFB
b
AFB
c
� � � �� =0.0998 � 0.0017
Pat Ward September 2004 4
LEP1 Electroweak Physics� OPAL have published new mea-
surements of � � , � �
� Use hadronic events with FSR
( � � � � � )
Enriched in up-type quarks
� � � � � � � � � �
� � � �� � � � � � � �
� Results:
� � = 300� �� � MeV
� � = 381� �� � MeV
� Good agreement with SM
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.1 0.2 0.3 0.4 0.5 0.6
Γhadrons
SMΓhadrons+γ
Γd-type [GeV]
Γ u-ty
pe [
GeV
]Jade E0, ycut=0.08
OPAL
� More precise than earlier measure-
ments (DELPHI, L3, OPAL)
Pat Ward September 2004 5
Two-fermion Production at LEP2
µ+
µ-
e-
e+
(Z/γ)*
Non-radiativeµ+
µ-
e-
e+Z0
γRadiative return
� Measure cross-sections and asym-
metries for inclusive and ‘non-
radiative’ events
� LEP combination of preliminary
measurements
Good agreement with SM
� limits on new physics, e.g. ��� ,
leptoquarks, RPV squarks, contact
interactions, extra dimensions
Cro
ss s
ectio
n (p
b)
√s
´/s
> 0.85
e+e−→hadrons(γ)e+e−→µ+µ−(γ)e+e−→τ+τ−(γ)
LEPpreliminary
√s
(GeV)
σ mea
s/σ S
M
1
10
10 2
120 140 160 180 200 220
0.8
0.9
1
1.1
1.2
120 140 160 180 200 220
Pat Ward September 2004 6
Two-fermion Production at LEP2� At LEP2 energies, � -exchange be-
comes important � can measure
� -Z interference � almost model-
independent determination of � �
in S-matrix fit
� e.g.DELPHI results
� � = 91.1831 � 0.0034 GeV
c.f. � � = 91.1863 � 0.0028 GeV
from standard LEP1 fit assuming
SM interferenceMZ (GeV/c 2)
jtot
had
DELPHI LEP1 data
DELPHI LEP1+LEP2 data
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
91.165 91.17 91.175 91.18 91.185 91.19 91.195
� OPAL, DELPHI have finalized two-fermion measurements; new
combination when all experiments have final results
� Expect small improvements to �� � � �
, but leptons dominated by statistics
Pat Ward September 2004 7
WW Cross-sections
W-
W+
e-
e+
Z
W-
W+
e-
e+
γ
W-
W+
e-
e+
ν
� 3 channels: � � � � � � �
� � � � � �� �
� � � �� � �� �
� LEP combination updated with fi-
nal values from ALEPH, L3
Final combination awaiting OPAL
final results
0
10
20
30
160 180 200
√s (GeV)
σ WW
(pb
)
YFSWW/RacoonWWno ZWW vertex (Gentle)only νe exchange (Gentle)
LEPPRELIMINARY
02/08/2004
Good agreement with theoretical ex-
pectations
Pat Ward September 2004 8
W Branching Ratios� LEP combination updated with final values from ALEPH, L3
02/08/2004
W Hadronic Branching Ratio
ALEPH 67.15 ± 0.40
DELPHI 67.45 ± 0.48
L3 67.50 ± 0.52
OPAL 67.91 ± 0.61
LEP 67.49 ± 0.28χ2/ndf = 15 / 11
66 68 70
Br(W→hadrons) [%]
Summer 2004 - LEP Preliminary
02/08/2004
W Leptonic Branching Ratios
ALEPH 10.81 ± 0.29DELPHI 10.55 ± 0.34L3 10.78 ± 0.32OPAL 10.40 ± 0.35
LEP W→eν 10.66 ± 0.17ALEPH 10.91 ± 0.26DELPHI 10.65 ± 0.27L3 10.03 ± 0.31OPAL 10.61 ± 0.35
LEP W→µν 10.60 ± 0.15ALEPH 11.15 ± 0.38DELPHI 11.46 ± 0.43L3 11.89 ± 0.45OPAL 11.18 ± 0.48
LEP W→τν 11.41 ± 0.22
LEP W→lν 10.84 ± 0.09
χ2/ndf = 6.8 / 9
χ2/ndf = 15 / 11
10 11 12
Br(W→lν) [%]
Summer 2004 - LEP Preliminary
� B(W � � � ) higher than average of B(W � � � ) and B(W � � � ) by 3 �
Pat Ward September 2004 9
Single Vector Boson Production� Zee
e-
e+
e-
e+
γZ
q
q--
0
0.5
1
180 190 200 210
√s (GeV)
σ Zee
ee (
pb)
WPHACT and GRACE
LEP PRELIMINARY
02/08/2004
� We�
e-
e+
e-
ν--
γW
q
q--
0
0.5
1
1.5
180 190 200 210
√s (GeV)
σ Weν
(pb
)
WPHACT and GRACELEP PRELIMINARY
03/08/2004
LEP combination updated with final measurements from ALEPH, L3
Pat Ward September 2004 10
Charged Triple Gauge Boson Couplings
W-
W+
Z
W-
W+
γ
� Measured using WW events:
� (WW), �� � � � , W decay angles
� Also We� and� � � channels
� Assuming C, P conservation and
gauge constraints: 14 � 3 cou-
plings: � � � � �� � �
� LEP combination: combine � ��
curves including correlated system-
atics
OPAL
0
250
500
750
1000
-1 0 1cosθw
Eve
nts/
bin
0
200
400
600
-1 0 1cosθ*
l
Eve
nts/
bin
0
200
400
600
-180 0 180φ*
lE
vent
s/bi
n
0
200
400
-1 0 1cosθ*
jet
Eve
nts/
bin
0
200
400
0 90 180φ* jet
Eve
nts/
bin W+W- → qq
–lν
–
l
data
SM (λ=0)
λ=+0.5
λ=-0.5
background
Pat Ward September 2004 11
Charged Triple Gauge Boson Couplings� L3, OPAL values final, ALEPH,
DELPHI values preliminary
� LEP combined results allowing
one free parameter:
� � � � � � � �� � �� � �� � �� �
� � � � � � �� � � �� � �� � �� � �
� � � � � � � � �� � �� � � � �� �
� Good agreement with Standard
Model
� Couplings measured with preci-
sion of 2–4%
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.9 1 1.1
g1Z
λ γ
0.75
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
0.9 0.95 1 1.05 1.1
g1Z
κ γ
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
-0.1 0 0.1
λγ
κ γ
95% c.l.
68% c.l.
2d fit result
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
0.9 1 1.1
g1Z
λ γ
0.75
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
1.25
0.9 0.95 1 1.05 1.1
g1Z
κ γ
0.8
0.85
0.9
0.95
1
1.05
1.1
1.15
1.2
-0.1 0 0.1
λγ
κ γ
95% c.l.
68% c.l.
2d fit result
LEP charged TGC Combination 2003
LEP Preliminary
� LEP combined confidence levels allow-
ing 2 free parameters
Pat Ward September 2004 12
Other Gauge Boson Couplings� Neutral Triple Gauge Boson couplings (ZZ � , Z � � ) are zero in SM
� Limits set from ZZ and Z � channels
� SM Quartic Gauge Couplings either zero or too small to be observed at LEP
� Limits set from WW � , Z � � and� � � � channels
0
0.5
1
100 120 140 160 180 200
OPAL DataSM (KK2F)a0/Λ
2=0.005 GeV-2
ac/Λ2=0.005 GeV-2
aZ
aZ
Eγ > 5 GeV|cosθγ| < 0.95cosθγq < 0.90|Mqq-MZ| < 3 ΓZ
√s [GeV]
σ(e+ e- →
Zγγ
γγ)
[pb]
-0.1
-0.05
0
0.05
0.1
-0.1 -0.05 0 0.05 0.1
a0/Λ2 [GeV-2]
a c/Λ
2 [G
eV-2
]
V
V
Best FitStandard Model
{a0 , ac{aW, aW}{a0 , ac{aZ, aZ}
Combined (aW=aZ) 95 % C.L.
Combined (aW=aZ) 68 % C.L.
OPAL
Pat Ward September 2004 13
W Mass Measurement� A principal aim of LEP2
� Comparison of direct mea-
surement with indirect deter-
mination from EW fits is test of
SM
� Measure by direct reconstruc-
tion of � � or �� � mass in
� � � � � �� � and
� � � � � � � channels
� Reconstruct event-by-event
mass using beam energy
constraint (kinematic fit) to
improve resolution
m /GeV
Eve
nts
m /GeV
Eve
nts
qqqq
qqlν
OPAL 183-209 GeV ∫ L dt = 677 pb-1
Signal
Combinatorial b/g
Other b/g
0
50
100
150
200
250
300
60 65 70 75 80 85 90 95 100 105
0
50
100
150
200
250
300
60 65 70 75 80 85 90 95 100 105
� Fit mass distribution � � � , using MC
to correct for bias
Pat Ward September 2004 14
W Mass Measurement� LEP preliminary values:
� � � � � �� � �
= 80.411 � 0.032(stat) � 0.030(sys) GeV
� � � � � � � �
= 80.420 � 0.035(stat) � 0.101(sys) GeV
80.0 81.0
MW[GeV]
LEP working groupχ2/dof = 29.6 / 37
ALEPH [1996-2000] 80.379±0.058
DELPHI [1996-2000] 80.404±0.074
L3 [1996-2000] 80.376±0.077
OPAL [1996-1999] 80.490±0.065
LEP 80.412±0.042
Summer 2003 - LEP Preliminary
� Systematics completely dominate
� � � � channel (and important in
� � �� � channel)
� Combined value:
� � = 80.412 � 0.042 GeV
� What has been happening in last
year to improve and finalize mea-
surements?
Pat Ward September 2004 15
W Mass Measurement� Main systematics in � � � � channel arise from final state interactions
� Separation of W decay vertices � 0.1 fm � hadronic scale � 1 fm
� W decays have space-time overlap and can exchange colour:
Colour Reconnection
� May also be Bose-Einstein Correlations
between like-sign particles from different
W’s
� Both effects may shift measured � � by
large amount ( � 100 MeV)
Not included in standard MC used to cali-
brate � � measurement
� Much effort to measure CR and BEC ef-
fects to estimate realistic errors
Pat Ward September 2004 16
Colour Reconnection
q1
q2
q_
1′
q_
2′
Normalq1
q2
q_
1′
q_
2′
Reconnected
� Effects estimated using phe-
nomenological models
� Look at particle flow between jets in
� � � � � � � events
� Compare particle flow in inter-W re-
gion with intra-W region
0.4
0.6
0.8
1
1.2
1.4
0 0.2 0.4 0.6 0.8 1φresc
fA+
B
N
/fC
+D
N
L3 a)
Particle flow
DataNo CRSKI (kI=3)SKI 100%
RN
� L3 results final, ADO preliminary
� Use largest reconnection probability compatible with data to set ��
� � �
Current LEP combination uses � = 2.1 in SK1 model ( � 55% reconnected)
� � � � � � � � � �
= 90 MeV
Pat Ward September 2004 17
Colour Reconnection� Soft particles most affected by FSI
� reduce effects with momentum
cuts or jet cone cuts
Reduces FSI error at expense of
statistical error, as jet directions
less precisely determined
� Final � � analyses will optimize jet
reconstruction
DELPHI preliminary SK1 curves
-100
0
100
200
300
400
500
600
10-1
1 10 102
Standard analysis1 GeV pcut2 GeV pcut3 GeV pcutCone R=1.00 radCone R=0.75 radCone R=0.50 radCone R=0.25 rad
SK1 Model parameter κ
∆mW
/ M
eV/c
2
κ = 0.66
Bose-Einstein Correlations
� BEC between like-sign pions well-established in Z decays
Do they occur between particles from different W’s?
� Currently contributes 35 MeV to � � � � � � � � �
(LUBOEI model)
Pat Ward September 2004 18
Bose-Einstein Correlations� Study using two-particle correla-
tions, normalized to ‘no BEC’
� Use � � � � � �� � events
to estimate intra-W correlations,
mixed events (or MC) for kine-
matic correlations
� � �� � � ��� �� � � � � �� � � � �
� �� �� � �� �
� LEP combined data:
� �� �� � �� �
� �� � � �� � �� � = 0.23 � 0.13
-4-202468
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
(a) OPALdatafull BECintra BECno BEC
Q [GeV]
∆ρ(±
±)
[GeV
-1]
-4-202468
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
(b) OPALdata full BECintra BECno BEC
Q [GeV]
∆ρ(+
−)
[GeV
-1]
� DELPHI, L3, OPAL results now final
� L3, OPAL compatible with no inter-W BEC, DELPHI sees significant effect
Pat Ward September 2004 19
Prospects for Final � Measurement� LEP measurements of � � should be finalized soon
� Expect improvements over preliminary measurement
� LEP beam energy uncertainty will decrease from 21 MeV to � 10 MeV
Final LEP beam energy paper recently submitted for publication
� Analyses in � � � � channel will be optimized to reduce total error
� Hadronization and detector effects will be better understood
� In absence of systematics, LEP statistical precision � 21 MeV
With reduced weight for � � � � channel, probably � 25 MeV
� Final total error will probably be in range 32–40 MeV, depending on FSI
results
Pat Ward September 2004 20
W Width Measurement� Fits to W mass distributions also
determine � �
� Preliminary LEP average:
� � = 2.150 � 0.091 GeV
1.5 2.0 2.5
ΓW[GeV]
LEP working groupχ2/dof = 19.7 / 24
ALEPH [1998-2000] 2.13±0.14
DELPHI [1997-2000] 2.11±0.12
L3 [1996-2000] 2.24±0.19
OPAL [1996-1998] 2.04±0.18
LEP 2.150±0.091
Summer 2003 - LEP Preliminary
Pat Ward September 2004 21
Standard Model Fit� Standard Model fit by LEPEWWG uses 17 inputs from LEP, SLD, Tevatron:
Z lineshape: � � , � � , �� � � � , � � , � � � ��
� polarisation: � �Polarised lepton asymmetry: � (SLD)
Heavy flavour: � , � � , � � � �� , � � � ��� , , �
Inclusive hadronic charge asymmetry
� � , � � , � �
� Updates since summer 2003:
� Tevatron � � : 178.0 � 2.7 � 3.3 GeV
� LEP1 heavy flavours
� Theory calculations including full two-loop corrections for � � and
��� � � � � � ��� (Awramik, Czakon, Freitas, Weiglein)
Shifts predicted value of � � from ��� � � � � � ��� alone by � 19 GeV
Pat Ward September 2004 22
Standard Model Fit� Fit �
� /dof = 15.8/13
� 67% correlation between
� � and � � �
� Largest contribution to ��
from � � � �� (2.4 � )
� � � � �� prefers large � � ,
whereas � � , � � and
lepton asymmetries prefer
small � �Measurement Fit |Omeas−Ofit|/σmeas
0 1 2 3
0 1 2 3
∆αhad(mZ)∆α(5) 0.02761 ± 0.00036 0.02769
mZ [GeV]mZ [GeV] 91.1875 ± 0.0021 91.1874
ΓZ [GeV]ΓZ [GeV] 2.4952 ± 0.0023 2.4966
σhad [nb]σ0 41.540 ± 0.037 41.481
RlRl 20.767 ± 0.025 20.739
AfbA0,l 0.01714 ± 0.00095 0.01650
Al(Pτ)Al(Pτ) 0.1465 ± 0.0032 0.1483
RbRb 0.21630 ± 0.00066 0.21562
RcRc 0.1723 ± 0.0031 0.1723
AfbA0,b 0.0998 ± 0.0017 0.1040
AfbA0,c 0.0706 ± 0.0035 0.0744
AbAb 0.923 ± 0.020 0.935
AcAc 0.670 ± 0.026 0.668
Al(SLD)Al(SLD) 0.1513 ± 0.0021 0.1483
sin2θeffsin2θlept(Qfb) 0.2324 ± 0.0012 0.2314
mW [GeV]mW [GeV] 80.425 ± 0.034 80.394
ΓW [GeV]ΓW [GeV] 2.133 ± 0.069 2.093
mt [GeV]mt [GeV] 178.0 ± 4.3 178.2
Summer 2004
Pat Ward September 2004 23
Standard Model Fit
80.2
80.3
80.4
80.5
80.6
130 150 170 190 210
mH [GeV]114 300 1000
mt [GeV]
mW
[G
eV]
Preliminary
68% CL
∆α
LEP1, SLD Data
LEP2, pp− Data
Good agreement between mea-
sured � � , � � and values pre-
dicted by fit excluding direct mea-
surements
0
1
2
3
4
5
6
10020 400
mH [GeV]
∆χ2
Excluded Preliminary
∆αhad =∆α(5)
0.02761±0.00036
0.02749±0.00012
incl. low Q2 data
Theory uncertainty
� � � 260 GeV 95% c.l.
Pat Ward September 2004 24
QCD and Two-photon Physics
Will discuss:
� � from event shapes
� � from 4-jet rate
� Unbiased gluon jets
� Coherence effects in Z � 3 jets
� Dead cone effect
� Inclusive jet/hadron production in
two-photon events
No time to cover
� Fragmentation functions and scal-
ing violation
� Fermi-Dirac and Bose-Einstein cor-
relations in Z decays
� b-quark mass effects
� Production of �� � , � in Z decays
� Pentaquark searches
Pat Ward September 2004 25
� � from Event Shapes
q-
q
e-
e+
g
� Event shapes sensitive to gluon
emission, e.g. thrust:
� � � �� �� �� � � �� ��� �
� Fit event shape distributions with
��
�� �
+ NLLA ( � � matching)
QCD predictions � �
� Final measurements from all ex-
periments
91 GeV133 GeV (×3)177 GeV (×9)197 GeV (×27)
OPAL
(1/σ
)⋅dσ/
d(1−
T)
PYTHIAHERWIGARIADNE
197 GeV
(MC
− d
ata)
/err
or
91 GeV
(1−T)
1
10
10 2
10 3
0 0.05 0.1 0.15 0.2 0.25 0.3
-5
0
5
0 0.05 0.1 0.15 0.2 0.25 0.3
-505
0 0.05 0.1 0.15 0.2 0.25 0.3
Pat Ward September 2004 26
� � from Event Shapes� Combinations by LEPQCD group
� Use variables which are infra-red
safe (soft gluon emission) and
collinear stable (collinear parton
branchings)
� Combination requires care because
of large correlated errors
� Treat hadronization and theory er-
rors as uncorrelated when calculat-
ing weights, but include 100% cor-
relation when calculating hadronisa-
tion and theory uncertainties on com-
bined �
PSfrag replacements
0.110
0.110
0.115
0.115
0.120
0.120
0.125
0.125
0.130
0.130
αs(MZ)
αs(MZ)
1 − T
1 − T
M2
H/s
M2
H/s
C
C
BT
BT
BW
BW
− ln(y3)
− ln(y3)
ALL
ALLPSfrag replacements
0.115
0.115
0.120
0.120
0.125
0.125
0.130
0.130
αs(MZ)
αs(MZ)
ALEPH
ALEPH
DELPHI
DELPHI
L3
L3
OPAL
OPAL
ALL
ALL
Pat Ward September 2004 27
� � from Event Shapes� Preliminary combiation of final re-
sults from all experiments
� Relative weights in global fit:
LEP1 46%
LEP2 54%
� � runs as expected from QCD
� Theory error includes variation of
renorm. scale: 0.5 �� � � 2,
log rescaling fact.: 2/3 � � � � 3/2,
matching scheme and kinematic
cutoffs
PSfrag replacements
αs(Q)
αs(Q)
Q (GeV)
Q (GeV)
100
100
150
150
200
200
0.100
0.100
0.105
0.105
0.110
0.110
0.115
0.115
0.120
0.120
0.125
0.125
αs(Q) fit at single energy
± σstat.
± σtotal
αs(Q) global fit± σstat.
± σstat.
± σtotal
± σtotal
� �
� � �
= 0.1202 � 0.0003(stat) � 0.0007(exp) � 0.0015(hadr) � 0.0044(theo)
Pat Ward September 2004 28
� � from 4-jet Rate
q-
q
q
q-
e-
e
q-
q
g
g
e-
e+
� New meas. of � from 4-jet rate
� DELPHI: Cambridge jet algorithm
� Fit to ��
� �
QCD prediction of DE-
BRECEN (Nagy, Trocsanyi) using ex-
perimentally optimized renormaliza-
tion scale
� OPAL: Durham jet finder
� Fit to ��
� �
+ NLLA QCD prediction
with� � =1
10-3
10-2
10-1
10-3
10-2
R4
fit range
DELPHI
xµ=1xµ
opt
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
10-3
10-2
CAMBRIDGECAMBRIDGE ycut
R4da
ta/R
4fit
CAMBRIDGECAMBRIDGECAMBRIDGE
Pat Ward September 2004 29
� � from 4-jet Rate
0.090.095
0.10.1050.11
0.1150.12
0.1250.13
100 150 200
DELPHI
αs from Cambridge4-jet rate1/logE fitQCD evolution
√s (GeV)
α s� DELPHI
� �
� � �
= 0.1175 � 0.0030
αS(MZ)=0.1208±0.0038
OPALPreliminary
Durham 4-Jet Rate
√s [ GeV ]
α S
0.08
0.09
0.1
0.11
0.12
0.13
80 100 120 140 160 180 200
� OPAL
� �
� � �
= 0.1208 � 0.0038
Pat Ward September 2004 30
Unbiased Gluon Jets with Jet Boost Algorithm� Jet boost algorithm (Eden, Gustafson) relates gluon jets in � � g events to
gg system � unbiased gluon jets
β=cos
=cos
α α
α
β α
α
(a)
αEg
E
E
q
q
−
� Decompose into 2
colour dipoles
E*
(b)
g
q*E
q*E−
� Boost each dipole into
back-to-back frame
Eg*q
EE
*q
*
(c)
−
� Recombine to give
event with gg struc-
ture
Pat Ward September 2004 31
Unbiased Gluon Jets with Jet Boost Algorithm� Method used by OPAL to mea-
sure gluon charged multiplicity
for 5 � � � � 20 GeV
� Results consistent with other
measurements, and most pre-
cise for this energy range
� Theoretical fits OK
� Also measure gluon fragmen-
tation function
4
5
6789
10
20
5 6 7 8 9 10 20 30 40Eg (GeV)*
(a)
<ngl
uon>
ch.
3NLO fitFixed αS fitHerwig (gg evts.)
OPAL data (boost)OPAL data (gincl)OPAL data (qqg-qq)CLEO data
-1-0.5
00.5
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
10-5
10-4
10-3
10-2
10-1
1
10
10 210 3
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
xE*
Dat
a-P
redi
ct.
σ data
1 N
dngl
uon
ch.
dxE*
Herwig (gg)DGLAP fit
OPAL data
Eg= 14.24 GeV*
(a)
Pat Ward September 2004 32
Inclusive Charged Particle Distributions<N
ch>
Ecm [GeV]
JADE
TASSO
ARGUS
AMY
HRS
MARKII
TPC
TOPAZ
ALEPH
3NLO nf = 3χ2/NDOF = 25.1/29
PYTHIAHERWIGARIADNE
0
5
10
15
20
25
30
0 25 50 75 100 125 150 175 200
� Mean charged multiplicity
Well-described by theory or MC
Ecm [GeV]
ξ*
ALEPH
TASSO
QCD MLLA + LPHD nf=3
χ2/NDOF=8.7/12
1.5
2
2.5
3
3.5
4
4.5
20 40 60 80 100 120 140 160 180 200
� Peak of �
� � � � � distribution
Sensitive to coherence effects
Pat Ward September 2004 33
Coherence Effects in 3-jet Events� New results from DELPHI
� Measure multiplicity in 30 � cone
perpendicular to event plane in 3-
jet events
� Compare with corresponding quan-
tity in 2-jet events
� Direct evidence for coherence ef-
fects in soft particle production
Pat Ward September 2004 34
Dead Cone Effect� QCD predicts gluon radiation
off heavy quark suppressed at
small angles
� DELPHI study using 2-jet
� � � � and � � � � events
� Remove particles associated
with b- or c-quark
� Compare angular distribution of
fragmentation particles in b- and
c-jets
� First DIRECT observation of
‘dead cone’ effect
θ, rad
(dN
b/dθ
)/(d
Nc/
dθ)
q direction estimated bythrustjetvertex
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Pat Ward September 2004 35
Two-photon Physics� Active area with many new
measurements � test QCD
� e.g. new DELPHI measure-
ment of total hadronic cross-
section at low ��
uses Very Small Angle Tag-
ger at � � 3-15 mrad
� Direct event-by-event recon-
struction of � � for double-
tagged events without un-
folding � small systematic
errors
250
300
350
400
450
500
550
600
650
700
0 20 40 60 80 100 120 W (GeV)
To
tal γ
γ cr
oss
-sec
tio
n (
nb
)
TWOGAM simulationDELPHI single tag
DELPHI double tag
OPAL
L3
DELPHI 189-206 GeV
� DELPHI measurements somewhat higher
than L3 and OPAL
Pat Ward September 2004 36
Inclusive jet production� L3 measurements of inclusive
jet production in two-photon
collisions for
� � � � 1
� � � � � 30 GeV
� �� � � 0.2 GeV�
� � � spectrum well-represented
by power law
� Data higher than NLO QCD
prediction (Frixione, Bertora)
10-3
10-2
10-1
1
10
10 2
20 40 60pt [GeV]
dσ /
dpt [
pb /
GeV
]
Data
NLO QCDPower law fit
L3
Pat Ward September 2004 37
Inclusive hadron production
10-3
10-2
10-1
1
10
10 2
10 3
10 4
5 10 15 20pt [GeV]
dσ /
dpt [
pb /
GeV
]
Data π±a)
NLO QCDScale dependenceDirect
L3
Wγγ > 5 GeV
� L3 measurements of inclusive ���
production in two-photon collisions
for
� � � � 1
� � � � � 5 GeV
� Also see excess at high � � com-
pared with NLO QCD calculations
(Binnewies, Kniehl, Kramer)� Similar disagreement seen in ��
Pat Ward September 2004 38
Inclusive hadron production� New DELPHI measurement of in-
clusive hadron production
� � � � 1.5
� � � � � 203 GeV
� Good agreement with NLO QCD for
� � � 6 GeV
� Some excess of data over NLO
QCD at high � � , but not at L3 level
Pat Ward September 2004 39
Exclusive Particle Production� Several measurements of exclusive
particle production
� e.g. L3 have measured ��
�� and
�� ��
� In range 1.1 � � � � 2.1 GeV,
1.2 � �� � 8.5 GeV�
� �� � � � �
� �� � � � � = 1.81 � 0.47 � 0.22
expect 2 from isospindσ
ee /
dQ2
[pb/
GeV
2 ]Q2 [GeV2]
ρ+ρ−
ρ0ρ0
QCD fit to ρ+ρ−
b) L3
10-3
10-2
10-1
1
10
10 2
1 10
� Fit to QCD expectation � � � �� � �
� � �� � � �� � � �gives � = 2.4 � 0.3 (2.5 � 0.4) for �
���
� �� ��
�
(expect � = 2)
Pat Ward September 2004 40
Summary� Since LEP finished running in 2000, the experiments have continued to
produce lots of new physics results covering a wide range of topics:
Fermion- and boson-pair cross-sections, gauge boson couplings, QCD,
two-photon physics....
� Some important LEP2 measurements still to be finalized, especially � �
� Important to utilize fully the plentiful, high-quality data produced at LEP
Pat Ward September 2004 41