Table Of ContentBose-Einstein correlations and color reconnection
in hadronic W
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P
n
a o
J RaimundStröhmer∗†
9 S
LMUMuenchen,Germany
1 E-mail: [email protected] (
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1 WereportonstudiesofBose-EinsteincorrelationsandcolorreconnectioninhadronicWpairpro- E
0 ductionatLEP.Bose-Einsteincorrelationsbetweenidenticalparticlesfromthedecayofdifferent
1 P
W’sarestudiedbycomparingtheparticlecorrelationinhadronicWpairdecaystothoseinevents
0
6 whichareconstructedfromthecombinationofthehadronicpartsoftwoeventswhereoneofthe 2
0 Wsdecaysleptonicaly. TheLEPcombinedresultofthestrengthoftheeffectisconsistentwith
0
/
x zeroandcanbeusedtolimitthesystematicuncertaintyduetoBose-Einsteincorrelationsonthe
e Wmassmeasurements. 0
-
p ColorreconnectionisexpectedtoaffecttheproductionofparticlesinhadronicdecaysofWpairs. 5
e
h Measurementsofinclusivechargedparticlemultiplicities, andoftheir angulardistributionwith )
: respect to the four jet axes of the events, are used to test models of color reconnection. The
v 3
i results are both consistent with models without color reconnection and models with moderate
X
0
colorreconnection.Theycanbeusedtoexcludemoreextrememodelsofcolorreconnectionand
r
a 4
therebylimittheuncertaintyduetocolorreconnectionontheWmassmeasurement.
InternationalEurophysicsConferenceonHighEnergyPhysics
July21st-27th2005
Lisboa,Portugal
∗Speaker.
†FortheLEPCollaborations.
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
Bose-EinsteincorrelationsandcolorreconnectioninhadronicW RaimundStröhmer
1. Introduction
InWpair events both Wbosons can decay hadronically. Inthis case twostrongly interacting
color singlet objects with considerable space time overlap are produced. It is interesting to study
whether the the two W bosons then decay independently or whether they influence each other. In
addition totheprinciple interestinthisquestionpossible interactions andcorrelations betweenthe
decayproductofthetwoWbosonsinfluencetheprecisionmeasurementoftheWmassatLEPand
aretreatedinthemassmeasurement assourceofsystematic uncertainty.
2. Bose-Einstein correlations
Bose-Einstein correlations (BEC)between identical bosons areawell-knownphenomenon in
P
highenergyphysics[1]. BECleadtoanenhancementoftheproductionofidenticalbosonsclosein
o
phase space. AtLEPBEChavebeenunambiguously established betweentheparticles originating
S
from one hadronically decaying W, representing so-called intra–W BEC [2]. The possible addi-
tional BECbetweentheparticles originating fromdifferent Wbosons, i.e. inter–WW BEC,hasto (
beseparated fromtheintra–W BEC.BECareusually presented intermsoftwo-particle densities, H
r (Q),measuredas
2 E
1 dN
r 2(Q)= pairs , P
N dQ
events
2
forthenumberN ofpairsofidenticalbosonswithfour-momenta p andp andQ=p−(p −p )2
pairs 1 2 1 2
inthenumberN ofeventsunderstudy. 0
events
Theobservedtwo-particledensitiesintheWpairdecaysisthencomparedwiththeexpectation 0
in the case of non inter–WW BEC which is constructed from W pairs where only one W decays
5
as:
)
2r W(Q)+2r WW(Q).
2 mix
3
The second term is a measure of the two-particle densities between particles from different W
0
bosons which decay independently. Is is estimated by artificially mixing the hadronic part of two
semileptonic W pair events. Care has to be taken that existing correlations between the two W 4
bosons are represented in the mixed events. Only events which polar angles pointing to the same
detector region (e.g. |q −q |≤75mrad or |(p −q )−q |≤75mrad)are combined and
W+ W− W+ W−
one event is then rotated so that both event are back to back. The standard selection for hadronic
W pair selection is then applied to the mixed events. Monte Carlo simulations and distributions
not sensitive to BEC are used to establish that these mixed events represent uncorrelated W pair
events. In order to maximize the sensitivity and to minimize systematic uncertainties different
ways to compare the measured two-particle density with that estimated for events without inter–
WWBEChavebeenused.
r WW(Q)
D r (Q)=r WW(Q)−2r W(Q)−2r WW(Q), d (Q)=D r (Q)/r WW(Q), D(Q)= 2
2 2 mix I mix 2r W(Q)+2r WW(Q)
2 mix
By either comparing the result for the data with the expectation from Monte Carlo without BEC
or by comparing the result from same-sign pairs with opposite-sign pairs uncertainties due to the
mixingprocedure canbereduced.
D(Q)
D r ′(Q)=D r (Q)−=D r (Q), D′(Q)=
no−BECMC
D (Q)
no−BECMC
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Bose-EinsteincorrelationsandcolorreconnectioninhadronicW RaimundStröhmer
LEP summer 2005
in
teALEPH D’ -0.05±0.22
r
-WALEPH Dr ’ -0.18±0.35
BALEPH R* -0.23±0.41
E
CDELPHI d 0.51±0.24
I
L3 D’ 0.08±0.21
L3 Dr 0.02±0.26
OPAL D 0.33±0.45
OPAL D’ 0.34±0.51
OPAL Dr -0.01±0.46
OPAL d -0.13±0.56
P
c 2/dof = 3.5/3
LEP 0.17±0.13 o
-1 0 1 2 3 4 S
fraction of model seen (
H
Figure1: Relativestrengthofinter–WBECcomparedtothestrengthofintra–WBEC.Themeasurements E
markedwitharrowsareusedforthecombination.
P
2
d(Q)=D(±±)/D(+−). R∗=(cid:0)Np++,−−/Np+−(cid:1)data/(cid:0)Np++,−−/Np+−(cid:1)MC
no−BEC 0
The measured distributions are then compared to Monte Carlo simulations with inter–W BEC
0
correlation. Thestrength ofthatcorrelation relative tothestrength ofintra–W BECisthenvaried
5
to determine which scaling factor best describes the measurements. Results for different observ-
)
ables from the four LEP experiments together with a combination are shown in Figure 1[3]. No
significant evidence forinter–W BECcorrelation wasfound. 3
0
3. ColorReconnection
4
Thedecay products ofthe twoWbosons inhadronic Wpair decays have asignificant space-
time overlap asthe separation of their decay vertices at LEP2energies issmall compared tochar-
acteristic hadronisation distance scales. It is therefore feasible that colored objects from both W
bosonsinteractduringthehadronisation. Withthecurrentunderstandingofnon-perturbative QCD,
such interference can be estimated only in the context of specific models [4, 5, 6]. In general this
models predict achanged charged multiplicity and achanged particle flowin theregions between
jets from different W bosons (inter-W) compared to the regions between jets from the same W-
boson (intra-W). The comparison of charged multiplicity in hadronic W pair decays to that in
semiletonic Wpairdecays isunfortunately notprecise enough toconstrain the color reconnection
models significantly. The definition of regions between different jets in 4-jet events is non-trivial
and slightly different approaches have been used bythe LEPexperiments. Figure 2(left) shows a
comparison of the ratio of particle flow in the inter- to the intra-W regions between the data and
different Monte Carlo Models without and with color reconnection[7]. In the SK-I model[4] the
strength of the color reconnection can be adjusted. One can clearly see that the extreme scenario
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Bose-EinsteincorrelationsandcolorreconnectioninhadronicW RaimundStröhmer
Rflow1.8 OPAL Data JETSET AR-1 r 1
1.6 (a) HERWIG Old JETSET
1.4 0.95
1.2 SKI model
0.9 ADLO (preliminary)
1
0.8
0 0.2 0.4 0.6 0.8 1 0.85
c R=c 1/c 0 0 0.2 0.4 0.6 0.8P 1
Rflow1.8 (b) Data SK-I, 100% CR AR-2 2c 4 reco
1.6 HERWIG CR SK-II
3
1.4
1.2 2 ADLO P
preliminary
1
1 o
0.8 0 0.2 0.4 0.6 0.8 c =c /c 1 00 0.2 0.4 0.6 0.8P 1 S
R 1 0 reco
(
H
Figure2: Left:Comparisonoftheratioofparticleflowinthe intertotheintraWregionsbetweenthedata
E
anddifferentMonteCarloModelswithout(top)andwith(bottom)colorreconnection.Right:Constrainton
thestrengthofcolorreconnectionfromtheLEPcombination P
2
of 100% CR (which is used as a benchmark and is not considered as a realistic scenario by the
0
authors) isexcluded. Themeasurement canbeused to determine themostlikely strength ofcolor
0
reconnection in the SK I model. The result of the LEP combination is shown in Figure 2 on the
right[3]. 5
)
References
3
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