colour reconnection and bose-einstein correlations in ww decays nuno anjos lip, lisbon delphi...
TRANSCRIPT
Colour Reconnection and
Bose-Einstein Correlationsin WW Decays
Nuno Anjos
LIP, Lisbon
DELPHI Collaboration
DIS 04, Štrbské Pleso, Slovakia, April 16th, 2004
Cover
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
2/20Nuno Anjos
Outline
o Cross Talk in WW 4 jets
o DELPHI at LEP2
o Colour Reconnection (CR)
o Particle Flow
o The mW Method
o Bose-Einstein Correlations (BEC)
o Conclusions
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
3/20Nuno Anjos
CR possible betweensoft particles
Cross Talk in WW 4 jets
++
00
Interference is possible!
Electroweak phase
Is there any interference between the two evolving hadronic systems from the decays of the W’s?
1s
Perturbative QCD
CR supressed
1s
Fragmentation
~ 0.1 fm < ~ 1 fm < ~ 10-100 fm
e+ W+
W-e-
Stable Hadrons BEC possible
CR and BEC
o Fundamental probes of the dynamics of hadronisation o Contribute to the W mass systematic error in the fully hadronic channel
Colour
Reconnection90 GeV/c2
Bose-Einstein Corr.
35 GeV/c2
Total Syst. 101 GeV/c2
Stat. 36 GeV/c2
Q
Q
q
q
_
_
Overlap
--
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
4/20Nuno Anjos
The DELPHI Detector @ LEP 2
e- e+
4 well separated, high multiplicity jets, balanced event, no qq
radiative return.
About 5,000 WW 4q events collected:
DELPHI detector at LEP2
4 jet topology
Year (1996) 1997 1998 1999 1999 total L
s(GeV)
161-172
183 189 192-202
203-209
183-209
L (pb-1) (20) 53 157 227 223 660
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
5/20Nuno Anjos
CR: Colour rearrangement between different colour singlets
Example: Hadronic decay of B mesons
B J/+ X
CR in WW decays: some hadrons cannot be assigned to either W
CR is suppressed in the perturbative phase 1) Group structure of QCD requires the exchange of 2 gluons
2)CR may be possible during hadronisation 1) Average multiplicity and particle distributions
2) W mass
Colour Reconnection
BR exp ~ 1%BR full CR ~ 3-5%BR no CR ~ 0.3-0.5%
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
6/20Nuno Anjos
CR Phenomenological Modelso Sjöstrand & Khoze (PYTHIA):
o Type I: (SK I) flux tubes
o Probrecon
= 1-exp(-I Vol. Overlap)
o Type II, II’ : vortex lines
o Gustafson & Häkkinen: minimizes string length
o Lönnblad(ARIADNE): CR between dipoles, min. string length
o Webber et al. (HERWIG): CR if smaller cluster sizes
SK-I
.18 1.26 2.34 4.9 100 I0.66
MW ~ O(100 MeV)
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
7/20Nuno Anjos
The Particle Flow Method
oSelect only events with well- defined regions: low efficiency
oCount the particles between each pair of jets
oCompare the Inside W’s and Between W’s particle flow
Data events183-189 GeV
Expected Events MC
1343 1286.01.0
Eff.[%] Pur.[%] Pairing Eff. [%]
22 – 15 75-70 ~79
All results are preliminary!
No CR
CR
Cross-Talk between W systems should result in some depletion and/or enhancement of soft particles in some phase space regions
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
8/20Nuno Anjos
Measuring the Particle Flow
Project all charged particles Pi in the plane ( jet 1, jet 2)
A
C
D B
Rescale the angles to have an
equal amount of phase-space
between the jets :
J,i J,i . (1/JK)
A B C D
Inside W’s and
Between W’s
Z qq()
WW qql
data at 189 GeV #( A+C )
#( B+D )( ) dd#( A+C )
#( B+D )
SK1 100%SK1 100%
no CR
SK1 100%
0.2 0.8
DELPHIpreliminary
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
9/20Nuno Anjos
The Ratio of Particle Flow
<R>189 = 0.979 0.032(sta) 0.030(sys)
In the systematic error, were considered: Bose-Einstein effects (0.7%) Fragmentation modelling (2.6%) Work Background subtraction/modelling (1.2%) Extrapolation to 189 GeV (1.4%)
R
8.0
2.0
8.0
2.0
DB regions
CA regions
rr
rr
dddn
dddn
R
DELPHI preliminary
Data: 183-209 GeV
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
10/20Nuno Anjos
Particle Flow Results
Comparison of LEP Particle Flow results
R
DELPHI preliminary
DELPHI Particle Flow results
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
11/20Nuno Anjos
The mW Method
MC prediction SK1
Bias of mW estimators:
Build mW estimators with low sensitivity to CR: mW(cone)
mW is unknown, but must be invariant
for different estimators
mW = mW(std)-mW(cone) contains information about
CR
Start with the standard mW analysis and mass estimator, mW(std)
DELPHI preliminary
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
12/20Nuno Anjos
mW ResultsData: 189-209 GeV
centr.
+1 sigma
MC prediction SK1
-1 sigma
2
Cross check :
mW(qqlv) = 8 56 MeV/c2
AR (noCR-CR) = 7 4 MeV/c2
HW (noCR-CR) = 20 4 MeV/c2
DELPHI preliminary
DELPHI preliminary
mw(std,Rcone= 0.5rad) = 59 35(sta) 21(sys) MeV/c2
MLBZ 11
Calibration 3
Background 3
BEI-BEA 8
BE0-BEI 16
Total Syst. 21
Stat. 35
Total 41
Syst.error
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
13/20Nuno Anjos
Combination of DELPHI Measurements
66.101.102.1)1(
SK
Correlation between the two methods is small
Combine both DELPHI measurements
2
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
14/20Nuno Anjos
)(/)()( 222 QQQR ref
Bose-Einstein Correlations
two-particle correlations
o Quantum-mechanical effect: Bose-Einstein statistics
o Symmetrisation of the wave function of n identical bosons
o Enhanced production of identical bosons close in phase-space
Inclusive two-particle density
Phase-space (projected into 1 dimension)
Normalised two-particle density
BEC in light quark Z decays identical to BEC inside a hadronically decaying W
Reference Sample: MC, Unlike sign, Mixed events
22221 4)( mMppQ
dQ
dn
NQ pairs
ev
1)(2
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
15/20Nuno Anjos
Measuring Inter-W BEC
Hadronic
events
Semi-hadronicEvents
Mix 2 Semi-hadronic events
Assuming independent WW decay
Observables
data-data comparison
BEC between separately hadronising strings?
No Inter-W BEC:0
DELPHI preliminary
No Inter-W BEC:1)(2)(2)( QQQ WWmix
WWW
)(2)(2)()( QQQQ WWmix
WWW
)(2)(2
)()(
QQD
WWmix
W
WW
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
16/20Nuno Anjos
Monte Carlo Simulation of BEC
- Andersson et al.• Based in Lund Area Law• No arbitrary input parameters• Reproduces BEC inside Z and W decays• No BEC between two strings • No mature MC
- LUBOEI• Has 2 input parameters: correlation strength and source size R• Reproduces BEC inside Z and W decays (BE ins)• Has a switch to allow inter-string BEC (BE full) mW35MeV• Implemented in PYTHIA (BE32 variant used here)
Global Models: Generate weights for a given event
Local Models: Reshuffle the 4-momentum of particles
- Kartvelishvili & Kvatadze
• Allows inter-string BEC (mW 15MeV)• Not tested yet
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
17/20Nuno Anjos
Measured BEC Distributions
like-sign
unlike-sign
D like-sign
D unlike-sign
550 pb-1 s = 189 – 209 GeV2567 qql events 3252 qqqq events
DELPHI preliminary
DELPHI preliminary
DELPHI preliminary
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
18/20Nuno Anjos
WW BEC Results
Measured correlation strength:
Systematic error:
Correlation strength
Sou
rce s
ize
Fit R and simultaneously
like-sign
Fixing R to BE full value
BE full
Data
-BE full-BE ins
DELPHI preliminary
DELPHI preliminary
= 0.241 0.075(sta) 0.038(sys)
)1)(1()( QeNQD RQ
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
19/20Nuno Anjos
LEP Combination of WW BEC Results
Combine observed fraction of BE full
Result of combination:23% of LUBOEI BE32
with BEC between W’s is observed
Corresponding W mass shift: 8 5 MeV/c2
DIS 2004, Štrbské Pleso, Slovakia, April 16, 2004
20/20Nuno Anjos
Conclusions
CR and BEC investigated in WWqqQQ events in DELPHI at LEP2
Colour Reconnection:o Two uncorrelated observables are studied:
Comparing the particle flow between jets ( R ) Modifying the kinematics of the jets ( mW[std,cone] )
o Preliminary results indicate small amount of Colour Reconnectiono The observables are not sensitive to the ARIADNE modelo Work continues on fragmentation error in the particle flow
Bose-Einstein Correlations:o Model independent measurement of inter-W BEC developedo Mixed reference sample built from semi-hadronic data eventso Inter-W BEC in like sign pairs found at 2.9leveloLEP combination: 23% BE32 inter-W, mW=8 5 MeV/c2