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University of GaziantepDepartment of Engineering Physics
August 2006
Page 1/18
A.Beddall, A.Beddall and A. Bingül
Department of Engineering PhysicsUniversity of Gaziantep, TURKEY
BPU 6, IstanbulAugust 2006
University of GaziantepDepartment of Engineering Physics
August 2006
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IntroductionHigh energy collisions of sub-atomic particles can result in events
containing a high multiplicity of hadronic particles.
An example was the production of Z0 Bosons at LEP
an e+e− collider with ECM = 91.2 GeV.
University of GaziantepDepartment of Engineering Physics
August 2006
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The transition:
quarks and gluons hadrons
can only be described by
phenomenological models.
The measurement of inclusive
particle production cross sections
of resonant states (e.g: ω or ρ )
improves the description of
the hadronization process.
LEP was ideal for studying
hadronization.
University of GaziantepDepartment of Engineering Physics
August 2006
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• This study describes the first ALEPH measurement of
– the production rate and
– the differential cross section
of the meson in hadronic decays of the Z boson.
• candidates are reconstructed from the decay channel:
• The results are compared with:
– JETSET 7.4 (a Monte Carlo event generator software)
– OPAL measurement (unique at LEP)
The work is still in progress …
University of GaziantepDepartment of Engineering Physics
August 2006
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The ALEPH Detector
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August 2006
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Hadronic Event SelectionTrack Cuts• at least 4 TPC hits• a polar angle 20o < θ < 160o
• a transverse impact-parameter |d0|<2 cm• a longitudinal impact-parameter |z0|< 5cm• A transverse momentum pt > 200 MeV/c
With these cuts a total
3,239,746 hadronic events
are selected with
efficiency: ε = 95 % purity : P = 97 %
from data recorded by ALEPH from 1991 to 1995 running periods.
Event Cuts:
• a minimum of 5 ‘good’ tracks
• a minimum of 15 GeV total ‘good’ charged energy
University of GaziantepDepartment of Engineering Physics
August 2006
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University of GaziantepDepartment of Engineering Physics
August 2006
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Track Selection candidates are reconstructed from the decay channel:
Selection:
All good tracks originating from IP are considered as pions.
The following cuts select pions from with
ε = 95% and P = 43%
• Impact parameters |d0|<0.3 cm and |z0|<3.0 cm
• −2 < χ(dE/dx) < 3 (with pion hypothesis)
University of GaziantepDepartment of Engineering Physics
August 2006
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Selection:Neutral Pions are built from the decay :
Selection:
The following cuts are applied:
• Each photon energy Eγ > 1 GeV
• Pion energy Eπ < 16 GeV
• Invariant mass of two photon m(γ1,γ2), should be within ± 3σ mass window around pion signal. σ is defined as HWHM.
• Poor purity of the pions are improved by a
ranking method
University of GaziantepDepartment of Engineering Physics
August 2006
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Selection:
The Ranking Method:
1. Neutral pions candidates are rankedaccording to a pion estimator
2. Signal candidates sharing photons with other candidates are removed depending on their relative rank
*** BEFORE RANKING ***
Pion Photon Truth# est. 1 2 info.
A 0.87 A1 A2 TRUE B 0.65 B1 B2 TRUEX 0.52 D1 E1 FALSEC 0.41 C1 C2 TRUEY 0.29 A1 B2 FALSED 0.33 D1 D2 TRUEZ 0.05 C2 E2 FALSE
For the ALEPH data
purity is improved
from 30 % to 55 %
*** AFTER RANKING ***
Pion Photon Truth # est. 1 2 info.
A 0.87 A1 A2 TRUE B 0.65 B1 B2 TRUE X 0.52 D1 E1 FALSE C 0.41 C1 C2 TRUE
University of GaziantepDepartment of Engineering Physics
August 2006
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Signal Extraction• The rate and cross sections of the meson is extracted
from the fitted two pion invariant mass (INVM).
• The data are analysed in nine different intervals of scaled energy, x = Eρ/Ebeam.
• Line Shape of resonances are a RBW:
with
University of GaziantepDepartment of Engineering Physics
August 2006
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The shape of the meson is more complex than a RBW due to
residual Bose-Einstein Correlation (BEC) distortion in INVM
Söding Model:
To include BEC, RBW shape must be multiplied by the factor:
where C is to be determined from the data.
C 1 for low x
C 0 for high x
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August 2006
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August 2006
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Rates & Cross Sections
Production Rate:
N : number of selected hadronic events
S : number of (fitted) signal
: width of the energy interval
ε : reconstruction efficiency defined as:
ε = = number of reconstructed rho-mesons in MC
number of generated rho-mesons in MC
nrecngen
Differential Cross Section:
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August 2006
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Initial Results
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August 2006
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August 2006
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Conclusion• Results obtained from JETSET 7.4 and ALEPH data fits presented and
compared with the OPAL measurements. Only statistical errors are considered.
• This measurement is not attempted at ALEPH due to very large backgrounds and difficulties with BECs
• Results show that extracted cross sections are agree with JETSET 7.4 predictions and OPAL measurements around the medium x values
• ALEPH data measurements require more work at low energy: problem with the signal line shape at high energy: problem with the background shape and the size of reflections
• This study will be completed when Systematic Uncertainties are also included to the final results
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August 2006
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D I S C U S S I O N
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