1. University of Gaziantep Department of Engineering Physics August 2006 Page 1/18 A.Beddall, A.Beddall and A. Bingül Department of Engineering Physics University of Gaziantep, TURKEY BPU 6, Istanbul August 2006

2. University of Gaziantep Department of Engineering Physics August 2006 Page 2/18 Introduction High energy collisions of sub-atomic particles can result in events containing a high multiplicity of hadronic particles. An example was the production of Z 0 Bosons at LEP an e + e − collider with E CM = 91.2 GeV.

3. University of Gaziantep Department of Engineering Physics August 2006 Page 3/18 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.

4. University of Gaziantep Department of Engineering Physics August 2006 Page 4/18 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 …

5. University of Gaziantep Department of Engineering Physics August 2006 Page 5/18 The ALEPH Detector

6. University of Gaziantep Department of Engineering Physics August 2006 Page 6/18 Hadronic Event Selection Track Cuts at least 4 TPC hits a polar angle 20 o < θ < 160 o a transverse impact-parameter |d 0 |<2 cm a longitudinal impact-parameter |z 0 |< 5cm A transverse momentum p t > 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

7. University of Gaziantep Department of Engineering Physics August 2006 Page 7/18

8. University of Gaziantep Department of Engineering Physics August 2006 Page 8/18 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 |d 0 |<0.3 cm and |z 0 |<3.0 cm −2 < χ(dE/dx) < 3 (with pion hypothesis)

9. University of Gaziantep Department of Engineering Physics August 2006 Page 9/18 Selection: Neutral Pions are built from the decay : Selection: The following cuts are applied: Each photon energy E γ > 1 GeV Pion energy E π < 16 GeV Invariant mass of two photon m(γ 1,γ 2 ), 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

10. University of Gaziantep Department of Engineering Physics August 2006 Page 10/18 Selection: The Ranking Method: 1.Neutral pions candidates are ranked according 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 TRUE X 0.52 D1 E1 FALSE C 0.41 C1 C2 TRUE Y 0.29 A1 B2 FALSE D 0.33 D1 D2 TRUE Z 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

11. University of Gaziantep Department of Engineering Physics August 2006 Page 11/18 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 = E ρ /E beam. Line Shape of resonances are a RBW: with

12. University of Gaziantep Department of Engineering Physics August 2006 Page 12/18 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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14. University of Gaziantep Department of Engineering Physics August 2006 Page 14/18 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 n rec n gen Differential Cross Section:

15. University of Gaziantep Department of Engineering Physics August 2006 Page 15/18 Initial Results

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17. University of Gaziantep Department of Engineering Physics August 2006 Page 17/18 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

18. University of Gaziantep Department of Engineering Physics August 2006 Page 18/18 D I S C U S S I O N ?