Exclusive _w63π_1hn0 [omega-pi-0] production with muons at COMPASS [Elektronische Ressource] / vorgelegt von Massimo Maria Bettinelli

Massimo M. Bettinelli

Sujets

• Physik

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• Publié par: ludwig-maximilians-universitat_munchen
• Publié le: 01 janvier 2010
• Nombre de lectures: 24
• Langue: English
• Poids de l'ouvrage: 4 Mo

Extrait

0Exclusive ωπ production with
muons at COMPASS
Massimo Maria BETTINELLI
Munchen,¨ 20100Exclusive ωπ production with
muons at COMPASS
Dissertation
an der Fakult¨at fur¨ Physik
der Ludwig–Maximilians–Universit¨at
Munchen¨
vorgelegt von
Massimo Maria BETTINELLI
aus
Gallarate (Italien)
Munchen,¨ den 2. Februar 2010Erstgutachter: Prof. Dr. Wolfgang Dunn¨ weber
Zweitgutachter: Prof. Dr. Otmar Biebel
Tag der mundlic¨ hen Prufung:¨ 25. M¨arz 2010Contents
Zusammenfassung xvii
1 Introduction 1
1.1 Kinematic of exclusive meson leptoproduction . . . . . . . . . 3
1.2 Vector meson decay angular distributions . . . . . . . . . . . 4
1.2.1 The photon spin density matrix . . . . . . . . . . . . . 6
1.2.2 The vector meson density . . . . . . . . . . . . 8
1.2.3 Vector meson angular distribution and SCHC . . . . . 9
2 The COMPASS experiment 11
2.1 The polarized muon beam . . . . . . . . . . . . . . . . . . . . 13
2.2 The p target . . . . . . . . . . . . . . . . . . . . . . . 14
2.3 Tracking detectors . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4 Particle identification with the RICH . . . . . . . . . . . . . . 18
2.5 Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.6 The Data Acquisition System . . . . . . . . . . . . . . . . . . 21
2.7 The event reconstruction with CORAL . . . . . . . . . . . . . 23
2.8 The analysis framework . . . . . . . . . . . . . . . . . . . . . 25
3 ECAL2 performance in exclusive ω production 27
3.1 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2 ECAL2 performance . . . . . . . . . . . . . . . . . . . . . . . 29
03.3 Contribution of calibration to the π width . . . . . . . . . . 33
04 Exclusive ωπ leptoproduction with muons 35
4.1 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.1.1 Event topology . . . . . . . . . . . . . . . . . . . . . . 35
04.1.2 Event with at least two π ’s . . . . . . . . . . . . . . . 37
04.1.3 The ω and ωπ selection . . . . . . . . . . . . . . . . . 40
04.1.4 Exclusive ωπ sample . . . . . . . . . . . . . . . . . . 43
4.1.5 Cut on the angle θ . . . . . . . . . . . . . . . . . . . . 44
4.1.6 Effect of the selection cuts on sample statistic . . . . . 45
4.1.7 Alternative procedures . . . . . . . . . . . . 47
4.2 Events within the target cells . . . . . . . . . . . . . . . . . . 47vi CONTENTS
5 Results 51
05.1 Kinematics of the exclusive ωπ sample . . . . . . . . . . . . 51
5.2 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
5.2.1 Non-ω-related events . . . . . . . . . . . . . . . . . . . 57
5.2.2 Target excitation . . . . . . . . . . . . . . . . . . . . . 60
05.3 ωπ mass dependence . . . . . . . . . . . . . . . . . . . . . . 63
5.4 Angular distributions . . . . . . . . . . . . . . . . . . . . . . . 65
6 The Monte Carlo simulation 75
6.1 The event generator . . . . . . . . . . . . . . . . . . . . . . . 76
06.1.1 The ωπ decay . . . . . . . . . . . . . . . . . . . . . . 78
6.1.2 The ω decay . . . . . . . . . . . . . . . . . . . . . . . 79
6.1.3 The beam scattering . . . . . . . . . . . . . . . . . . . 83
06.1.4 The ωπ production plane . . . . . . . . . . . . . . . . 84
6.1.5 Angular distributions . . . . . . . . . . . . . . . . . . 86
6.2 Detector response and MC event reconstruction . . . . . . . . 87
6.3 MC and real data comparison and acceptance corrections . . 88
6.3.1 The kinematic of reaction . . . . . . . . . . . . . . . . 88
6.4 Angular distributions: MC results . . . . . . . . . . . . . . . 93
6.5 Acceptance corrected data . . . . . . . . . . . . . . . . . . . . 97
07 Angular correlations and the spin-parity of the ωπ 99
7.1 (Partial) Partial Wave Analysis (PWA): the H(lmLM) mo-
ments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
±7.2 The spin matrix elements ρ . . . . . . . . . . . . . . . . . . 11000
−7.3 Extraction of the 1 contribution: the cosθ -fit method . . . 115H
27.3.1 Background contribution to|F | . . . . . . . . . . . . 1191
27.3.2 Correction to|F | . . . . . . . . . . . . . . . . . . . . 1241
P − 07.3.3 J =1 fraction in the ωπ final sample . . . . . . . 130
7.3.4 The λ-cut . . . . . . . . . . . . . . . . . . . . . . . . . 136
7.4 SCHC versus non-SCHC interpretation . . . . . . . . . . . . . 136
7.4.1 Φ angular distribution . . . . . . . . . . . . . . . . . . 139
7.4.2 ψ . . . . . . . . . . . . . . . . . 143V
7.4.3 The angle θ . . . . . . . . . . . . . . . . . . . . . . . . 147
7.4.4 The angle θ . . . . . . . . . . . . . . . . . . . . . . . 148A
8 Conclusion 153
A Year 2004 and 2006 results 157
B Wigner D-functions 163
2 ±C |F | dependence on ρ 1651 00
−2C.1 |F | dependence on ρ . . . . . . . . . . . . . . . . . . . . . 1661 00
2 +C.2 |F | dep on ρ . . . . . . . . . . . . . . . . . . . . . 1671 00Table of Contents vii
2D |F | values from cosθ -distribution fit 1691 H
E Non-ω background from λ fit 175
F cosθ distribution fit for λ>0.2 179H
2F.1 |F | values from fit . . . . . . . . . . . . . . . . . . . . . . . 1791
2F.2 Non-ω background contribution and|F | . . . . . . . . . . 1841 corr
−F.3 1 contribution . . . . . . . . . . . . . . . . . . . . . . . . . . 184
2 0G Angular distributions dependence on p and/or m(ωπ ) 189t
2H Dependence on Q 199
I Forward-backward asymmetry A 203FB
Danksagung 209viii Table of ContentsList of Figures
01.1 Exclusive ωπ leptoproduction with muons . . . . . . . . . . 2
01.2 The exclusive ωπ leptoproduction and subsequent decays . . 5
2.1 The COMPASS experiment . . . . . . . . . . . . . . . . . . . 12
2.2 Muon polarization as a function of the decay angle . . . . . . 14
2.3 Layout of the COMPASS BMS for muon beam . . . . . . . . 15
2.4 The SMC two-cells polarized target . . . . . . . . . . . . . . . 15
2.5 Principle of a Micromega detector . . . . . . . . . . . . . . . 16
2.6 Schematic cross section of a triple GEM detector . . . . . . . 17
2.7 COMPASS RICH-1: principle and artistic view . . . . . . . . 18
2.8 A counter of ECAL2 calorimeter . . . . . . . . . . . . . . . . 21
2.9 General architecture of the DAQ system . . . . . . . . . . . . 22
+ − 03.1 (π π π )-invariant mass . . . . . . . . . . . . . . . . . . . . 29
+ − 0 03.2 m(π π π ), E , reconstructed m(π ) signal and clustermiss
center distribution (period W37) . . . . . . . . . . . . . . . . 30
+ − 0 03.3 m(π π π ), E , m(π ) signal and clustermiss
center (period W26) . . . . . . . . . . . . . . . . 30
3.4 Cluster cell occupancy . . . . . . . . . . . . . . . . . . . . . . 32
03.5 Spread of the reconstructed m(π ) relative to the PDG value 34
4.1 Number of ECAL2 neutral clusters per event . . . . . . . . . 38
04.2 Number of reconstructed π per event in a typical run . . . . 39
4.3 2-photons invariant mass for events with at least four recon-
structed neutral clusters . . . . . . . . . . . . . . . . . . . . . 39
+ − 04.4 Invariant mass spectrum for identified π π π triplet . . . . 41
4.5 Number of ω candidates per event . . . . . . . . . . . . . . . 41
04.6 ωπ invariant mass vs. missing energy . . . . . . . . . . . . . 43
04.7 Number of ECAL2 neutral clusters for exclusive ωπ events . 44
04.8 Exclusive ωπ -invariant mass . . . . . . . . . . . . . . . . . . 46
+ − 0 04.9 Invariantmassspectrumfortheexclusive ω(π π π )π with
the acceptance dependence . . . . . . . . . . . . . . . . . . . 46
4.10 Distribution of the b.p.v. along the beam direction z and xy
section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48x LIST OF FIGURES
+ − 0 05.1 Invariant mass spectrum for the exclusive ω(π π π )π final
sample (not acceptance corrected) . . . . . . . . . . . . . . . 51
25.2 Q , Bjorken variable x and their correlation for the finalB
sample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
5.3 Virtual photon energy ν . . . . . . . . . . . . . . . . . . . . . 53
05.4 ωπ -momentum distribution in laboratory system . . . . . . . 53
∗5.5 Final sample γ p center-of-mass energy W . . . . . . . . . . . 54
5.6 Final †-distribution . . . . . . . . . . . . . . . . . . . 54
5.7 Final sample t. . . . . . . . . . . . . . . . . . . . 55
25.8 Final p . . . . . . . . . . . . . . . . . . . 56t
25.9 p -dependence of E . . . . . . . . . . . . . . . . . . . . . 57misst
5.10 Final sample λ-distribution . . . . . . . . . . . . . . . . . . . 59
+ − 0 05.11 λ-distribution for exclusive (π π π )π sample within the
ω-peak and in side bands . . . . . . . . . . . . . . . . . . . . 60
5.12 Invariant mass of the neutral and charged dipion system for
the final sample . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.13 Invariant mass M 0 for final sample. . . . . . . . . . . . . . 62Xπ
05.14 E for distinct ωπ -mass ranges . . . . . . . . . . . . . . . 64miss
5.15 Angular distributions for the final sample, not corrected for
acceptance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
5.16 (Continuation) . . . . . . . . . . . . . . . . . . . . . . . . . . 70
2 05.17 Finalsampleangulardistributionswithin1.1GeV/c <m(ωπ )<
21.4 GeV/c . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
2 05.18 Finalsampleangularwithin1.1GeV/c <m(ωπ )<
21.4 GeV/c . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
6.1 Photon energy spectra in the laboratory