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研究生:許軒豪
研究生(外文):Shiuan-Hal Shiu
論文名稱(外文):Photoproduction of Λ and Σ0 hyperons off protons with linearly polarized photons at Eγ=1.5–3.0 GeV
指導教授:林宗泰林宗泰引用關係章文箴郡英輝
指導教授(外文):Willis T. LinWen-Chen ChangHideki Kohri
學位類別:博士
校院名稱:國立中央大學
系所名稱:物理學系
學門:自然科學學門
學類:物理學類
論文種類:學術論文
論文出版年:2018
畢業學年度:106
語文別:英文
論文頁數:211
中文關鍵詞:K+介子光致產生微分反應截面光束不對稱Λ 超子Σ0 超子
外文關鍵詞:K+ mesonPhotoproductiondifferential cross sectionsphoton-beam asymmetryΛ hyperonΣ0 hyperon
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本論文詳細記述在SPring-8實驗設施中,測量γp->K+Λ以及γp->K+Σ0反應及其結果。除了當K+介子分布在前方角度時的微分反應截面(differential cross sections)量測外,我們亦提供了光束不對稱(photon-beam asymmetry)的量測結果。在入射光能量大於2.4 GeV時的光束不對稱是新的量測結果。
本實驗之資料取得時間於2007年10月6日起至同月18日止,並於同
年11月8日起至12月17日止。實驗資料之取得,係利用光子束能量介
於1.5至3.0 GeV之間之線性偏振標記光子束(linearly-polarized taggedphoton beams),撞擊液態氫之靶材,並探測分布於前方角度的K+介子。實驗用的探測儀器是LEPS探測儀(spectrometer)。
利用K+介子的未量測質量譜(missing-mass spectra),可以標定出Λ和Σ0的反應。藉由量測飛行時間,動量以及飛行距離我們可以重建K+介子之質量。我們可以藉由選定3倍標準差的區間,來作為K+介子的粒子標定。此處之標準差乃由動量相關之質量解析度得出。在高動量的量測區間中,我們利用預測背景譜線的方式來預測錯誤標定的π+介子對實驗數據可能造成的影響。而利用蒙地卡羅法產生之模擬數據與實驗數據之一致性亦經由校準t0來達成。
當入射光子束的能量提升時,γp->K+Λ以及γp->K+Σ0之微分反應截面都呈現緩慢的下降。在所有的量測能量範圍裡,Λ的微分反應截面都與量測到的K+介子角度有正相關,即角度越往前,其微分反應截面越高。此一正相關,為t-通道反應的典型特徵。而Σ0的微分反應截面則沒有與量測到的K+介子角度分布有顯著相關。此一現象代表了t-通道反應可能在此反應並不顯著,並且揭示s-通道之核子共振態(nucleon resonance) 反應可能在此有相當的重要性。
光束不對稱在兩個反應的所有探測區間中的結果都是正值,如此現象可能可以顯示K*為t-通道反應過程中的主要交換粒子。兩個反應的光束不對稱結果皆與入射光束能量有極度的正相關,即入射光束能量越大,其光束不對稱數值越大,其最大值截止於+0.6。在K+Σ0的反應中,其光束不對稱的測量結果皆大於K+Λ反應的結果。對比於基於Regge-trajectory 的t-通道模型以及基於核子共振態的模型之理論預測,顯示出t-通道反應在超子光致產生(hyperon photoproduction)過程中,於此能量範圍的產生機制提供了主要的貢獻,以及核子共振態
在此亦有不可忽略的貢獻。
This thesis presents measurements of the reactions γp->K+Λ and γp->K+Σ0 at SPring-8. In addition to differential cross sections, the photon-beam asymmetries were measured at forward K+ production angles. The photon-beam asymmetry in the range of Eγ> 2.4 GeV
were measured first time.
The data were collected from October 6th to October 18th, 2007 and November 8th to December 17th, 2007. Data were obtained at SPring-8 using a linearly-polarized tagged-photon beams in the range of Eγ= 1.5 - 3.0 GeV with a liquid hydrogen target. Particles produced at the target were detected with the LEPS spectrometer.
The Λ and Σ0 production was identified in K+ missing-mass spectra. The particle identification (PID) of the K+ is done by a 3σ cut on their reconstructed mass based on the measured time of flight, momentum and path length, where σ is the momentum dependent mass resolution. The degree of π+ contamination in the selected K+ increased for particles of larger momenta. A side-band method has been applied to estimate the miss-identified π+ in K+ at high-momentum region. The t0 correction has been applied to improve the consistency of the mean value of mass squared between Monte-Carlo and real data.
With increasing photon energy, the cross sections for both the γp->K+Λ and γp->K+Σ0 reactions decrease slowly. The forward peaking in the angular distributions of cross sections, a characteristic feature of t-channel exchange, is observed for the production of Λ in the whole
observed energy range. That Σ0 production did not show the forward peaking behaviour reflects a less dominant role of t-channel contribution and the importance of s-channel nucleon resonance contributions in this channel.
The photon-beam asymmetries are found to be positive for both reactions in all observed regions and this suggests the dominance of K* exchange in the t-channel . These asymmetries are found to increase gradually with the photon energy and have a maximum value of +0.6 for both reactions. The photon-beam asymmetries in K+Σ0 channel
is systematically higher than those in K+Λ channel. Comparison with theoretical predictions based on the Regge-trajectory in the t-channel and the contributions of nucleon resonances indicates the major role of t-channel contributions as well as non-negligible effects of nucleon resonances to account for the reaction mechanism of hyperon photoproduction in this energy region.
中文摘要-----i
Abstract-----iii
Acknowledgements / 誌謝-----v
目錄-----ix
圖目錄-----xi
表目錄-----xiii
1 Introduction-----1
1.1 The standard model-----3
1.2 Quantum Chromodynamics-----4
1.3 Constituent Quark Model and the Missing Resonances
Problem-----10
1.4 Kinematic variables-----15
1.5 Motivation-----18
2 Past Measurements and Theoretical Models-----21
2.1 Past measurements-----21
2.1.1 SAPHIR-----22
2.1.2 CLAS-----25
2.1.3 LEPS-----31
2.1.4 GRAAL-----38
2.1.5 Crystal Ball-----40
2.2 Theoretical Models-----42
2.2.1 Isobar Models-----43
2.2.2 Coupled Channel analysis-----45
2.2.3 Regge Models-----49
2.3 Current Work-----54
3 Experimental Setup-----55
3.1 Backward Compton scattering (BCS)-----56
3.2 Laser-electron photon beam-----59
3.2.1 SPring-8-----60
3.2.2 LEPS facility-----61
3.2.3 Laser system-----62
3.2.4 Tagging system-----64
3.2.5 Target-----64
3.3 LEPS spectrometer-----65
3.3.1 Upstream veto counter-----66
3.3.2 Trigger counter-----67
3.3.3 Vertex detector-----68
3.3.4 Dipole magnet-----69
3.3.5 e+e- blocker-----69
3.3.6 Drift chambers-----71
3.3.7 e+e- veto counter-----72
3.3.8 TOF wall-----72
3.3.9 Trigger-----74
3.4 Analysis overview-----75
4 Data Analysis-----79
4.1 Event selections-----80
4.1.1 Event selection conditions-----80
4.1.2 Event selection summary-----87
4.2 t0 Calibration-----88
4.2.1 Why we need t0 calibration?-----88
4.2.2 t0 calibration procedure-----92
4.3 Background Estimation-----95
4.3.1 Contamination fraction method-----98
4.3.2 Mirror method-----100
4.3.3 Side-band method-----103
4.4 Monte-Carlo-----106
5 Results and Discussions-----113
5.1 Differential cross sections-----113
5.1.1 Calculation of differential cross sections-----113
5.1.2 Results of differential cross sections-----116
5.2 Beam asymmetry (Σγ)-----118
5.2.1 Calculation of beam asymmetry (Σγ)-----118
5.2.2 Results of Photon-beam asymmetry (Σγ)-----122
5.3 Systematic error estimation-----122
5.4 Physics discussion-----126
6 Summary-----139
參考文獻-----141
Appendix A: The smallest χ2 distribution for all Time of
Flight slats-----147
Appendix B: Fitting results-----157
Appendix C: The comparision of the real data and the simulated Λ and Σ0 shape-----161
Appendix D: Check the Monte-Carlo acceptance efficiency-----169
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