Photoproduction of Λ and Σ0 hyperons off protons with linearly polarized photons at Eγ=1.5–3.0 GeV

Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/77082

Title:	Photoproduction of Λ and Σ0 hyperons off protons with linearly polarized photons at Eγ=1.5–3.0 GeV
Authors:	許軒豪;Shiu, Shiuan-Hal
Contributors:	物理學系
Keywords:	K+介子;光致產生;微分反應截面;光束不對稱;Λ 超子;Σ0 超子;K+ meson;Photoproduction;differential cross sections;photon-beam asymmetry;Λ hyperon;Σ0 hyperon
Date:	2018-05-04
Issue Date:	2018-08-31 13:56:30 (UTC+8)
Publisher:	國立中央大學
Abstract:	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.
Appears in Collections:	[Graduate Institute of Physics] Electronic Thesis & Dissertation

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