假根羽藻不同聚集态捕光色素蛋白复合物（LHC II）的纯化与功能分析

应用温和、非变性的液相色谱层析技术以及蔗糖密度梯度分级分离技术，从假根羽藻类囊体膜直接分离、纯化出5种不同聚集态的光系统II捕光色素蛋白复合物(LHC II)，即LHC II单体、同质三聚体、异质三聚体、寡聚体l和寡聚体2。这种分离、纯化LHC II的方法与传统的等电聚焦(IEF)电泳分离方法相比，具有蛋白质提取条件温和，纯化的蛋白质样品纯度高且数量大等优点，为进一步研究LHC II分子的晶体结构和功能创造了有利的条件。 SDS-PAGE电泳分析和MALDI-TOF质谱分析结果表明，纯化得到的LHC II单体和同质三聚体都具有一种分子量为24.3 kDa的脱辅基蛋白质，而异质三聚体和两种寡聚体除含有这个脱辅基蛋白外，还含有另一种分子量为23 kDa的脱辅基蛋白。 采用室温吸收光谱、低温荧光光谱及园二色(CD)光谱技术对LHC II单体、同质三聚体、异质三聚体、寡聚体l和寡聚体2内的叶绿素组成以及叶绿索之间的能量传递特性进行分析研究表明，在同一条组成LHC II的脱辅基蛋白多肽链上结合着Chl a二聚体，二聚体内的两个Chl a分子之间存在偶极子相互作用。在这5种LHC II亚复合物中均具有Chl a-Chl a相Chl biChl a→ Chl a能量传递途径，其中同质三聚体表现出最高的能量传递效率，而寡聚体的传能效率大大低于单体和三聚体，同时出现Chl a的淬灭现象。当LHC II脱辅基蛋白质高度聚集形成寡聚体时，其蛋白质上结合的Chlb大量减少，引起这两种寡聚体吸能和传能能力的急剧下降。 LHC II单体、同质三聚体、异质三聚体、寡聚体1和寡聚体2的二级结构数据表明，自由堆积的脱辅基蛋白的二级结构以8．折叠构象为主，当LHC II蛋白质有序聚集形成三聚体时，可能每条蛋白质多肽链上的叶绿素结合区域形成两个跨膜a-螺旋，对称排列在脂双层膜内，而非叶绿素结合区则形成一条跨膜a-螺旋。当LHC II蛋白质高度聚集形成寡聚体 时，LHC II蛋白质的二级结构似乎对其上结合着的叶绿素的影响不大，而LHC II寡聚体内蛋白质的高密度聚集引起的更高级的蛋白质相互之间的空间结构可能对叶绿素的影响起主要作用，特别是破坏Chlb结合位点。 依据上述的实验结果推测在类囊体膜内，LHC II可能通过其脱辅基蛋白聚集形式的转换，调控其光能吸收和激发能传递的过程和效率。

Signal and backgrounds for leptoquarks at the CERN LHC. II. Vector leptoquarks

We perform a detailed analyses of the CERN Large Hadron Collider (LHC) capability to discover first generation vector leptoquarks through their pair production. We study the leptoquark signals and backgrounds that give rise to final states containing a pair e+e- and jets. Our results show that the LHC will be able to discover vector leptoquarks with masses up to 1.8-2.3 TeV depending on their couplings to fermions and gluons. ©1999 The American Physical Society.

Zeitaufgelöste Fluoreszenzmessungen zur Faltung und Pigmentbindung des Lichtsammlerproteins LHC II aus Photosystem II

Zusammenfassung Der Lichtsammlerkomplex (LHCII) aus PhotosystemII hoeherer Pflanzen kann in vitro rekonstituiert werden. Es werden drei Reaktionszeiten (<10 s; <1 min; <10 min) aufgeloest. Dabei werden bei allen Reaktionszeiten Pigmente durch das Apoprotein gebunden. Chlorophylle (Chl a und Chl b) und Xanthophylle wirken limitierend auf die Rekonstitution. Chl a beschleunigt die zweite Reaktionszeit, ein ausgeglichenes Chl a/b-Verhaeltnis verkürzt die dritte Reaktionszeit. Ein molekularer Mechanismus als Interpretation dieser Effekte wird vorgeschlagen. Native Lipide verlaengern nichtspezifisch die Rekonstitution. Abiotische Faktoren haben einen spezifischen Einfluss auf die Rekonstitution. Spezifische Einfluesse der o. a. Bedingungen auf die thermische Stabilitaet des rekonstituierten LHCII wurden bestimmt.

高等植物光系统II捕光天线结构与功能的研究

高等植物光系统II的捕光天线蛋白（LHC II）在光能的吸收、传递和调节激发能在两个光系统之间的分配以及维持类囊体膜的垛叠等方面都起着重要的作用，因而得到了广泛的研究。目前普遍认为LHC II在植物体内是以三聚体的形式存在并行使功能的，但也有研究者发现了单体和寡聚体等多种形式的LHC II。本论文以菠菜为研究对象，采用改进的方法从类囊体膜中提取纯化了LHC II三聚体，对膜脂和色素在三聚体形成和蛋白空间结构中的影响，以及不同聚集态LHC II组成、结构和功能的差异进行了较系统的研究。此外，还将Lhcb2基因反向插入到烟草中，利用转基因植物来研究其生理功能。获得了如下结果： 1，采用改进的方法从菠菜类囊体膜中分离纯化了LHC II。与改进前比，其流程可以缩短2小时且产率也有明显的提高。SDS变性电泳和Triton X-100非变性电泳的检测结果表明，此样品纯度较高，是由三条分子量分别为29KD、28KD和26KD的多肽组成的异质三聚体。同时样品的吸收光谱和荧光光谱分析结果也与前人的报道一致。 2，分析了LHC II三聚体中的膜脂和脂肪酸组成及含量。与PSII相比，LHC II含有相同的四种膜脂：MGDG、DGDG、PG和SQDG。但LHC II中PG的含量是PSII的两倍，说明PSII中的PG主要富集在外周天线区域。同时PG中含有特异的反式十六碳一烯酸，且含量很高。用专一消化PG上Sn-2位脂肪酸链的磷脂酶A2（PLA2）处理LHC II三聚体，然后再加入PG重组的方法证明了含十六碳一烯酸的PG在三聚体结构的维持中起着至关重要的作用。去掉PG后， LHC II三聚体的结构受到了影响，部分解聚成了单体，同时其光谱特性也发生了变化，表现为叶绿素b分子的吸收峰及其激发的荧光发射峰都明显下降。回加PG则可使解聚的单体又重新聚集成三聚体。 3，分别用电洗脱和蔗糖密度梯度离心两种方法分离了LHC II三聚体、二聚体和单体。两者比较，电洗脱对样品的破坏较大，而蔗糖密度梯度离心更加温和，对蛋白上结合的色素影响不大。系统研究了不同聚集态LHC II的组成和光谱特性后发现，三种聚集态的LHC II有相同的多肽组成，并且都结合着5种色素，但是色素的含量差异较大。二聚体和单体中，叶绿素b和类胡萝卜素分子的含量比三聚体的低很多，此外，单体叶绿素a分子的含量也明显减少。对三种聚集态LHC II的各种光谱特性进行分析的结果表明，由于叶绿素b和类胡萝卜素分子含量较少，二聚体和单体中叶绿素b和类胡萝卜素的吸收均有所下降，而且从类胡萝卜素到叶绿素b以及从叶绿素b到叶绿素a的能量传递效率都低于LHC II三聚体，总体表现为三聚体 > 二聚体 > 单体。此外，不同单体之间叶绿素a到叶绿素a的能量传递也被破坏。推测三种聚集态LHC II在吸能、传能和结构上的差异，可能是植物适应不同外界环境的一种调控机制。 4，模拟体内过程，在体外将大肠杆菌中表达的Lhcb2蛋白和色素进行重组，以此来研究色素与蛋白组装过程中蛋白二级结构和色素结合状态的变化。结果表明色素在脱辅基蛋白的体外重折叠中至关重要。在与色素重组的过程中，蛋白二级结构中-螺旋含量逐渐上升并最终接近天然水平，而无规卷曲逐渐减少。从光谱的变化可以看出，色素分子与蛋白的结合经历了一个由无序到有序的过程，色素蛋白复合物的光谱信号由弱变强，重组得到的样品与天然LHC II十分相似。 5，为了更好地研究LHC II异质三聚体中单体可能具有的独特生理功能，建立了Lhcb2基因的反义抑制植物表达载体pBI-antiLhcb2，用根癌农杆菌介导法转化烟草，获得了转基因植株。酶切和PCR鉴定证明，Lhcb2基因已经成功地插入到烟草里。进一步的分子鉴定和生理生化功能分析还在进行中。

重金属离子镉对高等植物叶绿体光系统Ⅱ的影响的研究

本文研究了重金属离子Cd++、Pb++、Cu++、Zn++对高等植物（菠菜）离体叶绿体两个光系统光合电子传递功能的影响，以Cd++为代表重点研究了它对光系统II（PSII）的影响，首重讨论了Cd++在PSII的作用部位及作用方式。 Cd++、Pb++、Cu++、Zn++对高等植物叶绿体光合电子传递具有抑制作用，其中对PSII电子传递的抑制作用更为显著。PSII制剂的放氧活性比叶绿体的放氧活性对Cd++的毒害作用更加敏感。不同浓度的Cd++处理后，叶绿体全电子链的电子传递活性比放氧活性的降低速率快。这暗示着PSII氧化侧不是Cd++唯一的作用部位，在PSII电子传递链上还存在一个对Cd~(++)敏感的部位。 Cd++使叶绿体和PSII制剂的DCIP光还原活性降低;可变光受到抑制。加入PSII人工电子供体DPC仅使被抑制的DCIP光还原活性稍有恢复;而加入NH2OH对被抑制的可变荧光无影响。因此我们认为Ca++除了作用于PSII氧化侧外，还直接作用于PSII反应中心。这与Bazzaz和Govindje等提出的Cd++仅作用于PSII氧化侧的观点不同。我们认为可能是Cd++改变了叶绿体或PSII制剂类囊体膜的构型或内环境，使较多的PSII反应中心不易被光活化或使其钝化的缘故。 低温（-196℃）荧光发射光谱表明，Cd++使叶绿体的F686/F736及 F696/F736比值降低。另外，Cd++还可使叶绿体和PSII制剂的低温（-196℃）荧光激发光谱的F480/F436比值下降，使叶绿体表观吸收光谱变平。 温和的SDS-PAGE分析表明，Cd++处理后，叶绿体类囊体膜与PSII制剂类囊体膜中色素蛋白复合物LHC-II的部分寡聚体解聚成单聚体，叶绿体中LHC-II总量减少。进一步用梯度胶分析叶绿体类囊体膜的多肽组成。发现Cd++使属于LHC-II的多肽减少。表明Cd++引起了LHC-II的重新分配或它本身的解离。通过比较Cd++与Mg++对PSII活性的不同影响，我们认为Cd++使激发能不利于向PSII分配，从而破坏了两个光系统的协调合作。这或许是导致PSII电子传递活性显著降低的因素之一。

膜脂在类囊体不同功能膜区的分布及其与植物抗寒性的研究

一． 通过对黄瓜类囊体及其基粒片层、间质片层、PS II放氧颗粒和LHC II 复合体脂类成分的分析，得 到以下结果：各膜区均含有类囊体膜的五种脂类成分，在类囊体及其基粒片层、间质片层和PS II放氧颗粒中，MGDG含量最高，分别为42.5%、40.5%、46.3%、和35.7%，其次是DGDG，含量在31-35%之间。值得注意的是黄瓜类囊体间质片层MGDG含量高于基粒片层，而且DGDG/DGDG分子比也较高。这在其它植物材料中还未见报道。在黄瓜LHC II中，PG含量最高，为35.5%，约是类囊体膜PG含量的3倍。从基粒片层、PS II放氧颗粒到LHC II， PG含量呈逐渐增加的趋势，而在间质片层中，PG含量最低。SQDG除在LHC II中含量稍低外，在其它膜区中的分布没有明显的差异。脂类脂肪酸组成分析结果表明：MGDG主要含亚麻酸，含量在90%以上。DGDG也主要含亚麻酸，含量在90%左右，DGDG所含棕榈酸多于MGDG中的含量。SQDG中主要脂肪酸组分为棕榈酸和亚麻酸。不同膜区MGDG、DGDG和SQDG脂肪酸组成没有明显差异在PG中含量最高的脂肪酸是叶绿体特有的反式十六碳一烯酸（trans-16:1)。此外，PG还含有较多的棕榈酸、硬脂酸和油酸。在不同膜区PG的脂肪酸组成有较明显的差异。 根据以上结果，我们推测脂类除了形成膜的流动性基质外，还可能选择性地结合在膜蛋白周转形成特 异的脂质微区，通过膜脂膜蛋白的相互作用，以行使其特殊的生理功能。 二、通过比较两种不同抗寒性小麦品种在低温锻炼前后类囊体脂类及其脂肪酸成分、LHC II复合体及类囊体吸收光谱、低温荧光发射光谱，发现经低温锻炼后：（1）抗寒与不抗寒小麦品种类囊体PG的trans-16:1含量均明显降低，抗寒品种类囊体MGDG/DGDG比值也明显降低，而不抗寒品种这一比值变化不明显。（2）抗寒品种脂/色素比值明显增高，而不抗寒品种滑明显增加。（3）抗寒与不抗寒品种LHC II宏聚体含量均降低而单体含量增加。（4）抗寒与不抗寒品种类囊体吸收光谱四阶导数光谱A_(683)/A_(652)比值均升高。（5）不抗寒品种低温荧光发射光谱F_(685)/F_(738)比值上升，而抗寒品种这一比值没有变化。通过对上述结果的分析，我们认为低温锻炼过程中类囊体膜流动性增强是使抗寒品种抗寒力增强的主要原因之一，此外，MGDG含量降低对膜的稳定性可能起重要作用。trans-16:1含量的降低和LHC II寡聚体解聚可能是植物对于低温的一种适应性反应。

豌豆Lhcb2 基因的克隆、表达、功能分析及其克隆化蛋白与色素体外重组系统的建立

Lhcb2基因是LHCII基因家族中的一个重要成员。目前，对于其特性、结构和功能还不清楚。本文对豌豆Lhcb2基因的克隆、表达、功能及其在大肠杆菌中的表达产物与色素在体外的重组进行了比较系统的探讨，主要的结果如下： 1．采用RT-PCR技术，从豌豆幼叶中克隆了一个约800 bp的Lhcb2 cDNA。以特异探针进行Southern杂交的结果初步证明，Lhcb2基因以单拷贝形式存在于豌豆基因组中，这在文献中尚未见报道。 2．不同光照时间和温度对豌豆幼苗进行处理的RT-PCR，Northem blotting分析表明，Lhcb2基因转录水平上的表达受光照的控制，且明显地表现出对光照时间的依赖性。用400 μmolm-2.s-1的白光照射0～1.5小时Lhcb2基因未见表达，而在光照2小时以上则大量表达，推测该基因的表达可能要求某种(或某些)需光中间物的合成和积累。温度对Lhcb2基因的表达亦有显著的影响，相同的光照处理，4 ℃下Lhcb2基因的表达量比25℃下的表达量低一倍左右。 3．将豌豆Lhcb2基因反向插入植物表达载体pBIl21中，构建CaMV 35S启动子控制下反义Lhcb2基因的植物表达载体pBIantiLhcb2，通过农杆菌LBA4404介导，在Kan浓度为100 mg／L的筛选培养基上，获得120个抗Kan阳性植株。PCR检测表明至少有80个抗性植株为PCR阳性反应。Southern blotting分析表明，外源反义Lhcb2基因已整合到烟草基因组中。转基因植株在表型上主要表现为三大类型：叶色类似于未转基因的绿色植株、叶色发黄的植株、叶色发白甚至枯死的植株。这几类转基因植株光谱特性的分析表明，Lhcb2基因不仅影响光能的捕获，而且还可能参与激发能分配的调节作用。 4．将豌豆的Lhcb2基因亚克隆至原核表达载体pET-3d上，通过定点突变使其在大肠杆菌BL2l(DE3)中得到高效表达，其表达量约占大肠杆菌总蛋白的40％，并经纯化后获得了电泳纯的Lhcb2蛋白。应用改进的液氮／室温冻融重组方法将纯化的蛋白与色素进行体外重组，建立了高效的重组系统。实验结果表明重组后所获得的LHCB2单体与用生化方法从菠菜类囊体膜中分离纯化的天然LHC II单体相比较，其在部分变性胶的电泳行为，低温荧光发射光谱和激发光谱，以及室温吸收光谱和CD光谱的特征等方面都非常相似，说明大量表达的Lhcb2蛋白与色素已成功地重组，并具有与天然的LHCII单体相类似的组成和结构，这在国际上尚属首次。在此基础上又构建了N端和C端氨基酸残基缺失的突变体，并对这些缺失的氨基酸残基在LHCB2中的可能作用进行了初步的研究。结果表明，N端的前12个氨基酸残基、C端的前10个氨基酸残基和第11位的色氨基酸残基对LHCB2单体的形成不是必需的。 此外，从菠菜中纯化了LHCII，并对其多肽和色素组成及其光谱特性进行了比较系统的研究。同时对用不同浓度OGP和Mg2+处理所获得的不同聚集程度的LHCII的光谱特性进行了研究，并对Mg2+在其中的可能作用进行了初步的探讨。

Characterization of the main light-harvesting chlorophyll a/b-protein complex of green alga, Bryopsis corticulans

The main light-harvesting chlorophyll a/b -protein complex (LHC II) has been isolated directly from thylakoid membranes of shiphonous green alga, Bryopsis corticulans Setch. by using two consecutive runs of anion exchange and gel-filtration chromatography. Monomeric and trimeric subcomplexes of LHC 11 were obtained by using sucrose gradient ultracentrifugation. Pigment analysis by reversed-phase high performance liquid chromatography showed that chlorophyll a (Chl a), chlorophyll b (Chl b), neoxanthin, violaxanthin and siphonaxanthin were involved in LHC 11 from B. corticulans. The properties of electronic transition of monomeric LHC II showed similarities to those of trimeric LHC II. Circular dichroism spectroscopy showed that strong intramolecular interaction of excitonic dipoles between Chl a and between Chl b exist in one LHC II apoprotein, while the intermolecular interaction of these dipoles can be intensified in the trimeric structure. The monomer has high efficient energy transfer from Chl b and siphonaxanthin to Chl a similarly to that of the trimer. Our results suggest that in B. corticulans, LHC II monomer has high ordered pigment organization that play effective physiological function as the trimer, and thus it might be also a functional organization existing in thylakoid membrane of B. corticulans.

Structural analysis of peptides of PSII light-harvesting complexes in siphonous green algae, Codium fragile

peptide composition and arrangement of 4 major light-harvesting complexes LHCP1-3 and LHCP3, isolated from siphonous green algae (Codium fragile (Sur.) Hariot.) were investigated. LHCP1 showed five main peptides, 34.4, 31.5, 29.5, 28.2 and 26.5 kD in SDS-PAGE, the 34.4 and 31.5 kD peptides were never found in higher plants. LHCP3 contained the other four kinds of LHCP1 peptides except 34.4 kD, while LHCP3, consisted of only 28.2 and 26.5 kD peptides. We found that 34.4, 28.2 and 26.5 kD peptides were easy to decompose from LHCP1 when subjected to SDS-PACE without pretreatment. They might be located at the exterior of LHCP1, while the 31.5 and 29.5 kD peptides were at the central part. The 28.2 and 26.5 kD peptides often occurred in CPa, the center complex of PS II. They are possibly the LHC II peptides tightly associated with CC II. According to the results described above, a peptide map of LHCP1 was sketched.

From the LHC to future colliders

Discoveries at the LHC will soon set the physics agenda for future colliders. This report of a CERN Theory Institute includes the summaries of Working Groups that reviewed the physics goals and prospects of LHC running with 10 to 300 fb(-1) of integrated luminosity, of the proposed sLHC luminosity upgrade, of the ILC, of CLIC, of the LHeC and of a muon collider. The four Working Groups considered possible scenarios for the first 10 fb(-1) of data at the LHC in which (i) a state with properties that are compatible with a Higgs boson is discovered, (ii) no such state is discovered either because the Higgs properties are such that it is difficult to detect or because no Higgs boson exists, (iii) a missing-energy signal beyond the Standard Model is discovered as in some supersymmetric models, and (iv) some other exotic signature of new physics is discovered. In the contexts of these scenarios, the Working Groups reviewed the capabilities of the future colliders to study in more detail whatever new physics may be discovered by the LHC. Their reports provide the particle physics community with some tools for reviewing the scientific priorities for future colliders after the LHC produces its first harvest of new physics from multi-TeV collisions.

Light stop in the MSSM after LHC Run 1

The discovery of a Higgs boson with a mass of 126 GeV at the LHC when combined with the non-observation of new physics both in direct and indirect searches imposes strong constraints on supersymmetric models and in particular on the top squark sector. The experiments for direct detection of dark matter have provided with yet more constraints on the neutralino LSP mass and its interactions. After imposing limits from the Higgs, flavour and dark matter sectors, we examine the feasibility for a light stop in the context of the pMSSM, in light of current results for stop and other SUSY searches at the LHC. We only require that the neutralino dark matter explains a fraction of the cosmologically measured dark matter abundance. We find that a stop with mass below similar to 500 GeV is still allowed. We further study various probes of the light stop scenario that could be performed at the LHC Run-II either through direct searches for the light and heavy stop, or SUSY searches not currently available in simplified model results. Moreover we study the characteristics of heavy Higgs for the points in the parameter space allowed by all the available constraints and illustrate the region with large cross sections to fermionic or electroweakino channels. Finally we show that nearly all scenarios with a small stop-LSP mass difference will be tested by Xenon1T provided the NLSP is a chargino, thus probing a region hard to access at the LHC.

CMS physics technical design report, volume II: Physics performance

CMS is a general purpose experiment, designed to study the physics of pp collisions at 14 TeV at the Large Hadron Collider ( LHC). It currently involves more than 2000 physicists from more than 150 institutes and 37 countries. The LHC will provide extraordinary opportunities for particle physics based on its unprecedented collision energy and luminosity when it begins operation in 2007. The principal aim of this report is to present the strategy of CMS to explore the rich physics programme offered by the LHC. This volume demonstrates the physics capability of the CMS experiment. The prime goals of CMS are to explore physics at the TeV scale and to study the mechanism of electroweak symmetry breaking - through the discovery of the Higgs particle or otherwise. To carry out this task, CMS must be prepared to search for new particles, such as the Higgs boson or supersymmetric partners of the Standard Model particles, from the start- up of the LHC since new physics at the TeV scale may manifest itself with modest data samples of the order of a few fb(-1) or less. The analysis tools that have been developed are applied to study in great detail and with all the methodology of performing an analysis on CMS data specific benchmark processes upon which to gauge the performance of CMS. These processes cover several Higgs boson decay channels, the production and decay of new particles such as Z' and supersymmetric particles, B-s production and processes in heavy ion collisions. The simulation of these benchmark processes includes subtle effects such as possible detector miscalibration and misalignment. Besides these benchmark processes, the physics reach of CMS is studied for a large number of signatures arising in the Standard Model and also in theories beyond the Standard Model for integrated luminosities ranging from 1 fb(-1) to 30 fb(-1). The Standard Model processes include QCD, B-physics, diffraction, detailed studies of the top quark properties, and electroweak physics topics such as the W and Z(0) boson properties. The production and decay of the Higgs particle is studied for many observable decays, and the precision with which the Higgs boson properties can be derived is determined. About ten different supersymmetry benchmark points are analysed using full simulation. The CMS discovery reach is evaluated in the SUSY parameter space covering a large variety of decay signatures. Furthermore, the discovery reach for a plethora of alternative models for new physics is explored, notably extra dimensions, new vector boson high mass states, little Higgs models, technicolour and others. Methods to discriminate between models have been investigated. This report is organized as follows. Chapter 1, the Introduction, describes the context of this document. Chapters 2-6 describe examples of full analyses, with photons, electrons, muons, jets, missing E-T, B-mesons and tau's, and for quarkonia in heavy ion collisions. Chapters 7-15 describe the physics reach for Standard Model processes, Higgs discovery and searches for new physics beyond the Standard Model.

Diphoton signals for large extra dimensions at the Fermilab Tevatron and CERN LHC

We analyze the potentiality of hadron colliders to search for large extra dimensions via the production of photon pairs. The virtual exchange of Kaluza-Klein gravitons can significantly enhance this process provided the quantum gravity scale (MS) is in the TeV range. We studied in detail the subprocesses qq̄→γγ and gg → γγ taking into account the complete standard model and graviton contributions as well as the unitarity constraints. We show that the Fermilab Tevatron run II will be able to probe MS up to 1.5-1.9 TeV at 2σ level, while the CERN LHC can extend this search to 5.3-6.7 TeV, depending on the number of extra dimensions. ©2000 The American Physical Society.

Studi sul guadagno dei fotomoltiplicatori per il luminometro LUCID dell'esperimento ATLAS a LHC

Dall'inizio del 2013 il Large Hadron Collider (LHC) non produce dati (fase di shut down) per effettuare operazioni di consolidamento e manutenzione. Nella primavera del 2015 ritornerà in funzione con energia e luminosità maggiori. La fase di shut down è sfruttata anche per migliorare le prestazioni dei vari rivelatori che operano in LHC, così che possano acquisire dati in modo efficiente alla riaccensione. Per quanto riguarda il monitor di luminosità LUCID, che fa parte dell'esperimento ATLAS, sono stati sostituiti i rivelatori principali (i fotomoltiplicatori) e l'elettronica. Numerosi considerazioni rispetto alla durata dei nuovi sensori nell'ambiente di LHC tra il 2015 e il 2018, nonché sulla misura della luminosità durante il run II, hanno portato alla conclusione che i nuovi PMT dovranno operare ad un guadagno di math 10^5. E' stato dunque necessario, una volta identificati ed acquistati i nuovi sensori, qualificarne il funzionamento individuale e determinare la tensione di lavoro corrispondente al guadagno desiderato. La prima parte di tali misure, effettuate interamente a Bologna, riguarda la misura della corrente di buio e l'andamento della dipendenza del guadagno dei PMT dalla tensione applicata. La seconda parte riguarda invece lo studio di diversi metodi per le misure del guadagno assoluto, e la loro applicabilità durante la fase di funzionamento di LHC. Durante la presa dati, infatti, sarà essenziale essere in grado di monitorare continuamente il guadagno di ciascun sensore al fine di mantenerlo costante. Le misure di guadagno assoluto presentate in questa tesi sono state effettuate in parte a Bologna, sfruttando il metodo del singolo fotoelettrone, e in parte presso il CERN, utilizzando una sorgente radioattiva, il Bismuto 207.