高能对撞机上若干新物理的现象学研究

【作者】 孙昊；

【导师】 马文淦；

【作者基本信息】 中国科学技术大学 ， 粒子物理与原子核物理， 2008， 博士

【摘要】 规范场论的提出是人类向统一各种相互作用迈进的十分引人瞩目的一步,标准模型在这种背景下诞生并获得了极大的成功。迄今为止所有的低能实验现象与基于SU（3）（?）SU（2）（?）U（1）规范理论的标准模型相符合。然而标准模型理论的另一基石Higgs机制所预言的Higgs粒子至今没在实验上被发现。我们有理由相信标准模型并不是描述物质世界的终极理论,它有可能仅是其它更基本理论的一个低能有效理论。最小超对称标准模型（MSSM）,大额外维模型（LED）以及Littlest Higgs模型用不同的方法解决了标准模型的等级差问题,日益成为标准模型最具吸引力的几个扩展理论模型并被物理学家广泛接受。MSSM通过引入超对称性和超对称粒子来解决等级差问题。理论预言,在TeV能标下将会有标准模型以外的新物理出现,通过最近几年的研究人们希望能在高能对撞机上寻找到MSSM预言的（?）^±,（?）⁰等轻超对称粒子,所有同标准模型粒子相对应的超对称伴随子,也希望寻找到预言的Higgs粒子等。大额外维模型（LED）,用引入新的空间维数的方式来解决等级差问题。时空维数大于4的想法在弦论中非常普通,因而,大额外维模型可能是弦论的一个低能近似理论。这个理论将引力也纳入到标准模型的框架中,并且按照ADD模型的想法,在高维空间传播的无质量引力子可以等效于许多在四维时空传播的有质量的引力子,这使人们能够研究引力子与普通粒子的相互作用,特别是将来可能发现的Higgs玻色子的相互作用,为未来更高能标下可能发现的新物理做好理论准备。Littlest Higgs模型通过引入一系列与标准模型规范玻色子具有相同的量子数的重规范玻色子,以及一个重的Top夸克,以消除标准模型的规范玻色子圈和top夸克圈引起的Higgs粒子质量的平方发散。在LH模型中,Higgs机制仍然是一块不可或缺的基石,通过对Higgs三线性自耦合的测量,得以研究Higgs势并揭示对称性自发破缺和Higgs粒子质量产生机制的细节。由于top夸克和Higgs在粒子物理中占有重要的位置,top-Higgs Yukawa耦合正比于top夸克质量,且top夸克的质量很重,从而使得Yukawa耦合9t（?）h非常大,因此对t（?）h⁰联合产生过程的精确研究能很好的用来检验模型和发现新物理。未来的直线对撞机和强子对撞机的主要任务之一就是寻找各种预言的粒子,包括标准模型的Higgs粒子,扩展模型中的Higgs粒子,额外维模型中的引力子,最小超对称模型中的（?）^±,（?）⁰,squark,sleptons等等。要更好的在对撞机上进行实验探测,并对过程进行分析和讨论,我们不仅需要清楚了解对撞机原理及产生机制,而且要在理论上给出过程精确的产生信号。本文在不同模型下研究了部分新物理的产生效应:在强子对撞机上我们详细研究了（（?）₂⁰（?）₁^±）联合产生过程,通过对信号的分析,可以大致估计该过程产生截面测量精度在百分之几的量级,换言之,当我们圈修正的计算结果与标准模型预言值的偏差在百分之几个量级,高阶QCD以及电弱修正就有必要精确的计算出来,从而可以定性的提高预测新粒子的质量限的精确性以及适当缩小寻找新粒子的发现和排除域范围。此外,在考虑了高阶修正后的截面值能够很好的降低树图结果对重整化能标和因子化能标的依赖关系,从而为理论的分析和讨论减小了某些方面的不确定度;另外我们计算了在直线对撞机上测量了虚引力子交换过程t（?）h⁰的联合产生,在Littlest Higgs模型下该过程的QCD修正,通过该过程研究top物理,Higgs物理以及之间Yukawa耦合特性,研究其在加速器中的产生效应,从而用来检验模型和发现新物理。本论文的创新之处如下:·与通常的利用给胶子以及光子以虚拟质量来消除红外发散不同,本论文在计算NLO圈修正时采用维数正规化方案来消除和分离紫外发散以及红外发散,将该方法应用到QCD修正计算,并且将其应用到圈图的电弱修正计算,从而可以很容易地在解析结果中明确检验红外发散的消除,从而得到有限的结果。·本论文在计算实胶子以及实光子辐射修正时,采用两截断相空间分割的方法,这种方法是目前国际上进行NLO QCD计算时所采用的最新的也是精确度最高的计算方法之一,本人亦将该方法应用到电弱修正的计算中。·强子对撞机上（（?）₂⁰（?）₁^±）联合产生及其随后的三轻子衰变过程是寻找超对称粒子的非常重要的“金盘子”过程。本文首次完整计算了最小超对称模型下,强子对撞机上（（?）₂⁰（?）₁^±）联合产生过程,计算了该过程NLO QCD修正以及完整单圈电弱修正,得到了目前这个反应过程的最精确的理论预言,对未来LHC实验寻找和研究超对称粒子具有理论上的指导意义。·本文首次计算了最小超对称模型下,带电Higgs与底夸克,粲夸克的伴随产生过程,通过该味道改变过程研究味道改变机制和squark混合机制。如果存在squark混合机制,该过程将可能成为带电H^±产生的一个重要的反应道。我们给出了目前这个反应过程精确的理论预言,对未来LHC实验寻找H^±和标准模型以外的新物理有着重要的指导意义。·直线对撞机上t（?）h⁰的联合产生过程为Yukawa耦合强度的测量提供了很重要的手段,为比较不同模型下Yukawa耦合特性的差异,精确的理论计算十分必要。我们首次考虑了在不同初态极化方式下t（?）h⁰的联合产生,并给出该产生过程在虚引力子交换下的额外维效应;另外我们精确计算了该过程在Littlest Higgs withoutT-parity模型以及Littlest Higgs with T-parity模型下的修正结果,为在直线对撞机上精确检验LH/LHT效应提供了很好的理论指导。更多还原

【Abstract】 The naissance of the gauge theory is an extremely conspicuous step toward the unification of all kinds of interactions.The Standard Model was born in this background and has obtained enormous success.The present precise experimental data have shown an excellent agreement with the predictions of the Standard Model,which based on the SU（3）（?）SU（2）（?）U（1）gauge theory.But another foundation stone of the SM,the Higgs Mechanism,hasn’t been proved yet.However people believe that the SM has only been considered to be an effective theory of some underlying theory in low energy.The Minimal Supersymmetry Standard Model（MSSM）,the Large Extra Dimension Model（LED）and the Littlest Higgs Model（LH）have solved the hierarchy problem in different ways,and are widely considered as the most appealing extensions of the Standard Model.The MSSM settles the hierarchy problem by introducing supersymmetry and super particles.It predicts that the new physics beyond the Standard Model will appear at the TeV energy scale.We expect the lighter charginos（（?）^±）and neutralinos（（?）⁰）to be detected at the Tevatron and the Large Hadron Collider.We also expect all the SM particles’ supersymmetric partners and the predicted Higgs bosons to be found.The hierarchy problem can be overcome by adding new dimensions in Large Extra Dimension Model.It’s a common thought that the space-time dimensions are larger than 4 in String theory,so the LED can be considered as an effective theory of the String theory. The gravity is involved in this theory,and as Arkani-Hamed,Dimopoulos,and Dvali proposed,the picture of a massless graviton propagating in D dimensions is equivalent with the picture of massive Kaluza-Klein gravitons propagating in 4 dimensions.This makes it possible to study the interaction between the graviton and common SM particles, especially the interaction with Higgs bosons.The cancellations of the quadratic divergence in the Littlest Higgs model occur between particles with the same statistics:divergences due to gauge bosons are canceled by new gauge bosons and similarly for the heavy quarks.One of the most important task of present and future experiments is to search for Higgs boson and investigate its properties. Studying the properties of the Higgs potential will reveal details of mass-generation mechanism in spontaneously broken gauge theories,which can be obtained through measuring the Higgs boson self-interactions.Due to the coupling strength of the top-quark-Higgs Yukawa coupling is proportional to the top-quark mass,the top quark Yukawa coupling g_t（?）his very large and the cross section of t（?）h⁰ will be strongly enhanced.Besides,the top quark and Higgs boson play an important role in the particle physics.Therefore,the t（?）h⁰ associated production can be used to probe this coupling and further test of the predicted models and reveal new physics.One of the most important missions of future high energy experiments is to explore the electroweak symmetry break mechanism and search for predicted particles such as Higgs in SM,Higgs in other expanded models,graviton in Extra Dimension,（?）^±,（?）⁰,etc, predicted in MSSM.To investigate the discovering potential of colliders,not only a proper understanding of the production mechanisms is necessary,but also accurate theoretical predictions of the signature should be provided.In our paper,we have studied several production effects of new physics,including:investigate（（?）₂⁰（?）₁^±）associated production process at hardon colliders,one can expect the accuracy of the cross section measurement of the neutralino-chargino（（?）₂⁰（?）₁^±）associated production at the upgraded Tevatron can reach few percent.Therefore,for the precise experiments at TeV scale hadron colliders,both the higher order QCD and electroweak corrections should be considered in the theoretical predictions,and thereby one can improve experimental mass bounds and exclusion limits for the new particles.Moreover,the consideration of higher order QCD contributions can reduce the dependence of the cross sections on the renormalization and factorization scales in the LO.And the cross sections in NLO are under much better theoretical control than the leading order estimates.We also studied the t（?）h⁰ associated production processes at linear colliders in the large extra-dimension model and in the Littlest Higgs model and also its extension with T-parity up to NLO.Due to the Yukawa coupling is proportional to the top-quark mass which is very large and might be probed in experiments,the t（?）h⁰ associated production processes are very important.Through the study of the top physics,the Higgs physics and their Yukawa coupling,we can learn their production at the accelerators and further test the models and find new physics.The main innovations in this thesis are listed below:·Different from the traditional method of using a fictitious gluon（photon）mass to regularize the IR singularities,we use the dimensional regularization scheme to cancel and separate the UV divergences and IR singularities when we perform NLO QCD calculations and the full one-loop electroweak calculation.We can easily check the cancellation of IR singularities in the analytic expressions and get finite results. ·I use the Two Cutoff Phase Space Slicing Method to calculate the real gluon emission corrections and the real photon emission corrections.This method is one of the newest and most reliable methods of performing NLO QCD calculations which I also used to treat the full electroweak calculations for the first time.·The（（?）₂⁰（?）₁^±）associated production with their possible further decay to trilepton signature might be detected at hadron colliders.The trilepton signature which called "Gold-plated signature" can be the most promising channel for supersymmetric particle searches at hadron colliders.In this thesis,I calculated the（（?）₂⁰（?）₁^±）associated production at hadron colliders including NLO QCD and full one-loop electroweak corrections in MSSM.The results of my calculation are the most accurate results of this process so far.My results will be helpful for the experiments at hadron colliders to look for predicted supersymmetry particles.·In this thesis,I calculated a single charged Higgs boson production associated with a bottom-charm pair at CERN Large Hadron Collider.We conclude that the squark mixing mechanism in the MSSM makes the pp→（?）cH^-+X process a new channel for discovering a charged Higgs boson and investigating flavor changing mechanism.I present the most accurate results of this process so far.My calculation will be helpful for the LHC experiments to detect charged Higgs boson and look for physics beyond the Standard Model.·The process of t（?）h⁰ associated production at future colliders is an important instrument in measuring the Yukawa coupling strength.The precise calculation is necessary to distinguish the property of the Yukawa coupling in different models.For the first time we consider the t（?）h⁰ associated production at future linear colliders in both polarized and unpolarized collision modes in order to test the large extra-dimension effects.We also study the corrections to this process up to QCD next-to-leading order in the Littlest Higgs model with and without T-parity for the first time,and further use it to test the models and the LH/LHT effects.更多还原