Event selection criteria in nucleus-nucleus collisions

Several event selection criteria in ultrarelativistic nucleus-nucleus collisions are analysed in the framework of the dual parton model. First, central and semicentral collisions are compared. We conclude that semicentral processes are more appropriate to observe high energy density events, where the formation of a quark-gluon plasma could take place. Secondly, the dependence of the achieved energy density on the degree of centrality of the collision is studied. We analyse the implications of our results for future experiments at CERN.     

Invariant-mass distribution of top-quark pairs and top-quark mass determination

In this study, we investigate the invariant-mass distribution of top-quark pairs near the 2m_t threshold, which strongly influences the determination of the top-quark mass m_t. Higher-order non-relativistic corrections lead to large contributions, which are not included in the state-of-the-art theoretical predictions. A factorization formula is derived to resum such corrections to all orders in the strong-coupling, and necessary ingredients are calculated to perform the resummation at next-to-leading power. We combine the resummation with fixed-order results and present phenomenologically relevant numerical results. The resummation effect significantly increases the differential cross-section in the threshold region and makes the theoretical prediction more compatible with experimental data. We estimate that using our prediction in the determination of \begin{document}$m_t $\end{document} will lead to a value closer to the direct measurement result.     

Correlation-based method for improvement of NQR signals utilizing signal shape information

In the correlation method the signal shape information can be utilized for detection of weak signals, hidden in the noise. We tested the advantages, drawbacks and some practical features of the method applied to magnetic resonance detection. The influence of the signal width, shape and similarity to the template on the correlation function was studied by numerical simulation. The findings were applied to the optimization of the template for improved localization of a noisy¹⁴N nuclear quadrupole resonance (NQR) line in NaNO₂ during repetitive scans by a superregenerative spectrometer. The value of the correlation method in faster NQR intensity evaluation was also confirmed.     

一种通用的卫星导航信号码时延估计误差评估方法

卫星导航信号的码时延估计误差是决定系统服务性能的关键因素,迫切需要对多种不同调制与复用方式的导航信号进行全面的码时延估计误差性能评估,从而为后期的系统应用提供重要的选择依据.为此,本文提出了通用的码时延估计误差评估方法.首先,概括了导航接收机的码跟踪环路模型,根据是否匹配接收以及是否相干处理,将目前的导航接收机归纳为四种类型.其次,在假设码时延估计误差非常小的条件下,分别给出了匹配接收下相干处理和非相干处理时的估计误差以及相互之间的关系;推导了非匹配接收下非相干处理时的估计误差,并讨论了与相干处理时的关系.最后,推导了码时延估计误差的齐夫-扎凯界限,解决了估计误差不满足非常小这一假设条件时的评估.本文提出的评估方法均以导航信号的功率谱密度表示,为信号设计和接收机的研制提供了重要的理论指导,同时也给具体信号的评估带来了极大便利.仿真实验中对新一代典型导航信号的码时延估计误差做了有效评估.     

Event shape variables in supersymmetry searches at 7 TeV LHC

Supersymmetry (SUSY) signatures are probed at the Large Hadron Collider with 7?TeV energy in the framework of CMSSM with a new set of cuts based on event shapes and jet energy scales. It is showed that with our cuts, it is possible to probe a large portion of CMSSM parameter space in situations, where the SUSY cascade decay chain produces hard multijets + missing energy. We also extend our analysis to include other supersymmetries which produce hard multijets + missing energy.     

Status and prospects of top-quark physics

The top quark is the heaviest elementary particle observed to date. Its large mass of about 173 GeV/c² makes the top quark act differently than other elementary fermions, as it decays before it hadronises, passing its spin information on to its decay products. In addition, the top quark plays an important role in higher order loop corrections to standard model processes, which makes the top-quark mass a crucial parameter for precision tests of the electroweak theory. The top quark is also a powerful probe for new phenomena beyond the standard model.During the time of discovery at the Tevatron in 1995 only a few properties of the top quark could be measured. In recent years, since the start of Tevatron Run II, the field of top-quark physics has changed and entered a precision era. This report summarises the latest measurements and studies of top-quark properties and gives prospects for future measurements at the Large Hadron Collider (LHC).     

Probing top-quark operators with precision electroweak measurements

In the standard model effective field theory, operators involving the top quark are generally difficult to probe and can generate sizable loop contributions to electroweak precision observables measured by past and future lepton colliders. Could the high precision of electroweak measurements compensate for loop suppression and provide competitive reaches on these operators? Would the inclusion of these contributions introduce too many additional parameters for a meaningful global electroweak analysis to be performed? In this paper, we perform a detailed phenomenological study to address these two important questions. Focusing on eight dimension-6 operators that generate anomalous couplings between electroweak gauge bosons and third-generation quarks, we calculate their one loop contributions to \begin{document}$ e^+e^- \to f\bar{f} $\end{document} processes, both on and off the Z-pole, and the \begin{document}$ e^-e^+ \to WW $\end{document} process. A global analysis is performed with these eight operators and those that contribute to the above processes at tree level using measurements at the LEP, SLC, and several low energy experiments. We find that although current electroweak precision measurements are sensitive to the one-loop effects of top-quark operators, it is difficult to separate them from the operators that contribute at tree level, making a global analysis rather challenging. Under further assumptions (for instance, new physics contributes to only third generation quark operators and the S and T parameters), competitive reaches may be obtained in a global fit. Another important finding of our study is that the two operators that generate the dipole interactions of the bottom quark have a significant impact on the Z-pole measurements and should not be omitted. We also discuss the implications of the recently reported W-boson mass measurement at the CDF for our results. Finally, we estimate the reaches of future lepton colliders in probing top-quark operators with precision electroweak measurements.