Improving sensitivity to the unitarity triangle angle gamma with CLEO-c CP-tagged D0 decays. (Project proposal for Responsive RA call)

对B介子CP破坏的精确测量揭示了粒子物理学标准模型的基本参数，这些参数与费米子质量的产生和宇宙中物质相对于反物质的优势等领域有关，并以对新物理学的高度敏感性探索了CP破坏的标准模型描述。可以说，在未来十年中，CP破坏测量的最重要目标将是精确确定CP破坏参数γ。欧洲核子研究中心LHC的LHCb实验将首次能够进行如此精确的测量。测量伽马也将是可能的超级B工厂计划的核心。最有希望的伽马测量类别之一涉及衰变B+ -> DK+。在这个衰变链中，D是不稳定的，并进一步衰变，例如到Ks pi pi。有许多中间状态，通过它们D可以衰变到相同的最终状态。这些不同的衰变路径相互干扰，导致干涉图案，对于三体衰变，可以在二维Dalitz图中可视化。对于四体衰变，可视化更困难，但同样的原理适用。对于起源于B+/-衰变的D介子，干涉图样对B的衰变参数敏感，特别是参数γ。由于CP破坏，这些图案看起来不同，这取决于D是源自B+衰变还是B-衰变。分析这些模式原则上允许非常精确地确定伽马。然而，这需要在任何B衰变参数进入之前彻底理解D介子衰变和相应的干涉图案。预计这将是该方法伽马值精度的限制因素。完全理解D衰变需要达利茨图中每个点的两条信息：振幅和相位。通常，只能测量一个参数，即强度。理解强度模式的振幅和相位需要使用这些D衰变的模型。然后将该模型的参数拟合到强度分布。然而，这取决于模型的选择。这是上述伽马测量的最大系统不确定性。按照目前的价值，这种不确定性将限制LHCb和任何未来Super-B设施积累的数据的使用。在美国纽约州康奈尔大学进行的CLEO-c实验产生了处于量子关联态的D介子对，而在e+e- B工厂或LHC中则不是这种情况。对于相同的最终状态，该数据集允许Dalitz图的衰变模式的两个独立测量，“CP标记”和“风味标记”。原则上，这使得可以直接提取Dalitz空间中每个点的幅度和相位，而不依赖于任何模型。即使对于事件数量不足以完全独立于模型的方法的通道，现有的模型也将大大受益于量子相关数据所施加的独特约束。这将显着减少与我们对LHC B和其他地方的γ测量中D衰变的理解相关的系统误差，其中B->DK。然而，CLEO-c是一个资源非常有限的合作项目，目前没有足够的人力来分析对B物理实验中的伽马测量最重要的许多通道。因此，我们建议加入CLEO-c的Dalitz分析小组，在2008年春季数据采集结束前加入CLEO-c。这将使我们能够在2008/09年充分利用整个CLEO-c数据集进行拟议的测量。我们的项目的结论与应用的结果在LHCb的伽马测量。

Precision measurements of CP violation in B mesons shed light on fundamental parameters of the Standard Model of particle physics, related to areas such as the generation of fermion masses and the predominance of matter over antimatter in the universe, and probe the Standard Model description of CP violation with a high sensitivity to New Physics. Arguably the most important goal of CP-violation measurements over the next decade will be a precise determination of the CP-violation parameter gamma. The LHCb experiment at the LHC at CERN will, for the first time, be able to perform such precision measurements. Measuring gamma will also be central to the programme of a possible Super-B factory. One of the most promising classes of gamma measurements involves the decay B+ -> DK+. In this decay chain, the D is unstable and decays further, for example to Ks pi pi. There are many intermediary states via which the D can decay to the same final state. These different decay paths interfere with each other, causing an interference pattern that can, for 3-body decays, be visualised in a 2-dimensional Dalitz plot. For 4-body decays, the visualisation is more difficult, but the same principles apply. For D mesons that originate from B+/- decays, the interference patterns are sensitive to decay parameters of the B, in particular the parameter gamma. Due to CP violation, those patterns look different depending whether the D originated from a B+ decay or a B- decay. Analysing those patterns allows in principle a very precise determination of gamma. This however requires a thorough understanding of D meson decays and the corresponding interference patterns before any B decay parameters enter. This is expected to be the limiting factor in the precision on gamma with this method. Understanding the D-decays fully requires two pieces of information for each point in the Dalitz plot: the amplitude and the phase. Usually, only one parameter can be measured, the intensity. Understanding the intensity pattern in terms of amplitudes and phases requires the use of a model for those D decays. The parameters of this model are then fitted to the intensity distribution. However, there is a dependency on the choice of model. This is the largest systematic uncertainty for the aforementioned gamma measurements. At its present value, this uncertainty will limit the use of the data accumulated at LHCb and any future Super-B facility. The CLEO-c experiment operating at Cornell University, NY, USA produces pairs of D mesons in a quantum correlated state, which is not the case at the e+e- B factories or the LHC. This data set allows, for the same final state, two independent measurements of the decay patterns of the Dalitz plot, 'CP tagged' and 'flavour tagged'. This makes it possible, in principle, to directly extract both amplitude and phase for each point in Dalitz space, without any model dependence. Even for channels where event numbers are not sufficient for a completely model-independent approach, the existing models will benefit greatly from the unique constraints imposed by the quantum-correlated data. This will significantly reduce the systematic error associated with our understanding of D decays in gamma measurement with B->DK at LHCb and elsewhere. CLEO-c however is a collaboration with very limited resources and at present there is not enough manpower to analyse many of the channels that are most important for gamma measurements at B-physics experiments. We therefore propose to join the Dalitz analysis group of CLEO-c to spearhead these studies, joining CLEO-c well before the end of data taking in spring 2008. This will put us in a position in 2008/09 to fully exploit the entire CLEO-c dataset for the proposed measurements. Our project concludes with the application of the results to gamma measurements at LHCb.