2013年CMS物理研究进展研究报告

The Progress Report of CMS Physics Research in 2013

去年在欧洲核子中心大型强子对撞机上的CMS实验和ATLAS实验同时发现了质量为125 Ge V的新粒子,今年,也即2013年,CMS实验研究该粒子的性质,看到该粒子的衰变方式,信号强度,与其他粒子的相互作用强度以及自旋宇称都与理论预言的希格斯粒子一致,尽管目前还不能100%的确定,人们基本上相信该粒子就是上述理论家所预言的希格斯粒子。因此,做出预言的理论家获得今年的诺贝尔物理学奖。在希格斯粒子的发现以及性质测量中,我方做出了重要贡献。CMS实验是从双光子和4轻子这两个衰变道找到希格斯粒子的。CMS中国组参加了双光子道的分析。希格斯到双光子的产率很低,而本底的产率则要高12个量级,这好比在一大堆的沙子中找一颗金沙子,非常难。如何有效排除本底是成功的关键,中国组提出了最有效排除本底的办法,被CMS合作组采用。因此,在双光子道,我方解决了一个最关键的问题。在性质测量中,我方同时参加了双光子和4轻子这两个衰变道的分析。CMS利用H→双伽玛衰变道测量Higgs粒子的质量以及产生截面与理论预测之比,结果与标准模型在误差范围内一致。我方的主要贡献在于提供了多变量识别光子的方法,提供了信号与本底的干涉效应对信号强度的修正因子和光子能量的标定。CMS同时利用4轻子道测量Higgs粒子的质量,衰变宽度和自旋宇称,结果与标准模型预言一致。我们的主要贡献是负责了分析方案的设计、施行,代码的编写、调试等,我方在CMS的分析中对统计分析以及统计模型的建立做出了重要贡献。

IHEP CMS group made a good contribution for both discovery and property measurement. At CMS, Higgs boson was discoveredfrom two gamma and four lepton channel. IHEP group participated the two gamma channel analysis. As the production rate of Higgsto two gamma is very low, while the background rate is 12 magnitudes higher. To find Higgs in this channel is just like to dig outgold sand in a heap of normal sand. To reject the background effectively is the key for success. IHEP group provided the best methodto reject background, and method was accepted by CMS. So we solved a key problem in the two gamma channel. When measuring theproperty, IHEP group participated both two gamma and 4 lepton channel analyses. In the two gamma channel, the mass and productionrate was measured and the results are consistent with the Standard Model prediction. For the analysis we provided gamma identificationmethod, a production rate scale factor from the interference of signal and background and gamma energy calibration. In the 4 leptonchannel, the mass, production rate, couplings, spin and parity was measured. And the results all agree with the Standard Modelprediction. IHEP group provided the statistical model, analysis scheme, and performed code development and debug. Mingshui Chen fromour group is the convener of CMS Higgs combination and property measurement, and plays a key role in the Higgs property measurement.Now all the particles predicted were found, the Standard Model gained a great success. But it can not explain what is dark matter,nor dark energy, and has no idea for the asymmetry between mass and anti-mass. People believe it is only an effective theory under acertain energy scale. Something beyond must exist, and we are going to find it. CMS IHEP group tried to do it through two ways, oneis to search for new physics directly, the other is to check the Standard Model predictions, any violation could hint to a new physics.In the new particle search, we see an evidence of a new resonance at 17.6 GeV through double J/psi decay. It is not approved by CMS,and we are still working on it. For the Standard Model examining, we measured the two gamma final state differential production rate,and the results agree with the Standard Model prediction.