2013年CMS物理研究進展研究報告

陳國明

摘 要：去年在歐洲核子中心大型強子對撞機上的CMS實驗和ATLAS實驗同時發現了質量為125 GeV的新粒子，今年，也即2013年，CMS實驗研究該粒子的性質，看到該粒子的衰變方式，信號強度，與其他粒子的相互作用強度以及自旋宇稱都與理論預言的希格斯粒子一致，盡管目前還不能100%的確定，人們基本上相信該粒子就是上述理論家所預言的希格斯粒子。因此，做出預言的理論家獲得今年的諾貝爾物理學獎。在希格斯粒子的發現以及性質測量中，我方做出了重要貢獻。CMS實驗是從雙光子和4輕子這兩個衰變道找到希格斯粒子的。CMS中國組參加了雙光子道的分析。希格斯到雙光子的產率很低，而本底的產率則要高12個量級，這好比在一大堆的沙子中找一顆金沙子，非常難。如何有效排除本底是成功的關鍵，中國組提出了最有效排除本底的辦法，被CMS合作組采用。因此，在雙光子道，我方解決了一個最關鍵的問題。在性質測量中，我方同時參加了雙光子和4輕子這兩個衰變道的分析。CMS利用H→雙伽瑪衰變道測量Higgs粒子的質量以及產生截面與理論預測之比，結果與標準模型在誤差范圍內一致。我方的主要貢獻在于提供了多變量識別光子的方法，提供了信號與本底的干涉效應對信號強度的修正因子和光子能量的標定。CMS同時利用4輕子道測量Higgs粒子的質量，衰變寬度和自旋宇稱，結果與標準模型預言一致。我們的主要貢獻是負責了分析方案的設計、施行，代碼的編寫、調試等，我方在CMS的分析中對統計分析以及統計模型的建立做出了重要貢獻。

關鍵詞：大型強子對撞機 CMS 希格斯粒子

The Progress Report of CMS Physics Research in 2013

Chen Guoming

（Institute of High Energy Physics， Chinese Academy of Sciences）

Abstract：IHEP CMS group made a good contribution for both discovery and property measurement. At CMS， Higgs boson was discovered from two gamma and four lepton channel. IHEP group participated the two gamma channel analysis. As the production rate of Higgs to two gamma is very low， while the background rate is 12 magnitudes higher. To find Higgs in this channel is just like to dig out gold sand in a heap of normal sand. To reject the background effectively is the key for success. IHEP group provided the best method to reject background， and method was accepted by CMS. So we solved a key problem in the two gamma channel. When measuring the property， IHEP group participated both two gamma and 4 lepton channel analyses. In the two gamma channel， the mass and production rate was measured and the results are consistent with the Standard Model prediction. For the analysis we provided gamma identification method， a production rate scale factor from the interference of signal and background and gamma energy calibration. In the 4 lepton channel， the mass， production rate， couplings， spin and parity was measured. And the results all agree with the Standard Model prediction. IHEP group provided the statistical model， analysis scheme， and performed code development and debug. Mingshui Chen from our 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 a certain energy scale. Something beyond must exist， and we are going to find it. CMS IHEP group tried to do it through two ways， one is 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.

Key Words：LHC； CMS； Higgs Boson