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Higgs Boson Reconstruction and Detection Algorithm Analysis of the H0 → Z0 + Z0 → 2Μ+ + 2Μ− Decay Chain Using CMSSW

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Higgs Boson Reconstruction and Detection Algorithm Analysis of the H0 → Z0 + Z0 → 2Μ+ + 2Μ− Decay Chain Using CMSSW Higgs Boson Reconstruction and Detection Algorithm Analysis of the h0 ! Z0 + Z0 ! 2µ+ + 2µ− Decay Chain Using CMSSW Matthew Sanders∗ Washington and Lee University (Dated: August 20, 2008) Abstract The Compact Muon Solenoid is expected to become operational towards the end of 2008. When it does, our understanding of the world of elementary particles could change dramatically with the discovery of new particles such as the Higgs boson. The CMS collaboration had already developed a programming package called CMS Software (CMSSW) to aid in the discovery of the Higgs boson and other new physics. Using CMSSW researchers can develop and test algorithms designed to successfully detect the Higgs boson. In this project, I developed and tested one such algorithm to reconstruct the Higgs boson from four muons. I then compared the effectiveness of my algorithm to the effectiveness of a similar algorithm that also reconstructs the Higgs boson from four muons. Contents to satisfy three ends. They hope to probe physics at the TeV level, discover new physics beyond the standard I. Introdcution 1 model (i.e. supersymmetry and/or extra dimensions) and A. What is CMS? 1 discover the Higgs boson [1]. The latter of these goals has B. The Higgs Boson been the focus of my research. A Brief History and Itroduction 2 C. An Introduction to CMSSW 2 A. What is CMS? II. Higgs Boson Reconstruction Through Algorithm Development and Analysis 3 The CMS collaboration is a collection of researchers A. Summer Project from various educational and private research institu- h0 ! Z0 + Z0 ! 2µ+ + 2µ− 3 tions. Together they created the CMS detector and the B. Comparing Algorithms Without Detector CMSSW high energy particle physics analysis package. Background 7 While the researchers are spread across the world, the C. Comparing Algorithms With Detector CMS itself detector is located underground in Cessey, (Pseudo)Background 8 France. As was already aforementioned, CMS stands for D. Further Exploration 9 compact muon solenoid. However, if one was to look at 1. Look For the Presence of a picture of the CMS detector, he/she would find that Z0 ! µ+ + µ− + γ Decays 10 "compact" is a relative term (See Figure 1). Contrary to 2. h0 ! Z0 + Z0 ! 4l Decay Chains 10 3. Geometric Cuts 10 III. Conclusion 10 IV. Special Thanks 10 References 11 I. INTRODCUTION When the Large Hadron Collider at CERN becomes fully operational, CMS will be one of two general pur- FIG. 1: A picture of the CMS detector with dimensions. No- pose experiments at CERN whose goal is to search for tice how small the person is in comparison. [1, 2] new physics. The CMS collaborations hopes to use CMS its name, the CMS detector is actually quite large as it weighs over 12,500 metric tones, has a length of over 21 ∗Dr. John Conway and Milan Nikolic, thank you for the direction meters and a diameter of over 15 meters. and advice throughout my research.; Electronic address: sandersm@ The purpose of the CMS is detector is to look for new wlu.edu physics by through pp¯ collisions at energy levels greater 2 than the capabilities of any current particle accelerator. bilities of the LHC, CMS should easily be able to detect The expected center of mass energy produced by the LHC a Higgs boson with a mass of 115 GeV/c2 if it exists. is about 14TeV using a magnetic field of 3.8 Tesla [1]. The Higgs boson has been referred to as the "God Par- At these energies, physicists hope that the of the con- ticle" in both popular culture and the scientific commu- stituents of the hadrons in the pp¯ collisions will lead to nity [1] and for good reason. The discovery of the Higgs the observation of new particles such as supersymmet- boson could prove to be a victory for the Standard Model ric particles like squarks and sleptons and/or the Higgs of particle physics However, if the researchers of the CMS boson. Of particular importance to my research is the collaboration fail to discover the Higgs boson, then an possible discovery of the Higgs boson. overhaul of our entire understanding of the world of ele- As its name implies, CMS will look for new particles, mentary particles must ensue. like the Higgs boson, primarily through the reconstruc- tion of detected muons. That is not to say, however, that other particles cannot also be reconstructed. In fact, C. An Introduction to CMSSW using CMSSW, one can reconstruct particles using just about any particle that CMS is capable of detecting. For CMS Software, more commonly referred to as example one can reconstruct the Higgs boson from the CMSSW, is an open source software package designed decay chain h0 ! Z0 + Z0 ! 2l+ + 2l− using any com- + − + − + − by the members of the CMS collaboration for use with bination of µ µ , τ τ or e e . Figure 2 shows how the CMS detector. CMSSW 1.6.8 was the primary tool various particles are expected to behave in the CMS de- I used for my research. Within CMSSW 1.6.8 one will tector. find three powerful analysis tools. The fist of these tools is the Monte Carlo high en- ergy particle physics event generator. My research used a Monte Carlo event generator known as Pythia, but the CMSSW package contains other sources of Monte Carlo event generators such as ParticleGuns. Pythia is spe- cially designed to work with Tevatron and LHC appli- cations (like CMS){in essence, Pythia produces a digital CMS. The build of Pythia (Pythia 6.4) available for use with CMSSW 1.6.8 is written in F77 (Fortran), but a newer version (Pythia 8.1) written in C++ has recently finished its development stages and is now ready for full scale use with LHC applications such as CMSSW [4]. The second analysis tool contained in CMSSW is FIG. 2: Notice expected flight of the muons and how they ROOT. ROOT is a C/C++ interpreter with a graph- penetrate many more layers of the CMS detector than any ics engine designed to analyze and plot large amounts of other particle. [1, 2] data. In the case of use with CMSSW, ROOT is used in conjunction with a user designed algorithm to analyze the Monte Carlo high energy particle physics events out- put by Pythia and plot the meaningful data contained B. The Higgs Boson within these events in histograms for further analysis. A Brief History and Itroduction The third analysis tool available in CMSSW is actu- ally a software package in and of itself. The scramv1 The primary purpose of the CMS detector will be to build tool contains the compiler or interpreter for the user find new physics. A particle of particular importance to designed algorithm code which can be written in either the CMS collaboration is the Higgs boson. The Higgs bo- C++ (a compiled language) or Python (an interpreted son is the only particle predicted by the Standard Model language). The scramv1 build tool also contains the that has not yet been discovered. Physicists theorize that "mastermind" behind the CMSSW package. Scramv1 the Higgs boson gives mass to all elementary particles build tool is designed to ensure that the user designed that have mass through the action of the Higgs mecha- algorithm code, the Pythia output files and the ROOT nism. The existence of the Higgs boson was first theo- histogram files all work nicely together without the user rized by a group led by physicist Peter Higgs in 1964 [3]. having to supervise. The scramv1 build tool is also able Physicists theorize that there may be several Stan- to build a workspace within which one can build, execute dard Model Higgs bosons of different masses as well as and save all of his/her codes and work. Higgs bosons that extend beyond the Standard Model [3]. Because CMSSW is open source, it is constantly chang- Physicists have estimated that the lowest possible mass ing as researchers develop new analysis packages for use. a Higgs boson can have is 115GeV/c2{just outside the As an open source tool, the developers of CMSSW are range of detection given our current particle accelerators constantly updating the CMSSW package and publishing (i.e. the Tevatron) [3]. However, given the size and capa- analysis algorithms and results. To view these analysis 3 packages as well as to learn more about CMSSW, visit the CMS Twiki website [5]. II. HIGGS BOSON RECONSTRUCTION THROUGH ALGORITHM DEVELOPMENT AND ANALYSIS I came to the University of California, Davis high en- ergy physics department with very basic understanding of elementary particle physics and no idea about what CMS and CMSSW were. Therefore, the first weeks of research with the UC Davis high energy physics department fo- cused becoming accustomed to the various tools I would need to aid me in my research. I spent the first three weeks of my research program reading the framework tu- torials on the CMS Twiki page [5], running through the CMSSW 1.6.8 particle reconstruction tutorials supplied by UC Davis graduate student Milan Nikolic on the UC Davis high energy particle physics website [6] and tak- FIG. 4: "Miracle of the top quark" explained. [3] ing crash courses in C++ programming and ROOT. All of these tasks were supplemented with readings from an elementary particle physics text book [7]. boson candidate my algorithm had to: 1. Find all of the muons and anti-muons in an event. A. Summer Project 2 h0 ! Z0 + Z0 ! 2µ+ + 2µ− 2. Reconstruct the Z boson masses (91.1876 GeV/c ) using the best µ+µ− pairs.
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