S. N. Bose Scholars Program Report

Transcription

1 S. N. Bose Scholars Program Report An analysis of decay of Neutral Heavy Higgs to τ-τ+ in accordance with Higgs Doublet Model. (Under the guidance of Prof. Sridhara Dasu and Dr. Tapas Sarangi. In collaboration with Soubhik Kumar.) Praneeth Chilakalapudi Centre For Integrated Studies, Univ. Of Hyderabad Summer Intern at University of Wisconsin, Madison September 09, 2013 Abstract With the LHC being shut down for repair, particle physicists associated with the CMS detectors have the task of suggesting the kinds of improvisations that need to be made, so as to be able to perform experiments to explore beyond standard model theories. For doing so, they must first verify theoretically that the suggested experiments yield good results. Monte-Carlo simulations of decay process of the neutral heavy Higgs (H / A) to τ- τ + were analysed. 1

2 1 Introduction Particle colliders accelerate bunches of sub atomic particles over huge distances, making them gain high amounts of energy. Once that s done, the beams of particles are smashed into each other. With this, lots of different particles are generated. When trying to detect these fundamental particles, there s no way to actually see them. But if they can somehow be detected and matched with some theoretically calculated values, they can be identified. Do note that only the elastic and head on collisions are counted, and only the transverse momenta of the particles can be detected. Particles can generally be identified by their invariant masses. The invariant mass denotes the total energy and momentum of a particle, when at rest relative to the frame of reference and is always a constant; it is the magnitude of the Lorentz vector of the particle. When a particle decays completely into its final products, and we know one of its highly probable decay modes, we can detect all the particles that it decays to and add up the highest momentum particles Lorentz vectors, we can get the Lorentz vector of that particle before it dissociated. In other words the particle which has dissociated completely can be identified by taking the invariant mass from the vector sum of Lorentz vectors of the highest momentum decayed particles! If a histogram of such an invariant mass was plotted, a peak would be seen at the mass of the missing particle. And from this plot, the particle can be identified! Though this sounds simple, certain types of decays can complicate issues. It is known that a neutrino cannot be detected by the detectors. So, if at all a neutrino is generated in the decay, the energy carried by the neutrino cannot be estimated directly. However, if we summed up the total energy of the visible particles, and subtracted it from the total centre of mass energy of the particles, we d get the energy of the neutrinos (This energy referred to as MET- Missing Transverse Energy) and we d know how to compensate for the lost energy and modify the histograms accordingly. It so happens that certain modes of decay are more favoured by the particles; they have greater probability of occurrence than the others. Mathematically speaking, we can define a quantity called Luminosity as the number of events per crossection. And some events have low luminosity as compared to the others. So when making the plots for different plots (here, our signal and background) we scale them to equal luminosities and overlay them before we proceed to make further investigations. 2

3 2 Monte-Carlo event generations using PYTHIA When trying to see whether the theory correctly describes the experimental data, it is wise to theoretically predict the results and see if they match experimentally. Often, it so happens that the shapes of the graphs from experimentally observed data can be identified after looking at the theoretically predicted plots. PYTHIA, is one such software which theoretically generates the data. It simulates high energy 2-to-2 hard process collisions of elementary particles, and records the information of the multi-particle final states. PYTHIA (Version 8) was used as the event generator [1]. The generated results can be analysed by another open-source CERN related software called ROOT and plots were made. 3 H/A τ- τ + decay Our guide and mentor, Prof. Sridhara Dasu, is interested in exploring the possibilities of the Minimal Supersymmetric Standard Model (MSSM) Theory. The MSSM theory advocates the higgs doublet theory, which says that the higgs fields can be described as a doublet of column vectors. As a result of this model, there arises a need to include five kinds of Higgs Bosons: one light Higgs boson (h 0 ); two neutral heavy Higgs bosons (H,A) and two charged Higgs bosons(h + and H - ) into the model [2]. The aim of this analysis was to check theoretically for whether there was a way to detect the heavy higgs decay. It was previously ascertained that ascertained from experiments that when described by the tanβ and mass m A parameters, a graph such as the one shown below can plotted [3]. The unshaded region in the graph is the area which has not been explored yet (and we simulated collisions in this region). 3

4 The shaded regions in the above graph can most simply be explained as the plots for previously explored reasons where the MSSM signals of A 0 τ- τ + decay were not giving satisfactory outputs. 100K signal events (of 14TeV centre of mass energy proton-proton collisions) were generated and for neutral higgs of certain m A and tanβ (in the unshaded region) the histograms for invariant mass and MET were made. Also, made similar plots with the background events (Z τ _ τ + and h (standard model-like) τ _ τ +) and overlaid the signal plots on the backgrounds. Just as an example, some plots are being shown below: The adjacent figure is a plot of number of events (y-axis) versus generated (got directly from data) invariant masses (x-axis) for h τ-τ+, Z τ-τ+, and signal events for various masses while keeping tanβ fixed. The adjacent figure is a plot of number of events (y-axis) versus reconstructed (calculated backwards from data) invariant masses (x-axis) for h τ-τ+, Z τ-τ+, and signal events for various masses while keeping tanβ fixed. 4

5 4 Signal to Background Analysis The events that we re interested in looking for are called the signals. The events which occur simultaneously and interfere with the signals are called background or noise. It is a wise measure to devise cuts to eliminate as much background as possible and get a good signal to background ratio. Cuts were imposed on the transverse momenta of the produced leptons, and only those crossing a certain barrier were taken into account as an attempt to filter out select events. As you can see in the picture below (which has a histogram of number of events (y-axis) versus the transverse momentum of ττ (on x-axis)), the distribution of transverse momenta are different in the signal and background, and so we can probe only in those regions where signal to background ratio is high. As noted earlier, the neutrinos that are generated cannot be detected. As a result the peaks of invariant mass shift towards the left as can be seen from the Generated and Reconstructed Invariant plots on the previous page. Special methods such as Collinear Approximation and Missing mass calculator techniques have been devised to reconstruct accurately the mass of decaying resonances (as described in detail in Reference [4]) 5 Conclusions Analysis for the ττ decay from the neutral heavy higgs is the first of the large number of steps that need be taken to confirm the validity of the MSSM theory. A good understanding of particle physics and familiarity with programming is very necessary for this kind of a computational study of the decay channels of particles. Over the two months of the summer programme, was exposed to varied topics which form the basis of particle physics research. This experience gave me a strong foundation for a career in physics. 5

6 6 Acknowledgement I would like to thank Prof. Sridhara Dasu and Dr. Tapas Sarangi for giving me the opportunity to work under them and for patiently guiding me all throughout. I would also like to thank Soubhik Kumar, Stephane Cooperstein and Nate Woods for helping me out during times of need; and also other members of the CMS team at University of Wisconsin-Madison for the inspiring weekly meetings and the great experience. I express my gratitude to Prof. Aseem Ansari, the Indo-US Science and Technology forum and the Department of Science and Technology (Govt. of India) for having created and supported the S.N.Bose Scholars Program through which I have benefitted so much. 7 References [1] A Brief Introduction to PYTHIA 8.1 T. Sjöstrand, Stephen Mrenna, Peter Skands. arxiv: October 2007 [2] Theory and phenomenology of two-higgs-doublet models. G. C. Branco, P. M. Ferreira, L. Lavoura, M. N. Rebelo, Marc Sher, and Joao P. Silva. arxiv: v3 [hep-ph] 19 Dec 2011 [3] [4] A New Mass Reconstruction Technique for Resonances Decaying to τ τ. A.Elagina,P.Muratb, A.Prankoc, A.Safonov. arxiv: v2 [hep-ex] 22 Feb