A Simulated Study Of The Potential For The Discovery of the Supersymmetric Sbottom Squark at the ATLAS Experiment By Rishiraj Pravahan University of Texas at Arlington
Outline Why do we need Supersymmetry? 1. The Mass Problem 2. The Hierarchy Problem 3. How Supersymmetry Solves these problems What is ATLAS? 1. The Large Hadron Collider 2. The Detector specifications The Sbottom Squark 1. The particle zoo 2. Sbottom as a candidate for one of the lowest mass squarks.
Outline Continued How do we look for it? 1. Monte Carlo Simulations: ISAJET 2. The parameters for SUSY and SUGRA The Search Process 1. Finding a relationship between the Sbottom mass and the SUSY parameters 2. Studying the Cross Section for the production of the Sbottom for the parameter space 3. Finding the discovery reach of SUSY parameters according to the Sbottom mass and cross-section 4. Selection of the specific parametric points for calorimeter simulation 5. Study of the decay of the Sbottom 6. Simulating the calorimeter and measurement of counts
Supersymmetry! Supersymmetry is an elegant extension of the Standard Model which enables the laws of physics to be invariant under the exchange of Bosons with Fermions and Fermions with Bosons. It is a space-time symmetry that predicts the existence of a super-partner for every particle in the Standard Model. It is the only possible unique non-trivial extension of the known space-time symmetries.
The Mass Problem (Higgs Mechanism) The standard model cannot account for the masses of the W+, W-, Z and Quarks and Gluons This problem is solved by the Higgs Mechanism where a scalar potential term is added to the Lagrangian SUSY incorporates the Higgs
The Hierarchy Problem The mass of the Higgs must be in the range of the masses of the gauge bosons. The difference in the energy level of electroweak symmetry breaking and GUT is the hierarchy problem Supersymmetry helps in cancellation of the quadratically divergent terms in the radiative corrections to the Higgs Boson Mass and thus solves the problem
The Sbottom Squark Supersymmetry predicts a superpartner for every particle in the Standard Model The superpartner for the Bottom quark is the Sbottom Within present day theoretical calculations the Sbottom may be one of the lightest squarks, and thus one of the first to be found at the LHC The energy reach of ATLAS restricts the discovery of the Sbottom at a mass >1 TeV
How do we look for the Sbottom Before we can start looking for new physics we need to know how to look for the new particles the new physics predicts. We do so with the help of Monte Carlo simulation programs. ISAJET is one such MC simulator that can be used to simulate the conditions at the LHC I use ISASUGRA, the supergravity simulator for the various mass and parameter study. A toy calorimeter is then used to show events with high signal to background ratios.
Which Supersymmetry? I chose the simplest Supergravity inspired Minimal Supersymmetric Standard Model(MSSM) I assume that R-Parity is conserved 32 new particles are added We have five new parameters Sparticles are always pair produced The Lightest Supersymmetric Particle is stable
The Parameters of SUGRA inspired Supersymmetry M 0 the common scalar mass M 1/2 the common gaugino mass A 0 the trilinear coupling constant term tan( ) the ratio of Higgs vev s Sign of (the Higgs mixing parameter) Under these parameters all the couplings and masses can be derived using the 26 renormalization group equations.
The Search The mass of the Sbottom is determined mostly by the parameters M 0 and M 1/2 Thus an exhaustive study of the Sbottom mass and its relationship with M 0 and M 1/2 is done The Sbottom mass is shown for a the physical state SB1 (observable states SBR and SBL are mixtures of SB1 and SB2) 100 GeV < Mass(SB1) < 1000 GeV
The discovery limit in ATLAS for the various parameters have been studied in general and an initial estimated range of the parameters have been found My initial range of points, based on these estimates, are 1. 100 GeV < M 0 < 800 GeV 2. 100 GeV < M 1/2 < 800 GeV 3. tan Restriction of the Monte Carlo used) 4. A 0 =0 (Very weak influence) 5. 0 or >0 I Study the variation of Sbottom mass for all values of M 0 and M 1/2 for values of tan( )= 2 and 10 and both the signs of
Relationship between M 0, M 1/2 and Sbottom mass tan 2, <0, A 0 =0
Relationship between M 0, M 1/2 and Sbottom mass tan 10, <0, A 0 =0
Relationship between M 0, M 1/2 and Sbottom mass tan 2, >0, A 0 =0
Relationship between M 0, M 1/2 and Sbottom mass tan 10, >0, A 0 =0
Discovery range for the parameters For the points discussed one may infer that to restrict the Sbottom mass within 100 and 1000 GeV, M 1/2 < 500 GeV For values of M 0 >500 GeV, M 1/2 < 400 GeV This is in accordance with the general prediction for all squark masses in the msugra scenario, given by, m(squark) (M 0 2 + 6M 1/22 )
Cross section study The discovery of a particle depends on the number of events that occur in the given experiment An event is identified by a characteristic decay process The number of events n = x where, is the total Luminosity and is the cross-section measured in Barns
Cross section study Cont In my simulation I calculated the cross section of all possible events in which Sbottoms were produced With a luminosity of 1fb for a year of data at ATLAS, one can calculate the number of events for each parameter An estimate of the discovery reach of the parameters can then be made from the number of events
A study of the cross section and number of events for M 0 = 200 GeV, tan and m>0 M_half Cross Section in mb No of events 100 1.09E-08 1.09E+04 150 2.84E-09 2.84E+03 200 8.30E-10 8.30E+02 250 2.86E-10 2.86E+02 300 1.07E-10 1.07E+02 350 4.45E-11 4.45E+01 400 1.95E-11 1.95E+01 450 9.27E-12 9.27E+00 500 4.50E-12 4.50E+00 550 2.22E-12 2.22E+00 600 1.12E-12 1.12E+00 650 6.38E-13 6.38E-01 700 3.27E-13 3.27E-01
Cross Section vs. M0
A study of the cross section and number of events for M 1/2 = 200 GeV, tan 0 and m>0 M_0 Cross Section in mbarns No of events 100 1.01E-09 1.01E+03 150 9.14E-10 9.14E+02 200 8.30E-10 8.30E+02 250 7.22E-10 7.22E+02 300 6.00E-10 6.00E+02 350 4.81E-10 4.81E+02 400 3.86E-10 3.86E+02 450 2.96E-10 2.96E+02 500 2.25E-10 2.25E+02 550 1.75E-10 1.75E+02 600 1.30E-10 1.30E+02 650 9.86E-11 9.86E+01 700 7.41E-11 7.41E+01
Cross Section vs. M0
Conclusions from CS study Number of events decrease almost exponentially with increasing M 0 and M 1/2 For M 1/2 > 400 GeV the number of events is too low A similar study can restrict tan( )
The Decay of the Sbottom It is important to identify the possible ways the Sbottom can decay I did an exhaustive study of the decay products for the point ranges being discussed Major decay products are identified from their Branching Ratios
The five major Channels BTL W1SS- + TP BTL Z2SS + BT BTL GLSS + BT BTL W- + TP1 BTL Z1 + BT Out of these the first two are the most dominant in terms of branching ratios
Further decay Each of the first decays result into further decay channels depending on the parameters The final result of all the decays give us a certain number of Standard Model particle and the lightest supersymmetric particle (LSP) which is non interacting and shows up as missing energy Thus from the decay studies and the measurement of missing energy I determine the specific signatures of the Sbottom (in progress)
Example of further decays For a point of M 0 =100 GeV,M 1/2 =100 GeV, tan 2, >0, A 0 =0 BT1 Z2SS BT Z2SS Z1SS E- E+ Z2SS Z1SS MU- MU+ Z2SS Z1SS TAU- TAU+ W1SS TP W1SS+ Z1SS E+ NUE W1SS+ Z1SS MU+ NUM W1SS+ Z1SS TAU+ NUT W1SS+ Z1SS UP DB W1SS+ Z1SS CH SB
Calorimeter simulation and signal to background ratio The next step is to determine the signature of the Sbottom in the calorimeter and calculating the signal to background ratio I will select points for this study Given the high luminosity at ATLAS it is very likely that the signal to background ratio will be high and the Sbottom will be discovered there, if nature is Supersymmetric
Conclusions A study of the SUSY parameter and Sbottom mass determined the discovery limit A study of cross section determined how often the Sbottom will be produced at ATLAS A study of the decay of the Sbottom identified its signature A study of calorimetric readings will determine our capability to discover the Sbottom at ATLAS