Given the high precision of analysis techniques implemented at the LHC at Cern, there has been increasing opportunity to discover theories beyond the current model of fundamental physics. One such theory for physics “beyond the Standard Model” is known as Supersymmetry and proposes an additional symmetry to be added to space-time, allowing for a family of particles that are an exact duplicate (except for this quantity, labeled R) to those found in the Standard Model. Associated with the extension is a corresponding conservation law in supersymmetric interactions known as ‘R-parity’ (Martin 2010). R-parity conserving decays have been in high focus since they provide an explanation for the massive amount of dark energy foundr, estimated to be close to 73% (Lahanas 2007)(Garrett 2011). Since R needs to be conserved, the lightest supersymmetric particle (LSP for short) would not be able to decay to any other particle other than itself and would explain the massive amount of seemingly stable dark matter (Lahanas 2007). For this reason, there has been a large effort to look for data that resemble R-parity conserving modes, without much attention towards R-parity violating decays.
Over the past two years, I performed an analysis looking for data that resembles a signal that is consistent with a supersymmetric decay. My target process is a supersymmetric top decaying to oppositely charged W-bosons one of which decays to a positive muon and an anti-b quark, and the second decays to a b-quark and negatively charged muon. I chose to look for a particular decay that resembles a standard model interaction with well understood backgrounds because I assumed that the process would behave similarly, except for this additional symmetry which I ignored as part of the analysis. Therefore, I chose a process whose major backgrounds were well modeled using current Monte Carlo methods.
One variable that affects the rate at which this occurs is the mass of the supersymmetric top quark (Dolgov 2006). Since this quantity has not be measured, the goal of my research was to to calculate a cutoff mass at which I can say there is enough data to prove/disprove the theory at a mass lower than a certain value. I did this by performing a comparison between the expected numbers if the theory were true to the numbers found in the data sample. Since the probability at which the decay happens decreases as the supersymmetric top mass increase, there should be a point at which the numbers flip from being too high to too low. The point at which this flip occurs is the “cutoff” point where I can say below which the data does not have enough events to support the theory.
The data came from the latest set published by the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider at CERN in Switzerland. At the core of the design is a superconducting solenoid magnet that is 6m in diameter,13m long, and generates a 4T field which is used to determine the charge of the particle. On the outside of the magnet is an electromagnetic calorimeter (ECAL) which is designed to measure electromagnetic deposits. After the products of the decay have passed through the ECAL, they reach the hadronic calorimeter (HCAL) which absorbs most of the energy left in the collision. The particles that do make it through the HCAL are either neutrinos or muons. The muons are detected and collected in a separate configuration around the magnet composed of a drift tube and cathode-strip detector (AUFFRAY 2002)(Sguazzoni 2008).
The result of this analysis will provide future researchers with a better sense of possible values for the mass of the supersymmetric top and possibly allow for the creation of more specialized detectors that focus on higher mass regions than the calculated cutoff.
Measurement of ttbar(Aad 2014)
This paper provides a lot of important background information concerning the major backgrounds for the standard model version of my decay. Since it specifically focuses on ttbar production, it discusses many of the issues that I encountered while performing the analsysis. This paper will be useful in both the backbrgound section to provide appropriate info as well as guide me during my analysis of the data.
Stops and neutrino mass hierarchy (Marshall 2014)
This paper outlines the correlation between supersymmetric top decau paramters and the neutrino mass hierarchy. This paper will prove to be useful during the section dedicated towards future research as it shows a way for this analysis to provide guidance towards research in the future.
Previous serches for R-parity violation (Stoye 2010)
This paper discusses previous searches for R-parity violation and will be used in my background section to discuss the current state of ressearch in the field.
A general summary of susy searches at LHC (Paige 1999)
Whereas the previous paper discusses more current efforts to search for SUSY, this paper provides an outlook on the field when it was first being discuessed. By looking at the original papers, the original reasons for pursuing particular lines of reseaerch becomes much more clear. This paper will be referenced in my introduction for a description of the field before we knew where to look.
Dark matter considering MSSM (Trotta 2007)
This paper will also go in my introduction and provides a link between dark matter and the MSSM model. By providing this sort of information, it is clear that the hole I am filling with my research is not commonly considered due to the attractiveness of other theories, increasing the usefullness of my research.
Dark Matter Primer (Martin 2010)
This paper will be referenced in my introduction and provides a different viewpoint on the Dark Matter problem as it stands in modern astro physics. By highlighting this issue I am able to drive home the importance of my research.
LSP as DM Candidate (Dolgov 2006)
This paper describes the viability of the LSP as a dark matter candidate and will be referenced in my introduction to provide additional background material.
These two papers provide a detailed description of the two technologies that are used in the CMS detector. By looking at the details of the construction, I am able to notice possible shortcomings of the experiment and find ways in which it can be improved. This paper will be refernced in my introduction as well as briefly in the section concern data acquisition.