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Genigraphics® has been producing output from PowerPoint® longer than anyone in the industry; dating back to when we helped Microsoft® design the PowerPoint® software. US and Canada: 1-800-790-4001 Email: info@genigraphics.com [This sidebar area does not print.] but this object that has been identified as a jet may not be a jet at all but a misconstruction using the energy from the lepton. We must then remove events in which this false jet appears to ensure that we are working with a consistent sample Traditional Method: if a jet is within a certain distance from an object identified as a lepton, we remove the event. New Method: We propose a method of subtracting the 4-vector of the lepton from the 4- vector of the jet, then determining if the jet object still has adequate energy to be considered a jet. Z Boson Mass: Bare Muons Number of events methods of properly defining jets at the truth level in Monte Carlo simulations. My work contains two major pieces: 1.consistently defining truth level jets and leptons 2.Implementing a new method of jet overlap removal Estimating Efficiency and Background on W+b and Z+b Events with the ATLAS detector at CERN Lesya Horyn; Pierre-Hugues Beauchemin, PhD Tufts University, Summer Scholars References MotivationProcedure Successful definition truth leptons and jets at various levels Implementation of a new method of overlap removal This method is still being tested to see the improvement it will have We have realized various places for improvement since these histograms were created and we expect to see a more drastic difference between the two in the future. This method is relevant to a wide variety of studies in high energy physics. Truth level data of Monte Carlo simulations is often used as a control to compare efficiency of reconstruction algorithms. It has potential within the Tufts High Energy Physics group as well as within the ATLAS collaboration Conclusions The July 4, 2012 announcement of the discovery of a Higgs boson-like particle at the Large Hadron Collider (LHC) at CERN was an event of enormous significance for our generation. Much of the next run of the LHC will be focused on understanding the properties of the Higgs at a deeper level. To this end, it is essential to minimize factors that inhibit us from studying it. One of the main problems physicists face when studying physical phenomena at such high energy is called background, that is, events or particles not coming from the process of interest that are picked up by the detector. We can use current data from the ATLAS Detector at the LHC to to understand this background so that we can subtract it and refine our study. In this study, we address one element of this uncertainty and compare Truth Lepton Definition Figure 3: Final state radiation definitions 1.Truth Lepton Definition for various generator types Sherpa, Alpgen+Pythia, Alpgen+Herwig Dressed lepton 2. Truth Jet Definition3. Overlap removal test Bare lepton Born lepton Figure 2: Shows how jets are defined and the necessity of overlap removal A jet is a cone of particles produced by the hadronization of a quark. Because quarks carry a color charge and QCD confinement prohibits them from being seen independently, they decay to hadrons that can be directly observed and reconstructed to form an object we can study. When reconstructing jets, is possible to have another particle, in this case a lepton (an electron or muon), deposit its energy in the same area of the detector as the jet. Since we reconstruct objects based on the geometry of energy clusters in the detector, we would reconstruct a jet object and a lepton object, Jet Overlap Removal Figure 1: Depiction of one of the events studied in the ATLAS detector Z Boson Mass: Dressed Muons Number of events Z Boson Mass: Born Muons Number of events Z boson mass (GeV) I implemented methods for measuring these three levels of objects. The histograms to the right show how the mass of the reconstructed Z boson changes with leptons of varying states of final state radiation. Looking specifically at the left tail of the peak, we see that the mass from born and dressed leptons closely resemble each other. However, the mass from the bare leptons has a larger tail on the lower mass side of the peak due to the exclusion of the energy from FSR photons. Results To compare the effectiveness of one method versus the other, we compare a lepton decay (Z->ll) to a neutrino decay (Z->vv). Neutrinos do not interact with the detector so are only reconstructed from missing energy in the event. This makes a Z+neutrinos+jets event our control group, since overlap removal is not necessary. To compare the methods, we create a ratio plot of Z->ll/Z->vv. Above, are the two graphs of the same variable in the event and below is their ratio. If there is no difference between the two, we should see a straight line at 1. We can compare the ratio plots of the two different methods and we expect to see a better ratio using our new method. Acknowledgements I would like to thank the Tufts University Summer Scholars program and Steven J. Eliopoulos, A89 & Joyce J. Eliopoulos for their Endowed Fund for Undergraduate Research in Physics and Astronomy and their dedication to physics at Tufts University I would also like to thank my advisor, Dr. Beauchemin, and the entire Tufts physics department for endless support and encouragement. Lesya Horyn Tufts University, Physics 2015 Lesya.Horyn@tufts.edu 1.Beauchemin, P-H, V. A. Bednyakov, G. I. Lykasov, and Yu.Yu Stepanenko. "Search for Intrinsic Charm in Vector Boson Production Accompanied by Heavy Flavor Jets.” 2.Muller, Katharina. “Outcome of the WG: W&Z Precision Measurements.” Lecture 3.Beauchemin, P-H. “W/Z + jets in ATLAS.” Lecture. 2010. Figure 4: Z boson mass as reconstructed from born, bare, or dressed leptons Figure 5: Transverse momentum (Pt) of the highest Pt jet in each event, Z->ee vs Z->vv We transition from a reconstruction level discussion to a theory level discussion by studying the truth level data, removing detector effects. These results will be directly compared to theoretical predictions. There are however different level at which this can be done. These levels vary from having more dependence on the final state objects but more close to empirical data (Bare) to ones more independent of experimental conditions (Born).