Background Analysis Columbia University REU 2015

Transcription

1 Background Analysis Columbia University REU 2015 Kylee Branning Northern Michigan University Adviser: Dr. Kalliopi Iordanidou July 31, 2015 Abstract This study focuses on the development of data driven background estimation techniques to discriminate the backgrounds for VV resonance searches in the final state of lepton, neutrino, and two quarks. The background is composed of W +Jets, Z+Jets, t t, multijets, and standard model dibosons. Comparisons are made between data and Monte Carlo for an integrated luminosity of 6 pb 1 collected at 13 TeV center of mass energy. No discrepancy between data and Monte Carlo was found during this study. 1

2 Contents 1 Introduction CERN Large Hadron Collider ATLAS Particles Bosons and Jets Search for VV Resonances Studying the Background Introduction Background Composition Estimation Methods Control Regions Multijets W +Jets and t t Conclusion 21 4 Acknowledgments 22 References 23 2

3 1 Introduction 1.1 CERN The European Organization for Nuclear Research (CERN) founded in 1954 has been leading the study of high energy particle physics for years with experiments run by collaborations of scientists from institutes all over the world. Home to some of the world s most intricate scientific instruments including the Large Hadron Collider (LHC), CERN has accomplished many inconceivable feats from the creation of the World Wide Web to making Noble Prize winning discoveries. With the recent start of the 13 TeV experiments, CERN s main focus has been pushed towards the LHC; however, many other important experiments are still proceeding both on and off site [6]. 1.2 Large Hadron Collider Figure 1: Map of LHC ring to highlight the size and experiment detector locations [9]. The Large Hadron Collider (LHC) is currently the largest and most powerful particle accelerator in the world. First starting operation in 2010, the LHC uses superconducting magnets with accelerating structures to boost the energy of particles around a 27 kilometer underground ring. Traveling 3

4 close to the speed of light, two high energy particle beams move through the accelerator in opposite directions and separate beam pipes until they collide. The superconducting electromagnets chilled to C create a strong magnetic field to guide the beams around the ring [6]. Seven experiments at CERN, each of which is distinct and characterized by its detectors, make use of the LHC s detectors to analyze the collisions within these detectors caused by the accelerator. Four detectors, those of the larger experiments, are located in huge caverns that are underground along the LHC s ring [8]. 1.3 ATLAS Figure 2: ATLAS detector with cut out side and humans for size reference [5]. One of the biggest experiments at CERN is ATLAS. ATLAS, which stands for A Toroidal LHC ApparatuS, is a particle physics experiment that explores head-on collisions of protons at high energies in an attempt to make new discoveries such as new forces, the origin of mass, dark matter, and many others. Due to the recent discovery of the Higgs boson, data being collected now allows for a deeper examination of the boson s properties and the search for exotic physics or physics beyond the standard model. Beams of particles traveling around the LHC collide at the center of 4

5 the ATLAS detector forming new particles that expand from the point of collision in all directions. Around the point of collision are multiple different layers of detection. The layer closest to the particle beam is known as the inner detector and is used to track charged particles. The next layer is the calorimeters which measure the energy of easily stopped particles by absorbing it. Then there are the muon spectrometers which are used to accurately measure the momentum of the muons. In addition to these main detector layers, there is the magnet system which consists of two large superconducting magnet systems that bends charged particles allowing for their momenta to be measured. The first magnet, the solenoid magnet, is located around the inner detector layer and the second magnet, the toroidal magnet is located at the endcaps. After the collision, the different layers of detection record data including paths, momentum, and energy of the particles which then allows for each of the new particles to be identified. Since the ATLAS detector can experience a huge flow of events, it uses a trigger system. This allows for only certain events to be recorded and all other events are ignored [2]. 1.4 Particles Figure 3: Multiple groupings of particles [13]. The experiments at CERN are exploring particle physics which refers to the study of the most basic elements of matter. Particles can be classified into different groups, shown in Figure 3, including the fermions and the hadrons. These groups contain the basic particles that are the cornerstone for the ATLAS experiment. The fermion group is comprised of the lightest particles called the leptons. There are three generations of leptons shown in Figure 4 that are the pairs of electron - neutrino electron, muon - muon neutrino, tau - tau 5

6 neutrino. All of the leptons have a spin of 1/2. The electron, muon, and tau have a charge of -1 where as their associated neutrinos have a charge of 0. The neutrinos cannot be identified or recorded from the detector when taking data. Instead, they can be determined by looking at the energy. Since the total energy is known, it is able to distinguish the neutrinos from the missing energy [10]. Figure 4: Table of leptons with flavors, mass, and charge [7]. The hadron group consists of the heavier particles. This group is split into two more subgroups called the baryons and mesons. Within the baryon group are the protons and neutrons. Protons and neutrons, held together by the strong force, make up the nucleus of an atom [11]. These particles are composed of quarks. Quarks come in six different flavors shown in Figure 5, and they can successfully account for all known mesons and baryons (over 200) [12]. 6

7 Figure 5: Table of quarks with flavors, approximate mass, and charge [7]. For most kinds of particles there is also an antiparticle. The antiparticle is the particle with the same mass but an opposite charge including the electric charge, for instance, the antiparticle of an electron is a positively charged electron or a positron. Figure 6 below shows that how each of these particles interacts with different parts of the ATLAS detector. As can be seen, some particles, such as muons and neutrinos are able to pass through layers of the detector where as particles like the proton, neutron, photon, and electron are absorbed in the different layers of the detector. 7

8 Figure 6: Interaction of different particles with different layers of the ATLAS detector [5]. 1.5 Bosons and Jets Another interesting category of physics objects are the bosons. Within the boson group are the W and Z bosons which decay to particles and were first discovered at CERN. The W bosons can have either a +1 or a -1 charge, and it decays to a lepton and neutrino or decays hadronically to quarks. The Z boson has a 0 charge, and it decays to two leptons of the same flavor and opposite charge or decays hadronically to quarks. In addition to the bosons, the jets are another intriguing group of physics objects. The ATLAS group studies multiple different kinds of jets of many different cone sizes including regular jets which have a distance parameter 0.4 and the large-r jets which have a distance parameter of 1. Jets are sprays of particles seen in Figure 7 as the blue cone shapes. When dealing with boosted topology, multiple jets come together to form one large cone 8

9 shape which is known as a large-r jet. This can be seen in Figure 8 below. The large-r jets that studied by the ATLAS group can be created from when the W or Z boson decay hadronically. Figure 7: Image from the ATLAS detector that highlights all parts of the detector that had interaction during the collision [5]. Figure 8: Boosted topology forming large-r Jets from regular jets [1]. 9

10 1.6 Search for VV Resonances Our work with the ATLAS group at CERN is contributing to the search for Heavy Vector Triplet (HVT) decays. The search for HVT decays is said to be one of the most direct ways to find new physics at TeV scale. We are looking at VV resonance in our data samples. These resonances are W and Z bosons. Using specific selections and cuts in our code, we are able to detect the different kinds of bosons and determine which events are VV resonances [14]. Figure 9: Feynman diagram of VV Resonance and W and W /Z decay [5]. 2 Studying the Background 2.1 Introduction Here at CERN, my work focuses on VV resonance in the boosted topology where one of the V is a W which then decays to a lepton and neutrino and the other V decays hadronically forming a large-r jet. Our signal hypothesis is a heavy vector triplet. I have been studying the background of our data samples in a data driven way. Specifically, I am looking at W +Jets, multijets, t t, and standard model dibosons which are not studied in a data driven way by us. 2.2 Background Composition The samples I am using for my work are normalized to 6 pb 1 at 13 TeV. The samples and their generators can be seen in Table 1 below. 10

11 Events Cross Sections Filter Efficiency Generators Signal pb MadGraphPythia8 W + eν pb PowhegPythia8 W + µν pb PowhegPythia8 W + τν pb PowhegPythia8 W eν pb PowhegPythia8 W µν pb PowhegPythia8 W τν pb PowhegPythia8 Z ee pb PowhegPythia8 Z µµ pb PowhegPythia8 Z ττ pb PowhegPythia8 t t pb PowhegPythia Table 1: All of the data files and their generators are listed here as well as the number of initial events, cross sections, and filter efficiency which were used to scale the samples. Not listed is the data file. The data file has a luminosity of 6 pb Estimation Methods Since the standard model dibosons are very small and precision measurements are carried out by dedicated ATLAS groups, they are well known and understood backgrounds, and they are estimated from the Monte Carlo. The other estimation methods depend on the selection cuts that are made. For our finalized selection cuts I have a baseline selection as follows: jet p T greater than 200 GeV tight electron and medium muon which vetos events with more than one good lepton L1 trigger which is the lowest level trigger that is part of the trigger system that the detector uses to veto certain events MET greater than 100 GeV lepton and neutrino p T greater than 100 GeV at least one large-r jet with a p T greater than 200 GeV jet mass between 60 and 105 GeV 11

12 overlap removal between the electrons and the large-r jets overlap removal between the regular jets and the large-r jets Our mass for the jets is cut between 60 and 105 GeV in order for us to focus on the signal region. The mass between 60 and 110 GeV is where the W and Z masses are. Specifically, the W has a mass of GeV and the Z has a mass of GeV. Table 2 seen below shows the normalized event yields of our samples within the signal region. The data and total background are very much in agreement for this region. Normalized Event Yields Data 21 Signal ± W +Jets ± Z+Jets ± t t ± 0.14 Total Background ± 0.14 Table 2: These are the normalized event yields for each of the different parts of the background as well as the data for the signal region. The quoted uncertainties are statistical only. For the plots shown in Figure 10 through Figure 15, they show various distributions between the data, W +Jets, and t t for the signal region cuts to display what the topology and kinematics region looks like. Figure 10: Large-R jet p T for signal region. 12

13 Figure 11: Large-R jet η for signal region. Figure 12: Number of large-r jets for signal region. Figure 13: Large-R jet Mass for signal region. 13

14 Figure 14: Φ for lepton and large-r jets for signal region. Figure 15: Φ for neutrino and large-r jets for signal region. Figure 16 through 20 below show various distributions between the data, signal, and all backgrounds for the signal region. 14

15 Figure 16: Large-R jet mass for signal region. Figure 17: Lepton-Neutrino mass for signal region. Figure 18: Lepton-Neutrino and large-r Jet mass for signal region. 15

16 Figure 19: Number of Jets for signal region. Figure 20: MET for signal region. 2.4 Control Regions I have also looked at regions other than the signal region. This is known as the control region and is used for the background measurement. To make this control region, the signal region cuts are reverted to minimize the signal contamination. With the reverted cut for the jet mass, it is now required that the jet mass is to be either less than 60 GeV or greater than 105 GeV. With this, I intend to measure W /Z+Jets, t t, and multijets. Table 3 seen below shows the shows normalized event yields of the samples within the control region. In this table, the total background expects less events than what was found in the data sample. Figure 25 shows the MET for the control region and it can be seen that the low end of the MET is where this 16

17 small discrepancy between the data and total background is coming from. Normalized Event Yields Data 87 Signal ± W +Jets ± Z+Jets ± t t ± 0.18 Total Background ± 0.18 Table 3: These are the normalized event yields for each of the different parts of the background as well as the data for the control region. The quoted uncertainties are statistical only. Figure 21 through 25 below show various distributions for the control region. Figure 21: Large-R jet mass outside the signal region. 17

18 Figure 22: Lepton-Neutrino mass outside the signal region. Figure 23: Lepton-Neutrino and large-r Jet mass outside the signal region. Figure 24: Number of Jets outside the signal region. 18

19 Figure 25: MET outside the signal region Multijets The multijets are misidentified electrons and muons coming from semileptonic decay of hadrons within jets. The cut of the MET between 50 and 80 GeV is used in the plot shown in Figure 28 to validate the multijets. The case where there is more than 1 lepton in the events was checked as a possible control region; however, there were too low statistics. Figure 28: Lepton and Neutrino mass with MET between 50 and 80 GeV cut. When the selection for the MET between 50 and 80 GeV was made, no data excess was observed. This is from the low multijets contamination probably due to the tight electron and muon cuts. 19

20 In Run I, the analysis of VV resonance was performed using an integrated luminosity of 20.3 fb 1 collisions at 8 TeV collected by the ATLAS detector. There were 29 events and this means we are normalizing to 6 pb 1 at 13 TeV and with cross sections that gives us 6 ± 3 events [4] W +Jets and t t Then I had to distinguish between the W +Jets and t t. The W +Jets and the t t both have a W peak shape, so to distinguish between the W +Jets and the t t b tagging is used. B tagging is the identification of b jets which are jets containing b-hadrons. I used the b-veto with the good jet selection, overlap removal for regular and fat jets, and MV2c20 tagger greater than , to distinguish if an event had b jets. The MV2c20 cut corresponds to 80 percent efficiency, and it is an algorithm that determines if an event contains b-jets or not. If the event passed the b-veto than it is recognized it as the W +Jets where as if there is presence of b s, it is recognized as the t t [3]. Figures 26 and 27 below show the application of the b-veto and how it effects the number of jets. Figure 26: Number of Jets with the presence of b s. 20

21 Figure 27: Number of Jets with the no presence of b s. Table 4 below shows the data and Monte Carlo comparison for the b and no b cases. Normalized Event Yields Normalized Event Yields with b s without b s Data Signal ± ± W +Jets ± ± Z+Jets ± ± t t ± ± 0.11 Total Background ± ± 0.11 Table 4: These are the normalized event yields for each of the different parts of the background as well as the data for the case of presence of b jets and no b jets. The quoted uncertainties are statistical only. 3 Conclusion Data driven background techniques were developed for the study of VV resonances that result in the final state of a lepton, neutrino and two quarks. Specifically the W /Z+Jets and t t backgrounds are measured in control regions formed by reverting the signal region cuts. Additionally b-tagging is used to discriminate between the different background types. For the stan- 21

22 dard model dibosons, the estimation is based on the Monte Carlo. The multijets Run I extrapolation shows marginal quantum chromodynamics (QCD) contamination. Comparison has been made between data and Monte Carlo for 6 pb 1 at 13 TeV and no discrepancy was observed. 4 Acknowledgments I would like to thank the Columbia REU program and the National Science Foundation for providing students with opportunities like this. Also, I would like to thank Professor John Parsons for granting me this experience here at CERN. Most of all I would like to thank my adviser Dr. Kalliopi Iordanidou for helping me through every aspect of this research with patience, kindness, and understanding. 22

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24 [12] Nave, R. Quarks. Quarks. HyperPhysics, n.d. Web. 29 June [13] The Quantum Evolution of the Early Universe. Relevancy22. N.p., 1 Apr Web. 23 June [14] Santanastasio, Francesco. Search for Heavy Resonances Decaying to Bosons with the ATLAS and CMS Detectors. Sapienza, University of Rome and INFN), 4 July Web. 24