Search for Higgsinos in Final States with a Low-Momentum, Displaced Track
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Search for Higgsinos in final states with a low-momentum, displaced track von Moritz Wolf geboren am 04. März 1994 Masterarbeit im Studiengang Physik Universität Hamburg Institut für Experimentalphysik 2020 1. Gutachter: Prof. Dr. Peter Schleper 2. Gutachter: Jun.-Prof. Dr. Gregor Kasieczka Abstract Higgsinos are a class of supersymmetric particles that are of particular inter- est to searches at the LHC. They are featured in many SUSY models with masses on the order of the electroweak scale. A number of LHC searches tar- geting charged or neutral Higgsino decays in disappearing track and di-lepton searches, respectively, have set exclusion limits on the SUSY model parameters. However, a region in the parameter space with Higgsino mass differences be- tween 0:3 and 1:0 GeV remains constrained only by LEP results. This domain is of special interest from a phenomenological point of view as it is realized in natural SUSY scenarios. This thesis presents an analysis that establishes sensitivity to that region. In the examined signal models, the lightest chargino has a decay length in the detector of up to a few millimeters and its decay predominantly gives rise to a single, low-momentum pion. A soft and displaced track created by the pion is the crucial part of this analysis as it is used to enhance the signal sensitivity of a monojet-like analysis. Data corresponding to an integrated luminosity of 35:9 fb−1 collected by the CMS experiment in p proton-proton collisions at s = 13 TeV are analyzed. The observed event yields are consistent with the expected numbers of background events and ex- clusion limits are set in the plane of the chargino mass and mass difference of the Higgsino spectrum. For models with a mass splitting between the lightest chargino and the lightest neutralino of 0:8 GeV, charginos with masses up to 120 GeV are excluded. i Zusammenfassung Higgsinos sind supersymmetrische Elementarteilchen, die für Suchen nach neu- en Teilchen am LHC von besonderem Interesse sind. In vielen SUSY-Model- len werden für Higgsinos Massen nahe der elektroschwachen Skala vorherge- sagt. Zahlreiche LHC-Analysen konnten mithilfe von disappearing-track- und Di-Lepton-Suchen weite Teile des SUSY-Parameterraums ausschließen. Eine Region im Phasenraum mit Massendifferenzen der Higgsinos zwischen 0;3 und 1;0 GeV ist bis jetzt jedoch nur durch Resultate von LEP begrenzt. Aus phäno- menologischer Sicht ist dieser Bereich besonders interessant, da er Teil natür- licher SUSY-Szenarien ist. In dieser Arbeit wird eine Analyse vorgestellt, die in diesem Bereich sensitiv ist. Die untersuchten Signal-Modelle zeichnen sich dadurch aus, dass das leichteste Chargino eine mittlere Zerfallslänge von bis zu einigen Millimetern hat und der Zerfall in den meisten Fällen ein einzelnes Pi- on mit wenig Impuls hervorbringt. Wesentlich für diese Analyse ist eine stark gekrümmte und leicht versetzte Spur, die das Pion im Detektor hinterlässt. Sie wird benutzt, um eine Monojet-artige Analyse so zu erweitern, dass sie sensitiv auf den Signal-Prozess wird. Es werden Daten vom CMS-Experiment analysiert, die in Proton-Proton-Kollisionen bei einer Schwerpunktsenergie von p s = 13 TeV aufgezeichnet wurden und die einer integrierten Luminosität von 35;9 fb−1 entsprechen. Die beobachtete Anzahl von Ereignissen in dieser Analy- se deckt sich mit der erwarteten Anzahl von Untergrundereignissen. Es werden Ausschlussgrenzen in der Ebene der Chargino-Masse und der Massendifferenz des Higgsino-Spektrums festgelegt. Auf diese Weise können für Modelle mit einer Massendifferenz zwischen dem leichtesten Chargino und dem leichtesten Neutralino von 0;8 GeV Charginos mit Massen bis zu 120 GeV ausgeschlossen werden. ii Contents 1 Introduction1 2 Theoretical Background5 2.1 The Standard Model of Particle Physics . .5 2.1.1 Particle Content . .5 2.1.2 Mathematical Description . .8 2.1.3 Hints of Physics Beyond the SM . 14 2.2 Supersymmetry . 14 2.2.1 Natural SUSY and Light Higgsinos . 16 3 The CMS Experiment 19 3.1 The Large Hadron Collider . 19 3.1.1 Overview . 19 3.1.2 Proton-Proton Collisions . 21 3.2 The CMS Detector . 23 3.2.1 Coordinate System . 24 3.2.2 Tracking System and Magnet . 24 3.2.3 Calorimeters . 26 3.2.4 Muon System . 27 3.2.5 Trigger . 28 3.3 Particle Identification and Event Reconstruction . 29 3.3.1 Track Reconstruction . 29 3.3.2 Particle Flow Algorithm . 32 4 Analysis Strategy 35 4.1 Datasets . 37 iii Contents 4.2 Event and Track Observables . 38 5 Soft and Displaced Tracks 41 5.1 Helix Extrapolation . 41 5.2 Track-matching to Generated Particles . 42 5.3 Multivariate Classifier . 45 6 Analysis 53 6.1 Event Reconstruction and Selection . 53 6.2 Soft and Displaced Track Requirement . 55 6.3 Background Estimation . 63 6.4 Validation . 67 6.5 Predicted Event Yields and Uncertainties . 70 6.6 Observed Event Yields and Exclusion Limit . 72 7 Conclusion and Outlook 75 Appendix 79 List of Figures 81 List of Tables 83 Bibliography 85 iv 1 Introduction One of the strongest motivations to search for physics beyond the Standard Model (SM) is our current lack of a particle-based explanation for Dark Matter (DM). Mod- els incorporating supersymmetry (SUSY) extend the SM and predict the existence of new elementary particles that are linked to the already known particles. The lightest of those supersymmetric particles is, in numerous models, a viable DM candidate. A particularly interesting class of postulated particles are Higgsinos, supersymmetric partner particles of the SM Higgs boson. Higgsinos generally mix with gauginos to form mass eigenstates but in many cases suitable to explain DM, the lightest mass eigenstates are dominated by the Higgsino component. Many so-called natural SUSY models predict those Higgsinos to be relatively light, i.e. on the order of O (100 GeV). Higgsinos of such mass would be kinematically accessible to experiments at the LHC; however, their detection may prove challenging. The values of the masses of SUSY particles (the spectrum) in natural SUSY models 0 are typically such that two neutral and one charged Higgsino mass eigenstates, χe1, 0 ± χe2 and χe1 , are nearly degenerate but feature slight mass differences such that the lightest and heaviest of the three are electrically neutral, and the charged state takes an intermediate mass value. Therefore, when a charged Higgsino or heavier neutral Higgsino is produced, it decays to the lightest neutralino, which itself is invisible to the detector, along with additional low-momentum SM decay products that are possibly visible. To search for Higgsinos, the strategy strongly depends on the size 0 0 0 ± ± 0 0 of the mass differences, ∆m ≡ ∆m χe2; χe1 and ∆m ≡ ∆m χe1 ; χe1 . If ∆m is large enough, say ∆m0 > O (1 GeV), the decay often yields two leptons that can be reconstructed and used as a signal signature [1, 2]. In scenarios with more extreme 1 1 Introduction ± ± degeneracy, ∆m is very small, ∆m . 0:35 GeV, and the chargino becomes semi- stable and leaves a track in the detector before decaying. Disappearing tracks can hence be used to probe the SUSY parameter space with a very compressed mass spectrum [3, 4]. Figure 1.1 shows the current exclusion limits achieved with these ± 0 ± two search strategies in the plane of ∆m χe1 ; χe1 versus m χe1 . Also shown is the limit from direct searches conducted at the LHC’s predecessor, LEP. What is striking is that the parameter space with mass splittings between approximately 0:3 and 1 GeV remains constrained only by LEP results, as it is neither accessible to di-lepton searches at the LHC nor to disappearing track searches conducted so far. Figure 1.1: Current Higgsino exclusion limits obtained by ATLAS soft di-lepton and disappearing track searches and LEP limits. The region with mass split- tings between 0:3 and 1 GeV has not yet been probed at the LHC. [5] In this thesis, a strategy to search for Higgsinos with such a small mass splitting is presented, which targets the region that, so far, is lacking sensitivity. In this regime, the chargino decays predominantly, via on off-shell W boson, to a single pion [6]. The pion’s transverse momentum is of the order of the mass splitting, thus very soft, but often still reconstructable as a simple track. Notably, the chargino has a lifetime that leads to a decay length of up to a few millimeters and the pion is produced at a 2 vertex that is slightly displaced with respect to the primary interaction vertex. The soft and displaced track associated with the pion, as well as its kinematics, are used as handles to distinguish such events from SM background events. Figure 1.2 shows a Feynman diagram of the considered process with an additional jet from initial state radiation which enhances the sensitivity to the signal. Figure 1.2: Feynman diagram of a typical signal process. For mass splittings ± ∆m . 1 GeV the chargino decay is expected to yield a single pion. The LSP that is produced along with the chargino can be exchanged with another chargino or the second-lightest neutralino. [7] This thesis begins with a description of the theoretical background in Chapter 2. The experimental setup, namely, the CMS detector at the LHC, is described in Chapter 3. Chapter 4 summarizes the analysis strategy, while Chapter 5 takes a closer look at low-momentum displaced tracks. An analysis of data collected by CMS in 2016 is described in Chapter 6. Chapter 7 features the conclusions and an outlook. 3 2 Theoretical Background 2.1 The Standard Model of Particle Physics The Standard Model of particle physics (SM) describes all known elementary par- ticles, including those that carry three of the four known fundamental forces.