CMS-B2G-16-025

CMS-B2G-16-025 ; CERN-EP-2017-272
Search for pair production of excited top quarks in the lepton+jets final state
CMS Collaboration
29 November 2017
Phys. Lett. B 778 (2018) 349
Abstract: A search is performed for the pair production of spin-3/2 excited top quarks, each decaying to a top quark and a gluon. The search uses the data collected with the CMS detector from proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb$^{-1}$. Events are selected by requiring an isolated muon or electron, an imbalance in the transverse momentum, and at least six jets of which exactly two must be compatible with originating from the fragmentation of a bottom quark. No significant excess over the standard model predictions is found. A lower limit of 1.2 TeV is set at 95% confidence level on the mass of the spin-3/2 excited top quark in an extension of the Randall-Sundrum model, assuming a 100% branching fraction of its decay into a top quark and a gluon. These are the best limits to date in a search for excited top quarks and the first at 13 TeV.
Links: e-print arXiv:1711.10949 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Kinematic distributions of selected events with a single lepton and six or more jets of which exactly two are b tagged. Data events (points), simulated background processes (stacked histograms), and a simulated 800 GeV signal process (dashed line) are shown. Events selected in the $\mu $+jet final state are shown on the left while those in the e+jet final state are shown on the right. From upper to lower, the kinematic variables displayed are the lepton $ {p_{\mathrm {T}}} $, the jet $ {p_{\mathrm {T}}} $ and the $m_{\rm t+jet}$. The shaded region is the total uncertainty of the simulated background processes, which includes statistical and systematic uncertainties.
png pdf	Figure 1-a: Lepton $ {p_{\mathrm {T}}} $ distribution of selected events with a single muon and six or more jets of which exactly two are b tagged. Data events (points), simulated background processes (stacked histograms), and a simulated 800 GeV signal process (dashed line) are shown. The shaded region is the total uncertainty of the simulated background processes, which includes statistical and systematic uncertainties.
png pdf	Figure 1-b: Lepton $ {p_{\mathrm {T}}} $ distribution of selected events with a single electron and six or more jets of which exactly two are b tagged. Data events (points), simulated background processes (stacked histograms), and a simulated 800 GeV signal process (dashed line) are shown. The shaded region is the total uncertainty of the simulated background processes, which includes statistical and systematic uncertainties.
png pdf	Figure 1-c: Jet $ {p_{\mathrm {T}}} $ distribution of selected events with a single muon and six or more jets of which exactly two are b tagged. Data events (points), simulated background processes (stacked histograms), and a simulated 800 GeV signal process (dashed line) are shown. The shaded region is the total uncertainty of the simulated background processes, which includes statistical and systematic uncertainties.
png pdf	Figure 1-d: Jet $ {p_{\mathrm {T}}} $ distribution of selected events with a single electron and six or more jets of which exactly two are b tagged. Data events (points), simulated background processes (stacked histograms), and a simulated 800 GeV signal process (dashed line) are shown. The shaded region is the total uncertainty of the simulated background processes, which includes statistical and systematic uncertainties.
png pdf	Figure 1-e: $m_{\rm t+jet}$ distribution of selected events with a single muon and six or more jets of which exactly two are b tagged. Data events (points), simulated background processes (stacked histograms), and a simulated 800 GeV signal process (dashed line) are shown. The shaded region is the total uncertainty of the simulated background processes, which includes statistical and systematic uncertainties.
png pdf	Figure 1-f: $m_{\rm t+jet}$ distribution of selected events with a single electron and six or more jets of which exactly two are b tagged. Data events (points), simulated background processes (stacked histograms), and a simulated 800 GeV signal process (dashed line) are shown. The shaded region is the total uncertainty of the simulated background processes, which includes statistical and systematic uncertainties.
png pdf	Figure 2: The $m_{\rm t+jet}$ spectrum in the for data (points), the signal+background fit (green), the background component of the signal+background fit (blue), and the expected spectrum for a simulated 800 GeV signal process (red dashed) normalized to the integrated luminosity of data. Since there is no significant excess of signal found in data, the signal+background curve overlaps the background-only component. The distributions for the $\mu $+jets data are shown on the left while those for e+jets data are shown on the right. The probabilities of the Kolmogorov-Smirnov test between the data versus the signal+background model and between the data versus the background component are denoted by $K_{\text {all}}$ and $K_{\text {bkg}}$, respectively.
png pdf	Figure 2-a: The $m_{\rm t+jet}$ spectrum in the $\mu $+jet final state for data (points), the signal+background fit (green), the background component of the signal+background fit (blue), and the expected spectrum for a simulated 800 GeV signal process (red dashed) normalized to the integrated luminosity of data. Since there is no significant excess of signal found in data, the signal+background curve overlaps the background-only component. The probabilities of the Kolmogorov-Smirnov test between the data versus the signal+background model and between the data versus the background component are denoted by $K_{\text {all}}$ and $K_{\text {bkg}}$, respectively.
png pdf	Figure 2-b: The $m_{\rm t+jet}$ spectrum in the e+jet final state for data (points), the signal+background fit (green), the background component of the signal+background fit (blue), and the expected spectrum for a simulated 800 GeV signal process (red dashed) normalized to the integrated luminosity of data. Since there is no significant excess of signal found in data, the signal+background curve overlaps the background-only component. The probabilities of the Kolmogorov-Smirnov test between the data versus the signal+background model and between the data versus the background component are denoted by $K_{\text {all}}$ and $K_{\text {bkg}}$, respectively.
png pdf	Figure 3: The expected and observed 95% confidence level upper limits for the product of the production cross section of $ {{{\mathrm{t} {}^{}}} {{\mathrm{\bar{t}} {}^{}}}}$ and the square of the branching fraction, as a function of the $ {{\mathrm{t} {}^{}}} $ mass, for the combined lepton+jets analysis. The theoretical production cross section assuming a 100% $ {{\mathrm{t} {}^{}}} \to \mathrm{t} \mathrm{g} $ branching fraction is shown along with its uncertainties, described in Section xxxxx.

Tables
png pdf	Table 1: Expected numbers of selected events for the simulated signal process as a function of $ m_{{{\mathrm{t} {}^{*}}}}$. Also shown are the expected numbers of events predicted by the SM, together with the systematic uncertainties discussed in Section xxxxx and the uncertainties in the cross sections of the various processes, as well as the numbers of selected events observed in data.
png pdf	Table 2: Sources of systematic uncertainties and the methods used to evaluate their effect on the simulated signal sample. (s.d.: standard deviation, SF: correction scale factor)

Summary

A search has been conducted for pair production of spin-3/2 excited top quarks ${{\mathrm{t}{}^{*}}} $ in proton-proton interactions, with each $ {{\mathrm{t}{}^{*}}} $ decaying exclusively to a standard model top quark and a gluon. Events that have a single muon or electron and at least six jets, exactly two of which must be identified as originating from a bottom quark, are selected for the analysis. Assuming $ {{{\mathrm{t}{}^{*}}} {{\mathrm{\bar{t}}{}^{*}}} } $ production, the final-state objects are associated with the $ {{\mathrm{t}{}^{*}}} $ candidates in each event. No significant deviations from standard model predictions are observed in the $\mathrm{t}$+jet system, and an upper limit is set at 95% confidence level on the pair production cross section of $ {{{\mathrm{t}{}^{*}}} {{\mathrm{\bar{t}}{}^{*}}} } $, as a function of the $ {{\mathrm{t}{}^{*}}} $ mass. Interpreting the results in the framework of a spin-3/2 $ {{\mathrm{t}{}^{*}}} $ model, assuming a 100% branching fraction of its decay into a top quark and a gluon, $ {{\mathrm{t}{}^{*}}} $ masses below 1.2 TeV are excluded. These are the best limits to date on the mass of spin-3/2 excited top quarks and the first at 13 TeV.

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