CMSPASB2G16011  
Search for vectorlike quark pair production in final states with leptons and boosted Higgs bosons at $\sqrt{s}= $ 13 TeV  
CMS Collaboration  
September 2016  
Abstract: We present a search for pairproduced vectorlike top partners (``T quark'') using data from pp collisions at a centerofmass energy of $\sqrt{s}= $ 13 TeV that were recorded with the CMS detector in the 2015 datataking period. The search is carried out in the lepton+jets channel and is most sensitive for final states in which at least one T quark decays to a top quark and a Higgs boson. Since the final decay products tend to be collimated in the detector, jetsubstructure techniques are used to identify boosted Higgs boson decays to $\mathrm{ b\overline{b} }$. No deviation from the Standard Model (SM) background prediction is observed in the data and upper 95% confidence level (CL) exclusion limits on the $\mathrm{T\bar{T}}$ production cross section are calculated for multiple branching fraction scenarios of the T quark. Assuming a branching fraction of 100% for the $\mathrm{T}\rightarrow \mathrm{tH}$ decay, T quark masses below 860 GeV (870 GeV expected) can be excluded.  
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; 
Figures  
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Figure 1:
Representative Feynman diagram for production of a $\mathrm {T\bar{T}}$pair with one T quark decaying to tH. 
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Figure 2:
Left: Distribution of the number of btagged subjets of the ${p_{\mathrm {T}}} $leading Higgs candidate jet ($ {p_{\mathrm {T}}} > $ 300 GeV, $M_{\text{jet}} \in $ [60, 160 GeV]) without the subjet btag requirement. Right: Distribution of $ M_{text{jet}} $ of the ${p_{\mathrm {T}}} $leading Higgs candidate jet ($ {p_{\mathrm {T}}} > $ 300 GeV) with at least two subjet btags before applying the mass cut. Both distributions are shown after the event selection described in Sec. 5.1 and all corrections described in Sec. 6 are applied to the MC. For the signal, the $ M_{\mathrm{T}} = $ 1200 GeV mass point is shown and the distributions are normalized to the corresponding theory cross section times the number behind the legend entry. A branching fraction of 33% to all three decay channels is assumed. The blue curve contains $\mathrm {T\bar{T}}$ events with at least one Higgs boson in the decay chain, the red curve shows $\mathrm {T\bar{T}}$ events where this is not the case. 
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Figure 2a:
Distribution of the number of btagged subjets of the ${p_{\mathrm {T}}} $leading Higgs candidate jet ($ {p_{\mathrm {T}}} > $ 300 GeV, $M_{\text{jet}} \in $ [60, 160 GeV]) without the subjet btag requirement. The distribution is shown after the event selection described in Sec. 5.1 and all corrections described in Sec. 6 are applied to the MC. For the signal, the $ M_{\mathrm{T}} = $ 1200 GeV mass point is shown and the distribution is normalized to the corresponding theory cross section times the number behind the legend entry. A branching fraction of 33% to all three decay channels is assumed. The blue curve contains $\mathrm {T\bar{T}}$ events with at least one Higgs boson in the decay chain, the red curve shows $\mathrm {T\bar{T}}$ events where this is not the case. 
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Figure 2b:
Distribution of $ M_{text{jet}} $ of the ${p_{\mathrm {T}}} $leading Higgs candidate jet ($ {p_{\mathrm {T}}} > $ 300 GeV) with at least two subjet btags before applying the mass cut. The distribution is shown after the event selection described in Sec. 5.1 and all corrections described in Sec. 6 are applied to the MC. For the signal, the $ M_{\mathrm{T}} = $ 1200 GeV mass point is shown and the distribution is normalized to the corresponding theory cross section times the number behind the legend entry. A branching fraction of 33% to all three decay channels is assumed. The blue curve contains $\mathrm {T\bar{T}}$ events with at least one Higgs boson in the decay chain, the red curve shows $\mathrm {T\bar{T}}$ events where this is not the case. 
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Figure 3:
Postfit $S_{\mathrm {T}}$ distributions in the ${\mathrm{ t } \mathrm{ \bar{t} } }$ (left) and W+jets (right) control regions after applying all corrections and performing the maximumlikelihood fit described in the main text. The $\mathrm {T\bar{T}}$ signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. 
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Figure 3a:
Postfit $S_{\mathrm {T}}$ distribution in the ${\mathrm{ t } \mathrm{ \bar{t} } }$ control region after applying all corrections and performing the maximumlikelihood fit described in the main text. The $\mathrm {T\bar{T}}$ signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. 
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Figure 3b:
Postfit $S_{\mathrm {T}}$ distribution in the ${\mathrm{ t } \mathrm{ \bar{t} } }$ W+jets control region after applying all corrections and performing the maximumlikelihood fit described in the main text. The $\mathrm {T\bar{T}}$ signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. 
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Figure 4:
Postfit distributions of $S_{\mathrm {T}}$ in the 0H (top left), H1b (top right) and H2b (bottom) category after performing the maximumlikelihood fit described in Sec. 6. Signal samples are normalized to a cross section of 5 pb in the 0H and H1b categories and 1 pb in the H2b category. The $ \mathrm {T\bar{T}} $signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. 
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Figure 4a:
Postfit distribution of $S_{\mathrm {T}}$ in the 0H category after performing the maximumlikelihood fit described in Sec. 6. Signal samples are normalized to a cross section of 5 pb in the 0H and H1b categories and 1 pb in the H2b category. The $ \mathrm {T\bar{T}} $signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. 
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Figure 4b:
Postfit distribution of $S_{\mathrm {T}}$ in the H1b category after performing the maximumlikelihood fit described in Sec. 6. Signal samples are normalized to a cross section of 5 pb in the 0H and H1b categories and 1 pb in the H2b category. The $ \mathrm {T\bar{T}} $signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. 
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Figure 4c:
Postfit distribution of $S_{\mathrm {T}}$ in the H2b category after performing the maximumlikelihood fit described in Sec. 6. Signal samples are normalized to a cross section of 5 pb in the 0H and H1b categories and 1 pb in the H2b category. The $ \mathrm {T\bar{T}} $signal is normalized to the theory cross section times the number behind the legend entry and a branching fraction of 100% of $ \mathrm{ T \rightarrow tH } $ is assumed. 
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Figure 5:
Exclusion limits on the total crosssection of pairproduced T's with a branching fraction of 100% to tH. The theory cross section (dashed line) is computed at nexttonexttoleading order. 
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Figure 6:
Expected (left) and observed (right) upper mass limits in GeV for different combinations of $ \mathrm{ T \rightarrow tH } $ and $ \mathrm{ T \rightarrow tZ } $ branching fractions. The branching fraction $ \mathrm{ T \rightarrow bW } $ is, for each point in the triangle, 1 $$ BR($ \mathrm{ T \rightarrow tH } $) $$ BR($ \mathrm{ T \rightarrow tZ } $). 
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Figure 6a:
Expected upper mass limit in GeV for different combinations of $ \mathrm{ T \rightarrow tH } $ and $ \mathrm{ T \rightarrow tZ } $ branching fractions. The branching fraction $ \mathrm{ T \rightarrow bW } $ is, for each point in the triangle, 1 $$ BR($ \mathrm{ T \rightarrow tH } $) $$ BR($ \mathrm{ T \rightarrow tZ } $). 
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Figure 6b:
Observed upper mass limit in GeV for different combinations of $ \mathrm{ T \rightarrow tH } $ and $ \mathrm{ T \rightarrow tZ } $ branching fractions. The branching fraction $ \mathrm{ T \rightarrow bW } $ is, for each point in the triangle, 1 $$ BR($ \mathrm{ T \rightarrow tH } $) $$ BR($ \mathrm{ T \rightarrow tZ } $). 
Tables  
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Table 1:
Signal efficiencies in the three event categories for two example mass points, split into the six possible final states. Uncertainties include both systematic and statistical uncertainties on the MC samples. 
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Table 2:
Summary of all systematic uncertainties, their sizes, types and to which processes they apply. 
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Table 3:
Event counts in the three final categories after performing the maximumlikelihood fit described in Sec. 6. Uncertainties comprise both statistical and systematic uncertainties. For the $\mathrm {T\bar{T}}$signal, the theoretically predicted production cross section with a branching fraction of 100% $ \mathrm{ T \rightarrow tH } $ is assumed. 
Summary 
We present a search for pairproduced vectorlike T quarks analyzing data from pp collisions at a centerofmass energy of $ \sqrt{s} = $ 13 TeV. The data were recorded by the CMS detector during the 2015 datataking period and correspond to integrated luminosities of 2.6 fb$^{1}$ and 2.7 fb$^{1}$ in the electron and muon channel, respectively. The analysis requires at least one lepton in the final state and is optimized for the case that at least one of the T quarks decays to a Higgs boson and a top quark where the Higgs boson decays to $\mathrm{ b \bar{b} } $. Events are selected using substructure techniques to identify boosted Higgs bosons and the statistical interpretation of the results is conducted using $ S_{\mathrm{T}} $ as final discriminating variable. No excess above the Standard Model background is observed and upper 95% CL exclusion limits on the cross section of $ \mathrm{ T \bar{T} } $ production are calculated for various branching fraction scenarios. For a branching fraction of 100% $ \mathrm{ T \to tH } $, T quarks with masses up to 860 GeV can be excluded (870 GeV expected) which already exceeds the exclusion limits set by the 8 TeV analyses both by ATLAS and CMS [1618] for this decay mode. 
References  
1  ATLAS Collaboration  Observation of a new particle in the search for the Standard Model Higgs boson with the ATLAS detector at the LHC  PLB 716 (2012) 1  
2  CMS Collaboration  Observation of a new boson at a mass of 125 GeV with the CMS experiment at the LHC  PLB 716 (2012) 30  
3  M. Perelstein, M. E. Peskin, and A. Pierce  Top quarks and electroweak symmetry breaking in little Higgs models  PRD 69 (2004) 075002  
4  O. Matsedonskyi, G. Panico, and A. Wulzer  Light top partners for a light composite Higgs  JHEP 01 (2013) 1  
5  R. Contino, L. Da Rold, and A. Pomarol  Light custodians in natural composite Higgs models  PRD 75 (2007) 055014  
6  R. Contino, T. Kramer, M. Son, and R. Sundrum  Warped/composite phenomenology simplified  JHEP 05 (2007) 074  
7  D. B. Kaplan  Flavor at SSC energies: A new mechanism for dynamically generated fermion masses  Nuclear Physics B 365 (1991) 259  
8  M. J. Dugan, H. Georgi, and D. B. Kaplan  Anatomy of a Composite Higgs Model  Nucl. Phys. B254 (1985) 299326  
9  AguilarSaavedra, J. A.  Mixing with vectorlike quarks: constraints and expectations  EPJ Web of Conferences 60 (2013) 16012  
10  F. del Aguila, J. A. AguilarSaavedra, and R. Miquel  Constraints on Top Couplings in Models with Exotic Quarks  PRL 82 (1999) 1628  
11  The ALEPH, DELPHI, L3, OPAL, SLD Collaborations, the LEP Electroweak Working Group  Electroweak Measurements in ElectronPositron Collisions at WBosonPair Energies at LEP  hepex/1302txyz  
12  O. Eberhardt et al.  Impact of a Higgs Boson at a Mass of 126 GeV on the Standard Model with Three and Four Fermion Generations  PRL 109 (2012) 241802  
13  A. Djouadi and A. Lenz  Sealing the fate of a fourth generation of fermions  PLB 715 (2012) 310  
14  J. A. AguilarSaavedra, R. Benbrik, S. Heinemeyer, and M. P\'erezVictoria  Handbook of vectorlike quarks: Mixing and single production  PRD 88 (2013) 094010  
15  A. De Simone, O. Matsedonskyi, R. Rattazzi, and A. Wulzer  A first top partner hunter's guide  JHEP 04 (2013) 1  
16  CMS Collaboration  Inclusive search for a vectorlike T quark with charge $ \frac{2}{3} $ in pp collisions at $ \sqrt{s} $ = 8 TeV  PLB 729 (2014) 149  CMSB2G12015 1311.7667 
17  ATLAS Collaboration  Search for pair production of a new heavy quark that decays into a $ W $ boson and a light quark in $ pp $ collisions at $ \sqrt{s}= $ 8 TeV with the ATLAS detector  PRD 92 (2015) 112007  
18  ATLAS Collaboration  Search for production of vectorlike quark pairs and of four top quarks in the leptonplusjets final state in pp collisions at $ \sqrt{s}= $ 8 TeV with the ATLAS detector  JHEP 08 (2015) 1  
19  CMS Collaboration  The CMS experiment at the CERN LHC  JINST 3 (2008) S08004  CMS00001 
20  CMS Collaboration  Particleflow event reconstruction in CMS and performance for jets, taus, and $ E_{\mathrm{T}}^{\text{miss}} $  CDS  
21  CMS Collaboration  Commissioning of the particleflow event with the first LHC collisions recorded in the CMS detector  CDS  
22  CMS Collaboration  Performance of electron reconstruction and selection with the CMS detector in protonproton collisions at $ \sqrt{s}= $ 8 TeV  JINST 10 (2015) P06005  CMSEGM13001 1502.02701 
23  CMS Collaboration  Performance of CMS muon reconstruction in $ pp $ collision events at $ \sqrt{s}= $ 7 TeV  JINST 7 (2012) P10002  CMSMUO10004 1206.4071 
24  M. Cacciari, G. P. Salam, and G. Soyez  The anti$ k_t $ jet clustering algorithm  JHEP 04 (2008) 063  0802.1189 
25  M. Cacciari, G. P. Salam, and G. Soyez  FastJet user manual  EPJC 72 (2012) 1896  1111.6097 
26  M. Cacciari, G. P. Salam, and G. Soyez  The catchment area of jets  JHEP 04 (2008) 005  0802.1188 
27  CMS Collaboration  Determination of jet energy calibration and transverse momentum resolution in CMS  JINST 6 (2011) P11002  CMSJME10011 1107.4277 
28  P. Nason  A New method for combining NLO QCD with shower Monte Carlo algorithms  JHEP 11 (2004) 040  hepph/0409146 
29  S. Frixione, P. Nason, and C. Oleari  Matching NLO QCD computations with Parton Shower simulations: the POWHEG method  JHEP 11 (2007) 070  0709.2092 
30  S. Alioli, P. Nason, C. Oleari, and E. Re  A general framework for implementing NLO calculations in shower Monte Carlo programs: the POWHEG BOX  JHEP 06 (2010) 043  1002.2581 
31  S. Frixione, P. Nason, and G. Ridolfi  A Positiveweight nexttoleadingorder Monte Carlo for heavy flavour hadroproduction  JHEP 09 (2007) 126  0707.3088 
32  J. Alwall et al.  The automated computation of treelevel and nexttoleading order differential cross sections, and their matching to parton shower simulations  JHEP 07 (2014) 079  1405.0301 
33  J. Alwall et al.  MadGraph 5: going beyond  JHEP 06 (2011) 1  
34  T. Sjostrand, S. Mrenna and P. Skands  PYTHIA 6.4 Physics and Manual  JHEP 05 (2006) 026  hepph/0603175 
35  T. Sjostrand et al.  An Introduction to PYTHIA 8.2  CPC 191 (2015) 159177  1410.3012 
36  M. Czakon and A. Mitov  Top++: A Program for the Calculation of the TopPair CrossSection at Hadron Colliders  CPC 185 (2014) 2930  
37  M. Czakon, P. Fiedler, and A. Mitov  Total TopQuark PairProduction Cross Section at Hadron Colliders Through $ O(\alpha\frac{4}{S}) $  PRL 110 (2013) 252004  
38  M. Czakon and A. Mitov  NNLO corrections to top pair production at hadron colliders: the quarkgluon reaction  JHEP 01 (2013) 080  
39  M. Czakon and A. Mitov  NNLO corrections to toppair production at hadron colliders: the allfermionic scattering channels  JHEP 12 (2012) 054  
40  P. Barnreuther, M. Czakon, and A. Mitov  Percent Level Precision Physics at the Tevatron: First Genuine NNLO QCD Corrections to $ q \bar{q} \to t \bar{t} + X $  PRL 109 (2012) 132001  
41  M. Cacciari et al.  Toppair production at hadron colliders with nexttonexttoleading logarithmic softgluon resummation  PLB 710 (2012) 612  
42  NNPDF Collaboration  Parton distributions for the LHC Run II  JHEP 04 (2015) 040  1410.8849 
43  GEANT4 Collaboration  GEANT4: A simulation toolkit  NIMA 506 (2003) 250303  
44  CMS Collaboration  Measurement of the Inclusive $ W $ and $ Z $ Production Cross Sections in $ pp $ Collisions at $ \sqrt{s}=7 $ TeV  JHEP 10 (2011) 132  CMSEWK10005 1107.4789 
45  A. J. Larkoski, S. Marzani, G. Soyez, and J. Thaler  Soft Drop  JHEP 05 (2014) 146  1402.2657 
46  CMS Collaboration  Identification of bquark jets with the CMS experiment  JINST 8 (2013) P04013  CMSBTV12001 1211.4462 
47  CMS Collaboration  Identification of b quark jets at the CMS Experiment in the LHC Run 2  CMSPASBTV15001  CMSPASBTV15001 
48  CMS Collaboration  Search for $ \mathrm{t\bar{t}} $ resonances in boosted semileptonic final states in pp collisions at $ \sqrt{s}= $ 13 TeV  CMSPASB2G15002  CMSPASB2G15002 
49  N. Kidonakis  Top Quark Production  in Proceedings, Helmholtz International Summer School on Physics of Heavy Quarks and Hadrons (HQ 2013)  1311.0283 
50  M. Aliev et al.  HATHOR: HAdronic Top and Heavy quarks crOss section calculatoR  CPC 182 (2011) 1034  1007.1327 
51  P. Kant et al.  HatHor for single topquark production: Updated predictions and uncertainty estimates for single topquark production in hadronic collisions  CPC 191 (2015) 74  1406.4403 
52  T. Muller, J. Ott, and J. WagnerKuhr  theta  a framework for templatebased modeling and inference  
53  A. O'Hagan and J. J. Forster  Kendall?s Advanced Theory of Statistics. Vol. 2B: Bayesian Inference  Arnold, London  
54  R. Barlow and C. Beeston  Fitting using finite Monte Carlo samples  CPC (1993) 219  
55  CMS Collaboration  Search for pairproduced vectorlike B quarks in protonproton collisions at $ \sqrt{s} $ = 8 TeV  CMSB2G13006 1507.07129 

56  ATLAS Collaboration  Search for vectorlike $ B $ quarks in events with one isolated lepton, missing transverse momentum, and jets at $ \sqrt{s}= $ 8 TeV with the ATLAS detector  PRD 91 (2015) 112011 
Compact Muon Solenoid LHC, CERN 