CMS-PAS-B2G-16-011

CMS-PAS-B2G-16-011
Search for vector-like 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 pair-produced vector-like top partners (``T quark'') using data from pp collisions at a center-of-mass energy of $\sqrt{s}=$ 13 TeV that were recorded with the CMS detector in the 2015 data-taking 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, jet-substructure 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 & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Representative Feynman diagram for production of a $\mathrm {T\bar{T}}$ pair with one T quark decaying to tH.
png pdf	Figure 2: Left: Distribution of the number of b-tagged 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 b-tag 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 b-tags 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.
png pdf	Figure 2-a: Distribution of the number of b-tagged 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 b-tag 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.
png pdf	Figure 2-b: Distribution of $M_{text{jet}}$ of the ${p_{\mathrm {T}}}$ -leading Higgs candidate jet ( ${p_{\mathrm {T}}} >$ 300 GeV) with at least two subjet b-tags 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.
png pdf	Figure 3: Post-fit $S_{\mathrm {T}}$ distributions in the ${\mathrm{ t } \mathrm{ \bar{t} } }$ (left) and W+jets (right) control regions after applying all corrections and performing the maximum-likelihood 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.
png pdf	Figure 3-a: Post-fit $S_{\mathrm {T}}$ distribution in the ${\mathrm{ t } \mathrm{ \bar{t} } }$ control region after applying all corrections and performing the maximum-likelihood 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.
png pdf	Figure 3-b: Post-fit $S_{\mathrm {T}}$ distribution in the ${\mathrm{ t } \mathrm{ \bar{t} } }$ W+jets control region after applying all corrections and performing the maximum-likelihood 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.
png pdf	Figure 4: Post-fit distributions of $S_{\mathrm {T}}$ in the 0H (top left), H1b (top right) and H2b (bottom) category after performing the maximum-likelihood 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.
png pdf	Figure 4-a: Post-fit distribution of $S_{\mathrm {T}}$ in the 0H category after performing the maximum-likelihood 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.
png pdf	Figure 4-b: Post-fit distribution of $S_{\mathrm {T}}$ in the H1b category after performing the maximum-likelihood 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.
png pdf	Figure 4-c: Post-fit distribution of $S_{\mathrm {T}}$ in the H2b category after performing the maximum-likelihood 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.
png pdf	Figure 5: Exclusion limits on the total cross-section of pair-produced T's with a branching fraction of 100% to tH. The theory cross section (dashed line) is computed at next-to-next-to-leading order.
png pdf	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( $\mathrm{ T \rightarrow tZ }$ ).
png pdf	Figure 6-a: 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( $\mathrm{ T \rightarrow tZ }$ ).
png pdf	Figure 6-b: 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
png pdf	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.
png pdf	Table 2: Summary of all systematic uncertainties, their sizes, types and to which processes they apply.
png pdf	Table 3: Event counts in the three final categories after performing the maximum-likelihood 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 pair-produced vector-like T quarks analyzing data from pp collisions at a center-of-mass energy of

$\sqrt{s} =$ 13 TeV. The data were recorded by the CMS detector during the 2015 data-taking 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 [16-18] 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) 299--326
9	Aguilar-Saavedra, J. A.	Mixing with vector-like quarks: constraints and expectations	EPJ Web of Conferences 60 (2013) 16012
10	F. del Aguila, J. A. Aguilar-Saavedra, 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 Electron-Positron Collisions at W-Boson-Pair Energies at LEP		hep-ex/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. Aguilar-Saavedra, R. Benbrik, S. Heinemeyer, and M. P\'erez-Victoria	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 vector-like T quark with charge $\frac{2}{3}$ in pp collisions at $\sqrt{s}$ = 8 TeV	PLB 729 (2014) 149	CMS-B2G-12-015 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 vector-like quark pairs and of four top quarks in the lepton-plus-jets 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	CMS-00-001
20	CMS Collaboration	Particle-flow event reconstruction in CMS and performance for jets, taus, and $E_{\mathrm{T}}^{\text{miss}}$	CDS
21	CMS Collaboration	Commissioning of the particle-flow 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 proton-proton collisions at $\sqrt{s}=$ 8 TeV	JINST 10 (2015) P06005	CMS-EGM-13-001 1502.02701
23	CMS Collaboration	Performance of CMS muon reconstruction in $pp$ collision events at $\sqrt{s}=$ 7 TeV	JINST 7 (2012) P10002	CMS-MUO-10-004 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	CMS-JME-10-011 1107.4277
28	P. Nason	A New method for combining NLO QCD with shower Monte Carlo algorithms	JHEP 11 (2004) 040	hep-ph/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 Positive-weight next-to-leading-order Monte Carlo for heavy flavour hadroproduction	JHEP 09 (2007) 126	0707.3088
32	J. Alwall et al.	The automated computation of tree-level and next-to-leading 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	hep-ph/0603175
35	T. Sjostrand et al.	An Introduction to PYTHIA 8.2	CPC 191 (2015) 159--177	1410.3012
36	M. Czakon and A. Mitov	Top++: A Program for the Calculation of the Top-Pair Cross-Section at Hadron Colliders	CPC 185 (2014) 2930
37	M. Czakon, P. Fiedler, and A. Mitov	Total Top-Quark Pair-Production 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 quark-gluon reaction	JHEP 01 (2013) 080
39	M. Czakon and A. Mitov	NNLO corrections to top-pair production at hadron colliders: the all-fermionic 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.	Top-pair production at hadron colliders with next-to-next-to-leading logarithmic soft-gluon 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) 250--303
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	CMS-EWK-10-005 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 b-quark jets with the CMS experiment	JINST 8 (2013) P04013	CMS-BTV-12-001 1211.4462
47	CMS Collaboration	Identification of b quark jets at the CMS Experiment in the LHC Run 2	CMS-PAS-BTV-15-001	CMS-PAS-BTV-15-001
48	CMS Collaboration	Search for $\mathrm{t\bar{t}}$ resonances in boosted semileptonic final states in pp collisions at $\sqrt{s}=$ 13 TeV	CMS-PAS-B2G-15-002	CMS-PAS-B2G-15-002
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 top-quark production: Updated predictions and uncertainty estimates for single top-quark production in hadronic collisions	CPC 191 (2015) 74	1406.4403
52	T. Muller, J. Ott, and J. Wagner-Kuhr	theta - a framework for template-based 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 pair-produced vector-like B quarks in proton-proton collisions at $\sqrt{s}$ = 8 TeV		CMS-B2G-13-006 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