Search for pair and single production of new heavy quarks that decay to a $Z$ boson and a third-generation quark in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

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Search for pair and single production of new heavy quarks that decay to a $Z$ boson and a third-generation quark in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector JHEP 11 (2014) 104 19 September 2014
Contact: ATLAS Exotics conveners
e-print arXiv:1409.5500	pdf from arXiv
Inspire record	-
Figures Tables Auxiliary Material	-
Abstract
A search is presented for the production of new heavy quarks that decay to a $Z$ boson and a third-generation Standard Model quark. In the case of a new charge +2/3 quark ($T$), the decay targeted is $T \rightarrow Zt$, while the decay targeted for a new charge -1/3 quark ($B$) is $B \rightarrow Zb$. The search is performed with a dataset corresponding to 20.3 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Selected events contain a high transverse momentum $Z$ boson candidate reconstructed from a pair of oppositely charged same-flavor leptons (electrons or muons), and are analyzed in two channels defined by the absence or presence of a third lepton. Hadronic jets, in particular those with properties consistent with the decay of a $b$-hadron, are also required to be present in selected events. Different requirements are made on the jet activity in the event in order to enhance the sensitivity to either heavy quark pair production mediated by the strong interaction, or single production mediated by the electroweak interaction. No significant excess of events above the Standard Model expectation is observed, and lower limits are derived on the mass of vector-like $T$ and $B$ quarks under various branching ratio hypotheses, as well as upper limits on the magnitude of electroweak coupling parameters.
Figures
Figure 01a: A representative diagram illustrating the pair production and decay modes of a vector-like quark (Q=T,B). png (38kB) pdf (27kB)
Figure 01b: The √s=8 TeV LHC cross section versus quark mass for pair production, denoted by the solid line, as well as for the Tbq and Bbq single production processes, denoted by dashed lines. The pair production cross section has been calculated with Top++. The single production cross sections were calculated with protos and madgraph using different electroweak coupling parameters that are discussed in the text. png (136kB) pdf (9kB)
Figure 02a: Vector-like T quark branching ratios to the Wb, Zt, and Ht decay modes as a function of the T quark mass, computed with protos for an SU(2) singlet and two types of doublets. The X quark in an (X,T) doublet has charge +5/3. png (108kB) pdf (10kB)
Figure 02b: Vector-like B quark branching ratios to the Wt, Zb, and Hb decay modes for a singlet and two types of doublets. The Y quark in a (B,Y) doublet has charge -4/3. png (97kB) pdf (9kB)
Figure 03a: Representative diagrams illustrating the t-channel electroweak single production of a T quark via the Tbq process. png (238kB) pdf (169kB)
Figure 03b: Representative diagrams illustrating the t-channel electroweak single production of a B quark via the Bbq process. png (237kB) pdf (145kB)
Figure 04a: Unit-normalized distributions of signal sensitive variables employed in this analysis. The filled histograms correspond to SM backgrounds. Unfilled histograms correspond to signal, with solid (dashed) lines representing pair (single) production of SU(2) singlet T and B quarks with a mass of 650 GeV. The rightmost bin in each histogram contains overflow events. The lepton multiplicity distribution after a Z+≥2 central jets selection is shown for dilepton channel events. png (119kB) pdf (15kB)
Figure 04b: Unit-normalized distributions of signal sensitive variables employed in this analysis. The filled histograms correspond to SM backgrounds. Unfilled histograms correspond to signal, with solid (dashed) lines representing pair (single) production of SU(2) singlet T and B quarks with a mass of 650 GeV. The rightmost bin in each histogram contains overflow events. The p_T(Z) distribution is shown for dilepton channel events. png (149kB) pdf (16kB)
Figure 04c: Unit-normalized distributions of signal sensitive variables employed in this analysis. The filled histograms correspond to SM backgrounds. Unfilled histograms correspond to signal, with solid (dashed) lines representing pair (single) production of SU(2) singlet T and B quarks with a mass of 650 GeV. The rightmost bin in each histogram contains overflow events. The b-tagged jet multiplicity distribution is shown for dilepton channel events. png (107kB) pdf (14kB)
Figure 04d: Unit-normalized distributions of signal sensitive variables employed in this analysis. The filled histograms correspond to SM backgrounds. Unfilled histograms correspond to signal, with solid (dashed) lines representing pair (single) production of SU(2) singlet T and B quarks with a mass of 650 GeV. The rightmost bin in each histogram contains overflow events. The forward jet multiplicity distribution is shown for trilepton events. png (102kB) pdf (14kB)
Figure 05a: Unit-normalized distributions of the discriminating variables used for hypothesis testing, shown at the Z+≥2 central jets selection stage: H_T(jets+leptons) ditribution in the trilepton channel. The filled histograms correspond to SM backgrounds. Unfilled histograms correspond to signal, with solid (dashed) lines representing pair (single) production of SU(2) singlet T and B quarks with a mass of 650 GeV. The rightmost bin in each histogram contains overflow events. png (118kB) pdf (15kB)
Figure 05b: Unit-normalized distributions of the discriminating variables used for hypothesis testing, shown at the Z+≥2 central jets selection stage: the m(Zb) distribution in the dilepton channel. The filled histograms correspond to SM backgrounds. Unfilled histograms correspond to signal, with solid (dashed) lines representing pair (single) production of SU(2) singlet T and B quarks with a mass of 650 GeV. The rightmost bin in each histogram contains overflow events. png (130kB) pdf (16kB)
Figure 06a: The distribution of the Z boson candidate mass, m(Z), in dilepton channel events with ≥2 central jets and N_tag=1. Reference signals are displayed for BB and TT production assuming SU(2) singlet quarks with a mass of 650 GeV. The hatched bands in the upper and lower panels represent the total background uncertainty. png (128kB) pdf (19kB)
Figure 06b: The Z boson candidate mass, m(Z), in dilepton channel events with ≥2 central jets and N_tag≥2. Reference signals are displayed for BB and TT production assuming SU(2) singlet quarks with a mass of 650 GeV. The hatched bands in the upper and lower panels represent the total background uncertainty. png (148kB) pdf (22kB)
Figure 06c: The transverse momentum, p_T(Z), distribution in dilepton channel events with ≥2 central jets and N_tag=1. The Z+jets prediction is shown before the p_T(Z) correction described in the text is applied. Reference signals are displayed for BB and TT production assuming SU(2) singlet quarks with a mass of 650 GeV. The hatched bands in the upper and lower panels represent the total background uncertainty. png (149kB) pdf (22kB)
Figure 06d: The transverse momentum, p_T(Z), distribution in dilepton channel events with ≥2 central jets and N_tag≥2. The Z+jets prediction is shown after the p_T(Z) correction described in the text is applied. Reference signals are displayed for BB and TT production assuming SU(2) singlet quarks with a mass of 650 GeV. The hatched bands in the upper and lower panels represent the total background uncertainty. png (175kB) pdf (22kB)
Figure 07a: The H_T(jets) distribution after requiring p_T(Z)>150 GeV in dilepton channel events with N_tag=1. png (152kB) pdf (22kB)
Figure 07b: The H_T(jets) distribution after requiring p_T(Z)>150 GeV in dilepton channel events with N_tag ≥ 2. png (183kB) pdf (22kB)
Figure 07c: The final m(Zb) distribution after requiring p_T(Z)>150 GeV and H_T(jets)>600 GeV in events with N_tag=1. png (129kB) pdf (18kB)
Figure 07d: The final m(Zb) distribution after requiring p_T(Z)>150 GeV and H_T(jets)>600 GeV in events with N_tag ≥ 2. png (169kB) pdf (19kB)
Figure 08a: The distribution of the Z boson candidate mass, m(Z), in trilepton channel events. png (155kB) pdf (21kB)
Figure 08b: The central jet multiplicity distribution in trilepton channel events. png (109kB) pdf (18kB)
Figure 08c: The distribution of the Z candidate transverse momentum, p_T(Z), after requiring ≥2 central jets. png (122kB) pdf (18kB)
Figure 08d: The b-tagged jet multiplicity distribution after requiring ≥2 central jets and p_T(Z)>150 GeV. png (112kB) pdf (17kB)
Figure 09a: The H_T(jets+leptons) distribution in trilepton channels events with ≥2 central jets, p_T(Z)>150 GeV, and N_tag=0. png (137kB) pdf (19kB)
Figure 09b: The H_T(jets+leptons) distribution in trilepton channels events with ≥2 central jets, p_T(Z)>150 GeV, and N_tag≥1. png (139kB) pdf (19kB)
Figure 10a: The forward jet multiplicity distribution in dilepton channel events with ≥2 central jets, satisfying p_T(Z)>150 GeV, and N_tag=1. The predicted Tbq signal assumes a mixing parameter value of λ_T=2, while the predicted Bbq signal assumes a mixing parameter value of X_bB=0.5. png (134kB) pdf (16kB)
Figure 10b: The forward jet multiplicity distribution in dilepton channel events with ≥2 central jets, satisfying p_T(Z)>150 GeV, and N_tag ≥2. The predicted Tbq signal assumes a mixing parameter value of λ_T=2, while the predicted Bbq signal assumes a mixing parameter value of X_bB=0.5. png (139kB) pdf (16kB)
Figure 10c: The m(Zb) distribution following the final requirement of at least one forward jet in events with (c) N_tag=1. The predicted Tbq signal assumes a mixing parameter value of λ_T=2, while the predicted Bbq signal assumes a mixing parameter value of X_bB=0.5. png (149kB) pdf (18kB)
Figure 10d: The m(Zb) distribution following the final requirement of at least one forward jet in events with N_tag≥2. The predicted Tbq signal assumes a mixing parameter value of λ_T=2, while the predicted Bbq signal assumes a mixing parameter value of X_bB=0.5. png (156kB) pdf (17kB)
Figure 11a: The forward jet multiplicity distribution in trilepton channel events with ≥2 central jets, satisfying p_T(Z)>150 GeV, and N_tag ≥ 1. The predicted Tbq signal assumes a mixing parameter value of λ_T=2. png (88kB) pdf (15kB)
Figure 11b: The H_T(jets+leptons) distribution following the requirement of at least one forward jet in trilepton channel events. The predicted Tbq signal assumes a mixing parameter value of λ_T=2. png (124kB) pdf (16kB)
Figure 12a: Predicted pair-production cross section as a function of the heavy quark mass and 95% CL observed and expected upper limits for an SU(2) singlet B quark. png (138kB) pdf (13kB)
Figure 12b: Predicted pair-production cross section as a function of the heavy quark mass and 95% CL observed and expected upper limits for a B quark forming an SU(2) (B,Y) doublet with a charge -4/3 Y quark. png (143kB) pdf (13kB)
Figure 12c: Predicted pair-production cross section as a function of the heavy quark mass and 95% CL observed and expected upper limits for an SU(2) singlet T quark. png (145kB) pdf (13kB)
Figure 12d: Predicted pair-production cross section as a function of the heavy quark mass and 95% CL observed and expected upper limits for T quark forming an SU(2) (T,B) doublet with a charge -1/3 B quark. png (145kB) pdf (13kB)
Figure 13a: Expected limit (95% CL) on the mass of the B quark assuming the pair production hypothesis and presented in the (Wt,Hb) branching ratio plane. png (102kB) pdf (15kB)
Figure 13b: Observed limit (95% CL) on the mass of the B quark assuming the pair production hypothesis and presented in the (Wt,Hb) branching ratio plane. png (106kB) pdf (15kB)
Figure 14a: Expected limit (95% CL) on the mass of the T quark assuming the pair production hypothesis and presented in the (Wb,Ht) branching ratio plane. png (101kB) pdf (15kB)
Figure 14b: Observed limit (95% CL) on the mass of the T quark assuming the pair production hypothesis and presented in the (Wb,Ht) branching ratio plane. png (103kB) pdf (15kB)
Figure 15a: Upper limit (95% CL) on the single production cross section times branching ratio as a function of the heavy quark mass: σ(pp → Banti-{b}q) × BR(B → Zb). png (111kB) pdf (12kB)
Figure 15b: Upper limit (95% CL) on the single production cross section times branching ratio as a function of the heavy quark mass: σ(pp → Tanti-{b}q) × BR(T → Zt). png (112kB) pdf (12kB)
Tables
Table 01: Summary of the event selection criteria. Preselected Z boson candidate events are divided into dilepton and trilepton categories. The requirements on the number of central jets and the Z candidate transverse momentum are common to both channels, and for the pair- and single-production hypotheses. Other requirements are specific to a lepton channel or the targeted production mechanism. The last row lists the final discriminant used for hypothesis testing. png (29kB) pdf (52kB)
Table 02: Predicted and observed number of events in the dilepton channel after selecting a Z boson candidate and at least two central jets, exactly one of which is b-tagged. The number of events further satisfying p_T(Z)>150 GeV is listed next, followed by the number satisfying, in addition, H_T(jets)>600 GeV. The Z+jets predictions, as well as the total background prediction, are shown before and after the p_T(Z) spectrum correction described in the text. Reference BB and TT signal yields are provided for m_B/T=650 GeV and SU(2) singlet branching ratios. The uncertainties on the predicted yields include statistical and systematic sources. png (48kB) pdf (71kB)
Table 03: Predicted and observed number of events in the dilepton channel after selecting a Z boson candidate and at least two central jets, at least two of which are b-tagged. The number of events further satisfying p_T(Z)>150 GeV is listed next, followed by the number satisfying, in addition, H_T(jets)>600 GeV. Reference BB and TT signal yields are provided for m_B/T=650 GeV and SU(2) singlet branching ratios. The uncertainties on the predicted yields include statistical and systematic sources. png (32kB) pdf (77kB)
Table 04: Predicted and observed number of events in the trilepton channel, starting on the left with the selection stage of a Z boson candidate plus a third isolated lepton, followed by the yields after the additional requirements outlined in the text. The final column represents the signal region for testing the pair production hypotheses. Reference BB and TT signal yields are provided for m_B/T=650 GeV and SU(2) singlet branching ratios. The uncertainties on the predicted yields include statistical and systematic sources. png (31kB) pdf (49kB)
Table 05: Number of predicted and observed dilepton and trilepton channel events after the final selection for testing the single-production hypotheses, which includes a forward-jet requirement. The expected yield of Tbq and Bbq events is listed for SU(2) singlet T and B quarks with a mass of 650 GeV and for reference mixing parameters. The predicted contribution of pair-production events in the single-production signal regions is also provided. The uncertainties on the predicted yields include statistical and systematic sources. png (41kB) pdf (57kB)
Table 06: The fractional uncertainties (%) in the yields of signal and background events after the final dilepton channel selection for testing the pair production hypotheses. The signals correspond to SU(2) singlet T and B quarks with a mass of 650 GeV. The uncertainties are grouped into categories that are explained in more detail in the text. png (36kB) pdf (35kB)
Table 07: The fractional uncertainties (%) in the yields of signal and background events after the final trilepton channel selection for testing the pair production hypotheses. The signals correspond to SU(2) singlet T and B quarks with a mass of 650 GeV. The uncertainties are grouped into categories that are explained in more detail in the text. png (29kB) pdf (27kB)
Table 08: Observed (expected) 95% CL limits on the T and B quark mass (GeV) assuming pair production of SU(2) singlet and doublet quarks, and using the dilepton and trilepton channels separately, as well as combined. png (19kB) pdf (25kB)
Auxiliary figures and tables
Figure 01: Excluded regions of the (Hb,Wt) branching ratio plane for several B quark masses using the combined dilepton and trilepton channels under the BB pair production hypothesis. png (285kB) pdf (19kB)
Figure 02: Excluded regions of the (Ht,Wb) branching ratio plane for several T quark masses using the combined dilepton and trilepton channels under the TT pair production hypothesis. png (285kB) pdf (19kB)
Figure 03: An event with two electrons (with transverse momenta of 280 GeV and 42 GeV), two b-tagged jets (with p_T = 316 GeV and p_T = 29 GeV) and one forward jet (with p_T = 39 GeV) satisfying the dilepton selection targeting the electroweak single production of vector-like quarks. The two electrons reconstruct a Z boson candidate with p_T(Z) = 315 GeV. The invariant mass of the Z candidate and the highest-p_T b-tagged jet, m(Zb), is 659 GeV. png (2MB)
Figure 04: An event with three muons (with transverse momenta of 111 GeV, 67 GeV, and 57 GeV), one b-tagged jet (with p_T = 112 GeV) and one forward jet (with p_T = 87 GeV) satisfying the trilepton selection targeting the electroweak single production of vector-like quarks. Two of the muons reconstruct a Z boson candidate with p_T(Z) = 172 GeV. The scalar sum of the transverse momentum of all central jets and leptons, H_T(jets+leptons), in this event is 423 GeV. png (2MB)
Table 01: Upper limit (95% CL) on σ(pp → BB), assuming an SU(2) singlet B quark, as a function of the B quark mass using the dilepton and trilepton channel analyses. png (23kB) pdf (38kB)
Table 02: Upper limit (95% CL) on σ(pp → BB), assuming an SU(2) doublet B quark, as a function of the B quark mass using the dilepton and trilepton channel analyses. png (23kB) pdf (38kB)
Table 03: Upper limit (95% CL) on σ(pp → TT), assuming an SU(2) singlet T quark, as a function of the T quark mass using the dilepton and trilepton channel analyses. png (24kB) pdf (38kB)
Table 04: Upper limit (95% CL) on σ(pp → TT), assuming an SU(2) doublet T quark, as a function of the T quark mass using the dilepton and trilepton channel analyses. png (23kB) pdf (38kB)
Table 05: Lower limits (95% CL) on the B quark mass for various branching ratio sets using the combined dilepton and trilepton channels under the BB pair production hypothesis. png (38kB) pdf (31kB)
Table 06: Lower limits (95% CL) on the T quark mass for various branching ratio sets using the combined dilepton and trilepton channels under the TT pair production hypothesis. png (37kB) pdf (31kB)
Table 07: Absolute and fractional event yields in both the dilepton and trilepton channels for the BB signal, assuming SU(2) singlet B quarks with masses m_B=450,650,850 GeV. The Top++ cross section is provided for each mass point, and the total events produced corresponds to 20.3 fb^-1. The branching ratio BR(B → Zb) calculated with protos is also given. The number events with a reconstructed Z boson candidate is approximately equal to the total produced events multiplied by 2 × BR(B → Zb) × BR(Z → ll) × (ε A)_Z, where (ε A)_Z ≈ 0.5. png (68kB) pdf (60kB)
Table 08: Absolute and fractional event yields in both the dilepton and trilepton channels for the TT signal, assuming SU(2) singlet T quarks with masses m_T=450,650,850 GeV. The Top++ cross section is provided for each mass point, and the total events produced corresponds to 20.3 fb^-1. The branching ratio BR(T → Zt) calculated with protos is also given. The number of events with a reconstructed Z boson candidate is approximately equal to the total produced events multiplied by 2 × BR(T → Zt) × BR(Z → ll) × (ε A)_Z, where (ε A)_Z ≈ 0.5. png (67kB) pdf (60kB)
Table 09: Absolute and fractional event yields in the dilepton channel for the Bbq signal, assuming an SU(2) singlet B quark with masses m_B=500,650,850 GeV. The protos cross section is provided for each mass point, and the total number of events produced corresponds to 20.3 fb^-1. The branching ratio BR(B → Zb) calculated with protos is also given. The number of events with a reconstructed Z boson candidate is approximately equal to the total number of produced events multiplied by BR(B → Zb) × BR(Z → ll) × (ε A)_Z, where (ε A)_Z ≈ 0.5. png (46kB) pdf (60kB)
Table 10: Absolute and fractional event yields in both the dilepton and trilepton channels for the Tbq signal, assuming the SU(2) singlet T quark of the composite Higgs model (CHM) with masses m_T=450,650,850 GeV. The cross section is provided for each mass point, and the total events produced corresponds to 20.3 fb^-1. The branching ratio BR(T → Zt) calculated in the CHM is also given. The number of events with a reconstructed Z boson candidate is approximately equal to the total produced events multiplied by BR(T → Zt) × BR(Z → ll) × (ε A)_Z, where (ε A)_Z ≈ 0.5. png (73kB) pdf (61kB)
Table 11: Limit (95% CL) on σ(pp → Bbq) × BR(B→ Zb) as a function of the B quark mass using the dilepton channel analysis. png (29kB) pdf (39kB)
Table 12: Limit (95% CL) on σ(pp → Tbq) × BR(T→ Zt) as a function of the T quark mass using the dilepton and trilepton channel analyses. png (30kB) pdf (40kB)

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