arXiv subjects

ATLAS Collaboration

Publications and source records attributed to ATLAS Collaboration.

At least 1,027 records · Page 57Linked to original sources

arXiv2016

Searches for scalar leptoquarks in $pp$ collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

Searches for pair-produced scalar leptoquarks are performed using 20 fb$^{-1}$ of proton-proton collision data provided by the LHC and recorded by the ATLAS detector at $\sqrt{s}=8$ TeV. Events with two electrons (muons) and two or more jets in the final state are used to search for first (second)-generation leptoquarks. The results from two previously published ATLAS analyses are interpreted in terms of third-generation leptoquarks decaying to $bν_τ\bar{b}\bar{ν_τ}$ and $tν_τ\bar{t}\bar{ν_τ}$ final states. No statistically significant excess above the Standard Model expectation is observed in any channel and scalar leptoquarks are excluded at 95% CL with masses up to $m_{\mathrm{LQ1}}$ < 1050 GeV for first-generation leptoquarks, $m_{\mathrm{LQ2}}$ < 1000 GeV for second-generation leptoquarks, $m_{\mathrm{LQ3}} < $ 625 GeV for third-generation leptoquarks in the $bν_τ\bar{b}\bar{ν_τ}$ channel, and 200 $< m_{\mathrm{LQ3}} < $ 640 GeV in the $tν_τ\bar{t}\bar{ν_τ}$ channel.

hep-ex↗

arXiv2016

Search for high-mass diboson resonances with boson-tagged jets in proton-proton collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detector

A search is performed for narrow resonances decaying into $WW$, $WZ$, or $ZZ$ boson pairs using 20.3 fb$^{-1}$ of proton-proton collision data at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. Diboson resonances with masses in the range from 1.3 to 3.0 TeV are sought after using the invariant mass distribution of dijets where both jets are tagged as a boson jet, compatible with a highly boosted $W$ or $Z$ boson decaying to quarks, using jet mass and substructure properties. The largest deviation from a smoothly falling background in the observed dijet invariant mass distribution occurs around 2 TeV in the $WZ$ channel, with a global significance of 2.5 standard deviations. Exclusion limits at the 95% confidence level are set on the production cross section times branching ratio for the $WZ$ final state of a new heavy gauge boson, $W'$, and for the $WW$ and $ZZ$ final states of Kaluza--Klein excitations of the graviton in a bulk Randall--Sundrum model, as a function of the resonance mass. $W'$ bosons with couplings predicted by the extended gauge model in the mass range from 1.3 to 1.5 TeV are excluded at 95% confidence level.

hep-ex↗

arXiv2016

Measurements of fiducial cross-sections for $t\bar{t}$ production with one or two additional $b$-jets in $pp$ collisions at $\sqrt{s}$ = 8 TeV using the ATLAS detector

Fiducial cross-sections for $t\bar{t}$ production with one or two additional $b$-jets are reported, using an integrated luminosity of 20.3 fb$^{-1}$ of proton--proton collisions at a centre-of-mass energy of 8 TeV at the Large Hadron Collider, collected with the ATLAS detector. The cross-section times branching ratio for $t\bar{t}$ events with at least one additional $b$-jet is measured to be 950 $\pm$ 70 (stat.) $^{+240}_{-190}$ (syst.) fb in the lepton-plus-jets channel and 50 $\pm$ 10 (stat.) $^{+15}_{-10}$ (syst.) fb in the $e μ$ channel. The cross-section times branching ratio for events with at least two additional $b$-jets is measured to be 19.3 $\pm$ 3.5 (stat.) $\pm$ 5.7 (syst.) fb in the dilepton channel ($e μ$,\,$μμ$, and \,$ee$) using a method based on tight selection criteria, and 13.5 $\pm$ 3.3 (stat.) $\pm$ 3.6 (syst.) fb using a looser selection that allows the background normalisation to be extracted from data. The latter method also measures a value of 1.30 $\pm$ 0.33 (stat.) $\pm$ 0.28 (syst.)\% for the ratio of $t\bar{t}$ production with two additional $b$-jets to $t\bar{t}$ production with any two additional jets. All measurements are in good agreement with recent theory predictions.

hep-ex↗

arXiv2016

Measurement of the production cross-section of a single top quark in association with a $W$ boson at 8 TeV with the ATLAS experiment

The cross-section for the production of a single top quark in association with a $W$ boson in proton--proton collisions at $\sqrt{s}= 8$ TeV is measured. The dataset corresponds to an integrated luminosity of 20.3 fb$^{-1}$, collected by the ATLAS detector in 2012 at the Large Hadron Collider at CERN. Events containing two leptons and one central $b$-jet are selected. The $Wt$ signal is separated from the backgrounds using boosted decision trees, each of which combines a number of discriminating variables into one classifier. Production of $Wt$ events is observed with a significance of $7.7 σ$. The cross-section is extracted in a profile likelihood fit to the classifier output distributions. The $Wt$ cross-section, inclusive of decay modes, is measured to be $23.0 \pm 1.3$(stat.)$^{+3.2}_{-3.5}$(syst.)$\pm 1.1$(lumi.) pb. The measured cross-section is used to extract a value for the CKM matrix element $|V_{tb}|$ of $1.01 \pm 0.10$ and a lower limit of 0.80 at the 95\% confidence level. The cross-section for the production of a top quark and a $W$ boson is also measured in a fiducial acceptance requiring two leptons with $p_{\textrm{T}}>25$ GeV and $|η|<2.5$, one jet with $p_{\textrm{T}}>20$ GeV and $|η|<2.5$, and $E_{\textrm{T}}^{\textrm{miss}}>20$ GeV, including both $Wt$ and top-quark pair events as signal. The measured value of the fiducial cross-section is $0.85 \pm 0.01$(stat.)$^{+0.06}_{-0.07}$(syst.)$\pm 0.03$(lumi.) pb.

hep-ex↗

arXiv2016

Search for a high-mass Higgs boson decaying to a $W$ boson pair in $pp$ collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector

A search for a high-mass Higgs boson $H$ is performed in the $H\to{WW}\to\ellν\ellν$ and $H\to{WW}\to\ellν{qq}$ decay channels, using $pp$ collision data corresponding to an integrated luminosity of 20.3 fb$^{-1}$, collected at $\sqrt{s}=$ 8 TeV by the ATLAS detector at the Large Hadron Collider. No evidence of a high-mass Higgs boson is found. Limits on $σ_H\times \mathrm{BR}(H\to{WW})$ as a function of the Higgs boson mass $m_H$ are determined in three different scenarios: one in which the heavy Higgs boson has a narrow width compared to the experimental resolution, one a width increasing with the boson mass and modeled by the complex-pole scheme following the same behavior as in the Standard Model, and one for intermediate widths. The upper range of the search is $m_H=1500$ GeV for the narrow-width scenario and $m_H=1000$ GeV for the other two scenarios. The lower edge of the search range is $200-300$ GeV and depends on the analysis channel and search scenario. For each scenario, individual and combined limits from the two $WW$ decay channels are presented. At $m_H = 1500$ GeV, the highest mass point tested, $σ_H\cdot\mathrm{BR}(H\rightarrow WW)$ for a narrow-width Higgs boson is constrained to be less than 22 fb and 6.6 fb at the 95% CL for the gluon fusion and vector-boson fusion production modes, respectively.

hep-ex↗

arXiv2016

Search for flavour-changing neutral current top-quark decays to $qZ$ in $pp$ collision data collected with the ATLAS detector at $\sqrt{s}=8$ TeV

A search for the flavour-changing neutral-current decay $t\to qZ$ is presented. Data collected by the ATLAS detector during 2012 from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=8$ TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, are analysed. Top-quark pair-production events with one top quark decaying through the $t\to qZ$ ($q=u,c$) channel and the other through the dominant Standard Model mode $t\to bW$ are considered as signal. Only the decays of the $Z$ boson to charged leptons and leptonic $W$ boson decays are used. No evidence for a signal is found and an observed (expected) upper limit on the $t\to qZ$ branching ratio of $7\times 10^{-4}$ ($8\times 10^{-4}$) is set at the 95% confidence level.

hep-ex↗

arXiv2016

Measurement of four-jet differential cross sections in $\sqrt{s}=8$ TeV proton-proton collisions using the ATLAS detector

Differential cross sections for the production of at least four jets have been measured in proton-proton collisions at $\sqrt{s} = 8$ TeV at the Large Hadron Collider using the ATLAS detector. Events are selected if the four anti-$k_{t}$ R=0.4 jets with the largest transverse momentum ($p_{T}$) within the rapidity range $|y|<2.8$ are well separated ($dR^{\rm min}_{4j}>0.65$), all have $p_{T}>64$ GeV, and include at least one jet with $p_{T} >100$ GeV. The dataset corresponds to an integrated luminosity of 20.3 $fb^{-1}$. The cross sections, corrected for detector effects, are compared to leading-order and next-to-leading-order calculations as a function of the jet momenta, invariant masses, minimum and maximum opening angles and other kinematic variables.

hep-ex↗

arXiv2016

Measurement of the correlation between flow harmonics of different order in lead-lead collisions at $\sqrt{s_{NN}}$=2.76 TeV with the ATLAS detector

Correlations between the elliptic or triangular flow coefficients $v_m$ ($m$=2 or 3) and other flow harmonics $v_n$ ($n$=2 to 5) are measured using $\sqrt{s_{NN}}=2.76$ TeV Pb+Pb collision data collected in 2010 by the ATLAS experiment at the LHC, corresponding to an integrated lumonisity of 7 $μ$b$^{-1}$. The $v_m$-$v_n$ correlations are measured in midrapidity as a function of centrality, and, for events within the same centrality interval, as a function of event ellipticity or triangularity defined in a forward rapidity region. For events within the same centrality interval, $v_3$ is found to be anticorrelated with $v_2$ and this anticorrelation is consistent with similar anticorrelations between the corresponding eccentricities $ε_2$ and $ε_3$. On the other hand, it is observed that $v_4$ increases strongly with $v_2$, and $v_5$ increases strongly with both $v_2$ and $v_3$. The trend and strength of the $v_m$-$v_n$ correlations for $n$=4 and 5 are found to disagree with $ε_m$-$ε_n$ correlations predicted by initial-geometry models. Instead, these correlations are found to be consistent with the combined effects of a linear contribution to $v_n$ and a nonlinear term that is a function of $v_2^2$ or of $v_2v_3$, as predicted by hydrodynamic models. A simple two-component fit is used to separate these two contributions. The extracted linear and nonlinear contributions to $v_4$ and $v_5$ are found to be consistent with previously measured event-plane correlations.

hep-ex↗

arXiv2016

Measurements of four-lepton production in $pp$ collisions at $\sqrt{s}=$ 8 TeV with the ATLAS detector

The four-lepton ($4\ell$, $\ell = e, μ$) production cross section is measured in the mass range from 80 to 1000 GeV using 20.3 fb$^{-1}$ of data in $pp$ collisions at $\sqrt{s}=8$ TeV collected with the ATLAS detector at the LHC. The $4\ell$ events are produced in the decays of resonant $Z$ and Higgs bosons and the non-resonant $ZZ$ continuum originating from $q\bar q$, $gg$, and $qg$ initial states. A total of 476 signal candidate events are observed with a background expectation of $26.2 \pm 3.6$ events, enabling the measurement of the integrated cross section and the differential cross section as a function of the invariant mass and transverse momentum of the four-lepton system. In the mass range above $180$ GeV, assuming the theoretical constraint on the $q\bar q$ production cross section calculated with perturbative NNLO QCD and NLO electroweak corrections, the signal strength of the gluon-fusion component relative to its leading-order prediction is determined to be $μ_{gg}=2.4 \pm 1.0 (stat.) \pm 0.5 (syst.)\pm 0.8 (theory)$.

hep-ex↗

arXiv2016

Study of the $B_c^+ \to J/ψD_s^+$ and $B_c^+ \to J/ψD_s^{*+}$ decays with the ATLAS detector

The decays $B_c^+ \to J/ψD_s^+$ and $B_c^+ \to J/ψD_s^{*+}$ are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 fb$^{-1}$ and 20.6 fb$^{-1}$ of $pp$ collisions collected at centre-of-mass energies $\sqrt{s} = 7$ TeV and 8 TeV, respectively. Signal candidates are identified through $J/ψ\toμ^+μ^-$ and $D_s^{(*)+}\toϕπ^+(γ/π^0)$ decays. With a two-dimensional likelihood fit involving the $B_c^+$ reconstructed invariant mass and an angle between the $μ^+$ and $D_s^+$ candidate momenta in the muon pair rest frame, the yields of $B_c^+ \to J/ψD_s^+$ and $B_c^+ \to J/ψD_s^{*+}$, and the transverse polarisation fraction in $B_c^+ \to J/ψD_s^{*+}$ decay are measured. The transverse polarisation fraction is determined to be $Γ_{\pm\pm}(B_c^+ \to J/ψD_s^{*+})/Γ(B_c^+ \to J/ψD_s^{*+}) = 0.38 \pm 0.23 \pm 0.07$, and the derived ratio of the branching fractions of the two modes is $\mathcal{B}_{B_c^+ \to J/ψD_s^{*+}}/\mathcal{B}_{B_c^+ \to J/ψD_s^+} = 2.8 \,^{+1.2}_{-0.8} \pm 0.3$, where the first error is statistical and the second is systematic. Finally, a sample of $B_c^+\to J/ψπ^+$ decays is used to derive the ratios of branching fractions $\mathcal{B}_{B_c^+ \to J/ψD_s^+}/\mathcal{B}_{B_c^+ \to J/ψπ^+} = 3.8 \pm 1.1 \pm 0.4 \pm 0.2$ and $\mathcal{B}_{B_c^+ \to J/ψD_s^{*+}}/\mathcal{B}_{B_c^+ \to J/ψπ^+} = 10.4 \pm 3.1 \pm 1.5 \pm 0.6$, where the third error corresponds to the uncertainty of the branching fraction of $D_s^+\toϕ(K^+K^-)π^+$ decay. The available theoretical predictions are generally consistent with the measurement.

hep-ex↗

arXiv2016

Determination of the ratio of $b$-quark fragmentation fractions $f_s/f_d$ in $pp$ collisions at $\sqrt{s}=7$ TeV with the ATLAS detector

With an integrated luminosity of 2.47 fb$^{-1}$ recorded by the ATLAS experiment at the LHC, the exclusive decays $B_{s}^{0}\rightarrow J/ψϕ$ and $B_{d}^{0}\rightarrow J/ψK^{*0}$ of $B$ mesons produced in $pp$ collisions at $\sqrt{s}=7$ TeV are used to determine the ratio of fragmentation fractions $f_s/f_d$. From the observed $B_{s}^{0}\rightarrow J/ψϕ$ and $B_{d}^{0}\rightarrow J/ψK^{*0}$ yields, the quantity $\frac{f_s}{f_d}\frac{\mathcal{B}(B_{s}^{0} \rightarrow J/ψϕ)}{\mathcal{B}(B_{d}^{0} \rightarrow J/ψK^{*0})}$ is measured to be 0.199 $\pm$ 0.004(stat) $\pm$ 0.008(sys). Using a recent theory prediction for $\frac{\mathcal{B}(B_{s}^{0}\rightarrow J/ψϕ)}{\mathcal{B}(B_{d}^{0}\rightarrow J/ψK^{*0})}$ yields $\frac{f_s}{f_d} =$ 0.240 $\pm$ 0.004(stat) $\pm$ 0.010(sys) $\pm$ 0.017(th). This result is based on a new approach that provides a significant improvement of the world average.

hep-ex↗

arXiv2016

Search for vector-like $B$ quarks in events with one isolated lepton, missing transverse momentum and jets at $\sqrt{s}=$ 8 TeV with the ATLAS detector

A search has been performed for pair production of heavy vector-like down-type ($B$) quarks. The analysis explores the lepton-plus-jets final state, characterized by events with one isolated charged lepton (electron or muon), significant missing transverse momentum and multiple jets. One or more jets are required to be tagged as arising from $b$-quarks, and at least one pair of jets must be tagged as arising from the hadronic decay of an electroweak boson. The analysis uses the full data sample of $pp$ collisions recorded in 2012 by the ATLAS detector at the LHC, operating at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. No significant excess of events is observed above the expected background. Limits are set on vector-like $B$ production, as a function of the $B$ branching ratios, assuming the allowable decay modes are $B \rightarrow Wt/Zb/Hb$. In the chiral limit with a branching ratio of 100% for the decay $B \rightarrow Wt$, the observed (expected) 95% CL lower limit on the vector-like $B$ mass is 810 GeV (760 GeV). In the case where the vector-like $B$ quark has branching ratio values corresponding to those of an $SU(2)$ singlet state, the observed (expected) 95% CL lower limit on the vector-like $B$ mass is 640 GeV (505 GeV). The same analysis, when used to investigate pair production of a colored, charge 5/3 exotic fermion $T_{5/3}$, with subsequent decay $T_{5/3} \rightarrow Wt$, sets an observed (expected) 95% CL lower limit on the $T_{5/3}$ mass of 840 GeV (780 GeV).

hep-ex↗

arXiv2015

Search for flavour-changing neutral current top quark decays $t\to Hq$ in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

A search for flavour-changing neutral current decays of a top quark to an up-type quark ($q=u, c$) and the Standard Model Higgs boson, where the Higgs boson decays to $b\bar{b}$, is presented. The analysis searches for top quark pair events in which one top quark decays to $Wb$, with the $W$ boson decaying leptonically, and the other top quark decays to $Hq$. The search is based on $pp$ collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and uses an integrated luminosity of 20.3 fb$^{-1}$. Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of $b$-quark jets characteristic of signal events, and employs a likelihood discriminant that uses the kinematic differences between the signal and the background, which is dominated by $t\bar{t} \to WbWb$ decays. No significant excess of events above the background expectation is found, and observed (expected) 95% CL upper limits of 0.56% (0.42%) and 0.61% (0.64%) are derived for the $t\to Hc$ and $t\to Hu$ branching ratios respectively. The combination of this search with other ATLAS searches in the $H \to γγ$ and $H\to WW^*, ττ$ decay modes significantly improves the sensitivity, yielding observed (expected) 95% CL upper limits on the $t\to Hc$ and $t\to Hu$ branching ratios of 0.46% (0.25%) and 0.45% (0.29%) respectively. The corresponding combined observed (expected) upper limits on the $|λ_{tcH}|$ and $|λ_{tuH}|$ couplings are 0.13 (0.10) and 0.13 (0.10) respectively. These are the most restrictive direct bounds on $tqH$ interactions measured so far.

hep-ex↗

arXiv2015

Constraints on new phenomena via Higgs boson couplings and invisible decays with the ATLAS detector

The ATLAS experiment at the LHC has measured the Higgs boson couplings and mass, and searched for invisible Higgs boson decays, using multiple production and decay channels with up to 4.7 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=7$ TeV and 20.3 fb$^{-1}$ at $\sqrt{s}=8$ TeV. In the current study, the measured production and decay rates of the observed Higgs boson in the $γγ$, $ZZ$, $WW$, $Zγ$, $bb$, $ττ$, and $μμ$ decay channels, along with results from the associated production of a Higgs boson with a top-quark pair, are used to probe the scaling of the couplings with mass. Limits are set on parameters in extensions of the Standard Model including a composite Higgs boson, an additional electroweak singlet, and two-Higgs-doublet models. Together with the measured mass of the scalar Higgs boson in the $γγ$ and $ZZ$ decay modes, a lower limit is set on the pseudoscalar Higgs boson mass of $m_{A}>370$ GeV in the "hMSSM" simplified Minimal Supersymmetric Standard Model. Results from direct searches for heavy Higgs bosons are also interpreted in the hMSSM. Direct searches for invisible Higgs boson decays in the vector-boson fusion and associated production of a Higgs boson with $W/Z$ ($Z\to ll$, $W/Z \to jj$) modes are statistically combined to set an upper limit on the Higgs boson invisible branching ratio of 0.25. The use of the measured visible decay rates in a more general coupling fit improves the upper limit to 0.23, constraining a Higgs portal model of dark matter.

hep-ex↗

arXiv2015

Measurement of the $t\bar{t}W$ and $t\bar{t}Z$ production cross sections in $pp$ collisions at $\sqrt{s} = 8 \mathrm{\ Te\kern -0.1em V}$ with the ATLAS detector

The production cross sections of top-quark pairs in association with massive vector bosons have been measured using data from $pp$ collisions at $\sqrt{s} = 8 \mathrm{\ Te\kern -0.1em V}$. The dataset corresponds to an integrated luminosity of $20.3\ \mathrm{fb}^{-1}$ collected by the ATLAS detector in 2012 at the LHC. Final states with two, three or four leptons are considered. A fit to the data considering the $t\bar{t}W$ and $t\bar{t}Z$ processes simultaneously yields a significance of $5.0σ$ ($4.2σ$) over the background-only hypothesis for $t\bar{t}W$ ($t\bar{t}Z$) production. The measured cross sections are $σ_{t\bar{t}W} = 369^{+100}_{-91}$ fb and $σ_{t\bar{t}Z} = 176^{+58}_{-52}$ fb. The background-only hypothesis with neither $t\bar{t}W$ nor $t\bar{t}Z$ production is excluded at $7.1σ$. All measurements are consistent with next-to-leading-order calculations for the $t\bar{t}W$ and $t\bar{t}Z$ processes.

hep-ex↗

arXiv2015

A search for high-mass resonances decaying to $τ^{+}τ^{-}$ in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

A search for high-mass resonances decaying into $τ^{+}τ^{-}$ final states using proton-proton collisions at $\sqrt{s}= 8$ TeV produced by the Large Hadron Collider is presented. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 19.5-20.3 fb$^{-1}$. No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of $Z^{\prime}$ resonances decaying into $τ^+τ^-$ pairs as a function of the resonance mass. As a result, $Z^{\prime}$ bosons of the Sequential Standard Model with masses less than 2.02 TeV are excluded at 95% credibility. The impact of the fermionic couplings on the $Z^{\prime}$ acceptance is investigated and limits are also placed on a $Z^{\prime}$ model that exhibits enhanced couplings to third-generation fermions.

hep-ex↗

arXiv2015

Searches for Higgs boson pair production in the $hh\to bbττ, γγWW*, γγbb, bbbb$ channels with the ATLAS detector

Searches for both resonant and non-resonant Higgs boson pair production are performed in the $hh\to bbττ, γγWW^*$ final states using 20.3 fb$^{-1}$ of $pp$ collision data at a center-of-mass energy of 8 TeV recorded with the ATLAS detector at the Large Hadron Collider. No evidence of their production is observed and 95\% confidence level upper limits on the production cross sections are set. These results are then combined with the published results of the $hh\to γγbb, bbbb$ analyses. An upper limit of 0.69 (0.47) pb on the non-resonant Standard Model like $hh$ production is observed (expected), corresponding to 70 (48) times of the SM $gg\to hh$ cross section. For production via narrow resonances, cross section limits of $hh$ production from a heavy Higgs boson decay are set as a function of the heavy Higgs boson mass. The observed (expected) limits range from 2.1 (1.1) pb at 260 GeV to 0.011 (0.018) pb at 1000 GeV. These results are interpreted in the context of two simplified scenarios of the Minimal Supersymmetric Standard Model.

hep-ex↗

arXiv2015

A search for $t\bar{t}$ resonances using lepton-plus-jets events in proton-proton collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector

A search for new particles that decay into top quark pairs is reported. The search is performed with the ATLAS experiment at the LHC using an integrated luminosity of 20.3 fb$^{-1}$ of proton-proton collision data collected at a centre-of-mass energy of $\sqrt{s}=8$ TeV. The lepton-plus-jets final state is used, where the top pair decays to $W^+bW^-\bar{b}$, with one $W$ boson decaying leptonically and the other hadronically. The invariant mass spectrum of top quark pairs is examined for local excesses or deficits that are inconsistent with the Standard Model predictions. No evidence for a top quark pair resonance is found, and 95% confidence-level limits on the production rate are determined for massive states in benchmark models. The upper limits on the cross-section times branching ratio of a narrow $Z'$ boson decaying to top pairs range from 4.2 pb to 0.03 pb for resonance masses from 0.4 TeV to 3.0 TeV. A narrow leptophobic topcolour $Z'$ boson with mass below 1.8 TeV is excluded. Upper limits are set on the cross-section times branching ratio for a broad colour-octet resonance with $Γ/m =$ 15% decaying to $t\bar{t}$. These range from 2.5 pb to 0.03 pb for masses from 0.4 TeV to 3.0 TeV. A Kaluza-Klein excitation of the gluon in a Randall-Sundrum model is excluded for masses below 2.2 TeV.

hep-ex↗