Search

A search for resonant diboson production using a data sample corresponding to $4.7{\mathrm{fb}}^{-1}$ of integrated luminosity collected by the ATLAS experiment at the Large Hadron Collider in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ is presented. The search for a narrow resonance in the $WW$ or $WZ$ mass distribution is conducted in a final state with an electron or a muon, missing transverse momentum, and at least two jets. No significant excess is observed and limits are set using three benchmark models: $WW$ resonance masses below 940 and 710 GeV are excluded at 95% confidence level for spin-2 Randall–Sundrum and bulk Randall–Sundrum gravitons, respectively; $WZ$ resonance masses below 950 GeV are excluded at 95% confidence level for a spin-1 extended gauge model $W^{\prime }$ boson.

A generic search is presented for a heavy particle decaying to $WZ\to \ell \nu {\ell }^{\prime }{\ell }^{\prime }$ ($\ell$, ${\ell }^{\prime }=e$, $\mu$) final states. The data were recorded by the ATLAS detector in $\sqrt{s}=7\mathrm{TeV}$ $pp$ collisions at the Large Hadron Collider and correspond to an integrated luminosity of $1.02{\mathrm{fb}}^{-1}$. The transverse mass distribution of the selected $WZ$ candidates is found to be consistent with the standard model expectation. Upper limits on the production cross section times branching ratio are derived using two benchmark models predicting a heavy particle decaying to a $WZ$ pair.

This paper presents a search for a new heavy particle produced in association with a top or antitop quark. Two models in which the new heavy particle is a color singlet or a color triplet are considered, decaying, respectively, to $\overline{t}q$ or $tq$, leading to a resonance within the $t\overline{t}+\mathrm{\text{jets}}$ signature. The full 2011 ATLAS $pp$ collision data set from the LHC ($4.7{\mathrm{fb}}^{-1}$) is used to search for $t\overline{t}$ events produced in association with jets, in which one of the $W$ bosons from the top quarks decays leptonically and the other decays hadronically. The data are consistent with the Standard Model expectation, and a new particle with mass below 430 Gev for both $W^{\prime }$ boson and color triplet models is excluded at 95% confidence level, assuming unit right-handed coupling.

A search is presented for gluinos decaying via the supersymmetric partner of the top quark using events with two same-sign leptons, jets, and missing transverse momentum. The analysis is performed with $2.05{\mathrm{fb}}^{-1}$ of integrated luminosity from $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$ collected by the ATLAS detector at the LHC. No excess beyond the standard model expectation is observed, and exclusion limits are derived for simplified models where the gluino decays via the supersymmetric partner of the top quark and in the minimal supergravity and constrained minimal supersymmetric standard model framework. In those scenarios, gluino masses below 550 GeV are excluded at 95% C.L. within the parameter space considered, significantly extending the coverage with respect to existing limits. Depending on the model parameters, gluino masses up to 750 GeV can also be excluded at 95% C.L.

A search is presented for direct top squark pair production in final states with one isolated electron or muon, jets, and missing transverse momentum in proton-proton collisions at $\sqrt{s}=7\mathrm{TeV}$. The measurement is based on $4.7{\mathrm{fb}}^{-1}$ of data collected with the ATLAS detector at the LHC. Each top squark is assumed to decay to a top quark and the lightest supersymmetric particle (LSP). The data are found to be consistent with standard model expectations. Top squark masses between 230 GeV and 440 GeV are excluded with 95% confidence for massless LSPs, and top squark masses around 400 GeV are excluded for LSP masses up to 125 GeV.

A search for a heavy standard model Higgs boson decaying via $H\to ZZ\to {\ell }^{+}{\ell }^{-}\nu \overline{\nu }$, where $\ell =e$, $\mu$, is presented. It is based on proton-proton collision data at $\sqrt{s}=7\mathrm{TeV}$, collected by the ATLAS experiment at the LHC in the first half of 2011 and corresponding to an integrated luminosity of $1.04{\mathrm{fb}}^{-1}$. The data are compared to the expected standard model backgrounds. The data and the background expectations are found to be in agreement and upper limits are placed on the Higgs boson production cross section over the entire mass window considered; in particular, the production of a standard model Higgs boson is excluded in the region $340<m_H<450\mathrm{GeV}$ at the 95% confidence level.

This paper presents a measurement of the $W^{+}{W}^{-}$ production cross section in $pp$ collisions at $\sqrt{s}=7\mathrm{TeV}$. The leptonic decay channels are analyzed using data corresponding to an integrated luminosity of $4.6{\mathrm{fb}}^{-1}$ collected with the ATLAS detector at the Large Hadron Collider. The $W^{+}{W}^{-}$ production cross section $\sigma (pp\to W^{+}{W}^{-}+X)$ is measured to be $51.9\pm 2.0(\mathrm{stat})\pm 3.9(\mathrm{syst})\pm 2.0(\mathrm{lumi})\mathrm{pb}$, compatible with the Standard Model prediction of ${44.7}_{-1.9}^{+2.1}\mathrm{pb}$. A measurement of the normalized fiducial cross section as a function of the leading lepton transverse momentum is also presented. The reconstructed transverse momentum distribution of the leading lepton is used to extract limits on anomalous $WWZ$ and $WW\gamma$ couplings.

We report the first observation of a baryonic $B_s^0$ decay, $B_s^0\to p\overline{\mathrm{\Lambda }}{K}^{-}$, using proton-proton collision data recorded by the LHCb experiment at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$. The branching fraction is measured to be $\mathcal{B}(B_s^0\to p\overline{\mathrm{\Lambda }}{K}^{-})+\mathcal{B}(B_s^0\to \overline{p}\mathrm{\Lambda }{K}^{+})=[5.46\pm 0.61\pm 0.57\pm 0.50(\mathcal{B})\pm 0.32(f_s/f_d)]\times {10}^{-6}$, where the first uncertainty is statistical and the second systematic, the third uncertainty accounts for the experimental uncertainty on the branching fraction of the $B^0\to p\overline{\mathrm{\Lambda }}{\pi }^{-}$ decay used for normalization, and the fourth uncertainty relates to the knowledge of the ratio of $b$-quark hadronization probabilities $f_s/f_d$.

The $B^{+}\to D^{+}{K}^{+}{\pi }^{-}$ decay is observed in a data sample corresponding to $3.0{\mathrm{fb}}^{-1}$ of $pp$ collision data recorded by the LHCb experiment during 2011 and 2012. The signal significance is $8\sigma$ and the branching fraction is measured to be $\mathcal{B}(B^{+}\to D^{+}{K}^{+}{\pi }^{-})=(5.31\pm 0.90\pm 0.48\pm 0.35)\times {10}^{-6}$, where the uncertainties are statistical, systematic and due to the normalization mode $B^{+}\to D^{-}{K}^{+}{\pi }^{+}$, respectively. The Dalitz plot appears to be dominated by broad structures. Angular distributions are exploited to search for quasi-two-body contributions from $B^{+}\to D_2^{*}(2460{)}^0{K}^{+}$ and $B^{+}\to D^{+}{K}^{*}(892{)}^0$ decays. No significant signals are observed and upper limits are set on their branching fractions.

A search for the rare decay of a $B^0$ or $B_s^0$ meson into the final state $J/\psi \gamma$ is performed, using data collected by the LHCb experiment in $pp$ collisions at $\sqrt{s}=7$ and 8 TeV, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. The observed number of signal candidates is consistent with a background-only hypothesis. Branching fraction values larger than $1.5\times 10^{-6}$ for the $B^0\to J/\psi \gamma$ decay mode are excluded at 90% confidence level. For the $B_s^0\to J/\psi \gamma$ decay mode, branching fraction values larger than $7.3\times 10^{-6}$ are excluded at 90% confidence level; this is the first branching fraction limit for this decay.

The associated production of a $W$ boson with a jet originating from either a light parton or heavy-flavor quark is studied in the forward region using proton-proton collisions. The analysis uses data corresponding to integrated luminosities of 1.0 and $2.0{\mathrm{fb}}^{-1}$ collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, respectively. The $W$ bosons are reconstructed using the $W\to \mu \nu$ decay and muons with a transverse momentum, $p_{\mathrm{T}}$, larger than 20 GeV in the pseudorapidity range $2.0<\eta <4.5$. The partons are reconstructed as jets with $p_{\mathrm{T}}>20\mathrm{GeV}$ and $2.2<\eta <4.2$. The sum of the muon and jet momenta must satisfy $p_{\mathrm{T}}>20\mathrm{GeV}$. The fraction of $W+\text{jet}$ events that originate from beauty and charm quarks is measured, along with the charge asymmetries of the $W+b$ and $W+c$ production cross sections. The ratio of the $W+\text{jet}$ to $Z+\text{jet}$ production cross sections is also measured using the $Z\to \mu \mu$ decay. All results are in agreement with Standard Model predictions.

Using a $pp$ collision data sample corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, collected by the LHCb detector, we present the first search for the strangeness-changing weak decay ${\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-}$. No $b$ hadron decay of this type has been seen before. A signal for this decay, corresponding to a significance of 3.2 standard deviations, is reported. The relative rate is measured to be $$\frac{f_{{\mathrm{\Xi }}_b^{-}}}{f_{{\mathrm{\Lambda }}_b^0}}\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})=(5.7\pm 1.8_{-0.9}^{+0.8})\times {10}^{-4},$$where $f_{{\mathrm{\Xi }}_b^{-}}$ and $f_{{\mathrm{\Lambda }}_b^0}$ are the $b\to {\mathrm{\Xi }}_b^{-}$ and $b\to {\mathrm{\Lambda }}_b^0$ fragmentation fractions, and $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})$ is the branching fraction. Assuming $f_{{\mathrm{\Xi }}_b^{-}}/f_{{\mathrm{\Lambda }}_b^0}$ is bounded between 0.1 and 0.3, the branching fraction $\mathcal{B}({\mathrm{\Xi }}_b^{-}\to {\mathrm{\Lambda }}_b^0{\pi }^{-})$ would lie in the range from $(0.57\pm 0.21)\%$ to $(0.19\pm 0.07)\%$.

Decays of beauty baryons to the $D^{0}p{h}^{-}$ and ${\mathrm{\Lambda }}_c^{+}{h}^{-}$ final states (where $h$ indicates a pion or a kaon) are studied using a data sample of $pp$ collisions, corresponding to an integrated luminosity of $1.0{\mathrm{fb}}^{-1}$, collected by the LHCb detector. The Cabibbo-suppressed decays ${\mathrm{\Lambda }}_b^0\to D^{0}p{K}^{-}$ and ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{K}^{-}$ are observed, and their branching fractions are measured with respect to the decays ${\mathrm{\Lambda }}_b^0\to D^{0}p{\pi }^{-}$ and ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-}$. In addition, the first observation is reported of the decay of the neutral beauty-strange baryon ${\mathrm{\Xi }}_b^0$ to the $D^{0}p{K}^{-}$ final state, and a measurement of the ${\mathrm{\Xi }}_b^0$ mass is performed. Evidence of the ${\mathrm{\Xi }}_b^0\to {\mathrm{\Lambda }}_c^{+}{K}^{-}$ decay is also reported.

First observations of the decays ${\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_{(s)}^{-}$ are reported using data corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$ collected at 7 and 8 TeV center-of-mass energies in proton-proton collisions with the LHCb detector. In addition, the most precise measurement of the branching fraction $\mathcal{B}(B_s^0\to D^{+}{D}_s^{-})$ is made and a search is performed for the decays $B_{(s)}^0\to {\mathrm{\Lambda }}_c^{+}{\mathrm{\Lambda }}_c^{-}$. The results obtained are $$\begin{gathered} \mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}^{-})/\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_s^{-})=0.042\pm 0.003(\text{stat)}\pm 0.003(\text{syst}), \\ \left[\frac{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{D}_s^{-})}{\mathcal{B}({\overline{B}}^0\to D^{+}{D}_s^{-})}\right]/\left[\frac{\mathcal{B}({\mathrm{\Lambda }}_b^0\to {\mathrm{\Lambda }}_c^{+}{\pi }^{-})}{\mathcal{B}({\overline{B}}^0\to D^{+}{\pi }^{-})}\right]=0.96\pm 0.02(\text{stat)}\pm 0.06(\text{syst)}, \\ \mathcal{B}(B_s^0\to D^{+}{D}_s^{-})/\mathcal{B}({\overline{B}}^0\to D^{+}{D}_s^{-})=0.038\pm 0.004(\text{stat)}\pm 0.003(\text{syst)}, \\ \mathcal{B}({\overline{B}}^0\to {\mathrm{\Lambda }}_c^{+}{\mathrm{\Lambda }}_c^{-})/\mathcal{B}({\overline{B}}^0\to D^{+}{D}_s^{-})<0.0022[95\%\mathrm{C}\mathrm{.}\mathrm{L}\mathrm{.}], \\ \mathcal{B}(B_s^0\to {\mathrm{\Lambda }}_c^{+}{\mathrm{\Lambda }}_c^{-})/\mathcal{B}(B_s^0\to D^{+}{D}_s^{-})<0.30[95\%\mathrm{C}\mathrm{.}\mathrm{L}\mathrm{.}]. \end{gathered}$$Measurement of the mass of the ${\mathrm{\Lambda }}_b^0$ baryon relative to the ${\overline{B}}^0$ meson gives $M({\mathrm{\Lambda }}_b^0)-M({\overline{B}}^0)=339.72\pm 0.24(\text{stat})\pm 0.18(\text{syst)}\mathrm{MeV}/c^2$. This result provides the most precise measurement of the mass of the ${\mathrm{\Lambda }}_b^0$ baryon to date.

Violations of $CPT$ symmetry and Lorentz invariance are searched for by studying interference effects in $B^0$ mixing and in $B_s^0$ mixing. Samples of $B^0\to J/\psi K_S^0$ and $B_s^0\to J/\psi K^{+}{K}^{-}$ decays are recorded by the LHCb detector in proton-proton collisions at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. No periodic variations of the particle-antiparticle mass differences are found, consistent with Lorentz invariance and $CPT$ symmetry. Results are expressed in terms of the standard model extension parameter $\mathrm{\Delta }{a}_{\mu }$ with precisions of $\mathcal{O}({10}^{-15})$ and $\mathcal{O}({10}^{-14})\mathrm{GeV}$ for the $B^0$ and $B_s^0$ systems, respectively. With no assumption on Lorentz (non)invariance, the $CPT$-violating parameter $z$ in the $B_s^0$ system is measured for the first time and found to be $\mathcal{R}e(z)=-0.022\pm 0.033\pm 0.005$ and $\mathcal{I}m(z)=0.004\pm 0.011\pm 0.002$, where the first uncertainties are statistical and the second systematic.

Decays of the ${\mathrm{\Xi }}_b^{-}$ and ${\mathrm{\Omega }}_b^{-}$ baryons to the charmless final states $p{h}^{-}{h}^{\prime -}$, where $h^{(\prime )}$ denotes a kaon or pion, are searched for with the LHCb detector. The analysis is based on a sample of proton-proton collision data collected at center-of-mass energies $\sqrt{s}=7$ and 8 TeV, corresponding to an integrated luminosity of $3{\mathrm{fb}}^{-1}$. The decay ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{K}^{-}$ is observed with a significance of 8.7 standard deviations, and evidence at the level of 3.4 standard deviations is found for the ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{\pi }^{-}$ decay. Results are reported, relative to the $B^{-}\to K^{+}{K}^{-}{K}^{-}$ normalization channel, for the products of branching fractions and $b$-hadron production fractions. The branching fractions of ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{\pi }^{-}$ and ${\mathrm{\Xi }}_b^{-}\to p{\pi }^{-}{\pi }^{-}$ relative to ${\mathrm{\Xi }}_b^{-}\to p{K}^{-}{K}^{-}$ decays are also measured.

A combined search for the Standard Model Higgs boson with the ATLAS detector at the LHC is presented. The data sets used correspond to integrated luminosities from $4.6{\mathrm{fb}}^{-1}$ to $4.9{\mathrm{fb}}^{-1}$ of proton-proton collisions collected at $\sqrt{s}=7\mathrm{TeV}$ in 2011. The Higgs boson mass ranges of 111.4 GeV to 116.6 GeV, 119.4 GeV to 122.1 GeV, and 129.2 GeV to 541 GeV are excluded at the 95% confidence level, while the range 120 GeV to 560 GeV is expected to be excluded in the absence of a signal. An excess of events is observed at Higgs boson mass hypotheses around 126 GeV with a local significance of 2.9 standard deviations ($\sigma$). The global probability for the background to produce an excess at least as significant anywhere in the entire explored Higgs boson mass range of 110–600 GeV is estimated to be $\sim 15\%$, corresponding to a significance of approximately $1\sigma$.

This paper presents a study of the possible ordering of charged hadrons in the azimuthal angle relative to the beam axis in high-energy proton-proton collisions at the Large Hadron Collider (LHC). A spectral analysis of correlations between longitudinal and transverse components of the momentum of the charged hadrons, driven by the search for phenomena related to the structure of the QCD field, is performed. Data were recorded with the ATLAS detector at center-of-mass energies of $\sqrt{s}=900\mathrm{GeV}$ and $\sqrt{s}=7\mathrm{TeV}$. The correlations measured in a kinematic region dominated by low-$p_T$ particles are not well described by conventional models of hadron production. The measured spectra show features consistent with the fragmentation of a QCD string represented by a helixlike ordered gluon chain.

A measurement of the $CP$ asymmetry in $B^0\to K^{*0}{\mu }^{+}{\mu }^{-}$ decays is presented, based on $1.0{\mathrm{fb}}^{-1}$ of $pp$ collision data recorded by the LHCb experiment during 2011. The measurement is performed in six bins of invariant mass squared of the ${\mu }^{+}{\mu }^{-}$ pair, excluding the $J/\psi$ and $\psi (2S)$ resonance regions. Production and detection asymmetries are removed using the $B^0\to J/\psi K^{*0}$ decay as a control mode. The integrated $CP$ asymmetry is found to be $-0.072\pm 0.040(\mathrm{stat})\pm 0.005(\mathrm{syst})$, consistent with the standard model.

The time-dependent $CP$ asymmetry in $B_s^0\to J/\psi K^{+}{K}^{-}$ decays is measured using $pp$ collision data, corresponding to an integrated luminosity of $3.0{\mathrm{fb}}^{-1}$, collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV. In a sample of 96 000 $B_s^0\to J/\psi K^{+}{K}^{-}$ decays, the $CP$-violating phase ${\varphi }_s$ is measured, as well as the decay widths ${\mathrm{\Gamma }}_{\mathrm{L}}$ and ${\mathrm{\Gamma }}_{\mathrm{H}}$ of the light and heavy mass eigenstates of the $B_s^0–{\overline{B}}_s^0$ system. The values obtained are ${\varphi }_s=-0.058\pm 0.049\pm 0.006\mathrm{rad}$, ${\mathrm{\Gamma }}_s\equiv ({\mathrm{\Gamma }}_{\mathrm{L}}+{\mathrm{\Gamma }}_{\mathrm{H}})/2=0.6603\pm 0.0027\pm 0.0015{\mathrm{ps}}^{-1}$, and $\mathrm{\Delta }{\mathrm{\Gamma }}_s\equiv {\mathrm{\Gamma }}_{\mathrm{L}}-{\mathrm{\Gamma }}_{\mathrm{H}}=0.0805\pm 0.0091\pm 0.0032{\mathrm{ps}}^{-1}$, where the first uncertainty is statistical and the second, systematic. These are the most precise single measurements of those quantities to date. A combined analysis with $B_s^0\to J/\psi {\pi }^{+}{\pi }^{-}$ decays gives ${\varphi }_s=-0.010\pm 0.039\mathrm{rad}$. All measurements are in agreement with the standard model predictions. For the first time, the phase ${\varphi }_s$ is measured independently for each polarization state of the $K^{+}{K}^{-}$ system and shows no evidence for polarization dependence.