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1. chinaXiv:201609.00829 [pdf]
Khachatryan, Vardan and othersSubjects: Physics >> Nuclear Physics
| Results are presented from a search for supersymmetric particles in scenarios with a compressed mass spectrum. The data sample corresponds to 19.7 inverse femtobarns of proton-proton collisions recorded by the CMS experiment at sqrt(s) = 8 TeV. The search targets top squark pair production in scenarios with mass differences Delta m = m(top squark) - m(neutralino) below the W-boson mass and with top-squark decays in the four-body mode (top squark to b l nu neutralino), where the neutralino is assumed to be the lightest supersymmetric particle (LSP). The signature includes a high transverse momentum (pt) jet associated with initial-state radiation, one or two low-pt leptons, and significant missing transverse energy. The event yields observed in data are consistent with the expected background contributions from standard model processes. Limits are set on the cross section for top squark pair production as a function of the top squark and LSP masses. Assuming a 100% branching fraction for the four-body decay mode, top-squark masses below 316 GeV are excluded for Delta m = 25 GeV at 95% CL. The dilepton data are also interpreted under the assumption of chargino-neutralino production, with subsequent decays to sleptons or sneutrinos. Assuming a difference between the common lightest chargino/next lightest neutralino mass and the LSP mass of 20 GeV and a tau-enriched decay scenario, masses in the range m(chargino) < 307 GeV are excluded at 95% CL. |
2. chinaXiv:201609.00820 [pdf]
Reconstruction of hadronic decay products of tau leptons with the ATLAS experiment
Aad, Georges and othersSubjects: Physics >> Nuclear Physics
| This paper presents a new method of reconstructing the individual charged and neutral hadrons in tau decays with the ATLAS detector. The reconstructed hadrons are used to classify the decay mode and to calculate the visible four-momentum of reconstructed tau candidates, significantly improving the resolution with respect to the calibration in the existing tau reconstruction. The performance of the reconstruction algorithm is optimised and evaluated using simulation and validated using samples of?Z→ττ?and?Z(→μμ)+jets events selected from proton-proton collisions at a centre-of-mass energy?s√=8?TeV, corresponding to an integrated luminosity of 5 fb?1. |
3. chinaXiv:201609.00787 [pdf]
Search for excited leptons in proton-proton collisions at $\sqrt{s} =$ 8 TeV
Khachatryan, Vardan and othersSubjects: Physics >> Nuclear Physics
| A search for compositeness of electrons and muons is presented using a data sample of proton-proton collisions at a center-of-mass energy of sqrt(s) = 8 TeV collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 19.7 inverse femtobarns. Excited leptons (l*) produced via contact interactions in conjunction with a standard model lepton are considered, and a search is made for their gauge decay modes. The decays considered are l* to l + gamma and l* to l + Z, which give final states of two leptons and a photon or, depending on the Z-boson decay mode, four leptons or two leptons and two jets. The number of events observed in data is consistent with the standard model prediction. Exclusion limits are set on the excited lepton mass, and the compositeness scale Lambda. For the case M(l*) = Lambda the existence of excited electrons (muons) is excluded up to masses of 2.45 (2.47) TeV at 95% confidence level. Neutral current decays of excited leptons are considered for the first time, and limits are extended to include the possibility that the weight factors f and f', which determine the couplings between standard model leptons and excited leptons via gauge mediated interactions, have opposite sign. |
4. chinaXiv:201609.00782 [pdf]
Aad, Georges and othersSubjects: Physics >> Nuclear Physics
| A measurement of the correlations between the polar angles of leptons from the decay of pair-produced?t?and?t?quarks in the helicity basis is reported, using proton-proton collision data collected by the ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 4.6fb?1?at a center-of-mass energy of?s√=7TeV collected during 2011. Candidate events are selected in the dilepton topology with large missing transverse momentum and at least two jets. The angles?θ1?and?θ2?between the charged leptons and the direction of motion of the parent quarks in the?tt??rest frame are sensitive to the spin information, and the distribution of {\mbox{cosθ1?cosθ2}} is sensitive to the spin correlation between the?t?and?t??quarks. The distribution is unfolded to parton level and compared to the next-to-leading order prediction. A good agreement is observed. |
5. chinaXiv:201609.00737 [pdf]
Aad, Georges and othersSubjects: Physics >> Nuclear Physics
| A search for direct pair production of the supersymmetric partner of the top quark, decaying via a scalar tau to a nearly massless gravitino, has been performed using 20 fb?1?of proton-proton collision data at?s√=8?TeV. The data were collected by the ATLAS experiment at the LHC in 2012. Top squark candidates are searched for in events with either two hadronically decaying tau leptons, one hadronically decaying tau and one light lepton, or two light leptons. No significant excess over the Standard Model expectation is found. Exclusion limits at 95% confidence level are set as a function of the top squark and scalar tau masses. Depending on the scalar tau mass, ranging from the 87 GeV LEP limit to the top squark mass, lower limits between 490 GeV and 650 GeV are placed on the top squark mass within the model considered. |
6. chinaXiv:201609.00568 [pdf]
Aad, Georges and othersSubjects: Physics >> Nuclear Physics
| Measurements of the branching ratios of top quark decays into leptons and jets using events with?tt??(top antitop) pairs are reported. Events were recorded with the ATLAS detector at the LHC in pp collisions at a center-of-mass energy of 7 TeV. The collected data sample corresponds to an integrated luminosity of 4.6?fb?1. The measured top quark branching ratios agree with the Standard Model predictions within the measurement uncertainties of a few percent. |
7. chinaXiv:201609.00564 [pdf]
Aad, Georges and othersSubjects: Physics >> Nuclear Physics
| A search for the pair-production of heavy leptons (N0,L±) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels?N0→W±l??(?=e,μ,τ) and?L±→W±ν?(ν=νe,νμ,ντ) are considered. The analysis is performed using the final state that contains two leptons (electrons or muons), two jets from a hadronically decaying?W?boson, and large missing transverse momentum. The data used in the measurement correspond to an integrated luminosity of?20.3fb?1?of?pp?collisions at?s√=8?TeV collected by the ATLAS detector at the LHC. No evidence of heavy lepton pair-production is observed. Heavy leptons with masses below 325--540 GeV are excluded at the 95% confidence level, depending on the theoretical scenario considered. |
8. chinaXiv:201609.00242 [pdf]
Aad, Georges and othersSubjects: Physics >> Nuclear Physics
| An analysis is presented of events containing jets including at least one?b-tagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb?1?of?pp?collisions at?s√=8?TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events; the ones considered here are production of vector-like quarks, enhanced four-top-quark production, pair production of chiral?b′-quarks, and production of two positively charged top quarks. Eleven signal regions are defined; subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral?b′-quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models. |
9. chinaXiv:201609.00189 [pdf]
Khachatryan, Vardan and othersSubjects: Physics >> Nuclear Physics
| A search is presented for physics beyond the standard model in final states with two opposite-sign same-flavor leptons, jets, and missing transverse momentum. The data sample corresponds to an integrated luminosity of 19.4 inverse femtobarns of proton-proton collisions at sqrt(s) = 8 TeV collected with the CMS detector at the CERN LHC in 2012. The analysis focuses on searches for a kinematic edge in the invariant mass distribution of the opposite-sign same-flavor lepton pair and for final states with an on-shell Z boson. The observations are consistent with expectations from standard model processes and are interpreted in terms of upper limits on the production of supersymmetric particles. |
10. chinaXiv:201609.00141 [pdf]
Aad, Georges and othersSubjects: Physics >> Nuclear Physics
| The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider (LHC) are reported. The search is based on proton-proton collision data at a centre-of-mass energy?s√=8?TeV collected in 2012, corresponding to an integrated luminosity of 20 fb?1. No significant excess above the Standard Model expectation is observed. Limits are set on the parameters of a minimal universal extra dimensions model, excluding a compactification radius of?1/Rc=950?GeV for a cut-off scale times radius (ΛRc) of approximately 30, as well as on sparticle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV. |
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