Top PDF Search for scalar leptons at LEP with the L3 detector

Search for a heavy top quark partner in final states with two leptons with the ATLAS detector at the LHC

Search for a heavy top quark partner in final states with two leptons with the ATLAS detector at the LHC

Partners of the top quark are an ingredient of several models addressing the hierarchy problem of the Standard Model (SM). In order to stabilize the Higgs boson mass against divergent quantum corrections, these new particles should have masses close to the elec- troweak symmetry breaking energy scale, and thus be accessible at the LHC. One of these models is Supersymmetry (SUSY) [1–9] which naturally resolves the hierarchy prob- lem [10–13] by introducing supersymmetric partners of the known bosons and fermions. In the MSSM [14–18], an R-parity conserving minimal supersymmetric extension of the SM, the scalar partners of right-handed and left-handed quarks, ˜ q R and ˜ q L , can mix to form two mass eigenstates. In this paper a search for a scalar top ˜ t 1 which decays into a top quark and the lightest neutralino ˜ χ 0
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Search for light top squark pair production in final states with leptons and b jets with the ATLAS detector in √s =7 TeV proton proton collisions

Search for light top squark pair production in final states with leptons and b jets with the ATLAS detector in √s =7 TeV proton proton collisions

The effect of the jet energy scale (JES) uncertainty on the final event yield is calculated by shifting the p T of all jets up and down by p T and η dependent factors, which are 5–3% for jets with p T of 20–60 GeV. Repeating the analysis with these p T shifts applied to the MC simulation leads to variations on the final background es- timate of 6–10% depending on the signal region. The uncertainty due to the jet energy resolution (JER) is calculated by smearing the p T of each jet by factors depending on the jet p T and η . The smearing on a single jet is typically around 10%, and results in an overall uncertainty of 1–10%. Systematic uncertainties in the lep- ton identification efficiency amount to 1%. The uncertainty on the E miss T due to the energy scale of the clusters in the calorimeter not associated with jets and electrons is evaluated using the method described in Ref. [49], extended to include pile-up uncertainties. The effect is up to 9% on the total background estimate depending on the signal region. The uncertainty due to b-tagging is evaluated by varying the b-tagging efficiency and mis-tag rates within the uncertainties of the measured values [50–52], giving an effect of 1% in all signal regions. The uncertainty associated with pile-up re- weighting is evaluated by varying the number of interactions per bunch-crossing by 10%. The overall effect on the predicted back- ground yield is at most 3%.
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Further search for supersymmetry at s√=7  TeV in final states with jets, missing transverse momentum, and isolated leptons with the ATLAS detector

Further search for supersymmetry at s√=7  TeV in final states with jets, missing transverse momentum, and isolated leptons with the ATLAS detector

The SUSY models considered are minimal supergravity (MSUGRA) or constrained MSSM (CMSSM) [25,26], minimal gauge-mediated SUSY breaking (GMSB) [27–31] and a number of simplified models [32,33]. The MSUGRA/CMSSM model is characterized by five pa- rameters: the universal scalar and gaugino mass parameters m 0 and m 1=2 , a universal trilinear coupling parameter A 0 , the ratio of the vacuum expectation values of the two Higgs doublets tan, and the sign of the Higgsino mass parameter . In this analysis, the values of m 0 and m 1=2 are scanned, and the other parameters are fixed as follows: tan ¼ 10, A 0 ¼ 0 and is taken to be positive. A diagram showing the decay of the associated production of a squark and a gluino is depicted in Fig. 1(a). Other diagrams representa- tive for the SUSY models discussed in the following are shown in Figs. 1(b)–1(d).
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Search for R parity violating supersymmetry in events with four or more leptons in √s=7 TeV pp collisions with the ATLAS detector

Search for R parity violating supersymmetry in events with four or more leptons in √s=7 TeV pp collisions with the ATLAS detector

Figure 3. Observed and expected 95% CL limit contours for (a) simplified model and (b) MSUGRA/CMSSM. The expected and observed limits are calculated without signal cross-section uncertainty taken into account. The yellow band is the ± 1σ experimental uncertainty on the ex- pected limit (black dashed line). The red dotted lines are the ± 1σ signal theory uncertainty on the observed limit (red solid line). Linear interpolation is used to account for the discreteness of the signal grids. The exclusion contours are optimised by applying in each signal grid point the CL values from the more sensitive signal region (lowest expected CL). It should be noted that the y-axis does not start at zero for either plot. The LEP limit corresponds to the limit on the ˜ χ ±
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Search for third generation scalar leptoquarks in pp collisions at √s=7 TeV with the ATLAS detector

Search for third generation scalar leptoquarks in pp collisions at √s=7 TeV with the ATLAS detector

Tau leptons can decay either leptonically (to an electron or muon plus two neutrinos), or hadronically (typically to one or three charged hadrons, plus one neutrino, and zero to four neutral hadrons). Since the final state includes two taus, this leads to three possi- ble sub-categories of events: di-lepton, lepton-hadron and hadron-hadron. Of these, the lepton-hadron category has the largest branching fraction (45.6%), and the presence of one charged light lepton (` = e, µ) in the event is useful for event triggering and provides better rejection of the multi-jets background. Only the lepton-hadron decay mode is considered in this analysis, resulting in either an eτ had-vis bb + 3ν or µτ had-vis bb + 3ν final state, where
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Search for scalar top quark pair production in natural gauge mediated supersymmetry models with the ATLAS detector in pp collisions at √s = 7 TeV

Search for scalar top quark pair production in natural gauge mediated supersymmetry models with the ATLAS detector in pp collisions at √s = 7 TeV

This search uses data recorded between March and August 2011 by the ATLAS detector at the LHC. After the application of beam, detector, and data quality requirements, the dataset corresponds to a total integrated luminosity of 2 . 05 ± 0 . 08 fb − 1 [24,25]. To en- hance the sensitivity to the aforementioned SUSY scenarios, events are required to contain energetic jets, of which one must be iden- tified as a b-jet, large E miss T and two opposite-sign, same flavour leptons ( = e , μ ) with invariant mass consistent with the Z boson mass, m Z . This is the first search for scalar top quarks decay- ing via Z bosons in GMSB models. General searches for super- symmetric particles in events with a Z boson, energetic jets and missing transverse momentum have been reported by the CMS Collaboration [26]. Searches for direct stop pair production have been performed at the CDF and D0 experiments assuming different SUSY mass spectra and decay modes (see for example Refs. [27] and [28]).
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Search for supersymmetry at √s = 13 TeV in final states with jets and two same sign leptons or three leptons with the ATLAS detector

Search for supersymmetry at $$\sqrt{s}=13$$ s = 13  TeV in final states with jets and two same-sign leptons or three leptons with the ATLAS detector

Diboson processes with four charged leptons (), three charged leptons and one neutrino, or two charged leptons and two neutrinos are simulated using the Sherpa v2.1.1 generator [31], and are described in detail in Ref. [32]. The matrix elements contain the doubly resonant W W , W Z and Z Z processes and all other diagrams with four or six elec- troweak vertices (such as same-electric-charge W boson pro- duction in association with two jets, W ± W ± j j ). Fully lep- tonic triboson processes (W W W , W W Z , W Z Z and Z Z Z ) with up to six charged leptons are also simulated using Sherpa v2.1.1 and described in Ref. [32]. The 4 and 2+2ν processes are calculated at next-to-leading order (NLO) for up to one additional parton; final states with two and three additional partons are calculated at leading order (LO). The W W Z → 4 + 2 ν or 2 + 4 ν processes are calculated at LO with up to two additional partons. The 3 +1ν process is calculated at NLO and up to three extra partons at LO using the Comix [33] and OpenLoops [34] matrix element gener- ators and merged with the Sherpa parton shower [35] using the ME+PS@NLO prescription [36]. The W W W/ W Z Z → 3 + 3ν, W Z Z → 5 + 1ν, Z Z Z → 6 + 0ν, 4 + 2ν or 2 + 4ν processes are calculated with the same config- uration but with up to only two extra partons at LO. The CT10 [37] parton distribution function (PDF) set is used for all Sherpa samples in conjunction with a dedicated tuning of the parton shower parameters developed by the Sherpa authors. The generator cross-sections (at NLO for most of the processes) are used when normalising these backgrounds.
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Search for direct slepton and gaugino production in final states with two leptons and missing transverse momentum with the ATLAS detector in pp collisions at √s=7 TeV

Search for direct slepton and gaugino production in final states with two leptons and missing transverse momentum with the ATLAS detector in pp collisions at √s=7 TeV

The factor S T corrects for possible differences in jet-veto ef- ficiency between data and MC simulation. Good agreement is ob- served in separate samples of t ¯ t and Z / γ ∗ + jets events and so this factor is taken to be equal to 1.0, with an uncertainty of 6%. The transfer factor is evaluated before the m T2 requirement is applied in the signal region since this requirement is designed to eliminate all but the tail of the m T2 distribution for t ¯ t. The efficiency of this requirement is then evaluated using MC simulation for a looser se- lection (which is assumed not to change the m T2 shape) and used to obtain the final estimate in SR-m T2 . The efficiency of the m T2 re- quirement is found to be ∼ 2% in each channel for top events with an uncertainty of ∼ 50%. The uncertainty is largely dominated by MC statistical uncertainty, generator uncertainties and jet and lep- ton scales and resolutions.
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Search for light scalar top quark pair production in final states with two leptons with the ATLAS detector in √s =7 TeV proton–proton collisions

Search for light scalar top quark pair production in final states with two leptons with the ATLAS detector in √s =7 TeV proton–proton collisions

The stop is predominantly right-handed, but this has little effect on the acceptance and efficiency for the final interpre- tation. The chargino ( χ ˜ 1 ± ) mass is set to 106 GeV (above the present exclusion limit of 103.5 GeV [15]) and it is as- sumed to decay through a virtual W boson ( χ ˜ 1 ± → W ∗ χ ˜ 1 0 ). The choice of chargino mass is identical to that used in a previous study reported by the CDF experiment [16], thus allowing easy comparison of the CDF and ATLAS results. Stops within a mass range between 110 GeV and 160 GeV would be produced with relatively large cross sections—between 245 pb and 41 pb. In this search, dilep- ton final states ( = e, μ) are considered. Although these events could contribute to an anomaly in the measured t t ¯ cross section, the relative contribution would be small due to the low transverse momenta of the visible decay prod- ucts. Events are required to contain at least one energetic jet, large missing transverse momentum (E T miss ) and low trans- verse momenta (p T ) leptons, to target the light stop final
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Search for direct top squark pair production in final states with two leptons in pp collisions at √s = 8 TeV with the ATLAS detector

Search for direct top squark pair production in final states with two leptons in pp collisions at √s = 8 TeV with the ATLAS detector

Many sources of systematic uncertainty are considered when evaluating this back- ground. Like the probabilities themselves, the systematic uncertainties are also parame- terised as a function of the lepton and event variables discussed above. The parameterised uncertainties are in general dominated by differences in the measurement of the fake lep- ton probabilities obtained when using the two control regions above. The limited number of events in the CR used to measure the probabilities are also considered as a source of systematic uncertainty. The overall systematic uncertainty ranges between 10% and 50% across the various regions (control, validation and signal). Ultimately, in SRs with very low predicted event yields the overall uncertainty on the fake and non-prompt lepton back- ground yield is dominated by the statistical uncertainty arising from the limited number of data events in the SRs, which reaches 60-80% in the less populated SRs. In these regions, however, the contributions from fake and non-prompt lepton events are small or negligible. The predictions obtained using this method are validated in events with same-charge lepton pairs. As an example, figure 4 shows the distribution of m eff and m T2 in events
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NA62 and NA48/2 results on search for Heavy Neutral Leptons

NA62 and NA48/2 results on search for Heavy Neutral Leptons

The peak search is performed in steps of 1 MeV/c 2 in the signal region. For each bin of missing mass the Rolke- Lopez method for the case of a Poisson process in pres- ence of gaussian background is applied to find the 90% confidence intervals. No significance above three sigma is found in the whole region considered (Fig. 3 (right)). The upper limit on the number of observed events (N µN ) is con-

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Search for direct top squark pair production in final states with two tau leptons in pp collisions at √s=8 TeV with the ATLAS detector

Search for direct top squark pair production in final states with two tau leptons in pp collisions at √s=8 TeV with the ATLAS detector

Also at Louisiana Tech University, Ruston, LA, USA Also at Institucio Catalana de Recerca i Estudis Avancats, ICREA, Barcelona, Spain p Also at Graduate School of Science, Osaka Universi[r]

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Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 8 TeV pp collisions with the ATLAS detector

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 8 TeV pp collisions with the ATLAS detector

consists of a silicon pixel detector, a semiconductor microstrip detector, and a transition radiation tracker. The ID is surrounded by a thin superconducting solenoid providing a 2 T axial magnetic field. A high-granularity lead/liquid-argon (LAr) sampling calorimeter measures the energy and the position of electromagnetic showers within | η | < 3.2. Sam- pling calorimeters with LAr are also used to measure hadronic showers in the end-cap (1.5 < | η | < 3.2) and forward (3.1 < | η | < 4.9) regions, while an iron/scintillator tile calorime- ter measures hadronic showers in the central region ( | η | < 1.7). The muon spectrometer (MS) surrounds the calorimeters and consists of three large superconducting air-core toroid magnets, each with eight coils, a system of precision tracking chambers ( | η | < 2.7), and fast trigger chambers ( | η | < 2.4). A three-level trigger system [33] selects events to be recorded for offline analysis.
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Search for the direct production of charginos and neutralinos in final states with tau leptons in root s=13 TeV pp collisions with the ATLAS detector

Search for the direct production of charginos and neutralinos in final states with tau leptons in root s=13 TeV pp collisions with the ATLAS detector

The multi-jet contribution in the W -CR (W -VR) is esti- mated using the so-called OS–SS method by counting the number of events in data satisfying the same requirements as the W -CR (W -VR) but with the electric charge of the two leptons having the same sign (SS). Events from SM processes other than multi-jet production are subtracted from the data counts in the SS region using MC simulation. The OS–SS method relies on the fact that in the multi-jet background the ratio of SS to OS events is close to unity, while a significant difference from unity is expected for W + jets production. The latter is dominated by gu/gd-initiated processes that often give rise to a jet originating from a quark, the charge of which is anti-correlated with the W boson charge. Based on studies with simulated samples, a systematic uncertainty of 100% is assigned to the multi-jet estimate in the W -CR.
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Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s=7 TeV pp collisions with the ATLAS detector

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s=7 TeV pp collisions with the ATLAS detector

Three signal regions are then defined: two “Z -depleted” re- gions (SR1a and SR1b), with no SFOS pairs having invariant mass within 10 GeV of the nominal Z-boson mass; and a “ Z -enriched” one (SR2), where at least one SFOS pair has an invariant mass within 10 GeV of the Z-boson mass. Events in SR1a and SR1b are further required to contain no b-tagged jets to suppress con- tributions from b-jet-rich background processes, where a lepton could originate from the decay of a heavy-flavor quark. SR1b is designed to increase sensitivity to scenarios characterised by large mass splittings between the heavy gauginos and the LSP by requir- ing all three leptons to have p T > 30 GeV. In both SR1b and SR2, the transverse mass variable m T must take values greater than 90 GeV, where m T is constructed using the E miss T and the lepton not included in the lepton pair with invariant mass closest to the nom- inal Z -boson mass. The m T requirement is introduced to suppress background from W Z events. The SR1a/b regions target neutralino decays via intermediate sleptons or via off-shell Z bosons while SR2 targets decays via an on-shell Z boson. Table 1 summarises the selection requirements for the three signal regions.
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Study of b quark mass effects in multijet topologies with the DELPHI detector at LEP

Study of b quark mass effects in multijet topologies with the DELPHI detector at LEP

and b and light-quark samples were separated using the DELPHI flavour tagging methods, based on properties of the long-lived heavy B-hadrons. Experimental results were then corrected for detector and acceptance effects in two different ways, depending on the observable and topology, as explained in Sect. 3.3. Matrix element and event generator predictions were corrected for hadronisa- tion effects from the parton to the hadron level. The parton level is defined as the final state of the parton shower (in PYTHIA and HERWIG) or dipole cascade (in ARIADNE) in the simulation, before hadronisation. These corrections are discussed in Sect. 4.
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Measurements of jet rates with the anti-kt and SISCone algorithms at LEP with the OPAL detector

Measurements of jet rates with the anti-kt and SISCone algorithms at LEP with the OPAL detector

The simulated detector response data are then reconstructed with the same software as the real data to obtain simulated events at “detector level”. Events are labeled by the numbers of reconstructed jets at the values of y or of a given distribution. For example, with four increasing values of y an event could have the sequence {4322}, i.e. at y 1 4 jets, at y 2 3 jets and at y 3 and y 4 2 jets are found. The

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Beyond the standard model searches with b jets and hadronically decaying τ leptons with the ATLAS detector

Beyond the standard model searches with b jets and hadronically decaying τ leptons with the ATLAS detector

As previously stated, the ATLAS detector is one of two multi-purpose detectors on the LHC ring located approximately 93 m underground at Point 1. The detector has an almost cylindrical forward-backward symmetric design with a diameter of 25 m and an overall length of 44 m. The purpose of the ATLAS detector is to record the energies and trajectories of particles emerging from the collisions with the help of numerous sub-detector components. The inner detector (ID) surrounds the interaction point and is made up of high-resolution semiconductor pixels, silicon microstrip trackers (SCT) and a transition radiation tracker (TRT) all encompassed within a 2 T solenoidal magnetic field. In 2014, the ATLAS insertable B-layer (IBL) was introduced as a fourth layer to the present pixel detector around the beam pipe. This was to ensure tracking efficiency and precision throughout Run-2 and the future HL-LHC operations and protect against any degradation of the equipment over time.
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Search for heavy resonances with the ATLAS detector

Search for heavy resonances with the ATLAS detector

Searching for new physics scenarios in final states with one lepton plus missing transverse energy, two di ff erent extending theories are considered [3]. A first possible extension to the SM is provided by the Sequential Standard Model (SSM) predicting additional heavier vector gauge bosons with spin-1 outside the SM, denoted as W (charged current) and Z (neutral current). Their couplings to SM fermions are assumed to be identical to those of the SM bosons whereas couplings to W / Z (in the W case) are set to zero. The intrinsic width of the resonance increases linearly with the boson mass, and is assumed small compared to the detector resolution. An interference with the SM W or Z is neglected in the analysis.
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