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Register a new userPossible explanation of excess events in the search for jets, missing transverse momentum and a Z boson in pp collisions
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Ulrich Ellwanger
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We study to which extent SUSY extensions of the Standard Model can describe the excess of events of 3.0 standard deviations observed by ATLAS in the on-Z signal region, respecting constraints by CMS on similar signal channels as well as constraints from searches for jets and E^{miss}_T. GMSB-like scenarios are typically in conflict with these constraints, and do not reproduce well the shape of the E^{miss}_T distribution of the data. An alternative scenario with two massive neutralinos can improve fits to the total number of events as well as to the H_T and E^{miss}_T distributions. Such a scenario can be realised within the NMSSM.
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We present an updated search for the Higgs boson produced in association with a vector boson in the final state with missing transverse energy and two jets. We use the full CDF data set corresponding to an integrated luminosity of 9.45 fb${}^{-1}$ at a proton-antiproton center-of-mass energy of $sqrt{s}=1.96$ TeV. New to this analysis is the inclusion of a $b$-jet identification algorithm specifically optimized for $Hto bbar{b}$ searches. Across the Higgs boson mass range $90 le m_H le 150$ GeV$/c^2$, the expected 95% credibility level upper limits on the $V H$ production cross section times the $Hto bbar{b}$ branching fraction are improved by an average of 14% relative to the previous analysis. At a Higgs boson mass of 125 GeV$/c^2$, the observed (expected) limit is 3.06 (3.33) times the standard model prediction, corresponding to one of the most sensitive searches to date in this final state.
We present a search for new phenomena in events with two reconstructed $Z$ bosons and large missing transverse momentum, sensitive to processes $pbar{p}rightarrow X_2X_2 rightarrow Z Z X_1 X_1$, where $X_2$ is an unstable particle decaying as $X_2rightarrow ZX_1$ and $X_1$ is undetected. The particles $X_1$ and $X_2$ may be, among other possibilities, fourth generation neutrinos or supersymmetric particles. We study the final state in which one $Z$ boson decays to two charged leptons and the second decays hadronically. In data corresponding to an integrated luminosity of 4.2 fb$^{-1}$ from proton-antiproton collisions recorded by the CDF II detector at the Tevatron, with center-of-mass energy of 1.96 TeV, we find agreement between data and standard-model backgrounds. We calculate 95% confidence level upper limits on the cross section of the process $pbar{p}rightarrow X_2X_2 rightarrow Z Z X_1 X_1$ ranging from 50 fb to 1 pb, depending on the masses of $X_1$ and $X_2$.
Recent ATLAS data significantly extend the exclusion limits for supersymmetric particles. We examine the impact of such data on global fits of the constrained minimal supersymmetric standard model (CMSSM) to indirect and cosmological data. We calculate the likelihood map of the ATLAS search, taking into account systematic errors on the signal and on the background. We validate our calculation against the ATLAS determinaton of 95% confidence level exclusion contours. A previous CMSSM global fit is then re-weighted by the likelihood map, which takes a bite at the high probability density region of the global fit, pushing scalar and gaugino masses up.
We present the results of a search for dark matter production in the monojet signature. We analyze a sample of Tevatron pp-bar collisions at sqrt(s)=1.96 TeV corresponding to an integrated luminosity of 6.7/fb recorded by the CDF II detector. In events with large missing transverse energy and one energetic jet, we find good agreement between the standard model prediction and the observed data. We set 90% confidence level upper limits on the dark matter production rate. The limits are translated into bounds on nucleon-dark matter scattering rates which are competitive with current direct detection bounds on spin-independent interaction below a dark matter candidate mass of 5 GeV/c^2, and on spin-dependent interactions up to masses of 200 GeV/c^2.
We calculate the full one-loop electroweak radiative corrections, of ${cal O}(alpha^2alpha_s)$, to the cross section of single $Z$-boson inclusive hadroproduction at finite transverse momentum ($p_T$). This includes the ${cal O}(alpha)$ corrections to $Z+j$ production, the ${cal O}(alpha_s)$ corrections to $Z+gamma$ production, and certain QCD-electroweak interference contributions involving a single quark trace. We recover the QCD and purely weak corrections and study the QED corrections and the QCD-electroweak interference contributions for the first time. We also consider direct and resolved photoproduction in elastic and inelastic scattering. We present $p_T$ and rapidity distributions for the experimental conditions at the Fermilab Tevatron and the CERN LHC and assess the significance of the various contributions.
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