Event Topology Research Articles

On the generation and topology of flux transfer events

We use resistive magnetohydrodynamics (MHD) simulations to investigate the generation and topology of flux transfer events (FTEs) at Earth's dayside magnetopause under generic southward interplanetary magnetic field (IMF) conditions. We focus on three questions: (1) Can FTEs form spontaneously under steady solar wind conditions? (2) What is the role of magnetic reconnection in the generation of FTEs? (3) What effect do FTEs have on the magnetopause magnetic field topology? We find that in the simulations, FTEs form spontaneously under steady solar wind conditions. In contrast to the multiple X line model of Raeder (2006), dipole tilt is not required to produce FTEs in resistive MHD simulations. Further, FTEs are generated by flow vortices which begin to grow well before magnetic reconnection changes the magnetopause magnetic field topology. FTEs form under steady southward IMF conditions but not under steady northward IMF conditions, suggesting that the magnetopause becomes unstable when the IMF turns southward. In our simulations, this instability appears to be triggered by a movement of the flow stagnation point away from the magnetic separator, which modifies the subsolar stagnation point flow, thereby rendering it unstable. Finally, examination of the internal magnetic structure of the simulated FTEs reveals a complex topology resulting from multiple separator bifurcations driven by plasma flows within the FTEs.

Probing TeV gravity with the ATLAS detector

Models with compactified extra space dimensions offer a new way to address outstanding problems in and beyond the Standard Model. In these models, the strength of gravity is strongly increased at small distances, which opens up the possibility of observing quantum gravity effects in the TeV energy range reachable by the LHC. One of the most spectacular phenomena would be the production of microscopic black holes. Searches for black holes are foreseen in the ATLAS experiment with the start-up of data taking in 2009. We present feasibility studies for the triggering, selection and reconstruction of the black hole event topologies, the black hole discovery potential and their identification.

Using subsystemMT2for complete mass determinations in decay chains with missing energy at hadron colliders

We propose to use the MT2 concept to measure the masses of all particles in SUSY-like events with two unobservable, identical particles. To this end we generalize the usual notion of MT2 and define a new MT2(n,p,c) variable, which can be applied to various subsystem topologies, as well as the full event topology. We derive analytic formulas for its endpoint MT2{max}(n,p,c) as a function of the unknown test mass Mc of the final particle in the subchain and the transverse momentum pT due to radiation from the initial state. We show that the endpoint functions MT2{max}(n,p,c)(Mc,pT) may exhibit three different types of kinks and discuss the origin of each type. We prove that the subsystem MT2(n,p,c) variables by themselves already yield a sufficient number of measurements for a complete determination of the mass spectrum (including the overall mass scale). As an illustration, we consider the simple case of a decay chain with up to three heavy particles, X2 -> X1 -> X0, which is rather problematic for all other mass measurement methods. We propose three different MT2-based methods, each of which allows a complete determination of the masses of particles X0, X1 and X2. The first method only uses MT2(n,p,c) endpoint measurements at a single fixed value of the test mass Mc. In the second method the unknown mass spectrum is fitted to one or more endpoint functions MT2{max}(n,p,c)(Mc,pT) exhibiting a kink. The third method is hybrid, combining MT2 endpoints with measurements of kinematic edges in invariant mass distributions. As a practical application of our methods, we show that the dilepton W+W- and tt-bar samples at the Tevatron can be used for an independent determination of the masses of the top quark, the W boson and the neutrino, without any prior assumptions.

A general search for new phenomena at HERA

A model-independent search for deviations from the Standard Model prediction is performed using the full e±p data sample collected by the H1 experiment at HERA. All event topologies involving isolated electrons, photons, muons, neutrinos and jets with transverse momenta above 20 GeV are investigated in a single analysis. Events are assigned to exclusive classes according to their final state. A dedicated algorithm is used to search for deviations from the Standard Model in the distributions of the scalar sum of transverse momenta or the invariant mass of final state particles and to quantify their significance. Variables related to angular distributions and energy sharing between final state particles are also introduced to study the final state topologies. No significant deviation from the Standard Model expectation is observed in the phase space covered by this analysis.

High-pressure xenon gas electroluminescent TPC for 0-ν ββ-decay search

A high-pressure xenon gas TPC can provide both event topology information and optimized energy resolution for the detection of ββ decay in 136Xe. The result of optimization indicates that, at the 136Xe Q-value of 2480 keV, an energy resolution of δE/ E<5×10 −3 FWHM may be realizable, even at the 1000 kg scale. Signal detection by electroluminescence appears essential to realize this performance. A specific method for generation and detection of the electroluminescent signal and particle tracking in high-pressure xenon gas is advanced. Strengths and weaknesses of high-pressure xenon gas TPC detectors are evaluated and compared to detectors based on liquid xenon.

Heavy ion event generator HYDJET++ (HYDrodynamics plus JETs)

Heavy ion event generator HYDJET++ (HYDrodynamics plus JETs)

The magnetized steel and scintillator calorimeters of the MINOS experiment

The Main Injector Neutrino Oscillation Search (MINOS) experiment uses an accelerator-produced neutrino beam to perform precision measurements of the neutrino oscillation parameters in the “atmospheric neutrino” sector associated with muon neutrino disappearance. This long-baseline experiment measures neutrino interactions in Fermilab's NuMI neutrino beam with a near detector at Fermilab and again 735 km downstream with a far detector in the Soudan Underground Laboratory in northern Minnesota. The two detectors are magnetized steel-scintillator tracking calorimeters. They are designed to be as similar as possible in order to ensure that differences in detector response have minimal impact on the comparisons of event rates, energy spectra and topologies that are essential to MINOS measurements of oscillation parameters. The design, construction, calibration and performance of the far and near detectors are described in this paper.

Two-dimensional discrete wavelet analysis of multiparticle event topology in heavy-ion collisions

The event-by-event analysis of multiparticle production in high energy hadron and nuclei collisions can be performed using the discrete wavelet transformation. The ring-like and jet-like structures in two-dimensional angular histograms are well extracted by wavelet analysis both for toy models and in Monte Carlo simulated events of nucleus–nucleus collisions at LHC energies. For the first time the method is applied to the jet-like events. The jet positions are located quite well by the discrete wavelet transformation of angular particle distribution even in the presence of the underlying soft process background in nucleus–nucleus collisions at LHC energies.

Prospects of a search for a new massless neutral gauge boson at the ILC

Prospects to search for a new massless neutral gauge boson, the paraphoton, in ${e}^{+}{e}^{\ensuremath{-}}$ collisions at center-of-mass energies of 0.5 and 1 TeV are studied. The paraphoton naturally appears in models with Abelian kinetic mixing. A possible realistic model-independent lowest order effective Lagrangian contains magnetic interactions of the paraphoton with the standard model fermion fields. These interactions are proportional to the fermion mass and grow with energy, with however very weak paraphoton couplings to ordinary matter. At the ILC, a potentially interesting process to search for the paraphoton is its radiation from top quarks, so that the event topology of interest is a pair of acoplanar top quarks decaying to jets and missing energy. By combining many discriminating features of signal and background events, efficient paraphoton event selection is achieved, allowing us to set limits for the top-paraphoton coupling. Arguments in favor of the missing energy as the paraphoton with spin 1 are discussed.

General analysis of HERA II data

A model-independent search for deviations from the Standard Model prediction is performed in e±p collisions. Data collected in the years 2003-2007 corresponding to an integrated luminosity of about 340 pb-1 are analyzed. All event topologies involving isolated electrons, photons, muons, neutrinos and jets with high transverse momenta are investigated in a single analysis. Events are assigned to exclusive classes according to their final state. A statistical algorithm is applied to search for deviations from the Standard Model in the distributions of the scalar sum of transverse momenta or invariant mass of final state particles and to quantify their significance. A good agreement with the Standard Model prediction is observed in most of the event classes. No significant deviation is observed in the phase-space and in the event topologies covered by this analysis.

Searches for squarks and gluinos at CDF and DØ detectors

This contribution reports on preliminary measurements of searches for squarks and gluinos at CDF and DØ detectors in pp̄ collisions at s√ = 1.96 TeV using 1 fb-1 of data. The analyses are optimised for event topologies with multiple jets and large missing transverse energy in the final state. No excess with respect to the Standard Model predictions is observed and new limits on the gluino and squark masses are extracted in a mSUGRA scenario with R-parity conservation.

Modeling of the Architectural Studies for the PANDA DAT System

We present design studies of the DAQ and trigger system (DAT) for the PANDA detector proposed for the new FAIR facility at GSI. The broad physics program of PANDA requires a novel DAT system able to cope with high interaction rates (up to Hz) and to trigger on various event topologies simultaneously. We used SystemC as a modeling platform to investigate candidate architecture for the PANDA DAT system. We simulated the behavior of the complete system with simplified models of all the components. The model covers detector buffers connected via Ethernet to farms of computing nodes constituting two filtering levels and an event building level. We present results from modeling illustrating the impact of the key architectural choices and parameters on the overall performance.

Tests of QCD factorisation in the diffractive production of dijets in deep-inelastic scattering and photoproduction at HERA

Measurements are presented of differential dijet cross sections in diffractive photoproduction (Q^2<0.01 GeV^2) and deep-inelastic scattering processes (DIS, 4<Q^2<80 GeV^2). The event topology is given by ep-> e X Y, in which the system X, containing at least two jets, is separated from a leading low-mass proton remnant system Y by a large rapidity gap. The dijet cross sections are compared with NLO QCD predictions based on diffractive parton densities previously obtained from a QCD analysis of inclusive diffractive DIS cross sections by H1. In DIS, the dijet data are well described, supporting the validity of QCD factorisation. The diffractive DIS dijet data are more sensitive to the diffractive gluon density at high fractional parton momentum than the measurements of inclusive diffractive DIS. In photoproduction, the predicted dijet cross section has to be multiplied by a factor of approximately 0.5 for both direct and resolved photon interactions to describe the measurements. The ratio of measured dijet cross section to NLO prediction in photoproduction is a factor 0.5+-0.1 smaller than the same ratio in DIS. This suppression is the first clear observation of QCD hard scattering factorisation breaking at HERA. The measurements are also compared to the two soft colour neutralisation models SCI and GAL. The SCI model describes diffractive dijet production in DIS but not in photoproduction. The GAL model fails in both kinematic regions.

A linear RFQ ion trap for the Enriched Xenon Observatory

The design, construction, and performance of a linear radio-frequency ion trap (RFQ) intended for use in the Enriched Xenon Observatory (EXO) are described. EXO aims to detect the neutrinoless double-beta decay of 136Xe to 136Ba. To suppress possible backgrounds EXO will complement the measurement of decay energy and, to some extent, topology of candidate events in a Xe filled detector with the identification of the daughter nucleus ( 136Ba). The ion trap described here is capable of accepting, cooling, and confining individual Ba ions extracted from the site of the candidate double-beta decay event. A single trapped ion can then be identified, with a large signal-to-noise ratio, via laser spectroscopy.

Reconstruction of composite events in neutrino telescopes

Neutrino telescopes detect the emission of Cherenkov light resulting from the tracks and showers of charged and neutral current neutrino interactions. These tracks or showers are reconstructed using a corresponding probability density function (PDF) which depends on measured time and location of the detected photons. We call a composite event the mixed detector response due to the juxtaposition of more than one Cherenkov light source (track or shower). This paper presents the construction of a generic PDF corresponding to a composite hypothesis. This composite PDF is therefore useful to reconstruct an arbitrary event topology and to favor or discard a given event topology hypothesis.

Diffractive open charm production in deep-inelastic scattering and photoproduction at HERA

Measurements are presented of diffractive open charm production at HERA. The event topology is given by ep -> eX Y where the system X contains at least one charmed hadron and is well separated by a large rapidity gap from a leading low-mass proton remnant system Y. Two analysis techniques are used for the cross section measurements. In the first, the charm quark is tagged by the reconstruction of a D*(2010) meson. This technique is used in deep-inelastic scattering (DIS) and photoproduction. In the second, a method based on the displacement of tracks from the primary vertex is used to measure the open charm contribution to the inclusive diffractive cross section in DIS. The measurements are compared with next-to-leading order QCD predictions based on diffractive parton density functions previously obtained from a QCD analysis of the inclusive diffractive cross section at H1. A good agreement is observed in the full kinematic regime, which supports the validity of QCD factorization for open charm production in diffractive DIS and photoproduction.

Search for neutral MSSM Higgs bosons at LEP

The four LEP collaborations, ALEPH, DELPHI, L3 and OPAL, have searched for the neutral Higgs bosons which are predicted by the Minimal Supersymmetric standard model (MSSM). The data of the four collaborations are statistically combined and examined for their consistency with the background hypothesis and with a possible Higgs boson signal. The combined LEP data show no significant excess of events which would indicate the production of Higgs bosons. The search results are used to set upper bounds on the cross-sections of various Higgs-like event topologies. The results are interpreted within the MSSM in a number of “benchmark” models, including CP-conserving and CP-violating scenarios. These interpretations lead in all cases to large exclusions in the MSSM parameter space. Absolute limits are set on the parameter cosβ and, in some scenarios, on the masses of neutral Higgs bosons.

Optical time projection chamber for imaging of two-proton decay of /sup 45/Fe nucleus

We present a design of the gaseous detector for three-dimensional (3-D) measurement of the topology of a recently discovered radioactive decay of 45Fe nucleus which involves simultaneous emission of two protons from the ground state. Our apparatus, called an optical time projection chamber (OTPC), consists of several parallel wire-mesh electrodes inside a gaseous medium which form the conversion region and the multistage charge amplification structure. The gas mixture filling the chamber enables strong emission of ultraviolet (UV) photons during avalanche process. Ultraviolet photons from charge avalanches are converted into visible light by means of a wavelength shifter foil. A CCD camera located outside the detection volume records 2-D image of the decay process. Drift time of primary ionization charge toward the amplification stage provides the third coordinate. Correlation of 2-D image with drift-time structure allows 3-D reconstruction. We present results on: the light yield, the charge gain and the electron drift velocity for several gas mixtures consisting of: Ar, He, N2, and triethylamine vapor. A feasibility of the event topology reconstruction and the energy measurement is addressed

Multifragmentation of very heavy nuclear systems (III): Fragment velocity correlations and event topology at freeze-out

Kinetic energy spectra and fragment velocity correlations, simulated by means of stochastic mean-field calculations, are successfully confronted with experimental data for single multifragmenting sources prepared at the same excitation energy per nucleon in 32 A MeV 129Xe + natSn and 36 A MeV 155Gd + natU central collisions. Relying thus on simulations, average freeze-out times of 200–240 fm/ c are estimated. The corresponding spatial distributions of fragments are more compact for the lighter system ( ∼ 3 – 4 V 0 vs. ∼ 8 V 0 ).

On the magnetic topology of October/November 2003 events

We examine the topology of several magnetic ejecta events during October–November 2003. The Grad‐Shafranov reconstruction results from ACE magnetic and plasma data show magnetic flux rope configurations. Some are large scale, with sizes of tenths of an AU, typical of magnetic clouds. Some are of sizes hundredths of an AU. Their characteristic parameters are reported. Magnetic field data from Wind spacecraft are utilized to compare the prediction from numerical modeling with the actual measurement. Poor to good agreement is achieved. The deviations are discussed, particularly in the context of extremely high‐speed solar wind flows.