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CERN Document Server latest documents in PreprintsenTue, 09 Feb 2016 11:45:05 GMTInvenio 1.1.3.1106-62468cds.support@cern.ch360766719125https://cds.cern.ch/img/site_logo_rss.pngCERN Document Server
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Searches for high mass resonances in dilepton and diphoton final states
https://cds.cern.ch/record/2130629
The search for new physics at the TeV scale is one of the major aspects of the ATLAS and CMS experimental programmes. This includes a myriad of theoretical models involving resonances that can can decay to leptons or photons. An overview of such analyses is presented in this paper. All analyses presented here use the full 2012 dataset, corresponding to an integrated luminosity of $\approx$~20~fb$^{-1}$, depending on the channel, recorded in proton-proton collisions at $\sqrt{s} =$ 8 TeV with the ATLAS and CMS detectors at the CERN LHC. Unless otherwise stated, all electron, photon and muon identifications use a combination of shower shape and isolation criteria. No sign of new physics is observed and limits are set on the various model parameters.Charaf, OtmanTue, 09 Feb 2016 10:17:41 GMT24 Dec 2015CMS-CR-2015-374https://cds.cern.ch/record/21306292015374Searches for heavy stable charged particles and other exotic signatures with large ionization at the LHC
https://cds.cern.ch/record/2130628
The signature of the large ionization is used to search for signals of heavy stable charged particles or other new particles foreseen by models beyond the Standard Model. The related searches in proton-proton collisions at the Large Hadron Collider (LHC) are reviewed. No excess events were found. Results obtained by two LHC experiments ATLAS and CMS with data at a centre-of-mass energy of 7 and 8 TeV collected in 2011 and 2012 are presented in terms of exclusion limits on the mass and the production cross section of hypothetical particles.Kazana, MalgorzataTue, 09 Feb 2016 10:15:38 GMT01 Jan 2016CMS-CR-2016-003https://cds.cern.ch/record/21306282016003Particle Spectra in Minimum Bias events at 13 TeV
https://cds.cern.ch/record/2130627
Measurements of particle spectra at 13 TeV are presented for different event topologies.Grados Luyando, Juan ManuelTue, 09 Feb 2016 10:15:10 GMT25 Jan 2016CMS-CR-2016-017https://cds.cern.ch/record/21306272016017CMS Underlying Event and Double Parton Scattering Monte Carlo Tunes
https://cds.cern.ch/record/2130626
Three new PYTHIA 8 underlying event (UE) tunes are presented, one using the CTEQ6L1 parton distribution function (PDF), one using HERAPDF1.5LO, and one using the NNPDF2.3LO PDF; two new PYTHIA 6 UE tunes, one for the CTEQ6L1 PDF and one for the HERAPDF1.5LO, and one new HERWIG++ UE tune for the CTEQ6L1 PDF are also available. Simultaneous fits to CDF UE data at center-of-mass energies of 0.3, 0.9, and 1.96 TeV, together with CMS UE data at 7 TeV, check the UE models and constrain their parameters, providing thereby more precise predictions for proton-proton collisions at 13 TeV. In addition, several new double parton scattering (DPS) tunes are examined in order to investigate if the values of the parameters from fits to UE observables are consistent with the values determined from fitting DPS-sensitive observables. It is also examined how well the new UE tunes predict minimum bias (MB) events, jet, and Drell-Yan observables, as well as MB and UE observables at 13 TeV.Sunar Cerci, DenizTue, 09 Feb 2016 10:14:48 GMT31 Jan 2016CMS-CR-2016-019https://cds.cern.ch/record/21306262016019MPI and UE corrections to jet measurements and influence on PDF determination and $\alpha_s$
https://cds.cern.ch/record/2130625
The UE and MPI contributions and corrections to jet measurements will be presented, as well as an estimate on the uncertainties. In addition, the influence from initial and final state parton shower will be discussed.Cerci, SalimTue, 09 Feb 2016 10:14:30 GMT31 Jan 2016CMS-CR-2016-020https://cds.cern.ch/record/21306252016020Multi-jet measurements at CMS
https://cds.cern.ch/record/2130624
The cross section of hadronic jet production gives direct access to the basic QCD components like the strong coupling constant, $\alpha_s$, or the parton density functions. Measurements of correlations between jets allow to test pQCD resummation procedures or parton shower models. Recent CMS measurements on these topics are discussed in this review.Safronov, GrigoryTue, 09 Feb 2016 10:14:12 GMT02 Feb 2016CMS-CR-2016-023https://cds.cern.ch/record/21306242016023Search for supersymmetry in pp collisions at $\sqrt{s}=$ 8 TeV in final states with boosted W bosons and b jets using razor variables
https://cds.cern.ch/record/2130623
A search for supersymmetry in hadronic final states with highly boosted W bosons and b jets is presented, focusing on compressed scenarios. The search is performed using proton-proton collision data at a center-of-mass energy of 8 TeV, collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. Events containing candidates for hadronic decays of boosted W bosons are identified using jet substructure techniques, and are analyzed using the razor variables $M_\mathrm{R}$ and $R^2$, which characterize a possible signal as a peak on a smoothly falling background. The observed event yields in the signal regions are found to be consistent with the expected contributions from standard model processes, which are predicted using control samples in the data. The results are interpreted in terms of gluino pair production followed by their exclusive decay into top squarks and top quarks. The analysis excludes gluino masses up to 1.1 TeV for light top squarks decaying solely to a charm quark and a neutralino, and up to 700 GeV for heavier top squarks decaying solely to a top quark and a neutralino.CMS CollaborationTue, 09 Feb 2016 10:06:57 GMT09 Feb 2016CERN-EP-2016-008https://cds.cern.ch/record/21306232016008ATLAS/CMS Probing PDFs and soft QCD at the LHC
https://cds.cern.ch/record/2130620
LHC has delivered 30 fb$^{-1}$ of proton-proton collisions at center-of-mass energies of 7 and 8 TeV in 2010-2012 during the LHC Run I. Run II has started in 2015 and approximately 200 pb$^{-1}$ of proton-proton collisions at $\sqrt{s}=13$ TeV have been delivered by September. An important part of physics program in ATLAS and CMS experiments are precision studies of quantum chromodynamics (QCD). Parton density functions (PDFs) are an essential part of calculation of any process at hadron collider. Detailed studies of soft proton-proton interactions and production of soft particles allows to improve phenomenology of processes in non-perturbative QCD domain. A selection of measurements probing PDFs and soft QCD based on Run I data as well as early results from Run II are presented in this review.Safronov, GrigoryTue, 09 Feb 2016 09:51:08 GMT27 Dec 2015CMS-CR-2015-378https://cds.cern.ch/record/21306202015378Neutrino physics with the SHiP experiment
https://cds.cern.ch/record/2130615
Despite the Standard Model (SM) has been strongly confirmed by the Higgs discovery, several experimental facts are still not explained. The SHiP experiment (Search for Hidden Particles), a beam dump experiment at CERN, aims at the observation of long lived particles very weakly coupled with ordinary matter. These particles of the GeV mass scale, foreseen in many extensions of the SM, might come from the decay of charmed hadrons produced in the collision of a 400 GeV proton beam on a target. High rates of all the three active neutrinos are also expected. For the first time the properties and the cross section of the ντ will be studied thanks to a detector based on nuclear emulsions, with the micrometric resolution needed to identify the tau lepton produced in neutrino interactions. Measuring the charge of the tau daughters, will enable the first observation of the ν ̄τ and the study of its cross section.Buonaura, AnnaritaTue, 09 Feb 2016 08:42:08 GMT2016-09-09CERN-SHiP-PROC-2016-008https://cds.cern.ch/record/21306152016008Non-equilibrium simulations of thermally induced electric fields in water
https://cds.cern.ch/record/2130607
Using non-equilibrium molecular dynamics simulations, Bresme and co-workers recently demonstrated that water molecules align in response to an imposed temperature gradient. Employing the Wolf method to truncate the electrostatic interactions, they reported electric fields as high as $3.7\times10^8~\text{V/m}$ for a gradient of about $5.2~\text{K/\AA}$ [J. Chem. Phys. 139, 014504 (2013)]. Recently, however, Bresme and co-workers [J. Chem. Phys. 143, 036101 (2015)] advocated that the Wolf method overestimates the induced electric field by an order of magnitude. In this work, we investigate how thermally induced fields depend on the underlying treatment of long-ranged interactions. Our key findings are: Firstly, under identical conditions we find the peak field strength to be $2.8\times 10^7~\text{V/m}$ and $2.2\times 10^7~\text{V/m}$ for Ewald summation and the Wolf method, respectively. Our value for the short-ranged method is therefore an order of magnitude lower than the original value reported by Bresme and co-workers, who used an incorrect kernel in the calculation. Secondly and more seriously, we find that the Wolf method fails to predict molecular orientations, resulting in dipole densities with opposite sign to those computed using Ewald summation. By considering two different multipole expansions for molecules and slabs, we show that, for inhomogeneous polarisations, the quadrupole contribution can be significant and even outweigh the dipole contribution to the field. Thirdly, we propose a more accurate way of calculating the electrostatic potential and the field. In particular, we show that averaging the microscopic field analytically to obtain the macroscopic Maxwell field reduces the error bars by up to an order of magnitude. As a consequence, the simulation times required to reach a given statistical accuracy decrease by up to two orders of magnitude.Wirnsberger, PeterFijan, DomagojŠarić, AnđelaNeumann, MartinDellago, ChristophFrenkel, DaanTue, 09 Feb 2016 07:12:15 GMT08 Feb 2016arXiv:1602.02734https://cds.cern.ch/record/2130607['arXiv:1602.02734']arXiv:1602.02734A dynamical systems' approach for the contact-line singularity in thin-film flows
https://cds.cern.ch/record/2130606
We are interested in a complete characterization of the contact-line singularity of thin-film flows for zero and nonzero contact angles. By treating the model problem of source-type self-similar solutions, we demonstrate that this singularity can be understood by the study of invariant manifolds of a suitable dynamical system. In particular, we prove regularity results for singular expansions near the contact line for a wide class of mobility exponents and for zero and nonzero dynamic contact angles. Key points are the reduction to center manifolds and identifying resonance conditions at equilibrium points. The results are extended to radially-symmetric source-type solutions in higher dimensions. Furthermore, we give dynamical systems' proofs for the existence and uniqueness of self-similar droplet solutions in the nonzero dynamic contact-angle case.Belgacem, Fethi BenGnann, Manuel VKuehn, ChristianTue, 09 Feb 2016 07:12:15 GMT08 Feb 2016arXiv:1602.02733https://cds.cern.ch/record/2130606['arXiv:1602.02733']arXiv:1602.02733Extended formalism for simulating compound refractive lens-based x-ray microscopes
https://cds.cern.ch/record/2130604
We present a comprehensive formalism for the simulation and optimisation of CRLs in both condensing and full-field imaging configurations. The approach extends ray transfer matrix analysis to account for x-ray attenuation by the lens material. Closed analytical expressions for critical imaging parameters such as numerical aperture, vignetting, chromatic aberration and focal length are provided for both thin- and thick-lens imaging geometries.Simons, HughAhl, Sonja RosenlundPoulsen, Henning FriisDetlefs, CarstenTue, 09 Feb 2016 07:12:15 GMT08 Feb 2016arXiv:1602.02703https://cds.cern.ch/record/2130604['arXiv:1602.02703']arXiv:1602.02703Active colloids in complex fluids
https://cds.cern.ch/record/2130603
We review recent work on active colloids or swimmers, such as self-propelled microorganisms, phoretic colloidal particles, and artificial micro-robotic systems, moving in fluid-like environments. These environments can be water-like and Newtonian but can frequently contain macromolecules, flexible polymers, soft cells, or hard particles, which impart complex, nonlinear rheological features to the fluid. While significant progress has been made on understanding how active colloids move and interact in Newtonian fluids, little is known on how active colloids behave in complex and non-Newtonian fluids. An emerging literature is starting to show how fluid rheology can dramatically change the gaits and speeds of individual swimmers. Simultaneously, a moving swimmer induces time dependent, three dimensional fluid flows, that can modify the medium (fluid) rheological properties. This two-way, non-linear coupling at microscopic scales has profound implications at meso- and macro-scales: steady state suspension properties, emergent collective behavior, and transport of passive tracer particles. Recent exciting theoretical results and current debate on quantifying these complex active fluids highlight the need for conceptually simple experiments to guide our understanding.Patteson, Alison EGopinath, ArvindArratia, Paulo ETue, 09 Feb 2016 07:12:15 GMT08 Feb 2016arXiv:1602.02693https://cds.cern.ch/record/2130603['arXiv:1602.02693']arXiv:1602.02693Systematic Uncertainties in RF-Based Measurement of Superconducting Cavity Quality Factors
https://cds.cern.ch/record/2130602
Q0 determinations based on RF power measurements are subject to at least three potentially large systematic effects that have not been previously appreciated. Instrumental factors that can systematically bias RF based measurements of Q0 are quantified and steps that can be taken to improve the determination of Q0 are discussed.Holzbauer, J PPischalnikov, YuSergatskov, D ASchappert, WSmith, STue, 09 Feb 2016 07:12:15 GMT08 Feb 2016arXiv:1602.02689https://cds.cern.ch/record/2130602['arXiv:1602.02689']arXiv:1602.02689Internal energy of the classical two- and three-dimensional one-component-plasma
https://cds.cern.ch/record/2130601
We summarize several semi-phenomenological approaches to estimate the internal energy of one-component-plasma (OCP) in two (2D) and three (3D) dimensions. Particular attention is given to a hybrid approach, which reproduces the Debye-H$\ddot{\text{u}}$ckel asymptote in the limit of weak coupling, the ion sphere (3D) and ion disc (2D) asymptotes in the limit of strong coupling, and provides reasonable interpolation between these two limits. More accurate ways to estimate the internal energy of 2D and 3D OCP are also discussed. The accuracy of these analytic results is quantified by comparison with existing data from numerical simulations. The relevance of the KTHNY theory in locating melting transition in 2D OCP is briefly discussed.Khrapak, S AKhrapak, A GTue, 09 Feb 2016 07:12:15 GMT08 Feb 2016arXiv:1602.02677https://cds.cern.ch/record/2130601['arXiv:1602.02677']arXiv:1602.02677The happiness paradox: your friends are happier than you
https://cds.cern.ch/record/2130600
Most individuals in social networks experience a so-called Friendship Paradox: they are less popular than their friends on average. This effect may explain recent findings that widespread social network media use leads to reduced happiness. However the relation between popularity and happiness is poorly understood. A Friendship paradox does not necessarily imply a Happiness paradox where most individuals are less happy than their friends. Here we report the first direct observation of a significant Happiness Paradox in a large-scale online social network of $39,110$ Twitter users. Our results reveal that popular individuals are indeed happier and that a majority of individuals experience a significant Happiness paradox. The magnitude of the latter effect is shaped by complex interactions between individual popularity, happiness, and the fact that users cluster assortatively by level of happiness. Our results indicate that the topology of online social networks and the distribution of happiness in some populations can cause widespread psycho-social effects that affect the well-being of billions of individuals.Bollen, JohanGonçalves, Brunovan de Leemput, IngridRuan, GuangchenTue, 09 Feb 2016 07:12:15 GMT08 Feb 2016arXiv:1602.02665https://cds.cern.ch/record/2130600['arXiv:1602.02665']arXiv:1602.02665Optimisation of Simulations of Stochastic Processes by Removal of Opposing Reactions
https://cds.cern.ch/record/2130599
Models invoking the chemical master equation are used in many areas of science, and, hence, their simulation is of interest to many researchers. The complexity of the problems at hand often requires considerable computational power, so a large number of algorithms have been developed to speed up simulations. However, a drawback of many of these algorithms is that their implementation is more complicated than, for instance, the Gillespie algorithm, which is widely used to simulate the chemical master equation, and can be implemented with a few lines of code. Here, we present an algorithm which does not modify the way in which the master equation is solved, but instead modifies the transition rates, and can thus be implemented with a few lines of code. It works for all models in which reversible reactions occur by replacing such reversible reactions with effective net reactions. Examples of such systems include reaction-diffusion systems, in which diffusion is modelled by a random walk. The random movement of particles between neighbouring sites is then replaced with a net random flux. Furthermore, as we modify the transition rates of the model, rather than its implementation on a computer, our method can be combined with existing algorithms that were designed to speed up simulations of the stochastic master equation. By focusing on some specific models, we show how our algorithm can significantly speed up model simulations while maintaining essential features of the original model.Spill, FabianMaini, Philip KByrne, HelenTue, 09 Feb 2016 07:12:15 GMT08 Feb 2016arXiv:1602.02655https://cds.cern.ch/record/2130599['arXiv:1602.02655']arXiv:1602.02655Magnetization of laser-produced plasma in a chiral hollow target
https://cds.cern.ch/record/2130597
It is demonstrated that targets with a broken rotational symmetry may facilitate generation of a strong axial (poloidal) magnetic field. An intense laser beam irradiating such a target creates intense electron currents carrying vorticity and producing strong spontaneous magnetic fields. Combined with a laser electron acceleration, such targets may be used for generation and guiding of magnetized, collimated particle or plasma beams.Korneev, PhTikhonchuk, Vd'Humières, ETue, 09 Feb 2016 07:12:14 GMT08 Feb 2016arXiv:1602.02626https://cds.cern.ch/record/2130597['arXiv:1602.02626']arXiv:1602.02626Customized ion flux-energy distribution functions in capacitively coupled plasmas by voltage waveform tailoring
https://cds.cern.ch/record/2130596
We propose a method to generate a single peak at a distinct energy in the ion flux-energy distribution function (IDF) at the electrode surfaces in capacitively coupled plasmas. The technique is based on the tailoring of the driving voltage waveform, i.e. adjusting the phases and amplitudes of the applied harmonics, to optimize the accumulation of ions created by charge exchange collisions and their subsequent acceleration by the sheath electric field. The position of the peak (i.e. the ion energy) and the flux of the ions within the peak of the IDF can be controlled in a wide domain by tuning the parameters of the applied RF voltage waveform, allowing optimization of various applications where surface reactions are induced at particular ion energies.Schuengel, EDonko, ZHartmann, PDerzsi, AKorolov, ISchulze, JTue, 09 Feb 2016 07:12:14 GMT08 Feb 2016arXiv:1602.02624https://cds.cern.ch/record/2130596['arXiv:1602.02624']arXiv:1602.02624Capture into resonance and phase space dynamics in optical centrifuge
https://cds.cern.ch/record/2130595
The process of capture of a molecular enesemble into rotational resonance in the optical centrifuge is investigated. The adiabaticity and phase space incompressibility are used to find the resonant capture probability in terms of two dimensionless parameters P1,P2 characterising the driving strength and the nonlinearity, and related to three characteristic time scales in the problem. The analysis is based on the transformation to action-angle variables and the single resonance approximation, yielding reduction of the three-dimensional rotation problem to one degree of freedom. The analytic results for capture probability are in a good agreement with simulations. The existing experiments satisfy the validity conditions of the theory.Armon, TsafrirFriedland, LazarTue, 09 Feb 2016 07:12:14 GMT08 Feb 2016arXiv:1602.02604https://cds.cern.ch/record/2130595['arXiv:1602.02604']arXiv:1602.02604Estimates of the temperature flux-temperature gradient relation above a sea-floor
https://cds.cern.ch/record/2130594
The relation between the flux of temperature (or buoyancy), the vertical temperature gradient and the height above the bottom, is investigated in an oceanographic context, using high-resolution temperature measurements. The model for the evolution of a stratified layer by Balmforth et al. (1998) is reviewed and adapted to the case of a turbulent flow above a wall. Model predictions are compared to the average observational estimates of the flux, exploiting a flux estimation method proposed by Winters & D'Asaro (1996). This estimation method enables the disentanglement of the dependence of the average flux on the height above the bottom and on the background temperature gradient. The classical N-shaped flux-gradient relation is found in the observations. Model and observations show similar qualitative behaviour, despite the strong simplifications used in the model. The results shed light on the modulation of the temperature flux by the presence of the boundary, and support the idea of a turbulent flux following a mixing-length argument in a stratified flow. Furthermore, the results support the use of Thorpe scales close to a boundary, if sufficient averaging is performed, suggesting that the Thorpe scales are affected by the boundary in a similar way as the mixing length.Cimatoribus, Andrea Avan Haren, HansTue, 09 Feb 2016 07:12:14 GMT08 Feb 2016arXiv:1602.02599https://cds.cern.ch/record/2130594['arXiv:1602.02599']arXiv:1602.02599Kershaw closures for linear transport equations in slab geometry II: high-order realizability-preserving discontinuous-Galerkin schemes
https://cds.cern.ch/record/2130593
This paper provides a generalization of the realizability-preserving discontinuous-Galerkin scheme for quadrature-based minimum-entropy models to full-moment models of arbitrary order. It is applied to the class of Kershaw closures, which are able to provide a cheap closure of the moment problem. This results in an efficient algorithm for the underlying linear transport equation. The efficiency of high-order methods is demonstrated using numerical convergence tests and non-smooth benchmark problems.Schneider, FlorianTue, 09 Feb 2016 07:12:14 GMT05 Feb 2016arXiv:1602.02590https://cds.cern.ch/record/2130593['arXiv:1602.02590']arXiv:1602.02590On Super-Planckian thermal emission in far field regime
https://cds.cern.ch/record/2130592
We study, in the framework of the Landauer theory, the thermal emission in far-field regime, of arbitrary indefinite planar media and finite size systems. We prove that the flux radiated by the former is bounded by the blackbody emission while, for the second, there is in principle, no upper limit demonstrating so the possibility for a super-Planckian thermal emission with finite size systems.Biehs, Svend-AgeBen-Abdallah, PhilippeTue, 09 Feb 2016 07:12:14 GMT08 Feb 2016arXiv:1602.02587https://cds.cern.ch/record/2130592['arXiv:1602.02587']arXiv:1602.02587Non-Destructive Probing of Means, Variances, and Correlations of Ultracold Atomic System Densities via Qubit Impurities
https://cds.cern.ch/record/2130591
We show how impurity atoms can measure moments of ultracold atomic gas densities, using the example of bosons in a one-dimensional lattice. This builds on a body of work regarding the probing of systems by measuring the dephasing of an immersed qubit. We show this dephasing is captured by a function resembling characteristic functions of probability theory, of which the derivatives at short times reveal moments of the system operator to which the qubit couples. For a qubit formed by an impurity atom, in a system of ultracold atoms, this operator can be the density of the system at the location of the impurity, and thus, means, variances, and correlations of the atomic densities are accessible.Elliott, Thomas JJohnson, Tomi HTue, 09 Feb 2016 07:12:14 GMT08 Feb 2016arXiv:1602.02579https://cds.cern.ch/record/2130591['arXiv:1602.02579']arXiv:1602.02579The role of helicity in triad interactions in 3D turbulence investigated in a new shell model
https://cds.cern.ch/record/2130589
Fully developed homogeneous isotropic turbulence in 2D is fundamentally different from 3D. In 2D, the simultaneous conservation of both energy and enstrophy in the inertial ranges of scales leads to a forward cascade of enstrophy and a reverse cascade of energy. In 3D, helicity, the integral of the scalar product of velocity and vorticity, is also an inviscid flow invariant along with kinetic energy. Unlike enstrophy, however, helicity does not block the cascade of energy to small scales. Energy and helicity are not only globally conserved but also conserved within each non-linear triadic interaction between three plane waves in the spectral form of the Navier--Stokes equation (NSE). By decomposing each plane wave into two helical modes of opposite helicities each triadic interaction is split into a set of eight triadic interactions between helical modes (Waleffe 1992). Biferale et al. (2012) recently found that a subset of these interactions which render both signs of helicity separately conserved (i.e. enstrophy-like) leads to an inverse cascade of (part of) the energy. Motivated by this finding we introduce a new shell model obtained from the NSE expressed in the helical basis (Waleffe 1992). By analysing and integrating the new model we attempt to explain why the dual forward cascade of energy and helicity dominates in 3D turbulence.Rathmann, Nicholas MDitlevsen, Peter DTue, 09 Feb 2016 07:12:14 GMT08 Feb 2016arXiv:1602.02553https://cds.cern.ch/record/2130589['arXiv:1602.02553']arXiv:1602.02553