Transverse Mass Research Articles

From cold to hot nuclear matter

AbstractThe dynamics of partons and hadrons in relativistic nucleus‐nucleus collisions is analyzed within the Parton‐HadronString Dynamics (PHSD) transport approach which is based on a dynamical quasiparticle model for the partonic phase (DQPM) including a dynamical hadronization scheme with covariant transition rates. The PHSD approach is applied to nucleus‐nucleus collisions from FAIR/NICA to LHC energies. The traces of partonic interactions are found in particular in the directed and elliptic flow of hadrons and in their transverse mass spectra. Whereas at RHIC and LHC energies the dynamics is dominated by partonic degrees‐of‐freedom in the hot QGP, we find at FAIR/NICA energies a moderately hot but dense matter where chiral symmetry restoration and hadronic potentials appear to play a major role. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Semivisible Jets: Dark Matter Undercover at the LHC.

Dark matter may be a composite particle that is accessible via a weakly coupled portal. If these hidden-sector states are produced at the Large Hadron Collider (LHC), they would undergo a QCD-like shower. This would result in a spray of stable invisible dark matter along with unstable states that decay back to the standard model. Such "semivisible" jets arise, for example, when their production and decay are driven by a leptophobic Z' resonance; the resulting signature is characterized by significant missing energy aligned along the direction of one of the jets. These events are vetoed by the current suite of searches employed by the LHC, resulting in low acceptance. This Letter will demonstrate that the transverse mass-computed using the final-state jets and the missing energy-provides a powerful discriminator between the signal and the QCD background. Assuming that the Z' couples to the standard model quarks with the same strength as the Z(0), the proposed search can discover (exclude) Z' masses up to 2.5 TeV (3.5 TeV) with 100 fb(-1) of 14 TeV data at the LHC.

A one-dimensional modeling approach for dual-layer monolithic catalysts

This paper presents a model extension to dual-layer monolithic catalysts from a one-dimensional (1D) (Balakotaiah, 2008; Joshi et al., 2009) mathematical model used for single-layer monolithic catalysts. The novel transient non-isothermal model computes average concentrations inside the bulk flow and in each layer of washcoat at every computational grid point along the monolith channel. The model has the capability that physical (e.g. diffusivity, pore radius, and thickness) and chemical (reaction kinetics) properties of the two layers of the washcoat can be different from each other. The relatively low computational cost offers real-time simulations of multi-layer monolithic catalysts. Analytically derived expressions for transverse mass transfer coefficients between the bulk flow and washcoat layers are presented for single-layer and dual-layer catalysts with circular cross-section, which can be easily extended for multi-layer catalysts. In addition, three expressions are presented for calculation of three internal Sherwood numbers, which are used in the dual-layer model, based on the values of Thiele moduli of the washcoat layers. These expressions are especially useful for calculation of the mass transfer coefficients for dual-layer catalysts with non-circular cross-sections. Application limitations of the 1D multi-layer model with respect to physical and chemical operating conditions are discussed. The derived model is applied for an automotive aftertreatment catalyst and the results of this model are compared with the results of a detailed 2D model (selective catalytic reduction/platinum group metals dual-layer catalysts).

W′ at the LHC with √s = 14 TeV: Split universal extra dimension model

We study the search of new charged gauge boson W′ in the split universal extra dimension model by considering pp → W′ → lve with √s = 14 TeV at the Large Hadron Collider. The interference between the W′ and the W boson of the Standard Model can be used to probe W′ from the distribution shape of the transverse mass between the lepton and the neutrino from the decay of W′. We show that the LHC can cover a rather large range of masses of the Kaluza-Klein W boson for various values of the couplings.

Systematic study of charged-pion and kaon femtoscopy in Au + Au collisions atsNN=200GeV

We present a systematic study of charged pion and kaon interferometry in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=200 GeV. The kaon mean source radii are found to be larger than pion radii in the outward and longitudinal directions for the same transverse mass; this difference increases for more central collisions. The azimuthal-angle dependence of the radii was measured with respect to the second-order event plane and similar oscillations of the source radii were found for pions and kaons. Hydrodynamic models qualitatively describe the similar oscillations of the mean source radii for pions and kaons, but they do not fully describe the transverse-mass dependence of the oscillations.

On the tensoresistance of n-Ge and n-Si crystals with radiation-induced defects

A variation in the tensoresistance of n-Ge:Sb and n-Si:As crystals as a result of irradiation with γ-ray photons (60Co source) at fixed temperatures under conditions of the application of uniaxial elastic stress (0 ≤ X ≤ 1.2 GPa) along the main crystallographic direction is studied. It is found that, in the case of the deformation axis being in an asymmetric position relative to the isoenergetic ellipsoids, there is a maximum for the dependences of the tensoresistance ρX/ρ0 = f(X); an explanation as to the nature of the observed effect is suggested. Tensoresistance is revealed in unirradiated n-Si:As crystals in the case of the deformation axis being in a symmetric position relative to all isoenergetic ellipsoids; the value of the tensoresistance as a result of irradiation with γ-ray photons decreases. It is shown that this effect can be attributed to a variation in the mobility of electrons in the conduction band as a result of an increase in the transverse effective mass and the appearance of new deep-level centers under the effect of irradiation, respectively.

Search for W′ decaying to tau lepton and neutrino in proton–proton collisions at s=8 TeV

The first search for a heavy charged vector boson in the final state with a tau lepton and a neutrino is reported, using 19.7 inverse femtobarns of LHC data at sqrt(s) = 8 TeV. A signal would appear as an excess of events in kinematic regions where the standard model background is low. No excess is observed. Limits are set on a model in which the W' decays preferentially to fermions of the third generation. These results substantially extend previous constraints on this model. Masses below 2.0 to 2.7 TeV are excluded, depending on the model parameters. In addition, the existence of a W' boson with universal fermion couplings is excluded at 95% confidence level, for W' masses below 2.7 TeV. For further reinterpretation a model-independent limit on potential signals for various transverse mass thresholds is also presented.

Beam-energy-dependent two-pion interferometry and the freeze-out eccentricity of pions measured in heavy ion collisions at the STAR detector

We present results of analyses of two-pion interferometry in Au+Au collisions at sNN=7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeV measured in the STAR detector as part of the BNL Relativistic Heavy Ion Collider Beam Energy Scan program. The extracted correlation lengths (Hanbury-Brown–Twiss radii) are studied as a function of beam energy, azimuthal angle relative to the reaction plane, centrality, and transverse mass (mT) of the particles. The azimuthal analysis allows extraction of the eccentricity of the entire fireball at kinetic freeze-out. The energy dependence of this observable is expected to be sensitive to changes in the equation of state. A new global fit method is studied as an alternate method to directly measure the parameters in the azimuthal analysis. The eccentricity shows a monotonic decrease with beam energy that is qualitatively consistent with the trend from all model predictions and quantitatively consistent with a hadronic transport model.21 MoreReceived 30 January 2015DOI:https://doi.org/10.1103/PhysRevC.92.014904©2015 American Physical Society

Parton-hadron matter at finite temperature and chemical potential

The dynamics of partons and hadrons in ultra-relativistic nucleus-nucleus collisions is analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for the partonic phase (DQPM) including a dynamical hadronization scheme while reproducing lattice QCD results in thermodynamic equilibrium for the equation-of-state as well as transport coefficients like shear and bulk viscosities or the electric conductivity of the hot QCD medium. The PHSD model reproduces a large variety of observables from SPS to LHC energies, e.g. the quark-number scaling of elliptic flow, transverse mass and rapidity spectra of charged hadrons, dilepton spectra, open and hidden charm production, collective flow coefficients etc., which are associated with the observation of a strongly interacting QGP (sQGP). The ’highlights’ of the latest results from LHC energies are presented with a focus on the correlation between the average transverse momentum < pT > and the number of charged particles Nch in p-p, p-Pb and Pb-Pb collisions at midrapidity as observed by the ALICE Collaboration.

Effect of the jet production on pseudorapidity, transverse momentum and transverse mass distributions of charged particles produced in pp-collisions at Tevatron energy**Supported by Higher Education Commission (HEC) Government of Pakistan under Indigenous5000 PhD Scholarship Program Batch-IV

We investigate the effects of jet production on the following parameters: pseudorapidity, transverse momentum and transverse mass distributions of secondary charged particles produced in pp-collisions at 1.8 TeV, using the HIJING code. These distributions are analyzed for the whole range and for six selected regions of the polar angle as a function of the different number of jets. The obtained simulation results for these parameters are interpreted and discussed in connection to the increase observed in the multiplicity of secondary charged particles as a result of its multi-jet dependence, and are also discussed in comparison with the experimental results from the CDF Collaboration.

Search for physics beyond the standard model in final states with a lepton and missing transverse energy in proton-proton collisions ats=8 TeV

A search for new physics in proton-proton collisions having final states with an electron or muon and missing transverse energy is presented. The analysis uses data collected in 2012 with the CMS detector, at an LHC center-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of 19.7 inverse femtobarns. No significant deviation of the transverse mass distribution of the charged lepton-neutrino system from the standard model prediction is found. Mass exclusion limits of up to 3.28 TeV at a 95% confidence level for a W' boson with the same couplings as that of the standard model W boson are determined. Results are also derived in the framework of split universal extra dimensions, and exclusion limits on Kaluza-Klein W[KK,2] states are found. The final state with large missing transverse energy also enables a search for dark matter production with a recoiling W boson, with limits set on the mass and the production cross section of potential candidates. Finally, limits are established for a model including interference between a left-handed W' boson and the standard model W boson, and for a compositeness model.

The QGP dynamics in relativistic collisions

The dynamics of partons and hadrons in ultra-relativistic nucleus-nucleus collisions is analyzed within the Parton-Hadron-String Dynamics (PHSD) transport approach, which is based on a dynamical quasiparticle model for the partonic phase (DQPM) including a dynamical hadronization scheme while reproducing lattice QCD results in thermodynamic equilibrium for the equation-of-state as well as transport coefficients like shear and bulk viscosities or the electric conductivity of the hot QCD medium. The PHSD model reproduces a large variety of observables from SPS to LHC energies, e.g. the quark-number scaling of elliptic flow, transverse mass and rapidity spectra of charged hadrons, dilepton spectra, open and hidden charm production, collective flow coefficients etc., which are associated with the observation of a strongly interacting QGP (sQGP). The ‘highlights’ of the latest results from LHC energies are presented with a focus on observable effects from the non-equilibrium initial dynamics.

Deconstructed transverse mass variables

Traditional searches for R-parity conserving natural supersymmetry (SUSY) require large transverse mass and missing energy cuts to separate the signal from large backgrounds. SUSY models with compressed spectra inherently produce signal events with small amounts of missing energy that are hard to explore. We use this difficulty to motivate the construction of "deconstructed" transverse mass variables which are designed preserve information on both the norm and direction of the missing momentum. We demonstrate the effectiveness of these variables in searches for the pair production of supersymmetric top-quark partners which subsequently decay into a final state with an isolated lepton, jets and missing energy. We show that the use of deconstructed transverse mass variables extends the accessible compressed spectra parameter space beyond the region probed by traditional methods. The parameter space can further be expanded to neutralino masses that are larger than the difference between the stop and top masses. In addition, we also discuss how these variables allow for novel searches of single stop production, in order to directly probe unconstrained stealth stops in the small stop- and neutralino-mass regime. We also demonstrate the utility of these variables for generic gluino and stop searches in all-hadronic final states. Overall, we demonstrate that deconstructed transverse variables are essential to any search wanting to maximize signal separation from the background when the signal has undetected particles in the final state.

Measurement of a pipe belt conveyor contact forces and cross section deformation by means of the six-point pipe belt stiffness testing device

This paper presents an experimental study that investigated the influence of major pipe conveyor parameters, such as pipe diameter, belt width, transverse bending stiffness, line mass and position of the belt overlap on the load distribution between the individual idler rolls, as well as the ability of the belt to form a stable pipe. The measurements were conducted using a static six-point pipe belt stiffness testing device. The results confirmed that bigger pipe diameters require higher bending stiffness to ensure the belt forms a stable pipe. The pipe diameter can be changed by either varying the length of the overlap, or by selecting a different belt width and keeping the ratio pipe diameter versus the belt width constant. More flexible belts require larger overlap for stable pipe shape. If the ratio belt width versus pipe diameter is constant for all manufactured belt widths, higher stiffness is needed for wider belts.

Signatures of top flavored dark matter

We study the experimental signatures of top flavored dark matter (top FDM) in direct detection searches and at the LHC. We show that for a dark matter mass above 200 GeV, top FDM can be consistent with current bounds from direct detection experiments and relic abundance constraints. We also show that next generation direct detection experiments will be able to exclude the entire perturbative parameter region for top FDM. For regions of parameter space where the flavor partners of top FDM are not readily produced, the LHC signatures of top FDM are similar to those of other models previously studied in the literature. For the case when the flavor partners are produced at the LHC, we study their impact on a search based on transverse mass variables and find that they diminish the signal significance. However, when the DM flavor partners are split in mass by less than 120-130 GeV, the LHC phenomenology becomes very distinctive through the appearance of displaced vertices. We also propose a strategy by which all parameters of the underlying model can be experimentally determined when the flavor partners can be observed.

Lock-exchange flows in inclined pipes: the relevance of the Prandtl mixing length model

We examine the applicability of the Prandtl mixing length model to transverse momentum and mass flux in strongly confined, stably stratified turbulent shear flows. These fluxes were measured in the vertical diametral plane of lock-exchange flows in an inclined pipe by the simultaneous use of planar laser-induced fluorescence and particle image velocimetry at local Reynolds numbers ranging from Re = 580 to 1770 and Richardson numbers ranging from Ri = 0.26 and 1.6. Measurements indicate that the eddy diffusivities of mass and momentum are symmetric about the pipe axis, with their maximum at the axis. The corresponding Prandtl mixing lengths decrease with increasing distance from the pipe axis within the central 60% of the pipe cross-section. Within the range of experimental conditions, the mixing lengths at the axis increase linearly with Ri so that the corresponding turbulent Prandtl number Prt decreases with Ri. In contrast, Prt and the mixing lengths do not display a systematic dependence on Re. Comparison with unbounded and semi-bound shear flows suggests that the strong confinement imposed by the pipe wall may be constraining the integral length scale and Prandtl mixing lengths.

Effective mass diffusion and dispersion in random porous media

In this work, the effect of the stochastic nature of porous medium on porosity, effective diffusion and mass dispersion was investigated. To this end, a methodology to build idealized media composed by solid particles with random size has been introduced. Gaussian and uniform probability distribution functions were employed to design the size and orientation of rectangular cylinders and cubes. Predictions of effective parameters were performed by using numerical procedures based on the volume averaging method. From the porosity and effective diffusion analysis, it was found that a minimum number of particles minimizes fluctuations of predictions. Thus, a minimum sampling size to measure properties was inferred. The minimum number of particles depends on the probability distribution functions and the dimensionality of particles used (cylinders or cubes). The methodology to build porous media also allows the creation of anisotropic media, and its effect is marked over the longitudinal and transverse components of effective diffusivity tensor. But this is not the case for the mass dispersion tensor, where the flow direction is the main cause for anisotropy. In fact, the flow taking place at pore‐scale increases noticeable the randomness of predictions of mass dispersion, specifically when the number of particles or the particle Péclet number are increased. This effect is more significant on the transverse mass dispersion.

Running electroweak couplings as a probe of new physics

The energy dependence of the electroweak gauge couplings has not been measured above the weak scale. We propose that percent-level measurements of the energy dependence of $\alpha_{1,2}$ can be performed now at the LHC and at future higher energy hadron colliders. These measurements can be used to set limits on new particles with electroweak quantum numbers without relying on any assumptions about their decay properties. The shape of the high invariant mass spectrum of Drell-Yan, $p p \rightarrow Z^*/\gamma^* \rightarrow \ell^+ \ell^-$, constrains $\alpha_{1,2}(Q)$, and the shape of the high transverse mass distribution of $p p \rightarrow W^* \rightarrow \ell \nu$ constrains $\alpha_{2}(Q)$. We use existing data to perform the first fits to $\alpha_{1,2}$ above the weak scale. Percent-level measurements are possible because of high precision in theoretical predictions and existing experimental measurements. We show that the LHC already has the reach to improve upon electroweak precision tests for new particles that dominantly couple through their electroweak charges. The 14 TeV LHC is sensitive to the predicted Standard Model (SM) running of $\alpha_2$, and can show that $\alpha_2$ decreases with energy at $2-3 \sigma$ significance. A future 100 TeV proton-proton collider will have significant reach to measure running weak couplings, with sensitivity to the SM running of $\alpha_2$ at $4-5 \sigma$ and sensitivity to winos with masses up to $\sim$ 1.3 TeV at $2\sigma$.

Mean transverse mass of hadrons in proton–proton reactions

An energy dependence of the mean transverse mass $\langle m_T\rangle$ at mid-rapidity in proton-proton ($p+p$) reactions is studied within the ultra-relativistic quantum molecular dynamics (UrQMD). The UrQMD model predicts a nonmonotonous dependence of $\langle m_T\rangle$ on collision energy for several hadron species: for $\pi^+$, $p$, $K^+$, and $\Lambda$ the mean transverse mass has a maximum at the center of mass energy region $5\le \sqrt{s}\le 8$ GeV. These results are a consequence of an interplay of two contributions: 1) excitations and decays of the baryonic resonances $N^*$ and $\Delta$; 2) excitations and decays of the baryonic strings. The UrQMD results do not show any nonmonotonous dependence of $\langle m_T\rangle$ on $\sqrt{s}$ for $\pi^-$, $K^{-}$, and antiprotons. Whether a nonmonotonous dependence of $\langle m_T\rangle$ at mid-rapidity on the collision energy for $\pi^+$, $p$, $K^+$, and $\Lambda$ is relevant for real $p+p$ interactions will be soon checked experimentally by the NA61/SHINE Collaboration.

Modeling the Transverse Shell-side Mass Transfer in Hollow Fiber Membrane Contactors at Low Reynolds Numbers

A method for calculating the external mass transfer in a contactor with a transverse confined flow of a viscous incompressible liquid (gas) past hollow fibers at low Reynolds numbers is proposed. The method is based on the concept of regular arrays of parallel fibers with a well-defined flowfield. As a simplest model system, a row of parallel fibers is considered, for which dependences of a drag force and an efficiency of a solute retention on the inter-fiber distance, membrane mass transfer coefficient, Peclet and Reynolds numbers are computed. The influence of the fluid inertia on the mass transport is studied. It is shown that a linear Stokes equations can be used for as higher Re numbers, as denser is the fiber array. In this case the flow field is independent on the Re number, and analytical solutions for the flowfield and fiber sorption efficiency (fiber Sherwood number) can be used.