arXiv subjects

A. Gupta

Publications and source records attributed to A. Gupta.

At least 181 records · Page 10Linked to original sources

arXiv2017

Measurement of $D^0$ azimuthal anisotropy at mid-rapidity in Au+Au collisions at \sNN = 200\,GeV

We report the first measurement of the elliptic anisotropy ($v_2$) of the charm meson $D^0$ at mid-rapidity ($|y|$\,$<$\,1) in Au+Au collisions at \sNN = 200\,GeV. The measurement was conducted by the STAR experiment at RHIC utilizing a new high-resolution silicon tracker. The measured $D^0$ $v_2$ in 0--80\% centrality Au+Au collisions can be described by a viscous hydrodynamic calculation for transverse momentum ($p_{\rm T}$) less than 4\,GeV/$c$. The $D^0$ $v_2$ as a function of transverse kinetic energy ($m_{\rm T} - m_0$, where $m_{\rm T} = \sqrt{p_{\rm T}^2 + m_0^2}$) is consistent with that of light mesons in 10--40\% centrality Au+Au collisions. These results suggest that charm quarks have achieved local thermal equilibrium with the medium created in such collisions. Several theoretical models, with the temperature--dependent, dimensionless charm spatial diffusion coefficient ($2πTD_s$) in the range of $\sim$2--12, are able to simultaneously reproduce our $D^0$ $v_2$ result and our previously published results for the $D^0$ nuclear modification factor.

nucl-ex↗

arXiv2017

Elliptic flow of electrons from heavy-flavor hadron decays in Au+Au collisions at $\sqrt{s_{\rm NN}} = $ 200, 62.4, and 39 GeV

We present measurements of elliptic flow ($v_2$) of electrons from the decays of heavy-flavor hadrons ($e_{HF}$) by the STAR experiment. For Au+Au collisions at $\sqrt{s_{\rm NN}} = $ 200 GeV we report $v_2$, for transverse momentum ($p_T$) between 0.2 and 7 GeV/c using three methods: the event plane method ($v_{2}${EP}), two-particle correlations ($v_2${2}), and four-particle correlations ($v_2${4}). For Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 62.4 and 39 GeV we report $v_2${2} for $p_T< 2$ GeV/c. $v_2${2} and $v_2${4} are non-zero at low and intermediate $p_T$ at 200 GeV, and $v_2${2} is consistent with zero at low $p_T$ at other energies. The $v_2${2} at the two lower beam energies is systematically lower than at $\sqrt{s_{\rm NN}} = $ 200 GeV for $p_T < 1$ GeV/c. This difference may suggest that charm quarks interact less strongly with the surrounding nuclear matter at those two lower energies compared to $\sqrt{s_{\rm NN}} = 200$ GeV.

hep-ex↗

arXiv2017

Hubble Space Telescope Observations of BALQSO Ton 34 Reveal a Connection between the Broad Line Region and the BAL Outflow

Ton 34 recently transitioned from non-absorbing quasar into a BALQSO.Here, we report new HST-STIS observations of this quasar. Along with CIV absorption, we also detect absorption by NV+Ly alpha and possibly OVI+Ly beta. We follow the evolution of the CIV BAL, and find that, for the slower outflowing material, the absorption trough varies little (if at all) on a rest-frame timescale of 2 yr. However, we detect a strong deepening of the absorption in the gas moving at larger velocities (-20,000 - -23,000 km s-1). The data is consistent with a multistreaming flow crossing our line of sight to the source. The transverse velocity of the flow should be few thousand km s-1, similar to the rotation velocity of the BLR gas (2,600 km s-1). By simply assuming Keplerian motion, these two components must have similar locations, pointing to a common outflow forming the BLR and the BAL. We speculate that BALs, mini-BALs, and NALs, are part of a common, ubiquitous, accretion-disk outflow in AGN, but become observable depending on the viewing angle towards the flow. The absorption troughs suggest a wind covering only 20% of the emitting source, implying a maximum size of 10^-3 pc for the clouds forming the BAL/BLR medium. This is consistent with constraints of the BLR clouds from X-ray occultations. Finally, we suggest that the low excitation broad emission lines detected in the spectra of this source lie beyond the wind, and this gas is probably excited by the shock of the BAL wind with the surrounding medium.

astro-ph.GA↗

arXiv2017

Measurement of the cross section and longitudinal double-spin asymmetry for di-jet production in polarized $pp$ collisions at $\sqrt{s}$ = 200 GeV

We report the first measurement of the longitudinal double-spin asymmetry $A_{LL}$ for mid-rapidity di-jet production in polarized $pp$ collisions at a center-of-mass energy of $\sqrt{s} = 200$ GeV. The di-jet cross section was measured and is shown to be consistent with next-to-leading order (NLO) perturbative QCD predictions. $A_{LL}$ results are presented for two distinct topologies, defined by the jet pseudorapidities, and are compared to predictions from several recent NLO global analyses. The measured asymmetries, the first such correlation measurements, support those analyses that find positive gluon polarization at the level of roughly 0.2 over the region of Bjorken-$x > 0.05$.

hep-ex↗

arXiv2017

Coherent diffractive photoproduction of $ρ^{0}$ mesons on gold nuclei at RHIC

The STAR Collaboration reports on the photoproduction of $π^+π^-$ pairs in gold-gold collisions at a center-of-mass energy of 200 GeV/nucleon-pair. These pion pairs are produced when a nearly-real photon emitted by one ion scatters from the other ion. We fit the $π^+π^-$ invariant mass spectrum with a combination of $ρ$ and $ω$ resonances and a direct $π^+π^-$ continuum. This is the first observation of the $ω$ in ultra-peripheral collisions, and the first measurement of $ρ-ω$ interference at energies where photoproduction is dominated by Pomeron exchange. The $ω$ amplitude is consistent with the measured $γp\rightarrow ωp$ cross section, a classical Glauber calculation and the $ω\rightarrowπ^+π^-$ branching ratio. The $ω$ phase angle is similar to that observed at much lower energies, showing that the $ρ-ω$ phase difference does not depend significantly on photon energy. The $ρ^0$ differential cross section $dσ/dt$ exhibits a clear diffraction pattern, compatible with scattering from a gold nucleus, with 2 minima visible. The positions of the diffractive minima agree better with the predictions of a quantum Glauber calculation that does not include nuclear shadowing than with a calculation that does include shadowing.

nucl-ex↗

arXiv2017

Measurements of jet quenching with semi-inclusive hadron+jet distributions in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

The STAR Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum hadron trigger, in central and peripheral Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Charged jets are reconstructed with the anti-kT algorithm for jet radii R between 0.2 and 0.5 and with low infrared cutoff of track constituents ($p_T>0.2$ GeV/c). A novel mixed-event technique is used to correct the large uncorrelated background present in heavy ion collisions. Corrected recoil jet distributions are reported at mid-rapidity, for charged-jet transverse momentum $p_T^\mathrm{jet,ch}<30$ GeV/c. Comparison is made to similar measurements for Pb+Pb collisions at $\sqrt{s_{NN}}$ = 2.76 TeV, to calculations for p+p collisions at $\sqrt{s}$ = 200 GeV based on the PYTHIA Monte Carlo generator and on a Next-to-Leading Order perturbative QCD approach, and to theoretical calculations incorporating jet quenching. The recoil jet yield is suppressed in central relative to peripheral collisions, with the magnitude of the suppression corresponding to medium-induced charged energy transport out of the jet cone of $2.8\pm0.2\mathrm{(stat)}\pm1.5\mathrm{(sys)}$ GeV/c, for $10<p_T^\mathrm{jet,ch}<20$ GeV/c and R = 0.5. No medium-induced change in jet shape is observed for $R<0.5$. The azimuthal distribution of low-$p_T^\mathrm{jet,ch}$ recoil jets may be enhanced at large azimuthal angles to the trigger axis, due to scattering off quasi-particles in the hot QCD medium. Measurement of this distribution gives a 90% statistical confidence upper limit to the yield enhancement at large deflection angles in central Au+Au collisions of $50\pm30\mathrm{(sys)\%}$ of the large-angle yield in p+pcollisions predicted by PYTHIA.

nucl-ex↗

arXiv2017

Surface spin canting in Fe3O4 and CoFe2O4 nanoparticles probed by high resolution electron energy loss spectroscopy

High resolution electron energy loss spectroscopy (HR-EELS) is utilized to probe the surface spin canting in nanoparticles of two technologically important magnetic materials, i.e. Fe3O4 and CoFe2O4 (CFO). A soft experimental technique is developed that is capable of extracting EELS spectra with one atomic plane resolution recorded in a single frame. This yields information at different depth of the nanoparticle from the surface to the core regions with high signal to noise ratio and without beam damage. This enables comparing the fine structures between the surface and core regions of the nanoparticles. The results confirm earlier observations of uniformly oriented spin canting structure for CFO with additional information on atom site-selective spin canting information. In case of Fe3O4 preferred canting orientation forming core and shell structure is deduced. Unlike earlier reports based on polarized spin-flip neutron scattering measurement, it is possible to narrow down the possible canting angles for Fe3O4 (Td, Oh tilts 40°, 40°) and CFO (Td, Oh tilts 17°, 17°) from the experimental spectra combined with the first principle based calculation considering non-collinear magnetism. In addition, the role of Dzyaloshinskii-Moriya interaction in stabilizing the spin canting at the nanoparticle surface is discussed. The results demonstrate that HREELS can be a powerful technique to probe the magnetic structure in nano-dimensional systems and has advantages over neutron based techniques in terms of superior spatial resolution, site specific information and easy of sample preparation.

cond-mat.mtrl-sci↗

arXiv2016

Charge-dependent directed flow in Cu+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV

We present the first measurement of charge-dependent directed flow in Cu+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV. The results are presented as a function of the particle transverse momentum and pseudorapidity for different centralities. A finite difference between the directed flow of positive and negative charged particles is observed that qualitatively agrees with the expectations from the effects of the initial strong electric field between two colliding ions with different nuclear charges. The measured difference in directed flow is much smaller than that obtained from the parton-hadron-string-dynamics (PHSD) model, which suggests that most of the electric charges, i.e. quarks and antiquarks, have not yet been created during the lifetime of the strong electric field, which is of the order of, or less than, 1fm/$c$.

nucl-ex↗

arXiv2016

Upsilon production in U+U collisions at 193 GeV with the STAR experiment

We present a measurement of the inclusive production of Upsilon mesons in U+U collisions at 193 GeV at mid-rapidity (|y| < 1). Previous studies in central Au+Au collisions at 200 GeV show a suppression of Upsilon(1S+2S+3S) production relative to expectations from the Upsilon yield in p+p collisions scaled by the number of binary nucleon-nucleon collisions (Ncoll), with an indication that the Upsilon(1S) state is also suppressed. The present measurement extends the number of participant nucleons in the collision (Npart) by 20% compared to Au+Au collisions, and allows us to study a system with higher energy density. We observe a suppression in both the Upsilon(1S+2S+3S) and Upsilon(1S) yields in central U+U data, which consolidates and extends the previously observed suppression trend in Au+Au collisions.

nucl-ex↗

arXiv2016

Viscoelastic Multicomponent Fluids in confined Flow-Focusing Devices

The effects of elasticity on the break-up of liquid threads in microfluidic cross-junctions is investigated using numerical simulations based on the "lattice Boltzmann models" (LBM). Working at small Capillary numbers, we investigate the effects of non-Newtonian phases in the transition from droplet formation at the cross-junction (DCJ) and droplet formation downstream of the cross-junction (DC) (Liu & Zhang, ${\it Phys. Fluids.}$ ${\bf 23}$, 082101 (2011)). Viscoelasticity is found to influence the break-up point of the threads, which moves closer to the cross-junction and stabilizes. This is attributed to an increase of the polymer feedback stress forming in the corner flows, where the side channels of the device meet the main channel.

physics.flu-dyn↗

arXiv2016

Deformation and break-up of viscoelastic droplets in confined shear flow

The deformation and break-up of Newtonian/viscoelastic droplets are studied in confined shear flow. Our numerical approach is based on a combination of Lattice-Boltzmann models (LBM) and finite difference schemes, the former used to model two immiscible fluids with variable viscous ratio, and the latter used to model the polymer dynamics. The kinetics of the polymers is introduced using constitutive equations for viscoelastic fluids with finitely extensible non-linear elastic dumbbells with Peterlin's closure (FENE-P). We quantify the droplet response by changing the polymer relaxation time $τ_P$, the maximum extensibility $L$ of the polymers, and the degree of confinement, i.e. the ratio of the droplet diameter to gap spacing. In unconfined shear flow, the effects of droplet viscoelasticity on the critical Capillary number $\mbox{Ca}_{\mbox{\tiny{cr}}}$ for break-up are moderate in all cases studied. However, in confined conditions a different behaviour is observed: the critical Capillary number of a viscoelastic droplet increases or decreases, depending on the maximum elongation of the polymers, the latter affecting the extensional viscosity of the polymeric solution. Force balance is monitored in the numerical simulations to validate the physical picture.

cond-mat.soft↗

arXiv2016

Lattice Boltzmann Simulations of Droplet formation in confined Channels with Thermocapillary flows

Based on mesoscale lattice Boltzmann simulations with the "Shan-Chen" model, we explore the influence of thermocapillarity on the break-up properties of fluid threads in a microfluidic T-junction, where a dispersed phase is injected perpendicularly into a main channel containing a continuous phase, and the latter induces periodic break-up of droplets due to the cross-flowing. Temperature effects are investigated by switching on/off both positive/negative temperature gradients along the main channel direction, thus promoting a different thread dynamics with anticipated/delayed break-up. Numerical simulations are performed at changing the flow-rates of both the continuous and dispersed phases, as well as the relative importance of viscous forces, surface tension forces and thermocapillary stresses. The range of parameters is broad enough to characterize the effects of thermocapillarity on different mechanisms of break-up in the confined T-junction, including the so-called "squeezing" and "dripping" regimes, previously identified in the literature. Some simple scaling arguments are proposed to rationalize the observed behaviour, and to provide quantitative guidelines on how to predict the droplet size after break-up.

physics.flu-dyn↗

arXiv2016

Shared information between residues is sufficient to detect pair-wise epistasis in a protein

In a comment on our manuscript "Strong selection significantly increases epistatic interactions in the long-term evolution of a protein", Dr. Crona challenges our assertion that shared entropy (that is, information) between two residues implies epistasis between those residues, by constructing an explicit example of three loci (say A, B, and C), where A and B are epistatically linked (leading to shared entropy between A and B), and A and C also depend epistatically (leading to shared entropy between A and C), so that loci B and C are correlated (share entropy).

q-bio.PE↗

arXiv2016

$\rm{J}/ψ$ production at low transverse momentum in p+p and d+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

We report on the measurement of $\rm{J}/ψ$ production in the dielectron channel at mid-rapidity (|y|<1) in p+p and d+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV from the STAR experiment at the Relativistic Heavy Ion Collider. The transverse momentum $p_{T}$ spectra in p+p for $p_{T}$ < 4 GeV/c and d+Au collisions for $p_{T}$ < 3 GeV/c are presented. These measurements extend the STAR coverage for $\rm{J}/ψ$ production in p+p collisions to low $p_{T}$. The $ $ from the measured $\rm{J}/ψ$ invariant cross section in p+p and d+Au collisions are evaluated and compared to similar measurements at other collision energies. The nuclear modification factor for $\rm{J}/ψ$ is extracted as a function of $p_{T}$ and collision centrality in d+Au and compared to model calculations using the modified nuclear Parton Distribution Function and a final-state $\rm{J}/ψ$ nuclear absorption cross section.

nucl-ex↗

arXiv2016

Di-Jet Imbalance Measurements at $\sqrt{s_{NN}} = 200$ GeV at STAR

We report the first di-jet transverse momentum asymmetry measurements from Au+Au and p+p collisions at RHIC. The two highest-energy back-to-back jets reconstructed from fragments with transverse momenta above 2 GeV/c display a significantly stronger momentum imbalance in heavy-ion collisions than in the p+p reference. When re-examined with correlated soft particles included, we observe that these di-jets then exhibit a unique new feature -- momentum balance is restored to that observed in p+p for a jet resolution parameter of R=0.4, while re-balancing is not attained with a smaller value of R=0.2.

nucl-ex↗

arXiv2016

Near-side azimuthal and pseudorapidity correlations using neutral strange baryons and mesons in d+Au, Cu+Cu and Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV

We present measurements of the near-side of triggered di-hadron correlations using neutral strange baryons ($Λ$, $\barΛ$) and mesons ($K^0_S$) at intermediate transverse momentum (3 $<$ $p_T$ $<$ 6 GeV/$c$) to look for possible flavor and baryon/meson dependence. This study is performed in $d$+Au, Cu+Cu and Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV measured by the STAR experiment at RHIC. The near-side di-hadron correlation contains two structures, a peak which is narrow in azimuth and pseudorapidity consistent with correlations due to jet fragmentation, and a correlation in azimuth which is broad in pseudorapidity. The particle composition of the jet-like correlation is determined using identified associated particles. The dependence of the conditional yield of the jet-like correlation on the trigger particle momentum, associated particle momentum, and centrality for correlations with unidentified trigger particles are presented. The neutral strange particle composition in jet-like correlations with unidentified charged particle triggers is not well described by PYTHIA. However, the yield of unidentified particles in jet-like correlations with neutral strange particle triggers is described reasonably well by the same model.

nucl-ex↗

arXiv2016

Energy dependence of $J/ψ$ production in Au+Au collisions at $\sqrt{s_{NN}} =$ 39, 62.4 and 200 GeV

The inclusive $J/ψ$ transverse momentum ($p_{T}$) spectra and nuclear modification factors are reported at midrapidity ($|y|<1.0$) in Au+Au collisions at $\sqrt{s_{NN}}=$ 39, 62.4 and 200 GeV taken by the STAR experiment. A suppression of $J/ψ$ production, with respect to {\color{black}the production in $p+p$ scaled by the number of binary nucleon-nucleon collisions}, is observed in central Au+Au collisions at these three energies. No significant energy dependence of nuclear modification factors is found within uncertainties. The measured nuclear modification factors can be described by model calculations that take into account both suppression of direct $J/ψ$ production due to the color screening effect and $J/ψ$ regeneration from recombination of uncorrelated charm-anticharm quark pairs.

hep-ex↗

arXiv2016

Hybrid Lattice Boltzmann/Finite Difference simulations of viscoelastic multicomponent flows in confined geometries

We propose numerical simulations of viscoelastic fluids based on a hybrid algorithm combining Lattice-Boltzmann models (LBM) and Finite Differences (FD) schemes, the former used to model the macroscopic hydrodynamic equations, and the latter used to model the polymer dynamics. The kinetics of the polymers is introduced using constitutive equations for viscoelastic fluids with finitely extensible non-linear elastic dumbbells with Peterlin's closure (FENE-P). The numerical model is first benchmarked by characterizing the rheological behaviour of dilute homogeneous solutions in various configurations, including steady shear, elongational flows, transient shear and oscillatory flows. As an upgrade of complexity, we study the model in presence of non-ideal multicomponent interfaces, where immiscibility is introduced in the LBM description using the "Shan-Chen" model. The problem of a confined viscoelastic (Newtonian) droplet in a Newtonian (viscoelastic) matrix under simple shear is investigated and numerical results are compared with the predictions of various theoretical models. The proposed numerical simulations explore problems where the capabilities of LBM were never quantified before.

physics.comp-ph↗