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J. Adam

Publications and source records attributed to J. Adam.

At least 127 records · Page 7Linked to original sources
arXiv2018

Measurements of Dielectron Production in Au$+$Au Collisions at $\sqrt{s_{NN}}$= 27, 39, and 62.4 GeV from the STAR Experiment

We report systematic measurements of dielectron ($e^{\pm}e^{\pm}$) invariant-mass $M_{ee}$ spectra at mid-rapidity in Au+Au collisions at $\sqrt{s_{NN}}$ = 27, 39, and 62.4 GeV taken with the STAR detector at the Relativistic Heavy Ion Collider. For all energies studied, a significant excess yield of dielectrons is observed in the low-mass region (0.40$ < M_{ee} < 0.75$ MeV/$c^2$) compared to hadronic cocktail simulations at freeze-out. Models that include an in-medium broadening of the $ρ$-meson spectral function consistently describe the observed excess. In addition, we report acceptance-corrected dielectron-excess spectra for Au+Au collisions at mid-rapidity ($\left|y_{ee}\right|$ $<$ 1) in the 0$-$80% centrality bin for each collision energy. The integrated excess yields for $0.4 < M_{ee} < 0.75\ \textrm{GeV}/c^{2}$, normalized by the charged particle multiplicity at mid-rapidity, are compared with previously published measurements for Au+Au at $\sqrt{s_{NN}}$ = 19.6 and 200 GeV. The normalized excess yields in the low-mass region show no significant collision energy dependence. The data, however, are consistent with model calculations that demonstrate a modest energy dependence.

nucl-ex↗
arXiv2018

$J/ψ$ production cross section and its dependence on charged-particle multiplicity in $p+p$ collisions at $\sqrt{s}$ = 200 GeV

We present a measurement of inclusive $J/ψ$ production at mid-rapidity ($|y|<1$) in $p+p$ collisions at a center-of-mass energy of $\sqrt{s}$ = 200 GeV with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The differential production cross section for $J/ψ$ as a function of transverse momentum ($p_T$) for $0<p_T<14$ GeV/$c$ and the total cross section are reported and compared to calculations from the color evaporation model and the non-relativistic Quantum Chromodynamics model. The dependence of $J/ψ$ relative yields in three $p_T$ intervals on charged-particle multiplicity at mid-rapidity is measured for the first time in $p+p$ collisions at $\sqrt{s}$ = 200 GeV and compared with that measured at $\sqrt{s}$ = 7 TeV, PYTHIA8 and EPOS3 Monte Carlo generators, and the Percolation model prediction.

hep-ex↗
arXiv2018

Low-$p_T$ $e^{+}e^{-}$ pair production in Au$+$Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and U$+$U collisions at $\sqrt{s_{NN}}$ = 193 GeV at STAR

We report first measurements of $e^{+}e^{-}$ pair production in the mass region 0.4 $<M_{ee}<$ 2.6 GeV/$c^{2}$ at low transverse momentum ($p_T<$ 0.15 GeV/$c$) in non-central Au$+$Au collisions at $\sqrt{s_{NN}}$ = 200 GeV and U$+$U collisions at $\sqrt{s_{NN}}$ = 193 GeV. Significant enhancement factors, expressed as ratios of data over known hadronic contributions, are observed in the 40-80% centrality of these collisions. The excess yields peak distinctly at low-$p_T$ with a width ($\sqrt{\langle p^2_T\rangle}$) between 40 to 60 MeV/$c$. The absolute cross section of the excess depends weakly on centrality while those from a theoretical model calculation incorporating an in-medium broadened $ρ$ spectral function and radiation from a Quark Gluon Plasma or hadronic cocktail contributions increase dramatically with increasing number of participant nucleons. Model calculations of photon-photon interactions generated by the initial projectile and target nuclei describe the observed excess yields but fail to reproduce the $p^{2}_{T}$ distributions.

hep-ex↗
arXiv2018

Global polarization of $Λ$ hyperons in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV

Global polarization of $Λ$ hyperons has been measured to be of the order of a few tenths of a percent in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200 GeV, with no significant difference between $Λ$ and $\barΛ$. These new results reveal the collision energy dependence of the global polarization together with the results previously observed at $\sqrt{s_{_{NN}}}$ = 7.7 -- 62.4 GeV and indicate noticeable vorticity of the medium created in non-central heavy-ion collisions at the highest RHIC collision energy. The signal is in rough quantitative agreement with the theoretical predictions from a hydrodynamic model and from the AMPT (A Multi-Phase Transport) model. The polarization is larger in more peripheral collisions, and depends weakly on the hyperon's transverse momentum and pseudorapidity $η^H$ within $|η^H|<1$. An indication of the polarization dependence on the event-by-event charge asymmetry is observed at the $2σ$ level, suggesting a possible contribution to the polarization from the axial current induced by the initial magnetic field.

nucl-ex↗
arXiv2018

Correlation Measurements Between Flow Harmonics in Au+Au Collisions at RHIC

Flow harmonics ($v_n$) in the Fourier expansion of the azimuthal distribution of particles are widely used to quantify the anisotropy in particle emission in high-energy heavy-ion collisions. The symmetric cumulants, $SC(m,n)$, are used to measure the correlations between different orders of flow harmonics. These correlations are used to constrain the initial conditions and the transport properties of the medium in theoretical models. In this Letter, we present the first measurements of the four-particle symmetric cumulants in Au+Au collisions at $\sqrt{s_{NN}}$ = 39 and 200 GeV from data collected by the STAR experiment at RHIC. We observe that $v_{2}$ and $v_{3}$ are anti-correlated in all centrality intervals with similar correlation strengths from 39 GeV Au+Au to 2.76 TeV Pb+Pb (measured by the ALICE experiment). The $v_{2}$-$v_{4}$ correlation seems to be stronger at 39 GeV than at higher collision energies. The initial-stage anti-correlations between second and third order eccentricities are sufficient to describe the measured correlations between $v_{2}$ and $v_{3}$. The best description of $v_{2}$-$v_{4}$ correlations at $\sqrt{s_{NN}}$ = 200 GeV is obtained with inclusion of the system's nonlinear response to initial eccentricities accompanied by the viscous effect with $η/s$ $>$ 0.08. Theoretical calculations using different initial conditions, equations of state and viscous coefficients need to be further explored to extract $η/s$ of the medium created at RHIC.

nucl-ex↗
arXiv2018

Longitudinal Double-Spin Asymmetries for Dijet Production at Intermediate Pseudorapidity in Polarized $pp$ Collisions at $\sqrt{s}$ = 200 GeV

We present the first measurements of the longitudinal double-spin asymmetry $A_{LL}$ for dijets with at least one jet reconstructed within the pseudorapidity range $0.8 < η< 1.8$. The dijets were measured in polarized $pp$ collisions at a center-of-mass energy $\sqrt{s}$ = 200 GeV. Values for $A_{LL}$ are determined for several distinct event topologies, defined by the jet pseudorapidities, and span a range of parton momentum fraction $x$ down to $x \sim$ 0.01. The measured asymmetries are found to be consistent with the predictions of global analyses that incorporate the results of previous RHIC measurements. They will provide new constraints on $Δg(x)$ in this poorly constrained region when included in future global analyses.

hep-ex↗
arXiv2018

Longitudinal Double-Spin Asymmetries for $π^{0}$s in the Forward Direction for 510 GeV Polarized $pp$ Collisions

The STAR Collaboration reports measurements of the longitudinal double-spin asymmetry, $A_{LL}$, for neutral pions produced at forward directions in polarized proton-proton collisions, at a center-of-mass energy of $510$ GeV. Results are given for transverse momenta in the range $2<p_{T}<10$ GeV/$c$ within two regions of pseudorapidity that span $2.65<η<3.9$. These results are sensitive to the polarized gluon parton distribution function, $Δg(x)$, down to the region of Bjorken $x \sim 10^{-3}$. The asymmetries observed are less than $\pm 5 \cdot 10^{-3}$ in magnitude, and will help constrain the contribution to the spin of the proton from polarized gluons at low $x$, when combined with other measurements as part of a global analysis.

hep-ex↗
arXiv2018

Beam energy dependence of rapidity-even dipolar flow in Au+Au collisions

New measurements of directed flow for charged hadrons, characterized by the Fourier coefficient \vone, are presented for transverse momenta $\mathrm{p_T}$, and centrality intervals in Au+Au collisions recorded by the STAR experiment for the center-of-mass energy range $\mathrm{\sqrt{s_{_{NN}}}} = 7.7 - 200$ GeV. The measurements underscore the importance of momentum conservation and the characteristic dependencies on $\mathrm{\sqrt{s_{_{NN}}}}$, centrality and $\mathrm{p_T}$ are consistent with the expectations of geometric fluctuations generated in the initial stages of the collision, acting in concert with a hydrodynamic-like expansion. The centrality and $\mathrm{p_T}$ dependencies of $\mathrm{v^{even}_{1}}$, as well as an observed similarity between its excitation function and that for $\mathrm{v_3}$, could serve as constraints for initial-state models. The $\mathrm{v^{even}_{1}}$ excitation function could also provide an important supplement to the flow measurements employed for precision extraction of the temperature dependence of the specific shear viscosity.

nucl-ex↗
arXiv2017

Study of secondary neutron interactions with $^{232}$Th, $^{129}$I, and $^{127}$I nuclei with the uranium assembly "QUINTA" at 2, 4, and 8 GeV deuteron beams of the JINR Nuclotron accelerator

The natural uranium assembly, "QUINTA", was irradiated with 2, 4, and 8 GeV deuterons. The $^{232}$Th, $^{127}$I, and $^{129}$I samples have been exposed to secondary neutrons produced in the assembly at a 20-cm radial distance from the deuteron beam axis. The spectra of gamma rays emitted by the activated $^{232}$Th, $^{127}$I, and $^{129}$I samples have been analyzed and several tens of product nuclei have been identified. For each of those products, neutron-induced reaction rates have been determined. The transmutation power for the $^{129}$I samples is estimated. Experimental results were compared to those calculated with well-known stochastic and deterministic codes.

physics.acc-ph↗
arXiv2017

Mirror Position Determination for the Alignment of Cherenkov Telescopes

Imaging Atmospheric Cherenkov Telescopes (IACTs) need imaging optics with large apertures to map the faint Cherenkov light emitted in extensive air showers onto their image sensors. Segmented reflectors fulfill these needs using mass produced and light weight mirror facets. However, as the overall image is the sum of the individual mirror facet images, alignment is important. Here we present a method to determine the mirror facet positions on a segmented reflector in a very direct way. Our method reconstructs the mirror facet positions from photographs and a laser distance meter measurement which goes from the center of the image sensor plane to the center of each mirror facet. We use our method to both align the mirror facet positions and to feed the measured positions into our IACT simulation. We demonstrate our implementation on the 4 m First Geiger-mode Avalanche Cherenkov Telescope (FACT).

astro-ph.IM↗
arXiv2016

Systematic study of actinide and pre-actinide fission modes

In this work, we present new experimental data on mass distribution of fission fragments from $^{241}$Am proton-induced fission at $660$ MeV measured at the LNR Phasotron (JINR). The systematic analysis of several measured fragment mass distributions from different fission reactions available in the literature is also presented. The proton-induced fission of $^{241}$Am, $^{237}$Np and $^{238}$U at 26.5, 62.9 and 660 MeV was studied. The proton-induced fission of $^{232}$Th was studied at 26.5, 62.9 and 190 MeV. The fission of $^{208}$Pb also by a proton was investigated at 190, 500 and 1000 MeV. The fission of $^{197}$Au was studied for 190 and 800 MeV protons. Bremsstrahlung reactions with maximum photon energies of 50 and 3500 MeV were studied for $^{232}$Th and $^{238}$U. The framework of the Random Neck Rupture Model was applied in the analysis. The roles of the neutron excess and of the so called fissility parameter were also investigated.

nucl-th↗
arXiv2015

Measurement of the $ν_μ$ CCQE cross section on carbon with the ND280 detector at T2K

The Charged-Current Quasi-Elastic (CCQE) interaction, $ν_{l} + n \rightarrow l^{-} + p$, is the dominant CC process at $E_ν\sim 1$ GeV and contributes to the signal in accelerator-based long-baseline neutrino oscillation experiments operating at intermediate neutrino energies. This paper reports a measurement by the T2K experiment of the $ν_μ$ CCQE cross section on a carbon target with the off-axis detector based on the observed distribution of muon momentum ($p_μ$) and angle with respect to the incident neutrino beam ($θ_μ$). The flux-integrated CCQE cross section was measured to be $(0.83 \pm 0.12) \times 10^{-38}\textrm{ cm}^{2}$ in good agreement with NEUT MC value of ${0.88 \times 10^{-38}} \textrm{ cm}^{2}$. The energy dependence of the CCQE cross section is also reported. The axial mass, $M_A^{QE}$, of the dipole axial form factor was extracted assuming the Smith-Moniz CCQE model with a relativistic Fermi gas nuclear model. Using the absolute (shape-only) $p_μcosθ_μ$ distribution, the effective $M_A^{QE}$ parameter was measured to be ${1.26^{+0.21}_{-0.18} \textrm{ GeV}/c^{2}}$ (${1.43^{+0.28}_{-0.22} \textrm{ GeV}/c^{2}}$).

hep-ex↗
arXiv2015

Measurement of the $ν_μ$ charged current quasi-elastic cross-section on carbon with the T2K on-axis neutrino beam

We report a measurement of the $ν_μ$ charged current quasi-elastic cross-sections on carbon in the T2K on-axis neutrino beam. The measured charged current quasi-elastic cross-sections on carbon at mean neutrino energies of 1.94 GeV and 0.93 GeV are $(11.95\pm 0.19(stat.)_{-1.47}^{+1.82} (syst.))\times 10^{-39}\mathrm{cm}^2/\mathrm{neutron}$ and $(10.64\pm 0.37(stat.)_{-1.65}^{+2.03} (syst.))\times 10^{-39}\mathrm{cm}^2/\mathrm{neutron}$, respectively. These results agree well with the predictions of neutrino interaction models. In addition, we investigated the effects of the nuclear model and the multi-nucleon interaction.

hep-ex↗
arXiv2015

Measurement of the Electron Neutrino Charged-current Interaction Rate on Water with the T2K ND280 pi-zero Detector

This paper presents a measurement of the charged current interaction rate of the electron neutrino beam component of the beam above $1.5$~GeV using the large fiducial mass of the T2K $π^0$ detector. The predominant poriton of the $ν_e$ flux ($\sim$85 %) at these energies comes from kaon decays. The measured ratio of the observed beam interaction rate to the predicted rate in the detector with water targets filled is 0.89 $\pm$ 0.08 (stat.) $\pm$ 0.11 (sys.), and with the water targets emptied is 0.90 $\pm$ 0.09 (stat.) $\pm$ 0.13 (sys.). The ratio obtained for the interactions on water only from an event subtraction method is 0.87 $\pm$ 0.33 (stat.) $\pm$ 0.21 (sys.). This is the first measurement of the interaction rate of electron neutrinos on water, which is particularly of interest to experiments with water Cherenkov detectors.

hep-ex↗
arXiv2015

Measurements of neutrino oscillation in appearance and disappearance channels by the T2K experiment with 6.6E20 protons on target

We report on measurements of neutrino oscillation using data from the T2K long-baseline neutrino experiment collected between 2010 and 2013. In an analysis of muon neutrino disappearance alone, we find the following estimates and 68% confidence intervals for the two possible mass hierarchies: Normal Hierarchy: $\sin^2θ_{23}=0.514^{+0.055}_{-0.056}$ and $Δm^2_{32}=(2.51\pm0.10)\times 10^{-3}$ eV$^2$/c$^4$ Inverted Hierarchy: $\sin^2θ_{23}=0.511\pm0.055$ and $Δm^2_{13}=(2.48\pm0.10)\times 10^{-3}$ eV$^2$/c$^4$ The analysis accounts for multi-nucleon mechanisms in neutrino interactions which were found to introduce negligible bias. We describe our first analyses that combine measurements of muon neutrino disappearance and electron neutrino appearance to estimate four oscillation parameters and the mass hierarchy. Frequentist and Bayesian intervals are presented for combinations of these parameters, with and without including recent reactor measurements. At 90% confidence level and including reactor measurements, we exclude the region: $δ_{CP}=[0.15,0.83]π$ for normal hierarchy and $δ_{CP}=[-0.08,1.09]π$ for inverted hierarchy. The T2K and reactor data weakly favor the normal hierarchy with a Bayes Factor of 2.2. The most probable values and 68% 1D credible intervals for the other oscillation parameters, when reactor data are included, are: $\sin^2θ_{23}=0.528^{+0.055}_{-0.038}$ and $|Δm^2_{32}|=(2.51\pm0.11)\times 10^{-3}$ eV$^2$/c$^4$.

hep-ex↗
arXiv2015

Neutrino Oscillation Physics Potential of the T2K Experiment

The observation of the recent electron neutrino appearance in a muon neutrino beam and the high-precision measurement of the mixing angle $θ_{13}$ have led to a re-evaluation of the physics potential of the T2K long-baseline neutrino oscillation experiment. Sensitivities are explored for CP violation in neutrinos, non-maximal $\sin^22θ_{23}$, the octant of $θ_{23}$, and the mass hierarchy, in addition to the measurements of $δ_{CP}$, $\sin^2θ_{23}$, and $Δm^2_{32}$, for various combinations of $ν$-mode and \(\barν\)-mode data-taking. With an exposure of $7.8\times10^{21}$~protons-on-target, T2K can achieve 1-$σ$ resolution of 0.050(0.054) on $\sin^2θ_{23}$ and $0.040(0.045)\times10^{-3}~\rm{eV}^2$ on $Δm^2_{32}$ for 100\%(50\%) neutrino beam mode running assuming $\sin^2θ_{23}=0.5$ and $Δm^2_{32} = 2.4\times10^{-3}$ eV$^2$. T2K will have sensitivity to the CP-violating phase $δ_{\rm{CP}}$ at 90\% C.L. or better over a significant range. For example, if $\sin^22θ_{23}$ is maximal (i.e $θ_{23}$=$45^\circ$) the range is $-115^\circ<δ_{\rm{CP}}<-60^\circ$ for normal hierarchy and $+50^\circ<δ_{\rm{CP}}<+130^\circ$ for inverted hierarchy. When T2K data is combined with data from the NO$ν$A experiment, the region of oscillation parameter space where there is sensitivity to observe a non-zero $δ_{CP}$ is substantially increased compared to if each experiment is analyzed alone.

hep-ex↗
arXiv2014

Production of isomeric states in the deuteron-induced reaction of gold at incident energy 4 GeV

The independent cross section ratio for production of nuclei from 197Au targets irradiated with 4 GeV deuterons have been measured by off-line gamma-spectroscopy. On the basis of the measured independent cross section ratio of 198m, gAu the average intrinsic angular momentum of the primary nucleus was estimated by means of a simple statistical-model analysis based on the formalism developed by Huizenga and Vandenbosch.

nucl-ex↗