arXiv subjects

X. Chen

Publications and source records attributed to X. Chen.

At least 181 records · Page 10Linked to original sources

arXiv2020

RC-chain: Reputation-based Crowdsourcing Blockchain for Vehicular Networks

As the commercial use of 5G technologies has grown more prevalent, smart vehicles have become an efficient platform for delivering a wide array of services directly to customers. The vehicular crowdsourcing service (VCS), for example, can provide immediate and timely feedback to the user regarding real-time transportation information. However, different sources can generate spurious information towards a specific service request in the pursuit of profit. Distinguishing trusted information from numerous sources is the key to a reliable VCS platform. This paper proposes a solution to this problem called "RC-chain", a reputation-based crowdsourcing framework built on a blockchain platform (Hyperledger Fabric). We first establish the blockchain-based platform to support the management of crowdsourcing trading and user-reputation evaluating activities. A reputation model, the Trust Propagation \& Feedback Similarity (TPFS), then calculates the reputation values of participants and reveals any malicious behavior accordingly. Finally, queueing theory is used to evaluate the blockchain-based platform and optimize the system performance. The proposed framework was deployed on the IBM Hyperledger Fabric platform to observe its real-world running time, effectiveness, and overall performance.

cs.DC↗

arXiv2020

Measurements of $W$ and $Z/γ^*$ cross sections and their ratios in $p+p$ collisions at RHIC

We report on the $W$ and $Z/γ^*$ differential and total cross sections as well as the $W^+$/$W^-$ and $(W^+ + W^-)$/$(Z/γ^*)$ cross-section ratios measured by the STAR experiment at RHIC in $p+p$ collisions at $\sqrt{s} = 500$ GeV and $510$ GeV. The cross sections and their ratios are sensitive to quark and antiquark parton distribution functions. In particular, at leading order, the $W$ cross-section ratio is sensitive to the $\bar{d}/\bar{u}$ ratio. These measurements were taken at high $Q^2 \sim M_W^2,M_Z^2$ and can serve as input into global analyses to provide constraints on the sea quark distributions. The results presented here combine three STAR data sets from 2011, 2012, and 2013, accumulating an integrated luminosity of 350 pb$^{-1}$. We also assess the expected impact that our $W^+/W^-$ cross-section ratios will have on various quark distributions, and find sensitivity to the $\bar{u}-\bar{d}$ and $\bar{d}/\bar{u}$ distributions.

nucl-ex↗

arXiv2020

Distributed Machine Learning for Wireless Communication Networks: Techniques, Architectures, and Applications

Distributed machine learning (DML) techniques, such as federated learning, partitioned learning, and distributed reinforcement learning, have been increasingly applied to wireless communications. This is due to improved capabilities of terminal devices, explosively growing data volume, congestion in the radio interfaces, and increasing concern of data privacy. The unique features of wireless systems, such as large scale, geographically dispersed deployment, user mobility, and massive amount of data, give rise to new challenges in the design of DML techniques. There is a clear gap in the existing literature in that the DML techniques are yet to be systematically reviewed for their applicability to wireless systems. This survey bridges the gap by providing a contemporary and comprehensive survey of DML techniques with a focus on wireless networks. Specifically, we review the latest applications of DML in power control, spectrum management, user association, and edge cloud computing. The optimality, scalability, convergence rate, computation cost, and communication overhead of DML are analyzed. We also discuss the potential adversarial attacks faced by DML applications, and describe state-of-the-art countermeasures to preserve privacy and security. Last but not least, we point out a number of key issues yet to be addressed, and collate potentially interesting and challenging topics for future research.

cs.LG↗

arXiv2020

Measurement of inclusive J/$ψ$ polarization in p+p collisions at $\sqrt{s}$ = 200 GeV by the STAR experiment

We report on new measurements of inclusive J/$ψ$ polarization at mid-rapidity in p+p collisions at $\sqrt{s}$ = 200 GeV by the STAR experiment at RHIC. The polarization parameters, $λ_θ$, $λ_ϕ$, and $λ_{θϕ}$, are measured as a function of transverse momentum ($p_T$) in both the Helicity and Collins-Soper (CS) reference frames within $p_T< 10$ GeV/$C$. Except for $λ_θ$ in the CS frame at the highest measured $p_T$, all three polarization parameters are consistent with 0 in both reference frames without any strong $p_T$ dependence. Several model calculations are compared with data, and the one using the Color Glass Condensate effective field theory coupled with non-relativistic QCD gives the best overall description of the experimental results, even though other models cannot be ruled out due to experimental uncertainties.

hep-ex↗

arXiv2020

Neutron Star Extreme Matter Observatory: A kilohertz-band gravitational-wave detector in the global network

Gravitational waves from coalescing neutron stars encode information about nuclear matter at extreme densities, inaccessible by laboratory experiments. The late inspiral is influenced by the presence of tides, which depend on the neutron star equation of state. Neutron star mergers are expected to often produce rapidly-rotating remnant neutron stars that emit gravitational waves. These will provide clues to the extremely hot post-merger environment. This signature of nuclear matter in gravitational waves contains most information in the 2-4 kHz frequency band, which is outside of the most sensitive band of current detectors. We present the design concept and science case for a neutron star extreme matter observatory (NEMO): a gravitational-wave interferometer optimized to study nuclear physics with merging neutron stars. The concept uses high circulating laser power, quantum squeezing and a detector topology specifically designed to achieve the high-frequency sensitivity necessary to probe nuclear matter using gravitational waves. Above one kHz, the proposed strain sensitivity is comparable to full third-generation detectors at a fraction of the cost. Such sensitivity changes expected event rates for detection of post-merger remnants from approximately one per few decades with two A+ detectors to a few per year, and potentially allows for the first gravitational-wave observations of supernovae, isolated neutron stars, and other exotica.

astro-ph.HE↗

arXiv2020

Measurement of the mass difference and the binding energy of the hypertriton and antihypertriton

According to the CPT theorem, which states that the combined operation of charge conjugation, parity transformation and time reversal must be conserved, particles and their antiparticles should have the same mass and lifetime but opposite charge and magnetic moment. Here, we test CPT symmetry in a nucleus containing a strange quark, more specifically in the hypertriton. This hypernucleus is the lightest one yet discovered and consists of a proton, a neutron, and a $Λ$ hyperon. With data recorded by the STAR detector{\cite{TPC,HFT,TOF}} at the Relativistic Heavy Ion Collider, we measure the $Λ$ hyperon binding energy $B_Λ$ for the hypertriton, and find that it differs from the widely used value{\cite{B_1973}} and from predictions{\cite{2019_weak, 1995_weak, 2002_weak, 2014_weak}}, where the hypertriton is treated as a weakly bound system. Our results place stringent constraints on the hyperon-nucleon interaction{\cite{Hammer2002, STAR-antiH3L}}, and have implications for understanding neutron star interiors, where strange matter may be present{\cite{Chatterjee2016}}. A precise comparison of the masses of the hypertriton and the antihypertriton allows us to test CPT symmetry in a nucleus with strangeness for the first time, and we observe no deviation from the expected exact symmetry.

hep-ex↗

arXiv2020

Beam-Energy Dependence of the Directed Flow of Deuterons in Au+Au Collisions

We present a measurement of the first-order azimuthal anisotropy, $v_1(y)$, of deuterons from Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5, 14.5, 19.6, 27, and 39 GeV recorded with the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The energy dependence of the $v_1(y)$ slope, $dv_{1}/dy|_{y=0}$, for deuterons, where $y$ is the rapidity, is extracted for semi-central collisions (10-40\% centrality) and compared to that of protons. While the $v_1(y)$ slopes of protons are generally negative for $\sqrt{s_{NN}} >$ 10 GeV, those for deuterons are consistent with zero, a strong enhancement of the $v_1(y)$ slope of deuterons is seen at the lowest collision energy (the largest baryon density) at $\sqrt{s_{NN}} =$ 7.7 GeV. In addition, we report the transverse momentum dependence of $v_1$ for protons and deuterons. The experimental results are compared with transport and coalescence models.

nucl-ex↗

arXiv2020

Strange hadron production in Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}$ = 7.7, 11.5, 19.6, 27, and 39 GeV

We present STAR measurements of strange hadron ($\mathrm{K}^{0}_{\mathrm S}$, $Λ$, $\overlineΛ$, $Ξ^-$, $\overlineΞ^+$, $Ω^-$, $\overlineΩ^+$, and $ϕ$) production at mid-rapidity ($|y| < 0.5$) in Au+Au collisions at $\sqrt{s_{_{\mathrm{NN}}}}$ = 7.7 - 39 GeV from the Beam Energy Scan Program at the Relativistic Heavy Ion Collider (RHIC). Transverse momentum spectra, averaged transverse mass, and the overall integrated yields of these strange hadrons are presented versus the centrality and collision energy. Antibaryon-to-baryon ratios ($\overlineΛ$/$Λ$, $\overlineΞ^+$/$Ξ^-$, $\overlineΩ^+$/$Ω^-$) are presented as well, and used to test a thermal statistical model and to extract the temperature normalized strangeness and baryon chemical potentials at hadronic freeze-out ($μ_{B}/T_{\rm ch}$ and $μ_{S}/T_{\rm ch}$) in central collisions. Strange baryon-to-pion ratios are compared to various model predictions in central collisions for all energies. The nuclear modification factors ($R_{\textrm{CP}}$) and antibaryon-to-meson ratios as a function of transverse momentum are presented for all collision energies. The $\mathrm{K}^{0}_{\mathrm S}$ $R_{\textrm{CP}}$ shows no suppression for $p_{\rm T}$ up to 3.5 $\mathrm{GeV} / c$ at energies of 7.7 and 11.5 GeV. The $\overlineΛ$/$\mathrm{K}^{0}_{\mathrm S}$ ratio also shows baryon-to-meson enhancement at intermediate $p_{\rm T}$ ($\approx$2.5 $\mathrm{GeV} / c$) in central collisions at energies above 19.6 GeV. Both observations suggest that there is likely a change of the underlying strange quark dynamics at collision energies below 19.6 GeV.

nucl-ex↗

arXiv2020

Evolution of spin excitations from bulk to monolayer FeSe

The discovery of enhanced superconductivity (SC) in FeSe films grown on SrTiO3 (FeSe/STO) has revitalized the field of Fe-based superconductors. In the ultrathin limit, the superconducting transition temperature Tc is increased by almost an order of magnitude, raising new questions on the pairing mechanism. As in other unconventional superconductors, antiferromagnetic spin fluctuations have been proposed as a candidate to mediate SC in this system. Thus, it is essential to study the evolution of the spin dynamics of FeSe in the ultrathin limit to elucidate their relationship with superconductivity. Here, we investigate and compare the spin excitations in bulk and monolayer FeSe grown on STO using high-resolution resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations. Despite the absence of long-range magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped, dispersionless, and significantly hardened relative to the bulk counterpart. By comparing our RIXS results with simulations of a bilayer Hubbard model, we connect the evolution of the spin excitations to the Fermiology of the two systems. The present study reveals a remarkable reconfiguration of spin excitations in FeSe/STO, which is essential to understand the role of spin fluctuations in the pairing mechanism.

cond-mat.str-el↗

arXiv2020

Jiamusi pulsar observations: III. Nulling of 20 pulsars

Most of pulsar nulling observations were conducted at frequencies lower than 1400~MHz. We aim to understand the nulling behaviors of pulsars at relatively high frequency, and to check if nulling is caused by a global change of pulsar magnetosphere. 20 bright pulsars are observed at 2250~MHz with unprecedented lengths of time by using Jiamusi 66m telescope. Nulling fractions of these pulsars are estimated, and the null and emission states of pulses are identified. Nulling degrees and scales of the emission-null pairs are calculated to describe the distributions of emission and null lengths. Three pulsars, PSRs J0248+6021, J0543+2329 and J1844+00, are found to null for the first time. The details of null-to-emission and emission-to-null transitions within pulse window are first observed for PSR J1509+5531, which is a small probability event. A complete cycle of long nulls for hours is observed for PSR J1709-1640. For most of these pulsars, the K-S tests of nulling degrees and nulling scales reject the hypothesis that null and emission are of random processes at high significance levels. Emission-null sequences of some pulsars exhibit quasi-periodic, low-frequency or featureless modulations, which might be related to different origins. During transitions between emission and null states, pulse intensities have diverse tendencies for variations. Significant correlations are found for nulling fraction, nulling cadence and nulling scales with the energy loss rate of the pulsars. Combined with the nulling fractions reported in literatures for 146 nulling pulsars, we found that statistically large nulling fractions are more tightly related to pulsar period than to characteristic age or energy loss rate.

astro-ph.HE↗

arXiv2020

Isoscalar and isovector giant resonances in 44Ca, 54Fe, 64,68Zn and 56,58,60,68Ni

We have studied the uncharacteristic behavior of the measured values of the isoscalar and isovector centroid energies, ECEN, of giant resonances with multipolarity L=0-3 in 44Ca, 54Fe, 64,68Zn and 56,58,60,68Ni. For this purpose, we carried out calculations of ECEN within the spherical Hartree-Fock (HF)-based random phase approximation (RPA) theory with 33 different Skyrme-type effective nucleon-nucleon interactions. We have also determined the Pearson linear correlation coefficients between the calculated centroid energies and the various nuclear matter (NM) properties associated with each interaction and determined the sensitivity of ECEN to NM properties. We compared the calculated centroid energies of the giant resonances with experimental data and discuss the results. We note in particular, that we obtain good agreement between the calculated ECEN of isovector giant dipole resonance and the available experimental data.

nucl-th↗

arXiv2020

Determination of luminosity for in-ring reactions: A new approach for the low-energy domain

Luminosity is a measure of the colliding frequency between beam and target and it is a crucial parameter for the measurement of absolute values, such as reaction cross sections. In this paper, we make use of experimental data from the ESR storage ring to demonstrate that the luminosity can be precisely determined by modelling the measured Rutherford scattering distribution. The obtained results are in good agreement with an independent measurement based on the x-ray normalization method. Our new method provides an alternative way to precisely measure the luminosity in low-energy stored-beam configurations. This can be of great value in particular in dedicated low-energy storage rings where established methods are difficult or impossible to apply.

nucl-ex↗

arXiv2020

First measurement of $Λ_c$ baryon production in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV

We report on the first measurement of the charmed baryon $Λ_c^{\pm}$ production at midrapidity ($|y|$ $<$ 1) in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV collected by the STAR experiment at the Relativistic Heavy Ion Collider. The $Λ_c$/$D^0$ (denoting ($Λ_c^++Λ_c^-$)/($D^0+\bar{D^0}$)) yield ratio is measured to be 1.08 $\pm$ 0.16 (stat.) $\pm$ 0.26 (sys.) in the 0--20% most central Au+Au collisions for the transverse momentum ($p_T$) range 3 $<$ $p_T$ $<$ 6 GeV/$c$. This is significantly larger than the PYTHIA model calculations for $p+p$ collisions. The measured $Λ_c$/$D^0$ ratio, as a function of $p_T$ and collision centrality, is comparable to the baryon-to-meson ratios for light and strange hadrons in Au+Au collisions. Model calculations including coalescence hadronization for charmed baryon and meson formation reproduce the features of our measured $Λ_c$/$D^0$ ratio.

nucl-ex↗

arXiv2020

Sensitivity and Performance of the Advanced LIGO Detectors in the Third Observing Run

On April 1st, 2019, the Advanced Laser Interferometer Gravitational-Wave Observatory (aLIGO), joined by the Advanced Virgo detector, began the third observing run, a year-long dedicated search for gravitational radiation. The LIGO detectors have achieved a higher duty cycle and greater sensitivity to gravitational waves than ever before, with LIGO Hanford achieving angle-averaged sensitivity to binary neutron star coalescences to a distance of 111 Mpc, and LIGO Livingston to 134 Mpc with duty factors of 74.6% and 77.0% respectively. The improvement in sensitivity and stability is a result of several upgrades to the detectors, including doubled intracavity power, the addition of an in-vacuum optical parametric oscillator for squeezed-light injection, replacement of core optics and end reaction masses, and installation of acoustic mode dampers. This paper explores the purposes behind these upgrades, and explains to the best of our knowledge the noise currently limiting the sensitivity of each detector.

astro-ph.IM↗

arXiv2020

Results on Total and Elastic Cross Sections in Proton-Proton Collisions at $\sqrt{s} = 200$ GeV

We report results on the total and elastic cross sections in proton-proton collisions at $\sqrt{s}=200$ GeV obtained with the Roman Pot setup of the STAR experiment at the Relativistic Heavy Ion Collider (RHIC). The elastic differential cross section was measured in the squared four-momentum transfer range $0.045 \leq -t \leq 0.135$ GeV$^2$. The value of the exponential slope parameter $B$ of the elastic differential cross section $dσ/dt \sim e^{-Bt}$ in the measured $-t$ range was found to be $B = 14.32 \pm 0.09 (stat.)^{\scriptstyle +0.13}_{\scriptstyle -0.28} (syst.)$ GeV$^{-2}$. The total cross section $σ_{tot}$, obtained from extrapolation of the $dσ/dt$ to the optical point at $-t = 0$, is $σ_{tot} = 54.67 \pm 0.21 (stat.) ^{\scriptstyle +1.28}_{\scriptstyle -1.38} (syst.)$ mb. We also present the values of the elastic cross section $σ_{el} = 10.85 \pm 0.03 (stat.) ^{\scriptstyle +0.49}_{\scriptstyle -0.41}(syst.)$ mb, the elastic cross section integrated within the STAR $t$-range $σ^{det}_{el} = 4.05 \pm 0.01 (stat.) ^{\scriptstyle+0.18}_{\scriptstyle -0.17}(syst.)$ mb, and the inelastic cross section $σ_{inel} = 43.82 \pm 0.21 (stat.) ^{\scriptstyle +1.37}_{\scriptstyle -1.44} (syst.)$ mb. The results are compared with the world data.

hep-ex↗

arXiv2020

Tailoring Magnetism in Self-intercalated Cr1+δTe2 Epitaxial Films

Magnetic transition metal dichalcogenide (TMD) films have recently emerged as promising candidates to host novel magnetic phases relevant to next-generation spintronic devices. However, systematic control of the magnetization orientation, or anisotropy, and its thermal stability, characterized by Curie temperature (Tc) remains to be achieved in such films. Here we present self-intercalated epitaxial Cr1+δTe2 films as a platform for achieving systematic/smooth magnetic tailoring in TMD films. Using a molecular beam epitaxy (MBE) based technique, we have realized epitaxial Cr1+δTe2 films with smoothly tunable over a wide range (0.33-0.82), while maintaining NiAs-type crystal structure. With increasing δ, we found monotonic enhancement of Tc from 160 to 350 K, and the rotation of magnetic anisotropy from out-of-plane to in-plane easy axis configuration for fixed film thickness. Contributions from conventional dipolar and orbital moment terms are insufficient to explain the observed evolution of magnetic behavior with δ. Instead, ab initio calculations suggest that the emergence of antiferromagnetic interactions with δ, and its interplay with conventional ferromagnetism, may play a key role in the observed trends. To our knowledge, this constitutes the first demonstration of tunable Tc and magnetic anisotropy across room temperature in TMD films, and paves the way for engineering novel magnetic phases for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2020

Measurement of the central exclusive production of charged particle pairs in proton-proton collisions at $\sqrt{s} = 200$ GeV with the STAR detector at RHIC

We report on the measurement of the Central Exclusive Production of charged particle pairs $h^{+}h^{-}$ ($h = π, K, p$) with the STAR detector at RHIC in proton-proton collisions at $\sqrt{s} = 200$ GeV. The charged particle pairs produced in the reaction $pp\to p^\prime+h^{+}h^{-}+p^\prime$ are reconstructed from the tracks in the central detector, while the forward-scattered protons are measured in the Roman Pot system. Differential cross sections are measured in the fiducial region, which roughly corresponds to the square of the four-momentum transfers at the proton vertices in the range $0.04~\mbox{GeV}^2 < -t_1 , -t_2 < 0.2~\mbox{GeV}^2$, invariant masses of the charged particle pairs up to a few GeV and pseudorapidities of the centrally-produced hadrons in the range $|η|<0.7$. The measured cross sections are compared to phenomenological predictions based on the Double Pomeron Exchange (DPE) model. Structures observed in the mass spectra of $π^{+}π^{-}$ and $K^{+}K^{-}$ pairs are consistent with the DPE model, while angular distributions of pions suggest a dominant spin-0 contribution to $π^{+}π^{-}$ production. The fiducial $π^+π^-$ cross section is extrapolated to the Lorentz-invariant region, which allows decomposition of the invariant mass spectrum into continuum and resonant contributions. The extrapolated cross section is well described by the continuum production and at least three resonances, the $f_0(980)$, $f_2(1270)$ and $f_0(1500)$, with a possible small contribution from the $f_0(1370)$. Fits to the extrapolated differential cross section as a function of $t_1$ and $t_2$ enable extraction of the exponential slope parameters in several bins of the invariant mass of $π^+π^-$ pairs. These parameters are sensitive to the size of the interaction region.

hep-ex↗

arXiv2020

Measurement of D$^0$-meson + hadron two-dimensional angular correlations in Au+Au collisions at $\sqrt{s_{\rm NN}} = $ 200 GeV

Open heavy flavor hadrons provide unique probes of the medium produced in ultra-relativistic heavy-ion collisions. Due to their increased mass relative to light-flavor hadrons, long lifetime, and early production in hard-scattering interactions, they provide access to the full evolution of the partonic medium formed in heavy-ion collisions. This paper reports two-dimensional (2D) angular correlations between neutral $D$-mesons and unidentified charged particles produced in minimum-bias Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 200 GeV. $D^0$ and $\bar{D}^0$ mesons are reconstructed via their weak decay to $K^{\mp} π^{\pm}$ using the Heavy Flavor Tracker (HFT) in the Solenoidal Tracker at RHIC (STAR) experiment. Correlations on relative pseudorapidity and azimuth $(Δη,Δϕ)$ are presented for peripheral, mid-central and central collisions with $D^0$ transverse momentum from 2 to 10 GeV/$c$. Attention is focused on the 2D peaked correlation structure near the triggered $D^0$-meson, the {\em near-side} (NS) peak, which serves as a proxy for a charm-quark containing jet. The correlated NS yield of charged particles per $D^0$-meson and the 2D widths of the NS peak increase significantly from peripheral to central collisions. These results are compared with similar correlations using unidentified charged particles, consisting primarily of light-flavor hadrons, at similar trigger particle momenta. Similar per-trigger yields and widths of the NS correlation peak are observed. The present results provide additional evidence that $D^0$-mesons undergo significant interactions with the medium formed in heavy-ion collision and show, for the first time, significant centrality evolution of the NS 2D peak in the correlations of particles associated with a heavy-flavor hadron produced in these collisions.

nucl-ex↗