arXiv subjects

X. Chen

Publications and source records attributed to X. Chen.

At least 271 records · Page 15Linked to original sources

arXiv2018

Jet cross sections and transverse momentum distributions with NNLOJET

This talk discusses recent results for next-to-next-to-leading order (NNLO) QCD corrections to jet cross sections and transverse momentum distributions. The results are obtained in the NNLOJET code framework, which provides an implementation of the antenna subtraction method for the handling of infrared singular contributions at NNLO. We briefly describe the NNLOJET implementation, with particular emphasis on the construction of the real radiation phase space, which is tailored to ensure stability in all infrared sensitive regions.

hep-ph↗

arXiv2018

Beam-Energy Dependence of Directed Flow of $Λ$, $\barΛ$, $K^\pm$, $K^0_s$ and $ϕ$ in Au+Au Collisions

Rapidity-odd directed flow measurements at midrapidity are presented for $Λ$, $\barΛ$, $K^\pm$, $K^0_s$ and $ϕ$ at $\sqrt{s_{NN}} =$ 7.7, 11.5, 14.5, 19.6, 27, 39, 62.4 and 200 GeV in Au+Au collisions recorded by the STAR detector at the Relativistic Heavy Ion Collider. These measurements greatly expand the scope of data available to constrain models with differing prescriptions for the equation of state of quantum chromodynamics. Results show good sensitivity for testing a picture where flow is assumed to be imposed before hadron formation and the observed particles are assumed to form via coalescence of constituent quarks. The pattern of departure from a coalescence-inspired sum-rule can be a valuable new tool for probing the collision dynamics.

hep-ex↗

arXiv2017

Azimuthal transverse single-spin asymmetries of inclusive jets and charged pions within jets from polarized-proton collisions at $\sqrt{s} = 500$ GeV

We report the first measurements of transverse single-spin asymmetries for inclusive jet and jet + $π^{\pm}$ production at midrapidity from transversely polarized proton-proton collisions at $\sqrt{s} = 500$ GeV. The data were collected in 2011 with the STAR detector sampled from 23 pb$^{-1}$ integrated luminosity with an average beam polarization of 53%. Asymmetries are reported for jets with transverse momenta $6 < p_{T, jet} < 55$ GeV/c and pseudorapidity $|η| < 1$. Presented are measurements of the inclusive-jet azimuthal transverse single-spin asymmetry, sensitive to twist-3 initial-state quark-gluon correlators; the Collins asymmetry, sensitive to quark transversity coupled to the polarized Collins fragmentation function; and the first measurement of the "Collins-like" asymmetry, sensitive to linearly polarized gluons. Within the present statistical precision, inclusive-jet and Collins-like asymmetries are small, with the latter allowing the first experimental constraints on gluon linear polarization in a polarized proton. At higher values of jet transverse momenta, we observe the first non-zero Collins asymmetries in polarized-proton collisions, with a statistical significance of greater than $5σ$. The results span a range of x similar to results from SIDIS but at much higher $Q^{2}$. The Collins results enable tests of universality and factorization-breaking in the transverse momentum-dependent formulation of perturbative quantum chromodynamics.

hep-ex↗

arXiv2017

Mesonic String of Diquark-Quark Configuration at Finite Temperature

We investigate the distance and temperatures scale for which the string in baryonic quark configuration approaches the limiting behavior of mesonic strings in pure Yang-Mills SU(3) lattice gauge theory. We calculate and compare the numerical values of the Polyakov loop correlators and the width profile of both diquark-quark $(QQ)Q$ and mesonic $Q\bar{Q}$ strings. We find the diquark-quark configuration to exhibit an identical behavior to the mesonic string for the potential and energy-density width profile for temperature near the end of QCD plateau. In the vicinity of the deconfinement point; however, the symmetry in the energy-width profile with the meson does not manifest at both short and intermediate distance scales. Moreover, we observe significantly different numerical values for Polyakov loop correlators corresponding to each system. The splitting of the two identical system suggest that, in the high temperature region of the confined phase of QCD, the subsisted baryonic decouplet states overlap with the excited mesonic spectrum yielding the diquark-quark symmetry with the meson inexact in a small enough neighborhood of the critical point $T_{c}$.

hep-lat↗

arXiv2017

Dark Photon Search in the Mass Range Between 1.5 and 3.4 GeV/$c^2$

Using a data set of 2.93 fb$^{-1}$ taken at a center-of-mass energy $\sqrt{s}$ = 3.773 GeV with the BESIII detector at the BEPCII collider, we perform a search for an extra U(1) gauge boson, also denoted as a dark photon. We examine the initial state radiation reactions $e^+e^-\rightarrow e^+e^-γ_{\rm ISR}$ and $e^+e^-\rightarrow μ^+μ^-γ_{\rm ISR}$ for this search, where the dark photon would appear as an enhancement in the invariant mass distribution of the leptonic pairs. We observe no obvious enhancement in the mass range between 1.5 and 3.4 GeV/$c^{2}$ and set a 90% confidence level upper limit on the mixing strength of the dark photon and the Standard Model photon. We obtain a competitive limit in the tested mass range.

hep-ex↗

arXiv2017

Measurement of Singly Cabibbo Suppressed Decays $Λ_c^{+}\to pπ^{+}π^{-}$ and $Λ_c^{+}\to pK^{+}K^{-}$

Using 567 $pb^{-1}$ of data collected with the BESIII detector at a center-of-mass energy of $\sqrt{s}=$ 4.599 $GeV$, near the $Λ_{c}^{+}Λ_{c}^{-}$ threshold, we study the singly Cabibbo-suppressed decays $Λ_c^{+}\to pπ^{+}π^{-}$ and $Λ_c^{+}\to pK^{+}K^{-}$. By normalizing with respect to the Cabibbo-favored decay $Λ_c^{+}\to pK^{-}π^{+}$, we obtain ratios of branching fractions: $\frac{\mathcal{B}(Λ_c^{+}\to pπ^{+}π^{-})}{\mathcal{B}(Λ_c^{+}\to pK^{-}π^{+})}$ = $(6.70 \pm 0.48 \pm 0.25)\%$, $\frac{\mathcal{B}(Λ_c^{+}\to pϕ)}{\mathcal{B}(Λ_c^{+}\to pK^{-}π^{+})}$ = $(1.81 \pm 0.33 \pm 0.13)\%$, and $\frac{\mathcal{B}(Λ_c^{+}\to pK^{+}K^{-}_{\text{non-}ϕ})}{\mathcal{B}(Λ_c^{+}\to pK^{-}π^{+})}$ = $(9.36 \pm 2.22 \pm 0.71)\times10^{-3}$, where the uncertainties are statistical and systematic, respectively. The absolute branching fractions are also presented. Among these measurements, the decay $Λ_c^{+}\to pπ^{+}π^{-}$ is observed for the first time, and the precision of the branching fraction for $Λ_c^{+}\to pK^{+}K^{-}_{\text{non-}ϕ}$ and $Λ_c^{+}\to pϕ$ is significantly improved.

hep-ex↗

arXiv2017

Hamiltonian design to prepare arbitrary states of four-level systems

We propose a method to manipulate, possibly faster than adiabatically, four-level systems with time-dependent couplings and constant energy shifts (detunings in quantum-optical realizations). We inversely engineer the Hamiltonian, in ladder, tripod, or diamond configurations, to prepare arbitrary states using the geometry of four-dimensional rotations to set the state populations, specifically we use Cayley's factorization of a general rotation into right- and left-isoclinic rotations.

quant-ph↗

arXiv2017

Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB$_6$

The search for a Fermi surface in the absence of a conventional Fermi liquid has thus far yielded very few potential candidates. Among promising materials are spin-frustrated Mott insulators near the insulator-metal transition, where theory predicts a Fermi surface associated with neutral low energy excitations. Here we reveal another route to experimentally realise a Fermi surface in the absence of a Fermi liquid by the experimental study of a Kondo insulator SmB$_6$ positioned close to the insulator-metal transition. We present experimental signatures down to low temperatures ($\ll 1$ K) associated with a Fermi surface in the bulk, including a sizeable linear specific heat coefficient, and on the application of a finite magnetic field, bulk magnetic quantum oscillations, finite quantum oscillatory entropy, and substantial enhancement in thermal conductivity well below the charge gap energy scale. Thus, the weight of evidence indicates that despite an extreme instance of Fermi liquid breakdown in Kondo insulating SmB$_6$, a Fermi surface arises from novel itinerant low energy excitations that couple to magnetic fields, but not weak DC electric fields.

cond-mat.str-el↗

arXiv2017

Bulk Properties of the Medium Produced in Relativistic Heavy-Ion Collisions from the Beam Energy Scan Program

We present measurements of bulk properties of the matter produced in Au+Au collisions at $\sqrt{s_{NN}}=$ 7.7, 11.5, 19.6, 27, and 39 GeV using identified hadrons ($π^\pm$, $K^\pm$, $p$ and $\bar{p}$) from the STAR experiment in the Beam Energy Scan (BES) Program at the Relativistic Heavy Ion Collider (RHIC). Midrapidity ($|y|<$0.1) results for multiplicity densities $dN/dy$, average transverse momenta $\langle p_T \rangle$ and particle ratios are presented. The chemical and kinetic freeze-out dynamics at these energies are discussed and presented as a function of collision centrality and energy. These results constitute the systematic measurements of bulk properties of matter formed in heavy-ion collisions over a broad range of energy (or baryon chemical potential) at RHIC.

nucl-ex↗

arXiv2017

Lack of clustering in low-redshift 21-cm intensity maps cross-correlated with 2dF galaxy densities

We report results from 21-cm intensity maps acquired from the Parkes radio telescope and cross-correlated with galaxy maps from the 2dF galaxy survey. The data span the redshift range $0.057<z<0.098$ and cover approximately 1,300 square degrees over two long fields. Cross correlation is detected at a significance of $5.18σ$. The amplitude of the cross-power spectrum is low relative to the expected dark matter power spectrum, assuming a neutral hydrogen (HI) bias and mass density equal to measurements from the ALFALFA survey. The decrement is pronounced and statistically significant at small scales. At $k\sim1.5$ $ h \mathrm{Mpc^{-1}}$, the cross power spectrum is more than a factor of 6 lower than expected, with a significance of $14.8\,σ$. This decrement indicates either a lack of clustering of neutral hydrogen (HI), a small correlation coefficient between optical galaxies and HI, or some combination of the two. Separating 2dF into red and blue galaxies, we find that red galaxies are much more weakly correlated with HI on $k\sim1.5$ $h \mathrm{Mpc^{-1}}$ scales, suggesting that HI is more associated with blue star-forming galaxies and tends to avoid red galaxies.

astro-ph.CO↗

arXiv2017

Measurement of the integrated luminosity and the center-of-mass energy of the data taken by BESIII at $\sqrt{s} = 2.125$ GeV

To study the nature of the state $Y(2175)$, a dedicated data set of $e^+e^-$ collision data was collected at the center-of-mass energy of 2.125 GeV with the BESIII detector at the BEPCII collider. By analyzing large-angle Bhabha scattering events, the integrated luminosity of this data set is determined to be $108.49\pm0.02\pm0.85$ pb$^{-1}$, where the first uncertainty is statistical and the second one is systematic. In addition, the center-of-mass energy of the data set is determined with radiative dimuon events to be $2126.55\pm0.03\pm0.85$ MeV, where the first uncertainty is statistical and the second one is systematic.

hep-ex↗

arXiv2017

Power Regulation in High Performance Multicore Processors

This paper presents, implements, and evaluates a power-regulation technique for multicore processors, based on an integral controller with adjustable gain. The gain is designed for wide stability margins, and computed in real time as part of the control law. The tracking performance of the control system is robust with respect to modeling uncertainties and computational errors in the loop. The main challenge of designing such a controller is that the power dissipation of program-workloads varies widely and often cannot be measured accurately; hence extant controllers are either ad hoc or based on a-priori modeling characterizations of the processor and workloads. Our approach is different. Leveraging the aforementioned robustness it uses a simple textbook modeling framework, and adjusts its parameters in real time by a system-identification module. In this it trades modeling precision for fast computations in the loop making it suitable for on-line implementation in commodity data-center processors. Consequently, the proposed controller is agnostic in the sense that it does not require any a-priori system characterizations. We present an implementation of the controller on Intel's fourth-generation microarchitecture, Haswell, and test it on a number of industry benchmark programs which are used in scientific computing and datacenter applications. Results of these experiments are presented in detail exposing the practical challenges of implementing provably-convergent power regulation solutions in commodity multicore processors.

cs.DC↗

arXiv2017

Observation of $e^+ e^- \to ηY(2175)$ at center-of-mass energies above 3.7$\sim$GeV

The state $Y(2175)$ is observed in the process $e^+ e^- \to ηY(2175)$ at center-of-mass energies between 3.7 and 4.6$\sim$GeV with a statistical significance larger than $10σ$ using data collected with the BESIII detector operating at the BEPCII storage ring. This is the first observation of the $Y(2175)$ in this process. The mass and width of the $Y(2175)$ are determined to be ($2135\pm 8\pm 9$)~MeV/$c^2$ and ($104\pm 24\pm 12$)~MeV, respectively, and the production cross section of $e^+ e^- \to ηY(2175)\to ηϕf_{0}(980)\to ηϕπ^+ π^-$ is at a several hundred femtobarn level. No significant signal for the process $e^+ e^- \to η' Y(2175)$ is observed and the upper limit on $σ(e^+ e^- \to η' Y(2175))/σ(e^+ e^- \to ηY(2175))$ is estimated to be 0.43 at the 90\% confidence level. We also search for $ψ(3686) \to ηY(2175)$. No significant signal is observed, indicating a strong suppression relative to the corresponding $J/ψ$ decay, in violation of the "12\% rule."

hep-ex↗

arXiv2017

Determination of the number of $ψ(3686)$ events at BESIII

The numbers of $ψ(3686)$ events accumulated by the BESIII detector for the two rounds of data taking during 2009 and 2012 are determined to be $(107.0\pm0.8)\times 10^6$ and $(341.1\pm 2.1)\times 10^6$, respectively, by counting inclusive hadronic events, where the uncertainty is dominated by systematics and the statistical uncertainty is negligible. The number of events for the sample taken in 2009 is consistent with that of the previous measurement. The total number of $ψ(3686)$ events for the two data-taking periods is $(448.1\pm2.9)\times10^6$.

hep-ex↗

arXiv2017

Measurement of cross sections of the interactions $e^+e^-\rightarrow ϕϕω$ and $e^+e^-\rightarrow ϕϕϕ$ at center-of-mass energies from 4.008 to 4.600 GeV

Using data samples collected with the BESIII detector at the BEPCII collider at six center-of-mass energies between 4.008 and 4.600 GeV, we observe the processes $e^+e^-\rightarrow ϕϕω$ and $e^+e^-\rightarrow ϕϕϕ$. The Born cross sections are measured and the ratio of the cross sections $σ(e^+e^-\rightarrow ϕϕω)/σ(e^+e^-\rightarrow ϕϕϕ)$ is estimated to be $1.75\pm0.22\pm0.19$ averaged over six energy points, where the first uncertainty is statistical and the second is systematic. The results represent first measurements of these interactions.

hep-ex↗

arXiv2017

Search for the rare decay $D^+ \to D^0 e^+ν_e$

We search for the rare decay $D^+\to D^0 e^+ν_{e}$, using a data set with an integrated luminosity of 2.93 $\rm fb^{-1}$ collected at $\sqrt{s}=3.773$ GeV with the BESIII detector operating at the BEPCII storage rings. No signals are observed. We set the upper limit on the branching fraction for $D^+\to D^0 e^+ν_e$ to be $1.0\times 10^{-4}$ at the 90\% confidence level.

hep-ex↗

arXiv2017

Measurement of branching fractions for $ψ(3686)\rightarrowγη',γη$ and $γπ^{0}$

Using a data sample of $448\times10^{6}$ $ψ(3686)$ events collected with the BESIII detector operating at the BEPCII storage ring, the decays $ψ(3686)\rightarrowγη$ and $ψ(3686)\rightarrowγπ^{0}$ are observed with a statistical significance of $7.3σ$ and $ 6.7σ$, respectively. The branching fractions are measured to be $\mathcal{B}(ψ(3686)\rightarrowγη)=(0.85\pm0.18\pm0.05)\times10^{-6}$ and $\mathcal{B}(ψ(3686)\rightarrowγπ^{0})=(0.95\pm0.16\pm0.05)\times10^{-6}$. In addition, we measure the branching fraction of $ψ(3686)\rightarrowγη'$ to be $\mathcal{B}(ψ(3686)\rightarrowγη')=(125.1\pm2.2\pm6.2)\times 10^{-6}$, with improved precision compared to previous results.

hep-ex↗

arXiv2017

Very-High-Energy $γ$-Ray Observations of the Blazar 1ES 2344+514 with VERITAS

We present very-high-energy $γ$-ray observations of the BL Lac object 1ES 2344+514 taken by the Very Energetic Radiation Imaging Telescope Array System (VERITAS) between 2007 and 2015. 1ES 2344+514 is detected with a statistical significance above background of $20.8σ$ in $47.2$ hours (livetime) of observations, making this the most comprehensive very-high-energy study of 1ES 2344+514 to date. Using these observations the temporal properties of 1ES 2344+514 are studied on short and long times scales. We fit a constant flux model to nightly- and seasonally-binned light curves and apply a fractional variability test, to determine the stability of the source on different timescales. We reject the constant-flux model for the 2007-2008 and 2014-2015 nightly-binned light curves and for the long-term seasonally-binned light curve at the $> 3σ$ level. The spectra of the time-averaged emission before and after correction for attenuation by the extragalactic background light are obtained. The observed time-averaged spectrum above 200 GeV is satisfactorily fitted (${χ^2/NDF = 7.89/6}$) by a power-law function with index $Γ= 2.46 \pm 0.06_{stat} \pm 0.20_{sys} $ and extends to at least 8 TeV. The extragalactic-background-light-deabsorbed spectrum is adequately fit (${χ^2/NDF = 6.73/6}$) by a power-law function with index $Γ= 2.15 \pm 0.06_{stat} \pm 0.20_{sys} $ while an F-test indicates that the power-law with exponential cutoff function provides a marginally-better fit ($χ^2/NDF $ = $2.56 / 5 $) at the 2.1$σ$ level. The source location is found to be consistent with the published radio location and its spatial extent is consistent with a point source.

astro-ph.HE↗