Journals
2018 EN
STAR Collaboration · L. Adamczyk · J. R. Adams
+345 more
The azimuthal anisotropic flow of identified and unidentified chargedparticles has been systematically studied in Cu+Au collisions at$\sqrt{s_{_{NN}}}$ = 200 GeV for harmonics $n=$ 1-4 in the pseudorapidity range$|\eta|<1$. The directed flow in Cu+Au collisions is compared with therapidity-odd and, for the first time, the rapidity-even components of chargedparticle directed flow in Au+Au collisions at $\sqrt{s_{_{NN}}}$ = 200~GeV. Theslope of the directed flow pseudorapidity dependence in Cu+Au collisions isfound to be similar to that in Au+Au collisions, with the intercept shiftedtoward positive $\eta$ values, i.e., the Cu-going direction. The meantransverse momentum projected onto the spectator plane, $\langle p_x\rangle$,in Cu+Au collision also exhibits approximately linear dependence on $\eta$ withthe intercept at about $\eta\approx-0.4$, closer to the rapidity of the Cu+Ausystem center-of-mass. The observed dependencies find natural explanation in apicture of the directed flow originating partly due the "tilted source" andpartly due to the rapidity dependent asymmetry in the initial densitydistribution. Charge-dependence of the $\langle p_x\rangle$ was also observedin Cu+Au collisions, indicating an effect of the initial electric field createdby charge difference of the spectator protons in two colliding nuclei. Therapidity-even component of directed flow in Au+Au collisions is close to thatin Pb+Pb collisions at $\sqrt{s_{_{NN}}}$ = 2.76 TeV, indicating a similarmagnitude of dipole-like fluctuations in the initial-state densitydistribution. Higher harmonic flow in Cu+Au collisions exhibits similar trendsto those observed in Au+Au and Pb+Pb collisions and is qualitatively reproducedby a viscous hydrodynamic model and a multi-phase transport model. For allharmonics with $n\ge2$ we observe an approximate scaling of $v_n$ with thenumber of constituent quarks.
American Physical Society
Journals
2018 EN
STAR Collaboration · L. Adamczyk · J. K. Adkins
+340 more
We present measurements of three-particle correlations for various harmonicsin Au+Au collisions at energies ranging from $\sqrt{s_{{\rm NN}}}=7.7$ to 200GeV using the STAR detector. The quantity$\langle\cos(m\phi_1+n\phi_2-(m+n)\phi_3)\rangle$ is evaluated as a function of$\sqrt{s_{{\rm NN}}}$, collision centrality, transverse momentum, $p_T$,pseudo-rapidity difference, $\Delta\eta$, and harmonics ($m$ and $n$). Thesedata provide detailed information on global event properties like thethree-dimensional structure of the initial overlap region, the expansiondynamics of the matter produced in the collisions, and the transport propertiesof the medium. A strong dependence on $\Delta\eta$ is observed for mostharmonic combinations consistent with breaking of longitudinal boostinvariance. Data reveal changes with energy in the two-particle correlationfunctions relative to the second-harmonic event-plane and provide ways toconstrain models of heavy-ion collisions over a wide range of collisionenergies.
American Physical Society
Journals
2018 EN
STAR Collaboration · J. Adam · L. Adamczyk
+347 more
We report a new measurement of $D^0$-meson production at mid-rapidity($|y|$\,$$\,4\,GeV/$c$ incentral 0--10\% Au+Au collisions. At low $p_{T}$, the nuclear modificationfactors show a characteristic structure qualitatively consistent with theexpectation from model predictions that charm quarks gain sizable collectivemotion during the medium evolution. The improved measurements are expected tooffer new constraints to model calculations and help gain further insights intothe hot and dense medium created in these collisions.
American Physical Society
Journals
2018 EN
STAR Collaboration J. Adam · STAR Collaboration L. Adamczyk · STAR Collaboration J. R. Adams
+340 more
We present the first measurements of the longitudinal double-spin asymmetry$A_{LL}$ for dijets with at least one jet reconstructed within thepseudorapidity range $0.8 < \eta < 1.8$. 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 thejet 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 thepredictions of global analyses that incorporate the results of previous RHICmeasurements. They will provide new constraints on $\Delta g(x)$ in this poorlyconstrained region when included in future global analyses.
American Physical Society
Journals
2018 EN
STAR Collaboration J. Adam · STAR Collaboration L. Adamczyk · STAR Collaboration J. R. Adams
+340 more
The STAR Collaboration reports measurements of the longitudinal double-spinasymmetry, $A_{LL}$, for neutral pions produced at forward directions inpolarized 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 $2.65<\eta<3.9$. These resultsare sensitive to the polarized gluon parton distribution function, $\Deltag(x)$, down to the region of Bjorken $x \sim 10^{-3}$. The asymmetries observedare less than $\pm 5 \cdot 10^{-3}$ in magnitude, and will help constrain thecontribution to the spin of the proton from polarized gluons at low $x$, whencombined with other measurements as part of a global analysis.
American Physical Society
Journals
2018 EN
STAR Collaboration · J. Adam · L. Adamczyk
+345 more
The transverse spin transfer from polarized protons to $\Lambda$ and$\bar{\Lambda}$ hyperons is expected to provide sensitivity to the transversitydistribution of the nucleon and to the transversely polarized fragmentationfunctions. We report the first measurement of the transverse spin transfer to$\Lambda$ and $\bar{\Lambda}$ along the polarization direction of thefragmenting quark, $D_\mathrm{TT}$, in transversely polarized proton-protoncollisions at $\sqrt{s}=200\,\mathrm{GeV}$ with the STAR detector at RHIC. Thedata correspond to an integrated luminosity of $18\,\mathrm{pb}^{-1}$ and coverthe pseudorapidity range $\left|\eta\right| < 1.2$ and transverse momenta$p_{\mathrm{T}}$ up to $8\,\mathrm{GeV}/c$. The dependence on $p_\mathrm{T}$and $\eta$ are presented. The $D_\mathrm{TT}$ results are found to becomparable with a model prediction, and are also consistent with zero withinuncertainties.
American Physical Society
Journals
2018 EN
STAR Collaboration J. Adam · STAR Collaboration L. Adamczyk · STAR Collaboration J. R. Adams
+346 more
The longitudinal spin transfer $D_{LL}$ to $\Lambda$ and $\bar{\Lambda}$hyperons produced in high-energy polarized proton--proton collisions isexpected to be sensitive to the helicity distribution functions of strangequarks and anti-quarks of the proton, and to longitudinally polarizedfragmentation functions. We report an improved measurement of $D_{LL}$ fromdata obtained at a center-of-mass energy of $\sqrt{s}$ = 200 GeV with the STARdetector at RHIC. The data have an approximately twelve times largerfigure-of-merit than prior results and cover $|\eta|<$ 1.2 in pseudo-rapiditywith transverse momenta $p_T$ up to 6 GeV/c. In the forward scatteringhemisphere at largest $p_T$, the longitudinal spin transfer is found to be$D_{LL}$ = -0.036 $\pm$ 0.048 (stat) $\pm$ 0.013(sys) for $\Lambda$ hyperonsand $D_{LL}$ = 0.032 $\pm$ 0.043\,(stat) $\pm$ 0.013\,(sys) for $\bar{\Lambda}$anti-hyperons. The dependences on $\eta$ and $p_T$ are presented and comparedwith model evaluations.
American Physical Society
Journals
2018 EN
M. Aaboud · G. Aad · B. Abbott
+2853 more
A search for vectorlike quarks is presented, which targets their decay into a Z boson and a third-generation Standard Model quark. In the case of a vectorlike quark T ( B ) with charge + 2 / 3 e ( − 1 / 3 e ), the decay searched for is T → Z t ( B → Z b ). Data for this analysis were taken during 2015 and 2016 with the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 36.1 fb − 1 of p p collisions at √ s = 13 TeV . The final state used is characterized by the presence of b -tagged jets, as well as a Z boson with high transverse momentum, which is reconstructed from a pair of opposite-sign same-flavor leptons. Pair and single production of vectorlike quarks are both taken into account and are each searched for using optimized dileptonic exclusive and trileptonic inclusive event selections. In these selections, the high scalar sum of jet transverse momenta, the presence of high-transverse-momentum large-radius jets, as well as—in the case of the single-production selections—the presence of forward jets are used. No significant excess over the background-only hypothesis is found and exclusion limits at 95% confidence level allow masses of vectorlike quarks of m T > 1030 GeV ( m T > 1210 GeV ) and m B > 1010 GeV ( m B > 1140 GeV ) in the singlet (doublet) model. In the case of 100% branching ratio for T → Z t ( B → Z b ), the limits are m T > 1340 GeV ( m B > 1220 GeV ). Limits at 95% confidence level are also set on the coupling to Standard Model quarks for given vectorlike quark masses.
American Physical Society
Journals
2018 EN
Amaresh Sahu · Roger A. Sauer · Kranthi K. Mandadapu
The theory of irreversible thermodynamics for arbitrarily curved lipidmembranes is presented here. The coupling between elastic bending andirreversible processes such as intra-membrane lipid flow, intra-membrane phasetransitions, and protein binding and diffusion is studied. The forms of theentropy production for the irreversible processes are obtained, and thecorresponding thermodynamic forces and fluxes are identified. Employing thelinear irreversible thermodynamic framework, the governing equations of motionalong with appropriate boundary conditions are provided.
American Physical Society
Journals
2018 EN
Binita Nath · Gautam Biswas · Amaresh Dalal
+1 more
The present study focuses on the cross-stream migration of a neutrally buoyant two-dimensional drop in a Poiseuille flow in a channel under the influence of an electric field. In the absence of an electric field, the important nondimensional parameters describing this problem are the viscosity ratio (λ) between the drop fluid and the surrounding medium, the ratio of drop diameter to channel height (a^{*}), and the capillary number (Ca). The influence of all these parameters on drop migration is investigated. It is observed that a large drop moves slowly as compared to a smaller drop, but attains a steady shape at the center line of the channel. The increase in value of the capillary number enhances the cross-stream migration rate, while the increase in viscosity ratio reduces the tendency of the drops to move towards the channel center line. The presence of an electric field introduces additional interfacial stresses at the drop interface, which in turn alters the dynamics observed in the absence of an electric field. Extensive computations are carried out to analyze the combined effect of the electric field and the shear flow on the cross-stream migration of the drop. The computational results for a perfect dielectric indicate that the droplet migration enhances in the presence of an electric field. The permittivity ratio (S) and the electric field strength (E) play major roles in drop migration and deformation. Computations using the leaky dielectric model also show that for certain combinations of electrical properties the drop undergoes immense elongation along the direction of the electric field. The conductivity ratio (R) is again a vital parameter in such a system of fluids. It is further observed that for certain conditions the leaky dielectric drops exhibit rotation together with translation.
American Physical Society