Search Results (644 total)

Linear Thomson scattering of relativistic electrons with few-cycle laser pulses is theoretically analyzed, and the radiated energy spectra of electrons show that attosecond x-ray pulses can be supported. The influences of the carrier-envelope phase offset ϕ_CE of the incident laser pulses and of the initial phase as the electrons enter the field, ϕ_in, on the radiated x-ray pulses are studied, and the results demonstrate that a single attosecond pulse can be produced in linear Thomson scattering by using either a single-cycle laser pulse with fixed carrier-envelope phase offset ϕ_CE=0, or a few-cycle laser pulse with controlled initial phase at the electron entrance to the field ϕ_in.

We present a search for the decays B⁺→π⁺ℓ⁺ℓ^- and B⁰→π⁰ℓ⁺ℓ^-, where ℓ⁺ℓ^- is either a μ⁺μ^- or e⁺e^- pair, with a data sample of 657×10⁶ BB̅ pairs collected with the Belle detector at the KEKB e⁺e^- collider. Signal events are reconstructed from a charged or a neutral pion candidate and a pair of oppositely charged electrons or muons. No significant signal is observed and we set an upper limit on the isospin-averaged branching fraction B(B→πℓ⁺ℓ^-)<6.2×10^-8 at the 90% confidence level.

Using a 492  fb^-1 data sample collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider, we observe the decay B⁰→pp̅ K^*0 with a branching fraction of (1.18_-0.25^+0.29(stat)±0.11(syst))×10^-6. We study the decay dynamics of B⁰→pp̅ K^*0 and compare with B⁺→pp̅ K^*+. The K^*0 meson is found to be almost 100% polarized (with a fraction of (101±13±3)% in the helicity zero state), while the K^*+ meson has a (32±17±9)% fraction in the helicity zero state. The direct CP asymmetries for B⁰→pp̅ K^*0 and B⁺→pp̅ K^*+ are measured to be -0.08±0.20±0.02 and -0.01±0.19±0.02, respectively. In addition, we report improved measurements of the branching fractions B(B⁺→pp̅ K^*+)=(3.38_-0.60^+0.73±0.39)×10^-6 and B(B⁰→pp̅ K⁰)=(2.51_-0.29^+0.35±0.21)×10^-6, which supersede our previous measurements.

We present a measurement of the branching fraction B(D_s⁺→μ⁺ν_μ) using a 548  fb^-1 data sample collected by the Belle experiment at the KEKB e⁺e^- collider. The D_s momentum is determined by reconstruction of the system recoiling against DKγX in events of the type e⁺e^-→D_s^*DKX, D_s^*→D_sγ, where X represents additional pions or photons from fragmentation. This full-reconstruction method provides high resolution in the neutrino momentum and thus good background separation, equivalent to that achieved by experiments at the tau-charm factories. We obtain the branching fraction B(D_s⁺→μ⁺ν_μ)=[6.44±0.76(stat)±0.57(syst)]×10^-3, implying a D_s decay constant of f_Ds=[275±16(stat)±12(syst)]  MeV.

A search for mixing in the neutral D meson system has been performed using semileptonic D⁰→K^(*)-e⁺ν and D⁰→K^(*)-μ⁺ν decays. Neutral D mesons from D^*+→D⁰π_s⁺ decays are used and the flavor at production is tagged by the charge of the slow pion. The measurement is performed using 492  fb^-1 of data recorded by the Belle detector. From the yield of right-sign and wrong-sign decays arising from nonmixed and mixed events, respectively, we measure the ratio of the time-integrated mixing rate to the unmixed rate to be R_M=(1.3±2.2±2.0)×10^-4. This corresponds to an upper limit of R_M<6.1×10^-4 at the 90% C.L.

We study an extended velocity-selective coherent population trapping (VSCPT) model where vacuum-induced coherence (VIC) is considered. This effect takes place when two nonorthogonal electric dipole moments of a Λ-type three-level atom couple with a common continuous vacuum. First, we introduce the main results of a conventional VSCPT model in the nondegenerate and unsymmetrically driven situation. Then, we work out the dynamic equation of the system in the presence of VIC expressed in both internal and external degrees of freedom. We report on the generation of atomic external coherences due to VIC, which counteract the feeding action of spontaneous emission to the trapping state and indicate its limited function negative to the formation of the trapping state if the atomic center-of-mass motion is considered.

We report a new STAR measurement of the longitudinal double-spin asymmetry A_LL for inclusive jet production at midrapidity in polarized p+p collisions at a center-of-mass energy of sqrt[s]=200  GeV. The data, which cover jet transverse momenta 5<p_T<30  GeV/c, are substantially more precise than previous measurements. They provide significant new constraints on the gluon spin contribution to the nucleon spin through the comparison to predictions derived from one global fit to polarized deep-inelastic scattering measurements.

We present the first spin alignment measurements for the K^*0(892) and ϕ(1020) vector mesons produced at midrapidity with transverse momenta up to 5 GeV/c at sqrt[s_NN]=200 GeV at RHIC. The diagonal spin-density matrix elements with respect to the reaction plane in Au+Au collisions are ρ₀₀=0.32±0.04 (stat) ± 0.09 (syst) for the K^*0 (0.8<p_T<5.0 GeV/c) and ρ₀₀=0.34±0.02 (stat) ± 0.03 (syst) for the ϕ (0.4<p_T<5.0 GeV/c) and are constant with transverse momentum and collision centrality. The data are consistent with the unpolarized expectation of 1/3 and thus no evidence is found for the transfer of the orbital angular momentum of the colliding system to the vector-meson spins. Spin alignments for K^*0 and ϕ in Au+Au collisions were also measured with respect to the particle's production plane. The ϕ result, ρ₀₀=0.41±0.02 (stat) ± 0.04 (syst), is consistent with that in p+p collisions, ρ₀₀=0.39±0.03 (stat) ± 0.06 (syst), also measured in this work. The measurements thus constrain the possible size of polarization phenomena in the production dynamics of vector mesons.

We investigate theoretically the magnetic levels and optical properties of zigzag- and armchair-edged hexagonal graphene quantum dots (GQDs) utilizing the tight-binding method. A bound edge state at zero energy appears for the zigzag GQDs in the absence of a magnetic field. The magnetic levels of GQDs exhibit a Hofstadter-butterfly spectrum and approach the Landau levels of two-dimensional graphene as the magnetic field increases. The optical properties are tuned by the size, the type of the edge, and the external magnetic field.

We explore theoretically the feasibility of functionalizing carbon nanostructures for hydrogen storage, focusing on the coating of C₆₀ fullerenes with light alkaline-earth metals. Our first-principles density functional theory studies show that both Ca and Sr can bind strongly to the C₆₀ surface, and highly prefer monolayer coating, thereby explaining existing experimental observations. The strong binding is attributed to an intriguing charge transfer mechanism involving the empty d levels of the metal elements. The charge redistribution, in turn, gives rise to electric fields surrounding the coated fullerenes, which can now function as ideal molecular hydrogen attractors. With a hydrogen uptake of >8.4  wt % on Ca₃₂C₆₀, Ca is superior to all the recently suggested metal coating elements.

Using perturbation theory, intraband magneto-optical absorption is calculated for InAs/GaAs truncated pyramidal quantum dots in a magnetic field applied parallel to the growth direction z. The effects of the magnetic field on the electronic states as well as the intraband transitions are systematically studied. Selection rules governing the intraband transitions are discussed based on the symmetry properties of the electronic states. While the broadband z-polarized absorption is almost insensitive to the magnetic field, the orbital Zeeman splitting is the dominant feature in the in-plane polarized spectrum. Strong in-plane polarized magneto-absorption features are located in the far-infrared region, while z-polarized absorption occurs at higher frequencies. This is due to the dot geometry (the base length is much larger than the height) yielding different quantum confinement in the vertical and lateral directions. The Thomas-Reiche-Kuhn sum rule, including the magnetic field effect, is applied together with the selection rules to the absorption spectra. The orbital Zeeman splitting depends on both the dot size and the confining potential—the splitting decreases as the dot size or the confining potential decreases. Our calculated Zeeman splittings are in agreement with experimental data.

We report a study of the processes e⁺e^-→J/ψD^(*)D̅ ^(*). In J/ψD^*D̅ ^* we observe a significant enhancement in the D^*D̅ ^* invariant mass spectrum, which we interpret as a new charmoniumlike state and denote X(4160). The X(4160) parameters are M=(4156_-20⁺²⁵±15)  MeV/c² and Γ=(139_-61⁺¹¹¹±21)  MeV. We also report a new measurement of the X(3940) mass and width: M=(3942_-6⁺⁷±6)  MeV/c² and Γ=(37_-15⁺²⁶±8)  MeV. The analysis is based on a 693  fb^-1 data sample recorded near the Υ(4S) resonance by the Belle detector at the KEKB asymmetric-energy collider.

Using ψ(2S)→π⁺π^-J/ψ events in a sample of 14.0×10⁶ ψ(2S) decays collected with the BES-II detector, a search for the decay of the J/ψ to invisible final states is performed. No signal is found, and an upper limit at the 90% confidence level is determined to be 1.2×10^-2 for the ratio B(J/ψ→invisible) / B(J/ψ→μ⁺μ^-). This is the first search for J/ψ decays to invisible final states.

A number of physical processes involving quantum dots depend critically upon the “evanescent” electron eigenstate wave function that extends outside of the material surface into the surrounding region. These processes include electron tunneling through quantum dots, as well as interactions between multiple quantum dot structures. In order to unambiguously determine these evanescent fields, appropriate boundary conditions have been developed to connect the electronic solutions interior to the semiconductor quantum dot to exterior vacuum solutions. In standard envelope function theory, the interior wave function consists of products of band edge and envelope functions, and both must be considered when matching to the external solution. While the envelope functions satisfy tractable equations, the band edge functions are generally not known. In this work, symmetry arguments in the spherically symmetric approximation are used in conjunction with the known qualitative behavior of bonding and antibonding orbitals to catalog the behavior of the band edge functions at the unit cell boundary. This physical approximation allows consolidation of the influence of the band edge functions to two simple surface parameters that are incorporated into the boundary conditions and are straightforwardly computed by using numerical first-principles quantum techniques. These new boundary conditions are employed to analyze an isolated spherically symmetric semiconductor quantum dot in vacuum within the analytical model of Sercel and Vahala [Phys. Rev. Lett. 65, 239 (1990); Phys. Rev. B 42, 3690 (1990)]. Results are obtained for quantum dots made of GaAs and InP, which are compared with ab initio calculations that have appeared in the literature.

We report improved measurements of time-dependent CP violation parameters for B⁰(B̅ ⁰)→ψ(2S)K_S⁰. This analysis is based on a data sample of 657×10⁶ BB̅ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB energy-asymmetric e⁺e^- collider. We fully reconstruct one neutral B meson in the ψ(2S)K_S⁰ CP-eigenstate decay channel, and the flavor of the accompanying B meson is identified to be either B⁰ or B̅ ⁰ from its decay products. CP violation parameters are obtained from the asymmetries in the distributions of the proper-time intervals between the two B decays: S_ψ(2S)KS⁰=+0.72±0.09(stat)±0.03(syst), A_ψ(2S)KS⁰=+0.04±0.07(stat)±0.05(syst). These results are in agreement with results from measurements of B⁰→J/ψK⁰.

We report a study of semileptonic B decays to P-wave D^** mesons. Semileptonic decay to a D₂^* meson is observed for the first time and its product branching ratio is measured to be B(B⁺→D̅ ₂^*0ℓ⁺ν)×B(D̅ ₂^*0→D^-π⁺)=0.22±0.03(stat.)±0.04(syst.)%. The result is obtained using fully reconstructed B tags from a data sample that contains 657×10⁶BB̅ pairs collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider.

We present STAR results on the elliptic flow v₂ of charged hadrons, strange and multistrange particles from sqrt[s_NN]=200 GeV Au+Au collisions at the BNL Relativistic Heavy Ion Collider (RHIC). The detailed study of the centrality dependence of v₂ over a broad transverse momentum range is presented. Comparisons of different analysis methods are made in order to estimate systematic uncertainties. To discuss the nonflow effect, we have performed the first analysis of v₂ with the Lee-Yang zero method for K_S⁰ and Λ. In the relatively low p_T region, p_T≤2 GeV/c, a scaling with m_T-m is observed for identified hadrons in each centrality bin studied. However, we do not observe v₂(p_T) scaled by the participant eccentricity to be independent of centrality. At higher p_T,2≤p_T≤6 GeV/c,v₂ scales with quark number for all hadrons studied. For the multistrange hadron Ω, which does not suffer appreciable hadronic interactions, the values of v₂ are consistent with both m_T-m scaling at low p_T and number-of-quark scaling at intermediate p_T. As a function of collision centrality, an increase of p_T-integrated v₂ scaled by the participant eccentricity has been observed, indicating a stronger collective flow in more central Au+Au collisions.

We report an improved measurement of the B⁺→D⁺D̅ ⁰ and B⁰→D⁰D̅ ⁰ decays based on 657×10⁶ BB̅ events collected with the Belle detector at KEKB. We measure the branching fraction and charge asymmetry for the B⁺→D⁺D̅ ⁰ decay: B(B⁺→D⁺D̅ ⁰)=(3.85±0.31±0.38)×10^-4 and A_CP(B⁺→D⁺D̅ ⁰)=0.00±0.08±0.02, where the first error is statistical and the second is systematic. We also set the upper limit for the B⁰→D⁰D̅ ⁰ decay: B(B⁰→D⁰D̅ ⁰)<0.43×10^-4 at 90% CL.

We report on the observed differences in production rates of strange and multistrange baryons in Au+Au collisions at sqrt[s_NN]=200 GeV compared to p+p interactions at the same energy. The strange baryon yields in Au+Au collisions, when scaled down by the number of participating nucleons, are enhanced relative to those measured in p+p reactions. The enhancement observed increases with the strangeness content of the baryon, and it increases for all strange baryons with collision centrality. The enhancement is qualitatively similar to that observed at the lower collision energy sqrt[s_NN]=17.3 GeV. The previous observations are for the bulk production, while at intermediate p_T,1<p_T<4 GeV/c, the strange baryons even exceed binary scaling from p+p yields.

A distinct peak is observed in the π^±ψ^′ invariant mass distribution near 4.43 GeV in B→Kπ^±ψ^′ decays. A fit using a Breit-Wigner resonance shape yields a peak mass and width of M=4433±4(stat)±2(syst)  MeV and Γ=45_-13⁺¹⁸(stat)_-13⁺³⁰(syst)  MeV. The product branching fraction is determined to be B(B⁰→K^∓Z^±(4430))×B(Z^±(4430)→π^±ψ^′)=(4.1±1.0(stat)±1.4(syst))×10^-5, where Z^±(4430) is used to denote the observed structure. The statistical significance of the observed peak is 6.5σ. These results are obtained from a 605  fb^-1 data sample that contains 657×10⁶ BB̅ pairs collected near the Υ(4S) resonance with the Belle detector at the KEKB asymmetric energy e⁺e^- collider.

We present the results of a time-dependent Dalitz plot analysis of B⁰→π⁺π^-π⁰ decays based on a 414  fb^-1 data sample that contains 449×10⁶BB̅ pairs. The data were collected on the Υ(4S) resonance with the Belle detector at the KEKB asymmetric energy e⁺e^- collider. Combining our analysis with information on charged B decay modes, we perform a full Dalitz and isospin analysis and obtain a constraint on the quark mixing angle ϕ₂, 68°<ϕ₂<95° at the 68.3% confidence level for the ϕ₂ solution consistent with the standard model. A large standard-model–disfavored region also remains. The branching fractions for the decay processes B⁰→ρ^±(770)π^∓ and B⁰→ρ⁰(770)π⁰ are measured to be (22.6±1.1[stat]±4.4[syst])×10^-6 and (3.0±0.5[stat]±0.7[syst])×10^-6, respectively. These are the first branching fraction measurements of the process B⁰→ρ(770)π with the lowest resonance ρ(770) explicitly separated from the radial excitations.

By using ab initio electronic structure calculations within density functional theory, we study the structural, electronic, and magnetic properties of Si doped with a transition metal impurity. We consider the transition metals of the 3d series V, Cr, Mn, Fe, Co, and Ni. To get insight into the level filling mechanism and the magnetization saturation, we first investigate the transition metal-Si alloys in the zinc-blende structure. Next, we investigate the doping of bulk Si with a transition metal atom, in which it occupies the substitutional site, the interstitial site with tetrahedral symmetry, and the interstitial site with hexagonal symmetry. It is found that all of these transition metal impurities prefer an interstitial position in Si. Furthermore, we show that it is possible to interpret the electronic and magnetic properties by using a simple level filling picture and a comparison is made to Ge doped with the same transition metal atoms. In order to get insight into the effect of a strained environment, we calculate the formation energy as a function of an applied homogeneous pressure and we show that an applied pressure can stabilize the substitutional position of transition metal impurities in Si. Finally, the energies of the ferromagnetic states are compared to those of the antiferromagnetic states. It is shown that the interstitial site of the Mn dopant helps us to stabilize the nearest neighbor substitutional site to realize the ferromagnetic state. For doping of Si with Cr, a ferrimagnetic behavior is predicted.

We report improved measurements of time-dependent CP violation parameters for B⁰(B̅ ⁰)→J/ψπ⁰ decay. This analysis is based on 535×10⁶ BB̅ pairs accumulated at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. From the distribution of proper time intervals between the two B decays, we obtain the following CP violation parameters S_J/ψπ⁰=-0.65±0.21(stat)±0.05(syst)  and  A_J/ψπ⁰=+0.08±0.16(stat)±0.05(syst),which are consistent with standard model expectations.

Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR Collaboration presents a measurement of ρ⁰ and direct π⁺π^- photoproduction in ultraperipheral relativistic heavy ion collisions at sqrt[s_NN]=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross section of σ(AuAu→Au^*Au^*ρ⁰)=530±19(stat.)±57(syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The ρ⁰ transverse momentum spectrum (p_T²) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find σ_inc/σ_coh=0.29±0.03(stat.)±0.08(syst.). The ratio of direct π⁺π^- to ρ⁰ production is comparable to that observed in γp collisions at HERA and appears to be independent of photon energy. Finally, the measured ρ⁰ spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.