In 1929, H. Weyl proposed that the massless solution of the Dirac equation represents a pair of a new type of particles, the so-called Weyl fermions 1 . However, their existence in particle physics remains elusive after more than eight decades. Recently, significant advances in both topological insulators and topological semimetals have provided an alternative way to realize Weyl fermions in condensed matter, as an emergent phenomenon: when two non-degenerate bands in the three-dimensional momentum space cross in the vicinity of the Fermi energy (called Weyl nodes), the low-energy excitations behave exactly as Weyl fermions. Here we report the direct observation in TaAs of the long-sought-after Weyl nodes by performing bulk-sensitive soft X-ray angle-resolved photoemission spectroscopy measurements. The projected locations at the nodes on the (001) surface match well to the Fermi arcs, providing undisputable experimental evidence for the existence of Weyl fermionic quasiparticles in TaAs.The massless Dirac equation in the three-dimensional (3D) momentum space can be regarded as the overlap of two Weyl fermions with opposite chirality 1,2 . The Dirac fermionic quasiparticle is stable under the protection of some crystal symmetry in topological Dirac semimetals such as Na 3 Bi (ref. 3) and Cd 3 As 2 (ref. 4). However, a separated single Weyl node is much more robust and requires no protection of crystal symmetry. An isolated Weyl node is a sink or source of gauge field of Berry curvature, like a monopole in momentum space, and the chirality corresponds to its topological charge [5][6][7] . Weyl nodes appear in pairs of opposite chirality in a real material due to the 'No-go theorem' 8,9 . To obtain isolated Weyl nodes, the spin degeneracy of the electronic bands has to be removed by breaking either inversion symmetry or time-reversal symmetry. Although non-degenerate band crossing is not rare, finding a material with only Weyl nodes near the Fermi energy (E F ) is a big challenge. Recently, the noncentrosymmetric and non-magnetic transition-metal monoarsenide TaAs has been predicted to be a Weyl semimetal (WSM), and twelve pairs of Weyl nodes are expected in its 3D Brillouin zone (BZ; refs 10,11). Compared with other proposals 6,7,12-22 for realizing a Weyl state, the TaAs family features easy sample fabrication, a non-magnetic state and no fine-tuning of the electronic states is necessary, making experimental studies of Weyl semimetals possible. Many exotic properties induced by the Weyl nodes have been predicted and observed recently, such as surface states with Fermi arcs 23,24 and a negative magneto-resistivity 25,26 due to the chiral anomaly 27-29 . However, crucial evidence for Weyl nodes in the bulk states has not been observed. In this paper, by using soft X-ray angle-resolved photoemission spectroscopy (ARPES), which is sensitive to the bulk states, we report the first experimental observation of Weyl nodes in TaAs.TaAs crystallizes in a body-centred-tetragonal structure with the nonsymmorphic space group...

We consider the electromagnetic (EM) perturbative effects produced by the high-frequency gravitational waves (HFGWs) in the GHz band in a special EM resonance system, which consists of fractal membranes, a Gaussian beam (GB) passing through a static magnetic field.It is predicted, under the synchroresonance condition, coherence modulation of the HFGWs to the preexisting transverse components of the GB produces the transverse perturbative photon flux (PPF),which has three novel and important properties: (1)The PPF has maximum at a longitudinal symmetrical surface of the GB where the transverse background photon flux (BPF) vanishes; (2) the resonant effect will be high sensitive to the propagating directions of the HFGWs; (3) the PPF reflected or transmitted by the fractal membrane exhibits a very small decay compared with very large decay of the much stronger BPF. Such properties might provide a new way to distinguish and display the perturbative effects produced by the HFGWs. We also discuss the high-frequency asymptotic behavior of the relic GWs in the microwave band and the positive definite issues of their energy-momentum pseudo-tensor .

A coupling system between Gaussian type-microwave photon flux, static magnetic field and fractal membranes (or other equivalent microwave lenses) can be used to detect high-frequency gravitational waves (HFGWs) in the microwave band. We study the signal photon flux, background photon flux and the requisite minimal accumulation time of the signal in the coupling system. Unlike pure inverse Gertsenshtein effect (G-effect) caused by the HFGWs in the GHz band, the the electromagnetic (EM) detecting scheme (EDS) proposed by China and the US HFGW groups is based on the composite effect of the synchro-resonance effect and the inverse G-effect. Key parameters in the scheme include first-order perturbative photon flux (PPF) and not the second-order PPF; the distinguishable signal is the transverse first-order PPF and not the longitudinal PPF; the photon flux focused by the fractal membranes or other equivalent microwave lenses is not only the transverse first-order PPF but the total transverse photon flux, and these photon fluxes have different signal-to-noise ratios at the different receiving surfaces. Theoretical analysis and numerical estimation show that the requisite minimal accumulation time of the signal at the special receiving surfaces and in the background noise fluctuation would be ∼ 10 3 − 10 5 seconds for the typical laboratory condition and parameters of h r.m.s. ∼ 10 −26 − 10 −30 at 5GHz with bandwidth ∼1Hz. In addition, we review the inverse G-effect in the EM detection of the HFGWs, and it is shown that the EM detecting scheme based only on the pure inverse G-effect in the laboratory condition would not be useful to detect HFGWs in the microwave band. PACS numbers: 04.30Nk, 04.25Nx, 04.30Db, 04.80Nn a

The programming new e + e − collider with high luminosity shall provide another useful platform to study the properties of the doubly heavy Bc meson in addition to the hadronic colliders as LHC and TEVATRON. Under the 'New Trace Amplitude Approach', we calculate the production of the spin-singlet Bc and the spin-triplet B * c mesons through the Z 0 boson decays, where uncertainties for the production are also discussed. Our results show Γ ( PACS numbers: 12.38. Bx, 12.39.Jh, 14.40Lb, 14.40.Nd The B c meson is a double heavy quark-antiquark bound state and carries flavors explicitly. Since its first discovery at TEVATRON [1], B c physics is attracting more and wide interests. Recently, many progresses have been made for the hadronic production of B c meson at high energy colliders as LHC and TEVATRON. A computer program BCVEGPY for the direct hadronic production of B c meson has been presented in Refs. [2,3]. And it has been found that the indirect production of B c via top quark decays can also provide useful information on B c meson [4][5][6][7].Comparing with the hadronic colliders, an e + e − collider has its own advantages, mainly because of its lower background. As for the previous LEP-I experiment, no B c events have been found due to its lower collision energy and low luminosity [8,9]. However, if the luminosity of the e + e − collider can be raised up to L ∝ 10 34 cm −2 s −1 or even higher as programmed by the Internal Linear Collider (ILC) [10], then there might have enough events. Moreover, if the e + e − collider further runs at the Z 0 -boson energy, the resonance effects at the Z 0 peak may raise the production rate up to several orders. It has been estimated by Ref.[11] that more than 10 9∼10 Z 0 -events can be produced at ILC per year, which is about 3 ∼ 4 orders higher than that collected by LEP-I. Such a high luminosity collider is called as GigaZ [11] or a Z-factory [12]. Then it will open new opportunities not only for high precision physics in the electro-weak sector, but also for the hadron physics.The production of B c through Z 0 decays has been studied in Refs [8,9,13] with various methods. Since the process is very complicated, it would be helpful to have a cross check of these results. Furthermore, considering the forthcoming Z-factory, it may be interesting to know the theoretical uncertainties in estimating of B c production.For the purpose, we need to calculate the processc +b+c, whose Feynman diagrams are shown in Fig.(1). According to the NRQCD factorization formula * e-mail:wuxg@cqu.edu.cnc[14], the decay width for the process Z 0 → B ( * ) c + b +c can be written in the following factorization form:where the matrix element O H (n) is proportional to the inclusive transition probability of the perturbative state cb[n] into the bound states of B c . As for the two colorsinglet S-wave states cb[are related with the Bethe-Salpeter wave function at the origin that can be determined by the potential model [15][16][17][18][19][20]. dΓ(Z 0 → cb[n] + b +c) stands for the short-distan...

We investigate the hadronic production of the doubly heavy baryon Ξ bc at the large hadron collider (LHC), where contributions from the four (bc)-diquark states (bc)3 ,6 [ 1 S0] and (bc)3 ,6 [ 3 S1] have been taken into consideration. Numerical results show that under the condition of pT > 4 GeV and |y| < 1.5, sizable Ξ bc events about 1.7 × 10 7 and 3.5 × 10 9 per year can be produced for the center-of-mass energy √ S = 7 TeV and √ S = 14 TeV respectively. For experimental usage, the total and the interested differential cross-sections are estimated under some typical pT -and y-cuts for the LHC detectors CMS, ATLAS and LHCb. Main uncertainties are discussed and a comparative study on the hadronic production of Ξcc, Ξ bc and Ξ bb at LHC are also presented.

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We report the optical conductivity in high-quality crystals of the chiral topological semimetal CoSi, which hosts exotic quasiparticles known as multifold fermions. We find that the optical response is separated into several distinct regions as a function of frequency, each dominated by different types of quasiparticles. The low-frequency intraband response is captured by a narrow Drude peak from a high-mobility electron pocket of double Weyl quasiparticles, and the temperature dependence of the spectral weight is consistent with its Fermi velocity. By subtracting the low-frequency sharp Drude and phonon peaks at low temperatures, we reveal two intermediate quasilinear interband contributions separated by a kink at 0.2 eV. Using Wannier tight-binding models based on first-principle calculations, we link the optical conductivity above and below 0.2 eV to interband transitions near the double Weyl fermion and a threefold fermion, respectively. We analyze and determine the chemical potential relative to the energy of the threefold fermion, revealing the importance of transitions between a linearly dispersing band and a flat band. More strikingly, below 0.1 eV our data are best explained if spin-orbit coupling is included, suggesting that at these energies, the optical response is governed by transitions between a previously unobserved fourfold spin-3/2 node and a Weyl node. Our comprehensive combined experimental and theoretical study provides a way to resolve different types of multifold fermions in CoSi at different energy. More broadly, our results provide the necessary basis to interpret the burgeoning set of optical and transport experiments in chiral topological semimetals.

The production of the heavy (cc)-quarkonium, (cb)-quarkonium and (bb)-quarkonium states [(QQ ′ ) quarkonium for short], via the W + semi-inclusive decays, has been systematically studied within the framework of the non-relativistic QCD. In addition to the two color-singlet S-wave states, we also discuss the production of the four color-singlet P -wave states |(QQ ′ )( 1 P1)1 and |(QQ ′ )( 3 PJ )1 [with J = (0, 1, 2)] together with the two color-octet components |(QQ ′ )( 1 S0)8 and |(QQ ′ )( 3 S1)8 . Improved trace technology is adopted to derive the simplified analytic expressions at the amplitude level, which shall be useful for dealing with the following cascade decay channels. At the LHC with the luminosity L ∝ 10 34 cm −2 s −1 and the center-of-mass energy √ S = 14 TeV, sizable heavy-quarkonium events can be produced through the W + boson decays, i.e. 2.57 × 10 6 ηc, 2.65 × 10 6 J/Ψ and 2.40 × 10 6 P -wave charmonium events per year can be obtained; and 1.01 × 10 5 Bc, 9.11 × 10 4 B * c and 3.16 × 10 4 P -wave (cb)-quarkonium events per year can be obtained. Main theoretical uncertainties have also been discussed. By adding the uncertainties caused by the quark masses in quadrature, we obtain Γ W + →(cc)+cs = 524.8 +396.3 −258.4 KeV, Γ W + →(cb)+bs = 13.5 +4.73 −3.29 KeV, Γ W + →(cb)+cc = 1.74 +1.98 −0.73 KeV and Γ W + →(bb)+cb = 38.6 +13.4 −9.69 eV.