Full Momentum Research Articles

Causality violation and the speed of sound of hot and dense quark matter in the Nambu–Jona-Lasinio model

The Nambu--Jona-Lasinio model is widely used to study strong-interaction phenomena in vacuum and quark matter. Since the model is nonrenormalizable, one needs to work within a specific regularization scheme to obtain finite results. Here we show that a commonly used cutoff regularization scheme leads to unphysical results, such as superluminal speed of sound and wrong high-temperature behavior of the specific heat and other thermodynamical quantities. Such a troublesome feature of the cutoff regularization invalidates the model for temperature and baryon density values relevant to the phenomenology of heavy-ion collisions and compact stars. We show that the source of the problems stems from cutting off momentum modes in finite integrals depending on thermal distribution functions in the grand canonical potential. The problems go away when taking into account the full momentum range of those integrals. Explicit examples are worked out in the SU(2)-flavor version of the model.

Manipulating the Chirality of Moiré Metasurface by Symmetry Breaking

AbstractThe chirality of materials is of great significance in many fields. Metasurfaces with chirality have attracted great attention in the past decades because of their ability to miniaturize the device into the nanoscale. An important class of chiral metasurfaces is the Moiré pattern assembled by two layers of metasurfaces. However, Moiré metasurfaces usually have limited circular dichroism. Here, the origin and the limiting factor of the chirality of Moiré metasurfaces using the chiral coupled oscillator model is analyzed. It is demonstrated that, with the help of a non‐Hermitian Hamiltonian, the chirality can be improved by breaking the symmetry in the positive and negative k directions. The best result is achieved when both the two components have lowest transmission. In addition, the chiral anisotropy of a Moiré metasurface in full momentum space is studied, and it is shown that the chirality can be further improved in certain directions. The chiral anisotropy of Moiré metasurfaces may be used in applications such as optical information storage, structured color, unidirectional light coupling, sensitive chiral sensors, chiral nonlinear optics, chiral lasers, and Bose–Einstein condensation.

Effect of mesonic off-shell correlations in the PNJL equation of state

We study the meson contribution to the equation of state of the 2-flavor PNJL model, including the full momentum dependence of the meson polarization loops. Within the Beth-Uhlenbeck approach, we demonstrate that the contribution from the quark-antiquark continuum excitations in the spacelike region $\omega^2 - q^2 < 0$, i.e. the Landau damping, leads to an increase of the pressure for temperatures $\gtrsim 0.8\,T_c^\chi$ and a significant meson momentum cut-off dependence in the mesonic pressure and the QCD trace anomaly. We investigate the dependence of the results on the choice of the Polyakov-loop potential parameter $T_0$. From the dependence of the mesonic pressure on the current quark mass, by means of the Feynman-Hellmann theorem, we evaluate the contribution of the pion quasiparticle gas and Landau damping to the chiral condensate.

Electron-phonon coupling and spin fluctuations in the Ising superconductor NbSe2

Ising superconductivity, observed in NbSe2 and similar materials, has generated tremendous interest. Recently, attention was called to the possible role that spin fluctuations (SF) play in this phenomenon, in addition to the dominant electron–phonon coupling (EPC); the possibility of a predominantly triplet state was discussed and led to a conjecture of viable singlet–triplet Leggett oscillations. However, these hypotheses have not been put to a quantitative test. In this paper, we report first principle calculations of the EPC and also estimate coupling with SF, including full momentum dependence. We find that: (1) EPC is strongly anisotropic, largely coming from the K-{K}^{{prime} } scattering, and therefore excludes triplet symmetry even as an excited state; (2) superconductivity is substantially weakened by SF, but anisotropy remains as above; and, (3) we do find the possibility of a Leggett mode, not in a singlet–triplet but in an s++ – s± channel.

Nonperturbative strong coupling at timelike momenta

We compute the strong coupling constant of Landau gauge QCD in the full complex momentum plane, both directly and via spectral reconstruction. In particular, we consider the Taylor coupling given by the product of ghost and gluon dressing functions. Assuming spectral representations for the latter, we first show that also the coupling obeys such a representation. The subsequent spectral reconstruction of the coupling data, obtained from 2+1 flavour lattice QCD results for the ghost and gluon, is based on a probabilistic inversion of this representation using Gaussian process regression with analytically enforced asymptotics. In contradistinction, our direct calculation relies on earlier reconstruction results for the ghost and gluon spectral functions themselves, as well as data obtained in functional QCD. Apart from its relevance for studies of resonances or scattering processes, the calculation also serves as a non-trivial benchmark of our reconstruction approach. The results show remarkable agreement, testifying to the reliability of the method.

Overview of the PANDA detector design at FAIR

PANDA (antiProton ANihilation at DArmstadt) is the central experiment to fully exploit the physics research potential of antiproton beams at the international accelerator Facility for Antiproton and Ion Research in Europe (FAIR), currently under construction at GSI. Phase-space cooled high intensity antiproton beams up to 15 GeV/c will be provided by the High Energy Storage Ring (HESR) at FAIR to interact with PANDA internal proton or nuclear targets enabling a broad range of exciting studies in Particle and Nuclear Physics. The PANDA detector features two spectrometers, the Target Spectrometer with a superconducting solenoid magnet of 2 T around the interaction region with hermetic coverage and the Forward Spectrometer with a 2 Tm dipole magnet for coverage of the forward boosted particles. Several modern particle detector systems are employed in PANDA to provide excellent charged particle tracking, particle identification, calorimetry and muon detection, over the full momentum range in both spectrometers throughout the lifetime of the experiment. Focusing on the various PANDA detector systems we present an overview of recent developments, the detector construction progress and conclude with an outline for a phased deployment of PANDA at FAIR.

Decomposing the linear momentum transfer components in break-up fusion reactions: An experimental study of the F19+Tb159 system

In the present work, measurement and analysis of forward recoil range distribution of various reaction residues populated in the interaction of $^{19}\mathrm{F}$ projectile with $^{159}\mathrm{Tb}$ target nucleus have been performed. The aim is to disentangle the contribution of complete fusion (CF) and incomplete fusion (ICF) components at two different above-barrier energies $\ensuremath{\approx}83$ and 94 MeV. The recoil-catcher technique with off-line $\ensuremath{\gamma}$-ray spectroscopy has been performed. The complete and incomplete fusion events have been identified by full and partial linear momentum transfer, respectively. A single Gaussian distribution has been observed in case of residues populated via complete fusion processes. However, the recoil range distribution for $\ensuremath{\alpha}$-emitting channels was found to be deconvoluted into more than one Gaussian peak indicating the involvement of both full and partial linear momentum transfer components due to the presence of CF and ICF processes. The analysis of data suggests that projectile breaks up as soon as it comes near the vicinity of target nuclear field leading to different linear momentum transfer components. Further, the analysis done within the framework of universal fusion function indicates a significant break-up fusion probability of $^{19}\mathrm{F}$ projectile. As such, at lower energies $^{19}\mathrm{F}$ is found to behave similar to other $\ensuremath{\alpha}$-cluster beams.

Radial-momentum-resolved measurement of the tunneling ionization time in attoclock experiments

We studied the tunneling ionization time in the elliptically polarized laser field with the recently proposed phase-of-the-phase spectroscopy, wherein a weak linearly polarized second-harmonic field was introduced to the attoclock frame. By monitoring the oscillation of the photoelectron yield with the relative phase between the two fields, the ionization time of the photoelectrons can be determined without any effort on handling the Coulomb interaction between the escaping electron and the parent ion. This advantage enables us to resolve the ionization time in the full photoelectron momentum distribution. By solving the time-dependent Schr\"odinger equation, we showed that the ionization time depends on the radial momentum of photoelectron. The photoelectron with larger radial momentum ionizes earlier at the emission angle before the most probable emission angle, while this radial-momentum dependence is reversed at the emission angle after the most probable emission angle. These results are confirmed by our experiments.

Very low thrust station-keeping for low Earth orbiting satellites

Traditional station-keeping for Earth observation satellites with chemical thrusters generally involves maneuvers every couple months that are able to change significantly the semi-major axis and the inclination. These strategies do not scale down to very low thrust level (a few hundreds of μN) electrical thrusters. This paper presents both in-plane and out-of-plane strategies that spread corrections over very long arcs and discretize them to tiny maneuvers every couple orbits, taking into account mission-constraints on maneuvers locations. These strategies scale up to medium thrust strategies, filling the gap between propulsion technologies. The out-of-plane strategy although features a new no-deadband property and controls the full orbital momentum. All strategies allow control very close to the reference (a few hundreds meters in osculating parameters) and very low cost.

Loop-corrected Higgs masses in the NMSSM with inverse seesaw mechanism

In this study, we work in the framework of the Next-to-Minimal extension of the Standard Model (NMSSM) extended by six singlet leptonic superfields. Through the mixing with the three doublet leptonic superfields, the non-zero tiny neutrino masses can be generated through the inverse seesaw mechanism. While R-parity is conserved in this model lepton number is explicitly violated. We quantify the impact of the extended neutrino sector on the NMSSM Higgs sector by computing the complete one-loop corrections with full momentum dependence to the Higgs boson masses in a mixed on-shell-overline{text{ DR }} renormalization scheme, with and without the inclusion of CP violation. The results are consistently combined with the dominant two-loop corrections at mathcal{O}(alpha _t(alpha _s+alpha _t)) to improve the predictions for the Higgs mixing and the loop-corrected masses. In our numerical study we include the constraints from the Higgs data, the neutrino oscillation data, the charged lepton flavor-violating decays l_i rightarrow l_j + gamma , and the new physics constraints from the oblique parameters S, T, U. We present in this context the one-loop decay width for l_i rightarrow l_j + gamma . The loop-corrected Higgs boson masses are included in the Fortran code NMSSMCALC-nuSS.

Probing Charm Quark Dynamics via Multiparticle Correlations in Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

Multiparticle azimuthal correlations of prompt D^{0} mesons are measured in Pb-Pb collisions at a nucleon-nucleon center-of-mass energy of sqrt[s_{NN}]=5.02 TeV. For the first time, a four-particle cumulant method is used to extract the second Fourier coefficient of the azimuthal distribution (v_{2}) of D^{0} mesons as a function of event centrality and the D^{0} transverse momentum. The ratios of the four-particle v_{2} values to previously measured two-particle cumulant results provide direct experimental access to event-by-event fluctuations of charm quark azimuthal anisotropies. These ratios are also found to be comparable to those of inclusive charged particles in the event. However, hints of deviations are seen in the most central and peripheral collisions. To investigate the origin of flow fluctuations in the charm sector, these measurements are compared to a model implementing fluctuations of charm quark energy loss via collisional or radiative processes in the quark-gluon plasma. These models cannot quantitatively describe the data over the full transverse momentum and centrality ranges, although the calculations with collisional energy loss provide a better description of the data.

Deformed in-medium similarity renormalization group

In the $m$-scheme Hartree-Fock (HF) basis, we have developed an ab initio deformed single-reference in-medium similarity renormalization group (IMSRG) approach for open-shell nuclei. A deformed wave function may be more efficient in describing the deformed nucleus. The broken rotational symmetry can be restored using the angular momentum projection. However, a full angular momentum projection at the IMSRG level is still a challenge in both theory itself and computation. The angular momentum restoration mainly recaptures the static correlations, and in the present work we estimate the angular momentum projection effect by projecting the HF state as a leading-order approximation. As a test ground, we have calculated the deformed $^{8,10}\mathrm{Be}$ isotopes with the optimized chiral interaction ${\mathrm{NNLO}}_{\mathrm{opt}}$. The results are benchmarked with the no-core shell model and valence-space IMSRG calculations. Then we systematically investigate the ground-state energies and charge radii of even-even nuclei from light beryllium to medium-mass magnesium isotopes. The calculated energies are extrapolated to infinite basis space by an exponential form, and compared with extrapolated valence-space IMSRG results and experimental data.

Application of Deep Learning Model in the Avoidance of Investment Risk of Multinational Enterprises.

With the continuous improvement and development of the socialist market economic system, China's economic development has full momentum, but the domestic market is no longer sufficient to meet the needs of enterprise development. China has always focused on peaceful diplomacy, and the world market has a strong demand for Chinese products. This work aims to improve the accuracy of exchange rate forecasting. The risk factors that may be encountered in the investment process of multinational enterprises can be effectively avoided. Combining the advantages of Long Short-Term Memory (LSTM) and Convolutional Neural Network (CNN), the LSTM-CNN (Long Short-Term Memory-Convolutional Neural Network) model is proposed to predict the volatility trend of stocks. Firstly, the investment risk of multinational enterprises is analyzed, and, secondly, the principles of the used CNN and LSTM are expounded. Finally, the performance of the proposed model is verified by setting experiments. The experimental results demonstrate that when predicting the 10 selected stocks, the proposed LSTM-CNN model has the highest accuracy in predicting the volatility of stocks, with an average accuracy of 60.1%, while the average accuracy of the rest of the models is all below 60%. It can be found that the stock category does not have a great impact on the prediction accuracy of the model. The average prediction accuracy of the CNN model is 0.578, which is lower than that of the Convolutional Neural Network-Relevance model, and the prediction accuracy of the LSTM model is 0.592, which is better than that of the Long Short-Term Memory-Relevance model. The designed model can be used to predict the stock market to guide investors to make effective investments and reduce investment risks based on relevant cases. The research makes a certain contribution to improving the company's income and stabilizing the national economic development.

Shot-by-shot 250 kHz 3D ion and MHz photoelectron imaging using Timepix3

We demonstrate the application of event-driven Timepix3-based detectors in combination with a double-sided velocity-map-imaging spectrometer to record the full 3D momentum of charged particles at the free-electron-laser facility FLASH. We measured the extreme ultraviolet induced fragmentation of N2 using 250 kHz FLASH bursts with sub-pixel spatial resolution and up to 1.7 ns temporal resolution for photoelectrons. To further demonstrate the capabilities of this camera at even higher repetition rates we measured single-shot images of He(1s) photoelectrons for bursts with a repetition rate of 1 MHz. Overall, with the Timepix3 camera we overcome limitations of standard-camera technology for advanced-imaging experiments with requirements on high event-rates and high spatio-temporal resolution.

One-loop amplitudes in the worldline formalism

We summarize recent progress in applying the worldline formalism to the analytic calculation of one-loop N-point amplitudes. This string-inspired approach is well-adapted to avoiding some of the calculational inefficiencies of the standard Feynman diagram approach, most notably by providing master formulas that sum over diagrams differing only by the position of external legs and/or internal propagators. We illustrate the mathematical challenge involved with the low-energy limit of the N-photon amplitudes in scalar and spinor QED, and then present an algorithm that, in principle, solves this problem for the much more difficult case of the N-point amplitudes at full momentum in ϕ 3 theory. The method is based on the algebra of inverse derivatives in the Hilbert space of periodic functions orthogonal to the constant ones, in which the Bernoulli numbers and polynomials play a central role.

Observation of a Flat and Extended Surface State in a Topological Semimetal.

A flat band structure in momentum space is considered key for the realization of novel phenomena. A topological flat band, also known as a drumhead state, is an ideal platform to drive new exotic topological quantum phases. Using angle-resolved photoemission spectroscopy experiments, we reveal the emergence of a highly localized surface state in a topological semimetal BaAl and provide its full energy and momentum space topology. We find that the observed surface state is localized in momentum, inside a square-shaped bulk Dirac nodal loop, and in energy, leading to a flat band and a peak in the density of state. These results imply this class of materials as an experimental realization of drumhead surface states and provide an important reference for future studies of the fundamental physics of correlated quantum effects in topological materials.

The plain and simple parquet approximation: single-and multi-boson exchange in the two-dimensional Hubbard model

The parquet approach to vertex corrections is unbiased but computationally demanding. Most applications are therefore restricted to small cluster sizes or rely on various simplifying approximations. We have recently shown that the bosonization of the parquet diagrams provides interpretative and algorithmic advantages over the original purely fermionic formulation. Here, we present first results of the numerical implementation of this method by applying it to the half-filled Hubbard model on the square lattice at weak coupling. The improved algorithmic performance allows us to evaluate the parquet approximation for a 16times 16 lattice, retaining the full momentum and frequency structure of the various vertex functions. We discuss their symmetries and consider parametrizations of their momentum dependence using the truncated-unity approximation.Graphical abstract

Nonlinear diffusion of gluons

It is proposed to consider the fast thermalization of gluons in relativistic heavy-ion collisions as a diffusion process in momentum space. Closed-form analytical solutions of a nonlinear boson diffusion equation (NBDE) with constant drift and diffusion coefficients v, D and boundary conditions at the singularity are derived. The time evolution towards local central temperatures T≲600MeV through inelastic gluon scatterings in heavy-ion collisions is calculated for under- and overoccupied systems in the full momentum range. The results are consistent with QCD-based numerical calculations for gluon thermalization via inelastic gluon collisions.

Spectral function of fermions in a highly occupied non-Abelian plasma

We develop a method to obtain fermion spectral functions non-perturbatively in a non-Abelian gauge theory with high occupation numbers of gauge fields. After recovering the free field case, we extract the spectral function of fermions in a highly occupied non-Abelian plasma close to its non-thermal fixed point, i.e., in a self-similar regime of the non-equilibrium dynamics. We find good agreement with hard loop perturbation theory for medium-induced masses, dispersion relations and quasiparticle residues. We also extract the full momentum dependence of the damping rate of the collective excitations.

Unconventional spectral signature of Tc in a pure d-wave superconductor.

In conventional superconductors, the phase transition into a zero-resistance and perfectly diamagnetic state is accompanied by a jump in the specific heat and the opening of a spectral gap1. In the high-transition-temperature (high-Tc) cuprates, although the transport, magnetic and thermodynamic signatures of Tc have been known since the 1980s2, the spectroscopic singularity associated with the transition remains unknown. Here we resolve this long-standing puzzle with a high-precision angle-resolved photoemission spectroscopy (ARPES) study on overdoped (Bi,Pb)2Sr2CaCu2O8+δ (Bi2212). We first probe the momentum-resolved electronic specific heat via spectroscopy and reproduce the specific heat peak at Tc, completing the missing link for a holistic description of superconductivity. Then, by studying the full momentum, energy and temperature evolution of the spectra, we reveal that this thermodynamic anomaly arises from the singular growth of in-gap spectral intensity across Tc. Furthermore, we observe that the temperature evolution of in-gap intensity is highly anisotropic in the momentum space, and the gap itself obeys both the d-wave functional form and particle-hole symmetry. These findings support the scenario that the superconducting transition is driven by phase fluctuations. They also serve as an anchor point for understanding the Fermi arc and pseudogap phenomena in underdoped cuprates.