arXiv subjects

Yu Jia

Publications and source records attributed to Yu Jia.

At least 91 records · Page 5Linked to original sources

arXiv2019

The Ratio Law of the Structure Evolution and Stability for Ti$_n$O$_m$ ($n=3$-$18$, $m=1$-$2n$) Clusters

Most theoretical investigations about titanium oxide clusters focus on (TiO$_2$)$_n$. However, many Ti$_n$O$_m$ clusters with $m\neq 2n$ are produced experimentally. In this work, first-principles calculations are performed to probe the evolution of Ti$_n$O$_m$ clusters. Our investigations show that for $n=3$-$11$, there exist one relatively stable specie; while for $n=12$-$18$, there are two relatively stable species: Ti-rich and O-rich species. HOMO-LOMO calculations show that the gap can be tuned by changing the size and configurations of Ti$_n$O$_m$ clusters. Our investigation provides insights into the evolution of cluster-to-bulk process in titanium oxide.

physics.atm-clus↗

arXiv2019

$W$ radiative decay to heavy-light mesons in HQET factorization through ${\cal O}(α_s)$

Analogous to NRQCD factorization for heavy quarkonium exclusive production, in this work we propose to employ the heavy-quark-effective-theory (HQET) factorization, which has been predominantly applied to account for exclusive $B$ decays, to study the exclusive production of the heavy-flavored mesons. We take $W\to B(D_s)+γ$ as a prototype process. The validity of the HQET factorization rests upon the presumed scale hierarchy: $m_W\sim m_b\gg Λ_{\rm QCD}$. Through an explicit analysis at next-to-leading order in $α_s$ yet at leading order in $1/m_b$, we verify that the decay form factors can indeed be expressed as the convolution between perturbatively calculable hard-scattering kernel and the $B$ meson light-cone distribution amplitude (LCDA) defined in HQET. It is observed that the factorization scale dependence becomes reduced after incorporating the NLO perturbative correction. An interesting future investigation is to identify and resum large collinear logarithms of $m_W/m_b$ that arise ubiquitously in the fixed-order expressions of the hard-scattering kernel in HQET factorization.

hep-ph↗

arXiv2019

A factorization theorem connecting the light-cone distribution amplitudes of heavy-flavor mesons in QCD and HQET

The light-cone distribution amplitude (LCDA) of a heavy-light meson defined in heavy quark effective theory (HQET), is a fundamental nonperturbative input to account for innumerable $B$ meson exclusive decay and production processes. On the other hand, the conventional heavy-flavored meson LCDA defined in QCD, also ubiquitously enters the factorization formula for hard exclusive $B$ production processes. Inspired by the observation that these two LCDAs exhibit the identical infrared behaviors, yet differ in the ultraviolet scale of order $m_b$ or greater, we propose a novel factorization theorem for the heavy-light mesons, that the LCDA defined in QCD can be further expressed as a convolution between the LCDA in HQET and a perturbatively calculable coefficient function thanks to asymptotic freedom. This refactorization program can be invoked to fully disentangle the effects from three disparate scales $Q$, $m_b$ and $Λ_{\rm QCD}$ for a hard exclusive $B$ production process, particularly to facilitate the resummation of logarithms of type $\ln Q/m_b$ and $\ln m_b/Λ_{\rm QCD}$ in a systematic fashion.

hep-ph↗

arXiv2019

Meson-meson scattering in two-dimensional QCD

We extend the formalism pioneered by Callan, Coote and Gross to investigate the meson-meson scattering within the framework of 't Hooft model, i.e., the two-dimensional QCD in the $N_c\to \infty$ limit. We derive the analytic expressions for various two-body meson-meson scattering amplitudes, concentrating on those quark diagrams which may be identified as the meson-meson contact interaction vertex in the context of the mesonic effective lagrangian in $1/N_c$ expansion. We also carry out a detailed numerical study for the meson-meson scattering for various quark flavors, and observe the near-threshold enhancement in some channels. This may be viewed as the hint of the existence of the tetra-quark state below two-meson threshold.

hep-ph↗

arXiv2019

Band gap and band alignment prediction of nitride based semiconductors using machine learning

Nitride has been drawing much attention due to its wide range of applications in optoelectronics and remains plenty of room for materials design and discovery. Here, a large set of nitrides have been designed, with their band gap and alignment being studied by first-principles calculations combined with machine learning. Band gap and band offset against wurtzite GaN accurately calculated by the combination of screened hybrid functional of HSE and DFT-PBE were used to train and test machine learning models. After comparison among different techniques of machine learning, when elemental properties are taken as features, support vector regression (SVR) with radial kernel performs best for predicting both band gap and band offset with prediction root mean square error (RMSE) of 0.298 eV and 0.183 eV, respectively. The former is within HSE calculation uncertainty and the latter is small enough to provide reliable predictions. Additionally, when band gap calculated by DFT-PBE was added into the feature space, band gap prediction RMSE decreases to 0.099 eV. Through a feature engineering algorithm, elemental feature space based band gap prediction RMSE further drops by around 0.005 eV and the relative importance of elemental properties for band gap prediction was revealed. Finally, band gap and band offset of all designed nitrides were predicted and two trends were noticed that as the number of cation types increases, band gap tends to narrow down while band offset tends to go up. The predicted results will be a useful guidance for precise investigation on nitride engineering.

cond-mat.mtrl-sci↗

arXiv2019

Optimized predictions for $W \to B_c + γ$ by combining light-cone and NRQCD approaches

The rare process of $W$ radiative decay into the $B_c$ meson can serve as an interesting play ground for testing two influential perturbative QCD approaches: non-relativistic quantum chromodynamics (NRQCD) and ligh-cone (LC) factorization. Since the $B_c$ meson is a genuine heavy quarkonium which is composed of two different species of heavy quarks, it is natural to employ NRQCD factorization to tackle this exclusive quarkonium production process; on the other hand, since the mass of the $W$ boson is much greater than that of the $B_c$, the collinear factorization is also a viable approach. To fully disentangle the contributions from several distinct scales $M_W$, $m_Q$, $m_Q v$ ($v$ represents the typical velocity of the $c/\bar{b}$ quarks inside $B_c$), and $Λ_{\rm QCD}$, we carry out a joint analysis that combines both light-cone and NRQCD factorization approaches, and compute the order-$α_s$ correction to $W \to B_c+γ$. With the aid of the celebrated Efremov-Radyushkin-Brodsky-Lepage (ERBL) evolution equation, we resum the leading collinear logarithms $α_s \ln\! {M_W\over m_Q}$ to all orders in $α_s$. We also explore some phenomenological implications of our predictions.

hep-ph↗

arXiv2019

Near-the-origin divergence of Dirac wave functions of hydrogen and operator product expansion

There is a long-standing puzzle concerning the Coulomb solutions of the Dirac equation, i.e., what is the physics governing the weakly divergent near-the-origin behavior of the Dirac wave functions of the $nS_{1/2}$ hydrogen? As a sequel of our preceding work that aim to demystifying the universal near-the-origin behavior of the atomic Schrödinger and Klein-Gordon wave functions, the goal of this work is to demonstrate that, within the nonrelativistic effective field theory (NREFT) tailored for Coulombic atoms, the universal logarithmic divergence of the Dirac wave functions can be accounted by the perturbatively calculable Wilson coefficient emerging from the operator product expansion (OPE) of the electron and the nucleus fields. The cause is due to the relativistic kinetic correction and Darwin (zitterbewegung) term in the NREFT. With the aid of renormalization group equation, one can resum the leading logarithms to all orders in $Zα$ and recover the $r^{-Z^2α^2/2}$ anomalous scaling behavior exhibited by the Dirac wave function for the $nS_{1/2}$ hydrogen. It appears somewhat counterintuitive that these universal logarithmic divergences can not be accounted by the OPE set up in the relativistic QED. We are thereby enforced to conclude that the Dirac wave function must cease to be meaningful when $r$ is shorter than the electron's Compton wavelength.

hep-ph↗

arXiv2018

Near-the-origin divergence of Klein-Gordon wave functions for hydrogen-like atoms and operator product expansion

There have been some long-standing puzzles related to the Coulomb solutions of the Klein-Gordon and Dirac equations, namely how to understand the physics underlying the weakly divergent near-the-origin behavior of the $S$-wave wave functions for the hydrogen-like atoms. Taking the Klein-Gordon wave function as a simpler example, in this work we demonstrate that, with the aid of the renormalization group equation, this universal short-distance behavior can be successfully taken into account by the operator product expansion (OPE) formulated in the nonrelativistic effective field theory (EFT), which is tailored for Coulombic atoms. The key is to include the relativistic kinetic correction in the EFT. Somewhat counterintuitively, these universal near-the-origin logarithmic divergences can not be addressed by the OPE set up in the relativistic scalar QED. We conclude that the Klein-Gordon wave function at a length scale shorter than the electron Compton wavelength may cease to make physical significance.

hep-ph↗

arXiv2018

Mixed electroweak-QCD corrections to $e^+e^-\to μ^+μ^- H$ at CEPC with finite-width effect

The associated production of Higgs boson with a muon pair, $e^+e^-\to μ^+μ^- H$, is one of the golden channels to pin down the properties of the Higgs boson in the prospective Higgs factories exemplified by \textsf{CEPC}. The projected accuracy of the corresponding cross section measurement is about per cent level at CEPC. In this work, we investigate both ${\mathcal O}(α)$ weak correction and the ${\mathcal O}(αα_s)$ mixed electroweak-QCD corrections for this channel, appropriately taking into account the effect of finite $Z^0$ width. The $μ^+μ^-$ invariant mass spectrum is also predicted. The mixed electroweak-QCD correction turns out to reach 1.5\% of the Born-order result, and thereby must be included in future confrontation with the data. We also observe that, after including higher-order corrections, the simplified prediction for the integrated cross section employing the narrow-width-approximation may deviate from our full result by a few per cents.

hep-ph↗

arXiv2018

Gauge-invariant decomposition of meson energy in two-dimensional QCD

This work is driven by our curiosity towards some basic questions in QCD: how is the energy of a moving hadron partitioned among different gauge-invariant sectors in QCD Hamiltonian? How is the energy of an massless pion separated between quark and gluon sectors, particularly in the soft pion limit? Is it possible to decompose the celebrated Gell-Mann-Oakes-Renner (GOR) relation? To what extent can we justify the quark potential model from the field-theoretical mass decomposition for heavy quarkonium? Due to limitation of contemporary nonperturbative tools, we do not yet know answers to these questions in realistic QCD. In this work, we take the 't Hooft model (two-dimensional QCD in large-$N$ limit) as a prototype model that mimics some essential aspects of the true QCD. We investigate the gauge-invariant energy decomposition of a flavor-neutral meson that can carry an arbitrary momentum (including stationary case), with meson species ranging from massless pion to bottomonium. All the aforementioned questions can be addressed satisfactorily within this model, in particular some unexpected patterns related to pion are discovered for the first time in two-dimensional QCD. We hope that our study can offer some useful clues and stimulation for future investigations on hadron energy decomposition in realistic QCD.

hep-ph↗

arXiv2018

Next-to-leading-order QCD corrections to gluon fragmentation into ${}^1S_0^{(1,8)}$ quarkonia

Within the NRQCD factorization framework, we compute the next-to-leading-order QCD corrections to the gluon fragmentation into the ${}^1S_0^{(1,8)}$ Fock components of a quarkonium, at the lowest order in velocity expansion. We follow the operator definition of the fragmentation function advanced by Collins and Soper. The key technique underpinning our calculation is the sector decomposition method widely used in the area of multi-loop computation. It is found that the NLO QCD corrections have significant effects, and qualitatively modify the profiles of the corresponding leading-order fragmentation functions.

hep-ph↗

arXiv2018

Partonic quasidistributions in two-dimensional QCD

As a sequel of our preceding work [1] we carry out a comprehensive comparative study between the quasi parton distribution functions (PDFs), distribution amplitudes (DAs) and their light-cone counterparts for various flavor-neutral mesons, in the context of the 't Hooft model, that is, the two-dimensional QCD in the large $N$ limit. In contrast to the original derivation via diagrammatic techniques exemplified by Dyson-Schwinger and Bethe-Salpeter equations, here we employ the Hamiltonian operator approach to reconstruct the celebrated 't Hooft equation in light-front quantization, and Bars-Green equations in equal-time quantization. The novelty of our derivation is to employ the soft momentum cutoff as the IR regulator. As a virtue of this operator approach, the functional form of the quasi distributions can be transparently built out of the Bars-Green wave functions and the Bogoliubov angle with the aid of bosonization technique. Equipped with various bound-state wave functions numerically inferred in [1], we then investigate how rapidly the quasi distributions approach their light-cone counterparts with the increasing meson momentum. We observe that, light mesons' quasi distributions approach the light-cone distributions in a slower pace than the heavy quarkonia. Curiously, lattice simulations of quasi distributions in four-dimensional QCD also discover this feature. Furthermore, we also compute the partonic light-cone PDF and quasi-PDF to one-loop order in perturbation theory, again employing the momentum cutoff as the IR regulator. We explicitly verify one of the backbones underlying the large momentum effective field theory (LaMET), namely, both quasi-PDFs and light-cone PDFs in ${\rm QCD}_2$ indeed possess the same IR behavior at leading order in $1/P^z$

hep-th↗

arXiv2018

Inclusive $h_c$ production and energy spectrum from $e^+ e^-$ annihilation at Super $B$ factory

We calculate the next-to-leading order (NLO) radiative correction to the color-octet $h_c$ inclusive production in $e^+e^-$ annihilation at Super $B$ factory, within the nonrelativistic QCD factorization framework. The analytic expression for the NLO short-distance coefficient (SDC) accompanying the color-octet production operator $\mathcal{O}_8^{h_c}(^1S_0)$ is obtained after summing both virtual and real corrections. The size of NLO correction for the color-octet production channel is found to be positive and substantial. The NLO prediction to the $h_c$ energy spectrum is plagued with unphysical endpoint singularity. With the aid of the soft-collinear effective theory, those large endpoint logarithms are resummed to the next-to-leading logarithmic (NLL) accuracy. Consequently, further supplemented with the non-perturbative shape function, we obtain the well-behaved predictions for the $h_c$ energy spectrum in the entire kinematic range, which awaits the examination by the forthcoming Belle II experiment.

hep-ph↗

arXiv2017

Fragmentation function of gluon into spin-singlet $P$-wave quarkonium

Following the operator definition of the fragmentation function developed by Collins and Soper, we compute the gluon-to-$h_c$ fragmentation function at the lowest order in the velocity expansion in NRQCD factorization approach. Utilizing some modern technique developed in the area of multi-loop calculation, we are able to analytically deduce the infrared-finite color-singlet short-distance coefficient associated with the fragmentation function. The fragmentation probability for gluon into $h_c$ is estimated to be order $10^{-6}$.

hep-ph↗

arXiv2017

The Nature of Bonding in Bulk Tellurium Composed of One-Dimensional Helical Chains

Bulk tellurium (Te) is composed of one-dimensional (1D) helical chains which have been considered to be coupled by van der Waals (vdW) interactions. However, based on first-principles density-functional theory calculations, we here propose a different bonding nature between neighboring chains: i.e., the helical chains made of normal covalent bonds are connected together by coordinate covalent bonds. It is revealed that the lone pairs of electrons of Te atom participate in forming coordinate covalent bonds between neighboring chains. Therefore, each Te atom behaves as both electron donor to neighboring chains and electron acceptor from neighboring chains. This ligand-metal-like bonding nature in bulk Te results in the same order of bulk moduli along the directions parallel and perpendicular to the chains, contrasting with the large anisotropy of bulk moduli in vdW crystals. We further find that the electron effective masses parallel and perpendicular to the chains are almost the same each other, consistent with the observed nearly isotropic electrical resistivity. It is thus demonstrated that the normal/coordinate covalent bonds parallel/perpendicular to the chains in bulk Te lead to a minor anisotropy in structural and transport properties, distinct from a strong anisotropy observed in the typical two-dimensional (2D) vdW materials such as graphite and MoS$_2$.

cond-mat.mtrl-sci↗

arXiv2017

Quarkonium decay into photon plus graviton: a golden channel to discriminate General Relativity from Massive Gravity?

After the recent historical discovery of gravitational wave, it is curious to speculate upon the detection prospect of the quantum graviton in the terrestrial accelerator-based experiment. We carefully investigate the "golden" channels, $J/ψ(Υ)\toγ+\text{graviton}$, which can be pursued at \textsf{BESIII} and \textsf{Belle 2} experiments, by searching for single-photon plus missing energy events. Within the effective field theory (EFT) framework of General Relativity (GR) together with Nonrelativistic QCD (NRQCD), we are capable of making solid predictions for the corresponding decay rates. It is found that these extremely suppressed decays are completely swamped by the Standard Model background events $J/ψ(Υ)\to γ+ν\barν$. Meanwhile, we also study these rare decay processes in the context of massive gravity, and find the respective decay rates in the limit of vanishing graviton mass drastically differ from their counterparts in GR. Counterintuitive as the failure of smoothly recovering GR results may look, our finding is reminiscent of the van Dam-Veltman-Zakharov (vDVZ) discontinuity widely known in classical gravity, which can be traced to the finite contribution of the helicity-zero graviton in the massless limit. Nevertheless, at this stage we are not certain about the fate of the discontinuity encountered in this work, whether it is merely a pathology or not. If it could be endowed with some physical significance, the future observation of these rare decay channels, would, in principle, shed important light on the nature of gravitation, whether the graviton is strictly massless, or bears a very small but nonzero mass.

hep-ph↗

arXiv2017

Solving the Bars-Green equation for moving mesons in two-dimensional QCD

The two-dimensional QCD in the large $N$ limit, generally referred to as the 't Hooft model, is numerically investigated in the axial gauge in a comprehensive manner. The corresponding Bethe-Salpeter equation for a bound $q\bar{q}$ pair, originally derived by Bars and Green in 1978, was first numerically tackled by Li and collaborators in late 1980s, yet only for the {\it stationary} mesons. In this paper, we make further progress by numerically solving the Bars-Green equation for {\it moving} mesons, ranging from the chiral pion to charmonium. By choosing several different quark masses, we computed the corresponding quark condensates, meson spectra and their decay constants for a variety of meson momenta, and found satisfactory agreement with their counterparts obtained using light-cone gauge, thus numerically verified the gauge and Poincaré invariance of the 't Hooft model. Moreover, we have explicitly confirmed that, as the meson gets more and more boosted, the large component of the Bars-Green wave function indeed approaches the corresponding 't Hooft light-cone wave function, while the small component of the wave function rapidly fades away.

hep-ph↗

arXiv2017

Next-to-leading-order QCD corrections to $e^+e^-\to H+γ$

The associated production of Higgs boson with a hard photon at lepton collider, i.e., $e^+e^-\to Hγ$, is known to bear a rather small cross section in Standard Model, and can serve as a sensitive probe for the potential new physics signals. Similar to the loop-induced Higgs decay channels $H\to γγ, Zγ$, the $e^+e^-\to Hγ$ process also starts at one-loop order provided that the tiny electron mass is neglected. In this work, we calculate the next-to-leading-order (NLO) QCD corrections to this associated $H+γ$ production process, which mainly stem from the gluonic dressing to the top quark loop. The QCD corrections are found to be rather modest at lower center-of-mass energy range ($\sqrt{s}<300$ GeV), thus of negligible impact on Higgs factory such as CEPC. Nevertheless, when the energy is boosted to the ILC energy range ($\sqrt{s}\approx 400$ GeV), QCD corrections may enhance the leading-order cross section by $20\%$. In any event, the $e^+e^-\to Hγ$ process has a maximal production rate $σ_{\rm max}\approx 0.08$ fb around $\sqrt{s}= 250$ GeV, thus CEPC turns out to be the best place to look for this rare Higgs production process. In the high energy limit, the effect of NLO QCD corrections become completely negligible, which can be simply attributed to the different asymptotic scaling behaviors of the LO and NLO cross sections, where the former exhibits a milder decrement $\propto 1/s$ , but the latter undergoes a much faster decrease $\propto 1/s^2$.

hep-ph↗