Quark Part Research Articles

Effect of dark matter interaction on hybrid star in the light of the recent astrophysical observations

We have explored the effect of dark matter interaction on hybrid star (HS) in the light of recent astrophysical observational constraints. The presence of dark matter is assumed to be there in both the hadron as well as the quark sector. The dark matter particle interacts with both hadron and quark matter through the exchange of a scalar as well as a vector meson. The equation of state (EOS) of the hadron part is computed using the NL3 version of the relativistic mean field(RMF) model, whereas the quark part is taken care of using the well-known MIT Bag model with the vector interaction. We investigate the effect of the dark matter density and the mass of the dark matter particle on various observables like mass, radius, tidal deformability of the dark matter admixed hybrid star(DMAHS).In this study, we have noted an intriguing aspect that is the speed of sound in the DMAHS is insensitive to both the mass as well as the density of dark matter. We also observe a striking similarity in the variation of transition mass and its corresponding radius, as well as the maximum mass of neutron stars, with dark matter density and mass. We employ observational constraints from neutron stars to narrow down the allowed range of the parameters of dark matter.

Angular momentum distribution of a quark dressed with a gluon

We investigate the different decompositions of the angular momentum in QCD for a relativistic spin 1/2 composite state, namely a quark dressed with a gluon. We use light-front Hamiltonian perturbation theory, and in the light-front gauge, use the two-component framework by eliminating the constrained degrees of freedom. We also investigate the different decompositions of the angular momentum at the level of two-dimensional densities in the front form, including the effect of the so-called potential term. In this work, we consider the contribution coming from the quark part of the energy-momentum tensor. We contrast the different decompositions and also compare with other calculations in the literature. We also present the gravitational form factor related to the antisymmetric part of the energy-momentum tensor. Published by the American Physical Society 2024

Gravitational transition form factors of N → ∆ via QCD light-cone sum rules

We present the first direct calculation on the gravitational form factors (GFFs) of the N → ∆ transition using an analytic method, the QCD light-cone sum rules. The matrix element of the quark part of the energy momentum tensor current sandwiched between the nucleon and ∆ states are parameterized in terms of five independent conserved and four independent non-conserved GFFs, for calculation of which we use the distribution amplitudes (DAs) of the on-shell nucleon expanded in terms of functions with different twists. We present the results for two sets of light-cone input parameters. The results indicate that the behavior of the form factors with respect to Q2 are described by multipole fit functions. Our results may be checked by other phenomenological models including the Lattice QCD as well as future related experiments.

Twist-four gravitational form factor at NNLO QCD from trace anomaly constraints

It is known that the trace anomaly in the QCD energy-momentum tensor Tμν can be attributed to the anomalies for each of the gauge-invariant quark part and gluon part of Tμν, and their explicit three-loop formulas have been derived in the overline{textrm{MS}} scheme in the dimensional regularization. The matrix elements of this quark/gluon decomposition of the QCD trace anomaly allow us to derive the QCD constraints on the hadron’s gravitational form factors, in particular, on the twist-four gravitational form factor, {overline{C}}_{q,g} . Using the three-loop quark/gluon trace anomaly formulas, we calculate the forward (zero momentum transfer) value of the twist-four gravitational form factor {overline{C}}_{q,g} at the next-to-next-to-leading-order (NNLO) accuracy. We present quantitative results for nucleon as well as for pion, leading to a model-independent determination of the forward value of {overline{C}}_{q,g} . We find quite different pattern in the obtained results between the nucleon and the pion. In particular, for the nucleon, the present information from experiment and lattice QCD on the nonperturbative matrix elements arising in our NNLO formula allows us to obtain a prediction of the forward value of {overline{C}}_{q,g} at the accuracy of a few percent level.

Gravitational form factors of N(1535) in QCD

We calculate the gravitational form factors of the excited N(1535) state with the quantum numbers I(JP)=12(12−) via light cone QCD sum rules (LCSR). To this end, we consider the quark part of the energy-momentum tensor (EMT) current and use the general form of the nucleon's interpolating field as well as the distribution amplitudes (DAs) of N(1535). As both the nucleon and N(1535) couple to the same current, the N(1535)→N gravitational transition form factors are entered to the calculations as the main input parameters. First we revisit the transitional gravitational form factors of N(1535)→N, then extract the values of the form factors of the N(1535) excited state. We observe that the gravitational form factors of N(1535) in terms of Q2 are well described by the multipole fit function. As a byproduct, we also calculate the pressure and energy density at the center of N(1535) and estimate its mechanical radius.

Gravitational formfactors of the ρ , π , and K mesons in the light-cone QCD sum rules

Taking into account only the quark part of the energy-momentum tensor, the gravitational form-factors of the $ \pi $ and $ K $ mesons are calculated within the light-cone sum rules method using the distribution amplitudes of these pseudoscalar mesons. The $ Q^2 $-dependence of the relevant form-factors are obtained. Moreover, the mean square mass radii of $ \pi $ and $ K $ mesons are calculated and we find that $ \sqrt{\langle r_{\pi}^2 \rangle} = 0.31 {\rm\ fm} $ and $ \sqrt{\langle r_{K}^2 \rangle} = 0.24 {\rm\ fm} $, respectively. We compare our results for gravitational form-factors with the existing literature results. We obtain that our results on $ \sqrt{\langle r_\pi^2 \rangle} $ are in good agreement with the predictions of NJL model, AdS/QCD model, and other models as well as with the existing experimental data.

Gravitational form factors of hyperons in light-cone QCD

The quark parts of the gravitational form factors of hyperons are calculated by means of the light-cone QCD sum rule. In the calculations, the distribution amplitudes (DAs) of $\Sigma$, $\Xi$ and $\Lambda$ together with the general forms of their interpolating currents as well as the quark part of the energy-momentum tensor current are used. These form factors can provide information on their mass and distributions of the angular momentum, energy and pressure inside the hyperons. It is obtained that the $Q^2$ dependencies of the hyperon gravitational form factors are nicely characterized by a multipole fit function. Using the fits of these form factors, some mechanical properties such as the mechanical radius of the hyperons and the pressure and energy distributions at the center of these particles are obtained. The obtained results can help us in better understanding of the internal structures of these baryons and the QCD as theory of the strong interaction.

Gravitational transition form factors of N(1535)→N

We employ the quark part of the symmetric energy-momentum tensor current to calculate the transition gravitational form factors of the $N(1535) \rightarrow N$ by means of the light cone QCD sum rule formalism. In numerical analysis, we use two different sets of the shape parameters in the distribution amplitudes of the $ N(1535) $ baryon and the general form of the nucleon's interpolating current. It is seen that the momentum squared dependence of the gravitational form factors can be well described by the p-pole fit function. The results obtained by using two sets of parameters are found to be quite different from each other and the $N(1535) \rightarrow N$ transition gravitational form factors depend highly on the shape parameters of the distribution amplitudes of the $N(1535)$ state that parametrize relative orbital angular momentum of the constituent quarks.

Rare hyperon decays with missing energy

We explore the strangeness-changing decays of the lightest hyperons into another baryon plus missing energy within and beyond the standard model (SM). In the SM these processes arise from the loop-induced quark transition sto dnu overline{nu} and their branching fractions are estimated to be less than 10−11. In the presence of new physics (NP) the rates of these hyperon decays with missing energy could increase significantly with respect to the SM expectations because of modifications to the SM process or contributions from additional modes with new invisible particles. Adopting a model-independent approach and taking into account constraints from the kaon sector, we find that the current data on Kto pi nu overline{nu} do not permit sizable NP impact on the hyperon decays via underlying operators having mainly parity-even quark parts. In contrast, NP operators with primarily parity-odd quark parts are much less restricted by the existing bounds on K → invisible and Kto pi pi nu overline{nu} and consequently could produce substantially amplifying effects on the hyperon modes. Their NP-enhanced branching fractions could reach levels potentially observable in the ongoing BESIII experiment.

Three-loop formula for quark and gluon contributions to the QCD trace anomaly

In the QCD energy-momentum tensor Tμν, the terms that contribute to physical matrix elements are expressed as the sum of the gauge-invariant quark part and gluon part. Each part undergoes the renormalization due to the interactions among quarks and gluons, although the total tensor Tμν is not renormalized thanks to the conservation of energy and momentum. Recently it has been shown that, through the renormalization, each of the quark and gluon parts of Tμν receives a definite amount of anomalous trace contribution, such that their sum reproduces the well-known QCD trace anomaly, {T}_{mu}^{mu }=left(beta /2gright){F}^{mu nu }{F}_{mu nu }+mleft(1+{gamma}_mright)overline{psi}psi , and the corresponding formulas have been derived up to two-loop order. We extend this result to the three-loop order, working out all the relevant three-loop renormalization structure for the quark and gluon energy-momentum tensors in the (modified) minimal subtraction scheme in the dimensional regularization. We apply our three-loop formula of the quark/gluon decomposition of the trace anomaly to calculate the anomaly-induced mass structure of nucleons as well as pions.

Nucleon gravitational form factors from instantons: forces between quark and gluon subsystems

Using the instanton picture of the QCD vacuum we compute the nucleon {overline{c}}^Q(t) form factor of the quark part of the energy momentum tensor (EMT). This form factor describes the non-conservation of the quark part of EMT and contributes to the quark pressure distribution inside the nucleon. Also it can be interpreted in terms of forces between quark and gluon subsystems inside the nucleon. We show that this form factor is parametrically small in the instanton packing fraction. Numerically we obtain for the nucleon EMT a small value of {overline{c}}^Q(t) {overline{c}}^Q(0)simeq 1.4 cdotp {10}^{-2} at the low normalisation point of ∼ 0.4 GeV2. This smallness implies interesting physics picture — the forces between quark and gluon mechanical subsystems are smaller than the forces inside each subsystem. The forces from side of gluon subsystem squeeze the quark subsystem — they are compression forces. Additionally, the smallness of {overline{c}}^Q(t) might justify Teryaev’s equipartition conjecture. We estimate that the contribution of {overline{c}}^Q(t) to the pressure distribution inside the nucleon is in the range of 1-20% relative to the contribution of the quark D-term.

Properties of strange quark matter objects with two types of surface treatments

We study strange quark matter (SQM) objects ranging from strangelets to strange stars based on our recently proposed unified description. The important interface effects are investigated by adopting a constant surface tension as well as the multiple reflection expansion (MRE) method. It is shown that the properties of SQM objects are strongly affected by the different surface treatments. In the former case, strangelets are more compact, an electric dipole is predicted on the surface of the quark part, and a local minimum of the energy per baryon appears for unusually small values of the surface tension. In the latter case, on the other hand, an electric potential well is formed, and the energy per baryon decreases monotonically with the SQM object's size. It is found that the MRE scenario coincides with the constant-surface-tension one if realistic values are considered. However, the effects of quark depletion on the quark-vacuum interface cannot be solely described by a constant surface tension. Thus we conclude that the MRE scenario is more reasonable.

From strangelets to strange stars: a unified description

The conventionally separated treatments for strangelets and strange stars are now unified with a more comprehensive theoretical description for objects ranging from strangelets to strange stars. After constraining the model parameter according to the Witten–Bodmer hypothesis and observational mass–radius probability distribution of pulsars, we investigate the properties of this kind of objects. It is found that the energy per baryon decreases monotonically with increasing baryon number and reaches its minimum at the maximum baryon number, corresponding to the most massive strange star. Due to the quark depletion, an electric potential well is formed on the surface of the quark part. For a rotational bare strange star, a magnetic field with the typical strength in pulsars is generated.

Dark Matter and gauged flavor symmetries

We investigate the phenomenology of flavored dark matter (DM). DM stability is guaranteed by an accidental ${\mathcal Z}_3$ symmetry, a subgroup of the standard model (SM) flavor group that is not broken by the SM Yukawa interactions. We consider an explicit realization where the quark part of the SM flavor group is fully gauged. If the dominant interactions between DM and visible sector are through flavor gauge bosons, as we show for Dirac fermion flavored DM, then the DM mass is bounded between roughly $0.5$ TeV and $5$ TeV if the DM multiplet mass is split only radiatively. In general, however, no such relation exists. We demonstrate this using scalar flavored DM where the main interaction with the SM is through the Higgs portal. For both cases we derive constraints from flavor, cosmology, direct and indirect DM detection, and collider searches.

Crystalline chiral condensates as a component of compact stars

We investigate the influence of spatially inhomogeneous chiral symmetry-breaking condensates in a magnetic field background on the equation of state for compact stellar objects. After building a hybrid star composed of nuclear and quark matter using the Maxwell construction, we find, by solving the Tolman-Oppenheimer-Volkoff equations for stellar equilibrium, that our equation of state supports stars with masses around 2 $M_\odot$ for values of the magnetic field that are in accordance with those inferred from magnetar data. The inclusion of a weak vector interaction term in the quark part allows one to reach 2 solar masses for relatively small central magnetic fields, making this composition a viable possibility for describing the internal degrees of freedom of this class of astrophysical objects.

Dyson-Schwinger approach to Hamiltonian quantum chromodynamics

The general method for treating non-Gaussian wave functionals in the Hamiltonian formulation of a quantum field theory, which was previously proposed and developed for Yang-Mills theory in Coulomb gauge, is generalized to full QCD. For this purpose the quark part of the QCD vacuum wave functional is expressed in the basis of coherent fermion states, which are defined in terms of Grassmann variables. Our variational Ansatz for the QCD vacuum wave functional is assumed to be given by exponentials of polynomials in the occurring fields and, furthermore, contains an explicit coupling of the quarks to the gluons. Exploiting Dyson-Schwinger equation techniques, we express the various $n$-point functions, which are required for the expectation values of observables like the Hamiltonian, in terms of the variational kernels of our trial Ansatz. Finally, the equations of motion for these variational kernels are derived by minimizing the energy density.

Minimal lepton flavor violation implications of the b → s anomalies

The latest measurements of rare b → s decays in the LHCb experiment have led to results in tension with the predictions of the standard model (SM), including a tentative indication of the violation of lepton flavor universality. Assuming that this situation will persist because of new physics, we explore some of the potential consequences in the context of the SM extended with the seesaw mechanism involving right-handed neutrinos plus effective dimension-six lepton-quark operators under the framework of minimal flavor violation. We focus on a couple of such operators which can accommodate the LHCb anomalies and conform to the minimal flavor violation hypothesis in both their lepton and quark parts. We examine specifically the lepton-flavor-violating decays B → K(∗)ℓℓ′, B s → ϕℓℓ′, B → (π, ρ)ℓℓ′, and B d,s → ℓℓ′, as well as K L → eμ and K → πeμ, induced by such operators. The estimated branching fractions of some of these decay modes with μτ in the final states are allowed by the pertinent experimental constraints to reach a few times 10−7 if other operators do not yield competitive effects. We also look at the implications for B → K(∗)νν and K → πνν, finding that their rates can be a few times larger than their SM values. These results are testable in future experiments.

QCD at low energy: a many-body approach

A review is given on recent results in the treatment of an arbitrary number of orbital levels in low energy QCD. For the pure quark part, analytic results for the dominant part of the Hamiltonian are presented. Possible extensions, including dynamic gluons, are discussed.

Check of the gluon-reggeization condition in the next-to-leading order: Quark part

The bootstrap condition for gluon production in multi-Regge kinematics is considered in the next-to-leading order. Bootstrap conditions follow fromthe requirement that the Regge formof amplitudes in QCD be consistent with s-channel unitarity and are nonlinear relations between the Reggeized-gluon trajectory and vertex functions. Their fulfillment ensures the Reggeization of gluons—that is, the Regge form of both elastic and inelastic amplitudes. This condition is the only one that has not yet been verified. The demonstration of its fulfillment is the ultimate step in proving the Reggeization of gluons in the next-to-leading logarithmic approximation. In the present article, this is done for the quark part of the bootstrap condition.

Implication on the pion distribution amplitude from the pion-photon transition form factor with the newBABARdata

The new BABAR data on the pion-photon transition form factor arouses people's interest for the determination of the pion distribution amplitude. To explain the data, we take both the leading valence quark state's and the nonvalence quark state's contributions into consideration, where the valence quark part up to next-to-leading order is presented and the nonvalence quark part is estimated by a phenomenological model based on its limiting behavior at both ${Q}^{2}\ensuremath{\rightarrow}0$ and ${Q}^{2}\ensuremath{\rightarrow}\ensuremath{\infty}$. Our results show that to be consistent with the new BABAR data at the large ${Q}^{2}$ region, a broader amplitude other than the asymptoticlike pion distribution amplitude should be adopted. The broadness of the pion distribution amplitude is controlled by a parameter $B$. It has been found that the new BABAR data at low and high energy regions can be explained simultaneously by setting $B$ to be around 0.60, in which the pion distribution amplitude is closed to the Chernyak-Zhitnitsky form.