arXiv subjects

Neetika Sharma

Publications and source records attributed to Neetika Sharma.

At least 19 recordsLinked to original sources

arXiv2024

Exclusive diffractive J/psi and Psi(2S) production in dipole model using a holographic AdS/QCD light front wavefunction with longitudinal confinement

We use an anti-de Sitter/Quantum Chromodynamics (AdS/QCD) based holographic light-front wavefunction (LFWF) for vector meson, in conjunction with the dipole model to investigate the cross-sections data for the diffractive and exclusive J/psi and Psi(2S) production. We confront the experimental data using a new explicit form of the holographic LFWF, where the longitudinal confinement dynamics in light-front Schrodinger equation is captured by 't Hooft equation of (1+1)-dim, in large Nc approximation, in addition to the transverse confinement dynamics governed by the confining mass scale parameter, kappa in vector mesons. We obtain the LFWF parameters from fitting to the exclusive J/psi electro production data from electron-proton collision at HERA collider for m_c = 1.27 GeV. Our results suggest that the dipole model together with holographic meson LFWFs with longitudinal confinement is able to give a successful description for differential scattering cross-section for exclusive J/psi electro production for H1 and ZEUS data. We also predict the rapidity distributions of differential scattering cross-section and total photo production of J/psi and Psi(2S) states in proton-proton ultra-peripheral collisions(UPCs) at center of mass energy sqrt s = 7, 13 TeV. Using the minimum set of parameters, our predictions for the UPCs are in good agreement with the recent experimental observations of UPCs at ALICE and LHCb Collaborations.

hep-ph↗

arXiv2021

Extending light-front holographic QCD using the 't Hooft Equation

We show the 't Hooft Equation and the light-front holographic Schrödinger Equation are complementary to each other in governing the transverse and longitudinal dynamics of colour confinement in quark-antiquark mesons. Together, they predict remarkably well the light, heavy-light and heavy-heavy meson spectroscopic data. The universal emerging hadronic scale of light-front holography, $κ\approx 0.5$ GeV, controls the transverse dynamics of confinement in all mesons. In heavy-heavy mesons, it also coincides numerically with the 't Hooft coupling which governs longitudinal confinement, thus reflecting the restoration of manifest 3-dimensional rotational symmetry

hep-ph↗

arXiv2019

Non-thermal X-rays from Colliding Wind Shock Acceleration in the Massive Binary Eta Carinae

Cosmic-ray acceleration has been a long-standing mystery and despite more than a century of study, we still do not have a complete census of acceleration mechanisms. The collision of strong stellar winds in massive binary systems creates powerful shocks, which have been expected to produce high-energy cosmic-rays through Fermi acceleration at the shock interface. The accelerated particles should collide with stellar photons or ambient material, producing non-thermal emission observable in X-rays and gamma-rays. The supermassive binary star eta Carinae drives the strongest colliding wind shock in the solar neighborhood. Observations with non-focusing high-energy observatories indicate a high energy source near eta Carinae, but have been unable to conclusively identify eta Carinae as the source because of their relatively poor angular resolution. Here we present the first direct focussing observations of the non-thermal source in the extremely hard X-ray band, which is found to be spatially coincident with the star within several arc-seconds. These observations show that the source of non-thermal X-rays varies with the orbital phase of the binary, and that the photon index of the emission is similar to that derived through analysis of the gamma-ray spectrum. This is conclusive evidence that the high-energy emission indeed originates from non-thermal particles accelerated at colliding wind shocks.

astro-ph.HE↗

arXiv2017

Gravitational form factors and angular momentum densities in light-front quark-diquark model

We investigate the gravitational form factors (GFFs) and the longitudinal momentum densities ($p^+$ densities) for proton in a light-front quark-diquark model. The light-front wave functions are constructed from the soft-wall AdS/QCD prediction. The contributions from both the scalar and the axial vector diquarks are considered here. The results are compared with the consequences of a parametrization of nucleon generalized parton distributions (GPDs) in the light of recent MRST measurements of parton distribution functions (PDFs) and a soft-wall AdS/QCD model. The spatial distribution of angular momentum for up and down quarks inside the nucleon has been presented. At the density level, we illustrate different definitions of angular momentum explicitly for an up and down quark in the light-front quark-diquark model inspired by AdS/QCD.

hep-ph↗

arXiv2017

Predictions for Diffractive $ϕ$ Meson Production Using an AdS/QCD Light-front Wavefunction

We compute the rate for diffractive $ϕ$ electro-production using the Color Glass Condensate dipole model. The model parameters are obtained from fits to the most recent combined HERA data on inclusive deep inelastic scattering. As for the $ϕ$ meson, we use an AdS/QCD holographic light front wavefunction. Our predictions are compared to the available data collected at the HERA collider.

hep-ph↗

arXiv2016

Momentum transfer dependence of generalized parton distributions

We revisit the model for parametrization of momentum dependence of nucleon generalized parton distributions in the light of recent MRST measurements of parton distribution functions. Our parametrization method with minimum set of free parameters give a sufficiently good description of data for Dirac and Pauli electromagnetic form factors of proton and neutron at small and intermediate values of momentum transfer. We also calculate the GPDs for up and down quark by decomposing the electromagnetic form factors for nucleon using the charge and isospin symmetry and also study the evolution of GPDs to a higher scale. We further investigate the transverse charge densities for both the unpolarized and transversely polarized nucleon and compare our results with the Kelly's distribution.

hep-ph↗

arXiv2016

Diffractive $ρ$ and $ϕ$ production at HERA using an AdS/QCD holographic light-front meson wavefunction

We use an anti-de Sitter/Quantum Chromodynamics (AdS/QCD) holographic light-front wavefunction for the $ρ$ and $ϕ$ mesons, in conjunction with the Color Glass Condensate (CGC) dipole cross-section whose parameters are fitted to the most recent 2015 high precision HERA data on inclusive Deep Inelastic Scattering (DIS), in order to predict the cross-sections for diffractive $ρ$ and $ϕ$ electroproduction. Our results suggest that the holographic meson light-front wavefunction is able to give a simultaneous description of the $ρ$ and $ϕ$ production data provided we use a set of light quark masses with $m_{u,d}$ < $m_{s}$ ~ 0.14 GeV.

hep-ph↗

arXiv2016

Diffractive $ϕ$ electroproduction with a holographic meson wavefunction

We predict the cross-section for diffractive $ϕ$ electroproduction within the dipole model, using a holographic meson wavefunction for the $ϕ$. For the dipole cross-section, we use the Color Glass Condensate dipole model whose parameters are fitted to the latest 2015 combined HERA data on Deep Inelastic Scattering. Choosing a strange quark mass of $0.14$ GeV, we find good agreement with the available data.

hep-ph↗

arXiv2016

Hard gluon evolution of nucleon Generalized parton distributions in the Light-front quark model

We incorporate the perturbative evolution effects in the generalized parton distributions (GPDs) calculated in effective light-front quark model for the nucleon. The perturbative effects enters into formalism through the evolution of GPDs according to the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi-like (DGLAP) equation. We obtain the evolved GPDs in the momentum space and transverse impact parameter space. We observe that combining the light front quark model with the perturbative evolution effects, give the effective model for studying the phenomenological GPDs

hep-ph↗

arXiv2016

Phase separated behavior in Yttrium doped CaMnO3

The effect of electron doping on the structural, transport, and magnetic properties of Mn (IV) - rich Ca1-xYxMnO3 (x < 0.2) samples have been investigated using neutron diffraction, neutron depolarization, magnetization and resistivity techniques. The temperature dependence of resistivity follows the small polaron model and the activation energy exhibits a minimum for x=0.1 sample. A phase separated magnetic ground state consisting of ferromagnetic domains (~7microns) embedded in G-type antiferromagnetic matrix is observed in the sample, x = 0.1. The transition to the long range magnetically ordered state in this sample is preceded by a Griffiths phase. On lowering temperature below 300K a structural transition from orthorhombic structure (Pnma) to a monoclinic structure (P21/m) is observed in the case of x=0.2 sample. The ferromagnetic behavior in this case is suppressed and the antiferromagnetic ordering is described by coexisting C-type and G-type magnetic structures corresponding to the monoclinic and orthorhombic phases, respectively.<

cond-mat.mtrl-sci↗

arXiv2016

Eta Carinae's Thermal X-ray Tail Measured with XMM-Newton and NuSTAR

The evolved, massive highly eccentric binary system, eta Carinae, underwent a periastron passage in the summer of 2014. We obtained two coordinated X-ray observations with XMM-Newton and NuSTAR during the elevated X-ray flux state and just before the X-ray minimum flux state around this passage. These NuSTAR observations clearly detected X-ray emission associated with eta Car extending up to ~50 keV for the first time. The NuSTAR spectrum above 10 keV can be fit with the bremsstrahlung tail from a kT ~6 keV plasma. This temperature is Delta kT ~2 keV higher than those measured from the iron K emission line complex, if the shocked gas is in collisional ionization equilibrium. This result may suggest that the companion star's pre-shock wind velocity is underestimated. The NuSTAR observation near the X-ray minimum state showed a gradual decline in the X-ray emission by 40% at energies above 5 keV in a day, the largest rate of change of the X-ray flux yet observed in individual eta Car observations. The column density to the hardest emission component, NH ~1e24 cm-2, marked one of the highest values ever observed for eta Car, strongly suggesting increased obscuration of the wind-wind colliding X-ray emission by the thick primary stellar wind prior to superior conjunction. Neither observation detected the power-law component in the extremely hard band that INTEGRAL and Suzaku observed prior to 2011. If the non-detection by NuSTAR is caused by absorption, the power-law source must be small and located very near the WWC apex. Alternatively, it may be that the power-law source is not related to either eta Car or the GeV gamma-ray source.

astro-ph.SR↗

arXiv2015

Spin-Phonon Coupling and High-Temperature Phase Transition in Multiferroic Material YMnO3

We have carried out temperature-dependent inelastic neutron scattering measurements of YMnO3 over the temperature range 50 - 1303 K, covering both the antiferromagnetic to paramagnetic transition (70 K), as well as the ferroelectric to paraelectric transition (1258 K). Measurements are accompanied by first principles calculations of phonon spectra for the sake of interpretation and analysis of the measured phonon spectra in the room temperature ferroelectric (P63cm) and high temperature paraelectric (P63/mmc) hexagonal phases of YMnO3. The comparison of the experimental and first-principles calculated phonon spectra highlight unambiguously a spin-phonon coupling character in YMnO3. This is further supported by the pronounced differences in the magnetic and non-magnetic phonon calculations. The calculated atomistic partial phonon contributions of the Y and Mn atoms are not affected by inclusion of magnetic interactions, whereas the dynamical contribution of the O atoms is found tochange. This highlights the role of the super-exchange interactions between the magnetic Mn cations, mediated by O bridges. Phonon dispersion relations have also been calculated, in the entire Brillouin zone, for both the hexagonal phases. In the high-temperature phase, unstable phonon mode at the K point is highlighted. The displacement pattern at the K-point indicates that the freezing of this mode along with the stable mode at the Γ-point may lead to a stabilization of the low-temperature (P63cm) phase, and inducing ferroelectricity. Further, we have also estimated the mode Grüneisen parameter and volume thermal expansion behavior. The latter is found to agree with the available experimental data.

cond-mat.mtrl-sci↗

arXiv2014

Generalized parton distributions in the soft-wall model of AdS/QCD

We present a numerical analysis of helicity independent nucleon generalized parton distributions (GPDs) using the known formalism based on inclusion of higher Fock states in the soft-wall approach of the anti-de Sitter/QCD model. We calculate the momentum space GPDs by matching the electromagnetic form factors in the AdS model to the sum rules in QCD. We investigate their Mellin moments, transverse impact parameter GPDs, transverse mean square radius, and transverse width. We further extend this work to investigate the charge and anomalous magnetization densities for both unpolarized and transversely polarized nucleons. A comparison of results on density functions with phenomenological parametrization is also presented.

hep-ph↗

arXiv2014

Magnetic and dielectric behavior in YMn1-xFexO3 (x less than or equal to 0.5)

The role of doping Fe on the structural, magnetic and dielectric properties of frustrated antiferromagnet YMn1-xFexO3 (x less than or equal to 0.5) has been investigated. The neutron diffraction analysis shows that the structure of these polycrystalline samples changes from hexagonal phase (space group P63cm) to orthorhombic phase (space group Pnma) for x > 0.2. The frustration parameter decreases with Fe substitution. All the compounds are antiferromagnetic and the magnetic structure is described as a mixture of Γ3 and Γ4 irreducible representation (IR) in the hexagonal phase and the ratio of these two IRs is found to vary with Fe doping (x less than or equal to 0.2). A continuous spin reorientation as a function of temperature is observed in these samples. The magnetic ground state in the orthorhombic phase of the higher doped samples (x greater than or equal to 0.3) is explained by taking Γ1 (GxCyAz) representation of Pnma setting. In YMnO3 suppression of dielectric constant ε is observed below Tn indicative of magnetoelectric coupling. This anomalous behavior reduces in Fe doped samples. The dielectric constant is found to be correlated with the magnetic moment (M) obtained from neutron diffraction experiments and follows a M^2 behavior close to Tn in agreement with Landau theory.

cond-mat.mtrl-sci↗

arXiv2013

Magnetic and Structural properties of $MnV_{2}O_{4}$

Magnetization, neutron diffraction and X-ray diffraction of Zn doped MnV2O4 as a function of temperature have been measured and the critical exponents and magnetocaloric effect of this system have been estimated. It is observed, that with increase in Zn substitution the noncollinear orientation of Mn spins with the V spins decreases which effectively leads to the decrease of structural transition temperature more rapidly than Curie temperature. It has been shown that the obtained values of β, γ and δ from different methods match very well. These values do not belong to universal class and the values are in between the 3D Heisenberg model and mean field interaction model. The magnetization data follow the scaling equation and collapse into two branches indicating that the calculated critical exponents and critical temperature are unambiguous and intrinsic to the system. The observed double peaks in magneto-caloric curve of Mn0.95Zn0.05V2O4 is due to the strong distortion of VO6 octahedra.

cond-mat.str-el↗

arXiv2013

Quadrupole moments of low lying baryons with spin 1/2^+, spin 3/2^+, and spin 3/2^+ \to 1/2^+ transitions

The chiral constituent quark model (\chiCQM) with general parameterization (GP) method has been formulated to calculate the quadrupole moments of the spin 3/2^+ decuplet baryons and spin 3/2^+ \to 1/2^+ transitions. The implications of such a model have been investigated in detail for the effects of symmetry breaking and GP parameters pertaining to the two- and three-quark contributions. It is found that the \chiCQM is successful in giving a quantitative and qualitative description of the quadrupole moments.

hep-ph↗

arXiv2013

Magnetic moments of the low-lying {1/2}^- octet baryon resonances

The magnetic moments of the negative parity octet resonances with spin {1/2}: $N^*$(1535), $N^*$(1650), $Σ^*$(1620), and $Ξ^*$(1690) have been calculated within the framework of the chiral constituent quark model. In this approach, the presence of the polarized $q\bar{q}$ pairs (or the meson cloud, in other words) is considered by using the Lagrangian for Goldstone boson emission from the constituent quarks. Further, the explicit contributions coming from the spin and orbital angular momentum, including the effects of the configurations mixing between the states with different spins, are obtained. The motivation for these calculations comes from the recent interest in experimental measurement of the magnetic moment of the ${S_{11}(1535)}$ resonance and of similar calculations being done within lattice quantum chromodynamics approaches. Our results can be compared with those expected to come from these sources.

hep-ph↗

arXiv2012

Magnetic moments of the low-lying $J^P=\,1/2^-$, $3/2^-$ $Λ$ resonances within the framework of the chiral quark model

The magnetic moments of the low-lying spin-parity $J^P=$ $1/2^-$, $3/2^-$ $Λ$ resonances, like, for example, $Λ(1405)$ $1/2^-$, $Λ(1520)$ $3/2^-$, as well as their transition magnetic moments, are calculated using the chiral quark model. The results found are compared with those obtained from the nonrelativistic quark model and those of unitary chiral theories, where some of these states are generated through the dynamics of two hadron coupled channels and their unitarization.

nucl-th↗