arXiv subjects

Nahid Vasim

Publications and source records attributed to Nahid Vasim.

9 recordsLinked to original sources

arXiv2026

Determining the Proton's Gluon Cloud Geometry from HERA Data

The transverse shape of the proton's small-$x$ gluon distribution is determined from exclusive $J/ψ$ photoproduction at HERA. We derive analytic expressions for the coherent and incoherent diffractive cross sections in the hotspot model at leading twist, enabling a global fit to all 104 available H1 and ZEUS data points, with $|t|<30~{\rm GeV}^2$ and $40\leq W\leq 251~{\rm GeV}$, with $χ^2/{\rm ndf}=0.72$. The gluonic hotspots are resolved into a core of size 0.088(7) fm surrounded by an exponential halo of range 0.20(1) fm, consistent with the gluon-field correlation length and penetration length of the QCD vacuum modeled as a dual superconductor, and with the core-halo structure of flux tubes determined on the lattice. This shape is independent of $x_{I\!P}$ and of the assumed number of hotspots in the range $5\leq N_{hs}\leq 12$. We find the Ginzburg-Landau parameter of the QCD vacuum to be $κ=2.7(7)$, which corresponds to fluxtube geometry in a type-II dual superconductor. The calculated QCD string-tension from the extracted parameter values is in agreement with theoretical calculations on the lattice. The gluonic geometry evolves only through a slow transverse diffusion of the hotspot centres with $α'_{\rm eff.}=0.060(19)~{\rm GeV}^{-2}$, while the incoherent cross section at small $|t|$ is dominated by shot-noise fluctuations at the order of a few gluons per hotspot.

hep-ph↗

arXiv2026

Measuring Dual-Superconductor Length Scales in the QCD Vacuum at HERA

In the dual superconductor picture, the QCD vacuum is a condensate of chromomagnetic monopoles characterised by two lengths: the penetration depth $λ$ of the chromoelectric field and the coherence length $ξ$ of the condensate. These have previously been accessible only through lattice calculations of static flux tubes. We show that they can instead be measured in exclusive diffraction, where the transverse profile of a gluonic hotspot is the vacuum's response to a localised color source. We therefore use the Clem vortex profile to describe the hotspot geometry. Fitting Clem's vortex profile to all 104 available H1 and ZEUS exclusive $J/ψ$ photoproduction points, spanning $|t|<30~{\rm GeV}^2$ and $40\leq W\leq 251~{\rm GeV}$, we obtain $χ^2/{\rm ndf}=0.72$ and extract $λ=0.19(2)~{\rm fm}$, $ξ_v=0.087(13)~{\rm fm}$, and a Ginzburg--Landau parameter $κ=2.7(7)$, corresponding to the type-II regime. The corresponding vector mass $1/λ=1.03(11)~{\rm GeV}$ agrees with lattice determinations of the lightest gluelump. The resulting Ginzburg--Landau string tension implied by $λ$ and $κ$ reproduces the QCD string tension well; the matching requires $α_S=0.37(9)$ compared with $α_S(1/λ^2)=0.36(2)$ in the same LO scheme used for the dipole amplitude calculation.

hep-ph↗

arXiv2026

Small x dynamics of the unpolarised color dipole gluon TMD PDFs for all transverse momenta

A general solution of the Balitsky Kovchegov (BK) equation well describes the partonic behavior within the saturation regime and beyond this limit. This solution can potentially define the unpolarized color dipole gluon transverse momentum dependent (TMD) distribution in the small x regime for full k perp range. In this work, we Fourier-Bessel transform this general S matrix of the BK equation and, within the leading logarithmic approximation, obtain a closed form expression of unpolarised Gluon TMD. The distribution exhibits a smooth and well behaved k perp dependence from low to high transverse momenta. Numerical evaluation for different values of x accessible at the Electron Ion Collider (EIC), shows a characteristic inversion of the x ordering at the saturation scale. The result is in close agreement with the behavior observed in the unintegrated color dipole gluon distribution computed within the Bartels Golec Biernat Kowalski (BGK) model, suggesting that this feature is a model independent signature of gluon saturation.

hep-ph↗

arXiv2024

Spin-flip gluon GTMD $F_{1,2}$ at small-$x$

Until recently the spin-flip processes in the deep inelastic scatterings are thought to be suppressed in the high energy. We found a positive intercept for the spin-flip generalized transverse momentum-dependent parton distribution (GTMDs) ${\rm Re}(F_{1,2})$ as, \begin{eqnarray} {\rm Re}(F_{1,2}) \sim \left(\frac{1}{x}\right)^{{\bar α}_s\left(4\ln2-8/3\right)} \left(\cos 3ϕ_{kΔ} +\cos ϕ_{kΔ}\right). \nonumber \end{eqnarray} This is done by analytically solving the integro-differential evolution equation for ${\rm Re}(F_{1,2})$, recently proposed by Hatta and Zhou, in the dilute regime. Interestingly, the surviving solution corresponds to conformal spin $n=2$ and carries an explicit $\cos 3ϕ_{kΔ} + \cos ϕ_{kΔ}$ azimuthal dependence. As the imaginary part of $F_{1,2}$, is related to the spin-dependent odderon or gluon Siver function and scales as ${\rm Im}(F_{1,2}) \sim x^{0}$, the positive intercept for ${\rm Re}(F_{1,2})$, implies that it is expected to dominate over the gluon Siver function in the small-$x$ limit - and may directly impact the modeling of unpolarised GTMDs and associated spin-flip processes.

hep-ph↗

arXiv2022

Orbital Angular Momentum Distribution of Gluons at Small-$x$ : Analytic interpolation between Ji and Jaffe-Manohar

In this work, we first introduced a generalized Wilson line gauge link that reproduces both staple and near straight links along the light cone in different parameter limits. We then studied the gauge-invariant bi-local orbital angular momentum operator with such a general gauge link, in the framework of Chen et. al. decomposition of gauge fields. At the appropriate combination of limits, the operator reproduces both Jaffe-Manohar and Ji's operator structure and offers a continuous analytical interpolation between the two.

hep-ph↗

arXiv2022

Saturation Correction to Projectile -- some ${\cal{O}}(g^5)$ results within LCPT

We revisited the first saturation correction to projectile in nucleus-nucleus collisions \cite{Chirilli:2015tea} within diagrammatic light cone perturbation theory (LCPT). To get an analytic expression for the saturation correction in projectile authors in \cite{Chirilli:2015tea} calculated complete ${\cal{O}}(g^3)$ amplitudes, where the main results are given in transverse coordinate space. However to find the complete first saturation correction in the projectile, one needs to calculate both ${\cal{O}}(g^3)$ and ${\cal{O}}(g^5)$ gluon production amplitudes. We here present a detailed calculation of a sample graph at ${\cal{O}}(g^5)$ in diagrammatic light cone perturbation theory.

hep-ph↗

arXiv2021

Dynamics of QCD Matter -- current status

In this article, there are 18 sections discussing various current topics in the field of relativistic heavy-ion collisions and related phenomena, which will serve as a snapshot of the current state of the art. Section 1 reviews experimental results of some recent light-flavored particle production data from ALICE collaboration. Other sections are mostly theoretical in nature. Very strong but transient magnetic field created in relativistic heavy-ion collisions could have important observational consequences. This has generated a lot of theoretical activity in the last decade. Sections 2, 7, 9, 10 and 11 deal with the effects of the magnetic field on the properties of the QCD matter. There are several unanswered questions about the QCD phase diagram. Sections 3, 11 and 18 discuss various aspects of the QCD phase diagram and phase transitions. Recent years have witnessed interesting developments in foundational aspects of hydrodynamics and their application to heavy-ion collisions. Sections 12, 15, 16 and 17 of this article probe some aspects of this exciting field. Transport coefficients together with their temperature- and density-dependence, are essential inputs in hydrodynamical calculations. Sections 5, 8 and 14 deal with calculation/estimation of various transport coefficients (shear and bulk viscosity, thermal conductivity, relaxation times, etc.) of quark matter and hadronic matter. Sections 4, 6 and 13 deals with interesting new developments in the field. Section 4 discusses color dipole gluon distribution function at small transverse momentum in the form of a series of Bells polynomials. Section 6 discusses the properties of Higgs boson in the quark gluon plasma using Higgs-quark interaction. Section 13 discusses modification of coalescence model to incorporate viscous corrections and application of this model.

hep-ph↗

arXiv2019

Infrared renormalon effects in color dipole TMD PDF at small-$x$

The uncertainties from the infrared renormalons in the (color dipole) gluon distribution is estimated. It is shown that non-linear saturation effects at small-$x$ shift the first IR pole at the Borel plane from $2/β_2$ to $1/β_2$. As a consequence the estimated uncertainty is found to be ${\cal O}\left(Λ_{\rm QCD}^2\right)$ instead of ${\cal O}\left(Λ_{\rm QCD}^4\right)$.

hep-ph↗

arXiv2018

Unintegrated dipole gluon distribution at small transverse momentum

We derive analytical results for unintegrated color dipole gluon distribution function at small transverse momentum. By Fourier transforming the $S$-matrix for large dipoles we derive the results in the form of a series of Bells polynomials. Interestingly, when resumming the series in leading log accuracy, the results showing up striking similarity with the Sudakov form factor with role play of coupling is being done by a constant that stems from the saddle point condition along the saturation line.

hep-ph↗