arXiv subjects

Shivam Singh Kushwah

Publications and source records attributed to Shivam Singh Kushwah.

4 recordsLinked to original sources

arXiv2026

Charge Transport in Magnetized Holographic $\mathcal{M}$-QGP

We investigate DC transport in a top-down construction of thermal QGP-like theories using a holographic $\mathcal{M}$-theoretic background, incorporating quartic curvature corrections. The DC and Hall conductivities are computed from the Dirac-Born-Infeld (DBI) action of the corresponding type-IIA probe $D6$ flavor branes via the reality condition method proposed in arXiv:0708.1994, arXiv:0705.3870. We further analyze pair-production contributions in the presence and absence of an external magnetic field and work out regimes where thermal pair production dominates over charge carrier transport. These findings extend earlier AdS/CFT results to non-conformal, higher-derivative settings relevant to thermal QGP-like theories.

hep-th↗

arXiv2025

$T_c$, Photoproduction, Paramagnetic Anisotropic Plasma, IR Log-Gravitational-DBI Renormalization and $G_2$-Structure Induced (Almost) Contact 3-Structures in Hot Strongly Magnetic MQCD at Intermediate Coupling

After obtaining the flavor $D6$-brane gauge fields/their fluctuations in the type IIA dual of T>T_c QCD-like theories at intermediate coupling (via the ${\cal M}$-theory uplift's ${\cal O}(R^4)$ corrections) in the absence/presence of a strong magnetic field, we compute the photoproduction spectral function and get a nice agreement with gauged supergravity backgrounds arXiv:2204.00024 [hep-th]. We demonstrate from the EoS that the holographic dual, in principle, could correspond to several $T>T_c$ scenarios: anisotropic paramagnetic plasma transitioning via a smooth crossover to exotic matter as the universe cools, the stable wormhole, and a paramagnetic pressure/energy-anisotropic plasma. Given that $T>T_c$ QGP is expected to be paramagnetic, see Bali et al '14, the third possibility appears to be the preferred one. We also show that it is not possible that the anisotropic plasma leads to the formation of a compact star. IR renormalization of the DBI action requires a boundary Log-det-Ricci-tensor counter term. {\it Noting (i) photoproduction spectral function determined from gauge field fluctuations receiving ${\cal O}(R^4)$-corrections, if complexified, include a non-analytic-complexified gauge-coupling dependence, and correspond to Contact 3-Structures; (ii) pressure/energy density, etc. determined from world-volume gauge fields and not ${\cal O}(R^4)$-corrected, if complexified, are analytic in the complexified gauge coupling, and correspond to Almost Contact 3-Structures (AC3S) both induced from the $G_2$ structure of a closed seven-fold, we conjecture (i) the lack of $N$-path connectedness in the parameter space associated with AC3S and C3S arXiv:2211.13186[hep-th] to be equivalent to that gauge field fluctuations can not be finite, and (zero-instanton sector) ${\cal O}(R^4)$ non-renormalized gauge fields produce ${\cal O}(R^4)$-corrected gauge fluctuation; (ii) C3S--UV--> AC3S.

hep-th↗

arXiv2024

Bulk Viscosity, Speed of Sound and Contact Structure at Intermediate Coupling

In the context of strongly coupled thermal QCD-like theories, the bulk viscosity($ζ$)-to-shear viscosity($η$) ratio using the type IIA-theory dual of thermal QCD-like theories was shown to vary like $\frac{1}{3} - c_s^2$ (arXiv:1807.04713) ($c_s$ being the speed of sound), and the same ($\fracζη$) at weak coupling using kinetic theory and finite temperature field theory, was shown and is known to vary like $\left(\frac{1}{3} - c_s^2\right)^2$ (arXiv:1807.04713). The novelties of the results of this paper are that we not only show for the first time from ${\cal M}$ theory that at intermediate coupling, with the inclusion of the ${\cal O}(R^4)$ corrections, the result obtained for $\fracζη$ interpolates between the strong and weak coupling results in a way consistent with lattice computations of $SU(3)$ Gluodynamics (arXiv:0710.3717 [hep-lat]) within statistical errors (and within the temperature range permissible by our ${\cal M}$-theory uplift), but also observe that this behavior is related to the existence of Contact 3-Structures that exist only at intermediate coupling effected by the intermediate-$N$ "MQGP" limit of (arXiv:2211.13186[hep-th]). We also obtain an explicit dependence of $\fracζη$ on (fractional powers of) the temperature-dependent/running gauge coupling (and its temperature derivative), and verify that the weak-coupling result dominates at large temperatures. We further conjecture that the aforementioned fractional-power-dependence of $\fracζη$ on the gauge coupling is related to the lack of ``$N$-connectedness'' in the parameter space of Contact 3-Structures (as shown in arXiv:2211.13186[hep-th]).

hep-th↗

arXiv2024

Pole-Skipping and Chaos in Hot ${\cal M}$QCD

We address the question of whether thermal QCD at high temperature is chaotic from the ${\cal M}$ theory dual of QCD-like theories at intermediate coupling as constructed in arXiv: 2004.07259. The equations of motion of the gauge-invariant combination $Z_s(r)$ of scalar metric perturbations is shown to possess an irregular singular point at the horizon radius $r_h$. Very interestingly, at a specific value of the imaginary frequency and momentum used to read off the analogs of the ''Lyapunov exponent'' $λ_L$ and ''butterfly velocity'' $v_b$ not only does $r_h$ become a regular singular point, but truncating the incoming mode solution of $Z_s(r)$ as a power series around $r_h$, yields a ''missing pole'', i.e., $C_{n, n+1}=0,\ {\rm det}\ M^{(n)}=0, n\in\mathbb{Z}^+$ is satisfied for a single $n\geq3$ depending on the values of the string coupling $g_s$, number of (fractional) $D3$ branes $(M)N$ and flavor $D7$-branes $N_f$ in the parent type IIB set (arXiv:hep-th/0902.1540), e.g., for the QCD(EW-scale)-inspired $N=100, M=N_f=3, g_s=0.1$, one finds a missing pole at $n=3$. For integral $n>3$, truncating $Z_s(r)$ at ${\cal O}((r-r_h)^n)$, yields $C_{n, n+1}=0$ at order $n,\ \forall n\geq3$. Incredibly, (assuming preservation of isotropy in $\mathbb{R}^3$ even with the inclusion of higher derivative corrections) the aforementioned gauge-invariant combination of scalar metric perturbations receives no ${\cal O}(R^4)$ corrections. Hence, (the aforementioned analogs of) $λ_L, v_b$ are unrenormalized up to ${\cal O}(R^4)$ in ${\cal M}$ theory.

hep-th↗