arXiv subjects

Arnab Rudra

Publications and source records attributed to Arnab Rudra.

16 recordsLinked to original sources

arXiv2026

$\texttt{SMaSH}$ : Simplify Massive Spinor Helicity

We present $\texttt{SMaSH}$, a $\texttt{Mathematica}$ package to do spinor helicity computations in four spacetime dimensions $\href{https://github.com/aakash-kmr/SMaSH}{\text{(github)}}$. It can handle massive spinor helicity computations with explicit little group indices which is a novel feature. It can also handle massless as well as off-shell spinor helicity variables. It is designed to compute perturbative computations; it comes with predefined three point amplitudes and propagators for any masses and spins (arXiv:1709.04891). It can implement the high energy limit over an expression, check the discrete $\tt{C,P,T}$ transformations, compute contact terms and impose gauge invariance for any scattering process. We have shown the usage of such functions for computing gauge invariant Weinberg minimal amplitudes (arXiv:2506:12431, arXiv:2504:06343). The package can also generate both real and complex numerical kinematics for any $n$-point scattering for arbitrary masses and energy scales by implementing the $\tt{RAMBO}$ algorithm. It is also rich with basic spinor helicity manipulations like Schouten simplification, Clifford algebra manipulation, conversion between spinor helicity and Lorentz vectors, derivative w.r.t. spinors and their scalars, helicity scaling etc.

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arXiv2025

Compton amplitude and Contact term(s) in the Spinor Helicity formalism

In gauge theories, contact terms play an important role in ensuring gauge invariance. In the spinor helicity formalism, the choice of a gauge-fixing condition manifests itself in the form of the choice of reference vector to write the massless polarization vector(s). However, this choice must be irrelevant in any gauge-invariant observable. We use this principle to determine contact term for Electromagnetic Compton amplitude. We considered three-point function between two massive particles \& a photon to be one which is responsible for soft photon theorem/Coulomb and demonstrate that it is possible to use the above-mentioned principle to find the contact term for the tree-level Compton amplitude of two bosonic massive spinning particles and two photons. The final result does not suffer from any spurious poles.

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arXiv2025

Compton amplitude for massive bosons of arbitrary spin

In this work, we write down an analytic expression of electromagnetic tree-level Compton amplitude for a completely symmetric traceless (bosonic) higher spin particle in any dimension. Our analysis is restricted to the three-point function, which is unique in the Infrared and responsible for the Coulomb interactions/soft photon theorem. We propose an analogue of $R_ξ$ gauge in a theory of higher spin particles. We demonstrate that the theory is unitary only for $ξ=1,\infty$.

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arXiv2025

Three point interaction of Dirac fermions with higher spin particles and discrete symmetries

We constructed all possible kinematically allowed three-point interactions of two massless Dirac spinors with massive higher-spin bosons. In any $D$ spacetime, the interactions have been constructed using the projections of the higher spin irreducible representations of $Spin(D-1)$ over the product of two irreducible spinor representations of $Spin(D-2)$. Based on this analysis, we have further classified the space of theories involving two massless Dirac spinors and a single (or multiple) massive higher spin(s) based on the discrete symmetries: $C,\, R,$ and $ T$. We found that in any $D=2m+1/2m$, the interacting theories of a single massive higher spin have a \enquote{$m$} mod $2$ (or $D$ mod $4$) classification.

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arXiv2022

On-shell Supersymmetry and higher-spin amplitudes

We use on-shell Supersymmetry to constrain the three-point function of two massless particles and one massive particle in 3+1 dimensions. We use this information to write down the tree-level four-point function of massless particles for $\mathcal{N}=1$, $2$ and $4$ theories. In particular, we derive the expressions for four-photon/gluon amplitudes with massive higher spin exchange in theories with $\mathcal{N}=4$ Supersymmetry in 3+1 dimensions.

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arXiv2021

Spinning amplitudes from scalar amplitudes

We provide a systematic method to compute tree-level scattering amplitudes with spinning external states from amplitudes with scalar external states in arbitrary spacetime dimensions. We write down analytic answers for various scattering amplitudes, including the four graviton amplitude due to the massive spin $J$ exchange. We verify the results by computing angular distributions in 3 + 1 dimensions using various identities involving Jacobi polynomials.

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arXiv2021

Equivalence of JT Gravity and Near-extremal Black Hole Dynamics in Higher Derivative Theory

Two derivative Jackiw Teitelboim gravity theory captures the near horizon dynamics of higher dimensional near extremal black holes, which is governed by a Schwarzian action at the boundary in the near horizon region. The partition function corresponding to this boundary action correctly gives the statistical entropy of the near extremal black hole. In this paper, we study the thermodynamics of spherically symmetric four dimensional near extremal black holes in presence of arbitrary perturbative four derivative corrections. We find that the near horizon dynamics is again captured by a JT like action with a particular namely square of Ricci scalar higher derivative modification. Effectively the theory is described by a boundary Schwarzian action which gets suitably modified due to the presence of the higher derivative interactions. Near extremal entropy, free energy also get corrected accordingly.

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arXiv2019

Renormalisation in Open Quantum Field theory II: Yukawa theory and PV reduction

We compute Passarino-Veltman (PV) reduction for tensor loop integrals, that appear in open field theories. We apply these results to open-Yukawa theory and compute the self-energy correction of the fields. We found that non-local divergences show up in the one loop correction to the fermionic self-energy. These non-local divergences do not disappear even if the tree level theory is chosen to satisfy the trace preserving condition of the density matrix.

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arXiv2019

Leading soft theorem for multiple gravitini

We compute leading soft theorem for multiple gravitinos (and graviton) in an arbitrary theory of supergravity with an arbitrary number of finite energy particles with arbitrary mass and arbitrary spin by extending Sen's approach \cite{Sen:2017xjn} to fermionic symmetry. Our result is true for any compactification of type II and Heterotic superstring theory. Our result is valid at all orders in perturbation for four and higher spacetime dimensions.

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arXiv2019

APS $η$-invariant, path integrals, and mock modularity

We show that the Atiyah-Patodi-Singer $η$-invariant can be related to the temperature dependent Witten index of a noncompact theory and give a new proof of the APS theorem using scattering theory. We relate the $η$-invariant to a Callias index and compute it using localization of a supersymmetric path integral. We show that the $η$-invariant for the elliptic genus of a finite cigar is related to quantum modular forms obtained from the completion of a mock Jacobi form which we compute from the noncompact path integral.

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arXiv2017

Renormalization in Open Quantum Field theory I: Scalar field theory

While the notion of open quantum systems is itself old, most of the existing studies deal with quantum mechanical systems rather than quantum field theories. After a brief review of field theoretical/path integral tools currently available to deal with open quantum field theories, we go on to apply these tools to an open version of $ϕ^3$ + $ϕ^4$ theory in four spacetime dimensions and demonstrate its one loop renormalizability (including the renormalizability of the Lindblad structure).

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arXiv2017

Exploring Perturbative Conformal Field Theory in Mellin space

We explore the Mellin representation of correlation functions in conformal field theories in the weak coupling regime. We provide a complete proof for a set of Feynman rules to write the Mellin amplitude for a general tree level Feynman diagram involving only scalar operators. We find a factorised form involving beta functions associated to the propagators, similar to tree level Feynman rules in momentum space for ordinary QFTs. We also briefly consider the case where a generic scalar perturbation of the free CFT breaks conformal invariance. Mellin space still has some utility and one can consider non-conformal Mellin representations. In this context, we find that the beta function corresponding to conformal propagator uplifts to a hypergeometric function.

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arXiv2016

Type I/heterotic duality and M-theory amplitudes

This paper investigates relationships between low-energy four-particle scattering amplitudes with external gauge particles and gravitons in the E_8 X E_8 and SO(32) heterotic string theories and the type I and type IA superstring theories by considering a variety of tree level and one-loop Feynman diagrams describing such amplitudes in eleven-dimensional supergravity in a Horava--Witten background compactified on a circle. This accounts for a number of perturbative and non-perturbative aspects of low order higher derivative terms in the low-energy expansion of string theory amplitudes, which are expected to be protected by half maximal supersymmetry from receiving corrections beyond one or two loops. It also suggests the manner in which type I/heterotic duality may be realised for certain higher derivative interactions that are not so obviously protected. For example, our considerations suggest that R**4 interactions (where R is the Riemann curvature) might receive no perturbative corrections beyond one loop by virtue of a conspiracy involving contributions from (non-BPS) Z2 D-instantons in the type I and heterotic SO(32) theories.

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arXiv2014

Mass Renormalization in String Theory: Special States

String theory gives a well defined procedure for computing the S-matrix of BPS or a class of massless states, but similar calculation for general massive states is plagued with difficulties due to mass renormalization effect. In this paper we describe a procedure for computing the renormalized masses and S-matrix elements in bosonic string theory for a special class of massive states which do not mix with unphysical states under renormalization. Even though this requires working with off-shell amplitudes which are ambiguous, we show that the renormalized masses and S-matrix elements are free from these ambiguities. We also argue that the masses and S-matrix elements for general external states can be found by examining the locations of the poles and the residues of the S-matrix of special states. Finally we discuss generalizations to heterotic and superstring theories.

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arXiv2014

Mass Renormalization in String Theory: General States

In a previous paper we described a procedure for computing the renormalized masses and S-matrix elements in bosonic string theory for a special class of massive states which do not mix with unphysical states under renormalization. In this paper we extend this result to general states in bosonic string theory, and argue that only the squares of renormalized physical masses appear as the locations of the poles of the S-matrix of other physical states. We also discuss generalizations to Neveu-Schwarz sector states in heterotic and superstring theories.

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arXiv2014

String Perturbation Theory Around Dynamically Shifted Vacuum

In some string theories, e.g. SO(32) heterotic string theory on Calabi-Yau manifolds, a massless field with a tree level potential can acquire a tachyonic mass at the one loop level, forcing us to quantize the theory around a new background that is not a solution to the classical equations of motion and hence is not described by a conformally invariant world-sheet theory. We describe a systematic procedure for carrying out string perturbation theory around such backgrounds.

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