arXiv subjects

Anupam Mazumdar

Publications and source records attributed to Anupam Mazumdar.

At least 127 records · Page 7Linked to original sources

arXiv2018

Behavior of the Newtonian potential for ghost-free gravity and singularity-free gravity

In this paper we show that there is a universal prediction for the Newtonian potential for an infinite derivative, ghost-free, quadratic curvature gravity. We show that in order to make such a theory ghost-free at a perturbative level, the Newtonian potential always falls-off as 1/r in the infrared limit, while at short distances the potential becomes non-singular. We provide examples which can potentially test the scale of gravitational non-locality up to 0.004 eV.

gr-qc↗

arXiv2018

Quantum spreading of a self-gravitating wave-packet in singularity free gravity

In this paper we will study for the first time how the wave-packet of a self-gravitating meso-scopic system spreads in theories beyond Einstein's general relativity. In particular, we will consider a ghost-free infinite derivative gravity, which resolves the $1/r$ singularity in the potential - such that the gradient of the potential vanishes within the scale of non-locality. We will show that a quantum wave-packet spreads faster for a ghost-free and singularity-free gravity as compared to the Newtonian case, therefore providing us a unique scenario for testing classical and quantum properties of short-distance gravity in a laboratory in the near future.

gr-qc↗

arXiv2017

Do massive compact objects without event horizon exist in infinite derivative gravity?

Einstein's General theory of relativity is plagued by cosmological and blackhole type singularities Recently, it has been shown that infinite derivative, ghost free, gravity can yield non-singular cosmological and mini-blackhole solutions. In particular, the theory possesses a {\it mass-gap} determined by the scale of new physics. We will use this property to argue that it is possible to form a non-singular super-massive compact objects without having any event horizon in this class of theories.

gr-qc↗

arXiv2017

Observable tensor-to-scalar ratio and secondary gravitational wave background

In this paper we will highlight how a simple vacuum energy dominated inflection-point inflation can match the current data from cosmic microwave background radiation, and predict large primordial tensor to scalar ratio, $r \sim \mathcal{O}(10^{-3}-10^{-2})$, with observable second order gravitational wave background, which can be potentially detectable from future experiments, such as DECi-hertz Interferometer Gravitational wave Observatory (DECIGO), Laser Interferometer Space Antenna (eLISA), Cosmic Explorer (CE), and Big Bang Observatory (BBO).

astro-ph.CO↗

arXiv2017

A Spin Entanglement Witness for Quantum Gravity

Understanding gravity in the framework of quantum mechanics is one of the great challenges in modern physics. Along this line, a prime question is to find whether gravity is a quantum entity subject to the rules of quantum mechanics. It is fair to say that there are no feasible ideas yet to test the quantum coherent behaviour of gravity directly in a laboratory experiment. Here, we introduce an idea for such a test based on the principle that two objects cannot be entangled without a quantum mediator. We show that despite the weakness of gravity, the phase evolution induced by the gravitational interaction of two micron size test masses in adjacent matter-wave interferometers can detectably entangle them even when they are placed far apart enough to keep Casimir-Polder forces at bay. We provide a prescription for witnessing this entanglement, which certifies gravity as a quantum coherent mediator, through simple correlation measurements between two spins: one embedded in each test mass. Fundamentally, the above entanglement is shown to certify the presence of non-zero off-diagonal terms in the coherent state basis of the gravitational field modes.

quant-ph↗

arXiv2017

Baryogenesis, dark matter and inflation in the Next-to-Minimal Supersymmetric Standard Model

Explaining baryon asymmetry, dark matter and inflation are important elements of a successful theory that extends beyond the Standard Model of particle physics. In this paper we explore these issues within the Next-to-Minimal Supersymmetric Standard Model (NMSSM), by studying the conditions for a strongly first order electroweak phase transition, the abundance of the lightest supersymmetric particle (LSP), and inflation driven by a gauge invariant flat direction of MSSM made up of right handed squarks. We present the regions of parameter space which can yield successful predictions for cosmic microwave background (CMB) radiation, the observed relic density for the neutralino LSP, and successful baryogenesis constrained by collider measurements, such as the recent Higgs mass bound, branching ratios of rare, flavour violating decays, and the invisible Z decay width. We also explore where dark matter interactions with xenon nuclei would fall within current bounds of XENON100 and the projected limits for the XENON1T and LUX experiments.

hep-ph↗

arXiv2017

Gravitational waves at aLIGO and vacuum stability with a scalar singlet extension of the Standard Model

A new gauge singlet scalar field can undergo a strongly first-order phase transition (PT) leading to gravitational waves (GW) potentially observable at aLIGO and simultaneously stabilize the electroweak vacuum. aLIGO (O5) is potentially sensitive to cosmological PTs at $10^7$-$10^8$ GeV, which coincides with the requirement that the singlet scale is less than the Standard Model (SM) vacuum instability scale, which is between $10^8$ GeV and $10^{14}$ GeV. After sampling its parameter space, we identify three benchmark points with a PT at about $T\approx 10^7$ GeV in a gauge singlet extension of the SM. We calculate the nucleation temperature, order parameter, characteristic timescale, and peak amplitude and frequency of GW from bubble collisions during the PT for the benchmarks and find that, in an optimistic scenario, GW from such a PT may be in reach of aLIGO (O5). We confirm that the singlet stabilizes the electroweak vacuum whilst remaining consistent with zero-temperature phenomenology as well. Thus, this scenario presents an intriguing possibility that aLIGO may detect traces of fundamental physics motivated by vacuum stability at an energy scale that is well above the reach of any other experiment.

hep-ph↗

arXiv2016

Defocusing of Null Rays in Infinite Derivative Gravity

Einstein's General theory of relativity permits spacetime singularities, where null geodesic congruences focus in the presence of matter, which satisfies an appropriate energy condition. In this paper, we provide a minimal defocusing condition for null congruences without assuming any Ansatz-dependent background solution. The two important criteria are: (1) an additional scalar degree of freedom, besides the massless graviton must be introduced into the spacetime; and (2) an infinite derivative theory of gravity is required in order to avoid tachyons or ghosts in the graviton propagator. In this regard, our analysis strengthens earlier arguments for constructing non-singular bouncing cosmologies within an infinite derivative theory of gravity, without assuming any Ansatz to solve the full equations of motion.

gr-qc↗

arXiv2016

Nonlocal N=1 Supersymmetry

We construct $\mathcal{N}=1$ supersymmetric nonlocal theories in four dimension. We discuss higher derivative extensions of chiral and vector superfields, and write down generic forms of Kähler potential and superpotential up to quadratic order. We derive the condition in which an auxiliary field remains non-dynamical, and the dynamical scalars and fermions are free from the ghost degrees of freedom. We also investigate the nonlocal effects on the supersymmetry breaking and find that supertrace (mass) formula is significantly modified even at the tree level.

hep-th↗

arXiv2016

Non-thermal Axion Dark Radiation and Constraints

The Peccei-Quinn mechanism presents a neat solution to the strong CP problem. As a by-product, it provides an ideal dark matter candidate, "the axion", albeit with a tiny mass. Axions therefore can act as dark radiation if excited with large momenta after the end of inflation. Nevertheless, the recent measurement of relativistic degrees of freedom from cosmic microwave background radiation strictly constrains the abundance of such extra relativistic species. We show that ultra-relativistic axions can be abundantly produced if the Peccei-Quinn field was initially displaced from the minimum of the potential. This in lieu places an interesting constraint on the axion dark matter window with large decay constant which is expected to be probed by future experiments. Moreover, an upper bound on the reheating temperature can be placed, which further constrains the thermal history of our Universe.

hep-ph↗

arXiv2016

Towards understanding the ultraviolet behavior of quantum loops in infinite-derivative theories of gravity

In this paper we will consider quantum aspects of a non-local, infinite-derivative scalar field theory - a $\it toy \, model$ depiction of a covariant infinite-derivative, non-local extension of Einstein's general relativity which has previously been shown to be free from ghosts around the Minkowski background. The graviton propagator in this theory gets an exponential suppression making it $\it asymptotically \, free$, thus providing strong prospects of resolving various classical and quantum divergences. In particular, we will find that at $1$-loop, the $2$-point function is still divergent, but once this amplitude is renormalized by adding appropriate counter terms, the ultraviolet (UV) behavior of all other $1$-loop diagrams as well as the $2$-loop, $2$-point function remains well under control. We will go on to discuss how one may be able to generalize our computations and arguments to arbitrary loops.

hep-th↗

arXiv2016

High-Energy Scatterings in Infinite-Derivative Field Theory and Ghost-Free Gravity

In this paper, we will consider scattering diagrams in the context of infinite-derivative theories. First, we examine a finite-order higher-derivative scalar field theory and find that we cannot eliminate the external momentum divergences of scattering diagrams in the regime of large external momenta. Then, we employ an infinite-derivative scalar toy model and obtain that the external momentum dependence of scattering diagrams is convergent as the external momenta become very large. In order to eliminate the external momentum divergences, one has to dress the bare vertices of the scattering diagrams by considering renormalised propagator and vertex loop corrections to the bare vertices. Finally, we investigate scattering diagrams in the context of a scalar toy model which is inspired by a $\mathit{ghost-free}$ and $\mathit{singularity-free}$ infinite-derivative theory of gravity, where we conclude that infinite derivatives can eliminate the external momentum divergences of scattering diagrams and make the scattering diagrams convergent in the ultraviolet.

hep-th↗

arXiv2016

Generalised Boundary Terms for Higher Derivative Theories of Gravity

In this paper we wish to find the corresponding Gibbons-Hawking-York term for the most general quadratic in curvature gravity by using Coframe slicing within the Arnowitt-Deser-Misner (ADM) decomposition of spacetime in four dimensions. In order to make sure that the higher derivative gravity is ghost and tachyon free at a perturbative level, one requires infinite covariant derivatives, which yields a generalised covariant infinite derivative theory of gravity. We will be exploring the boundary term for such a covariant infinite derivative theory of gravity.

gr-qc↗

arXiv2016

Some inequalities bridging stringy parameters and cosmological observables

By demanding the validity of an effective field theory description during inflation, in this note we derive some peculiar inequalities among the three interesting stringy and cosmological parameters, namely the tensor-to-scalar ratio ($r$), the string coupling ($g_s$) and the compactification volume (${\cal V}$). In deriving these inequalities, we explicitly demand that the inflationary scale and the Hubble parameter during inflation are well below the Kaluza-Klein (KK) mass scale, string scale, and the four dimensional Planck mass. For the inflationary models developed within the framework of type IIB orientifold comapctification, we investigate the regions of parameters space spanned by the three parameters $(r, g_s, {\cal V})$ by satisfying our inequalities, and we find that the same can reduce the size of available parameter space quite significantly. Moreover, we comment on obtaining further constraints on the parameters by comparing gravitino mass ($m_{3/2}$) with the Hubble scale ($H$), which also provides a lower bound on tensor-to-scalar ratio ($r$), for the cases when $m_{3/2} <H$. We also illustrate the outcome of our bounds in some specific class of string(-inspired) models.

hep-th↗

arXiv2016

Consistent Higher Derivative Gravitational theories with stable de Sitter and Anti-de Sitter Backgrounds

In this paper we provide the criteria for any generally covariant, parity preserving, and torsion free theory of gravity to possess a stable de Sitter (dS) or anti-de Sitter (AdS) background. By stability we mean the absence of tachyonic or ghost-like states in the perturbative spectrum that can lead to classical instabilities and violation of quantum unitarity. While we find that the usual suspects, the F(R) and F(G) theories, can indeed possess consistent (A)dS backgrounds, G being the Gauss-Bonnet term, another interesting class of theories, string-inspired infinite derivative gravity, can also be consistent around such curved vacuum solutions. Our study should not only be relevant for quantum gravity and early universe cosmology involving ultraviolet physics, but also for modifications of gravity in the infra-red sector vying to replace dark energy .

gr-qc↗

arXiv2016

Higgs Stability and the 750 GeV Diphoton Excess

We study the implications of a possible unstable particle with mass $M_X<$ TeV for the Higgs stability, naturalness and inflation. We pay particular attention to the case $M_X\approx$ 750 GeV, suggested by recent results of ATLAS and CMS on diphoton final states, and work within the minimal model: we add to the Standard Model field content a pseudoscalar and a vector-like fermion carrying both color and electric charge. This can stabilize the electroweak vacuum without invoking new physics at very high energies, which would give an unnaturally large contribution to the Higgs mass. We also show that inflation can be obtained via a UV modification of General Relativity.

hep-ph↗

arXiv2016

Gravitational theories with stable (anti-)de Sitter backgrounds

In this article we will construct the most general torsion-free parity-invariant covariant theory of gravity that is free from ghost-like and tachyonic nstabilities around constant curvature space-times in four dimensions. Specifically, this includes the Minkowski, de Sitter and anti-de Sitter backgrounds. We will first argue in details how starting from a general covariant action for the metric one arrives at an "equivalent" action that at most contains terms that are quadratic in curvatures but nevertheless is sufficient for the purpose of studying stability of the original action. We will then briefly discuss how such a "quadratic curvature action" can be decomposed in a covariant formalism into separate sectors involving the tensor, vector and scalar modes of the metric tensor; most of the details of the analysis however, will be presented in an accompanying paper. We will find that only the transverse and trace-less spin-2 graviton with its two helicity states and possibly a spin-0 Brans-Dicke type scalar degree of freedom are left to propagate in 4 dimensions. This will also enable us to arrive at the consistency conditions required to make the theory perturbatively stable around constant curvature backgrounds. This will be included as a chapter in the book entitled "At the Frontier of Spacetime - Scalar-Tensor Theory, Bells Inequality, Machs Principle, Exotic Smoothness" (Springer 2016)

hep-th↗

arXiv2016

Possible resolution of the domain wall problem in the NMSSM

We discuss a possibility that the domain wall problem in the next-to-minimal supersymmetric standard model is alleviated without introducing a small explicit $Z_3$ breaking term by analyzing the evolution of the singlet scalar field within an inflationary paradigm. The singlet scalar field which explains the $μ$-term tracks a time-varying minimum of the effective potential after inflation and slowly rolls down to its global minimum if there exist sufficiently large negative Hubble-induced corrections on the effective potential for the singlet field, which arise through supergravity. As a consequence, the whole Universe is confined within a single domain during and after inflation, which prevents the formation of domain walls. This will further constrain the history of the early Universe along with the Higgs-singlet coupling.

hep-ph↗