Lepton Flavor Universal New Physics in B-Decays?
Abstract
"The flavor-changing neutral current (FCNC) semileptonic decays of B mesons offer a powerful avenue to indirectly probe New Physics (NP) beyond the Standard Model (SM). I will review the current status of the so-called B anomalies. While lepton flavor universality (LFU) ratios such as RK and RK∗ are in good agreement with SM predictions, notable deviations persist in individual branching fractions—for instance, B(B+ → K+μ+μ−) and B(B+ →K+e+e−), both of which deviate at the 4 − 5σ level. Additionally, the angular observable P'5 in B → K∗μ+μ− exhibits a 3.3σ deviation, and a 3.6σ discrepancy is reported in B(Bs → φμ+μ−). Although the recent measurement of Rφ is consistent with the SM, the individual branching fractions remain at odds with expectations. These persistent anomalies hint at NP contributions in both muon and electron sectors. Given that experimental bounds on b → see transitions are still relatively loose, NP might also manifest in the first-generation lepton sector. Motivated by these observations, we perform a global analysis of dimension-6 SMEFT operators, considering the possibility of NP
contributions not only in b → sμμ transitions but also in b → see transitions."
On the T-linear resistivity of cuprates: theory
Abstract
"By partitioning the electronic system of the optimally doped cuprates in two electronic components: (1) mobile electrons on oxygen sub-lattice; (2) localized spins on copper sub-lattice, and considering the scattering of mobile electrons (on oxygen sub-lattice) via generation of paramagnons in the localized sub-system (copper spins), we ask what should be the electron-paramagnon coupling function Mq so that T-linear resistivity results both in the low temperature limit (kBT << ~ωqcut) and in the opposite high temperature limit (qcut is a characteristic paramagnon cut-off wave vector). This ’reverse engineering approach’ leads to |Mq| scaling inversely with wave vector. We comment how can such exotic coupling emerge in 2D systems where short range magnetic fluctuations resides. In other words, the role of quantum criticality is found to be crucial. And the low temperature T-linear behaviour of resistivity demands that the magnetic correlation length scales as ξ(T) scales as 1/T which seems to be a reasonable assumption in the quantum critical regime of cuprates (that is, near optimal doping where T-linear resistivity is observed).
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"Modeling Superconductivity, Magnetism, and Quantum Phenomena in Complex Materials"
Abstract
We present a comprehensive exploration of quantum materials through state-of-the-art theoretical and computational techniques. Focusing on superconductivity, we delve into intra- and interband pairing mechanisms via Eliashberg formalism and model Hamiltonians, examining how pressure and doping drive Lifshitz transitions. The impact of magnetic impurities on superconducting states is analyzed using density functional theory (DFT) and Green’s function methods with potential applications in developing robust qubit platforms for quantum computing. We further investigate electronic structures through band dispersion, Fermi surfaces, and density of states to interpret experimental probes such as ARPES and IETS. We also highlight structure-property correlations under thermal perturbations, advanced crystal structure prediction using evolutionary algorithms like USPEX, and ongoing work on the optical and photocatalytic properties of mixed anionic systems, including thio-apatites with potential for solar-driven hydrogen production.
Probing collective response in the quantum system using Raman scattering
Abstract
"Raman scattering is an inelastic light scattering process in which energy is transferred from incident light to the system, and the strongest signal often appears in the absorption spectrum from the collective mode present in the quantum system. In the first part of the talk, I will present a novel effect arising from inversion symmetry broken spin-orbit coupling (SOC) on charge collective modes, plasmons, and how this could be studied using electronic Raman scattering. I will discuss that isolated plasmons could be strongly prominent in resonant eRS in the presence of spin momentum locking SOC in BiTeI, which were invisible previously due to standard $q^2$ suppression, where $q$ is the momentum transferred by the light.
The last part of the talk is on Raman response in superconductors. Here, I will discuss the electronic Raman response (eRS) of multiband SC in both A1g and B1g channels across the time reversal symmetry broken transition and the potential possibility of Leggett and Bardassis-Schreiffer modes in the observed spectrum, depending on the nature of the ground state. We will also see from our results that eRS could be used as a probe to detect spontaneously broken time reversal symmetry of superconductors such as s+is and s+id states."
Neutrinoless double beta decay in an realistic SU(5) Model
Abstract
Baryon number (B) and lepton number (L) are accidental global symmetries of the Standard Model (SM). Any observed violation of these quantum numbers would provide unambiguous evidence for physics beyond the SM. Grand Unified Theories (GUTs) offer a well-motivated framework to study such violations.
In this seminar, I will discuss the role of heavy scalar fields in mediating lepton number violation via neutrinoless double beta decay (0νββ) within the SU(5) framework. While the minimal SU(5) setup predicts extremely suppressed contributions to 0νββ due to the heavy scalar masses – as a consequence of the proton decay bound, we will show that this limitation can be circumvented by extending the model. Specifically, the introduction of a discrete ℤ3 symmetry and the inclusion of an additional 15-dimensional scalar representation allow for dominant contributions to the decay process. Such an extension not only remains consistent in yielding realistic fermion mass spectra but also leads to experimentally testable predictions in upcoming ton-scale 0νββ searches.
Electron-phonon coupling induced topological phase transitions in an α-T3 quantum spin Hall insulator
Abstract
We study the phenomenon of topological phase transitions induced by electron-phonon (e-ph) coupling in an α-T3 quantum spin Hall insulator that presents smooth tunability between graphene (α = 0) and dice (α = 1) lattice. Upon deriving an effective electronic model under suitable transformations, we come across different regimes of α, which host distinct topological transitions solely mediated through e-ph coupling, manifesting robust support from the bulk gap closing and the relative changes in the topological invariant together with the edge state features. The critical e-ph strengths of these transitions strongly depend on α. We also observe the evidence of an emergent second-order topological insulator (SOTI) phase in our system, which is characterized by the existence of corner modes and its topological marker. Interestingly, these corner modes are wiped out beyond a critical e-ph coupling (albeit different for different α), referring to a SOTI-trivial phase transition induced by the e-ph coupling.
Study of Neutrino Oscillation with non unitarity
Abstract
In this talk we will present the neutrino oscillation probabilities in presence of a non unitary mixing matrix . We will show the oscillation probabilities both in vacuum and including matter effects, Using the expressions of probabilities derived, we will show at which energies and baselines the signature of non unitary will be significantly different from standard scenarios.
Neutrinos in Cosmology
Abstract
In this talk, we will begin by introducing the basics of neutrinos and their significant role in cosmology. We will discuss the thermodynamics of the early universe and examine the Boltzmann equation and the process of neutrino decoupling. Moving forward, we will explore the nature of dark matter and investigate whether neutrinos could serve as viable dark matter candidates. We will then review cosmological constraints on neutrino masses. Finally, we will discuss the possibility of sterile neutrinos as dark matter. Throughout the talk, we aim to highlight how neutrinos influence key processes in the early universe and their relevance in modern cosmology.
Static structure factor and the dispersion of the Girvin-MacDonald-Platzman density mode for fractional quantum Hall fluids on the Haldane sphere
Abstract
We study the neutral excitations in the bulk of the fractional quantum Hall (FQH) fluids generated by acting with the Girvin-MacDonald-Platzman (GMP) density operator on the uniform ground state. Creating these density modulations atop the ground state costs energy since any density fluctuation in the FQH system has a gap stemming from underlying interparticle interactions. We calculate the GMP density-mode dispersion for many bosonic and fermionic FQH states on the Haldane sphere using the ground state static structure factor computed on the same geometry. Previously, this computation was
carried out on the plane. Analogous to the GMP algebra of the lowest Landau level (LLL) projected density operators in the plane, we derive the algebra for the LLL-projected density operators on the sphere, which facilitates the computation of the density-mode dispersion. Contrary to previous results on the plane, we find that, in the long-wavelength limit, the GMP mode accurately describes the dynamics of the primary Jain states."
Higher-Order QCD Corrections and Threshold Resummation for Processes in the Standard Model and Beyond at Hadron Colliders
Abstract
In this talk, we present our study on threshold resummation for various Standard Model processes at the Large Hadron Collider (LHC), including neutral and charged Drell-Yan production, Higgs boson production in association with a massive vector boson, and Higgs production via bottom quark annihilation. We perform resummation up to N³LO+N³LL accuracy in QCD, addressing the large logarithms that arise in the partonic threshold limit. Additionally, we analyzed the gluon fusion channel for ZH production, resumming contributions up to NLO+NLL accuracy in the Born-improved theory and combining them with Drell-Yan-type contributions. For on-shell ZZ pair production, we extend the resummation accuracy to NNLO+NNLL. After performing the threshold resummation, the theoretical uncertainties are reduced compared to fixed-order results. Furthermore, we investigate the two-loop corrections to the decay of a pseudo-scalar Higgs boson (A) into three partons, including higher-order terms in the dimensional regulator. These results are crucial for improving theoretical predictions for pseudo-scalar production with one jet at hadron colliders.
Hidden’ magnetism and a mechanism for it
Abstract
Recently a mysterious hidden magnetic memory, which manifests itself in the form of “spontaneous” vortices appearing in the superconducting state of 4Hb-TaS2, was reported [Nature, 607,692, 2022]. Motivated by this observation, we present a mechanism which leads to a similar phenomenology. The mechanism relies on spin-charge separation induced by strong electronic correlations in a flat-band tuned away from half filling, which is the expected picture of the T-layers in 4Hb-TaS2. For concreteness, we demonstrate the feasibility of this mechanism within a square lattice t-J model. Our results pave the way towards understanding the observed magnetic memory effect and may apply to a broader class of materials.
Towards Quantum Simulating QCD
Abstract
Being created from the Big Bang - evolved to families of sub-atomic particles - landing in an astrophysical environment -the dynamic properties of the strong interactions of nature are still unknown. Simulating the dynamics of quantum chromodynamics (QCD) is beyond the scope of even the most powerful supercomputers. Standing at the end of the first quarter of the century, the question is whether a quantum computer can (/will be able to) simulate nature. Okay, yes, it should. The visionary physicist Richard P. Feynman envisioned that "Nature isn't classical, dammit, and if you want to make a simulation of nature, you'd better make it quantum mechanical..." After almost half a century as quantum technology matures it appears to be close to a reality. Yet, as Feynman envisioned, the task remains challenging and requires substantial advancement in `qubitizing/quditizing’ nature - develop novel quantum algorithms and implement them on quantum hardware. In this talk, I'll briefly summarize the journey towards quantum simulating QCD - the challenges, advances and prospects.
From Monopole-Induced Berry Phase to Quadrupolar Berry Phase
Abstract
We explore the Berry phase associated with purely quadrupolar states (⟨ψ|S|ψ⟩ = 0) in spin-1 systems. By employing the Majorana stellar representation, we demonstrate the topological nature of the quadrupolar Berry phase, revealing that it takes values of either 0 or π, and establish its connection to the exchange of Majorana stars. Additionally, we investigate the dynamics of a state within the quadrupolar subspace under the influence of a static magnetic field. The time evolution governed by this Hamiltonian confines the system to the quadrupolar subspace, generating a geometric phase of the Aharonov-Anandan type, quantized to 0 or π. We also present a natural framework for understanding the topological properties of this subspace in terms of anti-unitary symmetries. Finally we will discuss a possible application of our findings to Holographic quantum codes and robust quantum phase gates.
LCSR application to D+ → π+l+l−
Abstract
Flavour Changing Neutral Currents (FCNCs) in the Standard Model (SM) arise only at loop level, making them important probes for New Physics (NP). However, unlike bottom FCNCs (for example b → s l+l−), the charm FCNCs (for example c → u l+l− ) are dominated by long distance (LD) effects due to strong GIM suppression, posing significant challenges. In this talk we will explore these challenges focusing on decays D+ → π+l+l− which can be realised as the combination of the singly Cabibbo suppressed (SCS) weak transitions with the electromagnetic emission of the lepton pair. We study these LD effects using the framework of Light Cone Sum Rules (LCSR) and make predictions for the differential widths for these decays. Our findings suggest that the weak annihilation contributions are dominant, with negligible loop contributions. Finally, I relate these decays to other Cabibbo favoured and doubly Cabibbo suppressed, D(s)+ → P l+l− (P = π, K... ) decays through flavour symmetries. As a byproduct, I further discuss Ds+ → π+l+l−, which is not an FCNC but shares the topologies with D+ → π+l+l−, and can therefore be useful in better understanding of LD dynamics involved.
Puzzles and predictions of the left right symmetric model
Abstract
We will show that O(1) leptonic CP violation generates too large a strong CP phase in one loop RGE running, and therefore the Minimal Left Right Symmetric Model (with triplet and bidoublet Higgses) is testable regardless of the scale of parity breaking by the following prediction: The neutrino experiments will not discover leptonic CP violation in the PMNS matrix. Moreover the lepton mass hierarchy can be understood in this model if the electron mass is radiatively generated in 2 loop RGE.
Non-Fermi Liquid Transport in Semimetals and Strongly Correlated Systems
Abstract
The Hall coefficient traditionally measures the density of charge carriers in metals, via Drude’s inverse carrier density relation. However, this relation may break down due to intriguing Fermi surface topology or strong electronic interaction. Using a recently developed thermodynamic formalism, we study deviations of the Hall coefficients from Drude's relation in (1) semimetals (e.g., Weyl, nodal-line) and (2) the Hubbard model. Our calculations explain the "Hall anomaly", characterized by a divergence of the Hall coefficient near half-filling and an abrupt sign change, observed in cuprate experiments. Finally, I will briefly discuss similar anomalies in thermopower, studied via the calculation of the Seebeck coefficient of strongly interacting systems.
In-In EFT, the In-Out way
Abstract
In-In correlators are the natural quantities in time dependent settings like cosmology or in non-equilibrium situations when the interest in not in scattering matrix elemnts but rather expectation values. The talk will describe an attempt to have an effective field theory (EFT) description for In-In correlators in terms of the more familiar In-Out formalism routinely used for S-matrix calculations.
Exploding Stars, Shapeshifting neutrinos, and the Synthesis of Heavy Elements
Abstract
How exactly do stars explode? Where and how are the elements that we observe in the cosmos synthesized? A common theme tying these questions together is the abundant presence of neutrinos – mysterious and elusive elementary particles – in these environments. In this talk, I shall describe how neutrinos can power these magnificent cosmic explosions, i.e., supernovae, and also aid in the synthesis of heavy elements thereafter. Particular attention will be given to a longstanding open question: the origin of proton-rich isotopes in nature. I will present some interesting results from my recent work, demonstrating how a once-popular solution to this conundrum still endures, despite a decade's worth of claims to the contrary. Finally, I shall briefly discuss some peculiar behaviors of neutrinos, such as their penchant for shapeshifting (flavor oscillations), or their potential to quantum-entangle as they interact with each other in these environments.
Anderson localization in QFT and hierarchies from nonlocality
Abstract
It was shown in [1710.01354] that disordered local interactions in theory space can localize mass eigenstates (analogous to Anderson localization in disordered lattice) enabling exponentially hierarchical couplings in QFT. In this talk, I shall show that such theories can also produce multiple massless modes. Subsequently, deterministic nonlocality can give rise to hierarchies that are qualitatively distinct from the original proposal.
Complete escape from localization on a hierarchical lattice: A Koch fractal with all states extended
Abstract
"An infinitely large Koch fractal is shown to be capable of sustaining only extended, Bloch-like eigenstates if certain parameters of the Hamiltonian describing the lattice are numerically correlated in a special way, and a magnetic flux of a special strength is trapped in every loop of the geometry. We describe the system within a tight-binding formalism and prescribe the desired correlation between the numerical values of the nearest-neighbor overlap integrals, along with a special value of the magnetic flux trapped in the triangular loops decorating the fractal. With such conditions, the lattice, despite the absence of translational order of any kind whatsoever, yields an absolutely continuous eigenvalue spectrum and becomes completely transparent to an incoming electron with any energy within the allowed band. The results are analytically exact. An in-depth numerical study of the inverse participation ratio and the two-terminal transmission coefficient corroborates our findings. Our conclusions remain valid for a large set of lattice models, built with the same structural units, but beyond the specific geometry of a Koch fractal, unraveling a subtle universality in a variety of such low-dimensional systems.
Reference: S. Biswas and A. Chakrabarti, Physical Review B 108, 125430 (2023)."