Thesis Colloquia

CHEP THESIS COLLOQUIUM

Name                 : Prabhat Solanki

Supervisor           : Prof. Biplob Bhattacherjee

Title                    :  Search for Long-Lived Particles at High Luminosity Large Hadron Collider and Beyond.

Date & Time         : 7th December 2023 @ 11:00 AM

Venue                 :  Lecture Hall 3, New Physical Science Building 

Abstract:-

Despite extensive searches, clear indications of new physics beyond the Standard Model (BSM) remain elusive. Traditionally, experimental searches at the Large Hadron Collider (LHC) during Phase I and phenomenological studies within BSM physics have focused on promptly decaying particles. More recently, however, the focus has shifted toward particles with macroscopic decay lengths, known as Long-Lived Particles (LLPs). These particles, predicted by several well-motivated BSM theories, such as R-Parity Violating (RPV) supersymmetry (SUSY), hidden valley models, and dark matter models, have become a compelling focal point for BSM physics searches. This thesis, focusing on the search for LLPs at the High Luminosity Large Hadron Collider (HL-LHC) and beyond, is divided into three parts, detailed as follows:

Firstly, considering the anticipated increase in pile-up events at the HL-LHC, the effective triggering of LLPs at the earliest analysis stage is critical for efficient searches. Our studies suggest that conventional Level-1 (L1) triggers, designed primarily for prompt particles, may be inadequate in the high pile-up scenario of the HL-LHC, necessitating dedicated L1 triggers for LLPs. We propose dedicated L1 triggers that utilize track data to identify LLP decaying to jets. We explore the prospect of utilizing Minimum Ionizing Particle (MIP) timing detector information at the HL-LHC for triggering LLP events with displaced jets. Our investigation extends to the utilization of Electromagnetic Calorimeter (ECAL) timing information to develop dedicated triggers for displaced jets. Our findings show that combining track and timing data available at L1 significantly enhances LLP detection capabilities at the trigger level over a wide range of decay lengths. Finally, we introduce a novel machine-learning strategy using a message-passing graph autoencoder employing edge convolution to refine L1 track-based triggering, achieving high signal acceptance rates for light LLPs.

The second part investigates the RPV SUSY model within the HL-LHC framework, focusing on the pair production of electroweakinos $\chi_2^0$ and $\chi_1^{\pm}$. We analyze a scenario where these decay into a Higgs and W boson, respectively, along with a long-lived Lightest Supersymmetric Particle (LSP), $\chi_1^0$, resulting in both prompt and displaced signatures. Integrating tracker, ECAL, and MIP Timing Detector (MTD) data and utilizing triggers developed in the first part of the thesis, we establish improved limits on the masses of electroweakinos, exceeding current experimental projections for LLPs that decay very late in the detector.

In the final part, we address the challenges and prospects of future collider experiments to detect light LLPs in the forward direction by proposing FOREHUNT, a novel forward detector for the 100 TeV FCC-hh. Tailored to detect light LLPs from B-meson decays and heavy neutral leptons, we propose several detector configurations optimized for such signatures. We compare the signal acceptance rate with other dedicated LLP detectors like MATHUSLA, CODEX-b, and DELIGHT. Our findings indicate that for LLPs with a decay length shorter than 10 m, FOREHUNT emerges as the premier detector choice.

CHEP THESIS COLLOQUIUM

Name                      : Tanay Pathak 

Supervisor               : Prof. Ananthanarayan. B

Title                         : Studies of Feynman and Related Integrals and their Hypergeometric connection using the   

                                            Method of Brackets, Algebraic Relations, and Singularity Analysis

Date & Time            : 06th Nov 2023 @ 03:30 pm

Venue                      :  Lecture Hall 3, New Physical Science Building 

Abstract:-

Feynman integrals are special kinds of integrals that arise in perturbative quantum field theory and are often very difficult to evaluate in terms of known functions of mathematical physics. Some of the methods used to evaluate other related integrals may be of use in the evaluation of Feynman integrals and vice versa.  

In this thesis work, we discuss one such method, the method of brackets to evaluate certain improper integrals of quadratic and quartic type. We further investigate integrals such as the Ising integrals, the Box integrals and the other related integrals. In the course of the study, we obtain multi-variable hypergeometric functions naturally and this leads us to carry out further analysis of these functions such as obtaining their connection formulas.  

We take Appell F2 as an example and obtain its analytic continuations. With the techniques developed for it, we further evaluate the analytic continuations of a more complicated function called Horn’s H1 . We then return to the evaluation of Feynman integrals and study the algebraic relations of the product of their propagators. We suitably modify the known technique so as to implement it in a computer algebra system for automated evaluation of such relations. In this investigation, we again find the presence of multi-variable hypergeometric functions and were also able to derive many elusive and non-trivial reduction formulas for the same. 

In connection with the numerical evaluation of the multi-variable hypergeometric series we also study the approximation theory focusing on the bi-variate series. We then further develop an automated Mathematica package for the evaluation of diagonal bi-variate approximants which are called the Chisholm approximants. We study their construction and their usage in the context of convergence acceleration and analytic continuation. We also show their utility using examples from various branches of physics such as particle physics and condensed matter physics.  

Finally, we return to the analysis of the Feynman integrals again and study their analytic properties. We study singularities associated with them without their explicit evaluation by compactifying the integrals in a projective space. We also extend the previous analysis in this regard, restricted to the singularities of second type to other types of singularities.

CHEP THESIS COLLOQUIUM

Name                      :  Abhijit Buddhadev Das

Supervisor               : Prof. B. Ananthanarayan. 

Title                         :  Some investigations into algebraic and geometric properties of Feynman integrals and related topics

Date & Time            :  19/06/2023, 2: 30 PM

Venue                      :  Lecture Hall 3 New Physical Science Building 

Abstract:-

We describe our investigations into various formal properties of Feynman integrals and of scattering amplitudes obtained from studying algebraic structures. There are variety of ways to study these properties. Out of which, we use method of regions and method of using Hopf algebra for studying Feynman integrals and a geometry based approach for scattering amplitudes. In the method of regions based approach, we use the ASPIRE program, which is based on the Landau singularities and the method of Power geometry to unveil the regions required for the evaluation of a given Feynman diagram asymptotically in a given limit. It also allows for the evaluation of scaling coming from the top facets. In this work, we relate the scaling having equal components of the top facets of the Newton polytope to the maximal cut of given Feynman integrals. We have therefore connected two independent approaches to the analysis of Feynman diagrams. In the second approach,  the method of using Hopf algebras for calculating Feynman integrals developed by Abreu et al. is applied to the two-loop nonplanar on-shell diagram with massless propagators and three external mass scales. We show that the existence of the method of cut Feynman diagrams comprising of the coproduct, the first entry condition and integrability condition that was found to be true for the planar case also holds for the nonplanar case; furthermore, the nonplanar symbol alphabet is the same as for the planar case. This is one of the main results of this work which have been obtained by a systematic analysis of the relevant cuts, using the symbolic manipulation codes HypExp and PolyLogTools . The obtained result for the symbol is crosschecked by an analysis of the known two-loop original Feynman integral result. In addition, we also reconstruct the full result from the symbol. This is the second main result of this paper. Finally, inspired by the recent work of Nima Arkani Hamed and collaborators who introduced the notion of positive geometry to account for the structure of tree-level scattering amplitudes in bi-adjoint $\phi^3$ theory, which led to one-loop descriptions of the integrands. Here we consider the one-loop integrals themselves in $\phi^3$ theory. In order to achieve this end, the geometrical construction offered by Schnetz for Feynman diagrams is hereby extended, and the results are presented. The extension relies on masking the loop momentum variable with a constant and proceeding with the calculations. The results appear as a construction given in a diagrammatic manner. The significance of the resulting triangular diagrams is that they have a common side amongst themselves for the corresponding Feynman diagrams they pertain to. Further extensions to this mathematical construction can lead to additional insights into higher loops. A mathematica code has been provided in order to generate the final results given the initial parameters of the theory.

 CHEP THESIS COLLOQUIUM

Name                      :  Mohd Siddique Akbar Alam Khan 

Supervisor               : Prof. B. Ananthanarayan. 

Title                         : Renormalization Group Summation at High Orders and Implications to the determination of some Standard Model Parameters.

Date & Time            :  20/06/2023, 2: 30 PM

Venue                      :  Lecture Hall 3 New Physical Science Building 

Abstract:-

Prediction from theories like Quantum Chromodynamics (QCD) are obtained by evaluating Feynman diagrams to high orders in perturbation theory. These theoretical predictions depend on the precise values of the various parameters. Higher-order results for various processes are already available in the literature.  With the availability of a large amount of data from experiments, it is possible to extract these parameters by comparing theoretical predictions with data. However, due to finite order information available from theory, predictions depend on the perturbative scheme used and sometimes suffer from large uncertainties. Therefore, it is necessary that free parameters in the perturbative scheme are optimized in such a way that the predictions are meaningful and precise.  

In the thesis, we have focussed on Renormalization Group Summed Perturbation Theory (RGSPT), a perturbative scheme based on the perturbative renormalization group, where large logarithms accessible to a given perturbative order are summed to all orders using the renormalization group equations. The resulting perturbative series is then less sensitive to the renormalization scale variations, which is one of the sources of uncertainties.

We apply this formalism to improve the predictions for the QCD static energy between heavy quarks, which are then compared with 2 flavor lattice QCD simulations, to obtain the Lambda parameter. Using various QCD sum rules, determinations of the strong coupling constant and masses of the strange, charm, and bottom quark and CKM parameter |Vus| are also performed.

      In addition to these determinations, the analytic continuation from the spacelike region to the timelike region, a process needed to match the theoretical calculations from the Feynman diagrams with experiments. This results in large π2- kinematical terms which can also be summed to all orders using RGSPT. Effects of summation of these kinematical corrections to various processes such as the R-ratios for the charm and bottom cases, hadronic higgs decays, as well as in continuum contributions to muon g-2 are discussed. Improvements obtained in this formalism are crucial for the Borel-Laplace sum rule determinations of QCD parameters.   As an application, the masses of the up, down, and strange quark masses are obtained.

Thesis  Colloquium

Name                      : Rhitaja Sengupta  

Supervisor        : Biplob Bhattacherjee

Title                         : Prompt and displaced signatures of physics beyond the Standard Model

Date & Time      : 25th April 2023 @ 2:30 pm

Venue                    :  Lecture Hall 3, New Physical Science Building 

Abstract:-

  The quest to understand our Universe’s fundamental particles and their interactions has led us to the Standard Model (SM) of particle physics. Despite successfully explaining the weak, electromagnetic, and strong interactions, the SM fails to explain many experimental observations and theoretical questions.

The solutions to these problems require new physics beyond the SM (BSM), motivating the hunt for any indication of BSM physics. We study various probes of BSM physics, ranging from deviations of precision measurements from SM predictions which capture indirect hints of new physics, to searching directly for BSM particles at colliders or dark matter experiments. We explore signatures from prompt decay of BSM particles having very small lifetimes and the exotic array of signatures arising from long-lived BSM particles (LLPs). While the former has been the primary focus of most BSM searches, the latter possibility has recently gained attention, demanding a more careful examination to ensure we are not missing any part of the BSM parameter space.

 The discovered Higgs boson is a leading portal connecting new physics particles to the SM particles, and this motivates our studies in the first part of the thesis. We begin by exploring the parameter space of the phenomenological Minimal Supersymmetric Standard Model (pMSSM), a well-motivated BSM model, with a neutralino thermal dark matter (DM) contributing to the invisible Higgs boson decay (M_DM ≤ M_h/2). We consider both positive and negative values of the higgsino mass parameter (µ) and track down the region of parameter space consistent with the current collider and astrophysical constraints. Our investigation shows that the recent experimental results put this scenario under severe tension.

 Experimental searches have largely constrained the parameter space with prompt BSM particles leading to conventional signatures in many popular BSM theories. We, therefore, shift to non-conventional displaced scenarios in our subsequent study of long-lived mediator particles being pair produced from the decay of the Higgs boson (M_LLP ≤ M_h/2) and their subsequent decay into standard model particles. We compute the projected sensitivity of using the muon spectrometer of the CMS detector at the high luminosity version of the Large Hadron Collider (HL-LHC) experiment for different production modes of the Higgs boson and various decay modes of the mediator particle, along with dedicated detectors for LLP searches like CODEX-b and MATHUSLA. Subsequently, we study the prospects at the hadronic future circular collider (FCC-hh), expected to reach a centre-of-mass energy of 100 TeV. We propose the DELIGHT detectors for dedicated LLP searches at the FCC-hh and study their sensitivity.

 The second part includes studies focusing on different kinds of LLP signatures using various simplified BSM scenarios as benchmarks. We demonstrate how the structure of collider detectors creates a distinction between the energy deposition pattern of prompt jets and displaced jets, where the latter comes from an LLP decay. Given the diverse applications of machine learning (ML) techniques and their promising results in various BSM searches, we study the usefulness of a convolutional neural network (CNN) in learning these differences and discriminating displaced jets from prompt jets, playing an essential role in LLP searches. Finally, we end on an optimistic note, which is motivated by the question of what can be said about the properties of the LLPs, like their mass and especially lifetime, once they are discovered. We discuss the challenges and remedies in estimating the mass and lifetime of an LLP, for a wide range of signatures, provided a few such LLP events are observed at HL-LHC.

Thesis colloquium

Name: Adithi Udupa

Title:   Transport and quantum criticality in topological systems and spin models

Date:  March 31st, 2023

Time:  2:30 pm

Venue: Multimedia Room in Physical Sciences Building

Research Supervisor:   Prof. Diptiman Sen

Abstract:

Topological insulators are a special class of materials which are insulators in the bulk but have gapless modes on the surfaces which conduct. They have been studied extensively both theoretically and experimentally because of their remarkable physical and mathematical properties. We study ballistic transport in the thin films of these materials in the presence of potential and magnetic barriers taking into consideration the hybridisation of the surface states. We also study the effects on the conductance in these materials by periodically driving the magnetic barrier, say, by shining light. We see that the conductance exhibits interesting features as a function of the driving parameters enabling us to have an elegant way of optically controlling the transport in the system.

Further we study the effects of periodic driving in a system of a spin-1/2 fermionic Hubbard model on a triangular lattice at half filling in the large interaction limit. Up to third-order perturbation in the hopping, we see that driving by an in-plane electric field gives rise to different two-spin couplings in the three directions as well as a chiral three-spin term. Consequently it allows for the effective spin model to have many interesting ordered and disordered phases. 

A three-spin interaction term is further studied in a simple 1-D spin chain with a transverse field. This system behaves differently from the usual transverse field Ising model in terms of the quantum criticality and integrability. We also find that the model shows signatures of quantum many-body scars.

Finally, we study a system of graphene interfaced with WSe_2 to enhance the strength of the spin-orbit coupling in graphene. This is important to cater to applications of graphene in spintronic devices. We analyse the experimental measurements of the band dispersion in this system and find the values of the different spin-orbit couplings.

Thesis colloquium

Name                      : Ahmadullah Zahed  

Supervisor         : Prof. Aninda Sinha

Title                         : New tools to constrain EFTs and CFTs

Date & Time      : 5th April 2023 at 2:30 pm

Venue                    :  Lecture Hall 3, New Physical Science Building 

Abstract:-

In this thesis, we develop new methods for the S matrix bootstrap in the  context of 2-2 scattering amplitudes and four-point correlators in  conformal field theories (CFTs). 

 For 2-2 scattering in quantum field theories, we consider manifestly  three-channel crossing symmetric dispersion relation (CSDR), unlike the  two-channel symmetric fixed-t dispersion relation. We give simple  derivations of certain known positivity conditions for effective field  theories, including the null constraints, which lead to two-sided bounds  and derive a general set of new nonperturbative inequalities. We derive  the analyticity domain of the CSDR analogously to the Lehmann-Martin  ellipse. We present a fascinating correspondence between an area of  mathematics called geometric function theory (GFT) and the scattering  amplitudes focusing on the case with $O(N)$ global symmetry. We obtain  two-sided bounds on Wilson coefficients of physical Pion amplitudes via  positivity and GFT. 

 Then we consider Bell correlations in light-by-light (LbyL) scattering  at low energies. The known contributions in the Standard Model (SM) lead  to Bell violation at all scattering angles except for a small transverse  region, leading to a fine-tuning problem. Incorporating a light  axion/axion-like particle (ALP) removes this problem and constrains the  axion-coupling--axion-mass parameter space. 

 In the second part of the thesis, we consider CSDR for Mellin amplitudes  of scalar four-point correlators in conformal field theories. This  allows us to rigorously set up the nonperturbative Polyakov bootstrap  for the conformal field theories in Mellin space, fixing the contact  term ambiguities in previous work. Our framework allows us to connect  with the conceptually rich picture of the Polyakov blocks being  identified with Witten diagrams in anti-de Sitter space. We also give  two-sided bounds for Wilson coefficients for effective field theories in  anti-de Sitter space. The derivation of the Polyakov bootstrap allows  rigorous epsilon expansion solely from bootstrap principles. 

Thesis colloquium

Name                      : Sabarnya Mitra  

Supervisor        : Prof. Prasad Hegde

Title                         : Unbiased Exponential resummation of lattice QCD Taylor series at finite chemical potential

Date & Time      : 14th March 2023  at 2:30 pm

Venue                     :  Lecture Hall 3, New Physical Science Building 

Abstract:-

The exponential resummation of Taylor series up to first N coefficients has been proposed in order to circumvent the computational hurdle of calculating high-order Taylor coefficients, with reliable precision in Lattice QCD. Unfortunately, this method causes apperance of biased estimates, which can lead to wrong results for higher values and orders of chemical potential $\mu$. We analyze these biased estimates order-by-order by applying cumulant expansion and perform a comparative study with Taylor series results, which involves calculations with unbiased powers only. We find that this method gives good agreement with Taylor series results, but at the expense of reweighting factor and all-order resummation. This motivates us to present a new formalism of unbiased exponential resummation which can exactly replicate Taylor series results up to a desired order in $\mu$. We illustrate this using pressure and number density results for baryon chemical potential $\mu_B$. We find this partially unbiased formalism produces an all-ordered Taylor series exactly, by exponentiating an infinite cumulant series or a series involving unbiased corrections to all orders in $\mu$.   

Thesis colloquium

Speaker:  Jyotirmoy Mukherjee

Title:  Entanglement properties of gauge theories and the graviton.

Advisor: Justin David.

Date, Venue:  14th Feb 2023 at 4pm,  LH-3

Abstract:

Entanglement entropy is emerging as a useful quantity to study critical phenomena in quantum systems, black holes physics, and holography. In this presentation, we discuss entanglement properties of gauge theories and the graviton in the ground state as well as excited states corresponding to  local quenches.

We show that the quantum entanglement entropy of the ground state of the free Maxwell field in d=4 dimensions, conformal p-forms, and conformal higher spins can be obtained from the partition function on the hyperbolic cylinder. We demonstrate that the entanglement entropy of linearized gravitons across a sphere coincides with that obtained from the partition function of Kaluza-Klein tower of traceless transverse massive spin-2 fields on the hyperbolic cylinder with the mass of the constant mode along S^1 direction saturating the  Brietenholer-Freedman bound in AdS_3.

We show that the Gauss law of gravity implies that certain radial components of Riemann tensors label the super-selection sectors for the graviton.  The classical or non-extractable part of the entanglement entropy is evaluated from the two-point functions of certain components of the Riemann tensors on S^2 which coincides with the logarithmic divergent piece of the `edge' partition function of the massless spin-2 field on the 4-sphere when written in terms of its Harish-Chandra character.

We develop a systematic procedure to evaluate the growth in entanglement entropies under local quenches created by free fields with spin, s ≤ 2. We show that in the zero-width limit of the quench, entanglement grows in time and then saturates at log(2) for free fields.  The growth profile is determined by order 2s + 1  polynomials in the ratio of the distance from the co-dimension-2 entangling surface and time. The polynomials corresponding to quenches created by the fields can be organized in terms of their representations under the SO(2)_T X SO(2)_L symmetry preserved by the presence of the co-dimension-2 entangling surface.