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Georgi Georgiev

Publications and source records attributed to Georgi Georgiev.

36 records · Page 2Linked to original sources
arXiv2022

Analysis of photoinjector transverse phase space in action and phase coordinates

Photoinjectors are the main high brightness electron sources for X-ray free electron lasers (XFEL). Photoinjector emittance reduction is one of the key knobs for improving XFEL lasing, so precise emittance measurement is critical. It's well known that rms emittance is very sensitive to low intensity tails of particle distributions in the phase space, whose measurement depend on the signal to noise ratio (SNR) and image processing procedures. Such sensitivities make the interpretations of beam transverse brightness challenging, leading to different emittance definitions to reduce the impact of tail particles. In this paper, transverse phase space is analyzed in action and phase coordinates for both analytical models and experiments, which give a more intuitive way to calculate the beam core brightness.

physics.acc-ph↗
arXiv2022

Slice energy spread measurement in the low energy photoinjector

Slice energy spread is one of the key parameters in free electron laser optimizations, but its accurate measurement is not straightforward. Two recent studies from high energy ($>$100 MeV) photoinjectors at SwissFEL and European XFEL have reported much higher slice energy spread than expected at their XFEL working points (200 - 250 pC). In this paper, a new method for measuring slice energy spread at a lower beam energy ($\sim$20 MeV) is proposed and demonstrated at the PhotoInjector Test facility at DESY Zeuthen (PITZ), and the results for 250 pC and 500 pC are much lower than those measured at high energy injectors.

physics.acc-ph↗
arXiv2022

Leaf: Multiple-Choice Question Generation

Testing with quiz questions has proven to be an effective way to assess and improve the educational process. However, manually creating quizzes is tedious and time-consuming. To address this challenge, we present Leaf, a system for generating multiple-choice questions from factual text. In addition to being very well suited for the classroom, Leaf could also be used in an industrial setting, e.g., to facilitate onboarding and knowledge sharing, or as a component of chatbots, question answering systems, or Massive Open Online Courses (MOOCs). The code and the demo are available on https://github.com/KristiyanVachev/Leaf-Question-Generation.

cs.CL↗
arXiv2021

Exposing Paid Opinion Manipulation Trolls

Recently, Web forums have been invaded by opinion manipulation trolls. Some trolls try to influence the other users driven by their own convictions, while in other cases they can be organized and paid, e.g., by a political party or a PR agency that gives them specific instructions what to write. Finding paid trolls automatically using machine learning is a hard task, as there is no enough training data to train a classifier; yet some test data is possible to obtain, as these trolls are sometimes caught and widely exposed. In this paper, we solve the training data problem by assuming that a user who is called a troll by several different people is likely to be such, and one who has never been called a troll is unlikely to be such. We compare the profiles of (i) paid trolls vs. (ii)"mentioned" trolls vs. (iii) non-trolls, and we further show that a classifier trained to distinguish (ii) from (iii) does quite well also at telling apart (i) from (iii).

cs.LG↗
arXiv2021

Feature-Rich Named Entity Recognition for Bulgarian Using Conditional Random Fields

The paper presents a feature-rich approach to the automatic recognition and categorization of named entities (persons, organizations, locations, and miscellaneous) in news text for Bulgarian. We combine well-established features used for other languages with language-specific lexical, syntactic and morphological information. In particular, we make use of the rich tagset annotation of the BulTreeBank (680 morpho-syntactic tags), from which we derive suitable task-specific tagsets (local and nonlocal). We further add domain-specific gazetteers and additional unlabeled data, achieving F1=89.4%, which is comparable to the state-of-the-art results for English.

cs.CL↗
arXiv2021

Generating Answer Candidates for Quizzes and Answer-Aware Question Generators

In education, open-ended quiz questions have become an important tool for assessing the knowledge of students. Yet, manually preparing such questions is a tedious task, and thus automatic question generation has been proposed as a possible alternative. So far, the vast majority of research has focused on generating the question text, relying on question answering datasets with readily picked answers, and the problem of how to come up with answer candidates in the first place has been largely ignored. Here, we aim to bridge this gap. In particular, we propose a model that can generate a specified number of answer candidates for a given passage of text, which can then be used by instructors to write questions manually or can be passed as an input to automatic answer-aware question generators. Our experiments show that our proposed answer candidate generation model outperforms several baselines.

cs.CL↗
arXiv2021

Integrability of the Trapped Ionic System

In this paper we will explore the 2D system describing trapped ionic system in the quadrapole field with a superposition of rationally symmetric hexapole and octopole fields for meromorphic integrability. We use the Lyapunov's and Ziglin-Morales-Ruiz-Ramis's classical methods for the proofs. The monodromy group of the normal variational equation (NVE), which is a Heun equation, is studied for commutativity. Some classical and modern results from the theory of transcendental numbers are used for the research.

math.DS↗
arXiv2019

Feature-Rich Part-of-speech Tagging for Morphologically Complex Languages: Application to Bulgarian

We present experiments with part-of-speech tagging for Bulgarian, a Slavic language with rich inflectional and derivational morphology. Unlike most previous work, which has used a small number of grammatical categories, we work with 680 morpho-syntactic tags. We combine a large morphological lexicon with prior linguistic knowledge and guided learning from a POS-annotated corpus, achieving accuracy of 97.98%, which is a significant improvement over the state-of-the-art for Bulgarian.

cs.CL↗
arXiv2019

Non-Integrability of Geodesic dynamics of Chazy-Curzon space-time

We study the integrability of the geodesic equations of the Chazy- Curzon space-time. It was established that for the equilibrium point $p_ρ=p_z=z=0$ and, $ρ_0 \in (1,\, 2)$, there are only periodic solutions, the Hamiltonian system, describing geodesic motion of Chazy-Curzon space-time has no additional analytic first integral. Our approach is based on the following: if the system has a family of periodic solutions around an equilibrium and if the period function is infinitely branched then the system has no additional analytical first integral.

math.DS↗
arXiv2017

Where Classification Fails, Interpretation Rises

An intriguing property of deep neural networks is their inherent vulnerability to adversarial inputs, which significantly hinders their application in security-critical domains. Most existing detection methods attempt to use carefully engineered patterns to distinguish adversarial inputs from their genuine counterparts, which however can often be circumvented by adaptive adversaries. In this work, we take a completely different route by leveraging the definition of adversarial inputs: while deceiving for deep neural networks, they are barely discernible for human visions. Building upon recent advances in interpretable models, we construct a new detection framework that contrasts an input's interpretation against its classification. We validate the efficacy of this framework through extensive experiments using benchmark datasets and attacks. We believe that this work opens a new direction for designing adversarial input detection methods.

cs.LG↗
arXiv2017

Non-integrability of a system with the Dyson Potential

In this paper it is shown that the Hamiltonian system with Dyson potential is analytical non-integrable and formal non-integrable. The approach is based on the following: when a system has a family of periodic solutions around an equilibrium and if the period function is infinitely branched then the system has non additional analytic first integral. We prove formal non-integrability using Ziglin-Moralez-Ruiz-Ramis theory.

math.DS↗
arXiv2017

The Case for Being Average: A Mediocrity Approach to Style Masking and Author Obfuscation

Users posting online expect to remain anonymous unless they have logged in, which is often needed for them to be able to discuss freely on various topics. Preserving the anonymity of a text's writer can be also important in some other contexts, e.g., in the case of witness protection or anonymity programs. However, each person has his/her own style of writing, which can be analyzed using stylometry, and as a result, the true identity of the author of a piece of text can be revealed even if s/he has tried to hide it. Thus, it could be helpful to design automatic tools that can help a person obfuscate his/her identity when writing text. In particular, here we propose an approach that changes the text, so that it is pushed towards average values for some general stylometric characteristics, thus making the use of these characteristics less discriminative. The approach consists of three main steps: first, we calculate the values for some popular stylometric metrics that can indicate authorship; then we apply various transformations to the text, so that these metrics are adjusted towards the average level, while preserving the semantics and the soundness of the text; and finally, we add random noise. This approach turned out to be very efficient, and yielded the best performance on the Author Obfuscation task at the PAN-2016 competition.

cs.CL↗
arXiv2016

Exponential Self-Organization and Moore's Law: Measures and Mechanisms

The question how complex systems become more organized and efficient with time is open. Examples are, the formation of elementary particles from pure energy, the formation of atoms from particles, the formation of stars and galaxies, the formation of molecules from atoms, of organisms, and of the society. In this sequence, order appears inside complex systems and randomness (entropy) is expelled to their surroundings. Key features of self-organizing systems are that they are open and they are far away from equilibrium, with increasing energy flowing through them. This work searches for global measures of such self-organizing systems, that are predictable and do not depend on the substrate of the system studied. Our results will help to understand the existence of complex systems and mechanisms of self-organization. In part we also provide insights, in this work, about the underlying physical essence of the Moore's law and the multiple logistic growth observed in technological progress.

physics.soc-ph↗
arXiv2015

Non-Integrability of Some Higher-Order Painlevé Equations in the Sense of Liouville

In this paper we study the equation $$ w^{(4)} = 5 w" (w^2 - w') + 5 w (w')^2 - w^5 + (λz + α)w + γ, $$ which is one of the higher-order Painlevé equations (i.e., equations in the polynomial class having the Painlevé property). Like the classical Painlevé equations, this equation admits a Hamiltonian formulation, Bäcklund transformations and families of rational and special functions. We prove that this equation considered as a Hamiltonian system with parameters $γ/λ= 3 k$, $γ/λ= 3 k - 1$, $k \in \mathbb{Z}$, is not integrable in Liouville sense by means of rational first integrals. To do that we use the Ziglin-Morales-Ruiz-Ramis approach. Then we study the integrability of the second and third members of the $\mathrm{P}_{\mathrm{II}}$-hierarchy. Again as in the previous case it turns out that the normal variational equations are particular cases of the generalized confluent hypergeometric equations whose differential Galois groups are non-commutative and hence, they are obstructions to integrability.

nlin.SI↗
arXiv2012

Multigap RPC for PET: development and optimisation of the detector design

Transforming the resistive plate chambers from charged-particle into gamma-quanta detectors opens the way towards their application as a basic element of a hybrid imaging system, which combines positron emission tomography (PET) with magnetic resonance imaging (MRI) in a single device and provides non- and minimally- invasive quantitative methods for diagnostics. To this end, we performed detailed investigations encompassing the whole chain from the annihilation of the positron in the body, through the conversion of the created photons into electrons and to the optimization of the electron yield in the gas. GEANT4 based simulations of the efficiency of the RPC photon detectors with different converter materials and geometry were conducted for optimization of the detector design. The results justify the selection of a sandwich-type gas-insulator-converter design, with Bi or Pb as converter materials.

physics.ins-det↗
arXiv2010

Computerized Stokes analysis of optically active polymer films

Optics labs are an integral part of the advanced curriculum for physics majors. Students majoring in other disciplines, like chemistry, biology or engineering rarely have the opportunity to learn about the most recent optical techniques and mathematical representation used in today's science and industry optics. Stokes analysis of polarization of light is one of those methods that are increasingly necessary but are seldom taught outside advanced physics or optics classes that are limited to physics majors. On the other hand biology and chemistry majors already use matrix and polarization techniques in the labs for their specialty, which makes the transition to matrix calculations seamless. Since most of the students in those majors postpone their enrollment in physics, most of the registered in those classes are juniors and seniors, enabling them to handle those techniques. We chose to study polymer samples to aid students majoring in other disciplines, especially chemistry and engineering, with understanding of the optical nature of some of the objects of their study. The argument in this paper is that it is advantageous to introduce Stokes analysis for those students and show a lab developed and taught for several years that has successfully, in our experience, done that. Measurements of oriented and unoriented polymer samples are discussed to demonstrate to students the effects of the molecular polarizability on the sample birefringence and the anisotropic Fletcher indicatrix in general.

physics.ed-ph↗
arXiv2010

The Least Action and the Metric of an Organized System

In this paper we formulate the Least Action Principle for an Organized System as the minimum of the total sum of the actions of all of the elements. This allows us to see how this most basic law of physics determines the development of the system towards states with less action - organized states. Also we state that the metric tensor can describe the specific state of the constraints of the system, which is its actual organization. With this the organization is defined in two ways: 1. A quantitative: the action I. 2. A qualitative: the metric tensor. These two measures can describe the level of development and the specifics of the organization of a system. We consider closed and open systems.

nlin.AO↗