1,721,616 research outputs found
Central exclusive production of top quarks
No full textThis summer student project at CERN was dedicated to exclusive production of top quarks. For proton colliders exclusive production means that colliding protons after the collision do not get destroyed but through photons (25% of times) or gluons (75% of times) loose some of their momentum and create in our case top and anti top quark pair (see Feynman diagram, figure 1). Due to the momentum loss, these protons are more bent in the magnetic field of the accelerator and escape the beam. There is no detector in the CMS to detect protons but as these protons are declined from the beam only slightly, it is possible to detect them in special detectors called Roman pots (RP) owned by the TOTEM experiment [1]. RP are aligned on both sides of the interaction point. From the position in the RP where the proton was detected and knowing how protons interact with the magnetic field bending the proton beam, it is possi-ble to calculate the fractional momentum loss ξ of a proton - how much momentum it lost after the collision. The proton interaction with the magnetic field can be described by the dispersion value Dx so that xRP = Dx(ξ)ξ If the energy loss of both protons is known, it is also possible to reconstruct the invariant mass of the tt¯system by a formula: mtt¯ = √sξ1ξ2, (1) where s is the centre of mass energy. The exclusive production is a new research field at CERN because it uses data from detectors owned by two different experiments - CMS and TOTEM. Only in a recent years a joint experiment called CMS-TOTEM Precision Proton Spectrometer (CT-PPS) was created and so far only one paper on the exclu-sive production has been published. It provided an evidence for the semi exclusive µµ− pair production -an event where only one of the protons was detected [2]. One of the problems for obtaining a good results for the exclusive top quark production is a large number of the background protons in the RP which could be solved by using the low pile up (low PU) data. The goal of this project was to make the first examination of data from the low PU run of 2018. At the time of doing this project the dispersion value Dx for the low pile up run of 2018 was not known but was to be calculated soon. This work would ensure the usage of these data for the search of central exclusive top quark production as soon as all of the information about the dispersion of the protons for this run would be received. The main tasks was to calculate the resolution of tt¯system for the 2018 data, improve the selection criteria, look for a shift of the PT for the central system at low PT scale and look if there are any problems with the data
Review of the measurements of the strong coupling constant in CMS at 13 TeV
No full textThe strong coupling constant is the least known of the coupling constants in the standardmodel. Nevertheless it appears in the calculations of cross sections of all the processes at the LHC. We present a review of the strong coupling constant measurements conducted at the CMS experiment, focusing on those performed at a center-of-mass of 13 TeV
Determination of the Diffusion coefficient from the theoretical calculations of the Brownian motion
No full textLiteratūrā atrodamas vairākas difūzijas koeficienta no Brauna kustības rēķināšanas metodes. Šajā darbā tiek apskatītas un ar skaitliskām simulācijām pārbaudītas pārklājošos, nepārklājošos un nekorelēto pārvietojumu metodes kā arī vairākas lineārās regresijas metodes, gan ar trajektorijas mērījumu kļūdu, gan bez. Tika atrasts, ka pārklājošos intervālu metode no apskatītajām sniedz vismazāko rezultāta kļūdu. Papildus tam tiek atrasta sakarība, kas saista izmērītās trajektorijas punktu skaitu ar iegūtā difūzijas koeficienta kļūdu un mērījuma kļūdu.In the literature it is possible to find several methods for computing the diffusion coefficient from a Brownian motion experiment. In this thesis the methods of overlapping, non-overlapping intervals and uncorrelated intervals as well as several methods of linear regression are being examined both with and without the measurement error. From all of the reviewed methods it was found that the method of overlapping intervals gives the smallest standard deviation from the true diffusion coefficient. Additionally a relationship between the number of measured trajectory points, the error of the obtained diffusion coefficients and the error of measurement is obtained
A Program for SU(N) Color Structure Decomposition into Multiplet Bases Using Wigner 3j and 6j Coefficients
No full textThe increased capacity of elementary particle accelerators raises the demand for the simulation data of the experiments. One of the bottlenecks in the simulations is the QCD color structure calculation, which is usually treated using non-orthogonal and overcomplete sets of bases. The computational cost could be decreased significantly if orthogonal bases, such as the multiplet bases, were used instead. However, no computation tool performing calculations using these bases is available yet. In this thesis, we present a Mathematica program as proof-of-principle demonstrating the color structure decomposition into the multiplet bases. For a given amplitude, the corresponding multiplet basis states can be created and the scalar product between the amplitude and each of the basis states can be evaluated whenever the required Wigner 6j coefficients are available. The program offers tools for visualization of the tensor expressions in the birdtrack notation as well as a syntax similar to how the tensor expressions would be defined on paper. The available functions and replacement rules allow performing operations on SU(Nc) tensor expressions including index contraction, tensor conjugation, and scalar product of tensors.It might be stunning to realize that researching the smallest constituents of the world we see requires building the largest constructions people have created. On the border of Switzerland and France one can find the greatest of the examples, the Large Hadron Collider, a more than eight kilometers in diameter large ring filled with a vacuum where scientists let bunches of around 100 million protons collide almost 40 million times per second. Despite all the great discoveries (you might have heard of the Higgs boson found in 2012, for example), particle physicists often claim that further explorations of the fundamentals of the Universe require building even larger colliders and increasing the number of collisions even more. However, something is often left out in this demand for faster, bigger, and stronger. In order to find new unknown physics, we must obtain just as much data from the computer simulations as we get from the experiments. Like with two fingerprints, we analyze all the different curves and shapes in the graphs obtained from these two data sets. Any discrepancy found in them would give a clue on where to search for discoveries such as new types of elementary particles, or even change our view on how our Universe works. My contribution to particle physics is connected to speeding up the methods of mathematical simulations of the collision data. A calculation of each clash of particles, like a collision of two asteroids, involves keeping track of a tremendous mess of collision products --- where they fly, how fast and how they interact with each other. What is even more complicated to calculate are predictions about what kind of particles get to be created in each of these collisions. This is determined by the laws of \textit{Quantum Field Theory}, a theory stating that all particles can be viewed as energetic bumps in some invisible fields spanning the whole Universe. The theory predicts how particles get created and destroyed in the interaction points like waves on a drum membrane when it is hit. %Some of the particles created in these tiny interaction points attract each other so strongly that they combine long before hitting the detectors. These particles are called quarks and gluons and this immensely strong force is called, well, the strong force. An even more peculiar feature of this force is that unlike the electromagnetic charge, the strong force charge comes in three types. Gluons and quarks interact differently depending on what charge they possess. Even though simplified methods to simulate particle collisions approximately exist, for exact calculations, all of the possible color charge combinations have to be considered. This makes one of the greatest bottlenecks of the whole simulation process and is a challenge that this thesis offers a possible solution to. Some of the particles created in these tiny interaction points attract each other so strongly that they combine long before hitting the detectors. These particles are called quarks and gluons and the force that creates these immensely strong bounds is called, well, the strong force. An even more peculiar feature of this force is that unlike the electromagnetic charge, the strong force charge comes in three types. Gluons and quarks interact differently depending on what charge they possess. Even though simplified methods to simulate particle collisions approximately exist, for exact calculations, all of the possible color charge combinations have to be considered. This makes one of the greatest bottlenecks of the whole simulation process and is a challenge that this thesis offers a possible solution to. In my thesis, I computationally implemented a new technique that uses abstract mathematical objects called multiplet bases that could potentially speed up the strong force calculations. We have already validated our method for the simplest collisions. However, the highest hope is to update the multiplet bases method to be able to calculate collisions where eight or more gluons and quarks appear and where the speed differences would become more significant. When this is achieved, it should immensely increase the capacity of simulating particle collisions. In this way, physicists would be able to search for even more complicated processes and in this complexity maybe some great discoveries hide
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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