98 research outputs found
Dark Matter from a Conformal Dark Sector
No full textWe consider theories in which a dark sector is described by a Conformal Field
Theory (CFT) over a broad range of energy scales. A coupling of the dark sector
to the Standard Model breaks conformal invariance. While weak at high energies,
the breaking grows in the infrared, and at a certain energy scale the theory
enters a confined (hadronic) phase. One of the hadronic excitations can play
the role of dark matter. We study a "Conformal Freeze-In" cosmological
scenario, in which the dark sector is populated through its interactions with
the SM at temperatures when it is conformal. In this scenario, the dark matter
relic density is determined by the CFT data, such as the dimension of the CFT
operator coupled to the Standard Model. We show that this simple and highly
predictive model of dark matter is phenomenologically viable. The observed
relic density is reproduced for a variety of SM operators ("portals") coupled
to the CFT, and the resulting models are consistent with observational
constraints. The mass of the COFI dark matter candidate is predicted to be in
the keV-MeV range.Comment: 50 pages incl. appendix. v2 fixes warm dark matter bound, and matches
journal versio
Conformal freeze-in from neutrino portal
No full textAbstract We study a scenario where a dark sector, described by a Conformal Field Theory (CFT), interacts with the Standard Model through the neutrino portal. In this setup, conformal invariance breaks below the electroweak scale, causing the theory to transition into a confined (hadronic) phase. One of the hadronic excitations in this phase can act as dark matter. In the “Conformal Freeze-In” cosmological framework, the dark sector is populated through interactions with the Standard Model at temperatures where it retains approximate conformal symmetry. The dark matter relic density depends on the CFT parameters, such as the dimension of the operator coupled to the Standard Model. We demonstrate that this model can reproduce the DM relic density and meet all observational constraints. The same neutrino portal interaction may also generate masses for the active neutrinos. The dark matter candidate could either be a pseudo-Goldstone boson (PGB) or a composite fermion with the quantum numbers of a sterile neutrino. In the latter case, the model is consistent with the current X-ray constraints, and may be detectable with future X-ray observations
Positivismo y antipositivismo en Argentina
No full textPositivism and Antipositivism in Argentina:
Positivism in Argentina was once a dominant influence on the thought of many intellectuals and, through them, on the very cultural formation of the country. This makes the approach of this book particularly interesting, as it subjects the currents of positivism and anti-positivism to a serious critical examination. By identifying the role played by both trends within Argentine thought, and furthermore by clarifying their influence, this book points out the derivations of positivism and anti-positivism not only in culture, but also in the local historical process. Its author, Berta Perelstein, was a pedagogue and communist intellectual whose contributions go beyond educational thought and reflection and extend into the great cultural debates, such as the one she proposes in this pioneer work, only now re-edited.PublishedEl positivismo en la Argentina tuvo en su momento una influencia preponderante sobre el pensamiento de muchos intelectuales y a través de ellos, en la propia formación cultural del país. De ahí que resulte por demás interesante el enfoque de esta obra que somete a las corrientes del positivismo y del antipositivismo a un serio examen crítico. Al ubicar el papel que tuvieron unas y otras dentro del pensamiento argentino, y más aún, al aclarar esas influencias, señala las derivaciones no solo en la cultura, sino también en nuestro proceso histórico. Su autora, Berta Perelstein, fue una pedagoga e intelectual comunista cuyos aportes desbordan el pensamiento y la reflexión educativa y se proyectan a los grandes debates culturales como el que propone en una obra precursora y hasta ahora no reeditada
Measurement Of Absolute Hadronic Branching Fractions Of D Mesons
No full textUsing 818 pb[-]1 of e+ e[-] collisions recorded at the [psi](3770) resonance with the CLEO-c detector at CESR, we determine absolute hadronic branching fractions of charged and neutral D mesons using a double tag technique. Among measurements for three D0 and six D+ modes, we obtain reference branching fractions B(D0 [RIGHTWARDS ARROW] K [-] [pi]+ ) = (3.906 ± 0.021 ± 0.062)% and B(D+ [RIGHTWARDS ARROW] K [-] [pi]+ [pi]+ ) = (9.157 ± 0.059 ± 0.125)%, where the first uncertainty is statistical, the second is systematic errors. Using an independent determination of the integrated luminosity, we also extract the cross sections [sigma](e+ e[-] [RIGHTWARDS ARROW] D0 D0 ) = (3.650 ± 0.017 ± 0.083) nb and [sigma](e+ e[-] [RIGHTWARDS ARROW] D+ D[-] ) = (2.920 ± 0.018 ± 0.062) nb at a center of mass energy, Ecm = 3774 ± 1 MeV
Techniques For The Detection Of A New Physics Signature With The Cms Experiment
No full textThis document investigates experimental and theoretical issues relating to the detection of physics beyond the Standard Model using the Compact Muon Solenoid (CMS). First, the structure of the CMS detector is reviewed. Particular attention is paid to the hardware and software components of the pixel tracking system. A description is presented of the algorithms that are used to reconstruct physics objects such as electrons and jets. Next, theoretical motivations are given for seeking new physics processes in the energy regime that is accessible by CMS. Two different theories, the Minimal Supersymmetric Standard Model and the Littlest Higgs model with T-parity, are introduced as extensions of the Standard Model. Their experimental signatures are considered, and a technique for discriminating between them using CMS observations is studied with Monte Carlo computer simulations. The remainder of the document focuses on a search for evidence of a new physics signature using CMS events that contain two electrons, two jets, and large missing transverse energy. A data-driven technique for estimating the QCD background due to fake electrons is developed, tested, and applied to this channel. The other background estimates are obtained from Monte Carlo simulations, and several sources of systematic uncertainty are surveyed. A correction factor to the Monte Carlo backgrounds is calculated from the electron recon- struction efficiencies in data and in simulation, which are measured using a tag and probe procedure. A statistical model is developed for propagating all of the background uncertainties to the calculation of the signal. Both Bayesian and semi-frequentist measures of significance are considered. The expected value of the signal is s = 0.238 ± 0.996 (stat) ± 0.304 (sys) events, and its 95% confidence interval is [0, 4.4]
Signals of Naturalness at the Large Hadron Collider
No full textThis thesis explores the hierarchy problem of the Standard Model of particle physics, its natural solutions, and the collider and cosmological signals of such theories. A review of the hierarchy problem is presented along with some of the model building tools used to solve it, followed by a complementary review of extra dimensional models which emphasizes their relation to the hierarchy problem. Several new ideas for addressing the hierarchy problem are presented in this thesis as well as their experimental manifestation. Aspects of models which exhibit neutral naturalness are explored, and it is shown that Twin Higgs models with a full mirror sector can be cosmologically viable. New signals of such a scenario include an excess contribution to the dark radiation energy density measured by the the cosmic microwave background and sterile neutrino signals at future beam dump experiments. The method of searching for long-lived Higgs boson decay products at the Large Hadron Collider is presented as the best strategy for constraining models of neutral naturalness. Another category of natural models with long-lived particles, supersymmetric models with R-parity violation, is explored and the leading bounds are derived. Finally, several ideas which were introduced to explain the infamous 750 GeV excess of 2015 are provided
Foraging Behaviour of the Particle Phenomenologist: A Broad Exploration of Physics Beyond the Standard Model
No full textDespite its numerous successes, the Standard Model (SM) of particle physics is known to be incomplete. In this thesis, we explore a variety of topics regarding physics beyond the SM (BSM). In the minimal supersymmetric Standard Model (MSSM), the leptons and Higgs have separate superpartners; however, it is possible to construct a model where the Higgs is the superpartner of one generation of left-handed leptons. In Chapter 2, we explore various implications of this Higgs-as-slepton model, in particular on electroweak precision and neutrino experiments. In Chapter 3, we use the model to explain various excesses reported in leptoquark and W' searches by the CMS experiment in 2014. These excesses were observed in electron but not muon channels, hence suggesting the violation of lepton universality, a hallmark of the model. In Chapter 4, we propose a supersymmetric left-right symmetric (SUSY LRS) model that can explain various excesses reported in W' searches by the CMS and ATLAS experiments around 2014-2015, but at the same time generate a large tree-level Higgs mass, hence resolving the usual issue of the Higgs mass being too small in the MSSM. In the most basic SUSY LRS model, a large Higgs mass also requires a large tan(beta) (the ratio of the two Higgs vacuum expectation values in the model), whereas the excesses seem to favour a smaller value. We resolve this tension by introducing heavy down-type vector-like quarks that mix with the light quarks; a large tan(beta) can now still remain consistent with the excesses. Lepton flavour models based on A4 symmetry are popular because they predict a tri-bimaximal mixing pattern for the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix, often regarded to be in good agreement with experimental measurements. However, recent results from the Daya Bay and RENO experiments have shown that the theta13 angle of the matrix is rather large, contrary to the predicted mixing pattern. We investigate in Chapter 5 whether a model based on the SO(3)-to-A4 symmetry-breaking pattern can accommodate these recent results. The enlargement to SO(3) is intended to motivate the A4 from a UV completion standpoint. We find that contrary to typical A4 models, the SO(3)-to-A4 model does generally predict a large theta13 due to mixing in the charged-lepton sector. In Chapter 6, we present a new mechanism for freeze-out dark matter which we call co-annihilation. This differs from most freeze-out mechanisms in that depletion is driven not by Boltzmann suppression, but by out-of-equilibrium decay that eventually "switches off''. Such a model is hard to probe using direct detection and collider production experiments, but should present enhanced indirect detection signatures. Hydrogen-antihydrogen (H-Hbar) oscillations are forbidden in the SM and hence represent signs of new physics. In the interstellar medium (ISM), any H atom that oscillates into Hbar may subsequently undergo annihilation with other atoms, producing gamma-rays that can be detected in astronomical surveys. While a bound on these oscillations was originally derived by Feinberg et al., we re-evaluate the bound in Chapter 7 using a more comprehensive theoretical framework, a multiphase description of the ISM, as well as recent gamma-ray data from the Fermi Large Area Telescope
Search for Supersymmetry with b-quark Jets and Missing Transverse Energy in pp Collisions at TeV
No full textWe present a search for supersymmetry in events with large , no leptons, at least three jets, and one or more -quark jets. We use a data sample corresponding to 4.98 fb of proton-proton collisions at TeV collected by the CMS experiment in 2011. The primary sources of standard model background are evaluated using data-driven techniques. We find good agreement between the data and the sum of the background predictions. The results are used to constrain the cross sections for the production of -quark-enriched final states in the context of supersymmetric models
Measurement of the top quark mass in the dileptonic decay channel at CMS
No full textThis dissertation presents a measurement of the top quark mass (Mt) in the dileptonic decay channel using data from proton-proton collisions at √s = 8 TeV recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 19.7 ± 0.5 fb−1. The analysis is based on three observables whose distributions are sensitive to the value of Mt. The Mbl invariant mass and MT2 'stransverse mass' observables are employed in a simultaneous fit to determine the value of Mt and an overall jet energy scale factor (JSF). In a complementary approach, the MT2-assisted on-shell reconstruction technique is used to construct an Mblv invariant mass observable that is combined with MT2 to measure Mt. The shapes of the observables, along with their evolutions in Mt and JSF, are modeled by a non-parametric Gaussian process regression technique. The sensitivity of the observables to the value of Mt is investigated using a Fisher information density function. The top quark mass is measured to be 172.22 ± 0.18 (stat) ± 0.91 (syst) GeV.
This dissertation also presents a missing transverse momentum (MET) significance variable, which is used to estimate the compatibility of the reconstructed MET with a zero nominal value. This variable may be used to discriminate between events containing real MET due to undetected particles and spurious MET due to object misreconstruction, finite detector resolution, or detector noise. The MET significance variable is tuned using data-driven techniques, and its performance is evaluated using the CMS Run 1 and Run 2 datasets
LOOKING FOR NEW PHYSICS: FROM DARK MATTER TO MACHINE LEARNING
No full text116 pagesEver since the Standard Model (SM) has been written down, countless efforts have been made to complement/extend it with new inputs from both theoretical and experimental sides. In this work, we provide such possible extensions from two perspectives: phenomenology (model-building) and data analysis. From the model-building perspective, dark matter has proven to be one of the most robust signs we have about new physics. As such, we propose new models involving dark matter, along with analyses of their properties and methods of probing them in experiments. An alternative approach of discovering new physics involves the use of more sophisticated data analysis methods based on modern machine learning models and techniques. On this front, we present works regarding improvements towards two existing proposals - 'Classification Without Labels' and 'Anomaly Detection with Density Estimation'
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