1,720,983 research outputs found
The flavor of a minimal composite S1 leptoquark and the b→cτν anomaly
No full textSeveral experiments have measured a deviation in B → D(∗) semileptonic decays, that point to new physics at the TeV scale violating lepton flavor universality. A scalar leptoquark S1, with a suitable structure of couplings in flavor space, is known to be able to solve this anomaly modifying b → cτν̄. In the context of composite Higgs models, we consider a theory containing H and S1 as Nambu-Goldstone bosons (NGBs) of a new strongly interacting sector, with ordinary resonances at a scale O(10) TeV. Assuming anarchic partial compositeness of the Standard Model (SM) fermions we calculate the potential of the NGBs that is dominated by the fermions of the third generation, we compute RD(∗) and estimate the corrections to flavor observables by the presence of S1. We find that the SM spectrum and mS1 ∼ TeV can be obtained with a NGB decay constant of order ∼ 5 TeV. We obtain a robust correlation between the main corrections to RD(∗), B K(∗)νν and gτ/gµ , that leads to a sever bound on RD(∗), roughly 2σ belowthe experimental value. Besides the bounds on the flavor observables gτW, BR(τ → µγ) and ∆mBs are saturated, with the first one requiring a coupling between resonances g∗≲ 2, whereas the second one demands mS1 ≳ 1.7 TeV, up to corrections of O(1).Fil: Da Rold, Leandro. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin
A vector leptoquark for the B-physics anomalies from a composite GUT
Full text linkA vector leptoquark at the TeV scale, mostly coupled to the fermions of the third generation, is the preferred option to explain the hints of lepton flavor universality violation in the decays of B-mesons. It seems interesting to assume that this leptoquark belongs to the same beyond the Standard Model sector that solves the hierarchy problem, since the third generation of fermions play the leading role in the instability of the Higgs potential. We present a composite Grand Unified Theory with resonances at the TeV that contains the required vector leptoquark and develops the Higgs as a pseudo Nambu-Goldstone boson. We show that anarchic partial compositeness of the Standard Model fermions can accommodate the couplings of Left-handed currents required by the B-anomalies, predicting very small couplings to the Right-handed currents without any additional hypothesis. By making use of an effective theory description of the strong dynamics, in terms of weakly coupled resonances, we are able to compute the corrections to B-physics, as well as the one-loop potential for the pseudo Nambu-Goldstone bosons. The theory has a rich phenomenology and a candidate for dark matter.Fil: Da Rold, Leandro. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Lamagna, Federico Agustín. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentin
Light custodians in natural composite Higgs models
No full textWe present a class of composite Higgs models arising from a warped extra dimension that can satisfy all the electroweak precision tests in a significant portion of their parameter space. A custodial symmetry plays a crucial role in keeping the largest corrections to the electroweak observables below their experimental limits. In these models the heaviness of the top quark is not only essential to trigger the electroweak symmetry breaking, but it also implies that the lowest top resonance and its custodial partners, the custodians, are significantly lighter than the other resonances. These custodians are the trademark of these scenarios. They are exotic colored fermions of electromagnetic charges 5/3, 2/3, and -1/3, with masses predicted roughly in the range 500-1500 GeV. We discuss their production and detection at the CERN LHC
Model for the singlet-triplet leptoquarks
Full text linkThe deviations of B-meson decays measured in RD(∗)τℓ and RK(∗)μe can be explained by the presence of two scalar leptoquarks, a singlet S1 and a triplet S3, mostly coupled to the third generation. We consider a theory of resonances, as an effective description of a strongly interacting theory, that generates the leptoquarks and the Higgs as Nambu-Goldstone bosons, with the rest of the resonances at a scale of order 10-30 TeV. We assume anarchic partial compositeness for the flavor of the SM fermions. Under this hypothesis, we study whether it is possible to reproduce the deviations in the B decays without being in conflict with flavor and electroweak bounds. We find a tension between RD(∗)τℓ and some flavor observables, dominated by flavor violating τ decays and ΔmBs, that require a tuning of order 10%-25%. We also compute the potential of the scalars showing that leptoquarks with masses O(2-3) TeV can be naturally expected in the model. We discuss briefly the phenomenology of the other resonances.Fil: Da Rold, Leandro. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; ArgentinaFil: Lamagna, Federico Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentin
A custodial symmetry for Zb\overline{\b}
No full textWe show that a subgroup of the custodial symmetry O(3) that protects delta rho from radiative corrections can also protect the Zbb coupling. This allows one to build models of electroweak symmetry breaking, such as Higgsless, Little Higgs or 5D composite Higgs models, that are safe from corrections to Z-> bb. We show that when this symmetry protects Zbb it cannot simultaneously protect Ztt and Wtb. Therefore one can expect to measure sizable deviations from the SM predictions of these couplings at future collider experiments. We also show under what circumstances Zb_R b_R can receive corrections in the right direction to explain the anomaly in the LEP/SLD forward-backward asymmetry A^b_{FB}
Vertex displacements for acausal particles: testing the Lee-Wick standard model at the LHC
Full text linkWe propose to search for wrong displaced vertices, where decay products of the secondary vertex move towards the primary vertex instead of away from it, as a signature for microscopic violation of causality. We analyze in detail the leptonic sector of the recently proposed Lee-Wick Standard Model, which provides a well motivated framework to study acausal effects. We find that, assuming Minimal Flavor Violation, the Lee-Wick partners of the electron, ℓ̃ e and ẽ, can produce measurable wrong vertices at the LHC, the most promising channel being qq̄ ℓ̃ eℓ̃ ee +e -jjjj. A Monte-Carlo simulation using MadGraph/MadEvent suggests that for M ℓ ≲450GeV the measurement of these acausal vertex displacements should be accessible in the LHC era.Fil: Alvarez, Ezequiel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Da Rold, Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Schat, Carlos Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina. Ohio University; Estados UnidosFil: Szynkman, Alejandro Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina. University of Montreal; Canad
Measuring <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mo stretchy="false">|</mml:mo><mml:msub><mml:mi>V</mml:mi><mml:mrow><mml:mi>t</mml:mi><mml:mi>d</mml:mi></mml:mrow></mml:msub><mml:mo stretchy="false">|</mml:mo></mml:math> at the LHC
No full textWe propose a direct measurement of the Cabibbo-Kobayashi-Maskawa element Vtd at the LHC. Making use of the imbalance between d and d quark content in the proton, we show that a nonzero Vtd induces a charge asymmetry in the tW associated production. The main backgrounds to this process - tt production, and tW-associated production mediated by Vtb - give charge-symmetric contributions at leading order in QCD. Therefore, using specific kinematic features of the signal, we construct a charge asymmetry in the dilepton final state which - due also to a reduction of systematic uncertainties in the asymmetry - is potentially sensitive to Vtd suppressed effects. In particular, using signal and background simulations up to the detector level, we show that this new observable could already improve the current direct upper bound on |Vtd| with existing LHC data. We also project that |Vtd| values down to ∼10 times the standard model prediction could be probed at the high-luminosity LHC.Fil: Alvarez, Ezequiel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de San Martín; ArgentinaFil: Da Rold, Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Estevez, Mariel. Universidad Nacional de San Martín; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Kamenik, Jernej F.. University of Ljubljana; Eslovenia. Jožef Stefan Institute; Esloveni
Beautiful mirrors for a pNGB Higgs
Full text linkWe consider one of the most significant deviations from the Standard Model: the forward-backward asymmetry of the b-quark measured at leptonic colliders. We investigate the possibility to solve this discrepancy by introducing new physics at the TeV scale. We focus on models where the Higgs is a pseudo Nambu-Goldstone boson of a new strongly coupled sector with a global SO(5) symmetry broken spontaneously to SO(4). Besides the usual top partners, we introduce bottom partners in the representations 16 and 4 of SO(5) and show that they can improve significantly the fit by correcting the (Formula presented.) couplings. We also estimate the corrections to the couplings at one-loop and obtain that the tree-level ones dominate and can give a reliable estimation. We find that the large shift required for (Formula presented.) leads to light custodians associated to the b-quark, similar to the top partners, as well as a rich phenomenology involving neutral interactions in the bottom-sector.Fil: Andres, Eduardo Carlos. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Da Rold, Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Davidovich, Iván Andrés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Area de Energía Nuclear. Instituto Balseiro; Argentin
Search for new physics in the third generation of quarks at the LHC
Full text linkEl quark top es posiblemente, junto al bosón de Higgs, una de las partículas menos estudiada y comprendida del Modelo Estándar. Menos estudiada porque, hasta el reciente anuncio del descubrimiento del tan buscado bosón de Higgs, fue durante mucho tiempo la última partícula descubierta (en el a˜no 1995 por los experimentos CDF y DØ del Tevatron). Además, comparativamente con otras partículas, su masa de 172 GeV la hace muy difícil de producir copiosamente en los colisionadores con el fin de realizar un estudio estadístico detallado de sus propiedades. Así, pese a los esfuerzos realizados, queda mucho por explorar sobre esta partícula. Y menos comprendida porque su masa es varios ordenes de magnitud más grande que la de cualquiera de los otros fermiones del Modelo Estándar. Con la llegada de la era del LHC (Large Hadron Collider) el quark top esta siendo estudiado como nunca antes, produciendo 80 millones de pares top-antitop por a˜no cuando alcance su luminosidad de dise˜no y examinando sus propiedades a una nueva escala de energías. Existen tanto argumentos experimentales como teóricos que impulsan, no sólo a explorar en profundidad las propiedades del quark top, sino también las de su compa˜nero electrodébil, el quark bottom. Por un lado, el quark top es el único fermión que posee un acoplamiento de orden ~ 1 con el bosón de Higgs. Desde un punto de vista teórico, ésto abre la posibilidad de que la tercera generación de quark juegue un papel fundamental en la ruptura de la simetría electrodébil. Más aún, muchas de las teorías que pretenden explicar el origen de la ruptura de la simetría predicen un fuerte acoplamiento entre la Nueva Física y estas partículas, lo cual hace muy importante su estudio. Por el otro lado, existe evidencia experimental de la posible presencia de Nueva Física en la asimetría entre la producción forward (con la dirección al haz de protones) y la producción backward (en dirección opuesta al haz de protones) tanto de pares top-antitop medida en el colisionador Tevatron, como de bottom-antibottom, medida en el LEP (Large Electron-Positron Collider). Los modelos con un sector fuertemente interactuante han sido una de las propuestas para explicar el mecanismo de la ruptura de la simetría electrodébil y solucionar el problema de la naturalidad de la masa del Higgs. En estos modelos, el bosón de Higgs es una partícula totalmente compuesta y su potencial está determinado por la dinámica del sector fuertemente interactuante. Además, éstos predicen la aparición de otras nuevas partículas compuestas en la escala TeV que interactúan fuertemente con la tercera generación de quarks, incluyendo nuevas resonancias fermiónicas de espín 1/2 y nuevos bosones de gauge masivos. Éstos últimos son producidos en colisionadores hadrónicos a través de aniquilación de quarks y decaen principalmente a un par top-antitop. Así, la producción de top-antitop vía fusión de gluones es background para las búsquedas de este tipo de resonancias. En esta tesis se estudiaron cortes en la se˜nal de producción de pares de top-antitop en el LHC que dan lugar a un aumento de la sensibilidad a la Nueva Física en las búsquedas de resonancias en el espectro de masa invariante producto del incremento de la proporción de eventos de aniquilación de quarks luego de dichos cortes. Tres variables de corte fueron consideradas en el análisis con el fin de mejorar las perspectivas de encontrar Nueva Física por medio de este tipo de búsquedas: Los momentos longitudinal y transversal del par top-antitop, y el ángulo de dispersión entre el haz de protones y el quark top. La existencia de una resonancia gluónica masiva es una de las principales propuestas de Nueva Física para explicar la desviación medida en la asimetría forward-backward en el Tevatron. En esta tesis se introdujo un modelo puramente fenomenológico donde el nuevo gluón masivo, con una masa entre 700 y 2500 GeV, interactúa con los quarks del Modelo Estándar con diferente magnitud. Si bien en el modelo no se supone ninguna teoría subyacente en particular, los resultados del Tevatron favorecen acoplamientos axiales entre el gluón masivo y los quarks, con magnitudes que van en aumento con la masa de los quarks. Así, emergen naturalmente escenarios con un nuevo sector compuesto. Finalmente, en los modelos de Higgs Compuesto es requerida la existencia de nuevas resonancias fermiónicas con cargas exóticas. En este contexto, un modelo que intenta encontrar una solución al problema de la asimetría forward-backward en la producción de quark bottom en el LEP predice una nueva resonancia de carga Q = −4/3. Esta resonancia es el compa˜nero compuesto más liviano del bottom y la principal se˜nal de Nueva Física. Con masas del orden de M ≥ 1,3 TeV, el principal mecanismo de producción de esta resonancia es la producción electrodébil simple y su decaimiento es a W−b. Se dise˜nó una estrategia de búsqueda para esta partícula en el LHC que explota las principales características cinemáticas de la producción simple para distinguir entre se˜nal y background: W− y b-jet, ambos de alto impulso transversal, y un jet liviano forward.The top quark is possibly, together with the Higgs boson, one of the particles less studied and understood of the Standard Model. Less studied because until the recent announcement of the Higgs boson discovery, it was long the last particle discovered (in 1995 by the experiments CDF and DØ at the Tevatron). Moreover and comparatively with other particles, because of its mass of 172 GeV it is hard to produce copiously in colliders in order to be able to perform a detailed statistical study of its properties. Thus, in spite of the effort done, there remains a lot to explore about this particle. Less understood because its mass is several orders of magnitude larger than any other of the Standard Model fermions. With the advent of the LHC (Large Hadron Collider) era the top quark is being studied as never before, producing 80 millions of pairs top-antitop a year when it reaches its maximum luminosity and testing its properties at a new scale of energies. There exist experimental and theoretical considerations that compel us to explore in depth the properties of the top quark but also those of its electroweak partner, the bottom quark. On one hand, the top quark is the only fermion which couples to Higgs with a coupling of order ~ 1. From a theoretical point of view, this opens the possibility that the third generation plays an important role in the mechanism of electroweak symmetry breaking. Moreover, many of the theories aiming to explain the origin of the symmetry breaking predict a strong coupling between the new physics and these particles, which makes their study very important. On the other hand, there exists experimental evidence of the possible presence of new physics in the asymmetry between the forward (in the direction of the beam of protons) and backward (in opposite direction to the beam of protons) production in top-antitop pairs measured in Tevatron as well as in bottom-antibottom measured in the LEP (Large Electron-Positron Collider). Models with a strongly interacting sector have been proposed to explain the electroweak symmetry breaking mechanism and to solve the naturalness problem in the Higgs mass. In these models, the Higgs boson is a totally composite particle and its potential is determined by the dynamics of the strongly interacting sector. Also, new composite particles arise at the TeV scale which interact strongly with the third generation of quarks, including new fermionic spin 1/2 resonances and new massive gauge bosons. The latter are produced at hadron colliders via quark annihilation and decay mainly to top-antitop pairs. Thus, the top-antitop production via gluon fusion is background for the searches of these kind of resonances. In this thesis cuts in the signal of top-antitop production in the LHC were studied, which results in an enhancement of the sensibility to the New Physics in the resonaces searches in the invariant mass spectrum because of the increment in the proportion of quark annihilation events after these cuts. Three cuts variables have been analyzed in order to improve the possibilities of finding new physics by means of these kind of resonances searches: the longitudinal and transverse momentum of the top-antitop pair and the dispersion angle between the proton beam and the top quark. The existence of a massive gluonic resonance is one of the main proposals of New Physics to explain the deviation measured in the forward-backward asymmetry at the Tevatron. A purely phenomenological model was proposed in this thesis, where the new massive gluon, with a mass between 700 and 2500 GeV, interacts with Standard Model quarks at different strength. Even though the model does not assume any underlying theory in particular, the results of Tevatron favor axial coupling between the massive gluon and the quarks, with strengths that grow with the mass of the quarks. Thus, a composite Higgs scenery naturally arises. Finally, the existence of new fermionic resonances with exotic charges is required in composite Higgs theories. In this context, a model aiming to solve the forward-backward asymmetry anomaly in the production of bottom quarks at LEP predicts a new resonance with charge Q = −4/3. This resonance is the lightest composite partner of the bottom and the main signal of New Physics. With masses M ≥ 1,3 TeV, the main production mechanism of this resonance is the single electroweak production and its decay is via W−b. In this thesis, a search strategy for this particle at the LHC was designed, exploiting the main kinematical features of the single production to distinguish signal over background: a hard W−, a hard b-jet and a forward light jet.Fil:Sanchez Vietto, Juan Ignacio. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina
Interpretation of LHC excesses in ditop and ditau channels as a 400-GeV pseudoscalar resonance
Full text linkSince the discovery in 2012 of the Higgs boson at the LHC, as the last missing piece of the Standard Model of particle physics, any hint of new physics has been intensively searched for, with no confirmation to date. There are however slight deviations from the SM that are worth investigating. The CMS collaboration has reported, in a search for heavy resonances decaying in tt¯ with a 13-TeV center-of-mass energy and a luminosity of 35.9 fb−1, deviations from the SM predictions at the 3.5σ level locally (1.9σ after the look-elsewhere effect). In addition, in the ditau final state search performed by the ATLAS collaboration at s = 13 TeV and L = 139 fb−1, deviations from the SM at the 2σ level have been also observed. Interestingly, both slight excesses are compatible with a new pseudoscalar boson with a mass around 400 GeV that couples at least to fermions of the third generation and gluons. Starting from a purely phenomenological perspective, we inspect the possibility that a 400-GeV pseudoscalar can account for these deviations and at the same time satisfy the constraints on the rest of the channels that it gives contributions to and that are analyzed by the ATLAS and CMS experiments. After obtaining the range of effective couplings compatible with all experimental measurements, we study the gauge invariant UV completions that can give rise to this type of pseudoscalar resonance, which can be accommodated in an SO(6)/SO(5) model with consistency at the 1σ level and in a SO(5) × U(1)P × U(1)X/SO(4) × U(1)X at the 2σ level, while exceedingly large quartic couplings would be necessary to account for it in a general two Higgs doublet model.Fil: Arganda Carreras, Ernesto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Da Rold, Leandro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; ArgentinaFil: Díaz, Daniel A.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Medina, Anibal Damian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; Argentin
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