arXiv subjects

R. Sekhar Chivukula

Publications and source records attributed to R. Sekhar Chivukula.

At least 55 records · Page 3Linked to original sources

arXiv2013

Same-Sign Dileptons from Colored Scalars in the Flavorful Top-Coloron Model

In this paper we study the phenomenology of color-octet and color-singlet scalars in the flavorful Top-Coloron model. We discuss the relevant production mechanisms at hadron colliders and the dominant decay modes, highlighting the most promising signatures for discovery, and derive bounds on the masses of the new scalars from LHC and Tevatron data. Of particular interest is the case in which color-octet scalars are pair produced and each decay to t cbar or tbar c, leading to a same-sign-dilepton final state. LHC data places a lower limit of 440 GeV on the octet mass in this scenario. The case of an octet lighter than the top, where the octet only decays into jets, has been tested by the Tevatron, which excludes the mass region from 50 to 125 GeV. The 8 TeV LHC is not yet sensitive to the observation of the color-singlet states, which are produced at rates much smaller than the octets. Nevertheless, the color-singlet pseudoscalar can be discovered at the 14 TeV LHC by analyzing the channel where it is produced from the decay of a color-octet vector boson.

hep-ph↗

arXiv2013

Hadron Collider Production of Massive Color-Octet Vector Bosons at Next-to-Leading Order

This paper completes the study of the next-to-leading order (NLO) QCD corrections to massive color-octet vector boson production at the LHC and Tevatron. The massive color-octet vector bosons are generically referred to as colorons. Building on our previous calculation of quark-initiated coloron production at NLO, we use the pinch technique to investigate coloron production via gluon fusion. We demonstrate that this one-loop production amplitude is finite, and find that its numerical contribution to coloron production is typically four orders of magnitude smaller than the contribution from quark annihilation. Coloron production via gluon fusion is therefore only relevant if the colorons are (nearly) fermiophobic. We then present extensive plots and tables of our full results for NLO coloron production at the Tevatron and the LHC.

hep-ph↗

arXiv2013

Coloron Models and LHC Phenomenology

This talk discusses the possibility of new physics within the strong gauge interactions, specifically the idea of an extended color gauge group that is spontaneously broken to QCD. After a brief review of the literature, three of our recent pieces of work on coloron phenomenology are summarized. First, some key results on coloron production to NLO at hadron colliders are described. Next, a method of using associated production of colorons and weak vector bosons to better determine coloron couplings is discussed. Finally, a new model that naturally realizes flavor physics is reviewed.

hep-ph↗

arXiv2013

Vector Bosons Signals of Electroweak Symmetry Breaking

We study the physics potential of the 8 TeV LHC to discover signals of extended gauge models or extra dimensional models whose low energy behavior is well represented by an SU(2) x SU(2) x U(1) electroweak gauge structure. We find that with a combined integrated luminosity of 40 fb^(-1), the first new Kaluza-Klein mode of the W gauge boson can be discovered up to a mass of about 400 GeV, when produced in association with a Z boson.

hep-ph↗

arXiv2013

A Flavorful Top-Coloron Model

In this paper we introduce a simple renormalizable model of an extended color gauge sector in which the third-generation quarks couple differently than the lighter quarks. In addition to a set of heavy color-octet vector bosons (colorons), the model also contains a set of heavy weak vector quarks. Mixing between the third-generation of quarks and the first two is naturally small, and occurs only through the (suppressed) mixing of all three generations with the heavy vector quarks. We discuss the constraints on this model arising from limits on flavor-changing neutral currents and from collider searches for the colorons and vector quarks, and discuss the prospects for discovery at the LHC.

hep-ph↗

arXiv2012

Discovering New Gauge Bosons of Electroweak Symmetry Breaking at LHC-8

We study the physics potential of the 8TeV LHC (LHC-8) to discover, during its 2012 run, a large class of extended gauge models or extra dimensional models whose low energy behavior is well represented by an SU(2)^2 x U(1) gauge structure. We analyze this class of models and find that with a combined integrated luminosity of 40-60/fb at the LHC-8, the first new Kaluza-Klein mode of the W gauge boson can be discovered up to a mass of about 370-400 GeV, when produced in association with a Z boson.

hep-ph↗

arXiv2012

Discovering Strong Top Dynamics at the LHC

We analyze the phenomenology of the top-pion and top-Higgs states in models with strong top dynamics, and translate the present LHC searches for the Standard Model Higgs into bounds on these scalar states. We explore the possibility that the new state at a mass of approximately 125 GeV observed at the LHC is consistent with a neutral pseudoscalar top-pion state. We demonstrate that a neutral pseudoscalar top-pion can generate the diphoton signal at the observed rate. However, the region of model parameter space where this is the case does not correspond to classic topcolor-assisted technicolor scenarios with degenerate charged and neutral top-pions and a top-Higgs mass of order twice the top mass; rather, additional isospin violation would need to be present and the top dynamics would be more akin to that in top seesaw models. Moreover, the interpretation of the new state as a top-pion can be sustained only if the ZZ (four-lepton) and WW (two-lepton plus missing energy) signatures initially observed at the 3? level decline in significance as additional data is accrued.

hep-ph↗

arXiv2012

Probing Color Octet Couplings at the Large Hadron Collider

Color-octet resonances arise in many well motivated theories beyond the standard model. As colored objects they are produced copiously at the LHC and can be discovered in early searches for new physics in dijet final states. Once they are discovered it will be important to measure the couplings of the new resonances to determine the underlying theoretical structure. We propose a new channel, associated production of $W,Z$ gauge bosons and color-octet resonances, to help determine the chiral structure of the couplings. We present our analysis for a range of color-octet masses (2.5 to 4.5 TeV), couplings and decay widths for the LHC with center of mass energy of 14 TeV and 10 ${\rm fb}^{-1}$ or 100 ${\rm fb}^{-1}$ of integrated luminosity. We find that the LHC can probe a large region of the parameter space up to very small couplings.

hep-ph↗

arXiv2012

Technicolor in the LHC Era

LHC searches for the standard model Higgs Boson in γγ or ττ decay modes place strong constraints on the light technipion state predicted in technicolor models that include colored technifermions. Compared with the standard Higgs Boson, the technipions have an enhanced production rate (largely because the technipion decay constant is smaller than the weak scale) and also enhanced branching ratios into di-photon and di-tau final states (largely due to the suppression of WW decays of the technipions). Recent ATLAS and CMS searches for Higgs bosons exclude the presence of technipions with masses from 110 GeV to nearly 2m_t in technicolor models that (a) include colored technifermions (b) feature topcolor dynamics and (c) have technicolor groups with three or more technicolors (N_{TC} > 3).

hep-ph↗

arXiv2012

Production of Massive Color-Octet Vector Bosons at Next-to-Leading Order

We report the first complete calculation of QCD corrections to the production of a massive color-octet vector boson. Our next-to-leading-order (NLO) calculation includes both virtual corrections as well as corrections arising from the emission of gluons and light quarks, and we demonstrate the reduction in factorization-scale dependence relative to the leading-order approximation used in previous hadron collider studies. We show that the QCD NLO corrections to coloron production are as large as 30%, and that the residual factorization scale-dependence is reduced to of order 2%. We also calculate the K-factor and the p_T spectrum for coloron production, since these are valuable for comparison with experiment. Our results apply directly to the production of the massive color-octet vector bosons in axigluon, topcolor, and coloron models, and approximately to the production of KK gluons in extra-dimensional models or color-octet technivector mesons in technicolor models.

hep-ph↗

arXiv2011

Topcolor in the LHC Era

Ongoing LHC searches for the standard model Higgs Boson in WW or ZZ decay modes strongly constrain the top-Higgs state predicted in many models with new dynamics that preferentially affects top quarks. Such a state couples strongly to top-quarks, and is therefore produced through gluon fusion at a rate that can be greatly enhanced relative to the rate for the standard model Higgs boson. As we discuss in this talk, a top-Higgs state with mass less than 300 GeV is excluded at 95% CL if the associated top-pion has a mass of 150 GeV, and the constraint is even stronger if the mass of the top-pion state exceeds the top-quark mass or if the top-pion decay constant is a substantial fraction of the weak scale. These results have significant implications for theories with strong top dynamics, such as topcolor-assisted technicolor, top-seesaw models, and certain Higgsless models.

hep-ph↗

arXiv2011

Technipion Limits from LHC Higgs Searches

LHC searches for the standard model Higgs Boson in di-photon or di-tau decay modes place strong constraints on the light top-pion state predicted in technicolor models that include colored technifermions. Compared with the standard Higgs Boson, the top-pions have an enhanced production rate (largely because the technipion decay constant is smaller than the weak scale) and also enhanced branching ratios into di-photon and di-tau final states (largely due to the suppression of WW decays of the technipions). These factors combine to make the technipions more visible in both channels than a standard model Higgs would be. Hence, the recent ATLAS and CMS searches for Higgs bosons exclude the presence of technipions with masses from 110 GeV to nearly twice the top-quark mass in technicolor models that (a) include colored technifermions (b) feature topcolor dynamics and (c) have technicolor groups with three or more technicolors. For certain models, the limits also apply out to higher technipion masses or down to the minimum number of technicolors. The limits may be softened somewhat in models where extended technicolor plays a significant role in producing the top quark's mass. Additional LHC data on di-tau and di-photon final states will be extremely valuable in further exploring technicolor parameter space.

hep-ph↗

arXiv2011

LHC Limits on the Top-Higgs in Models with Strong Top-Quark Dynamics

LHC searches for the standard model Higgs Boson in WW or ZZ decay modes place strong constraints on the top-Higgs state predicted in many models with new dynamics preferentially affecting top quarks. Such a state couples strongly to top-quarks, and is therefore produced through gluon fusion at a rate enhanced relative to the rate for the standard model Higgs boson. A top-Higgs state with mass less than 300 GeV is excluded at 95% CL if the associated top-pion has a mass of 150 GeV, and the constraint is even stronger if the mass of the top-pion state exceeds the top-quark mass or if the top-pion decay constant is a substantial fraction of the weak scale. These results have significant implications for theories with strong top dynamics, such as topcolor-assisted technicolor, top-seesaw models, and certain Higgsless models.

hep-ph↗

arXiv2011

The Flavor Structure of the Three-Site Higgsless Model

We study the flavor structure of the three-site Higgsless model and evaluate the constraints on the model arising from flavor physics. We find that current data constrain the model to exhibit only minimal flavor violation at tree level. Moreover, at the one-loop level, by studying the leading chiral logarithmic corrections to chirality-preserving Delta F = 1 and Delta F = 2 processes from new physics in the model, we show that the combination of minimal flavor violation and ideal delocalization ensures that these flavor-changing effects are sufficiently small that the model remains phenomenologically viable.

hep-ph↗

arXiv2011

Patterns of Custodial Isospin Violation from a Composite Top

In this paper we consider the effects of top quark compositeness on the electroweak parameters T-hat and S-hat and the Zbb coupling. We do so by using an effective field theory analysis to identify several promising patterns of mixing between standard model like and vector fermions, and then analyzing simple extensions of the Standard Model that realize those patterns. These models illustrate four ways in which an extended O(4) symmetry, which controls the size of radiative corrections to the observables discussed, may be broken. These models may also be viewed as highly-deconstructed versions of five-dimensional gauge theories dual to various strongly-interacting composite Higgs theories. We comment on how our results relate to extra-dimensional models previously considered, and we demonstrate that one pattern of O(4) breaking is phenomenologically favored.

hep-ph↗

arXiv2011

Simplified Models for LHC New Physics Searches

This document proposes a collection of simplified models relevant to the design of new-physics searches at the LHC and the characterization of their results. Both ATLAS and CMS have already presented some results in terms of simplified models, and we encourage them to continue and expand this effort, which supplements both signature-based results and benchmark model interpretations. A simplified model is defined by an effective Lagrangian describing the interactions of a small number of new particles. Simplified models can equally well be described by a small number of masses and cross-sections. These parameters are directly related to collider physics observables, making simplified models a particularly effective framework for evaluating searches and a useful starting point for characterizing positive signals of new physics. This document serves as an official summary of the results from the "Topologies for Early LHC Searches" workshop, held at SLAC in September of 2010, the purpose of which was to develop a set of representative models that can be used to cover all relevant phase space in experimental searches. Particular emphasis is placed on searches relevant for the first ~50-500 pb-1 of data and those motivated by supersymmetric models. This note largely summarizes material posted at http://lhcnewphysics.org/, which includes simplified model definitions, Monte Carlo material, and supporting contacts within the theory community. We also comment on future developments that may be useful as more data is gathered and analyzed by the experiments.

hep-ph↗

arXiv2011

Top-Higgs and Top-pion phenomenology in the Top Triangle Moose model

We discuss the deconstructed version of a topcolor-assisted technicolor model wherein the mechanism of top quark mass generation is separated from the rest of electroweak symmetry breaking. The minimal deconstructed version of this scenario is a "triangle moose" model, where the top quark gets its mass from coupling to a top-Higgs field, while the gauge boson masses are generated from a Higgsless sector. The spectrum of the model includes scalar (top-Higgs) and pseudoscalar (top-pion) states. In this paper, we study the properties of these particles, discuss their production mechanisms and decay modes, and suggest how best to search for them at the LHC.

hep-ph↗

arXiv2010

Technicolor and Lattice Gauge Theory

Technicolor and other theories of dynamical electroweak symmetry breaking invoke chiral symmetry breaking triggered by strong gauge-dynamics, analogous to that found in QCD, to explain the observed W, Z, and fermion masses. In this talk we describe why a realistic theory of dynamical electroweak symmetry breaking must, relative to QCD, produce an enhanced fermion condensate. We quantify the degree to which the technicolor condensate must be enhanced in order to yield the observed quark masses, and still be consistent with phenomenological constraints on flavor-changing neutral-currents. Lattice studies of technicolor and related theories provide the only way to demonstrate that such enhancements are possible and, hopefully, to discover viable candidate models. We comment briefly on the current status of non-perturbative investigations of dynamical electroweak symmetry breaking, and provide a "wish-list" of phenomenologically-relevant properties that are important to calculate in these theories

hep-lat↗