arXiv subjects

Stefano Di Chiara

Publications and source records attributed to Stefano Di Chiara.

26 records · Page 2Linked to original sources

arXiv2010

The Limits of Custodial Symmetry

We introduce a toy model implementing the proposal of using a custodial symmetry to protect the Z b_L bbar_L coupling from large corrections. This "doublet-extended standard model" adds a weak doublet of fermions (including a heavy partner of the top quark) to the particle content of the standard model in order to implement an O(4) x U(1)_X = SU(2)_L x SU(2)_R x P_LR x U(1)_X symmetry in the top-quark mass generating sector. This symmetry is softly broken to the gauged SU(2)_L x U(1)_Y electroweak symmetry by a Dirac mass M for the new doublet; adjusting the value of M allows us to explore the range of possibilities between the O(4)-symmetric (M to 0) and standard-model-like (M to infinity) limits.

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arXiv2009

The Limits of Custodial Symmetry

We introduce a toy model implementing the proposal of using a custodial symmetry to protect the Zbb coupling from large corrections. This "doublet-extended standard model" adds a weak doublet of fermions (including a heavy partner of the top quark) to the particle content of the standard model in order to implement an O(4) x U(1)_X = SU(2)_L x SU(2)_R x P_{LR} x U(1)_X symmetry that protects the Zbb coupling. This symmetry is softly broken to the gauged SU(2)_L x U(1)_Y electroweak symmetry by a Dirac mass M for the new doublet; adjusting the value of M allows us to explore the range of possibilities between the O(4)-symmetric (M to 0) and standard-model-like (M to infinity) limits. In this simple model, we find that the experimental limits on the Zbb coupling favor smaller M while the presence of a potentially sizable negative contribution to T strongly favors large M. A fit to all precision electroweak data shows that the heavy partner of the top quark must be heavier than about 3.4 TeV, making it difficult to search for at LHC. This result demonstrates that electroweak data strongly limits the amount by which the custodial symmetry of the top-quark mass generating sector can be enhanced relative to the standard model. Using an effective field theory calculation, we illustrate how the leading contributions to alpha T, alpha S and the Zbb coupling in this model arise from an effective operator coupling right-handed top-quarks to the Z-boson, and how the effects on these observables are correlated. We contrast this toy model with extra-dimensional models in which the extended custodial symmetry is invoked to control the size of additional contributions to alpha T and the Zbb coupling, while leaving the standard model contributions essentially unchanged.

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arXiv2008

Triplet Extended Supersymmetric Standard Model

We revisit an extension of the MSSM by adding a hypercharge-neutral, SU(2)-triplet chiral superfield. Similar to the NMSSM, the triplet gives an additional contribution to the quartic coupling in the Higgs potential, and the mass of the lightest CP-even Higgs boson can be greater than tne mass of the Z-bosn at tree-level. In addition to discussing the perturbativity, fine-tuning, and decoupling issues of this model, we compute the dominant 1-loop corrections to the mass of the lightest CP-even Higgs boson from the triplet sector. When the Higgs-Higgs-Triplet coupling in the superpotential is comparable to the top Yukawa coupling, we find that the Higgs mass can be as heavy as 140 GeV even without the traditional contributions from the top--s-top sector, and at the same time consistent with the precision electroweak constraints. At the expense of having Landau poles before the GUT scale, this opens up a previously forbidden region in the MSSM parameter space where both s-tops are light. In addition to having relatively small fine-tuning (about one part in 30), this leads to a gluo-philic Higgs boson whose production via gluon-gluon fusion at the LHC can be twice as large as the SM prediction.

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arXiv2008

Hypercharge-Universal Topcolor

We propose and discuss the phenomenology of a Topcolor-assisted Technicolor (TC2) model with a flavor-universal hypercharge sector. After discussing the symmetry breaking pattern and low-energy effective Lagrangian, we examine various experimental and theoretical constraints, finding that precision electroweak measurements yield the strongest bounds on the model. We perform a combined fit to all available $Z$-pole and LEP2 data and find that the goodness of fit for hypercharge-universal topcolor is comparable to that of the Standard Model. In contrast, TC2 models with a flavor non-universal hypercharge sectors are markedly disfavored by the data.

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arXiv2007

One Loop Corrections to the Rho Parameter in Higgsless Models

A large class of deconstructed Higgsless model is known to satisfy the tree-level experimental bounds on the electroweak precision parameters. In particular, an approximate custodial symmetry insures that the tree-level $ρ$ parameter is exactly one, for arbitrary values of the model parameters, and regardless of fermion delocalization. In this note we expand on previous work by considering the fermionic one-loop contributions to $ρ$, which are essentially due to loops with top and bottom modes. We analyze the dependence on the number $N$ of internal SU(2) sites in models with a ``flat background''. We find that the new-physics contribution rapidly increases with $N$, to quickly stabilize for large values of $N$. Experimental upper bounds on $ρ$ translate into lower bounds on the mass of the heavy fermions. These, however, are weakly correlated to $N$, and the three-site model (N=1) turns out to be already an excellent approximation for the continuum model ($N\to\infty$).

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arXiv2006

A Three Site Higgsless Model

We analyze the spectrum and properties of a highly-deconstructed Higgsless model with only three sites. Such a model contains sufficient complexity to incorporate interesting physics issues related to fermion masses and electroweak observables, yet remains simple enough that it could be encoded in a Matrix Element Generator program for use with Monte Carlo simulations. The gauge sector of this model is equivalent to that of the BESS model; the new physics of interest here lies in the fermion sector. We analyze the form of the fermion Yukawa couplings required to produce the ideal fermion delocalization that causes tree-level precision electroweak corrections to vanish. We discuss the size of one-loop corrections to b \to s γ, the weak-isospin violating parameter αT and the decay Z \to b \bar{b}. We find that the new fermiophobic vector states (the analogs of the gauge-boson KK modes in a continuum model) can be reasonably light, with a mass as low as 380 GeV, while the extra (approximately vectorial) quark and lepton states (the analogs of the fermion KK modes) must be heavier than 1.8 TeV.

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arXiv2005

Noncommutative QED corrections to e^+e^-\toγγγat linear collider energies

We compute the total cross section as well as angular and energy distributions for process $e^+e^-\toγγγ$ with both unpolarized and polarized beams in the framework of noncommutative quantum electrodynamics (NCQED). The calculation is performed in the center of mass of colliding electron and positron and is evaluated for energies and integrated luminosities appropriate to future linear colliders. We find that by using unpolarized beams it is possible to probe the Lorentz symmetry violating azimuthal dependence of the cross section. Furthermore, with polarized beams the left-right asymmetry of the CP violating NCQED amplitudes can be used to obtain bounds on the noncommutative scale $Λ_{NC}$ which exceed 1.0 TeV.

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arXiv2004

Noncommutative QCD corrections to the gluonic decays of heavy quarkonia

We compute the Noncommutative QCD (NCQCD) contributions to the three gluon decay modes of heavy quarkonia. For triplet quarkonia (ortho-quarkonia), the NCQCD correction to the QCD three gluon decay mode, like the standard model contribution, is infrared finite. In the case of singlet quarkonia (para-quarkonia), whose QCD three gluon decay mode has infrared singularities which are removed using one-loop corrections to the two gluon mode, we find that NCQCD contribution is also infrared finite. The calculations are performed in the weak binding limit and do not require the introduction of additional effective couplings.

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