Mass hierarchy and physics beyond the Standard Model

2000

Supersymmetric models with an inverted mass hierarchy (IMH: multi-TeV first and second generation matter scalars, and sub-TeV third generation and Higgs scalars) have been proposed to ameliorate phenomenological problems arising from flavor changing neutral currents (FCNCs) and CP violating processes, while satisfying conditions of naturalness. Models with an IMH already in place at the GUT scale have been shown to be constrained in that for many model parameter choices, the top squark squared mass is driven to negative values. We delineate regions of parameter space where viable models with a GUT scale IMH can be generated. We find that larger values of GUT scale first and second generation scalar masses act to suppress third generation scalars, leading to acceptable solutions if GUT scale gaugino masses are large enough. We show examples of viable models and comment on their characteristic features. For example, in these models the gluino mass is bounded from below, and effectively decouples, whilst third generation scalars remain at sub-TeV levels. While possibly fulfilling criteria of naturalness, these models present challenges for detection at future pp and e + e − collider experiments.

Nuclear Physics B, 1999

One of the drawbacks of conventional grand unification scenarios has been that the unification scale is too high to permit direct exploration. In this paper, we show that the unification scale can be significantly lowered (perhaps even to the TeV scale) through the appearance of extra spacetime dimensions. Such extra dimensions are a natural consequence of string theories with largeradius compactifications. We show that extra spacetime dimensions naturally lead to gauge coupling unification at intermediate mass scales, and moreover may provide a natural mechanism for explaining the fermion mass hierarchy by permitting the fermion masses to evolve with a power-law dependence on the mass scale. We also show that proton-decay constraints may be satisfied in our scenario due to the higher-dimensional cancellation of proton-decay amplitudes to all orders in perturbation theory. Finally, we extend these results by considering theories without supersymmetry; experimental collider signatures; and embeddings into string theory. The latter also enables us to develop several novel methods of explaining the fermion mass hierarchy via D-branes. Our results therefore suggest a new approach towards understanding the physics of grand unification as well as the phenomenology of large-radius string compactifications.

Springer Theses, 2015

This collection of studies on new physics at the LHC constitutes the report of the supersymmetry working group at the Workshop 'Physics at TeV Colliders', Les Houches, France, 2007. They cover the wide spectrum of phenomenology in the LHC era, from alternative models and signatures to the extraction of relevant observables, the study of the MSSM parameter space and finally to the interplay of LHC observations with additional data expected on a similar time scale. The special feature of this collection is that while not each of the studies is explicitely performed together by theoretical and experimental LHC physicists, all of them were inspired by and discussed in this particular environment.

Journal of High Energy Physics, 2007

In the standard model in universal extra dimensions (UED) the mass of the Higgs field is driven to the cutoff of the higher-dimensional theory. This reintroduces a small hierarchy since the compactification scale 1/R should not be smaller than the weak scale. In this paper we study possible solutions to this problem by considering five-dimensional theories where the Higgs field potential vanishes at tree level due to a global symmetry. We consider two avenues: a Little Higgs model and a Twin Higgs model. An obstacle for the embedding of these four-dimensional models in five dimensions is that their logarithmic sensitivity to the cutoff will result in linear divergences in the higher dimensional theory. We show that, despite the increased cutoff sensitivity of higher dimensional theories, it is possible to control the Higgs mass in these two scenarios. For the Little Higgs model studied, the phenomenology will be significantly different from the case of the standard model in UED. This is due to the fact that the compactification scale approximately coincides with the scale where the masses of the new states appear. For the case of the Twin Higgs model, the compactification scale may be considerably lower than the scale where the new states appear. If it is as low as allowed by current limits, it would be possible to experimentally observe the standard model Kaluza-Klein states as well as a new heavy quark. On the other hand, if the compactification scale is higher, then the phenomenology at colliders would coincide with the one for the standard model in UED.

EPJ Web of Conferences, 2017

We discuss the history of the extra dimensions hypothesis and the physics and phenomenology of models with large extra dimensions with an emphasis on the Randall-Sundrum (RS) model with two branes. We argue that the Standard Model extension based on the RS model with two branes is phenomenologically acceptable only if the inter-brane distance is stabilized. Within such an extension of the Standard Model, we study the influence of the infinite Kaluza-Klein (KK) towers of the bulk fields on collider processes. In particular, we discuss the modification of the scalar sector of the theory, the Higgs-radion mixing due to the coupling of the Higgs boson to the radion and its KK tower, and the experimental restrictions on the mass of the radion-dominated states.

1998

Recently, a new framework for solving the hierarchy problem has been proposed which does not rely on low energy supersymmetry or technicolor. The gravitational and gauge interactions unite at the electroweak scale, and the observed weakness of gravity at long distances is due the existence of large new spatial dimensions. In this letter, we show that this framework can be embedded in string theory. These models have a perturbative description in the context of type I string theory. The gravitational sector consists of closed strings propagating in the higher-dimensional bulk, while ordinary matter consists of open strings living on D3-branes. This scenario raises the exciting possibility that the LHC and NLC will experimentally study ordinary aspects of string physics such as the production of narrow Regge-excitations of all standard model particles, as well more exotic phenomena involving strong gravity such as the production of black holes. The new dimensions can be probed by events with large missing energy carried off by gravitons escaping into the bulk. We finally discuss some important issues of model building, such as proton stability, gauge coupling unification and supersymmetry breaking. * Actually λ corresponds to the vacuum expectation value of a dynamical scalar field. † In fact α G should be replaced by kα G , where k is the integer Kac-Moody level.

Proceedings of Corfu Summer Institute 2017 "Schools and Workshops on Elementary Particle Physics and Gravity" — PoS(CORFU2017), 2018

Proceedings of Proceedings of the Corfu Summer Institute 2011 — PoS(CORFU2011), 2012

Lowering the string scale in the TeV region provides a theoretical framework for solving the mass hierarchy problem and unifying all interactions. The apparent weakness of gravity can then be accounted by the existence of large internal dimensions, in the submillimeter region, and transverse to a braneworld where our universe must be confined. I review the main properties of this scenario and its experimental implications.

Journal of High Energy Physics, 2000

We study the theoretical correlation between the Higgs mass of the minimal standard model and the scale at which new physics is expected to occur. In addition to the classic constraints of unitarity, triviality and vacuum stability, we reexamine the constraints imposed by the precision electroweak data. We then pay particular attention to the constraint imposed by the absence of finetuning in the Higgs mass parameter (the Veltman condition). We find that the fine-tuning condition places a significant constraint on the new physics scale for the Higgs mass range 100 GeV < ∼ m h < ∼ 200 GeV mostly unconstrained by the classic constraints.

Physics Letters B, 1998

Recently, the LEP collaborations have reported a lower bound on a Standard Model-like Higgs boson of order 89 GeV. We discuss the implications of this bound for the minimal supersymmetric extension of the Standard Model (MSSM). In particular, we show that the lower bound on tan β, which can be obtained from the presently allowed Higgs boson mass value, becomes stronger than the one set by the requirement of perturbative consistency of the theory up to scales of order M GU T (associated with the infrared fixed-point solution of the top quark Yukawa coupling) in a large fraction of the allowed parameter space. The potentiality of future LEP2 searches to further probe the MSSM parameter space is also discussed.