Advances in High Energy Physics

Volume 2018, Article ID 3471023, 9 pages

https://doi.org/10.1155/2018/3471023

## Searches for Massive Graviton Resonances at the LHC

Institute for Nuclear Research, National Academy of Sciences of Ukraine, 47 Prosp. Nauki, Kiev 03028, Ukraine

Correspondence should be addressed to T. V. Obikhod; au.veik.rnik@dohkibo

Received 17 January 2018; Accepted 21 February 2018; Published 30 April 2018

Academic Editor: Marek Nowakowski

Copyright © 2018 T. V. Obikhod and I. A. Petrenko. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The publication of this article was funded by SCOAP^{3}.

#### Abstract

The Standard Model problems lead to the new theories of extra dimensions: Randall-Sundrum model, Arkani-Hamed-Dimopoulos-Dvali model, and model. In the framework of these models, with the help of computer program Pythia8.2, the production cross sections for Kaluza-Klein particles at various energies at the LHC were calculated. The generation of monojet events from scalar graviton emission was considered for number of extra dimensions (, 4, and 6) for the energy at the LHC 14 TeV. The graviton production processes through the gluon-gluon, quark-gluon, and quark-quark fusion processes are also studied and some periodicity was found in the behavior of the graviton mass spectrum. Production cross sections multiplied by branching fractions were calculated for the massive graviton, G, within Randall-Sundrum scenario and the most probable processes of graviton decay at 13 TeV, 14 TeV, and 100 TeV were counted.

#### 1. Introduction

The problems with theoretical explanation of vacuum energy as well as dark energy, dark matter, and cosmological constant problems are only the tip of the iceberg of problems in the modern theoretical physics. Some of them are(i)ordinary matter accounting for about 5% of mass energy in the Universe and no dark matter candidate in the Standard Model (SM),(ii)hierarchy problem,(iii)fine tuning of SM Higgs mass,(iv)no explanation for fermion masses and mixings and three family structures,(v)unification of strong, electroweak, and gravitational forces,(vi)compositeness of leptons and quarks,

It is an experimental fact that there is something we cannot explain within the SM.

As is known, vacuum is produced in the processes of phase transitions in Early Universe and the space-time structure of the physical vacuum exhibits the connection to the structure formation in our Universe. So, the understanding of Universe formation is deeply connected with the conception of the space-time. According to hierarchy formula [1], Plank energy can be reduced to the energy of about 10 TeV that is achieved at the LHC. So, the phenomena of the Universe formation at the early stages and the accompanying processes of particle physics could be studied at modern colliders. In spite of the fact that no new physics beyond the SM is discovered at the LHC at 13 TeV, the planned upgrading of the LHC to high luminosities and energies up to 100 TeV gives the possibility for the discovery of new physics. Among such searches for new physics, the most popular are the experimental searches for the Kaluza-Klein (KK) particles.

Historically, KK theory appeared as the unification of gravitational and electromagnetic interactions due to the proposition of a fifth dimension in addition to the usual four-dimensional space-time [2–4], which leads to the consideration of the concept of deformation of Riemannian geometry defined by extrinsic curvature of the space-time. The conclusions of this result are based, in particular, on the five-dimensional space from the paper [5]. Arkani-Hamed et al. proposed the solution to the hierarchy problem on the basis of the existence of new compact spatial dimensions. KK excitations in this extra dimensional space through the combined effect of all the gravitons became observable.

Today, the idea of additional space as the instrumentation of the unification of all four interactions is of interest not only in theoretical physics [6–8] but also in experimental searches at the LHC for exotic matter that deviates from normal matter [9].

Our paper is devoted to the searches for KK particles in three models of extra dimensions: Arkani-Hamed-Dimopoulos-Dvali (ADD) model, [6], Randall-Sundrum (RS) model [7, 8], and model [10]. Using computer program Pythia8.2 [11], within these three extra dimensional models, we have calculated(i)the production cross section of KK modes of massive gravitons and gauge bosons at energies from 14 TeV to planned 100 TeV,(ii)the graviton mass spectrum for three graviton, G, emission processes: (a) , (b) , and (c) at 14 TeV at the LHC,(iii)the graviton mass spectrum at 14 TeV at the LHC for numbers of extra dimensions ( = 2, 4, and 6) (for simplicity and brevity). Since the maximum of is equal to 6 for ADD model, it was of interest to look at the behavior of graviton mass spectrum at the extreme values of , from = 2 to = 6,(iv)the production cross section of graviton, , multiplied by branching ratios, Br (gluon-gluon (gg) pair), Br (leptons, are of any type, ), and Br (, Higgs boson) of the most probable processes of decay within RS model at 13 TeV, 14 TeV, and 100 TeV.

#### 2. Models of Extra Dimensions

In this section, we will observe three models of extra dimensions, ADD, RS, and , which are the base for our further calculations of KK particle properties. In the framework of M-theory [13], the metric of ADD model is as follows:where is the metric of (4 + )-dimensional space-time with compact extra dimensions, where the gravitational field propagates and the SM localized on a 3-brane embedded into the (4 + )-dimensional space-time, is (4 + )-dimensional Minkowski background and is the deviation of Minkowski metrics, is the fundamental mass scale, and is the number of extra dimensions. Masses of KK modes for ADD model are given byFive-dimensional metric of RS model with one extra dimension compactified to the orbifold, /, is with nonfactorizable geometry:Two 3-branes are located at points and of the orbifold with radius, , of . , and are four-dimensional coordinates and Minkowski metrics; the function inside the interval is equal to , dimensional parameter). In Figure 1 a nonfactorizable geometry with one spatial extra dimension is presented as a line segment between two four-dimensional branes, known as Planck and TeV brane.