الفهرس 
LEFT RIGHT SYMMETRIC MODELOnly 14 pages are availabe for public view
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Abstract
The Standard Model (SM) of particle physics is in an unification theory excellent agreement with most of the confirmed experimental results. However, there exist several forced arguments that show that the SM is only an effective low energy limit of a more fundamental underlying theory. Indeed, there are a number of phenomenological and theoretical outstanding issues in particle physics that can not be explained and the SM can not to address them adequately. Here, we may just mention some of them like the puzzles of dark matter and tiny neutrino masses [1, 2, 3, 4, 5], which can not be explained within the SM. One of the most popular extensions of the SM is the grand unified theory (GUT), where gauge symmetry SU (3)C × SU (2)L × U (1)Y for SM is extended to a bigger (simple or semisimple) group. Nonvanishing neutrino masses motivate us to the existence of right-handed neutrinos, and hence, all known fermions would have both left and right chirality. In this respect, the SM gauge group would be extended to the left-right (LR) symmetric gauge group, which is based on SU (3)C × SU (2)L × SU (2)R × U (1)B−L, where left chirality and right chirality are treated equally at high energy scales. In this class of models, the Majorana right-handed neutrinos are naturally heavy, and hence our small left-handed neutrino masses are generated through seesaw mechanisms.
In the conventional left-Right model proposed by Mohapatra et al. [6, 7, 8], the fermions
particle (including the right-handed neutrino) are assigned in left- or right-handed doublets unlike SM, and the following Higgs sector has been assumed: one bidoublet φ to construct the Yukawa couplings of quarks and leptons, in addition to a left- and right-handed scalar triplets δ for seesaw neutrino masses. The SU (2)R × U (1)B−L symmetry is broken down to U (1)Y , at a high energy scale, by the vacuum expectation value (VEV) vR of the neutral component of the right-handed triplet, while the VEVs of neutral components of the bidoublet χ1, χ2and the left-handed tripletχl contribute in breaking the electroweak symmetry, SU (2)L × U (1)Y , down to U (1)em. It was clear that the Higgs sector of this model is not minimal (is huge sector), with several neutral and singly and doubly charged components. Also, we note that the left-handed triplet was introduced only to preserve LR symmetry, although its VEV must be fine-tuned to a very small value to achieved the constraints from the observed neutrino masses to be very small. Moreover, the Higgs triplets may induce tree level flavor violating processes that contradict with the current experimental limits. Therefore, every different variants of the conventional LR model have been considered [9, 10, 11, 12, 13, 14].
Here, we consider an example of a LR model, with a Higgs sector consisting of one scalar right-handed doublet and one scalar bidoublet. In this case and for generate light neutrino masses of its order, we adopt the inverse-seesaw (IS) mechanism [15, 16, 17, 18, 19]. As known, inverse-seesaw mechanism requires introducing other singlet fermions that couple with right- handed neutrinos and have a small mass [∼ O(1) KeV], which may be generated radiatively. The IS mechanism is quite motivated by having the TeV scale LR model that can be discover in current and future colliders, while in the conventional LR model, the GUT scale is the typical
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scale of breaking LR symmetry, where heavy right-handed neutrino masses are generated. Moreover, in the limit of vanishing the above mentioned tiny mass, we will have massless left- handed neutrinos and the lepton number symmetry is restored. Thus, such a very small scale can be considered, according to ’t Hooft naturalness criteria [20], as a natural scale of a global symmetry (lepton number) breaking. We also argue that in this class of models the tree-level
flavor changing neutral current (FCNC) is under control by taken V R
L
CKM
. It turns out
that the right-handed doublet is essentially decoupled from the two Higgs doublets, generated from the bidoublet; hence the Higgs sector of this model will be same the scenario of two the Higgs doublet model [21, 22, 23]. We show that the lightest CP -even Higgs boson h, the SM- like Higgs boson, and the next lightest h𝘫 are generated from the neutral components of the bidoublet. For a wide range of the parameter space, one can show that the mass of the next lightest Higgs boson is of the order a few hundred GeVs also can be few TeVs.
in this paper we will analyze the discovery and probe the next lightest CP -even neutral Higgs boson, h𝘫, at the Large Hadron Collider (LHC). Our searches are performed by looking for resonant peaks in two processes, namely h𝘫 → hh → b¯bγγ and h𝘫 → ZZ → 4l (l = e, µ).
The analysis is based on three benchmark points, with mh𝘫 = 250 GeV, 400 GeV, and 600 GeV, for a center-of-mass energy √s = 14 TeV and Lint = 300 fb−1, and 3000 fb−1, respectively. After
imposing various sets of cuts to reduce backgrounds (B) from SM and improve the statistical significance (S/√B), where S refers to the signal, we find that the SM-like Higgs boson pair
production, with b¯bγγ final states, is the most promising channel for probing our heavy Higgs
boson h𝘫 at the LHC. The channel of the Z-pair production, decay to 4l is less significant as its cross section is small for mh𝘫 >∼ 300 GeV, and the associated background is large for mh𝘫 ’ 200 GeV. We show that to probe h𝘫 through this channel, Lint must be increased up to Lint = 3000 fb−1.
The thesis is organized as follows. In Chapter. 1 we make review for conventional Left- Right Symmetric Model (LRSM) and its sector interaction, and neutrino masses in this model and problem in LRSM. In Chapter. 2 we introduce the LR model with IS mechanism, its scalar sector and interaction. We show that the SM-like Higgs, h, and the next lightest h𝘫 are come from the real part of neutral components of the bidoublet sector.In Chapter. 3 we discuss searches for h𝘫 at the LHC. A detailed analysis and tools used for the SM Higgs pair production from h𝘫, and two SM Higgs decays into b¯bγγ, are provided. We show that the total cross section of this process is of order O(1) fb. In general the signal of this process is smaller than the background, however by selecting an appropriate set of cuts we can probe the signal with a reasonable significance. We also analyse a possible signature for h𝘫 decay to the Z-gauge boson pair production followed by the decays to 4l.
The main result of the thesis is published in the following paper:
K. Ezzat, M. Ashry, and S. Khalil. Search for a heavy neutral Higgs boson in a left-right model with an inverse seesaw mechanism at the LHC. Phys. Rev. D, 104(1):015016, 2021.