PHYS08200604017 Manimala Mitra - Homi Bhabha National Institute

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III seesaw model. Clearly, for our model with two Higgs doublets, the decay lifetime is almost 10 13 times smaller and hence we predict no displaced vertex for the heavy fermion decay. This can be used as a distinguishing signature between the two models. Another very important and unique feature of our model is the very long lifetime of ourneutralHiggsh 0 , whichcomesduetothesmallnessofsinα. Infact, sincesinα ∼ 10 −6 , thelifetime forh 0 inourmodel is10 12 times largercompared to thestandardmodel Higgs. In particular, the h 0 total decay rate is around 10 −15 GeV. This gives h 0 a rest frame lifetime of 4.97 cm. For a h 0 with hundred GeV of energy, the lifetime in the lab frame is should be few 10s of cm. Therefore, we expect a big gap between the decay vertices of the heavy fermion and the h 0 . This displaced h 0 decay vertex should be detectable at the LHC detectors ATLAS and CMS. Wewouldliketomakejustafewqualitativeremarksabouttheprospectsofdetecting the displaced h 0 vertex. The h 0 decay predominantly into b m¯bm pairs. While b-tagging is a very important and standard tool for collider experiments, and while both ATLAS [31] and CMS [32] have been developing algorithms for tagging the b, there is an additional complication with b-tagging in our model which should be pointed out here. Since the h 0 lifetime is a few 10sofcm in thelabframe, it isexpected to decay inside the silicon tracker of ATLAS and CMS. In particular, the pixel tracker of CMS and ATLAS which are only few cm from the center of the beam pipe, will miss the h 0 decay vertex. However, the silicon strip trackers would be useful in observing the b-jets. The tracks from the primary and secondary vertices of the b-hadron should be seen. In addition, one could use the two other standard tools for tagging the b-jets. Firstly, one could the tag the lepton in the jet coming from the semi-leptonic decays of the b-hadron. These leptons are expected to have smaller p T compared to the ones coming from W ± and Z decays, and hence this is called soft-lepton tagging [31,32]. More importantly, one could construct the invariant mass distribution of the 2 b-jets. This should give us a sharp peak corresponding to the h 0 . We therefore expect that ATLAS and CMS should be able to detect the displaced h 0 decay vertex. This would give a characteristic and unambiguous signal of our model. 3.8 Model Signatures at the LHC For notational simplicity, we change our convention in this section. Here, we do not use the subscript “m” any further to denote the fields in their mass basis. All the triplet fermions, standard model leptons and quarks fields written in this section are in the mass basis and we denote them simply by the notations Σ ±,0 , l ± , ν , b and so on. In the previous sections we have discussed in details the production and subsequent decays of the exotic fermions, aswell asthe decay branching fractionsof the intermediate Higgsinto final state particles. In this section we describe the signatures of the two Higgs doublet type-III seesaw model at the LHC. We will present a comprehensive list of final state particles and their corresponding collider signatures. 64
The most important characteristics of our model are the following: 1. Presence of µ-τ symmetry in Y Σ and M. This is expected to show-up in the flavor of the final state lepton coming directly from the Σ ±/0 decay vertex. 2. Presence of two CP even neutral Higgses (h 0 and H 0 ), one CP odd neutral Higgs (A 0 ), and a pair of charged Higgs (H ± ). 3. Predominant decay of the heavy fermions into light leptons, and h 0 , A 0 or H ± . Decays into H 0 and gauge bosons almost never happen. 4. Very short lifetime for the heavy fermion due to the very large Yukawa couplings. 5. Predominant decay of h 0 and A 0 into b¯b pairs 88-89% and 87% of the time, respectively. They decay also into τ¯τ 7-9% of the time. 6. Very large lifetime of h 0 . 7. The Higgs H ± decays into W ± h 0 and almost never into W ± H 0 . In what follows, we will use these model characteristics to identify the different final state channels atthecollider. Weidentify thepossiblechannels inthecollider forourmodeland calculate the respective effective cross-sections. The results are given in Tables 3.8, 3.9 and 3.10. We will also discuss some of the most important channels and the characteristic backgrounds, if any, associated with them. In this section we have only given results for effective cross-sections for the decay of Σ ±/0 1 with M Σ1 = 300 GeV. Results for the other heavy fermion generations can be similarly obtained. We have considered the light Higgs mass M h 0 = 40GeV, while calculating theeffective crosssections given inTable3.8, Table 3.9 and Table 3.10. For the other choice M h 0=70 GeV, the effective cross section does not change significantly. However, as an example we have also calculated the effective cross section for few of the significant channels which have large cross section and tabulate them in Table 3.12 for the case M h 0 = 70 GeV. 3.8.1 Signatures from Σ + Σ − decays We give in Table 3.8 the possible collider signatures coming from the decay of Σ + Σ − pairs, for our two Higgs doublet type-III seesaw model. In the last column we also give the corresponding effective cross-sections for these channels in units of fb. The final crosssectionscanbeobtainedonlyafterputtinginthevariouscutsandefficiencyfactors. These efficiency factors will have to be folded with the cross-sections given in Table 3.8 to get the final effective cross-sections for the various channels. We have not addressed these issues in this present work. Few clarifications on our notation is in order. Light charged leptons could be released in the final state through two ways: (i) from the decay of the 65
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Neutrinos and Some Aspects of Physi
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Declaration This thesis is a presen
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Acknowledgments First and foremost,
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iii
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trino masses are bounded within 0.1
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softly broken Z 2 symmetry, the mas
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tribimaximal mixing in the neutrino
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Bibliography [1] Two Higgs doublet
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Contents 1 Introduction 1 1.1 Stand
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6.2.2 Charged Lepton Masses and Mix
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Chapter 1 Introduction 1.1 Standard
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For positive µ 2 and λ, the Higgs
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type of Yukawa interaction between
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interaction with the Higgs as λ f
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where ˆΦ is the MSSM chiral super
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(2004); A. Ghosal, hep-ph/0304090;
Page 44 and 45: active flavor. In recent years, sev
Page 46 and 47: Figure 2.1: The possible neutrino s
Page 48 and 49: Majorana mass term breaks lepton nu
Page 50 and 51: The kinetic term of the Higgs tripl
Page 52 and 53: ight handed neutrino N R can be exa
Page 54 and 55: alternating group which is not a di
Page 56 and 57: The group contains two one-dimensio
Page 58 and 59: from neutral-current interactions i
Page 60: [31] K. S. Babu and R. N. Mohapatra
Page 64 and 65: of producing the heavy fermion trip
Page 66 and 67: where Σ ± Ri = Σ1 Ri ∓iΣ2 Ri
Page 68 and 69: while U ν diagonalizes m ν and ˜
Page 70 and 71: It is evident that the masses of th
Page 72 and 73: 0.001 0.001 0.0001 0.0001 U e3 1e-0
Page 74 and 75: if we neglect terms proportional to
Page 76 and 77: calculations for lepton flavor viol
Page 78 and 79: fb. Therefore, it is obvious that t
Page 80 and 81: Σ − -> e m 1 m h 0 Σ − -> e m
Page 82 and 83: We can also see that for Σ − m 1
Page 84 and 85: the sinα term. However, decay to h
Page 86 and 87: Σ − m 1 ->ν m1 W Σ 0 m 1 ->ν
Page 88 and 89: Γ 2HDM (Σ 0 m → ν m H 0 ) ≃
Page 90 and 91: Decay modes Σ ± m 1 Σ ± m 2 Σ
Page 94 and 95: Sl no Channels Effective cross-sect
Page 96 and 97: • The other dominant decay channe
Page 102 and 103: 3.9 Conclusions The seesaw mechanis
Page 104 and 105: Appendix A: The Scalar Potential an
Page 106 and 107: B: The Interaction Lagrangian B.1:
Page 108 and 109: C H0 ,L 1√ ll 2 (T 11Y † l S 11
Page 110 and 111: c Z,L ll c Z,L lΣ − c Z,L Σ −
Page 112 and 113: Bibliography [1] S. Weinberg, Phys.
Page 114: Lett. B 615, 231 (2005); Phys. Rev.
Page 118 and 119: violating λ ′′ coupling are co
Page 120 and 121: neutralino-neutrinomassmatrixandins
Page 122 and 123: 4.3 Symmetry Breaking In this secti
Page 124 and 125: +(M 2 +m 2 Σ )ũ2 + ∑ m 2˜ν i
Page 126 and 127: Here M 1,2 are the soft supersymmet
Page 128 and 129: is violated, we have neutrino-neutr
Page 130 and 131: (M 1,2 ,M,µ,Y Σi ,v 1,2 ,ũ,u i )
Page 132 and 133: In our model in addition to the sta
Page 134 and 135: 4.8 Conclusion In this work we have
Page 136 and 137: Y Σi √2 ĤuˆΣ 0 0c Rˆν L i
Page 138 and 139: while the parameter α 1 is α 1 =
Page 140 and 141: Bibliography [1] S. Roy and B. Mukh
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[15] M. C. Gonzalez-Garcia and J. W
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ν i A Σi • ˜ν i h,H,A × χ
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118
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of the form ⎛ ⎞ A B B m ν =
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triplet representation of A 4 , l =
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m 0 =0.016 m 0 =0.024 m 0 =0.032 2
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Higgs Neutrino mass matrix Eigenval
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5.3. The deviation of the mixing an
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Sin 2 θ 12 Sin 2 θ 13 Sin 2 θ 23
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Figure 5.5: Scatter plot showing th
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Figure 5.6: Same as Fig. 5.5 but fo
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mass matrix is then given as ⎛ m
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Bibliography [1] M. C. Gonzalez-Gar
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140
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142
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a way that the residual µ−τ sym
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where Λ is the cut-off scale of th
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m 2 . The eigenvectors are given as
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For λ ≃ 2 × 10 −2 ≃ λ2 c 2
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2 2 2 1 1 1 y 2 0 y 3 0 y 4 0 -1 -1
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0.08 0.25 0.06 0.2 10 -3 m νee (eV
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We show the values of |U e3 | ≡ s
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while the branching ratio for this
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6.4 The Vacuum Expectation Values I
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where we have defined B = (b+f 1 +f
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VEV alignments. Another way the VEV
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(2006); M. Maltoni, T. Schwetz, M.
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168
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metric standard model in chapter 1.
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mally two. However the R-parity vio
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sector contains the exact/approxima
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PHYS08200604017 Manimala Mitra - Homi Bhabha National Institute

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