arXiv subjects

Biswajoy Brahmachari

Publications and source records attributed to Biswajoy Brahmachari.

At least 19 recordsLinked to original sources

arXiv2016

Implications of the CMS search for W_R on Grand Unification

The CMS experiment at the Large Hadron Collider has reported a 2.8$σ$ excess in the $(2e)(2jets)$ channel around 2.1 TeV. Interpretation of this data is reconsidered in terms of the production of a right-handed weak gauge boson, $W_R$, of the left-right symmetric model and in an $SO(10)$ grand unified theory abiding by the Extended Survival Hypothesis. The left-right symmetric model can be consistent with this excess if (a) the heavy right-handed neutrino has a mass near $W_R$, or (b) if $g_L \neq g_R$, or (c) the right-handed CKM matrix is nontrivial. Combinations of the above possibilities are also viable. A $W_R$ with a mass in the TeV region if embedded in $SO(10)$ is not compatible with $g_L = g_R$. Rather, it implies $0.64 \leq g_R/g_L \leq 0.78$. Further, a unique symmetry-breaking route -- the order being left-right discrete symmetry breaking first, followed by $SU(4)_C$ and finally $SU(2)_R$ -- to the standard model is picked out. The $L \leftrightarrow R$ discrete symmetry has to be broken at around $10^{16}$ GeV. The grand unification scale is pushed to $10^{18}$ GeV making the detection of proton decay in ongoing searches rather unlikely. The $SU(4)_C$ breaking scale can be at its allowed lower limit of $10^6$ GeV so that $n - \bar{n}$ oscillation or flavour changing processes such as $K_L \rightarrow μe$ and $B_{d,s} \rightarrow μe$ may be detectable. The Higgs scalar multiplets responsible for $SO(10)$ symmetry breaking at various stages are uniquely identified so long as one adheres to a minimalist principle. We also remark, {\em en passant}, about a partially unified Pati-Salam model.

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arXiv2015

Constrained analytical interrelations in neutrino mixing

Hermitian squared mass matrices of charged leptons and light neutrinos in the flavor basis are studied under general additive lowest order perturbations away from the tribimaximal (TBM) limit in which a weak basis with mass diagonal charged leptons is chosen. Simple analytical expressions are found for the three measurable TBM-deviants in terms of perturbation parameters appearing in the neutrino and charged lepton eigenstates in the flavor basis. Taking unnatural cancellations to be absent and charged lepton perturbation parameters to be small, interrelations are derived among masses, mixing angles and the amount of CP-violation.

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arXiv2015

Testable constraint on near-tribimaximal neutrino mixing

General lowest order perturbations to hermitian squared mass matrices of leptons are considered away from the tribimaximal (TBM) limit in which a weak flavor basis with mass diagonal charged leptons is chosen. The three measurable TBM deviants are expressed linearly in terms of perturbation induced dimensionless coefficients appearing in the charged lepton and neutrino flavor eigenstates. With unnatural cancellations assumed to be absent and the charged lepton perturbation contributions to their flavor eigenstates argued to be small, we analytically derive the following result. Within lowest order perturbations, a deviation from maximal atmospheric neutrino mixing and the amount of CP violation in neutrino oscillations cannot both be large (i.e. $12$-$17 \% $), posing the challenge of verification to forthcoming experiments at the intensity frontier.

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arXiv2014

Kinetic mixing and symmetry breaking dependent interactions of the dark photon

We examine spontaneous symmetry breaking of a renormalisable U(1) x U(1) gauge theory coupled to fermions when kinetic mixing is present. We do not assume that the kinetic mixing parameter is small. A rotation plus scaling is used to remove the mixing and put the gauge kinetic terms in the canonical form. Fermion currents are also rotated in a non-orthogonal way by this basis transformation. Through suitable redefinitions the interaction is cast into a diagonal form. This framework, where mixing is absent, is used for subsequent analysis. The symmetry breaking determines the fermionic current which couples to the massless gauge boson. The strength of this coupling as well as the couplings of the massive gauge boson are extracted. This formulation is used to consider a gauged model for dark matter by identifying the massless gauge boson with the photon and the massive state to its dark counterpart. Matching the coupling of the residual symmetry with that of the photon sets a lower bound on the kinetic mixing parameter. We present analytical formulae of the couplings of the dark photon in this model and indicate some physics consequences.

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arXiv2012

Perturbative generation of theta_{13} from tribimaximal neutrino mixing

Solar and atmospheric neutrino oscillations are consistent with a tribimaximal form of the mixing matrix $U$ of the lepton sector. Exact tribimaximal mixing leads to $θ_{13}=0$. Recent results from the Daya Bay and RENO experiments have established a non-zero value of $θ_{13}$. Keeping the leading behaviour of $U$ as tribimaximal we perform a model-independent perturbative calculation to incorporate a non-vanishing $θ_{13}$. We identify the nature of the perturbation matrix and consider the possibility of the solar neutrino splitting also resulting from it. We calculate up to first order in perturbation theory and evaluate the deviations proportional to $\sin θ_{13}$ while including CP-nonconservation. Finally, we briefly discuss a gauge model where such an addition to the neutrino mass matrix arises through one-loop effects.

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arXiv2008

The A4 flavor symmetry and neutrino phenomenology

It has been shown that tribimaximal mixing can be obtained by some particular breaking pattern of the $A_4$ symmetry, wherein the extra $A_4$ triplet Higgs scalars pick up certain fixed vacuum expectation value (VEV) alignments. We have performed a detailed analysis of the different possible neutrino mass matrices within the framework of the $A_4$ model. We take into account all possible singlet and triplet Higgs scalars which leave the Lagrangian invariant under $A_4$. We break $A_4$ spontaneously, allowing the Higgs to take any VEV in general. We show that the neutrino mixing matrix deviates from tribimaximal, both due to the presence of the extra Higgs singlets, as well as from the deviation of the triplet Higgs VEV from its desired alignment, taken previously. We solve the eigenvalue problem for a variety of these illustrative cases and identify the ones where one obtains exact tribimaximal mixing. All such cases require fine-tuning. We show which neutrino mass matrices would be strongly disfavored by the current neutrino data. Finally, we study in detail the phenomenology of the remaining viable mass matrices and establish the deviation of the neutrino mixing from tribimaximal, both analytically as well as numerically.

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arXiv2008

Orbifold GUT model with nine Higgs doublets

We describe a non-supersymmetric orbifold GUT based on SU(5) symmetry. It is a modification of Kawamura's 5-D orbifold GUT model. The difference lies in the choice of Higgs scalars as we have allowed only 5-plets of SU(5) in the GUT scale. This variant was originally proposed by Brahmachari and Raychoudhuri. Proton decay problem and the doublet triplet splitting problems are solved by extra dimensional mechanism. The unification scale is around $5.0 \times 10^{13}$ GeVs. In low energy there are nine Higgs doublets. One at the 100 GeV region and eight others degenerate at around 1.4 TeV. It is an attractive non-supersymmetric extension of standard model with very rich collider physics phenomenology.

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arXiv2008

A 3 X 2 texture for neutrino oscillations and leptogenesis

In an economical system with only two heavy right handed neutrinos, we postulate a new texture for $3 \times 2$ Dirac mass matrix $m_D$. This model implies one massless light neutrino and thus displays only two patterns of mass spectrum for light neutrinos, namely hierarchical or inverse-hierarchical. Both the cases can correctly reproduce all the current neutrino oscillation data with a unique prediction $m_{ν_e ν_e} = \frac{\sqrt{Δm^2_{solar}}}{3}$ and $\sqrt{Δm^2_{atm}}$ for the hierarchical and the inverse-hierarchica cases, respectively, which can be tested in next generation neutrino-less double beta decay experiments. Introducing a single physical CP phase in $m_D$, we examine baryon asymmetry through leptogenesis. Interestingly, through the CP phase there are correlations between the amount of baryon asymmetry and neutrino oscillation parameters. We find that for a fixed CP phase, the hierarchical case also succeeds in generating the observed baryon asymmetry in our universe, plus a non-vanishing $U_{e3}$ which is accessible in future baseline neutrino oscillation experiments.

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arXiv2007

Upper bound on the mass scale of superpartners in minimal N=2 supersymmetry

If N=2 supersymmetry breaks to N=1 supersymmetry at an intermediate scale $m_2$ and then, later on, N=1 supersymmetry breaks and produces standard model at a scale $m_{susy}$ such that $m_2 > m_{susy}$, renormalization group evolution of three gauge couplings are altered above the scale $m_2$, changing the unification scale and the unified coupling. We show that when we enforce this general condition $m_2 > m_{susy}$ on the solutions of the renormalization group equations, the condition is translated into an upper bound on the scale $m_{susy}$. Using presently favored values of $α_1(m_z),α_2(m_z),α_3(m_z)$, we get $m_{susy} < 4.5 \times 10^9$ GeVs for the central value of $α_3(m_Z)$. When low energy threshold effect is present, this bound gets smeared yet remains generally stable in the $10^9-10^{10}$ GeV range. We also show that if we demand string unification instead of having an unified gauge theory, this constraint can be changed by exotic hypercharge normalizations.

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arXiv2006

Modified Zee mass matrix with zero-sum condition

We modify the Zee mass matrix by adding a real one parameter perturbation which is purely diagonal and trace-less. We show that in this way we can explain both solar and atmospheric neutrino oscillation data. There is a correlation between the deviation from strict maximality of $|U_{μ3}|= 1/\sqrt{2}$, with the emergence of a small but non-zero $U_{e3}$. We calculate how big a value can $U_{e3}$ get when we restrict ourselves within the allowed regions of solar and atmospheric neutrino masses and mixing angles. We also discuss the impact of a $S_2$ permutation symmetry on our mass matrix and show how a small $U_{e3} \ne 0$ can emerge when this $S_2$ permutation symmetry between the second and the third generation is broken.

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arXiv2006

A_4 symmetry and prediction of U_{e3} in a modified Altarelli-Feruglio model

We show that a modification of a recently proposed model by Altarelli and Feruglio with softly broken $A_4$ symmetry leads naturally to nonvanishing $U_{e3}$ with $θ_{13}\simeq 2^o - 4^o$. The observed mass squared differences for solar and atmospheric neutrinos have been fitted without fine tuning among model parameters. The predicted solar neutrino mixing angle is brought down from the tri-bimaximal prediction to be in concordance with the latest global analysis including experimental data from KamLAND and SNO.

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arXiv2005

A hyperbolic parametrization of lepton mixing for small $U_{e3}$

We use hyperbolic functions to parametrize lepton mixing matrix using only one input parameter $ϕ$. This matrix has three mixing angles as outputs therefore giving two predictions. In particular it predicts $U_{e3}=0$ besides predicting correct solar and atmospheric mixing. We confine us to real $ϕ$. For complex $ϕ$ mixing matrix is no longer unitary. Next we accentuate this unitary mixing matrix with an additional small parameter $δ$ while keeping unitarity of the matrix exact. When this second parameter is included, with this framework, one can handle small but non-zero $U_{e3}$. In the second case from two input parameters one obtains three mixing angles thus one prediction.

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arXiv2005

E_6 multiplets and unification in extra dimensions

We study the effect of all matter multiplets contained in {\bf 27} representation of $E_6$ GUT on gauge coupling unification in extra dimensions. Extra members of {\bf 27} multiplets of all three generations have their `zero modes' near $m_t$ such that they can be directly probed. From TeV scale onwards extra dimensions open up, theory becomes N=2 supersymmetric and gauge couplings unify or they do not, depending on how we distribute matter fields and gauge fields in bulk and brane. We find three such possible embedding which will lead to perfect gauge coupling unification below 100 TeV region for one extra dimension and lower than that if number of extra dimensions is larger.

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arXiv2004

Supersymmetric threshold corrections to Δm^2_{\odot}

For nearly degenerate neutrinos, quantum corrections can modify the tree-levelmasses via low energy supersymmetric threshold corrections comparable to the solar oscillation mass scale. We numerically calculate corrections to neutrino masses in minimal supergravity (mSugra) and Gauge Mediated Supersymmetry Breaking (GMSB) scenarios and identify parameter spaces in the high energy regime for which the solar neutrino mass splitting becomes too large compared to the LMA solution. We show that such considerations can give bounds on GMSB and mSugra models which can be useful. On the contrary, if we start from degenerate mass eigenvalues at the tree level, these threshold corrections being generation dependent, can also produce the required mass splitting at solar scale for regions of parameter space.

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arXiv2003

Single and double seesaw for quark-lepton and neutrino masses

A left-right model of quarks and leptons based on the gauge group $SU(3)_c \times SU(2)_L \times SU(2)_R \times U(1)_{B-L}$ is studied. Here the scalar sector consists of only two doublets namely (1,2,1,1) and (1,1,2,1) but familiar bidoublet (1,2,2,0) is removed. Quarks and charged leptons get mass from a single see-saw mechanism but neutrinos get mass from a double see-saw mechanism. In this type of models the heaviest right handed neutrino can be of order $10^{13}$ GeVs or less in a natural way, depending on the size of related Yukawa couplings.

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arXiv2003

CP violation and matter effect for a variable earth density in very long baseline experiments

The perturbative treatment of subdominant oscillation and the matter effect in neutrino beams/superbeams, propagating over long baselines and being used to look for CP violation, is studied here for a general matter density function varying with distance. New lowest order analytic expressions are given for different flavour transition and survival probabilities in a general neutrino mixing basis and a variable earth matter density profile. It is demonstrated that the matter effect in the muon neutrino (antineutrino) flavour survival probability vanishes to this order, provided the depletion, observed for atmospheric muon neutrinos and antineutrinos at super-Kamiokande, is strictly maximal. This result is independent of the earth density profile and the distance L between the source and the detector. In the general variable density case we show that one cannot separate the matter induced asymmetry from a genuine CP effect by keeping two detectors at distances $L_1$ and $L_2$ from the source while maintaining a fixed ratio $L_1/E_1 = L_2/E_2$. This needs to be done numerically and we estimate the asymmetry generated by the earth matter effect with particular density profiles and some chosen parameters for very long baseline neutrino oscillation experiments.

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arXiv2003

Truly Minimal Left-Right Model of Quark and Lepton Masses

We propose a left-right model of quarks and leptons based on the gauge group $SU(3)_C \times SU(2)_L \times SU(2)_R \times U(1)_{B-L}$, where the scalar sector consists of only two doublets: (1,2,1,1) and (1,1,2,1). As a result, any fermion mass, whether it be Majorana or Dirac, must come from dimension-five operators. This allows us to have a common view of quark and lepton masses, including the smallness of Majorana neutrino masses as the consequence of a double seesaw mechanism.

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arXiv2002

Gauge unification in 5-D SU(5) model with orbifold breaking of GUT symmetry

We consider a 5-dimensional SU(5) model wherein the symmetry is broken to the 4-dimensional Standard Model by compactification of the 5th dimension on an S^1/(Z_2 \times Z^\prime_2) orbifold. We identify the members of all SU(5) representations upto 75 which have zero modes. We examine how these light scalars affect gauge coupling unification assuming a single intermediate scale and present several acceptable solutions. The 5-D compactification scale coincides with the unification scale of gauge couplings and is determined via this renormalization group analysis. When SO(10) is considered as the GUT group there are only two solutions, so long as a few low dimensional scalar multiplets upto 126 are included.

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