arXiv subjects

S. Nandi

Publications and source records attributed to S. Nandi.

At least 145 records · Page 8Linked to original sources

arXiv2006

A crystallographic phase transition within the magnetically ordered state of Ce_2Fe_17

X-ray diffraction experiments were performed on polycrystalline and single-crystal specimens of Ce$_{2}$Fe$_{17}$ at temperatures between 10 K and 300 K. Below $T_{\mathrm{t}}$ = 118$\pm$2 K, additional weak superstructure reflections were observed in the antiferromagnetically ordered state. The superstructure can be described by a doubling of the chemical unit cell along the $\mathbf{c}$ direction in hexagonal notation with the same space group $R \bar{3} m$ as the room-temperature structure. The additional antiferromagnetic satellite reflections observed in earlier neutron diffraction experiments can be conclusively related to the appearance of this superstructure.

cond-mat.mtrl-sci↗

arXiv2006

A New Two Higgs Doublet Model

We propose an extension of the Standard Model by extending the EW symmetry to SU(2)_L x U(1) x Z_2 and introducing three SU(2)_L x U(1) singlet right handed neutrinos, N_R, and an additional Higgs doublet, phi. While the SM gauge bosons and the quarks and charged leptons acquire masses from the spontaneous breaking of SU(2)_L x U(1) symmetry at the electroweak scale, the neutrinos acquire masses from the spontaneous breaking of the discrete Z_2 symmetry at a scale of 10^{-2} eV. In addition to providing a new mechanism for generating tiny masses for the neutrinos, the model has interesting implications for neutrinoless double beta decay and the Higgs signals at high energy colliders, as well as in astrophysics and cosmology.

hep-ph↗

arXiv2006

Collider Implications of Multiple Non-Universal Extra Dimensions

We consider multiple TeV scale extra compact dimensions in an asymmetric string compactification scenario in which the SM gauge bosons can propagate into the TeV scale extra dimensions while the SM fermions are confined to the usual SM D3 brane. We calculate the contributions that the KK excitations of the gluons make to the multijet cross sections in proton proton collisions at the LHC energy. At very high pT, the dijet signal will either be enhanced significantly due to virtual g star exchanges or place a lower bound on the compactification scale of about 8 TeV. We find that the dijet signal is very sensitive to three parameters: the compactification scale, the string scale, and the number of extra dimensions. Thus, although the dijet signal is much more sensitive to KK effects, the dijet signal alone does not provide sufficient information to deduce the number of extra dimensions nor the compactification scale. However, the three jet signal, which is not sensitive to the string scale, can be analyzed in conjunction with the dijet signal to extract all three parameters. For proton proton collisions at the LHC energy, the three jet signal can be significantly enhanced by KK contributions for a compactification scale of about 4 to 5 TeV.

hep-ph↗

arXiv2006

Collider Implications of Universal Extra Dimensions

We consider the universal extra dimensions scenario of Appelquist, Cheng, and Dobrescu, in which all of the SM fields propagate into one extra compact dimension, estimated therein to be as large as $\sim (350$ GeV$)^{-1}$. Tree-level KK number conservation dictates that the associated KK excitations can not be singly produced. We calculate the cross sections for the direct production of KK excitations of the gluon, $\gs$, and two distinct towers of quarks, \qs and $\qt$, in proton-antiproton collisions at the Tevatron Run I and II energies in addition to proton-proton collisions at the Large Hadron Collider energy. The experimental signatures for these processes depend on the stability of the lowest-lying KK excitations of the gluons and light quarks. We find that the Tevatron Run I mass bound for KK quark and gluon final states is about 350--400 GeV, while Run II can push this up to 450--500 GeV at its initial luminosity and 500--550 GeV if the projected final luminosity is reached. The LHC can probe much further: The LHC will either discover UED KK excitations of quarks and gluons or extend the mass limit to about 3 TeV.

hep-ph↗

arXiv2006

A Model for Neutrino and Charged Lepton Masses in Extra Dimensions

We propose a model with one large submm size extra dimension in which the gravity and right-handed (RH) neutrino propagate, but the three Standard Model (SM) families are confined to fat branes of TeV^(-1) size or smaller. The charged leptons and the light neutrinos receive mass from the five dimensional Yukawa couplings with the SM singlet neutrino via electroweak Higgs, while the KK excitations of the SM singlet neutrino gets large TeV scale masses from the five dimensional Yukawa coupling with an electroweak singlet Higgs. The model gives non-hierarchical light neutrino masses, accommodate hierarchical charged lepton masses, and naturally explain why the light neutrino masses are so much smaller compared to the charged lepton masses. Large neutrino mixing is naturally expected in this scenario. The light neutrinos are Dirac particles in this model, hence neutrinoless double beta decay is not allowed. The model has also several interesting collider implications and can be tested at the LHC.

hep-ph↗

arXiv2005

Monojet and Single Photon Signals from Universal Extra Dimensions

The usual universal extra dimensions scenario does not allow for single production of first level Kaluza-Klein (KK) excitations of matter due to the KK number conservation. However, if the matter fields are localized on a fat brane embedded in a higher dimensional space, matter-gravitation interactions violate KK number, and the production of single KK excitations becomes possible. In this paper we analyze the production of a single KK matter excitation together with a graviton in the final state, and study the potential for discovery at the Tevatron and Large Hadron Collider.

hep-ph↗

arXiv2005

Orbifold Unification for the Gauge and Higgs Fields and Their Couplings

We present an orbifold GUT model in which the NMSSM Higgs trilinear couplings are unified with the three Standard Model gauge couplings. The model is constructed as an N=2 supersymmetric SU(8) gauge theory in six dimensions, which is reduced to the NMSSM with extra U(1) factors upon compactification. Such an unification is in good agreement with experiments. The predicted upper limit for the lightest CP-even neutral Higgs boson is somewhat larger than in the MSSM, and can be tested in the upcoming Large Hadron Collider.

hep-ph↗

arXiv2005

Hidden Gauge Symmetries: A New Possibility at the Colliders

We consider a new physics possibility at the colliders: the observation of TeV scale massive vector bosons in the non-adjoint representations under the Standard Model (SM) gauge symmetry. To have a unitary and renormalizable theory, we propose a class of models with gauge symmetry $\prod_i G_i \times SU(3)'_C \times SU(2)'_L \times U(1)'_Y$ where the SM fermions and Higgs fields are singlets under the hidden gauge symmetry $\prod_i G_i$, and such massive vector bosons appear after the gauge symmetry is spontaneously broken down to the SM gauge symmetry. We discuss the model with SU(5) hidden gauge symmetry in detail, and comment on the generic phenomenological implications.

hep-ph↗

arXiv2005

Coupling Unifications in Gauge-Higgs Unified Orbifold Models

Supersymmetric gauge theories, in higher dimensions compactified in an orbifold, give a natural framework to unify the gauge bosons, Higgs fields and even the matter fields in a single multiplet of the unifying gauge symmetry. The extra dimensions and the supersymmetry are the two key ingredients for such an unification. In this work, we investigate various scenarios for the unification of the three gauge couplings, and the Yukawa couplings in the Minimal Supersymmetric Standard Model (MSSM), as well as the trilinear Higgs couplings λand κof the Non-Minimal Supersymmetric Standard Model (NMSSM). We present an SU(8) model in six dimensions with N=2 supersymmetry, compactified in a T^2/Z_6 orbifold which unifies the three gauge couplings with λand κof NMSSM. Then, we present an SU(9) model in 6D, which, in addition, includes partial unification of Yukawa couplings, either for the up-type (top quark and Dirac tau-neutrino) or down-type (bottom quark and tau lepton). We also study the phenomenological implications of these various unification scenarios using the appropriate renormalization group equations, and show that such unification works very well with the measured low energy values of the couplings. The predicted upper bounds for the lightest neutral Higgs boson mass in our model is higher than those in MSSM, but lower that those in the general NMSSM (where the couplings λand κare arbitrary). Some of the predictions of our models can be tested in the upcoming Large Hadron Collider.

hep-ph↗

arXiv2004

6D Higgsless Standard Model

We present a 6D Higgsless Standard Model with a realistic gauge sector. The model uses only the Standard Model gauge group SU(2)xU(1) with the gauge bosons propagating in flat extra dimensions which are compactified on a rectangle. The electroweak symmetry is broken by boundary conditions, and the correct splitting between the W and Z boson masses can be arranged by a suitable choice of the compactification scales. The higher Kaluza-Klein excitations of the gauge bosons decouple from the low-energy theory due to dominant brane kinetic terms. The model has the following two key features compared to 5D models. The bulk kinetic couplings, responsible for electroweak symmetry breaking using mixed boundary conditions, are of order the electroweak scale. Moreover, the agreement with the precision electroweak parameters is improved compared to 5D warped or flat models. We also argue that the calculability of Higgsless models can be ameliorated in more than five dimensions.

hep-ph↗

arXiv2004

Unitarity in Higher Dimensions and Gauge Unification

Unitarity of the 4d standard model is ensured by the conventional Higgs mechanism with a fundamental spin-0 Higgs boson, responsible for gauge boson mass-generations. On the contrary Kaluza-Klein (KK) compactification of extra spatial dimensions can geometrically realize the gauge boson mass generation without invoking a fundamental Higgs scalar. We reveal that massive gauge boson scattering in the compactified theories is unitary at low energies, and the unitarity violation is {\it delayed} to the intrinsic ultraviolet (UV) scale of the higher dimensional gauge theory. We demonstrate that this is a generic consequence of the ``geometric Higgs mechanism'' (GHM), manifested via Kaluza-Klein equivalence theorem (KK-ET). We further show that the presence of many gauge KK states below the UV cutoff scale imposes strong bounds on the highest KK level (N_KK). Applying these bounds to higher-dimensional SUSY GUTs implies that only a small number of KK states can be used to accelerate gauge coupling unification, and suggests that the GUT scale in the 5d minimal SUSY SU(5) is above 10^{14} GeV.

hep-ph↗

arXiv2003

Model Building with Gauge-Yukawa Unification

In supersymmetric theories with extra dimensions, the Higgs and matter fields can be part of the gauge multiplet, so that the Yukawa interactions can arise from the gauge interactions. This leads to the possibility of gauge-Yukawa coupling unification, g_i=y_f, in the effective four dimensional theory after the initial gauge symmetry and the supersymmetry are broken upon orbifold compactification. We consider gauge-Yukawa unified models based on a variety of four dimensional symmetries, including SO(10), SU(5), Pati-Salam symmetry, trinification, and the Standard Model. Only in the case of Pati-Salam and the Standard Model symmetry, we do obtain gauge-Yukawa unification. Partial gauge-Yukawa unification is also briefly discussed.

hep-ph↗

arXiv2003

Test of Gauge-Yukawa Unification

Recently it has been proposed that, in the framework of quantum field theory, both the Standard Model gauge and Yukawa interactions arise from a single gauge interaction in higher dimensions with supersymmetry. This leads to the unification of the Standard Model gauge couplings and the third family Yukawa couplings at the GUT scale. In this work, we make a detailed study of this unification using the current experimental data, and find a good agreement in a significant region of the parameter space. Similar relations, required in Finite Grand Unification models, are also studied.

hep-ph↗

arXiv2003

Unification of Gauge, Higgs and Matter in Extra Dimensions

We consider the unification of gauge, Higgs as well as the matter fields in a 6D N=2 supersymmetric SU(8) gauge theory. The gauge symmetry SU(8) is broken down to SU(4) x SU(2)_L x SU(2)_R x U(1)^2 in 4D through T^2/Z_6 orbifold compactification, and the theory is reduced to 4D N=1 supersymmetric Pati-Salam model. The electroweak Higgs fields as well as the third family of fermions are unified in the 6D N=2 gauge multiplet. The 6D bulk gauge interaction provides both gauge and Yukawa interactions for the third family predicting α_1 = α_2 = α_3 = α_t = α_b = α_tau at the unification scale, in good agreement with experiment. Incorporation of the first and second family as well as other orbifolds are also briefly discussed.

hep-ph↗

arXiv2003

Gravity and Matter in Extra Dimensions

In this paper, we derive from the viewpoint of the effective 4D theory the interaction terms between linearized gravity propagating in N>= 2 large extra dimensions and matter propagating into one extra dimension. This generalizes known results for the interactions between gravity and 4D matter in ADD-type models. Although we assume that matter is described by an Universal Extra Dimensions (UED) scenario (with all fields propagating into the fifth dimension), we present our results in a general form that can be easily adapted to various other scenarios of matter distribution. We then apply our results to the UED model on a fat brane and consider some phenomenological applications. Among these are the computation of the gravitational decay widths of the matter KK excitations and the effect the width of the brane has on the interactions of gravity with Standard Model particles. We also estimate the cross-section for producing single KK excitations at colliders through KK number-violating gravitational interaction.

hep-ph↗

arXiv2003

Unity of elementary particles and forces in higher dimensions

We present a model in which elementary particles and forces are unified in the framework of quantum field theory in higher dimensions. The particles include gauge bosons, quarks and leptons, as well as the Higgs bosons and the forces include strong, weak, hypercharge as well as Yukawa interactions. The model is based on a simple group SO(16) in six dimensions with N=2 supersymmetry. The gauge symmetry, as well as supersymmetry is broken to SU(4) x SU(2)_L x SU(2)_R x U(1)^3 with N=1 supersymmetry upon compactification in four dimension on a T^2/Z_6 orbifold.

hep-ph↗

arXiv2003

Gauge Higgs Unification in the Left-Right Model

We construct a supersymmetric left-right model in four dimension with gauge-Higgs unification starting from a SU(3)_c x SU(4)_w x U(1)_{B-L} gauge symmetry in five dimension. The model has several interesting features, such as, the CKM mixings in the quark sector are naturally small while for the neutrino sector it is not, light neutrino masses can be generated via the seesaw mechanism in the usual way, and the model has a U(1)_R symmetry which naturally forbid dimension five proton decay operators. We also discuss the grand unification of our model in SO(12) in five dimensions.

hep-ph↗

arXiv2002

Collider Implications of Models with Extra Dimensions

We discuss the collider signals of large extra dimensions in which gravity as well as the SM particles propagate into the extra dimensions. These signals arise either from the production of Kaluza-Klein excitations of the SM particles and their subsequent decay, or from their off-shell exchanges. Depending on the scenario, the dominant signals are two high p_T jets + missing energy, two high p_T photons + missing energy and soft leptons, or a combination of photon + jet and missing energy. For the scenario in which only the gauge bosons propagate into the extra dimensions, Tevatron Run II (LHC) can observe such signals up to a compactification scale of about 2 TeV (7 TeV), while for the case of universal extra dimensions, the corresponding limits are about 600 GeV (3 TeV) respectively.

hep-ph↗