arXiv subjects

Amarjit Soni

Publications and source records attributed to Amarjit Soni.

At least 127 records · Page 7Linked to original sources

arXiv2008

Constraint on rho-bar, eta-bar from B to K*pi

A linear CKM relation, $\barη= \tanΦ_{3/2}(\barρ-0.24\pm 0.03)$, involving a $1σ$ range for $Φ_{3/2}$, $20^\circ < Φ_{3/2} < 115^\circ$, is obtained from $B^0\to K^*π$ amplitudes measured recently in Dalitz plot analyses of $B^0\to K^+π^-π^0$ and $B^0(t)\to K_Sπ^+π^-$. This relation is consistent within the large error on $Φ_{3/2}$ with other CKM constraints which are unaffected by new $b\to s\bar q q$ operators. Sensitivity of the method to a new physics contribution in the $ΔS=ΔI=1$ amplitude is discussed.

hep-ph↗

arXiv2008

The Little Randall-Sundrum Model at the Large Hadron Collider

We present a predictive warped model of flavor that is cut off at an ultraviolet scale O(10^3) TeV. This "Little Randall-Sundrum (LRS)" model is a volume-truncation, by a factor $y \approx 6$, of the RS scenario and is holographically dual to dynamics with number of colors larger by $y$. The LRS couplings between Kaluza-Klein states and the Standard Model fields, including the proton constituents, are explicitly calculable without ad hoc assumptions. Assuming separate gauge and flavor dynamics, a number of unwanted contributions to precision electroweak, $Z b\bar b$ and flavor observables are suppressed in the LRS framework, compared with the corresponding RS case. An important consequence of the LRS truncation, independent of precise details, is a significant enhancement of the clean (golden) di-lepton LHC signals, by O(y^3), due to a larger "$ρ$-photon" mixing and a smaller inter-composite coupling.

hep-ph↗

arXiv2008

Charged Higgs Boson Effects in the Production and Decay of a Heavy Majorana Neutrino at the LHC

We consider a new interaction between a heavy Majorana neutrino (N) and a charged Higgs boson (H+) and show that it can have drastic implications on lepton number violating (LNV) signal of same-sign dileptons at the LHC. The LNV signal of heavy Majorana neutrinos previously considered at the LHC, pp -> l+ N -> l+ l+ W-, may be overwhelmed by pp -> l+ N -> l+ l+ H-. With the subsequent decays H- -> b t(bar) or H- -> W- H^0, the heavy Majorana neutrino production leads to the spectacular events of l+ l+ b b(bar) + 2 jets. We also explore the case m_N < m_{H+}, where the decay H+ -> l+ N can become the dominant N-production mechanism at the LHC. In particular, we show that the process g b -> t H- followed by t -> b W+ and H- -> l- N -> l- l- W+ could lead to another type of spectacular events of l- l- b + 4 jets.

hep-ph↗

arXiv2008

New Physics at a Super Flavor Factory

The potential of a Super Flavor Factory (SFF) for searches of New Physics is reviewed. While very high luminosity B physics is assumed to be at the core of the program, its scope for extensive charm and tau studies are also emphasized. The possibility to run at the Upsilon(5S) is also very briefly discussed; in principle, this could provide very clean measurements of B_s decays. The strength and reach of a SFF is most notably due to the possibility of examining an impressive array of very clean observables. The angles and the sides of the unitarity triangle can be determined with unprecedented accuracy. These serve as a reference for New Physics (NP) sensitive decays such as B^+ ->tau^+ nu and penguin dominated hadronic decay modes, providing tests of generic NP scenarios with an accuracy of a few percent. Besides, very precise studies of direct and time dependent CP asymmetries in radiative B decays and forward-backward asymmetry studies in B-> X_s l^+ l^- and numerous null tests using B, charm and tau decays are also likely to provide powerful insights into NP. The dramatic increase in luminosity at a SFF will also open up entirely new avenues for probing NP observables, e.g. by allowing sensitive studies using theoretically clean processes such as B -> X_s nu nubar. The SFF is envisioned to be a crucial tool for essential studies of flavor in the LHC era, and will extend the reach of the LHC in many important ways.

hep-ph↗

arXiv2007

On Direct Verification of Warped Hierarchy-and-Flavor Models

We consider direct experimental verification of warped models, based on the Randall-Sundrum (RS) scenario, that explain gauge and flavor hierarchies, assuming that the gauge fields and fermions of the Standard Model (SM) propagate in the 5D bulk. Most studies have focused on the bosonic Kaluza Klein (KK) signatures and indicate that discovering gauge KK modes is likely possible, yet challenging, while graviton KK modes are unlikely to be accessible at the LHC, even with a luminosity upgrade. We show that direct evidence for bulk SM fermions, {\it i.e.} their KK modes, is likely also beyond the reach of a luminosity-upgraded LHC. Thus, neither the spin-2 KK graviton, the most distinct RS signal, nor the KK SM fermions, direct evidence for bulk flavor, seem to be within the reach of the LHC. We then consider hadron colliders with $\sqrt{s} =$ 21, 28, and 60 TeV. We find that discovering the first KK modes of SM fermions and the graviton typically requires the Next Hadron Collider (NHC) with $\sqrt{s} \approx 60$ TeV and ${\cal O}(1)$ ab$^{-1}$ of integrated luminosity. If the LHC yields hints of these warped models, establishing that Nature is described by them, or their 4D CFT duals, requires an NHC-class machine in the post-LHC experimental program.

hep-ph↗

arXiv2007

Charmless Three-body Decays of B Mesons

Charmless 3-body decays of B mesons are studied in the framework of the factorization approach. The nonresonant contributions arising from $B\to P_1P_2$ transitions are evaluated using heavy meson chiral perturbation theory (HMChPT). The momentum dependence of nonresonant amplitudes is assumed to be in the exponential form $e^{-α_{NR}} p_B\cdot(p_i+p_j)}$ so that the HMChPT results are recovered in the soft meson limit $p_i, p_j\to 0$. In addition, we have identified another large source of the nonresonant signal in the matrix elements of scalar densities, e.g. $ $, which can be constrained from the decay $\bar B^0\to K_SK_SK_S$ or $B^-\to K^-K_SK_S$. The intermediate vector meson contributions to 3-body decays are identified through the vector current, while the scalar meson resonances are mainly associated with the scalar density. Their effects are described in terms of the Breit-Wigner formalism. Our main results are: (i) All KKK modes are dominated by the nonresonant background. The predicted branching ratios of $K^+K^-K_{S(L)}$, $K^+K^-K^-$ and $K^-K_SK_S$ modes are consistent with the data within errors. (ii) Although the penguin-dominated $B^0\to K^+K^-K_{S}$ decay is subject to a potentially significant tree pollution, its effective $\sin 2β$ is very similar to that of the $K_SK_SK_S$ mode. However, direct CP asymmetry of the former, being of order -4%, is more prominent than the latter. (iii) For $B\to Kππ$ decays, we found sizable nonresonant contributions in $K^-π^+π^-$ and $\bar K^0π^+π^-$ modes, in agreement with the Belle measurements but larger than the BaBar result.

hep-ph↗

arXiv2007

LHC Signals for Warped Electroweak Neutral Gauge Bosons

We study signals at the Large Hadron Collider (LHC) for Kaluza-Klein (KK) excitations of the electroweak gauge bosons in the framework with the Standard Model (SM) gauge and fermion fields propagating in a warped extra dimension. Such a framework addresses both the Planck-weak and flavor hierarchy problems of the SM. Unlike the often studied Z' cases, in this framework, there are three neutral gauge bosons due to the underlying SU(2)_L x SU(2)_R x U(1)_X gauge group in the bulk. Furthermore, couplings of these KK states to light quarks and leptons are suppressed, whereas those to top and bottom quarks are enhanced compared to the SM gauge couplings. Therefore, the production of light quark and lepton states is suppressed relative to other beyond the SM constructions, and the fermionic decays of these states are dominated by the top and bottom quarks, which are, though, overwhelmed by KK gluons dominantly decaying into them. However, as we emphasize in this paper, decays of these states to longitudinal W, Z and Higgs are also enhanced similarly to the case of top and bottom quarks. We show that the W, Z and Higgs final states can give significant sensitivity at the LHC to 2 (3) TeV KK scale with an integrated luminosity of 100 invfb (1 invab). Since current theoretical framework(s) favor KK masses greater than about 3 TeV, luminosity upgrade of LHC is likely to be crucial in observing these states.

hep-ph↗

arXiv2007

Strong CP, Up-Quark Mass, and the Randall-Sundrum Microscope

In the Randall-Sundrum model, setting the ratio of up and down quark masses $m_u/m_d << 1$, relevant to the strong CP problem, does not require chiral symmetry or fine-tuning, due to exponential bulk fermion profiles. We point out that such geometric suppression of the mass of a fermion magnifies the masses of its corresponding Kaluza-Klein (KK) states. In this sense, these KK states act as "microscopes" for probing light quark and lepton masses. In simple realizations, this hypothesis can be testable at future colliders, like the LHC, by measuring the spectrum of level-1 KK fermions. The microscope can then provide an experimental test for the vanishing of $m_u$ in the ultraviolet, independently of non-perturbative determinations, by lattice simulations or other means, at hadronic scales. We also briefly comment on application of our microscope idea to other fermions, such as the electron and neutrinos.

hep-ph↗

arXiv2007

Signature of heavy Majorana neutrinos at a linear collider: Enhanced charged Higgs pair production

A charged Higgs pair can be produced at an ee collider through a t-channel exchange of a heavy neutrino (N) via e^+ e^- -> H^+ H^- and, if N is a Majorana particle, also via the lepton number violating (LNV) like-sign reaction e^\pm e^\pm \to H^\pm H^\pm. Assuming no a-priori relation between the effective eNH^+ coupling (ξ) and light neutrino masses, we show that this interaction vertex can give a striking enhancement to these charged Higgs pair production processes. In particular, the LNV H^-H^- signal can probe N at the ILC in the mass range 100 GeV < m_N < 10^4 TeV and with the effective mixing angle, ξ, in the range 10^{-4} < ξ^2 < 10^{-8} - well within its perturbative unitarity bound and the neutrinoless double beta decay (ββ_{0ν}) limit. The lepton number conserving (LNC) e^+ e^- \to H^+ H^- mode can be sensitive to, e.g., an O(10) TeV heavy Majorana neutrino at a 500 GeV International Linear Collider (ILC), if ξ^2 > 0.001.

hep-ph↗

arXiv2007

Warped Gravitons at the LHC and Beyond

We study the production and decay of Kaluza-Klein (KK) gravitons at the Large Hadron Collider (LHC), in the framework of a warped extra dimension in which the Standard Model (SM) fields propagate. Such a scenario can provide solutions to both the Planck-weak hierarchy problem and the flavor puzzle of the SM. In this scenario, the production via $q \bar{q}$ annihilation and decays to the conventional photon and lepton channels are highly suppressed. However, we show that graviton production via gluon fusion followed by decay to longitudinal $Z/W$ can be significant; vector boson fusion is found to be a sub-dominant production mode. In particular, the ``golden'' $ZZ$ decay mode offers a distinctive 4-lepton signal that could lead to the observation at the LHC with 300 fb$^{-1}$ (SLHC with 3 ab$^{-1}$) of a KK graviton with a mass up to $\sim 2$ ($\sim 3$) TeV for the ratio of the AdS$_5$ curvature to the Planck scale modestly above unity. We argue that (contrary to the lore) such a size of the curvature scale can still be within the regime of validity of the framework. Upgrades beyond the SLHC luminosity are required to discover gravitons heavier than $\sim 4$ TeV, as favored by the electroweak and flavor precision tests in the simplest such models.

hep-ph↗

arXiv2007

Footprints of the Beyond in flavor physics: Possible role of the Top Two Higgs Doublet Model

The B-factories results provide an impressive confirmation of the Standard Model (SM) description of flavor and CP violation. Nevertheless, as more data were accumulated, deviations in the 2.5-3.5 sigma range have emerged pointing to the exciting possibility of new CP-odd phase(s) and flavor violating parameters in B-decays. Primarily this seems to be the case in the time dependent CP asymmetries in penguin dominated modes (e.g. B -> phi (eta') Ks). We discuss these and other deviations from the SM and, as an illustration of possible new physics scenarios, we examine the role of the Top Two Higgs Doublet Model. This is a simple extension of the SM obtained by adding second Higgs doublet in which the Yukawa interactions of the two Higgs doublets are assigned in order to naturally account for the large top-quark mass. Of course, many other extensions of the Standard Model could also account for these experimental deviations. Clearly if one takes these deviations seriously then some new particles in the 300 GeV to few TeV with associated new CP-odd phase(s) are needed.

hep-ph↗

arXiv2007

Null tests of the Standard Model at an International Super $B$ Factory

In light of the results from the B factories, which clearly show that the Cabibbo-Kobayashi-Maskawa mechanism is the dominant source of the observed CP violation in K and B physics, only small deviations due to sources of CP violation beyond the Standard Model are likely. Therefore, in the quest for New Physics, null tests of the Standard Model become increasingly important. Motivated by these considerations, we describe a number of approximate null tests of the Standard Model. These tests provide several theoretically clean approaches to searching for new physics in the B system. We find that in many cases, the requisite sensitivity can only be achieved with an International Super B Factory, with luminosity around 10^36 cm^-2 s^-1.

hep-ph↗

arXiv2007

Clean Signals of CP-violating and CP-conserving New Physics in B->P V gamma Decays at B Factories and Hadron Colliders

In radiative B decays to final states containing one spin zero meson, one spin one meson and a photon, the photon polarization can be measured from the angular distribution of the final state. The P-odd, C-even components of this distribution lead to triple correlation asymmetries that are very sensitive to new physics as they are likely to receive only tiny contributions from the Standard Model. There are also (CP-conserving) forward-backward asymmetries to which there may be SM contributions of a few percent; nevertheless there is a data driven means to overcome the SM ``pollution''. These latter observables can be used to search for a class of New Physics which does not necessarily possess a new CP-odd phase, and wherein the current structure is different from the left-handed electroweak theory of the Standard Model. The mode $B^\pm \to K^\pm ϕγ$ is particularly useful due to the relatively large branching ratio and its distinctive final state but there are dozens of such final states in the decays of $B_u$, $B_d$ and $B_s$ mesons where the analysis is applicable. In many cases, after the decay of the spin one resonance, several of these channels lead to only charged mesons and a photon in the final state, so they may well be accessible in a hadronic environment in addition, of course, to $e^+ e^-$ (Super) $B$ Factories. In passing we also briefly explain why the CP-conserving forward-backward asymmetries in $B \to P V γ$ decays is a more reliable signal of new physics compared to the (CP-conserving) transverse polarization in $B \to V_1 V_2$ modes even though, of course, the latter may be somewhat more abundant.

hep-ph↗

arXiv2006

Massive neutrinos in a grounds-up approach with quark-lepton similarity

We examine neutrino oscillations in a two Higgs doublet model (2HDM) in which the second doublet couples only to the third generation right-handed up-fermions, i.e., to t_R and to N_3 which is the heaviest right-handed Majorana neutrino. The inherently large tan(beta) of this model can naturally account for the large top mass and, based on a quark-lepton similarity ansatz, when embedded into a seesaw mechanism it can also account for the observed neutrino masses and mixing angles giving a very small theta_13: -0.017 < theta_13 < 0.021 at 99% CL, and a very restrictive prediction for the atmospheric mixing angle: 42.9(deg) < theta_atm < 45.2(deg) at 99% CL. The large value of tan(beta) also sets the mass scale of the heaviest right-handed Majorana neutrino N_3 and triggers successful leptogenesis through a CP-asymmetry in the decays of the N_1 (lightest right-handed Majorana) which is tan^2(beta) enhanced compared to the CP-asymmetry obtained in models for leptogenesis with one Higgs doublet or in the MSSM. This enhancement allows us to relax the lower bound on M_{N_1} and consequently also the lower bound on the reheating temperature of the early universe.

hep-ph↗

arXiv2006

Majorana neutrinos and lepton-number-violating signals in top-quark and W-boson rare decays

We discuss rare lepton-number-violating top-quark and W-boson four-body decays to final states containing a same-charge lepton pair, of the same or of different flavors: t -> b W- li^+ lj^+ and W+ -> J J' li^+ lj^+, where i \ne j or i=j and J J' stands for two light jets originating from a (u-bar d) or a (c-bar s) pair. These ΔL=2 decays are forbidden in the Standard Model and may be mediated by exchanges of Majorana neutrinos. We adopt a model independent approach for the Majorana neutrinos mixing pattern and calculate the branching ratios (BR) for these decays. We find, for example, that for O(1) mixings between heavy and light Majorana neutrinos (not likely but not ruled out) and if at least one of the heavy Majorana neutrinos has a mass of ~100 GeV, then the BR's for these decays are: BR(t -> b li^+ lj^+ W-) ~ 10^{-4} and BR(W+ -> li^+ lj^+ J J') ~ 10^{-7} if m_N ~ 100 GeV and BR(t -> b li^+ lj^+ J J') ~ BR(W+ -> li^+ lj^+ J J') ~ 0.01 if m_N < 50 GeV. Taking into account the present limits on the neutrino mixing parameters, we obtain more realistic values for these BR's: BR(t -> b li^+ lj^+ W-) ~ 10^{-6} and BR(W+ -> li^+ lj^+ J J') ~ 10^{-10} for m_N ~ 100 GeV and BR(t -> b li^+ lj^+ J J') ~ BR(W+ -> li^+ lj^+ J J') ~ 10^{-6} for m_N < 50 GeV.

hep-ph↗

arXiv2006

Improved method for CKM constraints in charmless three-body B and B_s decays

Recently Ciuchini, Pierini and Silvestrini proposed a method for constraining CKM parameters in B->K pi pi and B_s-> K pi pi through phase measurements of amplitudes involving I=3/2 K*pi final states. We show that complementary information on CKM parameters may be obtained by studying the phases of Delta I=1 B->(K*pi)_{I=1/2}, B_s->(K*\bar K)_{I=1} and B_s->(\bar K* K)_{I=1} amplitudes. Hadronic uncertainties in these constraints from electroweak penguin operators O_9 and O_10, studied using flavor SU(3), are shown to be very small in B->K pi pi and B_s->K pi pi and somewhat larger in B_s-> K\bar K pi. The first processes imply a precise linear relation between \bar rho and \bar eta, with a measurable slope and an intercept at \bar eta=0 involving a theoretical error of 0.03. The decays B_s-> K pi pi permit a measurement of gamma involving a theoretical error below a degree. We note that while time-dependence is required when studying B^0 decays at the Upsilon(4S), it may not be needed when studying B_s decays at hadronic colliders.

hep-ph↗

arXiv2006

Determination of gamma from Charmless B --> M1 M2 Decays Using U-Spin

In our previous paper we applied U-spin symmetry to charmless hadronic B+- --> M0 M+- decays for the purpose of precise extraction of the unitarity angle gamma. In this paper we extend our approach to neutral B0 and Bs --> M1 M2 decays. A very important feature of this method is that no assumptions regarding relative sizes of topological decay amplitudes need to be made. As a result, this method avoids an uncontrollable theoretical uncertainty that is often related to the neglect of some topological diagrams (e.g., exchange and annihilation graphs) in quark-diagrammatic approaches. In charged B+- decays, each of the four data sets, P0 P+-, P0 V+-, V0 P+- and V0 V+-, with P=pseudoscalar and V=vector, can be used to obtain a value of gamma. Among neutral decays, only experimental data in the B0, Bs --> P- P+ subsector is sufficient for a U-spin fit. Application of the U-spin approach to the current charged and neutral B decay data yields: gamma=(80^{+6}_{-8}) degrees. In this method, which is completely data driven, in a few years we should be able to obtain a model independent determination of gamma with an accuracy of O(few degrees).

hep-ph↗

arXiv2006

Collider Signals of Top Quark Flavor Violation from a Warped Extra Dimension

We study top quark flavor violation in the framework of a warped extra dimension with the Standard Model (SM) fields propagating in the bulk. Such a scenario provides solutions to both the Planck-weak hierarchy problem and the flavor puzzle of the SM without inducing a flavor problem. We find that, generically, tcZ couplings receive a huge enhancement, in particular the right handed ones can be O(1%). This results in BR(t -> c Z) at or above the sensitivity of the Large Hadron Collider (LHC). At the International Linear Collider (ILC), single top production, via e^+ e^- -> t \bar c, can be a striking signal for this scenario. In particular, it represents a physics topic of critical importance that can be explored even with a relatively low energy option, close to the tc threshold. At both the LHC and the ILC, angular distributions can probe the above prediction of dominance of right-handed couplings.

hep-ph↗