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Namit Mahajan

Publications and source records attributed to Namit Mahajan.

At least 55 records · Page 3Linked to original sources
arXiv2008

CP asymmetry in $B^{+}\to K^+π^{0}$ and New Physics

The CP asymmetry in $B^0\to K^+π^-$ is expected to be similar to that in $B^+\to K^+π^0$. The experimental data however show $\sim 5σ$ difference between the two, leading to the so called $Δ{A}_{Kπ}$ puzzle. Employing sum rule(s) following from (approximate) flavour symmetry, we show that it is possible to accommodate the observed experimental values within the standard model (SM) for a narrow range of parameters. Sub-leading terms can bring the theoretical predictions in better agreement with the data. Resolution via modified electroweak penguin contributions is possible for a large CP violating phase generated by the new physics. However, the data on polarization in $B\to VV(T)$, $B_s$-$\bar{B}_s$ mixing (and large CP phase) and $B^+\toτ^+ν_τ$ rate can not be simultaneously accommodated within SM or new physics with only enhanced electroweak penguins. A plausible resolution to these, and not spoiling the $B\to Kπ$ rates and asymmetries, could be a general two Higgs doublet model.

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arXiv2007

Remarks on the $A_{SL}^s$ and $ΔΓ_s$ Studies at the Tevatron and Beyond

We give an assessment of the three approaches, undertaken by the D$\emptyset$ experiment, to probe CP violation in $B_s$ mixing without measurement of $B_s$-$\bar B_s$ oscillations or tagging: dimuon charge asymmetry $A_{SL}$, untagged single muon charge asymmetry $A_{SL}^s$, and lifetime difference in untagged $B_s \to J/ψϕ$ decay. The latter two approaches provide an alternative avenue, if not crosscheck, to the usual mixing-decay interference study in modes such as $B_s \to J/ψϕ$. Prospects at the Tevatron, LHC and (Super) B factories are discussed.

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arXiv2007

$B\to K^*\ell^+\ell^-$ Forward-backward Asymmetry and New Physics

The forward-backward asymmetry ${\cal A}_{\rm FB}$ in $B\to K^*\ell^+\ell^-$ decay is a sensitive probe of New Physics. Previous studies have focused on the sensitivity in the position of the zero. However, the short distance effective couplings are in principle complex, as illustrated by $B\to ρ\ell^+\ell^-$ decay within the Standard Model. Allowing the effective couplings to be complex, but keeping the $B\to K^*γ$ and $K^*\ell^+\ell^-$ rate constraints, we find the landscape for ${\cal A}_{\rm FB}(B\to K^*\ell^+\ell^-)$ to be far richer than from entertaining just sign flips, which can be explored by future high statistics experiments.

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arXiv2006

Probing Low-x QCD With Very High Energy Prompt Muons

We explore the possibility of utilizing the prompt muon fluxes at very high energies in order to discriminate various models/parametrizations of low-x QCD behaviour of hadronic cross-sections relevant at such energies. We find that the pair meter technique for measuring high energy prompt muons can be very efficient in such an endeavor. As a by product, it allows to cleanly probe the change in composition of the primary cosmic rays expected at high energies.

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arXiv2006

Finite Temperature Effects and Axion Cosmology

We investigate the impact of finite temperature effects on axions in the context of cosmology. The temperature dependence of the decay constant is modeled analogous to pions. For the two interesting cases considered here, we find that the temperature effects do lead to changes relevant for detailed and precise abundance and rate calculations. We also find that the axion decoupling temperature starts showing large deviations for larger values of the axion decay constant.

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arXiv2006

W-exchange/Annihilation amplitudes in LCSR - $B_d^0 \to D_s^-K^+$ as an example

We evaluate the W-exchange diagram within the framework of light cone sum rules (LCSR), taking $B_d^0 \to D_s^-K^+$ as an example. This decay mode, though proceeding via the W-exchange diagram only and therefore expected to be highly suppressed, has the branching ratio $3.2 \times 10^{-5}$. We estimate the W-exchange amplitude within LCSR, including soft gluon corrections, to twist-3 accuracy. The calculation naturally brings out the features which suggest that such an amplitude is expected to be small in cases when both the final state mesons are light, without relying on any kind of an assumption. With minor changes, it is also possible to have a rough estimate of the exchange/annihilation type contributions to other processes like $B \to ππ,~πK, KK$. We find that though it appears as if the sum rule method yields a fair agreement with the observed value, a careful analysis of individual terms shows that the method, in its present form, is inadequate to capture the correct physical answer for the case of heavy final states.

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arXiv2005

Revisiting 5D chiral symmetry breaking and holographic QCD models

We take a closer look at the recently discussed models of hadrons based on holographic ideas and chiral symmetry breaking in five dimensions. We study the two point correlator in detail and look at the field theoretic properties needed to be satisfied by the correlator. It is shown that the spectral density becomes negative, violating the basic requirements for the Kallen-Lehmann representation. We briefly discuss the implications of this violation of the positivity of the spectral density and also discuss possible origin of such a violation in these models. Put simply this means that such models are not physical descriptions of the hadron spectrum.

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arXiv2005

$B_d - \bar{B}_d$ mass difference in Little Higgs model

An alternate solution of hierarchy problem in the Standard Model namely, the Little Higgs model, has been proposed lately. In this work $B_d^0-\bar{B_d^0}$ mass difference in the framework of the Little Higgs model is evaluated. The experimental limits on the mass difference is shown to provide meaningful constraints on the parameter space of the model.

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arXiv2004

Littlest Higgs Model and Unitarity Constraints

The Littlest Higgs Model is constrained using the tree-level perturbative arguments, similar to those employed to obtain the Higgs mass bound. It is found that the perturbative unitaity violations set in at energy scales $\sim 1.5χf$, where $f$ is the analog of pion decay constant and sets the scale for the masses in the theory and also the strong interaction scale and $χ$ is a dimensionless parameter which together with $f$ sets the mass scale of heavy scalars in the theory.

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arXiv2004

$B\toρ$ form factors including higher twist contributions and reliability of pQCD approach

We discuss $\btorho$ form factors within the framework of perturbative QCD, including the higher twist contributions and study the validity of such an approach in calculating quantities such as form factors, which in principle and quite generally are thought to be completely non-perturbative objects and are expected to receive large contributions from the non-perturbative regime in the calculations. It is shown that including the Sudakov and threshold resummation effects, the general expectations of the pQCD approach are clearly met and the form factors do indeed receive most of the contribution from the perturbative region. We do not make an attempt to precisely evaluate the form factors but rather try to study the gross features and behaviour of the same. We also find that use of single wave function for the B-meson may actually underestimate various quantities. The results clearly indicate the validity and reliability of pQCD calculations, at least in this particular case.

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arXiv2004

CP violation in the decay mode $B\to πγγ$

Within the framework of Standard Model, the exclusive decay mode $B\to πγγ$ is studied. Although the usual short distance contribution is small compared to the similar $B\to Kγγ$ mode, the process offers the possibility of studying the CP violation, a feature absent in the $B \to K$ counterpart.

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arXiv2003

$t \to b W$ in NonCommutative Standard Model

We study the top quark decay to b quark and W boson in the NonCommutative Standard Model (NCSM). The lowest contribution to the decay comes from the terms quadratic in the matrix describing the noncommutative (NC) effects while the linear term is seen to identically vanish because of symmetry. The NC effects are found to be significant only for low values of the NC characteristic scale.

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arXiv2003

$B \to X_s +$ missing energy in models with large extra dimensions

We study the neutral current flavour changing rare decay mode $B\to X_s +$ missing energy within the framework of theories with large extra spatial dimensions. The corresponding Standard Model signature is $B\to X_s + ν\barν$. But in theories with large extra dimensions, it is possible to have scalars and gravitons in the final state making it quite distinct from any other scenario where there are no gravitons and the scalars are far too heavier than the B-meson to be present as external particles. We give an estimate of the branching ratio for such processes for different values of the number of extra dimensions and the effective scale of the effective theory.

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arXiv2002

Lepton polarization asymmetry in radiative dileptonic B-meson decays in MSSM

In this paper we study the polarization asymmetries of the final state lepton in the radiative dileptonic decay of B meson (\bsllg) in the framework of Minimal Supersymmetric Standard Model (MSSM) and various other unified models within the framework of MSSM e.g. mSUGRA, SUGRA (where condition of universality of scalar masses is relaxed) etc. Lepton polarization, in addition of having a longitudinal component (\pl), can have two other components, \pt and \pn, lying in and perpendicular to the decay plane, which are proportional to \ml and hence are significant for final state being $μ^+ ~ μ^-$ or $τ^+ \~τ^-$. We analyse the dependence of these polarization asymmetries on the parameters of the various models.

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