arXiv subjects

Ashutosh Kumar Alok

Publications and source records attributed to Ashutosh Kumar Alok.

72 records · Page 4Linked to original sources

arXiv2013

Decoherence free Bd and Bs meson systems

We study the impact of decoherence on B meson systems with specific emphasis on Bs. For consistency we also study the Bd mesons based on the most recent data. We find that the Bd mesons are 34 sigma away from total decoherence, while the Bs mesons are seen to be upto 31sigma away from total decoherence. Thus, our results prove, with experimental verity, that neutral meson systems are free from decoherence effects. Therefore, this provides a very useful laboratory for testing the foundations of quantum mechanics.

hep-ph↗

arXiv2012

b -> s Decays in a model with Z-mediated flavor changing neutral current

In the scenario with Z mediated flavor changing neutral current occurring at the tree level due to the addition of a vector-like isosinglet down-type quark d' to the SM particle spectrum, we perform a chi square fit using the flavor physics data and obtain the best fit value along with errors of the tree level Zbs coupling, U_{sb}. The fit indicates that the new physics coupling is constrained to be small: we obtain |U_{sb}| <= 3.40 X 10^{-4} at 3 sigma. Still this does allow for the possibility of new physics signals in some of the observables such as semileptonic CP asymmetry in Bs decays.

hep-ph↗

arXiv2011

New Physics in b -> s mu+ mu-: CP-Violating Observables

We perform a comprehensive study of the impact of new-physics operators with different Lorentz structures on CP-violating observables involving the b -> s mu+ mu- transition. We examine the effects of new vector-axial vector (VA), scalar-pseudoscalar (SP) and tensor (T) interactions on the CP asymmetries in the branching ratios and forward-backward asymmetries of Bs -> mu mu, B -> Xs mu mu, Bs -> mu mu gamma, B -> K mu mu, and B -> K* mu mu. In B -> K* mu mu, we also explore the direct CP asymmetries in the longitudinal polarization fraction f_L and the angular asymmetries A_T^{(2)} and A_{LT}, as well as the triple-product CP asymmetries A_T^{(im)} and A^{(im)}_{LT}. We find that, in almost all cases, the CP-violating observables are sensitive only to new physics which involves VA operators. The VA new physics may therefore be unambiguously identified by a combined analysis of future measurements of these CP-violating observables.

hep-ph↗

arXiv2011

Constraints on the Four-Generation Quark Mixing Matrix from a Fit to Flavor-Physics Data

In the scenario with four quark generations, we perform a fit using flavor-physics data and determine the allowed values -- preferred central values and errors -- of all of the elements of the 4X4 quark mixing matrix. In addition to the direct measurements of some of the elements, we include in the fit the present measurements of several flavor-changing observables in the K and B systems that have small hadronic uncertainties, and also consider the constraints from the vertex corrections to Z -> b bbar. The values taken for the masses of the fourth-generation quarks are consistent with the measurements of the oblique parameters and perturbativity of the Yukawa couplings. We find that |{\tilde V}_{tb}| >= 0.98 at 3sigma, so that a fourth generation cannot account for any large deviation of |{\tilde V}_{tb}| from unity. The fit also indicates that all the new-physics parameters are consistent with zero, and the mixing of the fourth generation with the other three is constrained to be very small: we obtain |{\tilde V}_{ub'}| < 0.06, |{\tilde V}_{cb'}| < 0.027, and |{\tilde V}_{tb'}| < 0.31 at 3sigma. Still, this does allow for the possibility of new-physics signals in Bd, Bs and rare K decays.

hep-ph↗

arXiv2010

Neutral Gauge Boson Contributions to the Dimuon Charge Asymmetry in B Decays

Recently, the D0 Collaboration measured the CP-violating like-sign dimuon charge asymmetry in neutral B decays, finding a 3.2sigma difference from the standard-model (SM) prediction. A non-SM charge asymmetry a_sl^s suggests a new-physics (NP) contribution to Bs-Bsbar mixing. In this case, in order to explain the measured value of a_sl^s within its 1sigma range, NP must be present in Gamma_12^s, the absorptive part of the mixing. In this paper, we examine whether such an explanation is possible in models with flavor-changing Z (ZFCNC) or Z' (Z'FCNC) gauge bosons. The models must also reproduce the measured values of the indirect CP asymmetry S_psi-phi in Bs -> J/psi phi, and Delta Gamma_s, the Bs-Bsbar width difference. We find that the ZFCNC model cannot reproduce the present measured values of S_psi-phi and a_sl^s within their 1sigma ranges. On the other hand, in the Z'FCNC model, the values of all three observables can be simultaneously reproduced.

hep-ph↗

arXiv2010

New-physics contributions to the forward-backward asymmetry in B -> K* mu+ mu-

We study the forward-backward asymmetry (AFB) and the differential branching ratio (DBR) in B -> K* mu+ mu- in the presence of new physics (NP) with different Lorentz structures. We consider NP contributions from vector-axial vector (VA), scalar-pseudoscalar (SP), and tensor (T) operators, as well as their combinations. We calculate the effects of these new Lorentz structures in the low-q^2 and high-q^2 regions, and explain their features through analytic approximations. We find two mechanisms that can give a significant deviation from the standard-model predictions, in the direction indicated by the recent measurement of AFB by the Belle experiment. They involve the addition of the following NP operators: (i) VA, or (ii) a combination of SP and T (slightly better than T alone). These two mechanisms can be distinguished through measurements of DBR in B -> K* mu+ mu- and AFB in B -> K mu+ mu-.

hep-ph↗

arXiv2010

SM with four generations: Selected implications for rare B and K decays

We extend our recent work and study implications of the Standard Model with four generations (SM4) for rare B and K decays. We again take seriously the several 2-3 $σ$ anomalies seen in B, $B_s$ decays and interpret them in the context of this simple extension of the SM. SM4 is also of course of considerable interest for its potential relevance to dynamical electroweak symmetry breaking and to baryogenesis. Using experimental information from processes such as $B \to X_s γ$, $B_d$ and $B_s$ mixings, indirect CP-violation from $K_L \to ππ$ etc along with oblique corrections, we constrain the relevant parameter space of the SM4, and find $m_{t'}$ of about 400-600 GeV with a mixing angle $| V_{t'b}^*V_{t's}|$ in the range of about (0.05 to 1.4)$\times 10^{-2}$ and with an appreciable CP-odd associated phase, are favored by the current data. Given the unique role of the CP asymmetry in $B_s \to ψϕ$ due to its gold-plated nature, correlation of that with many other interesting observables, including the semileptonic asymmetry ($A_{SL}$) are studied in SM4. We also identify several processes, such as $B \to X_s ν\barν$, $K_L \to π^0 ν\bar ν$ etc, that are significantly different in SM4 from the SM. Experimentally the very distinctive process $B_s\to μ^+μ^-$ is also discussed; the branching ratio can be larger or smaller than in SM, $(3.2 \to 4.2)\times 10^{-9}$, by a factor of ${\cal{O}}(3)$.

hep-ph↗

arXiv2009

The fourth family: a simple explanation for the observed pattern of anomalies in B-CP asymmetries

We show that a fourth family of quarks with $m_{t'}$ in the range of (400 - 600) GeV provides a rather simple explanation for the several indications of new physics that have been observed involving CP asymmetries of the b-quark. The built-in hierarchy of the 4$\times$4 mixing matrix is such that the $t'$ readily provides a needed {\it perturbation} ($\approx 15%$) to $\sin 2 β$ as measured in $B \to ψK_s$ and simultaneously is the dominant source of CP asymmetry in $B_s \to ψϕ$. The correlation between CP asymmetries in $B_s \toψϕ$ and $B_d\toϕK_s$ suggests $m_{t'} \approx$ (400 - 600) GeV. Such heavy masses point to the tantalizing possibility that the 4th family plays an important role in the electroweak symmetry breaking.

hep-ph↗

arXiv2009

B_s --> mu+ mu- decay in the R-parity violating minimal supergravity

We study B_s --> mu+ mu- in the context of the R-parity violating minimal supergravity in the high tan beta regime. We find that the lowest value of the branching ratio can go well below the present LHCb sensitivity and hence B_s --> mu+ mu- can even be invisible to the LHC. We also find that the present upper bound on Br(B_s --> mu+ mu-) puts strong constraint on the minimal supergravity parameter space. The constraints become more severe if the upper bound is close to its standard model prediction.

hep-ph↗

arXiv2009

Tension between scalar/pseudoscalar new physics contribution to B_s --> mu+ mu- and B --> K mu+ mu-

New physics in the form of scalar/pseudoscalar operators cannot lower the semileptonic branching ratio B(B --> K mu+ mu-) below its standard model value. In addition, we show that the upper bound on the leptonic branching ratio B(B_s --> mu+ mu-) sets a strong constraint on the maximum value of B(B --> K mu+ mu-) in models with multiple Higgs doublets: with the current bound, B(B --> K mu+ mu-) cannot exceed the standard model prediction by more than 2.5%. The conclusions hold true even if the new physics couplings are complex. However these constraints can be used to restrict new physics couplings only if the theoretical and experimental errors in B(B --> K mu+ mu-) are reduced to a few per cent. The constraints become relaxed in a general class of models with scalar/pesudoscalar operators.

hep-ph↗

arXiv2009

Bounds on Tensor operator contribution to B_s --> l+ l- gamma

We consider the effect of new physics interactions in the form of tensor operators on the branching ratio of B_s --> l+ l- gamma where l=e, mu. We find that the present data on the inclusive branching ratio B --> X_s l+ l- puts a strong constraint on the new physics tensor couplings and does not allow a large enhancement in the branching ratio of B_s --> l+ l- gamma beyond its standard model predictions. Large enhancement in the branching ratio of B_s --> l+ l- gamma due to new physics in the form of vector/axial-vector, scalar/pseudoscalar and magnetic dipole operators has already been ruled out. Thus we conclude that no new physics can provide a large enhancement in the branching ratio of B_s --> l+ l- gamma and hence it is expected to be observed in future experiments with a branching ratio close to its standard model expectation.

hep-ph↗

arXiv2009

CP asymmetry in the decays B --> (X_s, X_d) mu+ mu- with four generations

We estimate the CP asymmetry A_{CP}(q^2) in the decays B --> X_s mu+ mu- and B --> X_d mu+ mu- in the standard model (SM) with an additional fourth generation. We use a parametrization that allows us to explore the complete parameter space of the 4X4 quark mixing matrix, and constrain these parameters from the current data on B decays. We find that the enhancement in A_{CP}(q^2) depends strongly on the mass of the t', the up-type quark in the fourth generation. For m_t' around 400 GeV, the CP asymmetry in the high-q^2 region (q^2 > 14.4 GeV^2) can be enhanced by more than an order of magnitude for B --> X_s mu+ mu- and up to a factor of 6 for B --> X_d mu+ mu-. There is no enhancement in the low-q^2 region (1< q^2 <6 GeV^2). With increasing m_t', the A_{CP}(q^2) in the high-q^2 (low-q^2) region first decreases (increases) and then saturates at a value a few times the SM prediction. In the high-q^2 region of B --> X_s mu+ mu-, this saturation value may be up to 25 times the SM expectation.

hep-ph↗

arXiv2008

Large forward-backward asymmetry in B --> K mu+ mu- from new physics tensor operators

We study the constraints on possible new physics contribution to the forward-backward asymmetry of muons, A_{FB}(q^2), in B --> K mu+ mu-. New physics in the form of vector/axial-vector operators does not contribute to A_{FB}(q^2) whereas new physics in the form of scalar/pseudoscalar operators can enhance A_{FB}(q^2) only by a few per cent. However new physics the form of tensor operators can take the peak value of A_{FB}(q^2) to as high as 40% near the high-q^2 end point. In addition, if both scalar/pseudoscalar and tensor operators are present, then A_{FB}(q^2) can be more than 15% for the entire high-q^2 region q^2 > 15 GeV^2. The observation of significant A_{FB} would imply the presence of new physics tensor operators, whereas its q^2 dependence could further indicate the presence of new scalar/pseudoscalar physics.

hep-ph↗

arXiv2008

Probing extended Higgs sector through rare b --> s mu+ mu- transitions

We study the constraints on the contribution of new physics in the form of scalar/pseudoscalar operators to the average forward backward asymmetry of muons in B --> K mu+ mu- and the longitudinal polarization asymmetry A_{LP} of muons in B_s --> mu+ mu-. We find that the maximum possible value of allowed by the present upper bound on B(B_s --> mu+ mu-) is about 1% at 95% C.L. and hence will be very difficult to measure. On the other hand, the present bound on B(B_s --> mu+ mu-) fails to put any constraints on A_{LP}, which can be as high as 100% even if B(B_s --> mu+ mu-) is close to its standard model prediction. The measurement of A_{LP} will be a direct evidence for an extended Higgs sector, and combined with the branching ratio B(B_s --> mu+ mu-) it can even separate the new physics scalar and pseudoscalar contributions.

hep-ph↗

arXiv2007

Does the present data on B_s - bar B_s mixing rule out a large enhancement in the branching ratio of B_s --> mu+ mu- ?

In this letter, we consider the constraints imposed by the recent measurement of B_s - bar B_s mixing on the new physics contribution to the rare decay B_s --> mu+ mu-. New physics in the form vector and axial-vector couplings is already severely constrained by the data on B --> (K,K*) mu+ mu-. Here, we show that B_s - bar B_s mixing data, together with the data on K0 - bar K0 mixing and K_L --> mu+ mu- decay rate, strongly constrain the scalar-pseudoscalar contribution to B_s --> mu+ mu-. We conclude that new physics can at best lead to a factor of 2 increase in the branching ratio of B_s --> mu+ mu- compared to its Standard Model expectation.

hep-ph↗

arXiv2006

New physics upper bound on the branching ratio of B_s--> l+ l- and B_s--> l+ l- gamma

We consider the most general new physics effective Lagrangian for b--> s l+ l-. We derive the upper limit on the branching ratio for the processes B_s--> l^+ l- where l=e, mu, subject to the current experimental bounds on related processes, B--> (K,K*) l+ l-. If the new physics interactions are of vector/axial-vector form, the present measured rates for B--> (K,K*) l+ l- constrain B_s--> l+ l to be of the same order of magnitude as their respective Standard Model (SM) predictions. On the other hand, if the new physics interactions are of scalar/pseudoscalar form, B--> (K,K*) l+ l- rates do not impose any useful constraint on B_s--> l+ l- and the branching ratios of these decays can be as large as present experimental upper bounds. If future experiments measure B_s--> l+ l- to be > 10^{-8} then the new physics giving rise to these decays has to be of the scalar/pseudoscalar form. We also consider the effect of new physics on B_s--> l+ l- gamma subject to the present experimental constraints on B--> (K,K*) l+ l- and B--> K* gamma. New physics in form scalar/pseudoscalar, which makes a very large contribution to B_s--> l+ l-, makes no contribution at all to B_s--> l+ l- gamma due to angular momentum conservation. New Physics in the form of vector/axial-vector operators is constrained by the data on B--> (K,K*) l+ l- and new physics in the form of tensor/pseudo-tensor is constrained by the data on B--> K* gamma. In both cases, enhancement of B_s--> l+ l- gamma much beyond the SM expectation is impossible. In conclusion, present data on B-->(K,K*) transitions allow for large B(B_s--> l+ l-) but do not allow B(B_s--> l+ l- gamma) to be much larger than its SM expectation.

hep-ph↗

arXiv2006

New physics upper bound on the branching ratio of B_s --> l+ l- gamma

We consider the effect of new physics on the branching ratio of B_s--> l+ l- gamma where l=e,mu. If the new physics is of the form scalar/pseudoscalar, then it makes no contribution to B_s--> l+ l- gamma, unlike in the case of B_s--> l+ l-, where it can potentially make a very large contribution. If the new physics is in the form of vector/axial-vector operators, then present data on B-->(K,K^*)l+ l-, does not allow a large enhancement for B_s--> l+ l- gamma. If the new physics is in the form of tensor/pseudotensor operators, then the data on B-->(K,K^*)l+ l- gives no useful constraint but the data on B-->K^* gamma does. Here again, a large enhancement of B(B_s--> l+ l- gamma), much beyond the Standard Model expectation, is not possible. Hence, we conclude that the present data on b-->s transitions allow a large boost in B_s--> l+ l- but not in B_s--> l+ l- gamma.

hep-ph↗

arXiv2005

New physics upper bound on the branching ratio of B_s --> l+ l-

We consider the most general new physics effective Lagrangian for b --> s l+ l-. We derive the upper limit on the branching ratio for the processes B_s --> l+ l- where l=e, mu, subject to the current experimental bounds on related processes, B --> K l+ l- and B --> K* l+ l-. If the new physics interactions are of vector/axial-vector form, the present measured rates for B --> (K,K*) l+ l- constrain B(B_s --> l+ l-) to be of the same order of magnitude as their respective Standard Model predictions. On the other hand, if the new physics interactions are of scalar/pseudo-scalar form, B --> (K,K*) l+ l- rates do not impose any constraint on B_s --> l+ l- and the branching ratios of these decays can be as large as present experimental upper bounds. If future experiments measure B(B_s --> l+ l-) to be > 10^{-8} then the new physics giving rise to these decays has to be of the scalar/pseudo-scalar form.

hep-ph↗