arXiv subjects

Alexander L. Kagan

Publications and source records attributed to Alexander L. Kagan.

30 records · Page 2Linked to original sources

arXiv2004

Right-handed currents, CP Violation, and B->VV

Precision CP violation measurements in rare hadronic B decays could provide clean signatures of parity symmetric new physics, implying the existence of SU(2)_L x SU(2)_R x U(1)_{B-L} x P symmetry at high energies. New contributions to the weak scale Hamiltonian which respect parity to O(1%) in supersymmetric realizations are compatible with an SU(2)_R breaking scale M_R \le M_GUT, implying that sensitivity to the GUT scale is possible. The generic case of right-handed currents without left-right symmetry is also discussed. A detailed analysis of B -> VV polarization in QCD factorization reveals that the low longitudinal polarization fraction f_L (phi K*) \approx 50% can be accounted for in the SM via a QCD penguin annihilation graph. The ratio of transverse rates (Gamma_\perp /Gamma_\parallel) provides a sensitive test for new right-handed currents. CP violation measurements in B -> VV decays can discriminate between new contributions to the dipole and four quark operators.

hep-ph↗

arXiv2002

Constraints on Light Bottom Squarks from Radiative B-Meson Decays

The presence of a light b-squark (with mass about 4 GeV) and gluino (with mass about 15 GeV) might explain the observed excess in b-quark production at the Tevatron. Though provocative, this model is not excluded by present data. The light supersymmetric particles can induce large flavor-changing effects in radiative decays of B mesons. We analyse the decays B->X_s gamma and B->X_{sg} in this scenario and derive restrictive bounds on the flavor-changing quark-squark-gluino couplings.

hep-ph↗

arXiv2002

Isospin Breaking in B -> K^* gamma Decays

A calculation of the leading isospin-breaking contributions to the B -> K^* gamma decay amplitudes based on the QCD factorization approach is presented. They arise at order Lambda/m_b in the heavy-quark expansion and are due to annihilation contributions from 4-quark operators, the chromo-magnetic dipole operator, and charm penguins. In the Standard Model the decay rate for B^0 -> K^{*0} gamma is predicted to be about 10-20 % larger than that for B^- -> K^{*-} gamma. Isospin-breaking effects are a sensitive probe of the penguin sector of the effective weak Hamiltonian. New Physics models in which the hierarchy of B -> K^* gamma decay rates is either flipped or greatly enhanced could be ruled out with more precise data.

hep-ph↗

arXiv2002

Can the CP asymmetries in $B\to ψK_S$ and $B\to ψK_L$ differ?

In the standard model the CP asymmetries in $B\to ψK_S$ and $B\to ψK_L$ are equal in magnitude and opposite in sign to very good approximation. We compute the order $ε_K$ corrections to each of these CP asymmetries and find that they give a deviation from $\sin 2β$ at the half percent level, which may eventually be measurable. However, the correction to $a_{CP}(B\to ψK_S)+a_{CP}(B\to ψK_L)$ due to $ε_K$ is further suppressed. The dominant corrections to this sum, at the few times $10^{-3}$ level, come from the $B$ lifetime difference, and CP violation in $B- \bar B$ mixing and $B \to ψK$ decay. New physics could induce a significant difference in the $\sin(Δm_B t)$ time dependence in the asymmetries if and only if the "wrong-flavor" amplitudes $B \to ψ\bar K$ or $\bar B \to ψK$ are generated. A scale of new physics that lies well below the weak scale would be required. Potential scenarios are therefore highly constrained, and do not appear feasible. A direct test is proposed to set bounds on such effects.

hep-ph↗

arXiv2000

Implications of a Low sin(2 beta): A Strategy for Exploring New Flavor Physics

We explore the would-be consequences of a low value of the CP-violating phase $\sin2β_{ψK}$. The importance of a reference triangle obtained from measurements that are independent of $B$--$\bar B$ and $K$--$\bar K$ mixing is stressed. It can be used to extract separately potential New Physics contributions to mixing in the $B_d$, $B_s$ and $K$ systems. We discuss several constructions of this triangle, which will be feasible in the near future. The discrete ambiguity is at most two-fold and eventually can be completely removed. Simultaneously, it will be possible to probe for New Physics in loop-dominated rare decays.

hep-ph↗

arXiv1999

Trojan Penguins and Isospin Violation in Hadronic B Decays

Some rare hadronic decays of B mesons, such as B->pi K, are sensitive to isospin-violating contributions from physics beyond the Standard Model. Although commonly referred to as electroweak penguins, such contributions can often arise through tree-level exchanges of heavy particles, or through strong-interaction loop diagrams. The Wilson coefficients of the corresponding electroweak penguin operators are calculated in a large class of New Physics models, and in many cases are found not to be suppressed with respect to the QCD penguin coefficients. Several tests for these effects using observables in B->pi K decays are discussed, and nontrivial bounds on the couplings of the various New Physics models are derived.

hep-ph↗

arXiv1999

Large Delta(I)=3/2 Contribution to epsilon'/epsilon in Supersymmetry

We show that in supersymmetric extensions of the Standard Model gluino box diagrams can yield a large Delta(I)=3/2 contribution to s->d+q(bar)+q FCNC processes, which may induce a sizable CP-violating contribution to the I=2 isospin amplitude in K->pi+pi decays. This contribution only requires moderate mass splitting between the right-handed up- and down-squarks, and persists for squark masses of order 1 TeV. Taking into account current bounds on Im(delta_{sd}^{LL}) from K-K(bar) mixing, the resulting contribution to epsilon'/epsilon could be an order of magnitude larger than the measured value.

hep-ph↗

arXiv1998

The Phenomenology of Enhanced b->sg

Potential hints for New Physics enhanced b -> sg dipole operators are reviewed. Implications for inclusive kaon spectra, and corrresponding search strategies are discussed. Remarkably, all B meson rare decay constraints can be evaded. Large CP asymmetries are expected if new contributions to the dipole operators contain non-trivial weak phases. A critical comparison of eta' production in the Standard Model and in models with enhanced b -> sg is presented. A Standard Model explanation of the large B -> eta' Xs rate measured by CLEO poses a challenge, evidently requiring novel non-perturbative order of magnitude enhancement of gluon anomaly mediated contributions. However, in the case of enhanced b -> sg the existence of a cocktail solution is very likely.

hep-ph↗

arXiv1997

Final State Interactions and New Physics in B -> pi K Decays

Within the Standard Model, and if one assumes that soft rescattering effects are negligible, the CP asymmetry A^dir_CP (B^\pm -> π^\pm K) is predicted to be very small and the ratio R = BR(B_d -> π^\mp K^\pm)/BR(B^\pm -> π^\pm K) provides a bound on the angle γof the unitarity triangle, sin^2 γ\leq R. We estimate the corrections from soft rescattering effects using an approach based on Regge phenomenology, and find effects of order 10% with large uncertainties. In particular, we conclude that A^dir_CP \sim 0.2 and sin^2 γ\sim 1.2 R could not be taken unambiguously to signal New Physics. Using SU(3) relations, we suggest experimental tests that could constrain the size of the soft rescattering effects thus reducing the related uncertainty. Finally, we study the effect of various models of New Physics on A^dir_CP and on R.

hep-ph↗

arXiv1997

$η'$ production in B-decays: Standard Model vs. New Physics

Standard model factorization can account for the large rate for $B^\pm \to η' K^\pm$. The large rate for $B \to η' X_s$ is more problematic and requires a ``cocktail'' solution. Individual contributions from factorization, $b \to η' s g$, and ``intrinsic charm'' fall significantly short of the observed rate. In contrast, the observed $η'$ yields are easily explained in models with $B(b \to sg) \sim 15%$. Factorization can account for the entire semi-inclusive $η'$ yield without violating rare decay constraints. Implications for CP violation are discussed.

hep-ph↗

arXiv1997

Hints for Enhanced b -> sg from Charm and Kaon Counting

Previously, motivation for enhanced b -> sg from new flavor physics has centered on discrepancies between theory and experiment. Here two experimental hints are considered: (1) updated measurements of the charm multiplicity and BR(Bbar -> X_{ccbars}) at the Upsilon (4S) imply BR(B -> Xnocharm}) \approx 12.4 \pm 5.6 %, (2) the Bbar} -> K- X and Bbar -> K+/K- X branching fractions are in excess of conventional Bbar -> Xc -> KX yields by about 16.9 \pm 5.6 % and 18 \pm 5.3 %, respectively. JETSET 7.4 was used to estimate kaon yields from ssbar popping in Bbar -> Xcubard decays. JETSET 7.4 Monte Carlos for BR(Bbar -> Xsg) \sim 15 % imply that the additional kaon production would lead to 1 σagreement with observed charged and neutral kaon yields. The Ks momentum spectrum would be consistent with recent CLEO bounds in the end point region. Search strategies for enhanced b -> sg are discussed in light of large theoretical uncertainty in the standard model fast kaon background from b -> s penguin operators.

hep-ph↗

arXiv1994

Implications of TeV Flavor Physics for the ΔI =1/2 Rule and the B-meson Semileptonic Branching Ratio

Two of the outstanding discrepancies between weak interaction phenomenology and the standard model come in the large size of the $ΔI={1\over 2}$ enhancement in $K$ decays and in the small value of the $B$ semileptonic branching ratio. We argue that these discrepancies are naturally explained by chromomagnetic dipole operators arising from new physics at the TeV scale. These operators are closely connected to diagrams which contribute to the quark mass matrix, and we show how the proper enhancement of the hadronic decays of $s$ and $b$ quarks can be linked to generation of particular Cabbibo-Kobayaski-Maskawa mixing angles or quark masses. We confirm our model-independent analysis with detailed consideration of supersymmetric models and of technicolor models with techniscalars. This picture has additional phenomenological predictions for the $B$ system: The branching ratio of charmless nonleptonic $B$ decays should be of order 20\%, due to a large rate for $b\to sg$, while there are no dangerous new contributions to $b\rightarrow s γ$. Sizable contributions to $b \to d γ$ are a common feature of models incorporating this mechanism. In techniscalar models the $Z b \bar b$ coupling is enhanced, in association with sizable contributions to $b \to s μ^+ μ^-$.

hep-ph↗