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L. M. Sehgal

Publications and source records attributed to L. M. Sehgal.

At least 37 records · Page 2Linked to original sources
arXiv2000

Large Effects of CP- and T-Violation in K^0 Decay

The epsilon-impurity of the K_L wave-function gives rise to a huge CP-violating effect in the decay K_L -> pi^+ pi^- gamma which is hidden in the polarization state (Stokes vector) of the photon. One component of the Stokes vector is CP-odd and T-even, and may be identified with circular polarization. Another component is CP-odd and T-odd (``oblique polarization'') and reveals itself as a large asymmetry (14%) in the decay K_L -> pi^+ pi^- e^+ e^-. Striking time-dependent effects are predicted in the angular distribution of the pi^+ pi^- e^+ e^- system emanating from an initial K^0 or K^0bar state.

hep-ph↗
arXiv2000

Angular Distribution and CP Asymmetries in the Decays B->K^-pi^+e^-e^+ and B->pi^-pi^+e^-e^+

The short-distance Hamiltonian describing b->s(d)e^-e^+ in the standard model is used to obtain the decay spectrum of \bar{B}->K^-pi^+e^-e^+ and \bar{B}->pi^-pi^+e^-e^+, assuming the Kpi and pipi systems to be the decay products of K^* and rho respectively. Specific features calculated are (i) angular distribution of K^- (or pi^-) in the K^-pi^+ (or pi^-pi^+) centre-of-mass (c.m.) frame; (ii) angular distribution of e^- in the e^-e^+ c.m. frame; and (iii) the correlation between the meson and lepton planes. We also derive CP-violating observables obtained by combining the above decays with the conjugate processes B->K^+pi^-e^-e^+ and B->pi^-pi^+e^-e^+.

hep-ph↗
arXiv2000

Time Evolution of Decay Spectrum in K^0, K^0 bar -> pi^+ pi^- e^+ e^-

We consider the decay K^0 (K^0 bar) -> pi^+ pi^- e^+ e^- of a neutral K meson prepared in a state of strangeness +1 (-1). The time evolution of the state produces remarkable time-dependent effects in the angular distribution of the pi^+ pi^- e^+ e^- system. These effects are correlated with the time-dependence of the photon polarization in the radiative decay K^0 (K^0 bar) -> pi^+ pi^- gamma. We study, in particular, the CP-odd, T-odd term in the distribution d Gamma / d phi of the angle between the pi^+ pi^- and the e^+ e^- planes. We also give the spectrum in the case that the decaying meson is an incoherent mixture of K^0 and K^0 bar, and discuss the case of K_S regeneration in a K_L beam.

hep-ph↗
arXiv1999

Violation of Time Reversal Invariance in the Decays K_L -> pi^+ pi^- gamma and K_L -> pi^+ pi^- e^+ e^-

The origin of the large CP-odd and T-odd asymmetry observed in the decay K_L -> pi^+ pi^- e^+ e^- is traced to the polarization properties of the photon in the decay K_L -> pi^+ pi^- gamma. The Stokes vector of the photon S = (S_1, S_2, S_3) is studied as a function of the photon energy and found to possess CP-violating components S_1 and S_2 which are sizeable over a large part of the phase space, despite being proportional to the epsilon parameter of the K_L wave function. The component S_2 is T-even and manifests itself as a circular polarization of the photon, while S_1 is T-odd and gives rise to the asymmetry observed in K_L -> pi^+ pi^- e^+ e^-. The latter is shown to survive in the ``hermitian'' limit in which all unitarity phases are absent, and represents a genuine example of time reversal symmetry breaking in a CPT invariant theory.

hep-ph↗
arXiv1999

CP- and T-Violation in the Decay K_L -> pi+ pi- e+ e- and Related Processes

I review the theoretical basis of the prediction that the decay K_L -> pi+ pi- e+ e- should show a large CP- and T-violation, a prediction now confirmed by the KTeV experiment. The genesis of the effect lies in a large violation of CP- and T-invariance in the decay K_L -> pi+ pi- gamma, which is encrypted in the polarization state of the photon. The decay K_L -> pi+ pi- e+ e- serves as an analyser of the photon polarization. The asymmetry in the distribution of the angle phi between the pi+ pi- and e+ e- planes is a direct measure of the CP-odd, T-odd component of the photon's Stokes vector. A complete study of the angular distribution can reveal further CP-violating features, which probe the non-radiative (charge-radius and short-distance) components of the K_L -> pi+ pi- e+ e- amplitude.

hep-ph↗
arXiv1999

Angular Distribution of Decay Leptons from e^+e^- \to W^+W^- at Threshold

The reaction $ e^+e^- \to W^+W^-$ produces a $W$-boson pair with a non-trivial spin correlation at threshold. This correlation leads to a characteristic angular correlation between the leptons produced in $W^{\pm} \to \ell ^{\pm} ν$ (angles relative to the $e^-$ beam direction): $d σ/d \cos θ_+ d \cos θ_- \sim (1-\cos θ_+) (1+ \cos θ_-) (1+\cos θ_- \cos θ_+) $ . If only the $\ell^-$ is observed, its angular distribution is $d σ/ d \cos θ_- \sim (1+\cos θ_-)(3-\cos θ_-) $, implying a forward-backward asymmetry of 3/8. An analytic result is also given for the azimuthal correlation. These results are reproduced by a Monte Carlo program, that also enables us to study the effects of the $W$ decay width. The threshold behaviour, which stems from the dominance of $ν$-exchange, is contrasted with that due to $γ$- and $Z$-exchange, which is relevant for annihilation in the helicity state $e^-_Re^+_L$.

hep-ph↗
arXiv1998

Magnetic and axial vector form factors as probes of orbital angular momentum in the proton

We have recently examined the static properties of the baryon octet (magnetic moments and axial vector coupling constants) in a generalized quark model in which the angular momentum of a polarized nucleon is partly spin $\langle S_z \rangle$ and partly orbital $\langle L_z \rangle$. The orbital momentum was represented by the rotation of a flux-tube connecting the three constituent quarks. The best fit is obtained with $\langle S_z \rangle = 0.08\pm 0.15$, $\langle L_z \rangle = 0.42\pm 0.14$. We now consider the consequences of this idea for the $q^2$-dependence of the magnetic and axial vector form factors. It is found that the isovector magnetic form factor $G_M^{\mathrm{isovec}}(q^2)$ differs in shape from the axial form factor $F_A(q^2)$ by an amount that depends on the spatial distribution of orbital angular momentum. The model of a rigidly rotating flux-tube leads to a relation between the magnetic, axial vector and matter radii, $\langle r^2 \rangle_{\mathrm{mag}} = f_{\mathrm{spin}} \langle r^2 \rangle_{\mathrm{axial}} + \frac{5}{2} f_{\mathrm{orb}} \langle r^2 \rangle_{\mathrm{matt}}$, where $f_{\mathrm{orb}}/ f_{\mathrm{spin}} = \frac{1}{3}\langle L_z \rangle / G_A$, $f_{\mathrm{spin}} + f_{\mathrm{orb}} = 1$. The shape of $F_A(q^2)$ is found to be close to a dipole with $M_A = 0.92\pm 0.06$ GeV.

hep-ph↗
arXiv1997

CP Violation in Selected B Decays

We summarize the results of two papers in which we have studied CP violation in inclusive and exclusive decays b -> d e^+e^-. Two CP-violating effects are calculated: the partial rate asymmetry between b and \bar{b} decay, and the asymmetry between e^- and e^+ spectra for an untagged B/\bar{B} mixture. These asymmetries, combined with the branching ratio, can potentially determine the parameters (ρ, η) of the unitarity triangle. We also summarize a paper by Browder et al. on a possible CP-violating asymmetry in the inclusive reaction B -> K^-(K^{*-})X.

hep-ph↗
arXiv1997

CP Violation in the Exclusive Decays B -> pi e^+e^- and B -> rho e^+e^-

As a sequel to the calculation of the CP-violating asymmetry in the decay rates of b -> d l^+l^- and \bar{b} -> \bar{d} l^+l^-, we address in this paper the asymmetry in exclusive channels \bar{B} -> pi e^+e^- and \bar{B} -> rho e^+e^-, using form factors from two different models. In the invariant mass region 1GeV < \sqrt{s} < M_{J/psi}, the partial width asymmetry in the channel \bar{B} -> pi is -6% (-2%), and that in the channel \bar{B} -> rho, for one choice of form factors, is -5% (-2%), assuming CKM parameters eta = 0.34, rho = 0.3 (-0.3). We also calculate the forward-backward asymmetry A_{FB} of the e^- in the e^+e^- centre-of-mass system, and find average values (\bar{B} -> pi) \equiv 0, (\bar{B} -> rho)= -17 %, to be compared with the inclusive result (b -> d)= -9%. There is a CP-violating difference between A_{FB} and the corresponding asymmetry in the antiparticle channel \bar{A}_{FB}. Formulae are given that are applicable to any FCNC channel \bar{B} -> P_q (V_q) l^+l^-, q=s, d, with m_l \neq 0, including lepton spin effects. An approximate procedure is used to incorporate the rho, omega, and J/psi resonances.

hep-ph↗
arXiv1997

Energy Spectrum of Leptons from $e^+e^- \to W^+W^-$ in the Presence of Strong W^+_LW^-_L Interaction

The energy spectrum of leptons, produced by the decay of one or both W's in the reaction $e^+e^- \to W^+W^-$, is a significant probe of the helicity structure of this process. We calculate the energy spectrum $d σ/d E$ of a single decay lepton, as well as the two-dimensional energy distribution $d σ/d E_+dE_-$. We then consider a possible strong final state interaction in the longitudinally polarized state of the W^+W^- system, parametrized by a ρ-like resonance in the region of 1.8 TeV. We show that such a resonance produces measurable effects in the lepton spectrum at $\sqrt{s}=500$ GeV. The results are compared with those obtained from a non-resonant interaction approximated by a phase factor $e^{i δ}$ in the l=1 part of the W^+_LW^-_L amplitude.

hep-ph↗
arXiv1996

CP Violation in the Decay B \to X_d e^+ e^-

The decay $b\to d e^+e^-$ has an amplitude containing comparable contributions proportional to $V_{tb}V_{td}^{*}$, $V_{cb}V_{cd}^{*}$ and $V_{ub}V_{ud}^{*}$. These pieces involve different unitarity phases produced by $c\bar{c}$ and $u\bar{u}$ loops. The simultaneous presence of different CKM phases and different dynamical phases leads to a calculable asymmetry in the partial widths of $b\to d e^+e^-$ and $\bar{b}\to \bar{d} e^+e^-$. Using the effective Hamiltonian of the standard model, we calculate this asymmetry as a function of the $e^+e^-$ invariant mass. The effects of $ρ$, $ω$ and $J/ψ$ resonances are taken into account in the vacuum polarization of the $u\bar{u}$ and $c\bar{c}$ currents. As a typical result, an asymmetry of $- 5% (- 2%)$ is predicted in the nonresonant domain $1 GeV < m_{e^+e^-} < m_{J/ψ}$, assuming $η= 0.34$ and $ρ= 0.3 (-0.3)$. The branching ratio in this domain is $1.2\times 10^{-7} (3.3\times 10^{-7})$. Results are also obtained in the region of the $J/ψ$ resonance, where an asymmetry of $3\times 10^{-3}$ is expected, subject to certain theoretical uncertainties in the $b\to d J/ψ$ amplitude.

hep-ph↗
arXiv1996

Baryon Magnetic Moments and Proton Spin: A Model with Collective Quark Rotation

We analyse the baryon magnetic moments in a model that relates them to the parton spins $Δu$, $Δd$, $Δs$, and includes a contribution from orbital angular momentum. The specific assumption is the existence of a 3-quark correlation (such as a flux string) that rotates with angular momentum $\langle L_z \rangle$ around the proton spin axis. A fit to the baryon magnetic moments, constrained by the measured values of the axial vector coupling constants $a^{(3)}=F+D$, $a^{(8)}=3F-D$, yields $\langle S_z \rangle = 0.08 \pm 0.13$, $\langle L_z \rangle = 0.39 \pm 0.09$, where the error is a theoretical estimate. A second fit, under slightly different assumptions, gives $\langle L_z \rangle = 0.37 \pm 0.09$, with no constraint on $\langle S_z \rangle$. The model provides a consistent description of axial vector couplings, magnetic moments and the quark polarization $\langle S_z \rangle$ measured in deep inelastic scattering. The fits suggest that a significant part of the angular momentum of the proton may reside in a collective rotation of the constituent quarks.

hep-ph↗
arXiv1996

Lepton Polarization in the Decays $B\to X_sμ^+μ^-$ and $B\to X_sτ^+τ^-$

The effective Hamiltonian for the decay $b\to s l^+ l^-$ predicts a characteristic polarization for the final state lepton, which can serve as an important test of the underlying theory. The lepton polarization has, in addition to a longitudinal component $P_L$, two orthogonal components $P_T$ and $P_N$, lying in and perpendicular to the decay plane which are proportional to $m_l/m_b$, and therefore significant for the $τ^+τ^-$ channel. The normal polarization component $P_N$ is a $T$-odd effect connected with the nonhermiticity of the effective Hamiltonian, arising mainly from $c\bar{c}$ intermediate states. We calculate all three polarization components for the decay $B\to X_sτ^+τ^-$ as a function of the lepton pair mass, and find average values $ $ = -0.37, $ $ = -0.63, $ $ = 0.03. By comparison, the $μ^-$ polarization is $ $ = -0.77, $ = \approx 0$.

hep-ph↗
arXiv1996

Pair Production and Correlated Decay of Heavy Majorana Neutrinos in $e^+e^-$ Collisions

We consider the process $e^+e^-\to N_1N_2$, where $N_1$ and $N_2$ are heavy Majorana particles, with relative $CP$ given by $η_{CP}=+1$ or $-1$, decaying subsequently via $N_1,N_2\to W^{\pm}e^{\mp}$. We derive the energy and angle correlation of the dilepton final state, both for like-sign ($e^{\mp}e^{\mp}$) and unlike-sign ($e^-e^+$) configurations. Interesting differences are found between the cases $η_{CP}=+1$ and $-1$. The characteristics of unlike-sign $e^+e^-$ dileptons originating from a Majorana pair $N_1N_2$ are contrasted with those arising from the reaction $e^+e^-\to N\bar{N}\to W^+e^-W^-e^+$, where $N\bar{N}$ is a Dirac particle-antiparticle pair.

hep-ph↗
arXiv1995

Angular Distribution and Polarization of Photons in the Inclusive Decay $Λ_b\to X_s\,γ$

We study the angular distribution of photons produced in the inclusive decay of a polarized $Λ_b$, $Λ_b\to X_s\,γ$, using the technique of Heavy Quark Effective Theory. Finite non-perturbative corrections are obtained relative to the free quark decay $b\to sγ$. These corrections affect significantly the intensity and polarization of photons emitted at small angles relative to the $Λ_b$ spin direction.

hep-ph↗
arXiv1995

Tau Polarization in $ Λ_b \to X_c τ\barν$ and $B \to X_c τ\barν$

We discuss the longitudinal and transverse $τ$-polarization in inclusive decays of hadrons containing $b$-quarks. The calculation is performed by means of an OPE in HQET. Some mathematical difficulties in calculating transverse polarizations are explained. Numerical results are presented for longitudinal and for transverse polarizations, both in and perpendicular to the decay plane.

hep-ph↗