2,554 research outputs found

    Flow of entropy in the evolution of the B^0 anti B^0 system: Upper bound on CP violation from unidirectionality

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    We have previously studied the time-dependence of a B anti B meson mixture in terms of its density matrix. The requirement that the absolute value of the Stokes vector zeta(t) should evolve monotonically from its initial value zeta(0)=0 to its final value zeta(infinity)=1 was shown to lead to an upper bound on the CP violating overlap . In the present note we consider an entropy variable S as an alternative measure of mixing. We show that exactly the same upper bound emerges from the requirement that the flow of entropy is unidirectional (dS/dt<0). We compare the entropic current dS/dt with and without CP violation and identify certain physical features that appear when the bound on is violated.Comment: 8 pages, 4 figures. Typos corrected, comment on entropy added. To be published in Physical Review

    W+H+W^+-H^+ Interference and Partial Width Asymmetry in Top and Antitop Decays

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    We re-examine the question of a possible difference in the partial decay widths of tt and t\overline t, induced by an intermediate scalar boson H+H^+ with CPCP-violating couplings. The interference of W+W^+ and H+H^+ exchanges is analysed by constructing the 2×22\times 2 propagator matrix of the W+H+W^+-H^+ system, and determining its absorptive part in terms of fermion loops. Results are obtained for the partial rate difference in the channels tbl+νlt\to bl^+\nu_l and tbcst\to bc\overline s, which fulfil explicitly the constraints of CPTCPT invariance. These results are contrasted with those in previous work.Comment: 14 pages, report PITHA 94/1

    An upper limit on CP violation in the Bs0Bˉs0B^0_s-\bar{B}^0_s system

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    In a previous publication we noted that the time dependence of an incoherent B0Bˉ0B^0-\bar{B}^0 mixture undergoes a qualitative change when the magnitude of CP violation δ\delta exceeds a critical value. Requiring, on physical grounds, that the system evolve from an initial incoherent state to a final pure state in a monotonic way, yields a new upper limit for δ\delta. The recent measurement of the wrong charge semileptonic asymmetry of Bs0B_s^0 mesons presented by the D0 collaboration is outside this bound by one standard deviation. If this result is confirmed it implies the existence of a new quantum mechanical oscillation phenomenon.Comment: 7 pages, 2 figures, version submitted for publication (Physical Review

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

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    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^+.Comment: 19 pages, REVTeX, no figures. Equations (2.19a), (2.19b), (5.5)-(5.7) have been corrected; all results remain unchanged. These changes will appear in an Erratum submitted to Phys. Rev.

    Lepton Mass Effects in Single Pion Production by Neutrinos

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    We reconsider the Feynman-Kislinger-Ravndal model applied to neutrino-excitation of baryon resonances. The effects of lepton mass are included, using the formalism of Kuzmin, Lyubushkin and Naumov. In addition we take account of the pion-pole contribution to the hadronic axial vector current. Application of this new formalism to the reaction nu(mu) + p --> mu + Delta at E(nu) approx 1 GeV gives a suppressed cross section at small angles, in agreement with the screening correction in Adler's forward scattering theorem. Application to the process nu(tau) + p --> tau + Delta at E(nu) approx 7 GeV leads to the prediction of right-handed tau polarization for forward-going leptons, in line with a calculation based on an isobar model. Our formalism represents an improved version of the Rein-Sehgal model, incorporating lepton mass effects in a manner consistent with PCAC.Comment: 14 pages, 5 figures. Typos in eq. 9 and 27 corrected. Numbers in table I for coherent cross sections (RSA and RSC) corrected (normalization error). Figs 3 and 4 changed accordingly. These corrections also apply to the published version PRD 76, 113004 (2007

    CP Violation and Arrows of Time Evolution of a Neutral KK or BB Meson from an Incoherent to a Coherent State

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    We study the evolution of a neutral KK meson prepared as an incoherent equal mixture of K0K^0 and K0ˉ\bar{K^0}. Denoting the density matrix by \rho(t) = {1/2} N(t) [\1 + \vec{\zeta}(t) \cdot \vec{\sigma} ] , the norm of the state N(t)N(t) is found to decrease monotonically from one to zero, while the magnitude of the Stokes vector ζ(t)|\vec{\zeta}(t)| increases monotonically from zero to one. This property qualifies these observables as arrows of time. Requiring monotonic behaviour of N(t)N(t) for arbitrary values of γL,γS\gamma_L, \gamma_S and Δm\Delta m yields a bound on the CP-violating overlap δ=KLKS\delta = \braket{K_L}{K_S}, which is similar to, but weaker than, the known unitarity bound. A similar requirement on ζ(t)|\vec{\zeta}(t)| yields a new bound, δ2<1/2(ΔγΔm)sinh(3π4ΔγΔm)\delta^2 < {1/2} (\frac{\Delta \gamma}{\Delta m}) \sinh (\frac{3\pi}{4} \frac{\Delta \gamma}{\Delta m}) which is particularly effective in limiting the CP-violating overlap in the B0B^0-B0ˉ\bar{B^0} system. We obtain the Stokes parameter ζ3(t)\zeta_3(t) which shows how the average strangeness of the beam evolves from zero to δ\delta. The evolution of the Stokes vector from ζ=0|\vec{\zeta}| = 0 to ζ=1|\vec{\zeta}| = 1 has a resemblance to an order parameter of a system undergoing spontaneous symmetry breaking.Comment: 13 pages, 6 figures. Inserted conon "." in title; minor change in text. To appear in Physical review

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

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    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 Sz\langle S_z \rangle and partly orbital Lz\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 Sz=0.08±0.15\langle S_z \rangle = 0.08\pm 0.15, Lz=0.42±0.14\langle L_z \rangle = 0.42\pm 0.14. We now consider the consequences of this idea for the q2q^2-dependence of the magnetic and axial vector form factors. It is found that the isovector magnetic form factor GMisovec(q2)G_M^{\mathrm{isovec}}(q^2) differs in shape from the axial form factor FA(q2)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, r2mag=fspinr2axial+52forbr2matt\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 forb/fspin=13Lz/GAf_{\mathrm{orb}}/ f_{\mathrm{spin}} = \frac{1}{3}\langle L_z \rangle / G_A, fspin+forb=1f_{\mathrm{spin}} + f_{\mathrm{orb}} = 1. The shape of FA(q2)F_A(q^2) is found to be close to a dipole with MA=0.92±0.06M_A = 0.92\pm 0.06 GeV.Comment: 18 pages, 5 ps-figures, uses RevTe

    Pair production and correlated decay of heavy Majorana neutrinos in e+e- collisions

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    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 \eta_{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 \eta_{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
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