100 research outputs found
Commutators of lepton mass matrices associated with the seesaw and leptogenesis mechanisms
No full textThe origin of tiny neutrino masses and the baryon number asymmetry of the Universe are naturally interpreted by the canonical seesaw and leptogenesis mechanisms, in which there are the heavy Majorana neutrino mass matrix M-R, the Dirac neutrino mass matrix M-D, the charged-lepton mass matrix M-l, and the effective (light) neutrino mass matrix M-nu. We find that Im(det[(MDMD)-M-dagger, (MRMR)-M-dagger]), Im(det[MlMl dagger, M nu M nu dagger]), and Im(det[MlMl dagger, MDMD dagger]) can serve as a basis-independent measure of CP violation associated with lepton-number-violating decays of heavy neutrinos, flavor oscillations of light neutrinos, and lepton-flavor-violating decays of charged leptons, respectively. We first calculate these quantities with the help of a standard parametrization of the 6 x 6 flavor mixing matrix, then discuss their implications on both leptogenesis and CP violation at low energy scales. A comparison is also made with the weak-basis invariants of leptogenesis, as proposed by Branco et al.Astronomy & AstrophysicsPhysics, Particles & FieldsSCI(E)[email protected]
A Study of the decays of tau leptons produced on the Z resonance at LEP
No full textFrom the analysis of a data sample corresponding to an integrated luminosity of 4.63 pb-1 taken during the 1990 run of LEP at centre of mass energies between 88.2 GeV an 94.2 GeV, the tau decays tau- --> e-nu(e)nu(tau)BAR, tau- --> mu-nu(mu)nu(tau)BAR, tau- --> pi-(K-)nu(tau), tau- --> rho-nu(tau) and their charge conjugates have been studied. The following branching ratios have been measured; BR (tau- --> e-nu(e)nu(tau)BAR) = 18.6+/-0.8 (stat.)+/-0.6 (sys.)%, BR (tau- --> mu-nu(mu)nu(tau)BAR) = 17.4+/-0.7+/-0.6%, BR (tau- --> pi-(K-)nu(tau)) = 11.9+/-0.7+/-0.7%, BR (tau- --> rho-nu(tau)) = 22.4+/-0.8+/-1.3%, in good agreement with world averages. The measured electronic and muonic branching ratios lead to a measurement of the strong coupling constant, alpha(s)(m(tau)) = 0.26 +0.09/-0.12. Extrapolating the alpha(s) value from m(tau) to m(Z) yields alpha(s)(m(Z)) = 0.109 +0.012/-0.028. The average polarization P(tau) of taus produced in Z --> tau+tau- decays has also been measured using the above decay modes. The weighted mean of the polarizations obtained from the four decay modes is P(tau) = -0.24+/-0.07. This value of P(tau) gives, in the improved Born approximation, a ratio between the axial and vector coupling constants of the tau of v(tau)/a(tau) = 0.12+/-0.04, and hence a value of the effective electroweak mixing parameter sin2-theta(w)(m(Z)2) = 0.220+/-0.009
Bose-Einstein correlations in the hadronic decays of the Z0
No full textBose-Einstein correlations between pairs of like-sign charged particles produced in e+e- annihilations near the Z0 peak have been studied using data taken with the DELPHI detector at LEP. An enhancement is found in the production of pairs of identical pions of similar momenta, with respect to a reference sample. Under the hypothesis that the pions are emitted by a spherically symmetrical source with gaussian density, the data indicate a radius of the source of r = 0.62 +/- 0.04(stat.) +/- 0.20(syst.) fm. The large systematic uncertainty reflects the sensitivity of r to the choice of the reference sample
A MEASUREMENT OF B-MESON PRODUCTION AND LIFETIME USING DL- EVENTS IN Z(0) DECAYS
No full textA study of B meson decays into D l- X final states is presented. In these events, neutral and charged D mesons originate predominantly from B+ and B0 decays, respectively. The dilution of this correlation due to D** production has been taken into account. From 263700 hadronic Z0 decays collected in 1991 with the DELPHI detector at the LEP collider, 92 D0 --> K- pi+, 35 D+ --> K- pi+ pi+ and 61 D*+ --> D0 pi+ followed by D0 --> K- pi+ or D0 --> K- pi+ pi+ pi-, are found with an associated lepton of the same charge as the kaon. From the D0 l- and D*+ l-, the probability f(d) that a b quark hadronizes into a B- (or B0BAR),meson is found to be 0.44 +/-0.08 +/-0.09, corresponding to a total (B(s) + LAMBDA(b)) hadronization fraction of 0.12(-0.12)+0.24 .By reconstructing the energy of each B meson, the b quark fragmentation is directly measured for the first time. The mean value of the B meson energy fraction is: [X(E)(B)] = 0.695+/-0.015(stat.)+/-0.029(syst.) Reconstructing D-lepton vertices, the following B life-times are measured: tau(B) = 1.27(-0.18)+0.22(stat.)+/-0.15(syst.) ps, where bBAR --> D0 l- X, tau(B) = 1.18(-0.27)+0.39(stat.)+/-0.15(syst.) ps, where BBAR --> D+ l- X, T(B) = 1.19(-0.19)+0.25(stat.)+/-0.15(syst.) ps where BBAR --> D*+ l- X, and an average tau(B) = 1.23(-0.13)+0.14(stat.)+/-0.15(syst.) ps is found. Allowing for decays into D** l- vBAR, the B+ and B0 lifetimes are: tau(B+)= 1.30(0.29)+0.33(stat.)+/-0.15(syst. exp.) +/-0.05(syst. D**) ps, tau(B0)= 1.17(-0.23)+0.29(stat.)+/-0.15(syst. exp.) +/-0.05 (syst. D**) ps, tau(B+)/tau(B0) = 1.11(0.39)+0.51(stat.)+/-0.05(syst. exp.) +/-0.10(syst. D**) ps
SEARCH FOR HEAVY NEUTRAL HIGGS BOSONS IN 2-DOUBLET MODELS
No full textA search for pair production of neutral heavy Higgs bosons decaying into <b(b)over bar> has been carried out in a study of hadronic decays of the Z boson into four jet final states using data taken by DELPHI in 1991 and 1992. The two production mechanisms present in the two Higgs doublets scheme, bremsstrahlung production of hZ* and associated production of hA, may lead to four beauty jets well recognizable using the precise microvertex detector measurements. No evidence for a signal was found, leading to limits on BR(Z -->, hA --> 4b) from 3.5 to 5.5 x 10(-4) at 95% confidence level, depending on the mass of the ligthest Higgs. When combined with the results of the recent DELPHI standard Higgs search, this result allows the kinematical limit to be reached for the masses of h and A in the minimal supersymmetric extension of the Standard Model (MSSM) scheme. It also allows the tan beta less than or equal to 1 domain to be explored, and a region above the kinematic limit for direct hA production is constrained by considering virtual hA production. Results are also given in the general two-doublet scheme
MEASUREMENT OF THE PARTIAL WIDTH OF THE Z0 INTO B(B)OVER-BAR FINAL-STATES USING THEIR SEMILEPTONIC DECAYS
No full textThe spectra of prompt electrons and muons from the semi-leptonic decays of heavy hadrons produced in Z0 decays have been used to measure the coupling of the Z0 to b quarks weighted by the B hadrons mean semi-leptonic branching fraction, giving a value: BR(sl)b*GAMMA(bbBAR)/GAMMA(H)=0.0221+/-0.0015. The data has also been used to measure the value of the fragmentation parameter, defined in the context of the LUND PS Model, version 7.2, giving: epsilon(b)=(8(+5/-3)+/-2) 10(-3). The corresponding value of the mean fraction of the beam energy taken by a B hadron in the fragmentation of a b quark is: X(E)bBAR=0.69(+0.02/-0.03)+/-0.01. If the values of GAMMA(bbBAR) and GAMMA(H) are taken from the Standard Model, the following value is obtained for the mean semi-leptonic braching fraction of B hadrons: BR(sl)b=(10.1+/-0.7)%. Taking the value of GAMMA(bbBAR)/GAMMA(H) from an independent analysis of DELPHI data based on the use of the boosted sphericity product, a value: BR(sl)b=(10.1+/-1.3)% is obtained
MEASUREMENT OF THE AVERAGE LIFETIME OF B-HADRONS
No full textThe average lifetime of B hadrons produced in hadronic Z0 decays has been measured with the DELPHI detector at LEP. The measurement is based on the analysis of the impact parameter distribution of high p(t) muons and hadrons. The resulting mean B lifetimes are tau(B) = (1.30 +/- 0.10 +/- 0.08) ps and tau(B) = (1.27 +/- 0.04 +/- 0.12) ps respectively, giving a combined value of tau(B) = (1.28 +/- 0.10) ps. The hadronic sample was also used to measure the partial Z0 width GAMMA(bbBAR)/GAMMA(h) and gave a value of 0.222(-0.031)+0.033 +/- 0.017
MEASUREMENT OF THE TRIPLE-GLUON VERTEX FROM 4-JET EVENTS AT LEP
No full textFrom the combined data of 1990 and 1991 of the DELPHI experiment at LEP, 13057 4-jet events are obtained and used for determining the contribution of the triple-gluon vertex. The relevant variables are the generalized Nachtmann Reiter angle theta(NR)* and the opening angle of the two least energetic jets. A fit to their two-dimensional distribution yields C(A)/C(F)=2.12+/-0.35 and N(C)/N(A)=0.46+/-0.19, where C(A)/C(F) is the ratio of the coupling strength of the triple-gluon vertex to that of gluon bremsstrahlung from quarks, and N(C)/N(A), the ratio of the number of quark colours to the number of gluons. This constitutes a convincing model-independent proof of the existence of the triple-gluon vertex, since its contribution is directly proportional to C(A)/C(F). The results are in agreement with the values expected from QCD: C(A)/C(F)=2.25, and N(C)/N(A)=3/8
Determination of Z0 resonance parameters and couplings from its hadronic and leptonic decays
No full textFrom measurements of the cross sections for e+e- → hadrons and the cross sections and forward-backward charge-asymmetries for e+e- → e+e-, μ+μu- and τ+τ- at several centreof-mass energies around the Za pole with the DELPHI apparatus, using approximately 150000 hadronic and leptonic events from 1989 and 1990, one determines the following Z0 parameters: the mass and total width MZ = 91.177 ± 0.022 GeV. ΓZ = 2.465 ± 0.020 GeV, the hadronic and leptonic partial widths Γh = 1.726 + 0.019 GeV, Γl = 83.4 ± 0.8 MeV, the invisible width Γinv = 488 ± 17 MeV, the ratio of hadronic over leptonic partial widths RZ = 20.70 + 0.29 and the Born level hadronic peak cross section σ0 = 41.84 ± 0.45 nh. A flavour-independent measurement of the leptonic cross section gives very consistent results to those presented above (Γl = 83.7 ± 0.8 MeV). From these results the number of light neutrino species is determined to be Nv, = 2.94 ± 0.10. The individual leptonic widths obtained are: Γe = 82.4 ± 1.2 MeV, Γμ = 86.9 ± 2.1 MeV and Γτ = 82.7±2.4 MeV. Assuming universality, the squared vector and axial-vector couplings of the Z0 to charged leptons are: V̄l 2 = 0.0003 ± 0.0010 and Āl 2 = 0.2508 ± 0.0027. These values correspond to the electroweak parameters: ρeff = 1.003 ± 0.011 and sin2θW eff = 0.241 ± 0.009. Within the Minimal Standard Model (MSM), the results can be expressed in terms of a single parameter: sin2θW MS = 0.2338 ± 0.0027. All these values are in good agreement with the predictions of the MSM. Fits yield 43 < mtop < 215 GeV at the 95% level. Finally, the measured values of ΓZ and Γinv are used to derive lower mass bounds for possible new particles.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Search for Z0 decays to two leptons and a charged particle - anti-particle pair
No full textBased on a sample equivalent to 365 000 hadronic Z0 decays, the search in DELPHI data for pairs of leptons accompanied by a pair of charged particles is described. A total of 11 events were found in the electron channel, 9 in the muon channel and 7 in the tau channel. Results on lepton pairs with a radiated photon are also presented. The data from all channels are compatible with the expectations from standard processes. However, one event was found in the tau channel with an unusually high mass of the charged particle pair.0SCOPUS: ar.jinfo:eu-repo/semantics/publishe
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