178 research outputs found
Search for New Physics in Electron-Positron Interactions with Photons in the Final State
No full textThis thesis describes a measurement of single and multi-photon events with missing energy in electron-positron interactions. The results have been used to measure the neutrino pair-production cross section and to determine the number of light neutrino species. A search for various kinds of new physics processes involving photons and missing energy in the final state like the production of supersymmetric particles, a fermiophobic Higgs boson and massive gravitons has been carried out. The analysis has been performed using data collected with the L3 detector at LEP at centre-of-mass energies between 130 GeV and 190 GeV corresponding to a total luminosity of 265 1/pb. The cross section of the process electron positron to neutrino anti-neutrino photon (photon) for a photon energy larger than 5 GeV and a photon scattering angle with respect to the beam axis larger than 14 degrees at a centre-of-mass energy of 189 GeV has been measured to be (5.25 +- 0.22 +- 0.07) pb and has been extrapolated to a total cross section for neutrino pair-production yielding (58.3 +- 2.7) pb. The measured photon energy spectra have been used to derive the number of light neutrino species. The combined value determined from all data analysed in this thesis is 3.05 +- 0.11 +- 0.04, which is in good agreement with the hypothesis of three light neutrino types and supports the Standard Model with its three generations of particles. Since no indication for the appearance of new physics processes has been found, upper limits on cross sections of new particle production have been set and lower limits on particle masses have been derived assuming three neutrino generations. If the gravitino is the lightest supersymmetric particle and all other SUSY particles are too heavy to be created at LEP energies, gravitino pair-production could be the only process involving SUSY particles accessible at LEP. From an analysis of the process electron positron to gravitino gravitino photon, a lower limit of 8.9 times 10 to the power -6 eV has been determined at the 95% confidence level. Cross section limits have been set depending on the masses of the particles involved in neutralino gravitino production and neutralino pair-production and neutralino decay to gravitino and photon, and for the production of the lightest and second lightest neutralino and pair-production of the second lightest neutralino and its decay to lightest neutralino and photon. Within the framework of gauge-mediated supersymmetry breaking a lower limit on the neutralino mass of 88.2 GeV at the 95% confidence level has been derived under the assumption that the lightest neutralino is the second lightest supersymmetric particle. Fermiophobic Higgs bosons have been searched for investigating the process electron positron to Higgs Z to photon photon neutrino anti-neutrino. Cross section bounds on this process have been determined depending on the Higgs mass and limits on the branching ratio of the Higgs decaying into photons have been derived. A lower mass limit for such a fermiophobic Higgs boson is set to 89.9 GeV at the 95% confidence level. A search for the production of massive gravitons in quantum gravity models with extra spatial dimensions has been performed investigating the process electron positron to graviton photon. Lower limits on the energy scale ranging from 1018 GeV to 349 GeV have been derived at the 95% confidence level for two to ten extra dimensions. Their size has been limited to at most 0.46 mm and to at most 8.3 times 10 to the power -13 mm for two and ten extra dimensions, respectively
Measurement of inclusive production in two-photon collisions at LEP
No full textInclusive production of mesons in two-photon collisions was measured by the L3 experiment at LEP. The data were collected at a centre-of-mass energy GeV with an integrated luminosity of . Differential cross sections of the process are determined as functions of the transverse momentum and pseudorapidity of the mesons in the kinematic region GeV GeV and . The cross section integrated over this phase space domain is measured to be pb. The differential cross sections are compared with next-to-leading order perturbative QCD calculations.Inclusive production of D ∗± mesons in two-photon collisions was measured by the L3 experiment at LEP. The data were collected at a centre-of-mass energy s =189 GeV with an integrated luminosity of 176.4 pb −1 . Differential cross sections of the process e + e − → e + e − D ∗± X are determined as functions of the transverse momentum and pseudorapidity of the D ∗± mesons in the kinematic region 1 GeVInclusive production of mesons in two-photon collisions was measured by the L3 experiment at LEP. The data were collected at a centre-of-mass energy GeV with an integrated luminosity of . Differential cross sections of the process are determined as functions of the transverse momentum and pseudorapidity of the mesons in the kinematic region 1 GeV < p_{T}^{\mathrm{D^*}} < 5 GeV and \mathrm{|\eta^{D^*}|} < 1.4. The cross section integrated over this phase space domain is measured to be pb. The differential cross sections are compared with next-to-leading order perturbative QCD calculations
Measurement of the B-d(0)-(B)over-bar(d)(0) oscillation frequency RID C-4549-2008 RID C-5719-2008
No full textTime-dependent B-0-(B) over bar(0) mixing is studied using about two million hadronic Z decays registered by L3 in 1994 and 1995. For this study three techniques are used. Tagging of the b-quark charge at decay time is performed by identifying leptons from semileptonic B decays. The flavour of the b quark at production time is determined from the charge of the lepton in the opposite hemisphere or by using a jet-charge technique. The proper time of the B-particle decay is obtained by reconstructing the production and decay vertices or by a measurement of the lepton impact parameter. The combined result for the frequency of B-d(0) meson oscillations is Delta m(d) = 0.444 +/- 0.040 ps(-1)
Measurement of the oscillation frequency
No full textTime-dependent {\mathrm{B^{0}}\mbox{-}\bar{\mathrm{B}} ^{0}} mixing is studied using about two million hadronic Z decays registered by L3 in 1994 and 1995. For this study three techniques are used. Tagging of the b-quark charge at decay time is performed by identifying leptons from semileptonic B decays. The flavour of the b quark at production time is determined from the charge of the lepton in the opposite hemisphere or by using a jet-charge technique. The proper time of the B-particle decay is obtained by reconstructing the production and decay vertices or by a measurement of the lepton impact parameter. The combined result for the frequency of meson oscillations is \begin{displaymath} {\Delta m_d} = 0.444 \pm 0.040 \ \mathrm{ps}^{-1}. \end{displaymath} \end{abstract} \end{document
Measurement of the Weak Dipole Moments of the Lepton
No full textUsing the data collected by the L3 experiment at LEP from 1991 to 1995 at energies around the \Zo mass, a measurement of the weak anomalous magnetic dipole moment, ,~ and of the weak electric dipole moment, , of the lepton is performed. These quantities are obtained from angular distributions in , where is a or a . The results are: , and . This is the first direct measurement of
Study of the hadronic photon structure function at LEP
No full textThe hadronic photon structure function is studied in the reaction at LEP with the L3 detector. The data, collected from 1991 to 1995 at a centre-of-mass energy \sqrt{s} \simeq 91 \GeV, correspond to an integrated luminosity of 140~pb. The photon structure function is measured in the interval 1.2 \GeV^2 \leq Q^2 \leq 9.0 \GeV^2 and the interval . \FF shows a linear growth with . The value of the slope \alpha^{-1}\mathrm{d}\FF(Q^2)/\mathrm{d}\ln{Q^2} is measured to be
Measurement of the Spectroscopy of Orbitally Excited B Mesons at LEP
No full textWe measure the masses, decay widths and relative production rate of orbitally excited B mesons using million hadronic Z decays recorded by the L3 detector. B-meson candidates are inclusively reconstructed and combined with charged pions produced at the primary event vertex. An excess of events above the expected background in the \Bmeson\pi mass spectrum in the region 5.6-5.8~\GeV is interpreted as resulting from the decay \BdstarUD\rightarrow\BorBs tar\pi, where \BdstarUD denotes a mixture of B-meson states containing a \uquark or a \dquark quark. A fit to the mass spectrum yields the masses and decay widths of the \Bonestar and \Btwostar spin states, as well as the branching fraction for the combination of states. In addition, evidence is presented for the existence of an excited B-meson state or mixture of states in the region 5.9-6.0~\GeV.We measure the masses, decay widths and relative production rate of orbitally excited B mesons using 1.25 million hadronic Z decays recorded by the L3 detector. B-meson candidates are inclusively reconstructed and combined with charged pions produced at the primary event vertex. An excess of events above the expected background in the B\pi mass spectrum in the region 5.6-5.8 GeV is interpreted as resulting from the decay B_u,d^** -> B^(*)\pi, where B_u,d^** denotes a mixture of l=1 B-meson states containing a u or a d quark. A fit to the mass spectrum yields the masses and decay widths of the B_1^* and B_2^* spin states, as well as the branching fraction for the combination of l=1 states. In addition, evidence is presented for the existence of an excited B-meson state or mixture of states in the region 5.9-6.0 GeV
Photon structure functions and azimuthal correlations of lepton pairs in tagged collisions
No full textMeasurements of the QED structure of the photon based on the reaction ee to ee gamma /sup (*)/(P/sup 2/) gamma *(Q/sup 2/) to ee mu mu are discussed. This review is an update of the discussion of the results on the QED structure of the photon and covers the published measurements of the photon structure functions F/sub 2//sup gamma /, F/sub A//sup gamma / and F/sub B//sup gamma / and of the differential cross-section d sigma /dx for the exchange of two virtual photons. (10 refs)
The Q(2) evolution of the hadronic photon structure function F-2(gamma) at LEP RID C-2983-2009 RID C-4549-2008 RID C-5719-2008
No full textNew measurements at a centre-of-mass energy root s similar or equal to 183 GeV of the hadronic photon structure function F-2(gamma)(x) in the Q(2) interval, 9 GeV2 less than or equal to Q(2) less than or equal to 30 GeV2, are presented. The data, collected in 1997 with the L3 detector, correspond to an integrated luminosity of 51.9 pb(-1). Combining with the data taken at a centre-of-mass energy of 91 GeV, the evolution of F-2(gamma) With Q(2) is measured in the Q(2) range from 1.2 GeV2 to 30 GeV2. F-2(gamma) shows a linear growth with ln Q(2); the value of the slope alpha(-1)d F-2(gamma)(Q(2))/dlnQ(2) is measured in two x bins from 0.01 to 0.2 and is somewhat higher than predicted. (C) 1999 Published by Elsevier Science B.V. All rights reserved
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