1,851 research outputs found
Search for CP Violation in the Decay Z -> b (b bar) g
About three million hadronic decays of the Z collected by ALEPH in the years
1991-1994 are used to search for anomalous CP violation beyond the Standard
Model in the decay Z -> b \bar{b} g. The study is performed by analyzing
angular correlations between the two quarks and the gluon in three-jet events
and by measuring the differential two-jet rate. No signal of CP violation is
found. For the combinations of anomalous CP violating couplings, and , limits of \hat{h}_b < 0.59h^{\ast}_{b} < 3.02$ are given at 95\% CL.Comment: 8 pages, 1 postscript figure, uses here.sty, epsfig.st
Constraints on Spin-Dependent Dark Matter Scattering with Long-Lived Mediators from TeV Observations of the Sun with HAWC
We analyze the Sun as a source for the indirect detection of dark matter
through a search for gamma rays from the solar disk. Capture of dark matter by
elastic interactions with the solar nuclei followed by annihilation to
long-lived mediators can produce a detectable gamma-ray flux. We search three
years of data from the High Altitude Water Cherenkov Observatory and find no
statistically significant detection of TeV gamma-ray emission from the Sun.
Using this, we constrain the spin-dependent elastic scattering cross section of
dark matter with protons for dark matter masses above 1 TeV, assuming an
unstable mediator with a favorable lifetime. The results complement constraints
obtained from Fermi-LAT observations of the Sun and together cover WIMP masses
between 4 GeV and GeV. The cross section constraints for mediator decays
to gamma rays can be as strong as cm, which is more than
four orders of magnitude stronger than current direct-detection experiments for
1 TeV dark matter mass. The cross-section constraints at higher masses are even
better, nearly 7 orders of magnitude better than the current direct-detection
constraints for 100 TeV dark matter mass. This demonstration of sensitivity
encourages detailed development of theoretical models in light of these
powerful new constraints.Comment: 11 pages, 4 figures. See also companion paper 1808.05620. Accepted
for publication in Physical Review
First HAWC Observations of the Sun Constrain Steady TeV Gamma-Ray Emission
Steady gamma-ray emission up to at least 200 GeV has been detected from the
solar disk in the Fermi-LAT data, with the brightest, hardest emission
occurring during solar minimum. The likely cause is hadronic cosmic rays
undergoing collisions in the Sun's atmosphere after being redirected from
ingoing to outgoing in magnetic fields, though the exact mechanism is not
understood. An important new test of the gamma-ray production mechanism will
follow from observations at higher energies. Only the High Altitude Water
Cherenkov (HAWC) Observatory has the required sensitivity to effectively probe
the Sun in the TeV range. Using three years of HAWC data from November 2014 to
December 2017, just prior to the solar minimum, we search for 1--100 TeV gamma
rays from the solar disk. No evidence of a signal is observed, and we set
strong upper limits on the flux at a few TeV cm
s at 1 TeV. Our limit, which is the most constraining result on TeV
gamma rays from the Sun, is of the theoretical maximum flux (based
on a model where all incoming cosmic rays produce outgoing photons), which in
turn is comparable to the Fermi-LAT data near 100 GeV. The prospects for a
first TeV detection of the Sun by HAWC are especially high during solar
minimum, which began in early 2018.Comment: 14 pages, 6 figures. See also companion paper 1808.05624. Accepted
for publication in Physical Review
Search for R-Parity Violating Decays of Supersymmetric Particles in Collisions at Centre-of-Mass Energies near 183 GeV
Searches for pair-production of supersymmetric particles under the assumption that R-parity is violated via a single dominant , or coupling are performed using the data collected by the \ALEPH\ collaboration at centre-of-mass energies of 181--184~\gev. The observed candidate events in the data are in agreement with the Standard Model expectations. Upper limits on the production cross-sections and lower limits on the masses of charginos, sleptons, squarks and sneutrinos are de rived
Production of excited beauty states in Z decays
A data sample of about 3.0 million hadronic Z decays collected by the ALEPH experiment at LEP in the years 1991 through 1994, is used to make an inclusive selection of B~hadron events. In this event sample 4227 \pm 140 \pm 252 B^* mesons in the decay B^* \to B \gamma and 1944 \pm 108 \pm 161 B^{**} mesons decaying into a B~meson and a charged pion are reconstructed. For the well established B^* meson the following quantities areobtained: \Delta M = M_{B^*} - M_{B} = (45.30\pm 0.35\pm 0.87)~\mathrm{MeV}/c^2 and N_{B^*}/(N_B+N_{B^*}) = (77.1 \pm 2.6 \pm 7.0)\%. The angular distribution of the photons in the B^* rest frame is used to measure the relative contribution of longitudinal B^* polarization states to be \sigma_L/(\sigma_L + \sigma_T)= (33 \pm 6 \pm 5)\%. \\ Resonance structure in the M(B\pi)-M(B) mass difference is observed at (424 \pm 4 \pm 10)~\mathrm{MeV}/c^2. Its shape and position is in agreement with the expectation for B^{**}_{u,d} states decaying into B_{u,d}^{(*)} \pi^\pm. The signal is therefore interpreted as arising from them. The relative production rate is determined to be \frac{BR(Z \to b \to B_{u,d}^{**})}{BR(Z \to b \to B_{u,d})} = [27.9 \pm 1.6(stat) \pm 5.9(syst) \phantom{a}^{+3.9}_{-5.6}(model)]\%. where the third error reflects the uncertainty due to different production and decay models for the broad B_{u,d}^{**} states
Tau hadronic branching ratios
From 64492 selected \tau-pair events, produced at the Z^0 resonance, the measurement of the tau decays into hadrons from a global analysis using 1991, 1992 and 1993 ALEPH data is presented. Special emphasis is given to the reconstruction of photons and \pi^0's, and the removal of fake photons. A detailed study of the systematics entering the \pi^0 reconstruction is also given. A complete and consistent set of tau hadronic branching ratios is presented for 18 exclusive modes. Most measurements are more precise than the present world average. The new level of precision reached allows a stringent test of \tau-\mu universality in hadronic decays, g_\tau/g_\mu \ = \ 1.0013 \ \pm \ 0.0095, and the first measurement of the vector and axial-vector contributions to the non-strange hadronic \tau decay width: R_{\tau ,V} \ = \ 1.788 \ \pm \ 0.025 and R_{\tau ,A} \ = \ 1.694 \ \pm \ 0.027. The ratio (R_{\tau ,V} - R_{\tau ,A}) / (R_{\tau ,V} + R_{\tau ,A}), equal to (2.7 \pm 1.3) \ \%, is a measure of the importance of QCD non-perturbative contributions to the hadronic \tau decay widt
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