F. Barao

Experimental study of the triple-gluon vertex

Abstract In four-jet events from e+e− →Z0 →multihadrons one can separate the three principal contributions from the triple-gluon vertex, double gluon-bremsstrahlung and the secondary quark-antiquark production, using the shape of the two-dimensional angular distributions in the generalized Nachtmann-Reiter angle θ NR ∗ and the opening angle of the secondary jets. Thus one can identify directly the contribution from the triple-gluon vertex without comparison with a specific non-QCD model. Applying this new method to events taken with the DELPHI-detector we get for the ratio of the colour factor Nc to the fermionic Casimir operator C F : N c C F = 2.55 ± 0.55 ( stat. ) ± 0.4 ( fragm. + models ) ± 0.2 ( error in bias ) in agreement with the value 2.25 expected in QCD from Nc=3 and C F = 4 3 .

A measurement of the bb forward-backward asymmetry using the semileptonic decay into muons

The forward-backward asymmetry of bottom quarks is measured with statistics of approximately 80 000 hadronic Z0 decays produced in e+e− collisions at a centre of mass energy of √s≈Mz. The tagging of b quark events has been performed using the semileptonic decay channel b→X+μ. Because the asymmetry depends on the weak coupling, this leads to a precise measurement of the electroweak mixing angle sin2θw. The experimental result is AFBb = 0.115±0.043(stat.)±0.013(syst.). After correcting the value for the B0B0 mixing this becomes AFBb=0.161±0.060(stat.)±0.021(syst.) corresponding to sin2θWMS=0.221±0.011(stat.)±0.004(syst.).

Measurement of the Z0 branching fraction to b quark pairs using the boosted sphericity product

Abstract From a sample of about 120 000 hadronic Z 0 decays, using a technique based on a separation of the different event categories in the boosted sphericity product, the fraction of b b decays has been measured to be 0.219 ± 0.014 (stat)± 0.019 (syst). Using the DELPHI determination of the hadronic Z 0 width, this corresponds to a partial width τ b b = 378 ± 42 MeV (in good agreement with the standard model prediction of ∼-380 MeV). Combining this measurement with the determinations based on events with high p t leptons gives an estimate for the branching ratio of b into leptons at LEP of (11.2 ± 1.2)%, consistent with previous determinations.

The Ring Imaging Cherenkov detector of the AMS experiment: test beam results with a prototype

The Alpha Magnetic Spectrometer (AMS) to be installed on the International Space Station (ISS) will be equipped with a proximity Ring Imaging Cherenkov (RICH) detector for measuring the velocity and electric charge of the charged cosmic particles. This detector will contribute to the high level of redundancy required for AMS as well as to the rejection of albedo particles. Charge separation up to iron and a velocity resolution of the order of 0.1% for singly charged particles are expected. A RICH protoptype consisting of a detection matrix with 96 photomultiplier units, a segment of a conical mirror and samples of the radiator materials was built and its performance was evaluated. Results from the last test beam performed with ion fragments resulting from the collision of a 158 GeV/c/nucleon primary beam of indium ions (CERN SPS) on a lead target are reported. The large amount of collected data allowed to test and characterize different aerogel samples and the sodium fluoride radiator. In addition, the reflectivity of the mirror was evaluated. The data analysis confirms the design goals.

Cherenkov angle and charge reconstruction with the RICH detector of the AMS experiment

The Alpha Magnetic Spectrometer (AMS) experiment to be installed on the International Space Station (ISS) will be equipped with a proximity focusing Ring Imaging Cerenkov (RICH) detector, for measurements of particle electric charge and velocity. In this note, two possible methods for reconstructing the Cerenkov angle and the electric charge with the RICH, are discussed. A Likelihood method for the Cerenkov angle reconstruction was applied leading to a velocity determination for protons with a resolution of around 0.1%. The existence of a large fraction of background photons which can vary from event to event, implied a charge reconstruction method based on an overall efficiency estimation on an event-by-event basis.Abstract The Alpha Magnetic Spectrometer experiment to be installed on the International Space Station will be equipped with a proximity focusing Ring Imaging Cherenkov (RICH) detector, for measurements of particle electric charge and velocity. In this note, two possible methods for reconstructing the Cherenkov angle and the electric charge with the RICH are discussed. A Likelihood method for the Cherenkov angle reconstruction was applied leading to a velocity determination for protons with a resolution of around 0.1%. The existence of a large fraction of background photons which can vary from event to event implied a charge reconstruction method based on an overall efficiency estimation on an event-by-event basis.

ISOTOPE SEPARATION WITH THE RICH DETECTOR OF THE AMS EXPERIMENT

The Alpha Magnetic Spectrometer (AMS), to be installed on the International Space Station (ISS) in 2008, is a cosmic ray detector with several subsystems, one of which is a proximity focusing Ring Imaging Cherenkov (RICH) detector. This detector will be equipped with a dual radiator (aerogel+NaF), a lateral conical mirror and a detection plane made of 680 photomultipliers and light guides, enabling precise measurements of particle electric charge and velocity. Combining velocity measurements with data on particle rigidity from the AMS Tracker it is possible to obtain a measurement for particle mass, allowing the separation of isotopes. A Monte Carlo simulation of the RICH detector, based on realistic properties measured at ion beam tests, was performed to evaluate isotope separation capabilities. Results for three elements -- H (Z=1), He (Z=2) and Be (Z=4) -- are presented.

ISOTOPIC MASS SEPARATION WITH THE RICH DETECTOR OF THE AMS EXPERIMENT

The Alpha Magnetic Spectrometer (AMS) to be installed on the InternationalSpace Station (ISS) will be equipped with a proximity focusing Ring ImagingCerenkov detector (RICH). Reconstruction of theˇ Cerenkov angle and the electricˇcharge with RICH are discussed. A likelihood method for the Cerenkov angleˇreconstruction was applied leading to a velocity determination for protons with aresolution around 0.1%. The electric charge reconstruction is based on the countingof the number of photoelectrons and on an overall efficiency estimation on an event-by-event basis. The isotopic mass separation of helium and beryllium is presented.

VELOCITY AND CHARGE RECONSTRUCTION WITH THE AMS/RICH DETECTOR

The Alpha Magnetic Spectrometer (AMS), to be installed on the International Space Station (ISS) in 2008, will be equipped with a proximity focusing Ring Imaging CHerenkov detector (RICH). This detector will be equipped with a dual radiator (aerogel+NaF), a lateral conical mirror and a detection plane made of 680 photomultipliers and light-guides, enabling measurements of particle electric charge and velocity. A likelihood method for the Cherenkov angle reconstruction was applied leading to a velocity determination for protons with a resolution around 0.1%. The electric charge reconstruction is based on the counting of the number of photoelectrons and on an overall efficiency estimation on an event-by-event basis. Results from the application of both methods are presented.

AMS — A MAGNETIC SPECTROMETER ON THE INTERNATIONAL SPACE STATION

The Alpha Magnetic Spectrometer (AMS) is a particle detector, designed to search for cosmic antimatter and dark matter and to study the elemental and isotopic composition of primary cosmic rays, that will be installed on the International Space Station (ISS) in 2008 to operate for at least three years. The detector will be equipped with a ring imaging Cherenkov detector (RICH) enabling measurements of particle electric charge and velocity with unprecedented accuracy. Physics prospects and test beam results are shortly presented.

Determination of vertical bar V-cb vertical bar from the semileptonic decay B-0-gD*(-)l(+)nu

Semileptonic decays B --> D*(-)l(+)nu X were selected from a sample of 3.1 million hadronic Z decays collected by the DELPHI detector at LEP. A topological search for semileptonic B decays to re ...