MCP-PMT timing at low light intensities with a DRS4 evaluation board / Estudio de la respuesta temporal de dos MCP-PMT a bajas intensidades de la luz utilizando un DRS4

Abstract Positron emission tomography (PET) is one of the most important diagnostic tools in medicine, allowing three-dimensional imaging of functional processes in the body. It is based on a detection of two gamma rays with an energy of 511 keV originating from the point of annihilation of the positron emitted by a radio-labeled agent. By measuring the difference of the arrival times of both annihilation photons it is possible to localize the tracer inside the body. Gamma rays are normally detected by a scintillation detector, whose timing accuracy is limited by a photomultiplier and a scintillator. By replacing a photo sensor with a microchannel plate PMT (MCP-PMT) and a scintillator with Cherenkov radiator, it is possible to localize the interaction position to the cm level. In a pioneering experimental study with Cherenkov detectors using PbF 2 crystals and microchannel plate photomultiplier tubes MCP-PMT a time resolution better than 100 ps was achieved. In this work a DRS4 digital ring sampler chip was used to read out single photon output signals from two different MCP-PMTs (Hamamatsu R3809 and Burle 85001) with a sampling rate of 5×109 samples/s. The digitized waveforms were analyzed and a comparison between the two detectors timing response was made. The time resolutions achieved were (161 ± 2.21) ps and (220 ± 2.63) ps FWHM for the Hamamatsu and Burle MCP-PMT respectively. No significant variances were observed in the study of the behavior of the FWHM when both MCP-PMT were scanned.

Resumen La tomografía por emisión de positrones (PET) es una importante herramienta en el diagnóstico médico ya que permite la obtención de imágenes tridimensionales de los procesos funcionales en el cuerpo. La técnica está basada en la detección de los dos cuantos gamma de 511 keV originados en la aniquilación del positrón emitido por el radiofármaco administrado al paciente. Midiendo la diferencia en la llegada de los dos cuantos gamma es posible determinar la posición en la que ocurrió la aniquilación. En los equipos convencionales son utilizados detectores centellantes cuya respuesta temporal está limitada por el fotomultiplicador y el cristal centellante. Remplazando el fotomultiplicador por un PMT (MCP-PMT) y el cristal centellante por un detector Cherenkov, es posible localizar la posición en la que ocurrió la aniquilación con una exactitud a nivel de pocos centímetros. En previos resultados experimentales utilizando detectores Cherenkov con cristales de PbF 2 y MCP-PMT se alcanzó una respuesta temporal de menos de 100 ps. En este trabajo fue utilizado un chip DRS4 con una velocidad de procesamiento de las señales de 5×109 samples/s para la lectura de la salida de fotones únicos de los dos MCP-PMT estudiados (Hamamatsu R3809 y Burle 85001). Las señales digitalizadas fueron analizadas y se realizó una comparación entre la respuesta temporal obtenida para ambos MCP-PMT. El tiempo de respuesta obtenido en términos de FWHM fue de (161 ± 2.21) ps y (220 ± 2.63) ps para los MCP-PMT Hamamatsu y Burle respectivamente. No se detectaron variaciones significativas en el FWHM al escanearse la superficie activa de ambos MCP-PMT .

Search for CP violation in the decays D(s)+ --> KS0pi+ and D(s)+ --> KS0K+.

We have searched for CP violation in the charmed meson decays D((s))(+) --> K(S)(0)pi(+) and D((s))(+) --> K(S)(0)K(+) using 673 fb(-1) of data collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. No evidence for CP violation is observed. We report the most sensitive CP asymmetry measurements to date for these decays: A(CP)(D(+)-->K(S)(0)pi(+)) = (-0.71 +/- 0.19 +/- 0.20)%, A(CP)(D(s)(+) --> K(S)(0)pi(+)) = (+5.45 +/- 2.50 +/- 0.33)%, A(CP)(D(+) --> K(S)(0)K(+)) = (-0.16 +/- 0.58 +/- 0.25)%, and A(CP)(D(s)(+) --> K(S)(0)K(+)) = (+0.12 +/- 0.36 +/- 0.22)%, where the first uncertainties are statistical and the second are systematic.

Observation of a charmoniumlike enhancement in the gammagamma-->omega(J)/psi process.

We report the results of a search for a charmoniumlike state produced in the process gammagamma-->omegaJ/psi in the 3.9-4.2 GeV/c{2} mass region. We observe a significant enhancement, which is well described by a resonant shape with mass M=(3915+/-3+/-2) MeV/c{2} and total width Gamma=(17+/-10+/-3) MeV. This enhancement may be related to one or more of the three charmoniumlike states so far reported in the 3.90-3.95 GeV/c{2} mass region.

Observation of the doubly cabibbo-suppressed decay D_{s};{+}-->K;{+}K;{+}pi;{-}.

We report the first observation of the doubly Cabibbo-suppressed decay D_{s};{+}-->K;{+}K;{+}pi;{-} using 605 fb;{-1} of data collected with the Belle detector at the KEKB asymmetric-energy e;{+}e;{-} collider. The branching ratio with respect to its Cabibbo-favored counterpart B(D_{s};{+}-->K;{+}K;{+}pi;{-})/B(D_{s};{+}-->K;{+}K;{-}pi;{+}) is (0.229+/-0.028+/-0.012)%, where the first uncertainty is statistical and the second is systematic. We also report a significantly improved measurement of the doubly Cabibbo-suppressed decay D;{+}-->K;{+}pi;{+}pi;{-}, with a branching ratio B(D;{+}-->K;{+}pi;{+}pi;{-})/B(D;{+}-->K;{-}pi;{+}pi;{+})=(0.569+/-0.018+/-0.014)%.

Measurement of inclusive radiative B-meson decays with a photon energy threshold of 1.7 GeV.

Using 605 fb(-1) of data collected at the Upsilon(4S) resonance we present a measurement of the inclusive radiative B-meson decay channel, B-->X(s)gamma. For the lower photon energy thresholds of 1.7, 1.8, 1.9, and 2.0 GeV, as defined in the rest frame of the B meson, we measure the partial branching fraction and the mean and variance of the photon energy spectrum. At the 1.7 GeV threshold we obtain the partial branching fraction BF(B-->X(s)}gamma)=(3.45+/-0.15+/-0.40)x10(-4), where the errors are statistical and systematic.

Measurement of the time-dependent CP asymmetries in B0-->K(S)(0)rho(0)gamma decays.

We report the first measurement of time-dependent CP asymmetry in B(0)-->K(S)(0)rho(0)gamma decays based on 657 x 10(6) BB pairs collected with the Belle detector at the KEKB asymmetric-energy collider. We measure the CP-violating parameter S_{K_{S};{0}rho;{0}gamma}=0.11+/-0.33(stat)-0.09+0.05(syst) from a signal of 212+/-17 events. We also obtain the effective direct CP-violating parameter A{eff}=0.05+/-0.18(stat)+/-0.06(syst) for mK(S)0pi+pi- <1.8 GeV/c(2) and 0.6 GeV/c(2)