Observable implications of geometrical and dynamical aspects of freeze-out in heavy ion collisions

Using an analytical parameterization of hadronic freeze-out in relativistic heavy ion collisions, we present a detailed study of the connections between features of the freeze-out configuration and physical observables. We focus especially on anisotropic freeze-out configurations (expected in general for collisions at finite impact parameter), azimuthally-sensitive HBT interferometry, and final-state interactions between non-identical particles. Model calculations are compared with data taken in the first year of running at RHIC; while not perfect, good agreement is found, raising the hope that a consistent understanding of the full freeze-out scenario at RHIC is possible, an important first step towards understanding the physics of the system prior to freeze-out.Comment: 36 pages, 56 figures, 2 tables; version accepted for publication in PRC: some figures, references and discussion added; now also discusses classical versus quantum statistic

Space-time analysis of reaction at RHIC

Space-time information about the Au-Au collisions produced at RHIC are key tools to understand the evolution of the system and especially assess the presence of collective behaviors. Using a parameterization of the system's final state relying on collective expansion, we show that pion source radii can be tied together with transverse mass spectra and elliptic flow within the same framework. The consistency between these different measures provide a solid ground to understand the characteristics of collective flow and especially the possible peculiar behavior of particles such as Xi, Omega or phi. The validity of the short time scales that are extracted from fits to the pion source size is also addressed. The wealth of new data that will soon be available from Au-Au collisions at sqrt{s_{NN}} = 200 GeV, will provide a stringet test of the space-time analysis framework developped in these proceedings.Comment: Invited talk given at the SQM2003 conference (March 2003), to be published in Journal of Physics G. 10 pages, 3 figure

Stimulated Secondary Emission of Single Photon Avalanche Diodes

Large-area next-generation physics experiments rely on using Silicon Photo-Multiplier (SiPM) devices to detect single photons, which trigger charge avalanches. The noise mechanism of external cross-talk occurs when secondary photons produced during a charge avalanche escape from an SiPM and trigger other devices within a detector system. This work presents measured spectra of the secondary photons emitted from the Hamamatsu VUV4 and Fondazione Bruno Kessler VUV-HD3 SiPMs stimulated by laser light, near operational voltages. The work describes the Microscope for the Injection and Emission of Light (MIEL) setup, which is an experimental apparatus constructed for this purpose. Measurements have been performed at a range of over-voltage values and temperatures from 86~K to 293~K. The number of photons produced per avalanche at the source are calculated from the measured spectra and determined to be 40$\pm$9 and 61$\pm$11 photons produced per avalanche for the VUV4 and VUV-HD3 respectively at 4 volts over-voltage. No significant temperature dependence is observed within the measurement uncertainties. The overall number of photons emitted per avalanche from each SiPM device are also reported.Comment: 15 pages, 7 figure

Characterisation of SiPM Photon Emission in the Dark

In this paper, we report on the photon emission of Silicon Photomultipliers (SiPMs) from avalanche pulses generated in dark conditions, with the main objective of better understanding the associated systematics for next-generation, large area, SiPM-based physics experiments. A new apparatus for spectral and imaging analysis was developed at TRIUMF and used to measure the light emitted by the two SiPMs considered as photo-sensor candidates for the nEXO neutrinoless double-beta decay experiment: one Fondazione Bruno Kessler (FBK) VUV-HD Low Field (LF) Low After Pulse (Low AP) (VUV-HD3) SiPM and one Hamamatsu Photonics K.K. (HPK) VUV4 Multi-Pixel Photon Counter (MPPC). Spectral measurements of their light emissions were taken with varying over-voltage in the wavelength range of 450–1020 nm. For the FBK VUV-HD3, at an over-voltage of 12.1±1.0 V, we measured a secondary photon yield (number of photons (γ) emitted per charge carrier (e−)) of (4.04±0.02)×10−6γ/e−. The emission spectrum of the FBK VUV-HD3 contains an interference pattern consistent with thin-film interference. Additionally, emission microscopy images (EMMIs) of the FBK VUV-HD3 show a small number of highly localized regions with increased light intensity (hotspots) randomly distributed over the SiPM surface area. For the HPK VUV4 MPPC, at an over-voltage of 10.7±1.0 V, we measured a secondary photon yield of (8.71±0.04)×10−6γ/e−. In contrast to the FBK VUV-HD3, the emission spectra of the HPK VUV4 did not show an interference pattern—likely due to a thinner surface coating. The EMMIs of the HPK VUV4 also revealed a larger number of hotspots compared to the FBK VUV-HD3, especially in one of the corners of the device. The photon yield reported in this paper may be limited if compared with the one reported in previous studies due to the measurement wavelength range, which is only up to 1020 nm

ÉTUDE DES PROCESSUS D'ÉMISSION DES PIONS DANS LES COLLISIONS PB+PB À 17.3 AGEV, ET SON PROLONGEMENT AUX COLLISIONS AU+AU À 200 AGEV, À TRAVERS LA CARACTÉRISATION DES DÉTECTEURS AU SILICIUM À MICROPISTES DE L'EXPÉRIENCE STAR

Pions are the most abundant particle species produced in the ultra-relativistic heavy-ion collisions. They may carry informations about the formation of a quark gluon plasma (QGP). We have studied charged pions produced in the Pb+Pb collisions at 17.3 AGeV, using the WA98 charged particle spectrometers. We have built the ratio of the transverse mass distribution of the pi+ and pi-, and constructed the correlation function of the pi+-pi- pair. The ratio provides information about the coulomb interaction of the charged pions with a central charge which is directly related to the freeze out size of the system. The correlation function is sensitive to the relative distance between pions at freeze out. Comparing the experimental results predictions of two models, RQMD and NEXUS, we conclude that few pions are emitted in the early stage of the collision. Such a behavior is hardly compatible with the evolution of a system described only with hadronic degree of freedom.The Relativistic Heavy Ion Collider allows the study of gold nuclei collisions at 200 AGeV which may offer the opportunity of observing signatures of the QGP. If it is formed, its lifetime may be large enough to have a strong impact on many observables. The ones related to charged pions can be studied by the STAR experiment. The Silicon Strip Detector (SSD), the fourth layer of the STAR vertex tracker, will greatly help reconstructing the low transverse momentum pions. In order to extract physical signals, a precise understanding of the detector response is required. A 120 GeV/c pion beam was used to probe the behavior of the SSD detection modules. Based on this analysis, a detailed simulation of the detector was developed.Les pions sont les particules les plus abondamment produites dans les collisions d’ions lourds ultra-relativistes. Nous avons étudié la dynamique de leur émission afin de déterminer leur sensibilité à l’éventuelle formation d’un plasma de quarks et de gluons. A partir des données des collisions Pb+Pb à 17.3 AGeV étudiées dans l'expérience, WA98, au CERN, nous avons construit les rapports des distributions en masse transverse des pions négatifs et positifs, et la fonction de corrélation pi+-pi-. La comparaison des résultats expérimentaux aux prédictions de deux modèles différents, nous permet de conclure que peu de pions sont émis durant les 10 premiers fm/c de la collision. Ce comportement est difficilement explicable à l’aide d’une description du système en terme purement hadronique.Le collisionneur RHIC (Etats Unis) permet d’étudier les collisions Au+Au à 200 AGeV. La phase partonique, si elle est formée, devrait avoir une durée de vie importante ce qui rendra possible sa mise en évidence, sans ambiguïtés. Afin d'étudier précisément ces collisions une compréhension détaillée des modules de détection est nécessaire. Nous avons participé à la caractérisation des détecteurs au silicium à micropistes du Silicon Strip Detector (SSD) de l’expérience STAR, à travers l’étude de données prises avec un faisceau de pions de 120 GeV/c. Le SSD permettra d’étudier dans de bonnes conditions les pions de faible impulsion transverse, dans la continuité de notre étude à 17.3 AGeV

Development of a PET Scanner for Simultaneously Imaging Small Animals with MRI and PET

Recently, positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of cancer. Combined PET and X-ray computed tomography (PET-CT) scanners are now the modality of choice in cancer treatment planning. More recently, the combination of PET and magnetic resonance imaging (MRI) is being explored in many sites. Combining PET and MRI has presented many challenges since the photo-multiplier tubes (PMT) in PET do not function in high magnetic fields, and conventional PET detectors distort MRI images. Solid state light sensors like avalanche photo-diodes (APDs) and more recently silicon photo-multipliers (SiPMs) are much less sensitive to magnetic fields thus easing the compatibility issues. This paper presents the results of a group of Canadian scientists who are developing a PET detector ring which fits inside a high field small animal MRI scanner with the goal of providing simultaneous PET and MRI images of small rodents used in pre-clinical medical research. We discuss the evolution of both the crystal blocks (which detect annihilation photons from positron decay) and the SiPM array performance in the last four years which together combine to deliver significant system performance in terms of speed, energy and timing resolution.Medicine, Faculty ofScience, Faculty ofNon UBCPhysics and Astronomy, Department ofRadiology, Department ofReviewedFacult