by Gerardo Herrera Corral. Published: 24 June 2011

Detector construction

The VZERO (V0) system

The Mexican group1 in ALICE was responsible for the design, construction and operation of the V0A detector. This detector was crucial in the data analysis of the first events registered by the experiment and continues to play an important role as a trigger, online luminosity monitor, centrality measurement and beam gas rejection device.

The V0 system consists of two detectors V0A and V0C2 located at 330 cm and 90 cm from the interaction point respectively. Each detector is an array of 32 cells of plastic scintillator distributed in four rings forming a disk with 8 sectors. Hamamatsu photomultiplier tubes are installed close to the detector. In order to tolerate magnetic field, fine mesh tubes have been chosen.

The time resolution of the V0 system is better that 1 ns. It provides charge and time information.

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V0A detector system after installation in its final position

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Prototype of the V0A with optical fibers embedded in the scintillator

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Prototype of the V0A with optical fibers embedded in the scintillator

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The V0A flying to its final destination

The Cosmic Ray Detector

The Cosmic Ray detector was also designed and built by the Mexican group of ALICE. It is an array of 60 scintillator counters located in the upper part of the ALICE magnet.

The detector generates a single muon trigger to calibrate the Time Projection Chamber and other components of ALICE. It also generates a multi-muon trigger to study cosmic rays with the help of the Inner Tracking System and the Time Projection Chamber. The detector was intensively used during commissioning of several ALICE subsystems. As mentioned above the detector alignment was performed using cosmic rays during several months previous to the accelerator program.

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An artistic view of the Cosmic Ray Detector by Pavel Cugini

Major improvements of the detector electronics are underway. A new electronics card developed by Sergio Vergara and A. Vargas is being tested. The possibility to integrate signal digitization is being developed.

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A Cosmic Ray Detector ACORDE on the top of the magnet

Data Analysis Studies

The implementation of Mexico into the GRID has played and will continue playing a crucial role on the data analysis studies that are now conducted in Mexico. With the assistance and support of CERN a Regional Operation Center (ROC) was deployed recently. It has been developed as a distributed ROC based on Mexico, Brazil and Colombia. With these facilities the data analysis for the coming years is finding its way for the Mexican group.

Event Shape Analysis

An extensive effort has been invested in applying the concept of event shape analysis known from electron-positron colliding machines to hadron collisions in experiments of small rapidity acceptance. The analysis is producing interesting results for the transverse sphericity in high multiplicity proton - proton collisions.

The results will allow more detailed studies of the various generators. Specifically the sensitivity to the parton distribution functions and general dynamics of particle production. The Mexican student Antonio Ortiz is driving the analysis which is not far from its final form.

Cosmic Ray Studies

Making use of the Cosmic Ray Detector and the tracking system of the Time Projection Chamber as well as the Inner Tracking System we are exploring the possible existence of muon bundles in cosmic radiation.

The analysis of the data collected during several cosmic runs is now in progress. Events like the one shown in the picture are under study. This event contains more than 300 muons and there are no reports of another event like this. The event was found by Mario Rodriguez PHD student in Mexico.

The source and nature of multi-muon events is of interest to better understand cosmic radiation models.

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A unique event with more than 300 muons found by Mario Rodriguez in data from this year

J/? Production in Ultra-Peripheral Ion Collisions

Ultra peripheral ion collisions are those where the nuclei pass by each other with impact parameter larger than twice the radii of the ions. The number of photons scales like the square of the atomic number so that particle production in ultra-peripheral heavy ion collisions is dominated by electromagnetic interactions. The production of J/? in this reactions would provide interesting information on the production mechanism as well as on the nuclear gluon shadowing effects.

The analyisis is being conducted by Veronica Canoa - a posdoc in Mexico - together with members of the Orsay-France group.

Nuclei / anti-Nuclei production

The production of nuclei/anti-nuclei in ALICE is also being simulated and analyzed in p-p collisions and Pb-Pb. The study is based on the TPC and TOF momentum reconstruction and timing capabilities. A coalescence afterburner for ALIROOT, based on p-n minimum phase-space correlations, was implemented to estimate the expected nuclei/anti-nuclei yields for analysis strategy and efficiency correction purposes.

Detector upgrade activities

Very High Momentum Particle Identification

The separation of protons and kaons on a track by track basis is difficult. As there are physics topics of great relevance for which a good identification of protons at high momentum is important, a group of several institutions including Mexicans has started to work in a proposal to develop a Cherenkov detector that allows a 3s separation of pions and kaons up to 16 GeV/c and protons from 11 to 30 GeV/c momentum. The Research and Development plans for a VHMPID module has been approved by the collaboration in 2009. After evaluation of its results a decision will be taken on the convenience of integrating more modules to the experiment.

Test of CsI GEM in a Cherenkov counter with arrays of 10x10 cm detectors. In the center one can see the beam impact. The test was conducted by the VHMPID collab, and the analysis by Daniel Mayani, a mexican student.

Diffractive Physics in ALICE

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Test of CsI GEM in a Cherenkov counter with arrays of 10x10 cm detectors. In the center one can see the beam impact. The test was conducted by the VHMPID collab, and the analysis by Daniel Mayani, a mexican student.

A proposal to enhance the capabilities of ALICE to address several diffractive physics topics as well as in photon-photon interaction in both proton-proton and ion-ion collisions is being developed. In a first phase, beam shower counters have been placed in the forward direction on both sides of the interaction region.

Improvements to the V0 detector

The V0 system provides trigger signals at the L0 level. As the detector is in a magnetic field environment, fine mesh phototubes have been installed. The PMT however produces after-pulses which, considering the low levels of light collected by detector cell does become an important source of noise. A number of ideas to solve the problem are now being studied.

  • 1. The Mexican Group consists of:

    Benemérita Universidad Autónoma de Puebla (BUAP): I. Cortes, A. Fernández, M. I. Martínez, M. Rodriguez Cahuantzi, G. Tejeda Muñoz, S. Vergara, A. Vargas.

    Centro de Investigación y de Estudios Avanzados (CINVESTAV): V. Canoa Roman, G. Contreras Nuño, L. M. Montaño, Gerardo Herrera Corral

    Instituto de Ciencias Nucleares, UNAM: E. Cuautle, L. Díaz, T. Jiménez Bustamante, D. Madani, A. Ortiz Velásquez, G. Paic, E. Pérez, V. Peskov, X. Sánchez Castro

    Instituto de Física, UNAM: R. Alfaro, E. Almaraz, S. Aguilar, A. Belmont, E. Cruz Alaniz, L. González Trueba, V. Grabski, A. Martinez, A. Menchaca, A. Sandoval.

    Universidad Autónoma de Sinaloa (UAS): I. León Monzón, R. Gómez, P. Podesta.

  • 2. Designed and constructed by the Lyon group, overseen by Jean Yves Grossiord.