<?xml version="1.0" encoding="ISO-8859-1"?><article xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<front>
<journal-meta>
<journal-id>0035-001X</journal-id>
<journal-title><![CDATA[Revista mexicana de física]]></journal-title>
<abbrev-journal-title><![CDATA[Rev. mex. fis.]]></abbrev-journal-title>
<issn>0035-001X</issn>
<publisher>
<publisher-name><![CDATA[Sociedad Mexicana de Física]]></publisher-name>
</publisher>
</journal-meta>
<article-meta>
<article-id>S0035-001X2007001000033</article-id>
<title-group>
<article-title xml:lang="en"><![CDATA[Ultra high energy cosmic rays: present status and recent results]]></article-title>
</title-group>
<contrib-group>
<contrib contrib-type="author">
<name>
<surname><![CDATA[Villaseñor]]></surname>
<given-names><![CDATA[L]]></given-names>
</name>
<xref ref-type="aff" rid="A01"/>
</contrib>
</contrib-group>
<aff id="A01">
<institution><![CDATA[,University of Michoacan Institute of Physics and Mathematics ]]></institution>
<addr-line><![CDATA[Morelia Michoacan]]></addr-line>
<country>Mexico</country>
</aff>
<pub-date pub-type="pub">
<day>00</day>
<month>08</month>
<year>2007</year>
</pub-date>
<pub-date pub-type="epub">
<day>00</day>
<month>08</month>
<year>2007</year>
</pub-date>
<volume>53</volume>
<fpage>173</fpage>
<lpage>179</lpage>
<copyright-statement/>
<copyright-year/>
<self-uri xlink:href="http://www.scielo.org.mx/scielo.php?script=sci_arttext&amp;pid=S0035-001X2007001000033&amp;lng=en&amp;nrm=iso"></self-uri><self-uri xlink:href="http://www.scielo.org.mx/scielo.php?script=sci_abstract&amp;pid=S0035-001X2007001000033&amp;lng=en&amp;nrm=iso"></self-uri><self-uri xlink:href="http://www.scielo.org.mx/scielo.php?script=sci_pdf&amp;pid=S0035-001X2007001000033&amp;lng=en&amp;nrm=iso"></self-uri><abstract abstract-type="short" xml:lang="en"><p><![CDATA[Cosmic rays with energies above 10(20) eV are the highest-energy particles ever observed in the Universe. Their flux is so small that up to now only about two dozen such particles have been detected by past and present observatories. Their nature, the location of their sources and the mechanisms by which they are produced or accelerated constitute still a mystery worth of investigating by gathering much higher statistical samples of such events. The Pierre Auger Observatory is a large-area hybrid detector that is under construction to study such ultra high energy cosmic rays. At present the southern site is near completion and it already constitutes the largest scientific observatory ever built to study cosmic rays. In this paper we describe the latest status of ultra high energy cosmic ray research with an emphasis on the Pierre Auger Observatory.]]></p></abstract>
<abstract abstract-type="short" xml:lang="es"><p><![CDATA[Los rayos cósmicos con energías de 10(20) eV representan las partículas con mayor energía jamás observadas en el Universo. Su flujo es tan bajo que hasta ahora sólo se han detectado unas dos docenas de estas partículas con observatorios pasados y presentes. Su naturaleza, la localización de sus fuentes, y los mecanismos por los que son producidas o aceleradas continúan siendo un misterio que vale la pena investigar obteniendo muestras estadísticas mucho mayores de dichos eventos. El Observatorio Pierre Auger es un detector híbrido de gran área que se encuentra actualmente en construcción y que se dedicará a la observación de rayos cósmico de alta energía. En este momento, el observatorio en el sur esta prácticamente terminado y ya constituye el mayor observatorio de rayos cósmicos jamás construido. En este artículo se describe el estado actual de la investigación de rayos cósmicos de ultra alta energía haciendo énfasis en el papel del observatorio Pierre Auger.]]></p></abstract>
<kwd-group>
<kwd lng="en"><![CDATA[Cosmic rays]]></kwd>
<kwd lng="es"><![CDATA[Rayos cósmicos]]></kwd>
</kwd-group>
</article-meta>
</front><body><![CDATA[ <p align="center"><font face="verdana" size="4"><b>Ultra high energy cosmic rays: present status and recent results</b></font></p>     <p align="center"><font face="verdana" size="2">&nbsp;</font></p>     <p align="center"><font face="verdana" size="2"><b>L. Villase&ntilde;or</b></font></p>     <p align="center"><font face="verdana" size="2">&nbsp;</font></p>     <p align="justify"><font face="verdana" size="2"><i>Institute of Physics and Mathematics, University of Michoacan, Morelia, Michoacan, 58040, Mexico, e&#150;mail: <a href="mailto:villasen@ifm.umich.mx" target="_blank">villasen@ifm.umich.mx</a></i></font></p>     <p align="justify"><font face="verdana" size="2">&nbsp;</font></p>     <p align="justify"><font face="verdana" size="2">Recibido el 1 de mayo de 2006    <br>   Aceptado el 1 de noviembre de 2006</font></p>     <p align="justify"><font face="verdana" size="2">&nbsp;</font></p>     <p align="justify"><font face="verdana" size="2"><b>Abstract</b></font></p>     ]]></body>
<body><![CDATA[<p align="justify"><font face="verdana" size="2">Cosmic rays with energies above 10<sup>20</sup> eV are the highest&#150;energy particles ever observed in the Universe. Their flux is so small that up to now only about two dozen such particles have been detected by past and present observatories. Their nature, the location of their sources and the mechanisms by which they are produced or accelerated constitute still a mystery worth of investigating by gathering much higher statistical samples of such events. The Pierre Auger Observatory is a large&#150;area hybrid detector that is under construction to study such ultra high energy cosmic rays. At present the southern site is near completion and it already constitutes the largest scientific observatory ever built to study cosmic rays. In this paper we describe the latest status of ultra high energy cosmic ray research with an emphasis on the Pierre Auger Observatory.</font></p>     <p align="justify"><font face="verdana" size="2"><b>Keywords: </b>Cosmic rays.</font></p>     <p align="justify"><font face="verdana" size="2">&nbsp;</font></p>     <p align="justify"><font face="verdana" size="2"><b>Resumen</b></font></p>     <p align="justify"><font face="verdana" size="2">Los rayos c&oacute;smicos con energ&iacute;as de 10<sup>20</sup> eV representan las part&iacute;culas con mayor energ&iacute;a jam&aacute;s observadas en el Universo. Su flujo es tan bajo que hasta ahora s&oacute;lo se han detectado unas dos docenas de estas part&iacute;culas con observatorios pasados y presentes. Su naturaleza, la localizaci&oacute;n de sus fuentes, y los mecanismos por los que son producidas o aceleradas contin&uacute;an siendo un misterio que vale la pena investigar obteniendo muestras estad&iacute;sticas mucho mayores de dichos eventos. El Observatorio Pierre Auger es un detector h&iacute;brido de gran &aacute;rea que se encuentra actualmente en construcci&oacute;n y que se dedicar&aacute; a la observaci&oacute;n de rayos c&oacute;smico de alta energ&iacute;a. En este momento, el observatorio en el sur esta pr&aacute;cticamente terminado y ya constituye el mayor observatorio de rayos c&oacute;smicos jam&aacute;s construido. En este art&iacute;culo se describe el estado actual de la investigaci&oacute;n de rayos c&oacute;smicos de ultra alta energ&iacute;a haciendo &eacute;nfasis en el papel del observatorio Pierre Auger.</font></p>     <p align="justify"><font face="verdana" size="2"><b>Descriptores: </b>Rayos c&oacute;smicos. </font></p>     <p align="justify"><font face="verdana" size="2">&nbsp;</font></p>     <p align="justify"><font face="verdana" size="2">PACS: 95.55.Vj; 95.85.Ry; 98.70.Sa</font></p>     <p align="justify"><font face="verdana" size="2">&nbsp;</font></p>     <p align="justify"><font face="verdana" size="2"><a href="/pdf/rmf/v53s4/v53s4a33.pdf">DESCARGAR ART&Iacute;CULO EN FORMATO PDF</a></font></p>     ]]></body>
<body><![CDATA[<p align="justify"><font face="verdana" size="2">&nbsp;</font></p>     <p align="justify"><font face="verdana" size="2"><b>Acknowledgments</b></font></p>     <p align="justify"><font face="verdana" size="2">The author thanks the organizers of this Workshop for the invitation to give this talk and the Coordination of Scientific Research from University of Michoacan for supporting this project.</font></p>     <p align="justify"><font face="verdana" size="2">&nbsp;</font></p>     <p align="justify"><font face="verdana" size="2"><b>References</b></font></p>     <!-- ref --><p align="justify"><font face="verdana" size="2">1. K. Greisen,<i> Phys. Rev. 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