Pierre Auger: an observatory as vast as the horizon
"A journey to Malargüe, Argentina, where LIP contributes to one of the world’s largest scientific projects dedicated to the study of ultra-high-energy cosmic rays."
Not all science happens indoors. Some laboratories come with wind, dust and a horizon that seems to go on forever. One of them is in Malargüe, in Argentina’s Mendoza Province, where a LIP team made up of Pedro Assis, Raul Sarmento and Ruben Conceição travelled in late 2025 as part of the Pierre Auger Collaboration, one of the world’s largest scientific endeavours devoted to the study of cosmic rays: extremely energetic particles that reach Earth from space.
The Observatory has recently undergone a major upgrade, and its scientific programme has been extended until 2035, with the aim of uncovering more about the mysteries of ultra-high-energy cosmic rays. We now know that the origin and nature of these particles may be far more complex than researchers believed when the experiment was first conceived.
LIP has been part of this international collaboration since 2006, ensuring Portugal’s participation in a project that brings together more than 400 scientists, engineers, technicians and students from over 90 institutions across 18 countries. As Raul Sarmento explains, “being part of a collaboration like this is not just about analysing data remotely: it also means returning regularly to the site where the detectors are installed, following the experiment up close, and carrying out the technical work that makes the science possible”.
From Portugal to the Pampa Amarilla
For some, the journey to Malargüe begins in Braga; for others, in Lisbon. A day and a half later, it ends in a place unlike almost any other. Arriving by plane is only the first step: what follows is a long drive that gradually reveals the sheer scale of what lies ahead.
The town is crossed by Ruta Nacional 40, one of Argentina’s main roads, and serves as the gateway to the observatory region. But the road itself is not the most striking part — it is the landscape it cuts through: the Pampa Amarilla, the vast plain where the observatory is located. The observatory alone covers around 3,000 km², an area slightly larger than Luxembourg.
A site chosen with care
When a cosmic ray enters Earth’s atmosphere, it collides with air molecules and produces a shower of secondary particles that can spread over tens of kilometres. Detecting this phenomenon requires space — a great deal of it. The Pampa Amarilla was chosen for very specific scientific and logistical reasons: the region’s intermediate altitude, between 1,200 and 1,400 metres, is well suited to observing these particles before they dissipate completely, while the clear atmosphere and low levels of light pollution help researchers detect the signals they leave behind.
At the same time, Malargüe provides the essential infrastructure, roads, power and communications, needed to operate an experiment on this scale.
The collaboration meets regularly in Malargüe, close to the observatory, in order to maintain, as Raul Sarmento puts it, “a direct connection to the field, to the teams working there every day, and to the region itself”.
LIP’s contribution
During the collaboration meeting week, the days are intense. The schedule runs from 9 a.m. to 7 p.m., with presentations and discussions covering different areas of research. Breaks and informal moments also matter: they are where practical problems are discussed, ideas are sharpened and solutions begin to take shape. Sometimes even lunch is taken up with side meetings.
But a visit to the Pierre Auger Observatory is not limited to meetings. The LIP team’s work had two technical objectives: tasks related to data acquisition and the calibration of new detectors, namely scintillators. For these detectors to be used reliably in scientific analysis, they must first be calibrated against an auxiliary reference system. One of the team’s tasks was precisely to bring that system back into operation. They restored the electrical, gas and data transmission connections, everything needed for the calibration process to proceed reliably.
A window onto the invisible: LIP’s spark chamber
Of all the tasks carried out during this mission, one had the most immediate impact on those visiting the observatory: the maintenance of a spark chamber installed in the visitor centre and built by LIP. When a charged particle, such as a muon generated in a particle shower associated with cosmic rays, passes through the device, its path becomes visible. “Seeing a particle’s track directly changes the way people understand what we are doing,” explains Raul Sarmento. It is, therefore, a particularly valuable instrument for science communication.
The fact that this spark chamber was built in Portugal, in Coimbra, also makes it a very tangible example of LIP’s contribution to this project, not only through data analysis, but also through the instrumentation that underpins the science carried out at the observatory.
Malargüe: a town shaped by science
Malargüe is a small and relatively isolated town, but one closely bound up with one of the world’s largest scientific projects. Over more than two decades, that relationship has become visible in everyday local life: more than 90% of the observatory’s annual operating budget is spent locally in Malargüe, and its visitor centre has welcomed more than 178,000 people since it opened.
These figures come from a recent article on the observatory’s socioeconomic impact. But its presence can also be felt in other ways across the town, from taking part in the annual parade to, more recently, having two streets in Malargüe named after Alan Watson and David Nitz - respectively one of the observatory’s founders and one of its researchers.
