To Catch A Cosmic Ray

[Eustace Babcock]

The Pierre Auger Observatory in Argentina has spent almost ten years looking
for the source of ultra-high-energy cosmic rays -- but to no avail. Now the
observatory faces an uncertain future.
*BY* *KATIA* *MOSKVITCH*
The tank looks oddly out of place here on the windy Pampas of western
Argentina. Surrounded by yellow grass and spiky thorn bushes, the chest-high
plastic cylinder could be some kind of storage container -- were it not for the
bird-spattered solar panels and antennas on top.
More tanks can be seen in the distance, illuminated by a crimson Sun dropping
behind the far-off Andes. "Some locals think that the tanks influence the
weather: they make it rain or snow, or make a dry season," says Anselmo
Francisco Jake, the farmer who owns this stretch of land. "But I know they
don't. I know they catch cosmic rays."
Jake is right. There are 1,600 of these tanks, spaced over a
3,000-square-kilometre expanse that could fit all of Luxembourg with room to
spare. Together they comprise the Pierre Auger Observatory: a US$53-million
experiment to reveal the mysterious origins of ultra-high-energy cosmic rays,
the most energetic subatomic particles known to exist.
But for all its size, the array has fallen short. After almost ten years of
hunting, it has observed dozens of ultra-high-energy cosmic rays, but has not
managed to solve the mystery of where they come from. As a detector, "the
device worked twice as well as we expected," says project co-founder James
Cronin, a retired astrophysicist at the University of Chicago in Illinois. But
the particles seem to be coming from all over the sky, with too little
clustering for researchers to pinpoint the sources. "It's up to nature with
experiments like this one," he says.
Now, the Auger team is putting its hopes on a proposed upgrade that might
settle the question by improving Auger's resolution considerably. Five designs
are being evaluated internally by a committee of Auger physicists, who are
expected to present their final selection to the array's many funding agencies
in November. The trouble is, there is a sixth option, too. "In the worst-case
scenario, and I don't want to think about it, we may get shut down," says
Auger's deputy project manager, physicist Ingo Allekotte.
An upgrade would require an investment of roughly $15 million, and some argue
that the money would be put to better use elsewhere. "Although it was worth
building Auger, it was a gamble that unfortunately didn't yield much new
understanding," says Eric Adelberger, a physicist at the University of
Washington in Seattle. "Cosmic-ray physics has delivered very few surprises and
progress is terribly slow. Maybe it is time to move on."
That would be a blow to science -- and to Argentina, say Auger's supporters.
These flagship projects do more than just conduct research, says Pablo Mininni,
head of the physics department at the University of Buenos Aires. They also
raise awareness of physics and draw young people into the field. "Such a big
project deserves some continuity," he says.
Physicists have known for more than a century that Earth is continually
bombarded by charged particles from space -- many of which have energies that
are astonishing even by particle-physics standards. It is not uncommon for
cosmic rays to have hundreds or thousands of times the 7 trillion electron
volts (10¹² eV) soon to be achieved by the most powerful human-made particle
accelerator, the Large Hadron Collider (LHC) near Geneva in Switzerland.
Most of these particles are now thought to be protons and other light nuclei
originating far outside the Solar System, probably in cataclysmic stellar
explosions known as supernovas. But on very rare occasions, cosmic rays have
hit Earth's atmosphere at energies of 10¹⁸ eV or more. The most energetic
example on record -- the 'Oh-My-God particle' detected¹ on 15 October 1991 in
the skies above Utah -- had 3 ⨉ 10²⁰ eV, about 40 million times that of the
LHC. And therein lies a mystery: calculations suggest that the expanding shock
wave of a supernova detonation cannot accelerate charged particles beyond about
10¹⁷ eV. No one knows what physical process could accelerate particles to
higher energies -- or even what those particles might be (see /Nature/ *448,*
8-9; 2007).

*RULE-BREAKERS*
In 1992, Cronin, who shared the 1980 Nobel Prize in Physics for his work on
particle interactions, decided to find out. He, Alan Watson of the Univeristy
of Leeds, UK, and Murat Boratav of Pierre and Marie Curie University in Paris,
set out to build an observatory that -- they hoped -- could detect enough
ultra-high-energy cosmic rays to answer those questions.
Their sprawling, 1,600-detector design reflected two fundamental facts about
their quarry. The first is that the rays are exceedingly rare. Although their
low energy cousins come in at roughly a few particles per square centimetre per
second, the rates dive precipitously as the energy increases. Above 10²⁰ eV,
the cosmic-ray flux is less than one particle per square kilometre per century.
So the more detectors the researchers could deploy, the better their chances
would be of catching one.
The second fact is that 'primary' cosmic rays -- those that are coming in
from interstellar space -- never reach the ground. Instead, they smash into an
air molecule high in the atmosphere, producing a blast of photons, electrons,
positrons, muons and other collision products that then slam into other air
molecules. The result is an 'air shower': a cascade of lower-energy particles
that collectively follow along the track of the original cosmic ray. And that
calls for detectors over a very wide area, in the hope that the devices could
register enough of the air-shower particles as they hit the ground to
reconstruct the energy and direction of the original particle (see 'Celestial
messengers'). To help in the reconstruction, the physicists also planned to
surround the site with four clusters of fluorescence telescopes to scan the
skies over the array, mapping the faint streaks of blue and ultraviolet light
that the air-shower particles produce as they rip through the atmosphere.
Naming their observatory after Pierre Auger, the French physicist who
discovered air showers in 1938, the three scientists started going from
country to country knocking on doors. They gathered a cadre of high-level
physicists from around the world who wanted to join them. And those physicists,
in turn, used their connections to get funding from their own governments. In
short order, the United States agreed to help, as did Italy, Germany, France,
Argentina and several other countries.
At the same time, the Auger team was looking at potential sites in South
Africa, Australia and South America -- places that met their need for lots and
lots of empty, flat land with clear skies above. Nelson Mandela dearly wanted
the observatory to be based in South Africa. But the Auger team judged that the
nation did not have a strong-enough community of physicists to support the
project.
The Australian site had a different drawback: it was on land controlled by
the military, so collaborators from certain countries might not be able to work
there.
So in November 1995, Cronin, Watson and Boratav announced that the
observatory would be built in Pampa Amarilla, a plain some 1,400 metres above
sea level. Except for Malargüe, a mining town of 23,000 people just to the
southwest, the site was as empty as the Auger team could wish. Better still,
Argentina's then-president, Carlos Menem, was so excited by the idea of his
country hosting an international science project that he promised to support it
with the equivalent of US$10 million in Argentinian pesos. The province of
Mendoza, where the site is located, agreed to contribute another $5 million.
This largesse would prove to be a mixed blessing: in 2001, just as
construction was getting under way, Argentina experienced its biggest economic
crisis and government default in history. The peso instantly lost two-thirds of
its value, leaving the researchers to scramble for funding from other sources
to keep construction on schedule. It was one of Auger's biggest setbacks, say
Cronin. Another came in 2010, when US funding agencies declined the
researchers' request to build a sister observatory in Colorado, which would
have allowed them to look for the ultra-high-energy cosmic-ray sources across
the entire sky instead of just the Southern Hemisphere.

*AIR* *SHOWERS*
Still, the first 154 detectors of the Auger observatory were able to start
collecting data on 1 January 2004, with the rest of the detectors being
deployed in stages until the array was completed in 2008. Each of the plastic
tanks is filled 12,000 litres of purified water, which produces a streak of
light when an air-shower particle passes through, and is lined with phototubes
that can measure that light. The tank's antennas transmit the data to the
observatory's headquarters in Malargüe, where they are sent out for analysis to
some 350 researchers around the world.
Their first decade of data-taking has yielded a number of provocative
results, including hints that many of the highest-energy rays are actually
heavy nuclei such as that of iron, instead of the much more common protons².
"It was a surprising result that nobody had thought about," says Auger
spokesman Karl-Heinz Kampert, a physicist at the University of Wuppertal in
Germany. And if true, it might have something important to say about the
mysterious acceleration mechanism -- although no one is quite sure what. It
also threatened to undermine Auger's central quest: heavy nuclei tend to be
more strongly deflected by intergalactic magnetic fields than protons are, and
that could randomize their direction and make it impossible to trace the rays
back to their sources.
That concern seemed to have been put to rest in 2007. Working with
three-and-a-half years of data gleaned from 27 rays, Auger researchers reported
that the rays seemed to preferentially come from points in the sky occupied by
super massive black holes in nearby galaxies³. The implication was that the
particles were being accelerated to their ultra-high energies by some mechanism
associated with the giant black holes. The announcement generated a media
frenzy, with reporters claiming that the mystery of the origin of cosmic rays
had been solved at last.
But it had not. As the years went on and as the data accumulated, the
correlations got weaker and weaker. Eventually, the researchers had to admit
that they could not unambiguously identify any sources⁴. Maybe those random
intergalactic fields were muddying the results after all. Auger "should have
been more careful" before publishing the 2007 paper, says Avi Loeb, an
astrophysicist at Harvard University in Cambridge, Massachusetts.
The Auger physicists content that it would have made no sense to wait. "We
gave the statistical significance of what we observed, so scientists know how
to ponder the results," says team member Esteban Roulet, a physicist at the
Balseiro Institute in San Carlos de Bariloche, Argentina. "I think it is
important that the community gets the information we can gather in this way."

*MASSIVE* *UPGRADE*
Nonetheless, the mystery remains unsolved -- an impasse that the Auger team
wants to end with the hoped-for upgrade. The basic strategy is to get a better
measure of each primary cosmic ray's mass and thus distinguish the relatively
undeflected protons from the heavier particles, says Auger team member Alberto
Etchegoyen, a physicist working at Argentina's National Atomic Energy
Commission in Beunos Aires. "If nature is kind enough to us," he says, and if
there are enough protons among the ultra-high-energy cosmic rays to get
adequate statistics, "we'll be able to find the sources."
Currently, the mass is measured by Auger's fluorescence telescopes, which
watch how each air shower expands and deposits its energy as it descends
through the atmosphere. But the telescopes can operate only on clear, moonless
nights, which cuts down on their observing time. So instead, the team wants to
look within the showers to count muons: short-lived particles that behave like
heavy electrons. Because the muons in air showers tend to be produced most
copiously in collisions of heavier cosmic-ray particles, knowing their
abundance should tell the Auger physicists whether the incoming primaries are
protons or heavy nuclei.
The five upgrade proposals represent five different ways of identifying
muons, but all are based on the fact that muons tend to penetrate farther into
the water tank than other particles. Each scheme requires a different
combination of new electronics, new detectors and internal modifications for
all 1,600 tanks -- hence the $15-million cost of the upgrade. Supporters argue
that the investment is worthwhile, not least because the array currently has
little chance for ever getting statistics good enough to identify the sources,
yet still costs $1.7 million a year to run.
But the muon-detection schemes have yet to be proved in the field, and the
selection committee could still decide that the upgrade is not worth it -- and
perhaps even that Auger should be shut down. "This is a serious question," says
Kampert.
Cronin insists that it is much too soon to give up. Auger is exploratory, he
says. "I don't know how much we'll learn from it. But you don't learn anything
if you don't do something."
Besides, says Allekotte, scrapping Auger would be depriving Argentina of a
project that has greatly boosted the country's scientific capacity -- not least
by providing an incentive for young people to pursue physics. Tiny Malargüe now
has a university for first- and second-year undergraduate students where many
Auger engineers and scientists teach part-time. "One girl who started in 2012
was at first interested in maths, bu as she learned more and more about the
observatory and cosmic rays, she decided to switch to physics," says Marcos
Cerda, an Auger engineer and a physics lecturer at the university. "She's now
in her third year, doing a physics major at the University of Mendoza."
In addition, says Etchegoyen, there are many Argentinian students among the
roughly 360 who have already earned their PhDs doing research at Auger, or are
working towards one there. And now, he says, "two out of Auger's five upgrade
proposals -- design, prototype, construction, everything -- would be made in
Argentina. That would've been impossible at the beginning of Auger. We have
managed to grow a whole new generation of experimentalists linked to
international big physics."
Thanks to the observatory, "Argentina appeared on the map of global science,"
says the country's science minister Lino Barañao. He points to the Deep Space
Antenna 3 radio dish that the European Space Agency installed about 30
kilometres south of Malargüe to support space missions such as Mars Express,
Herschel and Planck. And he points to the Large Latin American Millimetre
Array, a radio telescope bing built in the north of Argentina in collaboration
with Brazil. The presence of Auger influenced the decisions to base both these
projects in Argentina.
So if the Auger upgrade does go ahead, Argentina hopes to gain even more
expertise, and add more capacity, says Barañao. And even if it doesn't, at
least it's left a legacy. "We're associated with producing soya beans, beef and
wine, but many countries can do that," he says. "Now we're also associated with
world-class astrophysics."
_______________________________________________________________________________
*Katia* *Moskvitch* /is/ /a/ /science/ /writer/ /in/ /London/ /and/ /an/
/International/ /Development/ /Research/ /Centre/ /fellow/ /at/ /Nature./

1. Bird, D. J. /et/ /al./ /Astrophys./ /J./ *424,* 491 (1994).
2. The Pierre Auger Collaboration /Phys./ /Rev./ /Lett./ *104,* 091101 (2010).
3. The Pierre Auger Collaboration /Science/ *318,* 938-943 (2007).
4. The Pierre Auger Collaboration /Astropart./ /Phys./ *34,* 314-326 (2010).


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[Harriet Merritt]

Two astronomical observing facilities won new leases of life last week. On 4
November, the University of California announced that it had reversed a 2013
decision to phase out funding for the Lick Observatory on Mount Hamilton,
California (see /Nature/ http://doi.org/w2m; 2014). Days earlier, the
University of Hawaii took ownership of the United Kingdom Infrared Telescope in
Hawaii. In 2012, the UK Science and Technology Facilities Council had announced
that it would close the telescope and concentrate funds on a much larger
planned telescope in Chile.