It is rare for a negative result in a physics experiment to
produce a high-flying paper for the Top Ten, but that’s
precisely what has happened this period, with #9, reporting no
contact in a dark matter search conducted at
Italy’s Gran Sasso National Laboratory (LNGS). This paper
is of great importance to both particle physics and cosmology
because it constrains the assumed particle physics properties of
the dark matter candidates required for cosmology.
Dark matter in the universe has been a problem for decades. In 1937
Fritz Zwicky, an astronomer at Caltech, published a paper about the
masses of galaxies. He asserted that invisible dark matter
(cool stars, cold dust) outweighed visible
matter (stars) by a factor of up to 100. His paper had zero
impact, possibly because Zwicky was a cantankerous maverick.
His paper received just one citation in the 20 years following
publication, yet in 2009 it scored 29 hits. Clearly, research
on dark matter has changed a lot in 70 years. The citation
rate of Zwicky’s classic shows how interest has
exploded.
A big change in how dark matter is perceived can be tracked by
looking at answers to the following question: What is dark matter?
Zwicky speculated that it was ordinary matter that was too cold to
emit of light. In the 1970s cosmologists still clung onto baryonic
matter, in the shape of neutron stars and stellar-mass
black holes.
The consensus today, however, is that dark matter must be
non-baryonic material, and this takes particle physics beyond the
standard model into the realm of supersymmetry.
Right now, dark matter candidates include weakly interacting
massive particles, or WIMPs, exotic relics of the Big Bang. The
hypothesis is that WIMPs engage with ordinary matter through the
weak interaction (one of the four forces of physics), which would
enable them to transfer kinetic energy to baryonic matter in
nuclear recoils from atomic nuclei. The principle underlying the
search described in #9 is to detect such recoils. Clearly such an
experiment can only be expected to work if the detector is shielded
from cosmic rays. That’s where Gran Sasso, or LNGS, comes
into play.
The XENON10 detector in operation, with some of
the control displays, July 2006.
LNGS is the largest underground laboratory in the world for
experiments in particle astrophysics. The facility has three
experimental halls, hacked out of the Gran Sasso Mountain, 120 km
from Rome. Above these chambers, 1400 m of dolomite rock reduces
the cosmic ray flux to 10-6 of the surface flux.
Currently the facility houses 15 experiments, one being XENON10,
which is dedicated to the direct detection of dark matter by
looking at the low energy recoils of Xe nuclei when zapped by
WIMPs.
In paper #9, lead author Jesse Angle and colleagues from the
XENON10 consortium describe their experimental set-up: liquid Xe at
–93 degrees C provides the target. Detection of a collision
between dark matter and Xe nuclei is achieved by photomultiplier
tubes, which are sensitive to scintillation in the liquid Xe. Their
technique is long established in particle physics: detect and
measure by scintillation the energy released in collision, using a
protocol to eliminate background events due to cosmic rays and
radioactive decay from local sources (dolomite rock contains very
little U and Th, which helps). The Nevis Laboratory at Columbia
University designed and built the detector.
An analysis of 58.6 live days of WIMP search data from XENON10
produced only 10 events in the WIMP-search window. However a
careful analysis of these candidate events led the team to the
conclusion that they had detected no WIMP interactions. This
negative result has a positive outcome: it impacts on the parameter
space for the minimal supersymmetric models of particle physics.
That’s because previously unexplored parameter space is
eliminated, setting new limits on the cross-section for
WIMP-nucleon interaction.
The situation in consensus cosmology is unchanged by this LNGS
result. Around 23% of the universe is non-baryonic dark matter,
about which we know nothing apart from the effect of its mass on
the dynamics of the universe. Paper #9, which describes just one of
many ongoing direct detection attempts, is highly cited partly
because it shows that detectors based on liquid Xe are improving
rapidly, and they are catching up on the cryogenic experiments. But
another underground facility will soon be on the case: the Large
Hadron Collider…
Dr. Simon Mitton is a Fellow of St. Edmund’s
College, Cambridge, U.K.
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