Antimatter Atoms Successfully Stored for the
First Time
November 18, 2010
An artist’s impression of an antihydrogen atom – a
negatively charged antiproton orbited by a positively charged
anti-electron, or positron – trapped by magnetic fields.
(Graphic by Katie Bertsche)
Atoms of antimatter have been trapped and stored for the first
time by the ALPHA collaboration, an international team of
scientists working at CERN, the European Organization for Nuclear
Research near Geneva, Switzerland. Scientists from the U.S.
Department of Energy’s Lawrence Berkeley National Laboratory
and the University of California at Berkeley have made key
contributions to the ongoing international effort.
ALPHA stored atoms of antihydrogen, consisting of a single
negatively charged antiproton orbited by a single positively
charged anti-electron (positron). While the number of trapped
anti-atoms is far too small to fuel the Starship
Enterprise’s matter-antimatter reactor, this advance
brings closer the day when scientists will be able to make
precision tests of the fundamental symmetries of nature.
Measurements of anti-atoms may reveal how the physics of antimatter
differs from that of the ordinary matter that dominates the world
we know today.
Large quantities of antihydrogen atoms were first made at CERN
eight years ago by two other teams. Although they made antimatter
they couldn’t store it, because the anti-atoms touched the
ordinary-matter walls of the experiments within millionths of a
second after forming and were instantly annihilated —
completely destroyed by conversion to energy and other
particles.
“Trapping antihydrogen proved to be much more difficult
than creating antihydrogen,” says ALPHA team member Joel
Fajans, a scientist in Berkeley Lab’s Accelerator and Fusion
Research Division (AFRD) and a professor of physics at UC Berkeley.
“ALPHA routinely makes thousands of antihydrogen atoms in a
single second, but most are too ‘hot’”—too
energetic—“to be held in the trap. We have to be lucky
to catch one.”
The ALPHA octupole being wound at Brookhaven.
The ALPHA collaboration succeeded by using a specially designed
magnetic bottle called a Minimum Magnetic Field Trap. The main
component is an octupole (eight-magnetic-pole) magnet whose fields
keep anti-atoms away from the walls of the trap and thus prevent
them from annihilating. Fajans and his colleagues in AFRD and at UC
proposed, designed, and tested the octupole magnet, which was
fabricated at Brookhaven. ALPHA team member Jonathan Wurtele of
AFRD, also a professor of physics at UC Berkeley, led a team of
Berkeley Lab staff members and visiting scientists who used
computer simulations to verify the advantages of the octupole
trap.
In a forthcoming issue of Nature now online, the ALPHA
team reports the results of 335 experimental trials, each lasting
one second, during which the anti-atoms were created and stored.
The trials were repeated at intervals never shorter than 15
minutes. To form antihydrogen during these sessions, antiprotons
were mixed with positrons inside the trap. As soon as the
trap’s magnet was “quenched,” any trapped
anti-atoms were released, and their subsequent annihilation was
recorded by silicon detectors. In this way the researchers recorded
38 antihydrogen atoms, which had been held in the trap for almost
two-tenths of a second.
The positions of the 38 real anti-atom annihilations (circles
and triangles) match predicted antihydrogen distribution (gray dots
in upper panel) but not the simulated distribution of bare
antiprotons (colored dots in lower panel). Charged bare antiprotons
would be steered to different clusters by different electric fields
(red right bias, blue left bias, green no bias), but anti-atoms are
neutral so their distribution is unaffected. (The violet star is an
energetic positron.)
“Proof that we trapped antihydrogen rests on establishing
that our signal is not due to a background,” says Fajans.
While many more than 38 antihydrogen atoms are likely to have been
captured during the 335 trials, the researchers were careful to
confirm that each candidate event was in fact an anti-atom
annihilation and was not the passage of a cosmic ray or, more
difficult to rule out, the annihilation of a bare antiproton.
To discriminate among real events and background, the ALPHA team
used computer simulations based on theoretical calculations to show
how background events would be distributed in the detector versus
how real antihydrogen annihilations would appear. Fajans and
Francis Robicheaux of Auburn University contributed simulations of
how mirror-trapped antiprotons (those confined by magnet coils
around the ends of the octupole magnet) might mimic anti-atom
annihilations, and how actual antihydrogen would behave in the
trap.
Learning from antimatter
Before 1928, when anti-electrons were predicted on theoretical
grounds by Paul Dirac, the existence of antimatter was unsuspected.
In 1932 anti-electrons (positrons) were found in cosmic ray debris
by Carl Anderson. The first antiprotons were deliberately created
in 1955 at Berkeley Lab’s Bevatron, the highest-energy
particle accelerator of its day.
At first physicists saw no reason why antimatter and matter
shouldn’t behave symmetrically, that is, obey the laws of
physics in the same way. But if so, equal amounts of each would
have been made in the big bang—in which case they should have
mutually annihilated, leaving nothing behind. And if somehow that
fate were avoided, equal amounts of matter and antimatter should
remain today, which is clearly not the case.
In the 1960s, physicists discovered subatomic particles that
decayed in a way only possible if the symmetry known as charge
conjugation and parity (CP) had been violated in the process. As a
result, the researchers realized, antimatter must behave slightly
differently from ordinary matter. Still, even though some
antiparticles violate CP, antiparticles moving backward in time
ought to obey the same laws of physics as do ordinary particles
moving forward in time. CPT symmetry (T is for time) should not be
violated.
One way to test this assumption would be to compare the energy
levels of ordinary electrons orbiting an ordinary proton to the
energy levels of positrons orbiting an antiproton, that is, compare
the spectra of ordinary hydrogen and antihydrogen atoms. Testing
CPT symmetry with antihydrogen atoms is a major goal of the ALPHA
experiment.
How to make and store antihydrogen
To make antihydrogen, the accelerators that feed protons to the
Large Hadron Collider (LHC) at CERN divert some of these to make
antiprotons by slamming them into a metal target; the antiprotons
that result are held in CERN’s Antimatter Decelerator ring,
which delivers bunches of antiprotons to ALPHA and another
antimatter experiment.
Antiprotons and positrons are brought into the ALPHA trap from
opposite ends and held there by electric and magnetic fields.
Brought together, they form anti-atoms neutral in charge but with a
magnetic moment. If their energy is low enough they can be held by
the octupole and mirror fields of the Minimum Magnetic Field
Trap.
Wurtele says, “It’s hard to catch
p-bars”—the symbol for antiproton is a small letter p
with a bar over it—“because you have to cool them all
the way down from a hundred million electron volts to fifty
millionths of an electron volt.”
In the ALPHA experiment the antiprotons are passed through a
series of physical barriers, magnetic and electric fields, and
clouds of cold electrons, to further cool them. Finally the
low-energy antiprotons are introduced into ALPHA’s trapping
region.
Meanwhile low-energy positrons, originating from decays in a
radioactive sodium source, are brought into the trap from the
opposite end. Being charged particles, both positrons and
antiprotons can be held in separate sections of the trap by a
combination of electric and magnetic fields—a cloud of
positrons in an “up well” in the center and the
antiprotons in a “down well” toward the ends of the
trap.
To join the positrons in their central well, the antiprotons
must be carefully nudged by an oscillating electric field, which
increases their velocity in a controlled way through a phenomenon
called autoresonance.
“It’s like pushing a kid on a playground
swing,” says Fajans, who credits his former graduate student
Erik Gilson and Lazar Friedland, a professor at Hebrew University
and visitor at Berkeley, with early development of the technique.
“How high the swing goes doesn’t have as much to do
with how hard you push or how heavy the kid is or how the long the
chains are, but instead with the timing of your pushes.”
The novel autoresonance technique turned out to be essential for
adding energy to antiprotons precisely, in order to form relatively
low energy anti-atoms. The newly formed anti-atoms are neutral in
charge, but because of their spin and the distribution of the
opposite charges of their components, they have a magnetic moment;
provided their energy is low enough, they can be captured in the
octupole magnetic field and mirror fields of the Minimum Magnetic
Field Trap.
Of the thousands of antihydrogen atoms made in each one-second
mixing session, most are too energetic to be held and annihilate
themselves against the trap walls.
Setting the ALPHA 38 free
After mixing and trapping—plus the “clearing”
of the many bare antiprotons that have not formed
antihydrogen—the superconducting magnet that produces the
confining field is abruptly turned off—within a mere
nine-thousandths of a second. This causes the magnet to
“quench,” a quick return to normal conductivity that
results in fast heating and stress.
“Millisecond quenches are almost unheard of,” Fajans
says. “Deliberately turning off a superconducting magnet is
usually done thousands of times more slowly, and not with a quench.
We did a lot of experiments at Berkeley Lab to make sure the ALPHA
magnet could survive multiple rapid quenches.”
From the start of the quench the researchers allowed
30-thousandths of a second for any trapped antihydrogen to escape
the trap, as well as any bare antiprotons that might still be in
the trap. Cosmic rays might also wander through the experiment
during this interval. By using electric fields to sweep the trap of
charged particles or steer them to one end of the detectors or the
other, and by comparing the real data with computer simulations of
candidate antihydrogen annihilations and look-alike events, the
researchers were able to unambiguously identify 38 antihydrogen
atoms that had survived in the trap for at least 172
milliseconds—almost two-tenths of a second.
Says Fajans, “Our report in Nature describes
ALPHA’s first successes at trapping antihydrogen atoms, but
we’re constantly improving the number and length of time we
can hold onto them. We’re getting close to the point where we
can do some classes of experiments on antimatter atoms. The first
attempts will be crude, but no one has ever done anything like them
before.”
“Trapped Antihydrogen,” by Gorm Andresen, Mohammad
Dehghani Ashkezari, Marcelo Baquero-Ruiz, Will Bertsche, Paul Bowe,
Eoin Butler, Claudio Lenz Cesar, Steve Chapman, Michael Charlton,
Adam Deller, Stefan Eriksson, Joel Fajans, Tim Friesen, Makoto
Fujiwara, Dave Gill, Andrea Gutierrez, Jeffrey Hangst, Walter
Hardy, Mike Hayden, Andrew Humphries, Richard Hydomako, Matthew
Jenkins, Svante Jonsell, Lars Jørgensen, Leonid Kurchaninov,
Niels Madsen, Scott Menary, Paul Nolan, Konstantin Olchanski, Art
Olin, Alex Povilus, Petteri Pusa, Francis Robicheaux, Eli Sarid,
Sarah Seif el Nasr, Daniel de Miranda Silveira, Chukman So, James
Storey, Robert Thompson, Dirk Peter van der Werf, Jonathan Wurtele,
and Yasunori Yamazaki, is available in advance online publication
of Nature. ALPHA is supported in part by the National
Science Foundation and the U.S. Department of Energy’s Office
of Science.
LBNL contact: Paul Preuss, 510-486-6249, paul_preuss@lbl.gov
SOURCE
