Alpha Antihydrogen Experiment

Abstract

ALPHA is an experiment at CERN, whose ultimate goal is to perform a precise test of CPT symmetry with trapped antihydrogen atoms. After reviewing the motivations, we discuss our recent progress toward the initial goal of stable trapping of antihydrogen, with some emphasis on particle detection techniques.

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ALPHA ANTIHYDROGEN EXPERIMENT
M.C. FUJIWARAa,b∗, G.B. ANDRESENc, M.D. ASHKEZARId,
M. BAQUERO-RUIZe, W. BERTSCHEf, C.C. BRAYe, E. BUTLERf,
C.L. CESARg, S. CHAPMANe, M. CHARLTONf, C.L. CESARg, J. FAJANSe,
T. FRIESENb, D.R. GILLa, J.S. HANGSTc, W.N. HARDYh, R.S. HAYANOi,
M.E. HAYDENd, A.J. HUMPHRIESf, R. HYDOMAKOb, S. JONSELLj,
L. KURCHANINOVa, R. LAMBOg, N. MADSENf, S. MENARYk, P. NOLANl,
K. OLCHANSKIa, A. OLINa, A. POVILUSe, P. PUSAl, F. ROBICHEAUXm,
E. SARIDn, D.M. SILVEIRAo, C. SOe, J.W. STOREYa, R.I. THOMPSONb,
D.P. VAN DER WERFf, D. WILDINGf, J.S. WURTELEe, AND Y. YAMAZAKIo
(ALPHA COLLABORATION)
aTRIUMF, 4004 Wesbrook Mall, Vancouver BC, V6T 2A3, Canada
bDepartment of Physics & Astronomy, University of Calgary, AB, T2N 1N4, Canada
cDepartment of Physics & Astronomy, Aarhus University, DK-8000, Denmark
dDepartment of Physics, Simon Fraser University, Burnaby BC, V5A 1S6, Canada
eDepartment of Physics, University of California, Berkeley, CA 94720-7300, USA
fDepartment of Physics, Swansea University, Swansea SA2 8PP, United Kingdom
gInstituto de F´ ısica, Universidade Federal do Rio de Janeiro, 21941-972, Brazil
hDepart. of Physics & Astronomy, Univ. of British Columbia, BC, V6T 1Z4, Canada
iDepartment of Physics, University of Tokyo, Tokyo 113-0033, Japan
jFysikum, Stockholm University, SE-10609, Stockholm, Sweden
kDepartment of Physics & Astronomy, York University, ON, M3J 1P3, Canada
lDepartment of Physics, University of Liverpool, Liverpool L69 7ZE, United Kingdom
mDepartment of Physics, Auburn University, Auburn, AL 36849-5311, USA
nNRCN-Nuclear Research Center Negev, Beer Sheva, IL-84190, Israel
oAtomic Physics Laboratory, RIKEN, Saitama 351-0198, Japan
∗E-mail: Makoto.Fujiwara@triumf.ca
ALPHA is an experiment at CERN, whose ultimate goal is to perform a precise
test of CPT symmetry with trapped antihydrogen atoms. After reviewing the
motivations, we discuss our recent progress toward the initial goal of stable
trapping of antihydrogen, with some emphasis on particle detection techniques.
1. Introduction
Hydrogen is the most abundant element in the Universe. Experimental and
theoretical studies of atomic hydrogen over the past century have helped
build the foundation of modern physics. Today, the energy level difference
between its 1s and 2s states is measured to a relative precision of 10−14,

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and the ground state hyperfine splitting to 10−12, making hydrogen one
of the best studied physical systems. On the other hand, the antimatter
counterpart of atomic hydrogen, namely antihydrogen, has been only re-
cently produced at low energies.1The goal of ALPHA (Antihydrogen Laser
Physics Apparatus) is to perform precision tests of CPT via spectroscopic
comparisons of hydrogen and antihydrogen atoms. In the longer term, we
envision extending our antimatter studies to the gravity sector.
According to the CPT theorem,2the energy levels of atoms and an-
tiatoms must be identical. Any difference would imply violation of funda-
mental assumptions in the theorem, which has been proven for pointlike
particles in a flat spacetime within the framework of local and relativistic
quantum field theory. Precision measurements of antihydrogen atoms thus
will potentially confront some of the most fundamental concepts in physics.
In the past decade, Kosteleck´ y and his coworkers have led intensive the-
oretical investigations on CPT and Lorentz violation.3Their model, the
so-called Standard-Model Extension (SME), is the most phenomenologi-
cally studied theory of CPT and Lorentz violation, and the parameters of
the theory have been extensively tested experimentally with laboratory sys-
tems using matter particles, as well as astrophysical sources. Yet, no direct
comparison of atomic and antiatomic systems4has been performed to date.
Such a measurement will provide a test of CPT and Lorentz violation that
is complementary to those using matter-only particles.
One prediction of the SME for hydrogen-antihydrogen comparisons4
is that for the same relative precision, microwave spectroscopy of hyper-
fine splitting would give a more sensitive test of CPT violation than laser
spectroscopy of the 1s-2s transition. Therefore both types of spectroscopic
measurements are worthwhile to be pursued. See Ref. 5 for a more detailed
discussion on fundamental physics motivations for antihydrogen studies.
2. ALPHA experiment
Antiatoms as previously produced at CERN, while nearly at rest, were not
confined and rapidly annihilated on the walls of the apparatus. In order
to probe matter-antimatter symmetry at the highest possible precision, it
is essential that the antiatoms be confined in vacuum to allow for detailed
interrogation via laser light or microwaves.
In a typical experimental cycle, a beam of 3×107antiprotons is delivered
from the Antiproton Decelerator (AD) every 100 s. Using a pulsed electric
field, roughly 50,000 antiprotons with energy less than 3 keV are trapped
in the catching trap, where they subsequently cool via Coulomb collisions

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with a preloaded cold electron plasma. The antiproton-electron mixture is
then sympathetically compressed via application of a rotating RF field, and
then transferred to the mixing trap. After removal of the electrons, we are
left with antiprotons at ∼300 K. In parallel, positrons are accumulated and
compressed in a buffer gas moderated Penning trap, then transferred to the
mixing trap, where they are further cooled and compressed. In this way,
3×104antiprotons and 4×106positrons are prepared prior to mixing. The
two species are then gently mixed by making use of a nonlinear dynam-
ics phenomenon, autoresonance.6If an antihydrogen atom formed during
the mixing procedure is cold enough, it will be confined in our Ioffe-type
multipolar magnetic trap.7This magnetic trap confines neutral antiatoms
via interaction of the antihydrogen magnetic moment with the magnetic
field gradients. The depth of the potential well, which uses state-of-the-art
superconducting technology, is limited to ∼0.5 K (or 50 µeV). Given the
currently achieved set of parameters, the expected rate for antihydrogen
trapping is low. A 3-layer silicon vertex detector, which surrounds the trap
region (with a total active area of 8000 cm2), plays a crucial role in in-
dentifying trapped antihydrogen and in rejecting the background. Another
novel feature is our ability to shut down the magnetic trap in ∼10 ms via
a controlled quench of the superconducting magnets. This further reduces
the cosmic background via temporal gating.
3. Recent progress
Progress towards antihydrogen trapping is faced with many unique chal-
lenges associated with the handling of antimatter particles, requiring de-
velopment of special techniques for particle manipulations. We have made
rapid progress since the startup of ALPHA. Our published achievements
include: (1) demonstration of trapped plasma stability in a combined Pen-
ning trap (for charged particles) and magnetic trap (for neutral atoms);8
(2) production of antihydrogen in a reduced magnetic field;9(3) develop-
ment of a technique for antiproton plasma diagnosis based on annihilation
detection;10(4) sympathetic radial compression of antiproton clouds;11(5)
observation of a new radial transport mechanism, induced by magnetic mul-
tipolar fields in a Penning trap;12(6) development of antiproton, positron,
and electron imaging with a microchannel plate/phosphor detector;13(7)
production (but not yet trapping) of antihydrogen in a multipolar antiatom
trap environment.14A key to the success in achieving these milestones has
been the development of sophisticated plasma diagnostic techniques, includ-
ing antiproton annihilation imaging via the Si vertex detector pioneered in

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the ATHENA experiment,15–17with which we have unique sensitivity to
particle loss processes.
Most recently, in 2009-2010, we made further important steps. First, we
demonstrated evaporative cooling of antiprotons clouds to temperatures of
order 10 K.18Evaporative cooling has been used widely for neutral cold
atoms in the context of Bose-Einstein condensate studies, but this is the
first time it has been accomplished for cold charged ions (with the exception
of electron beam ion traps at much higher (∼100 eV) temperatures), let
alone for antimatter particles.19Second, we have achieved, for the first time,
trap conditions and detection sensitivity where observation of antihydrogen
trapping could be realistically expected.
We have conducted an extensive search for trapped antihydrogen.20Sig-
natures of annihilations of antihydrogen released from the trap were sought
via detection of antiproton annihilations in the Si detector. In order to un-
ambiguously identify rare events against cosmic-ray background, we devel-
oped an analysis technique that minimizes experimenter bias. First, event
selection criteria (‘cuts’) were investigated using independent calibration
samples without directly analyzing the actual experimental data. (Using
the data themselves to optimize the cuts has resulted in numerous instances
of experimental bias in the history of particle physics).
Furthermore, our cuts were optimized for the best sensitivity via Monte
Carlo pseudo-experiments. Because of the statistical nature of our low event
rate experiment, the statistical significance one obtains in a single experi-
ment fluctuates from one experiment to another, according to the Poisson
distribution. However, by running a large number of pseudo-experiments,
we studied the effects of varying cuts where the results are averaged over a
number of trials. Thus, we have derived a set of cuts which would produce
a best statistical significance on average.21
After these detailed studies of the event selection criteria, the chosen
cuts were finally applied to the experimental data. We found 6 events that
are consistent with annihilations of trapped antihydrogen atoms. From our
cut studies, we estimated our cosmic background to be 0.14 events. Hence,
our observation has a significance of 5.6 σ against cosmic-ray background.
However, there is one other source of potential background, namely antipro-
tons which could be trapped in our magnetic trap via the magnetic mir-
ror effect. While detailed simulation studies indicated that this possibility
was highly unlikely, we could not experimentally rule out this background.
Nonetheless, the trapping conditions and detection sensitivity achieved in
these experiments are unprecedented, and observation of candidate events

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for trapped antihydrogen gives great promise for the techniques developed
by ALPHA. The details of the detector analysis will appear in Ref. 21.
4. Summary and prospects
Significant progress has been made towards establishing antihydrogen trap-
ping. In the meantime, we are actively preparing for the first spectroscopy
on antiatoms via microwaves.22Given our high efficiency for antihydrogen
annihilation detection, there is a realistic chance that initial spectroscopy
measurements could be performed even with a few trapped antiatoms. We
are entering a very exciting time for antihydrogen physics.
Acknowledgments
This work was supported by CNPq, FINEP/RENAFAE (Brazil), ISF
(Israel), MEXT (Japan), FNU (Denmark), VR (Sweden), NSERC,
NRC/TRIUMF, AITF (Canada), DOE, NSF (USA), EPSRC and the Lev-
erhulme Trust (UK).
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