Production and detection of cold antihydrogen atoms

Abstract

The production and observation of cold antihydrogen atoms have been recently reported by the ATHENA experiment at the CERN Antiproton Decelerator. The antiatoms were produced by mixing low-energy antiprotons and positrons in an electromagnetic trap. The antihydrogen detection is based on the observation of a characteristic signature in the annihilation of the neutral antiatoms on the trap walls by means of an imaging particle detector. An overview of the apparatus is given and the results obtained are discussed.

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Production and detection of
cold antihydrogen atoms
M. Amoretti*, C. Amsler†, G. Bonomi‡§, A. Bouchta‡, P. Bowek,
C. Carraro*, C. L. Cesar{, M. Charlton#, M. J. T. Collier#, M. Doser‡,
V. Filippiniq, K. S. Fine‡, A. Fontanaq**, M. C. Fujiwara††,
R. Funakoshi††, P. Genovaq**, J. S. Hangstk, R. S. Hayano††
M. H. Holzscheiter‡, L. V. Jørgensen#, V. Lagomarsino*‡‡, R. Landua‡,
D. Lindelo ¨f†, E. Lodi Rizzini§q, M. Macrı `*, N. Madsen†, G. Manuzio*‡‡,
M.Marchesottiq,P.Montagnaq**,H.Pruys†,C.Regenfus†,P.Riedler‡,
J. Rochet†#, A. Rotondiq**, G. Rouleau‡#, G. Testera*, A. Variola*,
T. L. Watson# & D. P. van der Werf#
*Istituto Nazionale di Fisica Nucleare, Sezione di Genova, and ‡‡Dipartimento
di Fisica, Universita ` di Genova, 16146 Genova, Italy
†Physik-Institut, Zu ¨rich University, CH-8057 Zu ¨rich, Switzerland
‡EP Division, CERN, CH-1211 Geneva 23, Switzerland
§DipartimentodiChimicaeFisicaperl’IngegneriaeperiMateriali,Universita ` di
Brescia, 25123 Brescia, Italy
kDepartment of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus
C, Denmark
{Instituto de Fisica, Universidade Federal do Rio de Janeiro, Rio de Janeiro
21945-970, and Centro Federal de Educac ¸a ˜o Tecnologica do Ceara, Fortaleza
60040-531, Brazil
#Department of Physics, University of Wales Swansea, Swansea SA2 8PP, UK
qIstituto Nazionale di Fisica Nucleare, Sezione di Pavia, and **Dipartimento di
Fisica Nucleare e Teorica, Universita ` di Pavia, 27100 Pavia, Italy
††Department of Physics, University of Tokyo, Tokyo 113-0033, Japan
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A theoretical underpinning of the standard model of fundamen-
tal particles and interactions is CPT invariance, which requires
that the lawsof physics be invariant under the combined discrete
operations of charge conjugation, parity and time reversal.
Antimatter, the existence of which was predicted by Dirac, can
be used to test the CPT theorem—experimental investigations
involving comparisons of particles with antiparticles are numer-
ous1. Cold atoms and anti-atoms, such as hydrogen and anti-
hydrogen, could form the basis of a new precise test, as CPT
invariance implies that they must have the same spectrum.
Observations of antihydrogen in small quantities and at high
energies have been reported at the European Organization for
Nuclear Research (CERN)2and at Fermilab3, but these experi-
ments were not suited to precision comparison measurements.
Here we demonstrate the production of antihydrogen atoms at
verylowenergybymixingtrappedantiprotonsandpositronsina
cryogenic environment. The neutral anti-atoms have been
detected directly when they escape the trap and annihilate,
producing a characteristic signature in an imaging particle
detector.
The current experiment, called ATHENA, seeks to compare the
frequency of the 1S–2S electronic transition (ground state to first
excitedstate)inhydrogenwiththatinantihydrogen.Thisfrequency
has been measured in hydrogen4to an accuracy of 1.8 parts in 1014.
Obtaining similar precisionin antihydrogenis possible inprinciple,
but only if very cold (of the order of a few kelvin) anti-atoms are
available.
Located adjacent to the antiproton decelerator5(AD) ring at the
CERN laboratory in Geneva, the ATHENA apparatus comprises
four main subsystems: the antiproton catching trap, the positron
accumulator, the antiproton/positron mixing trap, and the antihy-
drogen annihilation detector. All traps in the experiment are
variations on the Penning trap6, which uses an axial magnetic
field to transversely confine the charged particles, and a series of
hollow cylindrical electrodes to trap them axially (Fig. 1a). The
catching and mixing traps are adjacent to each other, and coaxial
with a 3T magnetic field from a superconducting solenoid. The
positron accumulator has its own magnetic system, also a solenoid,
of 0.14T. A separate cryogenic heat exchanger in the bore of the
superconducting magnet cools the catching and mixing traps to
about 15K. The ATHENA apparatus7features an open, modular
design that allows great experimental flexibility, particularly in
introducing large numbers of positrons into the apparatus—an
essential factor in the current work.
The catching trap8slows, traps, cools and accumulates antipro-
tons. To cool antiprotons, the catching trap is first loaded with
3 £ 108electrons, which cool by synchrotron radiation in the 3T
magnetic field. Typically, the AD delivers 2 £ 107antiprotons
having kinetic energy 5.3MeV and a pulse duration of 200ns to
the experiment at 100-s intervals. The antiprotons are slowed in a
thin foil and trapped using a pulsed electric field. The antiprotons
lose energy and equilibrate with the cold electrons by Coulomb
interaction. The electrons are ejected before mixing the antiprotons
with positrons. Each AD shot results in about 3 £ 103cold anti-
protons for interaction experiments.
The positron accumulator, based on a design detailed in ref. 9,
slows, traps and accumulates positrons emitted from a radioactive
source (1.4 £ 109Bq22Na). Accumulation for 300s yields 1.5 £ 108
positrons10, 50% of which are successfully transferred to the mixing
trap where they cool by synchrotron radiation. The mixing trap has
2.5 cm
B
Mixing trap electrodes
Si strip detectors
CsI crystals
511 keV
gamma
02
4
68 1012
–50
–100
–75
–125
Length (cm)
Trap potential (V)
Antiprotons
Positrons
Charged tracks
a
b
Figure 1 Central part of the ATHENA apparatus and trapping potential. a, Schematic
diagram, in axial section, of the ATHENA mixing trap and antihydrogen detector. The
cylindrical electrodes and the position of the positron cloud (blue ellipse) are shown. A
typical antihydrogenannihilation intothreechargedpionsand twoback-to-back511-keV
photons is also shown. The arrow indicates the direction of the magnetic field. The
detector active volume is 16cm long and has inner and outer diameters of 7.5cm and
14cm,respectively.b, Thetrappingpotentialisplottedagainstlengthalongthetrap.The
dashed line is the potential immediately before antiproton transfer. The solid line is the
potential during mixing.
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theaxialpotentialconfigurationofanestedPenningtrap11(Fig.1b),
whichpermitstwoplasmasofoppositechargetocomeintocontact.
In ATHENA, the spheroidal cloud of positrons can be characterized
byexciting and detecting axial plasma oscillations, as demonstrated
by ref. 12. Typical conditions are: 7 £ 107stored positrons, a radius
of 2–2.5mm, a length of 32mm, and a maximum density of
2.5 £ 108cm23.
Central to the observations reported here is the antihydrogen
annihilation detector13(Fig. 1a), situated coaxially with the mixing
region, between the outer radius of the trap and the magnet bore.
The detector is designed to provide unambiguous evidence for
antihydrogen production by detecting the temporally and spatially
coincident annihilations of the antiproton and positron when a
neutral antihydrogen atom escapes the electromagnetic trap and
strikesthetrapelectrodes.Anantiprotontypicallyannihilatesintoa
few charged or neutral pions14. The charged pions are detected by
two layers of double-sided, position-sensitive silicon microstrips.
The path of a charged particle passing through both microstrip
layers can be reconstructed, and two or more intersecting tracks
allow determination of the position, or vertex, of the antiproton
annihilation. The uncertainty in vertex determination is approxi-
mately 4mm (1j) and is dominated by the unmeasured curvature
ofthechargedpions’trajectoriesinthemagneticfield.Thetemporal
coincidencewindowisapproximately5ms.Thesolidanglecoverage
of the interaction region is about 80% of 4p.
A positron annihilating with an electron yields two or three
photons. The positron detector, comprising 16 rows, each row
containing 12 scintillating, pure CsI crystals15, is designed to detect
the two-photon events, consisting of two 511-keV photons that are
always emitted back-to-back. The energy resolution of the detector
is 18% full-width at half-maximum (FWHM) at 511keV, and the
photo-peak detection efficiency for single photons is about 20%.
The maximum readout rate of the whole detector is about 40Hz.
Ancillary detectors include large scintillator paddles external to the
magnet, and a thin, position-sensitive silicon diode through which
the incident antiproton beam passes before entering the catching
trap.
To produce antihydrogen atoms, we fill a positron well in the
mixing region with about 7 £ 107positrons and allow them to cool
to the ambient temperature (15K). The nested trap is then formed
around the positron well. Next, approximately 104antiprotons
(three AD shots) are launched into the mixing region by pulsing
thetrap from onepotential configuration (dashed lineinFig.1b)to
another (solid line). The mixing time is 190s, after which all
particles are dumped and the process repeated. Events triggering
the imaging silicon detector (three sides hit in the outer layer)
initiate readout of both the silicon and the CsI modules. We refer to
this procedure as ‘cold mixing’.
A second type of procedure, ‘hot mixing’, involves radio-
frequency heating of the axial motion of the positron plasma.
During the mixing cycle with antiprotons, the positrons are main-
tained at a temperature of several thousand kelvin. The positron
temperature is monitored using the plasma mode analysis tech-
nique of ref. 12. The two mechanisms of antihydrogen formation
expectedtobeimportantinATHENA,radiativerecombination and
three-body recombination16, should be effectively suppressed at the
hot mixing temperature, although the positrons and antiprotons
still interact.
For comparison and for investigating background processes, we
loaded antiprotons into the interaction region without positrons
and accumulated detector data. The resulting data set is referred to
as ‘antiproton only’.
For cold mixing, interaction of the two particle species is
immediately apparent, as the kinetic energy of the antiprotons is
rapidly reduced (,1s, an upper limit set by our current experi-
mentalmethod)tothelevelcorresponding tothepositronwell.The
cooling process is observed by lowering the wall of the antiproton
well to various levels and recording the antiproton loss on the
external scintillators. Such cooling has been reported previously17,
but with a factor of 300 fewer positrons. Our measurements of
antiprotons without positrons show no such cooling effect, also
indicating effective removal of electrons.
At the start of cold mixing, we observea rapid jump in the rate of
antiproton annihilation, rising to avalue that is tenfold higher than
that for hot mixing. The positron loss during hot mixing is
indistinguishable from that during cold mixing.
Subsequentobservationsarebasedonanalysisoftheannihilation
detector data. Antiproton annihilation vertices are reconstructed
from the silicon detector data using standard techniques18. Foreach
vertex, we search for ‘clean’ evidence of 511-keV photons in the
crystal data. A charged particle hit in a crystal, or an outer-layer
silicon hit lying in the footprint of a crystal, excludes that particular
crystal and its eight nearest neighbours. Next, we demand that
exactly two of the remaining crystals have hits in an energy window
around 511keV, and that there are no hits of any energy adjacent to
these two crystals. Energy calibration data, measured for each
individual crystal, are used in this test.
Tosearchforantihydrogeninthesampleofeventshavingavertex
and two ‘clean’ photons, we consider the opening angle, vgg,
between the lines connecting the vertex point to the geometric
Figure 2 Experimental data. a, The number of events passing the selection criteria is
plottedagainstthecosineoftheopeninganglevgg(seetextfordefinition).Thehistogram
is for cold mixing data (grey background). A total of 7,125 events, out of a sample of
103,270 reconstructed vertices, have two clean (but not necessarily back-to-back),
detected photons in the 511-keV energy window. The data represent 165 mixing cycles.
Filledtrianglesrepresenthotmixingdataandarescaledby1.6todepictthesamenumber
of mixing cycles. b, The opening angle distribution (grey histogram) for antiproton-only
data (99,610 vertices reconstructed, 5,658 clean events plotted). The filled circles
represent cold mixing data, analysed using an energy (Eg) window displaced upward so
as not to include the 511-keV photo peak; no angular correlation of photons is seen.
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centres of the two hit crystals. For an antihydrogen event, this angle
shouldbe1808(orcos(vgg) ¼ 21).Theopeningangledistribution
for reconstructed events detected during cold mixing is shown in
Fig. 2a. The equivalent plots for hot mixing (Fig. 2a) and anti-
proton-only (Fig. 2b) data are also shown. Compared with the hot
mixing and antiproton-only data samples, the cold mixing data
display a marked enhancement of events close to cos (vgg) ¼ 21,
indicating antihydrogen production. Analysis of the cold mixing
data using an energy window that does not include the 511-keV
peak in the crystal spectra yields no evidence for antihydrogen
(Fig. 2b).
The shape of the cos(vgg) distribution predicted byMonte Carlo
analysis of our detector for pure antihydrogen annihilation agrees
well with the cold mixing measurement. In particular, the width of
thepeakclosetocos(vgg) ¼ 21isthesameforourdataandfor the
Monte Carlo simulation. The dominant background for the experi-
ment is caused by antiproton annihilations that contain a neutral
pion, the decay of which produces electromagnetic showers in the
materialsurroundingthetrapandcanleadto511-keVsignalsinthe
CsIcrystals.Ourantiproton-onlydata(Fig.2b)ruleoutthisprocess
as a source of antihydrogen-like signal in our cold mixing data. The
shape of a Monte Carlo simulation of antiproton-only events also
agrees well with our measurement. Other sources of background,
such as cosmic rays and background from the accelerator complex
(AD injection and extraction) have also been excluded. The prob-
ability for accidental temporal coincidence of antiproton and
positron annihilation is negligible during mixing, as determined
from measured trigger rates for each part of the detector. Further-
more, Monte Carlo simulations of this background indicate no
enhancement in opening angle at 1808. Correlated loss due to
scattering of antiprotons on positrons outside of the spheroidal
cloud is ruled out by rate and dynamical considerations.
Further confirmation of antihydrogen production can be seen in
the cold mixing data if the distribution of all antiproton annihila-
tion vertex positions is projected onto a plane perpendicular to the
magnetic field (Fig. 3a). A clear image of the trap electrodes is
obtained, consistent with neutral anti-atoms annihilating on the
wall. This observation also implies that many more antihydrogen
atoms are produced than are actually detected through their
angularly correlated photons, a fact consistent with our Monte
Carlo analysis. The vertex distribution for mixing with heated
positrons has averydifferent shape and no evidence of annihilation
on the trap wall (Fig. 3b). Rather, the antiprotons annihilate in the
central region of the trap, probably owing to collisions with the
residual gas or with trapped ions in the positron plasma. The
average annihilation rate for hot mixing is about a factor of 4
smaller than that for cold mixing, whereas the peak ratio is about a
factor of 10 smaller, as observed above.
If we consider cold mixing data only in the range
cos(vgg) , 20.95, we have detected 131 ^ 22 events with a recon-
structed vertex and a pair of back-to-back, 511-keV photons above
a conservatively scaled antiproton-only background. With an
upper limit of the detection and reconstruction efficiency of
2.5 £ 1023, on the basis of Monte Carlo analysis, we estimate that
atleast50,000antihydrogenatomswerecreatedduringcoldmixing.
This can be compared with the total number of antiprotons mixed,
about 1.5 £ 106.
It is premature to discuss absolute production rates in our
experiment, but it is noteworthy that the time distribution of the
cos(vgg) , 20.95eventsparallelsthetotalannihilationrateduring
mixing. Further study is also needed to identify the mechanism of
antihydrogenformation.Wenotethatthepresentdetectionmethod
does not identify the quantum state of the produced antihydrogen
atoms. This is a question of considerable practical importance for
future experiments involving trapping of neutral anti-atoms, as the
demonstrated trapping techniques for hydrogen are only applicable
to ground-state atoms.
A
Received 27 August; accepted 3 September 2002; doi:10.1038/nature01096.
Published online 18 September 2002.
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annihilations are centred on a slightly smaller radius, in agreement with our Monte Carlo
simulations. (Some events appear to be outside of the trap radius owing to vertex
reconstruction errors.) b, The same plot as above, but for hot mixing. These data are
normalized to represent the same number of mixing cycles (165) as those in a.
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13. Regenfus, C. A cryogenic silicon micro strip and pure-CsI detector for detection of antihydrogen
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Acknowledgements
The authors comprise theATHENACollaboration. We wouldliketothank G. Bendiscioli,
S. Bricola, P. Chiggiato, J. Hansen, H. Higaki, A. Lanza, C. Marciano, O. Meshkov,
P. Salvini, G. Sobrero, B. Schmid and E. Søndergaard. We also thank the CERN AD team
and C. Surko, who provided essential advice. This work was supported by Istituto
Nazionale di Fisica Nucleare (Italy), Conselho Nacional de Desenvolvimento Cientı ´fico e
Technolo ´gico, Fundac ¸a ˜o de Amparo a Pesquisa do Estado do Rio de Janeiro (FAPERJ) e
Fundac ¸a ˜o CCMN/UFRJ (Brazil), Grant-in-Aid for Creative Basic Research of
Monbukagakusho (Japan), the Swiss National Science Foundation, the Danish Natural
Science Research Council, The UK Engineering and Physical Sciences Research Council
(EPSRC), The EU (Eurotraps Network), and the Royal Society. L.V.J., M.H.H., M.C. and
J.S.H. acknowledge the work of the late B. Deutch.
Competing interests statement
The authors declare that they have no competing financial interests.
Correspondence and requests for material should be addressed to J.S.H.
(e-mail: jeffrey.hangst@cern.ch).
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Quantum phase transition
in a common metal
A. Yeh*†, Yeong-Ah Soh*, J. Brooke*†, G. Aeppli*, T. F. Rosenbaum†
& S. M. Hayden‡
*NECResearchInstitute,4IndependenceWay,Princeton,NewJersey08540,USA
†The James Franck Institute and Department of Physics, The University of
Chicago, Illinois 60637, USA
‡H.H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK
.............................................................................................................................................................................
Theclassicaltheoryofsolids,basedonthequantummechanicsof
single electrons moving in periodic potentials, provides an
excellentdescriptionofsubstancesrangingfromsemiconducting
silicontosuperconductingaluminium.Overthelastfifteenyears,
it has become increasingly clear that there are substances for
which the conventional approach fails. Among these are certain
rare earth compounds1,2and transition metal oxides3,4, including
high-temperaturesuperconductors5,6.Acommonfeatureofthese
materialsiscomplexity,inthesensethattheyhaverelativelylarge
unit cells containing heterogeneous mixtures of atoms. Although
many explanations have been put forward for their anomalous
properties7, it is still possible that the classical theory might
suffice. Here we show that a very common chromium alloy has
some of the same peculiarities as the more exotic materials,
including a quantum critical point8, a strongly temperature-
dependent Hall resistance4,5and evidence for a ‘pseudogap’9.
This implies that complexity is not a prerequisite for unconven-
tional behaviour. Moreover, it should simplify the general task of
explaining anomalous properties because chromium is a rela-
tively simple system in which to work out in quantitative detail
the consequences of the conventional theory of solids.
Apart from complexity, a feature shared by almost all metals
whose properties are at odds with conventional theory is proximity
to magnetic instabilities10,11, that is, the materials are nearly mag-
netic, and when placed under pressure or their chemical formulae
are slightlychanged, they acquire magneticorder. Aunique point in
the associated pressure (or composition)–temperature phase dia-
grams is where the ordering temperatures vanish. Quantum rather
than thermal fluctuations destroy the order beyond this point,
which is therefore referred to as ‘quantum critical’.
We have selected chromium for our experiment because it is the
simplestmetalclosetoaquantumcriticalpoint.Indeed,itisabody-
centred cubic (b.c.c.) antiferromagnet, whose magnetism12can be
made to disappear with modest substitution of vanadium, its
neighbour in the periodic table. On increasing the vanadium
concentration x from 0, the Ne ´el temperature TN(Fig. 1a) and
ordered magnetic moment13(Fig. 1b) for Cr12xVxvanish continu-
ously at the quantum critical point with xc¼ 0.035. There is a
wealth of information14on this and other alloys of Cr, but,
remarkably for such a simple substance, basic quantities such as
the Hall resistance, which have engendered so much excitement for
other materials from semiconductor heterostructures to high-tem-
perature superconductors5, have not been studied systematically.
The Hall effect has until now received little attention in the context
of quantum phase transitions, and to our knowledge, our experi-
mentsarethefirsttoexaminethevariation oftheHalleffect near an
antiferromagneticquantumphasetransitioninCr(ref.15)orinany
metal.
Our experiments were performed on single crystals, and the
details are in the legend of Fig. 2. We plot in Fig. 2a the longitudinal
resistivity r(T) for several samples on each side of xc. A key
feature12,15is the “chromium anomaly” at TN, where the resistivity
increases owing to a loss of the carriers localized through magnetic
ordering. The anomaly rides on a background that consists of a
composition-dependent constant and a T-dependent term. The
former grows primarily with the disorder, which increases with
increased(random)alloying,althoughthereisasmallanomalyatxc
(ref. 16). The latter does not vary significantly with composition,
and, as demonstrated in the inset to Fig. 2a, appears to follow a
Figure 1 Magnetic ordering (Ne ´el) temperature and zero-temperature properties of
Cr12xVxnear its quantum critical point. a, Phase diagram for the isostructural dilution
series Cr12xVx, where the filled circles and open squares denote Ne ´el temperatures
obtained from magnetic susceptibility and Hall effect data, respectively. b, Ordered
magnetic moment measured by magnetic neutron diffraction13at 4.2K for Cr12xVx.
c, Low-temperature (T , 5K) carrier density obtained from inverse of the Hall number.
d, Low-temperature magnetic susceptibility.
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