Low Energy (Anti)atoms for Preision
Tests of Basi Physis
D.M. Silveira, O.Pereira, M. Veloso, and ClaudioL. Cesar
Instituto deFsia
UniversidadeFederaldoRiodeJaneiro
21945-970, RiodeJaneiro,RJ,Brazil
Reeivedon23Marh,2001
Reentadvanesintehniquestomanipulateandstudy,withhighpreision,atomihydrogen,from
onehand,andsuessfultrappingshemesforpositronsandantiprotons,fromtheotherhand,have
enouragedthe pursuitofexperimentsto testCPTviolation andtheWeak EquivalenePriniple
(WEP) through the omparison of hydrogen and antihydrogen. A desription of the hydrogen
trap and laser system being built in Rio, to trap and perform high resolution spetrosopy on
old hydrogen, is presented along with a disussion on the tehniques and experimental system
beingimplementedbytheATHENAollaborationatCERNtoprodueoldantihydrogen. Anew
tehniquetomakeaoldantihydrogenbeamisproposed.
I Introdution
CPTinvarianeanbederivedfromverygeneral
prin-iplesofrelativistiquantum eldtheoriesandits
on-servationtheoremhasbeenprovedinatspae-time[1℄.
The ombined operation of harge onjugation (C),
spae reetion (P)and timereversal(T) should
rep-resentan exatsymmetry of nature. By testing CPT
invariane,oneistestingtheorretnessofthe
desrip-tionofthemirosopiworldin termsoftheexhisting
eld theory; oneis testingtheStandard Model [2℄. In
whatonernsantimatter,itpreditsthatpartilesand
antipartiles shall haveequal masses and lifetimes, as
wellasequal(but opposite)hargesandmagneti
mo-ments. For (anti)atomi systems the preditions are
that the levels' energies and lifetimes should be the
samefortheonjugatespeies. Todate,noCPT
viola-tionhasbeenexperimentallyobserved. Preisiontests
of CPT violation inlude measurements of the
dier-enebetweenthemagnetimomentsoftheeletronand
positronandthedierenebetweentheharge-to-mass
ratio of the proton and antiproton. Nevertheless, the
mostpreisetest(10 18
auray)[3℄,thoughmodel
de-pendent,omesfromtheomparisonbetweenthemass
oftheneutralkaonandantikaon (seeFig.1).
Preisionmeasurementswiththesimplestantiatom
that ould be produed at low energies would
repre-senta morevigorous,model-independenttest of CPT
invariane. Moreover, the availability of these slow
antiatoms would prompt experiments to ompare the
gravitationalaelerationsofhydrogenand
antihydro-(WEP):forgiveninitialonditions,negletingspin
ef-fets, equalbodies follow the sametrajetories [4℄. If
thisprinipledoesnothold underexperimental
veri-ation, General Relativity will have to be revised. A
diretmeasurementofthegravitationalaelerationof
hydrogen and antihydrogenwould addressan old and
renewedquestiononthepossibilityofantigravity[5℄or
aWEP breakdown[6℄, an issuethat still laks
experi-mental observation. Thehallenge is to produe
suit-ablyoldantihydrogen.
Figure1. Comparison between preisiontests of CPT
in-variane. Seetextfordisussion.
Antihydrogen is believed to have been rst
pro-dued and deteted in 1996[2℄, in a team experiment
heldatCERN'sLEAR(LowEnergyAntiprotonRing).
The basi idea is that an antiproton passing through
eletron-apositronfromtheproduedpairandformafast
mov-ingantihydrogenatom. IntheCERNexperiment,the
xenonatoms(the target)wereshotfromagas-jet
noz-zlearossthepathoftheantiprotonsastheyirulated
aroundLEAR.Antihydrogenwasdetetedthroughthe
annihilation ofthe antiatoms' positronswith the
ele-tronsofasiliontarget,withtheemissionofa
hara-teristiphotonpairdetetedbyasetofsurroundingNaI
alorimeters. Additional measurements down the line
onthestrippedantiprotonseliminateallbut
antihydro-genevents. Thoughitwasnietoobserveantihydrogen
forthersttime,verylittleouldbedoneonthestudy
of its properties as the few antiatoms produed (nine
ounts ould be assigned to antihydrogen)travelled a
fewnanoseondsbeforeannihilation. Lateron,
antihy-drogenwasproduedinlargeramountsatFermilab[7℄.
Foraomprehensivestudyofitsproperties,
antihydro-gen has to be produed with muh less speed and in
muhgreaternumbers.
This is the hallenge posed to the ATHENA
ollaboration[4℄: the prodution of antihydrogen in a
novel way so as to get very low energy antiatoms
suitable for high preision measurements. All the
in-gredients to form antihydrogen are at hand. Sine
the early 80's steady progresshas been made in
ool-ing and trapping neutral atoms; in partiular,
ultra-old gaseous samples of spin-polarized atomi
hydro-genhavebeenstudiedwithhighpreisionthrough
two-photon Doppler-free laser spetrosopy on the 1S-2S
transition[8℄. Reentadvanes in apture, oolingand
trappingof positronsand antiprotons makeitfeasible
toproposereombinationshemeswiththeaimof
form-ingoldantihydrogen.
In this paper we address some of the
experimen-tal issues involved in this researh, its urrent status
and propose a new method to generate a old
anti-hydrogen beam. In setion II we present the urrent
status of trapped hydrogen researh and our
appara-tususingthebuergasloadingtehniqueandthelaser
systemfor spetrosopy. In setion III wedisuss the
ATHENAapproahandurrentstatus onantiprotons
andpositronstrappingandreombinationshemesfor
antihydrogenprodution. SetionIVdisussesthe
inte-grationofthese aspetsand physisexperiments,
on-ludingin setionV.
II Trapped atomi hydrogen
and laser spetrosopy
Bose-Einsteinondensation (BEC)wasthemain
driv-ingforebehindresearhwitholdhydrogen,asithad
beenpreditedthatspin-polarizedhydrogenwould
re-main in atomi form down to T = 0 K[9℄. The rst
magnetitrappingofatomihydrogenwasdoneatMIT
in1987[10℄usinganevauatedellooledbya 3
He-4
He
dilutionrefrigeratorandoveredwithsuperuidhelium
withinaninhomogeneousmagnetield.Moleular
hy-drogen,frozenonthewalls,isdesorbedanddissoiated
byaRFdisharge. Theresultinghydrogenatoms an
lose energy andboune o the weakly bindinghelium
lm,ontheellwalls. Theslowatomsarethen
magnet-ially trapped throughthe interation of its magneti
dipolemomentwiththeexternalinhomogeneouseld
B.Inthepreseneofexternalelds,theatoms' energy
levelssplitintohypernelevelswhoseenergyinreaseor
derease withtheapplied eld. SineMaxwell's
equa-tionsforbidthereationofapointofeldmaximumin
asoure-freeregion,one antrapatomswhose energy
inrease with inreasing eld in a point of eld
mini-mum;atomsinsuhstatesarealled`low-eldseekers'.
ThatsameyearthegroupatMIT was abletotest
the proposal[11℄ of fored evaporative ooling
ahiev-ing temperaturesas low as 1 mK[12℄. This tehnique
eliminatesthehighenergyatoms,whihareontinuosly
beinggenerated by ollisionswithin the thermal
sam-ple. The removal of exess energy auses the sample
tothermalizeateverloweringtemperatures. Typially
onelosesoneorderofmagnitudeinatomnumberwhile
gainingtwoordersofmagnitudein thedereaseof the
temperature. A newandlargertrapwasbuilt[13℄and
theMITgroupgottotemperaturesasoldas100mK
and densities ashigh as 5 x 10 13
m 3
. Bose-Einstein
ondensationouldnotbeseeninthesystemduetothe
lakofagooddetetor. Itwastheninvestedalarge
ef-fortin developingthelaser andoptial systemfor the
detetionofthetrappedatomsusingthe1S-2S
transi-tion,tobeusedasadetetionshemeforBECandfor
theinterestofspetrosopyonthisfundamentalatom.
A few years later, in 1995, the MIT group[8℄ was
able to perform laser spetrosopy on the trapped
atoms at400Kahievingareordhigh resolutionof
2 parts in 10 12
at the time. In a reent publiation,
CesarandKleppner[14℄showedhow,inthislowenergy
regime,oneanavoidtime-of-ightbroadening,whih,
alongwiththeseond-orderDopplereet,isthe
dom-inantbroadeningmehanismoftoday'sstate-of-the-art
hydrogen atomi beam[15℄. But either asa frequeny
standardorasaportablereferenetrapforomparisons
withantihydrogen,theMITtrapistooompliatedand
ostlyto beaviableoption. Other trappingstrategies
havetobedevelopedtomakeitlessomplexandmore
reliable.
Oneofthemostimportantparametersofamagneti
trap is itsdepth, the dierene betweenthe potential
(or the eld) on the edge and on the bottom of the
trap. Despite the amazing development of
superon-dutorsandpermanentmagnetstehnology,itremains
a hallenge to reate traps deeper than 3-4 T, whih
like hydrogen. It is lear that the atoms that are to
be trapped must undergosome kindof pre-oolingso
that itsenergyisloweredtoapointwherethegaseous
sampleanbeonnedbythemagnetitrap. Thishas
beenmadebasially byeither oftwotehniques: laser
ooling { whih works ne for alaline atoms with a
simpleshemeofenergylevelsandlaser-friendly,easily
aessible transitions { or thermalization with a thin
superuidliquid 4
Helm,whihiswellsuitedonlyfor
hydrogen. Despitehugeeorts inextending therange
of appliability of optial tehniques, most of the
ele-mentsoftheperioditable(aswellasmoleules)remain
untouhablebythosetehniques.
A. Buergas loading
Buer gas loading of magneti traps is an
alter-nativetehnique that proved to be widely appliable.
Proposed[16℄ and demonstrated[17℄ by John Doyle's
groupatHarvard,itwasemployedtomagnetiallytrap
largeamountsofmoleules[18℄,aresultneverahieved
before. The basi idea is to ool the gaseous sample
through elasti ollisions with the old buer gas in
the regime of large Knudsen's number, i.e., the mean
free path muh shorter thanthe elldimensions. The
buergas{typially 3
Heor 4
Hewhihhavereasonably
highvapourpressureinthesub-kelvinregion{is
main-tained in aryogeni ellat temperatures below 1 K.
In Doyle's trap this temperature is obtained through
a thermal link between the experimental ell and the
mixing hamberofadilutionrefrigerator.
AtomsanbeintroduedintotheellbyeitheraRF
dishargethatdesorbsatomsfrozenontheellwallsor
bylaser ablationof asolidlump. Forhydrogen, alow
temperature RF pulsed disharge would be preferred,
sinethe moleularhydrogensnowisspaed thorough
theellwalls. Atomiuxesof10 12
s 1
and
tempera-turesnogreaterthan10 3
Karetypialvaluesforsuh
disharges. The buer gas serves the purpose of
re-duing the atoms' kineti energy while keeping them
awayfromtheellwallswhere theywouldstik. Sine
the tehnique relies only on elasti ollisions and not
onyli optialtransitionsitisof generalappliation
and an be employed to trap large amounts of more
than70%oftheatoms (thosewith 1
B
), aswell
as moleules. Its eÆienydependsontheratioofthe
magneti energy depth to the sample's initial loading
temperature.
Althoughthedilutionrefrigeratorallowstheellto
bekeptattemperaturesaslowas100mK,itis
expen-sive, fairly large and it is not an user-friendly
equip-ment. Pumping on liquid 4
He one anreah beyond
thesuperuidtransitionat2.2K,or-point. Atypial
1 K pot ryostat regulates its temperature at 1.3 K,
where the vapour pressureis enoughto alloweÆient
theheatonduted bythesuperuidfrom hotterareas
and for ooling the ell. Estimating the eÆieny of
buergasloadingin a1 Kpot environmentonends
alowerperformanethanwiththedilutionrefrigerator
butstilladequatefortheseondphaseoftheATHENA
experiment, where thetrapped hydrogen will serveas
areferene for spetrosopi omparison with the
an-tihydrogen. Theadded benetsof the 1K pot are in
ost,omplexityand turnaroundtime.
B.The apparatus in Rio
InRio weare setting upan apparatus witha 1K
pot to perform buer gas loadingand magneti
trap-ping of paramagneti atomi and moleular speies {
inluding atomi hydrogen { with the aim of further
ooling the trapped samples through fored
evapora-tive ooling[11℄. The ell and the 1 K pot are
on-tainedinsidethemagnetitraponsistingofaspherial
quadrupoletype: twosuperondutingNbTioilsinan
anti-Helmholtzarrangement. Field is zeroat thetrap
enter and rises in all diretions; the trap enter is a
3Dmagnetield minimumandit'saroundthis point
thattheatoms remaintrapped. Themagnet askisa
titaniumsinglepiee,designedtowithstandthebrutal
repulsionfores ofapair of100A opposing oils,and
itisompletelyimmersed in liquidhelium. Thewhole
experiment(ell,1Kpot,superondutingmagnet,
liq-uidheliumreservoir)isontainedinaspeiallydesigned
ryostatasdepitedin Fig.II. It features aliquid
ni-trogenreservoir{toreduethermalondution tothe
liquidhelium{andahighlyonduting,lowemissivity
metallishield thermallyanhoredat 77K,to protet
theheliumenvironmentfromthe300Kblak-body
ra-diation. Optialaesstotheellisthroughaseriesof
optialwindows.
Buer-gasloaded,magnetiallytrappedatomi
hy-drogen is also very interesting from the metrology
point-of-view.Besidesbuildingasystemfordiret
om-parisonofthe1S-2Stransitioninhydrogenand
antihy-drogen,weplan tobuildanothersystemto beusedas
anoptialfrequenystandardbasedonthissame
tran-sition. Todate,themostpreiseoptialfrequeny
mea-surementwasmadeinhydrogenatomibeamsthrough
phase-oherent omparison to a old Cs atomi
foun-tainlok[15℄. Thetrappedhydrogen-basedsystemhas
potential for muh higher resolution than the atomi
beam-basedspetrometeratGarhing.
Magnetitrappingandforedevaporativeoolingof
atomispeies otherthanhydrogenoralaline-metal
-as well as moleules - are also being pursued in our
laboratory. Ourrstattempts shallemployhigh
mag-netimomentspeies,likeeuropium. Formostspeies,
ap-pulsed Nd:YAG laser soure. After loading and
trap-ping, foredevaporativeoolingshall be performed to
redue the sample's temperature from 1 Kto the K
range. Wearespeiallyinterestedinsympathetiev
ap-oration, whereoolingof onespeies forwhih
evapo-ration works ne leadsto oolingof another
(simulta-neouslytrapped)speieswhihisnotsuitedfor
evapo-ration.
Figure2. Depition ofthe 1Kryostat. See text for
dis-ussion.
C. The laser systemfor spetrosopy
Atomi hydrogen has proved to be a great
labo-ratory for tests of the most basi theories of Physis.
Reentspetrosopimeasurementsonthe1S-2S
tran-sitionhashadimpliationsonthemeasurementofthe
Lamb shift[19℄ and on orretions to QED[20℄. This
transition is aessed with laser radiation at 243 nm
whih anbe produed by frequeny doublingfrom a
dye laser at 486 nm as is done at MIT and will be
pursuedatATHENA,orbyfrequenyquadrupliating
fromaTi:saphirelaser,asweareimplementinginRio.
Thehoieforthislasersystemwasditatedbythe
needofexibilitytoaessdierentatomiand
mole-ularspeiesin ourownexperimental program. It
on-sists of a omerial (Coherent, In.) Argon Ion laser
whose output of up to 28 W is used to pump a
Co-herent899-21Titanium:saphirelaserwhihhasits
fre-quenydoubledtwie. TheTi:saphirelaserosilatesin
therange of700nm to 1100nm. Typialpeak power
at 15 W of pump power is 0.6 W, CW single mode,
at 972nm. For therst doublingstage weemployan
externalenhanementavityand aKNbO
3
rystal, as
shematially shownin Fig.3. PotassiumNiobate has
optial transparenyrangingfrom 400to 4500nm. It
presentsthelargestnon-linearoeÆientsofall
omer-iallyavailable inorganimaterial (d
31
=-15.8 pm/V,
d
32
=-18.3pm/V)and,withitsnegativebiaxial
stru-ture,issuitableforawidevarietyoftemperature-tuned
type I non-ritial phase-mathing (NCPM) and type
II,angle tuned, phase-mathing ongurations. It
es-beyondtheNd:YAGwavelength(1.064mm). Wewere
already generating blue light while sanning the
av-ityandwewilltryanewproposedmethodtolokthe
doublingavity[21℄(seeFig.3) afterthelaser's
\intra-avityassembly"isrepairedatthemanufaturer. This
laser systemwill serve for studying other atomi and
moleularspeiesaswell.
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Figure 3. Shematis of the rst optial frequeny
dou-blingavity,showingtheloksheme. Therystalatuseis
KNbO
3 .
The unusually long lifetime of the metastable 2S
level (122ms) makesit possible,at leasttheoretially,
for oneto measure the 1S-2Senergydierene with a
10 18
auray, provided the (anti)atomsare trapped
andold. Theexitationofthe1S-2Stransitionis
per-formed by two-photon absorption. The 2S state an
be quenhed by a small eletri eld whih auses a
mixing with the 2P state resulting in the emission of
aLyman
photon,whih istypiallydetetedina
mi-rohannel plate followed by an amplifying stage and
ountingsystem. Fortheantihydrogenoneanuse
dif-ferent shemes relying on the annihilation signal. As
proposed byP. Boweand J. Hangst[22℄ oneould use
photoionizationoftheexitedatomandeletrially
re-etbaktheantipartileswhilelettingthenon-exited
antiatomsgostraightontoanannihilationwall. Or,in
the ase of trapped antihydrogen, the Lyman
deay
alsoleads,sometimes,toahangeofspinstate,ausing
theatoms to be expelledfrom thetrapand the
anihi-lationsignaltobeseen,asproposedbyoneofus[23℄.
III Towards old antihydrogen
In the last years researhers working at aelerators
have sueded in developing new methods for
manip-ulating antipartiles. Antiprotons and positrons have
beenaptured,slowedandstoredforlongtimesin
ele-tromagneti traps; tehniques for reating antimatter
beams were also devised. Some of these methods are
InCERN'sLEAR,antiprotonshavebeenaptured
and ooledto meV energiesby suitablemethods. The
same shemes are used with CERN's new antiproton
mahine, the Antiproton Deelerator (AD). The
An-tiproton Deelerator (AD) started delivering
antipro-tons to the experiment by mid of 2000. Soon after
the ATHENA team reported apture of antiprotons.
After being slowed down in a beam ounter, the
an-tiprotons passafoildegraderwith adjustableeetive
thikness(asplitSiphotodiode-forrefereneand
en-teringofthebeam)andenter theappropriatePenning
trap, whih had been previously lled with eletrons.
Asantiprotonsosillatebakandforthinthetrapthey
loosetheirenergythroughCoulombollisionswiththe
eletron loud. This apture proess an be repeated
severaltimeswithoutlosingtrapped antiprotons, thus
enhaning thetotal numberof antiprotons to beused
intheexperiment. Withthedumpsignalfromthetrap
thefoilthiknessouldbevariedforoptimization. The
vauum onditions in this rst run of ATHENA was
not in agreat shapeand thelifetime ofthe
antiparti-les are diretlyaetedby anihilationwiththe
bak-ground gas. Typiallythesample ouldlast for hours
inaidealvauumonditions. Intheexperimental
on-dition shownthe sample lifetime wasredued to 10s.
Obviously this an be improved and one of the
ma-jor fators is the temperature of the old nose of the
experiment whih was high and this problem is being
addressedforthe2001run.
A seond important step towards the goals of
ATHENA is the prodution of slow positrons. These
low energy positrons (kineti energies in the MeV
range) are available in a ontrolled way either from
radioative soures or from pair prodution from
bremsstrahlung. Their energies an be further
re-dued uponpenetrationinto amoderator. Lowenergy
positronsarereadilyslowedtothethermallevelanda
frationofitisspontaneouslyemittedinvauumeither
as free positrons oras positronium. Positron
aumu-lation forATHENAhasbeenahievedbytheSwansea
group with the newly built positron aumulator
fol-lowingadesignbyC.Surko'sgroupatU.C.SanDiego.
This design uses a 22
Na soure onto whih surfaeis
deposited a thin layer of solid neon (moderation
eÆ-ienies as high as 10 3
), at ryogeni temperatures.
This system providesthe best demonstrated soureof
thermal positrons. After thesoure and following the
magneti eld lines the positrons enounter dierent
regionswithavaryingpressureofN
2
servingasa
ther-malizingbuergasandasadissipationmehanismfor
loading the positron Penning trap. After initial
trap-ping,buergaspressureisreduedtoallowlongstorage
times. As atypial value ahieved in 2000, an exess
of 4x10 6
positrons were trapped. The positrons
ou-tothewallandtheyquiklythermalizewith thewalls
temperature. Thissystemwill beimprovedbyoveran
order of magnitude this year mainly by the use of a
strongersoure.
The third major stepin the ATHENA
experimen-tal sheme is the transferof the oldpositrons to the
reombination trapnext to the antiproton trap. This
isahievedbyaneletrikikonthepositronsandthe
simultaneousopenningofavauumvalveandthe
puls-ingofthe\transfermagnet". Whiletherewasasignal
learlyindiatingthetransferofthepositronstothe
an-tiprotonregion,withaneÆieny ofabout25%,there
wasnoattempt toretrap thesepartiles. This proess
isbeingpursuednowatCERN.
Afourth majoringredientis theantipartile
anihi-lation detetor. Built with a onentri layer of 192
CsIrystals whose sintilation signals aredeteted by
aphotodiode, andtwolayersof Si-stripdetetors, this
detetor assembly, with its integrated eletronis an
workatliquidNitrogentemperatureandisableof
pro-viding vertex reonstrution for an anihilation event
with 100m radial resolution and 0.7 mm axial
reso-lution. This detetion system should provide the
ex-perimentwith anunambiguosantihydrogendetetion.
The detetor has been fully alibrated and no major
improvementisrequiredforthisyear'srun.
The major milestone eagerly pursued by the
ATHENAexperimentinthenextyearsistheformation
of low-energyantihydrogen. There are a few shemes
whih an be pursued for this reombination and we
still don't know of all the details on their outome.
Whateverreombinationmethodisused,itmustyielda
suÆientnumberofantiatomsforspetrosopy,
prefer-ablyatlowtemperatures(T>1Kshould beavoided
sine this would require prohibitevely high magneti
well depths). These atoms should also be produed
inthegroundstateorinalow-lyingexitedstateand
the whole proess should happen within a short time
period.
ThebasiideafromtheATHENAteamistoshoot
theantiprotonsinaontrollablefashiononthepositron
loudin suh awaythat they would stopin the
mid-dle of it(see Fig. III) . They would then drift slowly
throughthepositron loud, in anon-trapped
ongu-ration,untiltheyreombine orsapeand arereyled
bak to the antiproton trap. This proedure ould,
in priniple, be repeated many times. This method
will ertainly ensure the mixing of the two speies at
the lowest possible relative veloities therefore
garan-teeingthebestpossiblereombinationratefora
spon-taneousreombinationproess. Toformaboundstate
ofpositronandantiprotonstartingfromfreepartiles,
momentumandenergybalanerequiresthepreseneof
e +
+p!H+h ; (1)
There are a few unknowns in this sheme whih
will be studied experimentally. The rst is how
ef-iently an one yle bak the antiprotons and
eÆ-iently keep them old. The seond is how muh of
themagnetronmotionfrom thepositronloudwillget
transferedto the antiprotons and therefore get
trans-feredto theformedantihydrogen. Inpriniple,the
ro-tating energy of the positron plasma is muh larger
than its quoted temperature. If all this energy gets
transferred intokinetienergyofthereombined
anti-hydrogenthehaneofeverapturingenoughofthese
inamagnetitrapisminimal,duetothelowmagneti
energy. On the other hand, if a very small fration,
ornone,of this energyis transfered,theantihydrogen
willomeoutofthepositronloudinalldiretionsbut
withsmallenergy. Thisisnotanidealsituationforany
imaginableexperimentthatwillhavetorelyonlong
in-terationtimesandgooddetetionolletioneÆieny.
A muh better situation would be to have an atomi
beam. In viewof this desirewe haveproposed a
sim-pleshemetogenerateanantiatomibeamaswenow
explain.
A. Formingan atomibeam
With manyentimeters of trapdimensionwehave
enoughspaetoallowthepositronloudtotravelwith
aonstantspeed whiletheantiprotonsare oming
to-wards it. The mostprobablesituation for
reombina-tionwillbewhentheantiprotonsareatrestinthe
mov-ingpositronloudframe. Onetheneutralantiatomis
formeditisnotdraggedbythepositronsanymore.The
positrons are eletrially stopped upstream while the
antihydrogen mayontinuetravellingasaoldatomi
beam. One the non-reombined antiprotons are
re-apturedbakatitsoriginaltraptheproedureanbe
repeated again. One ouldexpet, therefore,a pulsed
atomibeamoperation. Ifoneonsidersa10Kenergy
atomi beamfor alaserexitation experiment,for
ex-ample,thetypialspeedrequiredforthepositronloud
andtheoutgoingantihydrogenatomsisabout400m/s.
If the whole proess is to last a few miliseonds, the
spaerequiredforthepositronloudtomoveisabout
halfameter. Theontrolledmovementofthepositron
loud ould bemade adiabatially by the use of
mul-tipleeletrodesand theontroleletronis istrivialto
buildsineitisaslowsystem.
Ifsuhabeamanbemade,afewimediate
exper-imentsome to mind. First weould easilystudy the
angular dispersionof the beam just by looking at the
anihilationsignalatafarawaywallloselysurrounded
positronloudandthelengthoftime,onewillertainly
beabletoestimateprettywelltheenergydistribution
of theformed antihydrogen. Thirdly,knowingthe
en-ergydistribution oneaneasily onsiderthefeasibility
of performing laser spetrosopy on the 1S-2S, using
thephotoionizationmethodmentionedabove. Finally,
oneanalsoattempttolookforantigravitybythe
de-viation oftheloudfrom thetrajetorythatwouldbe
initiallyfollowedbythepositronsandantiprotonsalong
the magneti eld lines, by simply turning o the
re-etion eletripotential.
Figure4. Oneofthepossiblereombinationshemestobe
usedfortheformationofantihydrogen.
IV Physis with old
antihydro-gen
As pointed out in the introdution, the main goal of
theATHENAexperimentis totestCPTinvarianeas
preisely as possible. This will be possible at a level
of parts in 10 10
as soon asATHENA sueds in
pro-duing a old atomi beam at the energy onditions
disussed aboveand withtheexitation anddetetion
ofafewhundredantiatoms. Higherpreisionisforseen
in phaseII,when theplan is to trapthe old
antihy-drogen. Lateron,aoolingshemewillbepursued.
Antihydrogenatoms produedatlowtemperatures
are very interestingfor gravitational tests. The
mea-surement of the gravitationalproperties of
antihydro-gen is a diret test of the Weak Equivalene
Prin-iple (WEP), the ornerstone of General Relativity.
The most stringent WEP test to date was performed
by omparing the aelerations of dierent types of
matter[24℄, with a preision of 1part in 10 11
, but no
direttestofWEPhaseverbeenperformedusing
anti-matter. Afamousexperiment[6℄hasmeasureddiretly
the gravitational aeleration of eletrons, but its
re-sults were found to be inonlusive beause harged
partiles are very sensitive to stray eletri elds. It
hasbeensuggested[5℄thatantimattermaybehave
dif-ferentlyfrom matter in agravitationaleld. A diret
test of WEP on antipartiles is of fundamental
rele-vane, and antihydrogen is the ideal andidate: not
only is it neutral but also muh more massive than
positrons.Thereareurrentlysomeproposalsfor
an-that, if any of these experiments should everbe
per-formed, it will not be as preise as CPT tests.
Nev-ertheless, it is of fundamental importane to observe
whether antimatterfalls downwith thesame
aelera-tion asmatterorwith adierent aelerationoreven
if antimatter is repelled from ordinary matter; in this
latterase,oneouldtalkaboutantigravity.
V Conlusion
In this paper we presented our ontributions to the
ATHENA experiment, whih has been running for
aboutoneyear. Thisollaborationisaimedat
produ-ingantihydrogenatlowenergiessuitableforhigh
prei-siontestsofCPTinvariane. Testswillrelyonthe
om-parisonbetweenthe1S-2S transition in hydrogenand
antihydrogen,studiedthroughDoppler-freelaser
spe-trosopy. Wealsodesribedtheideaofusingthebuer
gastehniqueforloadinghydrogenintomagnetitraps
usingasimple 4
He-pumpedsystem. Thissystemis
un-derimplementationinRio,inordertomakeaportable
referenehydrogentrapforomparisonwith
antihydro-gen. Theoretialimpliationsoftheexperimentinlude
notonlyCPTviolation{atestoftheStandardModel
{ but alsodiret tests of thegravitationalouplingto
antimatter,atestoftheWeakEquivalenePriniple,a
ornerstoneofGeneralRelativity.
JusthowwellwillATHENAdointhisomparison,
is too early a question. Nature and our sienti
re-soures should still have muh to sayaboutthis very
hallengingandinterestingexperiment!
Aknowledgements
Thisresearh wasin partsupported byCNPqand
PRONEX(Brazilian agenies), andphysisdisussions
with the MIT ultraold hydrogen group, J. Doyle at
Harvard, and with ATHENA ollaboration members
arethankfullyaknowledged.
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