A brief review of antimatter production

4.9 MB·English

by  Y. G. Ma

A brief review of antimatter production
Y. G. Ma ∗,1 J. H. Chen,1 and L. Xue1,2
1Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China
2 University of the Chinese Academy of Sciences, Beijing 100080, China
Inthisarticle,wepresentabriefreviewofthediscoveriesofkindsofantimatterparticles,including
positron(e),antiproton(p),antideuteron(d)andantihelium-3(3He). Specialemphasisisputonthe
discovery of the antihypertriton(3Λ¯H) and antihelium-4 nucleus (4He, or α) which were reported by
theRHIC-STARexperiment very recently. In addition, brief discussions about the effort to search
forantinucleiincosmicraysandstudyofthelongtimeconfinementofthesimplestantimatteratom,
antihydrogenarealsogiven. Moreover,theproductionmechanismofanti-lightnucleiisintroduced.
PACSnumbers: 25.75.Dw,13.85.Ni
3
1
0
1. Introduction exploration and the study of manmade matter such as
2
quarkgluon plasma (QGP)respectively, since the obser-

n 2. Observation of positron vation of anti-proton (p) [15] in 1955. The possibility of
a
the anti-gravity behavior between matter and antimat-
J 3. Observations of antiproton, antideuteron and
ter has been discussed somewhere else [16]. The recent
1 antihelium-3
progress regard the observation of antihypertriton (3H)
2 Λ¯
4. Observation of 3Λ¯H [17] and antihelium-4 (4He, or α) [18] nucleus in high
] energy heavy ion collisions [19–21] reported by RHIC-
x 5. Observation of 4He STARexperimentaswellasthe longtimeconfinementof
e
- antihydrogenatoms [22] based on an antiprotondeceler-
l 6. Effort to search for antinuclei in Cosmic rays ator facility by ALPHA collaboration have already cre-
c
u atedalotofexcitationinbothofthenuclearandparticle
7. Trap of antihydrogen atoms
n physics community. All of the measurements performed
[ 8. Production mechanism of antimatter light nuclei above have implications beyond the fields of their own.
1 Suchas,thestudyofhypernucleusinheavyioncollisions
v 9. Conclusions and perspectives is essential for the understanding of the interaction be-
2 tweennucleonandhyperon(YNinteraction),whichplays
0 an important role in the explanation of the structure of
9 I. INTRODUCTION neutron star. Furthermore, as we learned from heavy
4 ion collisions, the production rate for 4He produced by
.
1 Theidealofantimattercanbetracedbacktotheendof colliding the high energy cosmic rays with interstellar
0 1890s,when Schuster discussed the possibility of the ex- materials is too low to be observed. Even one 4He or
3
istenceofantiatomsaswellasantimattersolarsystemby heavier antinucleus observed in the cosmic rays should
1
hypothesisinhislettertoNaturemagazine[1]. However, be a great hint of the existence of massive antimatter in
:
v the modern concept of antimatter is originated from the the Universe. Finally, the successful trap of antihydro-
Xi negative energy state solution of a quantum-mechanical gen atoms leads to a precise test of the CPT symmetry
equation, which was proposed by Dirac in 1928 [2]. Two law,aswellasameasurementofthe gravitationaleffects
r
a yearslater,Chaofoundthattheabsorptioncoefficientof between antimatter and matter in the future.
hardγ-raysinheavyelementswasmuchlargerthanthat
Inthisarticle,wepresentareviewofkindsofantimat-
was to expected from the Klein-Nishima formula or any
ter particles experimentally, based on the time schedule
other[3,4]. This”abnormal”absorptionisinfactdueto
oftheir firsttime observations. The paperis arrangedas
theproductionofthepairofelectronanditsanti-partner,
follows, in section 2, we will take a look back to the dis-
so-called positron. Therefore Chao’s experiment is the
coveriesofpositronandantiproton. Insection3,thefirst
first indirect observation of the first anti-matter parti-
time observations of antideuteron and antihelium-3 will
cle, namely positron, in the history. Another two years
bediscussed. Section4presentsabriefreviewofthe for-
later, Anderson observed positron with a cloud chamber
mation and observation of 3H through their secondary
[5]. Antimatter nuclei such as d , 3H , 3He have been Λ¯
vertex reconstructions via decay channel 3H →3He +
widely studied in both cosmic rays [6–8] and accelera- Λ¯
π+ with a branch ratio of 25% in high energy heavy ion
tor experiments [9–14] for the purposes of dark matter
collisions. In Section 5, we have discussion of the parti-
cle identificationof4He nucleus by measuringtheir mass
value directly with the fully installed detector Time Of
∗Author to whom correspondence should be addressed: Flight (TOF) at RHIC-STAR. In Section 6, we discuss
[email protected] theeffortofsearchingforantinucleiincosmos. InSection
2
7,thelongtimetrapofantihydrogenatomsperformedby well as their curvatures in the chamber. Figure 1 shows
ALPHAexperimentisintroduced. Inthelastsection,we that, a positron reduces its energy by passing through a
have a brief discussion on antimatter nuclei production 6 mm lead plate in the cloud chamber. The track length
mechanism. Finally we give a simple summary and out- from the upper part of the cloud chamber can only be
look. interpreted with the observation of positron.
II. DISCOVERIES OF POSITRON AND
ANTIPROTON
In 1930, C. Y. Chao performed a few γ-ray scattering
experiments on different elements [3, 4]. γ-rays from Th
C after being filtered through 2.7cm of Pb were used as
the primary beam. Al and Pb were chosen as the repre-
sensitivesofthelightandtheheavyelements. ForAlthe
γ-scatteringis,withinexperimentalerror,thatpredicted
by the Klien-Nishima formula which assumes that the
removal of the energy from the primary beam is entirely
due to Comptonscatteringofthe extranuclearelectrons.
However,forPbadditionalscatteringrayswereobserved.
III. OBSERVATIONS OF ANTIPROTON,
The wavelength and space distribution of these are in-
ANTIDEUTERON AND ANTIHELIUM-3
consistent with an extranuclear scatterer [3, 4]. Later
on, this abnormal absorption was identified as the out-
comeofthe processofelectron-positronpairproduction.
Thereforethisexperimentwasalsothefirstexperimental
indictionofthefirstanti-matterparticle,positron,inthe Physicists had expanded their understanding of the
observation history for the antimatter. natural world, and took into consideration that every
———————————————————————– particle should have their antimatter partner after the
discovery of positron. They were able to expand their
knowledge of antimatter with the development of the
technology of accelerators, while the development of
Time-of-Flightdetector systemplayedanimportantrole
in the following identification of antimatter particles. In
theyear1955,O.ChamberlainandE.Segr`efromUniver-
sity of California reported their observation of antipro-
tons based on the Bevatron facility [15]. Antiprotons
were produced and scattered into the forward direction
by projecting a bunch of protonbeam to the copper tar-
getatBevatron. Byobservingtimesofflightforantipro-
tons,itmakesmoremeaningfulbythe factthatthe elec-
tronicgatetimeisconsiderablylongerthanthespreadof
observed antiproton flight times. The electronic equip-
mentacceptseventsthatarewithin±6millimicroseconds
of the right flight time for antiprotons, while the actual
antiprotontracesrecordedshowagroupingoffighttimes
to ± 1 or 2 millimicroseconds. Figure 2(a) and 2(b)
depicts a histogram of meson flight times and that of
Fig.1 Aphotographforthediscoveryofpositronfromreference antiproton flight times, respectively. Accidental coinci-
[5]. Thecurvaturestandsforacomicraypassingthrougha6mm dences accountfor many ofthe sweeps (about2/3of the
lead plate with a reduction of its energy. The length of the latter sweeps) during the runs designed to detect antiprotons.
part can only be interpreted with the appearance of antimatter A histogram of the apparent flight times of accidental
electron. coincidences is shown in Fig. 2(c). It will be noticed
that the accidental coincidences do not show the close
Two years later, C. D. Anderson identified 15 positive grouping of flight times characteristic of the antiproton
tracks by photographing the cosmic rays with a vertical or meson flight times. In total, 60 antiprotons were de-
WilsonchamberinAugust,1932[5]. Theunknownparti- tected, with their mass measured and found to be equal
cleswererecognizedasthe predictedantimatterelectron toprotonswithinuncertaintiesbasedonthedetectorsat
(positron)afteranalyzingtheirenergyloss,ionization,as Bevatron [15].
3
25 25 25
(a) (b) (c)
Negative Particles Negative Particles Negative Particles
20 P0 = 5.0 GeV/c 20 P0 = 5.4 GeV/c 20 P0 = 6.0 GeV/c
15 15 15
nts
u
o
C
10 10 10
5 5 5
0 0 0
10 15 20 25 30 35 40 4510 15 20 25 30 35 40 4510 15 20 25 30 35 40 45
Fig. 3 The evidence for the observation of antideuteron from
Ref. [9]. (a)(b)(c)showsthetime-of-flightdistributionforparti-
cles between counters S2 and S9 with 0.56 ns per channel. Taken
fromRef.[9].
1
p
10-2
10-4
o
ati 10-6 d
R
10-8
10-10 3He
Fig. 2 (a) Histogram of meson times used for calibration. (b) 10-120 1 2 3 4
Histogram of antiproton flight times. (c) Apparent flight times of Atomic Number
arepresentativeaccidentalcoincidences. Timesofflightareinunits
of10−9 s. Theordinatesshowthenumberofeventsineach10−10
Fig. 4 Atomic number dependence of the relative production of
secondintervals. TakenfromRef.[15].
antimatternucleiwithrespecttopionwithfixedmomentumat20
GeV/candθ equalto27mrad. TakenfromRef.[10].
Experiments with antimatter nuclei heavier than an-
tideuteronwereimplementedattheInstituteofHighEn-
ergy Physics, Russia. Five antihelium-3 [10] were pro-
Antideuteron was observed by Alternating Gradient duced by colliding the 70GeV proton beams with alu-
Synchrotron (AGS) [9] at BNL and later confirmed by minum target. The calculated charge is (0.99±0.03)2e,
Proton Synchrotron(PS) [23] at CERN in 1965, both of whilethemeasuredmassequalsto(1.00±0.03)3m . The
p
the experiments were implemented by colliding protons differentialproductioncrosssectionofantihelium-3 with
withberyllium. Thedatawascollectedwithasetoftime momentum equals to 20 GeV/c at θ = 27mrad was cal-
offlightsystemS S (210ft) andS S (170 ft) atAGS. culated and found to be 2.0×10−35cm2/sr·GeV/c per
1 10 2 9
Figure3(a),(b),(c)showthetraveltimeofantideuterons aluminumnucleus. Whilethedifferentialcrosssectionfor
between detectors S and S at different momentum re- π− is 1.5×10−25cm2/sr·GeV/c per aluminum nucleus.
2 9
gion. The time to channel calibration is 0.56 ns per Theratiobetweendifferentialcrosssectionofantihelium-
channel. The peak positions from Figure 3(a),(b),(c) 3 and pion is 1.3 × 10−10. Figure 4 shows an atomic
move from channel 21.5 at p = 6 GeV/c to channel 23.5 numberdependence ofthe ratios. The ratiosdecreaseby
at 5.4 GeV/c, and to channel 26 at 5.0 GeV/c, corre- a magnitude of four orders with each additional nucleon
sponding to ∆β/∆p = (1.6 ± 0.4) × 10−2(MeV/c)−1. added. By extrapolating the distribution to larger mass
The calculated mass equals to 1.86 GeV/c via formula area, the relative cross section of antihelium-4 is about
dβ/dp=β3m2/p3, and interpreted to be antideuteron. 10−14.
4
IV. OBSERVATION OF 3Λ¯H A 350 B 140
300 120
s250 s 100
unt200 unt 80
Co150 Co 60
100 signal candidates 40 signal candidates
Unlike the normal nuclei (anti-)nuclei, (anti- 50 rsoigtantaeld+ bbaacckkggrroouunndd fit 20 rsoigtantaeld+ bbaacckkggrroouunndd fit
)hypernucleus includes the (anti-)strange quark degree 0 0
2.95 3 3.05 3.1 2.95 3 3.05 3.1
of freedom, of which the typical one is Λ-hypernucleus. 3He + π- Invariant mass (GeV/c 2 ) 3 He + π+ Invariant mass (GeV/c 2 )
C D
Thesimplesthypernucleusobservedsofarishypertriton, 30
m) Expected 〈dE/dx〉 400 rigidity>1 GeV/c
wΛ-hhicyhpeirsocno.mpHoysepderonfucolneeusnepurtorvoind,esonaenpridoetoanl eannvdiroonne- eV/c 20 3πHe unts 300 33HHee
K o 200
ment to study the hyperon-nucleon (YN) interaction, 〉 (x 10 C 100
rwehspicohnsiisbleofinfunpdaarmt efnotralthinetebreinstdining noufclheyaprerpnhuycsliecis, 〈dE/d 00 1 2 3 4 0-0.4 -0.2 0 0.2 0.4
Rigidity (GeV/c) z( 3He)
and nuclear astrophysics. In previous research history,
quite lot hypernuclei have been discovered, even for the
Fig. 5 (A and B) Reconstructed invariant mass distribution of
observation of double-Λ hypernucleus [24], but still no 3Heandπ,opencirclesstandforthesignaldistribution,whilesolid
anti-hypernucleus was observed until the STAR collab-
lines are the rotated combination background. Blue dashed lines
oration announced the first anti-matter hypernucleus,
are the Gaussian (signal) plus double exponential (background)
i.e. 3H[17], in 2010. The observation of 3H can be
Λ¯ Λ¯ function fit to the distribution. (C) hdE/dxi as a function of
achieved by reconstructing their secondary vertex via
rigidity (p/|Z|) for negative particles, theoretical hdE/dxi value
3Λ¯H →3He + π+ [17, 19]. The data used for analysis for3He andπ arealsoplotted. (D) shows that aclean 3He and
was collected by STAR experiment at Relativistic 3He samplecanbeobtainedwithcut|z(3He)|<0.2. Takenfrom
Heavy Ion Collider (RHIC), using the cylindrical Time
theRef.[17].
Projection Chamber (TPC), which is 4 meters in the
diameter direction and 4.2 meters long in the beamline
direction [25]. TPC is able to reconstruct charged
A B 450
tracks between ±1.8 units of pseudo-rapidity with full 2.52 cτ = 5.5 ± 12..47 cm 400 PR180,1307(196P9)RD1,
raahsdihzcgEhioimwdi/eiusvdttyexhh.didaETilnbh/ayTednxPgdciliCoefff.roerwrreeilTtnnahttheigetnbahgatiendiivrdetehnsmettsiiarfitragaccnnaikeodtstinoifcvionzerraritosgdiufioidffsntietrtryahee.cennkFtesmrikggaciuynagrndnelseo4tbosCiecsf Count 103 χ2 01..55013 4χ25 c= τ61 .(07cχ8m28 =) 90.100811 3H lifetime (ps)Λ 122335050500000 81P9R(L12906,8)4N6P(1B9167660,()1927N609)P(B16977S,3T)AR
particles. Figure 5 is the distribution of a new variable, Λ 100 fSrTeeA RΛ f(rPeDeG Λ)
z3H=e Lannd(hd3EHe/d,xhi/ehrdeEhd/Edx/idBx)i,Bwhisichtheisexupseedctetdo vidaelunetifoyf 102 Λ35H 10 15 20 25 500-1PR1036, 61B(19624) 3 4DGalolict5zk,l e1,9 1692698 7
hdE/dxi. Approximately, a sample of 5810 3He and decay-length/(βγ) (cm) World data
2168 3He were collected for the secondary vertex
reconstruction of 3Λ¯H. Fig.6 (A)Theyieldsof3Λ¯H (solidsquares)andΛ(opencircles)
vs cτ distribution. The solid lines stand for the cτ fits, and the
insertplotdescribes χ2 distributionof the bestfits. (B) Compar-
isonbetween the presentmeasurement and theoretical calculation
Topological cuts including the distance between two
[26,27],aswellasthepreviousmeasurements[28–33]. Takenfrom
daughter tracks 3He and π+ (<1cm), distance of clos-
theRef.[17].
est approach (DCA) between 3H and primary vertex
Λ¯
(π<t1rcamck),(d>e0c.a8ycmle)n,gathreoefm3Λ¯pHlo(y>e2d.4tcomp),roamndottehtehDeCsiAgnoafl The measurement of 3ΛH (3Λ¯H) lifetime provides us an
effectivetooltounderstandtheY(Λ)-N(p,n)interactions
to background ratio. The invariant mass of 3H and
Λ [26, 27]. And, the secondary vertex reconstruction of
3Λ¯H were calculatedbase onthe conservationof momen- 3ΛH (3Λ¯H) makes us to be able to perform a calculationof
tum and energy in the decay process. The results are its lifetime, via equation N(t) = N(0)exp(−t/τ), where
shown in Figure 5A for 3ΛH and Figure 4B for 3Λ¯H . The t = l/(βγc), βγc = p/m, l is the decay length of 3ΛH,
successfully reproduced combinatorial background with p is their momentum, m is their mass value, while c is
a rotationstrategy can be described by double exponen- the speed of light. 3H and 3H samples are combined to-
Λ Λ¯
tialfunction: f(x)∝exp[−(x/p1)]−exp[−(x/p2)],where gether to get a better statistics, with the assumption of
x=m−m(3He)−m(π), and p1,p2 are the parameters. the same lifetime of 3ΛH and 3Λ¯H base on the CPT sym-
Finally,thesignalsarecountedbysubtractingthedouble metry theory. The measured yield is corrected with the
exponential backgroundof 3ΛH and 3Λ¯H . In total, 157 ± trackingefficiencyandacceptanceofTPC,aswellasthe
30 of 3H and 70 ± 17 of 3H are observed. reconstructionefficiencyof3H and3H. Then,thel/(βγ)
Λ Λ¯ Λ Λ¯
5
distributioncanbefittedwithanexponentialfunctionto tionaltotheparticleidentificationmethodbycombining
extract the lifetime parameter cτ. The best fitting with energyloss(hdE/dxi) andrigidityprovidedbyTPC,the
χ2 minimization method result in cτ = 5.5+2.7 ±0.08, observationof4Henucleus reliesonthemeasuredtravel-
−1.4
which corresponds a lifetime of 182+89±27 ps as shown ing time oftracks givenby the barrelTOF[39], whichis
−45
inFigure6showsacomparisonbetweenthepresentmea- composed of 120 trays, surrounding the TPC. With the
surement and theoretical calculation [26, 27], as well as barrelTOF,themassvalueofparticlescanbecalculated
the previous measurements [28–33]. It seems that the via m2 = p2(t2/L2−1) (where t and L are the time of
present measurement of 3H lifetime is consistent with flightandpathlengthoftracks,respectively)forparticle
Λ
calculationwithphenomenological3H wavefunction[26] identification. On the other hand, the online high level
Λ
and a more recent three-body calculation [27]. The re- trigger (HLT) was employed to perform preferential se-
sult is also comparable with the lifetime of free Λ within lection of collisions, which contains tracks with charge
errors. Ze = ±2e for fast analysis. The trigger efficiency for
4He is about 70% with respect to offline reconstruction,
with selection rate less than 0.4%. Figure 8 presents the
1.6 hdE/dxiversusrigidity(p/|Z|)distribution. Thecolored
bands stand for the helium sample collected by HLT. A
1.4
cut of the DCA less than 3 cm for negative tracks (0.5
3H
1.21 3He 3HΛ3eH × Λp 3HΛe × Λp cIsnemptahfroeartlepefdotsfpirtaoivnmeel3t,rH4aec4kHaste)tcihsaenuldsoeiwddamttoeosmraerejeenctitudmtehnetriefibgeaidocknag.nrWdouwhniedllel.
tio 3H a clear 4He signal is presented and centered around the
Ra 0.8 expected hdE/dxi value of 4He in the right panel.
0.6 Λ3H
3He × Λ 8800 80
0.4 p Negative Particles Positive Particles
0.02 4H1eΛ40H × Λps (GeV1)02 103 〉〈 (keV/cm) dE/dx 246246000000 4He3He 3H d p Kπ--246000π+K+pdH3 He3He4
NN 00-03.5 2 10 0.5 0 -0.5 01 2 30.5
p/|Z| (GeV/c)
Fig. 7 Particle ratios versus center of mass energy per nucleon-
nucleon collision. The data points besides this measurement are
Fig. 8 hdE/dxi as a function of p/|Z|) for negatively charged
taken from Refs. [36–38]. Only statistical error are presented in
particles(leftpanel)andpositivelychargedparticles(rightpanel).
theplot. TakenfromtheRef.[17].
The black curves represent the expected values for each particle
species. The lower edges of the colored bands correspond to the
High production rate of 3H (3H) due to equilibration
Λ Λ¯ HLT’sonlinecalculation of3σ below thehdE/dxi bandcenter for
amongstrangequarksandlightquarks(u,d) isproposed
3He. The grey bands indicate the hdE/dxi of deuteron, proton,
to be a signature of the formation of QGP [17, 34]. The
kaon,pionfromMinimumbiasevents at200GeV. Takenfromthe
baryonstrangenesscorrelationfactorcanbeextractedby
Ref.[18].
comparingtheyieldsof3H and3He. Ontheotherhand,
Λ
a recent calculation [35] indicates that the strangeness
population factor, S =3 H/(3He × Λ/p) is an effect The hdE/dxi of 3He (3He) and 4He (4He ) merge to-
3 Λ
tool to distinguish QGP phase and pure hadronic phase. getherathighermomentumregion,andnσdE/dx,whichis
Figure7 depicts the excitationfunction ofparticle ratios defined as n = 1 ln(hdE/dxi/hdE/dxiB) (R is the
σdE/dx R
forthe STARdataandotherpreviousmeasurement[36– resolution of hdE/dxi) is used for further particle iden-
38]. The value of S3 is near unity at RHIC energy,while tification. Figure 9 shows the combined particle identi-
thesamefactorisonly1/3atAGSenergy,indicatingthat fication with n and mass2/Z2 value distribution.
σdE/dx
thephasespacepopulationofstrangenessarecomparable Two clusters of 4He and 4He located at n = 0,
with light quarks at RHIC. σdE/dx
mass2/Z2 = 3.48 (GeV/c2)2 can be clearly separated
from 3He and 3He as well as 3H and 3H are presented in
the top panel and bottom panel. By counting 4He sig-
V. OBSERVATION OF 4He nal with the cuts window −2 < n < 3 and 2.82
σdE/dx
(GeV/c2)2 < mass2/Z2 < 4.08 (GeV/c2)2 as indicated
The STAR collaboration also reported its observation in the top panel, 16 4He candidates are identified. To-
of4Henucleus[18,20]inApril2011,with10billiongold- gether with 2 4He candidates detected by TPC alone in
gold collisions taken in the year 2007 and 2010. In addi- theyear2007whichispresentedinthefigure,184Hecan-
6
didates are observed by the STAR experiment. So far, or in BESS-PolarII data among 4.0 × 107 |Z|= 2 nuclei
4He is the heaviest antimatter nucleus observed in the from 1.0 to 14 GV [42]. They derived an upper limit of
world. Right after the public report of 4He from the 1.9 × 10−4 (m2 s srGeV/nucleon)−1 for the differential
STAR collaboration,the LHC-ALICE collaborationalso flux of cosmic-ray antideuterons, at the 95% confidence
claimed the observation of 4 4He particles [40]. level, between 0.17 and 1.15 GeV/nucleon at the top of
the atmosphere [41]. For antihelium, assuming antihe-
lium to have the same spectral shape as helium, a 95%
confidenceupperlimittothepossibleabundanceofanti-
44 helium relative to helium of 6.9 × 10−8 was determined
22
00 combining all BESS data, including the two BESS-Polar
-2-2 4He flights. With no assumed antihelium spectrum and a
-4-4 weighted averageof the lowest antihelium efficiencies for
-6-6 each flight, an upper limit of 1.0 × 10−7 from 1.6 to 14
-8-8
-1-010 3He GV was determined for the combined BESS-Polar data.
dx-1-212 3H Under both antihelium spectral assumptions, these are
E/-1-414 thelowestlimitsobtainedtodate[41]. Fig. 12showsthe
σd 44 2 4 6 8 10 12 newupperlimitsofantihelium/heliumfromtheBESSex-
n 22
periment[41]. It seemsverydifficult to huntanti-helium
00
-2-2 4He in cosmos.
-4-4
-6-6
-8-8
-1-010 3He
-1-212 3H
-1-414
-1-616 22 44 66 88 1100 1122
mass2/Z2 (GeV/c2)2
Fig. 9 Top(bottom)panelshowsthenσ versusmass2/Z2
dE/dx
distributionfornegative(positive)particles. Thehorizontaldashed
lines mark the nσ =0, while the vertical ones stand for the
dE/dx
theoretical massvaluesof3He(3He)and4He(4He). Thesignalsof
4Heand4Hearecountedinthecutswindowof−2<nσ <3.
dE/dx
and2.82(GeV/c2)2<mass2/Z2<4.08(GeV/c2)2.
VI. EFFORT TO SEARCH FOR ANTINUCLEI
IN COSMIC RAYS
As we discussed in above sections, most efforts on
hunting antinuclei were realized in high-energy nuclear
physics laboratories. Nevertheless, it is still a dream
to capture any antinucleus in cosmos. The search of
4He and heavier antinucleus in universe is one of the
majormotivations ofspaceshuttle basedapparatussuch
Fig. 10 Thenewupperlimitsofantihelium/heliumattheTOA
astheAlphaMagneticSpectrometer[6],boththeRHIC-
calculated assuming the same energy spectrum for He as for He
STARexperimentalresultandmodelcalculationprovide
with previous experimental results. The limit calculated with no
a background estimation of 4He for the future obser-
spectralassumptionisabout25%higher. TakenfromRef.[42].
vation in Cosmos radiations [18]. Recently, the effort
to search for the Cosmic-Ray Antideuterons and Anti-
helium by the Balloon-borne Experiment with Super-
conducting Spectrometer (BESS) collaborationhas been
VII. TRAP OF ANTIHYDROGEN ATOMS
made [41, 42]. However, neither Antideuterons candi-
date was found using data collected during four BESS
balloon flights from 1997 to 2000 [41], nor Antihelium Parallelto huge efforts of searching for the antimatter
candidate was found in BESS-Polar I data among 8.4 × nuclei in the cosmos and laboratory, some scientists are
106 |Z| = 2 nuclei from 1.0 to 20 GV (absolute rigidity) thinking how to build the antimatter atom and make it
7
trapped. Antihydrogen, the simplest form of antiatoms, [14, 50, 51]. The production probability is described by
whichistheboundstateofanantiprotonandapositron, Equ. (2), where Ed3N stands for the invariant yield of
d3p
was reported to be produced by the LEAR in 1996 [43]
nucleonsor (anti)nucleus, Z is the atomic number. And,
and at low energies at CERN (the European Organiza-
p ,p ,p are the momentum of (anti)nucleus, protons
A p n
tion for Nuclear Research) since 2002 [44, 45]. Antihy-
and neutrons, with p = A×p is assumed. B is the
A p A
drogenisofinterestforuseinaprecisiontestofnature’s
coalescence parameter.
fundamentalsymmetrieswhichisoneofthemostimpor-
tant projects of experiments in physics. In the standard
model of particle physics, all properties of a particular
physical process should be identical under the operation
of charge conjugation, parity reflection, and time rever- 1
sal (CPT). In nuclear physics, all properties of hydrogen
including the fine structure and hyperfine structure are
knownatahighprecision. Aprecisecomparisonbetween 10-1
o
antihydrogenandhydrogenspectrumisonethebestways ati
R
to measure the possible CPT violation. The first con-
finementofantihydrogenwitha traptime of172mswas 10-2 PHENIX data
STAR data
demonstrated by the ALPHA experiment in 2010 [46] Coalescence model
based on the antiproton decelerator facility. The most Thermal model
10-3
recent results with a confine time of 1000s was present
by the same collaboration [47], which is a big step to-
wards the measurement of the antihydrogen properties. p/p d/d Λ3H/Λ3H 3He/3He4He/4He4He/3He4He/3HeΛ3H/3HeΛ3H/3He
On the other hand, the mutual repulsive force between
antimatter and matter (anti-gravity) was proposed, and Fig. 11 The comparison of particle ratios between data and
manytheoreticalworkshavebeendone [16,48]since the model calculations. The data points are taken from the STAR
first observation of antimatter. However, no experimen- and the PHENIX experiments [13, 17, 18, 52]. The coalescent re-
talresultcanbe obtaineduntil now. The successfultrap sults are based on naive coalescence algorithm with a momentum
difference lower than 100MeV and a coordinator space difference
ofantihydrogenatomsprovidesamethodtomeasurethe
less than 2R (R is the nuclear force radius), while the thermal
gravitational effects by reducing the antihydrogen tem-
predicationistakenfrom[49]. TakenfromRef.[54].
peratures to the sub mK level in the way of adiabatic
cooling.
d3N gV
E A = E e−mpA/T, (1)
Ad3P (2π)3 A
A
VIII. PRODUCTION MECHANISMS OF
ANTIMATTER LIGHT NUCLEI
d3N d3N d3N
E A =B (E p)Z(E n)A−Z. (2)
Ad3P A pd3P nd3P
A p n
Antimatter particles including
e ,p ,d ,3He ,3H ,4He and antihydrogen atoms were Relative particle production abundance of (anti)nucleus
Λ¯
observed in the past eighty years based on different are explored based on thermal model and coalescence
kinds of sources anddetectors. Most of these antimatter model, and compared with data taken at RHIC. Fig-
particles were produced by nucleon-nucleon reactions, ure 11 shows the particle ratios of (anti)nucleus, both
where their production rate can be described by both thermal model and coalescence model can fit the antin-
thermodynamic model and coalescence model. In ucleus to nucleus ratios at RHIC energy. While
thermodynamic model, the system created is charac- the coalescence model has a better description for
terized by the chemical freeze-out temperature (T ), 3H/3He and 3H/3He than thermal model [54]. In a
ch Λ Λ¯
kinetic freeze-out temperature (T ), as well as the microscopic picture, both coalescent and thermal pro-
kin
baryon and strangeness chemical potential µ and µ , duction of (anti)nucleus predict an exponential trend
B S
respectively. (Anti)nucleus is regarded as an object for the production rate as a function of baryon num-
with energy E = Am (A is the atomic mass number, ber. The exponential behavior of (anti)nucleus produc-
A p
m is the mass of proton) emitted by the fireball [49]. tion rate in nuclear nuclear reaction has been mani-
p
The production rate are proportional to the Boltzmann fested in Figure 12, which depicts the invariant yields
factor e−mpA/T as shown in Equ. (1), where PA and (d2N/(2πpTdpTdy)) evaluated at the average transverse
g are the momentum and degeneracy of (anti)nucleus, momentum(p /|B|=0.875GeV/c)regionversusbaryon
T
V is the volume of the fireball. In coalescence picture, numberdistribution. Thesolidsymbolsrepresentresults
(anti)nucleusisformedbycoalescenceatthelaststageof reproducedbythecoalescencemodel,whichfitsthedata
the systemevolutionsincethereexistsstrongcorrelation points very well. By fitting the model calculation with
between the constituent nucleons in their phase space an exponential function e−r|B|, a reduction rate of 1692
8
(1285) can be obtained for each additional antinucleon [4, 5], antihelium-4 which is the heaviest antimatter nu-
(nucleon) added to antinucleus (nucleus), compared to cleusobservedsofar[18]aswellasanti-hypertritonwhich
1.6+1.0×103(1.1+0.3×103)fornucleusand(antinucleus) is the first antimatter hypernucleus [17]. With the in-
−0.6 −0.2
obtainedbytheSTARexperiment. Theyieldofnextsta- creasing of mass of antimatter particles, the difficulty
ble antinucleus (antilithium-6) is predicted to be reduce of observation becomes much larger. With the increase
by a factor of 2.6×106 compare to 4He , and is impossi- of accelerator technology and beam energy, the detec-
bletobeproducedwithincurrentacceleratortechnology. tion of heavier antimatter nuclei becomes feasible. Anti-
The excitation of (anti)nucleus from a highly correlated hypertriton and antihelium-4 are two good examples for
vacuum was discussed in reference [53]. This new pro- antimatter detection with the present relativistic heavy
duction mechanism can be tested with the measurement ion collision facility. In the viewpoint of antimatter pro-
of the production rate of (anti)nucleus, any deviation of duction, thermal model and coalescence model can ba-
theproductionrateof(anti)nucleusfromusualreduction sically describe the production yield of antimatter and
rate,mayindicatetheexistofthedirectexcitationmech- antimatter-matter ratio. In our recent calculation based
anism. On the other hand, the low production rate of on the hydrodynamic motivated BlastWave model cou-
4He antinucleus created by nuclear interaction indicate plewithacoalescencemodelatRHICenergy,wedemon-
that any observationof 4He or even heavier antinucleus strate that the current approach can reproduce the dif-
should be a great hint of the existence of large amount ferential invariant yields and relative production abun-
of antimatter somewhere in the Universe. dances of light antinuclei and antihypernuclei [54]. The
exponential behavior of the differential invariant yields
versus baryon number distribution is studied. By ex-
102 trapolating the distribution to B = -6 region, the pro-
2) 10 p p duction rate of 6Li in high energy heavy ion collisions
V
e 1 is about 10−16, and seems impracticably to be observed
G 10-1
2/y (c 1100--32 d d wSeitch.in5,ctuhrereonbtsearcvcaetleiornatoofr4tHecehnaonlodgye.veAnsheaadvdireerssaendtinin-
d 10-4
T uclei in Cosmic rays is a great hint of the existence of
dpT 1100--65 3He 3He massive antimatter in Universe. The model calculations
πpN/2 1100--87 4He SCToAaRle sdcaetnace 4He aconldliseixopnesrcimanenstiamlumlaetaesuthreeminetnetrsaicntihoingshbeentewrgeeynhheaigvhyeionn-
2d 10-9
10-10 ergy protons and interstellar materials. Thus our results
10-11-6 -4 -2 0 2 4 6 provide a goodbackgroundestimation for the future ob-
Baryon Number servationof 4He andevenheavierantinucleiin Universe.
Fig. 12 Invariant yields d2N/(2πpTdpTdy) of (anti)nucleus at
the average transverse momentum region (pT/|B|= 0.875GeV/c)
asafunctionofbaryonnumber(B). Theopensymbolsrepresents Acknowledgements This work is partially supported by the
thedatapointsextractedbytheSTARexperimentatRHICenergy, NSFC under contracts No. 11035009, 11220101005, 10875160,
while solid ones are reproduced by coalescence model. The lines 10805067 and 10975174, 11275250 and 10905085, the Knowledge
representtheexponential fitforourcoalescence resultsofpositive Innovation Project of Chinese Academy of Sciences under Grant
particles (right) and negative particles (left) with formulae−r|B|. No. KJCX2-EW-N01,andDr. J.H.Chenispartiallysupportedby
TakenfromRef.[54]. theShanghaiRisingStarProjectunderGrandNo. 11QA1408000.
IX. CONCLUSIONS AND PERSPECTIVES
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