Table Of ContentAnomalous Internal Pair Conversion
Signaling Elusive Light Neutral Particles
Fokke de Boer
NationaalInstituutvoorKernfysicaenHoge-EnergieFysica.
6
NL-1098SJAmsterdam,TheNetherlands
0
0
2 Abstract. In this paper we report on a systematic search for a neutral boson in the mass range
n between5and15MeV/c2inthedecayofhighlyexcitednuclei.Itssignatureisfoundadeviationin
a theangularcorrelationofthee+e− pairsfromconventionalinternalpairconversion(IPC)resulting
J from of its two-body decay kinematics. With an e+e− pair-spectrometer,a number of transitions
6 hasbeeninvestigatedinthea –nuclei8Be,12Cand16O,followinglightioninducedreactionsatlow
bombardingenergies,first atIKFinFrankfurtandduringthelast yearsatATOMKIin Debrecen.
2 Startlingly, in all isoscalar transitionsexcess e+e− pairs are foundat large angles with branching
v ratioswith respectto the totalyield rangingfrom10−2 to 10−6. If these deviationsareall related
9
tothetwo-bodydecayofanX-boson,thisobservationimpliespluralX-bosons.Ananalysisofall
4
angular spectra with a boson search program, yields a pandemoniumof more than ten candidate
0
bosons.
1
1
5
0
Introduction
/
h
p
- Elusiveneutral X-bosonscan beemitted in anuclear transitionwithintheconstraints
p
of spin-parity, isospin and energy-momentum conservation. Superallowed (L=0) emis-
e
h sion is expected for a pseudoscalar 0− particle in a 0−→0+ M0 transition, a scalar
v: particle0+ inan0+→0+ E0,anaxialvector1+ inan M1andavector1− particleinan
i E1transition.Allowed(L=1) emissionoccurs for0+ and 0− particlesin E1 and M1 re-
X
spectively,andfor1+ and1− inE1,M1butalsoinE2andM2transitions.Thesignature
r
a of X-boson emission is expected to be the two-body decay into e+e− pairs superposed
on conventional internal pair conversion (IPC) [1], but with a marked difference in the
distributionofthee+e− openingangle.
Since some time a search is carried out for such a neutral particle in high-energy
transitions in self-conjugate a -nuclei. Since 1990, experiments have been performed
at the ’small’ Van de Graaff accelerator of the Institut für Kernphysik in Frankfurt,
Germanyandsincetheyear2000attheAVFcyclotronandtheVandeGraaffaccelerator
at ATOMKI in Debrecen, Hungary. In all experiments the same compact spectrometer
was used, consisting of eight E-D E detectors, allowing a good compromise between
angulargranularity,stoppingpower,rangeand pair-efficiency [2].
A signal which couldoriginatefrom a bosonwithan invariantmass of9 MeV/c2 has
been observedintheM1transitionsdepopulatingthe17.64MeV, 1+ levelin8Beto the
ground-andfirstexcitedstate.InTable1ofrefs.[6,5,7],theresultsarecollectedwhich
arerelevanttothisanomalyforthea -nuclei4Heto20Ne,likethebranchingratioB ,the
X
width G , the coupling strength a and the m for the different transitions. It appears
X X X
AnomalousInternalPairConversion SignalingElusiveLightNeutralParticles 1
that isoscalar magnetic transitions and a possible 0−→0+ transition exhibit an excess
of e+e− pairs at large opening angles, whereas no deviations occur in isovectorE1 and
M1transitions.ThedeviationsinisoscalarM1transitionsindicateanisoscalarcharacter
for such a boson with spin-parity 0− or 1+, and coupling strength a proportional to
X
the isoscalar strength of the M1 transition. The sensitivity of these X-boson searches—
expressedintheratioofG X toG g –variesbetween 10−6 and10−2.
In order to test this X-boson scenario, we started a dedicated search of e+e− angu-
lar correlations in isoscalar magnetic transitions with inelastic proton scattering experi-
ments on 12C and 16O, the (p,a ) reaction on 19F and the (3He,pe+e−) reaction on 10B
and 14N targets. The first aim was to check the signal observed [7] for the isoscalar M1
groundstatetransitionfromtheT=0 levelat 12.71MeV in12C at ahighlevelofsignif-
icance and the second aim to look for the 9 MeV/c2 boson in the ’forbidden’ 0−→0+
(M0)transitionat10.96MeVin16O.Themereobservationofe+e− decaytotheground
stateimposesastraightforwardboundonan X-bosonemissionwidth.
Experiments
ExperimentsarecarriedoutatATOMKI,inDebrecen,Hungaryatthecyclotronandat
theVandeGraaffaccelerator. Thefirst fourexperiments(DEB1-DEB4)werededicated
to the 12C(p,p′e+e−)12C∗ and the the 16O(p,p′e+e−)16O∗ reaction at 16.5 MeV at the
cyclotron using only the six ’small’ detectors. For calibration the 19F(p,a e+e−)16O∗
reaction was used at 3.5 and 5.5 MeV. In the DEB5-run, the 14N(3He,pe+e−)16O∗
reaction was studied at 2.3 MeV together with the 19F(p,a e+e−)16O∗ reaction at 3.5
MeV using the complete eight detector set. The DEB7 experiment was performed in
an ’asymmetric’ arrangement with five telescopes (two large and three small ones)
and an additional detector for proton detection. The 10B(3He,pe+e−)12C∗ reaction
was measured followed by the 14N(3He,pe+e−)16O∗ reaction for comparison. These
reactions were found to favor the population of unnatural parity states [8, 9]. In this
experimentathinprotondetectorwithan11%efficiencywasimplementeddownstream.
In coincidence with the populating proton group, e+e− pairs from the different levels
couldbedistinguished.
Incomparisonwiththe p-capturespectraobtainedatIKFwherethedecayofonlyone
levelisselectedbythechoiceofthebombardingenergy,the(p,a ),(3He,p)and(p,p′)
reactions have several entrance channels in the final nuclei. The resulting e+e− sum-
spectra are composite, but nevertheless different sum-peaks can be distinguished. We
clearly observedeviationsin theangular correlation spectra of transitionswith energies
below9 MeV, the aimedboson mass,which energetically cannot decay by this particle.
In the particle scenario with (m ∼E·sin(w /2)), these deviations have to be explained
X
by the emission of different particles with lower mass. With the observation of several
deviations,a number of new particles has to be postulated. This also opens the exciting
possibilityofmultiplebosonemissioninonetransition.
Analysisof thepresumably compositespectra with thenumberof bosons,theirmass
and strength as free parameter is not trivial and time-consuming. We have solved this
problem in an indirect approach. We converted the angular spectrum in a ’quasi’-mass
AnomalousInternalPairConversion SignalingElusiveLightNeutralParticles 2
spectrum by comparing the ratio of experimental data-points, divided by the simulated
IPC distribution, with the ratio obtained by simulation of e.g. 108 bosons divided by
anequal numberofIPC’s. Normalizationwas obtainedby requiringanequal contentof
simulatedtoexperimentalX/IPCeventsbymultiplyingthetheoreticalratiowithafactor
u, the transmission or inverse efficiency for boson emission. Boson masses were fitted
fromthresholdtothetransitionenergyine.g64channelbinsusinga c 2 procedure. The
inverseofthe c 2 was takento bethetransitionprobability:P=(c 2/(N−1))−1, where
N isthenumberofdetectorcombinations.ThevaluePcan thenbeplottedasafunction
of assumed m , testing the likelyhoodof a particle with the given mass being present in
i
the spectrum. An upper limit for the branching ratio is obtained by multiplying P with
the’transmission’u.Forthreedetectorgeometries107or108eventshavebeengenerated
forsome40to70bosonmasses,withandwithoutenergydisparity(y)cuts,fortransition
energies of 6.05, 8.87, 10.96, 12.71 14.64, 17.23 and 17.64 MeV. Also for the IPC
(E0, E1, E2, M1 and M2) distributions,generally up to 109 events were simulated with
and without y-cuts. It was found that a proportional scaling between boson masses and
transitionenergiescouldbeapplied,exceptforthelowmasses,wherethetransitionfrom
backwarddecaytoforwardtwo-bodydecayatb =b isnotscaledcorrespondingly.
CM lab
Adetailed descriptionoftheprogramwillbepublishedelsewhere[13].
The IKF experiments revisited
To test the validity of the program we have first applied it to the earlier results
at IKF. Spectra from the 7Li(p,e+e−)8Be∗ reaction at 0.44 and 1.03 MeV and the
11B(p,e+e−)12C∗reactionat1.6MeVprotonenergywereconsidered.All28correlation
angles were used in the fits. The mass distributions perfectly confirm the 9 MeV/c2
boson hypothesiswith the peak at 9.23 MeV in Fig.1c. The second peak at 10.95 MeV
corresponds with the emission of the 9 MeV/c2 boson in the 14.64 MeV transition to
the first excited state, being at 9.09 MeV/c2 in the 14.64 MeV laboratory system. No
significant peak is seen at 9 MeV/c2 in Fig.1a. The P-value for 8Be at 9 MeV/c2 is
indeedafactorfivelargerthanfor12Casobservedintheangulardistributions.However,
wealsoseepeaks at thehighermassesof12.50and 14.23MeV/c2 masses.
A different picture is found for the isoscalar transition at 18.15 MeV. Here, the
dominant peaks are at 12.42 and 14.55 MeV/c2. They are a factor 20 stronger than
in the isovector M1, whereas the strength of the 9.23 MeV/c2 peak is about the same.
The masses in Fig.1 are indicated using the ground state transitions as two-body decay
frame.Applyingthetransformationto14.64,themassesbecome2.99,7.55,9.26,10.52
and 11.86 respectively and in Fig.1f (with 15.15) of 3.21, 4.78, 8.22, 10.37 and 12.15
MeV. The emission of the same 9 MeV/c2 boson in the 17.64 (9.10) and in the 14.64
MeV (9.23) was already suggested in [6] where approximatelythe samebranching was
deducedforthisboson.Figs.1b,1dand1fshowthebranchingratiosB forbosontog -
X
ray emission. Integration over 9 MeV/c2 yields respective values of 1.9·10−5, 1.2·10−4
and 4.0·10−4 within 1s in accord with the values of ≤2.3·10−5, (1.1±0.3)·10−4 and
(5.8±2.2)·10−4 given in [3, 6, 5, 7]. The mass distributions for the transitions after
applyingsymmetrycutstothespectra.showdramaticallylargervariationsthaninFig.1
AnomalousInternalPairConversion SignalingElusiveLightNeutralParticles 3
e+e- decay of the E1 and M1 transitions
from the 17.23 1- in 12C, 17.64 1+, T=1 and 18.15 1+, T=0 in 8Be
P 10-1 17.23 MeV, 1- a) BX 10-4 12C b)
9.34 9.23
14.20 10-5 3.20 14.20
3.20 -6 12+17-tot
10
5 10 15 5 10 15
P 10-1 17.64 MeV, 1+ c) BX 10-4 3.54 8Be d)
11.16
12.68
14.23 14.29
-5
10 9.10
9.23 12C
3.73 E1 10.9512.50 -6 14+17-tot E1
10
5 10 15 5 10 15
P 1 18.15 MeV, 1+ e) BX 8Be f)
-4
10 3.84
5.73
12.42
-1 3.84 13.21 12.42
10 5.73 9.85 14.55 14.55
15.72 9.85
15+18-tot
5 10 15 5 10 15
boson mass (MeV/c2) boson mass (MeV/c2)
FIGURE1. Figs. 1a,1cand1eshowtheprobabilitydistributionsofbosonsinthedecayofthe
17,231−,the17.641+andthethe18.15MeV1+levelsfollowingthe11B(p,e+e−)12C∗reaction
at 1.6 MeV and the 7Li(p,e+e−)16O∗ reactions at 0.44 respectively 1.03 MeV as measured
at IKF in 1992 and 1998 [3, 7]. Figs. 1b, 1d and 1e show the corresponding B -values i.e.
X
branchingratios for boson to g -ray emission. The dashed lines in Figs. 1c and 1d correspond
withtheE1distributionsfromFigs. 1aand1b.
at 9.0, 12.5 and 14.5 MeV/c2, in agreement with the expectations for two-body decay
where the symmetrical decay is enhanced with respect to the primarily asymmetrical
decayofM1-IPCatlargeangles.Integrationat9MeV/c2 yieldsB valuesofabouthalf
X
ofthoseobtainedfromFig.1,in agreement withtheexpectationsfortwo-bodydecay.
It appears that the boson search program adequately describes the IKF-results by
reproducingthepreviouslyfoundsignalat9MeV/c2.However,surprisingly,italsofinds
signals for particles at the masses of 12.5 MeV/c2 and 14.5 MeV/c2. In the calibration
runs: 19F(p,a e+e−)16O∗ at 0.72 and 0.84 MeV just below and above threshold for the
8.87 MeV 2− level, two boson candidates are observed at 5.0 and 5.38 MeV/c2 with
branchingratiosas largeas severalpercent.
AnomalousInternalPairConversion SignalingElusiveLightNeutralParticles 4
Back to ATOMKI and further
Because of the apparent feasibility of transformation of the angular spectra to mass-
spectra and the unbiased character of the program we have systematically converted
all available angular spectra from IKF and from ATOMKI and other sources into mass
spectra. The oldest investigated spectrum was the celebrated angular correlation mea-
surement of the E0 transitionat 6.05 MeV in 16O dated back to 1949 [10] and the most
recent one: thestudyof thee+e− decay ofthegiant resonancedecay at 22.23 MeV and
the15.11MeV isovectorM1decay in12Cat StonyBrook [11]in2003.
The 4.44, 6.05, 7.66, 8.87, 10.96 and 12.71 MeV transitions
In thisparagraph, wediscussthe mass-distributionsofsix different transitionsin 12C
and 16O, selected from results of the 12C(p,p′e+e−)12C∗ and the 16O(p,p′e+e−)16O∗
reactionsat 16.5MeV and fromthe19F(p,a e+e−)16O∗ reactionused forcalibration.
The mass-distributions for symmetrical (S) pairs are shown in Fig.2, with the P-
distribution in the left column and the branching ratio in the right. In all shown data,
disparitycuts of y≤0.5 have been applied. For every transition more than one spectrum
is available showing in essence the same features as Fig.2. In particular, for the 6.05
MeV E0 transition, data from twelve experiments have been analyzed. Eight of them
showstrong peaks at 5.0 and 5.3 MeV/c2. The four spectra where they are absent, stem
fromexperimentswhere themaximumcorrelation angleissmallerthan 1100.
The data are discussed from bottom to top. right to left. In the 12.71 MeV transition,
we clearly recognize the boson at 9 MeV/c2, the original goal for the experiments in
Debrecen.Thebranchingratioisabout8·10−4 inagreementwiththeresultsof[6,5,7].
However, there are two new candidates at 10.39 and 11.19 MeV/c2, which are stronger
thantheoneat 9 MeV/c2. TheM1 dataare compared (bottomleft) withameasurement
oftheangulardistributionoftheforbidden0+→0+ M0transitionat10.96MeVin16O,
where a strong signal is seen at 7.85 MeV/c2. The X/IPC value is as large as 0.8 and
for the (y≤1) cut (not shown here) it is even 2, suggesting that the decay via the 7.85
MeV/c2 bosonisthedominantdecay modeinthisM0 transition.Thetwopeaks at 8.99
and 9.84 MeV can be ascribed to reflection from bosons of 7.35 and 7.85 MeV/c2 both
occurring in thedecay of theneighboring8.87 MeV/c2 which couldexperimentallynot
be completely resolved from the 10.96 MeV sum-peak. It seems appropriate to assign,
from the superallowed decay, the 7.85 MeV/c2 as a pseudoscalar particle and for the
same reason the signals at 9.23 and 10.39 MeV/c2 as axial vector particles. A vector
characterwouldbepreferred forthe11.19MeV/c2 signal.
In the8.87MeVM2transitionweseeabosoncandidateat7.35withthespin1− and
possiblythe7.850−.The6.32MeV/c2 peakcouldbeduetoreflectionofthe7.85inthe
10.96 transition. However, the signals at 2.25. 3.38 and 5.04 MeV/c2 in this transition
are all viable boson candidates. Also in the mass-distributionsof the two E0 transitions
at 6.05 and at 7.65 MeV, multiple resonances are displayed. A candidate boson at 3.36
MeV/c2 isseeninallspectra,TheoccurrenceintheE0,E2andM2transitions(allowed
decay) could only be compatible with the spin-parity of 1− for this particle. With the
AnomalousInternalPairConversion SignalingElusiveLightNeutralParticles 5
4.44 (E2), 6.05 (E0), 7.65 MeV (E0), 8.87 MeV (M2), 10.96(M0), 12.71(M1)
4.44-S 4.44-S 6.05-S 6.05-S
10-1 1.31 2.429.83 3.66 10-4 1.341.88 2.783.29 10-1 4.505.05 10-2 4.45
4.04 2.55 4.09
P BX P 3.38 X/IPC 1.90 5.06
2.57
1.89 3.67 1.88
-5
10 2.48 -3
5.34 10 3.35
3.36
2 4 2 4 2.5 5 2.5 5
7.65-S 7.65-S 8.87-S 8.87-S
P 4.29 6.55 2.32 P
2.27 5.30 10-4 3.23 5.53.056 2.253.38 5.046.32 10-4
7.30
3.15 5.55 BX 6.33 -2 BX 7.75
10
-2 7.73.075 3.38 6.32
10 4.20
-5 2.25 5.04
10 -5
10
1.6 4.3 7 1.6 4.3 7 1.7 5 8.3 1.7 5 8.3
10.96-S 10.96-S 12.71-S 12.71-S
1
P 8.99 P
-4
10-1 X/IPC 3.19 9.25 10 10.48
3.35
6.26 10-1 10-2 6.15 BX
2.98 9.66
8.91 6.05 10.39 6.73
7.86 7.86 11.19 3.69 9.06
-2 -2
10 10
5 10 5 10 5 10 5 10
mass mass mass mass
FIGURE2. Fromtoptobottom(left)areshowntheprobabilitydistributionsofsymmetrical(S)
pairsinthe4.44(E2),6.05(E0),7.65(E0)and8.87(M2),10.96(M0)and12.71(M1)transitions
in 12C and 16O. Right are shown the correspondingbranching ratios for boson emission with
respect to E0-IPC (X/IPC) for the 6.05 MeV E0 and the 10.96 MeV (M0) transitions and to
g -raysfor the E2 at 4.44 MeV, the M2 at 8.87 and the M1 at 12.71 MeV. The B -value for the
X
7.65MeVE0isshownwithrespecttotheparallel0+→2+ 3.20MeVE2-g -ray.
simultaneous presence of the candidates below 4 MeV in the E0 and E1 transitions the
spin is restricted to 1−. Likewise, the resonance at 5 MeV/c2 must, from its presence
in the E0’s and the M2 transition, have spin-parity 1−. With its appearance in both E0
transitions and its absence in the M2, the 4.48 MeV/c2 candidate must have a scalar
0+ character since a 1− particle would also have shown up in the M2 transition. With
someoptimism,thisfigureindicatestheexistenceoftwoscalarX-bosonsat4.5and 5.3
MeV/c2 andsix vectorbosonsat2.5, 3.3,4.0,5.0, 6.3and 7.3MeV/c2.
AnomalousInternalPairConversion SignalingElusiveLightNeutralParticles 6
Conclusion and the future
It appears that in every isoscalar transition considered with energies from 4.44 to
12.71MeVdeviationsfromIPCareobserved.Thisanomalyisexplainedinanisoscalar
particlescenariowhichwouldrequireamultitudeofbosons.Thefindingcouldhavethe
bizarreconsequenceofanewtypeoflightneutralmatterinsteadofor/andthediscovery
ofa newgaugeboson.
A second generation of dedicated experiments is required to test this exciting pos-
sibility. The properties of every boson candidate have to be checked in great detail. A
first experiment in that direction [13] has been undertaken in the DEB7 run, where the
two-bodydecaypropertiesofthebosonswereverified,bymountingthecentraldetector
combination at an asymmetrical position. An alternative approach is being performed
[14] at ATOMKI to study the angular correlations with an order of magnitude higher
angularity,usingwirechambersin frontofthedE/dx-detectors.
Acknowledgement
This paper has been presented at the International Symposium on Exotic Nuclear
Systems,ENS’05,at ATOMKI, Debrecen, Hungary,20-25 June2005.
I am indebted to my colleagues: J. van Klinken (KVI, Groningen), H. Bokemeyer
(GSI, Darmstadt), K. Bethge, O. Fröhlich, K.A. Müller, K.E. Stiebing (IKF, Frankfurt),
R. van Dantzig, T.J.Ketel (NIKHEF, Amsterdam), K. Griffioen (William and Mary,
Williamsburg), A. van der Schaaf (Universität Zürich, Zürich), M. Csatlós, L. Csige,
Z.Gáczi, J. Gulyás,M.Hunyadi,A. Vitéz, A. Krasznahorkay Jr.and A. Krasznahorkay
(ATOMKI,Debrecen).
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