Spectroscopy results from Belle

0.27 MB·

by  Sookyung Choi

1
1
0 Spectroscopy results from Belle
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a
J

9
2

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x
e SookyungChoi(for the Belle Collaboration)
∗
-
p GyeongsangNationalUniversity
e E-mail: [email protected]
h
[

1 Wereportrecentresultsonthecharmoniumandcharmoniumlikestatesbasedonalargedatasam-
v
1 plerecordedatthe¡ (4S)and¡ (5S)resonanceswiththeBelledetectorattheKEKBasymmetric-
9 energye+e collider.
−
6
5
.
1
0
1
1
:
v
i
X
r
a
TheXthNicolaCabibboInternationalConferenceonHeavyQuarksandLeptons,
October11-15,2010
Frascati(Rome)Italy
Speaker.
∗
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
HQL2010 SookyungChoi(fortheBelleCollaboration)
1. Introduction
Starting with the observation of the h (2S) state, anumber of new states have been observed
c
byBelle. Someofthesehavebeenidentifiedasbeingthepredicted, butnot-yet-seen, charmonium
states, while others, designated by X, Y & Z, are considered to be candidates for new types of
charmoniumlike states such as hybrid cc¯-gluon states or multiquark states either of the molecular
type (cq¯c¯q) or the diquark-diantiquark (cqcq) type. Here, recent XYZ state-related measurements
arereported andexaminedinthecontextofpossible charmonium assignments.
2. The X(3872)
The X(3872) was discovered by Belle in the J/yp +p mass spectrum in exclusive B
−
→
KX(3872) decay [1] in a 140 fb 1 data sample; it was subsequently seen in three other exper-
−
iments. One curious fact about the X(3872) is the near equality of its mass and the m +m
D0 D 0
∗
threshold. ThelatestPDGworldaveraged mass[2]is3871.56 0.22 MeV,whiletheD0D 0 mass
∗
±
threshold is3871.79 0.30 MeV.TheCDFgroupreported thatonlytheJPC options 1++ and2 +
−
±
arecompatiblewiththeJ/y andpp helicityangledistributions [3]. Itisalsonowwellestablished
thatthepp systeminX J/yp +p comesfromr pp decays.
−
→ →
2.1 X(3872) g J/y (y )
′
→
TheobservationofX(3872)ing J/y (y )finalstatesensuresthatthecharge-conjugationparity
′
oftheX(3872)isC=+1. ThefirstreportedevidenceforX(3872) g J/y wasgivenbyBelle[4]
→
usinginB KX(3872)decaysina256fb 1 datasamplewithasignalsignificance of 4s .
−
→ ∼
BaBaralsoreportedX(3872)decaystobothg J/y andgy finalstatesintheB+ K+g J/y (y )
′ ′
→
process [5], with signal significances of 3.6 s and 3.5 s , respectively. Their measured product of
branching fractions are B(B+ K+X(3872)) B(X(3872) g J/y )= (2.8 0.8) 10 6 and
−
→ × → ± ×
B(B+ K+X(3872)) B(X(3872) gy )=(9.5 2.8) 10 6. The ratio of these branching
′ −
fractio→ns is BB((XX33887722→ggyJy′))×=3.4±1.4,w→hichislargefo±raDD×∗0 molecular state[6]
ThisyearBel→le studied theg J/y (y )finalstates using their full data sampleof772 106BB
′
×
events [7]. The B Kg J/y channel is dominated by B Kc ; c g J/y decays and this
c1 c1
→ → →
is used as a calibration reaction; the branching fraction for the well known B+ K+c decay is
c1
→
measuredtobe(4.94 0.35) 10 4,whichagreeswellwiththePDGvalue[2]. Bellealsoreported
−
± ×
firstevidence forB Kc (via c g J/y )with3.6s significance. Thebranching fraction for
c2 c2
B+→K+c c2 ismea→suredtobe(1.11→±0.37)×10−5. Theratioofbranchingfractions BB((BB++→KK++cccc21))
is 0.022 0.007, which is a measure of the factorization suppression factor for JPC = 2→++. In
±
the same g J/y final state but at higher masses, there is a clear X(3872) g J/y signal with
→
4.9 s significance. Figure 1 shows MgJ/y mass distributions in exclusive (a) B+ K+g J/y and
→
(b) B0 K0g J/y decay. The product of branching fractions for B KX and X g J/y is
→ S → →
measured to be (1.8 0.5) 10 6, which agrees with the BaBar result. However, there is no
−
± ×
significant signal forB+ K+X andX gy ′ decay. Figures1showtheMgy massdistributions
→ → ′
in B+ K+gy decay for (c) y ℓ+ℓ and (d) y J/yp +p decays. Anupper limit of the
′ ′ − ′ −
→ → →
productofbranchingfractionsisdeterminedtobeB(B+ K+X) (X gy )<3.4 10 6. The
′ −
→ × → ×
2
HQL2010 SookyungChoi(fortheBelleCollaboration)
Figure 1: g J/y mass distributions (top) for (a) B+ K+X(3872) and (b) B0 K0X(3872) candidate
→ → s
events with subsequent X(3872) g J/y decay. gy mass distributions (bottom) for B+ K+X(3872)
′
→ →
candidateeventswithsubsequentX(3872) gy decay,wherey ℓ+ℓ in(c)andp +p J/y in(d).
′ ′ − −
→ →
ratioofbranchingfractionsofits90%CLupperlimitis BB((XX→ggyJ/y′)) <2.1,incontradiction withthe
BaBarresult. →
2.2 X(3872) w J/y
→
In the D0D∗0 molecular model of Swanson [6], the JPC is assumed to be 1++, in which case
the D0D∗0 component is dominant, with small admixtures of w J/y and r J/y . In this model,
X(3872) p +p p 0J/y decays were predicted to occur at about half the rate for X(3872)
−
→ →
p +p J/y decay. Belleperformedasearchforthis3p J/y decaymode.
−
Figure2(b)showstheY(3940) [8]seeninw J/y massdistribution inB Kw J/y decay. In
→
the X(3872) mass region, which is at the right edge of the kinematic boundary, Belle observes a
signal inthe 3p massspectrum corresponding tothesub-threshold decay X(3872) w J/y . The
→
measurednumberofsignaleventswere12.4 4.1,fromwhichtheratioofthebranchingfractions
B(BX(3X87T328→7h2i→ps+wypJe−/aJyr/)yB)aBisadrerteearnmailnyezdedtoBbe1K.0w±J/0y.±5fi[4n]a.lstatesusingarelaxedomegamassselection[9]
→
0.5 < m3p < 0.9 GeV and saw a similar w signal. Using their reported branching fraction, we
oBbatBaianrtrheepocrotsmtbhiantetdheraMtio3pfrmoamssBseplelectarunmd BfraoBmarthtoe Xbe387B2(BX(3X87328J→7/2→pyw+wpJ−/Jfiy/n)ya)l=sta0t.e8s±is0s.u3p.pIrnesasdedditnioeanr,
→
itsupperkinematicboundary byacentrifugalbarrierfactorthatisconsistent withaP-wave. Their
P-wave (2−)fit(c 2/NDF=3.53/5 )tothe M3p massdistribution is favored overtheir S-wave (1+)
fit(c 2/NDF=10.17/5). Thiswouldbebadforamolecularinterpretation ofX(3872),however,this
corresponds toonlyabouta1.5s effect.
2.3 CharmoniumpossibilitiesofX(3872)
From the CDF angular analysis results [3], the only two possible JPC assignments for the X
3
HQL2010 SookyungChoi(fortheBelleCollaboration)
b)
16 a)
V
Me
Events/25 8
0
480 605 730
M(!+!-!0) (MeV)
Figure2:a)TheB-mesonsignalyieldsversus3p invariantmassintheX(3872)regionofM 3s <M <
X X
−
M +1s )forB Kp +p p 0J/y decay.b)B Kw J/y signalyieldsversusM(w J/y )
X −
→ →
are1++and2 +;allotherJPCvaluesareruledoutwithhighconfidence. Inthissection,wesurvey
−
charmonium possibilities forX(3872)withthesetwoJPC assignments.
Forthe1++assignment,thepossibleundiscoveredcharmoniumstateisc . FortheX(3872)=
c′1
c assignment, thefollowingpuzzlingquestions arise.
c′1
Sincethemassof c isnowknowntobe3930MeV[10],themassof c isexpected tobe
• c′2 c′1
3905MeV.Therefore, themassofX(3872)istoolowforittobethe c .
∼ c′1
Barnesetal. [11]estimatedthepartial widthforG (c gy ) 180keV,whileG (c
c1 ′ c1
• g J/y )∼14 keV. So, the ratio of of partial widths GG((ccc′1c1→′→ggyJ/y′)) s∼hould be much bigger t′h→an
unity. Therefore, the c assignment would be apossib′l→e option ifthe BaBarmeasurement
c1
′
isright,whichgivesalargepartialwidthforthegy mode.
′
G (X p +p J/y ) = (3.4 1.2) G (X g J/y ) is estimated to be about 45 keV using
−
• → ± × → G
the Barnes value for G gJ/y and the measured ratio of Gpp J/y . This partial width for X
gJ/y →
p +p J/y decayishugeforanisospin-violatingtransition;otherisospinviolatingtransitions
−
inthecharmonium systemarelessthan1keV(e.g.,G (y p 0J/y ) 0.4keV[2]).
′
→ ∼
For the 2 + assignment, the possible undiscovered charmonium state is the singlet D-wave
−
stateknownastheh (1D ). Forthisassignment, thefollowingquestions arise.
c2 2
Y. Jia et al. [12] estimated the partial widths to be G (h gy ) 0.4 keV and G (h
c2 ′ c2
• g J/y )∼9keV.ThisfavorstheBellemeasurement wheret→heratio∼GG((hhc2c2→ggyJ/y′)) issmaller.→
→
Usingthewellestablishedmassofitsmultipletpartner,M(y )=3770MeV,themassofthe
′′
•
h isestimatedtobe3837MeV. Thus,theX(3872)massishighforh .
c2 c2
G (X p +p J/y )=(3.4 1.2)(X g J/y ) is about 30 keV using the Jia g J/y width.
−
• → ± →
Thisisalsolargeforanisospin-violating transition.
For the 2 +(h ) assignment, the branching fraction for the B+ K+h is too high for a
− c2 c2
• →
non-factorizable decay. Other two B+ K+h and B+ K+c decay, which is also non-
c c2
→ →
factorizableandsuppressedbyanangularmomentumbarrier,arejustbarelyseeninthehuge
Belledatasample.
The branching fraction for h c2 DD∗ is expected to be small [13], but the averaged ratio
• frombothBelleandBaBaris G G(→X(X→ppDJD/∗y)) =9.5±3.1,whichishighfortheh c2.
→
4
HQL2010 SookyungChoi(fortheBelleCollaboration)
3. More X andY states near 3940MeV
Belle observed three states near 3940 MeV via three different production and decay chan-
nels[8,10,14]. Amongthesethree,theZ(3930)state,whichisproducedinthegg DDprocess,
→
isgenerallyconsideredtobethecharmoniumc state,eventhoughthemassM=3929 5 2MeV
c′2 ± ±
is somewhat lower than potential model predictions. The X(3940) is observed in the DD∗ mass
spectrum from double charmonium production ine+e J/y D D annihilation and theY(3940)
− ∗
→
is observed in the w J/y mass spectrum in B Kw J/y decays. The mass and width of the
→
X(3940) (Y(3940)) are measured to be M=3942+7 6 (3943 11 13) MeV and G =37+26 8
6± ± ± 15±
− −
(87 22 26)MeV.Althoughthemassesaresimilar,theX(3940)andY(3940)appeartobediffer-
± ±
entstates: theX(3940)(Y(3940))hasnotbeenseeninthew J/y (DD∗)finalstateinB X(Y)K
→
decays.
Itisimportanttosearchforw J/y (orDD∗)intwo-photoncollisions, whereitsspin-parityof
resonance ispreferentially constrained tobeJp=0 or2 . Belleobserved a7.7s enhancement in
± ±
thew J/y system[15]producedinthegg w J/y process;themassandtotalwidtharemeasured
→
to beM=3915 3 2 MeVand G =17 10 3 MeV.This state, denoted by X(3915), isprobably
± ± ± ±
relatedtooneofthethreeabove-mentioned statesinthe3.90-3.95GeVmassregion. Ifweassume
theX(3915)is0+ (2+)resonance, theproduct ofthetwo-photon decay widthandbranching frac-
tion to w J/y is determined to be G gg (X(3915))B(X w J/y )= 61 17 8(18 5 2) eV for
→ ± ± ± ±
Jp=0+(2+). Forcomparison,themeasuredproductofthetwo-photondecaywidthandbranching
fraction for Z(3930) DD is 180 50 30 eV. If the X(3915) is the Z(3930) (c ), the ratio of
branching fractions B→F(cc′2→w J/y ) is±large±for an above-open-charm-threshhold chacr′2monium state.
Also, for both the 0+BaFn(cdc′22→+DDo)ptions, if we assume that the gg partial width is 1 keV, which is
∼
typical for charmonium states, G w J/y would be of the order of 1 MeV,which islarge for charmo-
nium.
The G (Y w J/y ) partial width is also estimated to be large using the averaged prod-
3940
→
uct branching fraction from Belle and BaBar to be B(B+ K+Y ) B(Y w J/y ) =
3940 3940
→ × →
(5.0 0.8) 10 5 and the PDG averaged G (Y ) = 40+18 MeV. If we assume the maximum
± × − 3940 13
possible branching fraction for B KY is 10 10 4 (−the branching fraction for B KJ/y ),
3940 −
→ × →
thepartialwidthforG (Y w J/y )isdeterminedtobelargerthanorder1MeV,whichislarge
3940
→
forconventional charmonium.
4. TheY(4260)andY
b
The 1 Y(4260) state was first discovered by BaBar [16] and confirmed by Belle [17] in
−−
the J/yp +p in radiative e+e g Y(4260) process. Thepartial width forY p +p J/y is
− − ISR −
→ →
determinedtobelargerthan0.5MeVatthe90%CLlevelbyMoetal. [18],whichismuchlarger
than that for y p +p J/y . This large partial width is one of the remarkable properties of the
′ −
→
Y(4260)thathaveledtovariousexoticinterpretations ofitsquarkcontent. Aninterestingquestion
iswhetherornotthereexistcounterparts inthessand/orbbquarksystems.
Bellereportedananomalously largee+e ¡ (1,2S)p +p production crosssectionnearthe
− −
→
peakofthe¡ (5S)resonanceat√s 10.87GeVmeasuredwitha21.7fb 1datasample[19]. Ifthey
−
∼
assumethatthesignaleventsonlycomefromdecaysofthe¡ (5S)resonance,theirextractedpartial
5
HQL2010 SookyungChoi(fortheBelleCollaboration)
]
0mm[ 0.008 U (1Sp)p
s] / 0.006 U (2Sp)p
pp(nS) 0.004 U (3Sp)p
U[
s 0.002
0.000
10.75 10.8 10.85 10.9 10.95 11 11.05
s (GeV)
Figure 3: The ¡ (nS)pp (n=1,2 and 3) cross sections near the ¡ (5S) resonance show peak structure that
deviatesfromthepeak(verticalline)of¡ (5S)obtainedfromthehadroniccrosssections.
widths are 300 times larger than those for corresponding transitions from the ¡ (4S). Recently,
∼
Bellemeasuredtheenergydependenceofthee+e ¡ (nS)pp (n=1,2,3)productioncrosssection
−
→
using data accumulated at seven different cm energy points near the ¡ (5S) resonance. A new
common peak structure was observed for all three e+e ¡ (1,2,3S)p +p cross sections. A fit
− −
→
using Breit-Wigner resonance function with a common mass and width to these peaks, shown in
Fig. 3, gives a mass and width of M=10888.4+2.7 1.2 MeV and G =30.7+8.3 3.1 MeV[20],
2.6± 7.0±
whicharenotconsistent withanyknownbbsta−tesuchasthe¡ (10860). This−canbeconsidered to
beacandidate foraY -typestateinthebbsystem.
b
5. Charged Z+ states
Achargedcharmonium-likestatecouldnotbeacccharmoniumstate;itsminimalquarkstruc-
ture would have to be a ccud tetraquark arrangement. The charged Z(4430)+ state was first ob-
served by Belle as a peak in the p +y mass distribution in exclusive B Kp +y decays[21].
′ ′
→
It was confirmed by a subsequent reanalysis using a Dalitz plot formalism [22] that includes all
possible intermediate Kp resonances. This Dalitz analysis method was first employed in the ob-
servation of two other charged Z+ and Z+ states that are seen to decay to p +c final states in
1 2 c1
exclusive B Kp +c decays [23]. The Dalitz-plot analysis demonstrated that these Z states are
c1
→
notproduced byreflectionsfromanyknownandpossibly unknownresonances intheKp channel.
However,BaBarsearchedfortheZ(4430)+,butdidnotseeasignificant signal[24].
Acknowledgments
This work was supported by the Korea Research Foundation Grant funded by the Korean
Government(KRF-2008-313-C00177).
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7