Transcript New Results from BES

Glueball Searches and PWA
at BESIII
Shan JIN
Institute of High Energy Physics (IHEP)
[email protected]
PWA Workshop
Beijing, January 26, 2006
Outline
 Physics goal of hadron spectroscopy at BESIII
 What can we learn from BESII results?
 Selected topics on glueball searches
 PWA at BESIII (with discussions)
Physics goal of hadron
spectroscopy at BESIII
Multi-quark State, Glueball and Hybrid

Hadrons consist of 2 or 3 quarks：
Naive Quark Model：
Meson（ q q ）
Baryon（q q q）

New forms of hadrons:
•
Multi-quark states ：Number of quarks >＝ 4
•
Hybrids ： qqg，qqqg …
•
Glueballs ： gg， ggg …
How quarks/gluons form a hadron is far from being well understood.
Multi-quark states, glueballs and hybrids
have been searched for experimentally for
a very long time, but none is established.
J/ decays are an ideal factory to search for
and study light exotic hadrons:
 The production cross section of J/ is high.
 The production BR of hadrons in J/ decays are
one order higher than ’ decays (“12% rule”).
 The phase space to 1-3 GeV hadrons in J/
decays are larger than  decays.
 Exotic hadrons are naively expected to have
larger or similar production BR to conventional
hadrons in J/ decays.
 Clean background environment compared with
hadron collision experiments, e.g., “JP, I” filter.
Physics Goal (I)
With 1010 J/psi events, we hope to answer:
 Whether glueballs exist or not?
• Naively, we estimate in each exclusive decay mode:
BR( J /  G)  BR(G  hh) ~ 105
103
102
• If the eff. is about 20%, we would have 20000 events
for each decay mode
 we should observe a relative narrow (width:
50~200MeV) glueball if it exists.
Physics Goal (II)
 Is there any gluon content in hadrons – hybrid
mesons and baryons?
 Whether multiquark mesons and baryons exist
in the nature?
 Understanding conventional mesons and
baryons
How quark/gluon form a hadron?
QCD cannot “escape” from answering these
fundamental questions finally.
Difficulties (I)
 Theoretically (glueball as an example)
• Predictions on glueball masses from LQCD may be
unreliable due to quench approximation.
• No predictions on the widths so far (even the order).
• No prediction on the production rate (J/  G).
• Mix with qqbar mesons or even with 4q, qqg
mesons? (dirty?)
Difficulties (II)
 Experimentally:
• Data sample is not big enough (it is not a problem
for BESIII)
• No good way modeling background at low energy, in
many cases we have to study bck via data.
• Interferences among mesons make the mass/Dalitz
plots very complicated 
 PWA is a must for hadron spectroscopy at BESIII
 But PWA face many uncertainties (see the discussions on
PWA)
What can we learn from BESII results?
A number of unexpected new observations
at BESII
 A possible p p bound state:
pp
threshold enhancement in J /  p p
new observation of X(1835).
mass
and

J
/


pK

p


mass threshold enhancement in
 K   mass threshold enhancement in J /  pK  
  mass threshold enhancement in J/  
 New observation of a broad 1- - resonance in
J/  K+K- 0
Observation of an anomalous enhancement near
the threshold of pp mass spectrum at BES II
BES II J/pp
acceptance
weighted BW
M=1859 +3 +5 MeV/c2
10 25
 < 30 MeV/c2 (90% CL)
c2/dof=56/56
0
Phys. Rev. Lett. 91, 022001 (2003)
3-body phase space
0.1
0.2
M(pp)-2mp (GeV)
acceptance
0.3
Observation of X(1835) in
J       
Statistical Significance 7.7 
J       
The +- mass spectrum for  decaying into
+- and  
Mass spectrum fitting
The +- mass spectrum
for  decaying into
+- and  
7.7
N obs  264  54
m  1833.7  6.1  2.7MeV
BESII Preliminary
  67.7  20.3  7.7MeV
B( J   X ) B( X    )  (2.2  0.4  0.4)  104
Re-fit to J/p pbar including FSI
Include FSI curve from
A.Sirbirtsev et al.
( Phys.Rev.D71:054010, 2005 )
in the fit (I=0)
M = 1830.6  6.7 MeV
 = < 153 MeV @90%C.L.
In good agreement with X(1835)
M pp  2mp
A Possible ppbar Bound State
 X(1835) could be the same structure as ppbar
mass threshold enhancement.
 It could be a ppbar bound state since it
dominantly decays to ppbar when its mass is
above ppbar mass threshold.
 Its spin-parity should be 0-+: this would be an
important test  PWA is needed
Observation of an anomalous enhancement near
the threshold of p mass spectrum at BES II
BES II
J /  pK
3-body phase space
Phys. Rev. Lett. 93, 112002 (2004)
For a S-wave BW fit: M = 2075 12  5 MeV
Γ = 90  35  9 MeV
Observation of a strong enhancement near the
threshold of K   mass spectrum at BES II
NX*
BES II
J /  pK
M K  Λ (GeV/c 2 )
PS, eff. corrected
(Arbitrary normalization)
MKΛ  MK  M
 A strong enhancement is observed near the mass
threshold of MK at BES II.
 Preliminary PWA with various combinations of
possible N* and Λ* in the fits —— The structure Nx*has:
Mass 1500~1650MeV
Width 70~110MeV
JP
favors 1/2The statistics is not high enough to tell what it is.
The most important is:
It has large BR(J/ψ  pNX*) BR(NX* KΛ) 2 X 10-4 ,
suggesting NX* has strong coupling to KΛ. It could be
an KΛ bound/resonant state (5-quark system).
An  mass threshold enhancement is observed
background
M2()
X(1810)
M2()
19
 26
M  1812
 18 MeV/c
2
M()
  105 20  28 MeV/c2
JPC favors 0++
Phys. Rev. Lett., 96 (2006) 162002
It could be a multiquark/hybrid/glueball state.
New observation of a broad 1- - X(1580)
in J/  K+K- 0
X (1580
)
0

X (1580)
Background
i
i
 49  98
22  64
2
M    (157655 91 )  (818 23
)
MeV/c
133
2
2
J PC  1 
Phys. Rev. Lett. 97 (2006) 142002
How to understand broad X(1580)?
 Search of a similar structure in J/  KSK  will
help to determine its isospin.
 X(1580) could have different nature from
conventional mesons:
• There are already many 1- - mesons nearby.
• Width is much broader than other mesons.
• Broad width is expected for a multiquark state.
What do we learn from BESII results
 J/ψ decay is an ideal place to study
exotic structures.
 The statistics at BESII is not high
enough yet.
 We would expect: more unexpected
discoveries on hadron spectroscopy at
BESIII —— the more, the better !
Selected topics on glueball searches
Selected topics on glueball searches
 J/  , 
 2++ glueball candidates
 Where to search for the 0-+ glueball?
J/  , 
 These two processes are believed to be an ideal place
to tag the flavor of mesons.
 BESII studied these two channels (PLB549, 47(2004)):
'
f1 (1285)
 (1440)
M (GeV )
(1440) from
J/0KK bck
M (GeV )
J/  , 
 At BESII, we only observe ’, (1440) , f1(1285)
in J/  , no clear peak in J/  
 At BESIII:
• The background could be very high when searching
for other glueball candidates. Hope BESIII much
better detector can strongly suppress the
background  we will perform MC studies on this.
• PWA is needed.
2++ glueball candidates
 Lattice QCD predicts the 2++ glueball mass in
the range of 2.2~2.4 GeV
 (2230) was a candidate of 2++ glueball:
• It was first observed at MARKIII in J/KK
• It was observed at BES I in J/KK, , ppbar
• It was not observed at DM2.
BES-I (2230) Result
(2230)
The situation at BESII
J /  K  K 
J /  K s0 K s0
 The mass plots shows
no evident (2230) peaks
in J/KK, , ppbar,
which is different from
BESI.
 Difficult to draw firm
conclusion at present.
PWA is needed to draw
firm conclusion on it.
(2230) could be similar to f0(980)
at BESII
J/

BESII
M(+-)
 We saw a clear f0(980)
mass peak in J/ at
BESI, but we do not see
a clear f0(980) peak in the
mass plot at BESII.
 However, we need a
significant contribution
of f0(980) in PWA of
J/ at BESII
f 0 (980) ?
Other 2++ glueball candidates
 No other obvious good candidates have been
observed in J/psi radiative decays in the mass
range predicted by LQCD.
 What does it mean:
• LQCD prediction is not very reliable, or
• BR(J/  G)xBR(Ghh) is small ( <10-4 ) so that
we don’t have the sensitivity to observe it ( quite
possible ), or,
• The width of a glueball is very large ( ~1GeV,
E.Klepmt ).
Where to search for the 0-+ glueball?
 Lattice QCD predicts the 0-+ glueball mass in
the range of 2.3~2.6 GeV.
 (1440) and X(1835) were suggested being
possible candidates, but their masses are
much lower than LQCD predictions.
No 0-+ glueball candidate observed in the
mass range 2.3~2.6 GeV
X (1835)
 No evidence for a
relatively narrow state
( 100 ~ 200 MeV width )
above 2GeV in
J /  KK , , K * K *,
, ' ...
 Again:
M '
• LQCD reliable?
• Production rate could be
very low.
• Glueball width could be
very large.
PWA at BESIII
PWA is crucial to most analyses at BESIII
J /  K K 


0
 Not only to the spinparity determination of
new hadrons
 But also to the
measurement of decay
BR:
e.g.: BR(J/K*K)
BR(J/K*(1410) K)
From interference
Also for D decays…
 With huge data samples at BESIII, PWA is
possible.
 However , there are many difficult problems to
be solved.
 How to obtain robust PWA results with
high speed ?
Q1: How to speed up the PWA fit?
 We will have 200 times larger data sample at
BESIII:
• Typical size of a data sample at BESII: 10000 events.
Usually it takes 1- 3 years to publish one PWA result
(with more than 20 CPU fully used).
From previous talks at BESII, we have leant how
we obtained the PWA results, including how we
dealt with systematic uncertainties
• Naively, we would have 2M events for one data
sample at BESIII  The speed will be about 100
times slower  How many years do we need?
Q1: How to speed up the PWA fit?
 PWA procedure at BESII: Global fit
• The PWA input contains 4-momentum of all events (the whole
mass range).
• Various fits are tried with different combinations of the
possible components/resonances/amplitudes, finding
minimum –lnL of all these combinations.
 One possible solution at BESIII: bin-by-bin fit
• Divide the mass spectrum into many (~100) bins.
• In each bin, we only fit various JPC components without BW
structure.
• We can perform PWA fits for all bins on 100 CPU.
Bin-by-Bin Fit
 Advantages
• Model independent for each JPC component in each
mass bin.
• Phase shift measured automatically
• Fast
 Disadvantages
• Detail mass information lost
• The constraint on the phase in nearby mass bin lost.
MC Input/Output Checks of Bin-by-Bin Fit
 We have a working group to check whether the
bin-by-bin fit can reproduce the input values
based on extensive MC studies.
Here I would only show some examples 
Example 1: One 0++ resonance & one 2++ resonance
Generated KK mass plot in J/KK (160K evts)
M  1750MeV
  200MeV
All
int
0++
2++
M  1790MeV
  80MeV
Error bar: bin-by-bin fit result
Histogram: generated mass plot
0++
2++
MKK
Fit mass plot based on bin-by-bin fit
0++
Gen
Fit
Dif
2++
Gen
Nevent
65567
63874±814 2.2σ
Mass
1750
1753.3±1.6
Width
200
186.5±3.9
Fit
Dif
Nevent
38099
39322±814 1.5σ
2.1σ
Mass
1790
1690.1±1.1
0.1σ
3.5σ
Width
80
83.4±2.8
1.2σ
Example 2: Two 2++ resonances
Generated KK mass plot in J/KK (140K evts)
2++ A
All
2++ B
0++ is not significant
0++
2++
Gen
Fit
Dif
Nevent A
104514
100936±793 4.5σ
Mass A
1950
1947.5±0.6
4.2σ
Width A
150
144.3±1.6
3.6σ
Nevent B
7746
6759±375 2.6σ
Mass B
2100
Width B
40
2099.7±0.9
0.3σ
34.7±2.8
1.9σ
Example 3: Phase Shift Measurement
Input :
0++: m=2.5GeV, =0.2GeV, =60
2++: Phase Space =0
Output (fitting the phase
shift curve):
0++
=61.49±6.90
M(0++)=2.5012±0.005GeV
(0++)=0.206±0.012GeV
M(2++)=1.227±0.403GeV
(2++)=32.273±17.906GeV
2++
 Bin-by-bin approach looks promising…
 However 
Example 4: Two 2++ resonances (300K events)
Error bar: bin-by-bin fit result
Histogram: generated mass plot
Resonance 1:
M=1970 MeV
2++
Г=180 MeV
Resonance 2:
M=2040 MeV
Г=20 MeV
2++ component cannot be reproduced.
0++ component is significantly inconsistent with zero.
0++
 Given this MC study, we really worry
about bin-by-bin fit result (Fortunately,
BESII did not have official results based
on bin-by-bin analysis).
But in this case, global fit can well reproduce th
Inputs.
2++
 Global fit results are more reliable than bin-bybin fit.
 More study are needed to understand this.
 If bin-by-bin approach cannot obtain robust
PWA results, what’s other solutions to speed
up PWA fit? – Questions still remains.
Q2: How to treat background?
 MC
• No reliable inclusive generator at low energy.
 Sidebands
• More reliable than MC
• In some cases, maybe unreliable due to
kinematic limits.
Q3: How to parameterizethe intermediate states?
 e.g., the  mass shape in J/ process,
FSI…
In the  mass range
of (1760)
M
Q4: How to treat small components?
 From PWA of J/  K+K- 0 at BESII, we see
that small components could have big
interference so as to affect the results
significantly.
Q5:How can we include data/MC
differences in the PWA fit?
 Such differences may results in fake
resonances in PWA based on our MC studies.
 Example:
• Testing sample: J/+-0 with two intermediate
states (770) and (1450)
• Simulation of data/MC difference from MDC wire
resolution  two different algorithms at BESII
• We add in (1700), (2100) , 3(1690) one by one and
then check the significance ( -2lnL ) of each
resonance
Δs= -2lnL as a function of the size of data sample
1.3M J/+-0 events ~ BESII data sample
Q5:How can we include data/MC
differences in the PWA fit?
 When the data sample is as large as twice of the BESII
sample, (1700), (2100) , 3(1690) will have a
significance > 5  due to the data/MC difference from
MDC wire resolution
 They are fake resonances needed in PWA fit.
 So We need to find a way to include the data/MC
difference in our fit, or at least we need to make the
correct judgment that whether the resonances are from
the data/MC difference rather than physics resonance.
Q6: How to judge whether we have reached a
real minimum in the PWA fit rather than local
ones?
 Especially when there are many resonances
(parameters) in one fit.
 Currently at BESII, we just tried hundreds of
different combinations of initial values of the
fitted parameters  very time consuming.
Summary
 Based on the experience of BESII, we can expect much
more unexpected discoveries and much more
opportunities at BESIII on hadron spectroscopy.
 With huge data samples at BESIII, we hope to answer
some fundamental questions such as whether
glueball/hybrids/multiquarks exist or not.
 PWA is a crucial to most physics results at BESIII.
There are many difficult problems to be solved in PWA
—— Suggestions of the solutions are welcome!
谢
Thank
谢！
You!
NO strong dynamical threshold enhancement in
pp
cross sections (at LEAR)
 With threshold kinematic contributions removed,
there are very smooth threshold enhancements in pp
elastic “matrix element” and very small enhancement in
annihilation “matrix element”:
 much weaker than what BES observed !
|M|2
|M|2
BES
Both arbitrary normalization
BES
Both arbitrary normalization
| M elastic |  ~  elastic
2
M ( pp)  2mp
| M ann |2  ~ Plab ann
Any inconsistency? NO!
 For example: with
Mres = 1859 MeV, Γ = 30 MeV,
J=0, BR(ppbar) ~ 10%, an estimation based on:
 res
(2 J  1)
4 (c)2
Bin Bout 2

2
(2S1  1)(2S2  1) Ecm
 4m2p ( Ecm  mres )2  2 / 4
At Ecm = 2mp + 6 MeV ( i.e., pLab = 150 MeV ), in elastic
process, the resonant cross section is ~ 0.6 mb : much
smaller than the continuum cross section ~ 94  20 mb .
 Difficult to observe it in
experimentally.
pp cross sections
Pure FSI disfavored (I)
1.
Theoretical calculation (Zou and Chiang, PRD69 034004 (2003))
shows: The enhancement caused by one-pion-exchange
(OPE) FSI is too small to explain the BES structure.
2.
The enhancement caused by Coulomb interaction is
even smaller than one-pion-exchange FSI.
|M|2
BES
|M|2
BES
Both arbitrary normalization
Both arbitrary normalization
one-pion-exchange FSI
M ( pp)  2mp
Coulomb interaction
FSI Factors
Most reliable full FSI factors are from A.Sirbirtsev et al.
( Phys.Rev.D71:054010, 2005 )，which fit ppbar elastic cross
section near threshold quite well.
ppbar elastic cross section
near threshold
I=1 S-wave
P-wave
I=0 S-wave
M ( pp)  2mp
In ppbar collision, the background is much lager (I)
p
p
p
p
>>
p
p
p
p

J /
p
p
 J/ decays do not suffer large t-channel
“background” as ppbar collision.
In ppbar collision, the background is much lager (II)
In ppbar elastic scattering, I=1 S-wave dominant,
while in J/ radiative decays I=0 S-wave dominant.
ppbar elastic cross section
near threshold
I=1 S-wave
P-wave
I=0 S-wave
A.Sibirtsev, J. Haidenbauer, S. Krewald, Ulf-G. Meißner, A.W. Thomas,
Phys.Rev.D71:054010, 2005
So, the mechanism in ppbar collision is quite
different from J/ decays and the background is
much smaller in J/ decays
It would be very difficult to observe an I=0
S-wave ppbar bound state in ppbar collisions
if it exists.
J/ decays (in e+e- collider) have much
cleaner environment: “JP, I” filter
From B.S. Zou, Exotics 05:
`pp near threshold enhancement is very likely
due to some broad sub-threshold 0-+ resonance(s)
plus FSI.
From A. Sirbirtsev :
FSI factors should be included in BW fit.
Discussion on I=1 S-wave FSI
Pure FSI disfavored (III) — I = 1
Pure I=1 S-wave FSI is disfavored by more than 3 .
FSI + BW
Pure FSI
M = 1773  21 MeV
 = 0  191 MeV
 2 ln L  85 .3
 2 ln L  65 .8
M pp  2mp
I=0 dominant in J/  radiative decays
 Most I = 0 states have been observed in J/  radiative
decays with big production rate ( especially for 0-+
mesons ) such as , ’, (1440), (1760), f2(1270),
f2(1525), f0(1500), f0(1710).
 The only observed I=1 meson in J/  radiative decays
is 0 with low production rate 4*10– 5, e.g., no evidence
for (1800) in J/   3  process.
It is unlikely to be from (1800) .
I=1 S-wave FSI seems unlikely.
ppbar bound state in NNbar potential
 Paris NNbar potential:
( Paris 93, B. Loiseau et al., hep-ph/0411218, 0501112 )
• For 11S0 , there is a bound state:
E = - 4.8 - i 26.3 MeV
quite close to the BES observation.
 However, Julich NNbar model:
( A. Sibirtsev et al., hep-ph/0411386 )
• For 11S0 :
E = - 104 - i 413 MeV
seems quite far away from BES observation.
They both predict an 11S0 ppbar bound state,
although they are quantitatively different.
BES II Preliminary
J /  
0
J /    


0
No (1800)
J /  
J /  '   ()
NO strong dynamical threshold enhancement in
pp cross sections (at LEAR)
 With threshold kinematic contributions removed, there
are very smooth threshold enhancements in pp elastic
“matrix element” and very small enhancement in
annihilation “matrix element”:
 much weaker than what BES observed !
|M|2
|M|2
BES
Both arbitrary normalization
BES
Both arbitrary normalization
| M elastic |  ~  elastic
2
M ( pp)  2mp
| M ann |2  ~ Plab ann
The large BR to ppbar suggest it could
be an unconventional meson
 For a conventional qqbar meson, the BRs
decaying into baryons are usually at least one
order lower than decaying into mesons.
• There are many examples in PDG.
E.g.
BR(c  )  (2.6  0.9)%
BR(c  pp)  (1.3  0.4)  103
 So the large BR to ppbar (with limited phase
space from the tail of X(1860)) seems very hard to
be explained by a conventional qqbar meson.
Analysis of
'
 


J       (     )


X(1835)
5.1 
Analysis of
'
J       (   )


X(1835)
6.0 
Comparison of two decay modes
 Mass and width from J       (      )
m=1827.48.1MeV/c2 , =54.234.5MeV/c2
 
 Mass and width from J       (    )
m=1836.37.9MeV/c2 , =70.323.1MeV/c2
J      (     )
B( J   X ) B( X    )  (1.8  0.7)  104
 
 J       (    )
B( J   X ) B( X    )  (2.3  0.5) 104

 The mass, width and branching fractions obtained
from two different decay modes are consistent with
each other.
Similar enhancement also observed in
 '  pK
4 away from phase space.
This enhancement is NOT observed
in p  K process at SAPHIR
BES
J /  pK
Discussion on KΛ mass threshold
enhancement NX(1610)
 NX(1610) has strong coupling to KΛ:
• From BR( J /  pp)  2 103
(S&D-wave
decay) and J /  pNX (1600) is a P-wave
decay, we can estimate
BR( J /  pNX )  1.0 103
• From BESII,
BR( J /  pNX ) BR( N X  K) ~ 2 104
BR( N X  K)  20%
• The phase space of NX to KΛ is very small, so
such a big BR shows NX has very strong
coupling to KΛ, indicating it has a big hidden
ssbar component. (5-quark system)
Non-observation of NX in p  K
suggests an evidence of new baryon :
 It is unlikely to be N*(1535).
If NX were N*(1535), it should be observed
in p  K
process, since:
•  (p  N *  K)  BR( N *  p) BR( N *  K)
• From PDG, for the N* in the mass range
1535~1750 MeV, N*(1535) has the
largest BR( N *  p) , and from previous
estimation, NX would also have almost the
largest BR to KΛ.
 Also, the EM transition rate of NXto proton
is very low.
Clear  and  signals



recoiling against 
Side-bands do not have mass threshold enhancement

Side-bands
Partial Wave Analysis of J/  K+K- 0 events
Four decay modes are included :
1 component :
J /  ( X ,  (1700)) 0 , ( X ,  )  K  K 
K * (890)   K  0
J /   ( K * )  K  , ( K * )   K  0
where K *  K * (890), K * (1410)
Amplitudes are defined by
Covariant tensor formalism
K * (1410)   K  0
points : data
hist : PWA fit
B.S. Zhou and D.V. Bugg, Eur. Phys. J. A16, 537(2003)
 BW with energy-dependent width
1
BW ( s ) 
;
2
s  M R  i s R ( s )
M R2 p( s ) 2l 1
R ( s )  R ( M )
(
)
2
s p( M R )
2
R
J.H. Kuhn, A. Satamaria, Z. Phys. C48, 445 (1990).
1  component :
X  K K 
 (1700)  K  K 
PS
Angular distributions for events with M K
from PWA fit
points : data,
Figures on the right:
 (a),(c),(e) are polar angles
in lab. reference frame
 (b),(d),(f) are polar angles
in CM frames of
K  0 and K  K  respectively

K

 1.7 GeV / c 2
hist : PWA fit
Broad X cannot be fit with known mesons
or their interference
 It is unlikely to be (1450), because:
• The parameters of the X is incompatible with (1450).
• (1450) has very small fraction to KK. From PDG:
Br (  (1450)  K  K  )  1.6 103 (95%C.L.)
 It cannot be fit with the interference of (770) ,
(1900) and (2150):
• The log-likelihood value worsens by 85 (c2=170).
Summary (I)
 BES II has observed several strong mass threshold
enhancements in J/ decays.
 Why strong mass threshold structures are important?
Multiquark states may be only observable near mass
thresholds with limited decay phase space.
 Otherwise, it might be too wide to be observed as a resonance
since it can easily fall apart into two or more mesons.
I can see
f0(980)
I can see
broad 
under
other
peaks
broad
resonance or
phase space?
any broad
resonance
under other
peaks?
Summary (II)
 A very narrow and strong pp mass
threshold enhancement is uniquely
observed in J /  pp decays at BES II:
• It is NOT observed in B or Y(1S) decays.
• Its large BR to pp suggests it be a pp bound
state.
 
J




 
 X(1835) is observed in
It could be same structure as the ppbar
mass threshold enhancement, i.e., it could
be a ppbar bound state.