New Observations on Light
Hadron Spectroscopy at BESIII
Yanping HUANG
For BESIII Collaboration
Institute of High Energy Physics (IHEP)
ICHEP2010, Paris, Jul 21-28, 2010
Outline

Introduction

pp mass threshold study in J/y and y’
radiative decays

Confirmation of X(1835) and observation
of two new resonances in J / y    

 
J
/
y



Observation of X(1870) in

Summary
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Introduction

Multi-quark states, glueballs and hybrids have been
searched for experimentally for a very long time, but none
have been established.

However, during the past three years, a lot of unexpected
experimental evidence for hadrons cannot (easily) be
explained by the conventional quark model.
For example, at BESII:


pp threshold enhancement was observed in J /y   pp
 
X(1835) was observed in J /y     '
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The BEPCII/BESIII Project
Double-ring collider
Designed Luminosity: 1×1033 cm-2s-1
Record Luminosity : 3.2  1032cm-2s-1
6）
（10
200
J/y
150
y’
BESIII
CLEO
BESII
BESI
CBAL
MKIII
100
50
0
Beginning of 2004, construction starts
Apr. 14, 2009: ~106 M ψ’ events
(42.3pb-1 at 3.65GeV)
July 28, 2009: ~226 M J/y events
We have opportunities to confirm the existence of pp threshold
enhancement and X(1835) at BESIII… and for new observations!
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pp mass threshold study
in J/y and y’ radiative decays
Observation of pp mass threshold enhancement @ BESII
J / y   pp
3-body phase space
X(1860)
BESII
theoretical speculation:
 pp bound state (baryonium)
 FSI effect
 ……
0
0.1
acceptance
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0.3
Mpp-2mp (GeV)
0.2
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Several non-observations
(1S )   pp@CLEO
PRD 73 (2006) 032001
J / y   pp@BESII
EPJ C53 (2008) 15
y    pp @ BESII
PRL 99 (2007) 011802
No significant narrow strong
enhancement near threshold
(~2 if fitted with X(1860))
Mpp (GeV)
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pp Mass Spectrum and Dalitz Plot
y     J /y ,J /y   pp
3-body phase space
Narrow threshold enhancement is evident in
mass spectrum.
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pp
8
Mass spectrum fitting
y     J /y ,J /y   pp
J / y   pp
BESII
0
0.1
0.2
0.3
Mpp-2mp (GeV)
M=1859 +3 +5 MeV/c2
M=1861 +6 -13+7-26 MeV/c2
G < 30 MeV/c2 (90% CL)
G < 38 MeV/c2 (90% CL)
10 25
Chinese Physics C 34, 421 (2010)
PRL 91 (2003) 022001
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pp invariant mass spectrum and Dalitz plot
J / y   pp
BES III
Preliminary
Significant narrow threshold enhancement exists
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Mass spectrum fitting
J / y   pp
BES III
Fit result:
Preliminary
Mass=1861.6  0.8MeV / c2
G  8 MeV(90%CL)
Study on more sophisticated fits,
such as including FSI, is ongoing
BESII
M=1859
+3 +5
MeV/c2
10 25
G < 30 MeV/c2 (90% CL)
0
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0.1
0.2
0.3
Mpp-2mp (GeV)
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pp threshold mass spectrum in y’ radiative decay
y    pp
y    pp
BESII
BES III
Preliminary
PRL 99 (2007) 011802Mpp (GeV)
No significant narrow strong
enhancement near threshold
(~2 if fitted with X(1860))
No obvious narrow
threshold enhancement
Pure FSI interpretation of the narrow and pp
strong threshold enhancement is disfavored.
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Confirmation of X(1835) and
observation of two new structures in
 
J y     
Observation
of
X(1835)
in
 
J y      at BESII
BESII result (Stat. sig. ~ 7.7 ) :
M  1833.7  6.1( stat )  2.7( syst ) MeV
G  67.7  20.3(stat)  7.7(syst)MeV
PRL 95,262001(2005)

Confirmation of X(1835) is necessary with high statistic data
sample.

LQCD predicts the 0-+ glueball mass is ~2.3GeV.

A 0-+ glueball may have similar property as c (the main c decay
mode is ’).
 It
is important and interesting to study J /y     
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Mass spectrum of   

  

  
BES III
BES III
Preliminary
Preliminary
Combination for  to +- and 
 X(1835) and c is observed.
BES III
Preliminary
 Two additional structures at
M~2.1GeV and 2.3GeV
 There maybe some f1(1510).
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Fitting for the combined mass spectrum


Fitting with four resonances (acceptance weighted BW  gauss)
Three background components:
① Contribution from non-’ events estimated by ’ mass sideband
② Contribution from J /y   0   '( '   ) with re-weighting
method
③ Contribution from
“PS background”
1/ 2
3/ 2
5/ 2
fbkg ( x)  ( x  m0 )
 a0 ( x  m0 )
 a1( x  m0 ) , m0  2m  m
Red line: estimated contribution of ①+ ②
Black line: total background
BES III
Preliminary
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Stat. sig. is conservatively estimated:
fit range, background shape,
contribution of extra resonances
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
X(1835) resonance is confirmed at BESIII, but the
width is significantly larger than that measured at
BESII with one resonance in the fit.

Two new resonances, X(2120) and X(2370), are
observed with significances larger than 7.2 and
6.7 respectively.

PWA is needed not only to determine the spinparities of above three resonances, but also to make
more precise measurements on masses, widths and
BRs by considering possible interferences among
them.
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Observation of X (1870)  a0 (980) in
 
J y    
Analysis of J y     @BESIII


• With a0(980):
a0(980)
• Veto a0(980):
X(1870)
f1(1285)
’
(1405)
BES III
Preliminary
BES III
Preliminary
BES III
Preliminary
The f1(1285), (1410) and X(1870) primarily decay via a0(980)±
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Mass spectrum fitting


Fitting with three resonances (acceptance weighted BW  Gauss)
Background component described by Polynomial function
Fit result ( stat. sig.~ 7.7 )
M 1873  11MeV
BES III
Preliminary
G 82  19 MeV
Whether the X(1870) is the X(1835) or 2(1870), or a new resonance,
needs further study.
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Summary


X (1860)  pp is confirmed in J/y radiative decay, and
no obvious similar structure was observed in y’
radiative decay.
 
J
/
y


  ,
X (1835)      is confirmed in


and two new resonances, X(2120) and X(2370) are
observed with significance larger than 7.2 and 6.7
respectively.

A new process J/yX(1870)a0 is observed.

Whether or not the X(1860), X(1835) and X(1870) are
the same resonance, still needs further study.
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Summary on the previous
experimental results

The strong and narrow pp mass threshold
enhancement has only been observed in J/y
radiative decay, not in any other place so far.

Any model trying to interpret the mass threshold
enhancement should also answer why it is not
observed in other places, especially in y(2S) and
ϒ(1S) radiative decays as well as in J / y   pp
process.
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Mass spectrum fitting method

Fit function:
 signal: acceptance weighted S-wave BW
function:



q(2l 1) k 3
BW ( M ) 
( M 2  M02 )2  M 2G 2
q : the proton momentum in cms of ppb
k : the photon momentum
k: the ppb orbital angular momentum
 background shape: f bkg ( )  
1
2
 a1
3
2
 a2
5
2
a1 and a2 are obtain from a fit to an uniform phase space MC
sample
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pp threshold enhancement @ CLEOc
 CLEO-c does fit the same as BES and obtains:
agrees with BESII results [PRL91(2003)022011].
CLEO-c
preliminary
QWG2010
Z. Metreveli
 CLEO-c fits with three contributions:
BES considered these (2) and (3) as systematic errors.
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Event
selection for
 
J / y    (  ,      )

Initial selection criteria：

Ncharged=4，N>=2
 Nπ>2
 Kinematic fit(4C):
J / y   0       
J / y      
J / y       (   0 )
 24C (      )<40
 24C (     )< 24C ( K+K-   )

Final selection criteria：
 Reduce background from  0     :
m  m  0.04GeV
m  m  0.03GeV
0.72GeV  m  0.82GeV

Selection for  and  signal:
| M   m  | 0.2GeV
| M   m  | 0.018GeV
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Event selection for J / y    (    )

Initial selection criteria：



Ncharged=4，N>=2
Nπ>2
Kinematic fit(4C,5C):
 24C (      )<40
 25C (     )<40

Final selection criteria：
 selection for  and   signal:
| M   m | 0.03GeV
| M   m  | 0.01GeV
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Fitting for the combined mass spectrum
Fitting with one resonance as BESII
Fit result (Stat. sig.~20 ) :
M  1839.9  3.0( stat ) MeV
BES III
G 104.5  10.5( stat ) MeV
Preliminary
BESII result (Stat. sig. ~ 7.7 ) :
M  1833.7  6.1( stat )  2.7( syst ) MeV
G  67.7  20.3(stat)  7.7(syst)MeV
BESII
2010-7
X(1835) confirmed by BESIII
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Mass spectrum of   


Fit with one resonances + polynomial background:
  
BES III
Preliminary
BES III
Preliminary
Stat. sig. ~9
Stat. sig. ~18
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  
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Mass spectrum fitting


Fitting with three resonances (acceptance weighted BW  Gauss)
Background component described by Polynomial function
BES III
Preliminary
Study of Background component is ongoing
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