Transcript temporary title - Istituto Nazionale di Fisica Nucleare

Charmonium Spectroscopy
The charmonium system has often
been called the positronium of QCD.
Non relativistic potential models (with
relativistic corrections) and PQCD
make it possible to calculate masses,
widths and branching ratios to be
compared with experiment.
In pp annihilations states with all
quantum numbers can be formed
directly: the resonace parameters
are determined from the beam
parameters, and do not depend on
energy and momentum resolution
of the detector.
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The c (11S0)
Despite the recent measurements by E835 not much is
known about the ground state of charmonium:
• the error on the mass is still bigger than 1 Mev
• recent measurements give larger widths than
previously expected
A large value of the c width is difficult to explain in
terms of simple quark models. Also unusually large
branching ratios into channels involving multiple kaons
and pions have been reported.
A precision measurements of the c mass, width and
branching ratios is of the utmost importance, and it can
only be done in by direct formation in pp.
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The c(11S0)
M(c) = 2979.9  1.0 MeV
(c) = 25.5  3.3 MeV
T. Skwarnicki – Lepton Photon 2003
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The c (11S0)
• Two photon channel c   (weak branching ratio
BR(c)=310-4).
• Hadronic decay channels, with branching ratios
which are larger by several orders of magnitude.
– c +-K+K– c 2(K+K-)
– c 2(+-)
– c KK
– c  
– ...
• c  pp
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Expected properties
of the c(21S0)
• The mass difference  between the c and the  can be related to the
mass difference  between the c and the J/ :
 s (M  ) M  (   e e )
 
  65 MeV
2
 
 s ( M J / ) M J /   ( J /  e e )
2
• Various theoretical predictions of the c mass have been reported:
– M(c) = 3.57 GeV/c2 [Bhaduri, Cohler, Nogami, Nuovo Cimento A,
65(1981)376].
– M(c) = 3.62 GeV/c2 [Godfrey and Isgur, Phys. Rev. D 32(1985)189].
– M(c) = 3.67 GeV/c2 [Resag and Münz, Nucl. Phys. A 590(1995)735].
• Total width ranging from a few MeV to a few tens of MeV:
 (c)  5  25 MeV
• Decay channels similar to c.
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The c(21S0)
Crystal Ball Candidate
The first ´c candidate was
observed by the Crystal
Ball experiment:
e  e       X
By measuring the recoil 
they found:
M(c )  (3594  5) MeV / c2
(c )  8 MeV (95 % C.L.)
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The c(21S0)
E760/E835 search
Both E760 and E835
searched for the c in the
energy region:
2
E cm  (3570  3660) MeV
using the process:
Crystal Ball
p  p  c    
but no evidence of a signal
was found
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c(21S0) search in
 collisions at LEP
The c has been looked for by the
LEP experiments via the process:
e e  e e  ( )c
L3 sets a limit of 2 KeV (95 %C.L.)
for the partial width (c).
DELPHI data (shown on the right)
yield:
(c   )
 0.34 (90% C.L.)
(c   )
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The c(21S0) discovery by
BELLE
The Belle collaboration has recently
presented a 6 signal for BKKSK
which they interpret as evidence for
c production and decay via the
process:
with:
M  2978  2(stat ) MeV
  22  20(stat ) MeV
M  3654  6(stat ) MeV
  15  24(stat ) MeV
B  Kc ; c  KSK  
M(c )  3654  6  8 MeV / c2
(c )  55 MeV / c2
in disagreement with the Crystal Ball
result, but reasonably consistent with
potential model expectations.
(DPF 2002).
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Log-scale
  c(21S0)
BaBar 88 fb-1
Preliminary
M(c) = 3637.7  4.4 MeV
(c) = 19  10 MeV
T. Skwarnicki – Lepton Photon 2003
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The c(21S0)
In PANDA we will be able to identify the c in the following
channels:
• two photon decay channel c  .This will require a
substantial increase in statistics and reduction in
background with respect to E760/E835: lower energy
threshold, better angular and energy resolution, increased
geometric acceptance.
• The real step forward will be to detect the c through its
hadronic decays, such as K+K- and .
• c  pp
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The hc(1P1)
Precise measurements of the parameters of the hc are of
extreme importance in resolving a number of open questions:
• Spin-dependent component of the qq confinement potential. A
comparison of the hc mass with the masses of the triplet P
states measures the deviation of the vector part of the qq
interaction from pure one-gluon exchange.
• Total width and partial width to c+ will provide an estimate of
the partial width to gluons.
• Branching ratios for hadronic decays to lower cc states.
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Expected properties of the hc(1P1)
• Quantum numbers JPC=1+-.
• The mass is predicted to be within a few MeV of the center of
gravity of the c(3P0,1,2) states
M cog
M(  0 )  3M( 1 )  5M(  2 )

9
• The width is expected to be small (hc)  1 MeV.
• The dominant decay mode is expected to be c+, which should
account for  50 % of the total width.
• It can also decay to J/:
J/ + 0
violates isospin
J/ + +suppressed by phase space
and angular momentum barrier
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The hc(1P1) E760 observation
A signal in the hc region was seen
by E760 in the process:
p p  h c  J /   0
Due to the limited statistics E760
was only able to determine the mass
of this structure and to put an upper
limit on the width:
M (hc )  3526.2  0.15  0.2 MeV / c 2
(hc )  1.1 MeV (90%CL)
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The hc(1P1) E835 search
E835 has performed a search for
the hc, in the attempt to confirm
the E760 results and possibly
add new decay channels.
Data analysis is still under way
in various decay channels
• hc  c +   ()+
• hc  c +   ()+  (4K)+ 
• hc  J/+0  (e+e-)+()
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The hc(1P1)
It is extremely important to identify this resonance and study its
properties. To do so we need:
• High statistics: the signal will be very tiny
• Excellent beam resolution: the resonance is very narrow
• The ability to detect its hadronic decay modes.
The search and study of the hc is a central part of the experimental
program of the PANDA experiment at GSI.
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Charmonium States above
the DD threshold
The energy region above the DD threshold at 3.73 GeV is very poorly
known. Yet this region is rich in new physics.
• The structures and the higher vector states ((3S), (4S), (5S) ...)
observed by the early e+e- experiments have not all been confirmed
by the latest, much more accurate measurements by BES. It is
extremely important to confirm the existence of these states, which
would be rich in DD decays.
• This is the region where the first radial excitations of the singlet and
triplet P states are expected to exist.
• It is in this region that the narrow D-states occur.
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The D wave states
• The charmonium “D states”
are above the open charm
threshold (3730 MeV ) but
the widths of the J= 2 states
1
3
and
D2 are expected
D2
to be small:
1, 3
D2  DD
1, 3
D2  DD * forbidden by energy conservation
forbidden by parity conservation
• Only the  (3770) , considered to be largely 3 D1 state, has
been clearly observed
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The D wave states
• The only evidence of another D
state has been observed at Fermilab
by experiment E705 at an energy of
3836 MeV, in the reaction:
Li  J /    X
• This evidence was not confirmed
by the same experiment in the
 
reaction pLi  J /    X
and more recently by BES
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New State Observed by
Belle
BK (J/+-), J/µ+µ- or e+e-
Possible Interpretations
•D0D0* molecule
•(13D2) state
•Charmonium hybrid
•...
M = 3872.0  0.6  0.5 MeV
 2.3 MeV (90 % C.L.)
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Charmonium States above
the DD threshold
It is extremely important to identify all missing states above
the open charm threshold and to confirm the ones for which
we only have a weak evidence.
This will require high-statistics, small-step scans of the
entire energy region accessible at GSI.
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Radiative transitions of the
J(3PJ) charmonium states
The measurement of the angular distributions in the radiative decays
of the c states provides insight into the dynamics of the formation
process, the multipole structure of the radiative decay and the
properties of the cc bound state.
pp   c  J /  e e
Dominated by the dipole term E1. M2 and E3 terms arise in the
relativistic treatment of the interaction between the electromagnetic
field and the quarkonium system. They contribute to the radiative
width at the few percent level.
The angular distributions of the 2 and 2 are described by 4
independent parameters:
a 2 (  c1 ), a 2 (  c 2 ), B02 (  c 2 ), a 3 (  c 2 )
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Angular Distributions of the c states
• The coupling between the set of  states and pp is described
by four independent helicity amplitudes:
– 0 is formed only through the helicity 0 channel
– 1 is formed only through the helicity 1 channel
– 2 can couple to both
• The fractional electric octupole amplitude, a3E3/E1, can
contribute only to the 2 decays, and is predicted to vanish in the
single quark radiation model if the J/ is pure S wave.
• For the fractional M2 amplitude a relativistic calculation yields:
E
a 2 (  c1 )  
(1   c )  0.065(1   c )
4m c
3 E
a 2 ( c2 )  
(1   c )  0.096(1   c )
5 4m c
where c is the anomalous magnetic moment of the c-quark.
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c1(13P1) AND c2(13P2) ANGULAR
DISTRIBUTIONS
pp   c 2  J /   e  e 
Production amplitudes : B
2
0
Decay amplitudes : a , a
2
3
pp   c1  J /   e  e 
Production amplitudes : B  0
0
Decay amplitudes : a
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c1(13P1) AND c2(13P2) ANGULAR
DISTRIBUTIONS
B02
B(  c 2  pp, J z  0)

 0.13  0.08  0.01
B(  c 2  pp)
B (  0  pp )


 50 
 therefore
B(  2  pp, J z  0)


Interesting physics. Good test for models
055
a 3  0.02-00..044
 0.009
Predicted to be 0 or negligibly small
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c1(13P1) AND c2(13P2) ANGULAR DISTRIBUTIONS
 a2 (  c1 ) 


 0.02  0.34
 a2 (  c 2 )  E 835
McClary and Byers (1983)
predict that ratio is independent
a 2 (  c1 )  0.002  0.032  0.004 of c-quark mass and
anomalous magnetic moment
039
a 2 (  c 2 )  0.09300..041
 0.006
 a2 (  c1 ) 



 a2 (  c 2 )  theory
5 E (  c1  J /  )
 0.676
3 E (  c 2  J /  )
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Angular Distributions of the c
states
The angular distributions in the radiative decay of the 1 and
2 charmonium states have been measured for the first time
by the same experiment in E835.
While the value of a2(2) agrees well with the predictions of
a simple theoretical model, the value of a2(1) is lower than
expected (for c=0) and the ratio between the two, which is
independent of c, is 2 away from the prediction.
This could indicate the presence of competing mechanisms,
lowering the value of the M2 amplitude at the 1.
Further, high-statistics measurements of these angular
distributions are clearly needed to settle this question.
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