The c(21S0) Crystal Ball Candidate
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Transcript The c(21S0) Crystal Ball Candidate
Charmonium Spectroscopy:
Missing or Unconfirmed
States
Diego Bettoni
INFN – Sezione di Ferrara
International Workshop on Physics with
Antiprotons at GSI
GSI, June 6-8, 2002
Outline
• Introduction
• Unconfirmed or missing states
– The c(21S0)
– The hc(1P1)
– Charmonium states above the DD threshold
• Radiative transitions of the J(3PJ) charmonium states
• Proton e.m. form factors in the time-like region
• Conclusions
The charmonium spectrum
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 ) M2 ( e e )
65 MeV
2
s ( M J / ) M J / ( J / e e )
• 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.
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:
2
M(c ) (3594 5) MeV / c
(c ) 8 MeV (95 % C.L.)
The c(21S0)
E760/E835 search
Both E760 and E835
searched for the c in the
energy region:
E cm (3570 3660) MeV
using the process:
p p c
but no evidence of a signal
was found
2
Crystal Ball
The c(21S0)
E760/E835 limits
Upper limits on the product of the branching
ratios into the initial and final states can be set by
fitting the data to spin 0 resonance + a power law
background for different values of the total width.
From the combined E760/E835 data we get:
B.R.(c' pp) B.R.(c' ) 8.0 10 8 ( ' 5MeV)
c
B.R.(c' pp) B.R.(c' ) 5.0 10 8 ( ' 10MeV)
c
B.R.(c' pp) B.R.(c' ) 4.5 10 8 ( ' 15MeV)
c
In the restricted energy region 3589-3599 MeV:
B.R.(c' pp) B.R.(c' ) 5.6 10 8 ( ' 5MeV)
c
B.R.(c' pp) B.R.(c' ) 3.7 10 8 ( ' 8MeV)
c
c(21S0) search in
other channels
c
J /
0
c J /
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 )
The c(21S0)
BELLE candidate
The Belle collaboration has recently
presented a 6 signal for BKKSK
which they interpret as evidence for
c production and decay via the
process:
B Kc ; c KSK
with:
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).
M 2978 2(stat ) MeV
22 20(stat ) MeV
M 3654 6(stat ) MeV
15 24(stat ) MeV
The c(21S0)
The c is still waiting to be unambiguously identified.
To look for it in the two photon decay channel would 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 .
In addition to that, the comparison of the ratios
(c)/(c) and (cpp)/(cpp)
could shed light on the possible mixing of the c with a nearby
0+ glueball.
All this is ideally accomplished in direct pp formation at GSI !
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 qq 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 qq 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 cc states.
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
The hc(1P1)
E760 candidate
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)
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.
So far E835 has been unable to
confirm or deny the E760 result,
despite the presence of a clear
J/ signal in the hc region.
The hc(1P1)
Despite the considerable efforts of E760 and E835, the hc
continues to be seen by one experiment in only one channel.
It is extremely important to identify this resonance and
study its properties.
To do so we need:
• High statistics: the signal could be very tiny
• Excellent beam resolution: the resonance could be very
narrow
• The ability to detect its hadronic decay modes.
Once again, the proposed facility at GSI is the ideal place to
find and study this resonance.
Charmonium States above
the DD threshold
The energy region above the DD 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 DD 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.
The D wave states
• The charmonium “D states”
are above the open charm
threshold (3730 MeV ) but
the widths of the J= 2 states
3
D2 and 1D2 are expected
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
been clearly observed
3
D1 state, has
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
Charmonium States above
the DD 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 week evidence.
This will require high-statistics, small-step scans of the entire
energy region accessible at GSI.
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
cc 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 )
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, a3E3/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.
c1(13P1) AND c2(13P2) ANGULAR DISTRIBUTIONS
pp c 2 J / e e
Production amplitudes : B02
Decay amplitudes : a2 , a3
pp c1 J / e e
Production amplitudes : B0 0
Decay amplitudes : a2
c1(13P1) AND c2(13P2) ANGULAR DISTRIBUTIONS
2144 c1 events
cos 0.95
c1(13P1) AND c2(13P2) ANGULAR DISTRIBUTIONS
6028 c2 events
cos 0.95
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 )
therefore
50
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
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 ) 0.09300..041
0.006
a2 ( c1 )
a2 ( c 2 ) theory
5 E ( c1 J / )
0.676
3 E ( c 2 J / )
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.
Proton e.m. form factors
in the time-like region
The electromagnetic form factors of the proton in the time-like region
can be extracted from the cross section for the process:
pp e+eFirst order QED predicts:
4m 2p
d
2 2c 2
2
2
2 *
2 *
G
1
cos
G
1
cos
M
E
2 xs
s
d cos *
Data at high Q2 are crucial to test the QCD predictions for the asymptotic
behavior of the form factors and the spacelike-timelike equality at
corresponding values of Q2.
Proton magnetic form factor
PRELIMINARY
The dashed line is the PQCD fit:
GM
p
C
s
s 2 ln 2 2
The dot-dashed line represents the
dipole behavior of the form factor
in the spacelike region for the same
values of |q2|.
At the proposed facility at GSI it will
be possible to carry out the proton e.m.
form factors at the highest timelike q2.
Summary (I)
• Charmonium was discovered in e+e- annihilation: very
accurate measurements of the J/ and .
• The first pp experiment (R704 at the ISR) demonstrated
the feasibility of the technique.
• E760 and E835 have been very successful in producing a
wealth of new measurements:
High precision measurements of 1 and 2 masses and widths
High precision measurements of 2
Best measurements of 0 mass and width
Best measurements of 1 and 2 angular distributions
First observation of the c in pp annihilation, measurement of its
mass, total width and partial width to
– Observation of a signal in the hc region
– New limits on the c
– Measurement of the proton form factors at the highest timelike q2.
–
–
–
–
–
Summary (II)
Still there remains a lot to be done:
• Improve measurement of c mass (error still bigger than 2
MeV), width and branching ratios. Detect hadronic decay
channels.
• Identify unambiguously the c , measure its parameters
accurately, detect hadronic decay modes.
• Confirm/Find the hc(1P1)
• Find the states above the DD threshold
• Improve measurement of states angular distributions
• Measure the form factor of the proton at even higher q2.
• .......
The proposed experiment at GSI is the ideal facility to carry
out all these measurements !