Study of Charmonium States Study of Charmonium States formed informed inpp annihilations: pp annihilations: results from Fermilab E835results from Fermilab E835
Diego Bettoni
INFN – Sezione di Ferrara
for the Fermilab E835 Collaboration
International Conference
on Low Energy Antiproton Physics
Yokohama, March 3-7, 2003
OutlineOutline
• Introduction• Experimental Method• Physics results
– Charmonium decays to J/+X pp 1,2 J/ pp 0 J/
– Charmonium decays to c(11S0) 2 0
– Search for the c(21S0)– Search for the hc(1P1)– Radiative transitions of the J(3PJ) charmonium states– Proton e.m. form factors in the time-like region
• Summary and outlook
IntroductionIntroductionEver since its discovery in 1974 the charmonium system has
been considered a powerful tool for the understanding of the
strong interaction, and this is why it has often been called the
positronium of QCD.
Non relativistic potential models
+
relativistic corrections
+
perturbative QCD
make it possible to calculate masses, widths and branching
ratios to be compared with experiment.
s 0.3 2 0.2
WhyWhypp ?pp ?In e+e- annihilations only stateswith the quantum numbers ofthe photon JPC = 1-- can beformed directly via the processe+e- * cc. States withJPC 1-- are usually studiedfrom radiative decays, e.g. e+e- + In this case the measurementaccuracy ( for masses andwidths ) is limited by thedetector.
In pp annihilations all quantumnumbers are directly accessible.
The resonance parameters aredetermined from the beamparameters and do not dependon the detector energy andmomentum resolution.
The charmonium spectrumThe charmonium spectrum
Experimental MethodExperimental MethodThe accumulated beam of antiprotons(up to 51011p) is decelerated from theinjection momentum of 8.9 GeV/c to theformation energy of the resonance to bestudied. The beam momentum is thenvaried in small steps ( 150 KeV/c)and a scan across the resonance ismade. The formation of charmonium isdetected through e+ e – X and final states.The resonance
parameters (MR, R, BinBout) are extracted from the excitationcurve using a maximum likelihood fit.
4)(
4
)12(2
2
2
2
R
R
R
outin
BW
ME
BB
k
J
The E835 DetectorThe E835 Detector
pppp
KKKKpp
multipp
ccpp
XeeXJccpp
eeccpp
/
E835 eE835 e++ee–– EVENT SELECTION EVENT SELECTION
Hardware 1st level trigger: candidate events are
selected requiring two electrons (defined by
scintillators and Cerenkov counter concidences) and
two high invariant mass CCAL clusters (> 2 GeV).
Online software filter selects “golden” candidates by
requiring an inclusive invariant mass > 2.4 GeV
Event selection is based on electron id, event topology,
timing and kinematic fit.
• Electron ID is done using 8 variables from hodoscope counters dE/dx, Cerenkov photoelectrons, CCAL cluster shape variables. Background can be reduced to a few pb while keeping efficiency around 85%
• CCAL cluster timing (TDC) is used to reject pile-up clusters due to high intensity
•Event topology and kinematic fit are used to classify final states
eeJpp /2,1
1 2
Ecm(MeV) Ecm(MeV)
pp pp 00 J/ J/ + + e e++ee-- + +
46.19.00
0
0
10)3.01.4()(
1.00.18.9)(
2.04.04.3415)(
ppB
MeV
MeVM
E835 E835 event selection event selection
• Two high-energy deposits in the Central Calorimenter.
• Kinematical fit to hypothesis has > 5 % probability.
• |cos*|<(s) to improve signal to background ratio.
Backgrounds
pp 00 4; two of the photons missing (510-3 ).
pp 0 3; one of the photons missing (0.03 ).
Photons are lost either below the CCAL energy threshold of 20 MeV or outside
its acceptance (10o < < 70o).
Background from the above sources can be studied by first measuring the 00
and 0 cross sections, then using a Monte Carlo to predict their contribution to
the background.
cc(1(111SS00) )
]10)412()([
8.3)(
10)0.24.22()()(
0.24.20)(
0.11.21.2984)(
4
9.11.10.10.1
88.37.3
7.77.6
ppBwith
keV
BppB
MeV
MeVM
c
c
cc
c
c
PRELIMINARYPRELIMINARY
pp pp 22
003.0017.0)/(
)(
2
2
eeJBR
BR
pp pp 22
The results from previous experiments have beenre-calculated using the BR
values from 2002 PDG
The systematic differencebetween e+e- and ppexperiments has becomesmaller with the new PDGvalue for B(2J/)
pp pp 00 PRELIMINARY
pp pp 00 PRELIMINARY
800 10)55.018.152.6()()( BRppBR
using4
0 10)5.02.2()( ppBR 4
0 10)67.025.054.096.2()( BR
keVBR
sysstat
30.0
66.0
59.090.2)( 0
pp pp 00 PRELIMINARY
The results from previous experiments have beenre-calculated using the BR
values from 2002 PDG
• The mass difference between the c and the can be related to the mass difference between the c and the J/ :
• First candidate observed by the Crystal Ball
in the radiative decay of the .
• Recent discovery by BELLE in hadronic
B meson decays. Mass incompatible with
Crystal Ball.
The The cc(2(211SS00))
MeV65)ee/J(
)ee(
M
M
)M(
)M(2
/J
2
/Js
s
2
c c/MeV)53594()(M
2/)863654()( cMeVM c
• not seen in collisions at LEP
Both E760 and E835
searched for the c in the
energy region:
using the process:
but no evidence of a signal
was found
cc(2(211SS00) search) search
Crystal Ball
cpp
2
MeV)36603570(Ecm
cc(2(211SS00) E760/E835 limits) E760/E835 limits
)MeV8(107.3).(R.B)pp.(R.B
)MeV5(106.5).(R.B)pp.(R.B
'c
'c
8'c
'c
8'c
'c
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:
)MeV15(105.4).(R.B)pp.(R.B
)MeV10(100.5).(R.B)pp.(R.B
)MeV5(100.8).(R.B)pp.(R.B
'c
'c
'c
8'c
'c
8'c
'c
8'c
'c
In the restricted energy region 3589-3599 MeV:
0c /J /Jc
cc(2(211SS00) search in) search inother channelsother channels
• 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
• 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
Expected properties of the hExpected properties of the hcc((11PP11))
9)(M5)(M3)(M
M 210cog
A signal in the hc region was seen
by E760 in the process:
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:
The hThe hcc((11PP11))
E760 candidateE760 candidate
0c /Jhpp
)%90(1.1)(
/2.015.02.3526)( 2
CLMeVh
cMeVhM
c
c
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 hThe hcc((11PP11))
E835 searchE835 search
Radiative transitions of the Radiative transitions of the JJ((33PPJJ) )
charmonium statescharmonium statesThe 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.
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:
ee/Jpp c
)(a),(B),(a),(a 2c32c202c21c2
• 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:
where c is the anomalous magnetic moment of the c-quark.
Angular Distributions of the Angular Distributions of the cc states states
)1(065.0)1(m4
E)(a cc
c1c2
)1(096.0)1(m4
E
53
)(a ccc
2c2
c1c1(1(133PP11) AND ) AND c2c2(1(133PP22) ANGULAR DISTRIBUTIONS) ANGULAR DISTRIBUTIONS
eeJpp /1c
2
0
:amplitudesDecay
0 :amplitudes Production
a
B
eeJpp /2c
32
20
:amplitudesDecay
:amplitudes Production
a,a
B
2144 c1 events 95.0cos
c1c1(1(133PP11) AND ) AND c2c2(1(133PP22) ANGULAR DISTRIBUTIONS) ANGULAR DISTRIBUTIONS
6028 c2 events 95.0cos
c1c1(1(133PP11) AND ) AND c2c2(1(133PP22) ANGULAR DISTRIBUTIONS) ANGULAR DISTRIBUTIONS
01.008.013.0)pp(B
)0J,pp(BB
2c
z2c20
50
)0,(
)( therefore
2
0
zJppB
ppB
009.002.0a 055.0044.0-3
Predicted to be 0 or negligibly small
Interesting physics. Good test for models
c1c1(1(133PP11) AND ) AND c2c2(1(133PP22) ANGULAR DISTRIBUTIONS) ANGULAR DISTRIBUTIONS
004.0032.0002.0)(a 1c2
006.0093.0)(a 039.0041.02c2
34.002.0)(
)(
835E22
12
c
c
a
a
McClary and Byers (1983) predict that ratio is independent of c-quark mass and anomalous magnetic moment
676.0)/(
)/(
35
)(
)(
2
1
theory22
12
JE
JE
a
a
c
c
c
c
c1c1(1(133PP11) AND ) AND c2c2(1(133PP22) ANGULAR DISTRIBUTIONS) ANGULAR DISTRIBUTIONS
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.
Angular Distributions of the Angular Distributions of the cc states states
Proton e.m. form factorsProton e.m. form factorsin the time-like regionin 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+e-
First order QED predicts:
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.
*22
2*22
222
* cos14
cos12cos
E
pM G
s
mG
xs
c
d
d
The dashed line is the PQCD fit:
Proton magnetic form factorProton magnetic form factor
222 ln
ss
CG
p
M
s(GeV2)
Summary Summary 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 and 0
• 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.
Coming up from E835:
• Measurement of the decays
• Measurement of the e+e- angular distribution
• A new measurement of the 1 width
• Study of the 00,0 and final states
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.
• ..............
We look forward to carry out all these measurements atWe look forward to carry out all these measurements at
PANDA the proposed new experiment at the future GSI HESR. PANDA the proposed new experiment at the future GSI HESR.
OutlookOutlook