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