Charmonium Production with Antiproton Gas Jet Interactions at FNAL

1
Charmonium Production with Antiproton - Gas Jet
Interactions at FNAL

• Matteo Negrini
• University of Ferrara - INFN

for the E835 collaboration Fermilab, Ferrara,
Genova, Northwestern, Minnesota, Irvine, Torino
2
Why ?

• ee- annihilation ? only JPC1- states directly
  formed (J/y and y)
• gg fusion ? all C (J?1) states directly
  accessible
• B factory
• annihilation ? all the states directly
  formed through 2 or 3 gluons intermediate states

• Large hadronic background
• Detection of EM final states

3
FNAL Antiproton Accumulator
4
FNAL Antiproton Accumulator

• Antiprotons are accumulated until the desired
  current (50 mA) is reach. Then they are
  stochastically cooled and decelerated to the
  desired energy (continuous beam)
• Energy determined from the orbit length L and the
  revolution frequency f
• L from reference orbit (LREF) measured at the y
  DL/L 2?10-6
• f measured with precision Df/f 10-7
• The error on the beam energy measurement is
  sE?50-150 keV
• Energy spread after stochastic cooling sE?400
  keV. At the charmonium mass scale

5
Hydrogen Gas-Jet Target

• Jet of clusterized H2 molecules
• Density up to 1-41014 atoms/cm3
• Instantaneous luminosity up to 31031 cm-2s-1
• Luminosity is measured from elastic at
  90 with 3 precision
• Interaction region 5?5?7 mm3

6
Experimental technique

• Charmonium is formed in the complete annihilation
  of so the ECM is determined from the beam
  parameters
• The beam energy is moved to scan the resonance
• The number of events N of a given final state at
  energy E is obtained as
• L instantaneous luminosity
• G(E) beam energy distribution (gaussian)
• e detection efficiency
• The resonance cross section is obtained by
  deconvolution of the measured rate with the beam
  profile

7
Charmonium spectrum

• hc' observed by Crystal Ball with mass 3594 MeV,
  later observed by Belle with larger mass (3654
  MeV)
• hc (observed by E760 at M3526.2 MeV) needs
  confirmation

8
Historical review - 1

• R704 at CERN (1984)
• pioneered the technique and proved the
  feasibility
• 3 pb-1 collected on J/y and c1,2
• E760 at Fermilab (1990-1991) - 30 pb-1
• First observation of 1P1 resonance
• Precision measurement of cc1 and cc2 masses and
  widths
• First direct observation of hc in
  annihilations and mass measurement
• Search for hc
• Coupling to of J/y, y and c1,2
• Proton form factor in the time like region

9
Historical review - 2

• E835 at Fermilab (1996-1997) - 143 pb-1
• Measurement of c0 mass and width
• Measurement of c0,2 ???
• cc1 and cc2 angular distributions measurement
• Proton form factor in the time like region
• Precise measurement of the hc parameters
• Extensive search of the hc state
• Improved y branching ratios measurements
• Study of the ff final state

10
Historical review - 3

• E835-II at Fermilab (2000) - 113 pb-1
• Extensive study of c0 state
• Data taken in the 1P1 energy region to confirm
  resonance
• cc1 and cc2 mass and width measurement
• Measurement of y branching ratios
• Proton form factor in the time like region

11
The E835-II detector
Threshold Cerenkov counters (Separation of high
energy e from the hadronic background)
Electromagnetic calorimeter (12ltqlt68)
Luminosity monitor (Solid state detector. Counter
of elastic interactions at 90)
Charged tracking system (15ltqlt60)
12
Detector acceptance and resolution

• The detector has cylindrical symmetry around the
  beam axis with 2p azimuthal coverage
• Central calorimeter
• Acceptance 11 lt q lt 70
• Energy resol.
• Angular resol. sq?6mrad, sf?11mrad
• Forward calorimeter
• Acceptance 2 lt q lt 11
• Position resol. sx?5cm
• Inner tracking
• Acceptance 15 lt q lt 55
• Angular resol. sq?4mrad, sf?8mrad

13
Selection of J/y,y'?ee- final states

• Selection of high invariant mass ee- candidates
  from y'?ee- or J/yX?ee-X
• high energy deposition in calorimeter
• signal in the hodoscopes
• signal in Cerenkov
• Electron Weight (EW)
• Maximum likelihood method for the single
  electron selection based on calorimeter cluster
  shape and pulse height in Cerenkov and
  hodoscopes.
• Decay products reconstructed in the detector and
  final state classified with kinematic fit

EW1EW2gt1.5
Background
14
Example of y?ypp- event
Charged tracking Cerenkov calorimeter f view
Charged tracking q view
15
c0 mass and width measurement (E835-II)

• Luminosity 33 pb-1 (20 pb-1 on resonance) on
  17 energy points
• Selected channel radiative decay to J/y
• N. Selected events 400

Electron Weight
Kinematic Fit
16
c0 mass and width measurement (E835-II)
S. Bagnasco et al., Phys. Lett. B533 (2002) 237
17
c1 and c2 scan (E835-II)
Preliminary

• New measurement of mass and width of the c1 and
  c2
• Luminosity 6 pb-1 (c1) - 1 pb-1 (c2) - 11
  pb-1 (bkg)

18
Results for ? ccJ ? J/yg ? ee-g
T. A. Armstrong et al., Nucl. Phys. B 373 (1992)
35
M. Ambrogiani et al., Phys. Rev. Lett. 83 (1999)
2902
19
Electric dipole trasition (P?Sg)

• The value obtained for G(c0?J/yg), anomalously
  large in the past, is now consistent with the
  theory of electric dipole transitions

20
Angular distribution ? y' ? ee-
Preliminary
E835-I (2579 events)
E835-II (4822 events)
Combining the two data set
Measurement of the helicity 0 and 1 amplitudes
for the interaction
21
y' branching ratios
Preliminary

• y and J/y detected through their ee- decay.
  The observed channels are
• All the exclusive channels are fully
  reconstructed and selected with kinematic fits
• 14.3 pb-1 of data in the y energy region
  collected in year 2000
• 12.4 pb-1 on resonance ? 32000 events
• 1.9 pb-1 off resonance ? 60 events

22
y' branching ratios
Preliminary
T. A. Armstrong et al., Phys. Rev. D 55 (1997)
1153
M. Ambrogiani et al., Phys. Rev. D 62 (2000)
032004
23
y' branching ratios
Preliminary
J/yh
J/yp0p0
J/ypp-
ee-
24
Proton form factor in the time-like region

• Measurement of non resonant cross section for the
  process
• First order QED prediction
• At threshold GEGM (uniform angular
  distribution)
• At high s GE contribution negligible
• QCD asymptotic behavior

C and L free parameters
Upper limits at 90 C.L.
M. Andreotti et al., Phys. Lett. B 559 (2003) 20
25
gg final state selection

• Exactly 2 on-time clusters in the central
  calorimeter with high energy deposit and
  invariant mass within 20 of ECM
• No undetermined-time extra clusters with
  invariant mass within 35 MeV of the p0 mass
• 4C kinematic fit to gg
• cos(q) cut to improve signal to background
  ratio

26
gg background (feeddown)

• Background mainly from
• where one or more photons are missing because of
  acceptance or calorimeter energy thresholds
• Measurement of the cross section for the
  background processes and Monte Carlo
  determination of the background contribution
• Comparison with measured gg cross section for
  off-resonance points

27
hc?gg

• 18.9 pb-1 of data
• All the resonance parameters are measured in the
  gg channel

M. Ambrogiani et al., Phys. Lett. B 566 (2003) 45
28
hc??? search

• E760 hc search in the CBAL energy region (3594
  MeV)
• E835-I scan on a wider energy range

c2
T. A. Armstrong et al., Phys. Rev. D 52 (1995)
4839
M. Ambrogiani et al., Phys. Rev. D 64 (2001)
052003
29
c0?gg (E835-II)
Preliminary
Interference
No Interference
Taking from the PDG
30
p0p0 and hh event selection

• p0 and h detected through their gg decay
• 4g neutral events
• Search for p0 and h candidates cutting on
  invariant mass and opening angle of the photon
  pairs
• 4-C kinematic fit to

31
Study of ?p0p0 in the c0 region

• Interference between resonant and non resonant
  process enhancing the resonance signal

Interfering (helicity 0)
Non-Interfering (helicity 1)
Resonant
M. Andreotti et al., hep-ex/0308055, to be
published on PRL
32
Study of ? hh in the c0 region
Preliminary
33
Conclusions

• Charmonium states have been investigated at FNAL
  in proton antiproton annihilation obtaining...
• Precision measurement of mass and width of the
  charmonium states
• Measurement of branching ratios
• Study of radiative decays
• Search for states that need confirmation
• Interference between resonant and non-resonant
  multihadron production
• Proton form factor in the time-like region at
  s10-15GeV2