Recent results from FNAL E835 on the study of charmonium states in proton-antiproton annihilation

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Recent results from FNAL E835 on the study of
charmonium states in proton-antiproton
annihilation

• Matteo Negrini
• Università degli Studi di Ferrara - INFN
• Photon 2003 Frascati, 07-11 April 2003

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Charmonium spectroscopy in annihilation

• 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 electromagnetic final states

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Antiproton beam and target

• TARGET
• Hydrogen gas-jet target
• Hydrogen clusters density
• 1 41014 atoms/cm3
• The H2 target density can be tuned to obtain
  constant luminosity
• L INST 21031 cm-2s-1
• Target dimension ? 7 mm
• BEAM
• Antiprotons are accumulated until the desired
  current is reach. Then they are stochastically
  cooled and decelerated to the desired energy
  (continuous beam)
• The total CM energy can be determined directly
  from the antiproton beam parameter
• sE?400keV - sE /E?10-4
• Beam dimension ? 5 mm

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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)
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Experimental technique

• The beam energy is moved to scan the resonance
  (precision sE?250 keV)
• The number of events N 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

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Event selection where final state includes ee-

• Selection of electron / positron candidates
• 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.

EW1EW2gt1.5
Background

• Kinematic fit

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c0 mass and width
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
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c0 mass and width
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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

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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

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c0?gg
Preliminary
Taking from the PDG
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hc?gg
Preliminary

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

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Interference between ?c0 ?p0p0 and the
continuum
Preliminary
Measurement of the cross section for the
process in the c0 energy region
Interfering (helicity 0)
Non-Interfering (helicity 1)
Resonant
500000 p0p0 candidates
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c0 branching ratios
Preliminary
Using the PDG value
To be compared with the PDG
Using the result
To be compared with the PDG
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Electric dipole trasition (P?Sg)

• The value obtained for G(c0?J/yg), using the new
  total width and the BR measurements, is
  consistent with the theory of electric dipole
  transition

G(J/yg)exp (keV) k (MeV) G/k3 (MeV-2)
c0 13033 304 (4.61.2)?10-9
c1 29050 390 (4.90.8)?10-9
c2 38952 430 (4.90.7)?10-9
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ee- final states selection at the y

• y and J/y detected through their ee- decay
• All the exclusive channels are 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 ? 32862 events
• 1.9 pb-1 off resonance ? 66 events

Preliminary
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y branching ratios
Preliminary
J/yh
J/yp0p0
J/ypp-
ee-
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Conclusions

• Charmonium states are studied in proton
  antiproton annihilation detecting electromagnetic
  final states
• Extensive study of the cc0 ...
• Mass and total width
• gg width
• Interference in p0p0 decay
• ... and of the hc
• Mass and total width
• gg width
• New measurement of BR(y?J/y X) and BR(y?ee-)