Meson production with the

1

Meson production with the CELSIUS/WASA facility
in Uppsala
Karin Schönning, Dep. of Radiation Sciences,
Uppsala University
2

• Outline
• the CELSIUS/WASA experiment
• the pd ? 3He ? p0 reaction
• the pd ? 3He ? reaction
• the 3He trigger
• 3He identification
• candidates of the pd ? 3He ?
• reaction
• current analysis status
• summary and future prospects

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The CELSIUS/WASA experiment

• The physics topics studied at CELSIUS/WASA are
• meson production in nucleon-nucleon collisions
• ? decays

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The CELSIUS/WASA experiment

• Performing measurements on meson production and
• decay requires
• a high precision ion beam
• high luminosity
• capability of detecting charged and neutral
  particles
• in a solid angle close to 4p

and the WASA detector, a former integrated part
of the CELSIUS storage ring at the TSL facility
in Uppsala, Sweden, was (close to) fulfilling all
this!
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The CELSIUS/WASA experiment

• The CELSIUS (Cooling with Electrons and Storing
  of Ions from the Uppsala Synchrotron) was an 82 m
  storage ring that
• provided a proton beam
• with a maximum kinetic
• energy of 1450 MeV
• provided electron
• cooling up to a proton
• beam energy of 550 MeV

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The CELSIUS/WASA experiment

• The pellet target is an internal
• target that
• provides the high luminosity
• necessary for this kind
• of experiments
• is developed for a 4p detector
• geometry
• The pellet target system is based
• on the production of small frozen
• droplets of hydrogen or deuterium
• which are introduced through
• narrow tubes into the scattering
• chamber.

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The CELSIUS/WASA experiment
The WASA (Wide Angle Shower Apparatus) Detector
consists of
the Central Detector (CD) which measures
angles and energies of charged particles and
photons, mainly from meson decays the
Forward Detector (FD) which measures the
angles and kinetic energies of charged
particles,in particular scattered beam and
recoil particles
the Zero Degree Spectrometer that detects beam
and recoil particles at very small angles
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The CELSIUS/WASA experiment
PSB
MDC
FRI
Central detector
Forward detector
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pd ? 3He ? p0 at kinematic threshold
is interesting since we want to find out how
baryon resonances are involved in the production
mechanism. No known baryon resonance decays with
an appreciable fraction to a nucleon and an
?p-pair. Instead, it may be produced from two
different resonances the N(1535) and the
?(1232).
?(1232)
The cross section for the subprocess pn?pn? is 5
times larger than for the pp?pp?
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pd ? 3He ? p0 at kinematic threshold
At higher CM energies, a process involving an
intermediate a0(980) resonance is possible
?
a0(980)
p
3He
Measuring the cross section of the pd ? 3He ?
p0 reaction might shed light on the relative
importance of the mentioned processes .
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pd ? 3He ? p0 at kinematic threshold
can also give information on Final State
Interactions (FSI). When the ? and the p0 are in
a relative s-state, which is the case at the
kinematic threshold, they interact significantly.
The interaction is fully developed at an ?p0
invariant mass of 980 MeV with the formation of
the a0(980) scalar resonance. The a0(980) has
a width of 50-100 MeV and is likely not a regular
quark-antiquark state, but rather a dynamically
generated state of the ?p0 interaction. It could
also have some other, possibly exotic, feature.
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pd ? 3He ? p0 at kinematic threshold
At CELSIUS/WASA, the low energy tail (680 850
MeV) of the a0(980) will be probed. The low
energy ?p0 interaction can also have some other
dynamic feature, and we hope to learn more about
it from this experiment. Effects from the final
state interactions in the ?p0 system are
expected to show up in for example the ?p0
invariant mass distribution.
MC simulations of how FSI should influence the
?p0 invariant mass distributions. The curves show
the distribution with and without FSI.
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pd ? 3He ? at kinematic threshold
The amplitude squared of this reaction in
the backward direction was measured by the
Saturne collaboration 1 at 20 different beam
energies and it was found that it is very
suppressed near threshold. The reason for this
is debated

1 Wurzinger et al. Phys. Rev. C 51,
443
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pd ? 3He ? at kinematic threshold
Very little data on the total cross section
exist. With the WASA detector it is possible to
measure both channels (? ? p0pp- and ? ? p0?) at
two different beam energies, Ep 1450 MeV and Ep
1360 MeV, which corresponds to excess energies of
63 MeV and 19 MeV, respectively.
It is, with the WASA setup, also possible to
measure the angular distributions of the decay
products, which in turn can give information on
the ? polarisation
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pd ? 3He ? at kinematic threshold
Monte Carlo simulations show that the geometric
acceptance for the ? ? p0? channel is around 60
The total acceptance for the ? ? p0? channel,
including detector efficiencies, is around 20-25
The geometric acceptance of the ? ? p0pp-
channel is around 30 at 1360 MeV and around 20
at 1450 MeV
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The 3He trigger
The 3He trigger is based on the fact that 3He
deposit more energy than protons and deuterons in
the different detector layers of the Forward
Detector
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The 3He trigger
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The 3He trigger
We also tested different matching conditions in
the f angle between the high energy deposit hit
in the FWC and the hit in a downstream detector
layer. In December 2004 and in March 2005 we
required f matching between high energy deposit
in FWC and high energy deposit in the third layer
of the FTH. In March 2005, the condition above
combined with a requirement on at least one hit
in the FRH gave a relative 3He yield of 1.25
. Unfortunately, around 50 of the events
selected by the trigger were events with low
energy protons
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The 3He trigger
In May 2005 we required f matching between high
energy deposit in FWC and a hit in the FRH. The
approximate relative 3He yield was 1 . Still,
around 50 of the events selected by the trigger
were low energy proton events
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3He identification
pd data at 1450 MeV from March 2005
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Candidates of pd ? 3He ?
Candidates of the pd ? 3He ?, ? ? p0? reaction
have been reconstructed from data taken in June,
2004. A proton beam with a kinetic energy of
1450 MeV was used, and a deuterium target. The
trigger used at the time required high energy
deposit in the FWC, a matching hit in the third
layer of the FTH, at least two clusters in the CD
and no hits in the PS. The data were taken
during 3.5 shifts (1 shift 8 hours). Events
with one 3He in the Forward Detector 3? in the
Central Detector were analysed.
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Candidates of pd ? 3He ?

• The missing mass of the 3He was reconstructed
  when the following conditions were fulfilled
• there is one ?? combination in the event, with
  an invariant mass that is close to the p0 mass
  IM(??)-m(p0) lt 30 MeV
• the invariant mass of the 3 photons is close to
  the ? mass IM(???)-m(?) lt 100 MeV
• the missing mass of the 3He is relatively close
  to the invariant mass of the 3 photons
• lIM(???)-MM(3He)l lt 150 MeV
• the opening angle between the 3He missing
  momentum and the total momentum of the 3 photons
  is lt 25 degrees

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Candidates of pd ? 3He ?
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Candidates of pd ? 3He ?

• there is one ?? combination in the event, with an
  invariant mass
• that is close to the p0 mass IM(??)-m(p0) lt 30
  MeV
• the invariant mass of the 3 photons is close to
  the ? mass IM(???)-m(?) lt 200 MeV

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Candidates of pd ? 3He ?
Monte-Carlo simulations of the pd ? 3He 2p0
reaction, which should be the most important
background channel, show that this reaction
should give a much broader 3He missing mass
distribution than the one we see in our
data. The misidentification for 2p0
production using the cuts we have mentioned is of
the order of 1 .
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Current analysis status

• June 2004
• 3.5 shifts of data taking at 1450 MeV
• 35 000 3He candidates
• 100 ? ? p0? candidates
• December 2004
• 4 shifts of data taking at 1450 MeV
• 270 000 3He candidates
• Improved triggers that select the
• ? ? p0pp- channel as well
• Calibration needs to be understood
• before we can make a proper estimate of the
• number of ? candidates

The average luminosity was typically between
0.51030 and 3 1030 cm-2s-1
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Current analysis status

• March 2005
• 7.5 shifts of data taking at 1450 MeV
• 650000 3He candidates at 1450 MeV
• 3.5 shifts of data taking at 1360 MeV, but the
• 3He trigger was prescaled by a factor of 2-4
• Improved data aquisition system
• Analysis in progress
• May 2005
• 12 shifts of data taking at 1450 MeV
• 8 shifts of data taking at 1360 MeV
• Analysis is yet to be started

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Summary and future prospects

• å -the physics motivations for meson
• production, like pd ? 3He ? p0 and
• pd ? 3He ?, are great
• å the data so far carefully analysed taken
• during 3.5 eight-hour shifts - show promising
  
• results 100 ? ? p0? candidates
• å there are data from 35 more shifts to be
  analysed
• å I have more than two years left to spend on
  the analysis