Search for ' atoms with DIRAC

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Search for .
atoms with DIRAC

• Y. Allkofer
• University Zurich
• Doktorandenseminar
• Zurich, October 2005

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Outline

• Part 1 DIRAC
• What is DIRAC ?
• How to detect pK atoms
• DIRACs experiment
• existing setup
• upgrade for K identification
• Part 2 Our contribution
• The aerogel Cherenkov counter
• The cosmic ray test setup
• Data and simulation
• How to improve basic designs (layers, WLS...)

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What is DIRAC ?
Experiment
determined by DIRAC t 2.91 0.49 10-15s
-0.62
The dominant decay channel of pionium A2p
Theory
The scattering length can be predicted using QCD
ChPT, t 2.9 ?0.110-15 s
DIRAC provides a test for low energy QCD
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Open questions and Motivation

• SU(2) ChPT (Chiral perturbation theory) is in
  good agreement with the experiment

Experiment t 2.91 0.42 10-15 s
vs.
Theory t 2.9 ? 0.1 10-15 s
-0.62

• Is SU(3) (introducing the s-quark) Chiral
  perturbation an accurate theory ?

DIRACs new goal determine of the p0K0
lifetime in order to check SU(3) ChPT
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How to measure the lifetime of those atoms?
NA

• In the target atoms can
• decay in p0, K0
• be exited in a long-lived state
  (and decay afterwards)
• break up in p , K

ApK
24 GeV/c
undetected
NA-nA
nA
detected
DIRAC goal Measurement of PbrNA/nA
Pbr
Pbr 1 PDecay(t) Plong-lived state(t)
Determinate by the cross section for relativistic
atoms and the evolution of the atomic state
population
t (fs)
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How to measure nA ?

• time correlated events are fitted for
  MeV/c with a background function (BGF) where
  no atomic pairs are expected
• the fit parameter are used for
  MeV/c
• the difference between the data and the fit are
  the atomic pairs

data
BGF
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The DIRAC experiment
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The upgrade of the DIRAC setup for ApK detection

• In order to detect pK atomic pairs, one needs to
  discriminate between pions, protons and kaons
• 2 new Aerogel Cherenkov counters for kaon-proton
  separation
• 2 new heavy gas Cherenkov counters (C4F10) for
  pion-kaon separation

Zurich Universitys contribution (focus of part
2)
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Particle Identification Summary
e p K p
e p K p
e p K p
e
p
K
p
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Location of new Cherenkov detectors

• The kaon momentum from a AKp atom is 3.5 times
  higher than the pion momentum

K
1 lt pplt 3 GeV/c 4 lt pK lt 8.9 GeV/c
p-
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The Aerogel Cherenkov Counter
Goal K-p separation between 3.9-8 GeV/c
3 independent detectors are needed in each
spectrometer arm
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The Aerogel Cherenkov Counter
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Cosmic ray test setup
Hamamatsu R1584 (UV-window)
Event rate 0.1 Hz
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Results
n1.05
10 cm
Npe
Number of Cherenkov photons N(1-1/(bn)2)
As a first approximation, we can expect 17 p.e.
for the aerogel n1.008
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Simulation
Final design (15 by 40
cm2)
Npe
n1.008, b 1
Only 4 Npe are expected
Number of PMs vs. Npe
thickness
Can we improve the number of photo electrons ?
New designs ?
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Layers ?
aerogel only

• Advantage of the layer design
• reduce the path length between the light
  production and the PM
• good design to introduce Wave length shifter
  (WLS)

More aerogel is needed in order to investigate
the layer design for bigger thicknesses
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WLS ?
Simulation
Created photons
Photons reaching the PMs
Labs 10 cm _at_ 250 nm
Cherenkov light production distribution1/l
Light with high wavelength has less absorption
The best detection efficiency is in the range
250-400 nm
Shifting the light from UV to blue should improve
the light collection efficiency
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WLS ?
10 cm aerogel n1.05
aerogel layers and tetratex (without WLS)
Only aerogel
Intensity
POPOP (15150,15080)
TPB (88020)
pT (86470)
Other WLS
240
280
320
360
400
Wavelength (nm)
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Expected Npe for the final design

• For the final design
• n1.008
• box design
• size 15 x 40 cm2
• thickness 15 cm
• layers and p-terphenyl

• With the assumption
• the Cherenkov light production for the light
  aerogel (n1.008) is 17 of the heavy one
  (n1.05)
• b for the muons in the cosmic ray setup lt1
• the absorption length is the same for the 2
  aerogels
• the WLS shift all the photons in the region
  where the absorption length is in the order of
  the meter
  
  .
  of absorbed light is the same for a small box (10
  x 15 cm2) then for the final design

p (GeV/c) Npe 4.5 4.3 5 6.8
7 12.3 8 13.7 9 14.6
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Conclusion

• Stacking aerogel with the layer design improves
  the linearity between Npe and the thickness.
• p-terphenyl on Tetratex increases the Npe.
• How to apply the WLS ?
• Is it possible to evaporate the WLS on the
  Tetratex ?
• If yes, which thickness is the optimal one?
• Can we improve the detection efficiency by
  coating directly the PM glass with a WLS ?
• How is the time dependence on the quality of
  aerogel?

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What kind of p K- pairs are detected?
pK atoms NA (produced), nA (ionized)
Accidental pairs Nacc
Non Coulomb pairs NnC
Coulomb pairs, NC
Nreal
N total number of detected pK pairs
Nacc Nreal Nacc Nfree nA Nacc NC
NnC nA
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Pbr measurement NA

• Atom and Coulomb pairs are produced by the same
  mechanism
• ApK is a bound state (Coulomb wave function)
• C pairs are asymptotically free (Sommerfeld wave
  function)

ApK
p
K
Since NC is known using the fit parameters, we
have NA
Their relative yield is given by K NA/NC0.615