evidence for -atoms with dirac-ii yves allkofer university of zurich 20 th november 2008 r & d...
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Evidence for -atoms with DIRAC-II
Yves Allkofer
University of Zurich
20th November 2008
• R & D development
• Data taking
• Analysis and results
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Introduction to scattering length
ψ =exp(ikz)+f(θ ) ⋅exp(ikr)
r
Incoming (outgoing ) large distance wave function
atoms have 2 isospin contributions: 1/2 and 3/2
a01/2 and a0
3/2
Scattering length a0
-V0
r
sin(kr+)
sin(k’r)
ψ
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a 03
/2[m
-1]
theo
ry
a01/2 [m
-1]
S-wave scattering length ChPT
Dispersions relationsP.Buettiker et al,Eur.Phys.J C33 (2004) 409
V.Bernard et al.,Nucl.Phys. B357 (1991) 2757
m (a01/2 −a0
3/2 ) ; 0.238 ±0.002
m (a01/2 −a0
3/2 ) ; 0.269 ±0.015
K scattering lengths
P.Estabrooks et al.,Nucl.Phys.B133(1978)490
expe
rim
ent
m (a01/2 −a0
3/2 ) ; 0.475 ±0.070p
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K-atoms
• E 2S-2P = -0.3eV (em inter.) -1.1±0.1eV (strong inter.)
The decay channel of K-atoms:
J. Schweizer, Phys. Lett. B 587 (2004) 33• 1S = 3.7 ± 0.4 fs
rB
• rB (1S) = 1/()= 250 fm
• E (1S) = 22= -2.9 keV -9 eV (E strong inter.)
What we aim to measure
• Alternatively, DIRAC measures the scattering length through the lifetime of K-atoms.
DIRAC’s approach:
• DIRAC measures the number of ionized pairs so-called atomic pairs (competing process to the decay).
• The number of ionized pairs depends on .
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The DIRAC-II spectrometer
A
Pt
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One C4F10 module with n=1.0014
3 aerogel modules in left arm: 2 with n=1.015 and 1 with n=1.008
The Čerenkov detectors
e
Aerogel (coincidence)
Heavy gas (veto)
N2
(veto)
e K || p
e || K pe K p
Kp
One N2 module with n=1.0003 (1bar)
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Čerenkov radiation and aerogel
AEROGEL:• is identical to quartz with low density
= 30 - 300 mg/cm3
• has a refractive index between gas and solid radiator. n=1 + 0.21
• poor optical transmittance due to absorption and Rayleight scattering.
For a kaon/proton separation one has to choose a radiator for which the v(kaon)>c/n and v(proton)<c/n
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DIRAC’s requirement for K/p separation
n=1.015
n=1.008
6
4
8
GeV/c
Kaon spectrum
50 60 70 horizontal position (cm)
80
H1H2
L1 Issues :
• Distance between 2 PMs large
• For the n=1.008 module the light production is small
• kaons suppressed by a factor 6 compared to pions and protons
Simulation
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The cosmic ray test setup
LED
n=1.05
Results are used to tune the Monte-Carlo
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n=1.015
GEANT4 Simulation
n=1.008• strong impact position dependence due to absorption in aerogel
• low light yield due to a low refractive index
• Need for new designs
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Npen=1.05
Simulation
Vertical position (cm)N
pe
The pyramid design
The pyramid design cancels out the impact position dependence
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Labs =0. 1 m @ 250 nm Labs = 1 m @ 310 nm
0.1
1
10
Wavelength (nm)
250 350 450 550
Absorption length Labs (m)
Wavelength shifter?
Simulation
Wavelength (nm)
200 600
Created photons
Photons reaching the PMs
400 100 500300 700
Wavelength (nm)
Quantum efficiency (%)
28%
Shifting light from UV to blue improves the light collection
efficiency
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The aerogel counter• Aerogel with n=1.008
• 14 liters (250 pieces)
• Aerogel with n=1.015
• 24 liters (248 pieces)
Placement of aerogel
Budker/ Boreskov Institute Novosibirsk
Matsushita (Panasonic) Electric Works
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First run in 2007: detector performance(no beam in 2006 due to PS magnet breakdown)
• using the heavy gas and N2 Čerenkov as veto only kaons and protons are remaining.
• How to disentangle them? We need a pure proton and kaon beam!
Kaons (K-)
Kaon detection efficiency:
for L1: 85 - 90% for H1, H2: 95%
Proton rejection factor:
for L1: 10-15 for H1,H2: 20
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K atoms NA
(produced)Accidental pairs
Nacc
Non Coulomb pairs NnC
Coulomb pairs, NC
SignalBackground
Data analysis: event types
p-
Time correlated events
0 K0 K± mdecay ionization
detected
Atomic pairs
DIRAC looks for an excess of pairs with a very low relative momentum Q
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• electrons/positrons rejection (N2 Čerenkov counter, preshower detector)
• muons rejection (muon counter, preshower detector)
• pions rejection (heavy gas Čerenkov counter)
• proton rejections for K+- candidates (aerogel Čerenkov counter)
• |QL|< 20 MeV/c
• |QT|< 8 MeV/c
• 4 < P(kaon) < 8 GeV/c
• 1.2 < P(pion) < 2.1GeV/c
Event selections
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Background description
Data: accidental pairs
Monte-Carlo: Coulomb pairs
Monte-Carlo: non-Coulomb pairs
• Non-Coulomb and accidental pairs have the same shape and can be extracted directly from data using timing
• Coulomb pairs have an enhancement for low QL and are simulated
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Coulomb correlated -K+-pairsAccidental pairs Prompt pairs (accidental Coulomb
and non Coulomb pairs)|QL| distribution for time correlated events (Coulomb pairs, accidentals and non-Coulomb pairs) divided by accidental pairs.
Existence of Coulomb correlated K+ pairs is demonstrated without the use of Monte-Carlo.
NC=858±247 in signal region.
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Expected atomic-pairs
The number of Coulomb pairs (NC) and produced atoms (NA) are proportional:
NA =k·NC, k=0.237NA = 198 ± 57 (produced atoms)
Pion is the number atomic pairs (nA) divided by the number of produced atoms (NA):
Pion =nA
NA=
nA
k·NC=53%From theory
nA = 104 ± 30 (atomic pairs)
From the detection of 858 ± 247 Coulomb pairs one expect 104 ± 30 atomic pairs (without the use of MC).
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Coulomb correlated pairs
Non-Coulomb correlated pairs
Total background
143±53 detected K+ atomic pairs (104 expected).
Direct observation of K atomic pairs
A similar analysis leads to 29±15 detected K- atomic pairs (38 expected).
Residualsatoms
Signal region
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- K++K-
- K++ K- atomic pairs: 173 ±54
- K+
Atomic pairs Coulomb pairs
143±52 972 ±233
Atomic pairs Coulomb pairs
29±15 165 ±108
+ K-
- K+ + + K- atomic pairs
Pion =nA
NA=
nA
k·NC Pion=(64±25)%
atoms
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An atom while traveling through the target can either:
• be (de-)exited (Pex): (ex: 2S-->2P)
• be ionized (Pion):
• decay (Pdecay):
±K m → π ± + K m
±K
m→π
0+K
0(−−)
Pion=1-Pdecay-Pex
≥1.5·10-15 s with 84% confidence level (3.7±0.4 has been predicted).
Lifetime measurement
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Thanks to efficient particle identification from pioneering run 2007:
• observation of Coulomb correlation in K-pairs production. Atoms must also be produced.• first direct evidence for K-atoms production (173±54 detected atomic pairs).• first experimental estimation of a lower limit of K atoms lifetime (1.5 fs).
Summary
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DIRAC-II Collaboration]CERN Geneva, Switzerland
Santiago de Compostela University Santiago de Compostela, Spain
Czech Technical University Prague, Czech Republic
IFIN-HH Bucharest, Romania
Institute of Physics ASCR Prague, Czech Republic
JINR Dubna, Russia
Nuclear Physics Institute ASCR Rez, Czech Republic
IHEP Protvino, Russia
University of Messina Messina, Italy
SINP of Moscow State University Moscow, Russia
INFN-Laboratori Nazionali di Frascati Frascati, Italy
Basel University Basel, Switzerland
Trieste University and INFN-Trieste Trieste, Italy
Bern University Bern, Switzerland
Kyoto Sangyou University Kyoto, Japan
Zurich University Zurich, Switzerland (Y.Allkofer, c.Amsler, S.Horikawa, C.Regenfus, J.Rochet)
KEK Tsukuba, Japan
Tokyo Metropolitan University Tokyo, Japan
This work has been published in: Y.Allkofer et al., Nucl. Instr. Meth. A 582 (2007) 497, Y. Allkofer et al., Frascati Physics Series Vol. XLVI
(2008), Y. Allkofer et al., Nucl. In str. Meth. A 585 (2008)84