Evidence for -atoms with DIRAC-II

Yves AllkoferUniversity of Zurich 20th November 2008

• R & D development

• Data taking

• Analysis and results

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

a 03/

2 [m-1

]th

eory

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

rimen

t

m (a01/2 −a0

3/2 ) ; 0.475 ±0.070p

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/(a)= 250 fm

• E (1S) = a2/2= -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 .

The DIRAC-II spectrometerA

Pt

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)

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

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

The cosmic ray test setup

LED

n=1.05

Results are used to tune the Monte-Carlo

n=1.015GEANT4 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

Npen=1.05

Simulation

Vertical position (cm)N

pe

The pyramid design

The pyramid design cancels out the impact position dependence

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

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

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

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

• 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

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

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.

Expected atomic-pairsThe 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).

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

- 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

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

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

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, JapanTokyo 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