Search for η-mesic Hewith the WASA detector

Wojciech Krzemień,Paweł Moskal, Magdalena Skurzok
for WASA-at-COSY collaboration

Hadrons and Hadron Interaction in QCD 2015
05th of March 2015,
Yukawa Institute for Theoretical Physics, Kyoto University, Japan„Exotic” systems
Classical nucleus:
Bound state of nucleons
Hypernuclei:
Bound states of nucleons+ hyperion Λ,ΣMesic atoms and nuclei:
Bound state of nucleons+ meson π, K, η, η' ,ω, ..

Can a bound system be formed ?

Is interaction attractive or repulsive?

How strong is the interaction?

What type: strong + Coulombη interaction with nucleon
For low energies η­N interaction is dominated by N*(1535)/S11 resonance
MN+ Mη = 1486 MeV

1358 MeVΓ

MN* =1535 MeV

1685 MeVΓη interaction with nucleon
For low energies η­N interaction is dominated by N*(1535)/S11 resonance
MN+ Mη = 1486 MeV

1358 MeVΓ

MN* =1535 MeV

1685 MeVΓFirst η­mesic nuclei predictions (for A>12)

η-mesic nucleus = only strong interaction
For recent calculations see:E. Friedman, A. Gal and J. Mares, Phys. Lett. B725 (2013) 334.

Why η-mesic nuclei●New bound state of hadrons●Investigation η-N interactions●Studies of η quark structure
Binding energy and effective mass of η are sensitive to the gluon component of the flavour
singlet function |η0>
(more gluon content → more attractive binding → higher binding energy)(S.D. Bass, A.W. Thomas, Phys. Lett. B634 (2008))●

Study of in-medium properties of N*(1535) resonance:N-η system is strongly coupled with N*(1535) resonances. η-mesic nucleus as a probe for testing
different N* models
Jido, Oka, Hosaka, Nemoto, PTP106(01)873
Jido, Hatsuda, Kunirhiro, NPA671(00)471)
Garcia-Recio, Nieves, Inoue, Oset, PLB550(02)47
Jido, Nagahiro., Hirenzaki, PRC66(02)045202
Inoue, Oset, NPA710(02) 354

Experimental indications of existenceof a 4He-η bound system

dd → 4Heη

R. Frascaria et al., Phys. Rev. C 50 (1994) 573.
N. Willis et al., Phys. Lett. B 406 (1997) 14.
A. Wrońska et al., Eur.Phys.J. A26 (2005) 421­428.
A. Budzanowski et al., Nucl. Phys. A821, (2009) 193.

Experimental indications of existenceof the 3He-η bound system
dp → 3Heη

ANKE: T. Mersmann et al., Phys. Rev. Lett. 98 242301 (2007)
COSY-11: J. Smyrski et al., Phys. Lett B 649 258-262 (2007)

γ3He→ η3He

MAMI:

M. Pfeiffer et al., Phys. Rev. Lett. 92 252001 (2004)
F. Pheron et al., Phys. Lett. B709 21 (2012)

Enhancement independent of input channel → Strong 3He-η FSI

How to look for bound statesDirect method:
Search for some peak structure below the production threshold(e.g. in missing mass)

Indirect methods:
Based on cross-section behaviour above the threshold (sometheoretical model must be assumed)

dd → (η-4He)bound → N + π + 3He

dd → (η-4He)bound → N + π + 3He

η + N

N*(1535)

N + 

dd → (η-4He)bound → N + π + 3He

η + N

N*(1535)●

relative N-π angle in the CM :●low 3He momentum in the CM

N + 
θcm ~180˚

Signatures of the bound statep-π- opening angle in the CM frame

He momentum in the CM frame3

dd → (η-4He)bound → N + π + 3He

mBS = mA + m ­ EBE

Search for a resonance­like structurewith maximum below the η­4He production threshold

COSY accelerator in Juelich (Germany)
Beam:●
Unpolarized and polarized
protons or deuterons.
Energy range:●
Tp to 2.8 GeV●

Td to 2.3 GeV(maximum momentum: 3.7 GeV/c)
Cooling:●stochastic●
electron beam
Nb of particles stored: 1011

(COoler SYnchrotron)

Ramped beam techniqueWASA detector(Wide Angle Shower Apparatus)
Pellet target
acceptance ~4 π
symmetric with respect to φCentral Detector:
photons and charged particles(ΔE-p, ΔE-E)● Θ20-170central●“Forward” Detector:
charged particles (ΔE-ΔE, ΔE-E)● Θ3-18
forward●Experiments
June 2008

November-December 2010
Channels:

Channels:
dd → He p π
3dd → 3He p π--dd → 3He n π0 →3He n γγ

Normalization:

Normalization:

dd → He n3

dd → 3He n

Q: -51 to 22 MeV

Q: -70 to 30 MeV

P: 2.185 to 2.4 GeV/c

P: 2.127 to 2.422 GeV/c

Pbeam[GeV/c]

Q [MeV]

Pmax

Pmin

edpmraamebtime

Qmax

Qmin

Experiments
June 2008

November-December 2010
Channels:Channels:
dd → He p π
3dd → 3He p π--dd → 3He n π0 →3He n γγ

Normalization:

Normalization:

dd → He n
3dd → 3He n

Q: -51 to 22 MeV

Q: -70 to 30 MeV

P: 2.185 to 2.4 GeV/c

P: 2.127 to 2.422 GeV/c~20 x more statistics

Pbeam[GeV/c]

Q [MeV]

Pmax

Pmin

edpmraamebtime

Qmax
Unique COSY
Qmin

accelerator feature

WASA-at-COSY

π3pdd →(η-4He)bound→ 3He p π-HeWASA-at-COSYγ
3γdd →(η-4He)bound→ 3He n π0 → 3He γ γ (n)HeExcitation function(normalized and corrected for efficiency)

Phys. Rev. C 87, 035204 (2013)

Phys. Rev. C 87, 035204 (2013)

Upper limit of the maximum cross-section
for the reaction dd → (4He -η)bound→ 3He p πPhys. Rev. C 87, 035204 (2013)

Signal: Q , E BE ,  , A=A 22 2
Q−E BE   22Background:

BG=a 0a1 Qa 2 Q 2or

BG=a 0a1 Q

3He ions identification in Forward Detector(ΔE -ΔE and ΔE -E methods)

MCPion identification in the Central Detector
dd → 3He p π-DATA

dd → 3He n π0 →3He n γγ
DATAMCp π- identification

MCπ0 identification

Neutron identification(missing mass method)

dd → 3He n π0 →3He n γγ

„Signal-rich” vs „Signal-poor” area

red line (data): dd → 3He n π0
blue line (data): dd → 3He p πblack line (MC): dd → (4He -η)bound→ 3He n π0

Low 3He momentum in the CM expected from the boundstate decay.

Excitation function examples
preliminary

red line: dd → 3He n π0
blue line: dd → 3He p πblack line(MC): dd → (4He -η)bound→ 3He n π0preliminary

dd → 3He p π-

preliminary

dd → 3He n π0

Normalization
Total integrated luminosity determined in two independent
processes:●

dd → 3He +n●

dd → p + p (n n) quasi-free p + p collisions

Luminosity variation as function of beam energy based on quasi
free dd → p + p (n n).

preliminary

Overall reconstruction efficiency

Current analysis

Phys. Rev. C 87, 035204 (2013)

Current analysis Q range: from -70 to 30 MeV
Phys. Rev. C 87, 035204 (2013): from -50 to20 MeV

Efficiency flat as a function of excess energy !!!

Upper limit of the maximum cross-section
for the reaction dd → (4He -η)bound→ 3He p π-Current analysis

Phys. Rev. C 87, 035204 (2013)PRELIMINARY

Signal: Q , E BE ,  , A=A 22 2
Q−E BE   22Background:

BG=a 0a1 Qa 2 Q2orBG=a 0a1 Q

New experiment in May-June 2014
May-June 2014

Test plot from experiment

Channels:
p+d → (3He-η)bound→ p + p + p+ πp+d → (3He-η)bound → d + p + π0
p+d → (3He-η)bound → p + p + n + π0
Orbiting eta:
p+d → (3He-η)bound → 3He 6γ
Normalization:
pd → 3He η
pd → 3He π0

Pbeam[GeV/c]

Q [MeV]Pmax
Q: -50 to 20 MeV
P: 1.468 to 1.615 GeV/c

Pmin

edpm
ra

amebtime

Qmax

Qmin

Possible decay scenarios●

Absorption of orbiting η:
η → 2γ(inside nucleus)

Possible decay scenarios●

Absorption of orbiting η:
η → 2γ(inside nucleus)
γγ

Conclusions●

η­mesic nuclei not unequivocally confirmed experimentally so far,

●Search for a light mesic nuclei in η­3He and η­4He systems with WASA­at­COSY:Exclusive, high­acceptance measurement with ramped beam,●

New data set (η­3He) with perspectives to lower the current upper limit ~ 30 times,●

New experiments planned (η'­mesic nuclei): J­PARC, GSI, ELSA

ご清聴ありがとうございました。

Backup slides

Excitation functions

Phys. Rev. C 87, 035204 (2013)

Current analysis Q range: from -70 to 30 MeV
Phys. Rev. C 87, 035204 (2013)): from -50 to20 MeV

Nucleon momentum distribution in 4He
Used in old analysis

Used in new analysis
Analytical formula derived based on:
J. S. McCarthy et al., Phys. Rev. C 15, 1396
(1977).

More details in Magdalena Skurzok's Master thesis: arXiv:1009.5503v2

the difference was included as systematic uncertainty in previous results.
●
After discussions with theoreticians (S. Wycech, S. Hirenzaki, A. Nogga) we decided for
AV18 version:●H. Kamada et al., Phys. Rev. C64 044001 (“Benchmark test calculation of a four-nucleon bound state”)

Experimental indications of existenceof the 3He-η bound system
C.Wilkin et al., Phys. Lett. B654 (2007) 92

Full dots: COSY-ANKE(T.Mersmann et al., Phys. Rev. Lett. 98 242301-1-4(2007))

Empty circles: COSY-11(J.Smyrski et al., Phys. Lett B 649 258-262 (2007))Experimental indications of existenceof the 3He-η bound system
dp → 3Heη

Tensor analysing power T20 almost flat → independentof the input channel state S = ½ I S = 3/2

Khoukaz et al. Acta Phys. Pol. B (2014)

He-η vs 3He-η systems

4Machner et al. Acta Phys. Pol. B (2014).

Upper limits

PRELIMINARYSignal:
Background:

BG =a 0 +a 1 Q +a 2 Q2 Q , E BE ,  , A=A 22

 2
Q−E BE   22

Upper limits -different binding energy and width
PRELIMINARY

Predictions for He system

COSY accelerator in Juelich (Germany)
Beam:●
Unpolarized and polarized
protons or deuterons.
Energy range:●
Tp to 2.8 GeV●

Td to 2.3 GeV(maximum momentum: 3.7 GeV/c)
Cooling:●stochastic●electron beam
Nb of particles: 1011

(COoler SYnchrotron)

Ramped beam

Kinematical conditions

red line: dd → 3He n π0
blue line: dd → 3He p πblack line (MC): dd → (4He -η)→ 3He n π0Possible decay scenarios●

Via N* resonance decay :η + N → N* → N + π (inside nucleus)N*

Possible decay scenarios●

Via N* resonance decay :η + N → N* → N + π (inside nucleus)

Possible decay scenarios●

Via N* resonance decay :η + N → N* → N + π (inside nucleus)
πN*N

Possible decay scenarios●

Absorption of orbiting η:
η → 2γ(inside nucleus)γγ

Possible decay scenarios●

Absorption of orbiting η:
η → 2γ(inside nucleus)

Possible decay scenarios●

Non-resonant decay (absorption on two nucleons ):η + N N → NN (inside nucleus)

Possible decay scenarios●

Non-resonant decay (absorption on two nucleons ):η + N N → NN (inside nucleus)

Possible decay scenarios●

Non-resonant decay (absorption on two nucleons ):η + N N → NN (inside nucleus)

Possible decay scenarios●

Via N* resonance decay :η + N → N* → N + π (inside nucleus)●

Absorption of an orbiting η:
η → 6γ●(inside nucleus)

Non-resonant decay (absorption on two nucleons):
η + N N → NN (inside nucleus)

Dominant decay channels●

Via N* resonance decay:

p+d → (3He-η)bound→ p + p + p+ πp+d → (3He-η)bound → d + p + π0
p+d → (3He-η)bound → p + p + n + π0
p+d → (3He-η)bound → d + n + π+
Absorption of an orbiting η:
p+d → (3He-η)bound → 3He + 6γ
●Non-resonant decay (absorption on two nucleons ):p+d → (3He-η)bound → p + p + n●

p+d → (3He-η)bound → p + d

3π-p

He

WASA-at-COSY
Forward detector:
Scattering angle coverage 3°-18°
Scattering angle resolution 0.2°
Maximum energies for stopping π±/p/d/α 170/300/400/900 MeVTime resolution <3ns
Relative energy resolution
particles Tstop<T<2Tstop
3-8%
stopped particles T<Tstop 1.5-3%

WASA-at-COSY
4 π detector for charged and neutral particles

Central detector:
Max. stopping energy π ±/p/d
190/400/450 MeV
Angular resolution
chargedneutral

~1.2°
~5°

Relative energy resolution by SE
photons~8%
stopped charged particles ~3%