1. Marcin Zieliński Paweł Moskal Andrzej Kupść
WASA-at-COSY
Investigations of the η' meson hadronic decays with WASA-at-COSY facility
XIII LNF Spring School in Nuclear, Subnuclear and Astroparticle Physics
Marcin Zieliński
Paweł Moskal
Andrzej Kupść

2. This presentation Introduction
Studies of the η’ → π+π-π0 decay and direct production of 3 π

3. COoler SYnchrotron (COSY)
184 m long ring
proton beam (deutron also possible)
1011 protons per filling
max momentum p=3.65 GeV/c
p/p ≤ 10-4
beam cooling:
electron cooling
stochastic cooling
Up to 80% polarization

4. Detectors at COSY

5. Whay is η(η΄)  3π interesting?

6. η(η΄) 3π

7. Isospin violation

8. WASA: way from TSL (Uppsala) to COSY (Juelich)

9. WASA-at-COSY

10. Branching ratio Branching ratio is given by relation
Upper limit BR(η’ → π+π-π0 ) < 5% (Rittenberg 1962)
Recent theoretical prediction BR(η’ → π+π-π0 ) = 1% (Borasoy 2006)
In order to determine the Γ(η'→ π+π-π0 ) we have to measure the
branching ratio for the η'→ π+π-π0 which is equal to:

11. Goals With WASA we like to study decays of the η and η’
mesons like e.g.:
η’ -> π+ π- π (Upp. lim. BR = 5%)
η’ -> π0 π0 π (BR = 0.1%)
η -> π+ π- π (BR = 50%)
η -> π0 π0 π0
For these decays there exist a physical background from direct production of three pions via pp -> pp 3π reaction channel.

12. Total cross section for production of the η and η’ mesons
The cross section for the η’ meson production is about factor of thirty smaller in comparison to the η meson at the same excess energies.

13. Total cross section for production of the η and η’ mesons
For the η’ meson threshold region the total cross section for the direct production of the three pions increases by factor of about 700 in comparison to the η meson
The signal to background is expected to be worse by more then three orders of magnitude, making the investigations much more challanging experimentally.

14. Reaction identification (pp -> ppη’ -> ppπ+π- π0)
The measurments with WASA-at-COSY relay on registering of all particles in the final state and determination of their four-momenta.
The selection of events (identification) will be based on the missing and invariant mass techniques.
Data evaluation:
Selection of events where three pions were produced (invariant mass).
Separation into fractions which corresponds to direct production of three pions and to the decay of the η’ meson (missing mass).

15. Missing mass distribution
The letter „S” corresponds to the η’ signal, and letter „B” denotes the background in the slice under the signal.
The background under the the η’ peak originates from the direct production of pions which invariant mass is equal to the mass of the η’ meson.
The signal to background ratio on the missing mass will depend:
on the total cross section for the η’ meson production
on the differential cross section for direct three pion production
on the missing mass resolution

16. Total cross section for the η’ meson production
We can parametrize the total cross section using Faldt-Wilkin model:
Fit parameters:

17. Differential cross section for multimeson production
We have taken taken the missing mass spectra of the pp->ppX reaction determined by the
COSY-11 collaboration
The established differentiall cross section can be treated as an upper limit for the expected background from the pp->pp π+ π- π0

18. Missing mass resolution

19. The beam momentum optimization
Relative error of the BR determination nearly scales with the value of the assumed branching ratio.
The optimum accuracy is achieved for excess energies between 60 and 90 MeV independently of the BR magnitude.

20. Upper limit for the BR (η’ -> π+ π- π0 )

21. Summary
WASA at COSY studies symmetries and symmetry breaking
WaC successfully transfered from TSL to COSY
Detector upgrades mostly completed
Very prommising simulations and results form online analysis
Next step: long term production runs

22. Thank you !!!