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

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

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

Detectors at COSY

Whay is η(η΄)  3π interesting?

η(η΄) 3π

Isospin violation

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

WASA-at-COSY

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:

Goals With WASA we like to study decays of the η and η’ mesons like e.g.: η’ -> π+ π- π0 (Upp. lim. BR = 5%) η’ -> π0 π0 π0 (BR = 0.1%) η -> π+ π- π0 (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.

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.

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.

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).

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

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

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

Missing mass resolution

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.

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

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

Thank you !!!