Investigations of the charge symmetry breaking reaction dd − → απ 0 with the WASA-at-COSY experiment

The charge symmetry breaking reaction dd −−→ 4Heπ0 has been measured with the WASA-at-COSY experimental setup at a beam momentum of pd = 1.2 GeV/c (Q = 60 MeV). For the first time a signal for this reaction has been observed at an excess energy well above threshold. The determined preliminary total cross section is σ tot = (120 ± 20stat.) pb.


Introduction
In the framework of the Standard Model, isospin symmetry is broken by the mass and charge difference of quarks [1,2].From isospin symmetry breaking observables it is difficult to access these individual terms, because on the hadronic level, isospin symmetry breaking is dominated by the pion mass difference (∆π), which is almost a pure electromagnetic effect.Only the reactions, where these two sources of isospin symmetry breaking can be disentangled, open an experimental window to probe the u and d quark mass difference effects in hadronic processes [3].This can be achieved by using charge symmetry breaking (CSB) observables where ∆π does not contribute.Charge symmetry is a special case of isospin symmetry, corresponding to a rotation of 180 • in isospin space interchanging the up and down quarks.
Up to now, there were two successful measurements of CSB observables: one is the forwardbackward asymmetry (A f b ) in the np − − → dπ 0 reaction [4], the other is the π 0 production in the dd − − → 4 Heπ 0 reaction near threshold [5].Advanced theoretical calculations based on Chiral Perturabtion Theory (χ PT ) show that CSB effects manifest themselves in many different physical phenomena on the hadronic level [6,7].A consistent theoretical analysis of both measurements is currently in progress [8][9][10][11][12][13].It is shown within a plane wave calculation, that when all parameters are fixed the p-waves in the dd − − → 4 Heπ 0 reaction may be predicted parameter free to leading and nextto-leading order [11].Therefore, experimental input of p-wave contributions at higher excess energies is needed.

Experimental setup
The measurement of the CSB reaction dd − − → 4 Heπ 0 is the one of the primary objectives of the WASA-at-COSY experiment [16], which is located at the COoler SYnchortron COSY [15] at the a e-mail: maria.zurek@uj.edu.plInstitute for Nuclear Physics of Forschungszentrum Jülich, Germany.The CSB program started with an analysis of the charge symmetry conserving reaction dd − − → 3 Henπ 0 at p d = 1.2 GeV/c [14].The goal of this analysis was to get information about the main source of background for the signal reaction dd − − → 4 Heπ 0 and to provide experimental constraints for theoretical calculations of the dd initial state interactions.
The second step of the program is the investigation of the dd − − → 4 Heπ 0 reaction at p d = 1.2 GeV/c (E d = 350 MeV).In June 2008, in a high-luminosity two-weeks run the first data were collected.The main goal of this experiment was to obtain the total cross section and to extract a first angular distribution.

Data analysis and results
During the run a total integrated luminosity of about L ≈ 5 pb −1 was achieved.The reaction was tagged by two neutral clusters from the π 0 decay in the electromagnetic calorimeter of the Central Detector and a 4 He in the Forward Detector (a set of plastics scintillator layers and a tracking detector).
The identification of helium is based on the energy losses patterns dE − dE in the first layers of the Forward Detector -two layers of the Forward Window Counter and the first layer of the Forward Trigger Hodoscope.Because of the small kinetic energy, 4 He particles do not reach further layers of the Forwad Detector.The main sources of background are the charge symmetry conserving reaction dd − − → 3 Henπ 0 and the double radiative capture dd − − → 4 Heγγ.In order to suppress the background deriving from the 3 He isotopes and to ensure energy and momentum conservation, an overall kinematic fit was used.Two hypotheses were tested: dd − − → 4 Heγγ and dd − − → 3 Henγγ.No constraint on the γγ invariant mass was included to avoid the production of an artificial π 0 peak.Finally, a combined χ 2 cut was used to suppress background from the misidentified 3 He.Detector acceptance and all applied cuts efficiency for the signal reaction is about 20%.

Results
In Fig. 4 the 4 He missing mass spectrum is shown.On top of a flat background from the double radiative capture dd − − → 4 Heγγ, two distinct peaks are visible.One is linked to the signal, the second is caused by remaining misidentified 3 He from the dd − − → 3 Henπ 0 reaction.The background was fitted with distributions from event generators filtered by Monte-Carlo simulations.For dd − − → 4 Heγγ a homogeneous 3-body phase-space distribution was assumed, for the dd − − → 3 Henπ 0 channel the model obtained from the previous measurement [14] was used.The difference between data and fitted background is presented in the inset plot, which was fitted with a Gaussian function.The preliminary result for the total cross section of the dd − − → 4 Heπ 0 reaction is σ prel tot = (120±20(stat.))pb, assuming a 2-body phase space.For the double radiative capture reaction dd − − → 4 Heγγ it is σ prel tot = (960 ± 45(stat.))pb, based on the assumption of a 3-body phase space.
In addition, a first angular distribution for dd − − → 4 Heπ 0 at Q = 60 MeV was obtained, which is consistent with s-wave pion production.However, the limited statistics introduces large statistical and systematical uncertainties, which are currently under investagtion.

Future strategy
One of the challenges that made the dd − − → 4 Heπ 0 reaction analysis difficult was a poor 3 He − 4 He separation, solely based on the energy loss pattern from the first three layers of the Forward Detector.In order to improve that, a modified version of the WASA detector will be used for the next run (see Fig. 2).All layers of the Forward Detector between the Forward Proportional Chamber and the Forward Veto Hodoscope will be removed, which will provide time of flight as a new independent observable.In the new setup the Forward Veto Hodoscope will take over the role of the Forward Trigger Hodoscope as stopping layer.The simulation of the new setup verifies that this replacement does not affect the performance of the current detector.In addition, when time of flight is introduced, the 3 He − 4 He separation significantly improves.A high statistics data sample will be taken in a eight-weeks long run using this modified detector setup in spring 2014.The experimental goal is to reach about 20% statistical uncertainty for the anisotropy parameter of the angular distribution.This precision will allow a reasonable comparison with the theoretical calculations which are currently in progress.

Figure 2 .
Figure 2. The planned modification of the WASA-at-COSY detector setup.The upper part shows the removed Forward Detector layers between Forward Proportional Chamber and Forward Veto Hodoscope.The lower part presents the current WASA detector setup.The kinetic energy of 4 He needed to reach and stop in first or second layer of Forward Veto Hodoscope is indicated.