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Project C
Further Investigation of X(3872) and contribution to the hardware upgrade of BES III

Prof. Dr. Wolfgang Kühn and PD Dr. Sören Lange, Justus-Liebig-Universität Giessen

In this project, we propose to further investigate the X(3872) state and its relation to Y states, 1−− states. Most of the Y states were discovered in ISR radiation experiments at B-factories. Recently, BESIII has observed the decay Y(4260) → γX(3872), where X(3872) → J/ψπ+π- , with a statistical significance of more than 5σ. We propose to investigate more X(3872) decay modes, exploiting the existing BESIII data taken at and near the Y(4260) resonance as well as future runs with large integrated luminosity in this mass region.

Specifically, we propose to search for hadronic decays into light mesons, such as X(3872) → nπ or decays of X(3872) into pairs of vector mesons. Furthermore, we plan to investigate hadronic transitions such as Y(4260) → π+π-X(3872). In order to increase the X(3872) data sample, we plan to investigate the recoil mass method. Here, we do not require a complete reconstruction of X(3872) → J/ψ+π- , where the number of reconstructed X(3872) events is reduced by the product of the branching fraction of X(3872) → J/ψ+π- and the branching fraction of J/ψ → e+e- , μ+μ-. This method appears to be very successful in the reconstruction of Z(3900) and the hc .

In addition, we propose to measure the electronic width and potentially the total width of the X(3872) in a direct mass scan, exploiting the rare process e+e- → X(3872) which is possible via the exchange of two virtual photons. To test the feasibility of this approach, the BESIII collaboration has decided to search for e+e- → χc1 . Here, the formation cross section and the background for a well established cc state will be investigated. The electronic width of χc1 is unknown and theoretical estimates vary between 0.46 eV and 0.044 eV. Assuming a width of 0.1 eV, the resulting cross section would be about 15 pB which is well measurable within one week of BESIII running. If this experiment is successful, the same approach could be applied to the X(3872). Due to the unknown internal structure of this state, no reliable predictions for its electronic width are internal structure available at this time. If, however, e+e- → X(3872) can be observed, it might be possible to measure the so far unknown total width of this state using a mass scan around the peak energy.

Furthermore, we propose a contribution to the planed BESIII detector upgrades. Here, the group’s long-standing experience with FPGA based data acquisition - and trigger systems would be used for an upgrade of the zero-degree calorimeter (ZDC), opening the possibility of exclusive studies involving ISR processes over a wide kinematic range. Here the basic building block is the Compute Node, an FPGA based platform that our group has developed in close collaboration with IHEP Beijing (Prof. Zhen-An Liu). This platform is currently being used for the Belle II detector (Pixel Detector data acquisition and trigger system) as well as foreseen for the future PANDA experiment at FAIR.

Das Projekt hat zwei Ziele: (i) die Untersuchung des exotischen X(3872) ¿ Zustands durch Suche nach weiteren hadronischen Zerfällen sowie die Messung der elektronischen Breite und der totalen Breite an BESIII; (ii) Beiträge zum BESIII Detektor - Upgrade im Bereich von Datenerfassung und Trigger für das Null-Grad-Kalorimeter zur Untersuchung von ISR - Prozessen.