Contribution to the Tracking, PID and Muon Group I

tracking, PID and muon group

Takayuki Sumiyoshi Patrick Roudeau Ian Shipsey

Tracking makes a key contribution to the precision measurement of

the charm meson decay constants and semileptonic decays, particle ID plays a key role in measuring mixing. The requirement of implementing excellent tracking and PID in

a limited space is difficult.

The following two examples show how important excellent tracking is.

Charm Meson Decay Constants

Pseudoscalar meson decay constants can be determined by measuring lifetimes and

absolute branching ratios for the leptonic decays D, D if the relevant CKM matrix elements are assumed to be well known from unitarity. Charm lifetimes are well Figure 1 Square of the invariant mass of the neutrino candidate in D+. The shaded area is correctly tagged events.

measured at the 1-2% level. The analysis procedure is as follows. Reconstruct single hadronic tags. In the tagged sample require exactly one single primary tag in the event and exactly one charged track of correct sign in addition to the tracks used to reconstruct the primary tag. Backgrounds with neutral massive particles are reduced by a cut on the extra energy in the electromagnetic calorimeter. This quantity is the sum of energy depositions not caused by any neutral particle contributing to the primary tag, and well separted from any charge track. Events are accepted if the extra energy is less than 500 MeV.

The kinematic requirement used in this analysis is that the 4-momentum of the neutrino

Figure 2 Square of the invariant mass of the neutrino candidate in Ds. The shaded area is correctly tagged events.

as the only missing particle can be completely reconstructed from its recoil against the primary tag and the single track assumed to be a muon. Since we know that the total center-of mass energy is twice the beam energy and the total three-momentum must be zero we calculate

M()2 = [E(D)-E(]2 [P(D)- P( )]2

E(D) and p(D) are the components of the four vector of the single tag, after it has been constrained to the nominal D meson mass, the energy component set to the beam energy

and the 3-momentum kinematically fit to these parameters. For CLEO-c the single particle is not identified as a muon, this removes a large source of systematic error. The distribution in a simulation for 1 inverse femtobarn of data at CLEO-c is shown in the two figures below. The clear separation between signal and background in figre 1 and 2 is primarily due to the excellence of the momentum resolution.

Suggestions for BES Tracking

To achieve the physics goals of BES III it is essential that the kinematic separation shown in Figures 1 and 2 can be achieved. The BES III drift chamber design closely follows the design of the CLEO III drift chamber we therefore believe that if the chamber is carefully protoyped and constructed that the resolution achieved will be close to the designed resolution which is adequate to achieve the separation power of CLEO-.

However, the BES III drift chamber will operate in environment with a higher background rate due to the higher stored currents. The exact level of background is hard to predict. Due to early simulations of the background levels in the BESIII drift chamber the inner diameter is 7 cm. We belive it is important to instrument the region between the outer diameter of the beampipe and the inner diameter of the drift chamber. This could be done with a separate chamber or the main drift chambr coiuld be extended inwards.

Micro TPC

One way to deal with the very high rates expected is to change the technology for the inner layers. An ultra low mass tracking technology that has enjoyed considerable success in high rate environments is the Time Projection Chamber (TPC). We encourage BES to explore the possibility of building a micro-TPC to instrument the region between the beam pipe and r=7 cm. There is considerable interest in the particle physics community in extending TPC technology by developing Gas Electron Multiplier (GEM) or MICROMEGAS readout of a TPC. Prototypes of TPCs with advanced micro pattern gas detector readout have been built and encouraging results obtained. If BESIII agreed to evaluate this technology it would possibly attract international collaborators to BES.

Large TPC

It is possible to avoid the complications of having two tracking chambers if the drift chamber is replaced with a TPC from the beam pipe all the way out to 66 cm.