content_image/358096.jpg content_image/358097.jpg
have odd number of charged tracks. The total momentum, energy and one of the velocities 

are used to derive q , m$_{f}$$_{1}$ and m$_{f}$$_{2}$ . Another velocity v$_{r}$ ( f 2) = (1 − q ) P$_{n}$/ √ m 2 f 2 + ((1 − q ) P$_{n}$ ) $^{2}$( f 2) = v$_{m}$ ( f 2) is used to suppress the contribution from the first case. 

<img src='content_image/358096.jpg'>

To search for a pair of quarks f and f with opposite charge qe in events with only two charge tracks, 

are used to derive q and m$_{f}$ . Another velocity v$_{r}$ ( f ) = qP$_{n}$/ √ m 2 f + ( qP$_{n}$/c ) $^{2}$( f ) = v$_{m}$ ( f ) is used to suppress the contribution from the first case. 

<img src='content_image/358097.jpg'>

Search for events containing three free quarks or one free quark and one diquark can be done in the similar way. The two simplest examples are Δ $^{++}$uuu + others and Δ − ddd + others and their charge conjugation, where Δ decays to Nπ . 

To estimate the power of the technique above, it can be used to select fractional charge particles from a Monte Carlo sample set, which includes a signal sample and a background sample. In this article, 200 events of ψ (2 S ) → ddJ/ψ, J/ψ → e $^{+}$e − and 200 events of ψ (2 S ) → uuJ/ψ, J/ψ → e $^{+}$e − through BESIII detector simulation [11] construct the signal sample. The background sample is 40 million events of ψ (2 S ) inclusive decays, in which each decay branching ratio is from the particle data group (PDG). d ( d ) and u ( u ) carry 1/3 and 2/3 of the electron charge respectively. Their masses are assumed to be 0.005, 0.025, 0.045, 0.065 and 0.085 GeV . 

To pick up the signal events and suppress the background events, the events in the sample set are selected if they have four charge tracks with zero total charge. The scalar sum and the vector sum of the four tracks’ momenta are required to larger than 3.8 GeV and larger than 0.12 GeV to remove the background events with normal four tracks. Then a pair of opposite charge tracks’ momenta are adjusted by multiplying a factor of q so that the event 

