reading the bubble chamber image I have a bubble chamber image and some explanation from the textbook, however, I was still not sure how to read it clearly.
From What are the "x" marks in a bubble chamber image? I understood what's the x mark meant to the picture.


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*I just want to make sure, there was only one x mark because it's only a part of the image, right?(How many x marks were there in customs.)

*I didn't quiet understood what's big the circling at the center. Are those $e^+e^-$ pair? How come it broke into two traces? (Are those gamma ray? How come gamma ray also leaves trace in the bubble chamber, as it doesn't seem to carry any any charge.)

*What about the straight line crossed the big circle and then split into two ray? Are those $e^+e^-$ pair as well? or $\gamma e^+e^-$?

*Where is the electron carried by the hydrogen itself?

*Coming back to the big circle, why isn't the dense dark spot at the center? rather somehow shifted towards right?

*I noticed at the left of the big circle, there seem to be a three line intersection (two rays with the circle) at almost the same spot. Was it just a coincidence?

 A: 
I just want to make sure, there was only one x mark because it's only a part of the image, right?(How many x marks were there in customs.)

The X are fiducial marks, fixed on the space of the bubble chamber, front and back, so that the chamber can be reconstructed accurately, and then one can have   the x,y,z coordinates in real space of the tracks, using the same measuring machine.                       

I didn't quiet understood what's big the circling at the center. 

On the lower left you see a spiraling electron, which has been kicked out from thy hydrogen of the bubble chamber. The tiny dots of the tracks are also  kicked off electrons from the incoming pions , from below, but they are very low energy, just leaving an isonization track. Every now and then such an electron gets a bit more energy and can be seen spiraling down to rest. It spirals because of the magnetic field and the high probability of electrons interacting with the electrons of the hydrogen.
Thus one knows the direction of the beam, and the charge .

Are those e+e− pair? How come it broke into two traces? 

The central , where the pion track disappears, is also an electron because it turns the same way. The caption in you picture explains that it comes from the reaction $π^{-}+p$ -> $n +π^{0}$   and then , for this event, the $π^{0}$ goes to $e^+ + e^- +γ $. The $e^-$ is the spiraling track, the $e^+$ is the track leaving on the right, with much higher momentum so it does not spiral, and the $γ$ is not seen, but can be reconstructed from momentum conservation.
Gamma rays might create an e+e- pair, but it is not in this picture. See in this link for gamma in the list how it can produce a pair.

Where is the electron carried by the hydrogen itself?

Charged tracks leave ionization traces in liquid hydrogen.

Coming back to the big circle, why isn't the dense dark spot at the center? rather somehow shifted towards right?

It depends on the direction of the helix as the electron loses energy, with the magnetic field, which is perpendicular to the picture. it means there is an angle between the helix direction and the magnetic field.

I noticed at the left of the big circle, there seem to be a three line intersection (two rays with the circle) at almost the same spot. Was it just a coincidence?

Yes, what you call rays are pions going through without a strong interaction.
