[G12] Missing mass plot for pKK -- Advice needed

[Mukesh Saini]

Hi Dennis,

I am looking at Gamma  P  ->  P  K+  K-  (Missing Particle).

mm is the mass of this missing particle as calculate by clasEvent ~(missing mass 4 vector). The function that calculates the Invariant mass is represented by a tilde in the clasEvent.

E -> 4th or the 't' component of the energy-momentum 4 vector

P-> (Px, Py, Pz) the three momentum vectors

Kaons and Protons were selected using clasEvent from the PART bank. I have just ran the data over the skim1 - that is 1ckaon1ctrk.bos files

mm_p variables were the corresponding component of the mm (missing mass) 4 vector.

I didn't do any explicit selection on the photons. They are the photons that have supposedly triggered the event as selected by the clasEvent. So clasEvent selects the triggering photon.

There are no cuts on the plots. I imposed a timing cut "stVtime - vtime" < 1 ns, It doesn't clean up any. I lose about 5 % of the events. 

I am still trying to see what causes this. I hope I have answered all your queries. Let me know if you have others.

Thanks,
Mukesh


----- Original Message -----
From: Dennis Weygand <weygand at jlab.org>
Date: Friday, November 13, 2009 2:19 pm
Subject: Re: [G12] Missing mass plot for pKK -- Advice needed
To: Mukesh Saini <msaini at fsu.edu>
Cc: g12 at jlab.org

> I can make no sense of your email: you must provide more information:
> 
> Begin with the final state or topology you are investigating.
> Next: what was your selection criteria and what skims did you use?
> What is "E" and what is "P"?
> How exactly did you calculate the mm_p variables?
> How did you choose the photon? 
> 
> The obvious reason your events are not physical, i.e. the mm is 
> imaginary, is because your K's are really pi's, or you have chosen 
> the wrong photon.
> 
> Dennis
> 
> On Nov 12, 2009, at 5:38 PM, Mukesh Saini wrote:
> 
> > Hello all,
> > 
> > I am attaching a few plots and need help to understand the 
> negative masses we get. I am using ClasEvent and bos2ntuple code to 
> make my ntuples.
> > 
> > Code to make the plots:
> > 
> > root [30] ntp->Draw("sqrt( -mm_px**2 - mm_py**2 - mm_pz**2 + 
> mm_t**2 ) -mm","","")
> > root [31] ntp->Draw("sqrt( -mm_px**2 - mm_py**2 - mm_pz**2 + 
> mm_t**2 ) ","","")   
> > root [32] ntp->Draw("mm","","")                                   
>                 
> > 
> > As you can see majority of mm is less than 0,hence I lose a lot 
> of events when i make my cuts. The other way that I can calculate 
> sqrt(E**2 - P**2) seems to give me the wrong answer too (All values 
> positive even when I switch the signs on all terms inside the 
> sqrt). I am sure this is some famous functionality that I am just 
> unaware of. Please do let me know how to understand this and if 
> needed fix this. 
> > 
> > Thanks,
> > Mukesh.
> > 
> > Graduate Student,
> > Florida State University.
> > <mm.gif><mm_calc.gif><mm-
> mm.gif>_______________________________________________> G12 mailing 
> list> G12 at jlab.org
> > https://mailman.jlab.org/mailman/listinfo/g12
> 
> --
> Dennis Weygand
> weygand at jlab.org
> (757) 269-5926
> 
> 
> 
> 
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Graduate Student,
Florida State University.