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New pbarpSystem and pbardSystem implementations [message #13193]

Tue, 13 March 2012 16:10

StefanoSpataro
Messages: 2736
Registered: June 2005
Location: Torino

first-grade participant

From: *to.infn.it

Dear all,
few new implementations were done in EvtGen.

First all now, now you can set the spin of the initial pbar-p system, by using:

pbarpSystem0
pbarpSystem1
pbarpSystem2

The old pbarpSystem remains at the usual place, but please remember that it has spin 1 so you cannot use it with other models (i.e. the SVV used for eta_c).

Another note: if you use as initial state a particle, i.e. eta_c, in generation you will have an invariant mass resolution due to the particle width, and not to the beam momentum spread. For this reason we suggest to use pbarpSystem as initial state.

Morevoer, thanks to Albrecht we have a new model to simulate collisions antiproton-deuteron. In this case you have to use "pbardSystem" as initial state, and write a DEC file such as:

noPhotos
#
Decay pbardSystem
  1.0   p+  pbarnSystem		DeuteronSpectator 1.0 1.16;
Enddecay
#
Decay pbarnSystem
  1.0   pi- phi			PHSP;
Enddecay 
#
Decay phi
  1.0   K+ K-			VSS;
Enddecay
#
End

Maybe Albrecht could comment about the parameters to provide to the DeuteronSpectator model.

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Re: New pbarpSystem and pbardSystem implementations [message #13196 is a reply to message #13193]

Tue, 13 March 2012 17:16

Albrecht Gillitzer
Messages: 49
Registered: March 2004

continuous participant

From: *ikp.kfa-juelich.de

Hi,

Let me add some remarks concerning the pbar+d decay model:
It is based on one on-shell nucleon in the deuteron target not participating in the reaction (= the "spectator") while the other nucleon is off-shell to respect 4-momentum balance. The momentum distribution is parametrized with a Hulthen function.
Usually the struck (off-shell) nucleon is the neutron while the proton is the spectator (otherwise one would prefer a free proton target). However the decay model should also work for the latter case if the decay file is written accordingly.

As Stefano mentioned, there are two parameters in usage of the decay model:
Decay pbardSystem
1.0 p+ pbarnSystem DeuteronSpectator 1.0 1.16;
Enddecay
1st parameter: maximum internal momentum in the deuteron in GeV/c
2nd parameter: minimum mass of the residual pbar+n system for the specified decay (in this case M_phi+M_Pi-).

I plan to give a more detailed explanation in the next CM.

Albrecht

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