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Panda target and vertex cuts [message #18584]

Tue, 27 October 2015 15:50

Elisabetta Prencipe (2)
Messages: 214
Registered: February 2013

first-grade participant

From: *ikp.kfa-juelich.de

Dear all,

following a discussion started at the open-charm meeting, and today at the computing meeting, I would like to point you on a non-trial problem.
I am trying to reduce the DPM bkg in the simulation ppbar to Ds- Ds2317+. It turns out that a cut around the vertex position can be effective. However, I am currently using the /rho package, and I did not find a way to compute how effectively the target smearing can affect such a cut. It should be done at level of sim- macro.
Based on MC signal simulations, the vtx resolution ix x,y is in between 50 and 80 micrometer (it depends also on which vtx fitter one makes use of), and it is roughly double value for the z vtx position. However, to apply even a 5 sigma cuts around those distributions, it is not a clever idea, due to the fact that the we cannot assume IP(0,0,0): it is simply non realistic, due to the target smearing, which is still not taken in consideration.

Here is a phD thesis performed in 2011, from T. Randriamalala: http://www-brs.ub.ruhr-uni-bochum.de/netahtml/HSS/Diss/RandriamalalaTsit ohainaH/diss.pdf

At pag.58-61 studies are reported in this sense, for the pellet target. I point you at pag. 60, fig. 4.34.
If I understand it correctly, a cut at <= 2mm would be safe. Do you agree?

I do not find such a study related to the cluster-jet target, and/or related simulation for a physics channel. Anybody has some any clue?

Looking forward to hear from you,

Elisabetta

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Re: Panda target and vertex cuts [message #18588 is a reply to message #18584]

Tue, 27 October 2015 18:38

StefanoSpataro
Messages: 2736
Registered: June 2005
Location: Torino

first-grade participant

From: *to.infn.it

Please check the following wiki page:

https://panda-wiki.gsi.de/foswiki/bin/view/Computing/VertexSmearing

I.e., for the Central trackers TDR, it was implemented in the following way:

FairPrimaryGenerator* primGen = new FairPrimaryGenerator();
primGen->SetTarget(0., 0.5/2.355);
primGen->SmearVertexZ(kTRUE);
primGen->SmearGausVertexZ(kTRUE);
primGen->SetBeam(0., 0., 0.1, 0.1);
primGen->SmearVertexXY(kTRUE);
fRun->SetGenerator(primGen);

Of course the numbers should be updated.

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Re: Panda target and vertex cuts [message #18591 is a reply to message #18584]

Wed, 28 October 2015 09:30

Klaus Götzen
Messages: 293
Registered: June 2006
Location: GSI

first-grade participant

From: *gsi.de

Hi,

the PhD thesis indeed indicates, that the reactions come from this overlap volume with diameter 1.6mm (+-1 sigma region) in x,y directions and 3mm in z (thickness of the pellet stream). The integral of this 2-D gaussian in x,y between -2...2mm in both directions is something like 97%, which seems to be quite robust against small shifts of the center point (i.e. how well we know the absolute position of this volume in space, which depends on alignment). A shift of 1mm in both x and y still gives P=80%.

-> Your cut of +-2mm around (0,0,0) for the pellet target case might be fine I guess.

Concerning the cluster jet, you can take a look at p. 30, Fig. 5.8 in Target TDR (https://panda.gsi.de/oldwww/archive/TargetTDR/Targets_TDR.pdf). There is reported about a fermi like density function with a diameter of about 12-14mm. This would refer only to the spread in z-direction, so that you in that case can apply a tight cut only on the vertex distance to the z-axis, perhaps combined with a rather wide cut on the vertex z-position.

Best,
Klaus

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