[Halld-cal] forward shower systematics from MC, new timing comparison

[Shepherd, Matthew]

Hi Richard,

I believe the time walk is then expected.  

The time of an FCAL shower is supposed to be the time it takes a photon to get to the particular (x,y,z) associated with the shower.  We choose to do depth correction by modifying z of the shower rather than adjusting (x,y) for z at the FCAL face.  This results the the behavior that the time for higher energy showers is slightly higher because z is further from the vertex.

If one wants to check the timing, the correct thing to do is to compare the time difference:  t_FCAL - t_RF, where t_RF is the RF time of the bunch at the center of the target with the expected time:  | r_FCAL - r_target | / c.  Here r_FCAL is the 3D position the FCAL shower and r_vertex is the 3D position of the center of the target.

It will be difficult to learn anything from time of shower alone vs. energy.

Matt


> On Mar 21, 2019, at 2:52 PM, Richard Jones <richard.t.jones at uconn.edu> wrote:
> 
> Hello Matt,
> 
> It is whatever is returned from the method DFCALShower::GetTime().
> 
> -Richard Jones
> 
> On Wed, Mar 20, 2019 at 3:06 PM Shepherd, Matthew <mashephe at indiana.edu> wrote:
> 
> Richard,
> 
> The DFCALShower object (which is stored in REST) does not have a member called "shower RF time."  Without more information as to what you are plotting, I can't comment on it.
> 
> Matt
> 
> ---------------------------------------------------------------------
> Matthew Shepherd, Professor
> Department of Physics, Indiana University, Swain West 265
> 727 East Third Street, Bloomington, IN 47405
> 
> Office Phone:  +1 812 856 5808
> 
> > On Mar 19, 2019, at 4:24 PM, Richard Jones <richard.t.jones at uconn.edu> wrote:
> > 
> > Hello Matt,
> > 
> > I don't compute the shower RF time, I just read the value directly from the REST event record. I suppose someone else who is the expert on the in-memory FcalShower object that holds the corresponding field in its data can answer the question as to how it is computed.
> > 
> > -Richard J.
> > 
> > On Tue, Mar 19, 2019 at 3:37 PM Shepherd, Matthew <mashephe at indiana.edu> wrote:
> > 
> > Richard,
> > 
> > Can you explain exactly how you compute the expected RF time for these showers?  Specifically, do you extrapolate to the z position recorded in the shower or to the front face of the FCAL?
> > 
> > The photon should propagate to the (x,y,z) position of the shower at the speed of light.  If the evolution of z is not considered, then this will produce an apparent time walk with energy.
> > 
> > Matt
> > 
> > ---------------------------------------------------------------------
> > Matthew Shepherd, Professor
> > Department of Physics, Indiana University, Swain West 265
> > 727 East Third Street, Bloomington, IN 47405
> > 
> > Office Phone:  +1 812 856 5808
> > 
> > > On Mar 14, 2019, at 12:15 PM, Richard Jones <richard.t.jones at uconn.edu> wrote:
> > > 
> > > I have added a side-by-side comparison of the timing (Fcal only for the moment) from hdgeant and hdgeant4 to my MC shower systematics study. See slide 8 in the attached document, posted to docdb. The agreement between the two simulations is acceptable (30ps difference in the fit p0, probabably within MC systematics. The primary message of this plot is the large time-walk that exists in the simulated shower times after all corrections have been applied. This is the time recorded in REST within the fcalShower objects.
> > > 
> > > https://gcc01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fhalldweb.jlab.org%2Fdoc-private%2FDocDB%2FShowDocument%3Fdocid%3D3852&data=02%7C01%7Challd-cal%40jlab.org%7C18a234d9fe634c2a688f08d6aecf54c4%7Cb4d7ee1f4fb34f0690372b5b522042ab%7C1%7C0%7C636888602835383591&sdata=BjDgRhJnWLz%2FeRMYJczqKoLCqqBRa4TnpV4wJnziOmY%3D&reserved=0
> > > 
> > > -Richard Jones
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>