Fig 1. VPD-Minuit, d-Au 2008 events, minB events: ZDC East+VPD

Fig 2. VPD-PPV, p-p 2008 events, st_physics , no trig selection, mostly E & BEMC triggers

Hi Jan, Akio,

Xiaoping helped me to check the test 2008 pp production data Jan suggested. Please take a look at the attached plot. Basically what is plotted here is vz difference between vpd vertex and tpc vertex for different vpd hit configuration, similar to that in Akio's web page, but using the TOF electronics for vpd hit configuration selection.

So firstly we didn't see the ~30cm width gaussian component. The two narrow gaussian components are attributed to the VPD resolution. In the VPD timing resolution, we see a double gaussian structure, and with about a factor of 2-3 difference in widths. This is consistent with what we see here.  The largest ~50-60 cm gaussian component should be due to the fail of TPC VF and it is related to the beam vz distribution.

And secondly, the resolution in the (E,W)>(1,1) configuration is expected better than the configuration of (E,W)=(1,1). The "dilution" in vpd resolution with more hits seems not true to us.

Generally, we don't see an obvious issue on the VPD side. I am not sure if how the result will change when you use the DSM for selection. Or maybe your statistics is not good enough? Or the data are from some bad TOF runs?

Thanks and Best Regards

/xin

Jan Balewski wrote:
Hi Xin,
Those 2 analysis do not need to be contradicting.
There is much less pileup in dAu than in pp. There may be beam background in pp.

Can you investigate this effect in 2008 pp data from production requested by Matt  ~2 weeks ago, it is done, files are on data09,10.
http://www.star.bnl.gov/HyperNews-star/get/starprod/249/4/1/1/1/1/2.html
It is 3K daq files, 1M events w/ TPC , 1/4 of events have VPD vertex, ~90 % have TPC vertex produced by  fixed PPV.

Thanks
Jan



On Oct 1, 2008, at 11:40 PM, Xin Dong wrote:

Hi Akio,

Thanks for this message. Actually we always see the resolution will be improved if we require more VPD hits. I don't quite understand the ~25cm gaussian distribution at this step. Xiaoping helped me check the dAu data (we don't have TPC vertex in pptoftpx triggered data), you can find the distribution from the attached plot. It shows that with more VPD hit requirement, the vertex resolution is better. No 25cm-width gaussian contribution appears.

So  let me answer your questions directly, see them inline.
Akio Ogawa wrote:
Hello

I posted this yesterday to vertex mailing list. I'd like to make
sure you know this since you may be more intersted than us.

In zVPD-zTPC distribution at pp, we see 3 structures. See Fig 2 (2 gaussian fit) and Fig6 (3 gaussian fit) of
http://drupal.star.bnl.gov/STAR/blog-entry/rjreed/2008/sep/11/ppv-revision-1-29-high-luminosity-fmsslow-trigger-evaluation 

First is sigma ~3cm peak where TPC and VPD vertex matches. Quite
reasonable with resolutions of those two vertex finding.

2nd is sigma ~80cm, which is understandable if TPC and VPD picked
up 2 different vertex. Vertex distribution is ~gaussian with sigma
~60cm. If we pick 2 randomly and take difference, then sigma should be sqrt(2)*60cm ~ 85cm.

3rd one is the mistery. Sigma is around 25-30cm. So its much
narrower than random. Its hard for TPC to "miss" vertex by
10-20cm, since all track's DCA_z is <3cm.

Rosi changed selection of TPC vertex (more matched tracks) to
make TPC vertex better, she saw no difference in the structure.

Now if look at the plot at bottom of http://www.star.bnl.gov/protected/spin/akio/200806/index_8th.html

which is essentially same plot but divided by # of VPD hits.
This 3rd structure with sigma~25cm is most evident when both VPD-E
and VPD-W has 2 or more hits.

This suggests (at least to me) that when you have more than one hit
in VPD and taking time average, sometimes you are diluting VPD vertex resolution. This can be "real" (another collisions in the same crossing
or some beam halo hitting VPD) or detector effects (hot pmt, too
loose timing cut, etc).

Have you seen this?
==>Xin
No.
Is there way to get some more info from mudst?
==>Xin
The number of hits and Tdiff information should be available from MuDst. Tdiff cut may help some in resolution, but shouldn't create a 20cm gaussian peak.
(For example distance or rms of hits included in average?)
==>Xin
I don't quite understand, distance or rms of hits to what?
Is there some cut you tuned when you accepting hit to form average?
==> Xin
Yes. We have already removed the hits with non-physical timing information (out of trigger timing window, but for sure with 25ns resolution). And we always take the earliest hit.
(for exapmple maximum time difference?)
Is average weighted by ToT?
==> Xin
No. Supposedly the ToT dependence is calibrated. We just do simple average.
Have you tried taking earliest hit only?
==> Xin
Yes.

We will trying to see what the pp data look like. Thanks

/xin