Goal for the anti-counter

SVOBODA%7460@sn01.sncc.lsu.edu
Thu, 27 Apr 1995 10:11:11 -0600 (CST) 

Date: Thu, 27 Apr 1995 10:11:11 -0600 (CST)
From: SVOBODA%7460@sn01.sncc.lsu.edu
Subject: Goal for the anti-counter
To: DYE@buphyc.bu.edu, stone@buphyc.bu.edu, kearns@budoe.bu.edu,

From: Bob Svoboda
To:   fellow SuperK electrono-nerds
Subject: Anti-counter uses

	After the phone conf. on Wednesday, perhaps it is useful to re-visit
physics reasons for the anti in order to organize our thoughts on priorities
and specifications. Here is a "straw man" shot. Your comments/input are
welcome - as I propose we come up with some agreed upon statement so
we know better what the hell we are doing. 

	I see the main purpose of the anti-counter as: (1) tagging incoming
muons so that we can be sure that an event is fully contained-this will
make for much better fiducial volume definition in the central detector;
and (2) the solar neutrino analysis requires veto-ing out a cylinder
around muon tracks to reject spallation events. Thus the better the through-
going muon resolution the bigger the solar neutrino fiducial volume
for a given spallation background rate. The anti can provide a first
guess/independent determination of the entry point, though it is likely
the final fits will be done with the central detector.

	The reason I have been concerned about raising light collection
so much (e.g., not trashing the waveshifter plates, wanting reflectors
on the walls, worrying about girders, etc) is because I don't want to
see "holes" develop in the anti due to dead PMT's or unanticipated
shadowing. Such holes could bias the contained event sample due to a
non-uniform anti-counter efficiency.

	In IMB typically 5% or more of the PMT's were dead (really
dead, not just bad calibrations). Yes, I know we have addressed the problems
that contributed to this high rate (bad base resistors, mostly) - but
I expect we will see new ones (leaks, implosions, flashers...you name it).
We should not base the anti-counter performance on our being significantly
smarter in the 90's than we were in the 80's - there is too much evidence
to the contrary. By the way, trigger folks will have to contend with the
fact of FLASHERS in the ANTI effectively shutting the detector down
by saturating the DAQ system. This WILL happen (and it won't be pretty)
unless we plan on automatic corrective response from the computer/trigger
system.

	For the entry point recognition, it is very useful to know the
light levels. I presented curves of Q vrs distance from the entry point
at the collab. mtg at KEK last year. Mika has done a much better analysis
(though not yet complete) on entry-point recognition which she presented
last meeting again showing that Q is very useful. Thus, though we initially
considered not having any ADC's at all in our electronics, we set about
looking for cheap ways to get Q information. By the way, people should
stop laughing at me from getting AMY muon paddles and sending them to
the mine. Such muon paddles will give us our only real check of how
we are doing in supposedly one of the most important reasons for all
our work. We should make every efforts to install them from day 1. I plan
to spoend some time working on them on my next trip to Japan, but I need
help.

	Remember, we want Q basically to get a STARTING POINT for the
muon fit, the analysis does not crucially depend on this. Studies done
looking at calorimetry in the anti-counter (see meeting note from first
KEK meeting, from Maryland meeting, and from proposal) show that it is
only a little better than IMB-1 for detecting muons that decay and for
calorimetry of exiting tracks. That might be OK, but the calorimetry
and muon decay detection efficiency depends critically on the vertex location
and track direction. In short, getting Q for calorimetry for this doesn't
seem like a very good justification.


	Thus for the two MAIN purposes of the anti-counter (as I see it),
Q information is potentially VERY useful, but perhaps NOT CRITICAL to the
performance. Based on this I think we should look for a SIMPLE system
for providing Q information that can be built rapidly and cheaply.
All the information we have to date from MC studies indicates that simple
TOT is not so good since large errors would be common (though it has not been
shown that TOT with timing could not do the job of recognizing entry points).
In general, nobody likes the idea of large errors in the data that could
hopefully be dis-entangled later. We would all LIKE to rely on tried-and-true
integrated Q in some fashion, I think.

	What about secondary uses for the anti?

	I am interested in looking at downward-going muons that go through
the veto counter annulus and produce high energy pion that penetrates into
the central detector. The reason for this is that this has been suggested
as a background for upward-going "showers" in IMB. With SuperK we have
the possibility to measure the probability of such events provided we tag
those time that a muon goes through the annulus and does NOT produce a pion.
I Hawaii I presented a calculation that the rate of such events would be
roughly one every 45 minutes or so. This is the main reason why I think we
should have an anti-counter trigger from the start. We can write a paper
on the results. Q information might be useful for this study, since dual
hot spots would help to reject multiple-muon background.

	As a backup for Supernova detection in the event of central detector
DAQ choke? Maybe, but this sounds like a long shot to worry about spending
a lot of time and effort and money on. Better to spend time and effort
devising tests for the central detector electronics to minimize this
possibility, I think.

	For recognizing the entry point of UHE upward-going showers? This
seems like a good reason, but we get this for free with recognizing entry
points for muons. Also note that for AGN's, it is a very long shot that
by fitting event direction exactly we will be able to image the source
galaxy. 




	In summary:

	Main Purposes:

		(1) recognize entering/exiting particles to provide clean
		fully contained sample with minimal directional and
		vertex effects (as we saw in IMB).

		requirements: hit pattern associated in time with
		central detector trigger.

		(2) recognize entry point of muons as a starting point
		for muon fit for solar neutrino analysis.

		requirements: hit pattern plus timing and Q. Timing and
		Q calibration system. Muon paddle entry point calibration
		check.

	Secondary Purposes:

		(3) Upward-going pion probablility measurement.

		requirements: Same as for (2), with addition of an
		anti-counter trigger. Trigger need no be at low
		threshold.

		(4) UHE Upward-going event entry point recognition.

		requirements: same as for (2).

	Tertiary (Maybe) Possibilities:

		(5) calorimetry of exiting tracks

		(6) muon decay recognition

		(7) Supernova backup system