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DRAFT 0.1 - for discussion only

5 August 97


Rules for Construction of Level 3 Input Lines  - UNOFFICIAL
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AUTHORS:  The Level 3 Trigger Group



    The design requirements for the Level 3 trigger state that specific
rules for constructing the Level 3 input lines are to be created and
enforced.  This document is expected to evolve into the official
statement of those rules.

    This draft represents the current thinking of the Level 3 group on
what rules are needed.  As design work proceeds, we will gain more
experience with the implications of various definitions of Level 3 input
lines and modify or add to these rules as needed.  When we are confident
that we have a reasonable set of rules, a successor to this document wil
be officially released and placed under change control.


    Motivation
    ----------

    The Level 3 trigger uses its input lines as a first stage in event
filtering.  It is therefore imperative that the operation of each of
these lines be understandable and its efficiency measurable.  The rules
given here are believed to be necessary to achieve those goals.


    The Rules
    ---------

RULE 1: Each Level 3 input line must require one of a subset of the FCTS
output lines, i.e., "LineA OR LineB OR ..."

JUSTIFICATION: Each FCTS output line represents an independent decision.
It is appropriate that Level 3 base its own decisions on these earlier
decisions.  The simplest way to do this is by considering either a
single line or a logical OR of lines.


RULE 2: A Level 3 input line may require a Level-1-derived FCTS output
line to be asserted in conjunction with a more strict threshold
requirement on the corresponding GLT counter(s), i.e., "LineA AND
CounterZ."  In conjunction with Rule 1, this allows definitions of the
form:
    "(LineA AND CounterZ) OR (LineB AND CounterQ) OR LineC"

JUSTIFICATION: One may wish to run on a subset of events that passed a
particular Level 1 line.  The safest way to make such a selection is to
impose a stricter criterion on the relevant object count.


RULE 3: The Level 1 output lines (prior to FCTS prescaling) shall not be
considered.

JUSTIFICATION: A number of Level 1 output lines are prescaled to reduce
rate.  Ignoring the prescale is likely to lead to high rates for certain
types of events, and to biases that severely complicate the task of
understanding trigger efficiency and behavior.


RULE 4: Every Level 1 Accept that resulted from a Level 1-derived FCTS
output line must cause at least one Level 3 input line to be asserted.

JUSTIFICATION: This rule ensures that Level 3 will examine every event
it receives that Level 1 selected as a plausible physics candidate.
Note that this rule does not demand that every event cause an Level 3
input line to be asserted; FCTS may pass certain categories of events
that need not be examined by Level 3.


RULE 5: Any FCTS output line that is subject to prescaling may not be
combined with any other output line.

JUSTIFICATION: Combining prescaled lines that have different prescale
factors compromises our ability to understand trigger decisions and
efficiencies.  This rule averts that problem.