Hi all,
  I cannot attend the meeting tommorrow ( I have to be in IR-2 )
so I posted this small note:


Status of the KL veto with the IFR
----------------------------------

Looking at the feasibility of a KL veto to deplete Vcb events, the
following features emerged:

1. The KL momentum spectrum, as expected, is quite soft (peaking at
   700 MeV). This is a difficulty since the IFR 'prefers' high
   momentum KLs, as you can see in the following two plots::
     http://www.slac.stanford.edu/~denardo/klveto/genklmom.ps
   which shows the momentum of true KLin Vcb events and
     http://www.slac.stanford.edu/~denardo/klveto/matchklmom.ps
   which shows the same quantity when an IFR cluster 'matches' the true
   KL direction. Since GHIT based association does not work for KLs, the
matching is
   geometrical (a 200x200 mrad box in theta-phi).
   A good fraction of KL spectrum is above 500 MeV, then KL
   detection is not impossible.

2. Searching for a KL signal in the IFR without any constraint is
   limited by the precence of charged cluster splitoffs. These
   splitoffs are clusters due to charged hadrons for which
   the swimmer based tracks-clusters association
   in charged cluster reconstruction has (partially) failed.
   Defining as minimum opening angle the smallest among the 3D-angles
   between the KL candidate direction (given by the cluster centroid)
   and the direction evaluated at the EMC of all the charged tracks
   with p > 750 MeV, the following two plots show the distribution of
   this quantity for Vcb events with no true (generated) KL and with a KL
   candidate cluster 'matching' the true KL direction.

   no true KL plot:
     http://www.slac.stanford.edu/~denardo/klveto/openfake.ps
   matching KL plot:
     http://www.slac.stanford.edu/~denardo/klveto/openmatch.ps

3. Because of the beam backgrounds in the IFR endcaps,
   candidates in the endcaps with a clusters
   lasting too deeply have to be rejected..


>From point 2-3 a possible KL veto is

a     require quality clusters (3 layers at least, maybe more)
b     min opening angle grater than 500 mrad to avoid splitoffs
c     accept clusters in forward endcap only if the last layer hit is
      before the 14th.

Using this selection on Vub (~45000 events) and Vcb (~364000 events)
MC samples, I found that

  5332/60945 = 8.7% of Vcb events
  3462/42300 = 8.1% of Vub events


are rejected.  This happens because the fake clusters contamination is
still high (only 1919 out of 5332 are from real KL in Vcb events)

If we limit ourself to events with raw MX less than 1.5 GeV
the rejected events become
   964/7969 ~ 12% of Vcb events tagged
   1033/13028 ~8% of Vub events tagged
and true KL / fake KL ratio  is better (591/964)

The situation is not very exciting expecially if we note that
MX and MMiss^2  (the raw quantities as I get from recoilNtp.cc)
are correlated as shown in the following plot:

http://www.slac.stanford.edu/~denardo/klveto/mm2vsmx.ps
( KL tagged events in Vcb sample are shown )

The momentum stolen by the KL to the X system leaves trace on MMiss,
and the plot seems to suggest that a cut on MMiss could be enough

I am not saying that this is the end of the KL veto story ( I didn't look
at EMC and charged hadrons faking KL may be reduced somehow ),
but maybe a lower priority can be assigned to this task in order to
focus on more urgent ones.

I would have liked to talk with you about this (and the other
items in the agenda) at the meeting but I cannot attend ( I have to be
at the same time in IR-2 being the last day of before IFR endcap
doors will be closed).

See you
  Guglielmo