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K charged meeting 10/11/03
K tracking efficiency & geometrical acceptance : K(pK,K)
We use the tag in the handle emisphere to have in the signal
emisphere a “pure” beam of K+(K-)
The signal is flagged as Kaon with standard cut on momentum and
IP distance
Background to the signal is mainly due to early 3 body decay of the
K, whose secondary can mimic a K
We use the minimum distance between the signal track and the
extrapolated track from the handle as check parameter
The shape of the DR distribution for background is taken from MC
“ “ for signal is taken from MC and
from double tagged event
K charged meeting 10/11/03
K(pK,K) signal selection Four K definition cuts :1) q opposite to the “handle”2) 70 < PK < 130 MeV3) Rpca < 10 cm4) -20 < zpca < 20 cm
Once we found a “signal” K compute the distance of closest approch between the first hit of its track and the track extrapolated from the handle:
Handle K track
extrapolated
signal
We monitor the background contamination of the signal looking at the tracks minimum distance computed at the point of closest aproach.
K charged meeting 10/11/03
K track eff. = fit to r
r (cm)
BLUE K from MCRED K from 2 tagGREEN bck from MCThe fit to the distribution of the
distance of minimum approach between the signal track and the extrapolated track is made using MC and 2 tag shape for the signal and MC for the background shape
K charged meeting 10/11/03
K shape uncertainties
The r distribution in the K region is slightly overestimated by the fit with K shape from MC and underestimated by the fit with the K shape from 2 tag. The differences between the 2 fits gives the sistematic on the K shape
Fit – signal : MC shape
Fit – signal : 2tag shape
r (cm)
r (cm)r (cm)
signal
K charged meeting 10/11/03
K- versus time
We check the stability of K versus time. The 2001-2002 data were divided in chunk of 6 pb-1 each. The two different results account for the 2 different shape choice for the K contribution.
K-
IntLum/6 (pb-1)
2001
2002
Handle : K+
Signal : K-
K charged meeting 10/11/03
Sistematic : handle tag
Systematic on the K tracking eff. can be due to what happen in the opposite emisphere. Thus we measured the tracking efficiency with respect to the kind of handle tag
BLUE: K0
RED : KBLACK: all tag
All the variations seem to be within statistical error. There is no evidence for dependence of the eff from the handle tag.
K-
IntLum/5 (pb-1)
K charged meeting 10/11/03
K+ vs K
-
The nuclear interactions of K- in the beam pipe and in the DC wall reduce K
- in comparison to K
+ by more than 1 %
IntLum/6pb-1
K
BLUE = K+
RED = K-
K charged meeting 10/11/03
K+ with respect K and pK
K-
bin
Pbin
We divide the K in 6 bin in the range 30< K <90 and the K momentum in 6 bin in the range 70< pK<130 (Mev/c)
bin = 10 degPbin = 10 MeV/c
Nev
ents
bin
Pbin
K charged meeting 10/11/03
Summary
The K tracking efficiency times the geometrical acceptance K has been measured using the tag
tecnique at fraction of % level
The K has been measured independently for positive
and negative K
The sistematics due to the uncertainty on shape of the signal and due to tag bias have been evaluated
The K has been measured versus the time in step of
6pb-1
A memo is in preparation
K charged meeting 10/11/03
Tag Background evaluation
The use of the K+(K-) tag decay ( K and K0) allow us to select a pure K-(K+) beam. Eventual pollution of the tag reflects in a systematic underestimation of the absolute BR measured. We made a first attempt to estimated this background using a sample of 4 pb-1 of 2002 data
• We assumed that the background fraction in the events with one tag decay is small. •There is no background in the events where both K+ and K- undergo a tag decay (double tagged events) • We compare the single and double tag kinematic distribution: the differences can be due to the background ( and , to some extent, to slightly different acceptance ) •The statistical power of this analysis is limited by the rate of double tagged decay in K+K- events ( 10% of the total in the stream)
K charged meeting 10/11/03
Tag bck: Kinematic variables
The control variables was chosen both in the lab and in the center of mass
frame:
1. Momentum of the K charged secondary in the K frame2. Angle between the K flight path and the charged secondary in the K
frame3. Angle between the charge secondary and the K in the lab4. Number of clusters associated at the K decay product ( ≤1 for K and
≤3 for K0)5. Energy of the cluster associated to the charged secondary6. Time of flight of the charged secondary
Only the shape can be compared due to the different yelds of single and double tag events
K charged meeting 10/11/03
Charged secondary momentum in K frame
Red = difference of the 2 histoBlue = statistic uncertainty
Normalized comparison between single and double tag events
Linearscale
Logscale
Mev/cMev/c
Mev/c
K charged meeting 10/11/03
Cos() between K and secondary in K frame
Red = difference of the 2 histoBlue = statistic uncertainty
Linearscale
Logscale
K charged meeting 10/11/03
Cos() between K and secondary in lab frame
Linearscale
Logscale
K charged meeting 10/11/03
Number of secondary cluster associated
Ncluster ≤1 for K
Ncluster ≤3 for K0
Red = difference of the 2 histoBlue = statistic uncertainty
Linearscale
Logscale
K charged meeting 10/11/03
Energy of the cluster associated to the charged secondary
Linearscale
Logscale
MeV MeV
MeV
K charged meeting 10/11/03
Time of flight of the charged secondary
Linearscale
Logscale
ns
ns ns
K charged meeting 10/11/03
Background statistic estimator
To build a conservative background estimator I have to measure the deviation from statistic fluctuation of the difference of the two sets of histos. We define:
n) = abs [ his2tag(n) – his1tag(n) ]
For each bin I consider the quantity (n) = n) - (n) . This variable gives
the deviation of (n) from the statistical fluctuation and is > 0 if the bin is bigger then statistica fluctuation and < 0 is underfluctuate. The sum over all the bins of (n) is a upperlimit to the background.
n)2 = ( his1(n))2 + (his2(n))2
For bin n
K charged meeting 10/11/03
First results on 3 pb-1 of 2002
Variable ii ii ii
Pseccm 0.0219 0.0160 0.0059
coscm 0.0258 0.0301 -0.0043
coslab 0.0249 0.0298 -0.0050
Ncluass 0.0070 0.0038 0.0032
Eass 0.0318 0.0745 -0.0427
Tofass 0.0285 0.0491 -0.0205
Positive Tag
K charged meeting 10/11/03
First results on 3 pb-1 of 2002
Variable ii ii ii
Pseccm 0.0303 0.0161 0.0142
coscm 0.0280 0.0303 -0.0024
coslab 0.0267 0.0300 -0.0032
Ncluass 0.0085 0.0038 0.0047
Eass 0.0331 0.0715 -0.0384
Tofass 0.0304 0.0386 -0.0083
Negative Tag
K charged meeting 10/11/03
Backgroung on negative tag?
The difference between the 1 tag and the 2 tag distribution settles in the signal region.. True background ???
Conclusion:There is no evidence for a clear background contamination in the single tag events, at least at fraction of % level
We are working out a robust statistic estimator for the background level (or limit)
Work in progress..