K charged meeting 10/11/03 K tracking efficiency & geometrical acceptance : K (p K, K ) We use the tag in the handle emisphere to have in the signal

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(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 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 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- 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-


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+ 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+ 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


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


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)


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


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


Cos() between K and secondary in K frame


Linearscale

Logscale


Cos() between K and secondary in lab frame

Linearscale

Logscale


Number of secondary cluster associated

Ncluster ≤1 for K

Ncluster ≤3 for K0


Linearscale

Logscale


Energy of the cluster associated to the charged secondary

Linearscale

Logscale

MeV MeV

MeV


Time of flight of the charged secondary

Linearscale

Logscale

ns

ns ns


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


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


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


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..

Documents

K charged meeting 10/11/03 K tracking efficiency & geometrical acceptance : K (p K, K ) We use the tag in the handle emisphere to have in the signal