Like-sign CB in view of Hades acceptance
Pair efficiency correction in view of varying MDC efficiencies
Wolfgang Koenig @ Collab. Meeting XXVIII
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Pair efficiency correction day 108 versus day 122 (Au+Au)
Like sign geometrical combinatorial background & Hades
acceptance: An incomplete investigation
Summary & Outlook
Normalization of large invariant masses to pi0 Dalitz: missing sectors, missing efficiency
W.K. end of may 2014
Day108
Example: 5 or 6 sectors operational
Different sectors (large opang):
10 or 15 combinations: eff. = 66.7%
Same sectors (small opang):
5 or 6 combinations: eff. = 83.3%
Additional loss open pairs: 25 %
Normalization to pi0 Dalitz
needs correction
Day122
Day 108:
Ratio
close pairs killed/open pairs: 8.8
Ratio opening angle:
>50° / <25°: all: 2.0 (signal:0.254)
Gen 6
unlike
Day 122:
Ratio
close pairs killed/open pairs: 12.0
Ratio opening angle:
>50° / <25°: all: 1.57 (signal: 0.182)
Day 108 / Day 122:
Double Ratio (signal)
close / open pairs: 1.40
like
Day 108
Pi0 Dalitz (0-150 MeV):
signal/Bg = 74.0%
signal/events = 2.61 10-5
High Mass (>240MeV) :
signal/Bg = 9.14%
signal/events = 2.2 10-6
CB: arithmetic mean
geom. mean*0.99
Day 122
Pi0 Dalitz (0-150 MeV):
signal/Bg = 82.1%
signal/events = 1.62 10-5
High Mass (>240MeV) :
signal/Bg = 8.0%
signal/events = 0.97 10-6
Day108 / Day 122:
Pi0 Dalitz:
1.61 (geometry: 1.2)
High Mass (M>240MeV):
2.3 (geometry: 1.5)
CB: arithmetic mean
ToF / RPC ratio
2.0
Day 108
Day 122
ratio Day122 / day108
1.0
0.5
0.0
1
2
3
4
sector no
Single lepton counts: RPC:
104
5
6
, ToF: 1.6*
104
Left:
ratio day108/day122 for same sector
pairs (pion Dalitz)
Right:
ratio day108/day122 for different
sector pairs (large invariant masses)
Blue: calculation from single lepton
efficiencies
Red: experiment
ratio day108 / day122
ratio Tof/RPC
1.5
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Single lepton efficiency ratios
day122 / day108 sectorwise.
1
2
3
4
sector no
5
6
7all
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
calc. exp. close
calc. exp. open pairs
Consequences of Hades acceptance on
_
like sign geometrical background
+
At edges of the Hades acceptance:
Strong asymmetry in detection efficiency
as function of charge and momentum.
Different to solenoids
Combine:
Large  's : ++ dominates
Small  's: - - dominates
Small & large  's: + - dominates
Integrated over full acceptance:
Asymmetry nearly cancels
Following analysis:
Day 108 :start 17:00, day 109: stop 1:30
All 6 sectors operational
+
_
Geometrical mean:
to be calculated differentially
Taylor expansion:
sqrt(1+e) = 1+e/2 => correction = 1.0
1.0
0.9
geom./arithm. Mean
Ratio between geometric and arithmetic
mean as a function of the asymmetry
between ++ and – pairs.
Ratio P++/P-- = 1 + e
=> correction = sqrt(1+e) / (1+e/2)
1.1
0.8
0.7
0.6
0.5
0.4
0.3
1
3
5
7
9
11 13 15 17 19 21 23 25 27 29
ratio P++/P-- or P--/P++ (whatever is larger)
Like sign pair comparison as function of invariant mass (integrated over acceptance), gen 6
Arithmetic mean
Geometric mean
difference: 0.86%
Gen 7
+
+
-
+
5
Gen 6
1
0.5
Ratio between like-sign ++ and –
as function of opening angle
5
Gen 7
0.5
For theta > 75° the like sign combinatorial
background dominates above pi0 peak.
single leptons
Gen 6: 214237
Ratio 7/6: 1.54
Ratio
Gen 7/6: 1.9
Ratio
Gen 7/6: 2.7
geom. mean:0.36
The geometrical background for this angular
region is extremely asymmetric: factor 31.2
Gen 6: factor 17.4
CB needs to be corrected differentially
Asymmetry in like sign background as function of invariant mass (84°>theta >75°)
Arithmetic mean = 1.01*
mean geometrical mean
geom. mean = 0.36 *
mean geometrical mean
For theta < 25° the mean like
sign combinatorial background
dominates above pi0 peak.
Asymmetry in geometrical
background: factor 42 +- 16
Gen6: factor 16.3 +- 4.5
Gen 6: – : 114
++ : 7
Opposite sign compared to >75°
Small opening angles: small
invariant masses
_
_
Arithmetic mean = 1.01*
mean geometrical mean
geom. mean = 0.34 *
mean geom. mean
For theta < 25° & >75° the
unlike sign combinatorial
background dominates. Small
contamination from true e+epairs only. Negligible
contribution from pi0 peak
(large op-ang's)
No Asymmetry in geometrical
background
Like sign combinatorial bg
unusable to subtract
combinatorial unlike sign bg.
0
+
_
unlike / like
=2.35
For  >75° combined with a
fiducial  region (25°-70°) the
like sign geometrical
combinatorial background is too
small.
large asymmetry in like-sign
geometrical background
Ratio:
1.27
Ratio: 3.7
Like sign geometrical
combinatorial bg unusable to
subtract combinatorial unlike
sign bg. Arithmetic mean
better
+
_
+
Ratio: 1.27
Ratio: 3.7
Summary
Correction of time dependent missing sectors / reduced efficiency based on single
leptons possible but large difference between close (pion Dalitz) and open pairs (large
masses) needs to be taken into account (> 50%)
Huge difference between differential like-sign geometrical CB and mean geometrical
CB due to asymmetric acceptance.
Like-sign CB partially massively wrong. Particularly important when investigating pt or Y
bins.
Outlook
See Szymons talk
Correct for asymmetry in acceptance differentially in theta using simulations. Statistics ?
Give up on like-sign CB. Use event mixing in whole inv. mass range after clean up of
close pairs before mixing (correlations). Normalize to like-sign pairs within a fiducial
volume (30° - 70°) at intermediate inv. Masses (350 - 400 MeV)
Use fiducial volume at small invariant masses (sqrt(p1*p2) ) e.g. 30°-70° and increase
theta range when going up in mass (down in statistics)