detection of d +/- hadronic 3-body decays in the cbm experiment

V.Petracek TU Prague , UNI Heidelberg18.10.03 GSI

Detection of D+/- hadronic 3-body decays in the CBM experiment

● D+/- K B. R. c = 317 m , 25 AGeV Au-Au● Detector setup in simulation● Signal and Background simulation● Acceptance● S/B , Detection limit, Significance● Effects of pixel geometry and tracking● Other possible cuts


Detector setup in simulation

● 7 tracking stations

● First two used for track reconstruction

● acceptance 110 – 433 mrad

● No magnetic field

● Ideal P reconstruction

● Ideal PID

● Pixel position resolution = 20 m

● Pixel geom. ~ 60 x 300 m


Signal and Background simulation

● D+/- Kgenerated in kinematic decay generator

● Thermal Pt spectrum T = 175 MeV

● Gaussian rapidity with = 0.6

● Decay exponential in CMS , boosted to LAB

SIGNAL


Signal and Background simulation

● Pt and y parametrization based on uRQMD – same as used for D0

● <K-> = 13 / event (in 4)

● <> = 328 / event (in 4)

● Still only limited statistics

BACKGROUND


Acceptance● Geometrical acceptance for signal is

18% (all 3 products accepted)

● Geometrical acceptance for background tracks 0.462

● In acceptance <K> = 6 , <> = 151

K

Triplets


S/B , Detection limit, Significance● Main cut on track impact

parameter b

● Done before triples are made – reduces CPU time

● B estimate based on fit of the b distribution

● B triplets = all 3 tracks have b>b_max and invariant mass <1.8,2> GeV

b Signal b Background

4.79% 30%,23%,17%

8.16 10-3

2.75 10-3

1.1 10-3


S/B , Detection limit, Significance● Probability to 2K with b>b_max in event is given by

Be = (N)p3(1-p)N-2 N

K = 0.5N

KNNp

3(1-p)N-2

● Be = 1.1 10-2 , 9.37 10-4 , 7.6 10-5 (for b_max 0.2 , 0.25 ,0.3 mm)

● Combined with probability, that background triplet will be found in signal region we obtain B = 5.14 10-4 , 4.42 10-5 , 3.58 10-6 (for the respective b_max )

2

BACKGROUND

● B.R. 9% , <D0> ~10-2, <D+> ~0.3 <D0>, Geometrical acceptance 18%, 30%-17% of signal remains after the b_cut 0.2, 0.25, 0.3 mm

● S = 2.916 10-5 , 2.235 10-5 , 1.652 10-5 (for the respective b_max )

SIGNAL

● S/B = 0.056 , 0.505 , 4.16 (for the respective b_max )

● significance = 1.3 , 3.5 , 8 (for 1Mevt ) , for b_max=0.2mm is the detection limit ~6Mevt (for significance >3)

SIGNAL/BACKGROUND & SIGNIFICANCE


Effects of pixel geometry and tracking

● Pixel size 60 x 300 m

● Pixel orientation in first 3 layers optimizes resolution in non-bending plane ... precise determination of y-z projection of the displaced track

● In Stations 4-7 optimized for momentum resolution



● Implemented “Toy” tracking in idealized dipole magnetic field

● Studied influence of tracking and pixel size on SV reconstruction resolution and on reconstructed invariant mass



Pixel 60 x 300 morientation 3-4

Isotropic hit resolution 20 m




● Effect of multiple scattering dominates ● Mass resolution with ideal hit resolution is very similar● With this resolution would be possible to decrease width of the mass window and to improve S/B


Other possible cuts

Mean distance between tracks in their closest point.

This cut is effective on fake tracks



Other possible cuts

Triplet impact parameter

distance between PV and intersection of the line defined by reconstructed SV and total momentum 3-vector of products.

This cut is effectively momentum conservation cut smeared by position resolution of the SV

Maximum of this distribution ~

x(y) SV



Other possible cuts

Cuts in Dalitz plane

Better to avoid

Use with great care

Selection of resonant decays via K*(892) and K*(1430)


Conclusions

● Reconstruction of D 3-body hadronic decays is possible under idealized conditions considered in this study

● The main unknown factor is amount of fake tracks produced by tracking

● In the next phase will be used the tracking algorithm from Ivan Kisel

● There is still uncertainty in the fit of background b due to statistics – more background events are needed

detection of d +/- hadronic 3-body decays in the cbm experiment

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