Feasibility of the detection of D0 mesons in the NA61/SHINE experiment

P. Staszel and Yasir AliJagiellonian University

D0 → K- + π+ (BR=3.87%).

Outline

1. Introduction → Input 1) physics input 2) detector

2. Reconstruction → acceptance → realistic response

3. Cut strategy

4. Results on D0

5. Future strategy2

Input

1. We use AMPT (A MultiPhase Transport model) event generator→ generated 51k Pb+Pb events at √sNN = 17.2 GeV for 0-10% centrality

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0.01

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Input - cnt.

AMPT predicts 0.01 of <D0> + <D0> per central Pb+Pb event. → this seems to be under-predicted value, e.g. PYTHIA run for N-N and scaled to central Pb+Pb gives 0.21

We compensate low multiplicity by setting BR for D0 → K- + π+ to 1 (3.87%).

0.21 * 0.0387 ≃ 0.008 → our input over estimates scaled PYTHIA by 20%

Probability of having more than 1 K-π pair from D0 decay in one central collision < 10-4

P. Braun-Munzinger, J. Stachel, PLB 490 (2000) 196

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Vertex Detector in geant4 simulation

Each station consists of: silicon 200μm, gas 2mm, mylar foil 50 μmPositioned at 5, 10 and 15 cm downstream the target (Pb 25 μm)

target

InGrid1

InGrid2

InGrid3

VTPC1

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Detector overview in geant4 simulation

VTPCs filled with Ne-CO2 mixture, location and dimensions as in Na61 setup.Uniform magnetic field: 1.1 T in VTPC1 and 1.5 T in VTPC2

VTPC1

VTPC2

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Reconstruction1. Track distance in VTPC1 + VTPC2 > 1m2. require hit in each of the InGrid stations

Detector realistic effects: 1. momentum resolution dp/p = 1%

2. position resolution is 10 μm → hits are spread in y ans x around geant hit according to the Gaussian distribution ( σ = 10 μm). Track line is taken from the fit to the spread points.

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Cut strategy

● Combinatorial background is very large →need to to apply background suppression cuts.● The cuts should remove background and be friendly for signal.

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Background suppression strategy

List of cuts in the order they are applied

Single particle cuts:

1. track pT cut

2. track d cut (track impact parameter)

Two particle cuts:

3. cuts in Armenteros-Podolanski space to remove background from Ks and Λ

4. two track vertex cut Vz

5. reconstructed parent impact parameter cut D

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Background pT spectrum has maximum around ~ 0.2GeV/c, wheres maximum of signal distribution is at around 1 GeV/c→ cut on pT<0.4 as indicated

1. cut on pT

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2. cut on d

Relatively smooth shape of background at ~ 0 is due to uncertainly in reconstruction of track position and angle.Some uncertainly comes from multiple scattering. → cut on d<0.04 mm as indicated

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1. remove K, π if they don't make pair laying in D0 corridor or pair pT < 0.45 – signal safe cut

3. Armenteros-Podolanski analysis

2. remove π- and π+ if the pair is found in the Ks corridor

3. remove π- (π+) from p(pbar) pair if the pair is found in the Λ corridor

D0

Ks

Λ

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4. cut on Vz

→ cut on Vz < 0.5 mm as indicated

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5. cut on D

→ cut on D > 0.022 mm

Vz cut reduces background at D ~ 0, where the signal is located → Vz and D cuts are nicely complementary to each other

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Results

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Results - cnt.

S/B = 3.5 (from -3σ to 3σ )Cuts are not optimized yet!!!, we will optimized them to maximize S/noise ratio 17

Summary and outlook1. Cut values are not optimized yet. Optimization will lead to further improvement

2. We still miss realistic pid description→should be introduced in the next approach

3. Got an idea to merge presented vertex detector simulations with regular Na61 Monte Carlo → take the events reconstructed in the standard way and combine them with the output from geant4 (for the same physics input).→ In this way we got realistic acceptance, efficiency, pid and concentrate only on the vertex detector part.

4. We need to define the requirements for the vertex detector system

1. geometry,

2. resolution,

3. number of stations

4. pid information?

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Backup Slides

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Results: signal suppression

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Results - cnt.

10um -> 0

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