Stato della preparazione dell’analisi D+ → K-++

Elena Bruna, Massimo Masera, Francesco Prino

Università di Torino e INFN

Secondo convegno nazionale sulla fisica di ALICE, 30 Maggio – 1 Giugno 06, Vietri sul Mare

Elena Bruna 2

Outline Physics motivations for D+ analysis Selection of the signal

1. Single track cuts2. Secondary vertex finder3. How to combine the triplets4. Cuts on the triplets

First preliminary results of the D+ feasibility study on a sample of events:1. Perfect PID (given by the simulation)2. Experimental PID (combined)3. NO PID

Time-consumption problems, first ideas to avoid these problems → new analysis strategy

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Why also D+ (and Ds…)?

1.D0/D+ ratio: puzzle

• Expected to be 3.08 from spin degeneracy of D and D* and decay BR

• Measured by ALEPH@LEP to be 2.32

2.Different selection strategies due to:

D+ has a “longer” mean life“longer” mean life (c ~311m compared to ~123 m of the D0)

D+ fully reconstructable from a 3-charged body decay instead of the 2 (or 4) body decay of D0

Accurate measurement of the charm production cross section

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D+ → K-++ vs D0 → K-+

1. D+ has a “long” mean life“long” mean life (c ~311m compared to ~123 m of the D0)

2. D+ → K-++ has a relatively large branching ratiolarge branching ratio (BR=9.2% compared to 3.8% for D0 → K-+).

Advantages…

…drawbacks

1.1. Combinatorial backgroundCombinatorial background for this 3-body channel is larger than for D0 → K-+.

2. The average PPTT of the decay product is softersofter (~ 0.7 GeV/c compared to ~ 1 GeV/c for the D0)

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D± statistics

Nevents for 2·107 MB triggers

Ncc = number of c-cbar pairsIncludes shadowing EKS98Shadowing centrality dependence from Emelyakov et al., PRC 61, 044904

D± yield calculated from Ncc

Fraction ND±/Ncc (≈0.38) from tab. 6.7 in chapt. 6.5 of PPR

Geometrical acceptance and reconstruction efficiency

Extracted from 1 event with 20000 D± in full phase space

B. R. D± K = 9.2 %

bmin-bmax

(fm) (%) Nevents

(106)

Ncc / ev. D± yield/ev.

0-3 3.6 0.72 118 45.8

3-6 11 2.2 82 31.8

6-9 18 3.6 42 16.3

9-12 25.4 5.1 12.5 4.85

12-18 42 8.4 1.2 0.47

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Simulation and analysis strategy

Central Pb-Pb event (b<3.5 fm, dN/dy = 6000, √s=5.5 TeV)

~ 9 D+/D- in |y|<1

Too large statistics (108

events) would be required to study

the signal!!Signal and background events separately

generated with the Italian GRID

PT of the decay tracks is “soft” ~0.7-0.8 GeV/c

The magnetic field is low - 0.5 T - to allow the reconstruction of soft particles (~ 7000 in |y|<1) huge combinatorial background

A dedicated trigger for D+ → K-++ seems not possible.

Sim

ulat

ion

Good secondary vertex reconstruction capability (c (D+) ~ 300m resolution of 200m would be bad, 50m would be a dream…)Efficient system of cuts to discriminate the signal from the background

Ana

lysi

s

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1st step: Single track cutsCuts on PT, PTK, track impact

parameter (d0) on all the tracks for both signal and background events

Selection S/event B/event S/B

No cuts 0.1 109 10-10

Cuts:

PT = 0.5 GeV/c

PT K =0.7 GeV/c

d0 = 95 m

(8%)

0.008106 10-8

Choices of cuts which

maximise the Significance

Not optimized cut: we want to preserve D+ down to PT~1 GeV/c

Perfect PID is assumed

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2nd step: Combining K- pairs

Ki

…

1

2

j

…

IDEAIDEA: start from BKG pairs of K tracks (once the single track cut have been performed) and cut on the distance between the 2-track vertex and the primary one

K

Kj

both K pairs are required to pass the cut

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3rd step: Combining the Triplets

Single track cuts: applied Cut on the 2-tr vertex: applied

K are combined together according to the sign of their impact parameter

Signal Background

d0K x d01

d0K

x d

02

d0K

x d

02

empty

When (d0K x d01)<0 & (d0K x d02)<0. Due to the kinematics of the decay

Cut on d0…

d0K x d01

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ZOOM

BLACK: signal

RED: BKG K Triplets

BLACK: signal

RED: BKG K Triplets

P1 (x1,y1,z1)

SecondaryVertex (x0,y0,z0)

d1

Secondary Vertex Finder on the Triplets

BLACK: signal

RED: BKG K Triplets

Cut on the quality of the Vertex. Sigma:23

22

21

22 dddDsigma

track

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Cuts on the Triplets

1. Quality of the vertex: Sigma (prev. Slide)

2. Distance between primary and secondary vertices

The signal is peaked

at 1

cosθpoint

3.Cut on the cosine of the pointing angle defined by the PT of the D+ and the line connecting primary and secondary vertices

BLACK: signal RED: BKG K Triplets

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First strategy adopted for the analysis

For each variable (in the order Sigma, distance, cosθpoint):

1. loop on all the triplets (both signal and background)

2. Keep the value of the cut with Max Significance S/√(S+B) in the range |MINVK-MD+|<1

3. Use this value to maximize the Significance for the next cut variable

Different ranges of the reconstructed pT of the triplet

Perfect PID, PID and without PID

StatisticsStatistics used in this preliminary analysis:• BKGBKG: 500 HIJING events• SIGSIG: 8.5 X 105 reconstructed signal triplets

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Perfect PID – pT integrated

Chosen cuts:

Sigma=0.018 cm

Dist=1900 m

cospoint=0.995

Signif = 44±11

S/ev = 0.001

B/ev = 0.004

Sig

nif

ican

ceS

ign

ific

ance

Sig

nif

ican

ceS

ign

ific

ance

Sig

nif

ican

ceS

ign

ific

ance

SigmaDistance

cosθpointThe Significance is normalized to 107 central events

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SIGNAL – BKG at different cut levels

NO cuts

Single track

cuts

Dist 2

tr-ve

rt

sigm

a

Distan

ce p

rim-s

ecco

sθpo

int

NO cuts

Single track

cuts

Dist 2

tr-ve

rt

sigm

a

Distan

ce p

rim-s

ecco

sθpo

int

Number of SIGNAL triplets per event (full inv mass range)

Number of BACKGROUND triplets per event (full inv mass range)

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PID• Combined Bayesian PID

(ITS+TPC+TOF+TRD+HMPID) is used• Prior probabilities used in input:

P(p)=0.055P(K)=0.072P()=0.864P(e)=0.006P()=0.003

• PID in the ITS done with:Clusters from all the 4 layers (2 SDD+2 SSD)Convoluted Landau-Gaussian fits to the response

functions in each layerSee AliITSpidESD2 implemented in AliRoot

• Track tagged as type i when the corresponding combined Bayesian probability is P(i|track)>0.85

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PID – pT integrated

Chosen cuts:

Sigma=0.019 cm

Dist=2000 m

cospoint=0.995

Signif = 40±15

S/ev = 0.0007

B/ev = 0.002

Sig

nif

ican

ceS

ign

ific

ance

Sig

nif

ican

ceS

ign

ific

ance

Sig

nif

ican

ceS

ign

ific

ance Sigma

Distance

cosθpoint

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Without PID – pT integratedNo selection of tracks based on the particle identity

Chosen cuts:

Sigma=0.019 cm

Dist=1800 m

cospoint=0.999

Signif = 39±12

S/ev = 8 X 10-4

B/ev = 0.004

Sig

nif

ican

ceS

ign

ific

ance

Sig

nif

ican

ceS

ign

ific

ance

Sig

nif

ican

ceS

ign

ific

ance

Sigma

Distance

cosθpoint

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Sig

nif

ican

ceS

ign

ific

ance

ppTT integrated integrated

resultsresultsPerfect PIDPerfect PID PIDPID No PIDNo PID

Significance 44 ±11 40 ± 15 39 ± 12

Low pT under study:

1. Rebinning

2. Additional cuts

Preliminary global results

Analysis feasible also without PID, but more time consuming

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D+ elliptic flow: measurement perspectives

Error bars quite large Would be larger in a scenario with

worse event plane resolution (lower dNch/dy or v2)

May prevent to draw conclusions in case of small anisotropy of D mesons

v2 vs pT requires a semi-peripheral trigger

2·107 Minimum Bias events

Elena Bruna 20

Conclusions

The reconstruction of D+ → K-++ is a promising study.

The preliminary results show that the analysis is feasibleanalysis is feasible with a pretty good Significance.

More statistics is mandatory for a more accurate optimization of the cuts.

There still is room for optimization: work in progress.

Elena Bruna 21

Outlook

New cut strategy:

For each event fill 2 multi-dimesional matrices (one for Signal and one for BKG), each cell containing the number of triplets corresponding to all the possible combinations of cut variables. Es. with 5 cut variables and 30 steps for each variable 305 cells (~200MB per ev for Signal+BKG)

Sum all the multi-dimesional matrices (Signal and BKG) on all the events

Maximize the multi-dim matrix of the Significance

Apply LDA

pp studies to come on the PDC06 events and on italian production with parametrized TPC

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Backup slides

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DD++KK--+++ + BR = 9.2 %BR = 9.2 %

D± I(JP) = ½ (0-)

m = 1869.4 MeV/c2

c = 311.8 m

(PDG ’04)

D+→K-++ Non Resonant BR = 8.8 %

D+→K*0(892)+→K-++ Resonant BR = 1.3 %

D+→K*0(1430)+→K-++ Resonant BR = 2.3 %

D+→K*0(1680)+→K-++ Resonant BR = 3.8·10-3 %

Hadronic 3-charge-body decays of D+

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D

K

PPTT distributions of the generated particles (ONLY PYTHIA generation, NO propagation and reconstruction in

the detector)

(nonresonant events)

Mean = 1.66 GeV/c

Mean = 0.87 GeV/c

Mean = 0.67 GeV/c

Kinematics (1)

Knowledge of the PT shapes of the decay products important at the level of the selection strategy

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Non resonantNon resonant ResonantResonant

Sharp borders due to PYTHIA cut off on the tails of distributions

Dalitz Plots: Kinematics (2)

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Combining K- pairs /2Best Significance:

cut distance of the 2-tr vertex = 700 m

63% Signal triplets pass the cut

• S/ev=0.006

105 remaining BKG triplets per event.

Pt reconstructed D+

Mean=2.66 GeV/c

Low PT D+ still kept

S/B = 6X10-8 still low…

Full invariant mass range

Elena Bruna 27

Display D+ decay (made with the kinematics)

Impact parameter:

• Definition: segment of minimum distance of the (prolonged) track from the primary vertex

• Sign:

• + : primary vertex in the track “circle”

• - : primary vertex out of the track “circle”

K+

1-

2-

Primary vertex

Secondary vertex

Points on the 3 prolonged tracks defining the impact parameter d0

K+: d0>01

- : d0>02

- : d0>0

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Measurement of v2

• Calculate the 2nd order coefficient of Fourier expansion of particle azimuthal distribution relative to the reaction plane– The reaction plane is unknown.

• Estimate the reaction plane from particle azimuthal anisotropy: n = Event plane = estimator of the

unknown reaction plane

• Calculate particle distribution relative to the event plane

• Correct for event plane resolution– Resolution contains the unknown RP

– Can be extracted from sub-events

))(2cos(2 RPv

ii

iin w

w

n

cos

sintan1 1

))(2cos(' 22 v

RP

vv

2

22 2cos

'

Unknown reaction plane

Event plane resolution

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Worse resolution scenario

• Low multiplicity and low v2

Large contribution to error bar on v2 from event plane resolution

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Semi-peripheral triggerv2 vs. pT that would be obtained from 2·107 semi-peripheral events

( 6<b<9 )

pT limits N(D±)sel v2)

0-0.5 645 0.03

0.5-1 1290 0.02

1-1.5 1800 0.017

1.5-2 1650 0.018

2-3 2470 0.015

3-4 1160 0.02

4-8 1225 0.02

8-15 220 0.05