DD++KK--++++:reconstruction and :reconstruction and perspectives for the perspectives for the

measurement of the elliptic measurement of the elliptic flowflow

Elena Bruna

University of Torino

Terzo Convegno Nazionale sulla Fisica di Alice, 12-14 Nov 2007

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Outline

Exclusive reconstruction of D+ → K-++ in Pb-Pb and pp collisions:

Selection strategy

Results

pp: Results with more statistics w.r.t. previous analysis

pp: A look at the systematic errors due to the selection procedure

cut variables considered: distance primary-secondary vertices, cospoint

Perspectives for the measurement of D+ azimuthal distribution in Pb-Pb

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Selection strategy (1/4)Cuts on single tracks ( pT, d0

r)In Pb-Pb reduce the combinatorial background from 109 to 106 triplets per event keeping ≈7% of the signal

pT cut different for K and (identified by the charge sign: DD++KK--++++)

Cuts on K candidate pairs K and have opposite charge signCut on the distance between the vertex of the 2 tracks and the primary vertex

(m)

K

primaryvertex

cut (m)

Working point:

700m

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Selection strategy (2/4)

Cuts on K candidate triplets of tracksselection based on the products of impact parameters of the two K pairs 25% of BKG triplets rejected, ≈ all signal kept

Cuts on the secondary vertex quality:

BLACK: signal (K from D+)

RED: BKG (Kcombinatorics)

BLACK: signal (K from D+)

RED: BKG (Kcombinatorics)

σ (cm)

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Selection strategy (3/4)

Four selection variables:Distance between primary and secondary vertex (ddPSPS)

coscospointpoint

Sum of squared impact parameters s = s = dd0101

22+d+d020222+d+d0303

22

Max. pT among the 3 tracks

ppMM=Max{p=Max{pT1T1,p,pT2T2,p,pT3T3}}

SIG

BKG

d PS(m)

co

sp

oin

t

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Advantages:Advantages:Global maximization

Easy to add new selection variables

Selection strategy (4/4)

Significance S/(S+B) for 2<pT<3 GeV/c

(normalized to 107 PbPb events)

co

sp

oin

tCU

T

d CUT(m)

n-dimensional matrix proceduren-dimensional matrix procedure (n=4), i.e. one dimension for each cut variable (dPS, pM, cospoint, s):

Fill matrices Sijkl (signal) and Bijkl (background)

Each cell ijkl contains the number of triplets passing the set of cuts:

[d[dPS PS > > cutcutii AND p AND pMM > > cutcutjj ANDAND coscospoint point >>cutcutkk AND s > AND s > cutcutll ]]

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Results: D+K-++ in Pb-Pb (1)

Significance S/√S+B (normalized to 107 ev.) vs pT

Relative statistical error (S/S =1/√S) on D± vs pT (in 107 ev.) S/ev~10-3 , B/ev ~10-4

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Selection efficiency and pT spectra:

pT integrated ε (D+) ≈ 1.5% (Ideal PID), 0.6% (Real PID), 1% (no

PID)

Results: D+K-++ in Pb-Pb (2)

daughterstedreconstrucwithD

Dselected

3

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Signal statistics improved by a factor 2 w.r.t. Signal statistics improved by a factor 2 w.r.t. previous analysisprevious analysisSignificance and relative statistical error vs. D+ pT

S/ev~5 10-6 , B/ev ~5 10-6

Significance and relative statistical error (=1/S) normalized to 109 pp Minimum Bias events

Results: D+K-++ in pp (1)

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Results: D+K-++ in pp (2)

daughterstedreconstrucwithD

Dselected

3

Selection efficiency and pT spectra:

Selection efficiency (pT integrated) ε~ 4% (NO PID)

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Results: Pb-Pb vs pp(PID)

(no PID) S/S the same for Pb-Pb and pp for 2<pT<8 GeV/c

(PID)(no PID)

S/√S+B in one year

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Studies on D+ Invariant Mass

Signal Inv. Mass distribution fitted with a Gaussian function, in different D+ pT ranges

Signal normalized to 109 pp randomly generated according to a Gaussian function

The same for the Bkg triplets, but a polinomial function was used

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Invariant mass distributions

0<pT<2 GeV/c

pT>5 GeV/c3<pT<5 GeV/c

2<pT<3 GeV/c

Statistics enhanced in order to match 1 year of data taking

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Systematic errors on pp events

Two samples of events:1°) pp MB (85%) + pp with charm (14%) and beauty (1%) + events with cD+K (divided in 4 subsamples of pT

hard)• extract the D+ number with Gaussian fit to the invariant mass

distribution: Nsel

2°) only events with cD+K (divided in 4 subsamples of pT

hard)• extract the selection efficiency: ε

D+ reconstructed: Nrec=Nsel/

Nrec calculated for different values of the cut variables

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Cut on dprim-sec

Nsel vs d (norm to 109)COUNT

From FIT

COUNT

From FIT

vs d

Nrec vs d (norm to 109)

2<pT<3 GeV/c

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Cut on cospoint

Nsel vs cospoint (norm to 109)COUNT

From FIT

COUNT

From FIT

vs cospoint

Nrec vs cospoint (norm to 109)

2<pT<3 GeV/c

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Perspectives for D± elliptic flow

GOALGOAL: Evaluate the statistical error bars for measurements of v2 for D± mesons reconstructed from their K decay in Pb-Pb collisions v2 vs. centrality (b=collision impact parameter) v2 vs. pT in different centrality bins

HOWHOW: fast simulation to generate:NND+D+ ( (ppTT,,b)b) with an angular distribution

dND/d = vv22D,inD,in cos [2(-RP)]

For each D+: an event made of NNtrackstracks is superimposed

dN/d = vv22ev,inev,in cos [2(-RP)]

Inputs of the simulation: NND+D+ ( (ppTT,,b)b) , vv22D,inD,in, N, Ntrackstracks ,v ,v22

ev,inev,in

Outputs: vv22DD measured.

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v2 vs. pT6<b<9 fm

Large stat. errors on v2 of

D± → K in 2·107 MB events

How to increase the statistics? Sum D0→K and D±→K

•Sufficient for v2 vs. centrality

Semi-peripheral trigger•v2 vs. pT that would be obtained

from 2·107 semi-peripheral

events (e.g. 6<b<9 fm)

Semi-peripheral trigger

MB trigger

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Summary

D+ in PbPb and in pp:The results show that the DD++ → K → K--++++ analysis in Pb-Pb and pp is analysis in Pb-Pb and pp is feasiblefeasible with a pretty good Significance.The selection strategy based on n-dimensional matrices has been developed.

Recent updates: Significance in pp improved with enhanced statisticsInvariant mass studies give the expected spectra in 1 year of data taking No evident systematic error on the selection procedure from a first look: more statistics needed

D+ v2:Large stat. errors on v2 of D± → K in 2·107 MB eventsHow to increase the statistics?

Sum D0→K and D±→KSemi-peripheral trigger

Under study the methods for background subtraction

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Backup slides

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0<pT<2 GeV/c

pT>5 GeV/c3<pT<5 GeV/c

2<pT<3 GeV/c

Initial Nsel statistics (vs dprim-sec)

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Initial Nsel statistics (vs cospoint)0<pT<2 GeV/c

pT>5 GeV/c3<pT<5 GeV/c

2<pT<3 GeV/c

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Initial Bkg statistics with |M-Minv|<1 (vs d)

0<pT<2 GeV/c

pT>5 GeV/c3<pT<5 GeV/c

2<pT<3 GeV/c

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Results from RHIC

The Non-Photonic electrons RAA shows interesting results

Suppression is very large when compared to the expectation from radiative energy loss Other possible mechanisms?• Collisional EL, resonances, in

medium fragmentation…

First hint of strong charm v2 for pT<2 GeV/cVery important, need to disentangle charm from bottom

PHENIX nucl-ex/0611018STAR nucl-ex/0607012

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Results scaled to a lower multiplicity scenario for Pb-Pb

Results presented so far for Pb-Pb based on dNch/dy=6’000Ntracks=7000BKG/ev=N(N-1)(N-2)/3! ~ 6x1010

Extrapolations from RHIC results seem to favour a lower multiplicity scenario, dNch/dy=2’000

Ntracks~7000/3 ~2300BKG/ev ~ 2x109

Let’s consider the D+ pT interval: 0<pT<2 GeV/c:The results in the highest multiplicity scenario (dNch/dy=6’000) are not satisfactoryBKG/ev is downscaled by a factor ~30;SIG/ev not rescaledS/√S+B (normalized to 107 ev.) ~ 10 in case of Real PID: it is possible the study of the low-pT spectra

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Acceptance, reconstruction and particle identification efficiencies (~10%)Centrality selection (~7%)Parameters of the Woods-Saxon profile and nuclear density (~5%)Nucleon-nucleon inelastic cross section (~5%)Branching ratio D+K-++ (~3.6%)Feed-down from beauty:

NcD± = ND± - NbB D± Contamination

K= NbB D±/ NcD± =4%

Systematic uncertainities on D± measurements

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Feed-down from beauty

D+ from B are more displacedThe cut on distance between primary to secondary vertex increases the fraction of selected D+ coming from B decay

Histograms normalized to the same area

d~1000m KK =10%

Contamination K K vs d