T. Virgili II Convegno nazionale sulla fisica di ALICE 30/5 – 1/6 2006 Tiziano Virgili Università...

T. Virgili II Convegno nazionale sulla fisica di ALICE 30/5 – 1/6 2006

Tiziano Virgili Università di Salerno and INFN

Metodo (clusters e tracklets)

Correzioni e distribuzioni finali

Stima dell’errore sistematico

What next

Analisi di molteplicità e dN/dη a rapidità centrale


La molteplicità è una grandezza fondamentale che caratterizza l’evento.

La dN/dη è direttamente collegata alla densità di energia iniziale del sistema (Bjorken)

La misura della molteplicità costituirà un test base per tutti i modelli di interazioni Pb-Pb.

La distribuzione dN/dη offrirà numerose altre informazioni (fluctuazioni, scaling…)

La molteplicità costituirà uno dei primi osservabili misurati in ALICE, sia in interazioni Pb-Pb che p-p

=> Oggetto del primo articolo di fisica di Alice!



The detectors


-2< <2 (inner layer)

-1.4 < < 1.4 (outer layer)


Multiplicity with the SPDI Metodi:

Clusters: non occorre allineamento (risultati “on-line”), affidabile ad alta molteplicità. |η|<2.

(Occupancy < ~3%)

Tracklets: associazione dei clusters nei due layers con una retta passante per il vertice primario. Permette una migliore reiezione del rumore. |η|<1.5


Tracklets Reconstruction

Zprojection – Z cluster

Layer 1

Layer 2

Primary vertex

Fiducial window


Tracklets Reconstruction

φ layer 1– φ layer 2

Layer 1

Layer 2Primary vertex


Multiplicity with the SPD

I vantaggi rispetto alle tracce (TPC+ITS)

Maggiore accettanza in pseudorapidità (fino a |η|<2 per clusters)

Maggiore accettanza in pT (soglia SPD a 10 – 30 Mev)

Maggiore efficienza complessiva (tenendo conto dei canali morti, rumore etc.)

Minore fondo (decadimenti, interazioni secondarie, etc.).

Necessario il solo allineamento dei due layers di pixel.


Tracklets Reconstruction inThe framework of AliRoot

La nuova classe AliITSMultReconstructor per la ricostruzione della molteplicità è ora inserita nella nuova release di AliRoot

Per ogni tracklet viene salvato nell’ESD :

Zenithal angle θ

Azimuthal angle φ

Differenza tra φ1 – φ2 (determinati tramite layer 1 e layer 2)

(Utile per l’identificazione di carica e una grossolana separazione in classi di pT)


Systematic effects in Multiplicity reconstruction

From tracklets to “true” multiplicity

A number of corrections must be applied in order to reconstruct the “true” multiplicity and then the physical distributions:

Gemetrical correction (acceptance)

Background subtraction

Efficiency of tracklets reconstruction

Correction due to the event selection

(An internal note has been submitted)



Event selection

Pb-Pb events: the selection will affect the centrality scale and the associated systematic error will propagate on the centrality (the number of participants, or the fraction of cross section).

p-p events: in this case the selection will affect the average multiplicity, and the associated systematic error will propagate on the multiplicity. This effect is under study, in collaboration with Domenico Elia => 60000 events generated on Bari farm.



Geometrical correction

It depends on the longitudinal position of the vertex Zv.

It has been evaluated by generating about 300 Pb-Pb events (HIJING param.), and looking at the ratio between generated particles and hits in each η window.

In case of tracklets the condition that the particle produces hits in both layer is also applied.



Acceptance for layer 1.

3 “holes” at η=0, ±1.35

Total loss at Zv=0 : 2.5%



From the previous map a global correction factor can be extracted for any Zv: they are lower than 6% if |Zv|<5 cm.

layer 1



Acceptance for layer 2.

3 “holes” at η=0, ±0.86


Stronger dependence on Zv!



Global correction factors as a function of Zv: they are lower than 13% if |Zv|<5 cm.

layer 2



Acceptance for tracklets.




tracklets

Global correction factors as a function of Zv



In order to reconstruct the dN/dη distribution the full map is needed. Here the weights as a function of η for the tracklets in the case Zv=0 are shown.


dN/d reconstruction:

p-p events


dN/d reconstruction:

Pb-Pb events



Note that the effect of the magnetic field on the acceptance is small, it corresponds to a pt cutoff of about 10 MeV for layer 1 and 30 MeV for layer 2.

Its effect is more relevant on the tracklet reconstruction, as discussed in the PPR.


What is the error associatedto this correction?

There are two main sources of indetermination:

The statistic used in the simulation

The model used in the generation

The first one gives an error on the correction factor and therefore on the total reconstructed multiplicity of (about) 0.3%.

In order to evaluate the effect of the model used in the generation I have compared the previous results (HIJING) with those obtained with PYTHIA: the losses are in agreement within 10% and the contribution to the error on the rec. multiplicity is about 1%.

As the two models are radically different, this can be considered an upper limit to the indetermination introduced by the model itself.



Background subtraction

There are several sources of background:

“External background” (beam gas interactions, etc.).

Secondaries (decays, secondary interactions etc.)

Detector background (noise)

This last background will be negligible, as it is shown in PPR.

The external background can be in principle measured directly and subtracted. A detailed study has been started in collaboration with Domenico Elia.



Secondary particles

The fraction of clusters produced by secondaries are 4.7% and 5% of the total for layer 1 and 2. (HIJING).

By switching off the decays in the simulation this fractions become lower than 0.3% => almost all the secondaries come from the decays.

The error on this fractions is about 10%, and the error on the reconstructed multiplicity about 0.5%.



Reconstruction efficiency

This correction depends on the occupancy of the detector, and so on the multiplicity itself. At very high multiplicity the merging of the clusters produces a decrease of the efficiency.

This will not constitute a problem for p-p events!

The efficiency depends strongly on the size of the window where the cluster associations is allowed (fiducial window). In p-p this window can be very large, and the efficiency is close to 100%. In order to evaluate the efficiency I have considered the ratio between the reconstructed and generated multiplicity, after the previous corrections.



p-p events


Multiplicity Reconstruction



Residuals and cuts in Pb-Pb



Rec

onst

ruct

ed/g

ener

ated

dN

/dη



The error associated to this correction depends on:

the statistic used in the simulation

the model used in the generation.

This error can be evaluated as about 1% of the total mult.

The efficiency correction depends on :

the size of the association window

the intensity of the magnetic field

the multiplicity itself (if the detector occupancy is high)

the quality of the overall allignement. In p-p case the effect of misalignement will be negligible.



Minimum bias HIJING

(1000 events Pb-Pb)



The systematic error is reported

The statistical error is negligible!



The total multiplicity can finally be determined by applying all the previous corrections. The associated systematic error can be evaluated from the previous contributions as about 1.5% of the total multiplicity.

Note that the contributions due to the event selection (p-p case) and the background subtraction are still not considered.


CONCLUSIONI e WHAT NEXT

Gli strumenti di analisi sono essenzialmente pronti per produrre i primi risultati di fisica di ALICE.

Per ogni correzione è stato stimato il contributo all’errore sistematico totale.

Due contributi sono ancora mancanti, relativi alla selezione degli eventi (solo per p-p) e la sottrazione del background. Il lavoro è in progress, in collaborazione con Domenico Elia.

Ancora mancante un confronto completo con la molteplicità misurata con le tracce (TPC+ITS).


T. Virgili II Convegno nazionale sulla fisica di ALICE 30/5 – 1/6 2006 Tiziano Virgili Università...

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