open charm measurement in ceres ce renkov r ing e lectron pair s pectrometer

NA45/CERESNA45/CERES

V. Petráček, D. Adamová 1Catania, 12 December 2005

Open charm measurement in CERESCErenkov Ring Electron Pair Spectrometer

Motivation Experimental setup Signal reconstruction Background and background suppression Simulations Preliminary results


V. Petráček, D. Adamová 2Catania, 12 December 2005 3

Motivation I

Charm quark mostly produced from the initial fusion of partons (mostly gluons) Open charm production sensitive to initial parton distribution functions

Z. Lin & M. Gyulassy, PRC 51 (1995) 2177 light

2001, Dokshitzer and Kharzeev proposed “dead cone” effect => charm quark small energy loss, but in vacuumRecent: Heavy quark energy loss in medium, e.g.: Armesto et al, PRD 71, 054027, 2005; M. Djordjevic et al., PRL 94, 112301, 2005.



Motivation II

Charmed quarks produced in the initial hard parton scattering and at the very early stages of

the collision One of the methods for probing the

properties of the Quark-Gluon Plasma

essential as a baseline for J/ measurements

J/,suppressio

n

natural baseline B J/ background

Medium probes:early formation

energy loss

quarkonia open flavoursB,D

K

e



Motivation for Open Charm Analysis in CERES

CERES/NA45: one of the second generation heavy ion experiments at CERN SPS. Dedicated to the study of e+e- pairs in relativistic nuclear collisions. Precise Si Drift vertex telescope Pair of RICH detectors with thr=32 (e-veto, with P>4.5 GeV, K id) 2 azimuthal acceptance Acceptance in y 2.11-2.66 P measurement from deflection ('96), TPC (2000)

Main CERES features

Open charm reconstruction strategy

Reconstruction of 3-body decays of charged D mesons D K, D KK, Ds KK, D cascades via K*(892) and K*(1430) Better chances for D (c=317m) , low for Ds (c=167m) and D0 (c=124m)



Setup with TPC: 1999 and 2000




SD: event vertex, track vertex and angle

event z = 0.2 mmtrack = 0.2 mr = 2 mr




RICH's: electron identification




radial drift TPC: momentum and energy loss

p=2%1%*p/GeVm = 3.8 % for phi

dEdx = 10%



Detector setup: MWPC in 1996



Vertex telescope

Segmented Au target 8 x 25 m target foil Total rad. thickness 0.8% 3 mm sub-target separation2 x Radial Silicon Drift det.



5 s drift time 50 MHz sampling rate 360 segmented anodes 1 s before/after

protection

Radial Silicon Drift Detector SiDC



SiDC signal characteristics

Tracks from pile-up events can notspoil the SV reconstruction



Primary vertex z = 246 m SiDC 1,2 matching 0.42 mrad SiDC 1,2 matching mrad With single anode hit mrad

Good reason to separate in reconstruction and information

Primary vertex resolution and matching in SiDC telescope



Reconstruction of displaced track

Hit association Predictor from SiDC1 and SiDC2 hits PaDC hit association

(matching window 2.5 mrad)(or matching to TPC track)

Primary track suppression

Electron rejection Rejection of tracks with identical

segment in SiDC or PaDC Rejection of tracks containing split

hits in SiDC



Definition of SV track description elements

Track projection into the r-z plane Using <> for r-z

plane orientation from SiDC1,

SiDC2 hits



Relation between displaced track and projection Reconstruction of SV position using projections

3 pairs of tracks for each 3-vertex Each pair defines a

line as allowed SV position From these 3 lines, SV

can be determined

SV position was reconstructed without using the information on d SiDC1,2 This information is used later in candidate selection, where d measured is compared with dpredicted using the reconstructed SV position.



Selection criteria for SV candidates

Difference between predicted and measured d in SiDC1,2 Impact parameter Physical Dalitz

region For unknown mass

assignment physical region in Pseudo-Dalitz plot Momentum

conservation Product momenta Reconstructed D

momentum



Analysis framework

URQMD

Pa = 158 GeVT = 175 MeV *yCM = 2.95y = 1.2

Assumes no flow !



1996 data set and data handling

● Approx. 5380 data files of average size 0.4 MB 2.15 TB of data

● Altogether over 42 millions of events to be processed (31.7 done)

● Transferred from CERN CASTOR to Prague

● Stored partly on the computing cluster of CTU and partly on computing farm GOLIAS

● The analysis used the original CERES software adapted to Linux RedHat, kernel 2.4.22 (it was not quite easy!)

● The data files already processed archived on external USB discs to free space for the output files from the analysis, due to neverending fight over space on disc storage

● ¾ of data already processed in first and second analysis pass, last ¼ is currently under processing



Comparison of matching and dE/dx in MC and data

Data + OVL

FMC



Momentum resolution and shift (96)

20 GeV test from PV andmomenta of D decay products



Momenta of decay products and background

PD / 3

P track

K

Reconstructed

Reconstructed

Lower plots for zpd>0.15 cmMomentum cuts used in analysis



Invariant mass reconstruction (in KDG)

With ideal dP/P (and/or with =0.. ) is reconstructed mass independent on PD

DP/P parametrization 96 Invariant mass slope corrected dM/M 6.8% For dP/P resolution from 2000 is this

dependence negligible



Invariant mass reconstruction in Overlay MC

Very preliminary

Invariant mass - background subtracted



Effect of min. product momentum cut onreconstructed invariant mass peak

For Pprod >1.75 GeV, the admixture of fake tracks is largely eliminated

In the full analysisWe use Pprod >1.2 GeV



Spatial precision of SV reconstruction

Comparison of MC only with decay products and overlay MC All D decayed at SV z=0.4cmSV x,y determined from

direction of PD

z=242 m (FMC)z=305 m (OVL)



Definition of signal and background charge topologies for 3-body decays of Dproblem of random track mixing

Signal can be >>enhanced<< due to the random track admixture . Some SV candidates with random track

may pass selection criteriaContribution from Ds partial reconstruction

is unlikely to happen

Only the real D SV can produce sometimes more SV candidates in the same event. Background DOES NOT produce SV candidates with mass concentrated in D invariant mass peak!



Effect of primary track suppressing cut onD reconstruction probability



Set of cuts used in analysis

Zsv > 0.2 cm Impact parameter < 0.035 cm Sum of differences between measured and predicted dSiDC1,2 < 5.5 mrad Pprod <1.2 - 8> GeV Cut in pseudo-Dalitz plane m12+m23 < 2.25 GeV Product <140 - 240> mrad D <0 - 240> mrad



D family decays generated in Pythia with more then 3 detectable products in acceptance, fulfilling cuts

PD and mt(D)Distributions in LAB with z oriented || with PD and in CMS of decaying D



Momentum conservation, invariant mass and momentum transfer

Ideal dP/P used - for better understanding of reflections K mass assignment

assumedD produces the

left side band in inv. MassWith dP/P (96) this band

can not be distinguished from the main K peak



Mass spectra for D family decays - ideal dP/P

All ZSVDecays fulfilling cuts

(dominantly D) reconstructed in different mass assignments

D and D forming peak/band in corresponding mass assignments



Contributions from different D family members fulfilling cuts

D

Ds

D0

Signal is for Zsv > 0.2 cm dominated by DContribution from Ds <10%D0 contribution is negligible

Kmass assignment



Reconstructed invariant mass and impact parameter distributions

for different min. Zsv cuts

All Zsv Zsv>2mm Zsv>0.4cm

dP/P 2000



Rapidity and polar distributions of reconstructed D

Rapidity distributions in LAB and in LAB with z || PD

D in LAB



Comparison with data (5.5M events, Pb-Pb collisions at 158 AGeV)

<M> = 1.8 +/- 0.09 GeV



Opening angle distributions

Dalitz plot

Due to large Pdecay are opening angles relatively large. See above typical decay hit topology



dND/dz differentially and in residual distributioncorresponding reconstruction probability as function of

Zsv



D lifetime in CMS(D)Effect of min. Zsv cut on signal and background

Exponential decay with cm ?

Significance**2



2 analysis of D mass peak fit



Possible background from other 3-body decays

In the mass range 1.5 – 2.2 GeV may contribute and particles.

The former via decay to decaying to p so fast, that it seems like 3-vertex

The later produced in next target but with momentum vector as if it comes from PV

Both constraints supress admixtude of and deep below expected D yield



Distribution of SV and PV

Spikes due to sec. production are absent



Full data set used for analysis

● 20 M evt. with B+ orientation of magnetic field● 11.68 M evt. with reversed field polarity● Total 31.68 M evt.



Signal dependence on Zsv

Contributions from D and DsCannot be distinguished due todP/P – Peak continuum



Pt dependence of signal



Pt spectra of signal and background

Signal & Background integrated in the interval 1.5 – 2.2 GeV

Pt spectrum of signal is “harder” then for background

There is very little signal for lowPt (note that this is integral scan)Indication of radial flow of the heavy object



Zsv dependence of signal and background

Signal & backgroung integrated in the interval 1.5 – 2.2 GeV

Signal distribution is decreasing slowly in comparison with background Particles responsible for the signal are long living Signal dies out in region which is, according to simulations, in the reach of D+/-



Stability of reconstructed invariant mass

Reconstructed invariant mass of the signal is stable in all performed scans (Pt, Zsv)

Width of the mass distribution is consistent with simulation & smearing due to the M(PD)(…discussed earlier)



What can we say about the observed yield

We attribute signal to combined contribution of D and Ds 3-body decays

We observe signal rate ~2.5 10-4 /event

To be able to estimate reconstr. efficiency we need more simulations of D decays with , T corresponding to the observed Pt spectrum

Observed significance ~ 11-17



Next steps

•Reconstruction efficiency

• Yield

Improve MC statistics in Overlay mode

Understand precisely effect of random track admixture on reconstructed signal



Conclusions

A method of D meson 3-body decay reconstruction was presentedResults of the analysis of the CERES 1996 data set are

almost complete : the last quarter of events is under analysis currentlyReconstruction was tested in MC (both with decay products

only and with overlayed real event) A letter on the analysis to be assembled soon



CERES/NA45 collaboration



Backup slides



Physics motivation

One of the methods for probing the properties of QGP is the measurement of particles carrying heavy flavor quantum numbers.

Open charm as well as open beauty production allows one to investigate the mechanisms of heavy-quark production and their propagation in the hot and dense medium formed in high-energy heavy-ion collisions.

Charmed quarks can be produced in the initial hard parton scattering and then only at the very early stages of the collision, while the energy in parton rescattering is above the charm production threshold. The charm yield is not altered later.

Knowledge of the total production cross section for charm quarks is also essential as a baseline for J/ measurements.

open charm measurement in ceres ce renkov r ing e lectron pair s pectrometer

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