APS meeting, Feb. 13-17 2010 1

Performances studies of the silicon detectors in STAR towards

microvertexing of rare decays

Jonathan Bouchet,for the STAR CollaborationKENT STATE UNIVERSITY

1. Motivations2. STAR : detectors apparatus3. Method 4. Summary

APS meeting, Feb. 13-17 2010 2

Charmed mesons in Heavy Ion collisions

• Heavy flavor is supposed to be produced at the earlier stages of the collision via gluon fusion:– give insights of the medium created.

• If dense medium created, RAA = nuclear modification factor of charged hadrons :– =1 means no nuclear effect.– <1 means suppression.

APS meeting, Feb. 13-17 2010 3

Measurement : how to• indirect method

– Measurement of semi-leptonic decays of B,D mesons

• D0e++X , BR : 6.9 %• D+/- e+/-+X BR : 17.2%

• Direct method– Measurement of

hadronic decays through decay topological reconstruction

• D0 (D0)K-+(K+-)BR : 3.8 %• D+/-K BR : 9.2%

dAu :Phys. Rev. Lett. 94 (2005)AuAu :STAR arXiv:0805.0364CuCu: preliminary

RAA of non photonic electrons in AuAuSTAR Phys. Rev. Lett. 98 (2007) 192301

D0 reconstruction d+Au

• In order to disentangle between the b and c quarks contribution of the semi-leptonic measurement, a full topological reconstruction of the decaying particle is needed.

Challenging for charmed particles because of the small decay length (c (D0) ~ 124 µm).

APS meeting, Feb. 13-17 2010 4

STAR detector (in 2007)

The tracking system consisted in :•Time Projection Chamber (TPC) provides measurements of :

track momentum, particle identification (pId)•2 silicon detectors :

1 layer of silicon strip detectors (SSD) and 3 layers of silicon drift detectors (SVT) High spatial resolution.

Note : the silicon tracker wasn’t designed originally (in terms of distance and layer thickness) for charmed particle detection

SSD

SVT

TPC

APS meeting, Feb. 13-17 2010 5

Decay fitting

• Least square fit of the decay vertex [1] :• In 2 body decay, combination of 2 tracks + addition of the constraint that

they’re coming from a common point.• The Kalman fitter machinery allows the knowledge with high precision of

tracks near the primary vertex (by taking into account the MCS due to the silicon layers)

• Method : 1. Form pair of tracks and use TPC pId’s 2 .Compute the position of the secondary vertex associated to each pair.

2 = ∑ (qi-h(x,pi))T V-1i (qi-h(x,pi))

tracks i

measured track parameters

covariance matrixof measured track

parameters

[1]Decay Chain Fitting with a Kalman Filter,W. D. Hulsbergen (arxiv:physics,0503191)

Parametrisationof the track

(helix)

Vertex position

decay tree

3d path lengthfrom primary vertex to decay particle vertex

APS meeting, Feb. 13-17 2010 6

Comparison with simulation

• Left : correlation between reconstructed path length and MC• Right :

– There is no systematic sift (red crosses = mean) in reconstructed quantities.– The standard deviation (blue crosses) of the distribution (reco-MC) is flat at ~ 250 m ,

which is of the order of the resolution of (SSD+SVT).

Reco - MC [cm]

MC [cm] MC [cm]

Reco vs. MC [cm]

APS meeting, Feb. 13-17 2010 7

Real data:Scan of DCA* resolution• run 7 Au+Au@200GeV (MinBias trigger)• Cuts

– |zvertex |<5 cm : to have a clean sample– TPC hits> 15– || in SSD acceptance (||<1.2)– pT >0.1

• Fit function :

– Reflect the (detector+alignment) resolution and Multiple Coulomb Scattering.

– Fit done for 0.2 < 1/P < 5– resolution@1GeV (for tracks with

N=2,3,4) vs day number to study stability of DCA resolution.

Including the silicon detectors in the tracking improves the pointing resolution :

with 4 silicon hits, pointing resolution of tracks to the interaction point ~ 220 m at P = 1GeV/c

Real data

Real data

*: Distance of Closest Approach

APS meeting, Feb. 13-17 2010 8

Real data

*: Distance of Closest Approach

APS meeting, Feb. 13-17 2010 9

Example of cuts • The significance of the path

length, defined as = sLength/dsLength, where :– sLength = path from primary

vertex to decay particle vertex – dsLength = error associated,

can be used to select physical decays.

• From simulation, an enhancement of the D0 invariant mass is observed by applying a cut at > 2.

> 2

simulation

simulation

M(D0)

M(D0)

APS meeting, Feb. 13-17 2010 10

In real life …• AuAu@200GeV : 1M events.• A cut at pT > 1.5 GeV/c of daughter tracks of each D0

candidates is applied.

No cut > 2

Work is in progress

APS meeting, Feb. 13-17 2010 11

Summary• We presented a method using full track information to

obtain high precision of decay vertex.• agreed with simulation.• Work on going for cuts optimization.• Method is baseline for analysis involving the future

upgrade in STAR : Heavy Flavor Tracker. Low mass detector designed to identify mid-rapidity Charm and Beauty mesons and baryons through direct reconstruction and measurement of the displaced vertex with unprecedented pointing resolution.

APS meeting, Feb. 13-17 2010 12

STAR detector (current)

MRPC ToF barrelMRPC ToF barrel

100% ready for run 10100% ready for run 10

BBC

PMD

FPD

FMS

EMC barrel EMC End Cap

DAQ1000DAQ1000

FGT

Completed

Ongoing

MTD

R&DHFT

TPC

FHC

HLT

APS meeting, Feb. 13-17 2010 13

Cuts used in reconstruction code• EVENT level

– Primary vertex position along the beam axis : |zvertex| < 10 cm– Resolution of the primary vertex position along the beam axis: |zvertex|< 200µm

• TRACKS level– Number of hits in the vertex detectors :SiliconHits>1 (we want tracks with good

DCA resolution)– Momentum of tracks p >.3GeV/c – Pseudo-rapidity :||<1.2 (SSD acceptance)– dEdxTrackLength>40 cm– DCA to Primary vertex (transverse) DCAxy< .1 cm

• DECAY FIT level– Probability of fit >0.01 && |sLength|<.1cm– Particle identification : ndEdx :|nK|<2.5, |nπ|<2.5