Jaiby Joseph Ajish 11/9/2011

Charmed Meson measurements using a Silicon Tracker in Au+Au

collisions at √sNN = 200 GeV in STAR experiment at RHIC

Jaiby Joseph Ajish11/9/2011

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OutlineIntroduction

Why collide nuclei at high energies?RHIC and STAR

Physics at RHICImportant observationsHeavy quark sector

Charm measurement using Silicon TrackerSecondary vertexingProof of principle with Ks

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ResultsNew results using TOF detectorFuture

Why collide nuclei at high energies?

Quantum Chromo Dynamics (QCD) is the theory of strong interactions. A phase transition is predicted at high temperatures and/or densities.

Collisions of heavy ions at relativistic speeds creates extreme temperatures/densities: Nuclear Matter Quark-Gluon Plasma (deconfined

partonic matter)

Lattice QCD predicts the phase transition at:

Study the Strong Interaction at high temperatures/densities

Understand how matter behaved at the dawn of the Universe (~10-6 s)

Create and study the properties of the Quark-Gluon Plasma (QGP), a new phase of nuclear matter.

Phase Diagram

(Net Baryon) Density

Tc ~ 150 -170 MeV and ρ ~ 1GeV/fm33

RHIC BRAHMSPHOBOSPHENIX

STAR

AGS

TANDEMS

1 km

Relativistic Heavy Ion Collider (RHIC)

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exploring nuclear matter at extreme conditions over the last decade 2000-2010

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RHIC Collisions

Experimental approach to induce the QCD phase transition:

collide nuclei like Au+Au

How to vary the T ? the Volume ? vary energy, Nr of participant Nucleons

of Colliding Nuclei

Collision systems used at RHIC are:

Au+Au, Cu+Cu, d+Au and p+p at energies (7.7 GeV to 500 GeV for

p+p)

Initial Conditions Initial high Q2

interactions Partonic matter QGP

HadronizationFreeze-Out

STAR Detector view of the event

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STAR Detector (in 2007)

The tracking system consisted of : TPC : provides momentum, particle identification Silicon detectors :

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

higher spatial resolution : pointing resolution of 250µm in transverse direction (at 1GeV) was achieved.

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MRPC ToF barrel

BBC

PMD

FPD

FMS

EMC barrel EMC End Cap

DAQ1000

FGT

Completed

Ongoing

MTD

HFT

TPC

FHC

HLT

STAR Detector (in 2010)

1) Time of Flight –Full Barrel (Excellent Particle ID)2) Previous generation Silicon Detectors are removed and Heavy Flavor Tracker is

being built for exclusive charm measurement

• Hard Parton Scattering

• Jets and mini-jets (from hard-scattering of partons) 30 - 50 % of particle production high pt leading particles

• Extends into perturbative regime• Calculations reliable

hadrons

q

q

hadrons Leading particle

leading particle

schematic view of jet production

Physics @ RHIC New with Heavy Ions

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In p-p collisions hard parton scattering will lead to two jets emerging back-to-back with about equal energy

• Scattered partons that propagate through hot and dense nuclear matter will radiate (lose) energy in colored medium • interaction of parton with partonic matter

will lead to:• suppression of angular correlations

• suppression of high pT particles aka “parton energy loss” or “jet quenching” va

cuum

QGP

hadrons

q

q

hadrons leadingparticle

leading particle

schematic view of jet production

Physics @ RHIC New with Heavy Ions

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Physics @ RHIC Important observations (light quarks [u,d,s])

Suppression of angular correlations

Medium created at RHIC has very high opacity

In central Au+Au collisions the light hadrons in away-side jets are suppressed.

Not the case in p+p and d+Au

partons lose energy via gluon radiation

Energy loss depends on properties of medium (gluon densities, size) and properties of “probe” (color charge, mass)

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Physics @ RHIC Important observations (light quarks [u,d,s])

suppression of high pT particles

Medium created at RHIC has very high opacity

In addition, a measurement of energy loss of high pT partons using RAB shows significant suppression

Not the case in d+Au. NOT a cold nuclear matter effect.

partons lose energy via gluon radiation

Nuclear modification factor RAB

→ energy loss in partonic materRAB = (A-B pT spectra)/(p-p pT spectra *

“volume”)

Physics @ RHIC – Heavy Quark Sector Heavy flavor is mostly produced at the earlier stages of the collision via gluon fusion :

not affected by chiral symmetry restoration (i.e. mass is the same in-medium and vacuum) production cross-section found to binary scale

ideal (calibrated) probe the medium created in heavy ion collision

Look at the energy loss (RAA) [and elliptic flow (v2)] of heavy quarks.

Theoretical models predicted gluon radiative energy loss for heavy quarks to be smaller than of light quarks, which is not experimentally observed.

1) Non-photonic electrons (NPE) Method- decayed from charm and beauty hadrons

2) At pT ≥ 6 GeV/c,

RAA(NPE) ~ RAA(h±) !!!

3) Surprising Results: contradicts pQCD predictionschallenges our understanding of the energy loss mechanism

Needs Direct measurement of D and B mesons

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Measurement via Semi-leptonic (indirect) channels

Indirect measurement through Semi-leptonic decay channels:• D0 e+ + X (BR : 6.9 %)• D+/- e+/- + X (BR : 17.2%)✔ Large pT range.

✔Use of specific triggers✔ Relative contribution of electrons from B and D mesons are unknown.

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Measurement using azimuthal correlation of D mesons with e-

Azimuthal correlations of e-h and e-D0 can be utilized to disentangle the charm and bottom contributions

13 ✔ Triggers on high pT electrons

Measurement via hadronic (direct) channelsDirect measurement using a combinatorial method (combining K and π tracks)– Measurement of hadronic decay modes via invariant mass analysis.

• D0 (D0bar)K-+(K+-) BR : 3.8 %• D+/-K BR : 9.2%

TPC Only (Low pT)

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✔ C and B contributions separated.✔ Limited to low momentum range.✔ No triggers, no decay vertex reconstruction✔ Challenging due to small decay length Cu+Cu 200 GeV

Measurement using Silicon Vertex Detector and decay vertex fit

✔ SVT/SSD not designed (thickness, geometry) for charm measurement

✔ Full reconstruction/fit of the decay vertex by combining K and π tracks

– Some particle ID capabilities obtained from TPC dE/dx

Poor PIDChallenges:1. Very short lived particles (average decay-length

70 μm) coupled to marginal SVT resolution (the pointing resolution is about 250 μm/GeV)

2. Poor PID: Lack of TOF+SVT data sets, dE/dx has limited resolving power.

dE/dx bands of Kaon, Pion

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D0 decay length (Simulation)

For <pT> ~ 1GeV/c, βγ ~ 0.54average decay length ~ 65μm(in the transverse plane)un-boost in the collider!

Distance of Closest Approach resolution

STAR preliminary

Including the silicon detectors in the tracking improves the pointing resolution.with 4 silicon hits, the pointing resolution to the interaction point ~ 250 μm at P =

1GeV/c.

• run 7 Au+Au@200GeV (MinBias trigger).

• DCA resolution as a function of inverse momentum.

• Reflect the resolution and Multiple Coulomb Scattering.

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D0 Decay Topology

✿ We introduced the Full reconstruction/fit of the decay vertex

✿ We used the full track error matrix (inside the beam pipe) for best error estimates

✿ Cut Optimization is based on Monte Carlo studies

✿ Better resolution in secondary vertex position is achieved with the fit method compared to usual helix swimming methods

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Secondary vertex fit (MC Data - pure D0 Events)

• There is no systematic shift in reconstructed quantities.

• The standard deviation of the distribution is flat at ~ 250 m , which is of the order of the resolution of (SSD+SVT).

Mean of the difference reconstructed -MC

Rms of the difference reconstructed -MC

Correlation between the reconstructed decay and MC

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Reconstructed Quantities (example) (MC Data - pure D0 Events)

Invariant Mass Reconstructed pT Resolution

Decay Vertex Resolution ✿ Resolution : Inv Mass sigma ~ 13 MeV (0.7%, after a gauss fit) Decay Vertex Coordinates ~ 220 μm (transverse) ~ 200 μm (z-direction)

✿ The reconstructed parameters behave as expected with the current detector resolution.

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After cut

Test with K0s decay reconstruction (about 200x the decay distance of D0s):

c = 2.68 cm Signed decay length :

– an excess can be observed on the positive side of the decay length distribution, indicating the presence of long-lived decays.

– use the decay length significance L/L to improve the signal.– more appropriate because of the momentum dependence of the decay length.

Proof of principle with K0s

Before cut

After using a cut SL > 10, a clear peak at the K0S mass is observed.

backgroundSignal+background

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D0+D0bar Signal (in 2007 Data)

24 Million Au+Au @ 200 GeV/c events are used for this analysis.

3rd degree polynomial fit is used for background estimation.

Fit yielded an apparent signal significance of 10-σ (combined D0+D0bar signal)

Signal remained stable as cuts are varied.

Pol3 + gaus

Pol3

Gaussian Mean = 1864.19 ± 10 MeV

gaus fit

D0 and D0bar separately

Attempt to extract physics with polynomial fit method revealed some problems:

• A robust background estimation method needed to see if the peak observed was an artifact

• A Multi Variate analysis is in progress to measure signal using same sign background estimation method.

D0bar/D0 Ratio ~ 1.18 ± 0.24 ( compatible with unity indicating a vanishing μB)

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The fact that about a third of the SVT/SSD system was dead during Run-7, combined with the marginal resolution of the previous generation silicon detector and combinatorial background is challenging to our efforts.

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Ongoing Analysis with Multivariate Analysis (TMVA)

✿ TMVA is a tool for simultaneous optimization of many correlated cuts. ✿ Training samples for signal (pure D0) and background (`HIJING Au+Au’) are

provided. It then produces a classifier output with weight files for signal and background.

✿ After training, testing can be done with Data sample (MC Embedding/Real)

MC D0 Embedding 2007 Au+Au Data (1-2% of available data)

✿ Preliminary results looks promising but… work in progress.

Recent charm measurements with Time Of Flight (TOF) Detector

• STAR Time Of Flight (TOF) detector provides better particle ID (measures particle velocity β[β=v/c])

• dE/dx + TOF offers excellent K, π separation up to p ~ 1.5 - 2 GeV/c

• New results use ~ 250 Million Au+Au Events from year 2010 and ~ 210 Million p+p events from 2009

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Recent charm measurements with Time Of Flight (TOF) Detector

Corrected pT Spectra RAA

RAA shows charm suppression at ~ 4 GeV/c

Cross-section is found to binary scale, indicating its production via initial hard scattering at RHIC

Charm Cross Section

Future Heavy Flavor Tracker (HFT)• STAR is undergoing a detector upgrade for the unambiguous measurement of

charm – The Heavy Flavor Tracker (HFT)

– Low mass detector designed to identify mid-rapidity Charm and Beauty mesons and baryons through direct reconstruction, with unprecedented pointing resolution.

– CMOS sensors will provide single track resolution ~ 20-30 µm at <pT> ~ 1GeV/c.

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Key Measurements of the HFT(1) Energy loss of direct D0 - Rcp (2) Elliptic flow, v2

(3) Charmed Baryon to Meson Enhancement

The methods we have developed here are directly applicable in HFT

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Summary

For the 1st time in STAR, a secondary vertex reconstruction with full fit is developed using silicon vertex detectors.

Results obtained looks interesting, work in progress to wrap up the analysis with the most advanced tools in High Energy Physics – Multi Variate Analysis

Pioneering work that is directly applicable to the upcoming upgrade to STAR – Heavy Flavor Tracker (HFT)

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Thank you

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Back-Up

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Behaves like an ideal fluid

Substantial elliptic flow (v2) signal observed for a variety of particle species.

Rapid Thermalization

Physics @ RHIC Important observations (Light flavors)

Partonic Collectivity

v2 scaled by the number of valance quarks shows an apparent scaling

Development of anisotropy in the partonic stage of collision

Charm Cross-Section Comparison at 200 GeV

NLO Ref: R. Vogt, arXiv:0709.2531v1 [hep-ph]

STAR and PHENIX do not agree about total charm production x-section

Need precise, exclusive measurements33

Secondary Vertex Resolution Plots (x,y,z)Fit Method

(central region)σXY ~ 55μm σZ ~ 25μm

Helix Swimming Method – using global parameters

(central region)σXY ~ 150μm σZ ~ 135μm

Simulation results shows that a factor of two was gained in secondary vertex Resolution

X Y Z

Helix Swimming Method – using DCAGeometry

(central region)σXY ~ 140μm σZ ~ 125μm

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Strategy of ReconstructionSelect Event – Apply Event Level Cuts

Select TriggerCuts on Z-Vertex Position and its error

Loop over Tracks – Apply Track Level CutsNumber of Silicon Hits

Transverse DCA (DCAXY)Track Momentum etc.

Pair Association - D0 Candidate Level Cutsrapidity, Cosine of Kaon decay angle etc.

Decay Vertex Fit – Decay fit Level Cutsprobability of fit, decay length

error of decay length etc.

Particle Identification – Apply PID Cuts|nσK|, |nσπ|

Cuts are applied in the analysis code to reduce background and to increase the candidate pool

Output Saved for offline Analysis

Attempt to extract physics

✿ Uncorrected pT spectra:

✿ A normalized pT Spectra corrected for acceptance and efficiency would be used to:

- extract total charm cross-section, freeze out parameters etc.

- calculate energy loss RAA

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The results with a polynomial background estimate seem to be inconsistent.A robust background estimation method needed to see if the peak observed was an artifact

– a “same sign” background subtraction method was performed

Cuts in 1st Production

2007 Production MinBiasCuts in 2nd Production

EVENT leveltriggerId : 200001, 200003, 200013Primary vertex position along the beam axis : |zvertex| < 10 cmResolution of the primary vertex position along the beam axis: |zvertex|< 200µm

TRACKS levelNumber of hits in the vertex detectors :SiliconHits>2 (tracks with sufficient DCA resolution)Transverse Momentum of tracks: pT >.5GeV/cMomentum of tracks: p >.5GeV/c Number of fitted:TPC hits > 20 Pseudo-rapidity :||<1 (SSD acceptance)dEdxTrackLength>40 cmDCA to Primary vertex (transverse),DCAxy< .1 cm

EVENT leveltriggerId : 200001, 200003, 200013Primary vertex position along the beam axis : |zvertex| < 10 cmResolution of the primary vertex position along the beam axis: |zvertex|< 200µm

TRACKS levelNumber of hits in the vertex detectors: SiliconHits>1 Transverse Momentum of tracks:pT >.5GeV/c Momentum of tracks p >.8GeV/c Ratio TPC hits Fitted/Possible > 0.51Pseudo-rapidity :||<1.2dEdxTrackLength>40 cmDCA to Primary vertex (transverse),DCAxy< .2 cmRadius of first hit on track :

< 9 cm if number of silicon hits =2 < 13 cm else

• cut changed• new cut

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Cuts from Previous production

Continued..Cuts in New Production

DECAY FIT level

Probability of fit >0.1 && |sLength|<.1cm

Particle ID : ndEdx :|nK|<2, |nπ|<2

D0 candidate |y(D0)|<1|cos(*)|<0.8

DECAY FIT level

Probability of fit >0.01 && |sLength|<.1cm

Particle ID : ndEdx :|nK|<2.5, |nπ|<2.5

ndEdx :|nK|<2, |nπ|<2|cos(*)|<0.6DCA daughters < 300 µm

In both productions we made a pico file for further analysis.

Cuts Used for making a pico filePrevious Production New Production

|D0Eta|<1.85|Cos(θ*)<0.6

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Uncorrected pT Spectra

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Physics @ RHIC Important observations (Light flavors)

Partonic CollectivityPartonic Energy Loss

Behaves like an ideal fluidMedium created at RHIC has very high opacity

In central Au+Au collisions the light hadrons in away-side jets are suppressed.

Different for p+p and d+Au

In addition, a measurement of energy loss of high pT partons using RAB shows significant suppression

partons lose energy via gluon radiation

Substantial elliptic flow (v2) signal observed for a variety of particle species.

Rapid Thermalization

v2 scaled by the number of valance quarks shows an apparent scaling

Development of anisotropy in the partonic stage of collision

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‘Same sign’ background subtraction

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Heavy Quark Energy Loss Puzzle – NPE Method

Surprising results - - challenge our understanding of the energy loss mechanism - force us to RE-think about the elastic-collisions energy loss - Requires direct measurements of c- and b-hadrons.

1) Non-photonic electrons (NPE) decayed from - charm and beauty hadrons

2) At pT ≥ 6 GeV/c,

RAA(NPE) ~ RAA(h±) !!!

Contradicts naïve pQCD predictions

STAR: Phys. Rew. Lett, 98, 192301(2007)

and nucl-ex/0607012v3

Still the main method at RHIC

Measurement via Semi leptonic (indirect) channels Indirect measurement through

Semi-leptonic decay channels:• D0 e+ + X (BR : 6.9 %)• D+/- e+/- + X (BR : 17.2%)

✔ Large pT range. ✔ Relative contribution of electrons from B and D mesons are unknown.✔ Use of specific triggers

Measurement using azimuthal correlation of D mesons with e-

Azimuthal correlation of open charm mesons with non-photonic Electron can be utilized to disentangle the charm and bottom contributions[3]

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✔ Triggers on high pT electrons

Any information from direct reconstruction of D and B-mesons would help

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My contributions

Charm Analysis

QA, Problem fixing, Resolution studies

Detailed studies for ‘online’/‘offline’ Cut optimization

Data Productions (Micro/Pico-DSTs)

First observation charmed meson signal in real data (from 2007 Au+Au dataset)

Signal extraction, optimization, fitting, pT binning

Embedding QA, Study of Systematics and Physics Analysis

Service Work

acceptance of D-mesons with a prototype design for the HFT upgrade

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• This was a big surprise!!!• The fact that about a third of the SVT/SSD system was dead during Run-7, combined with

the marginal resolution of the previous generation silicon detector and combinatorial background limits our efforts.

• A final effort to measure the signal using a multivariate analysis is in progress.

Same cuts are used to produce this picture that were used in the polynomial fit case

An explanation for the non-consistent physics results

Red = SignalBlue = [(++) + (--)]

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Charm cross section vs √sNN

Kent State University, 9 November 2010, S. Kabana

4723/06/2010

Charm and beauty from e-D0 azimuthal correlations

sign(e) = sign(K)

sign(e) ≠ sign(K)Like Sign

Δϕ ~ π

Like SignΔϕ ~ 0

Unlike SignΔϕ ~ π

CHARM BEAUTYPYTHIA

A Mischke, Phys. Lett. B671, 361 (2009)