Jaiby Joseph Ajish 11/2/2011

Charmed Meson measurements using a Silicon Tracker in Au+Au

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

Jaiby Joseph Ajish11/2/2011

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OutlineQuick summary of my contributionsIntroduction

Why collide nuclei at high energies?RHIC, STAR

Physics at RHICImportant observationsHeavy quark sector

Charm measurement using Silicon TrackerSecondary vertexingStrategy of reconstructionProof of principle with Ks

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ResultsFuture

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My contributionsCharm Analysis

debugging the micro-vertexing code:- QA of the reconstructed parameters, fixing problems with dE/dx cut, and resolution studies.

Detailed Monte Carlo studies for online/offline cut optimization

Productions of Micro DSTs and 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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Why collide nuclei at high energies?

Quantum Chromo Dynamics (QCD) is the theory of strong interactions. QCD provides us with 2 important characteristics of quark-gluon interactions

(1) Asymptotic freedom – High energies, weakly interacting quarks and gluons(2) Confinement – No free quarks have been observed

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

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

Phase Diagram

Net Baryon Density

Tc ~ 150 -170 MeV and ρ ~ 1GeV/fm3

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RHIC BRAHMSPHOBOSPHENIX

STAR

AGS

TANDEMS

1 km

Relativistic Heavy Ion Collider (RHIC)

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RHIC Collisions

Collision systems used at RHIC are: Au+Au, Cu+Cu, d+Au and p+p at different 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 (see below).

• Scattered partons propagate through matter radiate energy (~ GeV/fm) in colored medium • interaction of parton with partonic matter• suppression of high pt particles

aka “parton energy loss” or “jet quenching”• suppression of angular correlation

vacu

umQG

P

• Hard Parton Scattering

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

• Extend into perturbative regime• Calculations reliable

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 flavors)

Partonic Energy Loss

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

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

partons lose energy via gluon radiation

Nuclear modification factor RAA

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

“volume”)

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

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

ideal to probe the medium created in heavy ion collision

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

Measuring collective motion (v2) of charm mesons will indicate whether thermalization is reached in the earlier steps of the collision.

There are unresolved charm cross-section discrepancies between STAR and PHENIX

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 using Silicon Vertex Detector and decay vertex fit

✔ In order to enhance physics capabilities, STAR used a 1-layer silicon strip (SSD) and 3-layer silicon drift (SVT) detectors which are placed inside the TPC.

✔ Full operation in year 2005 and 2007

✔ Was 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

Poor PID

Caveats:1. Very short lived particles: for a realistic D0

distribution average decay-length at <pT> ~ 1 GeV/c is 60-70 μm

2. Marginal resolution: At <pT> ~ 1GeV/c, the resolution achieved with hits on all silicon layers is ~ 250 μm

3. Poor PID: For p > 0.7GeV/c, the K, π bands overlap giving rise to large combinatorial background.

dE/dx bands of Kaon, Pion

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D0 Decay Topology

✿ Full reconstruction/fit of the decay vertex

✿ Introduction/Use of full track error matrix for best error estimates

✿ Optimization of cuts based on MC studies

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

Mean of the difference reconstructed -MC

Rms of the difference reconstructed -MC

Reco - MC [cm]

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

Invariant Mass Reconstructed pT Resolution

Decay Vertex Resolution ✿ Resolution : Inv Mass ~ 13 MeV (0.7%, after a gauss fit) Trans. Momentum ~ 17 MeV 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 :

K0S π+ π- (BR = 69.2%) ; c = 2.68 cm ; Mass = 0.497 MeV/c2

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.

background Signal+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.

Kinematic fit yields an improved signal of 10-σ for combined D0+D0bar signal.

Signal remains stable as cuts are varied.

Pol3 + gaus

Pol3

Gaussian Mean = 1864.19 ± 10 MeV

gaus fit

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Invariant Mass of D0 and D0bar separately

D0bar/D0 Ratio ~ 1.18 ± 0.24

Statistical thermal models predict vanishing baryonic chemical potential (μB) at RHIC energies ( ) The D0bar/D0 ratio obtained here is compatible with unity indicating a vanishing μB.

Attempts to extract physics✿ Uncorrected pT spectra:

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

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

- calculate energy loss RAA

✿ At this time, we do not have a proper embedding sample to do corrections

- sample has too few Silicon hits

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Some of 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

✿ The ratio of central to MB yield in |y|<0.5, scaled by the number of binary collisions:

✿ A ratio 1 is expected

✿ results from polynomial fit background is inconsistent with the binary collision scaling of charm.

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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 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

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Ongoing Analysis with Multivariate Analysis (TMVA)

✿ TMVA is a ROOT integrated machine learning technique. It uses classifiers to discriminate signal from background.

✿ We used the Boosted Decision Tree (BDT) classifier

✿ Training samples for signal (pure D0) and background (`same sign’) are provided. It will produce 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, work in progress to run over the whole data – which will be the final phase of this analysis.

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Recent charm measurements with Time Of Flight (TOF) Detector

• STAR Time Of Flight (TOF) detector provides better particle ID – measure particle velocity β

• 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 p+p events from 2009

Corrected pT Spectrum and RAA in AuAu

✿ Charm cross section shows scaling with number of binary collisions indicating charm production via initial hard scattering ✿ Suppression of charmed meson observed around ~ 4 GeV/c

D0 and D* in p+p 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)

Key Measurements of HFT include:(1) Rcp (2) Elliptic flow, v2

(3) Charmed Baryon to Meson Enhancement

The method developed here is a baseline for analysis involving the Heavy Flavor Tracker (HFT)21

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Thank you

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Back-Up

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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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Secondary Vertex fit – Simulation

✿ 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).

Reco vs. MC [cm] Mean of the difference reconstructed -MC

Rms of the difference reconstructed -MC

Reco - MC [cm]

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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

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

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

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

Results using STAR Time-Of-Flight (TOF) Detector(TOF+TPC offers better PID)TPC Only (Low pT)

✔ C and B contributions separated.✔ Limited to low momentum range.✔ No triggers, no decay vertex reconstruction✔ Challenging for charm mesons due to small decay length

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

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 measurements

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