Charming new results from STAR! NSD Staff Meeting, January 22, 2019 - - PowerPoint PPT Presentation

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Charming new results from STAR! NSD Staff Meeting, January 22, 2019 - - PowerPoint PPT Presentation

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Charming new results from STAR! NSD Staff Meeting, January 22, 2019 Sooraj Radhakrishnan Relativistic Nuclear Collisions, LBNL Relativistic Nuclear Collisions Ordinary Quark-Gluon nuclear matter Plasma (QGP) Nuclear matter

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‘Charm’ing new results from STAR!

NSD Staff Meeting, January 22, 2019 Sooraj Radhakrishnan Relativistic Nuclear Collisions, LBNL

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Sooraj Radhakrishnan

Relativistic Nuclear Collisions

Ordinary nuclear matter Quark-Gluon Plasma (QGP)

  • Nuclear matter transitions to QGP

phase at very high temperatures and densities

  • Study properties of QGP, evolution,

interactions with color charged probes, nature of phase transition, QCD phase diagram,..

  • Active experimental programs at Relativistic Heavy Ion Collider (RHIC) and

Large Hadron Collider (LHC)

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  • Charm quark energy loss:

D0 RAA and RCP [arXiv:1812.10224

(2018)]

  • Transport in QGP:

Elliptic (v2) [PRL.118.212301 (2017)] and directed (v1) flow of D0

  • Hadronization:

/\c production, Ds production

Sooraj Radhakrishnan

Charm (and bottom) quarks produced predominantly in initial hard scatterings: Ideal probes to study medium interactions and QGP properties Can study various aspects of charm quark evolution in the QGP

Heavy quarks in QGP

New results from STAR!

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  • Charm quark energy loss:

D0 RAA and RCP [arXiv:1812.10224

(2018)]

  • Transport in QGP:

Elliptic (v2) [PRL.118.212301 (2017)] and directed (v1) flow of D0

  • Hadronization:

/\c production, Ds production

Sooraj Radhakrishnan

Charm (and bottom) quarks produced predominantly in initial hard scatterings: Ideal probes to study medium interactions and QGP properties Can study various aspects of charm quark evolution in the QGP

Heavy quarks in QGP

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  • Hadronization implemented in PYTHIA via string fragmentation.

Hadronization: Λc production

  • In heavy-ion collisions the deconfined

quarks can hadronize via coalesence

  • Enhances baryon production

compared to string fragmentation

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  • Enhancement in B/M ratio at intermediate pT if hadronization by coalesence
  • Observed for light and strange flavor hadrons
  • Also important to understand charm hadron (eg: D0) modification and energy

loss in QGP and total charm cross-section

Hadronization: Λc production

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  • Heavy Flavor Tracker (HFT) installed for runs in 2014-2016
  • Phys. Rev. Lett. 118 (2017) 212301

Sooraj Radhakrishnan

  • Charged particle

tracks reconstructed with TPC (and HFT)

  • Particle

identification from ionization energy loss in TPC and time of flight from TOF detector

The STAR Detector

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  • HFT: 2 layers of Si pixels with MAPS and 2

layers of Si strips

  • Full azimuthal coverage
  • Provides excellent vertex position resolution

and allows reconstruction of charm hadron decays

  • Designed and constructed primarily at LBNL
  • Phys. Rev. Lett. 118 (2017) 212301

Sooraj Radhakrishnan

Heavy Flavor Tracker

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  • Use Supervised Learning Methods to improve signal to background separation
  • /\c reconstructed with the pKπ channe, Life time about 60 μm!
  • HFT improves S/B ratio for reconstructing /\c decay
  • Three body decay, huge combinatorial background in HI collisions

Λc signal reconstruction

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  • Boosted Decision Trees: Decision trees

recursively split the data into subsets. At each decision node a binary classification is made untill a classification a reached

  • ‘Boosting’ improves classification power and

reduce overtraining

Λc signal reconstruction

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  • Boosted Decision Trees: Decision trees

recursively split the data into subsets. At each decision node a binary classification is made untill a classification a reached

  • ‘Boosting’ improves classification power and

reduce overtraining

  • More than 50% signal significance improvement

with BDT

Rectangular Cuts (QM17) BDT Cuts (QM18)

Signal reconstruction Λc signal reconstruction

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Signal reconstruction Λc signal reconstruction

  • With statistics from 2016, signal significance of about 11 sigma
  • Allows measurement of pT and centrality dependence of /\c production in HI

collisions

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  • HFT detector description with fully misaligned

geometry incorporated into STAR GEANT for full event reconstruction and corrections for detector effects.

  • Tuning with data and cosmic data for hit

efficiency, hit resolution.

  • Also tune TPC performance also to reproduce

the high precision tracking

Modelling detector response

Data MC Embedding

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Embedding with HFT

  • Excellent description of detector response in simulations

D0 —> Kπ

Modelling detector response

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  • Large values of B/M ratio for charm hadrons, comparable to those of light and

strange flavor hadrons

  • Similar pT dependence as for light flavor hadrons

Results: Comparison to light flavor

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  • Significant enhacement of /\c/D0 ratio compared to p+p values from PYTHIA
  • PYTHIA with Color Reconnection enhances baryon production, but still

underpredicts data

χ2 to PYTHIA default = 23.86; P(χ2if true > χ2measured) = 2.7e-5 χ2 to PYTHIA CR = 7.74 ; P(χ2if true > χ2measured) = 0.052

Results: Model comaprisons

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  • Models with coalesence hadronization of charm quarks show similar

enhancement as in data

  • Coalesence models:

phase-space recombination of partons to hadrons

  • Quarks that dont

hadronize by coalesence hadronized by fragmentation

  • Models differ in choice of

spectra for light and charm quarks, Wigner functions for hadrons

Results - model comaprisons Results: Model comaprisons

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  • /\c/D0 ratio show increasing trend towards more central collisions, similar to

that for light and strange flavor hadrons

Results: Centrality dependence

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  • Charm quark energy loss:

D0 RAA and RCP [arXiv:1812.10224

(2018)]

  • Transport in QGP:

Elliptic (v2) [PRL.118.212301 (2017)] and directed (v1) flow of D0

  • Hadronization:

/\c production, Ds production

Sooraj Radhakrishnan

Charm (and bottom) quarks produced predominantly in initial hard scatterings: Ideal probes to study medium interactions and QGP properties Can study various aspects of charm quark evolution in the QGP

Heavy quarks in QGP

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  • Moving spectator protons induce extremely strong magnetic fields in

initial stages of HI collisions

  • Correlated in direction to the reaction plane

Directed flow of charm quarks

Charm quarks and intial magnetic fields in HI colliisions

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  • Charm quarks produced very early in

collisions when initial B field are significant

  • Also relaxation time large for charm

quarks

  • Results in v1 (directed flow) with opposite

slopes w.r.t rapidity for D0 and anti-D0

Directed flow of charm quarks

Charm quarks and intial magnetic fields in HI colliisions

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  • Significant directed flow (v1) predicted for charm quarks from flow!
  • Charge independent
  • ‘Tilted bulk’ in longitudinal direction, but HF quark production profile is

symmetric — first order density anisotropy

  • Viscous drag on c quarks by the expanding tilted bulk — generates D0 v1
  • Sensititive to initial tilt and viscous drag experienced by c quarks in medium

Directed flow from initial geometry

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  • Spectator neutrons pushed out along the impact parameter
  • Used to determine RP direction with Zero Degree Calorimeters

Measurement of D0 directed flow

  • D0 reconstrcuted at midrapidity using

HFT

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  • v1 measured by correlating D0 with the spectator plane from ZDC
  • Corrected for RP resolution

Measurement of D0 directed flow

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  • Evidence of non-zero v1 for D0 at mid-rapidity
  • Slope at mid-rapidity much larger than that for charged kaons

Results: D0 directed flow at mid-rapidity

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  • Magnitude of D0 v1 sensitive to initial tilt of the source
  • Can help constrain the model parameter

Results: Model comparisons

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  • Sensitive to temperature dependence of the drag coefficient
  • Together with D0 RAA and v2 can better constrain the tranport parameters

Results: Model comparisons Results: Model comparisons

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  • Negative slope for both D0 and anti-D0 v1
  • No significant difference observed at current precision (within ~1σ)
  • Magnitude of charge dependent signal predicted by Hydro+EM calculations

are also small

Results: Charge dependence

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  • Λc production in Au+Au collisions:
  • Significant enhancement of /\c/D0 ratio compared to p+p values from

PYTHIA

  • Evidence for coalesence hadronization of charm quarks
  • Large /\c production cross-section in HI collisions

Summary & Conclusions

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  • Directed flow of D0
  • Evidence of non-zero directed flow for D0

mesons

  • Magnitude much larger than for light flavor

hadrons

  • Can constrain c quark transport

coefficients and initial conditions in the longitudinal direction

  • No significant charge dependence
  • bserved, within uncertainties

Summary & Conclusions Summary & Conclusions

  • Future experiments (sPHENIX, ALICE ITS upgrade)
  • Improve precision and push to lower pT for /\c measurements
  • Differentiate between models
  • Predicted v1 signal from B field measurable at statistics projected for

sPHENIX

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Back Up

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Sooraj Radhakrishnan

Energy Loss [arXiv:1812.10224 (2018)]

  • Strong suppression of D0

mesons, increasing towards central collisions

  • Suppression smaller than light

flavor hadrons at intermediate pT

  • Most precise D0 measurements

in heavy-ion collisions, constrain the charm quark energy loss in the QGP

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Sooraj Radhakrishnan

Elliptic flow [PhysRevLett.118.212301 (2017)]

  • Pressure driven expansion of the QGP medium
  • Azimuthal anisotropies in the momentum

distribution of produced particles

  • Seeded by initial geometry of the fireball
  • QGP viscosity, transport properties
  • Charm quarks acquire flow from diffusion through QGP
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Results: Charm cross section

  • Enhancement for /\c and Ds and suppression for D0
  • But total charm cross-section is found to be consistent with p+p
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/\c cross-section

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String fragmentation vs cluster hadronization

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ALICE: arXiv:1712.09581

/\c production in p+p collisions

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  • Efficiency correction from data driven fast simulation (also used for D0)
  • Extensively validated with HIJING+GEANT simulations
  • Uses as inputs HFT ratio and dca resolution from data, TPC efficiency

and momentum resolution from Embedding εFS = εTPC x εHFT x εPID x εBDT

  • Corrections from Embedding:
  • Secondary protons from /\ are not matched in HFT.
  • Secondary tracks cause a broadened Dca tail in data
  • Primary vertex resolution effects not accounted for in FastSim
  • Uses /\c embedded into HIJING+ZB events to evaluate these

ε/\c = εFS x εsec x εvtx

Efficiency corrections

Au+Au 200 GeV 10-80% TPC x HFT x Topo x PID

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Comparison with rectangular cuts

  • Error bars are much reduced.
  • Cuts are from different tress with different efficiencies
  • BDT values are lower than that from rectangular cuts
  • BDT FastSim performance need to be validated with HIJING

Au+Au, 10-60%