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Pb-Pb ALICE



19 2013
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Introduction
1.Ultrarelativistic heavy-ion collisions (HIC) 2.Directed and elliptic flow 3.The purposes of the thesis research 4.Tools for the research: Collider Experiment: the ALICE detector Monte Carlo simulation of HIC: HYDJET++

·Phenomenological investigation of anisotropic flow in HYDJET++ model ·Summary · Directed flow measurement in experimental data · Summary

Results

Conclusions
2


Introduction
1.Ultrarelativistic heavy-ion collisions (HIC) 2.Directed and elliptic flow 3.The purposes of the thesis research 4.Tools for the research: Collider Experiment: the ALICE detector Monte Carlo simulation of HIC: HYDJET++

·Phenomenological investigation of anisotropic flow in HYDJET++ model ·Summary · Directed flow measurement in experimental data · Summary

Results

Conclusions
3


Ollitrault's suggestion (1992): transverse anisotropic collective effects are characterized via anisotropy in momentum distribution. It is natural to use Fourier series of azimuthal distribution with respect to reaction plane angle R:

Non-central collisions

flow coefficients, v

n

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Ollitrault's suggestion (1992): transverse anisotropic collective effects are characterized via anisotropy in momentum distribution anisotropy It is natural to use Fourier series of azimuthal distribution with respect to reaction plane angle R:

Non-central collisions

directed flow at lower energies

flow coefficients, v

n

Sensitive to EoS and phase transition Probes the system at early stages of evolution

Flow antiflow

Brachmann, et. al. , Phys.Rev. C 61 (2000) 024909.

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Elliptic flow at LHC

Directed flow predictions
Bleibel et.al., Phys Let B 659, 520 C200n) i et.al., Phys. Rev. C 84, 024914 , (2011) ( s er 8 a

y is rapidity center mass fluctuation

A different slope in comparison to existing RHIC data at top energy

v2 is described by hydrodynamics as almost perfect liquid

Fluctuation in initial overlap density even component of v1

E. Retinskaya, M. Luzum, J. -Y. Ollitrault, Phys. Rev. Lett. 108, 252302 (2012),

6


Introduction
1.Ultrarelativistic heavy-ion collisions (HIC) 2.The purposes of the thesis research 3. Directed and elliptic flow 4.Tools for the research: Collider Experiment: the ALICE detector Monte Carlo simulation of HIC: HYDJET++

·Phenomenological investigation of anisotropic flow in HYDJET++ model ·Summary · Directed flow measurement in experimental data · Summary

Results

Conclusions
7


Large Hadron Collider

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The ALICE experiment is one of 4 experiments in the LHC ring.

pp s=8 TeV in 2012 PbPb, sNN=2.76 TeV in 2010 9


A Large Ion Collider Experiment was build to study the physics of nuclear matter under extreme conditions of temperature and density.

TPC (Time Projection Chamber) : || < 0.8 , charge particle tracking ITS (Inner Tracking system ): Tracking, vertexing TOF (Time of Flight chamber) : particle identification VZERO (Forward scintillator arrays) : [-3.7 , -1.7] , [2.8 , 5.1], charged multiplicity measurement, centrality selection ZDC (Zero Degree Calorimeter): >8.8, 114 m from interaction point, sensitivity to spectators

10


Introduction
1.Ultrarelativistic heavy-ion collisions (HIC) 2.The purposes of the thesis research 3. Directed and elliptic flow 4.Tools for the research: Collider Experiment: the ALICE detector Monte Carlo simulation of HIC: HYDJET++

·Phenomenological investigation of anisotropic flow in HYDJET++ model ·Summary · Directed flow measurement in experimental data · Summary

Results

Conclusions

11


Directed flow measurements,
directed flow of produced particles TPC (Time Projection Chamber) measurements:
tracks in TPC, ||<0.9, with transverse momentum 0.15 i

||<0.9

TPC

B=0.5T

is charged particle multiplicity in a sector i

P Data sample: bPb collisions at sNN =2.76 TeV Events of 0-80% centrality, ~12

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Neutron ZDC, 4-tower calorimeter, was used

The centroid:

Ei tower energy; (xi, yi) tower center, , parameters

sensitivity to directed flow of spectators No/weak correlation along orthogonal directions:

Quality Assurance: A detailed study of ZDC performance Centroid reconstruction dependence on centrality, interaction vertex, run conditions
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Negative slope of v1()
Contrast to some of the theoretical predictions

Detailed study of systematic: Tracks reconstructed with TPC and ITS+TPC (non-uniform -acceptance) Different cuts on track reconstruction (number of TPC clusters, d.c.a.) Different event selection (z-vertex cut, three centrality estimators) SP vs EP method Comparison of different v1 terms Flattering of -distribution (due to final ZDC granularity ) Detector efficiency of track reconstruction eff(pt) from MC

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Decrease of v1 with collision energy Weaker centrality dependence at midrapidity than that at RHIC

pt dependence is similar to RHIC results Zero crossing around pt ~ 1.5 GeV/c For peripheral collision zero crossing point moves toward higher 15


A different slope of protons flow was observed at lower energies
034903)

(158 AGeV, NA49: Phys.Rev. C68 (2003)

Flattering of protons flow observed at RHIC 200GeV

(A.Tang, J. of Physics: Conf. Ser. 230 (2010) 012018)

ALICE PID :

Asymmetric -cut in the TOF detector to select high purity for , K , p. Additional 2 cut in the TPC dE/dx to clean the data.

Both charged pions (not shown here in the pic.), kaons have negative slope and consistent with all charged flow. Protons seems to be more flat. Stat. errors are large. Pions, kaons and all charged v1(pt) are compatible. A zero crossing in about the same pt-region
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y

beam

=7.98

VZERO

SPD: the two innermost layers of the tracker device, tracklets

i=1,8 : the sector's number Mi: charged particle multiplicity

Multiplicity in VZERO needs a correction for background particles. - SPD acceptance Highly non-uniform acceptance: careful 17 recentering procedure was


ybeam=7.98

uncorrected

Results of 3 subdetectors: the slope found to be negative in | |<5

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PHOBOS data: Phys Rev Let97, 012301 (2006) 252301 (2008)

STAR data: Phys Rev Let 101,

Universal trend when shifted to beam rapidity

Data follows the longitudinal scaling observed at RHIC

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We define:

Both components show little dependence on the collision centrality and change sign at p t 1.2-1.7 GeV/c.
A gap between A- and C-side is present Harmonic decomposition of 2particle correlations : the similar pt -shape for even part, but much larger magnitude ZDC is less sensitive to this observable.

5-80%

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To test the fluctuation origin:

The even pt -weighed v1 (in blue) vanishes. The mean px at midrapidity is zero participants and spectators do not correlate via momentum conservation.

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The center of mass rapidity fluctuates EbE:

Can potentially influence the odd harmonics!

We don't measure all particle due to acceptance limitation Try to estimate y
CM

of participant subsystem (B) via spectators subsystems (A, C) E, P:

ZDC doesn't measure all neutrons and protons a correction is needed model dependant study

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L.P. Csernai, V.K. Magas, H. Stocker, and D.D. Strottman, Phys. Rev. C 84, 02914 (2001)

incomplete spectator fragmentation charged fragments do not reach ZDC N of single neutrons are estimated by measured ZDC energy

A correspondence between cent and b are used from fluid dynamical model calculation A correction of measured N of single neutrons to the total number of spectators are made based on fluid dynamical model calculation. A correction of ZDC energy (momentum) is made
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y

CM

is found EbE and directed flow is recalculated with y

CM

shift

The mid-rapidity part ||<1 coincides well with directed flow measured without rapidity center of mass adjustment.

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V2 in VZERO

Method:

Reaction Plane

Detectors: 1) ZDC for first order RP 2) TPC for second order RP

DATA pass2

v2 =

wi cos(2 - 2 ) R

wi are VZERO multiplicities Gain equalization is done Correction for VZERO granularity is done The difference with TPC flow up to scale factor 2 ­ need to be studied Data show flat v2 up to 5 units of pseudorapidity

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MC correction of flow value
Rely on a correct description of the detector geometry, supporting material, secondary particles production

Aliroot with updated TPC, SPD thermal shield is used, without the beam-pipe/FMD alignment at the moment Different MC models were used with different dN/deta, dN/dpt, v2(eta, pt)

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3 models with different dN/deta, dN/dpt, v2(eta, pt)

Different secondaries/primaries ratio due to different amount of material along Z-axis

GeVSim M1 with more sharp pt distribution

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3 models with different dN/deta, dN/dpt, v2(eta, pt)

More flat distribution in GeVSim

Linear up to pt=2 or 3 GeV/c in GeVSim

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Correction from 3 models and the mean

Corrections are made as a direct ratio

v2(input) / v2(Vzero)

from the models above

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Data correction, elliptic flow
Example of 30-40% centrality MC correction doesn't remove a structure at C-side: Some other correction is needed ...

Comparison with FMD results from Alexander Hansen

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Directed flow of charged particles has been measured at midrapidity, || <0.8, and forward rapidity, 1.7 < || < 5.1, for Pb-Pb collisions at sNN = 2.76 TeV with the ALICE detector at LHC Charged pions, kaons, protons v1() has negative slope (as at RHIC): in contrast to some of the theoretical expectations Magnitude of v1 is about 3 times smaller than at top RHIC energy v1 (-y
b e am

) is consistent with a picture of longitudinal scaling observed at RHIC

The directed flow fluctuations (even component) are revealed by comparing flow of 2 sub-events Both odd and even v1(pt) crosses zero at pt~ 1.5 GeV/c

Even v1(pt) pt~ shape is close that obtained in 2-particle correlations analysis, the magnitude is about 50 times smaller. The directed flow stay unchanged in a midrapidity region after accounting for longitudinal fluctuations of CM rapidity.
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v4/(v2)2=1/2 : a universal prediction of hydrodynamics within some general approximation (particle momentum is parallel to fluid velocity). It does not depend on EoS, PID, centrality, rapidity, initial conditions.
N. Borghini, J.-Y. Ollitrault, PLB 642 (2006) 227

Experiment shows larger value
The objective: Study the effect of jets on the ratio Ratio for the hydro part of HYDJET++ in blue: close to ½, constant with pt Ratio for hydro+jet part in red triangles: Qualitative description of the rise of highpt tail

Jets result to increase by about 15% of the ratio v4/(v2)2=1/2 Eccentricity fluctuations can increase the ratio by factor 1.5 (Luzum, Ollitrault, ..PRC 2010)

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Anisotropic flow

ALICE week October 2008

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Extended rapidity range: Vzero rings of plastic scintillators with -granularity on 8 sectors.
Flow components:

Directed flow at forward rapidity

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Model parameters.

ALICE week October 2008

1. Thermodynamic parameters at chemical freeze-out: Tch , {µB, µS, 2. If thermal freeze-out is considered: Tth , µ-normalisation constant

µ

Q

}

3. Volume parameters: , , R · -maximal transverse flow rapidity for Bjorken-like parametrization 5. max -maximal space-time longitudinal rapidity which determines the rapidity interval [- max, max] in the collision center-of-mass system. 6. Impact parameter range: minimal bmin and maximal bmax impact parameters 7. Flow anisotropy parameters



(b),

(b)

PYTHYA+PYQUEN obligatory parameters
11. ptmin ­ minimal pt of parton-parton scattering in PYTHIA event (ckin(3) in /pysubs/)

Spectra

Width of the spectra allows one to fix max =3.3, Centrality dependence of multiplicity allows one to fix ptmin=3.4 GeV/c and =0.053

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Parameters of energy loss model in PYQUEN

ALICE week October 2008

(default, but can be changed from the default values by the user)

1. T0 - initial temparature of quark-gluon plasma for central Pb+Pb collisions at mid-rapidity at LHC: T0=1 GeV, at RHIC(200 AGeV) T0=0.300 GeV 2. tau0 - proper time of quark-gluon plasma formation at LHC: tau0=0.1 fm/c, at RHIC(200 AGeV) tau0=0.4 fm/c 3. nf - number of active quark flavours in quark-gluon plasma (nf=0, 1, 2 or 3) at LHC: nf=0, at RHIC(200 AGeV) nf=2 4. ienglu - flag to fix type of medium-induced partonic energy loss (ienglu=0 - radiative and collisional loss, ienglu=1 - radiative loss only, ienglu=2 - collisional loss only, default value is ienglu=0); ianglu - flag to fix type of angular distribution of emitted gluons (ianglu=0 - small-angular, ianglu=1 - wide-angular, ianglu=2 - collinear, default value is ianglu-0). ienglu=0
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Directed flow at lower energy
hys.Rev. C68 (2003) 034903

An odd function of rapidity A sign of the slope for different particle species changes with energy Þ Indicates a phase transition in some of the theoretical model

LHC predictions

y is rapidity center mass fluctuation

Bleibel et.al., Pys Let B 659, 520 (2008) Phys. Rev. C 84, 024914 (2011)

Csernai et.al.,
637 6

Much stronger signal than at RHIC Opposite slope vs. rapidity compared to RHIC energy


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