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First physics with heavy ions in ALICE
Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166 www.elsevierphysics.com
First physics with heavy ions in ALICE Luciano Ramelloa for the ALICE Collaboration a
Dipartimento di Scienze e Tecnologie Avanzate dell’Universit` a del Piemonte Orientale and INFN, 15100 Alessandria, Italy An overview of the soft physics observables addressed by the ALICE experiment with the first month of heavy ion (Pb+Pb) data at LHC is given. Emphasis is placed on centrality selection, charged multiplicity distributions, directed and elliptic flow, strange particle and resonance production at low and intermediate transverse momentum. Momentum correlations (HBT) and event-by-event physics are also addressed.
1. Introduction The ALICE experiment [1,2] at LHC has been designed specifically to cover a broad range of observables in heavy ion collisions at the c.m. energy of 5.5 TeV/nucleon pair. ALICE also has a substantial physics programme for protonproton collisions [3] both at the top LHC c.m. energy of 14 TeV and at 5.5 TeV, as well as a proton-nucleus physics programme. In this paper the physics observables which will be addressed with the first heavy ion collisions, i.e. the ”soft” physics ones, are outlined. The ”hard” probes are described in another paper [4]. For the studies presented here the HIJING event generator [5] has been used as an input to the AliRoot simulation and reconstruction package. The ALICE performance has been evaluated on the basis of the expected sample after the first Pb+Pb run of one month, namely 107 central collisions (10% most central collisions). Recent developments in the analysis strategy can be found in refs. [6] and [7].
trality classes can be defined, as shown in fig. 1. The experimental resolution on the impact parameter b and on the number of participant nucleons Npart is expected to be 1 fm and 15 nucleons, respectively.
2. Global event characterization The centrality of each collision will be determined by detecting spectator neutrons and protons on both sides of the interaction point with zero-degree calorimeters (ZDC). To remove the ambiguity due to the fragmentation of the projectile remnants, two zero-degree electromagnetic calorimeters (ZEM) will be used; from the correlation of signals in ZDC’s and ZEM’s several cen0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2007.11.103
Figure 1. Correlation between EZDC and EZEM in Pb+Pb collisions at LHC. The ALICE tracking detectors: ITS, TPC and FMD will provide the reconstructed charged particle distribution dNch /dη over the range −3.4 < η < 5.1 as a function of centrality. Fig. 2
162
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
dN/d η
shows the generated, reconstructed and corrected dNch /dη from tracklets in SPD layers 1 and 2 of the ITS. The height of the rapidity plateau, normalized to the number of participant pairs, is a crucial observable which will enable ALICE to test the parton saturation model [8] which describes current data up to RHIC energy. This model predicts a dNch /dη plateau level of 1800 (0-5% most central collisions), substantially lower than e.g. HIJING predictions (dNch /dη of 5000) which have been used in the design phase of ALICE. 4000 3500 3000 2500 2000 1500 1000 500 -3
Tracklets Tracklets (acceptance corrected) Generated
-2
-1
0
1
2
η3
Figure 2. Generated (histogram), reconstructed (triangles) and acceptance-corrected (open circles) dNch /dη distribution for Pb+Pb collisions at LHC.
3. Bulk properties mechanisms
and
hadronization
Details of the hadronization mechanism can be inferred from the measurement of yields, yield ratios and transverse momentum spectra of particles like p, K, π, hyperons and resonances. 3.1. Particle ratios Statistical hadronization models are able to reproduce yield ratios measured at SPS and RHIC energies with few parameters such as temperature T and baryochemical potential μB (see e.g. ref. [9]). At LHC such models predict a hadronization temperature of 170 MeV
and a chemical potential of ≈ 1 MeV. A nonequilibrium model [10] presents a different scenario with a supercooled (T ≈ 140 MeV) system and oversaturation of strangeness. The measurement by ALICE of several strange/nonstrange and multistrange/strange yield ratios should decide [2,11] between the two scenarios. 3.2. Transverse momentum spectra Yields and pT spectra of hadronic resonances, which can be destroyed and regenerated during the time between hadronization (chemical freezeout) and kinetic freezeout (see table 1), carry valuable information on the lifetime and other characteristics of the fireball. Table 1 The main hadronic resonances in order of increasing lifetime. Resonance mass width lifetime (MeV) (MeV) (fm/c) ρ 769 149 1.3 Δ 1232 118 1.7 980 40-100 2.6 f0 (980) 896 50 4.0 K ∗0 (896) Σ(1385) 1385 36 5.7 Λ(1520) 1520 16 13 ω 783 8.5 23 φ 1019.5 4.3 45 It is possible to reproduce the meson and baryon pT spectra and yields observed at RHIC in a wide range of pT by combining parton coalescence models (see e.g. [12]) at low and intermediate pT with perturbative QCD calculations at higher pT . An example is shown in fig. 3 (see P. Levai in [7]), with the predicted hard-soft overlap for pT of 4 ± 1 GeV/c for charged pions at LHC, well within the ALICE acceptance (see fig. 4). 3.3. Identification capabilities Several ALICE subsystems contribute to particle identification at midrapidity: TPC+ITS with dE/dx, secondary vertex and invariant mass reconstruction, the Time Of Flight system, PHOS,
163
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
2
Counts / (10 MeV/c )
6
×10 18 16 14 12 10 8 6 4 2 0 0.2
0.4
0.6
0.8
1
1.2 1.4 1.6 Invariant mass (GeV/c 2)
Figure 6. Invariant mass spectrum of π + π − pairs for 106 central Pb+Pb collisions. The fit (continuous line) includes contributions from ρ0 (770), ω, η + η and combinatorial background.
Counts / (1.25 MeV/c 2)
1500
1000
x103
2
TRD and HMPID. The pT ranges accessible with 107 central Pb+Pb collisions are shown in fig. 4. Examples of the ALICE decay reconstruction capabilities in Pb+Pb collisions are presented in figures 5 (Λ) and 6 (ρ).
shape used for the fit is a Breit-Wigner convoluted with a Gaussian representing the expected effective mass resolution of 1.23 MeV/c2 (due to the momentum resolution of the experiment). Entries / (1MeV/c )
Figure 3. Pion’s invariant pT spectra from the MICOR model and from pQCD at LHC.
60 50 40 30 20 10 0 0.98
0.99
1
1.01
1.02
+
1.03
1.04 2
Meff(K K -) (GeV/c )
500
0
(b)
1.08
1.1
1.12
1.14
1.16
Λ Invariant Mass (GeV/c2)
Figure 7. Invariant mass spectrum of K + K − pairs for 106 central Pb+Pb collisions and pT (K + K − ) > 2.2 GeV/c, after background subtraction.
Figure 5. Proton+pion invariant mass spectrum in central Pb+Pb collisions, with the Λ peak. In fig. 7 the opposite sign K + K − spectrum expected from 106 central collisions is shown after combinatorial background subtraction. The line-
4. Expansion dynamics and space-time structure More insight into the expansion dynamics of the fireball and its space-time structure can be
164
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
20
obtained by the study of anisotropic flow and of the momentum correlations (Hanbury-Brown and Twiss interferometry [13], also known as femtoscopy).
v2 %
Figure 4. pT ranges for meson and baryon indentification in the ALICE central barrel. 20 18 16 14 12 10
4.1. Flow In non-central collisions the initial energy density profile is azimuthally asymmetric in the transverse plane; this asymmetry can be quan2 −x2 tified by the spatial eccentricity = y y 2 +x2 and, with the hydrodynamic evolution of the system, produces an azimuthal asymmetry in momentum space [14]. The azimuthal distribution of produced particles is usually described by a Fourier expansion:
E × 1+2
+∞
1 d2 N d3 N = d3 p 2π pt dpt dy
vn (pt , y) cos[n(ϕ − ΨR )] ,
n=1
where ϕ and ΨR are the particle and reactionplane azimuths in the laboratory frame, respectively, and where coefficient v1 describes directed flow and coefficient v2 describes elliptic flow. The amount of elliptic flow v2 at LHC under different scenarios (hydrodynamics: v2 / = 0.22 or 0.33, dashed-dotted lines; Low Density Limit, continuous line) is shown in fig. 8 (see E. Simili in
8 6 4 2 0 0
0.2
0.4
0.6
0.8
/
1
Figure 8. Elliptic flow coefficient v2 (in %) vs. centrality (0=peripheral, 1=central) in Pb+Pb collisions at LHC under three different scenarios together with estimated non-flow contribution (see text).
[7]). The ALICE reconstruction capabilities with the Event Plane method have been investigated, leading to a satisfactory performance over most of the centrality range (the points in fig. 8 represent the non-flow background evaluated with HIJING). Also v1 and v4 will be measured by ALICE, resp. with ZDC’s and central barrel. 4.2. Femtoscopy (HBT) The correlation function of identical particles vs. their momentum difference q has been widely used at SPS and RHIC to study the size and lifetime of the emitting source. As the RHIC source
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
C(q inv )
parameters do not fully agree with expectations from otherwise successful hydrodynamical calculations (see e.g. [15]), it will be quite interesting to measure HBT radii at LHC. As an example of the ALICE capabilities, fig. 9 shows the two-pion correlation function from a supposedly Gaussian emitting source. The parameters λ and R have been extracted from a 2 R2 ), Gaussian fit f (qinv ) = 1 + λ · exp(−qinv 2 0 2 where qinv = | q| − (q ) . ALICE can measure HBT radii for several identical and non-identical pairs with good precision for the expected range ≤ 15 fm. 2
fit parameters sim. TPC rec. R(fm) 7.4 7.25 ± 0.04 λ 0.73 0.62 ± 0.01
1.8 1.6 1.4
χ 2 / ndf =2487 / 91
1.2 1 0.8 0.6
165
Figure 10. Left: Proton pT spectrum for a single HIJING event. The deduced temperature is 319 ± 13 MeV. Right: Generated and reconstructed proton temperature for 300 events.
6. Conclusions The ALICE physics potential for the first heavy ion run at LHC has been evaluated in detail [1,2]. With one month of Pb+Pb collisions (107 central events) several key measurements will be performed in a new energy regime. Event by event physics will become accessible for the first time.
0.4
REFERENCES
0.2 0
0.02
0.04
0.06
0.08
0.1
0.12 0.14 q inv [GeV/c]
Figure 9. Two-pion correlation function vs. qinv as obtained from simulation and reconstruction.
5. Event by event physics Thanks to the large multiplicity of charged particles identified by ALICE in single events (central collisions), many fluctuation signals such as pT , particle ratios, net charge, balance functions, Disordered Chiral Condensate (DCC) will be detectable at LHC. As an example, fig. 10 (see C. Zampolli in [6]) shows the proton pT spectrum from a single event and the reconstructed temperature (inverse slope) from 300 central events. ALICE has an expected resolution σT /T of 0.5% for charged pions, 6% for charged kaons and 7% for protons.
1. ALICE Collaboration, ALICE: Physics Performance Report, Volume I, J. Phys. G: Nucl. Part. Phys. 30 (2004) 1517-1763. 2. ALICE Collaboration et al., ALICE: Physics Performance Report, Volume II, J. Phys. G: Nucl. Part. Phys. 32 (2006) 1295-2040. 3. J.P. Revol, these proceedings. 4. F. Antinori, these proceedings. 5. X.N. Wang and M. Gyulassy, Phys. Rev. Lett. 86 (2001) 3496. 6. http://www.ct.infn.it/SPHIC06: Soft Physics in Ultrarelativistic Heavy Ion Collisions, Catania, Italy, Sept. 27-29, 2006. 7. http://qgp.uni-muenster.de/aliceweek/: Second ALICE Physics Week, Muenster, Germany, Feb. 12-16, 2007. 8. N. Armesto et al , PRL 94 (2005) 022002. 9. A. Andronic et al , NP A772 (2006) 167. 10. J. Rafelski et al , EPJ C45 (2006) 61. 11. B. Hippolyte, EPJ C49 (2007) 121. 12. V. Greco et al , PRL 90 (2003) 202302. 13. R. Hanbury Brown and R.Q. Twiss, Phil.
166
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
Mag. 45 (1954) 663. 14. P.F. Kolb and U. Heinz in R.C. Hwa (ed.), Quark gluon plasma, pages 634-714 (2003). 15. M. Lisa, nucl-th/0701058.
First physics with heavy ions in ALICE Luciano Ramelloa for the ALICE Collaboration a
Dipartimento di Scienze e Tecnologie Avanzate dell’Universit` a del Piemonte Orientale and INFN, 15100 Alessandria, Italy An overview of the soft physics observables addressed by the ALICE experiment with the first month of heavy ion (Pb+Pb) data at LHC is given. Emphasis is placed on centrality selection, charged multiplicity distributions, directed and elliptic flow, strange particle and resonance production at low and intermediate transverse momentum. Momentum correlations (HBT) and event-by-event physics are also addressed.
1. Introduction The ALICE experiment [1,2] at LHC has been designed specifically to cover a broad range of observables in heavy ion collisions at the c.m. energy of 5.5 TeV/nucleon pair. ALICE also has a substantial physics programme for protonproton collisions [3] both at the top LHC c.m. energy of 14 TeV and at 5.5 TeV, as well as a proton-nucleus physics programme. In this paper the physics observables which will be addressed with the first heavy ion collisions, i.e. the ”soft” physics ones, are outlined. The ”hard” probes are described in another paper [4]. For the studies presented here the HIJING event generator [5] has been used as an input to the AliRoot simulation and reconstruction package. The ALICE performance has been evaluated on the basis of the expected sample after the first Pb+Pb run of one month, namely 107 central collisions (10% most central collisions). Recent developments in the analysis strategy can be found in refs. [6] and [7].
trality classes can be defined, as shown in fig. 1. The experimental resolution on the impact parameter b and on the number of participant nucleons Npart is expected to be 1 fm and 15 nucleons, respectively.
2. Global event characterization The centrality of each collision will be determined by detecting spectator neutrons and protons on both sides of the interaction point with zero-degree calorimeters (ZDC). To remove the ambiguity due to the fragmentation of the projectile remnants, two zero-degree electromagnetic calorimeters (ZEM) will be used; from the correlation of signals in ZDC’s and ZEM’s several cen0920-5632/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2007.11.103
Figure 1. Correlation between EZDC and EZEM in Pb+Pb collisions at LHC. The ALICE tracking detectors: ITS, TPC and FMD will provide the reconstructed charged particle distribution dNch /dη over the range −3.4 < η < 5.1 as a function of centrality. Fig. 2
162
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
dN/d η
shows the generated, reconstructed and corrected dNch /dη from tracklets in SPD layers 1 and 2 of the ITS. The height of the rapidity plateau, normalized to the number of participant pairs, is a crucial observable which will enable ALICE to test the parton saturation model [8] which describes current data up to RHIC energy. This model predicts a dNch /dη plateau level of 1800 (0-5% most central collisions), substantially lower than e.g. HIJING predictions (dNch /dη of 5000) which have been used in the design phase of ALICE. 4000 3500 3000 2500 2000 1500 1000 500 -3
Tracklets Tracklets (acceptance corrected) Generated
-2
-1
0
1
2
η3
Figure 2. Generated (histogram), reconstructed (triangles) and acceptance-corrected (open circles) dNch /dη distribution for Pb+Pb collisions at LHC.
3. Bulk properties mechanisms
and
hadronization
Details of the hadronization mechanism can be inferred from the measurement of yields, yield ratios and transverse momentum spectra of particles like p, K, π, hyperons and resonances. 3.1. Particle ratios Statistical hadronization models are able to reproduce yield ratios measured at SPS and RHIC energies with few parameters such as temperature T and baryochemical potential μB (see e.g. ref. [9]). At LHC such models predict a hadronization temperature of 170 MeV
and a chemical potential of ≈ 1 MeV. A nonequilibrium model [10] presents a different scenario with a supercooled (T ≈ 140 MeV) system and oversaturation of strangeness. The measurement by ALICE of several strange/nonstrange and multistrange/strange yield ratios should decide [2,11] between the two scenarios. 3.2. Transverse momentum spectra Yields and pT spectra of hadronic resonances, which can be destroyed and regenerated during the time between hadronization (chemical freezeout) and kinetic freezeout (see table 1), carry valuable information on the lifetime and other characteristics of the fireball. Table 1 The main hadronic resonances in order of increasing lifetime. Resonance mass width lifetime (MeV) (MeV) (fm/c) ρ 769 149 1.3 Δ 1232 118 1.7 980 40-100 2.6 f0 (980) 896 50 4.0 K ∗0 (896) Σ(1385) 1385 36 5.7 Λ(1520) 1520 16 13 ω 783 8.5 23 φ 1019.5 4.3 45 It is possible to reproduce the meson and baryon pT spectra and yields observed at RHIC in a wide range of pT by combining parton coalescence models (see e.g. [12]) at low and intermediate pT with perturbative QCD calculations at higher pT . An example is shown in fig. 3 (see P. Levai in [7]), with the predicted hard-soft overlap for pT of 4 ± 1 GeV/c for charged pions at LHC, well within the ALICE acceptance (see fig. 4). 3.3. Identification capabilities Several ALICE subsystems contribute to particle identification at midrapidity: TPC+ITS with dE/dx, secondary vertex and invariant mass reconstruction, the Time Of Flight system, PHOS,
163
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
2
Counts / (10 MeV/c )
6
×10 18 16 14 12 10 8 6 4 2 0 0.2
0.4
0.6
0.8
1
1.2 1.4 1.6 Invariant mass (GeV/c 2)
Figure 6. Invariant mass spectrum of π + π − pairs for 106 central Pb+Pb collisions. The fit (continuous line) includes contributions from ρ0 (770), ω, η + η and combinatorial background.
Counts / (1.25 MeV/c 2)
1500
1000
x103
2
TRD and HMPID. The pT ranges accessible with 107 central Pb+Pb collisions are shown in fig. 4. Examples of the ALICE decay reconstruction capabilities in Pb+Pb collisions are presented in figures 5 (Λ) and 6 (ρ).
shape used for the fit is a Breit-Wigner convoluted with a Gaussian representing the expected effective mass resolution of 1.23 MeV/c2 (due to the momentum resolution of the experiment). Entries / (1MeV/c )
Figure 3. Pion’s invariant pT spectra from the MICOR model and from pQCD at LHC.
60 50 40 30 20 10 0 0.98
0.99
1
1.01
1.02
+
1.03
1.04 2
Meff(K K -) (GeV/c )
500
0
(b)
1.08
1.1
1.12
1.14
1.16
Λ Invariant Mass (GeV/c2)
Figure 7. Invariant mass spectrum of K + K − pairs for 106 central Pb+Pb collisions and pT (K + K − ) > 2.2 GeV/c, after background subtraction.
Figure 5. Proton+pion invariant mass spectrum in central Pb+Pb collisions, with the Λ peak. In fig. 7 the opposite sign K + K − spectrum expected from 106 central collisions is shown after combinatorial background subtraction. The line-
4. Expansion dynamics and space-time structure More insight into the expansion dynamics of the fireball and its space-time structure can be
164
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
20
obtained by the study of anisotropic flow and of the momentum correlations (Hanbury-Brown and Twiss interferometry [13], also known as femtoscopy).
v2 %
Figure 4. pT ranges for meson and baryon indentification in the ALICE central barrel. 20 18 16 14 12 10
4.1. Flow In non-central collisions the initial energy density profile is azimuthally asymmetric in the transverse plane; this asymmetry can be quan2 −x2 tified by the spatial eccentricity = y y 2 +x2 and, with the hydrodynamic evolution of the system, produces an azimuthal asymmetry in momentum space [14]. The azimuthal distribution of produced particles is usually described by a Fourier expansion:
E × 1+2
+∞
1 d2 N d3 N = d3 p 2π pt dpt dy
vn (pt , y) cos[n(ϕ − ΨR )] ,
n=1
where ϕ and ΨR are the particle and reactionplane azimuths in the laboratory frame, respectively, and where coefficient v1 describes directed flow and coefficient v2 describes elliptic flow. The amount of elliptic flow v2 at LHC under different scenarios (hydrodynamics: v2 / = 0.22 or 0.33, dashed-dotted lines; Low Density Limit, continuous line) is shown in fig. 8 (see E. Simili in
8 6 4 2 0 0
0.2
0.4
0.6
0.8
/
1
Figure 8. Elliptic flow coefficient v2 (in %) vs. centrality (0=peripheral, 1=central) in Pb+Pb collisions at LHC under three different scenarios together with estimated non-flow contribution (see text).
[7]). The ALICE reconstruction capabilities with the Event Plane method have been investigated, leading to a satisfactory performance over most of the centrality range (the points in fig. 8 represent the non-flow background evaluated with HIJING). Also v1 and v4 will be measured by ALICE, resp. with ZDC’s and central barrel. 4.2. Femtoscopy (HBT) The correlation function of identical particles vs. their momentum difference q has been widely used at SPS and RHIC to study the size and lifetime of the emitting source. As the RHIC source
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
C(q inv )
parameters do not fully agree with expectations from otherwise successful hydrodynamical calculations (see e.g. [15]), it will be quite interesting to measure HBT radii at LHC. As an example of the ALICE capabilities, fig. 9 shows the two-pion correlation function from a supposedly Gaussian emitting source. The parameters λ and R have been extracted from a 2 R2 ), Gaussian fit f (qinv ) = 1 + λ · exp(−qinv 2 0 2 where qinv = | q| − (q ) . ALICE can measure HBT radii for several identical and non-identical pairs with good precision for the expected range ≤ 15 fm. 2
fit parameters sim. TPC rec. R(fm) 7.4 7.25 ± 0.04 λ 0.73 0.62 ± 0.01
1.8 1.6 1.4
χ 2 / ndf =2487 / 91
1.2 1 0.8 0.6
165
Figure 10. Left: Proton pT spectrum for a single HIJING event. The deduced temperature is 319 ± 13 MeV. Right: Generated and reconstructed proton temperature for 300 events.
6. Conclusions The ALICE physics potential for the first heavy ion run at LHC has been evaluated in detail [1,2]. With one month of Pb+Pb collisions (107 central events) several key measurements will be performed in a new energy regime. Event by event physics will become accessible for the first time.
0.4
REFERENCES
0.2 0
0.02
0.04
0.06
0.08
0.1
0.12 0.14 q inv [GeV/c]
Figure 9. Two-pion correlation function vs. qinv as obtained from simulation and reconstruction.
5. Event by event physics Thanks to the large multiplicity of charged particles identified by ALICE in single events (central collisions), many fluctuation signals such as pT , particle ratios, net charge, balance functions, Disordered Chiral Condensate (DCC) will be detectable at LHC. As an example, fig. 10 (see C. Zampolli in [6]) shows the proton pT spectrum from a single event and the reconstructed temperature (inverse slope) from 300 central events. ALICE has an expected resolution σT /T of 0.5% for charged pions, 6% for charged kaons and 7% for protons.
1. ALICE Collaboration, ALICE: Physics Performance Report, Volume I, J. Phys. G: Nucl. Part. Phys. 30 (2004) 1517-1763. 2. ALICE Collaboration et al., ALICE: Physics Performance Report, Volume II, J. Phys. G: Nucl. Part. Phys. 32 (2006) 1295-2040. 3. J.P. Revol, these proceedings. 4. F. Antinori, these proceedings. 5. X.N. Wang and M. Gyulassy, Phys. Rev. Lett. 86 (2001) 3496. 6. http://www.ct.infn.it/SPHIC06: Soft Physics in Ultrarelativistic Heavy Ion Collisions, Catania, Italy, Sept. 27-29, 2006. 7. http://qgp.uni-muenster.de/aliceweek/: Second ALICE Physics Week, Muenster, Germany, Feb. 12-16, 2007. 8. N. Armesto et al , PRL 94 (2005) 022002. 9. A. Andronic et al , NP A772 (2006) 167. 10. J. Rafelski et al , EPJ C45 (2006) 61. 11. B. Hippolyte, EPJ C49 (2007) 121. 12. V. Greco et al , PRL 90 (2003) 202302. 13. R. Hanbury Brown and R.Q. Twiss, Phil.
166
L. Ramello / Nuclear Physics B (Proc. Suppl.) 177–178 (2008) 161–166
Mag. 45 (1954) 663. 14. P.F. Kolb and U. Heinz in R.C. Hwa (ed.), Quark gluon plasma, pages 634-714 (2003). 15. M. Lisa, nucl-th/0701058.