Introduction to Heavy Ion Physics at the LHC

Transcription

1 Introduction to Heavy Ion Physics at the LHC F. Noferini INFN Bologna/CNAF E. Fermi Centre, Rome ALICE Review 24/10/2016 1

2 Hadrons and confinement Gluon Proton Quark K0 meson Quark Anti- Quark Quarks are confined in nature forming bounding states of 3 quarks (BARYONS) or of quark-antiquark (MESONS). Baryons Anti-baryons Mesons In this way all the observable QCD states (HADRONS) are neutral with respect to the color charge. 24/10/2016 2

3 Asymptotic freedom At short distance (large transfer momenta) QCD running coupling is small lim S r 0 ( r) 0 allowing a perturbative approach, like in QED, to predict observables (i.e. cross sections). Unfortunatelly, this is not the case for large distances (small transfer momenta). 24/10/2016 3

4 Confinement QED QCD The potential between two quarks at large distances in QCD, is very different with respect to QED. The QCD fields are compressed in a string with a constant section The energy stored in the string per unit of length is constant V long kr with k ~ 1 GeV/fm and the total energy increase linearly with the quark separation. 24/10/2016 4

5 Predictions from lattice calculations Deconfinement is expected for high density over a critical temperature of 175 MeV. The strong coupling between two quarks is expected to vanish for T > T C. Heavy ion collisions are the suitable environment to probe it. Lattice calculations taken by: O.Kaczmarek and F.Zantow, Phys. Rev. D 71 (2005) /10/2016 5

6 Deconfinement and chirality Lattice QCD predicts that the chiral symmetry restoration and the transition to deconfinement happen (is it accidental?) at the same critical temperature. Data from 24/10/2016 6

7 From accelerators to the Big Bang Heavy ion collisions 24/10/2016 7

8 Time evolution of a heavy ion collisions 24/10/2016 8

9 Observables for the QGP Chemical probes Kinetic probes Strangeness enhancement and vector-meson production Effects on J/ and B-production High p T probes Electromagnetic probes Statistical models. m s T. Jet-medium interaction. Early stages of the interaction. Collective phenomena (flow) represent also a signature of the existence of a partonic medium and provide a way to probe the medium properties! 24/10/2016 9

10 Centrality in heavy ion collisions Impact parameter b = 0-5 fm: 0-10% b Central collisions b = fm 60-80% Pheripheral collisions Overlap region of two nuclei The centrality of the collisions can be also expressed in terms of the nucleons which partecipate to the collision (N part ) 24/10/

11 The ALICE detector and in 2015 Pb-Pb at 5.02 TeV (Run2) 24/10/

12 The Glauber model Semi classical model to describe AA collisions with a given impact parameter b Nuclei interaction described as an incoherent overlap of nucleon interactions The model is able to quantify Interaction probability The Number of binary nucleon-nucleon collisions (N coll ) The number of participant nucleons (N part ) Defined as the nucleons in the overlap region Size and anisotropy of the overlap region 24/10/

13 N part vs. b N part large ( A+B) for central collisions (b 0) N part 0 for peripheral collisions (b 2R) For a given b, N part increases with the size of the nuclei ( N part A) 24/10/

14 N coll vs. b N part large ( A+B) for central collisions (b 0) N part 0 for peripheral collisions (b 2R) For a given b, N part increases with the size of the nuclei (N coll AB, N coll <<AB) N ( b) AB T ( b) coll inel AB 24/10/

15 Centrality definition Alver, B. and Baker, M. and Loizides, C. and Steinberg, P., arxiv: [nucl-ex] (2008); The Glauber model allows also to correlate the detector multiplicity and the centrality of the collision. Distribution of the signal amplitudes summed over all the V0 scintillator tiles (histogram) compared with expectations from the Glauber model (line) 24/10/

16 Hadron PID Full azimuthal acceptance ITS TPC TOF TRD The PID capabilities of the detector were largely exploited during Run-1 in many analyses and collision systems (pp, ppb, PbPb). HMPID 0 < < 0.6, =0.31 Azimuthal acceptance completed for Run-2 Limited acceptance 24/10/

17 , K and p/p identification Particle Identification (PID) with TOF & TPC: Combination of allows to reach a particle separation in a wide p T range. TPC and TOF PID is complementary ALICE performance, arxiv: /10/

18 System size (femptoscopy) ALICE Collaboration (Aamodt, K. et al.), Phys. Lett., B696 (2011) 328 Volume of the fireball vs. charged track multiplicity density from two-pion HBT correlation: ALICE results compared to lower energy experiments The fireball spatial extent at decoupling is accessible via interferometry, a technique which exploits the Bose-Einstein enhancement of identical bosons close in phase space. The size of the medium at freezeout can be characterized by the correlators along the three axes providing the corresponding radii R out and R side (the size in the transverse plane) and R long (the size along z). 24/10/

19 Radial flow (hadron spectra) ALICE Collaboration (Abelev, B. et al.), Phys. Rev., C88 (2013) In an expanding medium particles are pushed to higher momenta during because of strong pressure gradient. Higher the mass higher the effect on p T The effect of the radial flow is quite evident in the p/ ratio at intermediate momenta! 24/10/

20 Radial flow (blast wave) 24/10/ ALICE Collaboration (Abelev, B. et al.), Phys. Rev., C88 (2013) p K f b b b f t b t b f t b t b t t b R t species t t N N N s T R r T m T p s K I d m rdr N p dp dn,,,,,,, : parameters 2 cos 2 1 / with cosh / sinh / 2 cos You can fit the hadron spectra simultaneously using the blastwave model (an approximation of hydro calculations) to extract the expanding properties. P. Huovinen, P.F. Kolb, U. Heinz, P.V. Ruuskanen and S. Voloshin, Phys. Lett. B503, 58 (2001).

21 Particle ratios ALICE Collaboration (Veldhoen, M et al.), Nucl. Phys., A (2013) ALICE Collaboration (Abelev, B. et al.), Phys. Rev. Lett., 111 (2013) In addition the enhancement in baryon production with respect to mesons is pronounced also in the strangeness sector, as for Λ/K ratio. This effect, named baryon anomaly, is strongly centrality dependent. 24/10/

22 Parton degrees of freedom in the QGP u,d,s cc suppression m s ~ T c Strangeness production 24/10/

23 Strangeness ALICE Collaboration (Adam, J. et al.), arxiv: [nucl-ex] (2015) The plateau in the multistrange-over-pion ratio for high multiplicity events is consistent with the prediction of some thermal models which expect a saturation to the grand canonical value when the size of the system is larger than several fm (the multiplicity is proportional to the volume, HBT). The rise in the low/intermediate multiplicity region may indicate that such a saturation is not still reached. 24/10/

24 dn/dφ Strong QGP: elliptic flow Peripheral collision Expansion is driven by a gradient of pressure There is a preferential direction for particle emission 2v 2 E 3 d N 3 dp 1 2 dn p dp dy t t 1 n1 2v n cos n R 24/10/

25 dn/dφ The elliptic flow ALICE Collaboration (Aamodt, K. et al.), Phys. Rev. Lett., 105 (2010) The integrated elliptic flow at the LHC is larger than at RHIC. The different behavior at low energy, where negative values are also measured, may be interpreted as a strong geometrical effect in such a regime: when the two nuclei pass each other the matter can not escape in the direction of the reaction plane because of the shield of the spectator nucleons. 2v 2 3 d N 1 dn E 1 2v cos n dp 3 2 p dp dy t t n1 n R 24/10/

26 Elliptic flow and a strongly interacting medium As mentioned an initial spacial anisotropy may result in an momentum anisotropy of the final momenta An analogy of what happens in heavy ion collisions is given by a system of Litium atoms at very low temperature The anisotropy is transfered to the momenta depending on the viscosity of the system (low visosity means collectivity large anisotropy in the final states) 24/10/

27 Identified v 2 at LHC ALICE Collaboration (Abelev, B. et al.), JHEP, 1506 (2015) 190 Collection of ALICE v2 for many speacies:, K, p, K 0 s, and ( and next slides). Mass splitting is clearly visible also for high mass particle like and. 24/10/

28 v 2 /n q > 0.9 v 2 scaled for the Number of v 2 /n q > 0.9 Constituent Quarks (NCQ) vs KE T /n q ALICE Collaboration (Abelev, B. et al.), JHEP, 1506 (2015) 190 (m T - m 0 )/n q (GeV/c 2 ) (m T - m 0 )/n q (GeV/c 2 ) 2 2 mt m p T KE T = Transverse Kinetic Energy = m T m 0 KE T /n q scaling: For low KE T /n q : the NCQ scaling is broken at the LHC For KE T /n q > 1 GeV/c: scaling holds at the level of 20% 24/10/

29 ALICE Collaboration (Krzewicki, M. et al.), J. Phys., G38 (2011) Triangular flow J. Phys. G: Nucl. Part. Phys. 38 (2011) v 3 exhibits similar particle mass dependence as that of v 2 The value of p T at which v 3 of all species cross looks similar to that for v 2 v 3 is quite sensitive to the input in the hydro models Note: 24/10/2016 v 3 {2} is without rapidity gap 29

30 Higher harmonics ALICE Collaboration (Noferini, F. et al.), EPJ Web Conf., 90 (2015) Nice agreement with IP-Glasma for v n {2} while MC-KLN and MC- Glauber do not describe the p T dependence of v 3 and v 4 The amplitude for different harmonics for very central collisions at intermediate p T confirms the dominant role of v 3 when geometrical anisotropies are reduced by a centrality selection. The choice of very central collisions allows to reduce the effects of the initial geometry and to emphasize the contribution of fluctuations. ALICE Collaboration (Aamodt, K. et al.), Phys. Lett., B708 (2012) /10/

31 Collectivity in ppb system A similar picture observed in Pb Pb is also observed in p Pb central collisions, with a clear mass scaling for protons and pions and a crossing point at p T 2 GeV/c. Hint of collectivity? ALICE Collaboration (Abelev, B. et al.), Phys. Lett., B726 (2013) /10/

32 Anti-alpha discovery STAR Coll., Nature 473 (2011) 353 J. Phys. G: Nucl. Part. Phys. 38 (2011) So far the heaviest anti-nucleus observed is the anti-alpha ( 4 He). 4 He was discovered for the first time by the STAR collaboration at RHIC and then observed also at the LHC by ALICE. 24/10/

33 Anti-nuclei production in AA collisions In high energy Pb-Pb collisions at the LHC a large amount of nuclei and anti-nuclei is produced. Yields of nuclei and anti-nuclei are very similar at the LHC (ALICE Coll., (2015) arxiv: ) Z 3 He 4 He p d t A t 4 He 3 He d p For lighter nuclei and antinuclei the production is large enough to measure with high precision their mass 24/10/

34 Mass differences ALICE CPT symmetry prediction Highest precision direct measurements of mass difference in the sector of nuclei Improvement by one to two orders of magnitude compared to previous measurements obtained more than 40 years ago ANT71: Nucl. Phys. B31 (1971) 235 DOR65: Phys.Rev.Lett 14 (1965) 1003 MAS65: Nuovo Cim. 39 (1965) 10 24/10/

35 CPT invariance tests Experimental limits are also used to constrain, for different interactions, CPT violating terms added to the SM Lagrangian in the Standard Model Extension (SME) (Rev. Mod. Phys. 83 (2011) 11) 24/10/

36 Hyper-nuclei with ALICE ALICE Collaboration (Adam, J. et al.), Phys. Lett., B754 (2016) 360 Not only nuclei but also hypernuclei (and other exotic states) are considered in ALICE. 24/10/

37 Summary The QGP (Quark Gluon Plasma) was largely explored at SPS and RHIC and now at LHC. Evidence of a thermal equilibrium (particle ratios vs. statistical models) The properties observed are consistent with a perfect fluid (RHIC) and strongly interacting QCD matter (Jet Quenching, elliptic flow) Several observables reveal a partonic nature of the produced matter (J/ѱ suppression, elliptic flow) 24/10/