UN PICCOLO BIG BANG IN LABORATORIO: L'ESPERIMENTO ALICE AD LHC

Transcription

1 UN PICCOLO BIG BANG IN LABORATORIO: L'ESPERIMENTO ALICE AD LHC Parte 1: Carlos A. Salgado Universidade de Santiago de Compostela

2 LHC physics program Fundamental Interactions Searches Higgs, SUSY, extradimensions... Increase energy density simple systems 2

3 LHC physics program Fundamental Interactions Searches Higgs, SUSY, extradimensions... Collective properties of the fundamental interactions Increase energy density simple systems Increase extended energy density less simple systems 2

4 LHC physics program Fundamental Interactions Searches Higgs, SUSY, extradimensions... Collective properties of the fundamental interactions Increase energy density simple systems Increase extended energy density less simple systems How?: Symmetries and breaking of the symmetries Chiral symmetry/confinement (ALICE) EW symmetry breaking: Higgs (CMS, ATLAS) CP violation (LHCb) 2

5 So, heavy ion collision to... Study QCD under extreme conditions High densities High temperatures 3

6

7 QCD matter 5

8 QCD QCD QCD is the theory of strong interactions. It describes interactions between hadrons (p, π,...) Asymptotic states. Normal conditions of temperature and density. Nuclear matter (us). Colorless objects. Frascati, May 2006 QGP and HIC p.4 6

9 QCD QCD QCD is the theory of strong interactions. It describes interactions between hadrons (p, π,...) Quarks and gluons in the Lagrangian Fundamental particles. charge=+2/3 u ( 5 MeV) c ( 1.5 GeV) t ( 175 GeV) charge=-1/3 d ( 10 MeV) s ( 100 MeV) b ( 5 GeV) Colorful objects. color = charge of QCD vector Similar to QED, but gluons can interact among themselves Frascati, May 2006 QGP and HIC p.4 6

10 QCD QCD QCD is the theory of strong interactions. It describes interactions between hadrons (p, π,...) Quarks and gluons in the Lagrangian Fundamental particles. charge=+2/3 u ( 5 MeV) c ( 1.5 GeV) t ( 175 GeV) charge=-1/3 d ( 10 MeV) s ( 100 MeV) b ( 5 GeV) Colorful objects. color = charge of QCD vector Similar to QED, but gluons can interact among themselves Gluons carry color charge This changes everything... Frascati, May 2006 QGP and HIC p.4 6

11 QCD QCD QCD is the theory of strong interactions. It describes interactions between hadrons (p, π,...) Quarks and gluons in the Lagrangian No free quarks and gluons: Confinement. Frascati, May 2006 QGP and HIC p.4 6

12 QCD QCD QCD is the theory of strong interactions. It describes interactions between hadrons (p, π,...) Quarks and gluons in the Lagrangian No free quarks and gluons: Confinement. Strength smaller at smaller distances: Asymptotic freedom. 0.4! s (µ) µ (GeV) Frascati, May 2006 QGP and HIC p.4 6

13 Picture In quantum field theory, the vacuum is a medium which can screen charge (quarks or gluons disturb the vacuum) 7

14 Picture In quantum field theory, the vacuum is a medium which can screen charge (quarks or gluons disturb the vacuum) Confinement isolated quarks or gluons = infinite energy 7

15 Picture In quantum field theory, the vacuum is a medium which can screen charge (quarks or gluons disturb the vacuum) Confinement isolated quarks or gluons = infinite energy Colorless packages (hadrons) vacuum excitations 7

16 Picture Picture In quantum field theory, the vacuum is a medium which can screen charge (quarks or gluons disturb the vacuum) In quantum field theory, vacuum is a medium Confinement which can isolated screen charge. quarks or gluons = infinite energy (quarks or gluons disturb vacuum). confinement = isolated quarks Colorless (gluons) = infinite packages energy (hadrons) vacuum excitations colorless packages (hadrons) = vacuum excitations. Masses masses: mass (GeV) qm (GeV) p 1 2m u + m d 0.03 π 0.13 m u + m d

17 String Picture String picture A way of visualizing a meson a q q pair join together by a string Colorless object Islamabad, March 2004 HIC and the search for the QGP - 1. QCD matter. p.6 8

18 String Picture String picture A way of visualizing a meson a q q pair join together by a string Colorless object The potential between a q q pair at separation r is V (r) = A(r) r + Kr Islamabad, March 2004 HIC and the search for the QGP - 1. QCD matter. p.6 8

19 String Picture String picture A way of visualizing a meson a q q pair join together by a string Colorless object The potential between a q q pair at separation r is V (r) = A(r) r + Kr Islamabad, March 2004 HIC and the search for the QGP - 1. QCD matter. p.6 8

20 String Picture String picture A way of visualizing a meson a q q pair join together by a string Colorless object The potential between a q q pair at separation r is V (r) = A(r) r + Kr When the energy is larger than m q + m q a q q pair breaks the string and forms two different hadrons. Islamabad, March 2004 HIC and the search for the QGP - 1. QCD matter. p.6 8

21 String Picture String picture A way of visualizing a meson a q q pair join together by a string Colorless object The potential between a q q pair at separation r is V (r) = A(r) r + Kr When the energy is larger than m q + m q a q q pair breaks the string and forms two different hadrons. In the limit m q the string cannot break (infinite energy) Islamabad, March 2004 HIC and the search for the QGP - 1. QCD matter. p.6 8

22 Chiral symmetry In the absence of quark masses the QCD Lagrangian splits into two independent quark sectors L QCD = L gluons + i q L γ µ D µ q L + i q R γ µ D µ q R For two flavors However, this symmetry is not observed Solution: the vacuum is symmetric under (i = u, d) L QCD SU(2) L SU(2) R 0 is not invariant 0 q L q R 0 0 chiral condensate Symmetry breaking Golstone s theorem = massless bosons associated: pions 9

23 So, properties of the QCD vacuum confinement chiral symmetry breaking 10

24 So, properties of the QCD vacuum confinement chiral symmetry breaking Is there a regime where these symmetries are restored? QCD phase diagram 10

25 Free quarks and gluons? Asymptotic freedom: quarks and gluons interact small distances increase large momentum increase temperature Phase transition? 11

26 Free quarks and gluons? Asymptotic freedom: quarks and gluons interact small distances increase large momentum increase temperature Phase transition? 11

27 How? Where? These phases could exist in several situations The early Universe some µs after the Big-Bang order of the transition has cosmological consequences In the core of neutron stars In experiments of heavy-ion collisions To concentrate a large energy density in a macroscopic region of space 12

28 real data from RHIC

29 real data from RHIC

30 Heavy-ion collisions, some history... o Landau (1953) applies hydrodynamics to hadronic collisions. Landau (1953) applies hydrodynamics to hadronic collisions. Assumptions Large amount of the energy deposited in a short time in a small region of space (little fireball) of the size of a Lorentz-contracted nucleus Created matter is treated as a relativistic (classical) perfect fluid Equation of state P = ɛ/3 The hydrodynamical flow stops when the mean free path becomes of the order of the size of the system: freeze out Normally, the condition is T m π 14

31 The essential measurement for hydro The Euler equation dβ dt = c2 ɛ + P P transverse plane x φ Outgoing particle y 15

32 The essential measurement for hydro The Euler equation dβ dt = c2 ɛ + P P ɛ = 3P = x P < y P transverse plane x φ Outgoing particle Elliptic flow normally measured by the second term in the Fourier expansion dn dφ v 2 cos(2φ) y More momentum in these directions Goal: to check the degree of thermalization of the created system. 15

33 One of the first measurements at RHIC As a function of the momentum projected in the trasverse plane ( ) p T v "! K! pbar " + K + p The effect of viscosity 0.1 v p T (GeV/c) p T (GeV/c) 0.1 " + "! " + "! K + K! K + K! p pbar p pbar hydro " hydro K hydro p p T (GeV/c) v 2 /n quark [data: STAR] p T /n quark (GeV/c) [Luzum, Romatschke 2008] 16

34 One of the first measurements at RHIC As a function of the momentum projected in the trasverse plane ( ) p T v "! K! pbar " + K + p The effect of viscosity 0.1 v Elliptic flow will be a 1st-day measurement in ALICE! p T (GeV/c) p T (GeV/c) 0.1 " + "! " + "! K + K! K + K! p pbar p pbar hydro " hydro K hydro p p T (GeV/c) v 2 /n quark p T /n quark (GeV/c) [Luzum, Romatschke 2008] [data: STAR] 16

35 the LHC in hydro Evolution of the temperature with time [simulations by V. Ruuskanen and H. Niemi] 17

36 the LHC in hydro Evolution of the temperature with time [simulations by V. Ruuskanen and H. Niemi] 17

37 the LHC in hydro Evolution of the temperature with time [simulations by V. Ruuskanen and H. Niemi] 17

38 QCD thermodynamics 18

39 QCD phase diagram Volume 59B, number 1 PHYSICS LETTERS Fig. 1. Schematic phase diagram of hadronic matter. PB is the density of baryonic number. Quarks are confined in phase I and unconfined in phase II. T tially more complex such as, e.g. those c We note finally t (phase transition or similar forms for the of state for high den first case we may ex come asymptoticall gas, while the limiti tremely "soft" equa has important astro References a hadron consists [Cabibbo of a bag and inside Parisi which 1975] quarks are confined. If many hadrons are present, space is divided into two regions: the "exterior" and the "interior". At low temperature the hadron density is low, and the "interior" is made up of disconnected islands (the [1] R. Hagedorn, Nu [2] R. Hagedorn and (1968) 169. [3] K. Johnson, Phys [4] A. Chodos et al., [5] G. Parisi, Quark i 19

40 QCD phase diagram First lattice calculation found a first order phase transition 19

41 QCD phase diagram First lattice calculation found a first order phase transition Including quark masses probably not a first order 19

42 QCD phase diagram First lattice calculation found a first order phase transition Including quark masses probably not a first order Present status: several different phases found. 19

43 QCD phase diagram QCD phase diagram We study this region #q # q First lattice calculations found a phase transition between ordinary First lattice matter calculation and QGP. found a first order phase transition Including quark masses probably not a first order Including quark masses maybe not a real phase transition Present status: several different phases found. 19 Frascati, May 2006 QGP and HIC p.15

44 QCD QCD thermodynamics I In the grand canonical ensemble, the thermodynamical properties are determined by the (grand) partition function Z(T, V, µ i ) = Tr exp{ 1 T (H i µ i N i )} where k B = 1, H is the Hamiltonian and N i and µ i are conserved number operators and their corresponding chemical potentials. The different thermodynamical quantities can be obtained from Z P = T ln Z V, S = (T ln Z) T Expectation values can be computed as, N i = T ln Z µ i O = TrO exp{ 1 T (H i µ in i )} Tr exp{ 1 T (H i µ in i )} Frascati, May 2006 QGP and HIC p.16 20

45 QCD QCD thermodynamicsii II In order to obtain Z for a field theory with Lagrangian L one normally makes the change it = 1/T, with this, the action 1/T is i dtl S = dτl E 0 and the grand canonical partition function can be written (for QCD) as 1/T Z(T, V, µ) = D ψdψda µ exp{ dx 0 d 3 x(l E µn )}, where N ψγ 0 ψ is the number density operator associated to the conserved net quark (baryon) number. Additionally, (anti)periodic boundary conditions in [0, 1/T ] are imposed for bosons (fermions) A µ (0, x) = A µ (1/T, x), ψ(0, x) = ψ(1/t, x) 0 V Frascati, May 2006 QGP and HIC p.17 21

46 QCD thermodynamics III In order to solve these equations Perturbative expansion (the coupling constant is small at large T) Lattice QCD Discretization in (1/T, V) space Contributions to the partition function are computed by random configurations of the fields Observables computed by averaging over these configurations Powerful computers are needed! 22

47 First First example: example: equation Equation of state of State (EoS) Naïve estimation:let s fix µ = 0, the pressure of an ideal gas (of massless particles) is proportional to the number of d.o.f: P NT 4. So, P π 3 T 4 ; P QGP ( ) T } {{ } } {{ } 4 quarks gluons So, one expects a large difference (factor 10) between the two phases. Lattice results (Karsch et al.) p/t p SB /T 4 [MILC Collaboration 2006] flavour 2+1 flavour 2 flavour T/T c Frascati, May 2006 QGP and HIC p.19 23

48 EoS and experiments In an ideal gas: ɛ SB = 3P 16 14!/T 4 RHIC! SB /T SPS 3 flavour 2 flavour 2+1-flavour T c = (173 +/- 15) MeV! c ~ 0.7 GeV/fm 3 LHC T [MeV] (Karsch et al.) Frascati, May 2006 This is just an estimate! QGP and HIC p.20 24

49 Phase transition: order parameters In order to know whether the change from a hadron gas to a QGP is a phase transition or a rapid cross-over order parameters are needed First order: discontinuity in the order parameter 25

50 Phase transition: order parameters In order to know whether the change from a hadron gas to a QGP is a phase transition or a rapid cross-over order parameters are needed Second order: discontinuity in the derivative 25

51 Phase transition: order parameters In order to know whether the change from a hadron gas to a QGP is a phase transition or a rapid cross-over order parameters are needed Cross-over: continuous function 25

52 x µ = 0]. Order parameters in QCD I Chiral symmetry restoration: for symmetry restoration. For m q = 0 the order parameter is the condensate chiral condensate is the order parameter 0 q L q R 0 0 Susceptibility: T χ m = m q = 0 0 q L q R 0 = 0 m q qq [Aoki et al 2006] May 2006 QGP and HIC p.23 26

53 Order parameters in QCD II Confinement: for m q the order parameter is the potential V (r) = A(r) r + Kr 14 V(R,T)/T 0 V(R)/T ! ! R T R T T < T c T > T c [Kaczmarek et al 2000] 27

54 However... When masses are taken into account the potential is screened even below T c V(r)/!" PG m=1.0 m=0.6 m=0.4 m=0.2 m=0.1 m=0.05 m= r!" [Karsch, Laermann, Peikert 2001] Light qq pair creation breaks the string 28

55 Finite baryochemical potential Lattice calculations very challenging at finite µ B [Fodor et al 2004] µ B Order of the transition depends on Possible critical point at experimental reach Still a lot of uncertainties exist 29

56 Where are the heavy-ion collisions? Statistical models fit particle abundances and obtain (T, µ B ) at freeze-out 200 T [MeV] µ q / T = T c, Bielefeld-Swansea T c, Forcrand, Philipsen (T c, µ c ), Fodor, Katz µ B [GeV] T f, J.Cleymans et. al (model dependent) 30

57 How can the created medium be characterized? Signatures Find an observable which depends on the properties of the medium 31

58 Soft probes Hydrodynamic behavior Check the degree of thermalization of the system Strangeness enhancement Remember that the chiral symmetric phase, up, down and strange quarks are expected to be almost massless Enhancement of strangeness production expected NA57 (Padova!) 32

59 A Asimple simple example: example: J/Ψ J/Ψsuppression Hard probes: charmonium suppression A J/Ψ is a c c bound state. A J/Ψ is a c c bound state. σ hh J/Ψ σ hh J/Ψ = f = i (x f 1, Q i (x 2 ) f 1, Q 2 ) f j (x 2, Q j (x 2 ) σ 2, Q 2 ) σ ij [c c] ij [c c] (x 1, x (x 2, Q 1 x 2 ) O([c c] 2, Q 2 J/Ψ) ) O([c c] J/Ψ) The potential is screened by the medium The potential is screened by the medium The long-distance part is modified O([c c] J/Ψ) 0 The long-distance part is modified O([c c] J/Ψ) 0 The J/Ψ production is suppressed [Matsui, Satz 1986] The J/Ψ production is suppressed [Matsui, Satz 1986] Kyoto, November 2006 Hard Probes to QGP p.6 Kyoto, November 2006 Introduzione alle collisioni nucleo-nucleo Hard Probes to QGP ad LHC p.6 33

60 A Asimple simple example: example: J/Ψ J/Ψsuppression Hard probes: charmonium suppression A J/Ψ is a c c bound state. A J/Ψ is a c c bound state. σ hh J/Ψ σ hh J/Ψ = f = i (x f 1, Q i (x 2 ) f 1, Q 2 ) f j (x 2, Q j (x 2 ) σ 2, Q 2 ) σ ij [c c] ij [c c] (x 1, x (x 2, Q 1 x 2 ) O([c c] 2, Q 2 J/Ψ) ) O([c c] J/Ψ) The potential is screened by the medium The potential is screened by the medium The long-distance part is modified O([c c] J/Ψ) 0 The long-distance part is modified O([c c] J/Ψ) 0 The J/Ψ production is suppressed [Matsui, Satz 1986] The J/Ψ production is suppressed [Matsui, Satz 1986] Kyoto, November 2006 Hard Probes to QGP p.6 Kyoto, November 2006 Introduzione alle collisioni nucleo-nucleo Hard Probes to QGP ad LHC p.6 33

61 A Asimple simple example: example: J/Ψ J/Ψsuppression Hard probes: charmonium suppression A J/Ψ is a c c bound state. A J/Ψ is a c c bound state. σ hh J/Ψ σ hh J/Ψ = f = i (x f 1, Q i (x 2 ) f 1, Q 2 ) f j (x 2, Q j (x 2 ) σ 2, Q 2 ) σ ij [c c] ij [c c] (x 1, x (x 2, Q 1 x 2 ) O([c c] 2, Q 2 J/Ψ) ) O([c c] J/Ψ) The potential is screened by [Enrico the medium Scomparin QM2006] The potential is screened by the medium The long-distance part is modified O([c c] J/Ψ) 0 The long-distance part is modified O([c c] J/Ψ) 0 The J/Ψ production is suppressed [Matsui, Satz 1986] The J/Ψ production is suppressed [Matsui, Satz 1986] Kyoto, November 2006 Hard Probes to QGP p.6 Kyoto, November 2006 Introduzione alle collisioni nucleo-nucleo Hard Probes to QGP ad LHC p.6 33

62 Jet What quenching is a jet? Islamabad, March 2004 HIC and the search for the QGP - 4. Status. p.28 34

63 What is a jet? Jet quenching high-pt partons produced with large virtuality (this is the short distance part) Islamabad, March 2004 HIC and the search for the QGP - 4. Status. p.28 34

64 What is a jet? Jet quenching virtuality is reduced by gluon radiation Islamabad, March 2004 HIC and the search for the QGP - 4. Status. p.28 34

65 What is a jet? Jet quenching virtuality is reduced by gluon radiation these radiated gluons form a cone: jet Islamabad, March 2004 HIC and the search for the QGP - 4. Status. p.28 34

66 Jet What quenching is a jet? What happens when this evolution takes place in the medium created in the collision?? Similar to a scattering experiment with the produced medium Islamabad, March 2004 HIC and the search for the QGP - 4. Status. p.28 35

67 R AA at 200 GeV p t Effects on high- particles Direct γ, π 0 and η in Au+Au R AA = dn AA /dp t N coll dn pp /dp t Direct γ R AA with measured p+p reference! 0-10% central events => R AA of η and π 0 consistent, both show suppression => R AA of γ is smaller than 1 at very high p T Photons don t interact (no effect) quarks and gluons do (suppression) 36

68 R AA at 200 GeV p t Effects on high- particles Direct γ, π 0 and η in Au+Au R AA = dn AA /dp t N coll dn pp /dp t Direct γ R AA with measured p+p reference! 0-10% central events photons Calibration mesons => R AA of η and π 0 consistent, both show suppression => R AA of γ is smaller than 1 at very high p T Photons don t interact (no effect) quarks and gluons do (suppression) 36

69 Summary LHC will continue to study symmetries at the fundamental level Heavy-ion collisions will study collective behavior of QCD QCD vacuum Confinement and chiral symmetry breaking Other states exists where symmetries are restored: Quark gluon plasma Signatures in heavy-ion collisions (some...) Hydrodynamic behavior (check degree of thermalization) Strangeness enhancement Heavy quarks and charmonium Jet quenching - particles at very large transverse momentum 37

70 Influence of the quark masses Two order parameters Influence of the quark masses (µ Two order parameters: m q = 0 Chiral condensate m q = Potential For physical masses, most likely cross-over Frascati, May 2006 For physical masses, most likely c 38