Direct Photons in Heavy-Ion Collisions

Transcription

1 Direct Photons in Heavy-Ion Collisions Outline Bjørn Bäuchle Table of contents Goethe-Uni Frankfurt, FIAS Disputation 13. Dezember 21 Motivation & Introduction The Model UrQMD, γ Results: Spectra and channels Summary Conclusions

2 Matter at Small Scales macroscopic Matter > 1 3 m Crystal Atoms Nucleus Nucleon 1 8 m 1 1 m Partons 1 14 m 1 15 m Goals of heavy ion physics < 1 2 m Find collective behaviour of nuclear matter Explore & test phase diagram of QuantumChromoDynamics

3 Phase Diagram of Strongly Interacting Matter T Quark Gluon Plasma Hadron Gas? Liquid/Gas 922 MeV Colour Super Conductor µ B Well-known: ground state, µ B - and T -behaviour

4 Phase Diagram of Strongly Interacting Matter T Quark Gluon Plasma µ B =?, T =?: Critical End Point µ B =, T 17 MeV: Cross-over Hadron Gas high µ B : Quarkyonic Matter? 1 st Order PT? Liquid/Gas 922 MeV Colour Super Conductor µ B Well-known: ground state, µ B - and T -behaviour Speculative: matter close to T C, high-µ B

5 General Experimental Setup Large nuclei (= heavy ions) are collided at high energies Something happens Debris is detected in the experiments Issues How do we get information about the hot phase from debris? Does the matter behave collectively? What are its properties?

6 General Experimental Setup Large nuclei (= heavy ions) are collided at high energies Something happens Debris is detected in the experiments Issues How do we get information about the hot phase from debris? Does the matter behave collectively? What are its properties? Look for non-hadronic particles not limited to late emission

7 Why Direct Photons? Interesting scattering in fireball Hadronic scattering products rescatter: information lost. Photons do not rescatter keep information! Plenty uninteresting photon sources Direct Photons......all photons which do not come from hadronic decays

8 UrQMD Classical propagation of hadrons QM scattering cross-sections UrQMD Ultra-Relativistic Quantum Molecular Dynamics Cross-sections fitted to data or calculated via detailed balance or parametrized via additive quark model Hard processes modelled by PYTHIA All hadrons from PDG up to m = 2.2 GeV Full microscopic collision history available

9 UrQMD+Hydro Microscopic initial state from transport (UrQMD) Map densities to hydrodynamic grid when nuclei have passed through each other Hydro propagation with variable Equation of State Back to transport when ǫ < ǫ crit in all cells at the same z. Rescatterings and decays with UrQMD

10 Equations of State Hadron Gas (HG-Eos) Includes all particles from UrQMD No phase transition Chiral Model (χ-eos) Chirally restored phase Only cross-over, T C 175 MeV Bag Model (BM-EoS) MIT Bag Model First order phase transition at T C = 17 MeV Below T c as Hadron Gas EoS

11 Photons from the model Both models π +π γ +ρ, π +ρ γ +π Only Cascade Cross-sections from Kapusta, Lichard & Seibert (PRD 44 (1991) 2774) π +π γ +η, π +η γ +π, π +π γ +γ Only Hydro Parametrizations from Turbide, Rapp & Gale (PRC 69 (24) 1493) π +K γ +K, π +K γ +K, ρ+k γ +K, K +K γ +π QGP-emission in χ-eos and BM-EoS-calculations And pqcd Photons from primordial N+N-scatterings are incoherently added!

12 The Goal of this Thesis Given the different versions of the model, what are the direct photon spectra from heavy ion collisions? how do they differ? how do the results compare to experimental data? what can we say about the matter from these calculations? what can this model say about the origin of direct photons?

13 Direct Photon Spectra: SPS w/o pqcd WA98 Pb+Pb 158 AGeV Cascade HG-EoS BM-EoS χ-eos 1 1 E dn d 3 p [GeV 2 ] Clear separation: pure hadronic HG, Cascade vs. hadronic+partonic χ, BM χ, BM hit data HG, Casc. undershoot data Pb+Pb Elab = 158 AGeV b < 4.5 fm ycm <.5 no pqcd p [GeV] Data: PRL 85, (2) 3595

14 E dn d 3 p [GeV 2 ] Direct Photon Spectra: SPS w/o pqcd Pb+Pb Elab = 158 AGeV b < 4.5 fm ycm <.5 no pqcd WA98 Pb+Pb 158 AGeV Cascade HG-EoS BM-EoS χ-eos p [GeV] Clear separation: pure hadronic HG, Cascade vs. hadronic+partonic χ, BM χ, BM hit data HG, Casc. undershoot data Misses pqcd-photons! Data: PRL 85, (2) 3595

15 Direct Photon Spectra: SPS with pqcd E dn d 3 p [GeV 2 ] Pb+Pb Elab = 158 AGeV b < 4.5 fm ycm <.5 with pqcd (Turbide et al.) WA98 Pb+Pb 158 AGeV Cascade HG-EoS BM-EoS χ-eos p [GeV] Not so clear separation: pure hadronic HG, Cascade vs. hadronic+partonic χ, BM χ, BM overshoot data (slightly) HG, Casc. hit data (kind of) Data: PRL 85, (2) 3595 pqcd: Turbide et al., PRC 69 (24) 1493

16 Direct Photon Spectra: RHIC UrQMD + pqcd Hybrid, HG-EoS + pqcd Hybrid, χ-eos + pqcd Hybrid, BM-EoS + pqcd pqcd Au+Au snn = 2 GeV E dn d 3 p [GeV 2 ] % % 1 Low-p -data from PHENIX Clear separation even with pqcd! χ, BM almost on data HG, Cascade way below Data: PRC 81 (21) pqcd: Gordon, Vogelsang, PRD 48 (1993) p [GeV]

17 Direct Photon Spectra: FAIR E dn d 3 p [GeV 2 ] U+U b < 5 fm Elab = 35 AGeV ycm <.5 Cascade Hybrid, HG-EoS Hybrid, χ-eos Hybrid, BM-EoS p [GeV] Prediction for FAIR No pqcd FAIR Possible to distinguish Same separation as before Yield low hard to measure

18 Channel composition: SPS Inclusive πρ γπ ππ γx πη γπ 1 2 E dn [GeV 2 ] dp Pb+Pb E lab = 158 AGeV y cm <.5 b < 4.5 fm Only cascade (no hydro) πρ dominates at p >.5 GeV ππ dominates at low p, dominates inclusive spectra! At high p, all contribute equal: strings! p [GeV]

19 Channel composition II p [GeV] Fraction of emission from this channel [%] FAIR SPS RHIC Hadron Gas-EoS πρ γπ ππ γx πη γπ πk γk K-resonance Elab= 35 AGeV U+U b < 5 fm Elab= 158 AGeV Pb+Pb b < 4.5 fm snn= 2 GeV Au+Au -2 % Hybrid calculation HG-EoS String contribution (high-p ) not at FAIR K-channels subleading Same for other EoS s p [GeV]

20 Summary What I didn t say (ask if interested) Cross-sections, rates; direct photon extraction methods; parameter tests; elliptic flow; baryon densities; emission times What I did say (a.k.a. The Summary) Successful hadronic model extended to photons Partonic channels enhanced w.r.t. hadronic Situation at SPS unclear due to pqcd At RHIC, partonic contributions necessary FAIR will allow to distinguish πρ biggest hadronic contribution

21 Thanks to all who supported me: Marcus, Horst, Henner; Johannes, Luisa, Stefan, Ulli; Marieluise, Peter, Melanie; Elvira, Marlene, Christoph, Gunnar, Thomas, Thomas, Martin, Jan; Alex, Thomas, Gabi, Guido, Christian; die Fachschaft, das Orchester und alle anderen. (And my sincerest apologies to my committee for this line) HGS-HIRe Helmholtz Graduate School for Hadron and Ion Research

22 Part I Backup-Slides

23 Total cross-section in UrQMD σ [mbarn] σ(π + π strings ) σ(π + π resonance ) σ(π + π hard scattering) 1 1 s [GeV] 1

24 Freeze-out distributions ttrans tstart [fm] Elab = 35 AGeV Elab = 158 AGeV snn = 2 GeV z [fm] z [fm] HG-EoS χ-eos BM-EoS 2 3 Pb+Pb b = fm 3 4 4

25 Distributions δ(λ λqgp)εd 4 x/ εd 4 x [%] Elab = 35 AGeV Elab = 158 AGeV snn = 2 GeV λ [%] 5 5 λ [%] χ-eos BM-EoS 8 9 Pb+Pb b = fm d 4 x(t) dn d 4 x (T) Elab = 35 AGeV Elab = 158 AGeV snn = 2 GeV 13 T [MeV] T [MeV] 22 HG-EoS χ-eos (hadronic) χ-eos (partonic) BM-EoS Pb+Pb b = fm

26 Origin of photons Photons in A+A Direct Photons Decay Photons. hard pre-/outofequilibrium thermal. prompt fragmentation QGP HG jet-γ-conversion medium-induced bremsstrahlung jetmedium mediummedium

27 Direct Photon Experiments Helios, WA 8, CERES (SPS) 1 upper limits WA 93 (SPS) no results WA 98 2 first measurements at SPS PHENIX 3 (RHIC) various results STAR preliminary results 1 Helios: Z. Phys. C 46, 369 (199); WA 8: Z. Phys. C 51, 1 (1991); PRL 76, 356 (1996); CERES: Z. Phys. C 71, 571 (1996) 2 PRL 85, 3595 (2) 3 e.g. PRL 94, (25)

28 Direct Photon Experiments Helios, WA 8, CERES (SPS) 1 upper limits WA 93 (SPS) no results WA 98 2 first measurements at SPS PHENIX 3 (RHIC) various results STAR preliminary results Photons from hadronic decays make 97 % of all photons Uncertainties in hadron yield significantly change direct photon yield EM-Calorimeters expensive, coverage therefore usually small 1 Helios: Z. Phys. C 46, 369 (199); WA 8: Z. Phys. C 51, 1 (1991); PRL 76, 356 (1996); CERES: Z. Phys. C 71, 571 (1996) 2 PRL 85, 3595 (2) 3 e.g. PRL 94, (25)

29 Experimental methods I: Subtraction method Principle Count number of π and η by invariant mass of γγ Other particles that decay to γ from m -scaling Subtract with double ratio R: γ direct = (1 R 1 )γ all ; ( ) γ all γ all R = γ background = /π ( ) measured γ all /π calculated Pros Relatively simple Ansatz m sampling easy WA98, PHENIX Cons Large Signal/Background-ratio, Background in high precision

30 Experimental methods II: HBT Principle π and η decay far outside 2-particle correlation function of decays narrow experimentally resolvable 2-particle corr. func. must come from direct photons get number of direct photons from its magnitude Pros Get rid of decay photons by cut WA98 Cons Only possible for very small p Decays inside or near fireball break method systematics hard to get PHENIX

31 Experimental methods III: Low-mass dileptons Principle If γ is possible, so is γ, γ e + e Number of photons N γ vs. number of dileptons N ee with mass M: d 2 N ee dm = 2α ( ) 1 1 4m2 e 3π M M m2 e M 2 SdN γ S: process dependent factor ( theory!), m e : Mass of e. Pros No need to measure γ directly independent! lower p than subtraction Cons Limited to M m η, p M Decays inside or near fireball break method

32 Photon production cross-sections 2m π m η+mπ m ρ+mπ m a1 m ρ +m π m a1 1 1 π ± π γρ π ± ρ γπ ± 1 π ± π γρ ± π ± ρ γπ π ± π γη π ρ ± γπ ± π ± η γπ ± πρ a 1 γπ 1 π ± π γγ 1 1 σ [mb].1 m ρ Breit-Wigner.1 m ρ fixed.1.1 σ [mb] s [GeV]

33 Photon production rates 1 dn T 3 d 4 x [fm 4 GeV 3 ] QGP / 3 hadronic rates πρ γπ ππ γρ πk γk πk γk ρk γk KK γπ T [MeV]

34 Comparison of rates: Transport vs. Hydro dr de [GeV 2 fm 4 ] 1 E 1 2π T = 15 MeV Box: V =(2 fm) 3, π/ρ/a 1 -gas Box calculations (UrQMD) thermal rates (Turbide et al.) ππ γρ πρ γπ E [GeV]

35 Comparison of yields: Transport vs. Hydro E dn d 3 p [GeV 2 ] Pure UrQMD Hybrid, Hadron Gas EoS p [GeV] Only common channels Pb+Pb 158 AGeV b < 4.5 fm y cm <.5

36 Influence of string ends UrQMD w/o string ends UrQMD with string ends pqcd-photons (Wong et al.) pqcd-photons (Turbide et al.) E dn d 3 p [GeV 2 ] E lab = 158 AGeV b < 4.5 fm, y cm < p [GeV]

37 ρ-mass (on-off)/off E dn d 3 p [GeV 2 ] p [GeV].6.8 m ρ fixed m ρ Breit-Wigner π ± π γρ ± π ± π γρ 1.4p [GeV].6 UrQMD Pb+Pb 158 AGeV b < 4.5 fm y cm <.5 π ± π γρ ± π ± π γρ

38 Previous work: Dumitru et al Dumitru et al. (UrQMD v1.) UrQMD v1.3, this work UrQMD v2.3, this work E dn d 3 p [GeV 2 ] p [GeV]

39 yield/standard setup Hydro-parameter I: t start Yield at p = 1 GeV Yield at p = 2 GeV Yield at p = 3 GeV AGeV t start [t ] t start [t ] t start [t ]

40 yield/standard setup Hydro-parameter II: ε crit Yield at p = 1 GeV Yield at p = 2 GeV Yield at p = 3 GeV ε crit [ε ] ε crit [ε ] ε crit [ε ] AGeV

41 Hydro-parameter III: Transition 2 yield/standard setup s NN = 2 GeV E lab = 45 AGeV E lab = 8 AGeV p [GeV] p [GeV] Elab = 158 AGeV

42 Direct Photon Spectra: SPS peripheral WA98 Pb+Pb 158 AGeV Cascade Cascade + pqcd 2 E dn d 3 p [GeV 2 ] Peripheral collisions: No hydro dominated by pqcd thermal contributions very small (quality of data only semi-good) pqcd: Turbide et al., PRC 69 (24) Pb+Pb Elab = 158 AGeV b > 12.5 fm ycm <.5 with pqcd (Turbide et al.) p [GeV]

43 Direct Photon Spectra: RHIC high-p E dn d 3 p [GeV 2 ] PHENIX-data UrQMD UrQMD+pQCD BM-EoS+pQCD Au+Au snn = 2 GeV min. bias 1 3-1% 1 1-2% % % % 1 8 High-p data from PHENIX: direct measurement everywhere: pqcd fits data (or upper limits, respectively) cascade much lower than pqcd Bag model fits central data (see next slide) % % p [GeV] pqcd: Gordon, Vogelsang, PRD 48 (1993) 3136

44 RHIC: lower energies, Cu+Cu E dn d 3 p [GeV 2 ] snn = 62.4 GeV snn = 13 GeV Au+Au Hybrid, HG-EoS Hybrid, χ-eos Hybrid, BM-EoS Upper curves: -2% 1 Lower curves: 2-4% 1 snn = 2 GeV 1 E dn d 3 p [GeV 2 ] Cu+Cu p [GeV] p [GeV] p [GeV] 3

45 Direct Photon flow: RHIC -2 % (p ) [%] v γ % Au+Au Cu+Cu Au+Au -2 % Cu+Cu -2 % -5-6 snn = 2 GeV -6 8 ycm < Cascade -4-6 Hybrid, HG-EoS -6 Hybrid, χ-eos -8 Hybrid, BM-EoS -8 Au+Au 2-4 % Cu+Cu 2-4 % p [GeV] p [GeV] Elliptic flow: 2 nd coefficient from azimuthal Fourier expansion Photons flow with matter until high p Large fluctuations at high p More flow in peripheral collisions Less flow in Cu+Cu Only thermal photons, lacks pqcd. Error bars in hydro calculations similar to cascade

46 Direct Photon flow: RHIC 2-4 % v γ 2 (p ) [%] -2 % Au+Au Cu+Cu Au+Au Cu+Cu Au+Au -2 % Cu+Cu -2 % -4-5 snn = 13 GeV -5 8 ycm < Cascade -4-6 Hybrid, HG-EoS -6 Hybrid, χ-eos -8 Hybrid, BM-EoS -8 Au+Au 2-4 % Cu+Cu 2-4 % p [GeV] p [GeV] -2 % (p ) [%] v γ % Au+Au -2 % Cu+Cu -2 % -4 snn = 62.4 GeV -4 8 ycm < Cascade -4-6 Hybrid, HG-EoS Hybrid, χ-eos -6-8 Hybrid, BM-EoS -8 Au+Au 2-4 % Cu+Cu 2-4 % p [GeV] p [GeV]

47 Direct Photon flow: SPS & FAIR SPS FAIR Cascade HG-EoS χ-eos BM-EoS 8 6 Cascade HG-EoS χ-eos BM-EoS v γ 2 (p )[%] Elab = 158 AGeV b < 4.5 fm, ycm < p [GeV] v γ 2 (p )[%] Elab = 35 AGeV b < 5 fm, ycm < p [GeV] N.B.: Uranium nuclei assumed spherical

48 Fraction of emission from this channel [%] Hadronic channel decomposition: χ & BM p [GeV] Chiral EoS πρ γπ ππ γx πη γπ πk γk K-resonance Elab= 35 AGeV U+U b < 5 fm Elab= 158 AGeV Pb+Pb b < 4.5 fm snn= 2 GeV Au+Au -2 % Fraction of emission from this channel [%] p [GeV] Bag Model EoS πρ γπ ππ γx πη γπ πk γk K-resonance Elab= 35 AGeV U+U b < 5 fm Elab= 158 AGeV Pb+Pb b < 4.5 fm snn= 2 GeV Au+Au -2 % p [GeV] p [GeV]

49 Fraction of emission from this channel [%] Channel decomposition Hadronic/Partonic/pQCD Chiral EoS p [GeV] prompt γ QGP emission hadronic emission Elab= 35 AGeV; U+U; b < 5 fm 3.5 Elab= 158 AGeV Pb+Pb b < 4.5 fm snn= 2 GeV Au+Au -2 % Bag Model EoS 8 Fraction of emission from this channel [%] p [GeV] prompt γ QGP emission hadronic emission 3.5 Elab= 35 AGeV U+U b < 5 fm Elab= 158 AGeV Pb+Pb b < 4.5 fm snn= 2 GeV Au+Au -2 % p [GeV] p [GeV]

50 E dn d 3 p [GeV 2 ] Stages contributions FAIR & SPS total U+U 1 1 early b < 5 fm 1 1 intermediate 1 Elab = 35 AGeV late ycm < Cascade only Hybrid, HG-EoS Hybrid, χ-eos Hybrid, BM-EoS p [GeV] p [GeV] E dn d 3 p [GeV 2 ] total Pb+Pb 1 early b < 4.5 fm intermediate 1 Elab = 158 AGeV late ycm < Cascade only Hybrid, HG-EoS Hybrid, χ-eos Hybrid, BM-EoS p [GeV] p [GeV]

51 High p E dn d 3 p [GeV 2 ] inclusive q coll > 3 GeV scoll > 4 GeV UrQMD Elab = 158 AGeV b < 4.5 fm ycm <.5 [GeV 1 ] dn d scoll inclusive p γ > 3 GeV q coll > 3 GeV UrQMD Elab = 158 AGeV b < 4.5 fm ycm < dn d 3 q coll [GeV 2 ] q coll p γ [GeV] 1 inclusive 1 1 p γ > 3 GeV scoll > 4 GeV UrQMD Elab = 158 AGeV b < 4.5 fm ycm < scoll [GeV] q coll [GeV]

52 Times FAIR temission [fm] Fraction of photons from this time [%] U+U b < 5 fm Elab = 35 AGeV all channels πρ γπ ππ γx πη γπ ycm < p [GeV] < tem < 5 fm 5 < tem < 1 fm 1 < tem < 15 fm 15 < tem < 2 fm 2 fm < tem Transport only U+U Elab = 35 AGeV b < 5 fm, ycm < p [GeV] dn dt [fm 1 ] Transport only Elab = 35 AGeV U+U b < 5 fm 1 ycm < t [fm] p <.5 GeV.5 < p < 1.5 GeV 1.5 < p < 2.5 GeV 2.5 < p < 3.5 GeV

53 Times SPS temission [fm] Fraction of photons from this time [%] all πρ γπ ππ γx πη γπ 158 AGeV b < 4.5 fm ycm <.5 Transport only p [GeV] < tem < 5 fm 5 < tem < 1 fm 1 < tem < 15 fm 15 < tem < 2 fm 2 fm < tem Transport only Pb+Pb Elab = 158 AGeV b < 4.5 fm, ycm < p [GeV] dn dt [1 3 fm 1 ] t3 t2 t < p < 1.5 GeV 2 < p < 2.5 GeV 3 < p < 3.5 GeV Only πρ γπ 158 AGeV b < 4.5 fm ycm <.5 Transport only t [fm]

54 Times 2 AGeV I Cu+Cu -2 % Au+Au -2 % Cu+Cu -2 % 1 1 snn = 2 GeV ycm < temission [fm] Au+Au -2 % 3 all πρ γπ 3 ππ γx πη γπ Au+Au 2-4 % Cu+Cu 2-4 % snn = 2 GeV 12 ycm < dn dt [fm 1 ] p <.5 GeV < p < 1.5 GeV Au+Au 2-4 % Cu+Cu 2-4 % < p < 2.5 GeV p > 2.5 GeV p [GeV] p [GeV] t [fm] t [fm]

55 Times 2 AGeV II Fraction of photons from this time [%] Au+Au -2 % Cu+Cu -2 % Au+Au 2-4 % Cu+Cu 2-4 % snn = 2 GeV 9 8 ycm < < tem < 5 fm 6 5 < tem < 1 fm 5 1 < tem < 15 fm 5 15 < tem < 2 fm 4 2 < tem < 3 fm 4 tem > 3 fm p [GeV] p [GeV]

56 Densities dn dρb [ρ 1 ] 1 1 all p 1 1 < p < 2 GeV 2 < p < 3 GeV p > 3 GeV Elab = 35 AGeV U+U b < 5 fm y < Au+Au -2 % 1 Cu+Cu -2 % snn = 2 GeV 1 ycm < ρb[ρ] dn dρb [ρ 1 ] 1 2 all p < p < 2 GeV 2 < p < 3 GeV 1 p > 3 GeV Elab = 158 AGeV Pb+Pb b < 4.5 fm y <.5 dn dρb [ρ 1 ] Au+Au 2-4 % Cu+Cu 2-4 % 1 all p.5 < 1 p < 1.5 GeV 1.5 < 1 1 p < 2.5 GeV 2.5 < p < 3.5 GeV < p < 4.5 GeV 1 2 p > 4.5 GeV ρb[ρ] ρb[ρ] ρb[ρ]

57 dσ dcosϑ 1 2σ Angular distributions: symmetric channels π π ± π ± π s = m ρ π ± π s m η π ± π γρ ± π ± π γγ s = mρ 3 s s s = 2mπ = 2 GeV =.7 GeV π s > mρ s = 1 GeV s = 1 GeV π ±

58 dσ dcosϑ 1 2σ Angular distributions: non-symmetric channels ρ / π π ± ρ γπ ± π ± ρ γπ s s = mπ + m ρ s s = mπ + m ρ = 1 GeV = 1 GeV s = 1.5 GeV s = 1.5 GeV π ρ ± γπ ± π ± η γπ ± s = mπ + m η m s π + m η 3.5 = 1. GeV s = 1.5 GeV 3. s = 1.5 GeV ρ / π