1. 14 Jan 2011 1 Adam Kisiel – CERN Pion femtoscopy at the LHC in p+p collisions at √s=0.9 and 7 TeV and Pb+Pb collisions at 2.76 ATeV Adam Kisiel CERN Politechnika Warszawska

2. 14 Jan 2011 2 Adam Kisiel – CERN Outline Physics motivation: – Femtoscopy: measuring the size of the hadron collision – Lessons from RHIC: k T dependence of the 3D radii as a signature of collective flow – How is p+p different from A+A? ALICE experiment – Data taking conditions – Event sample characteristics Results – Identifying scaling variables for the system size – Triply-differential analysis of the system sizes – Testing the multiplicity scaling of the radii – Comparing p+p and A+A results First results from the Pb+Pb run of the LHC

3. 14 Jan 2011 3 Adam Kisiel – CERN Heavy Ion collision evolution at RHIC time M. Chojnacki, W. Florkowski, PRC 74 (2006) 034905 Transverse plane HIC is expected to go through a Quark Gluon Plasma phase, where matter is strongly interacting – resulting in the development of collective motion Radial flow dominates, with elliptic flow as azimuthal modification

4. 14 Jan 2011 4 Adam Kisiel – CERN Which collectivity do we seek? A collective component is a “common” velocity for all particles emitted close to each other To that one adds “thermal” (random) velocity We expect specific “common” velocity – radial direction, pointing outwards from the center

5. 14 Jan 2011 5 Adam Kisiel – CERN Describing collectivity: hydrodynamics Hydrodynamics produces collective flow: common velocity of all particles The process drives the space-time evolution of the system The observables in the momentum sector: particle spectra, elliptic flow are well described with a variety of model parameters. Chojnacki M., Florkowski W. nucl-th/0603065, Phys. Rev. C74: 034905 (2006) Phys.Rev.C79:014902,2009 flow velocity

6. 14 Jan 2011 6 Adam Kisiel – CERN Measuring space-time extent: femtoscopy r |Ψ ( q,r)| 2 r1r1 r2r2 q=p 1 -p 2 p1p1 p2p2 Femtoscopy uses the correlation between two particles, reflected in the pair wave function Ψ. It comes from a combination of the quantum statistics (anti-)symmetrization, Coulomb and strong interactions. Using the Koonin-Pratt equation, one tries to learn as much as possible about the source (i.e. the source emission function S ), from the measured correlation C. The Koonin-Pratt Equation:

7. 14 Jan 2011 7 Adam Kisiel – CERN Directions and reference frames The analysis is usually performed in two ways: – 1-dimensional analysis vs. the invariant relative momentum q i n v. Can only extract system size averaged over all directions. – In the Longitudinally Co-Moving System in 3 directions: long – the beam axis, out – the pair momentum, side – orthogonal to the others. In LCMS the pair momentum in long vanishes. Gives access to three system sizes in these directions separately. Focus is on the transverse direction, where side is interpreted as “geometrical size” while out has additional components from emission duration. Long is used to extract total evolution time. The 3-dimensional correlation is usually shown either as 1-dimensional slices along the axes or as a set of spherical harmonics components. long out side p1p1 p2p2 q kTkT

8. 14 Jan 2011 8 Adam Kisiel – CERN Final state geometry via femtoscopy Sizes measured in heavy-ion collisions scale with the observed particle density The scaling is linear over a broad range of system types, collision energies and centralities Observed multiplicity is a final state observable, for initial state ones (such as N p a r t ) scaling is not as good M.Lisa et al, Ann. Rev. Nucl. Part. Sci. 55 311 (2005)

9. 14 Jan 2011 9 Adam Kisiel – CERN Dynamics via momentum dependence A particle emitted from a medium will have a collective velocity β f and a thermal (random) one β t As observed p T grows, the region from where such particles can be emitted gets smaller and shifted to the outside of the source velocity direction

10. 14 Jan 2011 10 Adam Kisiel – CERN RHIC Hydro-HBT puzzle First hydro calculations struggle to describe femtoscopic data: predicted too small R s i d e, too large R out – too long emission duration No evidence of the first order phase transition U. Heinz, P. Kolb, hep-ph/0204061 T. Hirano, K. Tsuda, nucl-th/0205043 Phys.Rev.C66:054905,2002.

11. 14 Jan 2011 11 Adam Kisiel – CERN Revisiting hydrodynamics assumptions Data in the momentum sector ( p T spectra, elliptic flow) well described by hydrodynamics, why not in space-time? Usually initial conditions do not have initial flow at the start of hydrodynamics (~1 fm/c) – they should. Femtoscopy data rules out first order phase transition – smooth cross-over is needed Resonance propagation and decay as well as particle rescattering after freeze- out need to be taken into account: similar in effects to viscosity S. Pratt, Phys. Rev. Lett. 102, 232301 (2009) Initial flow Cross-over EOS viscosity

12. 14 Jan 2011 12 Adam Kisiel – CERN Description of the soft sector at RHIC Dynamical model with hydrodynamical evolution, propagation of strong resonances Reproduces spectra, elliptic flow and femtoscopy Initial flow, smooth cross-over phase transition, resonance treatment naturally included Soft sector data at RHIC understood in detail W.Broniowski, W.Florkowski, M.Chojnacki, AK nucl-th/0801.4361; nucl-th/0710.5731

13. 14 Jan 2011 13 Adam Kisiel – CERN What about p+p? Puzzling scaling... STAR reports that 3D HBT radii scale in p+p in a way very similar to Au+Au m T dependence of 3D radii in Au+Au is taken as a signature of a flowing medium Is the scaling between p+p and Au+Au a signature of the universal underlying physics mechanism (collective flow) or a coincidence? STAR, arXiv (1004.0925)

14. 14 Jan 2011 14 Adam Kisiel – CERN First measurement from CMS First measurement of the femtoscopic radii from CMS shows the growth of the emitting system size with multiplicity. The radius is extracted using the exponential fitting function The background is constructed as an average of many methods The dependence on pair momentum is not shown, is mentioned to be flat Results are in agreement with expectations, but quantitative comparisons need more detailed data Phys.Rev.Lett.105:032001,2010.

15. 14 Jan 2011 15 Adam Kisiel – CERN The ALICE detector Not shown: ZDC (at ±116m) L3 Magnet T0/V0 Trigger TPC Tracking, PID (dE/dx) ITS Low p T tracking PID + Vertexing MUON μ-pairs TOF PID TRD Electron ID (TR) HMPID PID (RICH) @ high p T PHOS γ, π 0, jets PMD γ multiplicity ACORDE Cosmic trigger FMD Charged multiplicity Dipole EMCAL γ, π 0, jets h

16. 14 Jan 2011 16 Adam Kisiel – CERN Multiplicity at new energies Multiplicity increase from 0.9 TeV to 2.36 TeV and to 7 TeV greater than most predictions. Old Pythia tunes not reproducing the high tail of the multiplicity distributions. 0.9 TeV: dN ch /dη = 3.81 ± 0.01 (stat.) ± 0.07 (syst.) 2.36 TeV: dN ch /dη = 4.70 ± 0.01 (stat.) ± 0.11 (syst.) 7 TeV: dN ch /dη = 6.01 ± 0.01 (stat.) ± 0.20 (syst.)

17. 14 Jan 2011 17 Adam Kisiel – CERN Inclusive identified p T spectra Analysis ongoing on particle identification at low transverse momentum – necessary ingredient of the answer to the question about system collectivity

18. 14 Jan 2011 18 Adam Kisiel – CERN First look at system size at 0.9 TeV 250k minimum-bias triggered events analyzed (full 2009 sample) ALICE observes the growth of the system size with multiplicity The results are given as a function of pseudorapidity density and can be compared to world systematics The observed trend is qualitatively similar to world data, but quantitative comparisons are complicated by large differences in experimental acceptances and analysis methods R inv (fm) Phys. Rev. D82, 052001 (2010) arXiv:1007.0516

19. 14 Jan 2011 19 Adam Kisiel – CERN Pair momentum dependence First attempt in measuring pair momentum dependence limited by statistics and systematics Dominating systematic effect: appearance of “mini-jet” non-femtoscopic correlations ar large p T Integrated R inv shows flat behaviour, but only if the systematic effects are treated properly Discrepancy with data at lower (STAR) and higher (E735) energy is apparent, but acceptance (in particle momenta and event multiplicities) differs significantly Phys. Rev. D82, 052001 (2010) arXiv:1007.0516

20. 14 Jan 2011 20 Adam Kisiel – CERN ALICE p+p data taking in 2010 ALICE has been taking data continuously since March 2010. A high statistics sample of pp collisions at 0.9 TeV (~8M minimum bias triggers) has been collected About 100M minimum bias triggers at 7 TeV used for this analysis (out of 750M total recorded up to now) Used fully calibrated data collected in June 2010 All analysis done on the distributed computing environment: GRID Analyze primary particles in |η|<1.2, reconstructed by the combination of the ALICE Time Projection Chamber (TPC) and the Inner Tracking System (ITS) Particles identified with the TPC dE/dx method – small contamination by electrons at low p T, kaons at high p T. p T from 130 MeV/c up to 2 GeV/c Excellent two track resolution of the detector makes two- track effects negligible

21. 14 Jan 2011 21 Adam Kisiel – CERN Multiplicity binning ALICE performance √s = 7 TeV ALICE performance √s = 0.9 TeV CERN-PH-EP-2010-083

22. 14 Jan 2011 22 Adam Kisiel – CERN Multiplicity dependence of the correlation function Dependence of the CF on multiplicity visible, not strong Large holes in the acceptance in certain k T bins Consistent behavior for Spherical Harmonics and cartesian CFs Spherical Harmonics Cartesian CERN-PH-EP-2010-083

23. 14 Jan 2011 23 Adam Kisiel – CERN k T dependence of the correlation function Dependence on k T visible, stronger Acceptance holes in the CF depend directly on k T (kinematics cut effect) Additional structures appear in the correlation at higher k T ranges Extraction of the k T dependence of the radii complicated by the need to account for the background Spherical Harmonics Cartesian CERN-PH-EP-2010-083

24. 14 Jan 2011 24 Adam Kisiel – CERN Energy dependence of the correlation function Correlation functions for 0.9 TeV and 7 TeV, for same multiplicity and k T bins similar 3D shape ( C 2 0 and C 2 2 components) also consistent Checked all multiplicity/ k T bins - all show comparable similarity First important finding: the scaling variables are the total event multiplicity and pair momentum. Dependence on collision energy is small in comparison. CERN-PH-EP-2010-083

25. 14 Jan 2011 25 Adam Kisiel – CERN Additional structures in MC and data In MC there is no femtoscopic effect (symmetrization), all structures are a “background” Structures seen in C 0 0 (angle- averaged correlation) and in C 2 0 (slight transverse – longitudinal structure) The non-femtoscopic structures in data and MC are similar, in regions where they can be compared. They are the main source of the systematic uncertainty. CERN-PH-EP-2010-083

26. 14 Jan 2011 26 Adam Kisiel – CERN The baseline in Pythia (Perugia-0) Pythia Perugia-0 tune background fitted a 3D Gaussian peak with equal radii in LCMS Width of the peak varies by only 10% between multiplicity, k T bins Height of the peak grows with k T, falls with multiplicity Background at 0.9 TeV smaller than at 7 TeV CERN-PH-EP-2010-083

27. 14 Jan 2011 27 Adam Kisiel – CERN Better handle on the background Current hypothesis for the background origin: “mini-jets” “Mini-jets” are usually measured with un-triggered and triggered pair correlations vs. pseudorapidity Δη and azimuthal angle difference Δφ There is a direct connection between Δη, Δφ and components of relative momentum q inv Low Δη, Δφ correspond to low q inv (where femtoscopic effect is located), but this relation gets more diluted, as p T sum grows One expects that femtoscopic effect will be seen for identical- charge pairs, but not for non-identical, where only mini-jets and resonance pairs will contribute

28. 14 Jan 2011 28 Adam Kisiel – CERN Identical (positive) Non-identical Characterizing mini-jet correlations “femtoscopic effect” expected only in identical charge combinations, going away with increasing p T sum Mini-jet-like correlations expected to be stronger for non-identical ALICE performance ALICE pp@7TeV

29. 14 Jan 2011 29 Adam Kisiel – CERN Background cross-check – π+ π- Reasonable match between Pythia and data across multiplicity and k T bins Visible resonance structures (non-id not a suitable background for identical pairs, and vice- versa) Details of resonance structure not reproduced – a limitation of resonance treatment in Pythia. CERN-PH-EP-2010-083

30. 14 Jan 2011 30 Adam Kisiel – CERN Fitting a 3D CF The Koonin-Pratt equation: requires the functional form of the source S to be postulated. The 3D Gaussian is the most commonly used form. Coulomb K is factorized out of Ψ, it is then 1+cos(qr). This gives the femtoscopic part of the CF: where the radii R and q can be in Pair Rest Frame (PRF) or LCMS (this study) We need to add the non-femtoscopic background and normalization for the final version of the fit function:

31. 14 Jan 2011 31 Adam Kisiel – CERN 3D Fitting example: Gaussian Background B fixed to the values fitted to MC Fit undershoots at low q (Gaussian not good enough) Width reasonably described Out / side / long radii can be obtained both from SH and cartesian fits CERN-PH-EP-2010-083

32. 14 Jan 2011 32 Adam Kisiel – CERN Gaussian radii at 0.9 and 7 TeV Gaussian radii in LCMS grow with multiplicity, fall with k T Fall with k T very prominent for R long, develops with multiplicity for R out, less pronounced for R side The different evolution of R out and R side reflected in low R out /R side values – p+p vs. A+A scaling not obvious anymore. Radii comparable to STAR (EMCICs fit). Transverse size similar to the one in ALICE at similar multiplicity statistical errors only CERN-PH-EP-2010-083

33. 14 Jan 2011 33 Adam Kisiel – CERN Multiplicity dependence of gaussian radii Test if the data scales linearly with dN/dη 1 / 3 for all energies, separately for each k T bin. Multiplicity scaling is visible for all k T, all directions 0.9 TeV data follows a trend similar to 7 TeV data – confirming the finding with pure correlations comparison The scaling in the side and long directions similar, in out the gradient strongly depends on pair momentum CERN-PH-EP-2010-083

34. 14 Jan 2011 34 Adam Kisiel – CERN ALICE p+p vs. heavy-ions Heavy-ion data at ultra- relativistic energies show dN/dη scaling ALICE p+p does grow linearly with dN/dη 1 / 3, but not with the same offset and slope as heavy-ions Comparison at same dN/dη possible for the first time Violation of the scaling shows that initial geometry does play a role in the final freeze-out shape CERN-PH-EP-2010-083

35. 14 Jan 2011 35 Adam Kisiel – CERN Better functional form The Gaussian source function is clearly oversimplified – the correlation shape is not described in detail One can assume the emission function factorizes: Which factorizes the femtoscopic part of the correlation function Models predict that large fraction of pions comes from resonance decays. Exponential decay time is transformed into space in directions with non-vanishing pair momentum ( out and long ). Combination of many exponents from different resonances gives Lorentzian source function, producing Exponential correlation. Our function of choice is then:

36. 14 Jan 2011 36 Adam Kisiel – CERN 3D non-gaussian fit example Exponential- Gaussian- Exponential fit is best or second-best for all k T /multiplicity bins, χ 2 /Ndof comparable to 1.0 Specific features of the correlation function captured: – Steep rise of the CF at low q – Additional structures in the C 2 0 and C 2 0 SH components Spherical Harmonics Cartesian ALICE performanc e CERN-PH-EP-2010-083

37. 14 Jan 2011 37 Adam Kisiel – CERN Non-gaussian radii at 0.9 and 7 TeV Radii in out and side are not directly comparable to the Gaussian ones. The radii in side are the same as in the Gaussian fit within statistical error. Fit quality (judged by χ 2 /N dof ) is better. The λ parameter is higher by ~0.2 – a confirmation that the postulated functional form describes the true one better. Non-gaussian radii in LCMS grow with multiplicity, fall with k T, all dependencies are the same as for Gaussian radii. After changing the functional form to a better one physics message stays the same: it gives us confidence that the Gaussian form, used for comparisons to other experiments and to heavy-ions, is reasonable statistical errors only CERN-PH-EP-2010-083

38. 14 Jan 2011 38 Adam Kisiel – CERN Cross-checking 1D radii 1D R inv analysis (in Pair Rest Frame) done for comparison with previous ALICE and CMS publications New results consistent with published Development of k T slope with multiplicity also in R inv STAR pp @ 0.2 TeV CERN-PH-EP-2010-083

39. 14 Jan 2011 39 Adam Kisiel – CERN Predictions for collectivity at LHC Specific predictions for LHC: steeper m T dependence of the system size from larger flow, longer evolution gives larger size and the reversal of the azimuthal anisotropy in space-time. The R out /R side ratio is even smaller than at RHIC – due to a longer evolution time the freeze-out shape changes from inside-out to outside-in. LHC RHIC Phys.Rev.C79:014902,2009

40. 14 Jan 2011 40 Adam Kisiel – CERN arXiv:1011.3914 Moving to heavy ions The heavy-ion program in ALICE is progressing quickly. “First” measurements show (in comparison to RHIC): – Significant increase in event multiplicity – Decrease in R A A (stronger “high-p T suppression”) – Elliptic flow: similar v 2 (p T ), but stronger integrated v 2, due to a harder spectrum arXiv:1011.3916 arXiv:1012.1004

41. 14 Jan 2011 41 Adam Kisiel – CERN Correlations in Pb+Pb The correlation functions in Pb+Pb have been measured in the limited sample of events recorded at the beginning of November First analysis was performed only for top 5% central events The pair momentum dependence was the main focus of the paper Specifically the fall of radii with pair momentum as well as the overall increase of radii as compared to RHIC was expected arXiv:1012.4035, accepted for publication

42. 14 Jan 2011 42 Adam Kisiel – CERN Energy dependence Clear increase of the emitting region size and the system lifetime between the largest system to date (cental full energy RHIC) and the central LHC. This means that the “quark-gluon plasma”, if produced, lives longer and in a significantly bigger volume. The dependence with energy follows the multiplicity scaling – this will be tested in more detail with the full centrality dependence from ALICE arXiv:1012.4035, accepted for publication

43. 14 Jan 2011 43 Adam Kisiel – CERN Pair momentum dependence All the expected trends are observed: – Overall increase of the radii as compared to RHIC – Strong dependence of the radii on k T –R out / R side ratio even smaller than the one at RHIC arXiv:1012.4035, accepted for publication

44. 14 Jan 2011 44 Adam Kisiel – CERN Comparing to models The hydrodynamic models which were extensively tuned to reproduce the RHIC data work also at the LHC It is still not trivial to simultaneously reproduce the overall magnitude of the radii and the change between RHIC and the LHC. Only the models which introduce all the features important at RHIC (initial flow, cross- over phase transition, realistic equation of state, full inclusion of resonances) continue to work at the LHC. arXiv:1012.4035, accepted for publication

45. 14 Jan 2011 45 Adam Kisiel – CERN Summary ALICE has measured pion femtoscopic correlation functions at 0.9 TeV and 7 TeV, providing a link between multiplicities in p+p and heavy-ions at RHIC ALICE observes dependence of the correlation function on the multiplicity and strong dependence on the pair momentum Significant non-femtoscopic correlations are present, strong in 0.9 TeV, stronger in 7 TeV, developing with pair momentum, relatively well reproduced by Monte Carlo. Current hypothesis: mini-jet correlations. Obtained correlation functions visibly non-gaussian, combination of Exponential and Lorentzian functions in 3 dimensions necessary to better describe the full behaviour. Taking into account the underlying event correlations, ALICE sees radii growing with multiplicity, approximately linearly, with cube root of event multiplicity, in all 3 dimensions. 3D radii also fall with growing pair momentum. First results from the Pb+Pb run of the LHC show Quark Gluon Plasma that is bigger and lives longer than at RHIC, behaves hydrodynamically.

46. 14 Jan 2011 46 Adam Kisiel – CERN Backup slides

47. 14 Jan 2011 47 Adam Kisiel – CERN Therminator simulation Parameters used for simulation: – System: blast-wave with negative r-t slope (-0.2) – Transverse size of the system 1.5 fm – Emission proper time: 2 fm/c – Temperature: 165.6 MeV Multiplicity Flow profiles Collective momenta

48. 14 Jan 2011 48 Adam Kisiel – CERN p T spectra Exponential only for “no flow”, “no resonances” Flow modifies the spectra shape, the larger the mass, the larger the modification Resonances add complication, both to shape and overall normalization

49. 14 Jan 2011 49 Adam Kisiel – CERN 1D correlation and radii Full simulation (with resonances) gives 1D correlation shape very similar to the one in ALICE data (exponential) Gaussian and exponential shape give consistent radii “No flow” scenario gives radius rising with k T, “strong flow” scenario gives radius falling with k T

50. 14 Jan 2011 50 Adam Kisiel – CERN Understanding the details in 3D Primordial particles: fit with 3D Gaussian R o u t flat with no flow, falls with increasing flow R s i d e develops slope with flow, value changes slightly Both effects are most visible in the R o u t /R s i d e ratio R o u t /R s i d e ratio is less sensitive to the overall system size

51. 14 Jan 2011 51 Adam Kisiel – CERN Resonances and 3D Big change is CF shape: best fit is Exponential-Lorentzian- Lorentzian Resonances also make R o u t fall with k T – not a good signature of flow But the other effect from primordials is preserved: R o u t smaller with flow, R s i d e larger with flow Dramatic signal in R o u t /R s i d e : >1 means no flow, <1 means strong radial flow

52. 14 Jan 2011 52 Adam Kisiel – CERN Comparison to ALICE ALICE data seem to favor strong collectivity in the correlation function Collectivity develops with multiplicity, is stronger for 7 TeV than for 900 GeV no flow strong flow line of equal RMS

53. 14 Jan 2011 53 Adam Kisiel – CERN 3D radii individually Rout grows and then saturates in data. “Strong flow” simulation saturates at a similar value. Rside grows continuously with multiplicity in data. In “no flow” simulation Rside is similar to low multiplicity data, “strong flow” is consistent with higher multiplicity. Rlong in simulations is larger than in data – not surprising taking into account a very simplified dynamics in this direction (boost-invariance). This is the cause for Rinv being higher in simulations than in data. no flow strong flow

54. 14 Jan 2011 54 Adam Kisiel – CERN Components of the system size Femtoscopic size is actually a “variance” of the emission point distribution. The emission points are four-vectors, but the femtoscopic measurement can only access 3 components (the fourth is fixed, since the masses of particles are fixed). The time variance gets incorporated into the directions in which pair velocity does not vanish (out and long): Rout can be smaller than Rside if there is significant correlations between space and time in the emission process

55. 14 Jan 2011 55 Adam Kisiel – CERN Emission shapes vs. time – no flow OUT SID E

56. 14 Jan 2011 56 Adam Kisiel – CERN Emission shapes and times – with flow OUT SID E

57. 14 Jan 2011 57 Adam Kisiel – CERN Resonance decay Primordial From resonances All resonance βrβr β decay =p decay / E β tota l Resonance decay: – Fixed “parent” velocity (coming from flow) – Random decay velocity – Decay momentum matters more for light particles If p decay >m daughter than randomization from decay is strong (loss of x-p correlation), if not: the correlation is preserved

58. 14 Jan 2011 58 Adam Kisiel – CERN Freeze-out and treatment of resonances At freeze-out system is converted to particles via the von Neumann sampling of the Cooper-Frye formula Per-event particle multiplicities randomly generated with averages taken from the chemical model Resonance propagation and decay included after freeze-out, but not rescattering All resonances from the PDG (381 types) included Propagation and decay done particle by particle, in cascades All pion sources compared

59. 14 Jan 2011 59 Adam Kisiel – CERN Influence of resonances In AuAu simulations strong resonances produce two effects: – Source gets bigger by ~1 fm – Source becomes non- gaussian Both of these effects are on the order of a system size expected in pp. Will they dominate the signal? AK, W.Florkowski, W.Broniowski; Phys.Rev.C73:064902,2006 arXiv: nucl-th/0602039 no resonances full calculation

60. 14 Jan 2011 60 Adam Kisiel – CERN How to show 3D space via 1D plots? In traditional 3D cartesian representation one needs to integrate over a range in 1 or 2 directions to get a dependence of the CF on the third direction Statistics is diluted – symmetries of identical pair distributions are not exploited fully In spherical harmonics the 3D space is converted into infinite set of 1D q inv histograms (Y l m components) Pair symmetries make most of them vanish, only three important components remain: C 0 0 (the average correlation function), C 2 0 (difference between transverse and long), C 2 2 (difference between out and side) side long – along the beam side out -along pair momentum

61. 14 Jan 2011 61 Adam Kisiel – CERN Rescattering model p+p  , √ s = 14 TeV Fit to C(q inv ) with: Rescattering model predicts: – Growth of radius with multiplicity – Decrease of radius with pair momentum, more pronounced at higher multiplicity T. J. Humanic, Phys. Rev. C 76, 025205 (2007)

62. 14 Jan 2011 62 Adam Kisiel – CERN Resonance structures: Data vs. MC Different resonance widths ρ is shifted by 40 MeV, f 0, f 2 small in MC – Pythia does not take into account phasespace and rescattering “Underlying event” background similar MC good for baseline for identical pairs, not for non-identical “mixed” pairs “real” pairs ratio “mini-jet” background 1<M<15 k T (0.3- 0.4) 7 TeV data 1<M<15 k T (0.3-0.4) Pythia Coulomb subtraction K0SK0S ω ρ f 0 (?) f 2 (? ) η(?) ALICE performanc e simplified resonance cocktail fit

63. 14 Jan 2011 63 Adam Kisiel – CERN ρ 0 Mass in p+p at RHIC 40 MeV shift position ρ 0 peak measured by STAR in minimum bias p+p Minimum bias p+p 0.6 ≤ p T < 0.8 GeV/c STAR P. Fachini, STAR

64. 14 Jan 2011 64 Adam Kisiel – CERN Systematic error Main contribution to the systematic error is the determination of the underlying background: – Varying the background by +/- 10% – Varying the maximum of the fitting range up/down – Comparing fits to Spherical Harmonics and Cartesian functions – Still to do: Compare Pythia vs. Phojet Full error: ~10% in middle k T bins, 20% at lowest and highest k T bin. Multiply by ~2 at lowest and highest multiplicity.

65. 14 Jan 2011 65 Adam Kisiel – CERN Measurement mass and width wide resonances  exclusive reactions ONLY!  e + e -  π + π - ρ 0 and K *0  mass shift observed in p+p, d+Au, and Au+Au collisions at RHIC by STAR  even after phase space correction ρ 0 mass shift observed by previous experiments  NA27, OPAL, DELPHI, and ALEPH  even after phase space correction Phase space  does NOT reproduce mass shift measured ρ 0 mass shift multiplicity dependent  Evidence for π + π - Rescattering in p+p collisions Phase shift plus rescattering reproduces well STAR data Conclusions from RHIC P. Fachini, STAR

66. 14 Jan 2011 66 Adam Kisiel – CERN TPC entrance separation cut Residual splitting (?) remaining in the first bin after the anti- splitting cuts Include a cut on TPC entrance separation between two pions Need to fine-tune this cut, for now use 5 cm

67. 14 Jan 2011 67 Adam Kisiel – CERN How about rescattering models? Rescattering models also struggle to describe the femtoscopic data Problems similar to hydro: R s i d e too small (but with correct slope), R o u t too large Bleicher et al., nucl-th/0602032

68. 14 Jan 2011 68 Adam Kisiel – CERN Generalized 3D fit function One can assume the emission function factorizes: Which factorizes femtoscopic part of the correlation function Where S-C correspondence is as follows: S C

69. 14 Jan 2011 69 Adam Kisiel – CERN Data analysis 3D function representations: cartesian vs. spherical harmonics Real pairs Mixed pairs Numerator 3D histo Denominator 3D histo Correlation 3D histo For each q fit Y l m coef. Correlation Set of 1D Y l m (q) histos Numerator Set of 1D Y l m histos Denominator Set of 1D Y l m histos Correlation Set of 1D Y l m (q) histos + Y l m -Y l m covariances Phys.Rev.C80:064911,2009. arXiv:0901.3527 [nucl-th] Numerator 3D histo Denominator 3D histo Correlation 3D histo

70. 14 Jan 2011 70 Adam Kisiel – CERN HBT 1D radius 1D R i n v radii (in Pair Rest Frame), done with 1 st paper methods, consistent between published and new 900 GeV data 7 TeV radii similar but slightly larger Growth of R i n v with multiplicity at 7TeV continues

71. 14 Jan 2011 71 Adam Kisiel – CERN Multiplicity dependence in pp and AA Do the data scale with multiplicity in pp in the same way as they do in AA? What does it mean if they do? What does it mean if they do not? STAR pp@200 GeV is separated by a considerable multiplicity gap from the heavy-ion measurements ALICE multiplicity reach in pp @ 7TeV extends to the RHIC heavy-ion data. For the first time we can measure HBT radii in pp and AA at comparable multiplicity. ? ALICE pp @ 7TeV

72. 14 Jan 2011 72 Adam Kisiel – CERN Systematic error estimation Systematic error coming from the background estimation evaluated with: – Changing the width and magnitude by +/- 10% – Using Pythia and Phoject Change fitting range ALICE performanc e ALICE performanc e fit variation example syst. error

73. 14 Jan 2011 73 Adam Kisiel – CERN Gaussian radii at 900 GeV and 7 TeV Gaussian radii in LCMS grow with multiplicity, fall with k T Fall with k T very prominent for R long, develops with multiplicity for R out, less pronounced for R side Radii comparable to STAR (EMCICs fit) SH fit Cartesian fit

74. 14 Jan 2011 74 Adam Kisiel – CERN Non-gaussian radii at 900 GeV and 7 TeV Consistent with trends for Gaussian radii Fall with k T very prominent for R long, develops with multiplicity for R out and R side SH fit Cartesian fit

75. 14 Jan 2011 75 Adam Kisiel – CERN Resonance structure in detail (data) “real” pairs “mixed” pairs subtraction “real” pairs “mixed” pairs ratio K0SK0S ω ρ f 0 (?) f 2 (? ) “mini-jet” background 1<M<15 k T (0.12-0.2) 1<M<15 k T (0.3- 0.4) 1<M<15 k T (0.5- 0.6) η(?) “full” fit (“mini-jets” + resonances)

76. 14 Jan 2011 76 Adam Kisiel – CERN Systematic error from Pythia vs. Phojet Systematic error coming from using Pythia or Phojet for background estimation. ALICE performanc e

77. 14 Jan 2011 77 Adam Kisiel – CERN Systematic error from fitting range Systematic error coming from changing the maximum fitting range ALICE performanc e

78. 14 Jan 2011 78 Adam Kisiel – CERN Hypothesis for system evolution Several scenarios were proposed for the behavior of the system created in ultrarelativistic heavy ion collision Femtoscopy probes the dynamical picture of the collision by measuring the size- momentum and position- momentum correlations Superposition of elementary collisions Static source with temperature gradients Explosive source behaving collectively no space- momentum correlations size-momentum correlation also position- momentum correlation

79. 14 Jan 2011 79 Adam Kisiel – CERN pp vs. AuAu: puzzling scaling... STAR reports that 3D HBT radii scale in pp in a way very similar to AuAu m T dependence of 3D radii in AuAu is taken as a signature of a flowing medium Is the scaling between pp and AuAu a signature of the universal underlying physics mechanism or a coincidence? STAR, arXiV (1004.0925)