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Search for Effects of the QCD Color Factor in High-Energy Collisions at RHIC

Search for Effects of the QCD Color Factor in High-Energy Collisions at RHIC. Bedanga Mohanty LBNL. Outline. Motivation for nucleus-nucleus collisions Experiment set up High p T particle production in p+p collisions Search for color factor effects at RHIC Summary and Outlook.

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Search for Effects of the QCD Color Factor in High-Energy Collisions at RHIC

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  1. Search for Effects of the QCD Color Factor in High-Energy Collisions at RHIC Bedanga Mohanty LBNL Outline • Motivation for nucleus-nucleus collisions • Experiment set up • High pT particle production in p+p collisions • Search for color factor effects at RHIC • Summary and Outlook

  2. Such a big difference is not seen in molecules, atoms and nuclei Chiral Symmetry broken Consequence : valence quarks acquire dynamical masses - Constituent Quark Mass ~ 350 MeV Review of particle physics (PDG), Phys. Lett. B 592 (2004) Observations in Nature Quarks are confined inside hadrons

  3. A Question What will be the fate of nuclear matter when subjected to extremes of density and temperature ? Suggestion from the fundamental theory of strong interaction(QCD) • Restoration of chiral symmetry • De-confinement of quarks and gluons QGP ~ a (locally) thermally equilibrated state of matter in which quarks and gluons are deconfined from hadrons, so that color degrees of freedom become manifest over nuclear, rather than merely nucleonic, volumes. Is it possible to observe these QCD predicted transitions ?

  4. Lattice QCD Prediction e/T4 gparton ~ 47.5  ~ g (2/30) g ~ 3 T/Tc TC≈1708 MeV, eC≈1 GeV/fm3 Lattice QCD predicts a transition to Quark Gluon Plasma at high temperature How does the phase diagram look ? F. Karsch, Prog. Theor. Phys. Suppl. 153, 106 (2004)

  5. T > TC T~TC T < TC Critical Opalescence as observed in CO2 liquid-gas transition T. Andrews. Phil. Trans. Royal Soc., 159:575, 1869 The QCD Phase Diagram Sketch by W. Weise Chiral symmetry restored Chiral symmetry broken How can we create the QCD transitions in laboratory ? How can we study the QCD phase diagram ?

  6. Colliding two nuclei we expect to create the QCD transition in laboratory Tool : Nucleus-Nucleus Collisions Evolution of the system formed in Nucleus-Nucleus collisions J. D. Bjorken Physical Review D 27 (1983) 140

  7. Typical Experiment PMD Detectors for present analysis TPC and TOF • Physics driven detector • upgrade plan • - Heavy Flavor Tracker • - Forward Meson Spectrometer • - Muon Telescope Detector • - Data Acquisition What do we measure with these detectors …. K.H. Ackermann et al., Nucl. Instrum. Methds. A 499 (2003) 624

  8. Basic Measurements Spatial Position, Multiplicity, Momentum and Energy No. of events N  No. of events

  9. V0 decayvertices Ks p + + p - L  p + p - L  p + p + X-  L + p - X+L + p + W  L + K- “Kinks”: K  +  Resonances in invariant mass spectra dE/dx in TPC STAR Preliminary Au+Au 40% to 80% 0 f0K0S  K*0 Electron ID via p/E in EMC 0.2  pT  0.9 GeV/c Time of Flight (ToF) Particle Identification D0 

  10. Main work for PhD : • search for disoriented chiral condensates • signature for chiral symmetry restoration WA98 : PRC 64 (2001) 011901 (R) WA98 : PRC 65 (2002) 054912 WA98 : PRC 67 (2003) 044901 B. Mohanty et al., PRC 66 (2002) 044901 B. Nandi..B.Mohanty PLB 449 (1999) 109 B. Nandi B. Mohanty PLB 461 (1999) 142 B.Mohanty et al, Nucl. Phys. A 715 (2003) 339 B. Mohanty et al., I. J. Mod. Phys. A 18 (2003) 1067 B. Mohanty et al., I. J. Mod. Phys A 19 (2004) 1453 Efforts summarized in --- Photon Multiplicity Measurements at SPS Physics goals : Photon multiplicity Fluctuation in photon multiplicity Azimuthal anisotropy studies

  11. Observation of identified particle longitudinal scaling Photon Multiplicity Measurements at RHIC Two planes CPV+Preshower Gas detector of hexagonal cells Cell cross section : 1.0 cm2 Cell depth : 0.8 cm Gas used: Ar+CO2 in 70:30 Total number of cells : 82,944 Area of the detector : 4.2 m2 Distance from vertex : 540 cm Coverage: -2.3 to -3.8 in  with full STAR PMD : NIM A 499 (2003) 751 STAR : Physical Review Letters 95 (2005) 062301 STAR : Physical Review C 73 (2006) 034906

  12. Approach to Understand QCD Matter Attempt in the rest of the talk to relate fundamental aspect of QCD to some experimental observables • “Using fundamental aspects of QCD theory, similar to those I discussed in connection with jets, one can make quantitative predictions for the emission of various kinds of "hard" radiation from a quark­gluon plasma. We will not have done justice to the concept of a weakly interacting plasma of quarks and gluons until some of these predictions areconfirmed by experiment.” • Frank Wilczek • 2004 Nobel Laureate - discovery of asymptotic freedom in strong interactions, • Physics Today Vol 53, August 2000

  13. Heavy particles (charm and bottom) p+p collisions at STAR Jets hadrons q q hadrons Production of jet cross section calculable using pQCD High pT particle production if understood in p+p can act as solid baseline for physics in Au+Au Penetrating Probes : Nucleus-Nucleus Collisions Probes needs to be created here

  14. How partons fragment into hadrons How partons are distributed in hadrons we collide What is the probability that the partons will interact Parton Distribution Functions dominantly from deep-inelastic Scattering experiments uds-quark all c-quark LO parton processes b-quark s=91.2 GeV OPAL NLO parton processes Parton Fragmentation Functions determined from e+e- annihilations Parton-Parton Cross-Section (pQCD) Understanding : p+p Collisions

  15. Jet production in p+p Collisions at RHIC Jet production well explained by NLO pQCD calculations Let us look at high pT identified particle production in p+p collisions STAR : PRL 97 (2006) 252001

  16. p+p Collisions and pQCD at RHIC High pT particle production well explained by NLO pQCD calculations Ng(i)/ (Ng(i) + Nq(i)); i = , K, p… At high pT range measured : Significant gluon contribution to produced baryons Substantial quark contribution to produced mesons STAR : PLB 637 (2006) 161 S. Albino at al, NPB 725 (2005) 181

  17. p+p Collisions : Gluon Jet and Quark Jet PYTHIA 6.3 Gluon jet In PYTHIA, gluon jets produce baryon-meson splitting while quark jets produce mass splitting Data shows baryon-meson splitting. Supports gluon jet dominance at RHIC STAR : nucl-ex/0607033

  18. Summary from p+p Collisions Results at RHIC Observations • Jet Production well described by NLO pQCD • High pT particle production well described by NLO pQCD • At high mT : baryon-meson splitting Conclusions • Dominance of hard processes at high pT • Reference for understanding nucleus-nucleus collisions • Interesting species dependence of gluon contribution as a function of pT Let us now try to understand the nucleus-nucleus data …..

  19. Observations Leading particle Near side 2 AB 1 d N / dp d h T R = AB N 2 pp Dj d / dp d s h bin pTlp : 4 - 6 GeV/c pTasoc : 2 GeV/c - pTlp T Away side Interpretation : Energy loss of partons in a dense medium Interpretation : Matter exhibits collectivity, pattern at intermediate pT indicates partonic collectivity Particle Production in A+B Collisions STAR : PLB 637 (2006) 161 STAR : PRL 97 (2006) 152301 STAR : NPA 757 (2005) 102 STAR : PRL 91 (2003) 072304

  20. path length L One mechanism of energy loss : Medium induced gluon radiation Eg hard parton ~ 9/4 Eq 2 D E <q> ^ L  a C s Color factor: 4/3 for quarks 3 for gluons Energy Loss and QCD Suppression in high pT particle production is due to energy loss of partons in medium formed in nucleus-nucleus collisions An opportunity to relate experimental observable (of Eloss) to basic ingredient of QCD - Gauge Group through Color Factors PRL 85 (2000) 5535 nucl-th/0512076 NPB 483 (1997) 291

  21. CF ~ strength of a gluon coupling o a quark, CA ~ related to the gluon self coupling TF ~ strength of gluon splitting into a quark pair i,j represent fermion field indices and a,b gauge field indices Color Factors If Nc is the dimension of the group (Lie) - CA = Nc, CF = (Nc2-1)/2Nc and TF = 1/2 A and F represent adjoint and fundamental representations. QCD : For SU(3) : Nc = 3 CA = 3, CF = 4/3 Color factors reflect basic properties of QCD. They are therefore measured to prove SU(3) is the gauge group of QCD

  22. S = 0.119 SU(3) is the gauge group for QCD Measurement of Color Factors • Three basic vertices in four jet production in e+e- • Spin-1 or spin-1/2 particles in different configurations. Leads to different angular distributions in the final states. • Observed jet angular distributions are fitted to theoretical predictions with CA, CF, TF as free parameters. ALEPH : ZPC 76 (1997) 1 OPAL :EJPC 20 (2001) 601 What are the expectations of effect of color factor on observables in HI collisions ?

  23. Eg ~ 9/4 Eq Expectations Recall : Nucleus-Nucleus collisions produces a dense medium where gluons loose more energy than quarks. No such dense medium expected in p+p and d+Au collisions Then naïve expectation at high pT : pbar/p (pp or dAu) > pbar/p (central Au+Au) pbar/ (pp or dAu) > pbar/ (central Au+Au) Rcp() > Rcp (pbar+p) Do we see the color factor effect in experimental observables ?

  24. Observations In High pT Particle Production Anti-particle to particle ratio Baryon & meson NMF Anti-Baryon to meson ratio Observations different from expectation - Why particle ratios at high pT in Au+Au similar to d+Au and p+p ? Why  have similar RCP as p+pbar ? Where is the color factor effect ? STAR : PLB 637 (2006) 161 STAR : PRL 97 (2006) 152301 STAR : nucl-ex/0703040

  25. Model Comparison To Data Model calculation with partonic energy loss in heavy ion collisions + Color factor effect not consistent with measurements STAR : nucl-ex/0703040 Absence of color factor effect in data ? What could be the possible reasons ? What are the new probes to explore in future ? STAR : PLB 637 (2006) 161 STAR : PRL 97 (2006) 152301 Wang et al, PRC 70 (2004) 031901

  26. Possible Reasons • Gluon dominated initial conditions in heavy ion collisions at RHIC ? • Possibility of conversions between quark and gluon jets in the medium ? • Eg/Eq ~ 9/4 only apparent for the limit E/Ejet tending to zero ? • High S and a low Q2 regime at RHIC ? • Sensitive to different energy loss scenarios ? B. Mohanty (for STAR) nucl-ex/0705.9053

  27. Expt. data Conversions between q- and g- jets via both inelastic (qqbar -- gg) and elastic (gq(qbar) -- q(qbar)g) scatterings with thermal partons in the QGP Physics Possibilities : Quark and Gluon Jet Conversions NMF for partons W. Liu et al., nucl-th/0607047

  28. Summary of Search of Color Factor Effect Observations at high pT • Particle production suppressed and large collectivity • Anti-particle to particle, anti-baryon-to-meson ratios are similar in central, peripheral Au+Au, d+Au and p+p • Rcp of  is similar to Rcp of p+pbar Interpretations • A dense partonic medium which exhibits substantial collectivity • No simple answer to absence of color factor effect Outlook …..

  29. High pT ratio of heavy to light NMF ratio is sensitive to color factor effect Outlook - Heavy Flavor Sector Similar idea can also applied for R (/h) N. Armesto et al., PRD 71 (2005) 05427

  30. Suppression pattern in the away side of identified particle correlations. Outlook - PID di-Hadron Correlations Do we expect something like .. (This is not data) Choosing different particles may reflect varying contribution of quark and gluon at high pT. Will be interesting to see heavy-flavor correlations - probing quark energy loss.

  31. Future - Programs ALICE : Hard processes (~jets ) are produced at high rates Life time of QGP stage is expected to be longer Access low Bjorken-x ~ 10-5 Low Energy RHIC/CBM : Locate critical point in QCD phase diagram Look for Chiral Symmetry restoration A high baryon density environment could be key to understanding the above two physics possibilities

  32. The Creation Hymn (a time is envisioned when ..)There was neither non-existence nor existence then.There was neither realm of space nor the sky which is beyond.What was concealed? Where? In whose protection?There was no death then nor deathlessness. Of day and night there was no sign…….Darkness was hidden in the darkness in the beginning.Then that which was hidden by void and form less, That One, arose by the great power of heat, …….But That out of which creation has arisen, who really knows.The gods themselves are later than creation,Then who can tell whence it has arisen?(Rig Veda, X, 129) Thanks…. STAR Collaboration and RNC group (theory and experiment) for their support of my work and discussions So let us carry on this journey from understanding the origin of universe to understanding of fascinating objects of the universe ……lets do it together

  33. Back up slides

  34. A. Rose (STAR) : QM2006 Future Heavy Flavor Measurements in STAR Simple estimates show for 10K Ds we need 16 Weeks of data taking with HFT + upgrades in DAQ and Luminosity Addresses key issues yet to be resolved in HI Collisions : Search for Color factor effects Parton Eloss mechanism (Collisional vs. Radiative) Collectivity measurements - Thermalization

  35. Outlook - Energy Dependence Probing quark dominated jet production at lower energy to gluon dominated jet production at higher energy Similar Eloss for quarks and gluons QCD type (color charge effect) Eloss Wang et al., PRC 58 (1998) 2321 Wang et al., PRC 71 (2005) 014903

  36. Energy loss : absorption of fraction of partons (Dense core + pp like corona) or energy loss of every parton or some combination of the two Physics Possibilities : Eloss Scenarios T. Renk and K.J. Eskola hep-ph/0702096 What is the relative contribution of different energy loss scenario ?

  37. path length L One mechanism of energy loss : Medium induced gluon radiation Eg hard parton ~ 9/4 Eq 2 D E <q> ^ L  a C s Color factor: 4/3 for quarks 3 for gluons E C  dNg/dy L /AT D  s Eloss Formalism : BDMPS & GLV

  38. QCD

  39. QDC Phase Diagram

  40. Quark Gluon Plasma Relativistic plasma : Inter-particle distance Electric screening Magnetic screening Debye number : 1/g2T 1/gT 1/T “Coulomb” coupling parameter : S. Ichimaru, Rev. Mod. Phys. 54 (’82) 1071 QGP for g << 1 ( T >> 100 GeV )

  41. Chiral Symmetry

  42. Chiral symmetry

  43. Chiral Symmetry 1535 1520 1260 600 ≈ 290 ≈ 600 1232 ≈ 490 ≈ 470 magnetisation M 938 135 770 ferro magnetic chiral symmetry scalar mesons vector mesons nucleon para magnetic TCurie paramagnetic: full rotational symmetry ferromagnetic: rotational symmetry about 1 axis

  44. DCC A.A. Anselm et al., PLB 261(1991) 482 Rajagopal et al NPB 404 (1993)577

  45. Experiments on DCC Exotic events DCC ?? Null results Upper limit

  46. LHC

  47. Definition Jets

  48. Expectations from hadron beam 1 Energy loss by ionization only – Bethe Bloch Energy loss by charged particles Heavy particles m > mm Medium is characterized by Z/A Z/A ~ 1 • For MIPS : • energy loss ~ [1 +(mc/p)2 ] zp2 dE/dx • fixed p heavy particles loose more energy • p >> m, energy loss ~ constant value • Minimum ionization for singly charged particles ~ 1 – 2 MeV/gm cm2 Z/A ~ 0.5 Kinematical term Fermi Plateau Minimum ionizing particles (MIPs) Relativistic rise

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