1. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University James Nagle Lepton and Dilepton Production: Current Experimental Results AAAS Symposia Nuclear Matter at the Highest Energies and Densities

2. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University OutlineOutline What is the nature of nuclear matter at the highest energies and densities? What are leptons? How can we study nuclear matter using lepton and dilepton observables? What are the experimental results?

3. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Forces of Nature + +… Quantum Electrodynamics (QED) Field theory for electromagnetic interactions Exchange particles (photons) do not have electric charge Flux is not confined - U(r)  1/r and F(r)  1/r 2 Quantum Chromodynamics (QCD) Field theory for strong (nuclear) interactions Exchange particles (gluons) do have “color” charge Flux is confined - U(r)  r and F(r)  constant

4. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University QCD Confinement Quarks are the fundamental particles carrying color charge. However, we have never observed free quarks ! cc cccu c u J/  is a bound state of cc (hidden charm) Potential energy allows formation of qq pair. Now quarks confined in D mesons (open charm). Pulling quarks apart is like stretching a spring. D 0 mesonAnti-D 0 meson

5. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Studying Quark Confinement In a hot plasma of other quarks and gluons (Quark-Gluon Plasma), we expect a screening of the long range attractive force between quarks. This screening should suppress bound states such as J/ , but not change the total charm production (D mesons). r  V(r)/  Lattice QCD calculation

6. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Creating the Hot Plasma 1234 Collisions between heavy nuclei (Gold A=197) at relativistic velocities (v = 0.99995 x speed of light) deposit enormous energy and create approximately 10,000 quarks, antiquarks and gluons in a fireball. These are the highest energy nuclear reactions ever created on earth, but note that these collisions cannot chain react.

7. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University With and Without Plasma Goal: Fundamental understanding of the confinement of quarks in QCD and its modification at the highest energies and densities. Method: Compare the ratio of J/  to D mesons in a plasma and not in a plasma Proton+Proton collisions are the control with no Quark-Gluon Plasma cc Au+Au or Pb+Pb collisions produce charm in a Quark-Gluon Plasma cc Thus, look for a suppression in the ratio:

8. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Lepton =    = small Leptons are point-like particles that carry no color charge, and therefore have no strong interactions. How does this make them useful in studying strongly interacting nuclear matter? Vector mesons , J/ , … are unstable and can decay into dileptons. Once the leptons are created they travel through the dense plasma almost unaffected and carry out crucial information. J/  e + e -  or J/  +  -  e+e+ e-e-

9. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University D Meson Measurements D mesons decay very quickly. We can only observe them via their decay products. However, complete reconstruction is very difficult. For example D 0  K -  + D mesons can be measured via single leptons (electrons or muons) and also lepton pairs. K+K+ K-K- e+e+ e --  D *0 

10. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Intriguing Results at CERN “Strong evidence for the formation of a transient quark-gluon phase without color confinement is provided by the observed suppression of the charmonium states J/ ,  c, and  ’.” Maurice Jacob and Ulrich Heinz NA50 measures dimuons (  +  - ) in Pb-Pb and p-p collisions and observes Invariant Mass(GeV/c 2 ) J/  +  - mass = 3.1 GeV/c 2 Open Charm contribution D 0 D 0  +  - K + K -   CERN Press Release 2000 an open charm enhancement and a suppression in J/  production relative to model calculations.

11. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Unprecedented Energies New collider facility and four new experiments to study Gold-Gold collisions at over an order of magnitude higher energy than ever before for heavy nuclei.

12. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University PHENIX Experiment PHENIX is the only RHIC experiment specifically designed to measure leptons and dileptons. Electrons are measured by the two central spectrometers. Muons are measured by the two forward spectrometers.

13. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Collect the Data! Scale of the Problem Uncovering nature’s secrets is not easy. PHENIX example: over 500 people, over 10 countries tons of steel, specialized detectors thousands of custom signal processors transmitting over 5 Gigabytes per second

14. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Needle in a Haystack Over 5000 charged particles are produced in one Au-Au collision. Detectors need to find these rare leptons without mistaking other particles for a lepton at the level of one in 10,000. There is the electron.

15. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Ring Imaging Cherenkov Detector No particle can travel faster than the speed of light in vacuum (v < c). However, a particle can travel faster than the speed of light in a medium (c/n < v < c) where n=index of refraction. If it does a cone of light called Cherenkov radiation is produced similar to a sonic boom with the speed of sound. Since electrons are lighter than pions, at the same momentum, they have a much higher velocity and yield Cherenkov light in our detector. Giant mirror reflects light onto a array of thousands of photomultiplier tubes.

16. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Electron Tracking Measure trajectory of particles that bend through a magnetic field. This allows a momentum determination. All tracks Then measure the energy deposit in an electromagnetic calorimeter. Electrons and photons leave full energy, but hadrons (e.g. pions) leave only a small fraction. Thus check for energy-momentum measurement match for electrons. Electron enriched sample (using RICH) Energy/Momentum

17. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Single Electron Spectra PHENIX 1.2M Minimum bias events In August 2000, PHENIX recorded one million Au-Au collisions. With this data we can address charm production via single electrons. Transverse Momentum (GeV/c) Electron Yield per Event We are not done yet. Most of these electrons are not from open charm decays but from other hadron decays (e.g.  0,  ). These other contributions must be subtracted away.

18. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University  conversion Extracting the Charm Signal After accounting for other contributions, we see a clear electron signal most likely from open charm D mesons. Transverse Momentum (GeV/c) Electron Yield per Event Systematic Error Transverse Momentum (GeV/c)

19. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Both central and minimum bias data are consistent with theoretical calculations (PYTHIA) of expected charm production. Electron Physics Transverse Momentum (GeV/c) Electron Yield per Event

20. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Charm Energy Dependence Good agreement with extrapolation from proton-proton collisions at lower energies. Center-of-Mass Energy (GeV) Charm Yield per Collision

21. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Run II RHIC achieved full energy RHIC achieved ~50% of design beam intensities For example PHENIX recorded ~ 170 million events Days in the run Integrated Collisions Sampled

22. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Charm via Muons PHENIX Simulation With a new muon spectrometer, we can measure open charm (D mesons) and open beauty (B mesons) via their single muon decays. Transverse Momentum (GeV/c) Charm D 0  K +  -  Beauty B 0  D +  - 

23. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University J/  Measurements In the fall 2001, PHENIX recorded hundreds of J/  decaying to both e + e - and  +  -. Data analysis is underway. In the fall 2002, PHENIX will sample billions of Au-Au collisions to collect high statistics. Also, the STAR experiment will make a first measurement. We can then combine our open charm (D meson) measurement with our new J/  measurement to understand the change in the quark confining potential. PHENIX Simulation  +  - Invariant Mass(GeV/c 2 )

24. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Thermometer for the Plasma Upsilon  (bb) state is more strongly bound than J/  (cc) RHIC PHENIX Simulation  +  - Invariant Mass(GeV/c 2 ) Future upgrade to RHIC beam intensities should allow for Upsilon (bb) state measurements. b b c c Thus, the Upsilon should not be suppressed until much higher energy densities.

25. AAAS Symposia Nuclear Matter at the Highest Energies and Densities James Nagle Columbia University Conclusions Goal: Fundamental understanding of the confinement of quarks in QCD and its modification at the highest energies and densities. Method: Measurement of open charm and hidden charm (J/  ) production and study yields as a function of collision volume (which nuclei) and energy. Status: Measurements of charm (D mesons) via electrons are in hand. Critical J/  data analysis is underway. We are at the start of an exciting area of physics !