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Summer Student Lecture, June 17, 2010 – G. David, BNL

Heavy Ion Physics: an overview. G. David, BNL. With lots of inspiration by (and some outright plagiarism from) Bill Zajc, former spokesman of PHENIX. Summer Student Lecture, June 17, 2010 – G. David, BNL. Why?. Summer Student Lecture, June 17, 2010 – G. David, BNL.

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Summer Student Lecture, June 17, 2010 – G. David, BNL

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  1. Heavy Ion Physics: an overview G. David, BNL With lots of inspiration by (and some outright plagiarism from) Bill Zajc, former spokesman of PHENIX Summer Student Lecture, June 17, 2010 – G. David, BNL

  2. Why? Summer Student Lecture, June 17, 2010 – G. David, BNL

  3. History / 0 -- “In the beginning…” Nature’s first experiment with ultra-high density and temperature matter freely expanding in vacuum (quite succesful  ) RHIC’s attempt to duplicate it Summer Student Lecture, June 17, 2010 – G. David, BNL

  4. History / 1 -- how fundamental are “elementary” particles? 1950s  (slowly) growing number of elementary particles 1960s  (rapidly) growing number of “elementary” particles 1970s  thousands of elementary particles This just cannot be true! - should I really believe in thousands of different building blocks??? - and even if so, what is this exponential rise of their number (with mass)??? Summer Student Lecture, June 17, 2010 – G. David, BNL

  5. History / 2 -- QCD … The roaring 60s: - DIS (deep inelastic e-p scattering, probing on much smaller length-scale than the size of the proton)  there is a substructure (partons)! - could neatly explain the thousands of “elementary” particles assuming just a few building blocks (but has to assume strange properties) - frantic search for fractional charge  none found The victorious 70s: - quarks are confined by an interaction that is just the opposite of QED: the further apart the quarks, the stronger the attractive force! (Uhm, just like a rubber string…) - QCD (Quantum Chromodynamics) was born Summer Student Lecture, June 17, 2010 – G. David, BNL

  6. History / 3 -- …and Hagedorn Meanwhile… - increased energies and luminosities produce newer and newer particles the “density” of new states increases exponentially with mass It looks as if there were a “temperature” TH (numerically: 170MeV) above which one cannot “heat” the system, no matter how much energy is “pumped in”  “Hagedorn limiting temperature” Summer Student Lecture, June 17, 2010 – G. David, BNL

  7. The quest for free (unconfined) quarks Thus, confinement prevents us from seeing free quarks in the vacuum. However: what if we compress and heat up many nucleons in a sufficiently large volume for sufficiently long time such that the individual quarks “forget” where they belonged and roam freely? Summer Student Lecture, June 17, 2010 – G. David, BNL

  8. Direct photon Caveat: (almost) everything we see is indirect Even if we produce a system of free quarks and gluons, this lives only for extremely short times, ~10-23s – after that the quarks team up again in doublets and triplets (mesons and baryons, long-lived, observable particles) Ideally one would insert an external “probe” into the medium, but that’s impossible. The next best thing is to use “internal probes” – created in processes we know are happening in elementary particle collisions (like p+p). Therefore, understanding this baseline is crucial. One example: photon-jet pair production in p+p The rate can also be calculated and it factorizes - momentum distribution of partons before scattering (PDF) - parton-partonscattering cross-section - probability of the parton to hadronize into a particular species (and momentum) “fragmentation function” Summer Student Lecture, June 17, 2010 – G. David, BNL

  9. Direct photon In some more detail Prob. that a parton carries fraction x of the total proton momentum Probability that a u-quark “fragments” into a p+ carrying fraction z of the original quark momentum Parton-parton scattering Summer Student Lecture, June 17, 2010 – G. David, BNL

  10. Direct photon Warning So we have a decent understanding (and a theory, QCD) to interpret and calculate how do partons interact when confined in nucleons We will try to understand a system of free partons supposedly produced in collisions of relativistic nuclei by looking at the same processes All the time we should ask ourselves Are these still the same distributions? The only bright spot: this is almost certainly unchanged Isn’t this influenced by the medium? Are these still the same Feynman-graphs? Summer Student Lecture, June 17, 2010 – G. David, BNL

  11. A few words on jets, leading particles Jets (borne 1982)  in theory are all particles generated from a single parton  experimentally somewhat ill defined (“jets are a legal contract”) all particles within a cone and attributed (right or wrong) to the same parent parton In principle they have a well-defined axis, total energy, etc. in practice these quantities depend on the jet algorithm Sometimes we (casually) equate the jet with its leading particle (the one carrying the highest momentum) Summer Student Lecture, June 17, 2010 – G. David, BNL

  12. What? Summer Student Lecture, June 17, 2010 – G. David, BNL

  13. t Space-time and visualization of a nucleus-nucleus collision VNI Simulations: Geiger, Longacre, Srivastava, nucl-th/9806102 Summer Student Lecture, June 17, 2010 – G. David, BNL

  14. Different stages, different signals Initial hard (parton) scattering: jets High momentum hadrons, photons correlations Phase transition (constant T?) Hadronization, deexcitation Large pressures, anisotropies in particle emission Thermalized system of quarks, gluons Freely escaping e.m. probes: photons, dielectrons, quarkonia Tremendous yield of particles, 10-15,000/event over 4p Summer Student Lecture, June 17, 2010 – G. David, BNL

  15. But all we see are the end-products – except for photons (Electromagnetic probes, to be more correct, which, when created, come out almost freely because aem << as). And they are created in all stages of the collision! That surely is the magic bullet, the ideal probe, right? Unfortunately, they come equipped with their own detection problems, like very low production rates to begin with… Summer Student Lecture, June 17, 2010 – G. David, BNL

  16. g Parton energy loss – single particles We know at what rate are high transverse momentum (pT) hadrons produced in p+p collisions. These come from fragmentation of “hard scattered” partons – a very rare process (because it involves large momentum exchange) If the same hard scattering occurs in a nucleus-nucleus collision and a medium of deconfined partons is present, the scattered parton may re-interact and lose energy before leaving the medium and hadronizing  at the far end we observe less high pT hadrons than expected “jet quenching” observed with single particle distributions Summer Student Lecture, June 17, 2010 – G. David, BNL

  17. Parton energy loss – control experiment When a deuteron collides with a gold ion, you do not expect a partonic medium be formed. Therefore, if this deficit is really due to energy loss in the medium, there should be no deficit in d+Au collisions “Nuclear modification factor” – the observed yield divided by the properly normalized yield in p+p collisions Summer Student Lecture, June 17, 2010 – G. David, BNL

  18. Parton energy loss – is the normalization sane? Remember: we said that the photons do not interact with the medium. So they should not be “suppressed”, the nuclear modification factor should be around 1 Summer Student Lecture, June 17, 2010 – G. David, BNL

  19. Measuring the temperature Summer Student Lecture, June 17, 2010 – G. David, BNL

  20. Parton energy loss – path-length dependence In the average collision the nuclei overlap only partially (“almond-shape”). But this also means that the average pathlength for a parton in the medium is different in different directions: …and that’s exactly what we observe! Summer Student Lecture, June 17, 2010 – G. David, BNL

  21.   Au+Au Back-to-back correlations Summer Student Lecture, June 17, 2010 – G. David, BNL

  22. Pedestal&flow subtracted Parton energy loss – back-to-back jets Of course if oneparton scatters out of the beam direction, there should be anotherparton scattering in the opposite direction, with the same momentum such that you should see jets always in pairs (many caveats here…) So you should see jets (high pT leading particles) always in pairs except if one of them loses so much energy that it doesn’t look like a jet any longer! So: look for a jet somewhere, and look for a partner at the opposite side: While in p+p and d+Au we see the opposite jet, in central Au+Au it “disappears” Summer Student Lecture, June 17, 2010 – G. David, BNL

  23. Pedestal&flow subtracted Surface bias In the singles spectra we have seen that jets are suppressed, but they don’t disappear completely They can be produced near the surface such that one parton escapes almost without energy loss, but then the other loses most of its energy. In rare cases they can be produced “tangentially” and we observe both jets. Summer Student Lecture, June 17, 2010 – G. David, BNL

  24. Fine, but then where does the energy go? “Peripheral” (just grazing) collisions “Central” collisions (full overlap) The energy is “dissipated” into low (close to thermal) particle production both in f (azimuth) and in h (beam) direction Summer Student Lecture, June 17, 2010 – G. David, BNL

  25. -p 0 p ASSO TRIG Even stranger observables We have seen on the previous slide that in central events (where the opposite side high pT particles “disappear”) lower pT particles are enhanced at an angle (like a Mach-cone), producing what is called the “shoulder”. For the average event it is symmetric around p. Can we actually force a path-length dependence upon them? If so, what do we see? leads to (Esumi, QM’09) An asymmetry in the shoulder! Summer Student Lecture, June 17, 2010 – G. David, BNL

  26. Soft and hard: no strong separation Isn’t this interesting? All mesons are suppressed, about the same amount, independent of their mass – but baryons are less suppressed! One possibility: recombination Due to the steeply falling spectrum to produce a high pT particle is “most expensive” by fragmentation, easier from 2 medium-pT quarks and easiest from 3 low pT quarks Summer Student Lecture, June 17, 2010 – G. David, BNL

  27. RHIC Charmonia – a thermometer Charmonia are bound states of c-cbar pairs with increasing radii. If there is a medium with free color charges, then – depending on its density – it will “screen” the strong interaction above a certain distance. Once the “screening radius” is smaller than the radius of the charmonium, it “melts”, cannot survive any longer in the plasma! Watch which one survives  tell the temperature… Summer Student Lecture, June 17, 2010 – G. David, BNL

  28. Bulk behavior So far we discussed the high pT particles (the realm of perturbative QCD), which os about 0.1% of all  The rest 99.9%!) is in the “thermal” region, in fact, thermalized. What’s happening to them> Here is a typical collision: Fourier expansion of the distribution of produced particle angle wrt reaction plane (Df): Summer Student Lecture, June 17, 2010 – G. David, BNL

  29. The anisotropy parameter v2 Scales separately for baryons and mesons once the mass-effect is taken out (middle panel) Scales for all particles if in addition we divide by the number of constutient quarks (right panel) The system exhibits flow on the partonic level! Summer Student Lecture, June 17, 2010 – G. David, BNL

  30. pions protons PHENIX preliminary data PHENIX preliminary data pT (GeV/c) So, we created… a medium, which is hot, dense, appears to have partonic degrees of freedom, and in addition behaves like a nearly perfect fluid Calculations suggest small η/s value Summer Student Lecture, June 17, 2010 – G. David, BNL

  31. But where does it all start? RHIC low energy scan! Summer Student Lecture, June 17, 2010 – G. David, BNL

  32. Critical point Summer Student Lecture, June 17, 2010 – G. David, BNL

  33. First results from low energy scan STAR, PRC 81 (2010) 024911 Summer Student Lecture, June 17, 2010 – G. David, BNL

  34. Summary This was an introductory talk, meant to raise interest and to show how multi-faceted the RHIC heavy ion physics is. It didn’t do justice to many great observations, just to name a few - attempts at full jet reconstruction and measuring fragmentation functions in nnuclei - fluctuations of multiplicity, particle ratios, etc revealing the nature of phase transition - 3-particle correlations - photon-jet correlations, setting the energy scale of jets and allowing true “QGP tomography” - the infinitely rich (and puzzling) world of heavy quarks - hints of parity violation that may help to explain why the universe exists as we know it (overwhelmingly matter, not matter-antimatter) - direct photons, the “historians” of the collisions Etc. etc. However, one thing is clear: This is a very rich field, not only promising, but already delivering extremely important physics. Join – you won’t regret it! Summer Student Lecture, June 17, 2010 – G. David, BNL

  35. Backup Summer Student Lecture, June 17, 2010 – G. David, BNL

  36. n Particle identification / 1 – long lifetime (>10-8s) Examples: p, K, g, p, n, … Charge (if any!) and 4-momentum needed for PID 4-momentum can be determined from at least two of these quantities: energy 3-momentum velocity calorimetry tracking time-of-flight + pathlength or Cherenkov-effect Fully stop the particle Convert its energy to - light, charge… Collect and read out Follow path of charged particles in magnetic field – get momentum from curvature Time of flight s t1 v = s/(t1-t0) t0 p = 0.3Br Cherenkov + r1 - r2 cos(a) = 1/bn B Summer Student Lecture, June 17, 2010 – G. David, BNL

  37. Particle identification / 2 – still long lifetime (>10-8s) Why do we emphasize long lifetime? Because the detectors are fairly large, and the particle produced at the vertex has to survive until it reaches the detector! With 10-8s ct > 3m Example: hadron identification with momentum and time-of-flight measurement y axis: inverse of the momentum x axis: time-of-flight There are many other methods to identify long-lived particles Summer Student Lecture, June 17, 2010 – G. David, BNL

  38. h (0) Summer Student Lecture, June 17, 2010 – G. David, BNL

  39. Summer Student Lecture, June 17, 2010 – G. David, BNL

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