# Presentation: 935112

## Review of the CERN-SPS QGP Search and Open Questions

- James Nagle
- Columbia University
- 	(*late substitute)

**Workshop on Hard Processes and RHIC Physics ****Los Alamos Nat. Lab, 12-16 June, 2000**

## CERN Press Release

- _http://press.web.cern.ch/Press/Release00/PR01.00EQuarkGluonMatter.html_

## Top Quark Standard

- “Both CDF and D0 report a probability of less than one in 500,000 that their top quark candidates could be explained by background alone.”
- Followed by simultaneous publication in Physical Review Letters and accompanying detailed articles for review.

-  It is unrealistic to expect a statement like “there is a probability of less than one 	in 500,000 that these data are explained by non-plasma models alone.”
-  However, the same level of scrutiny is expected given the scientific importance.
-  There is no scientific paper on the CERN conclusions.  Thus I will use the 		presentation of U. Heinz (co-author with M. Jacob of the press 		statement) as a guide.

**March 2, 1995**

## Advance Warning !

- I have worked on AGS experiments E878, E864, E941 and now work on the PHENIX experiment at RHIC.
**I will give a critical analysis of the CERN-SPS results and press release**.
**The opinions expressed here are my own and others in the field may disagree.**

## Confinement

-  Normally quarks are bound together (confined) in hadrons
-  There are no observations of free individual quarks
-  However, QCD predicts that at high density (5 to 10 ) and high temperature (~ 150-200 MeV ~ **10****12**** ****o****F**), the quarks are no longer confined, but rather are “asymptotically free” and  form a **plasma of quarks and gluons**

- d

- u

- u

- u

- d

- d

- u

## Chiral Symmetry

- If the individual nucleons disappear, and the system is a plasma of individual quarks and gluons, one expects that the quarks will act as nearly massless objects.
- This transition to nearly massless quarks is called the restoration of approximate **Chiral Symmetry**.
- The up and down quark are expected to have masses of
- ~ 5 MeV, while the strange quark is reduced to a mass of       ~ 150 MeV.

**Up       Down    Strange  Charm   Bottom  Top**

## Phases of Matter

- We would like the understand the phases of nuclear matter, just like we understand the phases and phase transitions of water.
- Although lattice QCD gives us a theoretical guide, the figure on the left is more schematic.

## Lattice QCD

****

- (Karsch, Laermann, Peikert, 99’)

- (F. Karsch, hep-lat/9909006)

- Indication of phase transition to deconfined matter.
- Transition energy scale is of order ~ 0.6 GeV/fm3 or TC ~ 170 MeV at zero net baryon density.

## Key Points

** ****The evidence for this new state of matter is based on a multitude of different observations.**
** ****Many hadronic observables show a strong nonlinear dependence on the number of nucleons which participate in the collision.**
** ****Models based on hadronic interaction mechanisms have consistently failed to simultaneously explain the wealth of accumulated data. **
** ****On the other hand, the data exhibit many of the predicted signatures for a quark-gluon plasma.**
** ****Even if a full characterization of the initial collision stage is presently not yet possible, the data provide strong evidence that it consists of deconfined quarks and gluons.**

## Strangeness Enhancement

- Slide 4 from presentation of U. Heinz

**Strangeness Enhancement**

**J/******** Suppression**

**Thermal Radiation**

## Strangeness Enhancement

**“****A particularly striking aspect of this apparent ‘chemical equilibrium’ at the quark-hadron transition temperature is the observed enhancement, relative to proton-induced collisions, of hadrons containing strange quark.**
**Lead-lead collisions are thus qualitatively different from a superposition of independent nucleon-nucleon collisions. That the relative enhancement is found to increase with the strange quark content of the produced hadrons contradicts predictions from hadronic rescattering models where secondary production of multi-strange (anti)baryons is hindered by high mass thresholds and low cross sections. **
**Since the hadron abundances appear to be frozen in at the point of hadron formation, this enhancement signals a new and faster strangeness-producing process before or during hadronization, **_**involving intense rescattering among quarks and gluons**_**.****”**

## Mechanisms

- Quark-Gluon Plasma

- Hadron Gas

-  gluon fusion:
-  current quark mass:
-  threshold:
-  equilibration time:

-  hadron scattering:
-  constituent quark mass:
-  threshold:
-  equilibration time:
-  pion rescattering and
-   resonance:

- Contents of strange quarks relative
- to non-strange quarks in QGP is
- greatly enhanced compare with HG

## Hadron abundances are consistent with chemical equilibrium at a temperature of ~ 170 MeV.

- Slide 5 from presentation of U. Heinz

- Overall strangeness production is enhanced relative to  e+e- and p+p collisions.

- Multiply strange baryons and antibaryons show even greater enhancement relative to a scaling of p+p by the number of participating nucleons.

## Not just Pb-Pb....

**Thus the phase transition occurs just below 1 GeV/fm****3**** corresponding to a temperature of  170-180 MeV.**

- “Experimentally [strangeness enhancement] is found not only in lead-lead collisions, but even in sulfur-nucleus collisions.  This is consistent with estimates of the initial energy densities above the critical value of 1 GeV/fm3 even in those collisions.”

## Lower Energies

- F. Becattini et al., EPJ C5 (1998) 143

- s is largely determined by the K+/+ ratio and indicates similar strangeness enhancement at much lower energy at the BNL-AGS.

## Where is the phase transition?

- J.C. Dunlop and C.A. Ogilvie, Phys. Rev. C61:031901 (2000)

**p+p**

**Au+Au**

**Pb+Pb**

- Excellent data from E917 shows that strangeness enhancement is observed in Au + Au collisions down to threshold.
- Is this a different mechanism?  Is the phase transition at AGS or even BEVELAC energies?   What are the assumptions?

## Multiply Strange (Anti) Baryons

- E.Andersen et al. Phys.Lett. B449 (1999) 401
- CERN-EP/99-29

- “The multi-strange particle yields are proportional to Npart as would be expected if strange quarks are equilibrated in a deconfined and chirally symmetric quark gluon plasma.”

**WA97**

## Intriguing Result

- However, note that scaling with Npart is violated in many ways in A+A collisions as well as p + A.  Thus Npart scaling is not a good baseline.

**The large yields of ******** and ******** are difficult to explain with traditional hadronic rescattering due to low cross sections.  **
**Need to investigate other ideas (eg. Junctions / Junction loops)**

## J/ Suppression

- Vector meson J/
- bound state of a charm quark 			and anti-charm quark

- The pair feels an attractive force and can
- form the above bound state.  However, in the middle of a quark-gluon plasma the attractive force is screened.

## QCD Thermometer

- Hadrons with radii greater than ~ D will be dissolved (suppressed)
- Debye screening length D ~ 0.5 fm at a temperature T = 200 MeV

- As the temperature is raised above the critical temperature, one should see the sequential suppression of the various “onium” states

## NA50 Spectrometer

** ****Excellent muon identification**
** ****Triggering using hodoscopes**
** ****High flux of incident beam **
**	****~ 5 x 10****7**** ions / spill**
** ****Large Data Sample **
**            ****~ 2 x 10****5**** J/******** **

## NA50 Pictures

- Zero Degree Calorimeter

- Multiplicity Detector

- Active Target

- Muon Spectrometer

## NA50 Dimuon Mass Spectra

- Drell-Yan process is unaffected by nuclear medium (standard candle)

## Collision Geometry

**NA50**

- Impact parameter is not directly measurable, so it must be inferred by measuring correlated observables.

**Collision Geometry**

## Sequential Suppression

**“****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 at the CERN-SPS

**Discontinuity due to **c** melting**
**Drop due to **J/** melting**
**Using Drell-Yan as control**

## NA50 Web Page

## “We must conclude that the J/ suppression pattern observed in our data provides significant evidence for the deconfinement of quarks and gluons.”

**“****We must conclude that the J/******** suppression pattern observed in our data provides significant evidence for the deconfinement of quarks and gluons.”**

- NA50:  CERN-EP-2000-013, Accepted Phys. Lett. B

## Non-Monotonic Derivatives

- “A clear onset of the anomaly is observed.  It excludes models based on hadronic scenarios since only smooth behavior with monotonic derivatives can be inferred from such calculations”
-                 Phys. Lett. B 450, 456 (1999).

## Mathematical Definitions

- JLN, WAZ

## Plasma Theory

- D. Kharzeev, Nucl. Phys. A638, 279a (1998).

- Invoking a model of bubble nucleation, one is able to reproduce the suppression.   This implies a relatively strong 1st order phase transition.

## “Hadronic” Models

**? Matching peripheral data**

**? Matching central data**

**? Consistent with p+A,S+U and p+A (*Fermilab)**

- There is expected “hadronic” suppression of J/ due to:
-  pre-resonance absorption on target and projectile nucleons
-  * see next talk by E866/NuSea for more details
-  final state interactions with  , etc.
-  * cross sections not well known

## More “Hadronic” Models

- (Capella, Ferreiro and Kaidolov, hep-ph/0002300)

- Different modeling of ET production and detector response may play a significant role.

## Percolation

- H. Satz:  hep-ph/9908339

- Percolation model of H. Satz looks at localized parton densities and above a critical density assumes a strong 1st order phase transition (similar to bubble production).  Sequential melting of c and J/ seen.

## Thick Target Correction

- NA50 at the CERN-SPS

- _1998 data not included on plot:_
- With a 7% target in 1998 there was a “high contamination of Pb-air interactions, [but is] found to be negligable for ET > 40 GeV.
- Since the main goal of the 1998 run is to study the suppression pattern in central
- Pb-Pb collisions, we have limited the analysis to ET > 40 GeV.”

- _1996 data not included on plot:_
- “With a 30% target, it is conceivable that a spectator fragment from a first peripheral collision reinteracts downstream, resulting in measured values of ET and EZDC typical of central collisions.”

## More Targets

- Different targets 7%, 17%, 30%

- More ET
- Less EZDC
- More J/

- What are peripherals?

## Transverse Momentum

- <pt2>N = <pt2>pp + (N-1)  pt2

- Prior collisions broaden the transverse momentum spectrum (“Cronin effect”)

- S. Gavin et al., hep/9610432v2

## Number of Previous Collisions

**Suppression due to Deconfinement**

**Hadronic absorption with nucleons only**

- Plasma breaks up J/ formed at the core of the collision, which are the ones most likely to have the largest number of previous collisions (N)

- Target coordinates

- Projectile coordinates

## Data and Predictions

- D.Kharzeev, M.Nardi, H.Satz, Phys. Lett. B405, 14 (1997).
- JLN, M. Bennett, Phys. Lett. B465, 21 (1999).

- There is much more information in the full pT spectra, which has not been shown.
- Early predictions had suppression at low pT, since these objects spend more time in the plasma.
- Opposite effect of that shown here.

## Energy Density

- Lattice QCD predicts a phase transition at c ~ 0.6      GeV/fm3 or TC ~ 170 MeV.
- 	- S + S collisions reach  ~ 1 GeV/fm3
- 	- Plasma formation seen via strangeness enhancement
- Above  ~ 2 GeV/fm3 the c state melts
- 	- Percolation model indicates strong
- 	   1st order transition
- Above ~ 3 GeV/fm3 the J/ state melts
- 	- Most central Pb + Pb collisions reach
- 	   ~ 3.5 GeV/fm3 or T ~ 240 MeV

## Estimating Energy Density

- “*Highly Relativistic Nucleus-Nucleus Collisions:  The Central Rapidity Region*”,
- 				              J.D. Bjorken, Phys. Rev. D27, 140 (1983).
- Assumes ~ 1-d hydrodynamic expansion and  then boost invariance.

- Press release argues for 3-d hydrodynamic expansion, and  is unknown within a factor of 4.
- Energy density is large!  However, comparing this to a theoretical value from lattice QCD with no net baryon density is difficult.

## “new state of matter”

- “new state of matter”

- energy density ~ 20 x  nucl. matter

- chemical equilibrium

- strangeness enhancement

- J/ suppression

- Great success of the CERN heavy ion program !

- Slide 15 from presentation of U. Heinz

## Slide 16 from presentation of U. Heinz

## Conclusions

- The CERN program has
- Created nuclear matter at unprecedented densities
- Explored its properties in unprecedented detail
- Provided unprecedented challenges to the theoretical community
- The RHIC heavy ion community is ready to begin experiments with a set of detectors designed for the first _dedicated_ heavy ion collider
-      The variety, energy, uniqueness, promise and challenge of this program exceeds even that of the very impressive CERN era.

## Thank you

- Thanks to Peter Steinberg, Mike Bennett, Bill Zajc, and others
- I hope this exciting physics stimulates more discussion and more work (since this is a workshop).
-  Look into strangeness enhancement at all energies
-  Look into multi-strange baryon models
-  Look into percolation models and co-mover models
-  Look into thick target corrections
-  Look into predictions for transverse momentum spectra
-  ............
-  A long list should keep us busy this week

## “L” Parameter

- The NA50 extracted cross section is systematically low.
- The cross section is approximately ****** = 7-8 mb**.
- One expects = 2-3 mb in the color-singlet c-cbar state.
- There is evidence from p-pbar and p-A for color-octet state.

- Average path (L) is not a good variable for precision studies.

- M.J.Bennett and J.Nagle, Phys. Rev. C nucl-th/9812039

## J/ Suppression

- 1.  **Formation of Quark-Gluon ****	****Plasma and dissociation ****	****of the J********!**
- 2.  Interactions with hadronic 		co-movers ()
- 3.  Initial State Energy Loss 		 reducing J/ production
- 4.  Other Ideas....

- Glauber model with no c-cbar breakup

- Glauber model with breakup (=6-8 mb)

_**NA50 data**_

## Drell-Yan 

**Glauber Model** calculation gives an excellent description of NA50 (**1996**) Drell-Yan transverse energy spectra.  Includes fluctuations in ET production and calorimeter resolution.

**Linear Scale**

**Log Scale**

- M.C. Abreu et al.,  Phys.Lett. B450 (1999) 456

## Study physics in e+e- channel

- Study physics in e+e- channel
- After heroic efforts to
- Suppress Dalitz pairs
- Suppress conversions
- Understand background
- Then:
- Form M(e+e-) spectrum
- Divide by charged yield
- Compare to known sources
- Excess seen for
- 0.3 GeV < M(e+e-) < 0.7 GeV
-      from
-  annihilation?
- collision-broadening?
- density dependent masses?
- Chiral symmetry restoration?

## Antilambda Yields

## Chemical Equilibrium