# Presentation: 698759

## QCD in the Heat Bath:Results from PHENIX

- Thomas K Hemmick
- for the PHENIX Collaboration
- University at Stony Brook

## Overheard at APS ~1992:

- During a plenary RHI talk at APS, I wound up seated among “real” plasma physicists who made numerous comments about us:
- “These guys are stupid...”
- Always a possibility.
- “...why don’t they just shoot a laser through it and then they’d know if its plasma for sure!”
- Visible light laser...bad idea.
- Calibrated probe through QGP...good idea...
- ...but not new. (Wang, Gyulassy, ...)

## The “Calibrated” Plasma Probe

- Hard scattering processes and products:
- Occur at short time scales.
- Are “calculable” (even by experimentalists) in simple models (e.g. Pythia) with appropriate fudging:
- Intrinsic kT
- K scaling factor.
- Find themselves enveloped by the medium
- Are “visible” at high pT despite the medium
- Promise to be our laser shining (or not) through the dense medium created at RHIC.

## QCD and the Heat Bath

- I will make the following artificial divisions:
- QCD
- Charged Hadron + Neutral pion spectra at high pT.
- Heavy quark production measured via electrons at high pT.
- J/,  direct, +- coming in the future...
- Heat Bath
- Multiplicity, ET, ...
- Identified hadrons
- Singles, pairs
- Flow, Hadro-chemistry
- Caveat:
- Short mean free path means that the view of the heat bath is heavily influenced by the latest stage prior to breakup.
- Nonetheless, initial state information (azimuthal anisotropy) is apparent.
- Controlling parameter
- Nature’s variation of the “centrality” controls the heat bath.
- Size/Energy variation is better, but not yet available.

## Pioneering High Energy Nuclear Interaction eXperiment

- 11 	Countries
- 51 	Institutions
- 400+ Collaborators

## PHENIX from Above

## West Arm
tracking:
DC,PC1, PC2, PC3
electron ID:
RICH, 
EMCal
Photons
EMCal

- West Arm
- tracking:
- DC,PC1, PC2, PC3
- electron ID:
- RICH,
- EMCal
- Photons
- EMCal

- East Arm
- tracking:
- DC, PC1, TEC, PC3
- hadron& electron ID:
- RICH,TEC, TOF, EMC
- photons:
- EMCal

- PHENIX Setup During Year 2001 Run

**full heavy ion program and first spin physics including electron & muon pairs**

- South Arm
- tracking:
- MuTr
- muon ID:
- MuID

- Other Detectors
- Vertex
-         & centrality:
- ZDC, BBC,
- MVD

**accumulated **
**   ****~200 10****6 ****Au-Au **

- The main 2001 data production pass has begun.
- Physics results still from 2000 data set.

## Centrality Determination

- ZDC measures neutral energy (principally free spectator neutrons) along collision axis.
- BBC measures produced charged particles within +/-(3.0-3.9).
- Their joint response is divided into centrality slices :
- Each colored band is 5%.
- Central is at lower right.
- The PHENIX trigger accepts 92% of  minimum bias events.
- A Glauber calculation was used to deduce the numbers of participants (Npart) and binary collisions (Nbinary) for each centrality.

- 0-5%

- 5-10%

## Transverse Energy Production

- Transverse Energy production yields Bjorken Estimate for Energy Density as:
- For =1.0 fm/c, this yields ~4.6 GeV/fm3!
-  well above estimated critical value c~0.6-1.2 GeV/fm3

-  ~ 1.5X higher than at CERN

## Multiplicity and ET vs. Npart

- This model produces particles via parton-parton scattering.
- Production is affected by the initial-state gluon distribution which “saturates” as low x gluons fill available space.

- Multiplicity and ET vs. Npart

- Multiplicity and ET both rise faster than linearly with the number of participants.
- This has been parameterized as:

- The “Binary Scaling” contribution  varies , ~25%  in peripheral collisions~50% in central collisions
- <ET>/<Nch> ~ constant with centrality

- soft           hard

## Multiplicity and ET vs. s

- Nch and ET evolution very well described by *ln(****s)*
- Saturation model also quite reasonable here (and especially with the PHOBOS result...).

## Composition of the Heat Bath:  PID

- Charged hadron ID is achieved via high-res TOF measurement.

- True photon pairs show a peak that is absent from mixed sample.
- Subtracted spectrum can be fitted for 0 yield.

## Yield vs. Centrality

- Yield per participant for all species rises with Npart.
- Not surprising since the total Nch rises faster than Npart.
- Integral of , K, p yields in excellent agreement with dNch/d result.

**x1.2**

**x1.7**

- Surprising to me:
- Yields of K+, K-, p, p-bar rise faster with Npart than pion yields.
- The “faster than Npart” character of Nch production also involves a change in relative species abundances favoring K and nucleon production!

## Net and Total Baryon Density

- Typically we have measure and quote “net baryon” density (b—bbar).
- Some physical processes will more naturally depend upon the “total baryon” density:
- e.g. medium modifications to mesons are predicted to be same for baryonic and anti-baryonic dense matter (Rapp).
- Here we calculate both densities:

- Caveat:
- We don’t measure neutrons!
- Assume: --(n+p)part=A/Z--nprod = pprod

## <pT>: Energy Distribution in Bath

- <pT> variation with centrality roughly the same for all species:
- ~20% rise.
-  and K close to pp production for peripheral collisions.
- Proton differs from pp significantly at most peripheral bin.

## Hadron Spectra

- Hadron spectra stiffen with increasing hadron mass.
- Well known consequence of collective motion in the final state.
- Spectra well fitted by “flowing thermal Ansatz:

## Thermal Fit Error Contours

- For each TFO and , a best fit and 2 are found.
- 2min/dof = 34/40.
- Contours show 1, 2, 3...
- Similar T to CERN, higher flow velocity.

- Although the result of such a technique is only as good as the assumed functional form, the differences between experiments can be quite instructive.

## HBT

- HBT correlations also reflect underlying collective flow velocities via the kT dependence of the radius parameter.
- World systematics of HBT nearly universal trend of Rout, Rside???
- Rout~Rside???
- Only Rlong variation with s???
- A real dilemma.

## What’s the composition of h++h-?

- PID for charged hadrons run out prior to the pT required for hard processes.
- Plots w/o renormalization show the species content of hadrons leading into our high pT probe particles.

- The stiff slopes (and increased yield) of nucleons cause the high pT spectrum to be more than 50% nucleons!

## Where does hard scattering begin?

- Initial state Hard scattering should produce a p/ ratio which falls with pT.
- mT scaling (flow) will producing a rising p/.
- ISR:  Onset of hard processes noted above 2 GeV/c.
- RHIC:  No clear onset of hard scattering is found in the data up to pT = 3 GeV/c !!

**p****T**

**m****T**** scaling**

**pQCD**

**p/******** ratio**

**soft/hard transition**

**measured**

## Charged Hadron Spectra to High pT

- Charged hadron spectra in bins of centrality show a gradual loss in concavity as they evolve from peripheral to central collisions.
- The loss in concavity is opposite to the effect one might expect comes from the known stiff proton contribution.
- The protons have a significantly higher slope than pions and become the majority contributor in the range 2-3 GeV/c.

- To quantify the shape evolution we can use either pp data or peripheral collisions.

## Unfortunately, no pp data at s=130 GeV exist.
We choose to interpolate using ISR, UA1, and CDF data.
The data establish the parameters of the “Power Law Fit” as a function of s.

These are smoothly interpolated to produce the solid curve in the figure.

- We choose to interpolate using ISR, UA1, and CDF data.
- The data establish the parameters of the “Power Law Fit” as a function of s.
- These are smoothly interpolated to produce the solid curve in the figure.

- Interpolation of pp Data

## Spectral comparison

- Good fit to peri-collisions using interpolated power law.
- Power Law overpredicts the central collisions

******0**

- The discrepancy between the neutral pions and the interpolated power law is more severe than for the charged hadrons.

## Difference between h and 0

- We subtract the charged nucleon contribution from the all-charged hadron distribution.
- The result is in reasonable agreement with the neutral pion spectrum demonstrating that the charged part. contamination is principally nucleons.

## Raa

- We define the nuclear modification factor as:
- By definition, processes that scale with Nbinary will produce RAA=1.

- RAA is below 1 for both charged hadrons and neutral pions.
- The neutral pions fall below the charged hadrons since they do not suffer proton contamination

## RAA--Peripheral

- We can also reference RAA using measured peripheral collisions to avoid the systematic error in the pp interpolation.

- Again, the central data drops below 1.0 and shown no Cronin!

## RAA—0 at RHIC and CERN

- 	WA98 measurements of RAA for 0 show a dramatically different behavior than at RHIC.

## RAA influences from “ordinary” effects.

- x~2pT/s~0.02

- Cronin effect:  Multiple scatters of the projectile broaden the incoming pT
- RAA exceeds 1.0 at high pT e.g. at CERN.

- Shadowing?...No.

## HIJING prediction for 0

- One explanation for the RAA spectrum comes from multiple scattering from a colored medium.
- The HIJING model using 0.25 GeV/fm energy loss well reproduces the measurement.

## Electron Identification

- The Electromagnetic Calorimeter and RICH were used to select single electrons.
- Neither device has sufficient identification working alone.
- The combination shows a clear peak of signal electrons that both fire the RICH and deposit energy equal to their momentum in the EMcal.

## Electron Singles

- Identified electron spectra are shown on the left.
- Low pT single electrons dominantly come from 0 Dalitz decay:0->e+e-and other neutral meson decays.
- Our measurements of the 0, +, - spectra determine this background level.
- Other backgrounds can also be modeled and subtracted as an “electron cocktail”.

## Tuning Cocktail Spectra

- The pion spectra are fit to an exponential + power law.
- Dalitz decay is an internal photon conversion.
- External photons conversions are normalized using a GEANT simulation and modeled as an upscaling to the Dalitz.
- Heavier meson spectra are assumed to follow mT scaling.
- Heavier meson yields are normalized by the pp measured ratio at high Pt (consistent w/ thermal).

- mT scaling OK

## Decomposition of Electrons

- Green band indicates error (statistical+systematic) in background.
- Data Shows excess over Cocktail at high pT.

## Pythia (after tuning...)

- Akiba will show detail about the tuning of Pythia to describe open charm production in pp.
- The addition of open charm decays ala’ Pythia well describes the electron excess.

## Fit the excess as charm

- The subtracted spectrum can be used to normalize the charm yield and thereby the cross section.
- The result is comparable within large error bars to the Pythia calculation.

## Summary-1

- The RHIC medium is certainly intriguing and exciting:
- Particle production scales faster than Npart possibly signaling a hard Nbinary component.
- Kaon and Nucleon contributions grow in percentage with centrality.
- Low net baryon density but high total baryon density.
- Strong radial flow implied by relative slope constants of hadrons.
- Nucleons begin to be dominant contributors to pT spectra above 2 GeV/c.
- HBT Rout~Rside???
- Probes of the medium yield exciting results
- RAA below 1.  Especially dramatic for identified 0 which have no proton background.
- Charm = Pythia within huge errors.  Why?

## Summary-2

- PHENIX has produced a wealth of data and has much more in analysis.
- The RHIC era opens the hard scattering or QCD toolbox (Pandora box?) in RHI physics.
- My wife usually criticises my use of new tools:
- LMH:  “Don’t you ever read the manual?”
- TKH:  “Honey, these tools _don’t come with manuals_!”
- Maybe this week we’ll begin writing the manual for QCD in the RHIC era.

## END OF ALL SLIDES

- SLIDES BEYOND THIS POINT WILL NOT BE SHOWN

## Singles Spectra

## Singles==all

## Singles mt

## Yields

## <pt>

## Ratios

## Ratios2

## ID vs dN/dn

## Acceptance

## HBT Results

## HBT spectra

## Charged Particle PID

## Pi0 PID

## Flow

## Hard Spectra

## Hard Spectra 2

## Hard pi0 spectrum

## SQRT(s)

## SQRT(s)-2

## Raa

## Raa2

## Raa3

## Hard pp

## Suppression—Pt

## dE/dx-theory

## Lambda

## Electron PID

## Cocktail

## Electron Spectra

## Parameters

## Pythia

## Flow—v2

## Thermal

## Ralf

## Ralf2

## Ralf3