# Presentation: 400357

## Heavy Ions at the LHCTheoretical issues 

- Super-hot QCD matter
- What have we learned from RHIC & SPS
- What is different at the LHC ?
- Goals of HI experiments at the LHC

- LHC2003 WorkshopFermilab     April 30 – May 3, 2003

## QCD phase diagram

## From hadrons to QGP

- QCD equation of state from lattice QCD

## Signatures of a QCD phase change

- Effects of “latent heat” in (*E,T*) relation
- Enhancement of *s*-quark production
- Bulk hadronization
- Disappearance of light hadrons (*ρ*0)
- Thermal *l**+**l**-* and  radiation
- Dissolution of *Ψ**, **Y**  *bound states
- Large energy loss of fast partons (jet quenching)
- Net charge and baryon number fluctuations
- Collective vacuum excitations (DCC)

## Thermodynamics near Tc

## Hadrons at the boiling point ?

- Plateau is an indicator of latent heat of phase transformation.

## Strangeness enhancement...

- ...probes chiral symmetry restoration and deconfinement

## Jet Quenching

- Radiative energy loss

- can be described as medium effect on parton fragmentation:

## Scaled yield in Au+Au vs. p+p collisions

**Peripheral**

**Central**

## p /  ratio

- p /  ratio

- HIGH-ENERGY PHYSICS:Wayward Particles Collide With Physicists' Expectations
*Charles Seife*
**EAST LANSING --**At a meeting here last week, resear-chers announced results that, so far, nobody can explain. By slamming gold atoms together at nearly the speed of light, the physicists hoped to make gold nuclei melt into a novel phase of matter called a quark-gluon plasma. But al-though the experiment produced encoura-ging evidence that they had succeeded, it also left them struggling to account for the behavior of the particles that shoot away from the tremen-dously energetic smashups.

## Azimuthal anisotropy v2 

**p****y**

- Semiperipheral collision

- Collective flow behavior extends to higher *p*T for baryons (p,) than mesons (,K)

- Indicates early equilibration (< 1 fm/c)

## Quark recombination ?

## Recombination vs. Fragmentation

- Fragmentation...

- ... never competes with recombination for an exponential spectrum:

- ... but  wins out at large *p*T, where the spectrum is a power law ~ (*p*T)-b

- Recombination:

- For a thermal distribution:

## Model fit to hadron spectrum

- Also explains *p*/ ratio ≥ 1 below 5 GeV/c !

## Does v2 reflect parton flow?

- Recombination model suggests that hadronic flow reflects partonic flow (*n* = number of valence quarks)*:*

- Provides measurement of _partonic_ v2 !

## What’s different (better) at the LHC ?

- Higher energy density 0 at earlier time 0.
- Jet physics can be probed to *p*T > 100 GeV.
*b, c* quarks are plentiful, good probes.
- Increased lifetime of QGP phase (10-15 fm/c)  Initial state effects less important.
- QGP more dominant over final-state hadron interactions.

- Much larger “dynamic range” compared to RHIC

## Parton saturation at small x

- 

- After “liberation”, partons equilibrate and screen color force

## ECM dependence of dN/dy, dE/dy

- NLO pQCD with geometric parton saturation (Eskola et al. - EKRT)
- Probably overestimate of growth of *dN*/*dy*, *dE*/*dy *with *E*CM

## Quo vadis - J/ ?

*V*qq is screened at scale (*gT*)-1 heavy quark bound states dissolve above some *T*d.

- But deconfined *c*-quarks can recombine at hadronization and form new *J*/.

*J*/suppression ?

*J*/enhancement ?

## Jet quenching at the LHC

- I. Vitev, M. Gyulassy, PRL 89 (2002) 252301

## Hadron production at the LHC

## Photon tagged jets

- High-energy photon defines energy of the jet, but remains unaffected by the hot medium.

- Parton energy loss is measured by the suppression of the fragmentation function *D*(z) near z1 .

## Measuring the density

- R.J. Fries, BM, D.K. Srivastava, PRL 90 (2003) 132301

- Backscattering probes the plasma density and initial parton spectrum

## Summary

- SPS: First glimpse (“evidence”) of the QGP
- RHIC: Discovery of the QGP ?!
- LHC: (Quantitative) exploration of the QGP facilitated by plentiful hard probes !
- Specific questions:
- How does dE/dx depend on energy density?
- How is the fragmentation function modified?
- Are *c *and* b* quarks thermalized?
- Gluon saturation in nuclei at small *x*
- Higher twist effects