HEAVY ION COLLIDERS – THE NEXT GENERATION*
F. Pilat, BNL, Upton, NY 11973, USA
Abstract
High luminosity will extend the physics reach of heavy
ion colliders beyond the present quark-gluon plasma(QGP) production capabilities, into the domain of hard
scattering quarks and gluons in a QGP, and into thedetection of rare processes.
After a review of the physics potential of heavy ion
colliders, the fundamental luminosity limitations of suchmachines will be discussed, with particular emphasis on
the intra beam scattering mechanism.
Plans are being developed for a staged luminosity upgradeof RHIC: initially, the luminosity will be optimized byincreasing focusing and number of bunches, but aluminosity increase of over a factor ten will require higher
bunch intensity and lower emittance, the latter only
achievable via electron cooling of the RHIC beams at top
energy. Electron cooling at RHIC is feasible and a designfor the electron beam based on a re-circulation linac
already exists. The Heavy Ion option of the LHC is also
actively studied and planned at CERN, with the goal of ahigh energy heavy ion collider coming on line in thesecond half of this decade. Challenges and features of thismachine, including luminosity optimization, and recent
developments, are reviewed and discussed.

1 THE PHYSICS POTENTIAL
The experiments at the AGS and the SPS, as heavy-ion
fixed target machines, heralded a new era for nuclear
physics, and with the commissioning and early operationof RHIC, a dedicated heavy-ion collider, in summer of
2000, a new domain of physics has started to emerge, as
documented by early results presented at the Quark-Gluon
Matter Conference in January 2001.
RHIC is the first heavy-ion facility that can be truly
considered a nuclear-physics QCD machine, as it
produces nuclear matter at temperatures and densities
great enough to excite the expected phase transition from
confined quark-gluon states to a quark-gluon plasma
throughnucleon-nucleon(A-A)
collisions.
A
complementary physics program that studies QCD in highenergy proton-nucleus collisions (p-A) and the spin
structure of the nucleon in polarized proton-proton
collisions is also pursued.[1]The first priority for the next four to five years, as
discussed at recent Town Meeting organized by the APS
Division of Nuclear Physics to determine future directions
in nuclear physics [2], is to fully exploit RHIC and its
present physics potential. This means primarily
establishing signatures and dynamics of the phasetransition to QGP. To this end, systematic scans with AuAu collisions with a center of mass energy between 20

and 100 GeV/nucleons will be carried out together withcollision of lighter ion species.

Figure 1: Quark – Gluon plasma diagram
Though the exploitation of present facilities is theimmediate priority, existing experimental results from
AGS, SPS and RHIC and theoretical predictions, call for a
new generation of heavy-ion colliders. If a new state ofmatter is demonstrated, its properties have to besystematically studied and consequences for new physics
be drawn. Correlation of observables and processes ofextremely low cross section such as high mass and highpT events are the key, to move from soft to hard QCD
events.
To reach into the domain of perturbative QCD, one canpush the luminosity limit, and this is the plan for RHIC, orthe energy limit, as in the LHC Heavy-Ion. With center ofmass energy 30 times larger than RHIC, the LHC HeavyIon will open a new energy domain, where heavy-ion
collisions will happen in a “classical” QCD regime, andsemi-hard and hard processes will be the dominant feature.
Compared to RHIC, the ratio of the QGP state lifetime
versus the thermalization time is 10 times larger, so
parton dynamics are expected to dominate the expansion
and the collective features of the hadronic final state.
There is complementarity between the physics reach of aluminosity upgraded RHIC and the LHC Heavy-Ion.
In Section 2 I will discuss the factors determining theluminosity of a heavy ion collider and its limiting factors.
Specific plans for a RHIC luminosity upgrade arepresented in Section 3 with a timeline for its realization.
The LHC Heavy Ion program at CERN is presented in
Section 4 with particular emphasis on recent
developments that recently called for a design review.
Conclusions and future perspectives on heavy-ion
colliders are drawn in Section 5.

2 HEAVY ION COLLIDER LUMINOSITY
The luminosity of a collider can be usefully expressed interms of relevant beam parameters in the following way:L∝

f rev N b N 2
4π εβ *

To increase it, one can increase the number of bunches(Nb) in the machine, the bunch intensity (N), the focusing
at the interaction point and decrease the emittance, andlimits are reached in every case. Increasing the number ofbunches means reducing the bunch spacing: a limit of~100 nsec at RHIC is set by the data acquisition system of
the experiments. Decreasing β* leads to limits in theinteraction region final focus quadrupoles, at RHIC a
reduction from the design value of 2m to 1m is possible,
but beyond that stronger triplet quadrupoles are necessary.
The limits related to the increase of the phase spacedensity N/ε are of a more fundamental nature. Assuming
round beams and equal emittances the beam-beam
parameter can be written as:

ξ∝

r N

ε

where r is the classic radius of the ion. The beam-beamlimit can be reached by increasing phase space density.
The real limitation to the phase space density for ion
colliders though arises from Intra Beam Scattering (IBS),
which above transition causes beam growth bothtransversely and longitudinally.

Figure 3: RHIC Luminosity at collision as a function of
store time.

3 THE RHIC UPGRADE PROGRAM
The RHIC Upgrade program is summarized in Table 1,
where RDM (RHIC Design Manual) is the nominal
configuration, RDM+ (Phase 1) is a configuration inwhich machine parameters are pushed but no new“hardware” is required while RHIC II (or Phase 2) isbased on an active electron cooling system to counteract
the beam growth [3].

Initial ε (95%)
Final ε (95%)
β* at IPN bunches
N ions/bunch
b-b ξ
rms σ
Angular σPeak L
Average L
Timeline

Figure 2: Transverse beam size and normalized emittance
growth in RHIC as a function of store time due to IBS.
Figure 2 shows respectively the evolution of beam sizeand emittance in RHIC in the collision configuration.
Beam growth translates in loss of instantaneous andaverage luminosity due to IBS. IBS has been always
regarded as the ultimate luminosity limitation for heavy
ion colliders. Interestingly, new calculations for the LHC
evidenced that for LHC Lead option the luminosity islimited by the quench limit, a fact that will be discussed in
Section 4. For RHIC, IBS is still the limitation.

units

RDM

πµm
πµm
m

15
40
2
60
1
0.0016
216
108
0.8
0.2
2001

109
per IRµmµrad1027cm-1s-1
1027cm-1s-1
year

RDM
+
15
40
1
120
1
0.016
150
153
3.2
0.8
2003

RHIC
II
15
<6
1
120
1
0.004
95
95
8.3
7
2005+

Table 1: Scenarios and parameters for the RHIC Upgrade(RDM=RHIC Design Manual, RDM+=Phase 1 upgrade,
RHIC II= Phase 2 – with electron cooling)
In the Phase 1 Upgrade (RDM+) the number of bunches isdoubled as well as the focusing at the IP. The latter will
require careful optimization of the nonlinear correctors at
the triplet locations, but no new equipment will benecessary. This level of performance, that will increase
the luminosity by a factor 4 is expected to be reached in
2003.The further improvement of a factor 10 and 35
respectively in peak and average luminosity for RHIC II
is based on an electron cooling system.
(For RHIC II the luminosity is averaged over 5 h due toion burn-off).

Figure 3. The RHIC electron cooling system
Figure 3 describes the electron cooling system proposed
for RHIC [4], a collaboration of BNL and the Budker
Nuclear Physics Institute in Novosibirsk.
Electron cooling is well established and tested at low(er)energy machines. The electron cooler for RHIC will
operate for the first time on a high energy, bunched beam,in a collider. For a 100 GeV/n Au beam the required
electron energy is 54 MeV, with a peak current of ~3A
and average current of ~100 mA. The most efficient way
to produce the required electron beam is a high brightness
and high power energy-recuperating superconducting
linac. As can be seen in Figure 3, the gold and electron
beams transversely overlap over a 30 m drift, in the long
RHIC straight section. Electron are removed and theenergy recuperated, with an efficiency evaluated to ~95%.
.

No cooling1010 e-/bunch
3*1010 e-/bunch
1011 e-/bunch

e28

e27

e26
e-3

2h

Time (h)e-4Particular attention has to be paid to the cooling scenarios
to optimize cooling time and to counteract the effect ofion burn-off. As it can be seen from Figure 4, the coolingtime decreases as one increases the electron current andthe cooling efficiency increases as the beam becomes
smaller (self-enhancing effect). Typical cooling times for
RHIC are of ~1-2 hours. However, when the emittance isreduced, competing mechanisms set in that cause a loss ofions from the beam. The main effects causing ion losses
are ion recombination in the cooling section and collision
losses. Electrons from the beam can be captured by theion, leading to ion losses in the cooling section. Thiseffect can be cured by magnetized electron cooling. The
spiraling motion of the electrons in a solenoid magnet
greatly reduces the recombination effect. Magnetization
has been demonstrated in simulation and there are plans totest it experimentally at IUCF. Collision losses are due toe+-e- pair production, followed by e- capture by the ion,and electromagnetic dissociation (γ-nucleon event). The
total cross section is evaluated for RHIC at 212 barn.
Operation scenarios with stores of a few hours with fast
refills are considered to optimize the integrated luminosity.
A luminosity upgrade of a factor 35 over the design value
in average luminosity appears feasible with 120 bunches,
109 ions/bunch and electron cooling to counteract IBS
emittance growth. To increase the luminosity over afactor 35, going to even larger number of bunches (360and even 2520) may be possible with major upgrades of
the abort system, shielding and damping systems. In thecase of 360 bunches, the resulting bunch spacing is 35.5:
that would require an upgrade of the RHIC experiments
but is still modest in comparison with the bunch spacingof existing b-factories, the Tevatron and the LHC. Withlarger number of bunches, however, other beam dynamics
effects may play a role and need careful study andconsideration, such as electron cloud effects, impedance
and instabilities, and heat load to the cryogenics system.

e-5

4 THE LHC HEAVY ION PROGRAM
Figure 4. Luminosity and emittance vs. time(h) for
different electron currents

The Ion program has been an integral part of the LHC
from 1995. Recently the program for heavy-ion collision

at LHC has gone through a process of review andplanning for future directions [5].
Several new issues prompted this re-evaluation: the crosssection for nuclear effects in Pb-Pb collision have been
updated, the option of having more than one experiments
has been taken into consideration, and new operating
scenarios have been added. Moreover, a new limitation
for Pb-Pb collision related to ion losses has been shown
to be the luminosity limiting factor and not IBS.
The nominal parameters (after the review) for the LHC
lead program are listed in Table 2.
energy/nucleon [TeV/u]transverse normalized emittance [µm]β* at interaction point [m]rms beam size at IP [m]longitudinal emittance [eV sec/Q]
rms bunch length [m]rms energy spread [10-4]
bunch spacing [nsec]
number of bunches / ring
Filling time / ring [min]number of ions / bunch
IBS growth time [h]Luminosity half time [h]Initial luminosity [cm-2 s-1]

2.76
1.5
0.5
15
2.5
7.5
1.137
124.75608
9.8
6.8 10715
4.2
1 1027

Table 2: Nominal (revised) parameters for the LHC
Heavy Ion Project
Figure 7 [6] presents a scheme of the LHC Lead Collider
together with the chain of injectors.

An interesting development has been the re-evaluation of
nuclear cross section in Pb-Pb collisions, on the basis of
new experimental data. Table 3 summarizes the new crosssections.
Cross section Pb-Pb
hadronic
electromagnetic dissociationelectron capture

symbolσH
σemd
σec[barn]
8
225
204

Table 3. Cross sections for Pb-Pb collisions.
The total cross section of 437 barn is sufficiently larger
than the one previously used (280 barn) to justify a reoptimization of luminosity lifetime and operating
scenarios.
Another interesting new result is that the luminosity of the
Pb-Pb option is not limited by IBS. The ions lost toelectromagnetic dissociation and electron capture aredeposited in the LHC dispersion suppressor. [6] The
longitudinal density of energy deposition places a severe
limit on the luminosity in order not to exceed the quench
limit of the magnets (5 mW/cm-3). The luminosity limit is1027 cm-2s-1, which for 608 bunches sets a limit on thenumber of ion per bunch of 6.8 x107.
The present plan calls for the first lead beam to the SPS inearly 2006 and the first lead beam to the LHC in early
2007. There is a vision for further widening of the LHC
Ion program after the initial physics program with lead. A
second phase envisages collisions of lighter ions (type AA, not hybrid collisions). Candidates are Tin (Sn,50/120),
Krypton (Kr,36/84), Argon (Ar,18/40) and Oxygen (O,8/16). The luminosity with tin is also affected by quench
limit consideration, but the luminosity limit in this casecoincides with IBS. For all lighter ion options, theluminosity limitation arises from IBS. One of the ideasthat are considered is to optimize the luminosity byincreasing the focusing (β*), during the store.
There is another longer time scale upgrade possible withhybrid collisions of the type proton-nucleus and deuteronnucleus. Hybrid collisions of type A-B are not foreseen at
the moment. Physics at intermediate energies is also a
possibility that is considered.

Figure 7: Schematics of the LHC Lead Project

5 CONCLUSIONS
The focus o the injector upgrade is the development of
LEAR into LEIR, optimized for stacking and cooling of
the Pb ions.

There is exciting physics beyond the discovery and studyof quark-gluon plasma to justify the planning of a newgeneration of heavy-ion colliders.

RHIC’s potential will be fully exploited in the next few
years: an upgrade plan is being actively developed. A
two-phase approach calls for an increase in luminosity bya factor 4 by pushing the existing parameters by 2003 anda factor 10 (and respectively a factor 35 in averageluminosity) with the addition of an electron coolingsystem. The latter upgrade is planned the timescale of2005.
While RHIC will push its luminosity the LHC Heavy Ion
will come online on a timescale of 2007 and open an
entirely new energy domain in heavy-ion collisions.
RHIC and the LHC will ensure more than 2 decades ofheavy-ion physics and the physics results will show thedirection for the next to the next generation. A heavy-ion
option for the VLHC project, now on the drawing board
with 87.5 TeV protons in the ~230 km tunnel, should be
evaluated.

6 REERENCES[1] T.Roser, These Proceedings[2] Town Meeting on Nuclear Matter and Hadrons at
High Energies. http://www.bnl.gov/rhic/townmeeting/[3] I.Ben-Zvi, M.Brennan, M.Harrison, W.W.MacKay,
V.V.Parkhomchuck, S.Peggs, T.Roser, A.Stevens,
D.Trbojevic, “Luminosity Upgrade Options for RHIC”,
Draft document, Private communication[4] I.Ben-Zvi, V.V.Parkhomchuck, “Electron Cooling for
RHIC”, Draft document, Private communication.
[5] D. Brandt, “Review of the LHC Ion Programme”,
LHC Project Report 450.
[6] K.Schindl, Private Communication[7] B. Jeanneret, Private Communication