1. Ideas on future “heavy-ion physics” at the CERN SPS and SPS+
Physics case for fixed-target heavy-ion physics after LHC start-up, including comments on detectors, required R&D, cost, time scales
A very personal view, “biased” by a NA60-like perspective
Carlos Lourenço – CERN-PH POFPA, November 11, 2005

2. What makes the SPS special?
Energy range seems to be particularly interesting for studies of the QCD phase transition, according to some observations and theory understanding
Fixed-target allows for rather high luminosities, crucial for the study of rare processes, as long as production cross-sections are not too small (true for J/y and charm, not for  and beauty)
Comments specific to dimuon measurements (a la NA60):
Good acceptance and mass resolution
Good signal to background ratio
Good muon track offset resolution
Surely competitive w.r.t. RHIC and LHC experiments…

3. This is a data-driven field
Guidance from theory has been very important, of course, but significant progress in the field crucially depends on high accuracy measurements
An accurate measurement at the SPS can be more useful to push progress than a lousy measurement at higher (or lower) energies
Certain physics topics require a robust “reference baseline”, such as the “normal nuclear J/y absorption” defined from p-A data; at high energies (RHIC, LHC) we must redefine what “normal” is… How do we account for the beauty feed-down to J/y at RHIC? How do we separate QGP suppression from thermal regeneration at the LHC?
Each machine (SPS, RHIC, LHC, etc) needs p-A and light-ion running time; and comparing data collected at two or three different energies can be extremely informative (cf. RHIC)

4. The NA60 detector concept
2.5 T dipole field
Muon trigger and tracking:
muon spectrometer (NA10)
vertex tracker (silicon pixels)
beam tracker
magnetic field
iron wall
targets
hadron absorber
muon
other
Major technological breakthrough: rad-hard silicon pixel detectors
Matching in coordinate and momentum space
Improved dimuon mass resolution: from ~70 MeV to ~20 MeV at the w mass
Origin of muons can be accurately determined: distinguish from

5. Statistics and mass resolutionh w f
Background(s)
Opposite-sign muon pairs
Signal dimuons
NA60
CERES
From CERES to NA60:
More than 1000 times higher effective statistics (thanks to the dimuon trigger)
and better mass resolution by factor 2–3 (thanks to the vertex tracker)

6. Vertexing and muon offset resolution
Beam Tracker sensors
windows
7 targets
The interaction vertex is identified with better than 200 mm accuracy along the beam and 10–20 mm accuracy in the transverse plane 10 20 30
 (m)
Number of tracks
beam track vs. VT vertex dispersion
Resolution of the impact parameter of the muon tracks at the vertex: 40–50 mm (measured using J/y muons)
vertex resolution (assuming sBT = 20 mm)

7. NA60-like physics framework
Low-mass:
clear broadening of the r spectral function (related to chiral symmetry restoration?)
Intermediate-mass:
enhanced prompt dimuon production
(thermal radiation from the QGP phase?)
J/ production:
step-wise suppression
in In-In collisions (sign of QCD phase transition?)

8. Questions & answers: J/y suppression (I)
Question: Is the J/y really suppressed anomalously, as claimed by NA50 from Pb-Pb data?
Answer: Yes; seen by a second experiment (NA60) and in a different collision system (In-In)
Question: What is the physics mechanism behind the anomalous J/y suppression?
 compare the J/y suppression pattern measured in In-In collisions with the Pb-Pb pattern, as a function of several physics observables, and with theory predictions, previously tuned on the Pb-Pb data…
Answer: the measurements are consistent with expectations from a QCD phase transition and corresponding melting of the y’ and cc states at around the critical energy density
However: more and better data are needed to reach a conclusive interpretation…

9. J/y survival probability: SPS and RHIC
SPS pattern w.r.t. normal absorption curve; PHENIX pattern w.r.t. d-Au data; Bjorken e with t = 1fm/c at both energies
The open squares are × y’
using the NA50 Pb-Pb y’ pattern…
Should overlap with the J/y pattern if the y’ and the cc melt at the same energy density and if 40% of the observed J/y events in pp collisions are due to y’ and cc feed-down
J/y NA60 In-In
J/y NA50 Pb-Pb
J/y PHENIX Au-Au cc y’
J/y e
The measurements indicate that the y’ and the cc melt at around 1 GeV/fm3 of energy density while the (directly produced) J/y mesons are not suppressed, at least until e ~ 6 GeV/fm3
However: the existence of the two flat lines is not yet established: the PHENIX data points have big error bars (and may be shifted horizontally if we change t); the SPS points do not extend up to the expected plateau at 0.6 (requires higher energies) or down to the region below 1 GeV/fm3 (requires lower energies or lighter colliding nuclei, like Cu-Cu)

10. Questions & answers: J/y suppression (II)
Q: What is the normal nuclear absorption curve at the energy of the heavy-ion data?  study the J/y production in p-A collisions at 158 GeV
Q: What is the impact of the cc feed-down on the observed J/y suppression pattern?  study the nuclear dependence of cc production in p-A collisions
Questions being addressed by the 2004 p-nucleus data:
158 GeV data: will provide answers for the y, but surely not for the y’ or the y/DY
400 GeV data: will give a global ratio cc over J/y but probably not its nuclear dependence (despite the 40 MHz interaction rate; the beam time was limited by cooling system problems)

11. Questions & answers: thermal radiation
Q: Is there an intermediate mass region excess in HI collisions, as claimed by NA38/NA50?
A: Yes; also seen by NA60 in Indium-Indium collisions
Q: Is it compatible with the NA38/NA50 observation?
A: Yes; leaving the charm yield free describes the In-In data, with ~ 2 times more charm than needed by the p-nucleus dimuon data (of NA50)
Q: Is this validated by the muon offsets information?
A: No, the charm offset distribution is too flat to describe the excess spectrum…  We need more prompt dimuons
Q: How many more prompts do we need?
A: Two times more prompts than the expected DY yield (fixed in the high mass region)
Q: What is the mass shape of the excess and how does it scale with centrality?
A: The mass spectrum of the excess dimuons is steeper than DY and flatter than Open Charm; It seems that it scales faster than linearly with Npart
Q: What is the charm production cross-section in In-In?
A: Work in progress…
Is the IMR dimuon excess due to open charm enhancement? No!
Is it due to thermal dimuons from a QGP? Maybe!
Summary:

12. Questions & answers: low mass dileptons
Q: Are there in-medium modifications of the r meson, as claimed by CERES?
A: Yes; also seen by NA60 in Indium-Indium collisions
Q: Is it compatible with the CERES observation?
A: Yes; the new source has a faster than linear dependence on the charged particle multiplicity and is significantly higher for low pT than for high pT dileptons
Q: Do we see a shift of the r pole mass or a broadening of its width?
A: The measurements are compatible with r broadening but not with a r mass shift
Summary:
Is the low mass excess due to “Brown-Rho scaling”? No!
Are the observations related to chiral symmetry restoration? Not yet understood
Present In-In results are not limited by statistics or resolution but by systematical uncertainties: acceptances, efficiencies (trigger, matching, etc), Monte Carlo accuracy, etc.
The cocktail of hadron decays is also uncertain: BR(w→mm), w form factor, r/w interference, kinematical distributions, etc.  Importance of data from p-nucleus and light-ion collisions

13. Phase space coverage and acceptances
No acceptance for dimuons with transverse mass below ~ 500 MeV
In-In data
Event distribution in the M vs. pT plane
Low mass and pT dimuon acceptance could be improved by using a lighter hadron absorber, changing the dimuon trigger logic and decreasing the field of the MS magnet

14. Summary of the present situation
The NA60 accurate measurements already provided concrete and final answers to some of the questions which motivated the experiment
But the value of a scientific discovery is also measured by the number of important new questions it raises (Bernd Muller dixit)
Further high quality measurements should lead to a very significant step forward in our understanding of the QCD phase transition and chiral symmetry restoration
A new and significantly improved NA60-like experiment could be ready to take data from 2010 onwards (not in 2009), when ALICE will be smoothly running in stable conditions
Besides answering specific questions coming from previous results (at the SPS and at RHIC), the new experiment could also contribute, as a by-product, to explore new ideas (such as the search for the critical point of the QCD phase diagram)

15. A “lungimirant” wish list…
A clear “step-wise” J/y suppression pattern, with a well established understanding of the ccbar production in p-A collisions, the fractional contributions from higher state feed-downs, and the normal nuclear absorption cross sections for each charmonium state; complemented by a detailed study of open charm production rates Dream: measure cc production in HI collisions…
A clear confirmation of chiral symmetry restoration, through accurate data on the r spectral function as a function of mB and T (several collision systems and energies) Dream: measure the a1 spectral function in HI collisions…
A clear confirmation of thermal dimuon production, the “signature par excellence” of radiation from a QGP phase, with a well established understanding of the contribution from the hadronic phase and a solid link between the excesses seen in the low and intermediate mass ranges
Dream: complement with a simultaneous measurement of direct photons
Observation of anomalously high fluctuations in narrow ranges of T and mB (collision energy and system) in yields, particle ratios and average pT, of well identified particles (resonances) and in specific phase space windows… as an exploratory “by-product”
Dream: have a clear theoretical connection between such observations and the existence of the critical point

16. Concrete list of foreseeable future runs…
In the period 2010–2013:
Pb-Pb at 158 GeV/nucleon: improved study of charmonia w.r.t. NA50 data and completely new study of low and intermediate mass dimuons at the highest SPS energy densities
Pb-Pb at 50 GeV/nucleon: to measure the r spectral function at higher mB and lower T; ideally it should be complemented by shorter runs (~1 week) with other A-A and energy combinations
Cu-Cu at 158 GeV/nucleon: to establish the “normal J/y absorption” before new physics sets in and confirm the location of the critical energy density (e is not well defined in p-A)
Pb-Be at 158 GeV/nucleon: “inverse kinematics” experiment, to measure nuclear effects in charmonia production at negative xF (down to around -0.7)
Once the SPS+ is available:
Pb-Pb (or U-U ?) at the highest SPS+ energy: to confirm with high statistics that the J/y survival probability flattens out at 0.6, until it is further suppressed at the LHC (if regeneration does not take over)

17. Main detector components
Muon spectrometer:
Conceptually similar to NA60 but we cannot run in 2010 detectors built (by NA10) with technology developed before 1980…
Tracking chambers, readout and trigger electronics are anyway “dead” by now
NA10 was designed to study high mass DY and  production in p-A collisions; we need much better low mass dimuon acceptance and resolution, and coping with higher interaction rates and occupancies: an optimised design is desirable
Vertex Tracker:
Conceptually similar to NA60
Baseline scenario: fully made from ATLAS silicon pixel modules (40 MHz, low material budget, rad-hard)
Improved scenario: use pixel detectors with ~1 ns accurate time stamping, per hit; would imply R&D effort in collaboration with other projects ZDC:
New quartz fiber calorimeter; identical to those previously used (NA50/60, ALICE)

18. Very rough cost estimate
Muon spectrometer:
New MWPC tracking chambers: ~ 0.7 M€ (scaled down from ALICE muon-arm)
New readout and trigger electronics: ~ 0.5 – 1 M€
Vertex Tracker:
Using ATLAS-like modules: 0.5 – 1 M€ (if ATLAS people would help as in 2004)
ZDC:
Less than 150 k€ Total experiment:
Up to 5 MCHF in the “cheapest scenario” of using “existing technology”
Maybe twice as much using state-of-the-art detectors, if done in collaboration with other projects (LHC upgrades, etc)

19. Resources and time scale
A new collaboration needs to be formed…
Four NA60 groups are very interested; other groups must be found or confirmed
From Italy ~18 FTE permanent people (Torino: 11; Cagliari: 7) want to participate but are strongly involved in ALICE
 Time scale has to minimize impact on ALICE start-up:
Brainstorming and feasibility studies: 2006–2007
Design, construction, installation and testing: between 2007 and 2010
Running: from 2010 onwards, depending on smoothness of ALICE operation
Foreseeable duration of the project:
3 years of stable data taking with existing SPS machine
plus another 2 or 3 years if SPS+ available
Likely running scenario: 2 weeks of protons followed by 5 weeks of ions, per year, if NA48/3 is approved and gets “all” available proton beam time (otherwise we could take much more proton time)

20. Two final remarks
NA60 was successful, in terms of data quality and derived results, thanks to a very small group of exceptionally bright young people (experts in detectors, electronics, DAQ/DCS, and offline); some are still around for another couple of months…
Without these hard-working young people the new experiment will remain a dream; a positive encouragement by CERN towards such future ideas would be extremely important, and cannot wait much longer…
When NA10 was built, nobody thought the muon spectrometer would be running for 25 years… and doing several different and interesting physics studies
It is a good idea, for CERN, to have a state-of-the-art dimuon spectrometer, with accurate vertexing capabilities and able to survive very high interaction rates
You never know when someone will come with another brilliant idea and keeps it running for a longer period, producing unexpected physics results (cf. NA51)