Scientific Challenges for the Next Decade Urs Achim Wiedemann CERN, PH-TH Department Long Range Plan Rutgers Town Meeting 12 January 2007

assess the results obtained since the last LRP - do they address the goals identified then? - do they allow to identify novel goals of equally fundamental interest? assess the scientific outreach of the results obtained - are the results of interest/importance for other fields? - do they trigger questions/programs in other areas of nuclear science? To discuss the future of the field, start with an assessment: Then identify emerging opportunities: Whether and how to continue existing programs best? In the light of existing results, how should one focus/extend the supports of approved programs in the mid-term future? In the light of existing results, can we identify novel avenues of investigation, which address questions of fundamental interest?

RHIC results

Central Questions of the 2002 Long Range Plan 1)In relativistic heavy-ion collisions, how do the created systems evolve? Does the matter approach thermal equilibrium? What are the initial temperatures achieved? 2)Can signatures of the deconfinement phase transition be located as the hot matter produced in relativistic heavy-ion collisions cools? 3)What are the properties of the QCD vacuum and what are its connections to the masses of the hadrons? What is the origin of chiral symmetry breaking? 4)What are the properties of matter at the highest energy densities? Is the basic idea that such matter is best described using fundamental quarks and gluons correct? A long-term perspective should go beyond refining these questions!

1. Central (exp) Results of RHIC Program 1)Systems created in relativistic heavy-ion collisions display extremely strong form of collectivity (‘collective flow’). Consequences: a. collective behavior phenomenologically at least as important as local equilibration [HICs do more than studying the T-mu-plane!] -> understanding the collective dynamics is an essential requirement for any future ‘theory effort’. b. Arising novel questions include: - Is the observed collective flow close to maximal? This question is novel and fundamental, since i) ‘close to maximal’ can be quantified in terms of dissipative properties ii) dissipative properties are calculable from 1st principles of QCD iii) robustness criterion: the large size of the observed collective flow makes it experimentally feasible to establish dissipative corrections to fluid behavior. [ i)-iii) largely based on developments of last 5 yrs ] - How are collective modes generated by hard, well-localized perturbations, and how do they interact with them? [ question comes now into experimental reach (ridge, cone, …), and gives novel access to properties of hot and dense QCD matter.]

2. Central (exp) Results of RHIC Program 2) Systems created in relativistic heavy-ion collisions are extremely dense and strongly interacting with hard probes (‘jet quenching’). Consequences: a. RHIC results demonstrate that detailed experimentation of hot and dense matter with a large variety of hard probes is feasible. b. Central questions: how does a hard parton interact with and equilibrate in hot and dense QCD matter? RHIC has many options for further studies: - dependence of jet quenching on parton identity (vertex tagging for b-mesons!) - analysis of particle yields associated with high-pt triggers (cone, ridge, …) - varying penetration depth of probe (e.g. via photon triggers …) c. A connection of the abundant suppression pattern with 1st principles of QCD is feasible since: i) ‘strong suppression’ can be quantified, e.g. in terms of qhat and possibly other subleading parameters, important at lower pt. ii) qhat is calculable from 1st principles iii) robustness criterion: the large size of the jet quenching effect makes it experimentally feasible to arrive at a reliable determination despite many experimental and theoretical uncertainties.

3. Central (exp) Results of RHIC Program 3)Many bulk properties of the systems created in relativistic heavy-ion collisions are indicative of a suprisingly slow, tamed growth of parton distributions with log(s). (‘Parton saturation’). Consequences: a. RHIC results on bulk properties are generally consistent with expectations from ‘CGC-phenomenology’. b. RHIC may have access to further manifestations of small-x physics - in particular: d-Au with improved instrumentation at forward rapidity c. Non-linear QCD evolution is a fundamental and novel feature of QCD. It is calculable from first principles and it is experimentally testable. In the context of RHIC-phenomenology, there are many open questions in theory and experiment, but these should not prevent us from recognizing: RHIC data and phenomenology support a tamed growth of nuclear parton densities with log(s). This is indicative of parton saturation. Thus, RHIC has contributed significantly towards establishing a fundamental phenomenon of QCD, which lies outside the original scope of RHIC and which warrants further experimental and theoretical investigation.

My personal view on RHIC results: The fundamental task of ultra relativistic heavy ion physics is to connect properties of hot and dense matter to 1st principles of the underlying field theory, QCD. Historically, the first crisp connection between hot, strongly interacting matter and 1st principles of QCD were statements about the QCD phase transition, based on lattice QCD. But by now, this is only one amongst many connections. RHIC results have lead to a significant extension of the paradigm of what are the fundamental opportunities for connecting HI phenomenology to 1st principles of QCD: - strong collectivity opens opportunity for detailed characterization of dissipative properties of hot/dense matter, which are calculable from 1st principles of QCD - strong medium-modifications of hard probes allow to determine additional properties of hot/dense matter (such as qhat, a measure of the transverse color field strength), which are calculable from 1st principles of QCD. RHIC results have provided clear indications that the growth of parton distributions at small-x is tamed (e.g. extremely low multiplicity, strong suppression in d-Au at forward rapidity).

My personal recommendations I recommend that w.r.t. the RHIC HI program, the next LRP recognizes: RHIC has established that detailed experimentation of hot and dense QCD matter with a large variety of probes is possible. RHIC has discovered abundant and robust phenomena (e.g. collective flow, jet quenching, …), which are known to have a direct link to fundamental properties of QCD matter and which can be related to 1st principle calculations of QCD. It is known that the variety of the probes available for further exploring these abundant phenomena increases strongly with integrated luminosity and would profit from further upgrades in instrumentation. Qualitatively novel or qualitatively refined insights are likely to emerge from such a luminosity upgrade. To fully exploit the scientific opportunities arising from the significant (financial and intellectual) investment made into RHIC, a luminosity upgrade aiming for 10 times higher luminosity appears to be of the highest priority. To accomplish the task of relating RHIC data to 1st principles of QCD, a quantitatively controlled phenomenology of heavy ion collisions plays a crucial role. Theory must be strengthened to be able to meet this task.

Scientific Outreach

RHIC has widened the scope and outreach of nuclear physics A field is healthy and scientifically growing, as long as it attracts the interest of other communities, as long as it creates outreach: In the last 5 years, we have created significant outreach - to the wider QCD community (EXP and TH) - in particular to the small-x community (HI experiments + TH have helped to identify significant future experimental challenges for this diverse field) - to string theory In this specific sense, ultra-relativistic heavy ion physics is a healthy field, growing in depth and breadth up until today. Now turn to the possibility of further development and growth.

LHC A+A and p+A LHC is a confirmation machine for RHIC but it is conceivably also a ‘falsification machine’ for some of the models developed at RHIC this alone makes it a discovery machine. -> see example on next transparency. LHC is also a discovery machine in the simple sense: any increase of c.m. energy by factor 30 is likely to lead to significant discoveries.

1. Day LHC: event multiplicity at y=0 generic trends in - extended longitudinal scaling - self-similar trapezoidal shape Saturation models predict Armesto, Salgado, Wiedemann, PRL94 (2005) or Kharzeev, Levin, Nardi, NPA747 (2005) 609. Both consistent with main trends at RHIC, but … Extrapolations to LHC deviate from so-far generic trends in data From day 1 onwards, LHC will provide essential constraints for theory of RHIC HI collisions. PHOBOS, PRC74 (2006) ; W. Busza. N.Borghini, UAW to appear.

2. The interplay of RHIC and LHC Together, RHIC and LHC span an energy range of more than 3 units of magnitude (say 50 GeV TeV). How do the properties of hot and dense QCD matter evolve with energy from RHIC to LHC? Don’t trust the argument, that the high-temperature phase of QCD is featureless above T c. This may be true for the energy density, but: - Measurable quantities related to 1st principle calculations, such as are likely to satisfy non-linear small-x evolution equations (similar to Q s ) - QCD measures of deviations from conformality change strongly from T c to 3T c. This may provide crucial input for fundamental theoretical approaches (e.g. string theory), which start from perturbations of a conformal framework I advocate to identify this question now as a major scientific challenge of the next decade, to be explored in an interplay of RHIC and LHC. Consequence: RHIC luminosity upgrade also motivated by increasing kinematical reach for comparison with LHC.

3. Novel Questions for high-pt at the LHC Compilation: P. Jacobs Abundant yield of hard probes + robust signal ( medium sensitivity >> uncertainties) = detailed understanding of dense QCD matter One of many examples: Study internal structure of jets Borghini,Wiedemann, hep-ph/

4. adds to physics case for e-A Saturation, Pb Eskola et al. Hard Probes in Heavy Ion Collisions at the LHC: PDFs, Shadowing and pA, hep-ph/ e-RHIC

My personal view on LHC: 1. Results from the LHC heavy ion run will provide substantial novel tests for the key dynamical ideas (hydrodynamic behavior, hard parton propagation in matter, saturation) developed in the context of the RHIC heavy ion program. Consequence: Any theory initiative (even if it aims primarily at meeting the challenges of the RHIC heavy ion program), must aim at an unbiased use of all experimental constraints. The most successful theory efforts will work towards a phenomenological framework testable in the entire energy range spanning RHIC and LHC. 2. The properties of the hot and dense QCD matter produced at the LHC may differ from those produced at RHIC. We can state already now that testing QCD evolution of properties of hot and dense QCD matter is of fundamental interest and is experimentally testable in an interplay of RHIC and LHC. Consequence: We should recognize this novel opportunity. Rare hard high-pt probes provide the most versatile class of tools for characterizing properties of matter. Knowledge about these probes at RHIC can be improved significantly with a luminosity upgrade, which thus could enhance the interplay between RHIC and LHC significantly (in particular if operational during the LHC discovery era).

My personal view on LHC: 3. The LHC heavy ion program will have unprecedented access to processes at high momentum transfers (factor 10 increase in pt-range). This opens many novel opportunities (Q 2 -evolution of partons in hot matter? How does a parent parton equilibrate chemically/kinematically in QCD matter? When does it penetrate the matter? ….) Consequence: Similar to RHIC, the high-pt sector at LHC is likely to be one of the main areas, in which properties of QCD matter can be connected to 1st principles of QCD. From a US-perspective, it may thus be particularly interesting to enhance those high-pt experimental capabilities of LHC experiments, which lie outside the direct reach of RHIC (ALICE EMCal, novel high-pt PID devices?, CMS, ATLAS). 4. The LHC p+A and A+A program will have unprecedented access to processes at small Bjorken-x. LHC p+A allows us to compare systematically the rapidity dependence of observables with the -dependence (this is yet another important interplay of RHIC d+Au and LHC p+A). Consequence: LHC p+A can be expected to provide novel insight into the tamed growth of nuclear parton distributions and non-linear small-x QCD evolution. This makes an interesting testing ground for e-A.

Backup

RHIC has widened the scientific scope and outreach of nuclear physics a.Over the last 5 years, RHIC has provided arguably the most active and most versatile field of interplay between the theory of strong interactions (QCD) and experiment. Many physicists with research focus on perturbative QCD have shown interest in and contributed to the theory of heavy ion physics. This interdisciplinary aspect of heavy ion physics is likely to continue at LHC, in particular in the field of hard probes. b.Heavy ion theory has contributed strongly to the modern technical framework of non-linear QCD evolution and small-x physics. (10 years ago, there was no heavy ion physics presented at any small-x workshop, now there is no small-x workshop without a session on heavy ion physics!) c.Up until three years ago, string theory has been truly disjoint from heavy ion physics, now it is becoming a key player in addressing the novel theoretical challenges determined by RHIC (strong coupling techniques in the presence of strong collective dynamics), which are difficult to address by lattice QCD. This development was started by HI physicists, but it has reached Princeton, Perimeter Institute, Santa Barbara, ….