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Exploring the Properties of the Phases of QCD Research opportunities and priorities for the next decade

A White Paper for consideration in the 2015 NSAC Long Range Plan for

(title is tentative)

Please email comments and suggestions to both [email protected] and [email protected] ! 1 Executive Summary

Over the past decade, through a panoply of measurements made in heavy-ion collisions at the Relativistic Heavy Ion (RHIC) and the Large Collider (LHC), in concert with theoretical advances coming from calculations done using many different frameworks, we have obtained a broad and deep knowledge of what hot QCD matter does, but we still know little about how it works. These collisions create exploding little droplets of the hottest matter seen anywhere in the universe since it was a few microseconds old. We have increasingly quantitative empirical descriptions of the phenomena manifest in these explosions, and of some key material properties of the matter created in these “Little Bangs” which turns out to be a strongly coupled . However, we still do not know the precise nature of the initial state from which this liquid forms, and know very little about how the properties of this liquid vary across its diagram or how, at a microscopic level, the collective properties of this liquid emerge from the interactions among the individual and that we know must be visible if the liquid is probed with sufficiently high resolution. These findings us to the following Recommendation: The discoveries of the past decade have posed or sharpened questions that are central to understanding the nature, structure, and origin of the hottest liquid that the universe has ever seen and that is also one of the most strongly coupled forms of matter now known. To answer these questions we recommend, as our highest priority, capitalizing on the new capabilities planned during the last phase of the RHIC facility, continued strong U.S. participation in the LHC heavy-ion program, and continued investment in a broad range of theoretical efforts employing various analytical and computational methods. QCD theory and modeling, benefitting from continuous experimental guidance, have led to the development of a to describe the dynamic space-time evolution of the Little Bangs. Collective behavior observed via correlations among the particles produced in the debris of these explo- sions led to the discovery that, soon after the initial collision, dense QCD matter thermalizes at very high and forms a strongly coupled - (QGP). Surprisingly, this matter that filled the early universe turns out to be a liquid with a viscosity-to-entropy-density ratio (η/s) smaller than that of any other known substance and very near a limiting value that is characteristic of plasmas in infinitely strongly interacting gauge theories with a dual gravitational description. Despite continued progress, estimates of the η/s of QGP generally remain upper bounds, due to systematic uncertainties arising from an incomplete knowledge of the initial state. The unanticipated recent discovery that ripples in the near-perfect QGP liquid bring information about nucleonic and sub- nucleonic gluon fluctuations in the initial state into the final state has, however, opened new possibilities to study the dense gluon fields and their quantum fluctuations in the colliding nuclei via correlations between final state particles. The mapping of the transverse and longitudinal dependence of the initial gluon fluctuation spectrum will provide 1) a test for QCD calculations in a high gluon density regime and 2) the description of the initial state necessary to further improve the determination of η/s. The discovery of strongly coupled QGP poses many questions. How do its properties vary over a broad range of and ? What is the shortest length scale on which the liquid looks liquid-like? What is the smallest droplet of hot QCD matter whose behavior is liquid-like? An understanding of how the properties of the liquid emerge can only be gained by additionally probing the matter at varied, more microscopic, length scales and studying its production in systems of different size. Answering these and other questions requires an intensive modeling and computational effort to simultaneously determine the set of key parameters needed for a multi-scale characterization of the QGP medium and the initial state from which it emerges. This phenomenological effort requires broad experimental input from a diverse set of measurements, including 1) the completion of the heavy quark program to measure the diffusion coefficient of heavy quarks, 2) energy scans to map the of QCD and the dependence of transport coefficients on the temperature and chemical potential, 3) collisions of nuclei with varied sizes, including p+A and very high multiplicity p+p collisions, to study the emergence of collective phenomena, 4) the quantitative characterization of the electromagnetic radiation emitted by the Little Bangs and its spectral anisotropies, and 5) a detailed investigation of medium effects on the production rates and internal structure of jets of , for multi-scale tomographic studies of the medium. Together this program will illuminate how “more” becomes “different” in matter governed by the equations of QCD. The planned implementation of cooling of the RHIC beams opens new opportunities for the mapping and subsequent understanding of the phase diagram of strongly interacting matter as a function of temperature T and chemical potential µB, by exploring with higher luminosity collisions at lower beam energies in a second Beam Energy Scan (BES II). This phase diagram is a fundamental feature of QCD; it is the only phase diagram of matter controlled by non-Abelian gauge field interactions that is accessible to experiment. Since lower energy collisions produce matter with a lower initial temperature and a higher initial baryon chemical potential, they provide access to the QCD phase diagram away from the region of hot baryon-antibaryon symmetric matter that filled the early universe. Heavy ion collisions at top RHIC energies and at the LHC produce strongly coupled plasma with a low value of µb. As it cools, such matter hadronizes in a rapid but continuous crossover transition. There are many reasons to think that above some critical value of µB this QCD crossover becomes a first order , much like the liquid- crossover in highly pressurized water becomes a first-order transition below some critical pressure. Experimental evidence for a critical point that marks the beginning of a line of first order transitions in the QCD phase diagram would be a landmark achievement. The first phase of the RHIC Beam Energy Scan (BES I) in 2010-2011 used Au+Au collisions with center of mass energies from 7.7 to 39 GeV to cover the range of chemical potentials 110 ≤ µB ≤ 420 MeV. It led to a number of intriguing observations. A minimum seen in the net proton directed flow as a function of beam energy is suggestive of a minimum of the pressure gradient that might be caused by a first-order transition. Measurements of the moments of the net-proton event-by-event multiplicity distribution indicate a possible non-monotonic variation in the same energy range, as anticipated for matter that cools near a critical point on the phase diagram. Increasing deviations between the elliptic flows of and mesons and between particles and anti-particles, together with statistically limited evidence for a possibly small elliptic flow of φ mesons, may indicate a shortened QGP lifetime and an increased role of hadronic dynamics at lower collision energies. State-of-the-art dynamical modeling, using a hybrid approach that couples viscous fluid dynamics for the QGP liquid with a microscopic approach to the critical phase transition dynamics and the subsequent evolution of the hadronic phase, will be required to test these interpretations. A second Beam Energy Scan (BES II) planned for 2018- 2019, with significantly improved beam luminosity and upgraded detector capabilities, is needed to solidify the suggestive results from BES I with precision measurements in the targeted energy region identified in BES I. Unambiguous discovery of a critical point in BES II might necessitate and would warrant additional measurements at a later time to further quantify its properties. Studies of hard probes in heavy-ion collisions at RHIC and the LHC yield information about how energetic partons diffuse in transverse momentum space and lose energy as they slice through strongly coupled QGP, and about how their QCD branching processes are modified by the medium. Significant recent advances in theoretical modeling, within a perturbative QCD framework and by introducing in- sights from calculations done at strong coupling, have set the stage for tightening the determination of the transport coefficients describing transverse momentum diffusion and longitudinal drag. Experimen- tal capabilities for complementary detailed measurements of hard probes at RHIC and the LHC must be developed so that future measurements can combine and leverage the full ranges of jet kinematics and medium properties accessible at RHIC and the LHC, enabling a precise determination of these transport coefficients and their temperature dependence in the QCD cross-over region and above. Identifying the parton mass dependence of jet modification and energy loss, via measurements of heavy flavor particles and heavy flavor jets, hold the promise of separately quantifying the importance of different energy-loss mechanisms. More generally, detailed studies of how the structure of QCD jets is modified over a wide range of angular and energy scales by their passage through strongly coupled plasma will connect its macroscopic hydrodynamic description using a set of transport coefficients to a microscopic description in terms of quarks and gluons. As such, these jet measurements will provide unique microscopic tools for moving closer to a fundamental understanding of how a strongly coupled liquid can arise in an asymptotically free gauge theory. Such measurements require high luminosity operation and new instrumentation at RHIC and the LHC, and further development of theoretical tools for a direct comparison of calculations to the data. A key step in the early development of QCD was the understanding of the force between heavy, nearly static, quarks and antiquarks. Long before it was known that QGP is a liquid it was understood that one of its key attributes is that it must screen the static QCD force beyond some screening length that depends on its density and temperature. Precise and systematic measurements of quarkonium production hold the promise of determining this screening length. The effects of the screening of the fundamental quark-antiquark interaction are expected to be easier to discern in bottomonium production because it it less likely than charmonium to be produced at late times when the QGP droplet falls apart into hadrons, simply because there are many more charm quarks than bottom quarks diffusing within the QGP. A precise measurement of the medium modification of the production rates of all three Upsilon states will be available from the LHC by 2023. By 2021, the sPHENIX experiment will be making Upsilon measurements at RHIC with excellent mass resolution and statistical precision. The combination of these two data sets at quite different initial temperatures will provide strong constraints on models that relate quarkonium production rates to the screening length in hot QCD matter. Because the three Upsilon states have rather different sizes, the data will yield information on the screening of the simultaneously at different length scales. Electromagnetic radiation from the Little Bangs integrates over the electromagnetic spectral func- tion of hot QCD matter as it changes with position and time. On the one hand this provides information on how the temperature of the expanding fireball evolves, and on the other hand it opens a direct win- dow on how the degrees of freedom in the vector channel change with temperature. Recently first results from Pb+Pb collisions at the LHC and dilepton measurements by the collaboration, both at full RHIC energy and in the BES, have begun to augment the pioneering PHENIX measurements of direct photon and dilepton production in Au+Au collisions at RHIC. More precise future determina- tions of the low-mass dilepton spectrum are expected to lead to an improved understanding of chiral symmetry restoration at high temperature, while total yield measurements in the BES program will help to quantitatively determine the changing fireball lifetime at decreasing collision energy. At higher dilepton masses, the spectral slope incorporates information about the temperature history of the fire- balls, and how it changes with collision energy. More precise future measurements of the yields, slopes and anisotropic flow coefficients of direct photons emitted from RHIC and LHC collisions are highly anticipated and needed as input for a consistent dynamical theoretical description of electromagnetic radiation from the hot QCD and for a resolution of the so-called directed photon flow puzzle (the unexpectedly large elliptic flow of direct photons measured in the PHENIX and ALICE experiments). As a non-Abelian gauge theory, QCD possesses unusual topological gluon field configurations that have attracted strong theoretical interest. Theorists have identified several mechanisms by which anomalous symmetry breaking effects in hot QCD, by coupling the short but exceedingly strong mag- netic field pulse (of up to 1018 Gauss) generated by the electric current of the colliding nuclei via chromomagnetic and -electric fields to a real electric field, can cause interesting experimental effects in heavy-ion collisions with a color-deconfined initial state. The Chiral Magnetic Effect (CME) is a theo- retically predicted separation of electric charges along the magnetic field direction. The Chiral Vortical Effect (CVE) separates baryons and antibaryons along the direction of the orbital angular momentum between the colliding nuclei. The Chiral Magnetic Wave (CMW) describes a related coupled oscillatory motion between the electric and chiral charge densities and to a quadrupolar charge distribution in rapidity slices that have non-zero net charge. The magnitude of these effects is difficult to predict in a dynamical environment that realistically describes the evolution of the Little Bang. Experimental effects that share qualitative features with those predicted by these theories have been observed in A+A collisions over a range of collision energies ranging from the lowest RHIC energies to the LHC. Improved dynamical modeling, building on a recently developed magnetohydrodynamic approach that needs to be generalized to include viscous effects and a coupling to the microscopic hadronic dynamics after hadronization of the QGP, will be required to eliminate “trivial” background effects and ascertain the presence of the predicted anomalous topological phonemena in the experimental data. The experimental data are also expected to improve over the next few years as additional data are taken, especially during BES II.

RHIC and the LHC, together, provide an unprecedented opportunity to study the properties of QCD matter. While collisions at the LHC create temperatures well above those needed for the creation of QGP and may thus be able to explore the expected transition from a strongly coupled liquid to a weakly coupled gaseous phase at higher temperatures, the RHIC program enables unique research at temperatures close to the phase transition. Moreover, the unparalleled flexibility of RHIC allows for collisions between a variety of different ion species over a broad range in energy. The combined programs permit a comprehensive exploration of the QCD phase diagram, together with precise studies of how initial conditions affect the creation and dynamical expansion of hot QCD matter and of the microscopic structure of the strongly coupled QGP liquid. This varied program at two , covering three orders of magnitude in center of mass energy, has already led to an array of paradigmatic discoveries. Asymmetric Cu+Au collisions and collisions between deformed U+U nuclei at RHIC are helping to constrain the initial fluctuation spectrum and to eliminate some initial energy deposition models. The recently discovered unexpected collectivity of anisotropic flow signatures observed in p+Pb collisions at the LHC suggests that similar signatures seen in very-high-multiplicity p+p collisions at the LHC and in a recent re-analysis of d+Au collisions at RHIC might also be of collective origin. How collectivity develops in such small systems cries out for explanation. The unavoidable question “What is the smallest size and density of a droplet of QCD matter that behaves like a liquid?” can only be answered systematically by exploiting RHIC’s flexibility to collide atomic nuclei of any size over a wide range of energies. Future precision measurements made possible by increases in the luminosity of the LHC will quantify thermodynamic and microscopic properties of the strongly coupled plasma at temperatures well above Tc. At the same time, RHIC (in concert with the LHC) will be the only facility capable of providing the experimental lever arm needed to establish their temperature dependence and to extend present knowledge of the properties of deconfined matter to larger values of µb where a critical point and first order phase transition may be awaiting discovery. There is no single facility in the short- or long-term future that could come close to duplicating what RHIC and the LHC, operating in concert, will teach us about Nature. Both RHIC and the LHC are also capable of probing new, unmeasured physics phenomena at low longitudinal momentum fraction x. Proton-lead collisions at the LHC allow the study of previously unreachable in the search for parton saturation effects. However, a complete exploration of parton dynamics at low x will require an Electron-Ion Collider (EIC). Accordingly, a cost-effective plan has been developed for a future transition of the RHIC facility to an EIC. While a future EIC at RHIC will deliver crucially missing precise information on the nuclear parton distribution functions within the most desirable kinematic regime, forward rapidity studies in pA and AA collisions at RHIC and LHC provide access to low-x physics in a complementary kinematic range.