STAR Decadal Plan

1. STARDecadal Plan Carl Gagliardi Texas A&M University

2. The charge • A brief summary of the detector upgrades already (or soon to be) in progress … This can even be summarized in tabular form, and should be consistent with the latest RHIC Midterm Plan. • The compelling science goals you foresee for RHIC A+A, p+p, and d+A collisions that can only be carried out with additional upgrades (or replacements) of detector subsystems or machine capabilities (e.g., further luminosity or diamond size improvements). • Prioritized, or at least time-ordered, lists of the major (above $2M total project cost) and more modest (below $2M total project cost) new detector upgrades • Any plans or interest your Collaboration has in adapting your detector or detector subsystems (or detector R&D) to study electron-nucleon and electron-ion collisions with an eventual eRHIC upgrade. • The envisioned evolution of your Collaboration through the decade

3. Remarkable discoveries at RHICThe first six years • A+A collisions • Jet quenching • Perfect liquid • Number of constituent quark scaling • Heavy-quark suppression • Polarized p+p collisions • Large transverse spin asymmetries in the pQCD regime • d+A collisions • Possible indications of gluon saturation at small x

4. The discoveries continue • A+A collisions • First ever observation of an anti-hypernucleus • Azimuthal charged-particle correlations that may arise from local strong parity violation • Even b-quark production is suppressed in central Au+Au collisions • Polarized p+p collisions • Most precise constraints to date on the polarization of the gluons • Global analysis that combines DIS, SIDIS, and RHIC data • Contribution to proton spin from gluons with 0.05 < x < 0.2 is small • d+A collisions • Dramatic broadening of forward π0-π0 correlations in central d+Au • Clearest indication to date that the onset of gluon saturation is accessible at RHIC

5. New research areas • STAR and RHIC continue to perform very well under extreme conditions • Au+Au collisions at 7.7 GeV • Charge-sign separation to pT ~ 50 GeV

6. Key unanswered questions • What is the nature of QCD matter at the extremes? • What are the properties of the strongly-coupled system produced at RHIC, and how does it thermalize? • Where is the QCD critical point and the associated first-order phase transition line? • Are the interactions of energetic partons with QCD matter characterized by weak or strong coupling? What is the detailed mechanism for partonic energy loss? • Can we strengthen current evidence for novel symmetries in QCD matter and open new avenues? • What other exotic particles are produced at RHIC? • What is the partonic structure of nucleons and nuclei? • What is the partonic spin structure of the proton? • What are the dynamical origins of spin-dependent interactions in hadronic collisions? • What is the nature of the initial state in nuclear collisions?

7. STAR: a beautiful detector Electromagnetic Calorimetry: BEMC+EEMC+FMS (-1 ≤  ≤ 4) Tracking: TPC Particle ID: TOF Recent upgrades: DAQ1000 TOF have already begun to position STAR for the coming decade Full azimuthal particle identification over a broad range in pseudorapidity

8. How to answer these questions • Hot QCD matter: high luminosity RHIC II (fb-1 equivalent) • Heavy Flavor Tracker: precision charm and beauty • Muon Telescope Detector: e+μ and μ+μ at mid-rapidity • Trigger and DAQ upgrades to make full use of luminosity • Phase structure of QCD matter: energy scan • Electron cooling if lowest beam energies most promising • Near-term upgrades for p+p collisions • Forward GEM Tracker: flavor-separated anti-quark polarizations • Forward Hadron Calorimeter: strange quark polarization • Roman Pots phase II: search for glueballs • Nucleon spin and cold QCD matter: high precision p+p and p+A, followed by e+p and e+A • Major upgrade of capabilities in forward direction • Devote the beam time needed to do p+A, rather than d+A • Existing mid-rapidity detectors well suited for portions of the initial e+p and e+A program

9. Evolution of STAR Electromagnetic Calorimetry: BEMC+EEMC+FMS (-1 ≤  ≤ 4) Tracking: TPC Particle ID: TOF Additional upgrades: Muon Telescope Detector Trigger and DAQ Forward Upgrades Heavy Flavor Tracker (2013) Forward Gem Tracker (2011) Full azimuthal particle identification over a broad range in pseudorapidity

10. Local strong parity violation STAR, PRL 103, 251601 • Transitions between domains with different topological charge may induce parity violation in the dense matter • Similar transitions (at much higher energies) might have produced the matter-antimatter asymmetry in the early universe • Magnetic field in A+A plays a key role: chiral magnetic effect • Crucial to verify if parity violation is the correct explanation • Beam-energy dependence • Compare collisions of isobaric systems • U+U collisions: collisions with more v2 and less B field than Au+Au

11. Anti-quark and gluon polarization with 500 GeV p+p Assumes 600 pb-1 delivered @ P = 50% • W measurement will significantly reduce uncertainties on anti-quark polarizations • FGT essential for the forward W’s • Inclusive jet and di-jet ALL will extend our knowledge of gluon polarization to smaller x

12. Accessing strange polarization with Λ • STAR has performed initial ΛDLL measurements at mid-rapidity • Provides access to strange quark polarization • Most interesting with quite high pTΛ (trigger and stat limited) • Similar measurements at forward rapidity are very promising • Requires the Forward Hadron Calorimeter

13. Does charm flow hydrodynamically? Heavy Flavor Tracker: unique access to low-pT fully reconstructed charm Are charmed hadrons produced via coalescence? Heavy Flavor Tracker: unique access to charm baryons Would force a significant reinterpretation of non-photonic electron RAA Muon Telescope Detector: does J/Ψ flow? Properties of sQGP: charm

14. Precise measurements with TOF, DAQ upgrade Correlated charm in A+A Decorrelation? Order of magnitude uncertainty Address with: HFT: D0, displacement MTD: e-μ correlations Properties of sQGP: dileptons

15. Muon Telescope Detector: dissociation of Υ, separated by state At RHIC: small contribution from coalescence, so interpretation clean No contribution of Bremsstrahlung tails, unlike electron channel Properties of sQGP: Upsilon Υ What states of quarkonia is the energy density of matter at RHIC sufficient to dissociate? What is the energy density? RHIC

16. Is the mechanism predominantly collisional or radiational? Detailed, fully kinematically constrained measurements via gamma-hadron and full jet reconstruction Pathlength dependence, especially with U+U Does the mechanism depend on the parton type? Gluons: particle identification, especially baryons Light quarks: gamma-hadron Heavy quarks: Heavy Flavor Tracker and Muon Telescope Detector Does the energy loss depend on the parton energy and/or velocity? High precision jet measurements up to 50 GeV Vary velocity by comparing light quarks, charm, and beauty Mechanism of partonic energy loss

17. Jets: proven capabilities in p+p B.I. Abelev et al. (STAR Coll.), Phys.Rev.Lett. 97, 252001, 2006 SPIN-2010: Matt Walker/Tai Sakuma, for the collaboration Jets well understood in STAR, experimentally and theoretically

18. To date: jets and γ-hadron in A+A Untriggered: ~0.01 nb-1 Triggered: ~0.3 nb-1 Phys. Rev. C 82, 034909 • Beginning results from Run 7 indicative, but far from final word • Huge increase in significance with trigger upgrades+luminosity • Complementary to LHC • RHIC: quarks; LHC: gluons • Best place to do jets <~ 50 GeV

19. Sufficient statistical reach out to ~50 GeV for precision measurements Large unbiased datasets Trigger upgrades to lessen bias with walking jet patches Smearing of high momentum charged hadrons under control Corrections: need to calibrate level of smearing Hard cutoff in hadrons: small loss of jets that fragment hard Dominant uncertainty fluctuations in the underlying event Jet capabilities in A+A

20. Velocity dependence QED: different momenta, different mechanisms Just beginning the exploration of this space inQCD “Passage of Particles through Matter”, Particle Data Book b c light partons

21. What is the velocity dependence of energy loss? Key tools: heavy quarks with precise kinematic reconstruction Key technology: Heavy Flavor Tracker and Muon Telescope Detector Velocity dependence via heavy quarks STAR Preliminary

22. Cold QCD matter Hint that RHIC provides unique access to onset of saturation Compelling and necessary further measurements in future Kinematic constraints: large rapidity photons, Drell-Yan in p+A Same observables very important for our understanding of transverse spin asymmetries in p+p Beyond p+p and p+A: the Electron Ion Collider

23. Sivers effect sign change • In SIDIS, knock a colored quark out of a color-neutral nucleon • Quark was bound to the rest of the nucleon before the interaction • Final-state interaction that produces the interference is attractive • In Drell-Yan, a quark in one hadron annihilates with an anti-quark of the same flavor and opposite color from the other hadron • The rest of the “other hadron” has the same color charge as the incident quark • Initial-state interaction that produces the interference is repulsive • Leads to opposite signs for the Sivers effects in the two cases • A key test of the TMD approach • Similar sign change occurs for direct photon production within the Twist-3 approach

24. Other closely related measurements • AN for W production • Only measure the electron (or muon) • Can preserve significant sensitivity if measure just above the Jacobian peak with good pT resolution • Not clear whether we can achieve sufficient resolution or not • AN for Z production • Very easy to interpret • Very small cross section !!! • Not clear if this is practical or not

25. STAR as of 2014 MTD EMC Barrel MRPC ToF Barrel EMC End Cap FMS BBC Roman Pots Phase 2 FPD TPC FHC computing DAQ1000 COMPLETE Trigger and DAQ Upgrades FGT HFT Ongoing R&D

26. STAR forward upgrade To fully investigate the proton spin and cold QCD matter, STAR will move forward nucleus electron proton nucleus • Positive η: γ and Drell Yan • High precision tracking • e/h and γ/π0 discrimination • Baryon/meson via RICH(?) • Optimized for p+A and p+p • High momentum scale • Negative η: eRHIC • Optimized for low energy electrons (1~5 GeV) • Triggering, tracking, ID for -2.5 <~ η < -1 • Can we ID hadrons, too? • R&D necessary for optimal technology choice

27. STAR and eRHIC Phase 1 Current detector matches quite well to kinematics of eRHIC Particle ID, sufficent pT resolution, etc. at mid-rapidity High Q2 electron scattering Quark jets from low x events A spin example: Explore strange quark polarization at low x Existing DIS and SIDIS results appear inconsistent

28. Energy loss in cold QCD matter RAπ Hermes, Nucl. Phys.B 780, 1 (2007) Lc up to few 100 fm • Complementary probe of mechanism of energy loss • HERMES: mixture of hadronic absorption and partonic loss • Hadrons can form partially inside the medium • eRHIC: light quarks form far outside medium • Heavy quarks: unexplored to date. Low β: short formation time

29. Conclusions • RHIC and STAR have been extraordinarily successful in their first decade of operations • The STAR Collaboration has identified compelling physics opportunities for the coming decade • The STAR Collaboration has identified the detector upgrades required to address these opportunities • The path forward: • Early in the decade: physics with low mass in the central region • Mid decade: physics of the HFT and MTD • Later in the decade: physics in the forward region • End of the decade: early phase of eRHIC