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High p t Hadrons in STAR: Past, Present, and Future

High p t Hadrons in STAR: Past, Present, and Future. Michael L. Miller Yale University For the STAR Collaboration. What’s new?. What’s old?. STAR measurements from p+p Pt spectrum R AA at 200 GeV, R CP at 200 GeV Is 6 GeV a special number?. High-pt suppression for 2<pt<6. 1

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High p t Hadrons in STAR: Past, Present, and Future

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  1. High pt Hadrons in STAR: Past, Present, and Future Michael L. Miller Yale University For the STAR Collaboration

  2. What’s new? What’s old? • STAR measurements from p+p • Pt spectrum • RAA at 200 GeV, RCP at 200 GeV • Is 6 GeV a special number? • High-pt suppression for 2<pt<6.1 • Large and flat v2 for pt>2.2 • Pt>4 particle production in p+p, Au+Au consistent with jet fragmentation (same side correlation).2 • Disappearance of the away side jet.3 What’s coming? • What happened to the away side jet? • Musings on correlation method • Teaser: beyond correlations! Past, Present, Future • C. Adler et al. Phys. Rev. Lett. 89, 202301 (2002) • C. Adler et al.Phys. Rev. Lett. 90, 032301 (2003) • C. Adler et al. Phys. Rev. Lett. 90, 082302 (2003)

  3. The STAR Detector • Time Projection Chamber • ||<1.5, 0<||<2  • Central Trigger Barrel • Zero Degree Calorimeters • Beam Beam Counters • Scintillating tiles: 3.5<||<5

  4. 0.3 0.8 0 2 4 6 8 10 0 80 Multiplicity pt How to Measure Minbias p+p? • <pt> scales with mult. • Vertexing efficiency increases with mult. => bias! Mean pt GeV/c Analyze tracks from primary interaction without vertexing. Jon Gans (Yale) et al.

  5. Charged Pt Spectra at 200 GeV First measured p+p spectrum from STAR • Au+Au: • 7 centralities, • 11 GeV • p+p: • From STAR • 9 GeV

  6. RAA at 200 GeV RAA measured entirely at STAR • New: 6-10 GeV region • Central: RAA does not go to 1 • Errors dominated by p+p statistics • “Hints” at flat ratio at pt>6

  7. RCP at 200 GeV • High statistics measurement in Au+Au • Plateau • Factor 3-5 suppression at pt>6 • Below “naïve” Npart scaling

  8. RCP STAR preliminary Flavored RCP at 200 GeV • Baryon production longstanding puzzle • Peak value PID dependent • Argue: location of turnover changes? • At pt>6, common suppression • Statement: “more baryons than mesons” is valid only at intermediate pt

  9. Sqrt(s) Ratio: 200/130 • Well described by pQCD plus Cronin (X.N. Wang) • Ratio relatively insensitive to e-loss • Ratio relatively insensitive to p.d.f. • Is pQCD+Cronin a unique explanation?

  10. Correlation Functions: Open Questions • Correlation functions producing exciting results • Initial state or final state effects? • Initial State: • Nuclear kt? Shadowing? • Measure these with d+Au on disk! • Final State • E-loss? Pt-broadening? • More challenging to measure…

  11. py trigger px associated 2 GeV 4 GeV Correlation Method: Measurement • Statistical leading particle analysis • Histogram in 2-d • N:  vs.  • project Ntrigger: Total number of trigger particles: (4<pT<6) Once pt<2, particles disappear

  12. Correlation Method: Ref. Model • Compare Au+Au to p+p • Incorporate known sources of signal and dominant background • Au+Au correlations: • Jets • di-jets • elliptic flow • Combinatorial Background B*v2 Systematic dependence on v2 decreases with higher pt threshold

  13. pp measurement Fit B in non-jet region Add p+p to background term Correlation Method: Ref. Model Au+Au measurement Background term • Algorithm:

  14. Au+Au Measurement background p+p Measurement Quantify with Ratio

  15. Why is IAA< 1?   Disappearance of away side correlation What is IAA telling us? • Not an experimental artifact • Interesting physics? • Shadowing? • Initial state multiple scattering? • Final state effects?

  16. pp measurement Fit B in non-jet region Add p+p to background term As away side broadens, does it leak into fit region? Correlation Musings: Broadening Au+Au measurement Background term • Algorithm:

  17.   Correlations: What are they good for? • Away side correlation does disappear! • Intra-jet Correlations (Fragmentation): • Shapes, widths, charge dependent yields • At what pt does the away side reappear? • Measure of e-loss • RAA, RCP flat from 6<pt<11

  18. Correlations: What’s Coming? • Away side correlation does disappear! • Intra-jet Correlations (Fragmentation): • Shapes, widths, charge dependent yields • At what pt does the away side reappear? • Measure of e-loss • RAA, RCP flat from 6<pt<11 • Correlations using PID tagged triggers • Lambda, k0 • Correlations w.r.t. the reaction plane • d+Au correlations • Test Saturation model predictions • Much higher statistical reach than current p+p • Allow IAA measurement at higher pt • Lower combinatoric background • Reduce systematic error from v2 What’s beyond correlations?

  19. Full Jet Reconstruction • Challenges: • Jet definition • Jet finding in high-density • Jet energy calibration • Corrected data vs. experimentally filtered predictions? • STAR Capabilities • Multiple algorithms implemented (UA1, seedless-cone, midpoint-cone, kt-cluster, etc.) • Use p+p data sample as reference • Measure Fragmentation functions in p+p, d+Au • Measure pt where fragmentation functions no longer scale with jet pt • Compare “minbias” jets with “correlation” jets • Are they at high z? • Measure nuclear kt in d+Au

  20. A Glimpse of What’s Coming Raw STAR Preliminary Back to back and come in pairs!

  21. A Glimpse of What’s Coming Raw STAR Preliminary • Increase jet pt, tighten di-jet peak • Measure Nuclear kt in d+Au

  22. A Glimpse of What’s Coming Raw STAR Preliminary

  23. Conclusions • Correlations • Jet like correlations in near angle region • Provides strong constraint on particle production models • Disappearance of away side jet • Future • d+Au inclusives (RAA, RCP) • d+Au correlations (stats!, will address saturation/mini-jet prediction) • p+p and d+Au jets • STAR has measured p+p spectrum to 10 GeV • Ratio 200/130 • Well modeled by pQCD calculation plus Cronin • RAA • Suppressed for pt>2, flat from 6<pt<10 • RCP • Suppressed for pt>2, flat from 6<pt<12

  24. Backup slides

  25. 2 GeV 2 GeV 4 GeV 4 GeV Soft: Nch=7 Hard: Nch=4 Correlation Musings: trigger bias Why is IAA(Npart=0) < 1? Two fragmentation scenarios, pt conserved Centrality from Ncharge (at y=0) could bias most peripheral bin to harder fragmenting jets. IAA<1

  26. Reaction Plane Thrust Axis An Exercise for the Student Is there an easy, robust way to measure fragmentation changes in Au+Au? • Utilize all symmetries of the problem • Minimize threshold effect • Maximize signal to noise • Well defined near/away ratio? • Minimize v2 dependence • Look at all pt’s • Allow comparisons of Au-Au to p+p, centrality dependences, etc.

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