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Back-to-back Jets Tagged via 2+1 Correlation

Back-to-back Jets Tagged via 2+1 Correlation. Hua Pei Iowa State University. Outline. Motivation Method Current Analysis Result Summary. More central. arXiv:0801.4545v1. 0-20% Au+Au 20-40% 60-92%. STAR. 4<p T trig <6 GeV/c, 2<p T assoc <p T trig. STAR PRL 91 (2003) 072304.

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Back-to-back Jets Tagged via 2+1 Correlation

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  1. Back-to-back Jets Tagged via 2+1 Correlation Hua Pei Iowa State University RHIC-AGS Annual User Meeting 2008

  2. Outline • Motivation • Method • Current Analysis Result • Summary RHIC-AGS Annual User Meeting 2008

  3. More central arXiv:0801.4545v1 0-20% Au+Au 20-40% 60-92% STAR 4<pTtrig<6 GeV/c, 2<pTassoc<pTtrig STAR PRL 91 (2003) 072304 2-Particle Correlations Indicate Suppression We have been studying jets using the 2-particle correlation. Clear jet signal is observed on near-side even in the most central AA collisions. Strong suppression is observed on the away-side. RHIC-AGS Annual User Meeting 2008

  4. Theory Models Are There to Explain • We all agree: • Jet-quenching is due to partons lose energy in the hot and dense QCD medium • We don’t agree on: energy loss mechanisms • Mach cone (Stoecker, Casalderrey-Solana, Shuryak, Teaney, Ruppert, Muller, Renk) • Cherenkov gluon radiation (Dremin, Koch, Majumder, Wang) • Large angle gluon radiation (Vitev, Salgado) • Flow-induced deflection (Armesto, Salgado, Wiedemann) • Parton multiple scattering (Chiu, Hwa) • …. • Important to study the back-to-back jets simultaneously: use di-jets events. RHIC-AGS Annual User Meeting 2008

  5. STAR First Observations of Di-jets at RHIC 8<pT(trig)<15 GeV/c PRL 97 (2006) 162301 RHIC-AGS Annual User Meeting 2008

  6. Analysis Method • Select only events with 2 back-to-back (within azimuthal space) high-pT hadrons. • Then in the 2-D space, plot the azimuthal distribution of lower-pT particles around one of the two high-pT hadrons. • The flow background and underlying events are taken into consideration. RHIC-AGS Annual User Meeting 2008

  7. Trigger hadron Trigger hadron Medium The “2nd” hadron at high-pt Assoc h Assoc h Advantage of Using Di-jets Select events that have two high-pT hadrons back-to-back vs. Removes some events where hard-scattering occurs near surface but not tangential (large difference between path lengths) Path lengths comparable in dense medium. A.k.a., 2+1 correlations Shift distribution of hard scattering towards center of medium. Near-side parton travels through more medium RHIC-AGS Annual User Meeting 2008

  8. 1 _dN_ Ntrigd(Df ) T2A1_T1 T2A1 2 1 0 STAR STAR Preliminary -2 -1 2 0 1 3 4 5 Df (T2A1) A1 T1 T2 Di-jet trigger First 2+1 / T1T2_A Measurement by STAR T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c Di-jets in d+Au 200 GeV Minimum bias data • Difference in associated yield • Di-triggers sample higher energy jets From QM08 talk of Olga Barannikova RHIC-AGS Annual User Meeting 2008

  9. 1 _dN_ Ntrigd(Df ) 4 T2A1_T1 T2A1 2 0 STAR STAR Preliminary -2 -1 2 0 1 3 4 5 -2 A1 T1 T2 Di-jet trigger Df Di-jets Correlation in Most Central Au+Au T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c 200 GeV ZDC central 12% Au+Au From QM08 talk of Olga Barannikova • One high-pT trigger only: Away-side modification. • Di-jet trigger: jet peaks on both near and away side! From QM08 talk of Olga Barannikova RHIC-AGS Annual User Meeting 2008

  10. 200 GeV Au+Au & d+Au 1 _dN_ Ntrigd(Df ) STAR Preliminary Au+Au d+Au 3 2 1 STAR 0 -2 -1 2 0 1 3 4 5 Df Di-jets Correlation of Au+Au vs. d+Au T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c • We see similar shapes between d+Au and most central Au+Au • The suppression is less comparing to “normal” 2-particle correlation. • Was this due to tangential di-jets (surface-bias) or real punch-through jets, without much energy loss? RHIC-AGS Annual User Meeting 2008

  11. ZYAM normalization in Df T2A1_T1 T1A1_T2 1.0 1 _dN_ Ntrigd(Dh ) Au+Au 12% central |Df |<0.7 STAR 0.5 0 STAR Preliminary -1 -1.5 0 0.5 -0.5 1.5 1 Dh And Where does the Ridge Go? • Ridge is naturally considered as the energy lost (deposited) to the medium by jets. • di-jets statistically remove those surface-biased jets, and keep either tangential di-jets or those punch-through from center of medium, in either case two partons travel comparable path-lengths. • Lack of ridge, shall this remain us only tangential di-jets? T1: pT>5 GeV/c, T2: pT>4 GeV/c, A: pT>1.5 GeV/c RHIC-AGS Annual User Meeting 2008

  12. x10-3 Ntrig__ NevtNpart2/3 T1= 5 GeV/c 0.4 d+Au STAR Preliminary 0 100 0 Npart 200 300 0.015 #T1T2 pairs / #Single triggers #Di-Jets / #Single triggers 0.01 0.05 STAR Preliminary Npart 100 200 0 300 Tangential Bias of Di-jets • If the triggers are from tangential surface of medium, we expect a term related to the surface: ~ R2 ~ Npart2/3 T1: pT>5GeV/c T2: pT>4GeV/c RHIC-AGS Annual User Meeting 2008

  13. Model Calculation Renk, Phys. Rev. C 75, 054910 (2007) <E> deposition for back-to-back jets T1 Thorsten Renk, private comm. 2 density models From QM08 talk of Olga Barannikova T2 T1  & T2  energy deposition by T2  RHIC-AGS Annual User Meeting 2008

  14. Measurement: Di-jets Still Lose Energy arXiv:0801.4545v1 • PHENIX h-h correlation. This JAA is calculated on per-event basis instead of per-trigger. • At right-end of top-left panel (most central) , the JAA (pTA pTB) approaches the inclusive RAA2 (dash line), indicating an evident energy loss. • We need to further quantize the di-jets. RHIC-AGS Annual User Meeting 2008

  15. 1 _dN_ Ntrigd(Df ) 4 T2A1_T1 T2A1 2 0 STAR Preliminary -2 -1 2 0 1 3 4 5 -2 Df Di-jets Correlation Shape Evolution with Centrality in Cu+Cu 200GeV We show the near-side widths in di-jet correlation as function of centrality. Inside each bin, it goes from most central Cu+Cu (most left) towards peripheral until p+p (most right) Each bin represents a different pT region of “conditional” particle, or “T1”. QM08 talk of Hua Pei RHIC-AGS Annual User Meeting 2008

  16. 1 _dN_ Ntrigd(Df ) 4 T2A1_T1 T2A1 2 0 STAR Preliminary -2 -1 2 0 1 3 4 5 -2 Df Di-jets Correlation Yield Evolution with Centrality in Cu+Cu 200GeV We show the near-side yields in di-jet correlation as function of centrality. Inside each bin, it goes from most central Cu+Cu (most left) towards peripheral until p+p (most right) Each bin represents a different pT region of “conditional” particle, or “T1”. QM08 talk of Hua Pei RHIC-AGS Annual User Meeting 2008

  17. Discussion on Di-jets Correlation Centrality Dependence • Cu+Cu yields are higher than p+p at “non-required” conditional pT bin, is consistent to the Au+Au h-h correlation result (arXiv:0801.4545) • Cu+Cu or p+p yield increase with conditional pT is possibly due to the Q2 shift. That is, if we fixed the trigger pT and increase the away-side conditional pT, we are increasing the Q2 of di-jets, thus the parton energy on near-side, and naturally increase the yields. A similar discussion was given in PHENIX paper Phys. Rev. D 74, 072002 (2006) • On the other hand, the fact that Cu+Cu yields increase slower than p+p with conditional pT, indicates possible medium effect such as jet absorption. Existence of underlying events (random combination of T1 and T2) can also lower the yields especially in central AA. • Far-side yields was not shown here since I am still in development of removing the bias introduced by this “conditional” particle. RHIC-AGS Annual User Meeting 2008

  18. Outlook • People at RHIC are analyzing their Run 2007 Au+Au data with much more statistics (e.g., PHENIX has more than 3-times of what it got in 2004 Au+Au, and better flow measurement with the new RXNP detector). • Geometry dependence of di-jets (yields, ridge shapes, etc) can be extend to a broader range then. • Methods of removing underlying events will be improved. • Long-term scope: • (nearly) full di-jet reconstruction (STAR and PHENIX Upgrade)? • Direct g -jet correlation to ultimately remove the surface bias? Can be the silver bullet. RHIC-AGS Annual User Meeting 2008

  19. Summary • We already know from 2-particle correlation that jets are modified by medium, on both near and far side. Medium effect shows as jet suppression and varies on centrality and pT. • New analysis of 2+1 correlations:a new method of controlling jet source via the surface-bias, especially on exploring the jet suppression. • Preliminary results from both PHENIX and STAR show a di-jet structure, and statistically shift our observation to either tangential jets or punch-through (both sides) jets. • New technique allows further exploration of the “surface bias”, and will probe the medium in a more controlled way. RHIC-AGS Annual User Meeting 2008

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