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RHZYAM (Red Herring ZYAM) Why I think ZYAM is not the most important problem for us to be focusing on right now Or Dr. (Strange)Cole: How I learned to stop worrying and love ZYAM Why are we having this discussion? We’re here because of this feature in the data •Issue to put to rest: is it an artifact of the background subtraction procedure? My final (penultimate) word on subject BAC QM08 Seeing the “Mach Cone” without subtraction Pure correlation function, no subtraction. 0-5% centrality – large background but modest flow • Can see the distortion in the di-hadron correlation function without ever subtracting. – Of course, there is modulated background – Subtraction will decrease yield at ∆ϕ = π relative to π ± 1. Seeing the “Mach Cone” without subtraction(2) Use dependence of correlation function on angle of “trigger” wrt reaction plane “Trigger” here is an analysis cut on data • Characteristic variation depends on angle of trigger bin and resolution of reaction plane measurement. • In one bin, flow effects are weak. Seeing the “Mach Cone” without subtraction(3) PHENIX Run 5 4th reaction plane bin w/ weakest flow Pure correlation function, no subtraction. • Without subtraction, the “mach cone” is clear • Critical point: – Modification of ∆ϕ distribution at π ± ≈1 far more important than the behavior at ∆ϕ= π. Seeing the “Mach Cone” without subtraction(4) PHENIX Run 7 0-5 centrality correlation functions (not optimal plot but only approved corr. func. plot) • One important point to take away: – Dramatic change in shape of C2 vs ϕtrig. – Substantial errors in v2, v4, background would cause flow variations to swamp any jet / “mach cone” signal. Reaction Plane Dependence After Subtraction Centrality 0-5% • Many details to control in the subtraction. – v2(pt), v4(pt), Ψ resolution, background level • But, see same “Mach Cone” while flow-modulated background changes by 10x signal at ∆ϕ = π ± ≈1 Reaction Plane Dependence After Subtraction Centrality 30-40% • Flow modulation ~ 4-5 x larger here than in 0-5%. • Nonetheless, see ~ identical shape for “Mach Cone” Conical? flow – RP dependence (STAR) From parallel talk by A. Feng • PHENIX & STAR results on RP dependence in excellent qualitative agreement. Background Normalization • The problem: – How to find the level of the combinatoric background in di-hadron correlations. • In principle – can measure yield, v2 of each hadron, calculate background level. – “absolute normalization” – But, not so simple (e.g. multiplicity fluctuations). • ZYAM: – Use feature of jet C2 to determine/constraint background level. • Comparison used for systematic errors in Two-Component Model? But wait, wasn’t previous slide predicated on “two-component” assumption? • What is the two-component “model”? – Some hadrons correlated with jet (trigger hadron). – Some hadrons not correlated with jet (trigger hadron). ⇒These have the “inclusive” ϕ - Ψ distribution. • I refuse to accept that every hadron produced within ∆η=1 of a jet is affected by the presence of jet. – e.g. in 0-5% centrality – “jet” correlation/total ~ 0.01 • So how much is correlated? • We don’t know. Two-component model? (2) So, what to do? • One reasonable way to proceed: – Assume that the maximum # of hadrons consistent with C2 are uncorrelated w/ jet (minimal jet-medium interaction). – ZYAM assumption. • Pro’s: – ZYAM gives (~ unambiguous) estimate of background. – Relatively insensitive to details of modifications of jet shape. – Lower limit on conditional yield. • Con’s: – “Ad hoc” – We know it’s wrong (but how wrong?) – Makes it easy to generate “hidden” bias. Why ZYAM is wrong PHENIX paper (arXiv:0801.4545v1 [nucl-ex]) p-p di-hadron pair yield per event (NOT conditional yield) •Even in p-p collisions ∃ no region in ∆ϕ where di-hadron yield is zero “Underlying Event” Does it Matter? • Underlying event in p-p is ~ 1/100th of jet signal • Jet signal in Au-Au is ~ 1/100 of inclusive pairs. • Jet-associated underlying event ~ 10-4 inclusive pairs. – Would have to be enhanced by a factor of 100 to have a significant impact on background normalization. • Forget it. The underlying event (IS radiation, etc.) not relevant (yet) to this discussion. Unknown Jet-medium correlations? • So, ZYAM assumption is reasonable based on p-p data (also d-Au data). • But, what if there are unknown jet-medium correlations that contribute near ∆ϕ = π/2? – Then we will over-subtract. – If it does not follow flow pattern, we will subtract too much with the wrong shape. But, we see “Mach Cone” before subtraction where we can make flow modulation small. Everywhere else we get consistent results after subtraction. My conclusions • The “Mach Cone” is absolutely, positively not an artifact of background subtraction. • ZYAM is not correct, not ideal, would prefer something much better. – (e.g.) absolute subtraction – but also depends on the socalled “two-component” assumption. – No evidence from data for strong violation of “twocomponent” assumption. My final (I hope) word on subject BAC QM08