Direct photons and Jet correlation in HI

Integrated I AA (0.4<x E <1.0) similar with R AA ! more suppression, as peripheral  central similar to  0 R AA values!

Promising solution: direct  -jets qg Calibrated probe of the QGP – at LO, E T,  = Pre-quenched E T,jet No Surface Bias – clearer picture of jet fragmentation modification Hard process – pQCD calcs agree well with data  has no E- loss in medium! Size of medium  But D(z) is effectively softened in opposing jet

How to construct direct  -h yield i.Construct inclusive  -h yield ii.Construct decay  -h yield via: –Pair by pair weighted summation method convolutes all  0 -h pair contributions from higher p T Weight reflects the probability from kinematics for a  0 at given p T to decay into a photon in a given p T range iii.Subtraction via:

Assume two photon sources, (  = # photons in data sample):  p T =5-7 GeV/c  p T =9-12 GeV/c Direct photons: LO (compton, annihilation,…) NLO (bremsstrahlung, fragmentation) Hadronic decay photons:    2    2   each  ’  decay 00  Measuring direct  -jet yields (I)

Assume two photon sources, (  = # photons in data sample): Let Y = per-trigger conditional jet pair yield (1/N trig )dN pairs /d(  ): Write in terms of R   Measuring direct  -jet yields (I)

 direct -h ± in PYTHIA* (LO)  prompt -h ± (NLO)  frag -h ± p T  =7-9 GeV/c, p T h =2-5 GeV/c 200GeV Direct  -h: On near side, PYTHIA says only NLO  s contribute. *For amusement only. Not for serious quantitative predictions

Direct  -h yield: Au+Au Near side yields consistent with 0 or very small from fragmentation photons Away side yields also small, suppressed!?

Comparison btw Au+Au and p+p Any clue on away side modification in central AuAu

Away side Integrated yield p+p shows a systematic trend of having higher yields in the away side than Au+Au!

We expect that the proportion of direct photons is enhanced as the size of the medium increases, since R  is sensitive to suppression. If two different systems (e.g. Central Cu+Cu and mid-central Au+Au) suppress  0 s and  s by the same amount, then they should have comparable R  values. Component 2 of 3: “double ratio” R 

Double ratio R  measured in AuAu, but not yet in CuCu… Use scaling to map AuAu R   CuCu. We will refer to the quantity “(  direct /  decay ) Au+Au interp. ”

Full test of method in PYTHIA Black: correlations with “true” direct photon triggers (ID’ed in event record) Blue: direct photon correlations produced by subtraction method The bias toward a low jet yield is worse for lower p T photon triggers, where R is smaller. If one increases R by ~10%, the “true” correlation is roughly recovered. We use this 10% to gauge the systematic error from the subtraction method. Magenta: direct g-jet per-trigger yield, subtraction method as above, but with R  scaled up by 10% R  = 1.90 R  = 1.1*1.90 *Please note: This bias is due to the method, not uncertainty in R  !   -h +/- !=  decay -h +/- (can’t neglect ,… ) We are currently working hard to reduce this systematic error. Direct  -h pairs/trigger

Preliminary Cu+Cu Results: direct  -h +/- per-trigger jet pair yields: systematic from R  systematic from subtraction method

Cu+Cu  direct -h +/- vs.   -h +/- jet yields per-trigger jet pair yields: systematic from R  systematic from subtraction method

Direct photons in 200GeV Au+Au Blue line: N coll scaled p+p cross-section 900M events Reached up to 18GeV/c Qualitatively well described by NLO pQCD calculations

Direct photon R AA in 200GeV Au+Au Used p+p data is the denominator –NLO pQCD as denominator is shown as well for a reference For pT<10GeV, R AA is consistent with N coll scaled p+p reference. R AA seems to decrease at very high-p T Difference of NLO pQCD calcuation and p+p data affects quite a bit. R AA with pQCD R AA with p+p data

Comparison with some models Turbide et al. (Phys. Rev. C72 (2005) Private communication.) –AMY formalism for jet- quenching effect for fragmentation photons. –Systematically data points are below theoretical prediction. F. Arleo (JHEP 0609 (2006) 015) –High-p T suppression due to isospin effect, in addition to jet-quenching and shadowing. BDMPS for jet-quenching. –Medium induced jet-photon is not taken into account. –The suppression of very high- p T photon is well reproduced.??

What is expected from structure function? 100 x T Au+Au minimum bias Structure function ratios drops by ~20% from x=0.1 to 0.2? Eskola,Kolhinen,Ruuskanen Nucl. Phys. B535(1998)351

What is expected from isospin effect? Werner Vogelsang provided direct photon cross-section in p+p, p+n and n+n at 200 and 62.4GeV Minimum bias Au+Au can be calculated by:

What is R AA for pure hard scattering Consistent with F. Arleo’s work. Werner’s comment: The isospin effect has to be bigger than for  0, because for  0 the qg channel is proportional to g(x 1 )*(u+d+ubar+dbar)(x 2 ), among other things, which is flavor and isospin "blind"… 62GeV Direct photon would be a good measure of the effect –The effect could be seen in lower pT region, where the analysis is rather established Interplay of jet-photon conversion and isospin effect in 4-6GeV/c region would be, though One message: R AA <1 for direct photons does not necessarily mean that our message of “  0 suppression at high p T ” changes

Direct photon v 2 ~what we would see?~ p T >3GeV/c –N-N and jet fragmentation (v 2 >0) –Jet-photon conversion, in- medium bremsstrahlung (v 2 <0) –Higher the p T gets, lower the v 2 become. p T <4GeV/c –Thermal photon v 2 based on hydro calculation (roughly % centrality) Quark flow ~ photon flow –dilepton v 2 is also predicted, and is in the same order annihilation compton scattering Bremsstrahlung (energy loss) jet jet fragment photon v 2 > 0 v 2 < 0 Chatterjee et al. PRL 96, (2006) Turbide, et al., PRL96, (2006)

Direct photon v 2 in Au+Au collisions Hadron decay photon subtracted from inclusive photon v 2. Reached up to ~8GeV/c (~4GeV/c in Run2) Tends to be positive? PHENIX Preliminary  s NN =200GeV Au+Au PHENIX Preliminary  s NN =200GeV Au+Au Finalizing systematic errors..