Direct Photons in Nuclear Collisions: The Promise and the Peril Paul Stankus Oak Ridge National Laboratory CTEQ Summer School June 30, 2004

Broadening Your QCD Horizons CTEQ’s agenda of precision tests of QCD involves primarily perturbative calculations: fundamental interactions, pdf’s, electroweak processes, etc. However, QCD > pQCD ; that is, QCD is a grand and rich theory with many aspects beyond vacuum perturbation theory. In high-energy nuclear collisions we need to address: Perturbative Jet/parton re-interactions in dense medium; multi-parton processes Non-perturbative Equilibrium Lattice gauge QCD; thermal field theory Non-perturbative Non-Equilibrium In-medium hadronization; approach to equilibrium; classical fields (It’s not necessarily a job, but it would be an adventure.)

The (Original) Reason for High-Energy Nuclear Collisions Constant Energy Density Constant Temperature 170 MeV Lattice QCD predicts an “upper phase” of thermal QCD matter (QGP), with a sharp rise in the number of degrees of freedom (naively thought of as hadrons to quark and gluons d.o.f). One general goal of high- energy heavy-ion collisions has been to verify that this upper phase exists. A combination of energy density and temperature measurements could, in principle, verify the number of d.o.f.

Photons: More Sources, More Theory Turbide, Rapp, Gale EE Rate Hadron Gas Thermal T f QGP Thermal T i “Pre-Equilibrium”? Jet Re-interaction √(T i x√s) pQCD Prompt x√s Final-state photons are the sum of emissions from the entire history of a nuclear collision.

That depends on what you mean by “Direct”…. 00 Prompt Fragmentation Induced Thermal Radiation QGP / Hadron Gas EM & Weak Decay High-energy counts these High-energy nuclear counts these    

Thermal Photon Logic, ca 1985 QGP has chiral symmetry restored, quark masses ~0 -> QGP has lots o’ quarks flying around -> QGP radiates more than HG at same temperature (false!) -> Lots o’ thermal radiation is evidence for QGP Ah, for the good old days….

Why this is difficult Signal! Theoretical (or IRS) version Traditional experimental version Improved experimental version

A Word on Continuum Dileptons “The plot that launched a thousand papers.” Low-mass dileptons observed in S+Au at a rate above Dalitz decays. ( Eventually the number of papers exceeded the number of pairs in the excess, it is said.) In the 1990’s the thought that the low-mass excess indicated restoration of chiral symmetry generated much excitement. More detailed calculations now indicate that a dense hadron gas could produce the excess. R. Rapp hep-th/ CERES PRL 75, 1272 (1995)

Continuum Dileptons at High Masses Dileptons in the intermediate mass range M  <m  <M J/  are also good candidates for thermal radiation, though there is uncertainty on the contribution from associated open charm decays. In principle, a high-statistics measurement of intermediate-mass dileptons vs p T could be a better measure of thermal radiation than direct photons! But this avenue has not been thoroughly explored. NA50 Eur Phys J C14 (2000) 443 R. Rapp hep-th/

Early Fixed-Target Photon Results

Thermal Photon Logic, ca 1997 QGP has chiral symmetry restored, quark masses ~0 -> QGP has lots o’ quarks flying around -> QGP radiates more than HG at same temperature (false!) -> Lots o’ thermal radiation is evidence for QGP Experiment: Not much radiation, only limits. 2.Theory: QGP and HG radiate similarly at same T Final-state data does not constrain T, but rather energy density  -> At same , QGP has more d.o.f. than HG, higher  /T 4 -> At same , QGP has lower T -> At same , QGP radiates less than HG -> Lack of radiation is evidence for QGP!

Thermal Photon Rates Calculated Photon radiation from an equilibrated quark & gluon plasma. (Kapusta, Lichard, Seibert PRD ‘91) Includes lowest-order Compton and annih. graphs, and lowest order HTL cutoff (Braaten & Pisarski NP ‘88, PRL ‘89). A lowest-order pocket formula. (For fuller, higher- order version consult Gale & Haglin hep-ph/ ) Rate from thermal hadron gas also calculated. Version 1991, QGP and HG rates same at same T; after much work, version 2003 is basically the same conclusion. Reasonably rigorous -- but need to integrate over space-time!

Temperature Limits: Contact With Thermodynamics At Last Combination of high energy density and low temperature is evidence for high number of degrees of freedom -> QGP.

Pb+Pb: “Truly Heavy” Ion Collisions

Subtraction of decay photons depends critically on accurate  0 reconstruction. In low-multiplicity A+A collisions, similar to p+p collisions, the  0 peak stands out immediately (left). In high-multiplicity collisions, and especially at low p T, the extraction is extremely challenging, S/B<1% (center). Also, we must measure  ’s in- situ (right); they contribute about 15% to decay photons but we cannot presume  / . Why this is doubly difficult

A Spectrum at Last!

Some amount of k T required, but still can’t fill the whole spectrum Dumitru, et.al., PRC (01) WA98 Interpretation I: pQCD?

WA98 Interpretation II: k T or T? A nominal, complete scenario (above) under-predicts the observed photon rate. The gap can be closed either by increasing intrinsic k T effects (above, right), or by assuming a higher initial temperature (below, right). Thus, resolution of the thermal component depends on accurate separation of the prompt component.

Fixed-Target Results: Conclusions 1.The early days had more enthusiasm than rigor. 2.In S+Au upper limits on thermal photons were used to set limits on initial temperatures; weak evidence for high #d.o.f. 3.Direct photon spectrum (ie upper and lower limits) observed in heavier Pb+Pb collisions. 4.Thermal radiation from boosted Hadron Gas may dominate thermal radiation from cooler QGP. 5.Ambiguity between pQCD sources with intrinsic plus nuclear k T effects, and hotter thermal sources. More definitive pQCD calculations would be a great help. 6.Limiting initial temperatures in Pb+Pb possible, not yet done.

On To Collider Energies Klaus Reygers, QM ‘04 nucl-ex/ The change from SPS fixed-target to RHIC collider experiments is an increase of more than x10 in  s NN, from GeV to 200 GeV. But the range of p T accessible in p+p collisions (thus far) limits us to low x T, making  Direct /  0 low and the measurement difficult. (Great statistical increase in p+p data expected in 2005.) The preliminary direct photon measurement in p+p agrees with NLO pQCD calculation. Direct photons from p+p at RHIC

First RHIC A+A Photon Results Suppression of high-p T hadrons in nuclear collisions at RHIC greatly increases  Direct /  0, making direct photons much easier to measure! Justin Frantz, QM ‘04 nucl-ex/ Huge change in  /  0 ratio is dramatic confirmation of high-pT hadron suppression, independent of normalized p+p data. Trend with centrality (impact parameter) follows expected T AB (b) behavior (per-collision nucleon-nucleon luminosity).

RHIC Thermal Photons: The Plan We will greatly improve the direct photon measurements, both in p+p and Au+Au, compared to the preliminary (x20 in the can). Calibrate/normalize the pQCD prompt component at high p T. Investigate fragmentation and induced components through  +hadron correlations. Cross-check statistical subtraction method with other methods, for example decay sister tagging at high p T. Once pQCD components are constrained, try to interpret balance as upper limit on thermal sources.

A New Technique:  HBT D1 D2 pp d L R pp 1 2 h/R f pp f The Hanbury- Brown-Twiss method of boson interferometry -- works from stars to nuclei!

Direct Photons at Very Low p T Submitted to PRL, also hep-ph/ Credit: Dmitri Peressounko for WA98

Beyond Thermal Photons Jet+medium -induced direct photons Direct photon-tagged (and Z 0 -tagged) jet fragmentation Z 0 production and in-medium modification W production and parton measurements Beam-stopping bremstrahlung Investigate the approach to thermal equilibrium The traditional interest in thermal direct photons continues in RHIC and LHC nuclear collisions. But photon production, as well as W and Z production, touches on a wide range of physics topics beyond thermodynamics:

Yet another photon process Significant rate for scattered (anti)quarks to re-interact in the dense medium. Good old Compton and annihilation processes produce photons. Dominant 4-6 GeV/c? Fries, Mueller and Srivastava PRL

Photon-tagged Jets  Hadrons Observing jets and dijets through leading hadrons biases toward high fragmentation z, and also toward sources at the periphery. Tagging jets opposite isolated direct photon measures jet p T, and does not bias fragmentation or location of jet production. “Clean” measurement of medium effects on hadronization. Available with current statistics. Caveat: Requires identification of direct photons on photon-by- photon basis! (At LHC, tag jets with Z 0 ’s.)

W “Service Work” at RHIC RHIC will produce the first-ever s-channel W’s in collisions with nuclei (maybe already happened last year…). The W rate is very small, but identification is easy through high-p T single leptons; no competition from heavy flavor decays. √x 1 x 2 = 80/(200 or 250 or 500) assures high-x process. (Anti)quark pdf’s at high x and very high Q 2 -- interesting? Nuclear effects at at high x and very high Q 2 -- interesting? s-channel W’s produced way off-shell -- interesting? “Boutique” measurements possible at nuclear collider, for example ratio  (d+d  W + )/  (d+d  W - ) is a measure of proton-neutron charge symmetry violation.

Z 0 In-Medium Modification? ZZ Z self-energy diagram which is modified in presence of QGP (Kapusta & Wong PRD 62, Majunder & Gale PRC 65) leptons Z Z Collisional broadening? m Z MeV   MeV Homework:  (gZ->gZ)

The Biggest Puzzle at RHIC The greatest need for “new” QCD physics at RHIC is to describe the approach to equilibrium. We know the initial colliding state, and we can at least describe the earliest thermalized state. But there is no bridge between; no known or suggested mechanism can explain how local thermal equilibrium can be achieved in a time as short as ~1 fm/c. Whatever is going on in the pre-equilibrium stage, if there are quarks involved then we can in principle see thermal radiation. Interested? Try Serreau hep-ph/ “Out of Equilibrium Electromagnetic Radiation”, Boyanovski and Vega PRD 68, “Are direct photons a clean signal of a thermalized quark-gluon plasma?”

Summary 1.Thermal direct photons in high-energy nuclear collisions is a heartbreaking measurement: great fundamental promise, but very difficult experimentally and theoretically. 2.Results from fixed-target energy collisions have so far shown weak but basic evidence for the QGP EOS. Further thermal interpretation is possible but incomplete; more sophisticated pQCD rate calculations would be a big help. 3.At collider energies direct photon extraction is eased by dramatic suppression of hadrons. We expect solid and exciting measurements in the very near future! 4.Beyond thermal radiation, , W and Z production have many interesting roles to play in nuclear collisions at RHIC and LHC.