Transcript Direct Photons in Nuclear Collisions: The Promise and the
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 Temperature Constant Energy Density 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.
170 MeV
Rate
Photons: More Sources, More Theory
Hadron Gas Thermal
T
f QGP Thermal
T
i “Pre-Equilibrium”?
Jet Re interaction √(
T
i
x√s)
pQCD Prompt
x√s
E g Turbide, Rapp, Gale Final-state photons are the sum of emissions from the entire history of a nuclear collision.
That depends on what you mean by “Direct”….
p p r p Prompt Fragmentation High-energy counts these Induced Thermal Radiation QGP / Hadron Gas p 0 EM & Weak Decay 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
Theoretical (or IRS) version g Direct g Inclusive g Decays Signal!
Traditional experimental version g Direct p 0 g Inclusive p 0 g Decays p 0 Improved experimental version g Direct g Decays g g Inclusive Decays / / p p 0 0 1
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.) CERES PRL 75, 1272 (1995) 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/0201101
Continuum Dileptons at High Masses
Dileptons in the intermediate mass range M f p T could be a better measure of thermal radiation than direct photons! But this avenue has not been thoroughly explored. 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 1985 1. 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 e -> At same e , QGP has more d.o.f. than HG, higher e / T 4 -> At same e , QGP has lower T -> At same e , QGP radiates less than HG -> Lack of radiation is evidence for QGP! E g dR d 3 p g Rate per volume per time s p 2 Couplings and color factors T 2 e E g / T Planck g 2 E g T Infrared Cutoff from Hard Thermal Loop effective mass g 2 T A lowest-order pocket formula. (For fuller, higher order version consult Gale & Haglin hep-ph/0306098) 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). Reasonably rigorous -- but need to integrate over space-time! 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. Combination of high energy density and low temperature for high number of degrees of freedom -> QGP . is evidence Subtraction of decay photons depends critically on accurate p 0 low-multiplicity A+A collisions, similar to p+p collisions, the p 0 reconstruction peak stands out . In 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 / p . Dumitru, et.al., PRC 64 054909 (01) Some amount of k T required, but still can’t fill the whole spectrum k T T A nominal, complete scenario observed photon rate. The gap can be closed either by increasing intrinsic k T effects (above) under-predicts the (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 . 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 in heavier Pb+Pb collisions. lower limits) observed 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. The change from SPS fixed-target to RHIC collider experiments is an increase of more than x10 in s NN , from 17-19 GeV to 200 GeV. But the range of p T accessible in p+p collisions (thus far) limits us to low x T , making g Direct / p 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 Klaus Reygers, QM ‘04 nucl-ex/0404006 Suppression of high nuclear collisions at RHIC greatly increases g Direct / p 0 p T hadrons in , making direct photons much easier to measure! Huge change in g / p 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). Justin Frantz, QM ‘04 nucl-ex/0404006 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 g +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. R gg (D1, D2) (D1) (D2) D p D1 d D2 2 1 h/R Enhancement for L D p R Rpd L or d c L E R (D1, D2) (D1) (D2) 2 f 1+ f 1 D p The Hanbury Brown-Twiss method of boson interferometry - works from stars to nuclei! D p Very p T Submitted to PRL, also hep-ph/0403274 Credit: Dmitri Peressounko for WA98 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: • 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 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 PRL90 132301 g Caveat: Requires identification of direct photons on photon-by photon basis! (At LHC, tag jets with Z 0 ’s.) 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 location of jet production. p T , and does not bias fragmentation or “Clean” measurement of medium effects on hadronization. Available with current statistics. 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 s (d+d W + )/ s (d+d W ) is a measure of proton-neutron charge symmetry violation. 0 Z Z Z self-energy diagram which is modified in presence of QGP (Kapusta & Wong PRD 62, Majunder & Gale PRC 65) Z leptons Z Collisional broadening? m Z + 200 MeV Z + 100 MeV Homework: s (gZ->gZ) We know the initial colliding state, and we can at least describe the earliest thermalized state. But there is no bridge between equilibrium can be achieved in a time as short as ~1 fm/c. ; no known or suggested mechanism can explain how local thermal 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/0310051 “Out of Equilibrium Electromagnetic Radiation”, Boyanovski and Vega PRD 68,065018 “Are direct photons a clean signal of a thermalized quark gluon plasma?” 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, g , W and Z production have many interesting roles to play in nuclear collisions at RHIC and LHC. Early Fixed-Target Photon Results
Thermal Photon Logic, ca 1997
Thermal Photon Rates Calculated
5 9
2
ln
2.912
Temperature Limits: Contact With Thermodynamics At Last
Pb+Pb: “Truly Heavy” Ion Collisions
Why this is doubly difficult
A Spectrum at Last!
WA98 Interpretation I: pQCD?
WA98 Interpretation II:
or
?
Fixed-Target Results: Conclusions
On To Collider Energies
First RHIC A+A Photon Results
RHIC Thermal Photons: The Plan
A New Technique:
HBT
Direct Photons at
Low
Beyond Thermal Photons
Yet another photon process
Photon-tagged Jets
W “Service Work” at RHIC
Z
In-Medium Modification?
The Biggest Puzzle at RHIC The greatest need for “new” QCD physics at RHIC is to describe the approach to equilibrium.
Summary