# Presentation: 087952

## Electromagnetic Probes* in PHENIX

**Electromagnetic Probes* in PHENIX**

- Hard Probes 2006 – Asilomar, CA

**G. David (BNL)**
**for the PHENIX Collaboration**

- *also known as ***the silver bullet***

## Climbing the mountain – along a spiral 

- 10-15 years ago the ***main*** interest in direct photons:
-    - ***thermometer*** of the plasma and particularly of T**c**
-    - tool to calibrate ***jet energy*** (in ****-jet, modulo kT)
-         and all this feasible because it is a ***penetrating probe***
- But as it often does, Nature proved to be ***richer*** – and trickier
- She gave us a break (experimentally) by strongly ***suppressing hadrons***
-    - we learned ***pQCD agrees*** quite well p+p, d+Au, even Au+Au (T**AB** scaling)
-    - new techniques gave unusual ***precision*** even in the ***thermal region***
-      (which almost fell out of favor for a while due to small S/B)
-      but by the time we got there we also realized that “new” sources
-      make the ***interpretation ambiguous***
-       even more precision and new type of measurements needed to
-           differentiate (including the low p**T** p+p baseline)
- So now we know a lot, and we know much better ***what we don’t know***
-      – and ***how to learn about it*** in the next, higher round

***Climbing the mountain – along a spiral ***

## What will – and will not – be discussed  

***What will – and will not – be discussed  ***

**What PHENIX made public so far (publications or preliminary results)**
**The beauty (and curse) of electromagnetic probes**
**Neutral hadrons (briefly)**
**Direct photon results from p+p, d+Au and Au+Au**
**Dielectrons in the low and intermediate mass range**
**Immediate and mid-term future in PHENIX**
**Although electromagnetic probes, I will not discuss**
**   ****- single electrons (charm, Alan Dion, plenary)**
**   ****- dielectrons, dimuons (J/********, Abigail Bickley, plenary)**
**   ****- ********-jet correlations (Jan Rak / Mike Tannenbaum, plenary)**
**   ****- spin-related measurements **

##   0                                     e            e+e-

**AuAu**   62
-            130
-            200

**CuCu ** 22
-            62
-          200

**dAu** 200

**pp  **200

-   ******0**          ****             ****              **e**            **e****+****e****-**

**PRL 88 (2002)**

**PRL 88 (2002)**

**PRL 91 (2003)**

**PRL 91 (2003)**

**PRL 91 (2003)**

**PRL 94 (2005)**

**PRC 72 (2005)**

**PRL 94 (2005)**

**PRL 96 (2006)**

**PRD 71 (2005)**

**PRL 95 (2005)**

**PRL 96 (2006)**

**PRL 96 (2006)**

**QM 2005**

**DPG 2006**

**QM 2005**

**QM 2002**

**PRL 96 (2006)**

**PRL 96 (2006)**

**PRL 96 (2006)**

**QM 2005**

**QM 2005**

**JPS 2005**

***Electromagnetic probes in PHENIX – public results***

## Electromagnetic probes – the silver bullet

***Electromagnetic probes – the silver bullet***

- Penetrating probes emitted at all stages then surviving unscathed ( ******e** << ******s**)
- “Historians” of the heavy ion collision: encode all sub-processes at all times

**Different processes: different characteristic spectra  **

## Electromagnetic probes: the quicksilver bullet  

***Electromagnetic probes: the ******quick******silver bullet  ***

*log t*

*p****T***

- 1

- 10

- 10**7**

*(GeV)*

*(fm/c)*

- hadron
- decays

- hadron gas

- sQGP

- hard scatt

- jet Brems.

- All we can see
-    is the projection
-    to this axis
-    with the dashed
-    sources as
-    background
-    dominated up to
-    medium pT by this

- Penetrating probes emitted at all stages then surviving unscathed ( ******e** << ******s**)
- “Historians” of the heavy ion collision: encode all sub-processes at all times
- But for the very same reason their message is hard to decipher!

**Overlapping signals from different sources, separated in time  **

**Cartoon only: sources of ********,**
**   ****mean p****T**** vs time **
**   ****(dashed: hadrons)**

- jet-thermal

- jet fragmentation

## Yesterday’s signal is today’s background...  

**A short tribute to neutral hadrons**
**   ****- unavoidable first measurement before doing direct photons**
**   ****- turned out to be a major discovery at RHIC (suppression)**
**   ****- one of the important reasons why this conference was born**
**   ****- strong catalyst/constraint of theories (see Monday night shoot-out ******** ) **

**Pion suppression**
**   ****at different**
**   ****energies**

***Yesterday’s signal is today’s background...  ***

## Hadron suppression – validation of TAB (Ncoll) scaling

***Integrated***** R****AA**** of hadrons and photons vs. centrality**
**Interpretation (first order): initial state unaffected, there *****is***** a known process that**
**   ****scales from p+p as expected ******** hadron suppression is real and a final state effect**
**Fine print: scaling from p+p might work only by “accident”, with mechanisms suppressing**
**   ****and enhancing direct ******** balancing each other by chance (but at all p****T****???)**

**Azimuthal asymmetry,**
**   ****study of isolated vs.**
**   ****non-isolated ******** may**
**   ****shed light on this**
**Tremendous challenge**
**   ****in Au+Au**

**Improved precision (and statistics)**
**needed up to the highest pT possible**

***Hadron suppression – validation of T******AB****** (N******coll******) scaling***

## Pion suppression in different systems  

***Pion suppression in different systems  ***

**Same system size – same suppression**
**Inclusive R****AA**** doesn’t know about shape**
**of overlap region**

## (Non)suppression of different species  

***(Non)suppression of different species  ***

**Same suppression for ********, none for ********(watch out with the interpretation!) **

## Latest references from p+p  -  0 

**Result from run2**
**PRL91 (2003) 241803**
**Result from run5**
**preliminary**
**Comparison of ********0**** cross section **
**Next-to-leading order(NLO) pQCD**
**CTEQ6M + KKP or Kretzer**
**Calculated by W.Vogelsang**

***Latest references from p+p  -  ************0****** ***

**Higher precision and much higher p****T**** range**

## Latest references from p+p  -   

***Latest references from p+p  -  ************ ***

**PHENIX run3 preliminary result.**
**Recent Update down to 3GeV/c**
**Publication is coming soon.**
**NLO-pQCD calculation**
**Private communication with W.Vogelsang**
**CTEQ6M PDF.**
**Sum of direct ******** +**** ****Bremsstrahlung ******
**3 scales (****1/2****pT,1pT,2 pT)**

**Good agreement – tantalizing at low p****T**

## Direct photons: prompt vs. fragmentation 

***Direct photons: prompt vs. fragmentation ***

**Red curve****:**
**   ****prompt ******
**Blue curve****:**
**   ****fragmentation**
**   ****(30% of all at high p****T****)**
**Caveats:**
**   ****- questionable**
**     ****at low pT**
**   ****- purely theoretical**
**     ****(exp. cuts not applied)**
**   ****- real-life cuts always**
**     ****push the red curve**
**     ****higher **

**What *****fraction***** of pQCD photons are from the primordial hard scattering?**
**Testable in p+p – prompt photons are *****isolated ***** (little or no energy**
**   ****deposit in the neighborhood)**

**Important validation of pQCD (and feasible in p+p)**

## Isolated vs. non-isolated photons in p+p

**Primary (hard scattering) photons vs. photons associated with jets**
**   ****(fragmentation, FS hadron decay)**
**Calculable, and reasonable agreement with data even if the usual 0.5 cone is near**
**   ****PHENIX’s limit**

***Fraction of isolated/all photons, p+p, 200 GeV***

***Isolated vs. non-isolated photons in p+p***

**Clean: no additional source**
**from jet-medium interaction**
**Biased: finite acceptance**

## Preliminary

***x******T****** scaling***

- From QCD, if
- Q2-Scaling of PDF,FF
- No running coupling constant(s)
- n=constant。xT=2pT/s
- Can be express as two terms
- Interaction
- Structure
- If leading order n=4
- Next-to-leading order: n=4+

- xT-Scaling n=~5

## Direct  in d+Au

***Direct ************ in d+Au***

**PHENIX preliminary results**
**More data with lower pT region is under analysis.**
**NLO pQCD Calculation**
**p+p collisions**
**Calculated by W.Vogelsang**
**CTEQ6M**
**Scale(renormalization and factorization scale) ****0.5,1.0,2.0****pT**
**In comparison with d+Au**
**Averaged number of collisions (8.42) from the Glauber model was multiplied to the calculation.**

**Both p+p and d+Au consistent with NLO**
**   ****within errors**

## Direct  and 0 nuclear modification factor RdA in d+Au

***Direct ************ and ************0****** nuclear modification factor R******dA ******in d+Au***

**Both ******** and ********0**** consistent with 1  **

## “EMC effect” for gluons  

***“******EMC effect” for gluons  ***

- Consistent with 1  No modification within the error
- This is first measurement of ‘EMC effect’ for gluons

- Nuclear Modification Factor

## Direct  in Au+Au – combined results from two detectors

***Direct ************ in Au+Au – combined results from two detectors***

**PbSc and PbGl**
**Different systematics**
**New preliminary**
**   ****from Run-4**
**Up to 14 GeV**
**Good agreement with**
**   ****pQCD calculations**

## Direct  – excess ratios (double /0 ratios)

**New direct ******** results from Au+Au up to 18 GeV/c**
**Direct photon excess ratios, independent analyses from two detectors (PbSc, PbGl)**

***Direct ************ – excess ratios (double ************/************0****** ratios)***

## Isolated vs. non-isolated photons in A+A

***Isolated vs. non-isolated photons in A+A***

**Not done yet (and very hard), but another tool to probe medium properties**
**Assume fragmentation is *****known***** (p+p) and *****unchanged*****; decay ******** can be *****identified***
**May be “easier” in Cu+Cu**

- New processes (due to
-    the medium):
-    - jet-thermal conversion
-       isolated
-    - jet-Bremsstrahlung
-       non-isolated
- Sweet spot at 4-6 GeV?

**May become an independent**
**test of jet-medium interaction**

## Direct  excess ratio – what is the role of the medium?

***Direct ************ excess ratio – what is the role of the medium?***

**In 4-10 GeV range medium effects clearly necessary – but which mechanism(s)? **

- Turbide et al, PRC 72, 014906 (2005)
- Green: pQCD + QGP + HG
- Magenta: pQCD + jet-medium + QGP + HG

- Fries et al, PRC 72, 041902
- pQCD + jet-**** conversion

## Internal conversion – an independent tool for low pT  

***Internal conversion – an independent tool for low p******T****** ************ ***

- The “thermal window of opportunity” seems to be at low p**T** (~1-3 GeV/c)
-    right where calorimeter-techniques (subtraction, tagging, ...) become error-prone

**Absolute normalization very hard: take calorimeter inclusive ********   **

## Low to medium pT using internal conversion   

***Low to medium pT ************using internal conversion   ***

**D’Enterria, Peressounko nucl-th/0503054  - ********0**** = 0.15 fm/c, T = 570 MeV**

**Turbide, Rapp, Gale PRC 69 014903 (2004)**
******0**** = 0.33 fm/c, T = 370 MeV **

**Models with ********0**** = 0.15-0.5 fm/c, T = 300-660MeV agree within a factor of 2   **

## Photon azimuthal anisotropy - theory

**Turbide, Gale, Fries PRL 96, 032303 (2006)**

**Fragmentation photons**
**   ******** ****v****2**** > 0 (****red dots****)**
**   ****(diluted by primary ********)**
**Induced Bremsstr. +**
**   ****jet-******** conversion**
**   ******** ****v****2**** < 0 (black curve)**
**Assuming no E-loss **
**   ******** ****fragm. ******** isotropic**
**   ******** ****jet-******** conversion**
**       ****dominates v****2**
**   ******** ****v****2**** < 0 (****blue, dashed****)**

**Constraints on E-loss?**

**Direct + hadron decay ********anisotropy**
**   ****(including ********0**** and ******** from recombination**
**    ****gives reasonable agreement)**

***Photon azimuthal anisotropy - theory***

**High precision at high p****T**** needed! **
**How to tell the components apart?**

## Photon azimuthal anisotropy - data

***PRL 96, 032302 (2006)  -- 200 GeV Au+Au***

**v****2****(********,************) from identified ********0**
**   ******** ******** ****red**
**v****2****(********,inc) from inclusive**
**   ******** ******** ****black**
**Direct ******** excess**
**   ****R = N********inc****/N********had**
**   ****(PRL 94, 232301 (2005))**
**Rv****2****(********,********0****) – v****2****(********,inc)**
**   ******** ****v****2****(********,dir)(R-1) ******** blue**

***Photon azimuthal anisotropy - data***

**So far consistent with v****2****=0 for direct ******** **

## The first shot at an old idea – with dielectrons 

***The first shot at an old idea – with dielectrons ***

- “The simplest way to measure excess (supposedly thermal) photon production is
-    to check whether N**** scales with N**ch** or some higher power of it (N****)”
- Paraphrased: does the e**+**e**-** yield scale as ******0** Dalitz – or faster?

- 150-300 MeV

- 300-450 MeV

- 450-600 MeV

- 1.1-2.9 GeV

**PHENIX PRELIMINARY**

**PHENIX PRELIMINARY**

**PHENIX PRELIMINARY**

**PHENIX PRELIMINARY**

- Ratio of yields in different mass intervals to ******0** yield (0-100 MeV)

**Tantalizing – but**
**   ****higher precision**
**   ****needed – and **
**   ****why no excess**
**   ****above 1 GeV?**

## Dielectron spectrum, Au+Au

***Dielectron spectrum, Au+Au***

**Data and cocktail**
**   ****absolutely normalized**
**Cocktail: hadronic**
**Charm from PYTHIA**

**Low mass:**
**   ****hint of enhancement**
**Intermediate mass:**
**   ****single e suppression **

## The near future in PHENIX – electrons 

***The near future in PHENIX – electrons ***

## The near future in PHENIX – high rapidity   

**pre-**
**shower**

**shower-**
**max**

**Silicon pads**
**1.5x1.5 cm****2**

**Tungsten 3 mm (EM) & 15mm (HAD)**

**Depth:  42X****0 ****(1.6****λ****ABS****)**

***The near future in PHENIX – high rapidity ************  ***

## Conclusions  

***Conclusions  ***

**Solid ********-integrated results on ********0****, ******** in all systems, different energies**
**Solid baseline for ******** (p+p, d+Au) up from medium p****T**** – agrees with pQCD **
**    ****(prompt vs fragmentation about right, too)**
**In low p****T**** Au+Au indications of thermal radiation (from low mass dielectrons)**
**    ****but needs a solid p+p baseline (ongoing work)**
**    ****doesn’t constrain models sufficiently yet**
**In medium to high p****T**** Au+Au jet-medium interactions almost inevitable**
**    ****more precise ********-asymmetry measurements might differentiate**
**Vector-mesons will greatly improve in the next run**
**Upgrades, higher luminosities, energy scan**
**The future of photons is shining bright at RHIC **

## Backup – 0 and T range of thermal fits  

***Backup – ************0****** and T range of thermal fits  ***

**... ****to the PHENIX**
**AuAu low p****T**** photons**
**   ****- 0.15 fm/c, 590 MeV**
**     ****(d’Enterria, Peressounko)**
**   ****- 0.2 fm/c, 450-660 MeV**
**      ****(Srivastava et al.)**
**   ****- 0.5 fm/c, 300 MeV**
**      ****(Alam et al.)**
**   ****- 0.17 fm/c, 580 MeV**
**      ****(Rasanen et al.)**
**   ****- 0.33 fm/c, 370 MeV**
**      ****(Turbide et al.)**

## Dielectron spectrum, Au+Au, centrality dependence

***Dielectron spectrum, Au+Au, centrality dependence***

**0-10%**

**10-20%**

**20-40%**

**40-60%**

**60-92%**