# Presentation: 609962

## Electromagnetic Probes at RHIC-II

**Electromagnetic Probes at RHIC-II**

***G. David***
***Physics Department***
***BNL***
***Jan. 15, 2008 ***

##  This talk

- Roots:
-           Nov. 2004  Physics Working Group organized to explore
-           physics possibilities offered by RHIC-II  (actually: what **is** RHIC-II?)
- Trigger:
-           arXiv: nucl-ex/0611009 (RHIC-II electromagnetic probes
-              working group write-up, soon to be published, and Rachid read it... )
- Easy:
-           preaching to the choir (hopefully )
- Hard:
-           timing is awkward (QM’08 is around the corner
-            but several new results cannot be shown **now...**)
- The story line:
-           - same facility, same experiment(s)  reduced systematics
-           - exploration phase  precision, precision, precision
-           - rare probes, differential quantities
-           - role of electromagnetic probes in mapping the phase diagram
-           - **the dual use of luminosity: high statistics and/or species/energy scan**

** ****This talk**

## The promise (and trap) of electromagnetic probes 

***Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL***

*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!

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

- jet-thermal

- jet fragmentation

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

**The promise (and trap) of electromagnetic probes **

## From discovery to exploration 

- Instead, let me state what we are **missing** so far ( “exploration”)
*This is a partial list, with strong bias towards electromagnetic probes*.

- We didn’t map out the phase diagram, find (or disprove the existence of)
-   the critical point

- We don’t know what makes it thermalize incredibly fast

- We don’t know what happens to vector meson masses and yields
-    (is chiral symmetry (partially) restored?)

- We don’t have a statement yet on thermal photons and initial temperature

- We are just getting sensitive enough to differentiate between jet energy loss
-    models and constrain their free parameters

- We couldn’t disentangle yet different sources of medium p**T** direct photons

- We must get more **precise** (while staying **accurate**...  )

***Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL***

**From discovery to exploration **

## inclusion of syst errors

- Excess spectrum:

***2* analysis
- (ndf = 12, stat. errors only):
**2 (i.m.h.)  = 9.4   (*P* = 66.8%)
**2 (d.ρ-m.) = 34.6 (*P* = 0.0005%)

- (MC method) yields:
*P *(i.m.h.)    =   84.2%
*P *(d.ρ-m.)   =  10.9%

- CERES Pb-Au

**CERES – PB+Au, ******** mass**

## nontrivial changes of all three variables at dNch/dy>100 ?

- peak:            R=C-1/2(L+U)
- continuum:  3/2(L+U)

- nontrivial changes of all three variables at dNch/dy>100 ?

**NA60: centrality dependence of spectral shape**

## Combining M and pT of thermal dileptons to break hadron-parton duality?

- Strong rise of Teff with dimuon mass even persists for the pure in-medium part. Sudden drop for M>1 GeV now even more sharply defined

- Rise consistent with radial flow of a hadronic source (here →→)

- Drop signals sudden transition to low-flow source, i.e.  source of  partonic origin ?
- (here qq→)

- Disentangle pT spectra of thermal continuum (from peak), get slopes (steeper than peak)
- Flow is not directly measured (yet?)
- pT spectra do not depend on centrality, but do depend on mass

- Combining M and pT of thermal dileptons to break hadron-parton duality?

**NA60: the rise and fall of radial flow of thermal dimuons **

## H. Stocker calls it RHIC... 

- Mantra:
-   “same experiment,
-    same systematics”
-   “acceptance at y=0
-    unchanged, multiplicities
-    grow very slowly”
-   “evolution (ratios) have smaller
-    errors than individual points...”

**H. Stocker calls it ********RHIC... **

##  Energy/species scan is on everybody’s mind

** ****Energy/species scan is on everybody’s mind**

##  Energy/species scan is on everybody’s mind

** ****Energy/species scan is on everybody’s mind**

- (M. Gazdzicky)

##  FAIR

**The equation-of-state at high ********B**
-  collective flow of hadrons
-  particle production at threshold energies (open charm)

**QCD critical endpoint**
-   excitation function of event-by-event fluctuations (K/π,...)

** ****Onset of chiral symmetry restoration at high ********B**
-   in-medium modifications of hadrons (,, e+e-(μ+μ-), D)

** **mostly **new** measurements
** ****CBM Physics Book (theory) in preparation**** **

**Deconfinement phase transition at high ********B**** **
-  excitation function and flow of strangeness (K, , , , )
-  excitation function and flow of charm (J/ψ, ψ', D0, D, c)
-  charmonium suppression, sequential for J/ψ and ψ' ?

**Compressed Baryonic Matter @ FAIR – high ********B****, moderate T:**
- searching for the landmarks of the QCD phase diagram
-  first order deconfinement phase transition
-  chiral phase transition (high baryon densities!)
-  QCD critical endpoint
- in A+A collisions from 2-45 AGeV starting in 2015 (CBM + HADES)

** ****FAIR**

**(Claudia Hoehne)**

## Possible sweet spots for electromagnetic probes (QGP)

- IMR (1.5-3.0GeV) p**T** integrated dileptons

- 1.5-3.0GeV direct photons

- low mass,
- q**T**>2GeV/c
- dileptons

**Possible sweet spots for electromagnetic probes (QGP)**

- (compilation by R. Rapp)

##  Initial temperature, thermalization scale

** ****Initial temperature, thermalization scale**

- Observables come from: *production mechanisms folded with system evolution*
- You have to know/assume T**i**, T**c**, T**f**, v**1**, v**2**, EOS, ****...  (all interdependent to some level)
- Case in point:

- PHENIX preliminary low p**T** direct photon
-   spectra reasonably described by models
-   where ******i** ranges 0.15-0.5 (1.3!)fm/c,
-   and T**i** ranges 300-660MeV

- An interesting proposal:
-    while both real and virtual photon spectra
-    depend on T**i**, v**0**, T**f**, EOS (T**c** dep. negligible)
-    their ratio may depend only on T**i  ****(?!)**

##  Direct measurement of initial temperature?

**Alam, Sinha,  arXiv:0705.1591**

- The claim is quite interesting (and relatively “easy” to measure in PHENIX):
-    R**em** is sensitive only to the initial temperature, but (largely) independent of
-    flow, T**c**, **i**, form of the EOS, ...
- Note that this independence doesn’t seem to have a “deep” underlying reason
-    (or at least none the authors are aware of...)

** ****Direct measurement of initial temperature?**

## Thermal (?) photons by internal conversion 

**Thermal (?) photons by internal conversion **

##  Direct photons – high and low pT 

** ****Direct photons – high and low p****T**** **

**PRL 94 (2005) 232301**

**QM’05**

**QM’06**

## World data vs own data

**World data vs own data**

- The point:
-        **Same accelerator, same experiment, similar systematic errors**
-     more precise mapping of the evolution (even if errors are relatively large)

- **0** R**AA**, 62GeV Au+Au:
-    **0** points are the same, but the
-    reference changed from fit to world data
-    to our own p+p measurement

- New**0** R**AA**, 62GeV Au+Au compared to
-    suppression in 200GeV Au+Au
- If the new result survives, the physics
-    message changes quite a bit!

## PHENIX – “Isospin Effect” (?)

**PHENIX – “Isospin Effect” (?)**

- The isospin effect (charge difference between **uud** and **udd**) **SHOULD** be there,
-    but is this (and only this “trivial effect”) what we see?
- Or do we see in addition some genuine photon suppression?

**Only “primordial” photons should be unaltered,**
**   “****medium-induced” photons can be enhanced or suppressed**

**F. Arleo, JHEP09 (2006) O15**

**W. Vogelsang, NLO pQCD + isospin**

## What is the p+p reference?  calculation vs data 

- Photons in 200GeV p+p (Run-5)
- 0.5pT favored, but even this misses the shape

- Black circles: Run-5 data divided by
-    an empirical fit.
- Blue lines: NLO pQCD
-    (different both in magnitude and shape)

**What is the p+p reference?  calculation vs data **

- 20-30% deviation (only!) would be a reason to
-   celebrate 5-6 years ago, but now we are trying to
-   confirm / refute additional signals at that
-   level (like jet-photon conversion or isospin effect)

## Photon RAA (200 GeV) with fit to p+p data 

- Remember: integrated
-    R**AA** with NLO pQCD

- Most central collisions

***Is the high p******T***
***  ******suppression real?***
***Is it suppression at all? ***
***Are p+p data the right***
***  ******thing to normalize ***
***  ******photon R******AA******?***

**Photon R****AA**** (200 GeV) with fit to p+p data **

## Isospin Effect – xT scaling

- Unfortunately the suppression is seen in a region where we are very sensitive
-    to detector bias (cluster merging).
- Also, so far it was seen only in one of the detectors (the one more prone to merging)

- x**T** scaling to the rescue?

**The reason:**
**  ****certain known detector**
**  ****imperfections **
**  ****(like shower merging,**
**  ****nonlinearity...) are smaller!**
**  ****Yes, we do our best to correct**
**  ****for them but nothing beats**
**  ****not having the problem**
**  ****in the first place...**

**The catch:**
**  ****sources at intermediate p****T**** **
**  ****(like jet conversion)**
**  ****that are so far of unknown **
**  ****magnitude,**
**  ****come into play, too!**

**Isospin Effect – x****T**** scaling**

## So: is it real?  Stay tuned! 

**So: is it real?  Stay tuned! **

##  Photon flow (PHENIX)

** ****Photon flow (PHENIX)**

## The promise of flow and isolation

**Fragmentation: non-isolated**
**Bremsstrahlung: non-isolated**
**Jet-photon conversion: isolated**
**“****Primordial”: isolated**

**So if something**
**   ****like this were**
**   ****the truth, in**
**   ****principle you could**
**   ****try to ****disentangle**
**   ****the components**
**   ****like this:**

**1/ Get the NN part (including**
**    ****isospin effect)**
**2/ Get the jet-conversion**
**    ****(jet-th) part from isolated, v2<0**
**3/ Get the fragmentation from**
**    ****non-isolated, v2>0**
**4/ ... ******
**TALL ORDER, TO SAY THE LEAST**

**Note: assuming no energy loss**
**  ******** ****fragmentation ******** is isotropic**
**  ******** ****jet-******** conversion dominates v****2**** ******** v****2****<0**

**The promise of flow and isolation**

##  Source size – photon HBT (planned) 

** ****Source size – photon HBT (planned) **

**PRL 93, 162301 (2004)**

**Hard scattering (2GeV)**

**STAR proposal**

**Thermal (<600MeV)**

## A step forward: 0 RAA vs reaction plane 

**A step forward: ********0**** R****AA**** vs reaction plane **

**PRC 76 (2007) 034904**

- Double-differential R**AA** reveals
-    strong p**T** **and** reaction plane
-    (geometry) dependence
-     stronger constraint on
-         energy loss models
- But requires more statistics
-    (RXPN  better detector
-    is equivalent to higher statistics)
- Does this mean the era of
-    bulk R**AA** is over?
- Not quite!

##  Getting quantitative: statistical analysis

** ****Getting quantitative: statistical analysis**

**arXiv 0801.1665**

- Final results (Run-4)
-    on ******0** R**AA** (PHENIX)
- Does this bulk (****-integrated)
-    quantity really tell you something?
- Would it tell you something if the
-    errors on the last points were reduced?
- Important:
-    often increase in statistics not only
-    reduces your statistical error,
-    but opens up new ways to reduce
-    systematic errors as well!

##  Quantitative constraints on opacity (PQM)

**Experimental uncertainties only!**

**arXiv 0801.1665**

- PQM predictions (one specific implementation)
-  for various <q>  (red curve: best fit)

** ****Quantitative constraints on opacity (PQM)**

- Note: <q> is not cast in stone, it’s implementation
-    dependent; theoretical uncertainties (much) bigger
-    than experimental ones

- PQM: radiative loss, static medium, no IS mult. scat., no mod. PDF.

##  Quantitative constraints on gluon density (GLV)

** ****Quantitative constraints on gluon density (GLV)**

**Experimental uncertainties only!**

**arXiv 0801.1665**

- GLV predictions for various dN**g**/dy
-   (red curve: best fit)

- GLV: <L>, opacity exp., Bj. exp. medium, radiative only,
-       IS mult. scat., mod. PDF.

##  Quantitative constraints on gluon density (WHDG)

** ****Quantitative constraints on gluon density (WHDG)**

**Experimental uncertainties only!**

- WHDG predictions for various dN**g**/dy
-   (red curve: best fit)

**arXiv 0801.1665**

- WHDG: <L>, opacity exp., Bj. exp. medium, radiative and collisional,
-    no IS mult. scat., no mod. PDF.

## 0 RAA fitted with a simple straight line 

- 1, 2, 3**** uncertainty contours

- Slope consistent with zero:
-   m = 0.0017 +/-0.0035 (+/- 0.0070) c/GeV
-                       (1 and 2****)

******0**** R****AA**** fitted with a simple straight line **

**arXiv 0801.1665**

- With present experimental uncertainties the statement that single high p**T** ******0** is “fragile” to
-    opacity is not supported (more uncertainty in theories).
- This of course doesn’t mean that multi-differential observables should not be pursued.
-    But they also come at a price!

##  A case for (much) more statistics

- Five highest points contribute 70% of the total ******2**.
- If the fits are limited to 5-10GeV/c, ***p***-values increase to
- 55% (PQM), 36% (GLV) 17% (WHDG), 75% (linear fit)

**arXiv 0801.1665**

**Theoretical uncertainties are much bigger:**
**   ****the ball seems to be in the theorists’ court!**

** ****A case for (much) more statistics**

## Summary

**Summary**

- RHIC-II is apparently on its way, but in today’s world we shouldn’t let our guard down
- Dual use of luminosity: energy/species scan and rare probes
-    we are well positioned to find the CEP first
-    we are equally well positioned to nail down jet energy loss mechanisms
-    and much, much more
- Statistics not only extend range, reduce statistical errors, make rare probes accessible,
-    but often helps reduce systematic errors as well  precision
- We are in the exploration phase and the name of the game is precision
- But precision isn’t a goal for itself, either: the goal is to constrain (or refute) theories.
-    Sometimes it is best done with increased precision, other times with multi-differential observables
**We are in this boat together.  So far cooperation of theorists and experimentalists has been stellar.**
**   ****Let’s keep it this way!**

## 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 **

**Direct photons: prompt vs. fragmentation **

## 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***

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

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

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

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

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

## “EMC effect” for gluons  

**“****EMC effect” for gluons  **

## Phys. Dept. Seminar, Jan. 15, 2008 – G. David, BNL