# Presentation: 940584

## Direct photonsBasis for characterizing heavy ion collisions

- Takao Sakaguchi
- Brookhaven National Laboratory

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## We’ve seen lights from universe ever since people started dreaming, hoping dreams come true

***We’ve seen lights from universe ever since people started dreaming, hoping dreams come true***

***And, dreams came true in heavy ion collisions.***
**First Heavy ion collisions in universe (SN1987)**

**Jantar Mantar (Observatory)**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Outline

- Current landscape of photons
- “Much simpler, cleaner and precision measurement” to “much complicated, uncleaner and challenging measurement”
- High pT to Low pT
- High Energy to Nuclear physics
- Future exploration using direct photons

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Direct photons are emitted at all stages then surviving unscathed strongly ( s, almost transparent to medium)

*(fm/c)*

**Direct photon history**

- Direct photons are emitted at all stages then surviving unscathed strongly ( **********s**, almost transparent to medium)

***Cartoon from G. David, Hard Probe 2006***

**Good Write-up to read: nucl-ex/0611009**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Production Process
Compton and annihilation (LO, direct)
Fragmentation (NLO)

Often carry thermodynamical information of the state
Temperature, Degrees of freedom

- Compton and annihilation (LO, direct)
- Fragmentation (NLO)
- Often carry thermodynamical information of the state
- Temperature, Degrees of freedom

**Direct photon basics**

**Gordon and Vogelsang, PRD48, 3136 (1993)**** **

**Small Rate: Yield********s**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## High pT – how well it is described

- Finding of hard photons, whose yield is well explained by NLO pQCD, opened precision measurements
- Initial state condition is understood
***Also holds in Au+Au collisions***

**Aurenche et al., PRD73, 094007(2007)**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Two Methods in p+p 200GeV
Fraction of isolated/all photons, which is calculable from NLO pQCD. Isolation cut: ( 0.1*E > Econe(R=0.5) )

Looking at angular correlation between leading hadrons and photons

- Two Methods in p+p 200GeV
*Fraction of isolated/all photons, which is calculable from NLO pQCD. *Isolation cut: ( 0.1*E > Econe(R=0.5) )
*	**	*
- Looking at angular correlation between leading hadrons and photons

**Even fragment/prompt components are decomposed**

**PHENIX, ****PRL98, 012002 (2007)**

******(Isolated)/********(all direct)**

**See: Nguyen, Session XV**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Idea: (Direct  pT) = (away side jet pT)
 Momentum loss of jets can directly be measured

PHENIX: subtract -h, -h from (incl -h
STAR: subtract 0-h from (enriched -h so that near-side associated yield become zero
It would be nice to check the absolute yield by a spectrum measurement

**Many theoretical effort both on prediction and interpretation: ****X.N. Wang, ****et al.,**** PRC55****, 3047**** (1997****), F. Arleo, JHEP 0609 (2006) 015, T. Pietrycki et al., arXiv:0706.3442, etc..**

**See: Nguyen, Hamed, Session XV**

- Idea: (Direct  pT) = (away side jet pT)
-  Momentum loss of jets can directly be measured
- PHENIX: subtract -h, -h from (incl -h
- STAR: subtract 0-h from (enriched -h so that near-side associated yield become zero
- It would be nice to check the absolute yield by a spectrum measurement

**Application of “the probe” **-jet correlation

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Sources of Radiation in A+A (interaction of jet and medium)

_***Compton scattering of hard scattered and thermal partons***_** (Jet-photon conversion)**
- A recent calculation predicted yields for radiative and collisional E-loss case
***This itself probes the matter on similar way as jets do. New way to look at photons?***
_***Bremsstrahlung from hard scattered partons***_** in medium**

**C. Gale, NPA774(2006)335**

**Turbide et al., PRC72, 014906 (2005)**
**R. Fries et al., PRC72, 041902 (2005)**
**Turbide et al., arXiv:0712.0732**
**Liu et al., arXiv:0712.3619, etc..**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Direct photons in 200GeV Au+Au

- Isospin effect, in addition to jet-quenching(BDMPS) and shadowing.
- Jet-photon conversion is not taken into account
- Low pT region is underestimated because of lack of jet-photon conversion?

**Direct photons in 200GeV Au+Au**

- Remember the extended “highlight plots” from PHENIX
- Consistent with old published result up to ~ 12GeV/c
- Direct photons suppressed at very high pT?

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Is it (only) an isospin effect?

- Taking for example, the isospin effect: Direct photon cross-sections for
-  p+p, p+n and n+n are different because of different charge contents (eq2)
- Effect can be estimated from NLO pQCD calclation of p+p, p+n and n+n
- In low pT, quarks are from gluon split  no difference between n and p
- At high pT, contribution of constituent quarks manifests
- Minimum bias Au+Au can be calculated by:

**(********AA****/N****coll****)/********pp**** vs pT**

**(********AA****/N****coll****)/********pp**** vs xT**

***Same suppression will be seen in lower pT at ************s******NN******=62.4GeV***

***TS, INPC07, arXiv.org:0708.4265***

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Looks like there is an isospin effect (and/or PDF effect)

Question: p+p is a right reference to take?
Isospin effect is electric charge dependent, which affects to photons; 0 is color charge dependent
Therefore, e-loss models so far are still valid

- Question: p+p is a right reference to take?
- Isospin effect is **electric charge** dependent, which affects to photons; 0 is **color charge** dependent
- Therefore, e-loss models so far are still valid

**Also see: Miki, Session XV**

**The test: ****62GeV Au+Au direct photons**

**~18GeV/c@200GeV**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Direct photons in 200GeV Cu+Cu

**Direct photons in 200GeV Cu+Cu**

**See: Miki, Session XV**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Low pT is interesting, and difficult

**Low pT is interesting, and difficult**

_*Thermal radiation*_ from QGP (1<pT<3GeV)
- S/B is ~10%
- Spectrum is exponential. One can extract temperature, dof, etc..
_*Hadron-gas interaction*_ (pT<1GeV/c):  ()  (),  K*  K

**A compilation on photons, PRC 69(2004)014903**

***Also, many seminal works by Sinha, Alam, Nayak, Srivastava, Fries, Rapp et al.***

**Dilepton to Photon ratio gives T: Nayak et al., arXiv:0705.1591**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Low pT photons at SPS

- WA98 data can be either explained by kT-smearing or higher initial temperature
- Could not see pT>4GeV, where pQCD photons dominate
- Lack of information of kT prevents us from resolving the issue.

- WA98 data and theoretical interpretation, **PRC69(2004)014903**

**See: Baumann, Session XV**

***p+Pb, p+C data from WA98 have come!***

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Low pT photons at RHIC

**Low pT photons at RHIC**

- PHENIX applied internal conversion technique
- Real photons can convert to virtual photons
- Inv. mass shapes for Dalitz decay of mesons are calculable using Kroll-Wada formula
- If M<<pT, the ratio of observed inv. mass to expected is proportional to direct photon excess ratio
- Take ratio where 0 contribution is small  S/B increases

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Low pT photons in Au+Au and p+p are measured using internal conversion method

p+p agrees with NLO pQCD well, and Au+Au has excess over the calculation 

**Theoretical fit results are:**
**   ****- 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.)**

**Low pT photons in Au+Au and p+p**

**See: Dahms, Session XV**

- Low pT photons in Au+Au and p+p are measured using internal conversion method
- p+p agrees with NLO pQCD well, and Au+Au has excess over the calculation

***Shorter ************0****** increases total yield and temperature Result also strongly depends on EOS.***

**Lines are NLO pQCD**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Direct photon v2 ~a photon source detector~

**Direct photon v****2**** ****~a photon source detector~**

- Depending the process of photon production, angular distributions
-      of direct photons vary
- Jet fragmentation (v2>0), Jet-photon conversion, in-medium bremsstrahlung (v2<0), **Turbide, et al., PRL96, 032303(2006), etc..**
- Thermal photon v2 ~ Quark v2 (>0), (based on hydro calculation)
**Chatterjee, et al., PRL 96, 202302(2006), Kopeliovich et al., arXiv:0712.2829, etc..**

**Turbide et al., arXiv:0712.0732**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Direct photon v2 in Au+Au collisions

- Hadron decay photon subtracted from inclusive photon v2.
- Reached up to ~8GeV/c, no significance for pT<3GeV/c
- Tends to be positive?
- Uncertainty of meas/bkgd dominates error here.
- It is not due to v2 measurement of inclusive photons or 0
- Could be improved by internal conversion method?

**FYI, WA98 Result, EPCJ41(2005)287, no significance**
**See: Miki for PHENIX result, Session XV,  Raniwala for STAR result, Session XII**

**20-40% result is similar.**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Where do we go?

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Something we may have missed..

**Something we may have missed..**

- Initial multiple scattering effect should exist...?
- : kT broadening, 0: kT broadening + recombination.
- Detail study of p+A or d+A should help

*Russcher (STAR), QM06*

**STAR, d+Au********meas****/********bkgd**

**PHENIX, d+Au, R****AA**

*Peressounko (PHENIX), QM06*

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Hard scattering cross-section even goes higher
-Jet correlation analysis is the primary target
Key issue of the analysis is how clean tagged photon samples can be

- Hard scattering cross-section even goes higher
- -Jet correlation analysis is the primary target
- Key issue of the analysis is how clean tagged photon samples can be

**See: Loizides (CMS), Session XXIV**

**Input and Reconstructed fragmentation function from direct photon tagged events (0-10% Pb+Pb)**

**See: Morsch (ALICE), Session XXIV**

**Discrimination power of photons to hadrons**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## A calculation tells that even in low pT region(pT~2GeV/c), jet-photon conversion significantly contributes to total

What do we expect naively? (or guessively?)
Jet-Photon conversions  Ncoll  Npart  (s1/2)8  f(xT), “8” is xT-scaling power
Thermal Photons   Npart  (equilibrium duration)  f( (s1/2)1/4 )
Bet: LHC sees huge Jet-photon conversion contribution over thermal?

Together with v2 measurement, the “thermal region” would be a new probe of medium response to partons

**My LHC favorite**

- A calculation tells that even in low pT region(pT~2GeV/c), jet-photon conversion significantly contributes to total
- What do we expect naively? (or guessively?)
- Jet-Photon conversions  Ncoll  Npart  (s1/2)8  f(xT), “8” is xT-scaling power
- Thermal Photons   Npart  (equilibrium duration)  f( (s1/2)1/4 )
***Bet: LHC sees huge Jet-photon conversion contribution over thermal?***
- Together with v2 measurement, the “thermal region” would be a new probe of medium response to partons

**Turbide et al.,**
**arXiv:0712.0732**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## More future interest

- Determining Time evolution scenario by looking at forward direct photons
- Landau expansion will make ~ one order reduction at pT=4GeV/c at y=2 compared to y=0
*Nose-cone calorimeter upgrade in PHENIX (Covering 1<|y|<3)*
*Forward Photon Detector in STAR (Covering -3.7<y<-2.3)*

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Direct photons become indispensable probe for characterizing matter
Partons interacting with matter would produce photons with s. Cleaner probes, but harder observable

High pT direct photons are well calibrated, and become basis on evaluating all interactions of partons with medium
-jet correlation, etc.

Jet interacting with medium produces photons, and would be a useful tool for parton E-loss mechanism study
The process will be much useful in LHC era.

Isospin effect may have been observed in Au+Au.

low pT photon emission suggests a thermalized state.

Photon production at high rapidity would help determining the system expansion scenario

- Direct photons become indispensable probe for characterizing matter
- Partons interacting with matter would produce photons with s. Cleaner probes, but harder observable
- High pT direct photons are well calibrated, and become basis on evaluating all interactions of partons with medium
- -jet correlation, etc.
- Jet interacting with medium produces photons, and would be a useful tool for parton E-loss mechanism study
- The process will be much useful in LHC era.
- Isospin effect may have been observed in Au+Au.
- low pT photon emission suggests a thermalized state.
- Photon production at high rapidity would help determining the system expansion scenario

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## I would like to express my highest respect to those who devoted their efforts on founding direct photon measurements.

*I would like to express my highest respect to those who devoted their efforts on founding direct photon measurements.*

**Where there is an interaction, there is a photon**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## arXiv:0712.0732

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Comparison with some models

- Turbide et al. **(Phys. Rev. C72 (2005) 014906 + 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-pT 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-pT photon is well reproduced.

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Can’t we yet tell something more from it?

- NLO pQCD tells that there are direct photons and jet-fragment photons
- Direct photons should not be suppressed
- Fragment photons should be suppressed if jet is suppressed
- There should be an additional source!

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## What is expected from structure function?

**What is expected from structure function?**

**Au+Au minimum bias**

- Structure function ratios drop by ~20% from x=0.1 to 0.2?

- Eskola,Kolhinen,Ruuskanen Nucl. Phys. B**535**(1998)351

***gq ->************q is main contribution***

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## New! ~62GeV Au+Au Direct photons~

- Direct photon over NLO pQCD is consistent with unity at 62.4GeV
- p+p reference can not be well-defined. Needs a measurement at RHIC

*T. Sakaguchi, INPC07*

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Direct photon RAA in 200GeV Au+Au

**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, RAA is consistent with *N*coll scaled p+p reference.
- RAA seems to decrease at very high-pT (especially for central)
- Difference of NLO pQCD calculation and p+p data affects quite a bit.

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Are we in discovery? Single photons in Heavy Ion collisions ~Before RHIC~

- WA80, WA98 are the dedicated experiments for direct photon search in relativistic heavy Ion collisions.
- WA98 data can be either explained by kT-smearing or higher initial temeprature
- Any data did not see pT>4GeV, where pQCD photons dominate
- No information on kT does not allow us to resolve the issue.
- Recent data points at ~100MeV available from WA98.
- By analysis of correlation strength in interferometery

- WA98 data and theoretical interpretation
**PRC69(2004)014903**

- HELIOS **(*****Z. Phys. C46(1009)369*****)****,** CERES **(*****Z. Phys. C71(1996)571*****)****,** null result..

- Hope to hear new result from Christoph Bermann.

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Direct photons in p+p collisions

- p+p cross-section at s=200GeV is well established with PHENIX Year-5 data set.
- Statistically improved from the published Run3 result
- Reference for Au+Au collisions
- Data parameterized by a fitting function to interpolate to the pT of Au+Au points
- Measured p+p yield is higher than NLO pQCD calculation by more than 20%.

**Data/fit**

**Isobe et al., J.Phys.G34, S1015(2007)**.

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## 200GeV? ~A big discovery!~

- Suppression of 0 and  reduced background, and highlighted direct photons
- Ratio increases as centrality increases
- Direct photon yield for pT>6GeV/c is well described by NLO pQCD calculation
- NO direct photon suppression (initial state), and large 0 suppression (final state)

**[************]****measured**** / [************]****background****  = ********measured****/********background**

**S.S.Adler, et. al. (PHENIX Collaboration), PRL 94, 232301(2005)**

******measured ****/ ********background**

**Nuclear Modification factor**

******0**

**Direct photons**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## Other recent Theoretical interpretation

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## High Mass, High pT dilepton

- High Mass, High pT dileptons are very interesting!
- Contribution from virtual photons, Drell-Yan, jet-photon conversion, etc.
- Can be used as an alternative measure of direct photons
- Process:
- Annihilation q and qbar
- Conversion of real photons
- Advantage
- Combinatorial backgrounds can be absolutely normalized, and subtracted from measured invariant mass spectra

**Turbide et al., hep-ph/0601042**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

## WA98 results

- A dedicated experiments for direct photon search in relativistic heavy Ion collisions. (**PRL 85 (2000) 3595)**
- sNN=17.3GeV
- Data can be either explained by kT-smearing or higher initial temperature
- Any data did not see pT>4GeV, where pQCD photons dominate
- No information on kT does not allow us to resolve the issue.
- Recent data points at ~100MeV available from WA98.
- By analysis of correlation strength in interferometry,
**PRL93(2004)022301**

- WA98 data and theoretical interpretation
**PRC69(2004)014903**

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06

- T. Sakaguchi for QM08, Jaipur, India

- 2008-02-06