Flash Light spectroscopy
By Takao Sakaguchi
Electro-magnetic radiation is an excellent probe for extracting thermodynamical information in relativistic heavy ion collisions. They are emitted from all the stages of collisions, and don't interact strongly with medium once produced. Prompt direct photons produced in initial hard scattering, which are discovered in year 2002 run, showed that the high pT hadron suppression found at RHIC is a consequence of an energy loss of hard-scattered partons in the hot and dense medium. Recent direct photon data from year-2004 run lead us to more quantitative insight on interaction between hard scattered partons and bulk medium.
The electro-magnetic radiation is primarily produced through a Compton scattering of quarks and gluons (quark+gluon ->quark+gamma) and an annihilation of quarks and anti-quarks (quark+anti-quark -> gluon + gamma) as leading order processes, and the next leading process is dominated by bremsstrahlung (fragment) (quark+gluon -> quark+gluon+gamma). The mid-to-high pT region (pT>4GeV/c) is dominated by an interaction between hard scattered partons and the medium produced .
In year-2002 run, we were not able to reach above 12GeV/c at c.m.s (center of mass energy)=200GeV, where the nuclear modification factor (R_AA; the ratio of the yield in Au+Au collisions to that in p+p scaled by nuclear thickness function (T_AB) was consistent with unity, and thus concluded that direct photons are unmodified by the medium. The latest data shows a trend of decreasing at high pT (pT>14GeV/c) as shown in Fig.1.
One of several implications of the data has been an isospin effect. The isospin effect is an effect caused from the difference of the quark charge contents in neutrons and protons. The photon production cross-section at high pT is proportional to charge-squared of quarks, therefore the yield of photons would be different between n+p, p+p and n+n collisions. A gold ion consists of 79 protons and 118 neutrons, so the hard scattering cross-section for minimum bias Au+Au collisions would be weighted sum of p+P. n+p and n+n cross-section. Fig. 2(a) shows the R_AA purely expected from the fact. The calculation at c.m.s=200GeV is shown in red. There is ~15% drop at 18GeV/c caused by the effect. For a reference, the one at c.m.s=62.4GeV is also shown as blue. It is seen that the suppression is larger at the energy because the effect scales with xT(=2*pT/c.m.s) as shown in Fig. 2(b).
The calculation suggests that by looking at 62.4GeV result, we can quantify the isospin effect in Au+Au collisions.
Fig.3(a) shows the direct photon yield at c.m.s=62.4GeV over a NLO pQCD calculation scaled with T_AB, which just came out from PHENIX.
Since we donít have p+p direct photons measured at RHIC, we used well-defined NLO pQCD calculations instead. The result shows that the data is consistent with the calculation within error. However, we already know that the NLO pQCD calculation does not reproduce very well p+p data. We did an exercise to scale the data over NLO pQCD at c.m.s=200GeV to 62.4GeV, using xT scaling assumption (i.e., the same ratio would appear at same xT), which is shown in Fig.3(b).
If we consider this ratio, we could say that the direct photons at 62.4GeV is also suppressed as is seen in 200GeV.
This fact raised two issues. One is that we are getting into much advanced understanding of hard scattering process in relativistic heavy on collisions, therefore the process would be an excellent probe on investigating the matter produced. The other is that the reference we have been using to quantify the nuclear effect (we used p+p data for the reference so far) would not be appropriate, but the weighted p+p, p+n and n+n would be.
For more details, see recent publication of http://arxiv.org/abs/0705.1711.
The poster presentation made at INPC highlighted the result, and is awarded one of the two first NPA young scientist awards. The presentation will be published as a part of a NPA proceedings.  R. Fries et al., Phys. Rev. C72, 041902(2005).  F. Arleo, JHEP 0609(2006)015.  L.E. Gordon and W. Vogelsang, Phys. Rev. D48, 3136 (1993); priv. comm.
Biography: Takao Sakaguchi is an associate physicist with the BNL-PHENIX group. He is serving as a co-convener of Photon Physics working group in the PHENIX experiment. His interest has been focused on investigating early stage dynamics of heavy ion collisions through measurement of electro-magnetic probes.