1. Direct Photon HBT and future perspectives for dileptons in STAR Evan Finch-Yale University RHIC-AGS Users’ Meeting 2008

2. Direct Photon HBT: The Basics Direct Photons: probe all stages of collision and are sensitive to temperature evolution (though not trivially), but are swamped at low p T by (mostly π 0 ) decay photons. HBT: measurable correlations will exist only between direct photons (decay photon source is ~10 7 times larger) and are sensitive to source size evolution.  “The Dream”: d.o.f.  ε / T 4 But, a measurement is very challenging Statistically λ≈½(N γdirect / N γdecay ) 2 ~10 -3 and phase space considerations hurt as p T increases Systematically Without very good momentum resolution, π 0 peak can be hard to separate; other correlations to worry about

3. Calculations of radii vs. p T

4. D. Peressounko, PRC 67, 014905(2003) T. Renk, PRC 71, 064905(2005) D. K. Srivastava, PRC 67, 034905(2005) Comparison of different models’ radii evolution versus p T

5. Curious γ correlations at RHIC… STAR Au-Au (200 GeV) with TPC-EMC (G. Lin) STAR Au-Au (62 GeV) with EMC-EMC (D. Das) PHENIX (200GeV/c) D. Peressounko

6. Adding a (retractable) 0.1 radiation length cylinder of lead at r~42 cm (just inside the TPC inner field cage) to increase the number of photon conversions. A 1-week ‘test’ run in this configuration may give an initial (1-D) measurement of low-p T photon HBT. It would also give improved low-p T measurements of π 0, η, and help understand these previous measurements. Assuming the test run is successful, we would plan to follow with a longer run to push the measurement higher in p T. Under Consideration by STAR

7. Simple model to estimate sensitivity We assume this temperature evolution with time. Every piece of matter emits photons with Boltzmann p T spectrum corresponding to temperature at its proper time. Longitudinal Bjorken expansion. There is no transverse expansion (for simplicity). Also add π 0 background, noise in calorimeter, realistic tracking efficiencies for converter photons.

8. Simple model to estimate sensitivity Radii vs. time in our simple model…

9. “Expected” Signal from 1 week test run Note: where are π 0 s?

10. Low-p T η measurement from converter ‘test’ run Importance: Large enhancement of low p T η,η’ possible depending on details of U A (1) restoration. We expect to be able to measure the η yield to roughly 20%, (η enhancement of order 1 is suggested by Kapusta et.al., PRD 53,5028 (1996)). From 20 million SIMULATED central events assuming ‘thermal’ production and with a pessimistic tracking quality assumption.

11. Pushing to higher pT… If the ‘test’ run is a success, we may proceed with a longer run with converter present. Of course, the data volume increases substantially with increased pT and this will prove challenging. Using these same rough assumptions, the events needed are… 3-D & pT < 300 MeV350 million events 3-D & (300 < pT < 600) MeV800 million events 3-D & pT > 600 MeV20 billion events Turbide et. al., PRC 69, 014903 (2004) D. d’Enterria and D. Peressounko, Eur Phys J. C46, 451 (2006)

12. Switching gears to dileptons…

13. STAR Time Of Flight Run 9: 50-75% coverage Run 10: full coverage

14. STAR e + e - with Time Of Flight With 50% TOF coverage, and using TPC to reject some dalitz daughters, we expect that signal/background around the φ mass will be ~1/20. This would imply a measurement of φ  ee good to 10% with about 60M events and a low mass dilepton measurement from the next AuAu run that is of similar significance to PHENIX run 4 results.

15. STAR Heavy Flavor Tracker For dileptons: HFT reduces background from γ conversions in material and heavy flavor decays. ~Run 12: partial ~Run 13: full

16. With HFT and TOF… HFT: ~Run 12: partial ~Run 13: full

17. Muon Telescope Detector π MTD p T (GeV) Muon Detection efficiency

18. Muon Telescope Detector pKπ Detection efficiency p T (GeV)

19. Summary Photon HBT measurements, if possible, are capable of giving a unique probe into the temperature, size development of collisions system. STAR is considering a 1-week ‘test’ run with a photon converter in the ‘near’ future to attempt an initial photon HBT measurement + other soft photon physics. For the next AuAu run, STAR will have partial TOF, and a first look a the low mass dilepton spectrum. With the HFT added, STAR will have excellent dilepton capabilities.