# Presentation: 950699

## Direct Photon HBT and future perspectives for dileptons in STAR

- Evan Finch-Yale University
- RHIC-AGS Users’ Meeting 2008

## 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 pT by (mostly π0) decay photons.
**HBT**: measurable correlations will exist only between direct photons (decay photon source is ~107 times larger) and are sensitive to source size evolution.
- “The Dream”: d.o.f.  ε/T4

- But, a measurement is very challenging
- Statistically** **
**λ≈****1⁄2(N****γ****direct**** / N****γ****decay****) ****2**** ~10****-3**** **and phase space considerations hurt as pT  increases
- Systematically
- Without very good momentum resolution, π0 peak can be hard to separate; other correlations to worry about

## Calculations of radii vs. pT

## Calculations of radii vs. pT

- 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 pT

## 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

## 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-pT photon HBT.  It would also give improved low-pT measurements of π0, 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 pT.

- 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-pT photon HBT.  It would also give improved low-pT measurements of π0, 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 pT.

- Under Consideration by STAR

## Simple model to estimate sensitivity

- We assume this temperature evolution with time.
- Every piece of matter emits photons with Boltzmann pT spectrum corresponding to temperature at its proper time.
- Longitudinal Bjorken expansion.
- There is no transverse expansion (for simplicity).

- Also add π0 background, noise in calorimeter, realistic tracking efficiencies for converter photons.

## Simple model to estimate sensitivity

- Radii vs. time in our simple model...

## “Expected” Signal from 1 week test run

- Note: where are π0s?

## Low-pT η measurement from converter ‘test’ run

- Importance:  Large enhancement of low pT η,η’ possible depending on details of UA(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.

## 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  MeV		350 million events
- 	3-D  & (300 < pT < 600) MeV		800 million events
- 	3-D  &  pT > 600  MeV		20  billion events

- Turbide et. al., PRC 69, 014903 (2004)

- D. d’Enterria and D. Peressounko, Eur Phys J. **C46**, 451 (2006)

## Switching gears to dileptons...

## STAR Time Of Flight 

- Run 9: 50-75% coverage
- Run 10: full coverage

## 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.

## STAR Heavy Flavor Tracker

- For dileptons: HFT reduces background from γ conversions in material and heavy flavor decays.

- ~Run 12: partial
- ~Run 13: full

## With HFT and TOF...

- HFT:
- ~Run 12: partial
- ~Run 13: full

## Muon Telescope Detector

- π

- MTD

- pT (GeV)

- Muon Detection efficiency

## Muon Telescope Detector

- p

- K

**π**

- Detection efficiency

- pT (GeV)

## 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.