Direct Photon HBT and future perspectives for dileptons in STAR

Evan Finch-Yale University

RHIC-AGS Users’ Meeting 2008

Direct Photon HBT: The BasicsDirect 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≈½(Nγdirect / Nγdecay) 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.

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% coverageRun 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 φee 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

pKπ

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.