Crystal Detector R&D proposalfor Soft Photon Measurement in STAR

University of California at Los Angeles Joint Institute for Nuclear Research

Institute of Modern Physics at Lanzhou

Presented by Gang Wang (UCLA)

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Soft photons: 20 - 1000 MeV

• Physics motivations : New physics domain exploration Radiative decays of particles EM radiation from bulk medium

• Experimental opportunities : TPC +TOF +HFT +Crystal Detector

= unique combination, new particle measurement capability

Larger photon cross-section allows smaller acceptance

See next page for details…

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Physics motivations• Pre-equilibrium and thermal photons

Pre-equilibrium stage of the heavy ion collision and its evolution decidedly unexplored.

Thermal photon radiation from the QGP and HRG phases

• Radioactive decays of particles 0+ cJ/ +

J/ from feed-down decays

• EM radiation from bulk medium quark jets traversing the QCD color medium

high pT charged hadron – soft photon back-to-back correlation -> properties of the dense partonic matter

high angular momentum of bulk quark matter dipole radiation

Synchrontron radiation from possible formation of longitudeinal E/B fields

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Experiment: direct photons at WA98Direct Photon Production in 158 A GeV 208Pb+208Pb Collisions

Range:

0.5 < pT < 3.5 GeV/c

Yield of the direct photons at low pT extracted from the strength of the two-photon correlation.(See efforts by the Yale Group)

Low-Low-ppTT range [0.02, 1.0] GeV/ range [0.02, 1.0] GeV/cc is not accessible is not accessible

WA98, Phys. Rev. Lett. 93 (022301), 2004

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Experiment: direct photons at PHENIX

Probing the QCD equation of state with thermal photons in nucleus-nucleus collisions at RHICDavid d’Enterria and Dimitri Peressounko, nucl-th/0503054

Range: 1.0 < pT < 10.0 GeV/cDirect photon spectra obtained with the statistical subtraction method

Low-Low-ppTT range [0.02, 1.0] GeV/ range [0.02, 1.0] GeV/cc is not accessible is not accessible

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Soft photon measurement

Can not be measured by PHENIX, ALICE and STAR with present detectors

Predicted yield is large and even with a small crystal patch measurements seem to be possible at STAR in near future

How well (for how much) we can do that ? Needed R&D in FY07 to find out

ppTT coverage : from 20 to 1000 MeVcoverage : from 20 to 1000 MeV

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Crystals

Inorganic crystals is the only practical option for STAR,CsI(Tl) from IMP is the one we propose to investigate.

Crystal NaI(Tl)

CsI(Tl) BGO PbWO4

Density (g/㎝ 3) 3.67 4.51 7.13 8.28

Radiation length (㎝ ) 2.59 1.85 1.12 0.89

Molière radius (cm) 4.8 3.8 2.3 2.0

DE/dx (Mev/cm)(per mip) 4.8 5.6 9.2 13.0

Nucl. Int. length (㎝ ) 41.4 37 21.8 18

Refractive index (480 nm) 1.85 1.79 2.15 2.16

Peak emission λ (nm) 410 560 480 420-560

Relative light output 100 45(PMT)/140(PD)

15 0.01

Light yield temp.coef. (%/0C)

~0 0.3 -1.6 -1.9

Decay time (ns) 230 1000 300 10-50

Hygroscopic strong slight no no

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Monte-Carlo simulations

Crystal calorimeter:

Three projective “super-cubes” 9x20

Total 540 crystals in x 0.66x6o

Crystals: 2.54 cm x 2.54 cm x 25 cm

Crystal Detector is implemented within STARSIM framework. It is located in place of BEMC module 88 and TOF tray 88.

Crystal PatchCrystal Patch

Super-cube : 9x20 crystalsSuper-cube : 9x20 crystals

Crystal PatchCrystal Patch

STAR EAST VIEWSTAR EAST VIEW

Interaction pointInteraction point

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Crystal Detector: sample MC events

Hijing central Au +Au event: clean, two photons

Hijing central Au +Au event: hadronic background example

photonsphotons

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Occupancy and Energy Deposition

Occupancy data indicates that even in central Au +Au events it is possible to identify soft photons cleanly!

Background sources which contribute to detector occupancy : charged tracks and albedo from BEMC modules.

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Photon rates for low-pT region

Preliminary estimations show that we will need ~1M central events for ~3% error in yield measurements (low-pT region for all centrality bins)

Preliminary photon count rates for 0.02 < pT < 0.20 GeV are calculated using fully reconstructed Hijing events (starsim + BFC). Cluster finder algorithm efficiency is not included yet, charged hadron background removal is considered to be 100% efficient at this moment.

Small acceptance is compensated by large photon yield!

Expected energy resolution 5-10%, depends on cluster size: 3x3, 4x4 crystals and photo detector cutoffs.

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FY07, R&D Goals: MC simulationsComplete understanding of the efficiency and purity of

reconstructed soft photons in the Crystal Detector :• Various cluster finder algorithms investigation• Robust procedure for removal of non-photonic backgrounds

• Possible hardware veto layer for charged tracks?• Shower shape analysis possibilities and isolation cuts• Extended BEMC albedo studies

• Shielding, isolation cuts?Required dataset for detailed analysis :

• 50k fully reconstructed AuAu200 central events (Hijing) with most probable geometry settings (help from S&C group)

• Disk space is the most important issue here• Disk space required for studies is ~0.5 Tb

Various performance issues :• Pileup, leakages, optimizations etc…

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FY07, R&D Goals: Hardware Define crystal detector element.

• crystal, wrapping, coupling with Photo Detector, Photo-detector, readout• lots of things known from literature – need first hand experience.

Optimize light yield/uniformity. Investigate possibility of using TPC DAQ1000

• PSA/Altro chips for readout of PD. (Counting on help from Tonko)

Make preliminary design of a mechanical support structure and investigate integration into the STAR detector. (help from STSG)

Understand radiation conditions, required rad. hardness tests and monitoring scheme for crystals.

By the end of FY07 we want to define cost/schedule.

Requests:1. Acquire crystals from IMP. (in progress)2. Acquire PD from Hamamatsu.3. Acquire/Borrow radioactive sources 4. Set up a testing lab at BNL

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SummaryVery first and preliminary results look optimistic. More detailed R&D work is needed in FY07 and

we hope to reach a firm decision on feasibility of the soft photon physics with crystals in STAR

We need more collaborators!

Thanks to STAR Software team for their help!

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Backup slides

• Energy resolution: require a lot of explanation

• graph 3x3 20-200 MeV band (10% to as low as 3%?)

• Shower shape in crystals

3x3 – central fraction, leak from sides

put here light yield per MeV, realistic cutoff

How crystal patch can be compared with conversion on gas of TPC?