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An HBD for Low-Mass e + e - Measurements

An HBD for Low-Mass e + e - Measurements. Itzhak Tserruya Weizmann Institute, Israel RHIC Detector Advisory Committee Review BNL, Dec.19, 2002. Outline. Introduction Physics Motivation CERN highlights RHIC prospects Upgrade concept System specifications Limit from charm The HBD

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An HBD for Low-Mass e + e - Measurements

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  1. An HBD for Low-Mass e+e- Measurements Itzhak Tserruya Weizmann Institute, Israel RHIC Detector Advisory Committee Review BNL, Dec.19, 2002

  2. Outline • Introduction • Physics Motivation • CERN highlights • RHIC prospects • Upgrade concept • System specifications • Limit from charm • The HBD • Detector concept • R&D: results and plans • Conclusion

  3. Physics Motivation • Two fundamental issues of RHI collisions: - deconfinement and CSR. • Low-mass e+e-pairs are the best probe for CSR • Physics potential confirmed by CERN results • Prospects at RHIC are excellent • The RHIC program will be incomplete without a good measurement of low-mass e+e-pairs • PHENIX is the only experiment at RHIC that can perform such measurement

  4. Meson properties (m,) expected to be modified (?) * Best candidate: -meson decay ( = 1.3fm/c) Physics accessible through low-mass e+e- (I) • Chiral Symmetry Restoration Chiral symmetry spontaneously broken in nature. Quark condensate is non-zero: < qbarq >  300 MeV3  0 at high T and/or high baryon density Constituent mass  current mass Chiral Symmetry (approximately) restored. •  meson * simultaneous measurement of   e+ e- and  K+ K- powerful tool to evidence in-medium effects * strangeness enhancement

  5. No enhancement in pp nor in pA Main CERN Result Strong enhancement of low-mass e+e- pairs in A-A collisions (wrt to expected yield from known sources) Enhancement factor (m > 0.2 GeV/c2 ): 2.6 ± 0.2 (stat) ± 0.6 (syst)

  6. Interpretations Invoke: * +-  * e+e-(thermal radiation from HG) not enough to reproduce the data + * in-medium modifications of  (CSR): • dropping  meson mass(Brown et al) OR - broadening  spectral shape (Rapp and Wambach) In both cases baryon density is the key factor

  7. Low-Energy Run • Motivation: • At 160 GeV/u baryon density is the dominant factor for dropping masses • and spectral shape broadening At lower energies B • Softest point (P/) of equation of state • occurs at 30 GeV/u Lowest pressure gradients Larger lifetimes An even stronger? enhancement in Pb-Au collisions at 40 A GeV Dropping mass and collision broadening give very similar predictions Looking forward to the results from the high resolution and high statistics CERES run of 2000

  8. SPS (Pb-Pb) RHIC (Au-Au) dN( p ) / dy dN( p ) / dy 28.5 2.1 19.3 13.7 dN(  ) / dy dN(  ) / dy 12.9 1.9 17.3 12.7 Total baryon density* 97 102 * Produced baryons (4p + 2) + participating nucleons {(p-p) + 0.64(-)}A/Z Low-masse+e- Pairs: Prospects at RHIC  At SPS energies, both scenarios (-mass dropping and -width broadening) rely on a high baryon density at mid rapidity.  What can we expect at RHIC? Baryon density is almost the same at RHIC and SPS

  9. Low-mass e+e- Pairs: Prospects at RHIC R. Rapp nucl-th/0204003 • Strong enhancement of low-mass • pairs persists at RHIC • Contribution from open charm • becomes significant • Possibility to observe in-medium effects on the  ?

  10. Upgrade Concept Hardware * Compensate magnetic field with inner coil (foreseen in original design  B0 for r  50-60cm) * Compact HBDin inner region (to be complemented by a TPC). Specifications * Electron efficiency  90% * Double hit recognition  90% * Modest  rejection ~ 200 inner coil HBD/TPC Strategy * Identify signal electrons (low mass pairs) with p>200 MeV in outer PHENIX detectors * Identify low-momentum electrons (p<200 MeV) in HBD * Reject pair if opening angle < 200 mrad (for a 90% rejection). HBD/TPC

  11. Beating the Dalitz and conversions CB (Monte Carlo: Signal  meson, Background  conversions and 0 Dalitz ) • Inner detector: * perfect e-id  = 100 % • * perfect dhr = 0 mrad • *  rejection =  • * plus veto area Pair Signal  The number of tracks from 0 Dalitz and conversions is reduced by almost a factor of 20. Backgd tracks

  12. Effective at central arms The irreducible background from charm (I) Au-Au central (dN/dy) 0 = 350 e from charm e from 0 Dalitz e from conv. (X/X0 = 1%) e from conv. (X/X0 = 4%) 48 6.4 0.64 With 90% rejection of Dalitz and conversion tracks, open charm becomes the dominant background source and limits the quality of the measurement (for central collisions).

  13. The irreducible background from charm (II) EXODUS Au-Au central: (dNch/dy) = 650 (dN0 /dy) = 290 (dNccbar/dy)  4 Low-mass e+e- spectrum PHENIX central arm acceptance (S/B)-1 Ratio Total signal x charm signal * CB from charm + total CB Present PHENIX configuration Irreducible background

  14. Transmissive CsI photocathode • No photon feedback • Proximity focus  detect blob • Low granularity • Detector element: multi - GEM • High gain • Reduced ion feedback Detector • Concept: • Windowless Cherenkov detector. • Same radiator and detector gas. • preferred option CF4 • Large bandwidth and large Npe

  15. CsI Photocathode QE The most attractive option: • Transmissive photocathode • Relatively high QE CF4 & CsI: Very large bandwidth: 6 – 11.5 eV Very large N0 940: 40 pe in a 50cm long radiator (including transparency of mesh and first GEM) electron efficiency > 90%

  16. Double hit recognition: amplitude analysis • Single electron hits • 30 Npe • Poisson distribution of Npe • Exponential distribution of single electron charge • Double hit • 60 Npe • DHR > 90% possible at zero opening angle of the pair • Some redundancy provided by shape analysis • Npe is the most important factor

  17. Detector granularity • Pads • Shape: hexagonal • Dimension: app. size of the blob R=1.8cm in a 50cm long radiator • Threshold: defined by background Advantages: - low granularity 2000 pads - large charge per pad  10pe - low gain  104 - low cost Disadvantages: - larger capacitance - coarse pattern recognition (but does not affect the ability to recognize double hits with >90% efficiency). unresolved resolved double hits single hits

  18. HBD R&D • Hardware: * Detector configuration * Aging * Response to hadrons and electrons * CF4 scintillation * Front-end electronics • Simulations * More realistic Monte Carlo * How much Si can be tolerated in front of the HBD ? * Detector granularity * Effect of residual magnetic field

  19. Experimental Set-up (I) S1.S2  MIP C.S1.S2  “electron” Cosmic trigger C • pth 3.8 GeV • 1.30 m long • rate  1/min C: CO2 radiator S2 • can be pumped to 2 10-6 • directly coupled to detector CF4 Radiator S1 • can be pumped to 2 10-6 • several GEMs + MWPC • test with Fe55, UV lamp, ,  Detector Box

  20. Experimental Set-up (II) • All major components of the set-up (cosmic trigger, radiator tank, detector box, electronics, DAQ, gas system, gas analyser and monochromator) exist and are operational. • Program of systematic studies already started

  21. Detector Configuration • Requirement: stable operation at a gain of 104 Detector Options: 2 GEMs 3 GEMs 2 GEMs + MWPC 3 GEMs + MWPC Gas options: pure CF4 CF4 + Ne mixture other gases (CH4)

  22. Existence proof: CF4+GEM+CsI work! Detector Configuration • 2 GEMs: sparks observed at a gain of 2 104 • 3 GEMs: much more promising • Fe55 spark threshold at gains close to 105 • Am241 spark at total charge well in excess of 107 Fe55source Am241 source

  23. Aging Realistic Pessimistic Min bias (in PHENIX acceptance) e 3 x 40 6 x 40 h 80 x 4 160 x 4 Gain 104 5 104 Ion feedback 0.3 1 Interaction rate at design L / 4L (s-1) 1400 5600 Operation per year (s) 1 107 3 107 Years of operation 10 10 Detector area R=70cm (cm2) 1.3 104 CsI: Total ion charge in 10 years 2.3 C/cm2 0.9 mC/cm2 Photon and ion induced aging studies from literature: 20% QE loss after 100 – 10 000 C/cm2 GEM: Total charge in 10 years 7.7 C/cm2 0.9 mC/cm2 Radiation damage in 10 years 28 Rad 660 Rad • Negligible compared to requirements of COMPASS or HERA BUT we must do our own aging study for the combination CsI + GEM + CF4

  24. Scintillation of CF4 • CF4 scintillates at 160nm. • Two measurements in the literature: • * NIM A371, 300 (1996):  110 ph/MeV • * NIM A354, 262 (1995):  200 ph/MeV • Planned to be measured at BNL 2/2003 • Results of simple simulations: (using 200 ph/MeV, QE=0.3, Nch = 250) * signal/noise  10 * shades can reduce the noise by at least a factor of 3.

  25. Conclusions and Outlook • R&D well underway thanks to substantial institute contributions. • Institutes involved: • * Brookhaven National Laboratory * Stony Brook University * Tokyo University * Weizmann Institute of Science • Preliminary results encouraging • Major milestones: * 2003: demonstrate concept validity * 2004 : start HBD construction ( see C. Woody’s presentation for details)

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