Measurements of low mass e + e - pairs in p+p and Au+Au collisions with the HBD upgrade of the PHENIX detector Mihael Makek Weizmann Institute of Science for the PHENIX Collaboration HP2010, Eilat, Israel

Low mass dileptons The Hadron Blind Detector Status of the dielectron analysis Conclusion and outlook Outline: 3/16/20162M. Makek, HP2010

 Dileptons interact only electromagnetically  undisturbed path from the interaction region to the detectors  Probes for chiral symmetry restoration and in-medium modification of the low mass vector mesons  Results* from RHIC Run-4: yield in m ee = GeV/c 2 larger by a factor 4.7 +/- 0.4(stat.) +/- 1.5(syst.) +/- 0.9(model) compared to the expected hadronic contribution  S/B in this mass region is 1/200  combinatorial background should be reduced! Low mass dileptons 3/16/20163 *Phys.Rev.C81, (2010) M. Makek, HP2010

 The main sources of the combinatorinal background are from: π 0      e + e - π 0   e + e -  The magnetic field bends e + e - in opposite directions, one of them can go out of acceptance or can spiral in the magnetic field not reaching the tracking detectors Low mass dileptons 3/16/20164 ~12 m e+e+ e+e+ e-e- e-e- M. Makek, HP2010 comb. backg. pair

How to reduce the combinatorial background?  e + e - from π 0 Dalitz decays and conversion have a small opening angle (close pairs) unlike the pairs produced by resonance decays (open pairs)  Preserve the opening angle  create magnetic field-free region  Distinguish between the single electron hit from an open pair and the double electron hit from a closed (background) pair  the task of the HBD! Low mass dileptons 3/16/20165M. Makek, HP2010   π0π0 

The HBD is a windowless Cherenkov detector CF 4 radiator gas and active gas. L rad =50 cm triple GEMs for signal multiplication CsI photocathode proximity focus configuration hexagonal pad readout (pad side a = 1.55 cm) total radiation length: ~4.4% 0.9% (vessel) + 0.5% (CF 4 ) + ~3% (backplane) The HBD – the basic concept 63/16/2016M. Makek, HP2010 Cherenkov blobs e+e+ e-e-  p air opening angle B≈ m NIM A 546 (2005) , NIM A 523 (2004) m

Electron signals are relatively weak and rare (compared to hadrons) Hadron blindness: The HBD – hadron blindness 73/16/2016M. Makek, HP2010 a.Cherenkov light is formed only by e + or e -, threshold for  is 4 GeV/c b.the detector is operated in reverse bias mode to repel the ionization charge from dE/dx CsI (350 nm) primary ionization from dE/dx El. Field photo electron mesh GEMs pads

Run-9: p+p collisions at 500 GeV:  Recorded 0.01 nb -1 p+p collisions at 200 GeV Recorded 0.02 nb -1 Run-10: Au+Au collisions at 200 GeV:  Recorded 1.3 (1.1) nb -1 or 8.2B (7.0B) events in +/-30 (+/-20) cm vertex (5x data than in Run-4) Au+Au collisions at 62.4 GeV:  Recorded 0.1 nb -1 or 700M events in +/- 30 cm vertex Au+Au collisions at 39 GeV:  Recorded 0.04 nb -1 or 250M events Measured data sets 83/16/2016M. Makek, HP2010 PHENIX Integrated luminosity in Run GeV

The HBD performance: position resolution 93/16/2016M. Makek, HP2010 Position resolution: CA track projection( ,Z) – HBD cluster position( ,Z) Run 9, p+p For single pad hits: 2a/sqrt(12) ~ 1 cm ~1/p const For electrons (~3 pads):   = 8 mrad (0.5 cm)  intrinsic resolution)   = 1.05 cm ( 0.5 cm intrinsic, ~1 cm vertex )  (rad)  (cm)

The HBD performance: electron-hadron separation 103/16/2016M. Makek, HP2010 HBD charge: hadrons in forward vs. reverse bias Run 9, p+p HBD charge: electrons vs. hadrons Good electron-hadron separation! 20 pe

The HBD performance: single vs. double hit recognition 113/16/2016M. Makek, HP2010 Run 9, p+p HBD charge – single hitHBD charge – double hit Fully reconstructed  0 Dalitz pairs (m < 150 MeV/c 2 ) in the central arms. Matched to HBD into two separate clusters (open pairs) or one single cluster (close pairs) Single electron charge peaks at 20 pe Double electron charge peaks at ~40 pe  Good single to double separation, important for rejection of the combinatorial background

The average number of photo- electrons N pe in a Cherenkov counter: with:   (average Cherenkov threshold in the sensitive bandwidth of the detector)  bandwidth: 6.2 eV (CsI photocathode threshold) eV (CF 4 cut-off) The HBD performance: figure of merit N 0 and single electron detection efficiency 123/16/2016M. Makek, HP2010 N 0 ideal value795 cm -1 Optical transparency of mesh89 % Optical transparency of photocathode81 % Radiator gas transparency92 % Transport efficiency80 % Reverse bias and pad threshold90 % N pe measured20 N 0 measured value330 cm -1 The high photoelectron yield  excellent single electron detection efficiency:  Single electron efficiency using a sample of open Dalitz decays:  ~ 90 %  Single electron efficiency derived from the J/  region:  = 90.6  9.9 % 330 cm -1 The detector is kept almost H 2 O and O 2 free!

HBD occupancy grows due to scintillation (central events > 95 %) The average charge per cell subtracted (event-by-event basis) Occupancy reduced to ~30 % in the central events Efficiency estimate using embedding of MC with HBD data: 80% in the most central – 90% in the peripheral events The HBD performance: subtraction of the scintillation background (in Au+Au collisions) M. Makek, HP20103/16/ HBD module before subtraction: HBD module after subtraction: Run 10, Au+Au

143/16/2016M. Makek, HP2010 Estimate from Run-9 p+p: StepBckg. reduction factor 1 matching to HBD7.1  double hit cut close hit cut single pad cluster cut2 Run 9, p+p Pairs in Central Arms Pairs matched to HBD Pairs after HBD reject. Status of the dielectron analysis (m ee > 0.15 GeV/c 2 )

The HBD detector has been developed and installed It was successfully operated in the PHENIX set-up in Run-9 (p+p) and Run-10 (Au+Au) The data shows:  good position resolution  excellent electron detection efficiency  good separation between electrons and hadrons  good separation of single and double electron hits  successful handling of scintillation background Analysis of Run-10 Au+Au data in progress:  62.4 GeV data set produced  200 GeV data set production under way  Physics coming soon! Conclusion and Outlook 153/16/2016M. Makek, HP2010

BACKUP slides 163/16/2016M. Makek, HP2010

PHENIX detector 173/16/2016M. Makek, HP2010 Detector  Field PH Central Arms+/ °up to 1.15 Tm Inner and outer magnet coils producing field-free region for r < 55 cm

HBD design parameters 183/16/2016M. Makek, HP2010

Scintillation Ionization HBD gain determination 193/16/2016M. Makek, HP2010 Forward Bias Reverse Bias Zoom  Gain determination: Fit scintillation component with an exp fctn: 1/slope = G. = avrg no. of scintillation photons in a fired pad)  In p+p collisions  1  In Au+Au collisions, assuming the no. of scintillation photons per pad follows a Poisson distribution:

HBD in reverse bias mode 203/16/2016M. Makek, HP2010 Operating Point Electron vs. hadron detection efficiecy:Remaining hadron signal: The operating point chosen to minimize the hadron signal while keeping the optimal electron detection efficiency The remaining hadron signal comes from the collection of dE/dx ionization in the thin layer (100 microns) above the first GEM and from the ioniozation in the first transfer gap This signal is much lower than electron signal (1/20)! Remaning signal