Participation in the HADES experiment P.Fonte for the HADES RPC group Jornadas LIP 2014 Sixth framework programme

HADES RPC Group Current responsibilities Maintenance & Operation USC D. Belver P. Cabanelas E. Castro J.A. Garzón M. Zapata GSI W. Koenig TU Darmstadt G.Kornakov LIP A. Blanco N. Carolino O. Cunha P. Fonte L. Lopes A. Pereira P.Bordalo C. Franco S. Ramos L. Silva A.Oliveira C.Silva Maintenance & Operation Calibration IFIC-Valencia J. Diaz Participation in the physics analysis

Spectrometer concept § Geometry Fast particle identification Full azimuth, polar angles 18 o - 85 Pair acceptance » 0.35 About 80.000 detector channels Fast particle identification RICH CsI solid photo cathode, N ~ 80 , C 4 F 10 radiator TOF ( scintillator rods) RPC-TOF Pre Shower 18 pad chambers & lead converters Momentum measurement ILSE , super conducting toroid B r = 0.36 Tm MDC multi wire drift chamber, single cell resolution 100 m

Hexagonal geometry with 6 identical “sectors” RICH MDC I MDC II MDC III MDC IV Magnet TOF RPC Shower Hexagonal geometry with 6 identical “sectors”

Strong dilepton enhancement over hadronic cocktails

Unevenly distributed radially by a factor ~2 Heavy nuclei colisions Lots of particles (up to 30/sector) Unevenly distributed radially by a factor ~2 over the inner TOF area C+C Au+Au

RPC design criteria Operational parameter matched to the HADES overall performance Multi-hit capability hit-loss probability below 20% Minimum 150 channels/sector  effective cluster size resolution 100 ps () or better In principle not difficult for a good detector Potential problems: mechanics, crosstalk (hard to measure). rate capability up to 1 kHz/cm2 in some areas Not so easy for almost continuous beam efficiency above 95% for single hits In principle no problem, except for geometric coverage Area ~8/6 m2/sector Fast! no long R&D time available

Final design 2 layers of individual cells with partial overlap rows 187 cells/sector distributed in 29 rows and 6 columns, 3 on top and 3 on bottom 1122 cells total 124 different detectors with variable width, length and shape  no attempt at impedance matching

HADES cells Heat-tolerant materials Fully shielded 0.27 mm  4 gaps minimum for good efficiency Aluminum and glass 2mm electrods minimize amount of glass for maximum rate capability try to keep good mechanics Heat-tolerant materials Fully shielded Spring-loaded pressure plate Aluminium Glass HV & readout in the center

Assembly in gas box HV capacitor (~1cm2) HV distribution PCB 50 signal Feedthrough PCB Further shielding between feedthrougs

Readout scheme t1 t2 FEE

FEE electronics t1 t2 4 channel 3GHz amp+timing comparator + QtoWidth conversion Support motherboard (mechanics, services, signal routing)

TDC and DAQ (by GSI electronics group) TRB (Time Readout Board) 128 multihit TDC channels (CERN’s HPTDC)

Partially instrumented Time frame Concept validation, 2003 First prototype test, 2005 Partially instrumented Final prototype test, 2007 Fully instrumented

Time frame Final tests, 2009 Ready for beam, 2011

Data recorded, compared with previous beamtimes Very successful DAQ upgrade

Sub-threshold produced K- clearly visible: robust multihit performance

Mind the internal spacers of the RPCs!

Motivation The main goal of the LIP-HADES group is to study the mass properties of hadrons inside a very dense hadronic medium at low temperature This region of the phase-space, which is still poorly known, is explored in Hades through Au+Au collisions at 1.25 GeV/nucleon

Motivation Leptons are the ideal probe of the in-medium properties of hadrons: they are not disturbed by secondary interactions due to their lack of strong interaction The final goal is to compare the experimental e+e- (coming from the in-medium hadronic decays) mass spectrum with the predictions of models describing the heavy ion collisions The in-medium properties of the above mentioned mass spectrum are crucial for a better understanding of the mechanism responsible for the mass generation in hadrons

Flowchart of the method developed at LIP for the PID of leptons variables PID variables Signal Sample (lepton tracks) multivariate input Background Sample (hadron tracks) Multidimensional parame- trisation: all correlations are properly taken into account Dynamic Neural Network single network output (per track) Network Cut Accepted Tracks Rejected Tracks

Velocity distribution as a function of the momentum times polarity (before and after a multidimensional cut in the Neural Network response) All tracks Rejected tracks Accepted tracks The selection of relativistic electrons and positrons is evident The efficiency and purity of the lepton sample are both above 92% The selection of leptons with a low velocity or a high momentum is made possible by the use of a Neural Network

The invariant mass spectrum for dileptons Combinatorial background is subtracted e+e- Preliminary The physical background,   e+e-, was removed with the help of a Neural Network

Conclusion The HADES spectrometer was upgraded and had a very successful data taking with the Au-Au system. The new RPC TOF Wall performed flawlessly, in specs, and showing a robust multihit performance. There will be another production beamtime this year, either with  beam or with the Ag-Ag system. HADES will be part of the set of experiments of FAIR (the first one to run, actually).