A new Inner Tracker is going to be realized for the KLOE-2 experiment at DAΦNE. The tracker is based on GEM technology in order to improve vertices resolution near the interaction point, thanks to the low material budget and

consequently a low effect of multiple scattering. Here there follows a description of the detector and some measurements performed

Morello, G.

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DOI 10.1393/ncc/i2011-10795-yColloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010An innovative tracker for precision measurements at KLOE-2G. Morello for the KLOE-2 CollaborationUniversita` della Calabria and INFN, Gruppo collegato di Cosenza - Cosenza, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online l’8 Febbraio 2011)Summary. — A new Inner Tracker is going to be realized for the KLOE-2 ex-periment at DAΦNE. The tracker is based on GEM technology in order to improvevertices resolution near the interaction point, thanks to the low material budget andconsequently a low effect of multiple scattering. Here there follows a description ofthe detector and some measurements performed.PACS 29.40.Cs – Gas-filled counters: ionization chambers, proportional andavalanche counters.PACS 29.40.Gx – Tracking and position-sensitive detectors.1. – The KLOE experimentThe KLOE experiment [1] collected an integrated luminosity∫ Ldt ∼ 2.5 fb−1 at theFrascati φ-factory DAΦNE, an e+e− collider, at a center-of-mass energy of 1020MeV.The experiment achieved several precision results both in kaon and hadron physics: themeasurements of all branching ratios of KS , KL and K±, the study of scalar and pseu-doscalar mesons and e+e− → π+π− cross section, giving the low-energy hadronic con-tribution to the muon anomaly. A detailed description of the experiment can be foundin [1].2. – KLOE-2At KLOE-2 the interest will be focused on physics coming from the IR: KS decays,KS-KL interference, η, η′ and K± decays, multi-leptons events. Such a research requiresan additional tracker detector (Inner Tracker) [2, 3] between the beam pipe and theinner wall of the central tracking chamber. It will: 1) reduce the track extrapolationlength and improve the decay vertex reconstruction capability; 2) increase the geometricalacceptance for low momentum tracks; 3) improve track momentum resolution (fig. 1).The Inner Tracker must satisfy the following requests: σrφ ∼ 200μm and σZ ∼500μm, < 2%X0 material budget, 5 kHz/cm2 rate capability.c© Societa` Italiana di Fisica 221222 G. MORELLO for the KLOE-2 COLLABORATIONFig. 1. – Simulated resolution σpt/pt on the transverse momentum measurement as a functionof pT for KS → π+π− (left) and η → π+π−e+e− (right) with DC only (circles) and with theaddition of the IT information (triangles).3. – The Inner TrackerThe proposed solution is based on GEM technology: four layers of cylindrical triple-GEM with radii between 13 cm and 23 cm, 70 cm active length, XV strips readout witha stereo angle of 40◦ and a total radiation length of 1.5%X0. A full-scale CGEM pro-totype was built, with 15 cm radius and 35 cm active length. The construction processis described in [2]. The anode was patterned with 650μm pitch longitudinal strips. Thechamber was tested with X-rays and, once equipped with 16-channels prototype GAS-TONE ASIC chip, tested at CERN with 10GeV pion beam. The detector was flushedwith Ar : CO2 70 : 30 gas mixture and operated with the fields E = 1.5/2.5/2.5/4 kV/cmand with the GEM voltages 390/380/370V, corresponding to a gas gain of 2× 104. Thetracks reconstructed by two external tracker stations were compared to those recon-structed by the prototype [4]. The reconstruction efficiency is about 99.6% while theCGEM spatial resolution is ∼ 200μm, in good agreement with what expected from adigital readout of the strips. Since the IT will be equipped with XV patterned strips andit will operate in magnetic field, a dedicated test to address these issues was performedat CERN with 150GeV pion beam line. Five 10 × 10 cm2 planar triple-GEM detectorswere assembled with 650μm pitch strips. Four of them were equipped with standard XYreadout [5], while the fifth with the XV geometry. All the chambers were placed in mag-netic field adjustable up to 1.5T. The magnetic field affects the drift motion of electronsmaking a displacement between the track and the area where signals are induced and in-creasing the electron spread over the readout, as shown by GARFIELD simulations (fig. 2(left)). The gas mixture was Ar : CO2 70 : 30, with the fields 1.5/3/3/5 kV/cm and thevoltages 390/380/370V applied on GEM foils. In order to measure the displacement dx,the four XY chambers were likewise oriented, while the XV chamber was reversed. Thedistance between the reconstructed track and the point of the XV chamber is twice thedisplacement of the electrons in a chamber. The dx was measured for different magneticfields (fig. 2 (right)). The spatial resolution was measured as a function of the gain ofthe GEM and of the magnetic field (fig. 3 (right)). As shown in fig. 3 (left), the efficiencydrops as the magnetic field increases. This effect can be reduced working at higher gainwhich consequently involves a worse spatial resolution.AN INNOVATIVE TRACKER FOR PRECISION MEASUREMENTS AT KLOE-2 223Magnetic field (T)0 0.2 0.4 0.6 0.8 1 1.2 1.4Displacement (mm)-0.200.20.40.60.811.21.41.61.8X displacement as function of magnetic fieldExperimental valuesSimulated valueFig. 2. – GARFIELD simulation (left) and measurements of electron displacement as a functionof the magnetic field (right).Voltage (V)1060 1080 1100 1120 1140 1160Efficiency %5060708090100Efficiency vs Voltage (th=3.5 fC)B = 0TB = 0.3TB = 0.5TB = 0.75TB = 1TB = 1.35TMagnetic Field (T)0 0.2 0.4 0.6 0.8 1 1.2 1.4Resolution (mm)0.180.20.220.240.260.280.30.320.340.360.38Resolution X vs magnetic fieldFig. 3. – Efficiency as a function of voltage and magnetic field (left) and x resolution as a functionof the magnetic field (right).4. – ConclusionsThe beam tests performed on the KLOE-2 Inner Tracker prototype have demonstratedthe feasibility of this detector. The readout geometry was validated and the test inmagnetic field was successful in terms of achieved spatial resolution. The R&D phase isconcluded and the construction of the Inner Tracker has started.REFERENCES[1] Bossi F. et al. and KLOE Collaboration, Riv. Nuovo Cimento, 31 (2008) 531.[2] KLOE-2 Collaboration, Technical Design Report of the Inner Tracker for the KLOE-2experiment, ArXiv 1002.2572v1 and LNF-10/3(P) INFN-LNF Frascati (2010).[3] Di Domenico A. and KLOE-2 Collaboration, J. Phys.: Conf. Ser., 171 (2009) 012008.[4] Domenici D. et al., in 2009 IEEE Nuclear Science Symposium. Conference Records.[5] Balla A. et al., Nucl. Instrum. Methods A, 604 (2009) 23.

An innovative tracker for precision measurements at KLOE-2

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An innovative tracker for precision measurements at KLOE-2

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