NA62 is a fixed-target experiment located in the CERN experimental North Area. The beam is provided by SPS. During the first phase (2007 → 2010) the collaboration measured the ratio RK = BR(K → eν)/BR(K → μν). This

measurement is sensitive to Standard Model (SM) deviations; in particular it can test lepton universality. During the second phase, the collaboration aims to measure BR(K+ → π+ν¯ν). Due to the sensitivity of this BR to new physics, this decay has a strategic role in the search for physics beyond the SM. The hadronic contribution to the uncertainty is small and the SM prediction is precise: (8.5 ± 0.7)10−11. The apparatus is under construction and the first run is expected for spring 2013. The detector status, prototype tests, and signal sensitivity compared with background rejection will be discussed

Palladino, V.

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DOI 10.1393/ncc/i2011-11015-8Colloquia: IFAE 2011IL NUOVO CIMENTO Vol. 34 C, N. 6 Novembre-Dicembre 2011NA62 present statusV. Palladino on behalf of NA62 CollaborationINFN, Sezione di Roma “La Sapienza”piazzale Aldo Moro, Rome, Italy(ricevuto il 29 Luglio 2011; pubblicato online il 25 Ottobre 2011)Summary. — NA62 is a fixed-target experiment located in the CERN experimentalNorth Area. The beam is provided by SPS. During the first phase (2007 → 2010)the collaboration measured the ratio RK = BR(K → eν)/BR(K → μν). Thismeasurement is sensitive to Standard Model (SM) deviations; in particular it cantest lepton universality. During the second phase, the collaboration aims to measureBR(K+ → π+νν¯). Due to the sensitivity of this BR to new physics, this decay hasa strategic role in the search for physics beyond the SM. The hadronic contributionto the uncertainty is small and the SM prediction is precise: (8.5± 0.7)10−11. Theapparatus is under construction and the first run is expected for spring 2013. Thedetector status, prototype tests, and signal sensitivity compared with backgroundrejection will be discussed.PACS 11.30.Hv – Flavor symmetries.PACS 12.15.Hh – Determination of Cabibbo-Kobayashi & Maskawa (CKM) matrixelements.PACS 13.20.Eb – Decays of K mesons.1. – The physics caseIn the search for new physics (NP) effects beyond the Standard Model (SM), flavor-changing neutral-current processes are particurarly relevant. These processes are domi-nated by penguin and box diagrams and can sensitively test various NP scenarios. Kaonsrare and ultra-rare decays, such as KL → π0l+l− and K → πνν, are particularly cleansince no long-distance contributions from processes with intermediate photons are in-volved and hadronic matrix elements can be obtained from branching ratios (BR) ofleading K decays, such as K → πeν, via weak isospin rotation [1-4]. In the SM, the the-oretical expectation for the charged mode [5], BR(K+ → π+νν) = (8.5 ± 0.7) × 10−11,has a 7% non-parametric error. In contrast, possible NP contributions could change theBR by up to a factor of three in many scenarios [6]. The decay K → πνν may be sensi-tive to new physics in a wide range even in the absence of a signature of new particleswithin LHC reach. At present, seven K+ → π+νν events have been identified by thec© Societa` Italiana di Fisica 60NA62 PRESENT STATUS 61Fig. 1. – Schematic view of the NA62 detector.BNL E949/E787 stopped-kaon decay experiments [7]. The measured BR is compatiblewith the SM prediction, although with a large uncertainty: BR = (1.73+1.15−1.05) × 10−10.There is still plenty of room for possible NP effects [8].2. – The NA62 experimental techniqueThe aim of the NA62 experiment [9] is to detect about 100 K+ → π+νν events witha O(∼ 10%) signal acceptance and a background on the order of 10% in two years ofdata-taking. The design is inspired by years of experience with the NA48 apparatusand infrastructure. For NA62, the K12 beamline at the CERN SPS North Area willbe upgraded to increase the intensity by a factor of 50. In the final setup, a 400GeVSPS primary proton beam interacts into a beryllium target and produces an unseparated75GeV, 800MHz beam with ∼ 6% K+, corresponding to ∼ 5MHz kaon decays in a 60mlong fiducial volume. A transverse schematic view of the NA62 detector is shown in fig. 1.The guiding principles in the experiment design follow from the need to sustain ahigh-rate environment while guaranteeing high-resolution timing. Kaons are identifiedyby a CEDAR detector upstream to all other detectors.The goal is to identify a signal BR of ∼ 10−10 with a total background rejection ofthe order of 1012 against the leading K+ decay modes.Both kinematical and PID background rejection are implemented.Two- and three-body decay modes will be reduced by a factor of ∼ 104 by cutting onthe squared missing mass of reconstructed candidates. For this purpose, a fast up-streamtracker of every particle in the beam, the so-called Gigatracker (see subsect. 2.1), is usedto measure incoming K momentum. Downstream of a 60m long fiducial region for Kdecays, a straw-chamber magnetic spectrometer is used to measure with high-resolutiondaughter particle momenta.Further rejection of Kμ2,3,4 and Ke2,3,4 background will be obtained with a ring-imaging Cherenkov counter (see subsect. 2.2), used to identify in a efficient and non-destructive way daughter pions and separate them from muons and electrons. The π/μseparation is critical to achieve sufficient rejection for Kμ2 decays. For this purpose, ad-ditional information will be provided by a sampling calorimeter, the so-called MUon Veto(MUV), placed after the 27X0’s of the existing LKr NA48 electromagnetic calorimeter.62 V. PALLADINO on behalf of NA62 COLLABORATIONRejection of modes with π0’s and/or radiative photons will be provided by a her-metic, high-efficiency photon-veto system, covering from 0–50mrad γ emission angles(see subsect. 2.3.1). This has to provide a rejection factor of 108 against K+ → π+π0.Finally a series of guard rings are placed just after the 3th GTK station (see sub-sect. 2.1) in order to detect inelatic events that can produce a π+ and so mimic signalevents.A detailed description of the NA62 apparatus can be found in [9].In the following the main components of the detector and their contributions to themeasurement will be described.2.1. Fast Beam Particle Tracking . – The GigaTracKer(GTK) [10] will measure the3-momentum of each particle of the 800MHz beam with precision much better than thatgiven by the momentum bite of the beam. It will also provide a time measurement forsingle particles. This system is placed upstream, just before the decay volume. Threesilicon micro-pixel stations (called GTK1-2-3) with a total thickness of less than 0.5% X0each, will provide position measurement while particles traverse a magnetic achromat.A total of 18000, 300 × 300μm2 pixels for a sensitive area of 60 × 27mm2 will providespatial hit resolution of ∼ 100μm. Momentum will be measured with a fractional errorof ∼ 0.2%, corresponding to 150MeV resolution, while direction will be determined with12μrad angular resolution. In order not to cause station-by-station hit mismatch in morethan 1% of the cases, a hit time resolution better than 200 ps is required. The readouthas to sustain rates of up to 150 kHz. The R&D is almost completed, with two readout prototypes developed and compared, both with FE circuits in 130 nm IBM CMOStechnology.There is a drawback in this technological choise. In fact nuclear inelastic interactionat the last GTK station can produce pions or other particles that, if emitted at lowangle, can reach the STRAW tracker and mimic a signal decay, if no other track isdetected. A detector called CHANTI (CHarged ANTIcounter) is intended to identifyinelastic interactions in the GTK by tagging particles at higher angles with respect tothe beam.2.2. PID of decay products. – A RIng-imaging CHerenkov counter (RICH) [11], willprovide rejection for muons with less than 0.5% mis-ID probability for events not iden-tified by the MUV. More than three standard deviations of π/μ separation should beachieved in the K → πνν pion momentum range, 15 < pπ < 35GeV. Time determina-tion with a resolution better than 100 ps should be guaranteed, to efficiently match withGigatracker information. This performance will be obtained by using a 17m long, 3mdiameter tank, filled with 1 atm Ne gas acting as radiator. Mirrors at the downstreamside of the volume will focus rings of Cherenkov light into two separated regions on theupstream side. These are instrumented with 2000 photomultiplier tubes (PMT’s), each18mm wide.In a dedicated test beam for a prototype with ∼ 400 PMT’s a muon rejection betterthan 1% has been measured, with an overall pion loss of few per mil and a time resolutionbetter than 100 ps, these figures holding across the momentum range of interest.2.3. Efficient photon veto. – A system of different detectors will veto photons providinga rejection of 108 for π0 from K+ decay in a 60m long fiducial volume, allowing thebackground from K+ → π+π0 decays to be reduced to less than one part in 1012. Photonsemitted at very small angle, <∼ 1mrad, will be detected by compact calorimeters in theNA62 PRESENT STATUS 63Fig. 2. – NA62 schedule up to 2012.forward direction, with a required inefficiency of < 10−6 above 6GeV. In the angularrange between 1mrad and 8mrad, the existing NA48 LKr calorimeter will be re-used,profiting of a measured inefficiency < 10−5 for photons above 6GeV. At large angle,between 8mrad and 50mrad, a new system so-called Large Angle Veto (LAV) will provideγ detection with an inefficiency <∼ 10−4 above O(100MeV).2.3.1. The LAV system. After an intense R&D activity [12], the re-use of SF57 lead glassblocks from the dismounted OPAL barrel electromagnetic calorimeter, already instru-mented with R-2238 Hamamatsu phototubes, has been validated. The inefficiency mea-sured with dedicated test beams satisfies the requirements and is comparable with otheralternatives, including lead/scintillating-fiber or lead/scintillating-tile sampling calorime-ters. The LAV will be made of 12 stations of increasing diameter. Each station will becomposed of four or five layers (depending on station position), for a total depth of 29 to37 X0’s. Layers are staggered to guarantee that incident particles must encounter at leastthree blocks, corresponding to more than 20X0’s. A total of ∼ 2500 blocks will be used.Since high sensitivity to photons in the range from O(100)MeV to 20GeV is required,the front-end electronics must guarantee a wide dynamic range from 20mV (tipical MIPsignal) up to 10V on a 50 Ω load. A simple and cost-effective solution, easy to scale andto integrate with a common NA62 trigger and data acquisition infrastructure has beenadopted: a time-over-threshold discriminator, with two adjustable thresholds. Signalswill be clamped, split into two, amplified, and discriminated with two thresholds to allowslewing corrections. The LVDS digital output will allow accurate leading and trailingedge time determinations. From the time-over-threshold, a 10% resolution measurementof the charge will be made, allowing the LAV system to operate as a calorimeter as wellas a veto. Test beam results show that a fractional energy resolution ∼ 10%E (GeV)−1/2and a time resolution ∼ 300 psE(GeV)−1/2 are achievable.3. – Experiment statusThe experiment was presented to the CERN SPS Committee in 2005 and, after a longR&D program on the different sub-detectors, it was approved by the CERN ResearchBoard in December 2008. Multiple test beam have been performed for advanced pro-totypes or parts of single sub-detectors, as discussed above. Commissioning is on goingand a technical run is planned for the end of 2012 when the apparatus will be almostcomplete (see fig. 2), GTK will not partecipate, and only 2 out of 4 STRAW plane willbe ready. The first physics run is expected to take place in late 2013.64 V. PALLADINO on behalf of NA62 COLLABORATIONREFERENCES[1] Buras A. J., Gorbahn M., Haisch U. and Nierste U., Phys. Rev. Lett., 95 (2005)261805; J. High Energy Phys., 11 (2006) 002.[2] Isidori G., Mescia F. and Smith C., Nucl. Phys. B, 718 (2005) 319.[3] Buchalla G., D’Ambrosio G. and Isidori G., Nucl. Phys. B, 672 (2003) 387.[4] Mescia F. and Smith C., ArXiv:0705.2025 [hep-ph], update for Phys. Rev. D, 76 (2007)034017.[5] Brod J., Gorbahn M. and Stamou E., arXiv:1009.0947 [hep-ph], and references therein.[6] Isidori G., Mescia F., Paradisi P., Smith C. and Trine S., JHEP, 0608 (2006) 064[hep-ph/0604074].[7] Artamonov A. V. et al. (BNL-E949 Collaboration), Phys. Rev. D, 79 (2009) 092004,arXiv:0903.0030 [hep-ex].[8] From the FlaviaNet Kaon working group, http://www.lnf.infn.it/wg/vus/content/Krare BSM new.eps.[9] NA62 Collaboration, CERN NA62 note, NA62-10-07 http://na62.web.cern.ch/NA62/Documents/NotesDoc/na62-07-10.pdf.[10] Mazza G., Ceccucci A., Cortina E. et al., Nucl. Instrum. Methods A, 617 (2010) 558.[11] Bucci F., Collazuol G. and Sergi A., Nucl. Instrum. Meth. A, 623 (2010) 327.[12] Antonelli A. et al., J. Phys. Conf. Ser., 160 (2009) 012020 (Nucl. Phys. Proc. Suppl.,197 (2009) 224).

NA62 present status

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NA62 present status

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