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Physics Letters B 746 (2015) 44–47

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Physics Letters B 746 (2015) 44–47 Physics Letters B 746 (2015) 44–47 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb + Muon-induced background to proton decay in the p → K ν decay channel with large underground liquid argon TPC detectors ∗ J. Klinger , V.A. Kudryavtsev, M. Richardson, N.J.C. Spooner Department of Physics and Astronomy, University of Sheffield, Sheffield S3 7RH, UK a r t i c l e i n f o a b s t r a c t Article history: Large liquid argon TPC detector programs such as LBNE and LAGUNA-LBNO will be able to make Received 25 February 2015 measurements of the proton lifetime which will outperform Cherenkov detectors in the proton decay Received in revised form 9 April 2015 + channel p → K ν. At the large depths which are proposed for such experiments, a non-negligible source Accepted 23 April 2015 of isolated charged kaons may be produced in the showers of cosmogenic muons. We present an estimate Available online 27 April 2015 + of the cosmogenic muon background to proton decay in the p → K ν channel. The simulation of muon Editor: L. Rolandi transport to a depth of 4km w.e. is performed in the MUSIC framework and the subsequent propagation Keywords: of muons and secondary particles in the vicinity of a cylindrical 20 kt LAr target is performed using Proton decay Geant4. An exposure time of 100 years is considered, with a rate of < 0.0012 events/kt/year at 90% CL Nucleon decay predicted from our simulations. Muon-induced background © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license Liquid argon (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. LAr TPC 1. Introduction GUT models by suppressing or forbidding this decay mechanism. Extended GUT models can incorporate supersymmetry [7] in which → + An attractive framework in which to embed the Standard Model the favoured process for proton decay can be p K ν [8], whilst (SM) of particle physics is a Grand Unified Theory (GUT) in which allowing the SM couplings to unify at a single energy [3]. each of the three independent gauge coupling constants of the SM Amongst the next generation of high precision proton-decay de- symmetry group SU(3)C ⊗ SU(2)L ⊗ U(1)Y are unified at a high- tectors will be large liquid argon (LAr) time projection chambers energy scale, GUT. (TPC) such as LBNE or LAGUNA-LBNO [9,10]. These experiments Any GUT model of hadrons, leptons and gauge interactions will introduce LAr fiducial volumes of the order of tens of kt at would necessarily imply the violation of baryon-number conser- depths of at least 1.5 km. LAr TPC imaging will have a higher ef- → + vation at the GUT-scale [1]. If one pursues such a theory, the ficiency to measure the mode p K ν compared to Cherenkov fundamental observables of GUT-scale physics would be closely detectors, and as such they will outperform existing detectors such ± related to the stability of the proton. In particular, the lifetime of as Super Kamiokande [11]. Given the high efficiency for K parti- the proton and the dominant proton decay products could indicate cle identification in LAr TPC detectors, the largest background to + a preference towards a specific GUT model and could also provide the p → K ν signature will be from cosmogenic muons and neu- an insight into physics below the GUT-scale. trinos. In the simplest extension to the SM, one can extrapolate In this paper, we present the results of our simulation of the ± the three coupling constants to high energies such that the dif- number of backgrounds to this signal from K mesons produced ferent couplings become the same order of magnitude above in the showers of cosmogenic muons. We compare our results to 15 10 GeV [2]. One caveat with this procedure is that the couplings the study performed by Bueno et al. in Ref. [12]. Compared to the do not unify at a single energy scale [3,4] as one might expect latter study, we perform the first full Monte Carlo simulation of from the most simple GUT [5]. The observed stability of protons in particle production, transport and detection that includes cosmo- 0 + the decay mode p → π e [6] presents another problem for the genic muons and all secondary particles.1 We also consider a depth simplest GUT model. These problems can be avoided in extended 1 We define ‘primary’ particles as those which are present at the surface at the * Corresponding author. Earth, and ‘secondary’ particles as those which are produced at any depth below E-mail address: j.klinger@sheffield.ac.uk (J. Klinger). this. http://dx.doi.org/10.1016/j.physletb.2015.04.054 0370-2693/© 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. J. Klinger et al. / Physics Letters B 746 (2015) 44–47 45 simulation, all particle information and associated energy deposi- tions in the LAr volume are recorded. 3. Event selection In this section, we present event selection criteria which main- + tain high efficiency for selecting p → K ν signal events whilst rejecting cosmogenic muon-induced backgrounds. Kaons from free proton decay are expected to have a kinetic energy of 106 MeV. After accounting for the Fermi motion of a proton in a nucleus, the nuclear binding energy, and subsequent re-scattering of the kaon, the kinetic energy of a kaon emitted from a nucleus of argon is expected to be smeared across the Fig. 1. The distribution of muon energies on the top and side surfaces of the cuboid described in the main text, as sampled and simulated by MUSIC/MUSUN [14,15]. range 0 to 200 MeV [18]. In a LAr TPC detector, the kinetic en- 7 Atotal of 10 muons are considered in this distribution. ergy measurement of charged kaons is further broadened by the energy resolution, which we estimate following the procedure pre- and scale of the detector volume which is more applicable to the sented in Ref. [19]. In order to retain > 99% of the signal events LBNE and LAGUNA-LBNO detectors. we require that the energy deposition from a charged kaon due to Int ionisation and scattering, E K < 250 MeV. 2. Simulation framework By constraining the total energy of all proton decay products to Int the proton mass, we require that sum of E K and the energy de- In our model, a LAr TPC detector is positioned at a depth of Dec position of subsequent decay products, E K , should be less than 4 km w.e., which corresponds to the proposed depths of the LBNE 1GeV. Meanwhile energy deposition from other particles not as- and LAGUNA-LBNO detectors. We model a total mass of 20 kt of sociated with the kaon, its interactions or decay products, EOther, LAr, which is close to design specification of the LAGUNA-LBNO in the same event should be smaller than 50 MeV; otherwise this detector. The detector is modelled as a cylinder of LAr with a di- additional energy deposition would be clearly visible. ameter of 30 m and a height of 20 m. A cylindrical stainless steel We reject charged kaon tracks that originate from within 10 cm container, with walls of thickness 5cm, fully surrounds the LAr of a detector wall to ensure that the candidate signal events from cylinder. One can compare this to the study performed by Bueno proton decay are not mismatched with the events originating out- et al. [12] which reports a prediction of the background rate using side of the detector. This results in a decrease in efficiency of a 100 kt volume of LAr at a depth of 3kmw.e. simulated in the 2%. We also require that all tracks are not within 10 cm of the FLUKA [13] framework. detector walls, to ensure for proper energy evaluation and parti- The designs of large-scale LAr detectors include up to 2m of cle identification. Events containing such partially-contained tracks non-instrumented LAr which will separate TPCs from the physical are rejected. The requirement for no activity within 10 cm of the walls. This region of non-instrumented LAr defines a ‘wall’ for this detector wall is shown to reject 87% of events with no primary study; as no interaction or energy loss occurring in this volume muons in the fiducial volume. can be seen by the TPC and is unlikely to be caught by a light At the simulated depth of 4 km w.e., we find that muons pass detector, unless a very sophisticated light detection system is used. −1 through the LAr detector at a rate of 0.078 s , so the average time We do not model this region of the detector, and instead treat this between the two muons crossing the detector is approximately boundary as one of the physical walls of the detector. 12.8 s. Assuming that the maximum drift time, and hence the du- In this paper, the simulation of particle propagation is per- ration of the event record, will be about 10 ms, the probability of a formed in two stages. In the first stage, only muon transport muon crossing the detector within any 10 ms time window will be is considered and the interactions of secondary particles are ne- −3 approximately 10 . Rejecting events that contain a muon, where glected. In the second stage, all particles including secondary par- the muon’s track length is greater than 20 cm for clear identifica- ticles are fully simulated.
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