TThe detectors of the SHiP experiment at CERN
E. Graverini a , on behalf of the SHiP collaboration a Universit¨at Z¨urich
Abstract
SHiP is a proposed general purpose fixed target facility at the CERN SPS accelerator. The main focus will be the physics of theHidden Sector, i.e. search for heavy neutrinos, dark photons and other long lived very weakly interacting particles. A dedicateddetector, based on a long vacuum tank followed by a spectrometer and particle identification detectors, will allow probing a varietyof models with exotic particles in the GeV mass range. Another dedicated detector will allow the study of Standard Model neutrinocross-sections and angular distribution, and allow detection of light dark matter with world leading sensitivity.
Keywords:
SHiP, Detector techniques for Cosmology and Astroparticle Physics, Hidden Sector detectors
1. The SHiP experiment
Search for Hidden Particles (SHiP) is a proposed general pur-pose beam dump facility, aimed at searching for very weakly in-teracting long-lived particles, collectively referred to as the Hid-den Sector (HS). An integrated flux of 2 × protons collidingonto a heavy target will allow SHiP to probe a great variety ofNew Physics models, improving the current limits by several or-ders of magnitude in 5 years of nominal operation [1, 2]. HeavyNeutral Leptons (HNLs), right-handed partners of the StandardModel (SM) neutrinos, will be searched for in decays of beautyand charm mesons. The existence of such particles is stronglymotivated by theory, as they can simultaneously solve multipleproblems left open by the SM. In the Neutrino Minimal Stan-dard Model ( ν MSM), HNLs explain the baryon asymmetry ofthe Universe, account for the pattern of neutrino masses andoscillations and provide a dark matter candidate [3]. A redun-dant system of background tagging detectors will make SHiPa zero-background experiment. In addition, the peculiar lay-out of the SHiP facility also make it a very e ffi cient StandardModel neutrino factory. Therefore, a dedicated detector nick-named i SHiP will be installed upstream of the decay volume forhidden particles, aimed at studying the ν τ properties. The SHiPTechnical Proposal [1] and physics case [2] have been posi-tively reviewed by the CERN SPS Committee in 2015, and thecollaboration has been prompted to produce a ComprehensiveDesign Report. Email address: [email protected] (E. Graverini) H i d d e n S e c t o r d e c a y v o l u m e ν / i S H i P H i d d e n S e c t o r d e t e c t o r T a r g e t a n dh a d r o n a b s o r b e r μ s w e e p i n g m a g n e t s Figure 1: G eant
A scheme of the SHiP facility is illustrated in Figure 1. Adedicated beam line, extracted from the SPS using the sametransfer line as the other CERN North Area experiments, willconvey a 400 GeV / c proton beam. The beam will be stopped ina heavy Molybdenum-Tungsten target, which is followed by ahadron stopper and by a system of shielding magnets sweepingresidual muons away from the detector fiducial area.The target of the i SHiP detector will be placed in the muonfree area, and it will be composed of bricks of laminated leadand emulsions, interleaved with a scintillating fiber spectrome-ter and followed by a muon spectrometer, allowing to measurethe momentum and identify the flavour of scattering leptons.
Preprint submitted to Elsevier September 5, 2018 a r X i v : . [ phy s i c s . i n s - d e t ] S e p he main element of Hidden Sector detector will be a ∼
50 mlong decay volume, kept at a pressure of 10 − atm to minimizethe probability of ν interactions in air. The decay volume iscontained in a pyramidal frustum shaped vessel, with maximumtransversal dimensions of 5 ×
10 m . The length and shape of thevessel have been optimised by maximizing the acceptance tothe hidden particle decay products given the dimensions of themuon-free area achieved by the shielding magnet, which shapeand power were in turn optimized based on expected cost andperformance. The vessel walls enclose segmented 30 cm thicksections of liquid scintillator aimed at tagging particles comingfrom outside. An upstream straw detector, placed in vacuum ata distance of 5 m from the vessel entry lid, will identify eventsinitiated in the material upstream of the fiducial decay volume.The vacuum vessel will be followed by a detector for thecharged hidden particle daughters. Momentum information willbe provided by a tracking system placed in vacuum at the endof the decay volume, made of four stations of 5 m long strawtubes placed on either side of a 1 Tm magnet. A high-accuracytiming detector will be installed, aimed at reducing the yield ofcombinatorial events. Two technologies are being considered:plastic scintillating bars and multigap resistive plate chambers(MRPC). Both technologies can be based on existing and well-studied designs and can reach a time resolution of 100 ps [1].Particle identification will be provided by Shashlik electromag-netic and hadronic calorimeters, followed by a muon systemmade of four active layers interlaced with iron. Due to the small coupling expected between hidden parti-cles and the SM, production rates of 10 − or lower are ex-pected. The vast majority of SM particles produced in the targetare stopped by the hadron absorber or swept out of acceptanceby the muon shield; however, residual muons or other parti-cles can still enter the decay volume and produce a signal inthe HS detector. These events produce however a signal in theliquid scintillator tagger, too, and are therefore removed. Themain background to the hidden particle signal originates insteadfrom the scattering of SM particles (especially neutrinos) in thevicinity of the HS decay volume, producing long-lived neutralmesons such as K S . These particles nevertheless decay beforethe upstream straw tagger or have a vertex very close to the ves-sel walls, with the reconstructed candidate not pointing to thebeam target. Random combination of tracks can also mimic HSevents, but the use of a high resolution timing detector will re-duce this background to a negligible level. Thorough studiesreported in [1] have found no evidence of any irreducible back-ground, allowing to set an upper limit of 0.1 expected back-ground events during the whole 5 years SHiP run. With this level of background, the sensitivity of the SHiPdetector is illustrated in Figure 2 for the ν MSM, as a bench-mark New Physics model with light long-lived particles. Theexpected sensitivities to hidden particles exceed current limitsby several orders of magnitude, depending on the model [2].
HNL mass (GeV) -1
10 1 HN L c oup li ng t o S M U -11 -10 -9 -8 -7 -6 SHiP sensitivity to HNLs (90% C.L.)
BBN BAU / Seesaw BAU
PS191 NuTeV
SHiP
Figure 2: SHiP’s discovery potential in the parameter space of the ν MSM [1]. x0.05 0.1 0.15 0.2 0.25 0.3 0.35 + s i z e o f un c e r t a i n t y on s NNPDF3.0 NNLONNPDF3.0 NNLO + SHiP charmNNPDF3.0 NNLONNPDF3.0 NNLO + SHiP charm
Figure 3: SHiP contribution to the strange quark sea uncertainty bands [2].
The high intensity proton beam dump will produce a largeflux of neutrino of all three flavours impinging on the i SHiPtarget. The ¯ ν τ will be observed for the first time, and the crosssections of ν τ and ¯ ν τ will be measured with high precision. Sucha neutrino flux will allow to measure for the first time the F and F structure functions governing charged current ν τ inter-actions. In addition, neutrino-nucleon deep inelastic scatteringevents can improve our understanding of the flavour composi-tion of the nucleon, allowing to study the strange quark content.About 2 millions and 1 million events are expected from ν µ and ν e scattering, respectively, which would significantly im-prove our knowledge of the strange sea as shown in Figure 3and as documented in [2]. The i SHiP detector will also be ide-ally suited to detect the signal generated by light dark matterparticles scattering on the electrons of the emulsion target. Thebackground from neutrino scattering can be reduced to less than300 expected events during the whole SHiP run. SHiP is ex-pected to be sensitive in portions of the light dark matter pa-rameter space not accessible to previous experiments [2].
References [1] M. Anelli, et al., A facility to Search for Hidden Particles (SHiP) at theCERN SPS arXiv:1504.04956 .[2] S. Alekhin, et al., A facility to Search for Hidden Particles at the CERNSPS: the SHiP physics case arXiv:1504.04855 .[3] T. Asaka, S. Blanchet, M. Shaposhnikov, The nuMSM, dark matter andneutrino masses, Phys. Lett. B631 (2005) 151–156. arXiv:hep-ph/0503065 , doi:10.1016/j.physletb.2005.09.070 .[4] S. Agostinelli, et al., GEANT4: A Simulation toolkit, Nucl. Instrum. Meth.A506 (2003) 250–303. doi:10.1016/S0168-9002(03)01368-8 ..