Search for new physics with the SHiP experiment at CERN
SSearch for new physics with the SHiP experiment at
CERN
Oliver Lantwin ∗ Imperial College London, London, United KingdomE-mail: [email protected]
SHiP is a new general purpose fixed target experiment at the
CERN SPS designed to complement
LHC experiments in the search for new physics. In its initial phase, the 400 GeV proton beamextracted from the
SPS will be dumped on a heavy target with the aim of integrating 2 × potin 5 years. Shielded by an active muon shield, a dedicated detector, based on a long decay volumefollowed by a spectrometer and particle identification detectors, will allow probing a variety ofmodels with light long-lived exotic particles with masses below O ( ) GeV / c . The main focuswill be the physics of the so-called Hidden Portals, i. e. search for Dark Photons, Light scalars andpseudo-scalars, and Heavy Neutral Leptons. The sensitivity to Heavy Neutral Leptons will allowfor the first time to probe, in the mass range above the kaon mass, a coupling range for whichBaryogenesis and active neutrino masses could also be explained. A dedicated emulsion-baseddetector will allow detection of light dark matter in an unexplored parameter range. The European Physical Society Conference on High Energy Physics5–12 JulyVenice, Italy ∗ Speaker. c (cid:13) Copyright owned by the author(s) under the terms of the Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/ a r X i v : . [ h e p - e x ] D ec earch for new physics with the SHiP experiment at CERN
Oliver Lantwin
1. Introduction
Particle physics is faced with a puzzle. Even though there is experimental evidence fornew physics beyond the standard model ( SM ), with the exception of neutrino oscillations andindications from cosmology, so far all laboratory experiments have been incredibly consistent withthe predictions of the standard model. This leaves us clueless as to where the SM breaks down andbeyond standard model ( BSM ) physics takes over. And so far none of the predictions of popular
BSM models have been confirmed.There are two possibilities for why we did not see
BSM physics yet: The
LHC might not bepowerful enough to explore the scale of new physics, which could be just around the corner, butcould be out of reach of current and future technology as well. Alternatively, new physics could betoo weakly coupled to the SM to be seen at general purpose experiments.I focus on this second option here and will explain how Search for Hidden Particles (SHiP) [1]is designed to find these elusive particles, in particularly for super-weakly coupled new physics with m NP < O (
10 GeV ) .If there is super weakly coupled new physics, there generally is a portal that mediates betweenthe standard model and one or more hidden particles, i.e. the hidden sector ( HS ): L = L SM + L portal + L HS , where L SM is the SM Lagrangian, L HS is the Lagrangian of a possibly richly structured hiddensector, and L portal are the Lagrangian terms linking the two, i. e. those we could conceivably test forexperimentally. If these interaction terms do exist, their mathematical form is constrained by thefact that they, by definition, also involve SM fields. The simplest possible interactions of this kindare tabulated in table 1.As an example to motivate the SHiP design, we will consider the heavy neutral lepton ( HNL ) ofthe neutrino minimal standard model ( ν MSM) — a fermion portal. For details on the many othermodels, the reader is referred to our physics proposal ( PP ) [2]. The ν MSM [3] is a model with aminimal number of additional particles that can solve all of the experimental shortcomings of the SM . In it three right handed neutrinos N i are added to complete the SM : • A light N with mass O (
10 keV ) is essentially decoupled from N , , making it a dark mattercandidate. • Two heavy N , with masses O ( ) mix with the active neutrinos, effectively couplingthem weakly to the SM . They are the HNL . Through the mixing they set the active neutrinoPortal Interaction termScalar (e. g. dark scalar, dark Higgs) ( H † H ) φ Vector (e. g. dark photon) ε F µν F (cid:48) µν Fermion (e. g. heavy neutral lepton (
HNL )) H † NL Axion-like particle (
ALP ) aF µν ˜ F µν Table 1: Possible portal interactions1 earch for new physics with the SHiP experiment at
CERN
Oliver Lantwin masses via the see-saw mechanism, and via leptogenesis they can also explain the baryonasymmetry of the universe (which is converted from an asymmetry of the leptons to baryonsvia sphaelerons). Importantly for experimental studies, they can be produced in heavy flavourdecays, and can decay back to visible final states.With this benchmark model in mind, we can turn to how SHiP is designed to look for this andother portal models.
2. The SHiP experiment
An overview of the SHiP facility is shown in figure 1. The SHiP experiment is designed to lookfor two types of signatures predicted by many new physics models:1. Via decay to visible particles in hidden sector spectrometer,2. Via scattering off electrons or nuclei in nuclear emulsion.To ensure sensitivity to very weakly coupled new physics it is essential to maximise the intensitywhile minimising backgrounds. An intense proton beam from the super proton synchrotron (
SPS ) at400 GeV at the new beam dump facility (
BDF ) in the North Area will supply SHiP with 2 × protons on target over 5 years. It impinges on a very dense target of 12 × λ int , which ensures abundantheavy flavour production while reducing neutrino production from π and K decays. The yields of D -mesons and τ -leptons over 5 years are expected to be in excess of 10 and 10 respectively.Additionally, there will be a high yield of photons from bremsstrahlung, QCD processes and mesondecay, which allows the search for e. g. dark photons.The number of protons extracted from the
SPS will be similar to that provided for the
CERN neutrinos to Gran Sasso (
CNGS ) project, but with slow instead of fast extraction of the beam. Thiswill allow operation in parallel with the
LHC and other beam-lines at the
SPS .With enough intensity to possibly produce new particles, the crucial challenge becomes rejectingbackgrounds from SM processes, SHiP aims to be a zero background experiment for the visibledecay signature in the hidden sector spectrometer.The heavy target and the hadron absorber stop most SM particles, with the exception of muonsand neutrinos. Since the decay volume is under a vacuum to prevent neutrino interactions withinthe fiducial volume, the muons become the key problem. As muons lose very little momentum inmaterial, and the SHiP detectors need to be as close to the target as possible, active shielding isneeded to deflect the muons away from the detectors. This active shield is comprised of a system ofwarm electro-magnets, which first separate the muon charge and then deflect them to either side.To achieve the goal of zero background it needs to reduce the flux of muons in the detector by atleast six orders of magnitude, for the full kinematic range of muons produced, so up to p ∼
350 GeVand p T ∼ H CERN ’s SPS is planned for 2018 [5].2 earch for new physics with the SHiP experiment at
CERN
Oliver Lantwin
Target/Magnetised hadron absorberActive muon shieldEmulsion spectrometerDecay volume Hidden sector spectrometer p @ G e V πµ h n l m Figure 1: SHiPTo further reduce backgrounds from particles produced by muons passing through material,neutrino interactions in the surrounding structures and cosmic rays, the decay vessel is surroundedby background taggers, to detect any visible particles entering or exiting the vessel. A timingdetector will further suppress combinatorial background, while tracking is used for vertexing andimpact parameter measurement, further improving the rejection of fake signal candidates. Finallycalorimeters and a muon detector allow particle identification, allowing study of specific final states.Some of the expected signal final states are tabulated in table 2. Taken together these subsystemsare designed to redundantly reduce any possible backgrounds to negligible levels.To search for hidden sector particles via the complementary signature of scattering, which isparticularly important for e. g. light dark matter, and to study tau neutrinos, a detector based onnuclear emulsions is situated just downstream of the muon shield in front of the decay volume ofthe hidden sector spectrometer.
3. SHiP Sensitivity
This section will give a brief overview of the SHiP sensitivity to several classes of modelscompared to other current and future experiments. Please note, that these sensitivities are fromParticle Final states
HNL , neutralino (cid:96) ± π ∓ , (cid:96) ± K ∓ , (cid:96) ± ρ ∓ Vector, scalar, axion portals; goldstino (cid:96) ± (cid:96) ∓ HNL , neutralino, axino (cid:96) ± (cid:96) ∓ ν (cid:96) Axion portal, sgoldstino γγ Sgoldstino π π Table 2: Visible final-states by hypothesised signal3 earch for new physics with the SHiP experiment at
CERN
Oliver Lantwin before the current round of re-optimisation, i. e. these sensitivity curves correspond to the technicalproposal ( TP ) [1] configuration. (a) (b)(c) Figure 2: Sensitivity for different models at SHiP. (a)
HNL sensitivity at SHiP for ν MSM with U e : U µ : U τ = . m A (cid:48) m χ =
3. Source:Ref. [8]
HNL
For
HNL the available parameter space is limited theoretically by observations of the baryonasymmetry of the universe (
BAU ), the big bang nucleosynthesis (
BBN ) and a model-independentlimit for all see-saw models. The SHiP sensitivity for
HNL in this space is shown in figure 2a.The SHiP sensitivity is best up to about 3 GeV, which is above the charm kinematic limit, thanksto a significant contribution from B -meson decays. In this region it is unique and complementary tothe region that could be probed at the future circular collider ( FCC ) in e + e − mode. The SHiP sensitivity for dark scalars is shown in figure 2b. Again, SHiP covers a unique part ofthe parameter space, complementary to other experiments. For short lifetimes B -factories and LHCb4 earch for new physics with the SHiP experiment at CERN
Oliver Lantwin dominate. B -decays have a large contribution to the sensitivity achievable at SHiP. Note, that thereis a gap at c τ ∼ O ( m ) , where the lifetime is too short for SHiP and too long for the B -experiments,emphasising the importance for SHiP to be as close a possible to the target. (a) (b) Figure 3: Dark photon sensitivity at SHiP. (a) Sensitivity to visible final states. Source: Ref. [2]; (b)Detail of highlighted region. Source: Ref. [9]The SHiP sensitivity for dark photons is shown in figure 3. This estimate is based on a yieldof > γ at SHiP over 5 years. The dark photons here decay to visible final states. The SHiPsimulation includes production via QCD , bremsstrahlung and meson decays, with the respectivesensitivities shown separately in figure 3a, but not yet via electromagnetic showers, which arecurrently being implemented.The sensitivity of SHiP covers a broad region of the parameter space up to masses of a fewGeV and couplings down to 10 − . The upper boundary of the SHiP sensitivity is determined bythe detector’s distance to the target, as dark photons would decay before reaching the decay volume.However, other existing and future experiments can explore this region at higher couplings which isshown in more detail in figure 3b, complementing the region explorable at SHiP. For dark matter lighter than
WIMP s “direct detection” experiments quickly lose sensitivity, dueto the small recoil energy, which requires a very low energy threshold of the detectors. The twocommon approaches to hunt for light dark matter are via missing mass/energy searches ( ∝ U ) andvia scattering/recoil ( ∝ U ), which are complementary.At SHiP light dark matter is searched for indirectly via electron and nuclear recoil in nuclearemulsion. The main backgrounds here are electron recoils from ν e scattering, but differences in Missing energy searches assume a dark photon mediator and are thus insensitive to light dark matter produced byother mediators. earch for new physics with the SHiP experiment at CERN
Oliver Lantwin energy and angular spectra can be exploited to look for an excess consistent with light dark matter.The SHiP sensitivity for light dark matter is shown in figure 2c. Note that the sensitivity shown hereis preliminary, as cascade production of light dark matter is not yet implemented. Furthermore, thissensitivity projection only considers electron recoil, while nuclear recoils are not yet included. Eventhough, SHiP already has the best sensitivity for scattering, complementing
LDMX , which searchesfor light dark matter via missing energy at an electron beam.
4. Conclusion
There is plenty of unexplored parameter space in the dark sector new physics could hide in.SHiP is designed to be sensitive to many different final states for both decay and scattering, allowingit to probe a vast range of models.Currently SHiP is being re-optimised to improve the physics performance further while respect-ing cost constraints. In this context the sensitivities and backgrounds are currently being re-evaluatedand updated for new configurations. For the sensitivity updates in particular additional physicsmodels, production and decay channels are being added in close collaboration with the theoreticalcommunity.
References [1] SH I P C
OLLABORATION collaboration, M. Anelli et al.,
A Facility to Search for Hidden Particles (SHiP)at the CERN SPS , Tech. Rep. CERN-SPSC-2015-016. SPSC-P-350., CERN, Geneva, Apr, 2015.[2] SH I P C
OLLABORATION collaboration, S. Alekhin et al.,
A facility to Search for Hidden Particles at theCERN SPS: the SHiP physics case , Tech. Rep. CERN-SPSC-2015-017. SPSC-P-350-ADD-1., CERN,Geneva, Apr, 2015.[3] T. Asaka, S. Blanchet and M. Shaposhnikov,
The nuMSM, dark matter and neutrino masses , Phys. Lett.
B631 (2005) 151–156, [ hep-ph/0503065 ].[4] SH I P collaboration, A. Akmete et al.,
The active muon shield in the SHiP experiment , JINST (2017)P05011, [ ].[5] SH I P C
OLLABORATION collaboration, E. van Herwijnen,
Muon-flux measurements for SHiP at H4 ,Tech. Rep. CERN-SPSC-2017-020. SPSC-EOI-016, CERN, Geneva, Jun, 2017.[6] T. Spadaro,
Perspectives from NA62 , in
Physics beyond colliders workshop @ CERN (7 Sep 2016) , 2016,https://indico.cern.ch/event/523655/contributions/2246416/.[7] G. Krnjaic,
Probing Light Thermal Dark-Matter With a Higgs Portal Mediator , Phys. Rev.
D94 (2016)073009, [ ].[8] M. Battaglieri et al.,
US Cosmic Visions: New Ideas in Dark Matter 2017: Community Report , .[9] J. Alexander et al., Dark Sectors 2016 Workshop: Community Report , 2016, ,https://inspirehep.net/record/1484628/files/arXiv:1608.08632.pdf.,https://inspirehep.net/record/1484628/files/arXiv:1608.08632.pdf.