Zero Background approach
The "Zero Background Approach" is a key design principle of the SHiP experiment. In contrary to other experiments, SHiP does not try to detect a signal on top of a background, but aims to reduce the background to zero. Otherwise, it would not be possible to detect the extremely rare decays of hidden sector particles and the signal would be lost in the background.
For the seeked number of \(2 \cdot 10^{20}\) protons on target, the expected number of hidden sector particles produced is in between \(\mathcal{O}(10)\) and \(\mathcal{O}(10^3)\). Zero background means, that the expected number of background events in the signal region is less than \(0.1\) events for the entire run of the experiment. With this low number, the probability to observe a background event is
This would mean that over \(90\,\%\) of the datasets would not contain a single background event.
Background Sources
The SHiP collaboration has identified three main sources of background events that could mimic a hidden sector particle decay:
Muon Coincidence
- After the muon shield, a small number of muons will still reach the decay volume. If two muons happen to pass through the decay volume at the same time, they could be misidentified as a hidden sector particle decay, even without an interaction between them. It is schematically shown in Fig. 1. In order to reduce the number of these events, the decay volume is surrounded by a Surround Background Tagger (SBT) and an Upstream Background Tagger (UBT). These detectors are designed to detect any particle entering the decay volume and veto the event.
Fig. 1: A schematic of a muon coincidence event. Two muons pass through the decay volume at the same time and are misidentified as a hidden sector particle decay1.
Inelastic Muon Scattering
- Muons, deflected by the Muon Shield, can scatter inelastically with the material surrounding the decay volume and produce a secondary particle, which could be misidentified as a hidden sector particle decay. It is schematically shown in Fig. 2. To detect these background events, the Surround Background Tagger (SBT) is used. Furthermore, the vertex reconstruction rejects events that do not originate from within the decay volume.
Fig. 2: A schematic of a muon scattering event. A muon scatters inelastically with the material surrounding the decay volume and produces a secondary particle, which could be misidentified as a hidden sector particle decay1.
Neutrino-Interactions
- Neutrinos are not deflected by the Muon Shield and can interact with the material surrounding the decay volume. The resulting secondary particles could be misidentified as a hidden sector particle decay. It is schematically shown in Fig. 3.
Fig. 3: A schematic depiction of background, resulting from a neutrino interaction1.
Background Reduction
SHiP uses a combination of multiple methods, described in the other sections of this documentation, to achieve the "Zero Background Approach". Some of them are combined to provide a redundant background rejection. The main methods are:
- The Target is designed to remove all hadronic showers from the beam dump. It is additionally equipped with a Hadron Absorber.
- The Muon Shield deflects most of the muons away from the decay volume.
- The Decay Volume is filled with low pressure helium to reduce the number of neutrino interactions and to minimize the scattering of muons.
- The Surround Background Tagger (SBT) and the Upstream Background Tagger (UBT) detect any particle entering the decay volume and veto the event.
- The Downstream Spectrometer is equipped with a High Precision Timing Detector (HTPD) to reject events that do originate from muon coincidences. It vetoes events when the time difference between the two tracks is too large.
- The Reconstrution of the recorded events allows to reconstruct the vertex of the event and reject events that do not originate from within the decay volume. It also allows to reconstruct the invariant mass of the two tracks and reject events that do not match the expected mass of a hidden sector particle.
Overall, many Background Reduction methods are combined in the SHiP experiment.