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Poster excerpt 12: Neutrino Physics with the SHiP Experiment

March 4, 2015

(The following text is from Annarita Buonaura)

The discovery of the Higgs boson in 2012 provided a strong confirmation of the Standard Model (SM). Nevertheless, there are several experimental facts that the SM cannot account for, thus suggesting the existence of physics Beyond the SM (BSM).

The search for new physics can proceed in two different directions: either moving towards higher energy scales or towards lower interaction strengths. The energy frontier is the domain of the Large Hadron Collider (LHC) experiment at CERN, while the frontier of very weak couplings to the SM can be explored through the very high intensity achievable in a beam dump facility.

SHiP (Search for Hidden Particles) is a new experiment intended to operate in a beam dump facility at CERN.  The aim is to observe long lived and very weakly interacting particles with masses below 10 GeV predicted in many BSM theories and inaccessible otherwise.

Thanks to a data sample of 2×1017 D mesons (and >1015 ντ leptons), which can be accumulated in 5 years of SPS running, it will explore the so called Hidden Sector, looking for:

  • new particles which are singlets with respect to the SM gauge group
  • light sgoldstinos that appear in the breaking of simmetry of the SUSY theory
  • sterile neutrinos called Heavy Neutral Leptons (HNL) also foreseen in a minimal extension (νMSM1) of the SM which are heavy right handed partners of the SM neutrinos and can explain dark matter, baryon asymmetry of the universe as well as the origin of neutrino masses

The SHIP experiment will also have the opportunity to study the only missing tile in the Standard Model by observing for the first time tau anti-neutrinos.

The overall detector layout is shown in fig.1.

buonaura1

Fig.1

Protons accelerated at the SPS up to 400 GeV/c will hit a hybrid target (TZM molybdenum alloy and pure tungsten) producing a large number of charm mesons together with pions, kaons and short-lived light resonances. Since the subsequent decays of these latter particles would result in a large flux of muons and neutrinos, to minimise these decays, a very dense target followed by a 5m iron absorber is used. An active muon shield will reduce the extremely high muon rate. Decay vertices of particles from the hidden sector will then be searched in a downstream [1]vacuum vessel with an elliptical cross section of 5m width by 10m height and a length of 50 m long, followed by tracking chambers, electromagnetic calorimeter and a muon detector.

The ντ detector (fig.2) will be located immediately downstream of the active muon shield.

buonaura2Fig.2

It is an emulsion-based active neutrino target in a magnetic field followed by a muon magnetic spectrometer.

The neutrino detector is a modular target. The fundamental unit is made of a brick and a Compact Emulsion Spectrometer (CES) as shown in fig.3.

The brick employs the Emulsion Cloud Chamber (ECC) technology interleaving lead plates serving as passive material for neutrino interaction and emulsion films serving as tracking devices with micrometric resolution. The CES is made of a sandwich of light material plates (Rohacell) and emulsion films for a total length of 3.1 cm and it is designed in order to be able to perform charge measurement for hadrons with O(GeV) momentum, being the target placed in a magnetic field of about 1T.

buonaura3

Fig.3

The complete target will consist of more than one thousand of these units, thus reaching a total mass of about 9 tons. It will also be complemented by planes of electronic detectors to provide time stamp of the event, to link the information of the muon tracks in the target with those in the magnetic spectrometer and to measure the slopes of the tracks coming from the neutrino interaction.

The magnetic spectrometer is a warm iron dipole magnet instrumented with active layers to perform charge and momentum measurement of muons from τ decay and from νµ CC interactions. Its role in muon identification will be crucial for background rejection.

According to the estimate fluxes of active neutrinos arriving on the target, around 25000 ντ and 10000 anti-ντ charged current interactions are expected in the detector during the whole data taking. The anti-ντ will be seen for the first time, also thanks to the possibility given by the CES to distinguish among ντ and anti-ντ by reconstructing the electric charge of the τ decay products. The statistics will also be sufficient to perform cross section studies for ντ and anti- ντ, enabling the extraction of the F4 and F5 structure functions, never measured so far.

A large flux of νe and νµ is also expected. In particular, more than 68% of the νe flux will have energy above 20 GeV thus allowing studies of νe cross section at high energies. Other measurements will improve the current knowledge of fundamental quantities: the strange-quark content of the nucleon would be measured by means of the charmed hadron production in anti-neutrino interactions and the collection of a few millions of muon neutrinos will contribute to the studies of structure functions. Since νe production is dominated by charmed hadron decays, this measurement will also provide the normalization for the search for long lived particles from the hidden sector.

[1] T.Asaka, M.Shaposhnikov PL B620 (2005) 17

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