Vittorio Paolone on Minerva
Minerva is a dedicated neutrino-nucleus cross section experiment running on the NuMI line at Fermilab. It will perform detailed studies of interactions of neutrinos on a variety of nuclei, using low energy neutrinos.
The existing data between 1 and 20 GeV is poorly understood. It is mainly coming from bubble chamber data, wide band neutrino beams, low statistics samples, with large systematic errors from large uncertainties in the fluxes.
We care about cross sections for precise neutrino oscillation determinations. In oscillation experiments you need dm^2 L/E_beam = 1 to maximize the oscillation effect. With delta m^2 in the range of 2.4E-3 eV^2 and L of hundreds of kilometers, the beam energy comes out in the few GeV range. So we need precise cross sections there.
Disappearance oscillation measurements look at a distortion in the neutrino energy spectrum for E<5 GeV. The experiments however measure the visible energy, which depends on the flux, the cross section, and the detector response. A dependence is also clear on the particle multiplicities and particle type produced. The final state interactions do matter for these measurements, too.
For appearance oscillation measurements you try to measure theta_13, looking for electron neutrinos in muon neutrino beams.There are backgrounds from neutral pions arising in neutral current interactions; the decays of neutral pions produce two gammas, and one of them can easily mimic the electron. There is an intrinsic nu_e component in the beam to be aware of, too. It is thus critical to measure these background processes using the same nuclear targets.
MINERVA is positioned to resolve discrepancies between different experiments. NOMAD and MiniBoone show a disagreement in their cross section as a function of energy, measured in different energy range, as shown in the figure below.
The MINERVA collaboration consists of about 100 nuclear and particle physicists from 22 institutions in 7 countries. The detector is an active segmented scintillator with nuclear targets of carbon, iron and lead (helium and water targets are coming soon). Measuring the interactions of neutrinos with all targets in the same detector reduces systematic errors between them. There are 120 modules of four types: nuclear target, tracker, and E and Had calorimeters. Detectors are not magnetized, so muons are penetrating into MINOS to determine the sign of the muon.
The targets have parts made of carbon, lead, and iron in exagonal patterns. Five patterns are used. They have side “ears” with which they can be mounted on rails for easy assembly. Below are shown the different kinds of exagons making up the metal target.
The tracker modules are composed of an inner detector made of two layers of scintillator bars. Surrounding them there’s a “side” electromagnetic calorimeter, sampling the energy as it exits the detector.
The scintillating elements contain WLS fibers reading them out from within a central hole. These allow to get a very nice position resolution of 2.65 mm. There are 40 thousand such bars.
The detector has good tracking resolution, calorimetry, timing information within few nanoseconds, containment of events except muon measured in Minos. Particle ID is there, too.
Understanding the flux is extremely important. The beamline has a lot of instrumentation. To maximize the flux the target and the horns are movable. This can be used to tune the neutrino energy. They can thus select neutrino or antineutrino beams of various energies.
They started data taking in 11/2009 with 0.8E20 POT at low energy and collecting antineutrinos, when only 55% of the detector was built. Then, in 2/2010 they installed the rest of the detector, and then ran a low energy neutrino beam, gathering interactions from 1.2E20 POTs. Now they are running since November 2010 in low-energy antineutrino mode, so far 1.2E20 POTs again. In the spring 2011 they will start again with neutrinos. In the summer of 2012 the Fermialb accelerator will be shut down, and switch to medium energy.
Cross section errors: statistical errors are not a problem, the absolute cross sections are dominated by systematics. There are typical 30% errors on these fluxes, but the goal of Minerva is a 7% erro in the flux shape, and 10% in its normalization. They will normalize to the narrow band beam at CCFR at their high energy end. The event spectrum will be measured with quasi-elastic events.
The comparison of muon anti-neutrino data in Q^2 with Monte Carlo predictions (GENIE and GEANT) shows that the data undershoot the prediction (see right). They are trying to understand it in more detail now.
In conclusion, they are studying in precision the neutrino interactions in the low energy range (1-20 GeV), using a fine grained high resolution detector at the high-flux NuMI beam. They plan to improve our knowledge of neutrino cross sections at low Q^2, and their target dependence. This will reduce the systematics of many oscillation experiments.