Frequently Asked Questions

This page is a collection of questions that were asked at conferences.

Neutrino flux from charm vs pion + kaon

Q: How well can you constrain electron neutrino flux from charmed hadron decays from pp collisions?

A: Taking into account uncertainties in the correlation between the yield of charmed hadrons in a given eta region with the neutrinos in the measured eta region, it was evaluated that SND@LHC can measure the charmed-hadron production in pp collision with a statistical uncertainty of about 5%, while the leading contribution to the uncertainty is the systematic error of 35%.

Q: What is the source of the neutrinos?  How many events are expected from pion + kaon vs charm?

A:  Muon neutrinos are mainly produced in pion/Kaon decays for both FASERv and SND@LHC. About 50% of electron neutrinos interacting in FASERv come from charmed hadron decays. This fraction amounts to 90% in SND@LHC.

Neutrino energy range

Q: What is the expected range of neutrino energies ?

A: Between a few hundred GeV and a few TeV.

Energy resolution

Q. Why is the fractional energy resolution at the level of 20%? What is the main source of uncertainty?

A. The reported value (22%) is obtained using a counting method based on the correlation between the true energy and the number of registered hits. The application of CNN-based algorithms showed to further improve the resolution.

CC vs NC

Q. How do you distinguish νe NC and νμ NC ?

A. We do not. All we do is to check the ration between all flavour NC interactions and all flavour CC interactions to understand the detector systematics in separating NC and CC.

Q. In this boosted regime of high nu energies, in a νe interaction electromagnetic and hadron showers mix up. How well do you separate the NC interaction from νe CC interactions?

A.  Because of the micrometric accuracy of the emulsion you can topologically separate electrons from neutral pions by observing the displaced vertex associated with photon conversion. This information can be combined with the response of the electronic detectors, where simulations indicate there is room for distinguishing nueCC from NC events using the hit distribution in both SciFi and HCAL (see preliminary study reported by Cris (

But perhaps what this person was asking about was how to discriminate neutrino scattering events  from neutral hadron backgrounds. This was studied by FASERnu, they have determined the energy spectra of both cases and use a neural network to do the discrimination  (

New physics

Q. How can you distinguish between the elastic scattering of a new physics particle and a neutrino off protons?

A. A scattered proton will have a secondary vertex at the start of the hadron shower which is O(9 cm, one lambda) away from the primary vertex. A neutrino DIS event produces a cascade at the primary vertex. If one of the products in this cascade is a proton which produces another cascade, this event can be excluded by looking at the multiplicity at the primary vertex, which should be 1 in the case of the scattering of a new particle. For inelastic scattering one has to look at the energy spectrum of the neutrino, which is separated from that of a mediator for energies from 1 GeV (see TP page 98, fig. 73).

Q. How confident are you that you can use TOF to distinguish between neutrino and new physics interactions? Is the time resolution sufficient to take into account the 200ps LHC bunch size?

A. In the TP, the TOF was not used. It is an extra handle that can be applied to reject background. The significance of the use of the TOF depends on the particle mass and is given in Figure 72 on page 96 of the TP. The timing resolution of the scintillator bars is O(100 ps). However, TOF is not efficient for all scenarios, see arxiv:2104.09688.pdf, page 10.

Q. If you had a particle decaying to muon pairs, would you potentially be able to separate it from backgrounds if the decay occurred (a) before the initial scintillator plane, or (b) within the iron blocks?

A. These events have not been studied in detail, because the background has not been estimated. One source of background is a neutrino interaction when you produce a muon and a pion in the final state and you mis-identify the pion as a muon. Normally, at the LHC energies, this background is suppressed by the multiplicity of events being higher than 2 charged particles. It needs to be studied. We don't have a muon spectrometer so we won't be able to measure the invariant mass. If the decay happens in the emulsion brick we can at least precisely determine the multiplicity (in the iron we won't see nuclear fragments and other soft particles emerging) and the vertex with the muon slopes. Intuitively, the only chance we have to reduce the background is with events originated in the emulsion target. Even with these events, the lack of a muon momentum is a problem. This is one reason to go for an upgrade.


Comparison with FASERnu

Q. How is SND related or compared to FASERnu?

A. SND and FASERnu are complimentary in the sense FASER will observe neutrinos with eta>9, while SND looks at 7.2<eta<8.4. Actually, SND is making use of the background measurements performed for FASER, so this is already a nice example of cross-collaboration. There are relevant differences also in the detector layout.

Q. And concerning the energy spectrum? Are they different at all?

A. In the eta range explored by SND a large component of neutrinos reaching the detector come from the decay of charmed hadrons. This is not the case of FASERnu, which cannot perform charm production measurements.