NEUTRON PRODUCTION IN NEUTRINO-NUCLEUS INTERACTIONS WITH T2K

Srivastava, Asit

doi:10.5281/zenodo.17696176

NEUTRON PRODUCTION IN NEUTRINO-NUCLEUS INTERACTIONS WITH T2K

Srivastava, Asit

Publisher: Zenodo
DOI: 10.5281/zenodo.17696176
Repository: zenodo.org
Language: en

Abstract

Parallel talk presented at the XXI International Workshop on Neutrino Telescopes - Padova 29 September - 3 October 2025 (https://agenda.infn.it/event/44606/) Abstract: T2K is a long-baseline experiment which measures parameters of neutrino oscillations. This can be done by analyzing the interaction of neutrinos close to the point of beam production and 295 km downstream. The detector located near the beam source, called ND280, primarily measures the interactions of neutrinos with carbon nuclei. The particles produced as a result of the interactions deposit energy in ND280 which is used to characterize the incoming neutrino flux and neutrino cross-sections before oscillations occur. Detecting a neutron and analyzing those events is tied to the understanding of final state interactions (FSI) models. Charged-current neutrino quasielastic scattering, the dominant interaction in the energy range of T2K, can produce a neutron only via FSI. Thus, by selecting events with no mesons, but with at least one neutron, and a negative muon, a selection enhanced in FSI events is obtained. Comparing these events to the ones with no identified neutron provides a better understanding of FSI and nuclear effects. However, neutrons are not easy to detect and because of this, they provide uncertainties in identifying the interactions happening in the detector and measuring cross-sections. ND280 has a newly installed Super Fine-Grained Detector (SFGD) made of plastic scintillator cubes. The upgraded detector capable of better position resolution and 3D reconstruction opens up the possibilities of improving the efficiency of neutron detection. Neutrons often leave a cluster, which is basically a few hits in neighbouring cubes, or less likely, a track produced by a proton knocked out by the neutron. With the sub-ns timing resolution achievable in the SFGD, the neutron's kinetic energy can be reconstructed using the measured time-of-flight. The talk will discuss which set of variables have been identified to distinguish true neutron tracks/clusters from background, and on the event level, neutron producing events from events without neutrons. Funded by the Deutsche Forschungsgemeinschaft.

Full text

NEUTRON PRODUCTION IN NEUTRINO-NUCLEUS INTERACTIONS WITH T2K Asit Srivastava [email protected] XXI International Workshop on Neutrino Telescopes Neutron Production in Neutrino-Nucleus Interactions with T2K 2 • Long-baseline experiment (295 km) • Near detectors (ND280, INGRID, WAGASCI-BabyMIND) at 280m from J-PARC •�� peaks at 0.6 GeV at ND280 (2.5° off-axis) Phys.Rev.D 87,012001 (2013) The T2K Experiment Related Talks: S. King, L. Machado Neutron Production in Neutrino-Nucleus Interactions with T2K 3 • Super-FGD is made of ~ 2 million scintillator cubes of size 1 cm which provide sub-ns timing resolution. ToF TPCs FGDs Near Detector ND280 Upgrade • HA-TPC does the identification and reconstruction of particles traveling at high angles. • TOF, along with SFGD, can differentiate between incoming and outgoing particles Related Talks: G. Reina, M. Feltre, W. Okinaga-san Neutron Production in Neutrino-Nucleus Interactions with T2K 4 Typical T2K muon neutrino energy range https://doi.org/10.2172/1042577 Quasi-Elastic Scattering (CCQE) Delta Resonance Neutrino-Nucleus Interactions Related Talk: E. Sandford Neutron Production in Neutrino-Nucleus Interactions with T2K 5 • Particles produced after neutrino-nucleus interaction can undergo FSI. • This makes identification of specific interactions in detector difficult. • Identifying topology is a better choice as it reduces model dependence. Some topologies: CC0π, CC1π, CC-multi π, CC0πNn Final State Interactions (FSI) Picture Credits: T. Golan p Neutron Production in Neutrino-Nucleus Interactions with T2K 6 CCQE (�� → �−�) : With FSI 2p2h (�� → �−푛) : With or without FSI (�� → �−) : With FSI Delta resonance : With or without FSI Type of interaction % in CC0πNn (before FSI*) % in CC0πNn (after FSI*) CCQE 0% (0) 54.80% (1119) 2p2h 36.08% (245) 22.82% (466) Delta res. 59.65% (405) 21.11% (431) Others 4.27% (29) 1.27% (26) • Large increase in events due to CCQE and 2p2h undergoing FSI • CC0πNn topology can be used to test FSI models *FSI turned ON or OFF here is Nucleon FSI Interactions in CC0πNn Topology T2K WORK-IN-PROGRESS Neutron Production in Neutrino-Nucleus Interactions with T2K 7 Neutron candidate in an actual event One neutron candidate in this event S F G SFGD HATPC HATPC TPC1 TPC2 TPC3 FGDs Neutron Production in Neutrino-Nucleus Interactions with T2K 8 Vertex Muon Track Secondary Cluster Primary proton Selection of CC0π events • One muon track • No pion track • Any number of proton tracks Secondary Track Neutron Production in Neutrino-Nucleus Interactions with T2K 9 Reconstructing a neutron • Neutrons deposit energy away from the vertex. • Lever arm and Time-offlight are tools to identify a neutron and reconstruct its energy. Phys. Rev. D 101, 092003 (2020) Neutron Production in Neutrino-Nucleus Interactions with T2K 16 Initial purity of CC0πNn: ~ 30% With high BDT scores, ~ 90% purity is achievable. T2K WORK-IN-PROGRESS Event BDT Score CC-1�+ in the plot is a typo. It can be any of the three pions. Neutron Production in Neutrino-Nucleus Interactions with T2K 17 Distribution of neutron candidates (tracks + clusters) peaks around 2 - 4 neutron candidates. T2K WORK-IN-PROGRESS Number of neutron candidates CC-1�+ in the plot is a typo. It can be any of the three pions. Neutron Production in Neutrino-Nucleus Interactions with T2K 18 • ND280 upgrade has made reconstruction of particles with shorter tracks or smaller energies even better. • CC0πNn topology can be instrumental in testing FSI models. • Neutrons leave an energy deposit in the form of a track or a cluster away from the vertex. • To identify neutron events, BDTs are trained and they give high signal purity. • Currently, work is in progress to develop systematics and study model dependence in CC0πNn selection. Summary Thanks for your attention! Neutron Production in Neutrino-Nucleus Interactions with T2K 19 Neutron candidate in an actual event Two neutron candidates in this event SFGD HATPC HATPC TPC1 TPC2 TPC3 FGDs Neutron Production in Neutrino-Nucleus Interactions with T2K 20 Backup Neutron Production in Neutrino-Nucleus Interactions with T2K 21 • Neutrinos produced in a specific flavor state (��) with energy � • After traveling a distance �, �� decrease in number ⟹ �� disappearance �� increase in number ⟹ �� appearance • Sensitivity to maximum oscillation depends on �/� Neutrino Oscillations Neutron Production in Neutrino-Nucleus Interactions with T2K 22 • Neutrinos produced in a specific flavor state (��) with energy � • Oscillation probability depends on: Neutrino Oscillations Neutrino energy Travelling distance (“baseline”) PMNS mixing parameters Difference between mass states Parameters in PMNS matrix (�23, �13, �12, ��) Known Need to know What is measured Neutron Production in Neutrino-Nucleus Interactions with T2K 23 Neutrino Energy as Uncertainty C. Wret, NuSTEC2024 • For precise measurement of oscillation parameters, neutrino energy must be wellreconstructed. • One of the leading sources of uncertainty in neutrino energy reconstruction are neutrons (calorimetric) and interaction models (kinematic). • They have a high chance of escaping the detector without depositing any energy, hence, creating a problem in reconstructing neutrino energy. Neutron Production in Neutrino-Nucleus Interactions with T2K 24 Neutron Production in Neutrino-Nucleus Interactions with T2K 25 NEUTRINO-NUCLEUS INTERACTIONS Quasi-Elastic Scattering (CCQE) Two-particle two-hole process (2p2h) Delta Resonance �� → �−�� → �−푛 �� → �− �� → �−∆+→ �−�+� → �−�0 �� → �−Δ++ → �−�+ Neutron Production in Neutrino-Nucleus Interactions with T2K 32 CC0π pre-selected with secondary tracks (minimum distance from all primary tracks) • Background distribution peaks for tracks close to other tracks (~5 cm) and leads to impure neutron sample. • These are primarily due to muon hits not used in track reconstruction and delta electrons produced by primary muons. T2K WORK-IN-PROGRESS Correction: All the muons, not just TPC/HAT muon Neutron Production in Neutrino-Nucleus Interactions with T2K 33 Decision Tree Tree depth Root node Splitting Leaf Image Source Neutron Production in Neutrino-Nucleus Interactions with T2K 34 Problems with Single Decision Trees Single decisions tree are fast and easy but have problems: 1. Can lead to overfitting, especially if deep, which means difficult to generalize to new events. 2. Might not describe the data very well if shallow. 3. Instability: Small changes in data can change the whole tree. Neutron Production in Neutrino-Nucleus Interactions with T2K 35 Track and Cluster BDT Score T2K WORK-IN-PROGRESS T2K WORK-IN-PROGRESS Track BDT score is for all the secondary tracks (at least 3 cm from the reconstructed primary vertex) in all CC0π-selected events. Same for clusters. Signal means that the secondary object is connected to a primary neutron through one or more scatterings and that neutron has the same vertex as that of the muon. Neutron Production in Neutrino-Nucleus Interactions with T2K 36 There is at least one neutron candidate in these events. So, no cluster means that there is at least one track and vice versa. T2K WORK-IN-PROGRESS Number of neutron (track and cluster) candidates T2K WORK-IN-PROGRESS CC-1�+ in the plot is a typo. It can be any of the three pions.