Millennium Institute for Subatomic Physics at High-Energy Frontier, Saphir

We performed a search for a new generic $X$ boson, which could be a scalar ($S$), pseudoscalar ($P$), vector ($V$), or an axial vector ($A$) particle produced in the 100 GeV electron scattering off nuclei, ${e}^{\ensuremath{-}}Z\ensuremath{\rightarrow}{e}^{\ensuremath{-}}ZX$, followed by its invisible decay in the NA64 experiment at CERN. No evidence for such a process was found in the full NA64 dataset of $2.84\ifmmode\times\else\texttimes\fi{}{10}^{11}$ electrons on target. We place new bounds on the $S$, $P$, $V$, $A$ coupling strengths to electrons, and set constraints on their contributions to the electron anomalous magnetic moment ${a}_{e}$, $|\mathrm{\ensuremath{\Delta}}{a}_{X}|\ensuremath{\lesssim}{10}^{\ensuremath{-}15}--{10}^{\ensuremath{-}13}$ for the $X$ mass region $1\text{ }\text{ }\mathrm{MeV}\ensuremath{\lesssim}{m}_{X}\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{GeV}$. These results are an order of magnitude more sensitive compared to the current accuracy on ${a}_{e}$ from the electron $g\ensuremath{-}2$ experiments and recent high-precision determination of the fine structure constant.

Energetic quarks liberated from hadrons in nuclear deep-inelastic scattering propagate through the nuclear medium, interacting with it via several processes. These include quark energy loss and nuclear interactions of forming hadrons. One manifestation of these interactions is the enhanced emission of low-energy charged particles, referred to as grey tracks. We use the theoretical components of the BeAGLE event generator to interpret grey track signatures of parton transport and hadron formation by comparing its predictions to E665 data. We extend the base version of BeAGLE by adding four different options for describing parton energy loss. The E665 data we used consists of multiplicity ratios for fixed-target scattering of 490 GeV muons on xenon normalized to deuterium as a function of the number of grey tracks. We compare multiplicity ratios for E665 grey tracks to the predictions of BeAGLE, varying the options and parameters to determine which physics phenomena can be identified by these data. We find that grey tracks are unaffected by modifications of the forward production. Thus their production must be dominated by interactions with hadrons in the backward region. This offers the advantage that selecting certain particles in the forward region is unlikely to bias a centrality selection. We see a strong correlation between the number of grey tracks and the in-medium path length. Our energy loss model does not reproduce the suppression observed in the projectile region. We see an underprediction of the proton production rate in backward kinematics, suggesting that a stronger source of interaction with the nuclear medium is needed for accurate modeling. These results lay an important foundation for future spectator tagging studies at both Jefferson Laboratory and at the Electron-Ion Collider, where neutron and proton grey track studies will be feasible down to very small momenta.

The article gives a brief overview of the history of creation, the current state, and the prospects for the future development of the Data Center of the SAPHIR Millennium Institute in Santiago, Chile.