Search for millicharged particles in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$

We report on a search for elementary particles with charges much smaller than the electron charge using a data sample of proton-proton collisions provided by the CERN Large Hadron Collider in 2018, corresponding to an integrated luminosity of $37.5{\mathrm{fb}}^{-1}$ at a center-of-mass energy of 13 TeV. A prototype scintillator-based detector is deployed to conduct the first search at a hadron collider sensitive to particles with charges $\le 0.1e$. The existence of new particles with masses between 20 and 4700 MeV is excluded at 95% confidence level for charges between $0.006e$ and $0.3e$, depending on their mass. New sensitivity is achieved for masses larger than 700 MeV.

Figure 1

The position of the detector, shown as a blue rectangular volume, in an elevation view (left) and plan view (right). The dashed lines represent the projection of the center of the detector to the CMS IP.

Figure 2

A diagram of the detector components. The PMTs are not shown for the slabs or panels. All components are installed on an aluminum tube.

Figure 3

A photograph of the detector, with the panels removed from layer 1 so that the bars may be seen.

Figure 4

Time difference, measured in a combination of the beam-on and beam-off datasets, between layer 3 and layer 1 for beam (red) and cosmic (blue) muons that travel through the detector.

Figure 5

Cross section times branching ratios for production of different particles with $|\eta |<2$ decaying into ${\chi }^{+}{\chi }^{-}$ pairs, as a function of $\chi$ mass. For Drell-Yan the plotted cross section requires at least one of the $\chi \mathrm{s}$ to have $|\eta |<1$.

Figure 6

Simulation of the milliQan demonstrator response to a simulated cosmic ray shower event. The incident muon (red) interacts with the cavern walls to produce a shower. Electrons (black) and gamma rays (green) generated in the rock are a significant background source. The interactions of muons and shower particles with detector material produce scintillation photons (cyan).

Figure 7

Comparison of data and simulation $n_{\mathrm{pe}}$ distributions in events with a tagged throughgoing beam muon, for bars that do not contain a pulse consistent with originating from a muon, and are not neighboring any such bars. Note that only 783 out of 7363 tagged muons produced detectable showers entering this figure. Simulation events are categorized based on the material in which the particle(s) that produced the pulse originated.

Figure 8

Exclusion at 95% confidence level compared to existing constraints from colliders, CMS, ArgoNeuT and SLAC MilliQ [10, 11, 12, 13, 14, 15, 16, 27] as well as the indirect constraint from the CMB relativistic degrees of freedom [26].