Investigation of low-energy particle remnants in high-energy collisions at the LHC with a skipper-CCD detector

We deployed the Mobile Skipper Testing Apparatus <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mo stretchy="false">∼</a:mo> <a:mn>33</a:mn> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi mathvariant="normal">m</a:mi> </a:mrow> </a:math> away from the Compact Muon Solenoid collision point, the first skipper-CCD detector probing low-energy particles produced in high-energy collisions at the Large Hadron Collider. In this work, we search for beam-related events using data collected in 2024 during beam-on and beam-off periods. The dataset corresponds to integrated luminosities of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mn>113.3</e:mn> <e:mtext> </e:mtext> <e:mtext> </e:mtext> <e:msup> <e:mi>fb</e:mi> <e:mrow> <e:mo>−</e:mo> <e:mn>1</e:mn> </e:mrow> </e:msup> </e:math> and <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mn>1.54</g:mn> <g:mtext> </g:mtext> <g:mtext> </g:mtext> <g:msup> <g:mi>nb</g:mi> <g:mrow> <g:mo>−</g:mo> <g:mn>1</g:mn> </g:mrow> </g:msup> </g:math> for the proton-proton and Pb-Pb collision periods, respectively. We report observed event rates in a model-independent framework across two ionization regions: <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mo form="prefix">≤</i:mo> <i:mn>20</i:mn> <i:msup> <i:mi>e</i:mi> <i:mo>−</i:mo> </i:msup> </i:math> and <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"> <l:mo form="prefix">&gt;</l:mo> <l:mn>20</l:mn> <l:msup> <l:mi>e</l:mi> <l:mo>−</l:mo> </l:msup> </l:math> . For the low-energy region, we perform a likelihood analysis to test the null hypothesis of no beam-correlated signal. We found no significant correlation during proton-proton and Pb-Pb collisions. For the high-energy region, we present the energy spectra for both collision periods and compare event rates for images with and without luminosity. We observe a slight increase in the event rate following the Pb-Pb collisions, coinciding with a rise in the single-electron rate, which will be investigated in future work. Using the low-energy proton-proton results, we place 95% confidence level constraints on the mass-millicharge parameter space of millicharged particles. Overall, the results in this work demonstrate the viability of skipper-CCD technology to explore new physics at high-energy colliders and motivate future searches with more massive detectors.