About

Aaron Higuera Pichardo is an Assistant Research Professor in the Department of Physics and Astronomy at Rice University. He is a member of the Ken Kennedy Institute. His research focuses on exploring fundamental questions in particle physics related to matter asymmetry and understanding why Standard Model particles constitute only about one-sixth of the total matter in the Universe. His current work involves incorporating machine learning methods to enhance neutrino science, dark matter direct detection searches, and rare event searches such as neutrinoless double beta decay and nucleon decay.

Research topics

• Physics
• Particle physics
• Nuclear physics
• Astrophysics
• Astronomy
• Artificial Intelligence
• Computer Science
• Machine Learning
• Optics
• Environmental science
• Earth science
• Geology
• Mathematics
• Statistics
• Algorithm

Selected publications

• Challenging Spontaneous Quantum Collapse with the XENONnT Dark Matter Detector

  Physical Review Letters · 2026-01-22

  preprintOpen access

  We report on the search for x-ray radiation as predicted from dynamical quantum collapse with low-energy electronic recoil data in the energy range of 1-140 keV from the first science run of the XENONnT dark matter detector. Spontaneous radiation is an unavoidable effect of dynamical collapse models, which were introduced as a possible solution to the long-standing measurement problem in quantum mechanics. The analysis utilizes a model that for the first time accounts for cancellation effects in the emitted spectrum, which arise in the x-ray range due to the opposing electron-proton charges in xenon atoms. New world-leading limits on the free parameters of the Markovian continuous spontaneous localization and Diósi-Penrose models are set, improving previous best constraints by two orders of magnitude and a factor of five, respectively. For the strength and correlation length of the continuous spontaneous localization model, values in the originally proposed parameter ranges are experimentally excluded for the first time.

  PublisherDOI

• Model-independent searches of new physics in DARWIN with deep learning

  The European Physical Journal C · 2026-03-26

  preprintOpen access

  We present a deep learning pipeline to perform a model-independent, likelihood-free search for anomalous (i.e., non-background) events in the proposed next-generation multi-ton scale liquid xenon-based direct detection experiment, DARWIN. We train an anomaly detector comprising a variational autoencoder (VAE) and a classifier on high-dimensional simulated detector response data and construct a 1D anomaly score to reject the background-only hypothesis in the presence of an excess of non-background-like events. We use simulated validation data to determine the power of the method to reject the background-only hypothesis in the presence of a WIMP dark matter signal, without any model-dependent assumption about the nature of the signal. We show that our neural networks learn relevant features of the events from low-level, high-dimensional detector outputs, avoiding lossy and computationally expensive compression into lower-dimensional observables. Our approach is complementary to the usual likelihood-based analysis, in that it reduces the reliance on many of the corrections and cuts that are traditionally part of the analysis chain, with the potential of achieving higher accuracy and significant reduction of analysis time. We envisage the methodology presented in this work augmenting or complementing likelihood-based and other data-driven methods currently utilized in the DARWIN (and in the future, XLZD) analysis pipeline.

  PublisherOA PDFDOI

• First Search for Light Dark Matter in the Neutrino Fog with XENONnT

  Physical Review Letters · 2025-03-20 · 17 citations

  articleOpen access

  We search for dark matter (DM) with a mass [3,12] GeV/c^{2} using an exposure of 3.51 tonne year with the XENONnT experiment. We consider spin-independent DM-nucleon interactions mediated by a heavy or light mediator, spin-dependent DM-neutron interactions, momentum-dependent DM scattering, and mirror DM. Using a lowered energy threshold compared to the previous weakly interacting massive particle search, a blind analysis of [0.5, 5.0] keV nuclear recoil events reveals no significant signal excess over the background. XENONnT excludes spin-independent DM-nucleon cross sections >2.5×10^{-45} cm^{2} at 90% confidence level for 6 GeV/c^{2} DM. In the considered mass range, the DM sensitivity approaches the "neutrino fog," the limitation where neutrinos produce a signal that is indistinguishable from that of light DM-xenon nucleus scattering.

  PublisherOA PDFDOI

• XENONnT WIMP search: Signal and background modeling and statistical inference

  Physical review. D/Physical review. D. · 2025-05-19 · 3 citations

  articleOpen access

  The XENONnT experiment searches for weakly interacting massive particle (WIMP) dark matter scattering off a xenon nucleus. In particular, XENONnT uses a dual-phase time projection chamber with a 5.9-ton liquid xenon target, detecting both scintillation and ionization signals to reconstruct the energy, position, and type of recoil. A blind search for nuclear recoil WIMPs with an exposure of 1.1 ton-years (4.18 t fiducial mass) yielded no signal excess over background expectations, from which competitive exclusion limits were derived on WIMP-nucleon elastic scatter cross sections, for WIMP masses ranging from <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mn>6</a:mn> <a:mtext> </a:mtext> <a:mtext> </a:mtext> <a:mi>GeV</a:mi> <a:mo>/</a:mo> <a:msup> <a:mrow> <a:mi>c</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msup> </a:mrow> </a:math> up to the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>TeV</c:mi> <c:mo>/</c:mo> <c:msup> <c:mi>c</c:mi> <c:mn>2</c:mn> </c:msup> </c:math> scale. This work details the modeling and statistical methods employed in this search. By means of calibration data, we model the detector response, which is then used to derive background and signal models. The construction and validation of these models is discussed, alongside additional purely data-driven backgrounds. We also describe the statistical inference framework, including the definition of the likelihood function and the construction of confidence intervals.

  PublisherDOI

• WIMP Dark Matter Search using a 3.1 Tonne-Year Exposure of the XENONnT Experiment

  ArXiv.org · 2025-02-25

  preprintOpen access

  We report on a search for weakly interacting massive particle (WIMP) dark matter (DM) via elastic DM-xenon-nucleus interactions in the XENONnT experiment. We combine datasets from the first and second science campaigns resulting in a total exposure of 3.1 tonne-years. In a blind analysis of nuclear recoil events with energies above $3.8\,\mathrm{keV_{NR}}$, we find no significant excess above background. We set new upper limits on the spin-independent WIMP-nucleon scattering cross section for WIMP masses above $10\,\mathrm{GeV}/c^2$ with a minimum of $1.7\,\times\,10^{-47}\,\mathrm{cm^2}$ at $90\,\%$ confidence level for a WIMP mass of $30\,\mathrm{GeV}/c^2$. We achieve a best median sensitivity of $1.4\,\times\,10^{-47}\,\mathrm{cm^2}$ for a $41\,\mathrm{GeV}/c^2$ WIMP. Compared to the result from the first XENONnT science dataset, we improve our sensitivity by a factor of up to 1.8.

  PublisherOA PDFDOI

• WIMP Dark Matter Search Using a 3.1 Tonne-Year Exposure of the XENONnT Experiment

  Physical Review Letters · 2025-09-09 · 18 citations

  articleOpen access

  We report on a search for weakly interacting massive particle (WIMP) dark matter (DM) via elastic DM-xenon-nucleus interactions in the XENONnT experiment. We combine datasets from the first and second science campaigns resulting in a total exposure of 3.1 tonne-years. In a blind analysis of nuclear recoil events with energies above 3.8 keV_{NR}, we find no significant excess above background. We set new upper limits on the spin-independent WIMP-nucleon scattering cross section for WIMP masses above 10 GeV/c^{2} with a minimum of 1.7×10^{-47} cm^{2} at 90% confidence level for a WIMP mass of 30 GeV/c^{2}. We achieve a best median sensitivity of 1.4×10^{-47} cm^{2} for a 41 GeV/c^{2} WIMP. Compared to the result from the first XENONnT science dataset, we improve our sensitivity by a factor of up to 1.8.

  PublisherDOI

• Search for Light Dark Matter in Low-Energy Ionization Signals from XENONnT

  Physical Review Letters · 2025-04-25 · 18 citations

  articleOpen access

  We report on a blinded search for dark matter with single- and few-electron signals in the first science run of XENONnT relying on a novel detector response framework that is physics model dependent. We derive 90% confidence upper limits for dark matter-electron interactions. Heavy and light mediator cases are considered for the standard halo model and dark matter up-scattered in the Sun. We set stringent new limits on dark matter-electron scattering via a heavy mediator with a mass within 10-20 MeV/c^{2} and electron absorption of axionlike particles and dark photons for m_{χ} below 0.03 keV/c^{2}.

  PublisherOA PDFDOI

• Radon Removal in XENONnT down to the Solar Neutrino Level

  Physical Review X · 2025-09-30 · 5 citations

  preprintOpen access

  The XENONnT experiment has achieved an exceptionally low <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mmultiscripts><a:mrow><a:mi>Rn</a:mi></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>222</a:mn></a:mrow></a:mmultiscripts></a:mrow></a:math> activity concentration within its inner 5.9 tonne liquid xenon detector of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mo stretchy="false">(</c:mo><c:mn>0.90</c:mn><c:mo>±</c:mo><c:mn>0.02</c:mn><c:mtext> </c:mtext><c:mrow><c:mi>stat</c:mi></c:mrow><c:mo>±</c:mo><c:mn>0.07</c:mn><c:mtext> </c:mtext><c:mrow><c:mi>syst</c:mi></c:mrow><c:mo stretchy="false">)</c:mo><c:mtext> </c:mtext><c:mtext> </c:mtext><c:mi mathvariant="normal">μ</c:mi><c:mi>Bq</c:mi><c:mtext> </c:mtext><c:msup><c:mrow><c:mi>kg</c:mi></c:mrow><c:mrow><c:mo>−</c:mo><c:mn>1</c:mn></c:mrow></c:msup></c:mrow></c:math>, equivalent to about 430 <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mmultiscripts><h:mrow><h:mi>Rn</h:mi></h:mrow><h:mprescripts/><h:none/><h:mrow><h:mn>222</h:mn></h:mrow></h:mmultiscripts></h:mrow></h:math> atoms per tonne of xenon. This was achieved by active online radon removal via cryogenic distillation after stringent material selection. The achieved <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline"><j:mrow><j:mmultiscripts><j:mrow><j:mi>Rn</j:mi></j:mrow><j:mprescripts/><j:none/><j:mrow><j:mn>222</j:mn></j:mrow></j:mmultiscripts></j:mrow></j:math> activity concentration is 5 times lower than that in other currently operational multitonne liquid xenon detectors engaged in dark matter searches. This breakthrough enables the pursuit of various rare event searches that lie beyond the confines of the standard model of particle physics, with world-leading sensitivity. The ultralow <l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mrow><l:mmultiscripts><l:mrow><l:mi>Rn</l:mi></l:mrow><l:mprescripts/><l:none/><l:mrow><l:mn>222</l:mn></l:mrow></l:mmultiscripts></l:mrow></l:math> levels have diminished the radon-induced background rate in the detector to a point where it is for the first time comparable to the solar neutrino-induced background, which is poised to become the primary irreducible background in liquid xenon-based detectors.

  PublisherDOI

• Spectral Measurement of the $^{214}$Bi beta-decay to the $^{214}$Po Ground State with XENONnT

  ArXiv.org · 2025-10-06

  preprintOpen access

  We report the measurement of the $^{214}$Bi beta-decay spectrum to the ground state of $^{214}$Po using the XENONnT detector. This decay is classified as first-forbidden non-unique, for which theoretical predictions require detailed nuclear structure modeling. A dedicated identification algorithm isolates a high-purity sample of ground-state beta-decays, explicitly excluding events with associated gamma-rays emission. By comparing the measured spectrum, which covers energies up to 3.27 MeV, with several nuclear models, we find that the prediction based on the conserved vector current (CVC) hypothesis provides the best description of the data. Using this dataset, we additionally derive charge and light yield curves for electronic recoils, extending detector response modeling up to the MeV scale.

  PublisherOA PDFDOI

• The neutron veto of the XENONnT experiment: results with demineralized water

  The European Physical Journal C · 2025-06-24 · 3 citations

  articleOpen access

  Abstract Radiogenic neutrons emitted by detector materials are one of the most challenging backgrounds for the direct search of dark matter in the form of weakly interacting massive particles (WIMPs). To mitigate this background, the XENONnT experiment is equipped with a novel gadolinium-doped water Cherenkov detector, which encloses the xenon dual-phase time projection chamber (TPC). The neutron veto (NV) can tag neutrons via their capture on gadolinium or hydrogen, which release $$\gamma $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -rays that are subsequently detected as Cherenkov light. In this work, we present the first results of the XENONnT NV when operated with demineralized water only, before the insertion of gadolinium. Its efficiency for detecting neutrons is $$({82\pm 1}){\%}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mn>82</mml:mn> <mml:mo>±</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> <mml:mo>)</mml:mo> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> , the highest neutron detection efficiency achieved in a water Cherenkov detector. This enables a high efficiency of $$({53\pm 3}){\%}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mo>(</mml:mo> <mml:mrow> <mml:mn>53</mml:mn> <mml:mo>±</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> <mml:mo>)</mml:mo> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> for the tagging of WIMP-like neutron signals, inside a tagging time window of $${250}~{\upmu }\hbox {s}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>250</mml:mn> <mml:mspace/> <mml:mi>μ</mml:mi> <mml:mtext>s</mml:mtext> </mml:mrow> </mml:math> between TPC and NV, leading to a livetime loss of $${1.6}{\%}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mrow> <mml:mn>1.6</mml:mn> </mml:mrow> <mml:mo>%</mml:mo> </mml:mrow> </mml:math> during the first science run of XENONnT.

  PublisherOA PDFDOI

Frequent coauthors

• K. S. McFarland

  117 shared

• D. A. Martínez Caicedo

  South Dakota School of Mines and Technology

  111 shared

• J. K. Nelson

  111 shared

• L. Fields

  University of Notre Dame

  105 shared

• L. Aliaga

  Fermi National Accelerator Laboratory

  104 shared

• A. Norrick

  97 shared

• C. Marshall

  92 shared

• D. Thers

  IMT Atlantique

  91 shared

Education

• PhD, Physics

  Universidad de Guanajuato

  2014