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PIKIMO 20
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McKenna Hall 54756 N Notre Dame Ave, Notre Dame, IN 46556
Description
PIKIMO 20
The High Energy Physics group at the University of Notre Dame is excited to host the 20th PIKIMO meeting on Saturday, April 25th, 2026. The meeting will take place at McKenna Hall, room B01. Breakfast will start at 8:15am Eastern Time, with opening remarks at 8:50am and the first talk at 9:00am.
PIKIMO is a series of informal phenomenology meetings, with talks primarily given by graduate students and postdoctoral researchers. The scope of the meeting is particle phenomenology broadly defined, including particle and astroparticle phenomenology, amplitudes, and cosmology research.
Keynote Speakers
- Rachel Houtz (University of Florida)
- Carlos Blanco (Penn State)
A light breakfast, lunch, and coffee breaks will be provided during the meeting. To participate, please register for the meeting. If you wish to present a talk at the meeting, please submit an abstract by April 6, 2026 at 11:59pm.
Contact Itamar Allali at iallali@nd.edu with any questions.
Contact the organizers
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08:15
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08:50
Breakfast 35m
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08:50
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09:00
WelcomeConvener: Dr Itamar Allali (University of Notre Dame)
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09:00
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10:00
Morning Contributed TalksConvener: Dr Seth Koren
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09:00
Determination of the strong coupling constant from the new CTEQ-TEA global QCD analysis 15m
We present a new determination of the strong coupling constant $\alpha_s$ from a global QCD analysis CT25 of parton distribution functions (PDFs) that incorporates high-precision experimental measurements from Run-2 of the Large Hadron Collider together with a large sample of other measurements over a wide interval of energies. In addition to providing an up-to-date determination of $\alpha_s$ using NNLO calculations, we critically assess the quantification of uncertainty by exploring various statistical prescriptions including the dynamical tolerance and Bayesian hierarchical models.
Speaker: Kirtimaan Ajaykant Mohan -
09:15
Higgs decays to four leptons to $\mathcal{O}(1/\Lambda^4)$ in SMEFT 15m
We study the decays $h \to \ell \bar{\ell} \left(Z \to \ell' \bar{\ell'}\right)$ and $h\to\ell\bar{\nu}_\ell\nu_{\ell'}\bar{\ell'}$ within the SMEFT framework and including effects up to $\mathcal O(1/\Lambda^4)$, where $\Lambda$ is the new physics scale suppressing higher dimensional operators. To work to this order, we must include the square of dimension-six operators and the interference of dimension-eight operators with the Standard Model. We study angular asymmetries and other differential decay observables and determine which are most sensitive to $\mathcal O(1/\Lambda^4)$ effects. While new kinematic structures arising in higher dimensional operators have the potential to induce novel angular dependency, we find this does not occur for $h\to\ell\bar{\ell}\left(Z\xrightarrow{}\ell'\bar{\ell'}\right)$. For $h \to \ell \bar{\nu}_\ell \nu_{\ell'} \bar{\ell'}$, new angular dependencies do arise at $\mathcal O(1/\Lambda^4)$, though they require a fully reconstructible (meaning we can go to the Higgs rest frame) final state. For non-reconstructible final states such as $\ell \bar{\nu}_\ell \nu_{\ell'} \bar{\ell'}$, we must study Higgs production and decay together with the appropriate observables, which we find obscures the new angular effects.
Speaker: Mario Flores-Hernández (University of Notre Dame) -
09:30
Constraining Exotic Baryon Decays Through Energy Loss Limits in Pulsar Binary Timing 15m
Models that explain baryogenesis and the dark matter abundance often feature exotic decays of baryons. As in Elor et al. (2019), we minimally extend the Standard Model with a GeV scale dark Majorana fermion $\chi$ and a TeV scale Y scalar with hypercharge of either 2/3 or -1/3. For exotic neutron or hyperon decays, we can constrain quark flavor couplings $|y_{\chi i}y_{jk}|$ to this Y scalar by using energy loss limits in pulsar binaries. We explore the flavor coupling constraints in each ultraviolet completion.
Speaker: Stanford Broadwater (University of Kentucky) -
09:45
Hunting for New Physics with Tau Decays 15m
Baryon number violation (BNV) provides one of the most sensitive probes of physics beyond the Standard Model. In this work, we investigate rare BNV tau decays induced by effective field theory (EFT) operators, including scenarios with associated dark-sector final states. We demonstrate that any framework generating BNV tau decays necessarily induces proton decay, leading to stringent constraints from existing experimental bounds on proton lifetime. As a result, we find that the branching ratios ($\mathcal{B}$) for $\tau$ decays even after involving dark-sector particles are highly suppressed, of order $\mathcal{O}(10^{-30})$, rendering their direct observation extremely challenging at current facilities such as Belle II. We further explore proton–tau mixing operators and analyze the resulting multi-body proton decay channels mediated by semi-leptonic tau decays. The obtained spectra of the final state charged pions from those processes indicate that one can still search for them in Super-K/ Hyper-K.
Speaker: Sohini Pal (University of Cincinnati, Cincinnati, OH 45221, USA)
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09:00
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10:00
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10:15
Morning Coffee 1 15m
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10:15
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11:00
KeynoteConvener: Prof. Adam Martin (University of Notre Dame (US))
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10:15
Geometry and Amplitudes in the SMEFT 45m
Effective field theories contain redundancies associated with field redefinitions, which can make even simple calculations unnecessarily complicated. A useful way to organize these redundancies is to view them geometrically, treating field redefinitions as coordinate transformations. In this talk, I will describe how this perspective can be used in practice, focusing on a shortcut to scattering amplitudes in the Standard Model Effective Field Theory (SMEFT). Working in appropriate coordinates, many of the cancellations that typically arise are absent, and the resulting expressions take a significantly simpler form. I will also discuss how this viewpoint relates to the distinction between SMEFT and HEFT, where global features of the theory space become relevant.
Speaker: Prof. Rachel Houtz (University of Florida)
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10:15
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11:00
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11:30
Morning Coffee 2 30m
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11:30
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12:45
Morning Contributed TalksConvener: Dr Fengwei Yang
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11:30
Computing bubble wall velocity from entropy 15m
A precise determination of the bubble wall velocity $v_w$ is crucial for making accurate predictions of the baryon asymmetry and gravitational wave (GW) signals in models of electroweak baryogenesis (EWBG).
Working in the local thermal equilibrium approximation, we exploit entropy conservation to present efficient algorithms for computing $v_w$, significantly streamlining the calculation.
We then explore the parameter dependencies of $v_w$, focusing on two sample models capable of enabling a strong first-order electroweak phase transition: a $\mathbb{Z}_2$-symmetric singlet extension of the SM, and a model for baryogenesis with CP violation in the dark sector.
We study correlations among $v_w$ and the two common measures of phase transition strength, $\alpha_n$ and $v_n/T_n$.
Interestingly, we find a relatively model-insensitive relationship between $v_n/T_n$ and $\alpha_n$.
We also observe an upper bound on $\alpha_n$ for the deflagration/hybrid wall profiles naturally compatible with EWBG, the exact value for which varies between models, significantly impacting the strength of the GW signals.
In summary, our work provides a framework for exploring the feasibility of EWBG models in light of future GW signals.Speaker: Isaac Wang -
11:45
Constraining Dark First-Order Phase Transitions with CMB and Structure Formation Observations 15m
Cosmological first-order phase transitions (FOPT) can occur in a secluded dark sector at relatively late times before recombination. Since bubble nucleation occurs stochastically as the FOPT unfolds, different regions of the universe transition at different times. On large scales, this random fluctuation sources a scalar curvature perturbation with a dimensionless power spectrum that scales independently of the microscopic details of the FOPT. Even though the dark sector interacts with the Standard Model only gravitationally, the generated curvature perturbation can affect various cosmological and astrophysical observables. In this talk, we consider how such an FOPT can be constrained at different scales using CMB and structure formation measurements.
Speaker: Daven Wei Ren Ho -
12:00
Stochastic gravitational wave from cosmic string with non-standard cosmology 15m
Recent pulsar timing array observations have reported evidence for a common low frequency signal consistent with a stochastic gravitational wave background. In this work, we investigate the cosmic string interpretation of this signal in a non standard cosmological history featuring an early matter dominated era followed by a kination phase between the standard radiation dominated epoch. Previous studies have shown that kination can significantly modify the cosmic-string-induced gravitational wave spectrum, shifting characteristic features into the nanohertz range and producing a shape consistent with current PTA data. Building on this, we perform a Bayesian analysis to quantify the extent to which such a cosmological history is favored by the data. We find that the cosmic string scenario with kination is preferred over the standard radiation dominated cosmology, highlighting PTA observations as a probe of both cosmic strings and the early expansion history of the Universe.
Speaker: Taegyu Lee (Indiana University) -
12:15
Spectator Composes a Gravitational Canon 15m
We propose a general framework in which a phase transition is triggered during cosmic inflation by the slow-roll dynamics of a spectator field. The topological defects formed at the transition are inflated outside the horizon, reenter it after inflation, and can subsequently generate characteristic gravitational-wave (GW) signals. Quantum fluctuations of the spectator field modulate the timing of the transition, imprinting large-scale anisotropies in the resulting GW background. As an explicit realization, the spectator field may be identified with the Higgs field in a supersymmetric Standard Model. More generally, our framework applies to a wide class of spectator-modulated phenomena, providing a generic mechanism for producing anisotropic GW signals.
Speaker: Yunjia Bao (University of Chicago) -
12:30
Dissipative inflation in the CMB 15m
There is an increasing interest in the interplay between Open Quantum Systems and Inflationary Physics. A time-dependent background and limited access to the degrees of freedom invite the use of Open Quantum systems. It provides a more general framework than the unitary time evolution of a free state. In this talk, based on 2404.15416 and 2507.03103, we develop a local EFT for the scalar curvature perturbations subject to the dynamics of an open quantum system. We recover the scale-invariant power spectrum and then move on to the bispectrum. We study its amplitude and shape as a function of dissipation and other EFT parameters. Our results largely generalise the setup of warm inflation to general non-equilibrium cases. Finally, I will discuss the search for dissipative dynamics in the CMB bispectrum and the current constraints on the dissipation scale.
Speaker: Santiago Agui Salcedo (Kavli Institute for Cosmological Physics)
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11:30
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12:45
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14:00
Lunch 1h 15m
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14:00
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14:45
KeynoteConvener: Prof. Antonio Delgado (University of Notre Dame)
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14:00
AI-Driven Material Discovery for Next-Generation Directional Dark Matter Detectors 45m
The systematic discovery of optimal sub-GeV dark matter targets, such as anisotropic molecular crystals and nanomaterials, is bottlenecked by the computational cost of evaluating complex many-body scattering form factors. In this talk, I will present a novel machine learning framework designed to overcome this barrier by high-throughput screening through the vastness of material space. I will detail how we couple a generative material discovery model with SCarFFF, our GPU-accelerated electronic-structure computational framework, to perform fast validation of candidate materials. I will discuss how this pipeline allows us to efficiently identify next-generation detector targets sensitive to the directional dark matter wind and how these calculations have accelerated their deployment in recent experimental efforts.
Speaker: Prof. Carlos Blanco (Penn State)
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14:00
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14:45
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15:00
Afternoon Coffee 1 15m
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15:00
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16:15
Afternoon Contributed TalksConvener: Daven Wei Ren Ho
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15:00
Reshaping the Neutrino Fog: Evaluating WIMP-Xenon Scattering via Exact Shell-Model Response Functions 15m
The neutrino fog represents a fundamental irreducible sensitivity limit for dark matter direct detection arising from coherent elastic neutrino-nucleus scattering (CEvNS). We present a precision calculation of the neutrino fog for spin-independent (SI), spin-dependent proton-coupled (SDp), and spin-dependent neutron-coupled (SDn) WIMP interactions on a natural xenon target, derived by matching a relativistic effective field theory of WIMP interactions with quarks onto nucleon-level effective couplings at the hadronic scale, which are subsequently expressed in terms of a complete basis of Galilean-invariant non-relativistic effective operators governing the WIMP-nucleus interaction at the nuclear scale. Nuclear shell-model response functions replace the Helm form factor approximation for both the WIMP signal and the CEvNS neutrino background, yielding spectral differences that induce a systematic rescaling of the fog boundary for WIMP masses above 50 GeV across all three interaction channels. Steep vertical drops in the fog are observed in the 10–100 GeV mass range, whose origin is investigated in terms of kinematic degeneracy between the atmospheric neutrino background and the WIMP recoil spectrum.
Speaker: Julian Rovner (University of Cincinnati) -
15:15
A UVLF-Based Reassessment of Sub-keV Warm Dark Matter 15m
Warm dark matter (WDM) particles lighter than O(keV) are excluded at high significance because their early free-streaming strongly suppresses the formation of small-scale structure. However, recent work has suggested that introducing a small fraction of cold dark matter (CDM) with a blue-tilted isocurvature can counteract this suppression, potentially allowing lighter WDM candidates. In this talk, I will discuss new constraints on this “warm + cold-isocurvature” scenario. We combine galaxy ultraviolet luminosity functions from HST and JWST over redshifts 4 to 11 with cosmological data from the CMB, BAO, and supernovae. For pure WDM, we find a lower bound of m_{WDM} > 1.8 keV. When including a small CDM isocurvature component, this bound is relaxed significantly to m_{WDM} > 0.27 keV. This reflects a partial degeneracy where blue-tilted isocurvature fluctuations compensate for the suppression caused by WDM. Our results quantify how much such isocurvature contributions can hide the small-scale signatures of light dark matter and reassess the viability of sub-keV WDM candidates.
Speaker: Nicolas Lam -
15:30
Singlet Doublet Model Outside of Chemical Equilibrium 15m
The singlet-doublet fermion model of dark matter is an economical weak-scale dark matter model, which realizes the dark matter abundance through interactions with the weak bosons of the Standard Model. Depending on the size of the Yukawa couplings in the model, the dark matter relic abundance can be realized via freeze-out (including co-annihilation), co-scattering, freeze-in, or a super WIMP mechanism. We analyze the ways this model can realize the dark matter density with particular emphasis on the regime where the Yukawa couplings in the model are small and chemical equilibrium among particles is not guaranteed. Future work includes generalizing this to the case where particles fall out of kinetic equilibrium and solving the full momentum-dependent Boltzmann equation is required.
Speaker: Alexander Takla -
15:45
Searching for Ultralight Scalar Dark Matter with Clocks in Low Earth Orbit 15m
Ultralight dark matter with quadratic couplings to the Standard Model need not have a homogeneous local profile. Scattering on macroscopic bodies can distort the field configuration near Earth and, for sufficiently large couplings, the atmosphere acts as an efficient shield. I will discuss this effect in the context of clock searches and show that space-based platforms offer a natural way around it. When the orbital altitude exceeds the dark-matter de Broglie wavelength, the orbit-averaged field amplitude is restored to its unscattered value.
In addition, for $m_{\rm DM} \gtrsim 10^{-10}\,\mathrm{eV}$, when the de Broglie wavelength becomes shorter than Earth’s radius, the dark-matter profile near Earth develops a dipole structure. For clocks in low Earth orbit, this produces a characteristic temporal modulation that can enhance sensitivity and provide a nontrivial cross-check of the signal interpretation. I will show that optical clocks on the ISS can set leading bounds in some regimes, and that future orbiting nuclear clocks could extend the reach further into parameter space inaccessible to ground-based searches.
Speaker: Dawid Brzeminski -
16:00
New Searches for Dark Sectors via Deep Inelastic Scattering at the EIC 15m
Leveraging the joint QCD and QED factorization of DIS, with its improved sensitivity and reduced uncertainties in radiative corrections, opens new windows on the possibility of exotic particle emission, such as weakly coupled particles originating from a dark or hidden sector. Measurements of the cross-section would provide evidence through anomalous energy loss, as opposed to bump hunt or displaced vertex type searches, from modifications to the lepton distribution function to include the emission of new physics candidates. In this work, we focus on DIS with observed initial state QED radiation from the lepton, which provides a stronger framework to control for background sources of energy loss and allows us to take advantage of the asymmetric collider and the forward-backward detection sensitivity of the ePIC detector at the future EIC. We illustrate the sensitivity of our approach through the computation of the modified cross sections and specific observables in kinematics chosen for their sensitivity to initial-state electron radiation.
Speaker: Justin Cammarota (University of Kentucky)
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15:00
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16:15
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16:45
Afternoon Coffee 2 30m
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16:45
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18:00
Afternoon Contributed TalksConvener: Mario Flores-Hernandez
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16:45
Cosmological Aspects of Axion-Monopole Interactions 15m
We propose a minimal and natural dark-sector framework in which dark matter is composed of magnetic monopoles coupled to a light axion field. Through the Witten effect, the axion background induces electric charge on the monopoles, turning them into dyons that in turn modify the axion potential. This monopole-dependent axion mass provides a simple, radiatively stable mechanism for dark-sector interactions and allows the axion to act as a dynamical dark-energy component. The resulting slow evolution of the axion field at late times can naturally produce the polarization rotation associated with the reported CMB cosmic birefringence signal and may also accommodate the evolving dark-energy behavior suggested by recent DESI data. This framework offers a unified and economical explanation for multiple late-time cosmological hints.
Speaker: Hengameh Bagherian (University of Chicago) -
17:00
The W-Gluon Portal 15m
This talk will explore effective operator models where new light exotic (LEX) states interact with the Standard Model (SM) through the W-gluon portal. Operators included in this talk will contain LEX states with non-trivial representations under the SM gauge groups that couple to at least one W boson and one gluon. The W-gluon portal introduces new single production collider modes and phenomenological signatures at both the LHC and ep colliders. The talk will conclude by looking at the implications of adding a single new BSM operator to the SM Lagrangian. This simple operator, which contains a state in the (8, 3, 0) representation of the SM gauge groups, would require a new search at the LHC in order to reach a fairly significant discovery potential.
Speaker: Katherine Schwind (The Ohio State University) -
17:15
Mixed ’t Hooft anomalies in asymptotically and anomaly-free SU(N) chiral gauge theories for arbitrarily large N 15m
The presence of mixed ’t Hooft anomalies between zero and one-form symmetries could further constrain the possible low energy phases of gauge theories. By first constructing the full global symmetry group that acts faithfully on the fermions, we compute all the 't Hooft anomalies in the UV regime, which include mixed anomalies between one-form and zero-form symmetries, for a large class of previously overlooked 4d SU(N) chiral gauge theories that satisfy asymptotic freedom and gauge anomaly cancellation for arbitrarily large N. A three-loop beta function analysis suggests that some of these theories may develop a Banks–Zaks fixed point in the IR. As an alternative to ’t Hooft anomaly matching, we briefly discuss some conjectures that aim to constrain the possible low energy scenarios by examining the UV and IR degrees of freedom. This analysis further suggests that in some cases the theory flows to a weakly coupled CFT.
Speaker: Kort Beck (University of Illinois Urbana-Champaign) -
17:30
Hadronic Violation in 2HDM 15m
We present the first complete two-loop calculation of the electric and chromo-electric dipole moments of the light quarks and the gluon, as well as contributions to CP-violating lepton-quark interactions, in the unconstrained two-Higgs doublet model. We include the most general Yukawa interactions of the Higgs doublets with the Standard Model fermions up to quadratic order, and allow for generic phases in the Higgs potential. We pay particular attention to a consistent treatment of all fermionic contributions in the low-energy effective theory, including a consistent renormalization-group summation of all leading-logarithmic effects. This latter part of the work is independent of the specific UV model and can generally be applied to a large class of models that do not introduce new light degrees of freedom. A python implementation of our results is provided via a public git repository.
Speaker: Daniil Volkov
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16:45
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08:15
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08:50