arXiv subjects

Zhehan Qin

Publications and source records attributed to Zhehan Qin.

16 recordsLinked to original sources

arXiv2026

Cosmological Collider Signals From a Triangle Loop

Triangular loop signals appear frequently in cosmological collider models but their analytical forms remain unknown due to multiple technical challenges. Here, we determine for the first time the leading analytical cosmological collider signals generated by a massive scalar triangle loop in inflationary bispectrum and trispectrum. Combining a directional cutting rule with partial Mellin-Barnes representations, we obtain both the local and nonlocal trispectrum signals in a soft limit and derive the leading signal in the squeezed bispectrum. Remarkably, all these signals are reproduced by the corresponding bubble diagram with a single effective pinched coupling. Signals beyond leading order and cases with arbitrary internal masses, spins, and interactions can be generated within the same framework. Our approach also provides a systematic route to extracting analytical loop signals from general cosmological correlators.

hep-th↗

arXiv2026

On-Shell Bootstrap of Loop Inflation Correlators with Spectral Dispersion

We develop a new bootstrap strategy for cosmological correlators at loop level, which we call spectral dispersion. It is based on two conceptual observations that a correlator can be recovered from its on-shell data, also known as nonlocal signals, by analyticity up to local counterterms, and that the on-shell data for a loop process take the form of a discrete sum over quasinormal modes. Technically, our method combines the dS spectral decomposition with dispersion relations. Using this technique, we bootstrap new results in a simple and intuitive form for 3-point and 4-point correlators with 1-loop massive exchanges of scalar and vector bosons, either directly or derivatively coupled. Applications of this bootstrap technique to higher spins and higher-loop banana graphs with dS covariant dispersions but noncovariant couplings are also straightforward.

hep-th↗

arXiv2026

Fermionic Bubble Loop in Cosmological Collider Revisited: Exact signals from spectral and Mellin-Barnes methods

Fermionic degrees of freedom are essential ingredients in cosmological collider physics and are well motivated by many phenomenological models beyond the Standard Model, but their signals remain largely unexplored due to the difficulty of computing loop diagrams. In this work, we ask how fermionic bubble loops contribute to cosmological collider signals and provide an exact answer for arbitrary couplings. We develop two parallel analytical methods whose agreement provides a non-trivial check of the result. The first method is similar in spirit to spectral decomposition and is built directly from an identity for the product of propagators, which turns the bubble signal into an infinite sum of tree-level exchange signals. The second method is based on the Mellin-Barnes representation, where the result is reconstructed from the residues of distinct families of poles. We also show that the fermionic bubble can be generated from the scalar bubble by the action of appropriate differential operators. As a phenomenologically important application, we consider Yukawa interactions between fermions and the inflaton, finding that the resulting bispectrum signal vanishes identically. Through the spectral decomposition, this vanishing can be traced to a field redefinition of the associated tree-level counterparts.

hep-th↗

arXiv2026

Loop integrals in de Sitter spacetime: The parity-split IBP system and $\mathrm{d}\log$-form differential equations

We develop integration-by-parts (IBP) reduction and differential equations for massive loop integrals of cosmological correlators in de Sitter (dS) spacetime, demonstrating the feasibility of this approach. We identify a structural property of the dS IBP system: for an $n$-propagator family, it splits into $2^n$ closed subsystems classified by the parity of the propagator indices. We further formulate a Baikov representation for loop integrals in dS space and derive the corresponding dimensional recurrence relations. In flat spacetime, intersection theory shows that $\mathrm{d}\log$-form master integrands lead to $\mathrm{d}\log$-form differential equations. Motivated by fibration intersection theory, we conjecture that this construction extends to dS integrands involving Hankel functions. We verify this conjecture in the one-loop bubble family and determine the associated alphabet.

hep-th↗

arXiv2025

Open Effective Field Theory and the Physics of Cosmological Collider Signals

We examine the origin of the cosmological collider signal using the framework of open effective field theories. Focusing on the single exchange of a massive scalar field, we demonstrate that the trispectrum splits cleanly into its local and non-local components once the heavy-field propagators are decomposed in the Keldysh basis. Integrating out the massive degree of freedom yields a single-field effective field theory for the light scalar that necessarily contains both unitary operators and non-unitary contributions associated with dissipation and stochastic noise. We show that the leading local signal in parity-preserving theories arises from the unitary part of this effective field theory, whereas the non-local signal is intrinsically associated with its stochastic sector. The effective field theory coefficients themselves are a priori non-analytic in the external kinematics; however, this non-analyticity can be softened when a scale hierarchy - such as the heavy-mass expansion - is imposed, up to spurious contributions that ultimately cancel in observables. Finally, we establish a connection between the cosmological collider signal and entropy production, linking the observable non-local signal to intrinsic properties of the quantum state, including its degree of mixedness.

hep-th↗

arXiv2025

$λϕ^4$ as an Effective Theory in de Sitter

Effective field theories (EFTs) provide a powerful framework to parametrise unknown aspects of possible ultraviolet (UV) physics. For scalar fields in de Sitter space, however, new emergent phenomena can arise when the cut-off scale of the theory lies below the horizon scale $H$, as seen in the stochastic formalism of inflation. In this work, we study EFTs that, at leading order, reproduce the standard quartic theory in de Sitter, but with a variable cut-off identified with the mass of an integrated-out hidden sector. We perform the complete analytic computation for the tree- and loop-level matching between the effective $λϕ^4$ theory and two possible UV realisations. We find that when the cut-off is much larger than the horizon, the theory admits a unitary description, up to exponentially suppressed corrections. In contrast, when the cut-off is lowered below $H$, the system evolves into a mixed state and diffusive effects emerge. Nevertheless, at leading order, the EFT remains local and reproduces the same effective quartic coefficient as in the unitary regime. Furthermore, for the EFT matching at the loop-level, the effective quartic coupling changes sign and becomes negative as the cut-off decreases, in agreement with the result obtained from the stochastic formalism. In general, for cosmological EFTs, our findings highlight the role of non-unitary effects and illustrate their regimes of validity, within and beyond perturbation theory.

hep-th↗

arXiv2025

Primordial Stochastic Gravitational Waves from Massive Higher-Spin Bosons

Can a stationary stone radiate gravitational waves (GWs)? While the answer is typically "no" in flat spacetime, we get a "yes" in inflationary spacetime. In this work, we study the stationary-stone-produced GWs in inflation with a concrete model, where the role of stones is played by massive higher-spin particles. We study particles of spin-2 and higher produced by helical chemical potentials, and show that the induced GWs feature a scale-invariant and helicity-biased power spectrum in the slow-roll limit. Including slow-roll corrections leads to interesting backreactions from the higher-spin boson production, resulting in an intriguing scale-dependence of GWs at small scales. Given the existing observational and theoretical constraints, we identify viable parameter regions capable of generating visibly large GWs for future observations.

hep-ph↗

arXiv2025

The Exact and Approximate Tales of Boost-Breaking Cosmological Correlators

Cosmological correlators offer a remarkable window into the high-energy physics governing Universe's earliest moments, with the tantalising prospect of discovering new particles. However, extracting new physics from these observables requires both precise theoretical predictions of inflationary theories and accurate, analytical templates suitable for data analysis throughout parameter and kinematic spaces. In this paper, we extend the current analytical results by computing the most general boost-breaking seed correlator mediated by the tree-level exchange of a massive spinning particle. We derive the result using two complementary approaches, bootstrapping from boundary differential equations, and direct spectral integration. Both representations are packaged as a single partially resummed series that converges in all physical kinematics. Computing this correlator marks a milestone for carving out the space of all boost-breaking correlators, and therefore completes the tree-level catalogue. We then introduce a general procedure to obtain accurate approximations for cosmological collider signals based on the saddle-point method. This approach allows for a clear physical intuition of various signals hidden in correlators, as the bulk physics is made manifest through the location of these saddles in the complex time plane, which depend on the external kinematics. Evaluating the time integrals at these saddles yields results given as elementary functions that remain valid beyond soft limits and provide intuitive control over both the signal shape and amplitude. We demonstrate the power of this method in both de Sitter-invariant and boost-breaking scenarios, and uncover novel refined waveform and strength dependence for oscillatory signals from massive fields. We provide a complete cosmological collider shape template capturing all boost-breaking effects for upcoming cosmological surveys.

hep-th↗

arXiv2025

Cosmological Correlators at the Loop Level

Cosmological correlators encode rich information about physics at the Hubble scale and may exhibit characteristic oscillatory signals due to the exchange of massive particles. Although many 1-loop processes, especially those that break de Sitter (dS) boosts, can generate significant leading signals for various particle models in cosmological collider physics, the precise results for these correlators or their full signals remain unknown due to the lack of symmetry. In this work, we apply the method of partial Mellin-Barnes (PMB) representation to the calculation of cosmological correlators at the loop level. As a first step, we use the PMB representation to calculate four-point cosmological correlators with bubble topology. We find that both the nonlocal and local signals arise from the factorized part, validating the cutting rules proposed in previous work, and are free from UV divergence. Furthermore, the UV divergence originates solely from the background piece and can be manifestly canceled by introducing the appropriate counterterm, similar to the procedure in flat spacetime. We also demonstrate how to renormalize the 1-loop correlators in Mellin space. After a consistency check with known results for the covariant case, we provide new analytical results for the signals generated from a nontrivial dS-boost-breaking bubble.

hep-th↗

arXiv2024

Dispersive Bootstrap of Massive Inflation Correlators

Inflation correlators with massive exchanges are central observables of cosmological collider physics, and are also important theoretical data for us to better understand quantum field theories in dS. However, they are difficult to compute directly due to many technical complications of the Schwinger-Keldysh integral. In this work, we initiate a new bootstrap program for massive inflation correlators with dispersion relations on complex momentum planes. We classify kinematic variables of a correlator into vertex energies and line energies, and develop two distinct types of dispersion relations for both of them, respectively called vertex dispersion and line dispersion relations. These dispersion methods allow us to obtain full analytical results of massive correlators from a knowledge of their oscillatory signals alone, while the oscillatory signal at the tree level can be related to simpler subgraphs via the cutting rule. We further apply this method to massive loop correlators, and obtain new analytical expressions for loop diagrams much simpler than existing results from spectral decomposition. In particular, we show that the analyticity demands the existence of an "irreducible background" in the loop correlator, which is unambiguously defined, free of UV divergence, and independent of renormalization schemes.

hep-th↗

arXiv2023

Nonanalyticity and On-Shell Factorization of Inflation Correlators at All Loop Orders

The dynamics of quantum fields during cosmic inflation can be probed via their late-time boundary correlators. The analytic structure of these boundary correlators contains rich physical information of bulk dynamics, and is also closely related to cosmological collider observables. In this work, we study a particular type of nonanalytic behavior, called nonlocal signals, for inflation correlators with massive exchanges at arbitrary loop orders. We propose a signal-detection algorithm to identify all possible sources of nonlocal signals in an arbitrary loop graph, and prove that the algorithm is exhaustive. We then present several versions of the on-shell factorization theorem for the leading nonlocal signal in graphs with arbitrary number of loops, and provide the explicit analytical expression for the leading nonlocal signal. We also generalize the nonlocal-signal cutting rule to arbitrary loop graphs. Finally, we provide many explicit examples to demonstrate the use of our results, including an n-loop melon graph and a variety of 2-loop graphs.

hep-th↗

arXiv2023

Inflation Correlators at the One-Loop Order: Nonanalyticity, Factorization, Cutting Rule, and OPE

Inflation correlators with one-loop massive exchange encode rich information about the dynamics of the massive loop particles. Their nonanalytic behavior in certain soft limits leads to characteristic oscillatory pattern, which is the leading signal of many particle models of cosmological collider physics. In this work, we investigate systematically such nonanalyticity for arbitrary one-particle-irreducible (1PI) one-loop correlators in various soft limits. With the partial Mellin-Barnes representation, we present and prove a factorization theorem and a cutting rule for arbitrary 1PI one-loop inflation correlators, which is reminiscent of the on-shell cutting rule for flat-space scattering amplitudes. We also show how to understand this factorization theorem from the viewpoint of operator product expansion on the future boundary. As an application of the one-loop factorization theorem, we derive new analytic and exact formulae for nonlocal cosmological collider signals for massive one-loop four-point inflation correlators of all possible 1PI topologies, including the bubble, the triangle, and the box graphs. Finally, we show how to push the computation of nonlocal signals to higher orders in the momentum ratio.

hep-th↗

arXiv2023

Helical Inflation Correlators: Partial Mellin-Barnes and Bootstrap Equations

Massive spinning particles acquire helicity-dependent chemical potentials during the inflation from axion-type couplings. Such spinning fields can mediate sizable inflaton correlators which we call the helical inflation correlators. Helical inflaton correlators are approximately scale invariant, dS boost breaking, parity-violating, and are promising observables of cosmological collider physics. In this work, we present complete and analytical results for 4-point helical inflation correlators with tree-level exchanges of massive spinning particles, including both the smooth background and the oscillatory signals. We compute the bulk Schwinger-Keldysh integrals in two independent ways, including the partial Mellin-Barnes representation and solving bootstrap equations. We also present new closed-form analytical results for 3-point functions with massive scalar or helical spinning exchanges. The analytical results allow us to concretely and efficiently explore the phenomenological consequences of helicity-dependent chemical potentials. In particular, we show that the chemical potential can exponentially enhance oscillatory signals of both local and nonlocal types, but only affects the background in a rather mild way. Our results extend the de Sitter bootstrap program to include nonperturbative breaking of de Sitter boosts. Our results also explicitly verify the recently proposed cutting rule for cosmological collider signals.

hep-th↗

arXiv2023

Phase Information in Cosmological Collider Signals

Massive particles produced during the cosmic inflation can imprint in the primordial non-Gaussianities as characteristic oscillating functions of various momentum ratios, known as cosmological collider signals. We initiate a study of the phase of the oscillating signals which can be unambiguously defined and measured. The phase can provide useful new information about the spin and the couplings of the intermediate heavy particles that cannot be obtained from the signal frequency and angular dependences alone. We also present new analytical results for full nonlocal signals from two typical 1-loop processes, enabling precise determination of the signal phase away from the squeezed limit.

hep-th↗

arXiv2023

Closed-Form Formulae for Inflation Correlators

We derive exact and closed-form expressions for a large class of two-point and three-point inflation correlators with the tree-level exchange of a single massive particle. The intermediate massive particle is allowed to have arbitrary mass, spin, chemical potential, and arbitrary nonderivative or derivative couplings to external inflaton modes. We also allow the coupling coefficients to have arbitrary complex power dependences on the conformal time. Our results feature closed-form expressions involving only familiar special functions and without any infinite sums. This is achieved by an improved bootstrap method with a suitable change of variables. Our results cover a wide range of cosmological collider models and can be directly used for future phenomenological studies. Our results can also be used as basic building blocks for constructing more complicated inflation correlators.

hep-th↗

arXiv2022

Fracton Topological Order at Finite Temperature

As new kinds of stabilizer code models, fracton models have been promising in realizing quantum memory or quantum hard drives. However, it has been shown that the fracton topological order of 3D fracton models occurs only at zero temperature. In this Letter, we show that higher dimensional fracton models can support a fracton topological order below a nonzero critical temperature $T_c$. Focusing on a typical 4D X-cube model, we show that there is a finite critical temperature $T_c$ by analyzing its free energy from duality. We also obtained the expectation value of the 't Hooft loops in the 4D X-cube model, which directly shows a confinement-deconfinement phase transition at finite temperature. This finite-temperature phase transition can be understood as spontaneously breaking the $\mathbb{Z}_2$ one-form subsystem symmetry. Moreover, we propose a new no-go theorem for finite-temperature quantum fracton topological order.

cond-mat.str-el↗