Search

The signal strength of primordial gravitational waves experiencing an epoch of early scalar domination is reduced with respect to radiation domination. In this paper, we demonstrate that the specific pattern of this reduction is sensitive to the coupling between the dominant field and gravity. When this coupling is zero, the impact of early matter domination on gravitational waves is solely attributed to the alteration of the Hubble parameter and the scale factor. In the presence of non-zero couplings, on the other hand, the evolution of primordial gravitational waves is directly affected as well, resulting in a distinct step-like feature in the power spectrum of the gravitational wave as a function of frequency. This feature serves as a smoking gun signature of this model. In this paper, we provide an analytical expression of the power spectrum that illustrates the dependence of power spectrum on model parameters and initial conditions. Furthermore, we provide analytical relations that specify the frequency interval in which the step occurs. We compare the analytical estimates with numerical analysis and show they match well., Comment: 19 pages, 7 figures, 1 table, and 2 appendices

We introduce the Anarchic Axion, a class of axion models which solves the Strong CP problem within current nEDM constraints with a lighter than usual QCD axion, thus populating new parameter space that ongoing and future experiments target. The Anarchic Axion is driven light by a soft breaking of the Peccei-Quinn symmetry, which also predicts a residual neutron electric dipole moment. We introduce a novel measure to quantify the tuning required for large deviations from the usual QCD axion band. In addition to motivating searches for unusually light axions, this work establishes a new target for axion effective field theory., Comment: 5+2 pages. 3 figures. 1 table. v2: minor comments added, version published in Phys. Rev. D Letters

In this paper, we examine the evolution of light and feebly interacting non-abelian dark gauge bosons dark matter in the early universe. We specifically work on a multi-component dark matter scenario with a $SU(2)_{_R}$ gauge symmetry, spontaneously broken by a scalar $\phi$. For sufficiently light $\lesssim O( \rm{MeV})$ and feebly interacting $(g_{_R} \lesssim 10^{-10})$ gauge bosons, $W^3_{_R}$ and $W^{\pm}_{_R} \equiv W_{_R}^1 \pm i W_{_R}^2$ become the dark matter candidates. In this study, the portal between the dark sector and the standard model sector is provided by the right-handed electron charged under $SU(2)_{_R}$. We show that in the region of the parameter space where $g_{_R} \sim 10^{-12}$, the dark gauge boson $W^3_{_R}$ can be produced efficiently via the freeze-in mechanism, however, this mechanism fails to produce $W^{\pm}_{_R}$ sufficiently. For smaller dark gauge coupling, the relic density of three dark matter components can be obtained via the vector misalignment mechanism. Vector misalignment has already been discussed in the case of dark photon dark matter. In this study, we extend the arguments to non-abelian gauge bosons. After discussing the evolution of $W_{_R}$s in the early universe, we study the model's constraints and show the parameter space's allowed region., Comment: 17 pages, 5 figures

The Standard Model of Particle Physics cannot explain the observed baryon asymmetry of the Universe. This observation is a clear sign of new physics beyond the Standard Model. There have been many recent theoretical developments to address this question. Critically, many new physics models that generate the baryon asymmetry have a wide range of repercussions for many areas of theoretical and experimental particle physics. This white paper provides an overview of such recent theoretical developments with an emphasis on experimental testability., Comment: Contribution to Snowmass 2021. Solicited white paper from TF08. v2: includes additional contributions

We propose a search for long lived axion-like particles (ALPs) in exotic top decays. Flavour-violating ALPs appear as low energy effective theories for various new physics scenarios such as t-channel dark sectors or Froggatt-Nielsen models. In this case the top quark may decay to an ALP and an up- or charm-quark. For masses in the few GeV range, the ALP is long lived across most of the viable parameter space, suggesting a dedicated search. We propose to search for these long lived ALPs in $t\bar{t}$ events, using one top quark as a trigger. We focus on ALPs decaying in the hadronic calorimeter, and show that the ratio of energy deposits in the electromagnetic and hadronic calorimeters as well as track vetoes can efficiently suppress Standard Model backgrounds. Our proposed search can probe exotic top branching ratios smaller than $10^{-4}$ with a conservative strategy at the upcoming LHC run, and potentially below the $10^{-7}$ level with more advanced methods. Finally we also show that measurements of single top production probe these branching ratios in the very short and very long lifetime limit at the $10^{-3}$ level., Comment: v2: matches published version; 18 pages, 7 figures, 2 tables

We leverage the CP violation in charged $B$ meson decays to generate the observed baryon asymmetry and dark matter at $\mathcal{O}(10 \text{ MeV})$ temperatures. We realize this in two scenarios: $B_c^+$ Mesogenesis and $B^+$ Mesogenesis. In the first, CP violating $B_c^\pm$ decays to $B^\pm$ mesons are followed by decays to dark and Standard Model baryons. In the second, CP violating $B^\pm$ decays to lighter charged mesons are accompanied by the latter's decays to dark and Standard Model leptons, which then scatter into the baryon asymmetry. $B_c^+$ Mesogenesis is actively being probed at Belle and LHCb, while $B^+$ Mesogenesis can be tested at colliders and sterile neutrino searches., Comment: 11+7 pages, 6 figures; v2: minor revisions (no change to results)

In this paper, we study the simultaneous evolution of baryon asymmetry and hypermagnetic field amplitude assuming an early matter domination. We contrast our results to the conventional case where radiation domination during early universe is assumed. We show that the baryon asymmetry and the hypermagntic field amplitude can change by orders of magnitude if we assume a non-standard history of cosmology. That is because the Hubble rate determines which processes are efficient. We find that a change in Hubble rate can have a significant impact on when the weak sphalerons become active. As a result of a change in the evolution of baryonic asymmetry, alters the evolution of hypermagnetic field amplitude. It is known that if the hypermagnetic field amplitude is large enough, it can save the baryon asymmetry from diminishing. We show that whether a small seed of hypermagnetic field amplitude can be amplified to a large enough value will strongly depend on the history of cosmology., Comment: 15 pages, 10 figures

In this paper, we explore the evolution of baryon asymmetry as well as the hypermagnetic field in the early universe with an assumption that the flavon of the Froggatt-Nielsen carries an asymmetry. Through the decay of the flavon to Standard Model fermions, this asymmetry is transferred to fermions, where the right-handed electron keeps its asymmetry while its Yukawa interaction is out of thermal equilibrium. Through the existence of the flavon, we can ensure that the freezing-in temperature of the right-handed electron is closer to the electroweak phase transition than the Standard cosmology scenario. With this trick, the asymmetry in the right-handed electron is saved for a longer time. Moreover, the injection of the asymmetry to the right-handed electron is gradual, which helps the preservation of the asymmetry in the right-handed sector significantly. Due to the intimate relationship between fermion number violation and the helicity of the hypermagnetic field, some of the asymmetry is used to amplify the hypermagnetic field which itself helps to preserve the remnant asymmetry through keeping the Yukawa processes out of thermal equilibrium. We find the sweet region of the parameter space that can produce the right asymmetry in the baryons while generating a large hypermagnetic field by the time of the electroweak phase transition., Comment: 18 pages, 9 figures

According to the Standard Model (SM), we expect to find a proton for each decaying neutron. However, the experiments counting the number of decayed neutrons and produced protons have a disagreement. This discrepancy suggests that neutrons might have an exotic decay to a Dark Sector (DS). In this paper, we explore a scenario where neutrons decay to a dark Dirac fermion $\chi$ and a non-abelian dark gauge boson $W'$. We discuss the cosmological implications of this scenario assuming DS particles are produced via freeze-in. In our proposed scenario, DS has three portals with the SM sector: (1) the fermion portal coming from the mixing of the neutron with $\chi$, (2) a scalar portal, and (3) a non-renormalizable kinetic mixing between photon and dark gauge bosons which induces a vector portal between the two sectors. We show that neither the fermion portal nor the scalar portal should contribute to the production of the particles in the early universe. Specifically, we argue that the maximum temperature of the universe must be low enough to prevent the production of $\chi$ in the early universe. In this paper, we rely on the vector portal to connect the two sectors, and we discuss the phenomenological bounds on the model. The main constraints come from ensuring the right relic abundance of dark matter and evading the neutron star bounds. When dark gauge boson is very light, measurements of the Big Bang Nucleosynthesis impose a stringent constraint as well., Comment: 20 pages, 6 figures

A notable feature of UV freeze-in is that the relic density is strongly dependent on the highest temperatures of the thermal bath, and a common assumption is that the relevant 'highest temperature' should be the reheating temperature after inflation $T_\text{RH}$. However, the temperature of the thermal bath can be significantly higher in certain scenarios, reaching a value denoted T max , a fact which is only apparent away from the instantaneous decay approximation. Interestingly, it has been shown that if the operators are of sufficiently high mass dimension then the dark matter abundance can be enhanced by a 'boost factor' depending on ($T_\text{max}/T_\text{RH}$) relative to naive estimates assuming instantaneous reheating. We highlight here that in non-standard cosmological histories the critical mass dimension of the operator above at which the instantaneous decay approximation breaks down, and the exponent of the boost factor, depend on the equation of state $\omega$ prior to reheating. We highlight four examples in which the dark matter abundance receives a significant enhancement in the context of gravitino dark matter, the moduli portal, the Higgs portal, and the spin-2 portal (as might arise in bimetric gravity models). We comment on the transition from kination domination to radiation domination as a motivated example of non-standard cosmologies., Comment: 22 pages, 5 figures

In this paper, we explore the constraints that the LHC can place on a massive gauge boson $X$ that predominantly couples to the third generation of fermions. Such a gauge boson arises in scenarios where the $B-L$ of the third generation is gauged. We focus on the mass range $10 \leq m_X \lesssim 2\,m_W$, where current constraints are lacking, and develop a dedicated search strategy. For this mass range, we show that $b \bar b \tau^+ \tau^-$, where at least one of the $\tau$s decay leptonically is the optimal channel to look for the $X$ at the LHC. The QCD production of $b$ quarks, combined with the cleanliness of the leptons coming from the decay of the $\tau$ allow us to detect $X$ gauge boson with couplings of $g_X \sim(0.005-0.01)$, for $m_X < 50 \ \text{GeV}$, and a coupling of $O(0.1)$ for heavier $X$ gauge boson with $100 \ \text{fb}^{-1}$ of integrated luminosity. This is about a factor of 2-10 improvement over previous constraints coming from the decay of $\Upsilon \to \tau^+ \tau^-$. Extrapolating to the full HL-LHC luminosity of $3000 \ \text{fb}^{-1}$, the bounds on $g_X$ can be enhanced by another factor of $\sqrt{2}$ for $m_X < 50 \ \text{GeV}$., Comment: 28 pages, 12 figures

In this paper, we explore a dark sector scenario with a gauged $SU(2)_R$ and a global $U(1)_X \times \mathbb{Z}_2$, where the continuous symmetries are spontaneously broken to a global $U(1)_D$. We show that in various regions of the parameter space we can have two, or three dark matter candidates, where these dark matter particles are either a Dirac fermion, a dark gauge boson, or a complex scalar. The phenomenological implications of this scenario are vast and interesting. We identify the parameter space that is still viable after taking into account the constraints from various experiments. We, also, discuss how this scenario can explain the recent observation by DAMPE in the electron-positron spectrum. Furthermore, we comment on the neutrino mass generation through non-renormalizable interactions between the standard model and the dark sector., Comment: PRD version, 26 pages, 6 figures

Cold gas clouds recently discovered hundreds of parsecs from the center of the Milky Way Galaxy have the potential to detect dark matter. With a detailed treatment of gas cloud microphysical interactions, we determine Galactic Center gas cloud temperatures, free electron abundances, atomic ionization fractions, heating rates, cooling rates, and find how these quantities vary with metallicity. Considering a number of different dark sector heating mechanisms, we set new bounds on ultra-light dark photon dark matter for masses $10^{-22}-10^{-10}$ eV, vector portal dark matter coupled through a sub-MeV mass boson, and up to $10^{60}$ GeV mass dark matter that interacts with baryons., Comment: PRD version, 30 pages, 5 figures

We demonstrate that dark matter heating of gas clouds hundreds of parsecs from the Milky Way Galactic center provides a powerful new test of dark matter interactions. To illustrate, we set a leading bound on nucleon scattering for 10-100 MeV mass dark matter. We also constrain millicharged dark matter models, including those proposed to match the recent EDGES 21 cm absorption anomaly. For Galactic center gas clouds, galactic fields' magnetic deflection of electromagnetically charged dark matter is mitigated, because the magnetic fields around the Galactic center are poloidal, as opposed to being aligned parallel to the Milky Way disk. We discuss prospects for detecting dark matter using a population of Galactic center gas clouds warmed by dark matter., Comment: 2 figures, 7 pages

We investigate the prospect of the LHC for discovering new physics effects via new strategies in the same-sign top pair and same-sign top pair associated with a $W$ boson signatures. Significant enhancement in production of same-sign top quarks (plus a $W$ boson) is a joint property of several models beyond the standard model. We concentrate on the leptonic (electron and muon) decay of the top quarks and study the exclusion reach of the LHC data for a simplified model approach where top quark flavor changing could occur through a $Z'$ or a neutral scalar $\phi$ exchange. Less background contributions and clean signature are the advantages of the leptonic decay mode of the top quarks in the same-sign production processes. A combination is performed on both same-sign top pair and same-sign top pair plus a $W$ boson production modes which enables us to reach a large fraction of the model parameter space. Assuming the couplings of new physics of the order of $10^{-2}$, the mass of a flavor changing $Z'$ or a neutral scalar above 1 TeV could be excluded. We propose a momentum dependent charge asymmetry and angular observables in the same-sign top process which provide the possibility of separation of new physics signal from the SM backgrounds as well as discrimination of the flavor changing $tuX$ from $tcX$, where $X = Z',\phi$., Comment: 24 pages, 16 figures and one table

Dark matter (DM) is added to the Froggatt-Nielsen (FN) mechanism, and conditions for its successful freezeout identified. Requesting the FN scale $\Lambda_{\text{FN}}$ to be the cutoff of the theory renders freezeout scenarios surprisingly few. Fermionic DM is typically charged under $U(1)_{\text{FN}}$, with the dominant annihilation channel a CP-even flavon + CP-odd flavon. A minimal case is when the DM-flavon coupling strength is $\mathcal{O}(1)$, with several implications: (1) the DM mass is $\mathcal{O}$(100 GeV - 1 TeV), thanks to the WIMP coincidence, (2) requiring perturbativity of couplings puts a lower $and$ upper limit on the flavor scale, 2 TeV $\lesssim \Lambda_{\text{FN}} \lesssim 14~$TeV, on account of its relation to DM mass and couplings, (3) DM is a "secluded WIMP" effectively hidden from collider and direct detection searches. Limits on the masses of dark matter and mediators from kaon mixing measurements constitute the best constraints, surpassing Xenon1T, Fermi-LAT, and the LHC. Future direct detection searches, and collider searches for missing energy plus a single jet/bottom/top, are promising avenues for discovery., Comment: 14 pages, 6 figures; v2: typos corrected, references added, version accepted for publication in PRD

In this paper, we explore the discriminatory power of the matrix element method (MEM) in constraining the $L_\mu-L_\tau$ model at the LHC. The $Z'$ gauge boson associated with the spontaneously broken $U(1)_{L_\mu-L_\tau}$ symmetry only interacts with the second and third generation of leptons at tree level, and is thus difficult to produce at the LHC. We argue that the best channels for discovering this $Z'$ are in $Z \to 4\mu$ and $2\mu+\displaystyle{\not}E_T$. Both these channels have a large number of kinematic observables, which strongly motivates the usage of a multivariate technique. The MEM is a multivariate analysis that uses the squared matrix element to quantify the likelihood of the testing hypotheses. We find that with $300 \, \text{fb}^{-1}$ of integrated luminosity, we are sensitive to the couplings of $ g_{Z'} \gtrsim 0.002 ~ g_1$ and $M_{Z'} < 20 \text{ GeV}$, and $g_{Z'} \gtrsim 0.005 g_1$ and $20\, \text{GeV}

In this paper we examine the constraints dedicated LHC multi lepton searches can place on $Z'$ bosons coming from gauged muon number minus tau number, $L_{\mu}-L_{\tau}$. As the $L_{\mu}-L_{\tau}$ gauge boson does not couple to proton constituents or electrons at tree level, the current bounds are fairly loose, especially for $M_{Z'} \gtrsim 1\, \rm GeV$. For $2m_{\mu} < M_{Z'} < M_Z/2$ we develop search strategies using the $pp \rightarrow Z \to 4\, \mu$ channel. The cleanliness of the final state, combined with the fact that $pp \to Z \to 4 e$, $Z \to 2e\,2\mu$ can be used as background control samples, allow us to spot $L_{\mu}-L_{\tau}$ $Z'$ with couplings $\mathcal O(10^{-3})$ times the Standard Model couplings. For lighter $Z'$, we propose the mode $pp \rightarrow 2\mu + \displaystyle{\not} E_T$. The presence of missing energy means there is a wider set of backgrounds to consider in this final state, such as Drell-Yan production of leptonically decaying $\tau$ pairs, however we find these can be controlled with careful cuts. Combining the $4\, \mu$ and $2\,\mu + \displaystyle{\not} E_T$ modes, we find that with $\sim3 \, \text{ab}^{-1}$ of integrated luminosity we are sensitive to couplings $g_{Z'} \gtrsim 0.005\, g_1$ and $0.5\, \rm GeV \le M_{Z'} \le 40\, \rm GeV$ and $g_{Z'} \gtrsim 0.001\, g_1$ for $M_{Z'} < 2\, m_{\mu}$. This region includes the parameter space where $L_{\mu}-L_{\tau}$ models can ameliorate the muon $g-2$ anomaly. We repeat these analyses at a future $e^+e^-$ Z-factory, where we find improved sensitivity. Specifically, given $2.6\, \rm ab^{-1}$ of luminosity, we can exclude $g_{Z'} \gtrsim 0.001\, g_1$ for $2\, m_{\mu} \le M_{Z'} \le M_Z/2$., Comment: version accepted for publication in PRD

Pulsars apparently missing from the galactic center could have been destroyed by asymmetric fermionic dark matter ($m_X = 1-100$ GeV) coupled to a light scalar ($m_{\phi}= 5-20$ MeV), which mixes with the Higgs boson. We point out that this pulsar-collapsing dark sector can resolve the core-cusp problem and will either be excluded or discovered by upcoming direct detection experiments. Another implication is a maximum pulsar age curve that increases with distance from the galactic center, with a normalization that depends on the couplings and masses of dark sector particles. In addition, we use old pulsars outside the galactic center to place bounds on asymmetric Higgs portal models., Comment: 12 pages, 3 figures

If dark matter is thermally decoupled from the visible sector, the observed relic density can potentially be obtained via freeze-in production of dark matter. Typically in such models it is assumed that the dark matter is connected to the thermal bath through feeble renormalisable interactions. Here, rather, we consider the case in which the hidden and visible sectors are coupled only via non-renormalisable operators. This is arguably a more generic realisation of the dark matter freeze-in scenario, as it does not require the introduction of diminutive renormalisable couplings. We examine general aspects of freeze-in via non-renormalisable operators in a number of toy models and present several motivated implementations in the context of Beyond the Standard Model physics. Specifically, we study models related to the Peccei-Quinn mechanism and Z' portals., Comment: 24 pages. v2: Published version, correction to equilibration constraints. v3. Minor numerical error in eq.2.16 corrected and propagated, conclusions unchanged

In this paper we present a new search technique for electroweakinos, the superpartners of electroweak gauge and Higgs bosons, based on final states with missing transverse energy, a photon, and a dilepton pair, $\ell^+\,\ell^- + \gamma + \displaystyle{\not} E_T$. Unlike traditional electroweakino searches, which perform best when $m_{\widetilde{\chi}^0_{2,3}} - m_{\widetilde{\chi}^0_1}, m_{\widetilde{\chi}^{\pm}} - m_{\widetilde{\chi}^0_1} > m_Z$, our search favors nearly degenerate spectra; degenerate electroweakinos typically have a larger branching ratio to photons, and the cut $m_{\ell\ell} \ll m_Z$ effectively removes on-shell Z boson backgrounds while retaining the signal. This feature makes our technique optimal for `well-tempered' scenarios, where the dark matter relic abundance is achieved with inter-electroweakino splittings of $\sim 20 - 70\,\text{GeV}$. Additionally, our strategy applies to a wider range of scenarios where the lightest neutralinos are almost degenerate, but only make up a subdominant component of the dark matter -- a spectrum we dub `well-forged'. Focusing on bino-Higgsino admixtures, we present optimal cuts and expected efficiencies for several benchmark scenarios. We find bino-Higgsino mixtures with $m_{\widetilde{\chi}^0_{2,3}} \lesssim 190\,\text{GeV}$ and $m_{\widetilde{\chi}^0_{2,3}} - m_{\widetilde{\chi}^0_1} \cong 30\,\text{GeV}$ can be uncovered after roughly $600\,\text{fb}^{-1}$ of luminosity at the 14 TeV LHC. Scenarios with lighter states require less data for discovery, while scenarios with heavier states or larger mass splittings are harder to discriminate from the background and require more data. Unlike many searches for supersymmetry, electroweakino searches are one area where the high luminosity of the next LHC run, rather than the increased energy, is crucial for discovery., Comment: Updated to published version. Reference adde, discussion of other models expanded, and typos fixed. revtex4-1, 29 pages, 9 figures, and 3 tables