arXiv subjects

JiJi Fan

Publications and source records attributed to JiJi Fan.

At least 55 records · Page 3Linked to original sources

arXiv2018

Clockwork Axions in Cosmology: Is Chromonatural Inflation Chrononatural?

Many cosmological models rely on large couplings of axions to gauge fields. Examples include theories of magnetogenesis, inflation on a steep potential, chiral gravitational waves, and chromonatural inflation. Such theories require a mismatch between the axion field range and the mass scale appearing in the $a F \widetilde{F}$ coupling. This mismatch suggests an underlying monodromy, with the axion winding around its fundamental period a large number of times. We investigate the extent to which this integer can be explained as a product of smaller integers in a UV completion: in the parlance of our times, can the theory be "clockworked"? We argue that a clockwork construction producing a potential $μ^4 \cos(\frac{a}{j F_a})$ for an axion of fundamental period $F_a$ will obey the constraint $μ< F_a$. For some applications, including chromonatural inflation with sub-Planckian field range, this constraint obstructs a clockwork UV completion. Alternative routes to a large coupling include fields of large charge (an approach limited by strong coupling) or kinetic mixing (requiring a lighter axion). Our results suggest that completions of axion cosmologies that explain the large parameter in the theory potentially alter the phenomenological predictions of the model.

hep-th↗

arXiv2018

Experimental Targets for Photon Couplings of the QCD Axion

The QCD axion's coupling to photons is often assumed to lie in a narrow band as a function of the axion mass. We demonstrate that several simple mechanisms, in addition to the photophilic clockwork axion already in the literature, can significantly extend the allowed range of couplings. Some mechanisms we present generalize the KNP alignment scenario, widely studied as a model of inflation, to the phenomenology of a QCD axion. In particular we present KSVZ-like realizations of two-axion KNP alignment and of the clockwork mechanism. Such a "confinement tower" realization of clockwork may prove useful in a variety of model-building contexts. We also show that kinetic mixing of the QCD axion with a lighter axion-like particle can dramatically alter the QCD axion's coupling to photons, differing from the other models we present by allowing non-quantized couplings. The simple models that we present fully cover the range of axion-photon couplings that could be probed by experiments. They motivate growing axion detection efforts over a wide space of masses and couplings.

hep-ph↗

arXiv2017

Jet observables and stops at 100 TeV collider

A future proton-proton collider with center of mass energy around 100 TeV will have a remarkable capacity to discover massive new particles and continue exploring weak scale naturalness. In this work we will study its sensitivity to two stop simplified models as further examples of its potential power: pair production of stops that decay to tops or bottoms and higgsinos; and stops that are either pair produced or produced together with a gluino and then cascade down through gluinos to the lightest superpartner (LSP). In both simplified models, super-boosted tops or bottoms with transverse momentum of order TeV will be produced abundantly and call for new strategies to identify them. We will apply a set of simple jet observables, including track-based jet mass, N-subjettiness and mass drop, to tag the boosted hadronic or leptonic decaying objects and suppress the Standard Model as well as possible SUSY backgrounds. Assuming 10% systematic uncertainties, the future 100 TeV collider can discover (exclude) stops with masses up to 6 (7) TeV with 3 inverse attobarns of integrated luminosity if the stops decay to higgsinos. If the stops decay through gluinos to LSPs, due to additional SUSY backgrounds from gluino pair production, a higher luminosity of about 30 inverse attobarns is needed to discover stops up to 6 TeV. We will also discuss how to use jet observables to distinguish simplified models with different types of LSPs. The boosted top or bottom tagging strategies developed in this paper could also be used in other searches at a 100 TeV collider. For example, the strategy could help discover gluino pair production with gluino mass close to 11 TeV with 3 inverse attobarns of integrated luminosity.

hep-ph↗

arXiv2017

Cooling in a Dissipative Dark Sector

The possibility of a subdominant component of dark matter dissipating energy could lead to dramatic new phenomenology such as the formation of a dark disk. One rigorous way to assess this possibility and settle the debate on its feasibility is to include the dissipative dark component in a numerical hydrodynamical simulation. A necessary input to such a simulation is a prescription including energy dissipation rates of different processes and rates of processes that change the number densities of dark ions and atoms. In this article, we study the simplest dissipative dark sector which consists of a dark electron and proton, both charged under a dark gauged U(1). We present approximate analytic formulas for energy loss rates due to Compton scattering, bremsstrahlung, recombination, collisional ionization and collisional excitation as well as the rates of number density change. We also include the heating rate due to photoionization. The work serves as the first step to realize a numerical simulation including a dissipative dark sector, which hopefully can shed more light on the formation and properties of a dark disk originating from dark matter self-interactions.

astro-ph.GA↗

arXiv2017

Deciphering the MSSM Higgs Mass at Future Hadron Colliders

Future hadron colliders will have a remarkable capacity to discover massive new particles, but their capabilities for precision measurements of couplings that can reveal underlying mechanisms have received less study. In this work we study the capability of future hadron colliders to shed light on a precise, focused question: is the higgs mass of 125 GeV explained by the MSSM? If supersymmetry is realized near the TeV scale, a future hadron collider could produce huge numbers of gluinos and electroweakinos. We explore whether precision measurements of their properties could allow inference of the scalar masses and $\tan β$ with sufficient accuracy to test whether physics beyond the MSSM is needed to explain the higgs mass. We also discuss dark matter direct detection and precision higgs physics as complementary probes of $\tan β$. For concreteness, we focus on the mini-split regime of MSSM parameter space at a 100 TeV $pp$ collider, with scalar masses ranging from 10s to about 1000 TeV.

hep-ph↗

arXiv2016

Ultralight Repulsive Dark Matter and BEC

Ultralight scalar dark matter with mass at or below the eV scale and pressure from repulsive self-interaction could form a Bose-Einstein condensate in the early Universe and maybe in galaxies as well. It has been suggested to be a possible solution to the cusp/core problem or even to explain MOND phenomenology. In this paper, I initiate a study of possible self-interactions of ultralight scalar dark matter from the particle physics point of view. To protect its mass, the scalar dark matter is identified as a pseudo Nambu-Goldstone boson (pNGB). Quite a few pNGB models with different potentials such as the QCD axion and the dilaton lead to attractive self-interactions. Yet if an axion is a remnant of a 5D gauged U(1) symmetry, its self-interactions could be repulsive provided the masses and charges of the 5D matter contributing to its potential satisfy certain constraints. Collective symmetry breaking could also lead to a repulsive self-interaction yet with too large a strength that is ruled out by Bullet Cluster constraints. I also discuss cosmological and astrophysical constraints on ultralight repulsive dark matter in terms of a parametrization motivated by particle physics considerations.

hep-ph↗

arXiv2016

Experimental Considerations Motivated by the Diphoton Excess at the LHC

We consider the immediate or near-term experimental opportunities offered by some scenarios that could explain the new diphoton excess at the LHC. If the excess is due to a new particle $X_s$ at 750 GeV, additional new particles are required, providing further signals. If connected with naturalness, the $X_s$ may be produced in top partner decays. Then a $t'\bar t'$ signal, with $t'\to t X_s$ and $X_s\to gg$ dominantly, might be discovered by reinterpreting 13 TeV SUSY searches in multijet events with low MET and/or a lepton. If $X_s$ is a bound state of quirks, the signal events may be accompanied by an unusual number of soft tracks or soft jets. Other resonances including dilepton and photon+jet as well as dijet may lie at or above this mass, and signatures of hidden glueballs might also be observable. If the "photons" in the excess are actually long-lived particles decaying to photon pairs or to electron pairs, there are opportunities for detecting overlapping photons and/or unusual patterns of apparent photon-conversions in either $X_s$ or 125 GeV Higgs decays. There is also the possibility of events with a hard "photon" recoiling against a narrow isolated HCAL-only "jet", which, after the jet's energy is corrected for its electromagnetic origin, would show a peak at 750 GeV.

hep-ph↗

arXiv2016

Pseudoscalar Portal Dark Matter and New Signatures of Vector-like Fermions

Fermionic dark matter interacting with the Standard Model sector through a pseudoscalar portal could evade the direct detection constraints while preserving a WIMP miracle. We study the LHC constraints on the pseudoscalar production in simplified models with the pseudoscalar either dominantly coupled to b quarks or tau leptons and explore their implications for the GeV excesses in gamma ray observations. We also investigate models with new vector-like fermions that could realize the simplified models of pseudoscalar portal dark matter. These models yield new decay channels and signatures of vector-like fermions, for instance, bbb, btautau, and tautautau resonances. Some of the signatures have already been strongly constrained by the existing LHC searches and the parameter space fitting the gamma ray excess is further restricted. On the other hand, the pure tau-rich final state is only weakly constrained so far due to the small electroweak production rate.

hep-ph↗

arXiv2015

Stealth Supersymmetry Simplified

In Stealth Supersymmetry, bounds on superpartners from direct searches can be notably weaker than in standard supersymmetric scenarios, due to suppressed missing energy. We present a set of simplified models of Stealth Supersymmetry that motivate 13 TeV LHC searches. We focus on simplified models within the Natural Supersymmetry framework, in which the gluino, stop, and Higgsino are assumed to be lighter than other superpartners. Our simplified models exhibit novel decay patterns that differ significantly from topologies of the Minimal Supersymmetric Standard Model, with and without $R$-parity. We determine limits on stops and gluinos from searches at the 8 TeV LHC. Existing searches constitute a powerful probe of Stealth Supersymmetry gluinos with certain topologies. However, we identify simplified models where the gluino can be considerably lighter than 1 TeV. Stops are significantly less constrained in Stealth Supersymmetry than the MSSM, and we have identified novel stop decay topologies that are completely unconstrained by existing LHC searches.

hep-ph↗

arXiv2015

Possible Futures of Electroweak Precision: ILC, FCC-ee, and CEPC

The future of high-precision electroweak physics lies in e+e- collider measurements of properties of the Z boson, the W boson, the Higgs boson, and the top quark. We estimate the expected performance of three possible future colliders: the ILC, FCC-ee (formerly known as TLEP), and CEPC. In particular, we present the first estimates of the possible reach of CEPC, China's proposed Circular Electron-Positron Collider, for the oblique parameters S and T and for seven-parameter fits of Higgs couplings. These results allow the physics potential for CEPC to be compared with that of the ILC and FCC-ee. We also show how the constraints on S and T would evolve as the uncertainties on each of the most important input measurements change separately. This clarifies the basic physics goals for future colliders. To improve on the current precision, the highest priorities are improving the uncertainties on the W mass and effective weak mixing angle. At the same time, improved measurements of the top mass, the Z mass, the running of alpha, and the Z width will offer further improvement which will determine the ultimate reach. Each of the possible future colliders we consider has strong prospects for probing TeV-scale electroweak physics.

hep-ph↗

arXiv2015

Precision Natural SUSY at CEPC, FCC-ee, and ILC

Testing the idea of naturalness is and will continue to be one of the most important goals of high energy physics experiments. It will play a central role in the physics program of future colliders. In this paper, we present projections of the reach of natural SUSY at future lepton colliders: CEPC, FCC-ee and ILC. We focus on the observables which give the strongest reach, the electroweak precision observables (for left-handed stops), and Higgs to gluon and photon decay rates (for both left- and right-handed stops). There is a "blind spot" when the stop mixing parameter Xt is approximately equal to the average stop mass. We argue that in natural scenarios, bounds on the heavy Higgs bosons from tree-level mixing effects that modify the bottom Yukawa coupling together with bounds from b to s gamma play a complementary role in probing the blind spot region. For specific natural SUSY scenarios such as folded SUSY in which the top partners do not carry Standard Model color charges, electroweak precision observables could be the most sensitive probe. In all the scenarios discussed in this paper, the combined set of precision measurements will probe down to a few percent in fine-tuning.

hep-ph↗

arXiv2015

Vacuum stability bounds on Higgs coupling deviations

We analyze the constraints imposed by Higgs vacuum stability on models with new fermions beyond the Standard Model. We focus on the phenomenology of Higgs couplings accessible at the Large Hadron Collider. New fermions that affect Higgs couplings lead to vacuum instability of the Higgs potential. Above the scale of vacuum instability, bosonic states must stabilize the potential, implying a cut-off to the pure fermion model. Conservatively tuning the models to produce the maximal cut-off for a given Higgs coupling effect, we show that observing a deviation in the $Htt$, $H$-diphoton, or $H$-digluon coupling, larger than 20$%$, would require that new bosons exist in order to stabilize the Higgs potential below about 100 TeV. For generic parameter configurations, and unless the new fermions are made as light as they can possibly be given current experimental constraints, observing a 10$%$ deviation in any of these couplings would suggest an instability cut-off below 10-100 TeV. Similarly, if new bosons are absent up to a high scale, then a deviation in the $Hbb$ or $Hττ$ coupling, larger than about 20$%$, should be accompanied by a sizable deviation in the $Zbb$ or $Zττ$ couplings that can be conclusively tested with electroweak precision measurements at planned lepton colliders.

hep-ph↗

arXiv2014

Heavy Gravitino and Split SUSY in the Light of BICEP2

High-scale supersymmetry (SUSY) with a split spectrum has become increasingly interesting given the current experimental results. A SUSY scale above the weak scale could be naturally associated with a heavy unstable gravitino, whose decays populate the dark matter (DM) particles. In the mini-split scenario with gravitino at about the PeV scale and the lightest TeV scale neutralino being (a component of) DM, the requirement that the DM relic abundance resulting from gravitino decays does not overclose the Universe and satisfies the indirect detection constraints demand the reheating temperature to be below 10^9 - 10^{10} GeV. On the other hand, the BICEP2 result prefers a heavy inflaton with mass at around 10^{13} GeV and a reheating temperature at or above 10^9 GeV with some general assumptions. The mild tension could be alleviated if SUSY scale is even higher with the gravitino mass above the PeV scale. Intriguingly, in no-scale supergravity, gravitinos could be very heavy at about 10^{13} GeV, the inflaton mass scale, while gauginos could still be light at the TeV scale.

hep-ph↗

arXiv2014

Direct and indirect detection of dissipative dark matter

We study the constraints from direct detection and solar capture on dark matter scenarios with a subdominant dissipative component. This dissipative dark matter component in general has both a symmetric and asymmetric relic abundance. Dissipative dynamics allow this subdominant dark matter component to cool, resulting in its partial or total collapse into a smaller volume inside the halo (e.g., a dark disk) as well as a reduced thermal velocity dispersion compared to that of normal cold dark matter. We first show that these features considerably relax the limits from direct detection experiments on the couplings between standard model (SM) particles and dissipative dark matter. On the other hand, indirect detection of the annihilation of the symmetric dissipative dark matter component inside the Sun sets stringent and robust constraints on the properties of the dissipative dark matter. In particular, IceCube observations force dissipative dark matter particles with mass above 50 GeV to either have a small coupling to the SM or a low local density in the solar system, or to have a nearly asymmetric relic abundance. Possible helioseismology signals associated with purely asymmetric dissipative dark matter are discussed, with no present constraints.

hep-ph↗

arXiv2014

Non-thermal Histories and Implications for Structure Formation

We examine the evolution of cosmological perturbations in a non-thermal post-inflationary history with a late-time matter domination period prior to BBN. Such a cosmology could arise naturally in the well-motivated moduli scenario in the context of supersymmetry (SUSY) -- in particular in models of Split-SUSY. Sub-horizon dark matter perturbations grow linearly during the matter dominated phase before reheating and can lead to an enhancement in the growth of substructure on small scales, even in the presence of dark matter annihilations. This suggests that a new scale (the horizon size at reheating) could be important for determining the primordial matter power spectrum. However, we find that in many non-thermal models free-streaming effects or kinetic decoupling after reheating can completely erase the enhancement leading to small-scale structures. In particular, in the moduli scenario with wino or higgsino dark matter we find that the dark matter particles produced from moduli decays would thermalize with radiation and kinetically decouple below the reheating temperature. Thus, the growth of dark matter perturbations is not sustained, and the predictions for the matter power spectrum are similar to a standard thermal history. We comment on possible exceptions, but these appear difficult to realize within standard moduli scenarios. We conclude that although enhanced structure does not provide a new probe for investigating the cosmic dark ages within these models, it does suggest that non-thermal histories offer a robust alternative to a strictly thermal post-inflationary history.

hep-ph↗

arXiv2014

A New Look at Higgs Constraints on Stops

We present a simple new way to visualize the constraints of Higgs coupling measurements on light stops in natural SUSY scenarios beyond the MSSM, which works directly in the plane of stop mass eigenvalues (with no need to make assumptions about mixing angles). For given stop mass eigenvalues, the smallest value of $X_t$ that can bring the correction to the $h \to gg$ and $h\to γγ$ couplings into agreement with data is computed. Requiring that this $X_t$ is consistent--i.e. that the chosen mass eigenvalues can be the outcome of diagonalizing a matrix with a given off-diagonal term--rules out the possibility that both stops have a mass below $\approx$ 400 GeV. Requiring that $X_t$ is not fine-tuned for agreement with the data shows that neither stop can be lighter than $\approx$ 100 GeV. These constraints are interesting because, unlike direct searches, they apply no matter how stops decay, and suggest a minimum electroweak fine-tuning of between a factor of 5 and 10. We show that a multi-parameter fit can slightly weaken this conclusion by allowing a large Higgs coupling to $b$-quarks, but only if a second Higgs boson is within reach of experiment. Certain models, like $R$-symmetric models with Dirac gauginos, are much more strongly constrained because they predict negligible $X_t$. We illustrate how the constraints will evolve given precise measurements at future colliders (HL-LHC, ILC, and TLEP), and comment on the more difficult case of Folded Supersymmetry.

hep-ph↗

arXiv2013

Probing Charged Matter Through Higgs Diphoton Decay, Gamma Ray Lines, and EDMs

Numerous experiments currently underway offer the potential to indirectly probe new charged particles with masses at the weak scale. For example, the tentative excess in Higgs diphoton decay and the tentative gamma-ray line in Fermi-LAT data have recently attracted attention as possible one-loop signatures of new charged particles. We explore the interplay between such signals, dark matter direct detection through Higgs exchange, and measurements of the electron EDM, by studying the size of these effects in several models. We compute one-loop effects to explore the relationship among couplings probed by different experiments. In particular, models in which dark matter and the Higgs both interact with charged particles at a detectable level typically induce, at loop level, couplings between dark matter and the Higgs that are around the level of current direct detection sensitivity. Intriguingly, one-loop Higgs diphoton decay and DM annihilation into two photons, two-loop EDMs, and loop-induced direct detection rates are all coming within range of existing experiments for approximately the same range of charged particle masses, offering the prospect of an exciting coincidence of signals at collider, astrophysical, underground and atomic physics measurements.

hep-ph↗

arXiv2013

A Simple Recipe for the 111 and 128 GeV Lines

Recently evidence for gamma ray lines at energies of approximately 111 and 128 GeV has been found in Fermi-LAT data from the center of the galaxy and from unassociated point sources. Many explanations in terms of dark matter particle pairs annihilating to gamma gamma and gamma Z have been suggested, but these typically require very large couplings or mysterious coincidences in the masses of several new particles to fit the signal strength. We propose a simple novel explanation in which dark matter is part of a multiplet of new states which all have mass near 260 GeV as a result of symmetry. Two dark matter particles annihilate to a pair of neutral particles in this multiplet which subsequently decay to gamma gamma and gamma Z. For example, one may have a triplet of pseudo-Nambu-Goldstone bosons, charged pions and a neutral pion, where the charged pions are stabilized by their charge under a new U(1) symmetry and the slightly lighter neutral pion state decays to gamma gamma and gamma Z. The symmetry structure of such a model explains the near degeneracy in masses needed for the resulting photons to have a line-like shape and the large observed flux. The tunable lifetime of the neutral state allows such models to go unseen at direct detection or collider experiments that can constrain most other explanations. However, nucleosynthesis constraints on the neutral pion lifetime fix a minimum necessary coupling between the new multiplet and the Standard Model. The spectrum is predicted to be not a line but a box with a width of order a few GeV, smaller than but on the order of the Fermi-LAT resolution.

hep-ph↗