# NPAC (Nuclear/Particle/Astro/Cosmo) Forum

electron-positron pairs in the presence of an external electric field is one of the first non-trivial predictions of QED, due to Heisenberg and Euler, and later formalized by Schwinger. This non-perturbative effect has implications and analogies in QCD and gravitational physics. However, the effect is so weak that it has still not been directly observed. But recent experimental progress in ultra-intense laser systems has brought us

tantalizingly close to seeing this non-perturbative particle production. I will review the current status and describe some new theoretical ideas aimed at making this process accessible.

*University of Connecticut*

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These particles are well motivated by particle physics theories beyond the Standard Model, and the discovery of WIMPs would have enormous impact on both astrophysics and particle physics. WIMPs, if they exist, would occasionally interact with normal matter. With a mass in the range of 1 to 1000 times the mass of the proton, and moving at speeds relative to the Earth on the order of 200 km/s, WIMPs would only deposit a small amount of energy when scattering with nuclei.

Detectors that are low in radioactivity and sensitive to small energy depositions can search for the rare nuclear recoil events predicted by WIMP models. In recent years, several new efforts on direct dark matter detection have begun in which the detection material is a noble liquid. Advantages include: large nuclear recoil signals in both scintillation and ionization channels, good scalability to large target masses, effective discrimination against gamma ray backgrounds, easy purification, and reasonable cost.

*Yale University*

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Coupling dark matter (DM) to dark energy (DE) is one of the most promising way to build a unified description of the invisible sector of cosmology. However, such DM-DE couplings make the mass of the DM particles varying, therefore breaking the universality of free fall (Galileo's equivalence principle). Doing so, the strong equivalence principle, stating the universality of gravitational binding energy, does not hold anymore, particularly where DM is profuse like in the large-scale universe. Such mass-varying DM particles therefore constitute an Abnormally Weighting type of Energy (AWE Hypothesis) and their cosmological abundance induces modifications of gravity on large-scales, modifications that can explain cosmic acceleration. I will present here how this AWE hypothesis can be naturally achieved in terms of the explicit symmetry breaking of a global U(1) symmetry in particle physics. In this context, the U(1)-charged complex scalar usually splits into a mass-varying pseudo-Nambu-Goldstone boson (axion-like DM particles) while the vacuum expectation value of the complex scalar has to be stabilised, usually through the use of the potential (spontaneous symmetry breaking mechanism). We show here that the cosmological relaxation toward the equivalence principle can play this crucial stabilising role and explain in addition the observed cosmic acceleration. Stabilisation of the new scalar is achieved through non-minimal gravitational couplings. We will show several remarkable cosmological predictions of this idea and emphasize some interesting applications on neutrino physics for the gravitational symmetry breaking of lepton number symmetry.

*GAMASCO, University of Namur (FUNDP) and LUTH, Observatory of Paris.*

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Observed properties of UHECR showers are difficult to reconcile with simulated showers and arrival direction information, and no choice of composition seems compatible with all observations. At the moment, one or more improbable or very improbable statistical fluctuations, or a new physics threshold at about 100 TeV, seem to be the only viable explanations. I will also describe new, much better constraints on the Galactic Magnetic Field. The proposal that a large fraction of UHECRs may come from Centaurus A -- which, at about 3.5 Mpc is the closest plausible source -- can be excluded, unless extragalactic magnetic fields are much larger than generally thought.

*New York University*

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(including photons) is described by an effective field theory (EFT)

that includes only contact interactions. I will describe this EFT,

how it improves upon the standard (non-physical) description, how it

echoes the Danilov treatment, and how its predictions compare to

existing (presently under-constraining) measurements.

*Duke University*

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the entire overhead sky. The large field-of-view combined with<br>

the long observation time makes Milagro the most sensitive<br>

instrument available for surveys and especially for the study<br>

of large, low surface brightness sources. In this talk I will<br>

present recent results from Milagro including the identification<br>

of several new TeV sources associated with Fermi BSL (bright<br>

source list) objects within the Galactic plane. The success of<br>

Milagro has lead to the proposed High Altitude Water Cherenkov<br>

(HAWC) Observatory. HAWC will be built at a high altitude site<br>

(4100m a.s.l.) in central Mexico. The increased elevation, along<br>

with the re-optimization of the design will lead to a 15x<br>

sensitivity improvement compared to Milagro.

*University of Maryland, College Park*

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*University of Milan Bicocca*

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Coupling dark matter (DM) to dark energy (DE) is one of the most promising way to build a unified description of the invisible sector of cosmology. It also glimpses beyond the concordance model LCDM in which DM and DE are assumed physically unrelated. However, such DM-DE couplings make the mass of the DM particles varying, therefore breaking the universality of free fall (Galileo's equivalence principle). Doing so, the strong equivalence principle, stating the universality of gravitational binding energy, does not hold anymore, particularly where DM is profuse like in the large-scale universe. Mass-varying DM therefore induces modifications of gravity. This gravitational feedback on ordinary matter can explain cosmic acceleration, which is then interpreted as the observable signature of the violation of the equivalence principle on cosmological scales. To embrace the various physics of DM-induced violation of the equivalence principle, we have developped a generalisation of Brans-Dicke tensor-scalar theories of gravitation, dubbed the Abnormally Weighting Energy (AWE) Hypothesis. In this approach, the variation of the inertial mass of DM particles induces a running of the gravitational coupling strength on cosmological scales that is observable in the late-time cosmic acceleration. Besides of describing both DM and DE, the AWE hypothesis allows measuring the density paramters of baryons and dark matter from the Hubble diagram *alone*, and its predictions are consistent with the independent cosmological tests of Cosmic Microwave Background (CMB) and Big Bang Nucleosynthesis (BBN). This interpretation also shed new light on the coincidence problem. We will end this seminar by showing how this mechanism could interestingly be applied to the physics of neutrino mass generation and mass-varying neutrinos by turning the spontaneous symmetry breaking of lepton number symmetry into a gravitational symmetry breaking.

*GAMASCO, University of Namur (FUNDP); Louvain U; Paris Observatory;*

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*Ohio State University*

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U(1)'-extended MSSM with/without CP violation.<br>

Unlike the MSSM, the EWPT can be strong first order without a light stop.<br>

In such a case, the singlet-like Higgs bosons and the charged Higgs bosons<br>

play an important role. It is found that at least two Higgs bosons should be<br>

less than 300 GeV for the strong first order phase transition.<br>

Depending on the charged Higgs boson mass, the lightest Higgs boson can be<br>

as large as 220 GeV. It is also found that the CP violating phase in the Higgs sector<br>

do not weaken the strength of the first order EWPT.<br>

*National Central University, Taiwan*

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