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DOE OSTI2013

Is the cosmic microwave background telling us that dark matter is weaker than weakly interacting?

If moduli, or other long-lived heavy states, decay in the early universe in part into light and feebly interacting particles (such as axions), these decay products could account for the additional energy density in radiation that is suggested by recent measurements of the CMB. These moduli decays will also, however, alter the expansion history of the early universe, potentially diluting the thermal relic abundance of dark matter. If this is the case, then dark matter particles must annihilate with an even lower cross section than required in the standard thermal scenario (sigma v < 3x10^-26 cm^3/s) if they are to make up the observed density of dark matter. This possibility has significant implications for direct and indirect searches for dark matter.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Heavy quark diffusion in strong magnetic fields at weak coupling and implications for elliptic flow

In this paper, we compute the momentum diffusion coefficients of heavy quarks, κ ∥ and κ ⊥ , in a strong magnetic field B along the directions parallel and perpendicular to B, respectively, at the leading order in QCD coupling constant α s . We consider a regime relevant for the relativistic heavy ion collisions, α s eB << T 2 << eB, so that thermal excitations of light quarks are restricted to the lowest Landau level (LLL) states. In the vanishing light-quark mass limit, we find κ LO ⊥ ∝ α 2 s TeB in the leading order that arises from screened Coulomb scatterings with (1+1)-dimensional LLL quarks, while κ ∥ gets no contribution from the scatterings with LLL quarks due to kinematic restrictions. We show that the first nonzero leading order contributions to κ LO ∥ come from the two separate effects: 1) the screened Coulomb scatterings with thermal gluons, and 2) a finite light-quark mass m q . The former leads to κ LO,gluon ∥ ∝ α 2 s T 3 and the latter to κ LO,massive ∥ ∝ α s (α s eB) 1/2 m 2 q . Based on our results, we propose a new scenario for the large value of heavy-quark elliptic flow observed in RHIC and LHC. Namely, when κ ⊥ >> κ ∥ , an anisotropy in drag forces gives rise to a sizable amount of the heavy-quark elliptic flow even if heavy quarks do not fully belong to an ellipsoidally expanding background fluid.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Constraints on large- x parton distributions from new weak boson production and deep-inelastic scattering data

A new set of leading twist parton distribution functions, referred to as "CJ15", is presented, which take advantage of developments in the theoretical treatment of nuclear corrections as well as new data. The analysis includes for the first time data on the free neutron structure function from Jefferson Lab, and new high-precision charged lepton and W-boson asymmetry data from Fermilab, which significantly reduce the uncertainty on the d/u ratio at large values of x.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗