Tuesday, February 7, 2023

A New Top Quark Mass Measurement

The latest top quark mass measurement at the Large Hadron Collider (LHC) is on the low side relative to previous measurements and the global average (which is 172.69 ± 0.30 from direct measurements), and is fairly precise despite using a fairly complex set of decay products to measure it. 

The new measurement is 1.93 sigma from the global average, so the new measurement is just barely consistent with the global average. In contrast, many other recent LHC measurements of the top quark mass have been high (almost two sigma high in at least one case) relative to the global average.
The mass of the top quark is measured in 36.3 fb−1 of LHC proton-proton collision data collected with the CMS detector at s√ = 13 TeV. The measurement uses a sample of top quark pair candidate events containing one isolated electron or muon and at least four jets in the final state. For each event, the mass is reconstructed from a kinematic fit of the decay products to a top quark pair hypothesis. A profile likelihood method is applied using up to five observables to extract the top quark mass. The top quark mass is measured to be 171.77 ± 0.37 GeV. This approach significantly improves the precision over previous measurements.
CMS Collaboration, "Measurement of the top quark mass using a profile likelihood approach with the lepton+jets final states in proton-proton collisions at s√ = 13 TeV" arXiv:2302.01967 (February 3, 2023).

Monday, February 6, 2023

Dark Matter Still Has Nothing On MOND In The Milky Way

The more complex dark matter particle mass models of the Milky Way, perform not better in describing what we see with other telescopes than the far simply MOND model when it comes to the Milky Way's rotation curve.
We use data from the Gaia DR3 dataset to estimate the mass of the Milky Way (MW) by analyzing the rotation curve in the range of distances 5 kpc to 28 kpc. 
We consider three mass models: the first model adds a spherical dark matter (DM) halo, following the Navarro-Frenk-White (NFW) profile, to the known stellar components. The second model assumes that DM is confined to the Galactic disk, following the idea that the observed density of gas in the Galaxy is related to the presence of more massive DM disk (DMD), similar to the observed correlation between DM and gas in other galaxies. The third model only uses the known stellar mass components and is based on the Modified Newton Dynamics (MOND) theory. 
Our results indicate that the DMD model is comparable in accuracy to the NFW and MOND models and fits the data better at large radii where the rotation curve declines but has the largest errors. For the NFW model we obtain a virial mass M(vir)=(6.5±0.3)×10^11M⊙ with concentration parameter c=14.5, that is lower than what is typically reported. In the DMD case we find that the MW mass is M(d)=(1.6±0.5)×10^11M⊙ with a disk's characteristic radius of Rd=17 kpc.
Francesco Sylos Labini, et al., "Mass models of the Milky Way and estimation of its mass from the GAIA DR3 data-set" arXiv:2302.01379 (February 2, 2023) (accepted for publication in The Astrophysical Journal).