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Recently, I have read that there is a hypothesis that dark matter might consist of a particle which is its own anti-particle and when two of them meet they annihilate to form an electron-positron pair. This cannot be mediated by a virtual photon since only charged particles interact directly with photons and dark matter cannot be charged since it would not then be dark, but visible instead. So could it be mediated by a virtual Z^0 particle which would then decay into an electron-positron pair? JRSpriggs (talk) 19:25, 5 April 2025 (UTC)[reply]
According to our article on W and Z bosons, the Z boson, unlike the two W bosons, is not involved in the absorption or emission of electrons or positrons. Also, still according to the article, the exchange of a Z boson between particles (called a neutral current interaction) leaves the interacting particles unaffected. This still leaves a hypothetical role for virtual W bosons. As far as I could readily see, the recent study that drew some media attention does not address the issue. Since the idea of dark matter annihilation is an old one, perhaps earlier publications do, but those I looked at, such as this one, seemed to assume no mediation was needed. The Wikipedia reference desk is not really a venue for speculation on which particles might mediate in hypothetical interactions between hypothetical particles. ‑‑Lambiam07:47, 6 April 2025 (UTC)[reply]
According to that article, "Z bosons decay into a fermion and its antiparticle.". The electron is a fermion, so Z^0 could decay into an electron and a positron. If the emission of a Z^0 by the dark matter particle leaves the dark matter particle unaffected, then one can merely make the out-going dark matter particle into an in-coming anti-particle (time reversal) which by hypothesis is the same as an in-coming dark matter particle. So your own source material supports my position. JRSpriggs (talk) 17:29, 6 April 2025 (UTC)[reply]
If you are referring to conventional passenger trains, the floor level is well above the tops of the wheels, which are generally on bogies. Same for most flatbed freight wagons. Given the need for bogies to turn, you'd complicate the internal layout, with very little benefit in terms of extra height. And with regard to passenger transport, you generally build new stock to suit existing platform heights - you wouldn't want a step down into the carriage.
There may possibly be trams and/or metro stock where the floor is lower than the tops of the wheels, but as far as mainline rail goes, the loading gauge allows plenty of height. AndyTheGrump (talk) 21:54, 5 April 2025 (UTC)[reply]
AndyTheGrump, that's correct for the trams. Here in Melbourne, the older trams are high-floor (so you have to climb a few steps to get inside), so there's no "bump", but in newer low-floor trams, everything's much lower down, and a good deal of space is occupied by the "bumps". See image of low-floor tram and images one and two of high-floor trams. Unfortunately I don't have interior images conveniently available. Nyttend (talk) 03:18, 8 April 2025 (UTC)[reply]
The initial lines of the NYC Subway were (mostly) constructed as cut-and-cover tunnels, like the initial lines of the London Underground of the 1860s. Compared to the later bored tube lines in London, there was less need for a small profile, so it was decided to keep them compatible with existing above ground lines. PiusImpavidus (talk) 08:49, 8 April 2025 (UTC)[reply]
To add to the above, one exception to this seems to be for double-decker coaches, which are often built with the lower floor dropped down below the top of the wheels, between the bogies. Presumably the benefit of extra passenger space is seen to outweigh the obvious accessibility issues. AndyTheGrump (talk) 22:11, 5 April 2025 (UTC)[reply]
Oh yeah, I've been on those but not in so long I forgot (bi-levels don't fit in my city's subway & direct (non-transfer) suburban or longer journeys from it cause of the tunnels) Sagittarian Milky Way (talk) 01:09, 6 April 2025 (UTC)[reply]
Thanks. Apparently Americans call it a "truck", but I wouldn't have known it by that name either, so that wouldn't have helped. I guess Andy knows more about trains than I do. --Trovatore (talk) 02:58, 6 April 2025 (UTC)[reply]
I just looked at my car, and it seems typical for smallish compact, sedan, and similar models. The wheels set further forward and backward of the passenger compartment. But in the engine compartment and in the rear luggage compartment (and typically extending under the rear seats) are indeed humps. SUVs and hatchbacks often have a hump in the rear-most compartment. In some cases, the rear hump is hidden by using the space between them a covered compartment such as for a spare tire. DMacks (talk) 20:17, 6 April 2025 (UTC)[reply]
Bilevel rail cars may feature designs that accommodate the wheels and their supporting apparatus ('bogies' or 'trucks') in different ways. For example, these Bombardier bilevel coaches have a single intermediate-height passenger area above the trucks at either end (look at the windows on the side), with a lower floor in the middle of the car.
Trams, streetcars, and light-rail vehicles also come in low-floor versions. Here's an interior shot of a Flexity tram during manufacturing. The 'boxes' over the wheelsets are quite obvious; in the final assembled vehicle, each box would have back-to-back pairs of seats on top. TenOfAllTrades(talk) 16:34, 6 April 2025 (UTC)[reply]
