![down quark down quark](https://www.researchgate.net/profile/Michael-Creutz/publication/344622332/figure/fig6/AS:946023107141643@1602560893632/A-down-quark-mass-induces-an-effective-mass-for-the-up-quark.png)
But in other combinations they contribute different masses. Happily, both the proton and the neutron 'end up' with the charge they should have. The masses quoted are model dependent, and the mass of the bottom quark is quoted for two different models. We also said that a neutron has two 'down' quarks and one 'up' quark, so it has a total charge of (-1/3) + (-1/3) + (+ 2/3) = 0. We said earlier that a proton has two 'up' quarks and one 'down' quark, so it has a total charge of (+2/3) + (+2/3) + (-1/3) = +1.
![down quark down quark](https://media.indiedb.com/images/members/5/4078/4077050/profile/quarks.png)
![down quark down quark](https://media.springernature.com/full/springer-static/image/art%3A10.1038%2F551040a/MediaObjects/41586_2017_Article_BF551040a_Fig1_HTML.jpg)
The decay of the down quark is involved in the decay of the neutron and in beta decay in general. The decay of the up quark above is important in the proton-proton cycle of nuclear fusion. Is this consistent with what we know about protons and neutrons? Remember that protons carry an electrical charge of +1 while neutrons carry no electrical charge. The most common of the quark transformations are those of the up and down quarks which are the constituents of ordinary matter in the form of protons and neutrons. There are many possible colour charge combinations. An 'up' quark has a charge of +2/3 and a 'down' quark has a charge of -1/3. A proton is a baryon made of two up (u) quarks and one down (d) quark. Now nuclear physicists in Japan, Russia and the US have discovered a particle that contains two up quarks, two down quarks and a strange antiquark. The down and anti-up quarks form a negative pion which is absorbed by a nearby proton and then the anti-up quark annihilates with an up-quark in the recipient proton, which then absorbs the remaining d-quark, converting the proton from uud to udd, i.e. Quarks carry fractional electrical charges. Protons are made up of two 'up' quarks and one 'down' quark while neutrons are made up of two 'down' quarks and one 'up' quark. Protons and neutrons are each composed of three quarks. What is the charge of an up quark and the charge of a down quark? How many quarks make up a proton and a neutron?