Volume 105B, number 2,3 PHYSICS LETTERS 1 October 1981 COMMENT ON QUARK SEARCHES M.I. STRIKMAN Leningrad Nuclear Physics Institute, Gatchina, Leningrad 188350, USSR Received 27 July 1981 We argue that due to a small difference in the current masses of u- and d-quarks, white quarks with screened colour could be bound electromagnetically only to a medium size nucleus, but not to the iron nucleus, studied in theexperiments of Marinelli and Morpurgo. We indicate also that existing beam-dump experiments can be used to reach higher sensitivity in order to search for quarks in hadronie collisions. In recent years there was a steady increase of inte- rest in experimental searches of objects with fraction- al charges (see e.g. refs. [1-3]). Basically, this is due to the experimental discoveries of the last decade and the success of QCD and the theory of electroweak in- teractions which have established the existence of quarks and gluons but have not yet elucidated the mechanism of the colour screening. It appears that in hunting such exotic objects as fractional-charge par- ticles one should concentrate on a search for the par- ticles which existence does not contradict to the pre- sent knowledge of the structure of QCD. The most popular viewpoint is that in QCD the colour charge of the quark can be neutralized by quarks and thus free states with fractional charges could not exist. However, in principle the colour charge of the quark could be screened via vacuum polarization without screening of the electric charge. This idea has been suggested recently by Gribov in analogy with his analysis of massless quantum electro- dynamics and it appears tO be one of the few possibi- lities left for discussio n of experiments like in refs. [ 1-3 ] in the framework of QCD*I. (Several analogues could be useful. One is the Debye screening of an ar- bitrary charge in a plasma. One more example is mass- less QED where according to the recent analysis of Gribov [5,6] the charge of the massless electron is ,1 An other possibility, related to the violation of QCD at large distances was discussed in ref. [4]. dynamically screened up to zero). Thus, there probably could exist particles with fractional baryon and electric charges but with zero colour charge - in the following we shall call them white quarks (WQ). Once the hypothesis of WQ is made rather general features of QCD could be used to find out the peculiar properties of WQ useful in mak- ing the quark searches more effective. We shall discuss below two types of experiments - searches for frac- tional charges in stable matter [1,2] and production of quarks in beam-dump experiments. We give argu- ments that there exists a natural mechanism of Coulomb capture of WQ's by medium nuclei like nio- bium studied in ref. [1 ] which is absent in the case of iron investigated in ref. [2]. We shall argue also that due to the small absorption cross section for WQ the existing high-energy beam-dump experiments could be used to improve the sensitivity of quark searches in hadronic interactions by orders of magnitude. Let us first discuss the experiments [1,2]. It is im- portant for the comparison of the experiments in ref. [1 ] and in ref. [2] that though the masses of WQ are large the difference of masses of u- and d-WQ is small - about 2 MeV* 2. Indeed, one can view the WQ (in analogy tO the ,2 The presented arguments can be applied more or less also to other exotic objects, e.g. a bound state of a light quark and massive colour bosons or a zero electric charge heavy quark [7]. 230 0 031-9163/81/0000-0000/$ 02.75 © 1981 North-Holland