It has no specific equivalent in English universities, however "research doctors" is quite common.

I just done some research on the web for you, hoping that it may come to some use. You may take a look for yourself as it gives the explanation both in Italian and English:

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Doctors of Philosophy (abbreviated Ph.D. (American English) or PhD (British EnglishIn the English-speaking world it has become the most common denomination for a research doctorate and applies to graduates in a wide array of disciplines in the sciences and humanities.

The dottorato di ricerca (doctorate of research) is the highest Italian academic degree, the equivalent of a Ph.D..

The dottorato is a relatively recent addition to the Italian academic landscape, having been instituted in 1980.

I am adding explanations that I have found in an italian website.
"I dottorati di ricerca sono corsi universitari post laurea che si propongono di fornire una formazione specialistica finalizzata allo svolgimento di attività di riecerca di elevato livello. Hanno la durata di 3 o 4 anni e prevedono frequenza obbligatoria."
www.uomini.it/indice_analitico/1280.htm


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Note added at 28 mins (2007-09-14 09:49:23 GMT)
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"Il titolo di Dottore di ricerca si sonsegue con il superamento di un esame finale che consiste nella discussione della Tesi di Dottorato e dà accesso al mondo della Ricerca Scientifica prevalente in ambito academico ma, secondo una tendenza che va sempre più rafforzandosi anche nelle strutture produttivi e in centri di ricerca autonomi."
The reference is mentioned above.

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Note added at 30 mins (2007-09-14 09:51:23 GMT)
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Correction: the title is specialized researchers.

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Note added at 39 mins (2007-09-14 10:00:00 GMT)
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The term "specialized researchers" gives 13,700 Google hits. Here is an example of its usage:
"We Specialize in Producing Hyaluronic Acid And Have The Group Made Up of Specialized Researchers, Advanced Facilities..."
www.sulekhab2b.com/Company/Search/sodium-hyaluronate.htm

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Note added at 1 hr (2007-09-14 10:51:56 GMT)
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I am adding 3 examples that demonstrate that there are specialized researchers not only in the medical field but in other fields as well.
1. "Walter W. Muller, a specialized researcher in ancient Arabian History."
www.wikipedia.org/wiki/Year_of_the_Elephant
2. "specialized researcher in the nuclear energy field. C.J. DORAN. Methods for Atmospheric Radiometry."
http://taylorandfrancis.metapress.com/index/9HARXWV2GMHT90V1...
3. "However, this piece most likely would appeal to the specialized researcher in the arena of the United Nations Conferences and international law..."
www.vsos.umd.edu/gvpt/subpages/reviews.riles.htm

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Note added at 1 hr (2007-09-14 10:59:35 GMT)
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Correction: The last link is
www.bsos.umd.edu/gvpt/lpbr/subpages/reviews/riles.htm

This is appropriate for the UK, at least

Post-doctoral Research Assistant - INSTITUTE OF ASTRONOMY ...
No. (to be assigned by AAS) Post-doctoral Research Assistant INSTITUTE OF ASTRONOMY, CAMBRIDGE UNIVERSITY, UK Madingley Rd Cambridge, CB3 0HA ...
www.ast.cam.ac.uk/~optics/cirpass/job.html - 4k - Cached - Similar pages

Post-doctoral Fellow/ Research Assistant. GOLDSMITHS COLLEGE ...
Post-doctoral Fellow/ Research Assistant GOLDSMITHS COLLEGE, UNIVERSITY OF LONDON ... of Cambridge, commenced the AHRC four-year interdisciplinary research ...
doctorants.mshparisnord.org/actualite/article.php3?id_article=382 - 10k -

Template Oxford RA advert
Post Doctoral Research Assistant in the John Adams Institute to work on non-scaling FFAGs for Research, Oncology and Medicine ...
www.physics.ox.ac.uk/pp/jobs/emma-ra-ad.htm - 3k - Cached - Similar pages
[ More results from www.physics.ox.ac.uk ]

[PDF]
Job Descriptions Job Title Post-Doctoral Research Assistant for ...
File Format: PDF/Adobe Acrobat - View as HTML
Post-Doctoral Research Assistant. for European Union SIMAC project. (Semantic Interaction with Music Audio Contents). The SIMAC project is concerned with ...
www.elec.qmul.ac.uk/department/vacancies/2003/0312-simac.pd... -

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Note added at 9 hrs (2007-09-14 19:14:37 GMT)
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or Postdoctoral

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Note added at 2 days23 hrs (2007-09-17 09:01:30 GMT) Post-grading
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these are people who have completed their doctorate/gained a Ph.D. status, and are working in research within a university, so able to collaborate alongside professors, etc., with commercial firms and so on, as per context...