I do not have anything regarding specifically sexual abuse, but regarding neglect, which can be considered a form of abuse. But only regarding mental impact (not physical).
In the book "Brain Rules for Baby", updated and expanded version, by John Medina, chapter "Relationship", section "Limited time to establish perceptions of safety", there is the following text about massively neglected children:
In 1966, [...] the dictator Nicolae Ceausescu banned both contraception and abortion and taxed those who were childless [...]. As the birth rate rose, so did poverty and homelessness. Children were often simply abandoned. Ceausescu's response was to create a gulag of state orphanages, with children warehoused by the thousands. [...] The scenes in these orphanages were shocking. Babys were seldom held or given deliberate sensory stimulation. Many were found tied to their beds, left alone for hours or days, with bottles of gruel propped haphazardly into their mouths. Many infants stared blankly into space. Indeed, you could walk into some of these hundred-bed orphanages and not hear a sound. Blankets were covered in urine, feces, and lice. The childhood mortality rate in these institutions was sickening [...]. One remarkable study involved Canadian families who adopted some of these infants and raised them back home. As the adopted children matured, researchers could easily divide them into two groups. One group seemed remarkably stable. Social behaviour, stress responses, grades, medical issues - all were indistinguishable from healthy Canadian controls. The other group seemed just as remarkably troubled. They had more eating problems, got sick more often, and exhibited increasingly aggressive antisocial behaviours. The independent variable? The age of adoption. If the children were adopted before the fourth month of life, they acted like every other happy kid you know. If they were adopted after the eighth month of life, they acted like gang members. The inability to find safety through bonding, by a specific age in infancy, clearly caused immense stress to their systems. [...]
The study on which this text is based is found in the book's references:
Fisher, L. “Problems Reported by Parents of Romanian Orphans Adopted to
British Columbia.” Int’l J. of Behav Dev 20, no. 1 (1997): 67-82.
Although sexual abuse is not mentioned, there seems to me a bigger picture that also concerns the scope of your question. I put it like that:
If the abuse happened very early in life (before 4 months old), it might not have any (lasting) effect on mental development at all.
Although, if you think about it, doing something to the child (abuse) might have a different effect than not doing anything to/with the child (neglect). They will probably imprint differently in the memory of the child. On the other hand, it is known that memories of the earliest part of life do not survive long, they will at some point be completely deleted/overwritten.
Regarding your specification "inappropriate touching":
It is known that infants need touch for proper brain development. My guess is that at infancy, there is no such thing as "inappropriate touching" because infants have no notion of being a self and of what is appropriate. Of course there is touching that leads to pain and touching that does not, and there are probably body regions which, when touched, give a more pleasurable experience than others. But other than that, I guess that in that age, touch is simply touch.