Monomer, polymer, chemically amplified negative resist composition, and resist pattern forming process

A monomer having the formula (A1):

• wherein a1 is 1 or 2, a2 is 0, 1 or 2,
• RA is hydrogen, fluorine, methyl or trifluoromethyl,
• R1 is halogen or a C1-C20 hydrocarbyl group which may contain a heteroatom,
• R2 and R3 are each independently hydrogen, a C1-C15 saturated hydrocarbyl group or C6-C15 aryl group, the hydrocarbyl group may be substituted with a hydroxy or C1-C6 saturated hydrocarbyloxy moiety, the aryl group may have a substituent, excluding that R2 and R3 are hydrogen at the same time, R2 and R3 may bond together to form a ring with the carbon atom to which they are attached, some —CH2— in the ring may be replaced by —O— or —S—, and
• W1 is hydrogen, a C1-C10 aliphatic hydrocarbyl group, C2-C10 aliphatic hydrocarbylcarbonyl group or optionally substituted C6-C15 aryl group.