Evaluation of [11C]RO6924963, [11C]RO6931643, and [18F]RO6958948 as Tracers for Positron Emission Tomography (PET) Imaging of Tau in Healthy and Alzheimer's Disease (AD) Participants

Inclusion Criteria: Inclusion Criteria for All Participants * Agreement to use highly effective contraception measures * If participants are on any concomitant medication, the indication and dosage of these medicines should be stable for at least 4 weeks prior to study start with the expectation that no relevant changes in use or dose will occur throughout the study * Body mass index (BMI) between 18 and 32 kilograms per square meter (kg/m\^2) * Weight less than or equal to (\</=) 300 pounds (lb) Inclusion Criteria for Healthy Control Participants * Healthy "young" control participants aged 25-40 years or healthy "elderly" control participants aged greater than or equal to (\>/=) 50 years * Normal cognitive function, including a normal Mini Mental State Examination (MMSE) score as judged by the investigator * Healthy control participants who participate in Part 2B: must be less than (\<) 195 centimeter (cm) (6 feet, 5 inches) tall in order to accommodate the whole body scanning Inclusion Criteria for Participants with a Diagnosis of Probable AD * Diagnosis of probable AD, according to the National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association criteria * Participants aged \>/= 50 years * A study partner able to accompany the participant to all visits and answer questions about the participant * MMSE score between 16 and 26, inclusive Exclusion Criteria: Exclusion Criteria for All Participants * History or presence of a neurological diagnosis other than AD that may influence the outcome or analysis of the scan results; examples include but are not limited to stroke, traumatic brain injury, space occupying lesions, non-Alzheimer's tauopathies, and Parkinson's disease * Participants with a medical history that includes known autosomal dominant AD mutations in amyloid precursor protein (APP) or presenilin (PS1, PS2) or mutations in genes that cause other types of autosomal dominant familial dementia * History or presence of any clinically relevant hematological, hepatic, respiratory, cardiovascular, renal, metabolic, endocrine, or central nervous system disease or other medical conditions that are not well controlled, may put the participant at risk, could interfere with the objectives of the study, or make the participant unsuitable for participation in the study for any other reason in the opinion of the principal investigator * Clinically relevant pathological findings in physical examination, electrocardiogram, or laboratory values at the screening assessment that could interfere with the objectives of the study * Known history of clinically significant infectious disease including acquired immunodeficiency syndrome (AIDS) or serological indication of acute/chronic hepatitis B or C or human immunodeficiency virus infection * Pregnancy or lactation * Unsuitable veins for repeated venipuncture * Current symptoms of allergy and/or severe allergy to drugs in medical history * Alcohol consumption that averages \>3 drinks daily or regular smoker (\>10 cigarettes, \>3 pipefuls, or \>3 cigars per day) * Coffee (or tea) consumption \>10 cups per day or methylxanthine-containing drinks \>1.5 liters per day (L/day) * Have received an investigational medication within the last 3 months or 5 times (x) the elimination half-life, whichever is longer, prior to Day 1 (i.e., enrollment) Exclusion Criteria Related to Trial Procedures * Presence of pacemakers; aneurysm clips; artificial heart valves; ear implants; foreign metal objects in the eyes, skin, or body, or any other circumstance (e.g. claustrophobia) that would contraindicate a magnetic resonance imaging (MRI) scan * For participants of Part 1 and Part 2A, any contraindications to arterial cannulation Exclusion Criterion for Participants with Probable Alzheimer's Disease * Has received treatment that targeted amyloid-beta or tau within the last 24 months