OSLC REST API: How to set a "drop down list" attribute programmatically, for ex: "Severity", for a RTC WI.

Below is the code used to construct Work item programmatically. 

     String sStartOSLC = "<oslc_cm:ChangeRequest";
  String sEndOSLC = "</oslc_cm:ChangeRequest>";
        String xmlNS =  " xmlns:dc=\"http://purl.org/dc/terms/\"   xmlns:rtc_cm=\"https://jazz.net/xmlns/prod/jazz/rtc/cm/1.0/\"  xmlns:rdf=\"http://www.w3.org/1999/02/22-rdf-syntax-ns#\"  xmlns:oslc_cm=\"http://open-services.net/xmlns/cm/1.0/\" xmlns:oslc_cmx=\"http://open-services.net/ns/cm-x#\">";
        String sType = "<rtc_cm:type rdf:resource=\""+sType_data+"\"/>";       
        String sTitle = "<dc:title>"+"A New Defect for OSLC_5"+"</dc:title>";
        String sSeverity = "<oslc_cmx:severity>"+"severity.literal.l3"+"</oslc_cmx:severity>";       
        String sPriority = "<oslc_cmx:priority>"+"priority.literal.l3"+"</oslc_cmx:priority>";
        String sProjectArea = "<dc:projectarea>"+"Test RTC (Project)"+"</dc:projectarea>";
        String steamArea = "<dc:teamarea>"+"Test RTC (Project)"+"</dc:teamarea>";
        String sDescrption = "<dc:description>"+"OSLC Rest Service Test Defect Creation_5"+"</dc:description>";       
              
          String sOslcXML = sStartOSLC+xmlNS+sTitle+sType+sSeverity+sPriority+sProjectArea+steamArea+sDescrption+sEndOSLC;
        HttpEntity myEntity = new StringEntity(sOslcXML);
  httppost.setEntity(myEntity);

Using the above code I am able to create a workitem programmatically. The problem is only Text area fields like Title, Description are populated but it could not populate Drop down values like Severity, Owned By etc., 

Could anybody help me find out why dropdown fields like Severity, FiledAgainst are not set programmatically