Answer the following question: Given the following context:  After a tour of the UK performing Dark Side, Pink Floyd returned to the studio in January 1975 and began work on their ninth studio album, Wish You Were Here. Parsons declined an offer to continue working with them, becoming successful in his own right with the Alan Parsons Project, and so the band turned to Brian Humphries. Initially, they found it difficult to compose new material; the success of The Dark Side of the Moon had left Pink Floyd physically and emotionally drained. Wright later described these early sessions as "falling within a difficult period" and Waters found them "tortuous". Gilmour was more interested in improving the band's existing material. Mason's failing marriage left him in a general malaise and with a sense of apathy, both of which interfered with his drumming.Despite the lack of creative direction, Waters began to visualise a new concept after several weeks. During 1974, Pink Floyd had sketched out three original compositions and had performed them at a series of concerts in Europe. These compositions became the starting point for a new album whose opening four-note guitar phrase, composed purely by chance by Gilmour, reminded Waters of Barrett. The songs provided a fitting summary of the rise and fall of their former bandmate. Waters commented: "Because I wanted to get as close as possible to what I felt ... [that] indefinable, inevitable melancholy about the disappearance of Syd."While Pink Floyd were working on the album, Barrett made an impromptu visit to the studio. Thorgerson recalled that he "sat round and talked for a bit, but he wasn't really there". He had changed significantly in appearance, so much so that the band did not initially recognise him. Waters was reportedly deeply upset by the experience. Most of Wish You Were Here premiered on 5 July 1975, at an open-air music festival at Knebworth. Released in September, it reached number one in both the UK and the US.  answer the following question:  What is the last name of the band member who was upset when a past band member visited the studio and no one initially recognized him?
Answer:
Waters