Fixed Target Program in STAR Fixed-target running allows much higher rates without e-cooling at lower energies Top of  B range now ~ 400 MeV; extends  B on up to ~ 800 MeV BES analysis is focused on evolution of trends with √ s NN – with fixed target, very broad range: [few tens] <  B <800 MeV (!) Minimal impact on concurrent operation in collider mode

4.0 cm diameter Be Beam Pipe Al Beam Pipe  =1.0  =1.5  =2.0 T of BBC Place fixed target here (+2m ) Location of Fixed Target EAST Blue West Yellow

3 inch (7.62 cm) Aluminum beam pipe 38 mm gap beam Au targets Geometry with a slot rather than circular hole avoids an accidental evaporation of part of Au target in the event of a pre-fire of an abort kicker: kicker magnet, used to dump the beam, deflects in horizontal plane

Au beam Au beam halo Au target produced particles in collider mode produced particles in fixed-target mode beam pipe fixed-target events taken while waiting for collider mode collisions Concurrent running in STAR

What is composition of Au beam halo at BES energies? Red stars: 4.5 GeV Au+Al fixed-target charged pion multiplicity. Agreement with Glauber supports interpretation that with cut for central events, halo ions are mostly Au or mostly close in mass to Au. [From B. Haag for STAR collab., QM 12 poster] Final answer to this question remains a work in progress. Recent investigation by C-AD indicates that “distant” halo in past runs could not have been Au. In future runs, deliberate tuning of halo would be possible. Physics of interest in BES can tolerate a known variation in the Z of the incident beam.

Summary STAR will have ideal coverage (from mid-rapidity to target rapidity) in fixed-target mode, which is sufficient for various BES studies (proposal document provides various specifics, while more detailed analysis of limitations remains in progress). Main detectors TPC and TOF tested, work in progress on EEMC/BEMC, and trigger tracking, vertexing, PID reasonable (maybe improved with optimization). An internal fixed target can be used to study collisions between beam halo and target at injection energy, which will provide collisions at approximately √s NN of 5 GeV (data point missing from existing BES data). If successful – this may open a way for fixed target runs with other beams used in BES program in collider mode experiments (√s NN = 3.5 and 3 GeV,  B up to 800 MeV) Minimal or no impact on beam geometry Concurrent running mode