1 S1,N1 Counters RIKEN/RBRC Itaru Nakagawa

2 Paddle right next to beam pipe

3 STAR Vernier Scan PHENIX Vernier ScanSTAR Vernier Scan Barely change PHENIX background counters Only minor rates increase observed in triplet counters

4 Beam Position at PHENIX during STAR Vernier Scan mm 500  m STAR Vernier Scan Some increase of triplet rates during STAR can be understood due to orbit drift in triplets at PHENIX to compensate beam steering at STAR

5 Vernier Scan South S1 behaves pretty much like S4~S6 (shielded) counters. Rates goes down as steer beams away from collision unlike triplet counters whose rates shoots up by steered beam hits beam pipe. S1 triplets Shielded counters

6 Vernier Scan North N1 behaves similarly to S1 Counter.

7 Hypothesis (1) Beam halo is developed by collision and it circulates RHIC ring several times before it becomes backgrounds in PHENIX –If so, then 1/2 of the backgrounds generated in PHENIX area should be responsible in STAR collision – Rates PHENIX #1&4~6 background counters gets much less than 1/2 once PHENIX is not in collision, but STAR is still in collision. –Backgrounds are generated right after collision. Mostly downstream of IP ? Then beam halo is effectively quenched after collimator? Counters #4~6 are well shielded from backgrounds come from upstream beam line. Thus their rates are believed to be dominated by collision ralated.

8 Hypothesis (2) The tunnel shielding is not shielding well around beam pipe. Large flux of shallow scattering beam pipe scraped particles at triplets get into IR through the shield hole around the beam pipe. –If so, then S1,N1 counters re-located in MuID square hole should see backgrounds generated at triplets and therefore should behave like triplet counters. –S1, N1 counters behave pretty much like shielded counters #4 ~ #6 which are well shielded from triplet origin backgrounds, rather than triplet counters. –Materials such as cryostat wall are thick enough to absorb 100 GeV protons?

9 From these facts, can we identify the fraction of rates originated from IR in S1,N1 counters? The rates in S1,N1 are the sum of rates come from incoming and outgoing flux. When steers beam, rates go down but one can draw the scenario outgoing rates drop defeats increasing beam scraping background rates from triplet. Can we conclude S1, N1 rates are dominated by outgoing collision related products? Indeed beam halo profile could be quenched by not colliding after steering, so the characteristics of flux comes from incoming beam could be also quenched. So I am not sure if I can rush to conclude like above. However, the effect of small effect seen from STAR collision indicates backgrounds generated by incoming beam halo is negligible. Isn’t it?

10 Additional Shielding around beam pipe in tunnel helps? If flux go though MuID square hole is dominated by outgoing, the additional shielding around beam pipe may not be too much of help.

11 Paddle on the floor

12 Vanier Scan Collimiator offHorizontal SteerVertical Steer 1/14 When beams are steered Triplet Counters shoots up Inner Counters goes down Triplet sees incoming beam Inner Counters see collision

13 Conclusion CollisionMy HypothesisMeasured PHENIX & STAR11 STAR1/21/14 The beam halo is developed by collision. If the beam halo is able to go around the RHIC, then the backgrounds should remain about 1/2 once beams are steered off at PHENIX, since STAR is still under collision. 1.Beam halo developed at STAR affects very small at PHENIX 2.It may exclude the scenario the backgrounds caused by beam halo goes around RHIC. May be not that simple. 1. may be just a coincidence of betatron tune. 1.Backgrounds are generated by outgoing beam?

14 How N1 should behave if it is dominated by incoming beam? Should be rather proportional to triplet counters. N4,N5,N6 are well shielded. If N1 is not well shielded, it should behave just like triplet counters.