IP Crossing Angles in PP at 250 GeV Run11

Crossing angles have the potential to reduce the maximum luminosity

Error source for luminosity calibration 14 vernier scans in run11 analyzed

8 PHENIX scans 6 STAR scans Compared DX and Q1 BPMs

Data set of interest: all vernier scans of run11

8 PHENIX scans

6 STAR scans

Beam sizes between 0.15 and 0.18 mm (from vs)

Effective cross section:

Rmax

depends on crossing angles

Use DX BPMs to calculate crossing angles and Q1 BPMs to check/confirm results.

Orbit thru IP8 (LISA)

IR beam position at PHENIX

Beam position at IR extrapolated from the two sides

Expect all identical (/plane), close to 0

The difference should be 0 (head-on collisions!) regardless of absolute positions

Vertical Q1 results seems to be the “outlier” (with a constant offset of 0.6 mm) => which BPM?

Error +/- 0.2 mm on position (=> 0.025 mrad)

Individual BPMs around PHENIX Even with a crossing

angle DX and Q1 BPMs should line up: Q1-7, DX-7, DX8, Q1-8

They line up in the yellow horizontal plane (minus the 1st 2 stores), still reasonable in blue horizontal

Convincing line-up in yellow vertical plane

Candidate: blue vertical Q1 BPMs (7 o'clock side?)

IR beam position at STAR As before:

Difference between blue and yellow should be 0

Horizontal Dx and Q1 BPMs agree (? : one exception)

Vertical Q1 BPMs have a constant offset of ~1.5 mm

Seems to come from the yellow Q1 BPMs

Use difference b-y as error: +/- 0.1 mm, +/- 1.5 mm seems excessive

?

Individual BPMs around STAR

Horizontal

Vertical

“good”

“bad”

Candidate: yellow vertical 5 o'clock side

Using DX and Q1 BPMs to calculate crossing angles

PHENIX “collision” angle =

blue-yellow crossing angle

Bunch length between 4 ns and 11 ns

Resulting correction: 0.96 to 0.99

Using correction factor:

1/sqrt(1+(sigZ/sigY*phi)^2)

Crossing Angles contd.

STAR Collision

angle = blue – yellow

Bunch length from 4 ns to 10 ns (FWHM)

0.86-0.99 reduction factor !

Old angle scan (#7112)

Angle measurements from DX BPMs

SigY result is within range of beamsizes measured in run5 using vernier scans!

Effect is small because of relatively large beams.

Summary

• Crossing angles have the potential to reduce maximum luminosity (up to 10%). Largest single error for cross section evaluation.

• DX BPMs look 'reasonable' while vertical Q1 BPMs seem to have a problem. (keep in mind though that physical offsets in the DX BPMs are by design the same for blue and yellow! This indicates an offset problem in the Q1 BPMs but a more generic disagreement, level of 100-200 microns, in the DX).

• Effect of crossing angles on lumi varies strongly depending on bunch length and beam size.

• Suggest to do angle scans routinely before fixing the target orbit for collisions at store.

Example of a quick angle scan with 100 GeV pp in run12

Vertical scan caused some background (should not be done automatic!)Horizontal angle found to be ~0.03 mrad from optimum, effect small with run12 beam conditions.

Setting up PHENIX for 0.12 mrad horizontal crossing angles

Crossing angle changes cause beam motion of 34 microns/microrad at the Q3 magnet with this lattice (here 4.5 mm)

Background needs to be controlled while changing the angle

Collimatoradjustments

Can't go in one step, collimators need adjustments at several angles in between (risk of beam abort due to permit and/or detector trips)Stopped at an angle where I could maintain same background conditions for STAR