Beam Angle Scan

 During the RHIC low luminosity fill on March 5, 2012, the angle of the beams was changed (direction and particular beam not indicated to me). I was told that this occurred during Run 13065043, and according the Electronic ShiftLog, this was a 200 μrad angle change. At these small angles, using radians for the angle θ = sinθ = tanθ = slope. I used run 13065041 as a comparison for before the angle was changed and found the following:

Cuts: events with offline trigger ID=11, primary vertex has at least one daughter with a matching BEMC hit, |v_Z|<100 cm, r=√(x²+y²)<4 cm.

Results (errors are statistical only; statistics are number of vertices passing the cuts; units are [cm] or unitless):

run	statistics	x0	dx/dz	y0	dy/dz
13065041	49971	0.06417 ± 0.00051	0.001385 ± 0.000016	0.06132 ± 0.00077	0.000057 ± 0.000015
13065043	101085	0.06400 ± 0.00050	0.001381 ± 0.000013	0.06098 ± 0.00057	0.000030 ± 0.000014
Differences		0.00016 ± 0.00071	0.000004 ± 0.000021	0.00034 ± 0.00096	0.000026 ± 0.000021

So while looking for a change in slope on the order of 200 μrad = 0.0002 in slope, I see no significant change in slope in the horizontal (dx/dz) plane to with an error which is an order of magnitude better precision than that angle (~0.000021, or ~21 μrad), and a change in the vertical plane which is only slightly over 1σ from zero (no change).

 

-Gene

 

Other details:

Processing details: I used the following BFC chain:
"VtxSeedCalG B2012 ITTF ppOpt l3onl emcDY2 fpd ftpc ZDCvtx NosvtIT NossdIT trgd hitfilt VFMinuit3 BEmcChkStat Corr4"
And I processed all events from these files:
/star/data03/daq/2012/065/13065043/st_physics_13065043_raw_4010001.daq
/star/data03/daq/2012/065/13065043/st_physics_13065043_raw_5010001.daq
/star/data03/daq/2012/065/13065041/st_physics_13065041_raw_3010001.daq
/star/data03/daq/2012/065/13065041/st_physics_13065041_raw_4010001.daq
/star/data03/daq/2012/065/13065041/st_physics_13065041_raw_5010001.daq

The choice of offline trigger ID=11 excludes laser events in run 13065041, which had offline trigger ID=1, though only a single laser event passed the other cuts on BEMC matches and |v_Z|.

The choice of requiring a single BEMC match was motivated by these plots of BEMC matches vs. vertex rank (run 13065041 in the first plot, then run 13065043 in the second plot), showing that vertices without any BEMC matches are characteristically different (note: the plots saturate such that greater than 7 BEMC matches is set equal to 7). I saw no reason for otherwise excluding low-ranked, or low-multiplicity vertices. Low-quality events are anyhow considered by using the vertex-finer-assigned errors on reconstructed position ε_x,y when fitting for the BeamLine.

 

Here are plots of x and y vs. z (note the logarithmic color scale, units are [cm]), weighted by 1/ε_x and 1/ε_y respectively, and the histogram profile (including errors-on-the-mean) overlaid in black (note that there is a cut on r=√(x²+y²)<4 cm, which is used in the profile determination; i.e. the profiles are not made from the drawn 2D plots which would inherently restrict restriction to |x,y|<0.8 cm).

Run 13065041:

 

Run 13065043:

UU193

 Cuts used:

• st_centralpro files where available (all but the first 4 fills)
• rank > -3.8
• highest ranked vertex in the event (index==0)
• # of daughters > 1000 (mult>1000)

Here are the results for 61 fills:

I was a little concerned about the notably different intercepts for the second fill (16779), but this was a very low luminosity fill with 12x12 bunches and looking at the individual vertices I found the half mm shift to be consistent across all z, independent of luminosity or multiplicity, so I left this point in. I don't think there's any UPC data until the fourth fill (16780) anyhow.

It should be noted that some luminosity depedence is still believed to exist in this data, most likely from remaining uncorrected luminosity-dependent distortions in the TPC. This has been documented elsewhere, but the below plot shows the intercepts from the above data color-coded by luminosity to demonstrate that a correlation remains at the level of ~0.05 cm (half a mm). NB: available for luminosity in the ntuples was the average of the ZDC coincidence rates from events used, though some of the ZDC-based rates are also known to have issues, and were not used in the TPC luminosity-dependent distortion corrections.

 

-Gene