Relative Luminosity - Thoughts From STAR Some Background Possible Causes of Rel. Lum. Diffs. Some Possibly Related Observations A Few Comments and Suggestions H. Spinka RSC Meeting 27 Jan Special thanks to J. Hays-Wehle, J. Seele, E. Aschenauer, and the STAR spin group

Some Background Relative luminosities for different spin states are important for high statistics spin measurements, such as inclusive jet production at STAR. Several years ago, problems with agreement of the relative luminosities from the STAR BBCs (beam-beam counters) and ZDCs (zero degree calorimeters) were found. For the 2-spin longitudinal asymmetry, R 3 = L ++ / L +-

Some Background (cont.) The preceding equations assumed σ ++ = σ -- and σ +- = σ -+. The problem with relative luminosities was that R 3,BBC ≠ R 3,ZDC for east-west BBC coincidences and east-west ZDC coincidences. Analysis of the run9 relative luminosity data for STAR was performed by Jim Hayes-Wehle, Joe Seele, et al. at MIT. –This used the accidentals and multiple hit corrections initially developed at CDF and D0 (first employed at STAR by Steve Trentalange for run6 relative luminosities). –J. Bantly et al., Fermilab technical report Fermilab-TM-1995 (June, 1997 unpublished). –D. Cronin-Hennessy et al., NIM A443, 37 (2000). The corrections took care of some, but not all, of the relative luminosity differences.

Some Background (cont.) Defining Then –ε(R 1 ) = R 1 = (L ++ + L -+ ) / (L +- + L -- ) –ε(R 2 ) = R 2 = (L ++ + L +- ) / (L -+ + L -- ) –ε(R 3 ) = R 3 = (L ++ + L -- ) / (L +- + L -+ ) –ε(R 4 ) = R 4 = L ++ / L -- –ε(R 5 ) = R 5 = L -+ / L -- –ε(R 6 ) = R 6 = L +- / L -- –ε(R 7 ) = R 7 = L ++ / L +- –ε(R 8 ) = R 8 = L -+ / L +- –ε(R 9 ) = R 9 = L ++ / L -+ –All statistical uncertainties are smaller than for the >900 runs used. Most of these asymmetries are clearly nonzero. All ratios are for E-W ZDC coinc. and E-W BBC coinc.

Some Background (cont.) Various other comparisons were made to BBC hits in the east detectors only or ZDC hits in the west detectors only, etc. –The best agreement was for BBC east-west coincidences and ZDC east singles. Various combinations of “scalar configurations” were also considered for the definition of east hits/singles or coincidences – this allows correction for some possible hardware problems. Documentation of this work has begun. Limited work on run 11 relative luminosities has occurred (Zilong Chang at TAMU).

Some Background (cont.) False asymmetries in inclusive jet production are shown above. –From a talk by Pibero Djawotho at DIS2011. –Uses BBC east-west coincidences for relative luminosity normalization, and finds that the false asymmetries are consistent with zero. –Note A L = (σ + - σ – )/(σ + + σ – ) and A LL us = (σ +– - σ –+ ) / (σ +– + σ –+ ), etc. –Suggests that the problem is with the ZDCs.

Possible Causes of Rel. Lum. Diffs. A number of possible causes for the differences in east- west coincidence relative luminosities were considered. These analyses have generally been performed by only one person and not checked. Beware!!! Possible cause 1: Nonzero A LL in the BBC and/or ZDC. –This would lead to ε(R 4 ) = 0 and ε(R 8 ) = 0, which are not true. –Conclude this is not the sole cause of the differences in relative luminosities. Possible cause 2: Nonzero A L in the BBC and/or ZDC. –Again, it would be expected that ε(R 8 ) = 0, which is not true. –Conclude this is not the sole cause of the differences in relative luminosities.

Possible Causes of Rel. Lum. Diffs. (cont.) Possible cause 3: Nonzero A L and A LL in the BBC and/or ZDC. –Again, it would be expected that ε(R 8 ) = 0, which is not true. –Conclude this is not the sole cause of the differences in relative luminosities. Possible cause 4: Nonzero transverse beam spin component, offset of the (projected) beam at the luminosity monitor, and a nonzero A N for the physics process(es) detected in the ZDC (or BBC). –This would lead to ε(R 4 ) – ε(R 5 ) – ε(R 6 ) = 0, which is not observed. –Conclude this is not the sole cause of the differences in relative luminosities. –Note an offset, not angle, is assumed. Does it matter???

Possible Causes of Rel. Lum. Diffs. (cont.) The beam position is monitored for each run at STAR. –Changes of ~ a mm are clearly seen. New ZDCs were installed before run11. –These were more carefully aligned. –With beam and ZDC position changes, it would be interesting to see if the pattern of the observed ε(R i ) changed from run9 to run11. –There may be evidence of changes when the rotators were adjusted as well – need to check. 200 → 500 GeV Reversal of STAR B field pp2pp

Possible Causes of Rel. Lum. Diffs. (cont.) Possible cause 5: Beam “migration” into following bunches. –It was assumed all bunches had the same intensity on average. Also, it was assumed a small fraction of the beam in one bunch would “migrate” to the following bunch. Effects at the abort gaps were ignored. –Assuming migration fractions a, b for the blue beam +, - bunches and c, d for the yellow beam +, - bunches, then for spin patterns “5” and “9”, –and for patterns “6” and “10”, –But ε(R 6 ) ≈ 0, so ε(R 4 ) should equal ε(R 5 ), which it does not. –Conclude this is not the sole cause of the differences in relative luminosities.

Some Possibly Related Observations The blue beam was always injected into RHIC first. –Generally it had a shorter lifetime. The beams interacted continuously with the H-jet target. –Bending in the DX magnet after this target may have led to a preferential loss of scattered beam for one spin state due to the nonzero A N for scattering on the jet. –This could have differed for the two beams because of differences in phase space or emittance. There may be after-pulsing or ringing of phototube signals for some of the ZDC and/or BBC counters. –This might result in undesired accidentals, and may extend beyond the following bunch (as described for bunch “migration” effects).

Some Possibly Related Observations (cont.) A possible cause of differences in relative luminosity could be a longitudinal polarization profile to the beam. –This was investigated by Sasha Bazilevsky for run9, 250 GeV running. No hint of a long. profile!! Thanks Elke Different acceptance for BBCs and ZDCs.

Some Possibly Related Observations (cont.) A possible cause of differences in relative luminosity could be a longitudinal polarization profile to the beam. –This was investigated by Dima Smirnov for run11, 250 GeV running. Thanks Elke

Some Possibly Related Observations (cont.) The ZDC and BBC acceptance of events along the beam is expected to be different. –Caused by different distances to the interaction region and angular regions covered. –This could have given systematic differences if the spin state sign was correlated with bunch length or to polarization direction change from front to middle to back of bunches. For transverse beam everywhere in RHIC, the blue beam makes a left bend after the STAR interaction region and the yellow beam makes a right bend. –Because of the Siberian Snakes, which flip the spin from up to down and vice versa, the only other region “equivalent” to STAR would be at 12 O’clock. The other 4 regions would all be opposite. –This may have led to more losses of beam scattered from residual gas in one spin state and beam, etc.

Some Possibly Related Observations (cont.)

I believe that for all spin patterns used in run9 (and run11?) –Blue beam had … or offset by 4 starting after the abort gap –Yellow beam had … or offset by 2 Then the effects of beam migration from one bunch to the following bunch would be different in the two beams. –For Blue, the + → – and – → + transitions occur about 3 times more frequently than the + → + or – → –. –For Yellow, the transitions occur approximately equally frequently! Do such migrated beam particles preferentially end up at the front or back of a bunch? –The acceptance for events along the beam of the ZDCs and BBCs are probably different – this could lead to different relative luminosities.

Some Possibly Related Observations (cont.) Another suggestion is that there is a non-negligible A LL in one or both of the luminosity monitors (based on HERMES results) – from Elke. Thanks Elke

Some Possibly Related Observations (cont.) Note studies of BBC – ZDC coincidences by Alice Bridgeman and Dave Underwood gave indications of double diffractive dissociation from measured asymmetries in run9, 500 GeV running. Thanks Elke

Some Possibly Related Observations (cont.) During longitudinal beam running at STAR or PHENIX, there are large horizontal transverse spin components between the DX magnets and the spin rotators. –This is near the ZDCs! –The beam size is also relatively large in the low - β quadrupoles, between the DX magnets and spin rotators, as well. This could allow for scattering from beam pipe walls, etc. –An offset of the beam relative to the up-down center of the ZDCs would be required to give an effect on the measured relative luminosity (as in the previous calculation for transverse spin effects). –Significant changes in the relative luminosity asymmetries might be observed with changes in rotator current – we have observed such changes, I believe. –The ZDC spacing is nearly the same as the bunch spacing, so secondaries from scattering off gas, etc. would arrive “in time”.

A Few Comments and Suggestions Perhaps try to correlate differences in relative luminosities between PHENIX and STAR –Probably concentrate on run9, 200 GeV runs. –Were the signs and magnitudes of the various ε(R i ) the same? –Use some other methodology? Consider adding 4 more spin patterns so that Blue would have etc. or offset by 2 and Yellow would have etc. or offset by 4. –This is just the reverse of the 4 spin patterns used in run9. –These spin patterns should be run for a sizeable number of fills. Cross check calculations mentioned in this talk, and extend them to include multiple effects.

A Few Comments and Suggestions (cont.) A number of years ago, it was found that the bunch lengths and transverse widths varied significantly for one or two fills studied. (C-A/AP/206) –It is unclear whether, or to what extent, these differences persist. –There may be systematic effects that could be uncovered by studying in detail the bunch properties correlated to the spins for many fills. These could conceivably be the cause of the differences in relative luminosity. Perhaps some period (a fill??) could be devoted to measurements with the beam offset at slightly different positions / angles within both STAR and PHENIX. –The effects on the relative luminosity asymmetries, ε(R i ), could be tested.

Backups

The Problem STAR physicists have analyzed relative luminosities from the BBCs and ZDCs. –These are needed to normalize physics asymmetries. –Various quantities (singles, coincidences) have been studied, and corrections for accidentals and multiple interactions (D0/CDF – type) have been made. –However, there have been differences among some of the relative luminosities observed. Attempts to explain these differences have not been successful so far: –A LL in the BBCs and/or ZDCs –A L in the BBCs and/or ZDCs –A N in the ZDCs and transverse spin components in the beam and beam offset at the ZDC(s) –Various combinations and other hypotheses H. Spinka RSC Meeting 28 Jan. 2011

A Couple Observations I believe that for all spin patterns used in run9 –Blue beam has … or offset by 4 starting after the abort gap –Yellow beam has … or offset by 2 Then the effects of beam migration from one bunch to the following bunch would be different in the two beams. –For Blue, the + → – and – → + transitions occur about 3 times more frequently than the + → + or – → –. –For Yellow, the transitions occur approximately equally frequently! Do such migrated beam particles preferentially end up at the front or back of a bunch? –The acceptance for events along the beam of the ZDCs and BBCs are probably different – this could lead to different relative lums. An alternate explanation may be PMT (either/or both ZDC / BBC) after-pulsing. Or both effects may be occurring.

The Request To help understand the cause of the differences in some relative luminosities, it would be good to have additions to the beam conditions from run9. –Among many effects tested, the spin pattern seems to show the largest impact on the differences in relative luminosity. Other parameters studied include: STAR magnet field direction Early vs. late in a fill Removing single bunches. –One possibility would be to add 4 more spin patterns so Blue would have etc. or offset by 2 and Yellow would have etc. or offset by 4. –This is just the reverse of the 4 spin patterns used in run9. –The effects on the relative luminosities could be observed, and perhaps the origin of the differences determined.

Some Possible Explanations (cont.) Consider the possibility of a transverse beam spin component plus beam offset at the ZDC. –There are solid angles times efficiency times cross sections for each beam L or R of the nominal beamline. –Maybe I have the signs correct???