Luminosity leveling is performed in the ALICE experi-ment of the Large Hadron Collider (LHC) in order to limitthe event pile-up probability, and ensure a safe operation forthe detectors. It will be even more important during Run3 when 50 KHz Pb ion-Pb ion (Pb-Pb) collisions will bedelivered in IP2. On the ALICE side, it is handled by theALICE-LHC Interface project, which also ensures an onlinedata exchange between ALICE and the LHC. An automated luminosity leveling algorithm was developed for the proton-proton physics run, and was also deployed for the Pb-Pb run with some minor changes following experience gained. The algorithm is implemented in the SIMATIC WinCC SCADA environment, and determines the leveling step from measured beam parameters received from the LHC, and the luminosity recorded by ALICE. In this paper, the softwarearchitecture of the luminosity leveling software is presented,and the performance achieved during the Pb-Pb run and Vander Meer scans is discussed.peer-reviewe

De Cataldo, Giacinto

Franco, Antonio

Hostettler, Michi

ICALEPCS2019

Valentino, Gianluca

Visnyei, Orsolya

English

OAR@UM

PERFORMANCE OF THE ALICE LUMINOSITY LEVELING SOFTWAREARCHITECTURE IN THE Pb-Pb PHYSICS RUNG. Valentino∗, University of Malta, Msida, MaltaG. De Cataldo1, A. Franco1, M. Hostettler, CERN, Geneva, SwitzerlandO. Visnyei, Hungarian Academy of Sciences, Budapest, Hungary1also at INFN, Bari, ItalyAbstractLuminosity leveling is performed in the ALICE experi-ment of the Large Hadron Collider (LHC) in order to limitthe event pile-up probability, and ensure a safe operation forthe detectors. It will be even more important during Run3 when 50 KHz Pb ion-Pb ion (Pb-Pb) collisions will bedelivered in IP2. On the ALICE side, it is handled by theALICE-LHC Interface project, which also ensures an onlinedata exchange between ALICE and the LHC. An automatedluminosity leveling algorithm was developed for the proton-proton physics run, and was also deployed for the Pb-Pb runwith some minor changes following experience gained. Thealgorithm is implemented in the SIMATIC WinCC SCADAenvironment, and determines the leveling step from mea-sured beam parameters received from the LHC, and theluminosity recorded by ALICE. In this paper, the softwarearchitecture of the luminosity leveling software is presented,and the performance achieved during the Pb-Pb run and Vander Meer scans is discussed.INTRODUCTIONA Large Ion Collider Experiment (ALICE) [1] is opti-mized to study the collisions of nuclei at ultra-relativisticenergies, which provide a phase of matter known as thequark-gluon plasma. It is one of the major experiments ofthe CERN Large Hadron Collider (LHC) [2], which is a27-km long particle accelerator designed to collide protonsand heavy ions with center-of-mass energies of √𝑠 = 14 TeVand 2.759 TeV/nucleon, respectively. The LHC Interfaceproject [3] (LHC_IF) ensures the online data exchange be-tween the LHC and ALICE, as well as the safe operation ofthe complex experiment-collider interface.One of its main tasks is to coordinate the luminosity lev-elling, which is necessary in order to limit the event pile upprobability to a few percent. In the 2018 heavy-ion run, theLHC could provide up to 6×1027 cm−2s−1 of instantaneousluminosity, however in the ALICE experiment this valuehad to be reduced through luminosity levelling to a value of1×1027 cm−2s−1 by applying a beam-beam separation in thehorizontal plane of up to several 𝜎 (beam size units). Whencollisions between the beams are established and the LHC isin stable beams mode, a controlled and automatic luminosityramp up kicks in to reach the target luminosity defined byALICE. During the remainder of the fill, minor corrections∗ gianluca.valentino@um.edu.mtare applied to the beam separation in order to maintain theluminosity at a constant value. Luminosity levelling wasalso required during special very low luminosity runs in or-der to measure and map the distortions in the ALICE TimeProjection Chamber with the two 𝐵-field polarities.In this paper, following an description of the luminositylevelling procedure, an overview of the software architectureis provided, and the operational experience with levelling inthe Pb-Pb physics run in 2018 is presented and discussed.LUMINOSITY LEVELLING PROCEDURELuminosity levelling via beam separation was first carriedout in ALICE with proton-proton collisions in May 2011 [4],and for the first few years of operation made use of a fixedstep size in 𝜎 to displace the beams until the target luminos-ity was reached. In 2015, due to the higher beam intensityand the running in main-main beam mode, the fixed stepsize was deemed no longer viable due to the long and notalways convergent levelling procedure. As a result, the LHCmachine operator was required to manually steer the beamsuntil the target luminosity was reached, resulting in a lossof data taking time and luminosity overshooting which candamage ALICE’s detectors. Therefore, the luminosity level-ling procedure was upgraded in 2016 to calculate a dynamiclevelling step size based on an inversion of the luminosityformula and taking into account measured beam and ma-chine parameters such as the bunch intensities, number ofcolliding bunches and 𝛽∗ (which defines the beam size atthe ALICE experiment) [5].The following intervals of validity for Δ𝛿 with the corre-sponding values of Δ𝑑 were defined:if Δ𝛿 ≥ 0.52; Δ𝑑 = 0.5if Δ𝛿 ≥ 0.27 and Δ𝛿 < 0.52; Δ𝑑 = 0.25if Δ𝛿 ≥ 0.07 and Δ𝛿 < 0.27; Δ𝑑 = 0.06if Δ𝛿 < 0.07; Δ𝑑 = 0.025SOFTWARE ARCHITECTUREThe LHC_IF control software is implemented in theSIMATIC WinCC SCADA environment [6] and comprisesover 200 control processes, called managers. LHC_IF sub-scribes to the beam conditions published by the LHC viathe data interchange protocol (DIP) [7]. Depending on theparticle type (protons or heavy ions), the geometric beamProceedings of ICALEPCS2019, New York, NY, USA - Pre-Release Snapshot 11-Oct-2019 14:00 UTCMOMPR0060ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Data Analytics!"#$%&#'#$"("')*%!"#$%&#'(#)&'&()*+,-.'&$/(+'0,(*+!(-1+(#(.20+#,-./"%&#'#$"("')*%#"1!(.+34+$3//&5&#2+!"#$%()+&#+678*+9:*++$.3))&#2+-#2/(*;(<(//&#2+)'(,+=56#)'-#'-#(3")+/"1&#3)&'0*>-.2(' /"1&#3)&'0>&1(+-<(.-2&#2+34+&#)'-#'-#(3")+/"1&#3)&'0+-#5+!"#$%+&#'(#)&'&()?31,"'-'&3#+34+=@")&#2+&#<(.'(5+/"1&#3)&'0+43.1"/-A('(.1&#-'&3#+34+=54.31+/33BC",+'-!/(!"#$%&'()*)((+,-'./,01/('23,3-)10",".1"2%3'4$%5670",".1"2%3'4$%8597:;"/(%(4%567Figure 1: Schematic of the software architecture of the luminosity levelling procedure implemented in the SIMATICWinCCSCADA environment.emittance is computed via the well-known formula from thebeam energy, atomic number and atomic mass. The 1 𝜎beam size is then obtained via the square root of the productof the 𝛽∗ at IP2 received from the LHC and the geometricbeam emittance. It is assumed that the beams are round, i.e.the beam size is identical in both the horizontal and verticalplanes.To prevent oscillation in the Δ𝛿 calculation and, conse-quently, in the Δ𝑑, the instantaneous luminosity and thebunch intensities are averaged over the previous 5 s and 10 srespectively. A hysteresis interval of ± 0.005 𝜎 is appliedon each Δ𝛿 threshold. This hysteresis corresponds to theresidual instantaneous fluctuation of Δ𝛿. Finally, before itspublication, a check on the stability of Δ𝑑 over 5 s is alsoapplied. An overview of the processes and software architec-ture of the luminosity levelling procedure as implementedin the WinCC SCADA environment is shown in Fig. 1.Together with Δ𝑑, the other parameters leading to itsdetermination are also logged as data points with a 1 Hzsampling rate for offline analyses. These include Δ𝛿, thetime-averaged luminosity and bunch intensities, and all theparameters received from LHC. The panel through whichLHC_IF project members can view the status of the luminos-ity levelling is shown in Fig. 2. Apart from an instantaneousnumeric display of the most relevant parameters, a trendview showing their evolution over time is also provided. Thename of the current luminosity source used in ALICE andthe status of the levelling (“ACTIVE” and “ENABLE” flags)are also visible. From this panel, experts can also modifythe thresholds used to determine the Δ𝛿.EXPERIENCEWITH LEVELLING INPb-Pb PHYSICSThe Pb-Pb physics run at LHC commenced on 8th Novem-ber 2018 during Fill 7427, and lasted until 3rd December2018 during Fill 7494. A new optics was put in place in AL-ICE, with the smallest ever 𝛽∗ of 0.5 m. Initially, the sameluminosity levelling routine operational in the proton runwas used, with the appropriate corrections to take into ac-count the heavy ion particles. However, early on during therun, it was apparent that the luminosity in ALICE was lowerthan expected, and the luminosity levelling procedure wasslow to converge and level the beams to the target luminosity.Following some beam tests performed by LHC operation,the cause of this was put down to a beam deformation andreduced overlap at the IP introduced by strong local beta-tron coupling in IR2. This had the effect of increasing theapparent beam size at IP2 by approximately 50%. A cor-rection with skew-quadrupoles was implemented to solvethis issue during the ALICE polarity reversal, which tookplace mid-way during the physics run. Due to the issue withthe beam deformation, the steps proposed by the automaticluminosity levelling procedure, which assumed a nominal 1𝜎 beam size of 16𝜇m, resulted in smaller effective step sizesin terms of real beam 𝜎, leading to a very slow convergenceto reach the target luminosity and hence loss of physics datafor ALICE. As the problem was only understood much laterin the physics run, the ALICE Run Coordination took thedecision to ask the accelerator operation to perform the lev-elling using a backup routine [8], rather than use the stepsproposed by ALICE.An example of such a fill in which the levelling was per-formed in this manner is shown in Fig. 3. The step size(Δ𝑑 computed by ALICE based onΔ𝛿 is shown in magenta,however the actual steps applied by LHC are shown in yel-low. This plot also shows the difference between the Δ𝛿calculated assuming a nominal value of 𝜎, and using a 𝜎value which is 50% larger. Although there are differencesbetween these two values (which increase with beam separa-Proceedings of ICALEPCS2019, New York, NY, USA - Pre-Release Snapshot 11-Oct-2019 14:00 UTCData AnalyticsMOMPR0061ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Figure 2: View of the luminosity levelling Graphical User Interface in the LHC_IF WinCC expert application.tion), the plot demonstrates that as expected, once the targetluminosity is reached then Δ𝛿 = 0.The step applied by LHC is logged in units of mm, andtherefore in order to compare the step size applied with thechange in Δ𝛿 to validate the performance of the ALICE lev-elling routine (as was also done for proton-proton physics),the LHC step size was converted to units of 𝜎 assuminga 1-𝜎 value greater than nominal by a 50%. The result isshown in Fig. 4, with the error bars representing the stan-dard deviation. A linear fit applied to the data shows thatthe average change in Δ𝛿 scales less linearly with the stepsize applied by LHC than was observed for levelling duringproton-proton physics. This may be due to the fluctuationsin the apparent beam size at the IP over the entire heavy-ionphysics run either due to emittances or the strong betatroncoupling as mentioned earlier.Van der Meer scans [9], in which the beams are transver-sally scanned across each other to determine the cross-section, have already proven to be a useful diagnostic tool inorder to evaluate the performance of the luminosity levellingroutine during proton-proton operation. The Van der Meerscans performed in ALICE during the heavy-ion run wereanalysed, and are shown in Fig. 5. At the moment when thebeams are colliding head-on, 𝛿𝑖𝑛𝑠𝑡, which is the beam sepa-ration with respect to the absolute luminosity, should be zero,which is in fact the case. The values of 𝐿0 and 𝐿𝑖𝑛𝑠𝑡 havebeen calculated assuming the measured beam emittance.CONCLUSIONThe LHC operation uses luminosity levelling via beamseparation to limit the event pile up in ALICE and ensure safeand correct operation of its detectors. In the 2018 Pb-Pb run,a procedure for calculating the levelling steps developed byALICE for the previous proton-proton run and implementedin the framework of the ALICE-LHC Interface software ar-chitecture was deployed. However, due to initially unknownissues with the effective beam size at IP2, the automatic lev-elling procedure which guarantees convergence to the targetluminosity without luminosity overshooting was replacedby a feedback loop running on the LHC side. In this paper,an overview of the software architecture used to implementthe ALICE levelling routine was presented, together withsome passive results obtained during the Pb-Pb physics run.In order to detect issues with the LHC beams which couldaffect the luminosity levelling procedure earlier, a possiblesolution is to conduct scans during the intensity ramp-upphase when the beam intensities are still low and it is possi-ble to collide the beams head-on without causing issues forALICE.ACKNOWLEDGEMENTSThe authors would like thank Jorg Wenninger for discus-sions concerning LHC operation during the Pb-Pb physicsrun.Proceedings of ICALEPCS2019, New York, NY, USA - Pre-Release Snapshot 11-Oct-2019 14:00 UTCMOMPR0062ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.Data Analytics0 100 200 300 400 500Time (s)0.00000.00020.00040.00060.00080.0010L)m nos (y [Hz/μbarn]LiLref0.00.51.01.52.0Levell ng separa( on [σ]F ll: 7471Δδ (nom nal σ)Δδ (meas)red σ)Ac( ve flagALICE proposed level s(epLHC appl ed s(ep s zeFigure 3: Luminosity levelling during Fill 7471, showing the step sizes proposed by ALICE and the actual steps applied byLHC.0.0 0.1 0.2 0.3 0.4 0.5Step size in units of σ a  lied by LHC0.00.10.20.30.40.5Average change in Δδ in units of σ measured by ALICELinear fit: m=0.81Δ c=0.0Figure 4: Linear fit applied to the averaged change in Δ𝛿measured following a step made by LHC.0 2000 4000 6000 8000Time (s)0.00000.00020.00040.00060.00080.0010Luminosity [Hz/μbarn]LiL00.00.51.01.52.02.53.03.54.0Absolute beam separation [σ]Fill: 7440δinstNominal separationFigure 5: Horizontal and vertical Van der Meer scans inALICE, showing the comparison between the nominal beamseparation and the 𝛿𝑖𝑛𝑠𝑡 beam separation measured by AL-ICE, as well as the luminosity.REFERENCES[1] “ALICE technical proposal for a large ion collider experimentat the CERN LHC”, ALICE Collaboration, Geneva, Switzer-land, Tech. Rep. CERN/LHCC/95-71 LHCC/P3, 1995.[2] O. S. Bruning et al., “LHC design report”, Geneva, Switzer-land, Tech. Rep. CERN-2004-003-V1, 2004, vol. 1.[3] G. de Cataldo, The ALICE-LHC Interface, ALICE Mat-ters, http://alicematters.web.cern.ch/?q=content/node/954, 2016.[4] F. Follin, D. Jacquet, “Implementation and experience withluminosity levelling with offset beam”, Proc. ICFA Mini-Workshop Beam-Beam Effects Hadron Colliders, pp. 183-187,2013, doi:10.5170/CERN-2014-004.183[5] G. de Cataldo, G. Valentino, A. Franco, R. Alemany, F. Follin,M. Hostettler, J. Wenninger, “An upgraded luminosity levelingprocedure for the ALICE experiment”, IEEE Transactions onNuclear Science, vol. 66, no. 5, pp. 763-770, May 2019,doi:10.1109/TNS.2019.2907227[6] SIMATIC WinCC SCADA, https://w3.siemens.com/mcms/human-machine-interface/en/visualization-software/scada/pages/default.aspx[7] W. Salter, “LHC Data Interchange Protocol (DIP) Definition”,document no. 457113, EDMS, 2004.[8] M. Hostettler et al., “Online luminosity controland steering at the LHC”, in Proc. ICALEPCS2017, Barcelona, Spain, pp. 989-993, 2017, doi:10.18429/JACoW-ICALEPCS2017-TUSH201[9] S. White, R. Alemany-Fernandez, H. Burkardt, M. Lamont,“First luminosity scans in the LHC”, in Proc. InternationalParticle Accelerator Conference, Kyoto, Japan, MOPEC014,pp. 486-488, 2010.Proceedings of ICALEPCS2019, New York, NY, USA - Pre-Release Snapshot 11-Oct-2019 14:00 UTCData AnalyticsMOMPR0063ContentfromthisworkmaybeusedunderthetermsoftheCCBY3.0licence(©2019).Anydistributionofthisworkmustmaintainattributiontotheauthor(s),titleofthework,publisher,andDOI.

Performance of the ALICE luminosity levelling software architecture in the Pb-Pb physics run

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Performance of the ALICE luminosity levelling software architecture in the Pb-Pb physics run

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