Sharing ATLAS data and research with young students

Transcription

1 Nuclear Physcs B Proceedngs Supplement 00 (2014) 1 7 Nuclear Physcs B Proceedngs Supplement Abstract Sharng ATLAS data and research wth young students Maken Pedersen, Fard Ould-Saada, Magnar K. Bugge on behalf of the ATLAS collaboraton and IPPOG Internatonal Masterclasses maken.pedersen@fys.uo.no Unversty of Oslo In recent years the Internatonal Masterclasses (IMC) featured the use of real expermental data as produced by the Large Hadron Collder (LHC) and collected by the detectors. We present ATLAS-based educatonal materal usng these data allowng hgh-school students to learn about propertes of known partcles and search for new phenomena. The ambton to brng to the classrooms mportant LHC dscoveres s realsed usng the recent dscovery of the Hggs boson. Approxmately 10% of the ATLAS dscovery data are made avalable for students to search for the Hggs boson: 2 fb 1 at 8 TeV for the Z path, and 1 fb 1 at 7 TeV for the W path, n the 2014 verson of IMC. The Hggs study samples consttute one thrd of the total sample ncludng Z, W and other low mass resonances. The educatonal materal s tuned and expanded to follow LHC heartbeats. Keywords: ATLAS, IPPOG, outreach, partcle physcs 1. Introducton The Internatonal Masterclasses (IMC) [1, 2, 3] s a partcle physcs outreach program run by the Internatonal Partcle Physcs Outreach Group (IPPOG) [4]. The am of the program s to provde an opportunty for 15- to 19-year old school students to dscover partcle physcs through hands-on measurements wth real LHC data. Several dfferent measurements are avalable, of whch two nvolve ATLAS data. These two, the so-called W path [5, 6] and Z path [7, 8] measurements, are the subjects of ths artcle. The goal of the Z path measurement s to rely on the nvarant mass concept to dentfy and measure propertes of known partcles, such as the Z boson, nferred from the decay products, pars of leptons. When a heavy gauge boson Z wth mass 1 TeV s mxed wth the real data, the smulated sgnal shows up n the d-lepton mass dstrbuton, to the surprse of students, who realze that they have mastered a dscovery tool. They go on and apply the same technque to d-photons and pars of d-leptons to search for the Hggs boson. To help the buld-up and dsplay of the nvarant mass dstrbutons, we developed OPloT, a scalable, php-based web plottng tool for submsson and automatc combnaton of all measurements performed. Ths allows for prompt access of results for further dscusson wthn nsttutes and durng vdeoconferences. The W path deals wth the structure of the proton by comparng the numbers of W + and W, and the search for the Hggs nto a par of W bosons by measurng the angle between the leptons stemmng from the W bosons. 2. Physcs background The Standard Model (SM) descrbes the fundamental nteractons (except gravty) between the matter partcles, the leptons (such as the electron and the muon) and quarks, as medated by certan force carrer partcles. The electromagnetc force s medated by the photon, the weak force by the W ± and Z 0 bosons, and the strong force by 8 gluons.

2 Maken Pedersen, Fard Ould-Saada, Magnar K. Bugge / Nuclear Physcs B Proceedngs Supplement 00 (2014) At the LHC, protons are collded head on at very hgh energy, and the partcles emergng from the collsons are measured by nstruments such as the ATLAS detector. The recorded data are used to mprove our understandng of nature at the smallest dstance scales. Many pp nteractons are n fact really nteractons between ndvdual proton consttuents. Such nteractons are referred to as hard scatterngs. As the proton conssts of up and down quarks held together by gluons and as gluons can splt nto quark-antquark pars, the proton conssts effectvely of both the up and down quarks (called valence quarks), gluons, and addtonal quarks and antquarks from gluon splttngs (called sea quarks). The Parton Densty Functons (PDFs) descrbe how often each of the varous proton consttuents wll go nto a hard scatterng wth a gven momentum fracton x (the fracton of the proton momentum carred by the consttuent). They summarze the proton structure as seen on a gven energy scale. The W ± and Z 0 bosons may be produced n pp collsons va hard scatterngs between quarks and antquarks. For example, an up quark and a down antquark may nteract to form a W + boson, or a down quark and an up antquark may nteract to form a W boson. An up or down quark may nteract wth ts respectve antquark to form a Z 0 boson. The W ± and Z 0 bosons are short lved (wth lfetmes of the order s), and decay n practce mmedately after ther creaton. They can therefore only be observed va ther decay products, and decays nto leptons are deal for such detecton. The decays of the W ± and Z 0 bosons nto leptons provde characterstc expermental sgnatures. The decay of the Z 0 boson nto a par of oppostely charged leptons gves such a par among the partcles emergng from the collson (among the fnal state partcles), where each lepton wll typcally have a large transverse momentum (perpendcular to the beam lne). The decay of a W + or W boson nto a charged lepton and a neutrno gves one charged lepton wth a large transverse momentum. In addton, there wll be a momentum mbalance n the transverse plane due to the neutrno, whch s not measured drectly by the detector. Ths momentum mbalance motvates the calculaton of the mssng transverse momentum, whch can be thought of as an ndrect measurement of the neutrno s transverse momentum. Wth a much smaller probablty, the Hggs boson may also be produced n a pp collson, manly through gluon-gluon fuson va ntermedate states nvolvng the heavy top quark. The Hggs boson can decay to fnal states wth two photons or four leptons va ntermedate states of heavy partcles, W + W and tt, or Z 0 Z 0 respectvely. The Hggs boson decays to fnal states wth two photons or four leptons are golden channels for the detecton of the Hggs boson at the LHC. Another mportant channel s the Hggs decay to a fnal state wth two oppostely charged leptons and two neutrnos, whch happens va a W + W ntermedate state. The branchng fractons of a partcle quantfy how often the partcle wll decay nto each of the dfferent possble sets of decay products. The Z 0 boson decays 6% of the tme to an electron-postron par (e + e ) or a muon par (µ + µ ). The Hggs boson decays to two photons or four leptons (e + e and/or µ + µ ) 0.2% and 0.01% of the tme respectvely. There are many processes occurrng frequently n pp collsons that nvolve nether Z 0 nor Hggs bosons, and these consttute backgrounds to Z 0 and Hggs searches. Lookng for the Z 0 boson and n partcular the Hggs boson s therefore a bt lke lookng for the proverbal needle n the haystack. A good decay channel for a Z 0 or Hggs boson search must therefore not only have a szeable branchng fracton, but must also provde an expermental sgnature whch s as easly as possble dstngushed from those of the most frequent background processes. The demand for such a clean expermental sgnature favours decays nto leptons and photons. 3. The W path and the structure of the proton The electrc charge of a W boson produced n a pp collson depends on the speces of quark and antquark that entered nto the hard scatterng. An up quark and a down antquark wll produce a W + boson, whle a down quark and an up antquark wll produce a W boson. Ths follows smply from the conservaton of electrc charge. Furthermore, the decay of a W boson nto a charged lepton and a neutrno leads to a fnal state charged lepton of the same charge as the W boson. Hence, by studyng the charges of fnal state charged leptons from W boson decays n pp collsons, one can learn about the content of quarks and antquarks nsde the proton,.e. about the structure of the proton. In the W path measurement, the students learn how to dentfy collson events where a W boson may have been produced (a W event ) wth a subsequent decay nto a charged lepton (electron or muon) and a neutrno. As mentoned before, such events wll n general contan one charged lepton wth a large transverse momentum and large mssng transverse momentum caused by the neutrno. Ths s exactly what the students need to look for to dentfy W events when they are analyzng the events one by one n the event dsplay program MINERVA [9]. The students then count the number of

3 Maken Pedersen, Fard Ould-Saada, Magnar K. Bugge / Nuclear Physcs B Proceedngs Supplement 00 (2014) Fgure 1: Example results from the 2014 W path Masterclasses. The charge rato R ± = N W + /N W s shown wth one pont for each partcpatng nsttute. The average, takng nto account all submtted data, s 1.33 ± 0.20, whch can be compared to the offcal ATLAS number 1.50 ± dentfed W events where the fnal state charged lepton s postve (.e. the number of W + events), N W +, and the correspondng number of events where the charged lepton s negatve, N W. The fnal result of the analyss s the charge rato N W +/N W (fg. 1), a number whch s senstve to the structure of the proton. Snce the proton has two valence up quarks and only one valence down quark, a frst nave approxmaton to the charge rato s N W +/N W = 2. The presence of sea quarks complcates the pcture, and brngs the charge rato down to a value between 1 and 2. As prevously mentoned, the structure of the proton s summarzed by the PDFs, so a charge rato measurement can be used to test our knowledge of or constran the PDFs of the proton. Such a measurement s an mportant physcs result. The students dong the W path measurement are thus performng a measurement whch s close to an actual mportant physcs measurement performed by the ATLAS collaboraton. In the W path measurement, the students also look for events where two oppostely charged W bosons may have been produced wth subsequent decays nto charged leptons and neutrnos. Such events are characterzed by two oppostely charged fnal state leptons wth large transverse momenta and large mssng transverse momentum caused by the two neutrnos. The producton and decay of the Hggs boson may produce such a sgnature, and the students look at the dstrbuton of the angle between the charged leptons n the transverse plane to search for the Hggs boson. The W path measurement s performed usng 7 TeV pp collson data consstng of about 1000 W cand- Fgure 2: Example results from the 2014 W path Masterclasses. The dstrbuton of the angle between the charged leptons n the transverse plane n W + W canddate events s shown. An excess s seen at low values, but the excess s much larger than that expected from the Hggs sgnal. dates, 700 W + W canddates, and 3300 background events (jets and Z canddates). Combnaton of results from ndvdual student groups s performed usng an onlne spreadsheet accessble va the W path web ste [5]. Fgs. 1 and 2 show some results of the 2014 W path Masterclasses. A typcal W path Masterclass day proceeds much as descrbed for the Z path n secton The Z path and the nvarant mass technque The Z path measurement deals wth the nvarant mass technque for partcle dentfcaton and dscovery, whch wll now be presented. As mentoned, the W and Z bosons are short lved. The exstence and propertes of these and other short lved partcles must be nferred from measurements of ther decay products, and n ths context, the nvarant mass s a very useful concept. Consder a massve short lved partcle decayng nto several lghter partcles. The energy E of the short lved partcle s related to ts momentum p and mass m by E 2 = p 2 c 2 + m 2 c 4, (1) where c s the speed of lght n vacuum. Assume now that the energes and momenta of the decay products are measured. Conservaton of energy and momentum mples that E = E and p = p, (2) where the sums run over the decay products and E (p ) s the energy (momentum) of decay product number. Equatons (1) and (2) lead to 1 2 m = c 4 E 1 2 c p 2. (3)

4 Maken Pedersen, Fard Ould-Saada, Magnar K. Bugge / Nuclear Physcs B Proceedngs Supplement 00 (2014) The expresson on the rght hand sde s known as the nvarant mass, and can be calculated for any set of measured fnal state partcles. In the case that the fnal state partcles are the decay products of a short lved partcle, the nvarant mass s equal to the mass of the short lved partcle. One can search for short lved partcles by plottng dstrbutons of nvarant masses of fnal state partcles n pp collsons at the LHC. Short lved partcles wll gve rse to peaks n such dstrbutons. The wdth of a peak n an nvarant mass dstrbuton depends on the natural wdth of the correspondng short lved partcle and the expermental resoluton. In the lmt of perfect expermental resoluton, the wdth of each peak n an nvarant mass dstrbuton would be the correspondng partcle s natural wdth, whch s nversely proportonal to ts lfetme: the larger the wdth, the hgher the decay probablty, and the shorter the lfetme. The students dong the Z path measurement learn how to dentfy electrons and postrons, muons, and photons n the ATLAS detector. They look for events contanng () two oppostely charged leptons, () two photons, or () two pars of oppostely charged leptons,.e. four charged leptons n total. The goal of the measurement s to produce nvarant mass dstrbutons and look for peaks correspondng to short lved partcles. The analyss performed by the students follows closely the general procedure used n many mportant physcs analyses performed by the ATLAS collaboraton. In the dstrbuton of the nvarant mass of pars of oppostely charged leptons, the students may dscover the Z boson as well as the J/ψ and Υ mesons, each consstng of a quark and an antquark bound together by the strong force. The peaks correspondng to these partcles wll be around 90 GeV/c 2, 3 GeV/c 2, and 10 GeV/c 2 respectvely. In addton to these well known partcles, the students may dscover a new, heaver, verson of the Z boson, called the Z. The latter s expected n theores nvolvng hypothetcal new weak nteractons. Smulated events wth the producton and decay nto leptons of ths partcle have been mxed n wth the real data gven to the students. Ths gves the students the possblty of really dscoverng somethng new and unexpected, and allows them to see how a new partcle could be dscovered at the LHC. The dstrbutons of the nvarant mass of two photons and four charged leptons are senstve to the producton and decay of the Hggs boson because of ts decays to these fnal states. 5. The Z path Masterclass When attendng a standard Z path Masterclass event, the students spend one full day at ther local unversty. The program begns n the mornng wth lectures on both theoretcal and expermental aspects of partcle physcs. In the theoretcal lectures, the partcles and forces of the SM are ntroduced. The expermental lecture ntroduces the nvarant mass technque and explans how one can learn about short lved partcles by studyng ther decay products. Furthermore, t deals wth the expermental detecton of partcles usng a partcle detector. The structure of the ATLAS detector s ntroduced, and the students learn how dfferent partcles are seen n the detector. After lunch, the students proceed wth the actual practcal measurement. Before they begn, there s a short demonstraton where some key elements of the mornng lectures are repeated. In partcular, the procedures for dentfyng electrons and postrons, muons, and photons n ATLAS events are revewed wth some examples. Pars of students share a computer, and proceed to analyze ther own set of real LHC collson events recorded by ATLAS. Tutors are avalable for questons and gudance. In the late afternoon, there s a results sesson. Frst, the results obtaned by the students at the gven unversty are dscussed n a plenary sesson. Fnally, the students take part n a vdeo conference wth all the other unverstes that partcpated n the IMC on that gven day. The conference s led by moderators based at CERN. It ncludes dscusson of the results obtaned by all the unverstes, a quz, and a queston sesson where the students can ask the moderators about anythng, for example what t s lke to be a scentst and to be workng at CERN The measurement For the actual measurement, each group of two students s assgned a unque (except for ntentonal duplcaton of four lepton events) dataset contanng 50 LHC collson events recorded by ATLAS. Some detals on the event mxture are gven n table 1. The students analyze the events one by one by nspectng them vsually n the event dsplay program HY- PATIA [10, 11]. For each event, the students should decde whether t could fall nto one of the categores mentoned n secton 4. If so, the students select the partcles they beleve to be electrons/postrons, muons, or photons, and HYPATIA calculates the nvarant mass. After the students have analyzed all ther 50 events, a plan text fle contanng the calculated nvarant masses

5 Maken Pedersen, Fard Ould-Saada, Magnar K. Bugge / Nuclear Physcs B Proceedngs Supplement 00 (2014) Event type Number of events Fracton n mxture Z l + l % J/ψ l + l % Υ l + l % Z l + l % Four lepton 40 5% Two photon % Table 1: Detals on the event mxture for the 2014 Z path Masterclasses. All the real data (everythng except for the Z events) are selected from runs (perod B12-C6, 2 fb 1 ) recorded by ATLAS n June and July The numbers of events used from each category are shown, and correspond n general to the relevant fractons except n the case of four lepton canddate events, whch are replcated many tmes n the students datasets to allow all students the possblty of dscoverng such events. For all categores of l + l events, there s a democratc dvson between electron and muon pars. s exported. The students upload ths fle to the onlne plottng tool OPloT [12], where they can look at the nvarant mass dstrbutons they have obtaned. The results uploaded to OPloT are used for combnaton plots shown n the afternoon results sesson at the unversty and n the vdeo conference. Fgure 3: A HYPATIA event dsplay showng two projectons of the ATLAS detector, as well as the lego plot (top rght) showng the dstrbuton of measured energy n an unfolded calormeter geometry. Ths event contans both an e + e and a µ + µ par Event dentfcaton n HYPATIA Fg. 3 shows a HYPATIA event dsplay. We see two projectons of the ATLAS detector. In the transverse projecton (top left), the beam lne s perpendcular to the plane of the paper, whle n the longtudnal projecton (bottom), the beam lne s horzontal and n the plane of the paper. In the trackng detectors (grey), we see reconstructed tracks correspondng to the trajectores of charged partcles. In the electromagnetc (green) and hadronc (red) calormeters, yellow dots correspond to measured energy deposts. In ths event, there are two pronounced clusters of energy deposts n the electromagnetc calormeter. Snce there are also tracks n the trackng detector pontng n the drecton of these clusters, we dentfy ths event as contanng an e + e par. Only muons pass through to and are detected n the muon spectrometer, the outermost part of the detector, and we see also a µ + µ par n the event. An example of a typcal two photon event s shown n fg. 4. Photons depost energy n the electromagnetc calormeter, but do not leave tracks n the trackng detector. A photon may convert nto an e + e par when nteractng wth the materal of the trackng detector. In ths case, there wll be two tracks close together n the trackng detector pontng towards an energy cluster n the Fgure 4: A HYPATIA event dsplay of a typcal event contanng two photons. Only the longtudnal projecton s shown. electromagnetc calormeter. The students can calculate the nvarant mass of the two tracks, whch should be compatble wth zero f they are ndeed the result of a converted photon The Oslo Plottng Tool (OPloT) The Oslo onlne Plottng Tool (OPloT) s developed specfcally for the analyss, combnaton, and presentaton of results for the Z path Masterclass. Immedately after uploadng ther fle, the students can study ther own results n the form of nvarant mass hstograms. They can nteractvely change the nvarant mass axs range, choose between lnear and logarthmc bnnng, and set the number of bns. The nvarant mass of two charged leptons, two photons, and four charged leptons can be vewed ndvdually and together.

6 Maken Pedersen, Fard Ould-Saada, Magnar K. Bugge / Nuclear Physcs B Proceedngs Supplement 00 (2014) OPloT makes nvarant mass dstrbutons where the data from many students are combned. In partcular, one can choose to combne all the student data from a gven unversty on a gven day, or all the student data from all unverstes takng part n the IMC on a gven day. The former combnaton s used for the plenary dscusson of results locally at each unversty, whle the latter combnaton s used when results are dscussed n the vdeo conference at the end of the day. Fg. 5 shows the nvarant mass dstrbutons resultng from the combnaton of all submtted results from all unverstes takng part n the IMC on the 14th of March The two lepton nvarant mass dstrbuton n fg. 5(a) shows clear evdence of the J/ψ and Υ mesons as well as the Z boson. It s also clear that the students have dscovered a new partcle wth a mass of 1 TeV/c 2. Although ths s because of the smulated events mxed n wth the real data as mentoned earler, t allows the students to see what the dscovery of a new partcle may look lke. Obvously, t s explaned to the students durng the results sesson that the peak at 1 TeV/c 2 s due to the smulated events. We also observe a smooth contnuum dstrbuton between the peaks. Ths must be comng prmarly from msdentfcaton by the students. It s nterestng to note that e + e events domnate completely the regons between the peaks, as expected from the fact that an electronlke expermental sgnature s more easly mmcked by hadrons, of whch there are always plenty n pp collson events. The four lepton (e + e e + e, e + e µ + µ, or µ + µ µ + µ ) nvarant mass dstrbuton n fg. 5(b) shows that the students are very eager to look for such events, and possbly that they should be more crtcal n ther partcle dentfcaton. In fact, only 40 four lepton events were selected and mxed nto the event samples for the 2014 Masterclasses, whle the students have dentfed more than 200. We note the domnance of events of the type e + e µ + µ, whch could be due to the msdentfcaton of hadrons as electrons and postrons n events wth a real muon par. The two photon nvarant mass dstrbuton n fg. 5(c) looks pretty much as expected, but the statstcal fluctuatons are clearly too large for a small peak due to the Hggs boson to be dscovered. It s mportant that the students understand that ths s a lmtaton of the sze of the data sample, and that the two photon nvarant mass dstrbuton was n fact a key ngredent n the Hggs dscovery at the LHC. Ths s dscussed n the plenary results sesson and the vdeo conference, and to ad the dscusson, one can n OPloT choose to dsplay smulated data correspondng to dfferent data sample szes (a) The combned two lepton nvarant mass dstrbuton. (b) The combned four lepton nvarant mass dstrbuton. (c) The combned two photon nvarant mass dstrbuton. Fgure 5: The nvarant mass dstrbutons resultng from the combnaton of all submtted results from all unverstes takng part n the IMC on the 14th of March The two lepton (a), four lepton (b), and two photon (c) nvarant mass dstrbutons are shown. The nterested reader can vew these and other results nteractvely va the Moderator tab n OPloT [12]. The students fnd too many four lepton events. Whle the four lepton events are replcated many tmes n the students data, ths fact s not causng the four lepton excess as OPloT automatcally removes the overlap.

7 Maken Pedersen, Fard Ould-Saada, Magnar K. Bugge / Nuclear Physcs B Proceedngs Supplement 00 (2014) events are both much smaller n the four lepton case. Fgure 6: The two photon nvarant mass dstrbuton presented by OPloT when set to dsplay smulated background and Hggs sgnal correspondng to an ntegrated lumnosty of 25 fb 1. n order to show how a Hggs peak becomes more apparent as the amount of data ncreases. The smulated data correspondng to a large data sample (25 fb 1 of ntegrated lumnosty, whch can be compared to the 2 fb 1 avalable to the students 1 ) s shown n fg. 6. Here, t should be possble to convnce oneself that the peak due to the Hggs boson would be vsble even f t were the same colour as the background. Even though examples are not shown here, there are further nterestng possbltes n OPloT for the two photon nvarant mass dstrbuton. One can choose to compare the students dstrbuton to smulated background and sgnal dstrbutons correspondng to the sze of the data sample analyzed by the students. Dong so for the results of the 14th of March 2014, we fnd that the students have only dentfed about half as many two photon events as expected. A sgnfcant part of the msmatch between expected and observed event counts s assumed to be due to student groups whch do not manage to analyze all ther 50 events wthn the avalable tme. The student data can also be replaced by the correct dstrbuton, resultng from the selecton of events usng ATLAS software analyss procedures. Also n the four lepton nvarant mass dstrbuton, one can choose to dsplay smulated Hggs sgnal n OPloT. One can see the expected sze of a Hggs sgnal n ths dstrbuton, and dscuss n the plenary sesson and vdeo conference how t could be used to dscover the Hggs boson. The students should understand that also the four lepton nvarant mass dstrbuton was a key ngredent n the Hggs dscovery at the LHC, but that t looks very dfferent from the two photon dstrbuton, as the expected numbers of background and sgnal 1 In the 2014 Z path data, about 60 Hggs events are expected n the two photon nvarant mass dstrbuton. In the four lepton dstrbuton, between one and two Hggs events are expected, but there are several canddates n the range GeV. 6. Summary and outlook The ATLAS W and Z path Masterclass measurements have been presented. Whle the W path deals wth both the structure of the proton and the search for the Hggs boson, the Z path s devoted completely to searches for short lved partcles and mass and wdth measurements. The nterpretaton of the Z path results n terms of short lved partcles has been revewed, and whle evdence of the Hggs boson could not be observed n the students dstrbutons, the two photon and four lepton nvarant mass dstrbutons were both dscussed n terms of the Hggs search at the LHC. The W path data sample wll be mproved n the near future to ncrease the event dentfcaton success rate and decrease the complexty. The Z path measurement s also stll evolvng. In the future we hope to brng new dscoveres to the publc. Among new features to be mplemented n the Z path, a sgnal of gravton resonances n d-lepton, d-photon mass dstrbutons, and the explotaton of mssng transverse momentum to study dlepton nvarant mass endponts of supersymmetrc partcles. Fnally, a scrpt s already avalable for more advanced unversty students to loop through the full Z path dataset. References [1] U. Blow and M. Kobel, Internatonal Masterclasses brngng LHC data to school chldren, EPJ Web of Conferences 71 (2014) [2] F. Ould-Saada, Internatonal Partcle Physcs Masterclasses Brngng LHC data nto the Classroom, PoS ICHEP2012 (2013) 559 [3] Internatonal Masterclasses, [4] Internatonal Partcle Physcs Outreach Group (IPPOG), [5] K. Jende et al., ATLAS W path measurement, [6] U. Blow et al., ATLAS W path real data from the LHC for hgh school students, EPJ Web of Conferences 71 (2014) [7] M. Pedersen et al., ATLAS Z path measurement, [8] M. K. Bugge et al., ATLAS Masterclasses W and Z path physcs and presentaton of the Z path measurement, EPJ Web of Conferences 71 (2014) [9] T. McLaughlan et al., MINERVA, [10] S. Vouraks, Brngng hgh energy physcs to the classroom wth HY.P.A.T.I.A., EPJ Web of Conferences 71 (2014) [11] Ch. Kourkoumels et al., HYPATIA, [12] V. Morsbak et al., OPloT,