Heavy Flavour measurements in Pb − Pb collisions with the upgraded ALICE Inner Tracking System
HHeavy Flavour measurements in Pb–Pb collisions with theupgraded ALICE Inner Tracking System ∗ D. Andreou on behalf of the ALICE Collaboration
CERN, 1211 Geneva 23, Switzerland,Nikhef, Amsterdam, The NetherlandsABSTRACT: During the second LHC long shutdown (LS2) the In-ner Tracking System (ITS) of ALICE (A Large Ion Collider Experiment)will be replaced by seven layers of CMOS Monolithic Active Pixel Sensors(MAPS). The latest innovations in silicon imaging technology allow for theconstruction of large, ultra-thin silicon wafers which can further improvethe capabilities of the ALICE tracker. The research and development stud-ies towards the construction of a novel vertex detector have started. Thedetector installation has been proposed for the third LHC long shutdown(LS3) during which the three innermost layers shall be replaced by threecylindrical layers of large curved CMOS wafers. This upgrade (ITS3) willfurther improve the impact parameter resolution and the tracking efficiencyof low momentum particles. The innermost layer will be positioned closerto the interaction point and the material budget will be reduced down to0.05 % X per layer.Monte Carlo simulations of a simplified ITS3 geometry within the ITS2 de-sign indicate an improvement in the impact parameter resolution and thetracking efficiency, which are of crucial importance for measurements ofheavy-flavour hadrons. This contribution shows the improved performancefor the example of the Λb, for which the significance of its measurementis extracted based on these MC simulations. A significant improvement byalmost a factor of three in the low momentum region compared to the ITS2is observed.PACS numbers: 12.38.Mh, 29.40.Gx, 25.75.-q, 25.75.Cj
1. Introduction
The ALICE experiment at the CERN Large Hadron Collider (LHC) isstudying the properties of the Quark Gluon Plasma (QGP). Important infor-mation can be extracted by studying the behaviour of heavy flavour particles ∗ Presented at Excited QCD 2020 (1) a r X i v : . [ nu c l - e x ] A p r eQCD˙Andreou˙Dimitra printed on May 1, 2020 after their journey in the QGP. Future opportunities on high-density QCDmeasurements after LHC long shutdown 2 (LS2) have been presented [1].The main topics that will be explored through the heavy flavour measure-ments are the characterization of the macroscopic long wavelength QGPproperties with high precision and the investigation the parton dynamicsresponsible for the QGP properties. More specifically, the focus will beon putting strong constraints on the transport coefficients of heavy quarkswhich will help to identify the leading interaction mechanisms of heavyquarks and derive their level of thermalization. This will be done throughhigh precision measurements of transverse momentum anisotropies and ofthe nuclear modification factors of heavy flavoured hadrons, which are usedto constrain the heavy quark diffusion coefficient and its dependence ontemperature. The new measurements of D and B mesons and of charm andbeauty baryons will not only give important information about the ther-malization of heavy quarks in the medium but will also indicate to whichdegree the process of recombination of heavy quarks with lighter quarkscontributes to the hadronisation process. Recent studies have shown thatrecombination is expected to be dominant in the low momentum region inPb–Pb collisions since the increased amount of partons allows the heavyquarks to hadronise with light ones through recombination [1]. The hadro-nisation through recombination results in the increased production rate ofmany heavy flavour species, the measurements of which are currently lim-ited by statistics. With the increase of the instantaneous luminosity in thefollowing years and with the usage of cutting edge detector technologiesmany heavy flavour measurements will be improved and new ones will bemade possible.
2. The ALICE Inner Tracking System upgrades
During LS2 a major upgrade of the ALICE Inner Tracking System istaking place. The 6 layers of the ITS1 with three different technologies(pixel, drift, strip) will be replaced by 7 layers of CMOS Monolithic ActivePixel Sensors (MAPS) as demonstrated in the detector overview design ofITS2 in Figure 1 [2]. The small thickness of the MAPS (50 µm in the innerbarrel) along with other modifications in the mechanical design reduce thematerial budget from 1.14% X down to 0.35% X per layer in the InnerBarrel. The innermost layer of the ITS is placed closer to the interactionpoint and the continuous read-out rate capability will be increased from1 kHz to 100 kHz in Pb–Pb collisions. All the above modifications contributeto the improvement of the impact parameter resolution and the trackingefficiency in the low momentum region.In order to further improve the tracking capabilities during Run 4, AL- QCD˙Andreou˙Dimitra printed on May 1, 2020 Fig. 1: Detector overview of the ALICE ITS2. Copyright CERN, reusedwith permission.ICE has started a research and development effort towards a further upgradeof the ITS, the ITS3, during the third LHC long shutdown [3]. During theITS3 upgrade the three innermost layers of the ITS2 will be replaced bythree ultra-light cylindrical layers of silicon pixel detectors. In an effort toreduce further the material budget, large scale ultra-thin silicon wafers willbe produced through the process of stitching and will operate as single sen-sors. The silicon wafers will be thinned down to 20-40 µ m, a thickness atwhich the flexible nature of silicon allows the bending of the silicon wafers.The wafers will be wrapped around the beam pipe. The periphery of thesensor, the interface responsible for the sensor configuration and serial data,will be positioned outside the fiducial volume eliminating the need of exter-nal devices in the active volume.Fig. 2: Detector design of the inner layers of the third ALICE Inner TrackingSystem. Copyright CERN, reused with permission.In this way in the area of interest there will be only silicon and lowmass open-cell carbon foam cells for the mechanical support of the sensors(Fig. 2). The water cooling that is present in the ITS2 will be replacedby air cooling, possible by reducing the power consumption of the sensorsto 20 mW/c m . With all the above modifications the material budget willdecrease from 0.35% X to 0.05% X per layer. Additional modifications eQCD˙Andreou˙Dimitra printed on May 1, 2020 will be done on the beam pipe, the thickness of which will be reduced from800 µ m to 500 µ m and the inner radius from 22 mm to 18 mm. This willallow the innermost layer of the ITS to be moved closer to the interac-tion point. The detector upgrade along with the installation of the newbeam pipe will improve the impact parameter resolution and the trackingefficiency in the low momentum regions, as indicated in Figure 3. The point-ing resolution and the tracking efficiency were studied with a Fast MonteCarlo Tool (FMCT) which takes into account multiple scattering, secondaryinteractions and detector occupancy, but ignores the energy loss of the par-ticle in the detector and in the beam pipe. The pointing resolution andthe tracking efficiency were estimated considering a standalone ITS (solidlines) and the combined case of the ITS plus the Time Projection Chamber(TPC) (dashed lines) for ITS2 and ITS3. The distribution of the pointingresolution indicates an improvement by a factor two for p T (cid:39) GeV /c withthe ITS3+TPC information. The results of the FMCT were confirmed by afull Monte Carlo study that was done for a standalone ITS2 and ITS3 andis demonstrated in the plot with circles. r p o i n t i n g r e s o l u t i o n [ m ] ITS2 standaloneITS2+TPCITS2 standalone (full MC)ITS3 standaloneITS3+TPCITS3 standalone (full MC) T r a c k i n g e ff i c i e n c y [ % ] ITS2 standaloneITS2+TPCITS3 standaloneITS3+TPC
Fig. 3: Pointing resolution in r φ plane (left) and tracking efficiency (right)with the ITS2 (blue) and the ITS3 (red). Copyright CERN, reused withpermission.
3. Prospects on heavy flavour measurements
The better pointing resolution and tracking efficiency offered by theITS3 will improve many heavy flavour measurements that are currentlylimited and enable new ones. Of particular interest is the study of theenhancement of the baryon to meson ratios of many heavy flavour speciesexpected from recombination observed in Pb–Pb collisions compared to p-pcollisions. Preliminary studies have shown an enhancement of the Λ c /D and D s /D ratios and there are predictions for other strange mesons and QCD˙Andreou˙Dimitra printed on May 1, 2020 beauty baryons, namely for the B s and Λ b [1]. However, the measurementsthat quantify the degree of the enhancement are limited for most of thespecies. Based on MC simulations it was demonstrated that with the ITS3the measurements of all the above particles will be improved compared toITS2.Λ b is a beauty baryon with udb quarks and mass 5.62 GeV /c [4]. Thedecay channel that was studied is the Λ b → Λ + c + π − with Λ + c → π + + p + K − .The branching ratio (BR) of the Λ b in this channel of interest is very small, BR Λ b = 0 . BR Λ + c = 6 . SignalSignal + Background of measuring the Λ b particle. - · (cm) c L vertex s E n t r i e s / b i n · ALICE Simulation = 5.5 TeV NN s Pb-Pb, , 0-20% -1 = 10 nb int L c < 9 GeV/ T p £ - · (cm) c L vertex s E n t r i e s / b i n · ALICE Simulation = 5.5 TeV NN s Pb-Pb, , 0-20% -1 = 10 nb int L c < 9 GeV/ T p £ Fig. 4: Signal (black) and background (red) distributions of the σ vertex Λ c with the ITS2 (left) and the ITS3 (right). Copyright CERN, reused withpermission.The distributions of the topological variables of the decay were studiedfor both cases and optimal cuts were selected in order to keep the signaland reject a significant amount of background for the maximization of thesignificance. As shown in Fig. 4 the effect of the improvement is very large inthe signal and background distributions of the track dispersion around thedecay vertex, the σ vertex of Λ c . The signal distribution is narrower makingthe signal to background separation in the ITS3 case larger and allowingfor tighter cuts in the selection. The significance of measuring the Λ b in0-20% centrality at 5.5 TeV in Pb–Pb collisions was calculated in four p T bins and scaled to luminosity L = 10 nb − (further assumptions are detailedin the reference) [5]. As shown in Fig. 5 with the ITS3 the significance ofmeasuring the Λ b particle will significantly increase, especially in the low eQCD˙Andreou˙Dimitra printed on May 1, 2020 momentum region where it is estimated to improve by almost a factor ofthree. c (GeV/ b Λ T, p S i gn i f i c an c e ALICE Simulation = 5.5 TeV NN s Pb-Pb, , 0-20% -1 = 10 nb int L ) + π - pK → +c Λ ( - π +c Λ→ b Λ Upgrade
ITS3ITS2
ALI−SIMUL−332192
Fig. 5: Significance measurement of Λ b particle with the ITS2 (black) andthe ITS3 (red). Copyright CERN, reused with permission.
4. Conclusions
The upgrade of the ALICE ITS3, with the latest innovations in siliconimaging technology, will improve the impact parameter resolution and thetracking efficiency of low momentum particles. The effect of these improve-ments is reflected by estimates for the measurements of the heavy flavourparticles, strange mesons and charm and beauty baryons, in a wide mo-mentum range. The significantly improved reconstruction will enable highprecision measurements of transverse momentum anisotropies and of nu-clear modification factors, revealing new information on the processes ofhadronization and thermalization of heavy quarks.REFERENCES [1] Citron, Z. et al,
Future physics opportunities for high-density QCD at the LHCwith heavy-ion and proton beams , CERN-LPCC-2018-07. [2] ALICE Collaboration,
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Letter of Intent for an ALICE ITS Upgrade in LS3 , CERN-LHCC-2019-018. [4] M. Tanabashi et al. (Particle Data Group), Phys. Rev. D 98, 030001 (2018)and 2019 update.[5] ALICE collaboration,
Conceptual Design Report for the Upgrade of the ALICEITS ,,