PPhysics performance studies for the ALICE innertracker upgrade
Johannes Stiller for the ALICE Collaboration
Physikalisches Institut, University of Heidelberg, Im Neuenheimer Feld 226, 69120 HeidelbergE-mail: [email protected]
Abstract.
During the second long shutdown of the LHC in 2018, the ALICE Collaborationplans to install an upgrade of the ALICE Inner Tracking System (ITS) in the central barrel withseven layers of silicon detectors starting at 2.2 cm radial distance from the interaction regionand a material budget as low as 0.3 % radiation length per layer. A single-hit resolution of 4 µ mand a readout rate capability of up to 50 kHz in Pb–Pb collisions will allow new and uniquemeasurements in the heavy-quark sector, i.e. charm and beauty. Using detailed Monte Carlosimulations of pp and Pb–Pb collisions we study the performance for heavy-flavor detection withan upgraded ITS in the following benchmark analyses: Charm meson and baryon production,i.e. D → K − π + and Λ +c → pK − π + , and beauty meson and baryon production, i.e. displacedvertices of B + → D π + and Λ b → Λ +c π − .
1. Introduction
A Large Ion Collider Experiment (ALICE) [1] is designed to study strongly interacting matterat extreme energy densities in high-energy nucleus-nucleus (AA) collisions at the CERN LargeHadron Collider (LHC). The individual subsystems of the detector are arranged in a centralbarrel and a forward muon spectrometer, which are specifically designed to study Pb–Pb as wellas pp and p-Pb collisions as reference. The main tracking devices in the central barrel are, inthe order from closest to furthest away from the interaction region, the Inner Tracking System(ITS), the Time Projection Chamber (TPC), the Transition Radiation Detector (TRD) andthe Time Of Flight (TOF) system. At central rapidity and down to low transverse momenta( p T ), ALICE has unique particle identification (PID) capabilities as well as high tracking andvertexing precision. These allow for a detailed characterization of the Quark-Gluon Plasma(QGP). Within this scope the light quark sector is extensively studied by ALICE. For example,the charged particle production in Pb–Pb collisions was determined via the nuclear modificationfactor (R AA ) and it was observed that the suppression of high- p T particles strongly depends onevent centrality [2]. For the first time, higher harmonic anisotropic flow was measured for chargedparticles in Pb–Pb collisions [3], giving insight into initial spatial anisotropy and fluctuations.Further, a large enhancement in the strange baryon/meson-ratio was observed [4], which may beexplained by coalescence models. In addition, ALICE has performed several measurements inthe heavy-flavor sector. A strong suppression of D meson production in Pb–Pb at LHC energieswas measured for the first time down to p T > /c [5]. In comparison, models predict ahierarchy in the energy loss, ∆E g > ∆E c > ∆E b , for example due to the dead cone effect [6].Furthermore, data from Pb–Pb collisions indicates a non-zero elliptic flow of D mesons [7], a r X i v : . [ nu c l - e x ] M a y hich suggests that charm quarks might participate in the collective expansion of the system.However, to achieve deeper insight, especially within the scope of rare heavy quark production,high-statistics, precision measurements at very low p T are required.
2. Future LHC and ALICE upgrade strategy
The so-called
Phase 0 of the LHC schedule was completed in the beginning of 2013. After thefirst long shutdown from 2013 to 2014, in which the ALICE detector completion is scheduled,the
Phase 1 program will then start in 2015 until 2017. In this time period, a total of 1 nb − ofPb–Pb at √ s NN = 5.5 TeV and further pp and p–Pb reference data are planned to be collected.In the second long shutdown from 2017 until 2018, significant detector upgrades are planned.Beginning in 2018 with the Phase 2, the LHC will increase its luminosity with Pb beams upto an interaction rate of about 50 kHz, which corresponds to an instantaneous luminosity of6 × cm − s − . With the high-rate upgrade a total amount of data of more than 10 nb − (Pb–Pb) and more than 6 pb − (pp reference) will be available in ALICE. The ALICE upgradeconcept [8] aims for new high-precision AA measurements of rare probes at low p T . Among theseare measurements of D mesons to zero p T , charm and beauty baryons (accessible for the firsttime) and the measurement of beauty via displaced D → K π (accessible for the first time) toalmost zero p T . The latter is limited by the statistical uncertainty obtained, which is much largerthan the systematic uncertainty from theory-driven methods. Within this concept, the ALICEupgrade entails a new, high-resolution and low-material ITS. This detector yields significantimprovements for vertexing and tracking at low p T , namely of the pointing resolution (up to afactor of 5 better) as well as the tracking efficiency and the momentum resolution. Further, forlow- p T heavy-flavor measurements, a continuous readout is planned at a rate of up to 50 kHzfor Pb–Pb and several MHz for pp collisions, with High Level Trigger data compression basedon charge clusters stemming from particle tracks. This is necessary because of the very hightrigger rates for the most interesting signals, as background candidates that pass the selectionswould also fire the trigger. In case of a conventional trigger and due to the small signal-over-background ratio (S/B) of many signals, a minimum p T threshold would be required to stay ataffordable rates, i.e. for Λ +c with p T > /c gives a rate of 16 kHz. Such a cut at triggerlevel prevents the measurement at very low p T . Besides a new ITS readout, an upgrade of thereadout systems of most of the detectors of the central barrel to a pipelined readout is foreseen,including a complete replacement of the TPC readout chambers with gas electron multipliers(GEMs) at the endcaps.
3. ITS upgrade concept
The rareness and the low S/B ratio of the desired signals at low p T pose strong constraints onthe design of the upgraded ITS [9]. In order to achieve the required improvement of the impactparameter resolution, the radial position of the first layers of silicon detectors will be moved closerto the interaction point (39 mm →
22 mm). Depending on the final design, the total materialbudget X / X per layer will be reduced from presently 1.14 % to as low as 0.3 %. The overallpixel size improves from currently 50 µ m (r φ ) × µ m (z) to O (20 µ m × µ m) or O (50 µ m × µ m) for monotlithic and state-of-the-art hybrid pixels respectively. An improved trackingefficiency and p T resolution at low p T is achieved by adding an additional detection layer (7 intotal) and increasing the pixel granularity. The radial extension increases to 22 −
430 mm. Inaddition it will be possible to quickly insert or remove the detector for yearly maintenance. Twodesign options have been studied for the upgraded ITS, either consisting of seven layers of pixeldetectors or of three inner layers of pixel detectors and four outer layers of strip detectors. Thefirst option yields better standalone tracking efficiency and p T resolution, whereas the seconddesign has better standalone PID capabilities. Here, only the first option will be addressed. . Physics performance of the upgraded ITS The reconstruction of D → K π decays, which was performed on Pb–Pb data with goodprecision, is used as a benchmark to quantify the improved performance of the new ITS. Inorder to estimate the ITS upgrade performance, a so-called Hybrid simulation approach wasused. This is a simple scaling of the residuals of impact parameters (d , r φ , d , z ) and transversemomentum based on their true values from Monte Carlo (MC) simulations. The scaling factorsare the corresponding ratios of the ”upgrade to current” resolution for these parameters. (GeV/c) t p0 2 4 6 8 10 12 14 16 S / B -4 -3 -2 -1 Current ITSUpgraded ITS + ! - K " D Centrality 0-20% = 2.76 TeV NN sPb-Pb, Figure 1.
Comparison of the D signal-over-background ratio for current and upgradedITS [9]. (GeV/c) t p0 2 4 6 8 10 12 14 16 v ! K " D Pb-Pb, 30-50% events Figure 2.
Estimated statistical uncertaintyon v of prompt and secondary D mesons forL int = 10 nb − [9].As visible in Fig. 1, the S/B ratio of the measurement improves substantially due to theimproved separation of the signal topology from the background. In both scenarios, thesame selection cuts were applied and a similar amount of signal was selected. In the range0 < p T < /c the expected signal and background yields were scaled from pp FONLLpredictions at 2.76 TeV, as the D meson productions have never been measured in central Pb–Pb collisions in this p T range. The spread of the R AA (varied between 0.3 and 1) and of theFONLL prediction are the major sources of the rather large uncertainty. The cut efficiencieswere determined using MC simulations. Scaling to an integrated luminosity of 10 nb − , astatistical significance above one hundred is measured at any p T , which leads to higher precisionand extended p T range of the measurements of v and R AA . The former is shown in Fig. 2for prompt D and secondary D from B, whose relative contributions are determined from theanalysis of D displacement from the primary vertex. Here, v was calculated based on the in-plane and out-of-plane yields with respect to the Event Plane (EP) direction, neglecting higherharmonics. Only statistical uncertainties are shown, as most of the systematic uncertaintiescancel in the ratio. Further, first performance studies on the D +s production in Pb–Pb collisionswith the upgrade ITS show, that the measurements of R AA and v would largely benefitfrom the improved tracking precision and high statistics provided by the high-rate upgradeof the ALICE central barrel. Another benchmark case is the measurement of charmed baryons(Λ +c → pK − π + ), which strongly relies on the excellent ALICE PID capabilities with TPC andTOF. However, a Λ + c signal was not observed in Pb–Pb data due to the large combinatorialbackground. Here, because of the short mean proper decay length of the Λ +c , c τ ≈ µ m, veryhigh spatial resolution is needed in order to cleanly identify the secondary vertex. With theimproved detector the most effective cut variables are the cos ( θ p ), the decay length and therequirement of a minimum p T of the three decay tracks. The pointing angle θ p describes theangle between the straight connection line of primary and secondary vertex and the direction ofhe reconstructed momentum vector of the decaying particle. As shown in Fig. 3, the relativestatistical uncertainty improves significantly for all p T . Here, three different scenarios areconsidered, namely the current and upgraded ITS with no high-rate capability (0 . − ) andthe upgraded ITS with high-rate capability (10 nb − ). In the high-rate scenario, Λ c productionshould be measurable down to p T = 2 GeV /c . Based on this new measurement, for examplethe enhancement (Λ +c / D) Pb − Pb / (Λ +c / D) pp can be determined, as shown in Fig. 4. The pointsare drawn on a line (dashed), which captures the trend and magnitude of the Λ / K ratio.Systematic uncertainties, which mainly come from feed-down from Λ b decays, are displayed asboxes. Two model calculations are presented to indicate the expected sensitivity of the baryonto meson ratio in the measurement. The tagging of Λ +c also allows for the full reconstruction ofΛ b → Λ +c π − . (GeV/c) t p0 1 2 3 4 5 6 7 8 9 10 R e l a t i v e s t a t i s t i c a l un c e r t a i n t y -2 -1 + ! - pK " c =5.5 TeV NN sPb-Pb, centrality 0-20% events (No high-rate) $ Current ITS, 1.7 events (No high-rate) $ Upgrade ITS, 1.7 events (High-rate) $ Upgrade ITS, 1.7
Figure 3.
Comparison of statistical precisionfor different cases of ITS performance andintegrated luminosity [9]. (GeV/c) t p0 1 2 3 4 5 6 7 8 9 10 pp / D ) c ! / ( P b - P b / D ) c ! E nhan c e m en t, ( param (2.76 TeV) S0 /K ! ALICE Ko et al. (200 GeV)TAMU, Rapp et al. (2.76 TeV) = 5.5 TeV NN sPb-Pb, central events (10/nb) " Upgraded ITS
Figure 4.
Enhancement of the Λ + c /D ratioin central Pb–Pb with respect to pp collisionsfor L int = 10 nb − [9].
5. Conclusion and outlook
The ALICE experiment shows outstanding performance in studying strongly interacting matter.For a deeper understanding of, e.g., heavy-flavor energy loss mechanisms, azimuthal anisotropyand in-medium hadronization, the ALICE upgrade concept foresees to build a new, high-resolution, low-material ITS in the second long shutdown of the LHC. As a fundamentalcomponent of the ALICE upgrade concept, this detector will open the door for a set of new highprecision measurements of rare probes at very low p T . Among these are the measurement of opencharm mesons down to zero p T , charmed baryons, beauty via displaced D → K π from B decaysand low-mass di-leptons. In addition, extensive studies are ongoing for the full reconstructionof B + → D π + (D → K + π − ) and Λ b → Λ +c π − (Λ +c → pK π ). References [1] ALICE Collaboration,
ALICE: Physics Performance Report, Volume I , J.Phys.
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