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Systematic Error Related to the Transport Model Systematic Error Introduced by the Material Budget Uncertainties Using Geant3 interface (GHEISHA) to transport (anti-)protons through the central detectors Variation of the detector materials and the gas mixtures from 90% up to 110% of the nominal material budget Systematic error is estimated by taking the difference between the lowest and highest values of the obtained particle yields after reconstruction. Corresponding results on the systematic error for the particle ratio and asymmetry yield 2.3% max for 20% material uncertainty (P>0.525 GeV/c). For the more reasonable material budget uncertainty of 5% (10%) the systematic error for P>0.525 GeV/c is 0.4% (0.6%). Systematic Error due to Beam Gas Events Scattering of beam particles with the residual gas inside the beam pipe (mainly C, H, and O nuclei) is a problem at LHC due to the high beam intensity. Long drift time of the TPC (88 s) makes it sensitive even for far out-of-time events. Simple beam-gas events are efficiently rejected, but coincidences of beam-gas events with beam-beam reactions are problematic. An estimated beam-gas interaction rate of 12 kHz/m and an experimental area of roughly ±20 m results in an integrated rate of 500 kHz which compares to 200 kHz for pp collisions, only. Many additional background protons (less anti-protons) will be produced. Simulation: p-O fixed target collisions at 7 TeV on top of PYTHIA 14 TeV pp collisions. Beam-gas event rate varied from 12 kHz/m (worst case scenario) to 1 kHz/m. Baryon Number Transport Mechanisms at LHC with the ALICE Experiment Baryon-Antibaryon Measurements in Nucleus- Nucleus Collisions Aim: Understand transport of baryon number (BN) from beam-rapidity to mid-rapidity Gain knowledge about baryon energy loss and nuclear stopping Different models predict different net-baryon densities at mid-rapidity –Quark-Gluon String Model  small (~2%) net-baryon density at mid-rapidity. Contradicted by HERA  and RHIC [3,4,5] measurements. two new approaches based on string junctions –Baryon number is carried by valence quarks , joined by strings connected at a string junction (SJ) baryon transport to mid-rapidity allowed, but exponentially suppressed –Baryon number is carried by gluonic field  baryon number transport allowed over large rapidity gaps: asymmetry (A, see definition below) at mid-rapidity for protons ~ 5% Challenges: ALICEs central detectors acceptance allows to measure asymmetries only in a region where the predicted differences are small. Study of systematic errors of great importance! Definitions: –Ratio: relative differences –Asymmetry: absolute differences –Systematic Error: half the difference between the extreme values of R or A P. Christakoglou and M. Oldenburg Effect of Variations of Event and Track Cuts Event and track cuts reduce the overall event sample and the number of found (anti-)protons. Even carefully chosen cuts reduce not only the background but also remove part of the signal. The final result is affected by those cuts. Even though the overall error due to these cut variations stays below 1%, this is the largest contributor on the systematic error. These studies have to be repeated once real data are available. CutLower value Upper value Step size Nominal value Vertex z-position±5 cm±15 cm2 cm±10 cm Max. distance of closest approach to the primary vertex (DCA) 2 6.5 0.5 3 Minimum number of TPC track clusters 401001070 BN/3 A y=0ybyb -y b BN A y=0ybyb -y b A y=00.9 -0.9 Three types of cuts were varied, in order to see how much the final result is changing: –Event quality cuts. –(Primary) track quality cuts –Particle identification quality cuts. Applying ALICEs standard event and track quality cuts leaves us with only 0.1% of (anti- )protons originating from beam-gas interactions. ITS refit cut is most important, but even without it we still exclude 98% of the background (anti-)protons. The resulting systematic error on both the ratio and the asymmetry is well below 1%. Using published anti-proton+nuclei cross sections  to estimate interaction cross sections in the ITS (Inner Tracking System) and the ALICE TPC. Comparing different transport codes –Geant3 with Gheisha interface –Geant3 with Fluka interface –Fluka stand alone by using a flat input momentum and pseudo-rapidity distribution Calculate reconstruction efficiency: Systematic error obtained by evaluating the differences between the efficiencies Large differences between the survival probability for Geant3/Gheisha and Fluka triggered a detailed search in literature to understand the compliance between experimental data (input)  and the results obtained. Results: Fluka gives the better description of the macroscopic cross-section. Even though the above estimated error (comparing Geant3 with Fluka) was on the order of 2-4%, based on Fluka alone we estimate the error of the cross section to be about 200 mb, which translates to 0.8% absolute in asymmetry. 2 layers of Silicon 4 layers 6 layers 6 layers of Silicon + TPC gas P [GeV/c] protonsanti-protons protonsanti-protons Summary and Outlook Systematic Effects: Error on asymmetry 0.8%.Transport Model: Comparing the cross-sections with experimental data we conclude Fluka is the better transport code. Error on asymmetry 0.8%. 0.6% absolute errorMaterial Budget: For 10% material uncertainty we obtain 0.6% absolute error on the asymmetry. error below 1%Beam Gas Events: Normal track and event selection cuts result in an error below 1% for the asymmetry. error of the asymmetry around 1%Variation of Event and Track Cuts: Largest contribution to the overall error of the asymmetry around 1%. Has to be re-evaluated with real data. 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