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 [1] small (~2%) net-baryon density at mid-rapidity. Contradicted by HERA [2] and

RHIC [3,4,5] measurements. two new approaches based on string junctions– Baryon number is carried by valence quarks [6], 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 [7] baryon number transport allowed over large rapidity gaps: asymmetry (A, see definition below) at mid-rapidity for protons ~ 5%

• Challenges: ALICE’s 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

€

σ syst. =1

2Rmax − Rmin( )

€

R =Np

N p

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.

Cut Lower value

Upper value

Step size

Nominal value

Vertex z-position ±5 cm ±15 cm 2 cm ±10 cm

Max. distance of closest approach to the primary

vertex (DCA)2σ 6.5σ 0.5σ 3σ

Minimum number of TPC track clusters

40 100 10 70

BN/3BN/3

BN/3

A

y=0 yb

-yb

BN

A

y=0 yb

-yb

A

y=0 0.9-0.9

• 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 ALICE’s 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%.

€

ε =Nprimariesrec.

Nprimariesgen.

• Using published anti-proton+nuclei cross sections [8] 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) [8] 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.

σ

2 layers of Silicon

4 layers

6 layers

6 layers of Silicon + TPC gas

P [GeV/c]P [GeV/c]P [GeV/c]

protons anti-protons

€

p +Ne

€

p +Ne

€

p + A

protons anti-protons

Summary and Outlook

• Systematic Effects:• Transport Model: Comparing the cross-sections with experimental data we

conclude Fluka is the better transport code. Error on asymmetry 0.8%.Error on asymmetry 0.8%.• Material Budget: For 10% material uncertainty we obtain 0.6% absolute error0.6% absolute error on

the asymmetry.• Beam Gas Events: Normal track and event selection cuts result in an error error

below 1%below 1% for the asymmetry.• Variation of Event and Track Cuts: Largest contribution to the overall error of the error of the

asymmetry around 1%asymmetry around 1%. Has to be re-evaluated with real data.

• Future: Studies will be extended to other Baryons, e.g. Lambdas.

[8]

[8] Bendiscioli and Kharzeev, Riv. Nuovo Cim.17N6 (1994) 1.2

[1] G. Cohen-Tannoudji, A. E. Hssouni, J. Kalinowski, R. Peschanski, Phys. Rev. D19 (1979) 3397; A. B. Kaidalov, Phys. Lett. B52 (1982) 459; A. Capella, J. Tran Thanh Van, Phys. Lett. B114 (1982) 450.

[2] C. Adloff et al. [H1 Collaboration], published in the proceedings of ICHEP98, Vancouver, Canada July 1998.

[3] I. G. Bearden et al. [BRAHMS Collaboration], Phys. Rev. Lett. 87 (2001) 112305; I. G. Bearden et al. [BRAHMS Collaboration], Phys. Rev. Lett. 90 (2003) 102301; I. G. Bearden et al. [BRAHMS Collaboration], Phys. Lett. B607 (2005) 42-50.

[4] C. Adler et al. [STAR Collaboration], Phys. Rev. Lett. 86 (2001) 4778.

[5] B.B. Back et al. [PHOBOS Collaboration], Phys. Rev. C71 (2005) 021901.

[6] G. C. Rossi, G. Veneziano, Nucl. Phys. B123 (1977) 507; G. C. Rossi, G. Veneziano, Phys. Rep. 63 (1980) 149.

[7] B. Z. Kopeliovich, Sov. J. Nucl. Phys. 45 (1987) 1078; B. Z. Kopeliovich, B. G. Zakharov, Phys. Lett. B211 (1988) 221; B. Z. Kopeliovich, B. G. Zakharov, Sov. J. Nucl. Phys. 48 (1988) 136; B. Z. Kopeliovich, B. Povh, Z. Phys. C75 (1997) 693; B. Z. Kopeliovich, B. G. Zakharov, Z. Phys. C43 (1989) 241; B. Z. Kopeliovich, B. Povh, Phys. Lett. B446 (1999) 321.

MDO Production ©2008

€

A = 2N p −N p

N p + N p

–

–