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When Bunches Collide

When Bunches Collide. USCMS Hans Wenzel and Nick Leioatts. Terminology. Pt – momentum transverse to the beam axis Pseudorapidity – angle of a particle relative to the beam axis Rapidity – angle and momentum of a particle relative to the beam axis. Background. Beam width 20um

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When Bunches Collide

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  1. When Bunches Collide USCMS Hans Wenzel and Nick Leioatts

  2. Terminology • Pt – momentum transverse to the beam axis • Pseudorapidity – angle of a particle relative to the beam axis • Rapidity – angle and momentum of a particle relative to the beam axis

  3. Background • Beam width 20um • Arbitrary detector coordinate system • Other methods of reconstruction • must interrupt beam

  4. Purpose (Beam Position) • Find the origin of collisions without having to reconstruct the primary vertex • Align/Measure Resolution of Detectors • Important Input for many physics analyses • B lifetime • Improvement of momentum measurement (beam constrained track fit) • Input for the trigger • (SVT, HLT)

  5. Purpose (Beam Profile) • Useful feedback for the accelerator group (Measurement of beta-function) • Used for alignment of the Detector • Important resolution input for physics analyses

  6. Methods • Impact Parameter: D – closest approach of track to (0,0) • Phi(φ) – angle at minimum approach • Z0 – Z value at minimum approach

  7. D-φ Correlation D(φ0,Z0) = x0(sinφ0) + ax(sinφ0 )(Z0) – y0(cosφ0) - ay(cosφ0 )(Z0) ax, ay : x and y slope of beam • X0, Y0 : position of beam at Z = 0

  8. How many tracks? 1000 tracks needed for 1 micron precision

  9. (longer luminous region) 1000 tracks give 0.5 micron/cm precision

  10. β function • Width of the beam given Z • “Hourglass Effect” • Negligible in LHC

  11. Reconstruction Fit Beam Position D-phi fit Adjust track parameters to fitted beam position Fit Beta Function and longitudinal beam distribution Data Generation • Generation (MC) • Pythia generated minimum bias events • Pt filter (1.5 GeV/c) • Distribute according to betafunction • Calculate track parameters • Smear track parameters with resolution function • Displace beam • Generation (Accelerator) • Not done yet

  12. Rapidity and Pt of Pythia MC events • Pseudorapidity : η = -ln(tan(θ/2)) • Rapidity : y = (ln/2)*((E+pz)/(E-pz)) • Pt = √(ρx2+ρy2)

  13. Beam Profile

  14. Average abs(D0) vs Z correlation closest impact parameter (D0) given various errors in z

  15. What are we fitting? Fit Parameters: ε: Emittance β* Z0: Longitudinal beam Displacement σz: width of Longitudinal beam distribution

  16. σDtr(Pt) = 10μm + (10/Pt) 10000 tracks give 2cm (6%) precision

  17. Conclusions • Beam properties can be reconstructed from their tracks (no vertex fitting necessary) • This allows position and width to be measured without disrupting the beam • Important input for various physics analysis • Measurement tool for aligning and checking the alignment of tracking detectors and measuring the tracking IP resolution function.

  18. Future • Create accurate models of SVX and CMS detectors for analysis (Acceptance) • Apply to actual data (CDF)

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