1 / 62

Precision Tracking for ep Physics in the HERA-II Run

Precision Tracking for ep Physics in the HERA-II Run. Rainer Mankel DESY Hamburg Seminar, Oxford, 30-May-2006. Outline. Physics Motivation ZEUS Tracking System Track Reconstruction Chain Commissioning & Alignment First Look at Physics Signals. All material shown is preliminary. HERA.

thora
Télécharger la présentation

Precision Tracking for ep Physics in the HERA-II Run

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Precision Tracking for ep Physics in the HERA-II Run Rainer Mankel DESY Hamburg Seminar, Oxford, 30-May-2006

  2. Outline • Physics Motivation • ZEUS Tracking System • Track Reconstruction Chain • Commissioning & Alignment • First Look at Physics Signals All material shown is preliminary R. Mankel, Precision Tracking

  3. HERA • HERA ep collider at DESY: a unique machine • HERA collides protons and electrons/positrons at s=319 GeV • HERA-II run features upgraded luminosity and polarization • presently the only operating collider in Europe Charged current cross section as function of epolarization R. Mankel, Precision Tracking

  4. Heavy Flavor Physics at HERA • Heavy flavor production constitutes a considerable part of the DIS cross section at HERA, in particular at high pT • Dominant mechanism: boson-gluon fusion • direct access to the gluon • Appropriate description of the way charm & beauty are produced is vital to understanding of proton & photon • QCD: heavy quark mass stabilizes theoretical predictions Charm production in DIS by boson-gluon fusion R. Mankel, Precision Tracking

  5. Heavy Flavor Signatures • Charm hadrons & their decay products • D0, D+, D*, J/; e,  • small branching ratios  high extrapolation factor • need to disentangle charm & beauty • Lifetime tagging • exploit displacements of tracks & vertices • exclusive or inclusive  little extrapolation • requires very precise tracking in vicinity of interaction point  micro-vertex detector R. Mankel, Precision Tracking

  6. ZEUS HERA-II Data Samples • Initially, HERA-II running had suffered from unstable machine operation & harsh background conditions • After introduction of additional experiment shielding in 2003, the first “serious” HERA-II data-taking proceeded from Nov 2003 (start of “2004 run”) • Three major data samples: • 2004 e+ 42.3 pb1 (38 with MVD) • 2005 e 142 pb1(132 with MVD, no STT) • 2006 e e+ 45 pb1(ongoing) • 2005 dataset recently reprocessed with improved MVD alignment R. Mankel, Precision Tracking

  7. The ZEUS Tracking System R. Mankel, Precision Tracking

  8. ZEUS Detector (CAD View) R. Mankel, Precision Tracking

  9. Straw Tube Tracker (STT) Micro-Vertex Detector (MVD) The ZEUS Tracking System Central Tracking Detector (CTD) e p R. Mankel, Precision Tracking

  10. The Central Tracking Detector • Cylindrical drift chamber • Nine superlayers (five axial + 4 stereo) with eight layers each • drift cells tilted by 45o with respect to radial direction • coordinate resolution ~160 m R. Mankel, Precision Tracking

  11. The Straw Tube Tracker • 2 superlayers of straw chambers in the forward region (5o-25o) • 12 layers per superlayer • oriented in four stereo views • 7.5 mm straw diameter, Ar/CO2 • During 2005, STT was switched off due to insufficient cooling no results from 2005 run • will not cover STT in detail in this presentation • STT cooling has been upgraded, participates in 2006 running R. Mankel, Precision Tracking

  12. The Micro-Vertex Detector (MVD) The forward section: • 4 wheels • each composed of 2 layers of 14 Si detectors • in total 112 hybrids, 50k channels The barrel section: • 30 ladders • each composed of 5 modules of 4 Si detectors • in total 300 hybrids, >150k channels The rear section: • Cooling pipes and manifolds • Distribution of FE, slow control and alignment cables R. Mankel, Precision Tracking

  13. The MVD Barrel • Single-sided n-doped silicon, 300 m thick, p+ strip implants, 20 m pitch • Readout pitch 120 m (capacitive coupling) • R and Z sensors are ganged • Helix3.0 analog R/O chip (Heidelberg/NIKHEF) • Five modules are mounted on a carbon fiber support structure to form a ladder • Si planes, hybrids and cabling are located on the 3 planes of the ladder • 30 ladders arranged in three cylinders around elliptical beam pipe R. Mankel, Precision Tracking

  14. The Layout of the MVD Barrel • Major part of azimuthal acceptance covered by three cylinders of ladders ( six measurements per track) • Optimal use of available space between beam pipe & CTD Mechanical view Tracking view R. Mankel, Precision Tracking

  15. The MVD Forward Wheels • The four forward wheels have trapezoidal shape detectors with two different sizes to accommodate the beam pipe • Each two layers of single sided detectors, same pitch and construction as in barrel • strips cross at angle of 26o • Same electronics and connectivity as in barrel R. Mankel, Precision Tracking

  16. Material & Simulation x • Very detailed material model of MVD and surrounding elements • Due to the layout, traversed amount of material in beam pipe and MVD strongly  dependent z R. Mankel, Precision Tracking

  17. The Track Reconstruction Chain Coordinate reconstruction Track pattern recognition Track fitting Vertex finding Vertex fitting Higher level analysis R. Mankel, Precision Tracking

  18. MVD Cluster Finding • Cluster algorithm is one of the crucial items determining tracking resolution • Present reconstruction uses centre-of-gravity algorithm • obtained 25-35 m resolution for vertical incidence • Alternative algorithms are under study • head-tail • three-strip-algorithm • eta algorithm R. Mankel, Precision Tracking

  19. Track Finding • ZEUS uses a combined track pattern recognition of MVD and CTD • not merely an extension of CTD tracks into the MVD • improved efficiency • complex multi-pass procedure • also MVD standalone tracks • Main challenge: “ganging” of barrel MVD strips • 50% of clusters are ghosts • Extension of combined tracking into forward area under development R. Mankel, Precision Tracking

  20. Track Finding (cont’d) • Example: seed creation in barrel and forward MVD R. Mankel, Precision Tracking

  21. The Track Fit direction of flight  • Using the Kalman filter method with smoother to account for multiple scattering and ionization energy loss on MVD part of trajectory • Also performs rejection of outlier hits purification of track • Working on extension of Kalman filter into forward region (MVD+CTD+STT) production vertex  direction of filter R. Mankel, Precision Tracking

  22. Fitted curvature - true curvature estimated error Track Fit (cont’d) “pull”= • Initial version of track fit showed slight shifts in invariant masses • explained by insufficient correction for ionization energy loss • visible in normalized parameter residuals (pulls) • Improved track fit • appropriate correction for ionization energy loss • improved multiple scattering correction • improved mean and width improved version of track fit pull=1.2 old version of track fit old version of track fit pull=1.5 R. Mankel, Precision Tracking

  23. Impact Parameter Resolution From simulated events: Parameterization: • Averaged over azimuth angle • better than 100 m for pT>1 GeV • Better than 50 m at large momentum • good performance of reconstruction chain R. Mankel, Precision Tracking

  24. Primary Vertex Reconstruction • Primary vertex finder/fitter based on Kalman filter technique • typical resolution in simulated photoproduction events ~90 m • strongly depends on event type • Topology of heavy flavor events poses additional challenges to primary vertex finder • long-lived particles  outliers • need robust method • presently implementing deterministic annealing filter (DAF) for vertex Residual of primary vertex x position R. Mankel, Precision Tracking

  25. Commissioning & Alignment R. Mankel, Precision Tracking

  26. Commissioning HERA-II Tracking • Initially, many components of the tracking software were essentially untested with real data • Differences in comparison with MC might have come from many, usually convoluted sources: • inadequacy of MC (material, resolutions, …) ? • alignment uncertainty ? • performance of pattern recognition & track fit ? • quality of vertex assignments ? • It was essential to devise methods that are suitable to isolate such effects from the data R. Mankel, Precision Tracking

  27. MC Data Testing Material Reality Versus Simulation Secondary interactions in MVD • Using photon conversions and secondary interactions • Uncovers carbon fiber shield & cooling pipes that were not included in this earlier version of MC • now included in MC geometry • Geometry description in MC rather complete R. Mankel, Precision Tracking

  28. The Beam Spot • By design, in HERA-II the beams have gaussian widths of ~110 m horizontally and ~30 m vertically • ZEUS measures the beam spot position offline by mean of reconstructed primary vertex • This measurement shows that the beam spot position can undergo sizable movements (~100 m) even within a fill R. Mankel, Precision Tracking

  29. Beam Spot Width • Knowledge of the beam spot width is essential for physics, but also important to disentangle detector resolutions in real data • An elegant (new?) method is the study of impact parameter correlations in pairs of tracks in the same event track 1 track 2 beam spot int. point • In the correlation, all track-specific resolutions cancel R. Mankel, Precision Tracking

  30. Measurement of Beam Spot Width • Powerful constraint for impact parameter & decay length analysis R. Mankel, Precision Tracking

  31. Testing Track Resolutions in Real Data • General problem in testing track resolutions: • in real data the result is often biased by vertex finding algorithms, or the prevalence of background • search for ways to check resolution independent of vertex reconstruction • study of decays D*+ D0+  (K+) +, • clean sample of D0  K+ , guaranteed to emerge from the same spot • control background with wrong charge combinations R. Mankel, Precision Tracking

  32. Testing Track with D* Tagged Events • D* tagged events allow to investigate the distance-of-closest-approach (DCA) of the helices of K– and + • DCA resolution-per-track (DCA/√2) is a measure related to the track spatial resolution • independent of vertexing • averaged over longitudinal (Z) and transverse (DH) resolutions • Difference between real data & MC, persists at large momenta • material description not at fault • improve alignment (initial alignment) R. Mankel, Precision Tracking

  33. Alignment of the MVD • Quality of MVD alignment is crucial aspect for spatial resolution • From survey, position of sensors within ladders is expected to be known within 5 m. Absolute positions & orientations of ladders & wheels, however, are less well known. • Main sources of in-situ MVD alignment are • MVD laser alignment • alignment with cosmic muons • alignment with tracks from ep collisions R. Mankel, Precision Tracking

  34. Laser Alignment • 5 laser beams (780 nm, 5 mW), 7 sensors per beam • Double-sided sensors measure position to ~10 m • Purpose: • monitor global alignment and possibly distortions of MVD • identify unstable conditions R. Mankel, Precision Tracking

  35. MVD Laser Alignment (cont’d) Machine studies (several injections) • Due to its sensitivity, laser alignment records effects from ramping of HERA magnets during injection • During data-taking conditions, laser alignment shows high stability of MVD/CTD geometry • Important warning system @ BMVD1 Laser coordinate (m) @ FMVD wheel 3 @ FMVD flange GO/GG magnets (current) time [s] R. Mankel, Precision Tracking

  36. Alignment with Cosmic Muons • Advantages: • clean signature. Achievable samples ~100k events (1-2 weeks of dedicated running) • tracks passing through whole height of detector  typically 6 hits (r)+6 hits (z) on track • Method: • for each ladder in barrel, determine residuals of hits with tracks (fitted under exclusion of the very hits of this particular ladder) • local least squares fit determining 6 alignment parameters (3 shifts + 3 rotations) for ladder • apply for all ladders, iterate, combine with global alignment R. Mankel, Precision Tracking

  37. Alignment with Cosmic Muons (cont’d) • Based on ~100k good cosmic tracks • Considerable reduction of residual widths, down to ~50 m • Principal limitation: • ladders on sides of barrel are not well covered • forward wheels cannot be aligned at all R. Mankel, Precision Tracking

  38. Using Inclusive Impact Parameter Distributions to Check Alignment track • Study impact parameter with respect to beam spot  independent of vertex reconstruction • Inclusive selection of tracks (fiducial in CTD, pT>3 GeV) gives very clean impact parameter distributions • Expectation (if perfect alignment): • narrow distributions for horizontal tracks • wider distributions for vertical tracks Q DH =0o Beam spot projection narrow track Q DH =90o Beam spot projection wide R. Mankel, Precision Tracking

  39. Impact Parameter “Radar Map” • significant excess in impact parameter resolutions in certain azimuth ranges • correlation with ladders that are least accessible to cosmics alignment • need alignment method that covers whole detector MC DATA r: visible impact parameter resolution [m] : track azimuth at level of cosmic alignment R. Mankel, Precision Tracking

  40. Alignment with ep Collisions • Tracks from ep collisions form the largest quantitative basis for alignment • select about 1 M tracks per ~10 M ep events • Compared to cosmic muon alignment, far less redundancy at MVD level (only ~6 hits instead of ~12 per track) • compensate this by using beam spot and CTD segment as additional constraint • not feasible to use unbiased residuals. Must take correlations into account • High granularity of alignment parameters • 2 shifts + 3 rotations per individual sensor • about 3000 alignment parameters • Simultaneous global fit of all track and alignment parameters • millions of free parameters • use fitting engine “millepede” (by V. Blobel) • Rather complementary to cosmic alignment R. Mankel, Precision Tracking

  41. The ep Alignment Factory • Track selection parallelized on farm (1-2 days) • Actual fit (“aligner”) takes 10-20 minutes Job 1 Track/Hit sample Job 2 Track/Hit sample Alignment Constants Aligner … … Job 100 Track/Hit sample ROOT format R. Mankel, Precision Tracking

  42. Alignment Constants: Snapshot • Clear correlations of modules within ladder • no evidence for significant shifts within ladder • high precision of construction & survey • r: indications for ladder-level rotations (sub-mrad) • possibly some indications of sag, twist or warp effects? • Typical alignment accuracy ~20 m Note: error bars exclude multiple scattering R. Mankel, Precision Tracking

  43. 18 17 19 16 6 7 5 20 15 1 8 0 21 4 14 22 9 29 13 3 2 23 10 28 12 11 24 27 25 26 Hit Residuals • Significant improvement from ep track alignment in critical areas 34 m 22 m R. Mankel, Precision Tracking

  44. Impact Parameter Resolution After ep Track Alignment ep alignment + refit ep alignment + pattern recognition Cosmics alignment • Reclustering/refitting with new constants already gives substantial improvement • Full reprocessing gives further improvement, probably due to resolution of alignment-related pattern recognition glitches R. Mankel, Precision Tracking

  45. First Look at Physics Signals in Reprocessed 2005 Data R. Mankel, Precision Tracking

  46. pT>3 GeV D*+ m(D0+) - m(D0) D0 with D* Mass Tag • InD*+ D0+  (K+) +,mass difference tag ensures clean sample of D0 • study decay length resolution in the real data • background estimate from wrong charge combinations, used for subtraction • in addition, D0 lifetime tag may serve in analysis to purify D* signal R. Mankel, Precision Tracking

  47. 2D Decay Length of D0 • Wrong charge combinations allow separation of signal and background • Background symmetric and roughly gaussian • Signal displays marked asymmetry (as expected) Signal Wrong charge background R. Mankel, Precision Tracking

  48. The Decay Length Distribution D0 proper time • exponential • c=123 m • The decay length distribution is essentially a convolution of four elements: D0 Lorentz boost • pT/m • distribution can be measured Decay length resolution • track contribution Decay length resolution • beam spot contribution R. Mankel, Precision Tracking

  49. D0 Decay Length Spectrum assuming c=123 m (=PDG) • Mean value of decay length distribution OK • lifetime correct • Width of peak very sensitive to decay length resolution • From fit: • (l2D) = (185  14) m • beam spot subtracted • entirely determined from the data R. Mankel, Precision Tracking

  50. Using D0 Lifetime Tag for D* Tag Anti-Tag • Selection based on significance of D0 decay length can be used to enhance or suppress the D* signal • Can obtain almost background-free D* signal R. Mankel, Precision Tracking

More Related