The heavy ionising particles in CMS tracker

1. The heavy ionising particles in CMS tracker • Hip effect description. • The PSI beam test. • Measurements of hip rate and induced dead time. • Impact on Cms tracker performances. Benjamin TROCME (IPN Lyon) on behalf of CMS collaboration

2. CMS tracker and hip: introduction • The Cms all silicon tracker: • ~16k modules with ~75k read out chips(Apv). • 128 strips/Apv, 4 or 6 Apvs per module. • Inelastic hadronic interactions generate hips: • Up to 1000 mips signal in a 500mm Si detector. • Well known effect, whose rate is predicted by theory of energy deposition in silicon. • Impact on Apv first observed in 120GeV p test beam at Cern in october 01. Lhc Test beam

3. The hips and their effects on Apvs • Inverter supply common to 128 strips: • In case of hip: sharing of the current ''consumed'' by few hit strips. • Hip topology: • High signal shared between few strips. • Downward saturated baseline in the remaining strips of the apv. • Apv may remain saturated during ~100-700 ns 150 ns 250 ns 225 ns 200 ns 175 ns 75 ns 125 ns 100 ns 300 ns 50 ns 25 ns 275 ns 350 ns 0 ns 375 ns 650 ns 625 ns 600 ns 325 ns 550 ns 525 ns 575 ns 475 ns 450 ns 425 ns 400 ns 500 ns Raw data(ADC) Pedestal 300 Digital zero 100 1 128 Strips

4. PM1 / PM2 The PSI test beam • Goal:estimate hip rate and induced Apv deadtime. • 12 modules of different thicknesses and with different Rinv. • Two trigger modes: • (PM1+PM2)1 data read out:hip rate estimate. • (PM1+PM2+!PM3) (enriched hips sample)30 data read out spaced by 25ns: Apv recovery following. 300MeV pions(+/-) 20ns bunched beam PM1 / PM2 PM3

5. The PSI test beam (cont’d) • For the first time, use of Xdaq framework: • Stable with high performances. • Efficient online monitoring. • Quasi online analysis with Orca (C++ reconstruction framework). Beam profile S/N~20 Outer barrel module Endcap module

6. Hip CMN/CMN0 Hip caracterisation • Basic method: • Common Mode Noise:event by event Apv noise. • Because of limited digitizer dynamic: CMN > -CMN0. • An Apv is said ''hipped'' if: CMN/CMN0 < -0.8 • A 2nd method makes use of baseline flatness: • An Apv is said ''hipped'' if:Rms(Raw data)<1 • Not considered here but leads to similar results. • Warning:all numbers are preliminary. Pedestal CMN

7. Hip rate measurement • Distribution normalised to number of good trigger,incident pions. • Hip rate increases with: • Thickness. • Rinv (100W by default). Hip rate Thickness:500mm Thickness:320mm Cut on CMN/CMN0

8. Induced dead time estimate • Principle: • Use of data sample with 30 consecutive readouts. Only 6 detectors considered (500mm thick). • Looks for a hipped Apv in a detector. • In following samples,looks for tracks intercepting hipped Apv. • Compute tracking efficiency • Normalise it by regular efficiency (i.e non hipped Apv).

9. etracking 00 00 00 00 00 00 00 etracking 00 00 00 00 00 00 00 Time t(ns) Induced dead time estimate(cont'd) • Average Apv dead time: t = S(1-etrack.) x t = 165ns Corrected Not corrected

10. Impact on physic • Hip effect included in Cms full simulation: • 2 input parameters:hip rate r and induced dead time t. • Simple simulation:when a hip occured,all apv data are lost during t. • At high luminosity:1.6 (0.2) equivalent hadron per bunch crossings in first (last) layer of silicon. • Track angle with detector is taken into account. • Pessimistic parameters choice:

11. Impact on physic(cont'd) • High P tracks (>1Gev) from B jets: • Impact on HLT performances:

12. Conclusion • Due to the presence of an inverter supply common to 128 strips,the Apv chip can be disabled by heavy ionising particle. • The occurrence of such events and the induced deadtime has been studied during a dedicated testbeam. • Preliminary studies showed a very limited impact on physic performance.