Time of CDF (II)

Transcription

1 TOF detector lecture, 19. august 4 1 Time of CDF (II) reconstruction/simulation group J. Beringer, A. Deisher, Ch. Doerr, M. Jones, E. Lipeles,, M. Shapiro, R. Snider, D. Usynin calibration group E. Palencia, J. Piedra, L. Scodellaro, I. Vila, R. Vilar Outline: TOF Offline Calibration TOF Reconstruction Validation/TOF Performance Physics with TOF

2 TOF detector lecture, 19. august 4 The TOF-Detector 4 cm 4 cm 79.5 cm 14 cm z 16 scintillator bars ( cm 3 ), as cylinder directly outside of the COT 1 photomultiplier-tube at each end PMT + readout electronics measure time when collected charge is above threshold integrates the charge within a time window ( 1 ns)

3 TOF detector lecture, 19. august 4 3 Contribution to Time Measurement in the TOF System t cable t = d /s speed t tof discriminator TAC start scintillator bar PMT d = L/ + z (L=8cm) C s : common stop TAC stop > C s TDC t walk correction = α /sqrt(q) TDC = C s - (t + tof + t s + t c + t w )

4 TOF detector lecture, 19. august 4 4 TOF Calibration Online Calibration Pedestal subtraction and TAC calibration Time Walk Effect (slewing correction) Due to dependence of discriminator time on pulse height ( ns) Speed of Signal Propagation in Bars Time depend on z postion of hit along the bar ( ns) Channel-to-Channel Time Offset Includes cable length and clock signals ( 1 ns) Residual Correction in z Position along the bar ( ps) (Design TOF resolution: 1 ps)

5 TOF detector lecture, 19. august 4 5 TOF Calibration: Effective Speed of Light Time difference East - West vs. z t E t W = t E t W (t w E (Q) t ww (Q)) + t ce t cw s: speed of signal propagation + z/s Time walk dependence almost cancels in time difference of east/west. The effective speed of light comes from the fitted slope. Channel-by-Channel offset doesn t affect the slope, just causes a shift. The effective speed of light is fitted again after the time walk correction. (ns) - t west t east 1 Run number range: C = ±. cm/ns C z Bar number 45 (East) z (cm)

6 TOF detector lecture, 19. august 4 6 TOF Calibration: Time Walk Correction Time Walk Correction Due to leading edge discriminator: Larger pulses fire the discriminator at earlier time than smaller pulses During calibration event t is not yet known. Effect studied with tracks passing through two neighbouring bars (A,B) t A = t A + t s A (z) + t ca α A Q ; t B = t B + t s B (z) + t cb α B Q ; α A α B 1 t A t B α ( 1 ); QA QB (ns) - t 46 t 45 Run number range: α 45 = 79. ± Pair bar numbers (East) θ 45-46

7 TOF detector lecture, 19. august 4 7 TOF-Occupancy TOF occupancy is about 3% (average b-event) Two thirds of the TOF signals are not related to a reconstructed track significant background even for isolated tracks Occupancy is present in data and MC It is related to real particles in the detector

8 TOF detector lecture, 19. august 4 8 TOF-Track Matching extrapolate track into tof bar calibrate time according to z position of intersection measured charge and time on east and west side after calibration should be the same cut on χ = (t E t W ) σ E +σ W + (Q E Q W ) σ Q charge has to be calibrated for attenuation length single track matching efficiency: 74% (average b-event) single track matching purity: 96% (average b-event)

9 TOF detector lecture, 19. august 4 9 Arrival-Time Resolution Like to study arrival time resolution and track-matching independent of t. Isolate calibration and matching effects from t computation. Study time difference of two muon legs of J/Ψ event (K S ππ,...) t = (tof µ1 tof exp.µ1 ) (tof µ tof exp.µ ) s! = t µ1 t s 1 c 1 + m µ p 1 t µ t s c s 1 + m µ p! Errors on momentum (p) and arclength (s) are negligible compared to timing resolution. Arrival time resolution: σ t = σ t / σ t = 156/ ps = 11 ps arrival arrival Entries Mean -.51 RMS.9 Underflow 817 Overflow 73 Integral 1.681e+5 χ / ndf 35.9 / 94 Prob p 741 ± 6.1 p ±.47 p.1563 ±.4 p ± 5.3 p ±.354 p5.45 ± (tofmu1-exp.tofmu1)-(tofmu-exp.tofmu)

10 TOF detector lecture, 19. august 4 1 Efficiency-Purity Definition Efficiency: all J/Ψs where both muon fulfill matching cut divided by number of J/Ψs with both muons fiducial tof tracks Purity: J/Ψ in narrow gaussian divided by all J/Ψs which pass matching cuts Combined χ of time and charge works best Default cut: χ 14 Single track efficiency: 74% Single track purity: 96% Efficiency Τ Q χ Purity 7% of all tracks are fiducial absolute tof efficiency about 5%

11 TOF detector lecture, 19. august 4 11 A Closer Look at the Tails (1) Bad Muons: (T of µ1 T of exp.µ1 ) (T of µ T of exp.µ ) 5ps & T of µ1/ T of exp.µ1/ T of µ/1 T of exp.µ/ early arrival times late arrival times tof exp. tof [ns] We measure more often too late than too early times

12 TOF detector lecture, 19. august 4 1 A Closer Look at the Tails () Early tails: Another particle crosses the bar before the track we are interested in time measurement is triggered too early Late tails: Another particle crosses the bar while the gate is still open. Additional charge makes signal look larger as it is. Time Walk Correction too small corrected time is too late Inefficiency: Another particle has triggered or polluted either the time or the charge measurement

13 TOF detector lecture, 19. august 4 13 t Computation The CDF are long: σ z 3 cm t = σ z /c 1 ns >> σ tof ( 11 ps) t [ns] Need to calculate t event by event (vertex by vertex) to achieve good resolution. Need at least two tof tracks excluding the track of interest in order to compute unbiased t.

14 TOF detector lecture, 19. august 4 14 t Likelihood-Fit L = P(π)*L(π) + P(K)*L(K) + P(p)*L(p) Likelihood fit function L(π/K/p) consists of two gaussian distributions: mean: t = arrival time - expected time (π/k/p) narrow gaussian corresponding to tof resolution ( 11 ps) broad gaussian to take misreconstructed/mismatched tracks into account (6 ps) ratio narrow/broad gaussian:.9:.1 weight Likelihood with particle probability obtained by COT de/dx and apriori probability (.8:.1:.1)

15 TOF detector lecture, 19. august 4 15 Define a Metric for t Studies Split number of tracks per vertex into two subsets with n/ tracks Compute t for both subsets Plot the difference, fit the distribution with double gaussian resolution: size of narrow gaussian divided by purity: square-root of the fraction of vertices where t 1 t 3σ (t1 t) [ns] (all tracks)

16 TOF detector lecture, 19. august 4 16 t Resolution/Purity number of tracks Resolution [ps] Purity number of tracks The t resolution is a strong function of number of tracks used in the fit (statistical 1/ N) Average t resolution is about 5 ps number of tracks Average t purity is about 99%.

17 TOF detector lecture, 19. august 4 17 TOF Resolution tof contains contributions from arrival time and t resolution TOF measured expected, unbiased unbiased early arrival time or late t K looks like π Entries Mean.899 RMS.188 χ / ndf / 94 p 7139 ± 8. p1.159 ±.4 p.1186 ±.4 p ± 6.6 p4.189 ±.51 p5.479 ±.75 time of flight [ns] π/p 1 late arrival time or early t π looks like K [ns] σ = 119 ps (design value: 1 ps) 4.5% in tails ( tof-exp.tof.3 ns) 1 K/p.8.6 Κ/π.4 σ pt [GeV/c] Separation of different particle species is a function of tof resolution BUT tails cause background e.g. for kaon-tagging or CHAMPS search

18 TOF detector lecture, 19. august 4 18 Protons: Λ pπ pull_proton pull_pion pull_kaon pull_proton pull_pion pull_kaon GeV pt.5gev.5gev pt 3.5GeV mass [GeV/c ] Λ pπ candidates, mass[gev] pull_proton pull_pion pull_kaon pt.5gev p t [GeV/c] 5 proton pt[gev/c] Armin Scheurer (Karlsruhe)

19 TOF detector lecture, 19. august 4 19 Soft Pions: D D ( Kπ)π s D * D ( K π) π s 1 hyp. 8 π K hyp. 1 hyp. 8 π K hyp. Events/ 1. MeV pull pt.5gev pull.5gev pt.75gev D Mass [GeV] D candidate, mass[gev] 3 5 π hyp. K hyp π hyp. K hyp p t [GeV] pull.75 pt 1.GeV pull pt 1.GeV soft pion pt[gev/c] Kurt Rinnert (Karlsruhe)

20 TOF detector lecture, 19. august 4 Combining TOF and de/dx PID TOF works better for low momentum tracks, COT de/dx works for very low or high momentum. separation of protons/pions (Λ pπ) N / 1 MeV/c Λ b Λ c( pk π )π + mass_all L~pb M=5,61.3 ±.7 MeV/c N = 33 ± 35 σ = 5 ± 3 MeV/c [GeV/c ] M(Λ c π - ) Mass of potential Λ b candidates N / 1 MeV/c mass_pid 7 M=5,61.1± 1.9 MeV/c 6 N = 53 ± 1 σ=.7±.1 MeV/c 5 4 LH p >.3-1 L~pb [GeV/c ] M(Λ c π - ) Mass with combined PID for proton Paula Squillacioti (Pisa) Dmitry Litvintsev (FNAL)

21 TOF detector lecture, 19. august 4 1 PID CDF Background Rejection : e.g. penta quark search (proton) CHAMPS search (looking for very massive and slow particles) Monopole search (highly ionizing particles, curving along the B field (z)), TOF trigger B-Flavour Tagging B Opposite-Side-Kaon Tagging: It is more likely that a B meson contains a K than a K + in the final state Same-Side-Tagging (B s ) B s is likely to be accompanied close by a K + from fragmentation b c s q u K B b c s q u K b b s B s s + Κ u u

22 TOF detector lecture, 19. august 4 Summary TOF calibration and reconstruction are interwoven TOF inefficiency and impurity are caused by high occupancy in the detector (3%, 1% track related) t has to be computed event-by-event TOF separation power does not only depend on resolution but on tails of the measured arrival time and the t computation: σ 1 ps, fraction of tails ( tof-tof exp 3 ps) about 4.5%. TOF works well for low momentum (up to about 1.6 GeV) and is complemented by de/dx for higher momentum. PID is started to be CDF Further development of TOF reconstruction code is going on