Strangeness Experimental Techniques

1. Strangeness Experimental Techniques An expert is a man who has made all the mistakes which can be made, in a narrow field.-Niels Bohr

2. The Goal To design the Ultimate Strangeness Experiment What we need: To be able to measure charged and neutral decays Lots strangeness created. Measurements at low and high pt. Measurements at mid and high rapidity. • Only 5 charged particles are sufficiently stable to reach most detector: • Pions, Kaons, Protons, Electrons and Muons • (+1) Photon: the only neutral particle which • can be efficiently detected.

3. V0s Reconstruction? Strangely enough most strange particles are neutral or decay into something neutral • Strange Hadrons • K0S →p+p- (494 MeV/c2, 2.7cm, 68.6%), • L→pp- (1.12 GeV/c2, 7.9cm, 63.9%), • Ξ-→Lp- (1.32 GeV/c2, 4.9cm, 99.9%), • W-→LK- (1.67 GeV/c2, 2.5cm, 67.8%),

4. Finding V0s  - DCA Daughters  0short  + Primary Vtx DCA Daughter To PrimVtx V0 decay length K p- p- p- p- p- p- p- p- p- p- p- p- p- p- p- p- p- p- p- p- X- X- X- X- X- X- X- X- X- X- p p p p p p p p p p L L L L L L L L L L proton Primary vertex pion

5. Extracting the particle yields Consider each of the possible final states in turn Calculate the parent mass as a function of (y,pT) Count the number in the mass peak and correct for reconstruction losses Invariant mass distributions X+

6. The Podolanski-Armenteros plot Unique identification of two-body decaying particles by studying the division, between the daughters, of the parent's mtm vector. a - the fractional difference in momentum of the daughters pt - the mtm component of the +ve daughter transverse to the line of flight of the parent All possible values are constrained to lie along a curved locus specific to the mass of the parent a characterizes the decay asymmetry, <a> - daughter mass difference

7. In case you thought it was easy… After Before

8. Acceptance and Efficiency

9. Fine way to calibrate a detector! • Large peaks at 2 o’clock and 8 o’clock • TPC pad row “floating”

10. Event mixing method Background subtracted K+K- invariant mass distribution dN/dM [MeV-1] f K+K- K+K- invariant mass distribution (11% central events) dN/dM [MeV-1] Same event distribution Mixed event distribution Invariant mass [GeV/c2] • The measurement of hadronic resonances. • These particles are short-lived compared to the reaction time. • Resonances are reconstructed using a combinatorial technique. Consider all track combinations and calculate the invariant mass. • The background is calculated using positive tracks from one event mixed with the negative tracks of another event. STAR Preliminary

11. Kink reconstruction • In this method, one of the decay daughters is not observed. • Main background is from p-decay, which has a smaller Q-value • A cut on the decay angle (momentum dependent) removes the p contamination • Remaining background from multiple scattering and split tracks. • Find ~ few kink decays per event. Approx. 10% of a real event showing a reconstructed kaon decay K± m ±n(64%) or K± p ±p0(21%). Lifetime ct = 3.7 m Decay angle (degrees) Kaon limit Pion limit Parent momentum (GeV/c)

12. So now we know how to reconstruct them. • First question what kind of accelerator do we want?

13. Collider vs Fixed Target • Collider: • Higher energy • Lab frame == CM frame • Less focussed particles Fixed Target: Higher rates Known z vertex Boost gives longer ct Go with the Collider

14. Beam @ RHIC Complex

15. Beam at SPS Complex Heavy Ion LINAC 1mm carbon foil Pb+53 4.2A MeV Fixed Target Experiments Forward emission at mid-rapidity Experimental Areas Fix target experiments natPb ~ 450 mg/cm2 107 PbPb collisions per burst Proton Synchroton 4.25A GeV 1mm Al foil Pb+82 Super Proton Synchroton Pb+82 160A GeV Electron Cyclotron Resonance Lead Plasma is bombarded with an e beam Pb+27 2.5A KeV PS Booster Pb+53 95.4A MeV The experiments PS Booster North Area NA LINAC3 ECR PS SPS West Area WA

16. Needle in the Hay-Stack! How do you do tracking in this regime? Solution: Build a detector so you can zoom in close and “see” individual tracks high resolution Clearly identify individual tracks Good tracking efficiency Pt (GeV/c)

17. What’s the Main Tracking Device? Advantages of a TPC (why there are 7 at RHIC) • Large highly segmented tracking volume at low cost • Permits over sampling, a big plus • Simplifies tracking code • Improves position/momentum resolution • Improves dE/dx resolution • Design simplicity, an empty volume of gas • Low mass – reduced multiple coulomb scattering

18. Disadvantages • Slow readout – can’t be used in level 0 trigger • Two track resolution limited with classic design – (although improvements possible with radial magnification)

19. How a TPC works 420 CM • Tracking volume is an empty volume of gas surrounded by a field cage • Drift gas: Ar-CH4 (90%-10%) • Pad electronics: 140000 amplifier channels with 512 time samples • Provides 70 mega pixel, 3D image

21. Silicon Tracker? Lots of material – Not so good – Lots of scattering

22. Charge Determination Magnetic field Tracking detectors Trajectory NA50 & NA60, PHOBOS, BRAHMS, ALICE PHENIX, STAR, ALICE, CMS, CERES, NA49, NA57 In or Out of Magnetic Field

23. In a non-uniform magnetic field the drift velocity is not strictly perpendicular to the pad-row Find the drift velocity by solving the Langevin equation A(t) is a stochastic damping term, resulting from collisions in the gas t is the mean time between collisions Charge transport correction (ExB) The Solution: where: Horrible mess!!!!!!! Place in nice uniform Field

24. Now have Main detector Want low momentum tracks , near primary vertex Need fine pixelation

25. Fine Vertex Determination ALICE ITS SSD SDD SPD Lout=97.6 cm Rout=43.6 cm • 6 Layers, three technologies (keep occupancy ~constant ~2%) • Silicon Pixels (0.2 m2, 9.8 Mchannels) • Silicon Drift (1.3 m2, 133 kchannels) • Double-sided Strip Strip (4.9 m2, 2.6 Mchannels)

26. Position Sensitive Silicon Detector Strip Pad/Pixel Drift 1eh/3.6eV, 300mm 25000 e

27. Cluster reconstruction Each pad-row crossing results in charge deposited in several pad-time bins. These are joined to form clusters which have certain characteristics The position is calculated as the weighted mean of the cluster charge in the pad-time directions The coordinates are determined by: The mean pad position (x) The mean time bin x drift velocity (y) The pad row position (z) Total charge can be used for particle identification (see later) Charge cluster reconstruction

28. Step-by-step Find charge clusters in all TPCs Apply charge transport correction Track following in the main detector Start where the track density is lowest First find high momentum Form initial 3 point tracks seeds Use (local) slope to extrapolate the track Tracks are propagated to inner detectors No momentum measurement outside magnetic field Assume all tracks are primary Momentum assignment based on trajectory Use trajectory to define a “road” in detector Search for non-primary vertex tracks Do track following as a separate step Momentum determined from curvature of tracks Track reconstruction Row: n-5 n-4 n-3 n-2 n-1 n seed Track following method Row: n-5 n-4 n-3 n-2 n-1 n Track road method

29. Calibration - Lasers Using a system of lasers and mirrors illuminate the TPC Produces a series of >500 straight lines criss-crossing the TPC volume • Determines: • Drift velocity • Timing offsets • Alignment

30. Momentum measurement ~ constant B Proportional to p Constant Measurement : X+ P3 L=3m, s=10cm r=11 m, B=0.5T p=1.7 GeV/c Uncertainty: s L P2 P1 R

31. Calibration – Cosmic Rays Determine momentum resolution dp/p < 2% for most tracks

32. Reconstruction losses can be divided into two types: Geometrical Acceptance Consequence of limited detector coverage Straightforward correction, calculated by Monte Carlo Reconstruction Efficiency Particles in acceptance but not reconstructed Possible reasons for loss: Hardware losses Detector resolution Merged/split tracks Reconstruction algorithm Efficiency correction Needs a detailed understanding of the detector response Embedding Method Tune Monte-Carlo simulation to reproduce the data Cluster characteristics Number of space-points on tracks Embed a few simulated tracks into real events Tracking Efficiency

33. Energy loss Bethe showed that energy loss is strictly a function of b =v/c and the properties of the medium Including relativistic effects, the Bethe-Bloch equation is where, The Bethe-Bloch equation

34. Experimental factors that affect the measured charge Temperature Controlled to better than ± 0.1o C Pressure TPCs are operated at “atmospheric” pressure Ionisation varies by 0.6% mbar-1 monitored and normalised to 970 mbar Correction for O2 and H2O Both highly electronegative Results in linear charge loss with drift distance (few % in TPCs) Effective path length (dx) - depends on the track crossing angle Two angles: one in pad direction, the other in drift direction Equalise the electronic response Electronics and gas gain correction In practice an absolute gain calibration is difficult to obtain Inter-sector calibration (relative gain correction) Corrections

35. Practical considerations All energy loss distributions have inherently large spread Primary ionisation Number of primary electrons = DE /W W = energy to produce e--ion pair Follows Poisson statistics Secondary ionisation Energy distribution of primary electrons ~ E-2 If E > W they can produce further ionisation Convolution produces Landau distribution TPC samples dE/dx from this distribution Use truncation to better estimate the mean Reduces sensitivity to fluctuations Typically drop ~20% highest dE/dx samples Truncation ratio must be optimised experimentally What happens in higher density media ? Fluctuations are reduced … but ... Height of rise decreases (probability for large DE increases) Momentum resolution worsens (multiple scattering) Comment on dE/dx measurements

36. Two methods Pulse the sense wires above the padrow Induces charge on all pads simultaneously Easy and quick to perform, but ... Measures electronics response at maximum load Doesn’t measure the gas gain Measure response to 83mKr added to the detector gas (ALEPH) Simultaneous calibration of electronics and gas gain variations 9 keV peak used to calibrate to MIP peak in data Provides linearity check up to several MIPs, ( depends on dynamic range of electronics) MIP = Minimum Ionising Particle Electronics and Gas Gain Calibration

37. PID Techniques(1) - dE/dx Resolution: dE/dx No calibration 9 % With calibration 7.5% Design 6.7% Even identified anti-3He ! dE/dx PID range: ~ 0.7 GeV/c for K/ ~ 1.0 GeV/c for K/p

38. Still need high momentum PID

39. Requirements Time measurement between two scintillation counters (or similar) For p > 1 GeV/c, very good time resolution and long flight path For example The time difference between two particles, m1 and m2, over a flight path, L, is which for p2 » m2c2 becomes NA49 Experiment The flight path is 13 m The time resolution st= 60 ps At 6 GeV/c: p-K separation = 2 st K-p separation = 6 st Time-of-Flight method

40. Now Have the Ideal Strangeness Detector! A magnetic field for charge and momentum determination A TPC for main tracking An Innner silicon detector for high precision vertexing and low momentum tracking A TOF for high momentum PID Tracking at high and mid-rapidity with large acceptance Sound Familiar?

41. Year 2000, The STAR Detector Time Projection Chamber Silicon Vertex Tracker * FTPCs Endcap Calorimeter Vertex Position Detectors Barrel EM Calorimeter + TOF patch year 2001, year-by-year until 2003, installation in 2003 Magnet Coils TPC Endcap & MWPC ZCal ZCal Central Trigger Barrel RICH * yr.1 SVT ladder

42. Other Stuff

43. Energy Loss: Bethe-Bloch

44. Calorimeters PbWO4 X0 0.89 RM 2 cm lI 19.5 cm n 2.16 Res 3% at >3GeV • Electromagnetic Calorimeters • e- and g deposit their total energy in the Calorimeter • Hadronic calorimeter (may be in the future at mid-rapidity) • Zero Degree Calorimeters are largely used • High Multiplicity : • Small RM ~ 2-5 cm • Distance 4-5 m from IP • Spectrometer • Sampling Calorimeters: • cheap (acceptance) • Lead+Scintillator • Homogenous Calorimeters : • Resolution, • LeadGlass, PbWO4 PHOS in ALICE & ECAL in CMS

45. Triggering/Centrality • “Minimum Bias” ZDC East and West thresholds set to lower edge of single neutron peak. REQUIRE: Coincidence ZDC East and West • “Central” CTB threshold set to upper 15% REQUIRE: Min. Bias + CTB over threshold ~30K Events |Zvtx| < 200 cm Spectators – Definitely going down the beam line Participants – Definitely created moving away from beamline Several meters Spectators Zero-Degree Calorimeter Participants Impact Parameter Spectators

46. RHIC ZDC

47. V0 Efficiency Decay Length Daughter DCA Pos. Daughter DCA DCA to PV Neg. Daughter DCA Mult.

48. Kinematics Invariant cross-section Lorentz Transformations

49. Why Rapidity? • Kinematical reason: • The shape of the rapidity distribution, dn/dy, is invariant y* = y + y0 • Dynamical reason: • The invariant cross-section can be factorized

50. Pseudo-Rapidity gSPS =9, q>>6o // gRHIC =100, q>>1.6o // gLHC =2750, q>>0.02o Maximun Rapidity