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Low Momentum Measurements and Particle Identification

Low Momentum Measurements and Particle Identification. Jan Fiete Grosse-Oetringhaus, CERN PH/ALICE CTEQ - MCnet Summer School 2010 Lauterbad, Germany. Content. Methods Low p T tracking Particle identification (PID) Capabilities at the LHC experiments ALICE, ATLAS, CMS, LHCb

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Low Momentum Measurements and Particle Identification

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  1. Low Momentum Measurements and Particle Identification Jan Fiete Grosse-Oetringhaus, CERN PH/ALICE CTEQ - MCnet Summer School 2010 Lauterbad, Germany

  2. Content • Methods • Low pT tracking • Particle identification (PID) • Capabilities at the LHC experiments • ALICE, ATLAS, CMS, LHCb • Physics Results • Analysis Methods • Comparisons to Monte Carlos Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  3. |h| < 2 7 TeV Pythia D6T Motivation Count • Low momentum measurements • Especially at low p MC generators depend on phenomenological QCD-inspired models • Low p production in soft (small Q2) regime • pQCD cannot be applied • The underlying event is mostly soft • Need to understand especially low pT to assess background right • Particle identification • Reconstruction of masses of well-known resonances  assess performance of tracking • Input for the description of fragmentation (strangeness) • Hadrochemistry (statistical model) allows to extract freeze-out temperatures and baryochemical potential in heavy-ion collisions (pp needed as baseline) pT (GeV/c) 7 TeV, Pythia D6T Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  4. Low Momentum Tracking • LHC detectors are operated with magnetic field • Particles with low momentum have a small bending radius  might not reach outer parts of detector • Multiple Coulomb interaction scattering results in wider distribution at low pT (q ~ (bp)-1)  hits in the detector might not be associated to the same track • Detector components close to the beam line are used for low momentum tracking • Silicon detectors in all LHC experiments Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  5. Layer 1 Layer 2 Low Momentum Tracking Approaches CMS, JHEP 02 (2010) 041 • Single cluster measurement • Difficult to associate particles with primary vertex • Cluster shape can be used to separate primaries and secondaries • Tracklets • Uses at least two layers • Straight line fits • Tracks • At least 3-4 layers • Primary vertex constraint • Magnetic field taken into account Pixel cluster z length h Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  6. ATLAS 3 pixel layers (5, 8.9 and 12.2 cm) Normal tracking algorithm at low pT with modified thresholds for seeding and less number of required hits Track needs at least 5 silicon hits at low pT pT > 100 MeV/c CMS 3 pixel layers (4.4, 7.3 and 10.2 cm) Single cluster in first layer pT > 30 MeV/c Shape of cluster rejects secondaries Tracklet: 2 out of 3 layers Track: additionally the strips are used pT > 150 MeV/c Low Momentum Tracking ALICE • Tracklet method • 2 pixel layers digital read-out (3.9 and 7.6 cm) • pT > 50 MeV/c • Single cluster • Tracking • 6 layers (4 with dE/dx) • pT > 150 MeV/c (TPC) LHCb • VELO detector ~5.5mm from the beam • Reconstruct primary and secondary vertices • VELO-only tracks at low momentum are straight lines no momentum information • Forward detector: low pT tracks can still have a large p Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  7. ALICE Tracklets ALICE Tracking Efficiency SPD physics efficiency for primaries (2009 configuration 80% active) arxiv:1007.0719 pT (GeV/c) Low pT Tracking Efficiency Efficiency CMS Tracklets QCD-007-01 ATLAS Tracking CONF-2010-046 Geometrical efficiency pT (GeV/c) Efficiency p K p Algorithmic efficiency pT (GeV/c) pT (GeV/c) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  8. Particle Identification • With detector signals • Specific energy loss and dE/dx measurement • Time of flight • Cherenkov radiation • Transition radiation • By spectra and topology • Invariant mass spectra • Missing energy ( neutrinos, new particles at LHC) • V0 vertex reconstruction • Cascades (secondary and tertiary vertices) • E.g. X pL  ppp and W  KL  Kpp • Kinks (decay in the tracking volume) • E.g. K- m-nm Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  9. Specific Energy Loss • Particles passing through matter loose energy mainly by ionization • Average energy loss can be calculated with the Bethe-Bloch formula • Identify particle by measuring energy deposition and momentum • Not necessarily unique in all regions • The single energy loss by (primary) ionization depends on E-2 • Most of the times the energy loss is small, but a small probability exists to have a large energy loss • Landau tail of the energy loss distribution  Truncated mean used Lippmann, to be published Ionization signal Probability Momentum p (GeV/c) Energy loss Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  10. b TOF Momentum p (GeV/c) Time Of Flight • Although particles have practically speed of light, particles with the same momentum have slightly different speed due to their different mass • Precise measurement of the flight time between the interaction and arrival in a detector allows to determine the mass, and thus the particle type • Needed precision, e.g. for a particle with p = 3 GeV/c, flying length 3.5 m • t(p) ~ 12 ns, t(K) – t(p) ~ 140 ps • Detector without drift volume needed, dispersion usually spoils time resolution MRPCs (multigap resistive plate chambers) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  11. charged particle e1 < e2 Transition Radiation • Transition Radiation (TR) is produced when a particle traverse the boundary between materials with different dielectric constants • The probability is dependent on g • Particles with p = 1 GeV/c • g(e)/g(p) = 2000/7 • Very suitable todifferentiate e and p • Probability for a TR photon ~1% on a boundary crossing • Stacks of foils used • TR detectors usually consist of a radiator (to create TR) and a wire chamber to detect TR Image from Christoph Rembser ALICE TRD module Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  12. Cherenkov Radiation • Radiation emitted by a particle traveling faster than the speed of light (possible in a medium, refraction index n) • A Cherenkov cone under a certain „Cherenkov angle“ is emitted • cos(qC) = 1/(nb) • Threshold b > bt = 1/n • Threshold Cherenkov detectors are used to distinguish two types of particles • One which is above the threshold • And one below • Ring Imaging Cherenkov detectors (RICH) resolve the ring and thus qC • Particle momentum measurement + Cherenkov angle allows to calculate the mass and thus identify the particle Cherenkov light in the Advanced Test Reactor, Idaho Source: Wikipedia Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  13. Maximum Likelihood Method • Detector provides deposited charge (energy loss) signal • How to conclude the probability for the particle to be of a certain type? • Example: Two particle separation (e/p) and just charge information • Create a probability distribution for a particle to deposit certain charge: P(E|e), P(E|p) • Test beam or simulation • For a given charge E, the likelihood for the particle to be an electron is:Le(E) = P(E|e) / (P(E|e) + P(E|p)) • Detectors with more layers • Calculate P for all layers • Multiplication if uncorrelated Pions Electrons Probability Image from Wilk, 2004 Charge • See backup slides for • Detector performance described by conditional efficiencies • PID with more than two particle species / more than one detector  Bayesian PID Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  14. ATLAS Focus on g, e, µ PID by combination of tracking and calorimeter information Large muon system dE/dx with the silicon detector Transition radiation tracker for electron identification CMS Focus on g, e, µ PID by combination of tracking and calorimeter information Large muon system dE/dx with the silicon detector PID @ LHC In the following slides: Some examples of PID detectors and methods from the LHC experiments ALICE • Focus on K/p/p and p/e separation • µ and g detectors • Largest TPC in the world • About all available methods for PID used • Partly with small acceptance LHCb • Especially K/p separation needed up to 100 GeV/c • Muon system • PID by combination of tracking and calorimeter information • Large RICH system (K/p/p) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  15. TR photon electrons ATLAS Transition Radiation Tracker (TRT) • Part of the inner detector • Located at 0.5 m < r < 1.1 m with lengths of 1.44 m + endcaps • 370 000 straw tubes • Single particle crosses up to 35 tubes • Provides tracking information and electron identification • Sustains high occupancy and radiation close to the LHC beam line Images from Christoph Rembser Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  16. TRT identified W Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  17. Combined PID • Combines the information from the different detector systems to conclude the particle type • E.g. an electron leaves a signal in the tracker and electromagnetic calorimeter, while a photon leaves a signal only in the electromagnetic calorimeter • CMS • The electromagnetic calorimeter has 26 radiation lengths • Essentially everything reaching the hadronic calorimeter is a hadron p± Lippmann, to be published Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  18. PID in CMS Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  19. PID in CMS Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  20. particle ALICE Time Projection Chamber (TPC) • A TPC is essentially a large volume filled with an ionizing gas • Large sensitive volume • Low amount of material • The ALICE TPC is the largest in the world (90 m³) • Charged particles create electron clusters • These drift towards the readout region • 2 Spatial + 1 time coordinate • Up to 160 clusters per track • No influence of Landau tail in energy loss distribution • Tracking capability of ~500M pixels E E readout + gating grid central electrode readout + gating grid Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  21. dE/dx TPC Particle Identification with the ALICE TPC Kink: K± m±nm(bar) Rigidity = Momentum p / Charge q Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  22. LHCb RICH • Detection elements outside of acceptance to reduce amount of material  mirrors to deflect Cherenkov photons • Gas (+silica aerogel) used for radiators (n very close to 1  large Cherenkov cone angle) • Two RICH systems (up to three cones per track) LHCb RICH-1 simulated event Powell, CERN-THESIS-2010-10 Lippmann, to be published Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  23. Particle Identification with the LHCb RICH MKK MKK Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  24. ALICE PID Alice uses ~ all known techniques! p/K TPC + ITS (dE/dx) K/p e /p p/K TOF e /p K/p HMPID (RICH) p/K K/p 0 1 2 3 4 5 p (GeV/c) p/K K/p TPC (rel. rise) p /K/p TRD e /p PHOS g /p0 1 10 100 p (GeV/c) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  25. Low pT Results Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  26. Multiplicity 2.36 TeV dNch/dh • Pseudorapidity density dNch/dh • Average number of charged particles as function of pseudorapidity h = - ln tan q/2 • Inclusive distributions in pT • Below the pT cut off, correction from MC • Possibility to publish values for tracks above a certain pT • Less model dependent • Can be compared with MCs • Often not with phenomenological models h arXiv:1004.3514 points at the same energy slightly shifted arXiv:1004.3514 Significantly larger increase from 0.9 to 7 TeV than in MCs Increase (%) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  27. Phojet Provides a good description at 900 GeV Fails at 2.36 and 7 TeV Comparison to Monte Carlo P(Nch) P(Nch) P(Nch) 0.9 TeV 7 TeV 2.36 TeV arXiv:1004.3034 arXiv:1004.3034 arXiv:1004.3514 Multiplicity Multiplicity Multiplicity • Pythia Atlas CSC • Fails at 0.9 TeV • Reasonably close at 2.36 and 7 TeV but deviations around 10-20 • Pythia D6T and Perugia-0 far from the distribution at all energies Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  28. Momentum Distributions ALICE ALICE ATLAS CMS arXiv:1007.0719 arXiv:1007.0719 ATLAS-CONF -2010-046 pT (GeV/c) pT (GeV/c) pT (GeV/c) Different h-regions show different hardness of spectrum Pythia D6T and Perugia-0 reproduce dNch/dpT shape (but not the multiplicity)  Low pT region difficult to model with MCs Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  29. Mean pT vs. multiplicity (2) CONF- 2010-46 <pT> (GeV/c) <pT> (GeV/c) <pT> (GeV/c) arXiv:1007.0719 arXiv:1007.0719 Multiplicity Multiplicity Multiplicity E.g. Perugia-0 which was tuned to <pT> vs multiplicity at another energy works well for the data starting at 0.5 GeV/c, but not for those including particles down to 0.15 GeV/c (at 900 GeV) Very soft particle production important to measure Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  30. MC Tunes Move Fast • Already new Pythia tune “AMBT1” • Fine for data starting > 500 MeV/c (used for tuning) • Discrepancy with new measurement > 100 MeV/c dNch/dh dNch/dh  More about these distributions in the Minimum Bias Lecture by William Bell h ATLAS-CONF-2010-46 Ös (GeV) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  31. PID Results Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  32. dE/dx TPC 350 MeV < pT < 400 MeV Count Reconstruction of Yields • Expected signal from analytical form*based on Bethe-Bloch curve mean expected signal • Energy loss distribution in a pT slice • Fit one Gaussian per particle type * Blum, Rolandi, Particle Detection with Drift Chambers, Springer Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  33. Spectra from different detectors consistent Levi (Tsallis) function fits the dataat low pT Sum of fits (p+K+p) matches well with dNch/dpT (all charged) result Fit also allows to extract integrated yields ITS dE/dx TPC dE/dx TOF Identified ParticleAnalysis Concept positives pT (GeV/c) pT (GeV/c) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  34. 0.2 GeV 0.5 GeV Hadron Yields • Yields of p, K, p as function of pT (here for pos. particles, similar for neg.) • Pions reasonably described by Phojet, Pythia D6T, Perugia-0 • D6T describes inclusive distribution well at low pT, but not pions • Kaon yield underestimated at pT ~ 1-2 GeV/c • Proton yield underestimated except by Pythia D6T p+ K+ p Data Phojet Pythia ATLAS-CSC Pythia D6T Pythia Perugia-0 900 GeV pT (GeV/c) pT (GeV/c) pT (GeV/c) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  35. Reconstruction of V0s • Particles with a long-enough lifetime decay with a distance from the primary vertex • E.g. weak decaying particles, K0S, L (called V0) • In the analysis (identified) oppositely charged particles are combined • Have to have a common origin (small DCA) • Origin reasonably far away from the primary vertex (~mm) • Combined momentum points to the primary vertex Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  36. Reconstruction of a Cascade p p X L p Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  37. Invariant Mass Spectra • Invariant mass of pairs of identified particles • Fit with Gaussian and background function • Assess background shape with MC • In pT bins • Extract yields as function of pT K0S pp ATLAS-CONF-2010-033 |h| < 1.2 (barrel detector region) Mpp pT (GeV/c) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  38. Example: X Invariant Mass Spectra CMS-QCD -10-007 MLp ATLAS-CONF -2010-032 MLp Schukraft, ICHEP MLp Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  39. X + X → L p X + X → L p Strange Particle Yields L → p p • Yields of K0S, L, X as function of pT and rapidity • Pythia 6 (D6T, ATLAS-CSC, Perugia-0), Pythia 8 and Phojet • Underestimate overall yields • Underestimate increase from 0.9 to 7 TeV • Discrepancy increases with increasing particle mass, strangeness and pT CMS-QCD-10-007 Yield K0S → pp Rapidity y Yield K0S → pp L → p p Ratio MC / Data Schukraft, ICHEP pT (GeV/c) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  40. string junction M M M B B B Antiproton-to-Proton Ratio • Who is the carrier of the baryon number? • Different approaches based on theory • QGSM considers the baryon as a bound quark-diquark state. Baryon number (BN) transport implies breaking the diquark pair No BN transport to mid-rapidity • Gluonic mechanism aka string junction (SJ). BN transport implies the stopping of the junction  BN transport even with large rapidity gaps • Veneziano: exponentially suppressed with rapidity • Kopeliovich: constant with rapidity d u u u d SJ u Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  41. Results show no pT dependence for both energies Results are compared with model predictions with different BN transport mechanisms PYTHIA tunes, standard prescription for transferring the BN over large rapidity intervals (D6T, ATLAS-CSC, Perugia-0), describe the data well Perugia-SOFT underestimates the data points HIJING-B clearly underestimates the pT dependence (particularly at the lower energy) Antiproton-to-Proton Ratio (2) arXiv:1006.5432 pbar/p ratio 900 GeV pbar/p ratio 7 TeV pT (GeV/c) Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  42. Energy dependence of the ratio parameterized based on the contribution of different diagrams describing the p(bar) production (pair production at mid-rapidity and BN transfer) Intercept of the Pomeron set to 1.2 based on fit on the energy dependence of the multiplicity Junction intercept set to 0.5 Good description of the (high energy) points Little room for any additional diagrams which transport baryon number over large rapidity gaps Antiproton-to-Proton Ratio (3) arXiv:1006.5432 pbar/p ratio Dy Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  43. Many More to Come… Counts / 100 MeV Counts / 25 MeV Counts / 100 MeV MKpp Mee MKp MKp 2<pT<2.5 GeV/c Mgg J/Y In the ee or µµ channel D0, D+ , D* , DS Via its secondary vertex p0 With photons in the calorimeters B mesons Mµµ MDp Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

  44. Summary • Low pT tracking is an algorithmic challenge • Only few detector layers close to the beam line available • Large influence of material • Particle identification can be performed using dedicated detector signals, topologies and invariant mass spectra • Low pT (minimum bias) physics as well as particle spectra need phenomenological models • Measurements needed to constrain soft part of the event and fragmentation • Only first glance at available data, many analyses on the way Stay tuned!  Credits to Stephanie Hansmann-Menzemer, Christian Lippmann, Werner Riegler, Kim Vervink, Cris Jones, Patrick Janot, Christof Roland, Ferenc Sikler Thank you for your attention! Low pT Measurements and Particle ID at LHC - Jan Fiete Grosse-Oetringhaus

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