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Aruna Kumar Nayak Thesis Supervisor : Prof. Tariq Aziz

Measurement of production cross section of Z boson with associated b-jets and Evaluation of b-jet energy corrections using CMS detector at LHC. Aruna Kumar Nayak Thesis Supervisor : Prof. Tariq Aziz. 1. Overview. Standard Model of Particle Physics

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Aruna Kumar Nayak Thesis Supervisor : Prof. Tariq Aziz

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  1. Measurement of production cross section of Z boson with associated b-jetsandEvaluation of b-jet energy corrections using CMS detector at LHC Aruna Kumar Nayak Thesis Supervisor : Prof. Tariq Aziz Synopsis Seminar 1

  2. Overview • Standard Model of Particle Physics The reach of LEP and Tevatron • The Large Hadron collider CMS detector Ability of CMS detector : physics objects reconstruction • Cross section measurement of bbZ, Z → ll process • Evaluation of b-jet energy corrections • Study on cosmic muon charge ratio using CRAFT data • Jet plus tracks algorithm : performance study using Test beam data • Higgs boson search in CP violating MSSM like model

  3. The Standard Model SM Building blocks The SM is based on SU(3)C X SU(2)L X U(1)Y gauge symmetry Strong : QCDElectroweak Gluon W±, Z /  SU(2)L X U(1)Y U(1)Q Electroweak Symmetry Breaking (Higgs mechanism), Responsible for generating particle mass

  4. The SM : Symmetry Breaking The potential is of the form The 2nd choice does the spontaneous breaking of gauge symmetry The strength of the interactions of the particles with the Higgs field determines the mass of the particles e.g. in case of Z boson :

  5. Success of Standard Model Success : W, Z were discovered at SPS, CERN in 1980s Tevatron, Fermilab discovered Top quark (the heaviest among all) in 1995 Most of the SM parameters, like masses and gauge couplings have been measured very precisely at LEP, CERN and matching well with SM predictions Yet Unknown : The only parameter yet unknown in SM is the mass of Higgs boson : the fundamental ingredient of the model Limits to the Higgs boson mass : From Experiments : Indirect limit : LEP precision EWK measurements : 191 GeV upper limit (at 95% CL) LEP-II direct limit : 114 GeV lower limit From Theory : bound from Triviality and Vacuum stability LEP EWK page hep-ph/0503172

  6. Is There any New Physics? • THE SM has quite a few shortcomings, e.g. : • The SM is silent about the Gravitational force (the 4th fundamental force) • It does not explain the pattern of fermion masses • In SM, the higher order corrections to the Higgs boson mass diverges, unless a fine adjustment of the parameters is performed. • Possible candidates for New Physics : Supersymmetry : predicts the existence of a super partner for each SM particles (with spin difference ½) , Extra dimension etc… • The LHC can explore all the possibilities upto TeV scale and can answer some of the unknowns. Also precision EWK measurements, mtop etc. One of the EWK measurements : cross section of Z + b-jets production

  7. The large Hadron Collider ~27 KM ring, The LEP tunnel proton-proton collision : 14 TeV CM energy 25 ns bunch crossing : 2808 bunches with ~1011 protons in a bunch Design luminosity : 1034 cm-2s-1 => 100 fb-1/year

  8. The Compact Muon Solenoid Detector (CMS) • Design Objectives : • Example of few important Higgs discovery modes : • H →gg • H → ZZ → 4 m • H → ZZ → 4 e • H → ZZ → 2e and 2m • Very good and redundant • Muon detection system • The best possible measurement of e/ • Good resolution of hadronic jets and • missing transverse energy • High quality central tracking Total weight : 14500 t Diameter : 14.60 m Length : 21.60 m Magnetic Field : 4 Tesla Size of 1 event : 1 MB 100 events / second (stored in tape)

  9. Detector Components (I) Magnet : The choice of magnetic field is key to the design of any HEP detector in collider experiments. CMS Magnet : Superconducting Solenoid Field strength : 3.8 Tesla, Length : 13 m, Inner R = 2.95 m operating current : 20 kA Advantage :Compact and small detector good resolution in inner tracking, good muon momentum resolution Tracker : Geometry : r ~ 110 cm, L ~ 540 cm, || < 2.4 66 million pixels, 9.6 million silicon strips Pixel : r ~ 10 cm, Particle flux ~ 107/s, size of pixel : 100 m X 150 m occupancy : 10-4 /pixel/bunch crossing spatial resolution : ~10 m in r- and ~20 m in r-z Strip : 20 < r < 55 cm, size : 10 cm X 80 m occupancy : 2-3% / bunch crossing TIB resolution : 23-34 m in r- and 230 m in z r > 55 cm, size : 25 cm X 180 m occupancy : 1% /bunch crossing TOB resolution : 35-52 m in r- and 530 m in z TID : 3 disks TEC : 9 disks, 120 cm < z < 180 cm

  10. Detector Components (II) ECAL : Compact, Hermatic, homogeneous, 61200 lead tungstate (PbWO4), X0 = 0.89 cm, Rm = 2.2 cm Fast : 80% light yield within 25 ns Radiation hard : 10 Mrad Barrel (EB) :Rin ~ 129 cm, 36 Super modules 0 < || < 1.479, Each crystal 0.0174 X 0.0174 (, ), front face ~ 22 X 22 mm2, L = 230 mm (~25.8 X0) Endcap (EE) : Zin ~ 314 cm, 1.479 < || < 3.0 , crystal : 28.6 X 28.6 mm2, L = 220 mm ( 24.7 X0) Preshower (ES) :2 layers of Si strip (1.9 mm pitch), behind lead (2X0 , 3X0) The energy resolution is of the form S : stochastic term, N : noise term, C : constant

  11. Detector Components (III) HCAL : Layers of plastic scintillator tiles, stacked within layers of absorbers (brass). Light read out using WLS fibre. WLS fibres are connected to clear fibres outside the tiles. Barrel (HB) :32 towers, || < 1.4, 2304 towers in total 0.087 X 0.087 (, ) , 15 brass plates of 5cm, 2 steel external plates, front scint. plate 9 mm, others 3.7 mm Eencap (HE) :14  towers, 1.3 < || < 3.0, outer 5 towers :  ~ 0.087,  ~ 50 , Inner 8 towers :  ~ 0.09-0.35,  ~ 100 Forward (HF) : steel/quarz fibre calorimeter. 3.0 < || < 5.0, Zin ~ 11.2 m 13  towers ~ 0.175,  ~ 100 Outer (HO) : Plastic scintillator, 10 mm, 2 layers in ring 0 separated by an iron absorber of thickness 18 cm, 1 layer each in ring +/- 1,2. towers size same as HB. || < 1.26 . Increases the effective thickness of HCAL to 10 .

  12. Detector Components (IV) Muon :Drift Tube (barrel), Cathode Strip Chambers (endcap), Resistive Plate Chambers. Barrel (MB) :|| < 1.2, low radiation, low muon rate, low residual magnetic field. 4 station : MB1-MB4, 12 sectors , single point resolution 200 m each station : 100 m  precision (1 mrad in direction). Endcap (ME) :|| < 2.4 , high muon rate, higher magnetic field as well. 486 CSCs in 2 endcaps, trapezoidal shape, 6 gas gaps in each chamber, strip resolution 200 m,  resolution 10 mrad. RPC provides fast response (few ns) and good time resolution. But has coarser position resolution w.r.t DT and CSC. Use to identify correct bunch crossing. RPC and DT, CSC provide independent and complementary information for L1 trigger.

  13. Detector Components (V) Example of a Level-1 Jet Trigger CMS Trigger : L1 Trigger : Electronics modules e.g. Look up Tables (RAM, ASIPs) L1 Rate : ~25 kHz (at 2X1033 cm-2s-1) L1 decision time < 1 s HLT :computer farm, partial reconstruction of physics objects HLT Rate : ~100 Hz

  14. Physics Objects Reconstruction : Electrons Reconstructed from the information of Tracker and ECAL Electron Id : Robust (cut based) Electron Id (to discriminate against Jets) H/E < 0.115(barrel), 0.150(endcap), shh < 0.0140(barrel), 0.0275(endcap) Dfin < 0.090(barrel), 0.092(endcap), Dhin < 0.0090(barrel), 0.0105(endcap) H/E : Hadronic to electromagnetic energy deposit ratio. Dfin : f difference between the electron supercluster and the electron track at vertex Dhin : h difference between the electron supercluster and the electron track at vertex

  15. Physics Objects Reconstruction : Electrons Isolation : track isolation S (pT(track)/pT(electron))2 < 0.02 (in cone 0.02-0.6, track pT > 1.5 GeV, ) (This isolation criteria is only for Z measurement study) Efficiency calculated by matching MC electron to Reco electron in 0.1 cone Electrons from Z decay

  16. Physics Objects Reconstruction : Muons Reconstructed from the information of Tracker and Muon Chamber Isolation : S pT(tracks) (0.3 cone) < 3 GeV Efficiency calculated by matching MC muon to Reco muon in 0.1 cone Muons from Z decay

  17. Physics Objects Reconstruction : Jets Jets are reconstructed from calorimeter energy using IterativeCone algorithm of cone size 0.5  dependent & pT dependent corrections are used. Reconstruction efficiency of jets Vs generated Jet pT and h for Z + bb events.

  18. Cross section Measurement of pp → Z+bb, Z→l process CMS PAS EWK-08-001 CMS AN-2008/020 CMS CR-2008/105 (CMS approved result) Dominant at LHC ~ 15% of bbZ total s • Measurement of Zbb production is an important test of QCD calculation • Background to Higgs discovery channels at LHC, like SM H → ZZ → 4l, SUSY bbF, F →tt (mm) • bbZ measurement can help reduce the uncertainty in SUSY bbH calculation • Z + 1 b-jet has been measured both at CDF & D0 • Z + 2-bjet may be observed for the 1st time • The possibility of observing and measuring the production of Z + 2 b-jet at LHC has been studied aiming at early 100 pb-1 of CMS data at 14 TeV center of mass energy. 18

  19. Cross section and Event generation Signal llbb (Zbb) : CompHEP events with pT(b) > 10 GeV, |h|(b) < 10 , mll > 40 GeV, |h|(l) < 2.5 were generated and fully simulated in CMS with 100 pb-1 calibration and mis-alignment Cross section calculated using MCFM, NLO s (llbb) = 45.9 pb , l = e, m, t PDF : CTEQ6M, scale mR = mF = MZ LO cross section calculated using PDF : CTEQ6L1 and same values for scale K (NLO) = 1.51 19

  20. Cross section and Event generation Backgrounds tt~ + n jets, n >= 0 : Generated using ALPGEN Cross section normalized to NLO inclusive tt~ cross section 840 pb llcc + n Jets, n>= 0 (Zcc) : Generated using ALPGEN Normalized on NLO s (using MCFM) 13.29 pb, k factor = 1.46 with cuts : pT(c) > 20 GeV, |h|(c) < 5, mll > 40 GeV ll + n Jets, n >= 2, (Zjj) : Generated using ALPGEN Normalized on NLO s (using MCFM) 714 pb , k factor = 1.02 with cuts : pT(j) > 20 GeV, |h|(j) < 5, mll > 40 GeV All events are passed through full CMS detector simulation and reconstruction chain, with appropriate alignment and calibration uncertainties corresponding to early 100 pb-1 of integrated luminosity. 20

  21. Primary Event selections Trigger selection : single isolated electron or muon Level-1 threshold 12 GeV, 7 GeV & High-Level threshold 15 GeV, 11 GeV Corresponds to low luminosity period L = 1032cm-2s-1 Lepton Selection : Two high pT, isolated, opposite charged leptons |h|(e) < 2.5, |h|(m) < 2.0, lepton pT > 20 GeV Jet Selection : Two or more jets with corrected ET > 30 GeV , |h| < 2.4 Jet corrected using Monte Carlo jet energy correction (as described earlier) 21

  22. b-Jet Tagging Lepton, jet selections + double b-tagging with b-discriminator > 0. b-discriminator of 2nd highest discriminator jet Jets are tagged using “Track Counting b-tagging” Which uses the 3-dimentional impact parameter significance , of 3rd highest significance track, as the b-tagging discriminator i.e. No. track (3D IP significance cut) >= 3 Effective to supress the Z+jets backgrounds. 22

  23. b-tag efficiency b-tagging efficiency for b, c, light jets after applying cut on b-discriminator > 2.5 23 *statistical error bars are not shown

  24. ETmiss selection Lepton, jet selections + double b-tagging with b-discriminator > 0 Missing ET reconstructed from calorimeter and corrected for Jet Energy scale and muons. Type-1 ETx,ymiss = - (ETx,ycalo + Sjets(ETx,ycorr – ETx,yraw)) Muon corr. = - (Smuons (px,y – Ex,y(calo. deposit))) Effective to supress the tt~+jets backgrounds Cut ETmiss < 50 GeV 24

  25. Event Selection details Two Leptons, pT > 20 GeV, |h|(e) < 2.5 , |h|(m) < 2.0 Two or more Jets , ET > 30 GeV , |h| < 2.4 Two b-tagged Jets Missing ET < 50 GeV Initial and final cross sections after all selections *More details for each selection cuts are in backup

  26. Expected Measurement for 100 pb-1 events scaled to 100 pb-1 Purity of b-tagging in Zbb events The points are the result of random selection of exactly 100 pb-1 of data 26

  27. Expected Measurement for 100 pb-1 Z →ee final state Z →mm final state

  28. tt~ background Estimation Dilepton mass region Signal : 75-105 GeV(Z) Side band : 0-75 GeV & 105 – above (no Z) NZ(tt) = (eZ(tt)/enoZ(tt)) X NnoZ(tt) • DNZ(tt)/NZ(tt)= 1/√NnoZ(tt) NZ(tt) = expected no. of tt~ events in signal region NnoZ(tt) = measured no. of tt~ events out side signal region eZ(tt) = selection efficiency of tt~ in signal region enoZ(tt) = selection efficiency of tt~ outside signal region DNZ(tt) = uncertainty of the expected number of tt~ events in the signal region. Uncertainty on eZ(tt)/enoZ(tt) is negligible compared to the statistical uncertainty on NnoZ. Assuming side band contains only tt~ background. Other possible backgrounds are negligible 28

  29. Systematics Uncertainty due to Background and double b-tagging. NZbb and DNZbb are determined as follows. NZbeforeb-tag = NZjj + NZcc + NZbb NZafterb-tag = elX NZjj + ec X NZcc + eb X NZbb Where, NZbeforeb-tag = measured number of Z/g* → ll events after all selections except b-tagging under Z mass peak (75-105 GeV). Contribution of tt~ is negligible (~1%). NZafterb-tag = measured number of Z/g* → ll events after all selections including b-tagging with tt~ subtracted NZjj is unknown number of ll+jets (u, d, s, g) events before double b-tagging. NZcc is unknown number of Zcc events before double b-tagging. NZbb is unknown number of Zbb events before double b-tagging. eb, ec, elare the efficiency of double b-tagging for Zbb, Zcc and Z+jets events ( Ratio of number of events before and after double b-tagging) (after all selections except b-tagging) 29

  30. Systematics Contd ...... Reduce the no. of variables to two using the Ratio where is ratio of selection efficiencies Solving the equations The Uncertainties on NZbb is calculated from uncertainties of NZafter b-tag (uncertainty due to tt~ subtraction), dR and uncertainties on eb, ec, el *Calculation of systematics due to JES and MET scale and others are in backup 30

  31. Total Uncertainty on Measurement Total cross section is expected to be measured in 100 pb-1 of data with uncertainty ds = +21%, - 25% (syst.) , +/- 15% (stat.) 31

  32. Evaluation of the b-jet energy corrections from data using bbZ, Z->ll process CMS Note-2007/014 CMS AN-2006/106 (CMS approved result) Why do we need It : b-Jets in final state of many processes at LHC b quark fragmentation function is different than light quark and gluon Production and decay of heavy hadrons in the b-jet Part of the energy will be carried by neutrinos in semi-leptonic decays. c1pxb1 + c2pxb2 = -pxZ c1pyb1 + c2pyb2 = -pyZ c1 = (pyZpxb2-pxZpyb2) / (pxb1pyb2-pyb1pxb2) c2 = (pyZpxb1-pxZpyb1) / (pxb2pyb1-pxb1pyb2) c1 and c2 are mere scale factors Assumption : Exact pT balance in the event (but there is effect of radiated jets) Jets reproduce the parton direction : Effect of detector, Algorithm In Ideal case It will be exactly 1 32

  33. Applying to Generator level Jets Ideal : ISR off in PYTHIA ISR Effect Df = f separation between Jet and quark The error in direction measurement of one jet affects the other. Ctrue = ET(jet)/ET(quark)

  34. Detector level Jets Very much similar Selections, 10 fb-1 of data (LO cross section used for Zbb sample) 1000 total events after selections 75% signal and 25% background (detail in backup) Jet veto improves pT balance Selected events with DR > 1.2 ET and h of veto jets

  35. Measured pT balance between di-b jets and di-leptons The effect of background on pT balance is small ( < 1 %) (if we fit around the peak) Physics meeting, CMS Week

  36. Extraction of energy corrections Because of ISR, Z boson and two b quarks are not perfectly balanced in the transverse plane. Jet veto does not reduce completely this effect. When the jet deviates from the original b-quark direction that error propagates in the pT balance equation and gives a wrong correction coefficient

  37. Getting the functional form: 10 fb-1 “data” 10 fb-1 “data” S and S+B points are within ~ 2 s stat errors Physics meeting, CMS Week

  38. How correction function works on bbZ events : As a first test, the b-jets in the same gg->bbZ process has been corrected using this correction function. The plot shows pT ratio of Z boson to that of combined two b-jets, dashed plot is for uncorrected jets and solid plot is for corrected jets. The correction restores the pT balance and also makes the distribution narrower compared to uncorrected jets. Physics meeting, CMS Week

  39. How correction function works on h->bb in tth, h->bb, W->ln events : - restore Higgs boson mass to nominal value - improve resolution by ~ 25 % Physics meeting, CMS Week

  40. b JES Uncertainty Fit Uncertainty with 10 fb-1 of data Uncertainty of Mbb Mbb = 122.0 ± 8 (syst) GeV Generated Mbb = 120 GeV Physics meeting, CMS Week

  41. Cosmic Muon Charge Ratio(ongoing) • Cosmic muon Charge ratio : 90% of proton in cosmic ray Production of more p+ and K+ in Shower than p- and K-. • Data used : 300 M triggered events taken last year in CMS : 100 M good events (tracker used in the run) • Studying cosmic physics is not CMS aim : not designed for it. • But it helps understanding the detector by measuring this which has been measured very precisely in earlier dedicated experiments and also confirms CMS capability. 41

  42. Cosmic Muon Charge Ratio(ongoing) Example of a cosmic muon passing CMS detector • Muon Selection for Charge Ratio studies • Global muon two Leg, f < 0 (downward) • pT (at PCA) > 10 GeV, pT = 1/C (curvature) C = (1/2)(q1/pT1 + q2/pT2) at Point of Closest Approch (PCA) • Does not share tracker track • No. CSC Hits, TEC Hits = 0 • No. of DT Hits (per leg) >= 20 • No. of TOB Hits (per Leg) >= 5 • No. of DT SL2 (Z) Hits >= 3 • Net q = Sign(q1/pT1 + q2/pT2)

  43. Charge ratio Vs Zenith Angle pT measured at PCA from the curvature of two Legs a : Zenith angle measured at the entry point (CMS detector surface)

  44. Muon selection : 2 Leg Barrel muons f (muon) < 0., same track charge for both leg , # of total track hits >= 25, 15 for upper and Lower legs. Track propagation The Lower Leg track is propagated to the closest approach to the 1st hit point (inner most point as convention) of the upper Leg track, using SteppingHelixPropagator in opposite to momentum. The difference of the measured angle (f, q, zenith angle) at the entry point are studied. Cosmic Muon Angular Resolution(ongoing) Point of measurement Upper Leg Lower Leg

  45. f Resolution GLB muon Df = f (Extp Lower Leg) – f (Upper Leg) Data MC 10GeV Fitted with double gaussian function May be due to difference in magnetic field map 45

  46. a (Zenith angle) Resolution GLB muon Da = a (Extp Lower Leg) – a (Upper Leg) Data MC 10GeV 46

  47. Selection Efficiency from data Using Tag & Probe (ongoing) Muon Selection Tag Muon : Lower Leg < 0, pT (at PCA) >= 10, no. DT Hits >= 20, no. of TOB Hits >= 5, No. of CSC Hits = 0 , no. of TEC Hits >= 0 Compatible lower tracker track and lower Stand alone muon track Probe Muon : Upper Leg f < 0, pT (at PCA) >= 10, no. DT Hits >= 20, no. of TOB Hits >= 5, No. of CSC Hits = 0 , no. of TEC Hits >= 0 Compatible upper tracker track and upper Standalone muon track Q(lower leg) * Q(upper leg) > 0. Probe Tag

  48. Efficiency from Tag & Probe MC Data < 2% difference in most of the bins.

  49. Jet Plus Tracks performance study using Test Beam 2007 data CMS AN-2008/111 Main JPT steps: subtract average response of “in-calo-cone” tracks from calo jet E and add track momentum. - add momentum of “out-of-calo-cone” tracks (1,2,3 on figure) to jet E Particle Energy Response : ECAL (7 X 7 crystal ) HCAL (3 X 3 Tower) Without Zero Suppression ECAL Calibration using 100 GeV electrons HCAL Calibration using muon and wire source Jets are made from Charged pions only, by randomly picking 6 pi of 5 GeV, 4 pi of 6 GeV 2 pi of 7 GeV, 1 pi of 8 GeV True Jet Energy : 76 GeV ( pT = 28 GeV, eta = 1.653) Track Correction : for each particle subtract average (EE+HE) response and add true energy. Calo Correction : multiply each Jet energy by True energy / Emeanraw 49

  50. Higgs search at CMS in CPV MSSM Model (133 GeV) (51 GeV) CMS AN-2008/025 arxiv:0803.1154 (hep/ph) (part of 2007 Les houches study) Because of the suppressed H1ZZ coupling, LEP could not exclude the presence of a light Higgs boson at low tanb (~ 3.5 to 10) Because of the suppressed H1VV coupling one of the pseudo-scalar Higgs state is very light Since there is correlation between the mass of charged Higgs and that of the pseudo-scalar Higgs state in MSSM, => a light charged Higgs, with MH+< Mtop . The traditional decay mode H+->tn is suppressed over an order of magnitude. (LEP Higgs working group, hep-ex/0602042) • M(H1) = 51 GeV, M(H+) = 133 GeV, M(top) = 175 GeV • F(CP) = 90o, tan(b) = 5 • * BR = 2 * 840 pb * 0.01 (BR(t->bH+) • * 0.567 (BR(H+->H1W) * 0.99 (BR(t->bW)) • * 0.92 (BR(H1->bb) = 8.675 pb Main backgrounds : tt + >= 2jets & ttbb+jets 50 (Ghosh, Godbole, Roy hep-ph/0412193)

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