Measurement of the top quark mass and couplings at Linear Colliders

Transcription

1 Measurement of the top quark mass and couplings at Linear Colliders Ignacio García García IFIC (UV-CSIC) On behalf of the ILC Physics and Detector Study and CLICdp

2 OUTLINE 1. Introduction International Linear Collider (ILC) Compact Linear Collider (CLIC) Motivation 2. Top quark mass measurement at threshold Precision in the measurement of the top quark mass The tt threshold Event generation, detector simulation and reconstruction Measurement of the m t and αs at CLIC and ILC 3. Top quark electroweak couplings Top quark electroweak couplings at ILC Event generation, detector simulation and reconstruction Observables Sensitivities for the top electroweak couplings at ILC 4. Summary and conclusions 2

3 International Linear Collider (ILC) ILD SiD ILD and SiD detectors are optimised for: Particle Flow Algorithm (PFLOW) Electron positron collisions Superconducting acceleration technology Ecms tuneable between 200 GeV and 500GeV, posible upgrade to 1TeV Integrated L = 500 fb -1 (2 years of running) Beams are polarised: P(e - ) ± 80%, P(e + ) ± 30% About 31 km site length The energy of charged hadrons will be measured by the tracking detectors The energy of photons will be measured by the electromagnetic calorimeter The hadronic calorimeter is then used only to measure the energy of neutral hadrons 3

4 Compact Linear Collider (CLIC) Drive Beam decelerator, 24 sectors of ~900 m TA BC2 e main linac, 12 GHz, 100 MV/m, 21 km BDS 2.75 km IP BDS 2.75 km e + main linac BC2 TA CLIC_CDR 48.3 km Main Beam Electron-positron collider in the multi-tev energy range Fe Yoke ILD SiD About 48 km site length The c.o.m. energy: s = 3 TeV (default design) 500 GeV TeV Luminosity: L = 2x10 34 cm -2 s -1 Intense R&D in the CLIC collaboration to fully develop two-beam acceleration at high gradients Fig. 3.1: Longitudinal cross section of the top quadrant of CLIC_ILD (left) and CLIC_SiD (right). A CLIC_ILD and CLIC_SiD detector concepts have been developed from the ILD and SiD detector concepts for ILC Modifications motivated by the more challenging experimental conditions at CLIC and by the higher collision energy 4

5 Motivation arxiv: Top quark mass A small change in M h and M t can drastically modify the conclusions regarding vacuum stability Pole top mass Mt in GeV Instability Meta-stability 1,2,3 s Stability M t must be characterised well Higgs mass M h in GeV Top quark electroweak couplings Learn about BSM physics from the deviations observed on Higgs and top EW couplings. LHC cannot achieve enough accuracy in the measurement of the coupling deviations -> ILC accuracies are needed to access to fully significant deviations arxiv:

6 Precision in the measurement of the top quark mass Hadron colliders achieve precisions in the measurement of the top mass of ~ 0.76 GeV Historical result, first ever LHC/Tevatron Combination [arxiv: ] At linear colliders there are two techniques to determine the mass of the top quark 1. Direct reconstruction of top from its decay products (above threshold) Experimentally well-defined but the generated mass is not well-defined theoretically and nonperturbative corrections could be substantial 2. A scan of the top pair production threshold High degree of precision using a theoretically well-defined top mass (1S mass, can be transformed into other mass schemes). Precise top mass measurement with well-controlled theory uncertainties See details about the 1S mass scheme: arxiv:hep-ph/ v2 6

7 The tt threshold cross section [pb] tt threshold - 1S mass 174 GeV TOPPIK NNLO ISR only CLIC350 LS only CLIC350 LS+ISR Top mass input 174 GeV in the 1S mass scheme and α s = NNLO calculations provided by the code TOPPIK Corrections for ISR and luminosity spectrum s CLIC [GeV] These corrections result in a smearing of the cross section peak at threshold The smearing is due to the statistical efficiency, reduced by the luminosity spectra. Not affected by systematics. 7

8 Event generation, detector simulation and reconstruction e + e - tt production at threshold CLIC@352 GeV m t = GeV Γ t = 1.37 GeV 1. Generated events (signal + background) Pythia: e + e - tt, WW, ZZ WHIZARD: e + e - qq, qqe + e -, qqeν + beam backgrounds are included 2. Simulation of the detector Full simulation with high level of realism 3. Reconstruction Standard algorithms Kinematic fitting: Grouping W-bosons and b-jets into top quarks Simulated electron positron collision producing several jets in the ILD-like detector at CLIC Katja Seidel, Frank Simon, Michal Tesar, Stephane Poss Eur. Phys. J. C73 (2013)

9 Top mass measurement in a threshold scan The cross-section depends on the top mass, so measuring the cross-section the top mass can be extracted (Also the α s, Yukawa coupling, top width ) Inclusion of higher-order QCD contributions are needed for a correct description of the cross section Determination of event selection efficiency and background contamination Threshold scan with 10 energy points spaced by 1 GeV from 344 GeV to 353 GeV with an integrated luminosity of 10 fb -1 9

10 Threshold scans at ILC and CLIC cross section [pb] 0.8 tt threshold - 1S mass GeV TOPPIK NNLO + ILC350 LS + ISR simulated data: 10 fb /point top mass ± 200 MeV cross section [pb] 0.8 tt threshold - 1S mass GeV TOPPIK NNLO + CLIC350 LS + ISR simulated data: 10 fb /point top mass ± 200 MeV ILC ILC CLIC detector s [GeV] CLIC CLIC s [GeV] The cross section for ILC rises faster due to the luminosity peak is narrower But it does not result in a significant difference of the precision of the top quark mass measurement 10

11 Measurement of the top mass and αs at CLIC and ILC Statistical uncertainty of top mass around 30 MeV (CLIC ~ 20% larger than ILC due to different luminosity spectrum) In addition: Experimental and theoretical systematics, and uncertainties from the conversion to the MS mass scheme. Total uncertainty below 100 MeV within reach. 100 MeV α s α s σ σ 1 σ 1 σ [ GeV; ] [ GeV; ] CLIC CLIC ILC ILC CLIC detector top mass [GeV] top mass [GeV] 11

12 Top quark electroweak couplings at the ILC The process e + e - tt involves only ttz 0 and ttγ primary vertices A way to describe the current at the ttx vertex: See details in: arxiv.org/abs/hep-ph/ ttx µ (k 2,q,q) =ie µ ef X 1V (k 2 )+ 5F ex 1A (k 2 ) + (q q) µ ef X 2m 2V (k 2 )+ 5F ex 2A (k 2 ) t where: V = Vector coupling A = Axial coupling X = Z,γ F 1V γ Z F 1A 2V γ γ * F 2V F 1V Z F 1A Z F 2V Z /γ Non CP violating top quark couplings 12

13 e + e - tt semi-leptonic channel at ILC@500 GeV 1. Generated events (signal) WHIZARD: Generate 6 fermions final state e + e - qqbblν Pythia: Parton shower and hadronisation M.S. Amjad, M. Boronat, et al LC-REP WbWb and beam backgrounds added Beams are polarised (2 samples) 1. e L )e+ R ) P(e - ) -80%, P(e + ) +30% 2. e R )e + L ) P(e - ) +80%, P(e + ) -30% 2. Simulation of the detector Full realistic simulation of the ILD detector concept Three-dimensional image of a 500-GeV tt event simulated in the ILD detector 13

14 Event generation, detector simulation and reconstruction 3. Reconstruction Standard algorithms for event selection Signal reconstruction: combination of b quark jet and W boson that minimises the following equation d 2 = mcand. m t m t 2 Ecand. + E cand. E beam 2 p 2 + b 68 cos bw p b cos bw 2 Efficiency of selection 51.9% for P,P = -1,+1 (Left-handed electrons) 55.0% for P,P = +1, -1 (Right-handed electrons) 14

15 Observables Total cross section (σ) Standard model values The Forward-Backward Asymmetry (AFB top ) The slope of the distribution of the helicity angle (λt) But actually there are 6 independent observables = 3 observables x 2 polarisations So we can obtain the following CP conserving 6 couplings of the top to Z and γ σ (+) A FB (+) λ hel (+) (+ = e R )" ' $ # σ ( ) A FB ( ) λ hel ( ) ( = e L )%$ F γ $ 1V ( Z )$ F 1V * F1Aγ = 0 because of the gauge invariance Z F 1A 2V γ γ * F 2V Z Z F 1A F 2V " $ # %$ 15

16 ) Momentum of b jet at top rest frame. (b) Angle between b-jet and W. re 2: Distributions of the momentum of the b quark jet in the centre-of-mass frame of1000 quark, p b and the cosine of the angle bw between the b quark and the W boson. Measurement of observables 0-1 The entire selection retains 53.5% signal events for the configuration P, P = The cross section +1 and 56.5% for the configuration P, P 0 = +1, 1. 0 The -0.5 e-re+l e boosts ethe crosslorentz sectiontransformation can be measured to L R Measurement of the forward backward cos(θtop) lepton into the rest system of the t quar Figure 6: Reconstructed forward backward asymmetry compared with the prediction 2 Reconstructed with cut on χ should give a very precise knowledge of cos. To determine helicity an helof a on 2 < 15 for the event generator WHIZARD after the application the beam polaris % (stat.1,++1lumi) - Whizard asymmetry the angle lepton needs intogenerator betext. known. leptonic decays of for thethe P, P 0 =of the as explained the NoteFor thatthe no correction is applied 0 = +1, 1 to this analysis (10-15%), the charged lepton which significantly polarisations P, Pcontribute 2000 t Garcı a For the determination of the forward-backward asymmetry AFare num lepton approximately collinear and therefore the method remains valid B, the The helicity angle The Forward-Backward Asymmetry of events in the hemispheres of the detector w.r.t. the polar angle of the t quark 1000 unted, i.e. 6.1 Analysis of the distribution 1 helicity d 1angle + t cos 1 cos hel N (cos > 0) N (cos < 0) hel t = = + (2FR 1) AF B =. (13) dcos N (cos > 0) + N (cos < 0) 0 hel Based on the selection introduced in Sec. 4 0the angular of th distribution 1, the polar angle of the t quark is calculated from the decay products in the cos( θtoppolarised ) lepton in the rest frame of the t quark is shown in Fig. 7 for fully b onic decay branch. The direction measurement depends on the correct associa1 for t(stat. 1 for tl + syst.) t = ~4% R t =+ syst.) of the b quarks to the jets 2% of the(stat. hadronic b quark decays. The analysis is carried Figure 6: Reconstructed forward backward asymmetry with the predictio separately for a left-handed polarised electron beam and for a This right handed po- 1 angular distribution is therefore linear and verycompared contrasted between tl a e e event generator WHIZARD after the application of a on < 15 for the beam ereltwo di erent situations have to be distinguished, R In practice there tl (beware that here L and Rpola mea ed beam. Therefore, see alsowill be a mixture of terl- eand P, P = 1, +1 as explained in the text. Note that no correction is applied for th R L eler and right handed helicities) and will have a value between -1 and +1 depe 3: t Generator - Whizard 0.8P 0 = +1, 1 polarisations P, 2 Reconstructed with cut on χ Reconstructed on the composition of the t quark sample Generator - Whizard 9 According to0.6 [16], the angle hel is measured in the rest frame of the t quark the z-axis defined by the direction of motion of the t quark in the laboratory. A 1 d 1 + t cos 1 cos investigatio 2000 hel hel cussed in [4] this definition unique hel is not =but+some (2FR detailed 1) 0.4 dcos of = 2 reproduced in this note havehel shown that2the choice2 of [16] seems optimal. The o ForwardJBackward5asymmetry:5AFB& able cos hel is computed from the momentum of the t quark decaying semi-lepton " 1000 into a lepton, a0.2 b quark and a neutrino. If ISR e ects (with the photon lost The5ForwardJBackward5Asymmetry& = 1simply for tr assume 1energy for tl momentum conserv t = beam pipe) are neglected, one tcan N (cosθ > 0) N (cosθ < 0) A = "1)<)AFB)<)1, N (cosθ > 0) + N (cosθ < 0) 0 & This, by means of0 the energy-momentum of the quark decayingbetween hadronical is therefore linear andt very contrasted tl This angular distribution " & deducing thewill energy-momentum the ttlquark decaying semi-leptonica Infor practice there be a mixture of trofand (beware that here L and R m cos(θtoptop5 ) lows t cos( θ ) The5 sign5 of5 the5 is5 the5 and right handed helicities) and t will have a value between hel -1 and +1 dep one5of5the5lepton& e e θ on the composition of the t quark sample. For5t5we5change5θ5to5θ + π" 13 Reconstructed forward backward asymmetry compared with the prediction bypolar the angle of the decay lepton t Figure 7: in theinrest frame of the t quark. According to [16], the angle hel is measured the rest frame of the t qua 2 rator WHIZARDICHEP after Valencia the application of a on < 15 for the beam polarisations & 2-9 July the z-axis defined by the direction of motion of the t quark in the laboratory. top top top top FB TAE Benasque + I.García IFIC (Valencia) 10

17 Sensitivities for the electroweak couplings [1] arxiv:hep-ph/ /fb at 500 GeV yields 1-2 orders of magnitude better sensitivity than the LHC (300/fb at 14 TeV) Coupling SM value LHC [1] L = 300 fb 1 ef 1V 0.66 ef Z 1V 0.23 ef Z 1A ef 2V ef Z 2V e + e [ILC DBD] L = 500 fb 1 P, P 0 = ±0.8, e LHC studies (Snowmass 2005) Present study denoted as ILC DBD 17

18 Summary Top mass at threshold Top electroweak couplings Statistical uncertainty of top mass around 30 MeV (CLIC ~ 20% larger than ILC due to different luminosity spectrum). Total uncertainty below 100 MeV in reach, expected to be dominated by theory systematics. Log scale 18

19 Conclusions In a threshold scan, the top mass can be determined in a theoretically well defined way, using 1S mass scheme These studies confirm the expectation that a linear e + e - collider will be capable of measuring the mass of the top quark with 30 MeV error Polarisation allows to double the number of observables It is a powerful tool for analysis because it also allows full separation between axial and vectorial couplings and between ttz and ttγ vertices In LC with polarised beams we can measure with accuracies one or two orders of magnitude better than LHC 19

20 THANK YOUR FOR YOUR ATTENTION 20

21 BACKUP SLIDES 21

22 Slide by Steinar Stapnes, CERN Tunnel implementations (laser straight) Central MDI & Interaction Region Ties Behnke, ILC - ILD

23 Ties Behnke, ILC - ILD 50 23

24 tt decay modes e + e tt givesthreedifferentfinalstates:1 1 Fullyleptonic(10.3%)1 2jets+2leptons+2neutrinos" " " SemiDleptonic(43.5%)1 4jets+lepton+neutrino" Fullyhadronic(46.2%)1 6jetsatfinalstate" 24