Colour Reconnection and Its Effect on Precise Measurements at the LHC

Transcription

1 Colour Reconnection and Its Effect on Precise Measurements at the LHC Torbjörn Sjöstrand Department of Astronomy and Theoretical Physics Lund University Sölvegatan 14A, SE Lund, Sweden ISMD 2013, Chicago, 17 September 2013

2 The Structure of an Event An event consists of many different physics steps, which have to be modelled by event generators: Torbjörn Sjöstrand Colour Reconnection slide 2/17

3 The Density of Particle Production multiplicities in nondiffractive events (8 TeV LHC) strings crossing y = 0 primary hadrons in y < 0.5 charged particles in y < Prob(n) String width hadronic width Overlap factor 10! Torbjörn Sjöstrand Colour Reconnection slide 3/17 n

4 Reconnection in B decays Colour operators in B decay some η c : A. Ali, J.G. Körner, G. Kramer, J. Willrodt, Z. Phys. C1 (1979) 269 B J/ψ µ + µ good way to find B mesons: H. Fritzsch, Phys. Lett. B86 (1979) 164, soon confirmed by experiment g cc J/ψ production mechanism in pp ( colour octet ) H. Fritzsch, Phys. Lett. B67 (1977) 217 more complicated to test (at the time, later confirmed ) Torbjörn Sjöstrand Colour Reconnection slide 4/17

5 Reconnection at SppS T.S. and M. van Zijl, Phys.Rev. D36 (1987) 2019 p (n ch ) sensitive to colour flow. long strings to remnants comparable n ch /interaction p (n ch ) flat. shorter extra strings for each consecutive interaction p (n ch ) rising. Torbjörn Sjöstrand Colour Reconnection slide 5/17

6 Interconnection at LEP 2 e + e W + W q 1 q 2 q 3 q 4 reconnection limits m W precision! perturbative δm W 5 MeV : negligible! (killed by dampening from off-shell W propagators) nonperturbative δm W 40 MeV : inconclusive. (but more extreme models from other authors ruled out) Bose-Einstein δm W 100 MeV : full effect ruled out. (but models with 40 MeV barely acceptabe) V.A. Khoze & TS, PRL 72 (1994) 28; L. Lönnblad & TS, EPJ C6 (1999) 271 Torbjörn Sjöstrand Colour Reconnection slide 6/17

7 Colour rearrangement models for LEP 2 Colour rearrangement studied in several models, e.g. Scenario II: vortex lines. Analogy: type II superconductor. Strings can reconnect only if central cores cross. Scenario I: elongated bags. Analogy: type I superconductor. Reconnection proportional to space time overlap. q q g q W q W g W + W + q q q q In both cases favour reconnections that reduce total string length. g g (schematic only; nothing to scale) Torbjörn Sjöstrand Colour Reconnection slide 7/17

8 Each MPI, even when soft, is a color spark Final distributions crucially depend on color space Multiple colour charges extracted from beams by MPIs: Colour assignment at hadron colliders Different mod make different an Sjöstrand & PS, JHEP 03(2004)053 Ambiguities from N C = 3 and spatial correlations? N C = builds too high remnant charge (forward particle flow)! Random walk in colour space, with restoring force? Junction topologies when 2 valence quarks kicked out! # of strings Torbjörn Sjöstrand Colour Reconnection slide 8/17

9 Reconnection at the LHC p (n ch ) effect alive and kicking: [GeV] T p GeV pp Soft QCD (mb,diff,fwd) Average p T ATLAS Pythia 8 Pythia 8 (no CR) vs N ch (N ch > 1, p T ATLAS_2010_S Pythia > 0.5 GeV/c) Ratio to ATLAS N ch Reconnection important also for other generators, e.g. Herwig++ mcplots.cern.ch Rivet 1.8.2, 200k events 0.5 Torbjörn Sjöstrand Colour Reconnection slide 9/17

10 Colour rearrangement models for the LHC Space time models too complicated simplified (in Pythia) Common aspect: reduce string length λ = ln(m 2 ij /m2 0 ) multiplicity Ingelman, Rathsman: reduce mij 2; Generalized. Area Law In total 12 scenarios in Pythia 6, mainly annealing: P reconnect = 1 (1 χ) n MPI 2 4 m13 m 24 < m 12 m 34 with χ strength parameter. Random assignment by P reconnect for each string piece. Choose new combinations that reduce λ (with restrictions). Pythia 8 still only primitive: let each MPI either form a separate system, or attach its partons to a higher-p MPI where it gives minimal λ increase. Much room for improvement. 1 3 Torbjörn Sjöstrand Colour Reconnection slide 10/17

11 The Mass of Unstable Coloured Particles MC: close to pole mass, in the sense of Breit Wigner mass peak. t, W, Z: cτ 0.1 fm < r p. t b W t At the Tevatron: m t = ± 0.51 ± 0.71 GeV = PMAS(6,1) At the LHC (CMS): m t = ± 0.33 ± 0.96 GeV = 6:m0? Now severely limited by colour reconnection uncertainty Torbjörn Sjöstrand Colour Reconnection slide 11/17

12 Top mass shift in Pythia 6 Studies for the Tevatron. Green bands: old virtuality-ordered showers. Blue bands: new p -ordered showers. In total ±1.0 GeV, whereof ±0.7 GeV perturbative, and ±0.5 GeV nonperturbative. NON-PERTURBATIVE QCD EFFECTS AND THE TOP MASS AT THE TEVATRON Fit scaled: Jet Energy Scaling. m fit top {}}{ Pythia v6.416 Tune A Tune A-CR Tune A-PT Tune DW Tune BW S0 S1 S2 NoCR m scaled top {}}{ Δm t (M.Sandhoff and P.Z Skands, FERMILAB-CONF T;) Fig. 5. Comparison of calibration offsets obtained for each model. The column o D. Wickeshow and the P.Z. results Skands, obtained EPJ C52 before(2007) JES rescaling, 133, Nuovo the right Cim. column B123 (squares) (2008) S1 after statistical precision due to the finite number of generated events is at the level Torbjörn Sjöstrand Colour Reconnection slide 12/17

13 Kinematics dependence of mass determinations Dependence*of*Top*Mass*on*Event* Kinema2cs* CMS^PAS^TOP^12^029* color*recon.* 2D - <m t > [GeV] CMS preliminary, 4 2 s = 7 TeV, lepton+jets -1 Data (5.0 fb ) MG, Pythia Z2 MG, Pythia P11 MG, Pythia P11noCR MC@NLO, Herwig NEW* 2D ISR/FSR* m t 0-2 b^quark*kin.* [GeV] data - MG Z p [GeV] T,t,had! First#top#mass#measurement#binned#in#kinema3c#observables.#! Addi2onal*valida2on*for*the*top*mass*measurements.**! With*the*current*precision,*no*mis^modelling*effect*due*to* " color*reconnec2on,*isr/fsr,**b^quark*kinema2cs,*difference* between*pole*or*ms~*masses.* E. Yazgan (Moriond 2013) Torbjörn Sjöstrand Colour Reconnection slide 13/17 10*

14 Top mass shift in Pythia 8 Semileptonic top decay. Find jets with anti-k, R = 0.5, p min = 20 GeV. Request n jet 4. Find two jets closest to m W. Kill if m 12 m W > 5 GeV. Find third jet closest to m t. Kill if m 123 m t > 20 GeV. t/w decay after before CR: δn ch = 0.26 ± 0.09 δm t = ± GeV t/w decay after no CR: δn ch = ± 0.09 δm t = ± GeV top mass shift (8 TeV LHC) no top rec with top rec no rec at all t transverse momentum (GeV) Torbjörn Sjöstrand Colour Reconnection slide 14/17

15 Reconnection and collective flow - FIG. 2. Illustration of the color reconnection in the string fragment color connected to the projectile and target remnants (a). The seco - Transverse boosts collective particle vecocity. More common with reconnection. the event multiplicity in η < 2.4. The parameter wh 0.5 pp s=7 TeV 0.2 controls CR is the reconnection range, RR, which ent ALICE, preliminary in the probability to merge a hard scale ˆp T system w 0.15 Pythia 8, pp s=7 0.4 Pythia 8, tune 4C one of a harder scale, (2.085 RR) 2 TeV Tune 4C /((2.085 RR) 2 +ˆp NLO, Phys. Rev. D 82, (2010) 0.1 The tune 4C uses the valuetune RR 4C, = 1.5 N which < 5 gives a go mpi description of p T as a function Tune 4C, ofn multiplicity 20 mpi [9] Tune 4C, w/o CR Top panel of Fig. 3 shows the (p + p)/(π + + π ) ra in the p T -interval, p T < 6GeV/c, formb(pythia tune 4C) pp collisions - at 7 TeV. The distribution sho 1.5 (p+p)/(π + +π ) a clear bump around GeV/c. The result for simu (2 φ )/(π +π ) 1.4 tions using RR = 0 (without CR) indicates a complet 0 different behaviour, (Λ+Λ)/(2 KS for p T larger than 1.8 GeV/c 1.3 ) ratio stays flat. + - Therefore - (K +K )/(π the origin of the peak is + +π ) tributed to CR. The result also indicates that for p T > 1.1 GeV/c, the particles inside hard jets are not sensitive color reconnection. The peak is enhanced if one cons ers 1 events with increased MPI activity. In the figure p (GeV/c) T 0.9 plotted the results for events with more than 20 MP in this case the peak is also pushed up to higher p A. Ortiz Velasquez et al., T FIG. 1. (Color online) Proton to pion ratio from pp collisions at 0.8 characteristic effect of flow. The curve crosses the M Phys. s =7TeV.ALICEdataarecomparedtoresultsfrom Rev. Lett. 111 (2013) curve at 1.8 GeV/c and the maximum ratio reaches PYTHIA 8 Tune 4C, as well as NLO QCD calculation [13]. at03 GeV/c. 1 On2the contrary, 3 4in events 5 with less tha MPI s the ratio behaves like in simulations p (GeV/c) T without C Torbjörn Sjöstrand Colour Reconnection The evolution of the ratio with the number slide 15/17 of MPI ) + π (p + p) / (π + ) + π + (p + p) / (π (RR=1.5)/(RR=0.0)

16 Summary and Outlook Reconnection well established, from B J/ψ to p (n ch ). Missed chance for clean tests at LEP 2. Multitude of algorithms for Pythia 6 uncertainty band. Predict (possibility of) significant effects on m t. To do: develop new reconnection algorithms in Pythia8. Want more detailed understanding of space time picture combined with colour algebra. High string density will preclude any definitive answers? Breakthrough from new precision differential data? Far future: high-luminosity e + e collider? Addendum: new CMS PAS TOP , Study of the underlying event, b-quark fragmentation and hadronization properties in tt events shows colour reconnection impact on underlying activity as function of p tt and φ tt. Torbjörn Sjöstrand Colour Reconnection slide 16/17

17 Let s aim for more than a flying circus... Torbjo rn Sjo strand Colour Reconnection slide 17/17