QCD at the Large Hadron Collider
Jon Butterworth SLAC Summer Institute SLAC August 2018 Outline • Total cross section and parton densities • The soft stuff • Jet production • Jet substructure • Heavy Flavour production • Measuring the coupling • Quark and gluon matter • Future prospects
Aug 2018 JMB: QCD@LHC SSI 2 Outline • Total cross section and parton densities • The soft stuff • Jet production • Jet substructure • Heavy Flavour production • Measuring the coupling • Quark and gluon matter • Future prospects
Aug 2018 JMB: QCD@LHC SSI 3 Aug Aug 2018 JMB: QCD@LHC SSI QCD@LHC JMB: 4
Modelling the Underlying Event Aug Aug 2018 JMB: QCD@LHC SSI QCD@LHC JMB: 5
Modelling the Underlying Event To the TeV scale and beyond…
Pic from Sherpa/F.Krauss
Aug 2018 JMB: QCD@LHC SSI 6 Measuring multiparton scattering
ATLAS: arXiv:1301.6872
Aug 2018 JMB: QCD@LHC SSI 7 Measuring multiparton scattering
CMS: arXiv:1312.5729
Aug 2018 JMB: QCD@LHC SSI 8 So what about jets then?
Aug 2018 JMB: QCD@LHC SSI 9 ‘Hard’ QCD • When quarks and gluons make a break for it… Jets!
Aug 2018 JMB: QCD@LHC SSI 10 ‘Hard’ QCD • When quarks and gluons make a break for it… Jets!
Aug 2018 JMB: QCD@LHC SSI 11 ‘Hard’ QCD • When quarks and gluons make a break for it… Jets!
Aug 2018 JMB: QCD@LHC SSI 12 What is a Jet? • Protons are made up of quarks and gluons. • Quarks and gluons are coloured and confined – we only ever see hadrons. • A jet of hadrons is the signature of a quark or gluon in the final state. • The gross properties (energy, momentum) reflect the properties of the quark or gluon, and stand out above the rest of the event. • Jets are defined by an algorithm which can be applied to data and theory so we compare like-with-like.
Aug 2018 JMB: QCD@LHC SSI 13 Jet finder(s)
• Form a list – (of particles, tracks, clusters or partons) • Merge the smallest pair • Iterate Cacciari, Salam, Soyez • Anti-kt : p = -1 (Default R=0.4 or 0.6) JHEP 0804:063,2008 – Stable, “circular” jets built around highest pT regions Dokshitzer, Leder, Morretti, • Also used: Webber (JHEP 08 (1997) – Cambridge/Aachen: p = 0. Based only on angular 01; Wobisch and Wengler information hep-ph/9907280
– kt : p = 1. Merging scales are physically meaningful Catani et al Phys Lett B269 (1991); Nucl. Phys. B406 (1993); Ellis and Soper Phys Rev D48 (1993). Aug 2018 JMB: QCD@LHC SSI 14 Dijet cross sections arXiv:1711.02692
NNLO calculation: J. Currie et al, arXiv:1611.01460, arXiv:1704.00923
Aug 2018 JMB: QCD@LHC SSI 15 Dijet cross sections arXiv:1711.02692
NNLO calculation: J. Currie et al, arXiv:1611.01460, arXiv:1704.00923
Aug 2018 JMB: QCD@LHC SSI 16 Total cross sections
PDG 2018
Aug 2018 JMB: QCD@LHC SSI 17 Rapidity gaps/intervals between jets
arXiv:1710.02586
Aug 2018 JMB: QCD@LHC SSI 18 Rapidity gaps/intervals between jets
Aug 2018 JMB: QCD@LHC SSI arXiv:1407.5756 19 Rapidity gaps/intervals between jets
Aug 2018 JMB: QCD@LHC SSI 20 arXiv:1407.5756 Dijet cross sections • Vector boson fusion, vector boson scattering, and tag jets – see Fabrizio’s slides yesterday – jet veto problem – Color coherence from Q&A
Aug 2018 JMB: QCD@LHC SSI 21 Vector Boson Fusion
Aug 2018 JMB: QCD@LHC SSI arXiv:1401.7610 22 Jets + Electroweak bosons
Aug 2018 JMB: QCD@LHC SSI 23 To the TeV scale and beyond… • Precision theory, exclusive calculations, fixed orders & resummed & matched
Aug 2018 JMB: QCD@LHC SSI 24 To the TeV scale and beyond… • Precision theory, exclusive calculations, fixed orders & resummed & matched
• Vertices as, O(0.1)
Aug 2018 JMB: QCD@LHC SSI 25 To the TeV scale and beyond… • Precision theory, exclusive calculations, fixed orders & resummed & matched
• Vertices as, O(0.1) • Soft or collinear kinematics log(scales) O(10)
Aug 2018 JMB: QCD@LHC SSI 26 To the TeV scale and beyond… • Precision theory, exclusive calculations, fixed orders & resummed & matched
• Vertices as, O(0.1) • Soft or collinear kinematics log(scales) O(10)
• aslog terms O(1), must be resummed (exponentiated)
Aug 2018 JMB: QCD@LHC SSI 27 To the TeV scale and beyond… • Precision theory, exclusive calculations, fixed orders & resummed & matched
• Vertices as, O(0.1) • Soft or collinear kinematics log(scales) O(10)
• aslog terms O(1), must be resummed (exponentiated) • Still need fixed- order as outside these enhanced Aug 2018 JMB: QCD@LHC SSI regions 28 To the TeV scale and beyond…
W
n
e
Aug 2018 JMB: QCD@LHC SSI 29 To the TeV scale and beyond…
Pic from Sherpa/F.Krauss
Aug 2018 JMB: QCD@LHC SSI 30 To the TeV scale and beyond…
• Into the unknown… – Well-defined, precise measurements and calculations. – High multiplicities – High boosts, even for electroweak-scale objects
Aug 2018 JMB: QCD@LHC SSI 31 To the TeV scale and beyond… • New feature… the boost • Perturbative QCD between about 1 GeV (hadronisation) and LHC kinematic limit (highest pT jet formation)
Aug 2018 JMB: QCD@LHC SSI 32 To the TeV scale and beyond… • New feature… the boost • Perturbative QCD between about 1 GeV (hadronisation) and LHC kinematic limit (highest pT jet formation) • Electroweak symmetry breaking scale lies between these scales • Electroweak physics “inside” jets (H,W,Z,t – collimated decay products). • Need to account for this in measurments and searches
Aug 2018 JMB: QCD@LHC SSI 33 Example: High pT Higgs and Vector Boson
• By requiring that the Higgs and Vector Boson have a high transverse momentum, we lose a factor of ~20 in cross section – However, much of this would have failed other analysis cuts anyway – Background cross sections fall by a bigger factor (typically t-channel not s- channel) • W/Z and H are all central “mono”-Jet – Better b-tagging, better jet resolution mH • W/Z and H decay products collimated b – Simpler topology, fewer combinatorials H – Difficult for tops to fake this W • Z neutrinos becomes visible u – High missing ET • JMB, Davison, Rubin, Salam, Phys. Rev. Lett. 100, 242001 (2008) l
Aug 2018 JMB: QCD@LHC SSI 34 Sub-jet analysis
1. Start with Higgs candidate jet (highest pT jet in acceptance) with mass m) 2. Undo last stage of clustering (reduce radius to R12) J J1, J2
3. If max(m1,m2) < 2m/3 Call this a “mass drop”. This fixes the optimal radius for reconstructing the Higgs decay. Keep the jet J and call it the Higgs candidate. Else, go back to 2
4. Require Y12 > 0.09 Dimensionless rejection of asymmetric QCD splitting Else reject the event
5. Require J1, J2 to each contain a b-tag Else reject the event
Aug 2018 JMB: QCD@LHC SSI 35 Sub-jet analysis
6. Define Rfilt = min(0.3, Rbb/2) Make use event-by-event of the known Higgs decay radius Angular ordering means this is the characteristic radius of QCD radiation from Higgs products Stuff outside of this is likely to be underlying event and pileup.
7. Recluster, with Cambridge/Aachen, R = Rfilt 8. Take the 3 hardest subjets and combine to be the Higgs b, anti-b and leading order final state gluon radiation 9. Plot the mass
Aug 2018 JMB: QCD@LHC SSI 36 Improved subjet analysis
Aug 2018 JMB: QCD@LHC SSI 37 Improved subjet analysis
Aug 2018 JMB: QCD@LHC SSI 38 Improved subjet analysis
Aug 2018 JMB: QCD@LHC SSI 39 Improved subjet analysis
Aug 2018 JMB: QCD@LHC SSI 40 Improved subjet analysis
Aug 2018 JMB: QCD@LHC SSI 41 Improved subjet analysis
Aug 2018 JMB: QCD@LHC SSI 42 Combined particle-level result
• Note excellent Z peak for calibration • 5.9 s; potentially very competitive • bb branching information critical for extracting Higgs properties • “Measuring the Higgs sector” Lafaye, Plehn, Rauch, D.Zerwas, Duhrssen, arXiv:0904.3866 [hep-ph] • Studies with full simulation supported this.
Aug 2018 JMB: QCD@LHC SSI 43 Aug 2018 JMB: QCD@LHC SSI 44 Aug 2018 JMB: QCD@LHC SSI 45 Additional (2009-2018): • Many more and better techniques • Reduce impact of pile up • Quark/gluon separation • Study quark/gluon medium Aug 2018 JMB: QCD@LHC SSI 46 Jet Substructure • Many improvement since the first ideas • Excellent summaries: – Experiment: arXiv:1803.0699 – Theory, and Machine Learning: arXiv:1709.04464 • Obviously useful for tagging; but needs to be under good theoretical control or we won’t understand what we have measured
Aug 2018 JMB: QCD@LHC SSI 47 Jet Substructure • Obviously useful for tagging; but needs to be under good theoretical control or we won’t understand what we have measured • Example: Non-global logarithms, QCD parton radiates out of, then back into, the jet – Dasgupta and G. P. Salam, arXiv: hep-ph/0104277 – Generally present at NLL accuracy, but possible to defined variables which are insensitive to them – Softdrop jet mass: M. Dasgupta et al., arXiv: 1307.0007; A. J. Larkoski et al., arXiv: 1402.2657.
Aug 2018 JMB: QCD@LHC SSI 48 Jet Substructure
• Find jet with anti-kT, recluster constituents with Cambridge/Aachen • Go back down the clustering history, apply criteria to proto-jet
• If not satisfied, remove the smaller pT branch, continue on the higher. • If satisfied, terminate and calculate mass of remaining jet Aug 2018 JMB: QCD@LHC SSI 49 Jet Substructure
• Measured by ATLAS, arXiv:1711.08 341
Aug 2018 JMB: QCD@LHC SSI 50 CMS boosted Higgs search
arXiv:1709.05543
Aug 2018 JMB: QCD@LHC SSI 51 Boost 2012 Experimental Summary: Sal Rappoccio
Aug 2018 JMB: QCD@LHC SSI 52 Heavy Flavour • Gluon splitting to bb is a difficult QCD feature which needs to be studied and understood • B-tagged dijets, but limited by jet radius • Angular distribution between B-hadrons
CMS arXiv:1102.3194 ATLAS arXiv:1705.03374
Aug 2018 JMB: QCD@LHC SSI 53 “Top” stuff QCD radiation in top events has impact on mass and cross section measurements
arXiv:1610.09978
Aug 2018 JMB: QCD@LHC SSI 54 “Top” stuff • Boosted top differential cross section, built from final-state particles
Aug 2018 JMB: QCD@LHC SSI 55 Strong coupling 2 H1 multijets at low Q : EPJC 67:1 (2010)
H1+ZEUS (NC, CC, jets) H1-prelim-11-034, ZEUS-prel-11-001 (2011)
* ZEUS incl. jets in g p : NPB 864:1 (2012)
H1 multijets at high Q2 : arXiv 1406.4709 (2014) The parameter of QCD… CDF Incl. Jets : PRL 88:042001 (2002)
D0 incl. jets : PRD 80:111107 (2009)
D0 ang. correl. : PLB 718:56 (2012)
Malaescu & Starovoitov (ATLAS Incl. Jets 7TeV) EPJC 72:2041 (2012)
ATLAS N32 7TeV : ATLAS-CONF-2013-041 (2013)
ATLAS TEEC 7TeV : PLB 750:427 (2015)
CMS R32 7TeV : EPJC 73:2604 (2013)
CMS tt cross section 7TeV : PLB 728:496 (2014)
CMS 3-Jet mass 7TeV : EPJC 75:186 (2015)
CMS Incl. Jets 7TeV : EPJC 75:288 (2015)
CMS Incl. Jets 8TeV : JHEP 03:156 (2017)
CMS R32 8TeV : CMS-PAS-SMP-16-008 (2017)
World Average : Chin. Phys. C 40:100001 (2016)
0.1 0.12 0.14 0.16 0.18 0.2
aS(M ) Aug 2018 JMB: QCD@LHC SSI 56 Z High Density, Heavy Ions .. All collective flow Jurgen Schukraft in “SM@50”, Case Western signatures seen in pp & pA Reserve, June 2018 .. Other ‘QGP’ signals emerge (strangeness, HBT, Y(2S), The only exception is “Jet Quenching”. … continuous & smooth down to dN/dy ≈ 0 (MinBias pp)
Christopher McGinn, Boost 2018
Aug 2018 JMB: QCD@LHC SSI 57 The Future? • Boosted objects are here to stay, and many involve decays to (sub)jets. Maintaining resolution on this will be an experimental challenge. • Much potential precision LHC physics (HL upgrade) requires improved QCD precision and understanding • Electroweak & QCD mixed parton showers/resummation probably needed, especially for multi-TeV jets • Connecting total cross sections and high density QCD to the QCD lagrangian may be in reach
Aug 2018 JMB: QCD@LHC SSI 58