Hard Scattering, Jets and Jet Quenching

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Quark-Gluon Plasma Physics

8. Hard Scattering, Jets and Jet Quenching

Prof. Dr. Klaus Reygers Heidelberg University SS 2017 Theoretical description of High-pT particle production: Perturbative QCD

■ Scattering of pointlike partons described by QCD perturbation theory (pQCD) ■ Soft processes described by universal, phenomenological functions ‣ Parton distribution function from deep inelastic scattering ‣ Fragmentation functions from e+e- collisions ■ Particle production dominated by hard scattering for pT ≳ 3 GeV/c ‣ However, 99% or so of all particle from soft processes

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 2 Jet quenching in heavy-ion collisions

p+p A+A

q q

q q q q

q q

A-A collision: shower evolution in the medium, energy loss of the leading parton

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 3 Jet quenching history

It is now believed that radiative energy loss (gluon bremsstrahlung) is more important than elastic scattering

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 4 Collisional vs. radiative parton energy loss

Collisional energy loss: Radiative energy loss:

■ Elastic scatterings with medium ■ Inelastic scatterings within the constituents medium ■ Dominates at low parton momenta ■ Dominates at higher momenta

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 5 Analogy: Energy loss of charged particles in ordinary matter http://pdg.lbl.gov /g] 2 µ+ on Cu 100 µ−

⟩ Bethe Radiative

x Anderson- Ziegler /d

E Radiative E d effects µc ⟨ 10 reach 1% Scharff Radiative Lindhard- Minimum losses ionization Nuclear losses Without δ Mass stopping power [MeV cm 1 4 5 0.001 0.01 0.1 1 10 100 1000 10 10 βγ

0.1 1 10 100 1 10 100 1 10 100 [MeV/c] [GeV/c] [TeV/c] Muon momentum μ+ on Cu: Radiational energy loss ("bremsstrahlung") starts to dominate over collisional energy loss ("Bethe-Bloch") for p ≫ 100 GeV/c QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 6 Basics of radiative parton energy loss (1)

■ Energy loss E in a static medium of length L for a parton energy E → ∞:

2 2 µ 3 for gluon jets E ↵s CF qLˆ qˆ = CF = / (4/3 for quark jets µ2 : typical momentum transfer from medium to parton per collision

: mean free path length in the medium BDMPS result, Nucl. Phys. B 483, 291, 1997

■ Landau-Pomeranchuk-Migdal (LPM) effect ‣ Parton scatters coherently off many medium constituents: destructive interference ‣ Reduces radiative energy loss

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 7 Basics of radiative parton energy loss (2) 3-momentum and energy of the radiated parton Formation time (or length) of a radiated gluon: (“time for the fast parton to get rid of its virtuality”) ! 1 zcoh = tcoh 2 2 ' kT ' !✓

The gluon acquires additional transverse momentum if it scatters with medium constituents within its formation time (or formation length zc): µ2 k2 qzˆ = z T ' coh coh ! ! This results in a medium-modiﬁed formation length: z coh ' k2 ' qˆ T r

>zcoh : incoherent multiple scattering

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 8 Basics of radiative parton energy loss (3)

dE/dz increases linearly with medium thickness L as long as L is smaller then the coherence length zcoh

zcoh coherence thickness length of the medium

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 9 Ncoll-scaled π0 yields in pp compared to Au-Au

3 –2

) 10 NN

c 0 TAA = Ncoll /inel 2 Au-Au → π X @ 200 GeV [0–10%] 10 h i 0 0–10% “increase in parton luminosity” p-p → π X @ 200 GeV × TAA 10 0–10% per collision when going from NLO × T [CTEQ6, KKP, μ =p –2p ] AA i T T pp to AA” dy (GeV/ 1 [W.Vogelsang] T 10–1

N/dp –2 2 10 Without a medium, ] d –3 T 10 hadron yields for p π 10–4 pT ≳ 2-3 GeV are –5 1/[2 10 expected to scale with Ncoll 10–6 10–7 –8 Observation: 10 Clear suppression w.r.t. 10–9 Ncoll scaling 0 2 468 10 12 14 16 18 20 p (GeV/c) T

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 10 Discovery of Jet Quenching at RHIC (ca. 2000–2003)

dN/dpT R = |A+B , AB T d /dp h ABi⇥ inv T|p+p where T = N /NN h ABi h colli inel

■ Hadrons are suppressed, direct photons are not ■ Evidence for parton energy loss

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 11 A simple explanation for the rather ﬂat RAA at RHIC: a constant fractional parton energy loss

1 dN 1 0 π spectrum without energy loss: n pT dpT / pT

π0 spectra at RHIC energy (√sNN = 200 GeV) described with n ≈ 8

Constant fractional energy loss:

pT "loss := ,i.e.,pT0 =(1 "loss)pT pT

Resulting RAA:

n 2 1/(n 2) R =(1 " ) " =1 R 0.2 for R 0.25 AA loss ) loss AA ⇡ AA ⇡ RAA depends on the parton energy loss and the shape of the pT spectrum

In this simplistic model the constant RAA ≈ 0.25 implies a constant fractional energy loss of about 20% in central Au+Au collisions at 200 GeV

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 12 Single-particle RAA in Pb-Pb at the LHC: Qualitatively similar observation as for RHIC energies

2 ± ± h , p-Pb s = 5.02 TeV, NSD (ALICE) ■ No suppression for γ, pPb h , Pb-Pb (ALICE) NN ± R γ, Pb-Pb s = 2.76 TeV, 0-10% (CMS) +- 0 1.8 h , Pb-Pb (CMS) NN W , Z in Pb-Pb

, W±, Pb-Pb s = 2.76 TeV, 0-10% (CMS) sNN = 2.76 TeV, 0-5% NN 1.6 0 Z , Pb-Pb s = 2.76 TeV, 0-10% (CMS) ■ No suppression of

PbPb NN

R 1.4 hadrons in p-Pb 1.2 ■ Strong suppression of hadrons in Pb-Pb 1 0.8 0.6 0.4

0.2 0 0 10 20 30 40 50 60 70 80 90 100 p (GeV/c) or mass (GeV/c2) T

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 13 Medium properties from charged hadron RAA(pT)

■ Fit of various models to RAA(pT) at RHIC and the LHC ■ Jet transport parameter loss for radiative energy loss at the highest temperatures reached (for Eparton = 10 GeV, QGP thermalization at τ0 = 0.6 fm/c):

Jet Coll., Phys.Rev. C90 (2014) 014909

■ Result relies on standard hydro description of the medium evolution ■ Conjectured relation to η/s: T 3 ⌘ weakly-coupled QGP: K 1 Majumder, Müller, Wang, = K ⇡ PRL, 99 (2007) 192301 strongly-coupled QGP: K 1 qˆ s ⌧

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 14 π, K, p RAA: Suppression independent of hadron species for pT ≳ 8 GeV/c

ALICE, arXiv:1506.07287 AA R 1

ALICE 0-5% Pb-Pb sNN=2.76 TeV π++π- 0.8 K++K- p + p 0.6 Leading-parton energy 0.4 loss followed by fragmentation in QCD 0.2 vacuum (as in pp) for pT,hadron > 8 GeV/c? 0 2 4 6 8 10 12 14 16 18 20 p (GeV/c) ALI−DER−86145 T

■ RAA(p) > RAA(K) ≈ RAA(π) for 3 < pT < 8 GeV/c

■ Similar p, K and π RAA for pT > 8 GeV/c

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 15 D meson RAA: Charm quark energy loss similar to quark and gluon energy loss Eg > Eu,d,s > Ec > Eb ALICE, arXiv:1203.2160 2 color factor dead cone eﬀect AA

R ALICE (gluon emission suppressed 1.8 0-20% centrality at forward angles for slow 1.6 Pb-Pb, s = 2.76 TeV quarks) NN ■ Strong suppression also for D 0 + *+ 1.4 Average D , D , D , |y|<0.5 mesons (which cannot be Charged particles, |η|<0.8 1.2 CMS non-prompt J/ψ, |y|<2.4 explained by shadowing) 1 ■ Suppression of D mesons and 0.8 pions (surprisingly?) similar

0.6 ‣ Pions mainly from gluons ‣ Dead cone effect for c and b 0.4 ■ Still hint for expected 0.2 hierarchy? 0 0 2 4 6 8 10 12 14 16 18 ‣ However, need to carefully p (GeV/c) ALI−PUB−14258 t consider also the steepness of the initial parton spectra

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 16 Centrality dependence of RAA for pions and D mesons

ALICE, arXiv:1506.06604 1.4

AA ■ Npart dependence similar for

R Pb-Pb, s = 2.76 TeV NN pions and D mesons π± (ALICE) 8

0.6

50-80% 0.4

40-50% 30-40% 0.2 20-30% 10-20% π± shifted by +10 in 〈N 〉 0-10% part 0 0 50 100 150 200 250 300 350 400 〈N 〉 ALI−PUB−93568 part

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 17 Evidence for smaller energy loss for b quarks than for c quarks

ALICE, arXiv:1506.06604 1.4

AA b quark energy loss via

R Pb-Pb, sNN = 2.76 TeV D mesons (ALICE) 8

0.6

50-80%* 0.4

40-50% Evidence for the expected 0.2 30-40% 20-30% energy loss hierarchy between (*) 50-100% for non-prompt J/ψ 10-20% 0-10% c and b quarks 0 0 50 100 150 200 250 300 350 400 〈N 〉 ALI−DER−93729 part

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 18 B-MesonFlavor R DependenceAA(pT) of Parton Energy Loss

PbPb 2.76, 5.02 TeV b dijet and inclusive dijet balance xJ = pT,2 / pT,1 Large double ratio if b jets are less quenched No quark ﬂavor dependence of RAA(pT) for pT ≳ 10 GeV/c

Indication for quark ﬂavor dependence at lower pT from PbPb B → J/ψ + X b→J/ψ

D0 h± B+ CMS, 1705.04727

• Appearance of the flavor dependence at QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 19 pT < 10 GeV between b→J/ψ and other particles

Ta-Wei Wang (Talk) Kurt Jung (Talk) • b dijet are not more symmetric than dijet D0 CMS-PAS-HIN-16-001 Charged hadrons • Similar b jet quenching and inclusive jet arXiv: 1611.01664 J/ψ arXiv: 1610.00613 B+ CMS-PAS-HIN-16-011 Submitted to JHEP Submitted to EPJC Jing Wang (Poster) Gian Michele Innocenti (Poster) B dijet CMS-PAS-HIN-16-005

Yen-Jie Lee (MIT) Highlights from CMS 12 Studying jet quenching with jets: Large dijet energy asymmetries in Pb-Pb

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 20 Jet suppression in Pb-Pb at √sNN = 2.76 TeV: RAA ≈ 0.5 in central collisions

ATLAS, arXiv:1411.2357 0 - 10 % 30 - 40 % 60 - 80 %

AA Interestingly, there anti-k R = 0.4 jets -1 R ATLAS t 2011 Pb+Pb data, 0.14 nb -1 sNN = 2.76 TeV 2013 pp data, 4.0 pb is not much of a pT 1 dependence of the jet suppression 0.5

| y | < 2.1 0 40 60 100 200 400 p [GeV] T

AA -1 2011 Pb+Pb data, 0.14 nb anti-kt R = 0.4 jets R -1 2013 pp data, 4.0 pb sNN = 2.76 TeV 1

0.5

80 < p < 100 GeV | y | < 2.1 T 0 0 50 100 150 200 250 300 350 400 N part QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 21 6 First measurement: jet RAA in 5 TeV PbPb

n 0-10% central jet RAA measured in 100 - 1 TeV

n Clear increase with pT and flattens above 200-300 GeV n Consistent with 2.76 TeV results, but much reduced uncertainty Pb-Pb at √sNN = 5.02 TeV: Jet RAA up to pT = 1 TeV n Thanks to the 5 TeV pp reference data taken with same detector condition

ATLAS-CONF-2017-009, http://cds.cern.ch/record/2244820

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 22 Summary/questions jet quenching

■ Collisional and radiative parton energy loss ‣ Radiative energy loss expected to be dominant for light quarks ■ Evidence for expected quark mass dependence of the energy loss ■ QCD inspired models are capable of reproducing many features seen in the data ‣ Medium properties can be constrained ■ What’s next? ‣ First generation of models focussed on leading-particle energy loss (“medium-modiﬁed fragmentation function”) ‣ Need to describe full parton shower evolution in the medium ‣ Can one eventually describe parton energy loss based on ﬁrst principles? ‣ Can one connect heavy-quark energy loss to string theory via the gauge/gravity duality?

QGP physics SS2017 | K. Reygers | 8. Hard Scattering, Jets and Jet Quenching 23