Recent Jet Measurements in ALICE - QCD 2021 Hannah Bossi (Yale University) for the ALICE Collaboration July 5th, 2021 (Remote)
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Recent Jet Measurements
in ALICE
Hannah Bossi (Yale University) for the ALICE Collaboration
QCD 2021
July 5th, 2021 (Remote)
What is a jet?
In hard scatterings, partons scatter off of one
another with a high momentum transfer.
A jet is the spray of particles that results from
the fragmentation and hadronization of a
parton.
Hard
Scattering
Production of high pT partons calculable in
perturbative QCD (pQCD).
Parton Shower Hadronization
Jets are sensitive to physics information from many physics scales →great object to study
these different processes!
Hannah Bossi (Yale University) QCD 2021 1
Experimental jets
Jets are not a perfect proxy for the dynamics of the parent parton.
→Different physics effects can move energy into or outside of the jet cone.
Parton Shower Hadronization Underlying Event
R R
R
2
∼ln(1/R) ∼1/R ∼R
Effects such as hadronization and the underlying event are non-perturbative.
Each of these effects scales differently with the cone radius R; jets can be used to
study these effects! CMS, JHEP 12 (2020) 082
Hannah Bossi (Yale University) QCD 2021 2
Some usages of jet measurements
Studying processes of a Probing the Quark-Gluon
Testing Fundamental QCD
specific origin Plasma in Heavy-ion
R Collisions
q q
q q
0
D
Ex: Boosted objects (Higgs, Look at modifications of
Both perturbative andp+p A+A
BSM), quark vs. gluon jets, jets and their substructure
non-perturbative effects!
heavy-flavor jets from the vacuum case.
Jet measurements provide a wealth of physics information!
Hannah Bossi (Yale University) QCD 2021 3
The ALICE detector
Dedicated heavy-ion experiment at the LHC.
Reconstructs jets at mid-rapidity ( | η | < 0.7)
in pp, p—Pb and Pb—Pb collisions.
Utilizes high precision
tracking detectors to
measure charged particle
jets over a wide range in
jet pT.
Full jets combine charged
particle information with
neutral particle information
measured in the
electromagnetic calorimeter.
ALICE is great for jet measurements, especially measurements of jet substructure!
Hannah Bossi (Yale University) QCD 2021 4
Jet axis differences
Cal, Neill, Ringer, Waalewijn JHEP 04 (2020) 211
Standard - groomed
Standard Axis: anti-kT jet with E-scheme recombination.
Groomed Axis: groom jets with Soft Drop (SD) algorithm
Larkoski et al. JHEP 05 (2014) 146
Less sensitive to soft radiation than standard (SD
selects hard splittings, soft splittings groomed away).
pT,2
z > zcutθ β z= zg
pT,1 + pT,2 θg
1 −zg
Observable is sensitive to soft effects and is
calculable! (good to study hadronization & UE effects)
More aggressive grooming, higher probability for misalignment.
Hint of tension between data and MC for standard vs. groomed axes. Useful for
tuning MCs!
Hannah Bossi (Yale University) QCD 2021 5
The primary Lund jet plane pT,2 = zpT,jet
ln(kT)
Representation of the phase space of jet splitting pT,1 = (1 −z)pT,jet
in terms of the angle and momentum fraction of
the split. kT = pT,2 sin(ΔR)
Soft, Wide Angle Radiation
H
ar
z=
d,
C
ol
lin
1
ea
rR
ad
Collimated Wide Angle Symmetric Split Asymmetric Split
ia
tio
n
Primary Lund plane is filled with the
splittings off of the hardest prong!
ln(R/ΔR)
At leading order emissions populate the Lund plane uniformly →
running of the coupling sculpts the plane!
Different regions sensitive to perturbative/non-perturbative effects!
Hannah Bossi (Yale University) QCD 2021 6
The primary Lund jet plane ALICE-PUBLIC-2021-002
ALICE Preliminary Charged-particle jets anti-k T R = 0.4
pp s = 13 TeV |ηjet | < 0.5, 20 < p ch
T, jet
< 120 GeV/c
ΔR
0.4 0.35 0.3 0.25 0.2 0.15 0.1
1.5
Measurement of the fully
(1/ N jets )d2N emissions/(dln( k T /GeV)dln( R / ΔR ))
ln(k T/GeV)
1
1 0.8
corrected Lund plane density
0.5 0.6
0 0.4 Compliments measurement by
−0.5 0.2
ATLAS which was made at higher pT
( > 675 GeV/c vs. 20 –120 GeV/c )
−1 0
0 0.2 0.4 0.6 0.8 1 1.2 1.4 ATLAS, Phys. Rev. Lett. 124 (2020) 22, 222002
ALI-PREL-480020
ln(R / ΔR )
Lund plane covers the full phase space! → Can use projections to make
discriminative differential measurements when compared to theory!
Hannah Bossi (Yale University) QCD 2021 7
The primary Lund jet plane ALICE-PUBLIC-2021-002
Project onto the ln(kT) axis in two different regions!
ALICE Preliminary Charged-particle jets anti-k T R = 0.4
pp s = 13 TeV |ηjet | < 0.5, 20 < p ch
T, jet
< 120 GeV/c
ΔR
0.4 0.35 0.3 0.25 0.2 0.15 0.1
1.5
(1/ N jets )d2N emissions/(dln( k T /GeV)dln( R / ΔR ))
ln(k T/GeV)
1
1 0.8
A B
0.5 0.6
0 0.4
−0.5 0.2
−1 0
0 0.2 0.4 0.6 0.8 1 1.2 1.4
ALI-PREL-480020
ln(R / ΔR )
ALI-PREL-479228
A: Wide Angle B: Collimated
Tension in hard collimated splittings in the perturbative region (kT > 1 GeV /c)!
Model uncertainties dominant (Herwig vs. PYTHIA in the response + matching
purity corrections)
Hannah Bossi (Yale University) QCD 2021 8
Dead cone effect with heavy-quark Lund Plane
Projections of the Lund plane of heavy-quarks make the
θ first direct measurement of the dead cone possible!
Pattern of the parton shower is expected to depend on the
Dead cone angle: θ0 = m/E mass of the initiating parton.
Gluon spectrum emitted
from a heavy quark
suppressed in within a cone
of m/E from the emitter →
dead cone effect!
Suppression of splittings at
0
low angles for D jets
compared to inclusive jets!
ALI-PUB-493419
ALICE, arXiv: 2106.05713
Hannah Bossi (Yale University) QCD 2021 9Jets as a probe of the QGP
High pT parton interacts
with the QGP resulting in jet
energy loss and jet
substructure modification
q
(jet quenching).
q
q q
Jets are an excellent probe
of the QGP as they
experience its full evolution!
Use pp, where jets are
measured in vacuum, as a
p+p
p+p A+A
Heavy-Ions (A+A) reference for no QGP.
Measurement of jets in heavy-ions is difficult due to the large underlying event.
Hannah Bossi (Yale University) QCD 2021 10ML background estimator
Conventional approach: Apply a minimum pT q q
requirement on the leading track of the jet, correct the q q
jet for the background with a pedestal subtraction.
corr raw
pT,jet = pT,jet −ρ ⋅ Ajet
ML approach: Use ML (shallow NN) to make an improved correction of the jet pT for the
p+p track bias).
background with the aid of jet properties (no leading A+A
Jet Properties Unfold for
ML
(Including constituent Corrected Jet pT fluctuations and
detector effects
properties)
R.Haake, C. Loizides Phys. Rev. C 99, 064904 (2019)
Hannah Bossi (Yale University) QCD 2021 11Applying to data - jet RAA
Expected signature of jet quenching is the
suppression of jet yields at a given pT.
We quantify this through the nuclear
modification factor RAA:
1 d 2Njet
PbPb
|cent
RAA = scaled
=
Nevent dpTdy
2 pp
d σjet
⟨TAA⟩ dp dy
T
RAA < 1 → Suppression
RAA = 1 → No Modification
RAA > 1 → Enhancement
See significant jet suppression down to 40 GeV/c with reduced systematic uncertainties!
(q/g fragmentation bias included in systematics).
ML result is relatively robust to the explored biases!
Hannah Bossi (Yale University) QCD 2021 12Groomed jet splittings in Pb—Pb θg =
Rg
R
=
Δη 2 + Δϕ 2
R
Goal is to probe the extent to which the
colored medium resolves the color-structure
of the jet! ALICE-PUBLIC-2020-006
Decoherence: medium resolves split, more
energy loss (more partons undergoing energy
loss) (Fully Decoherent ~ Lres = 0)
Coherence: medium does not resolve split,
less energy loss (Fully Coherent ~Lres = ∞)
Lres Lres
ALI-PREL-352930
Suppression of wide angle splittings, favored by
models with decoherence. Could this also be Pablos et al. JHEP (2020) 044 Caucal et al. JHEP (2019) 273
coherence with a high quark fraction? Yuan et al. arXiv:1907.12541 JETSCAPE arXiv: 1903.07706
Hannah Bossi (Yale University) QCD 2021 13Subjet fragmentation
Cluster jets with radius R and subjets with radius
r < R. Look at the fragmentation of the subjet. R = 0.4 r = 0.1
pT,subjet
zr = r
pT,jet R
Hint of hardening at intermediate zr with a hint of
modification as zr → 1.
In this region, the sample is closer to a purely
quark-jet sample.
ALI-PREL-490655
Could this be due to modification of the q/g fraction?
Hannah Bossi (Yale University) QCD 2021 14Conclusions
Jets offer a wealth of experimental observables which can help investigate
many different physics questions.
ALICE is well suited to measure jets and their substructure at low/intermediate pT
at the LHC.
Recent jet measurements report new discriminative observables which can be
measured differentially → good for isolating physics effects!
Many more new and exciting measurements on the way! Stay tuned!
Hannah Bossi (Yale University) QCD 2021 15Recent ALICE jet measurements
Jet axis differences
Generalized jet angularities (with and without grooming)
Lund jet plane ALICE-PUBLIC-2021-002 ALICE-PUBLIC-2020-002
0
Groomed jet substructure measurements of charm jets tagged with D mesons
Direct observation of the dead-cone effect in QCD arXiv: 2106.05713
Groomed jet substructure and differential jet cross section at s = 13 TeV
Hardest kT splittings
Inclusive jet measurements with ML techniques
= pp only
Inclusive and leading subjet fragmentation
= Pb—Pb and pp
N-subjettiness arXiv: 2105.04936
Groomed jet splittings ALICE-PUBLIC-2020-006
Jet acoplanarity Many more interesting results and more on the way!
Hannah Bossi (Yale University) QCD 2021 16Backup
Exact scalings
Parton Shower Hadronization Underlying Event
R R
R
Scaling dictated by the QCD splitting 2CA(μ1) 1 2
(δpT)q ≈− (δpT)q ≈ ΛUER
function at leading order πR 2
Derived by taking
For quarks
αs pT 1 3 Where ΛUE is the
αs(μ) = μ1δ(μ −μ1)
(δpT)q = −Cf ln( )(2ln(2) − ) + . . . energy density per unit
π R 8 Where μ1corresponds rapidity.
For gluons
(δpT)g = −
αs pT 1 43 7
ln( )[CA(2ln(2) − ) + TRnf ] + . . . .
to scale at which αs is
π R 96 48 infinite (Landau pole) CMS, JHEP 12 (2020) 082
Hannah Bossi (Yale University) QCD 2021What is a jet? - A experimentalist’s answer
Energy from jets gets deposited in the detector and is then
reclustered using jet clustering algorithms with a specified
distance parameter, related to the cone radius R. R
A common choice of jet finding algorithm at the LHC is the
anti-kT algorithm.
arXiv: 0802.1189
A jet is the result of a jet finding algorithm!
Hannah Bossi (Yale University) QCD 2021Jet axis differences Cal, Neill, Ringer, Waalewijn JHEP 04 (2020) 211
Check the difference between jet axes for different definitions!
Standard Axis: anti-kT jet with E-scheme recombination.
Groomed Axis: groom jets with Soft Drop (SD) algorithm
Larkoski et al. JHEP 05 (2014) 146
Less sensitive to soft radiation than standard (SD
selects hard splittings, soft splittings groomed away).
pT,2
β z= zg
z > zcutθ pT,1 + pT,2 θg
1 −zg
Winner Take All Axis (WTA): jet axis is given by its leading constituent.
Insensitive to soft radiation at leading order.
Observable is sensitive to soft effects and is calculable! (good to study hadronization &
UE effects, tuning of MC generators)
Hannah Bossi (Yale University) QCD 2021Jet axis differences
Standard - groomed Standard - WTA
Standard and groomed axes
are strongly aligned.
WTA axes showing the largest
differences (larger ΔRaxis
range).
PYTHIA and Herwig also
predict similar trend.
Hint of tension between data and MC for standard vs. groomed axes.
Will be useful for tuning MC!
Hannah Bossi (Yale University) QCD 2021Generalized angularities JHEP 11 (2014) 129
Phase space of observables to κ β
pT,i ΔRi,jet
( pT,jet )( )
probe the pT and angular κ
λβ ≡
distribution of constituents with R
relative weightings κ and β.
Distributions are self-normalized.
Agreement
with theory
predictions
much stronger
κ = 1, β = 3 κ = 1, β = 1.5 in perturbative
dominated
regime!
ALI-PREL-364992 ALI-PREL-365016
Non-perturbative dominated regime Perturbative dominated regime
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