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Review of final-state structureof pp interactions at the LHC
Paolo Gunnellini
DESY (Deutsches Elektronen Synchrotron), Hamburg
QCD at cosmic energiesKalchida (Greece)
May 2016
Paolo Gunnellini QCD at cosmic energies May 2016 1
Outline
1 Final states at the LHC2 Generalities on Monte Carlo event
generators3 Generator tuning and machinery
Available tunesOur understanding of QCD exp. dataCurrent issues and incompatibilities
4 Comparison between predictionsand measurements
Diffractive selectionsHard QCD events and multijet scenariosVector-boson final statesTop-antitop final states
5 Universality of tunes?6 Summary and conclusions
DISCLAIMER: Mainly results from ATLAS and CMS!
Paolo Gunnellini QCD at cosmic energies May 2016 2
Hard scattering at the LHC
To simulate proton-proton collisions, physicists generally use
Monte Carlo event generators
Ingredients of the hard scattering
→ Factorization theorem→ Parton distribution functions→ Matrix element calculation
..starting from lowest order in αS butmight include additional real and
virtual corrections
σab→F (Q2) =∫
dx1dx ′1 f 1a (x1,Q
2) f 2b (x ′1,Q
2) σ̂ab(x1, x′1,Q
2)
Total Cross Section Partonic Cross Section
Parton Distribution Functions
with x = longitudinal proton momentum fraction carried by the partonQ2 = scale of the scattering
Paolo Gunnellini QCD at cosmic energies May 2016 3
The underlying event at the LHC
Paolo Gunnellini QCD at cosmic energies May 2016 4
Final states at the LHC
What can be produced in proton-proton collisions?
From soft to hard..
Soft particles
Jets
Heavy flav. jets
Vector boson
Top quark
Vector boson pair
Higgs
.......
pp
80 µb−1
W
35 pb−1
Z
35 pb−1
t̄t tt−chan WW H
total
VBF
VH
tt̄H
Wt
2.0 fb−1
WZ
13.0 fb−1
ZZ ts−chan t̄tW t̄tZ
σ[p
b]
10−1
1
101
102
103
104
105
106
1011Theory
LHC pp√s = 7 TeV
Data 4.5 − 4.9 fb−1
LHC pp√s = 8 TeV
Data 20.3 fb−1
LHC pp√s = 13 TeV
Data 85 pb−1
Standard Model Total Production Cross Section Measurements Status: Nov 2015
ATLAS Preliminary
Run 1,2√s = 7, 8, 13 TeV
Huge collection of measurements about these final states..in this talk only
some of them with focus on our understanding of the experimental results!
Paolo Gunnellini QCD at cosmic energies May 2016 5
MC generator types
Pure matrix element (ME) simulationMC integration of cross section + PDFNo hadronization nor UE
Useful for theoretical studies, no exclusive events generated
Event generatorsCombination of ME and parton showersGenerator for fixed-order ME combined with leadinglog (LL) parton shower MC
Exclusive events, useful for experimentalists
Paolo Gunnellini QCD at cosmic energies May 2016 6
MC generator types
Paolo Gunnellini QCD at cosmic energies May 2016 7
MC generator types
Paolo Gunnellini QCD at cosmic energies May 2016 8
MC generator types
Paolo Gunnellini QCD at cosmic energies May 2016 9
Details of the Monte Carlo generators
Example: multijet final state
PYTHIA8 and HERWIG++: LO MC generators with extra jets from PS & MPI
SHERPA, MADGRAPH: matrix element with N-jets (extra real emission)
POWHEG: matrix element with a hard emission @ NLO (real & virtual)
PYTHIA, HERWIG SHERPA MADGRAPH@LO POWHEG
Paolo Gunnellini QCD at cosmic energies May 2016 10
The Underlying Event at the LHC
Hard scattering
Initial and Final StateRadiation
Multiple Parton Interactions(MPI)
Beam-beam remnants
Hadronization
In general, the UE is asofter contribution but..some MPI can be hard!
Double PartonScattering
[TeV]s
0.04 0.1 0.2 1 2 3 4 5 10
[mb]
eff
σ
5
10
15
20
25
30
35
40 CMS (W + 2 jets)ATLAS (W + 2 jets)CDF (4 jets)
+ 3 jets)γCDF ( + 3 jets)γCorrected CDF (
+ 3 jets)γD0 (UA2 (4 jets - lower limit)AFS (4 jets - no errors given)
PA= σA
σpptot
PB = σB
σpptot
σDPSAB ∝ m
2PAPBσ
pptot
σDPSAB = m
2σAσB
σeff
σeff � σpptot
Need for correlations!Paolo Gunnellini QCD at cosmic energies May 2016 11
How do we deal with that?
Montecarlo event generators (PYTHIA, HERWIG, SHERPA..)
Parameters need to be adjusted (tuned) to describe data
MPI
Primordial kT
Parton shower
Hadronization
e.g. p0T = pref
T · (E/Eref )ε
Proton matter distribution profileColour reconnection
e.g. Width of the gaussian used for modelling theparton primordial kT inside the proton
e.g. Strong coupling valueRegularization cut-offUpper scale
e.g. Length of fragmentation stringsStrange baryon suppression
How does one tune all these?Choice of parameter ranges and sensitive observablesPredictions for different parameter choices and interpolation of the MC responseData-MC difference and minimisation over parameter space
Paolo Gunnellini QCD at cosmic energies May 2016 12
Some ”official” tunes from the authors..
PYTHIA 8
HERWIG++
Monash Tune - PDF: NNPDF2.3LO (EPJ C74 (2014) 8)
UE-EE-5C - PDF: CTEQ6L1 (JHEP 1310 (2013) 113)
PYTHIA 8 Monash HERWIG++ UE-EE-5C(soft) MPI UE pp(p̄) data at various
√s UE pp(p̄) data at various
√s
Value of measured σeff
Primordial kT pT spectrum of lepton pair pT spectrum of Z boson
from Z decays in hadronic collisions in hadronic collisions
Parton shower Event shapes in pp̄ interactions Jet multiplicity, jet rates and
(taken from previous tune) shapes at various colliders
Hadronization Particle multiplicities in hadronic Particle production at
Z decays in e+e− collisions various colliders
General approach is a ”factorized” tuning procedure with only some ofthe components investigated
N.B. For the DPS simulation, generators normally useparameters of soft MPI and extrapolate to harder scales
Paolo Gunnellini QCD at cosmic energies May 2016 13
Can they be refined?
How well do they describe observables at different energy?
→ Nch and psumT as a function of the leading charged particle
TRANS MIN: sensitive to MPI
TRANS MAX: sensitive to MPI and PS
TRANS DIF: sensitive to PS
TRANS AVE: sensitive to MPI and PS
PURPOSE: Tuning MPI and colourreconnection parameters
PTmax Direction
Δφ
“TransMAX” “TransMIN”
“Toward”
“Away”
“Toward-Side” Jet
“Away-Side” Jet
Jet #3
Leading Object Direction
Δφ
“Toward”
“Transverse-1” “Transverse-2”
“Away”
Paolo Gunnellini QCD at cosmic energies May 2016 14
Results of the energy-dependence tuning
Charged particle mult. in the MAX reg. @ 0.9 (left) and 7 (right) TeV
b
b
b
b
bb b b b b
bb
bb
b
CDF datab
PYTHIA8 Tune 4CCUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9TransMAX charged-particle density
√s = 900GeV
(1/Nevents)dNch/d
ηd
φ
2 4 6 8 10 12 14 16 18 20
0.6
0.8
1
1.2
1.4
pmaxT [GeV]
MC/data
b
b
b
b
bb b b b b
bb
b b
b
CDF datab
HERWIG++ Tune UE-EE-5CCUETHppS1
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
TransMAX charged-particle density√s = 900GeV
(1/Nevents)dNch/d
ηd
φ
2 4 6 8 10 12 14 16 18 20
0.6
0.8
1
1.2
1.4
pmaxT [GeV]
MC/data
b
b
b
b
b
bb
bb b
b b bb b
b
b
CMS datab
PYTHIA8 Tune 4CCUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
TransMAX charged-particle density√s = 7 TeV
(1/Nevents)dNch/d
ηd
φ
5 10 15 20 25
0.6
0.8
1
1.2
1.4
pmaxT [GeV]
MC/data
b
b
b
b
b
bb
bb b
b b bb b
b
b
CMS datab
HERWIG++ Tune UE-EE-5CCUETHppS1
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
TransMAX charged-particle density√s = 7 TeV
(1/Nevents)dNch/d
ηd
φ
5 10 15 20 25
0.6
0.8
1
1.2
1.4
pmaxT [GeV]
MC/data
New tunes!PYTHIA 8 (CUETP8)
HERWIG++ (CUETHpp)
with various PDFs
Better constrain of theenergy extrapolation
CR changes with thechoice of the PDF
Rising part andplateaux region are
well predictedby the new tunes
(arXiv 1512.00815)
Paolo Gunnellini QCD at cosmic energies May 2016 15
Tune performance at the new energy
2.5− 2− 1.5− 1− 0.5− 0 0.5 1 1.5 2 2.5
η /
dch
N d⋅
evN
1/
1
1.5
2
2.5
3
3.5
4
DataPYTHIA 8 A2PYTHIA 8 MonashHERWIG++ UE-EE5EPOS LHCQGSJET II-04
| < 2.5η| > 500 MeV, T
p 1, ≥ chn
= 13 TeVsPreliminary ATLAS
η2.5− 2− 1.5− 1− 0.5− 0 0.5 1 1.5 2 2.5
MC
/ D
ata
0.8
1
1.2
5 10 15 20 25 30
>φ dη
/dch
N2<
d
0
0.5
1
1.5
2
2.5 PreliminaryATLAS
= 13 TeVs
Transverse region
|< 2.5η > 0.5 GeV, |T
p
> 1 GeV leadT
p
DATA (uncorrected) EPOS
PYTHIA 8 A14 PYTHIA 8 A2HERWIG++ EE5 PYTHIA 8 Monash
[GeV]leadT
p5 10 15 20 25 30
MC
/Dat
a
0.70.80.9
11.11.21.3
0
1
2
3
4
5
6
7
8
-3 -2 -1 0 1 2 3
dN
ch/d
ηη
pp √s = 13 TeV inelastic
CMS
dataPYTHIA8 CUETP8S1EPOS LHC
(GeV)T
Leading Track p2 4 6 8 10 12 14 16 18 20 22 24
data
/MC
0.4
0.6
0.8
1
1.2
1.4
1.6
HIST_data_syserror
Stat. UncertaintyStat. + Syst. Uncertainty
HIST_data_syserror
2 4 6 8 10 12 14 16 18 20 22 24
(G
eV)
)φ∆(∆η∆/⟩
T pΣ⟨
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8 CMSPreliminary
(13 TeV)-1281 nbdata
Pythia8 + CUETP8M1
Pythia8 + Monash
EPOS
Herwig + CUETHS1
transMIN
√s = 13 TeV
TOP:dN/dη
ATLAS-CONF-2015-028,
PLB751 (2015)
BOTTOM:Nch vs plead
T
ATLAS-PHYS-2015-019,
CMS-FSQ-15-007
Current tunesreproduce the
data at 13 TeV!
May the energydependence of
the MPI beimproved in the
generators?
p0T = pref
T · (E/Eref )ε
Paolo Gunnellini QCD at cosmic energies May 2016 16
What about other observables? (arXiv 1512.00815)
Min. Bias observables X
bb b
b b b b bb b
ALICE datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
1
2
3
4
5
6
7Charged-particle multiplicity,
√s = 7 TeV
dNch/d
η
-1 -0.5 0 0.5 1
0.6
0.8
1
1.2
1.4
η
MC/data
Incl. jet cross sections
Forward region
Z-boson observables
UE vs pjetT
DPS observables
Paolo Gunnellini QCD at cosmic energies May 2016 17
What about other observables? (arXiv 1512.00815)
Min. Bias observables X
bb b
b b b b bb b
ALICE datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
1
2
3
4
5
6
7Charged-particle multiplicity,
√s = 7 TeV
dNch/d
η
-1 -0.5 0 0.5 1
0.6
0.8
1
1.2
1.4
η
MC/data
Incl. jet cross sections
Forward region X
b
bb b
b bb b b
b bb
b bb b b
bTOTEM datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Charged-particle multiplicity,√s = 7 TeV
dNch/d|η|
5.4 5.6 5.8 6 6.2 6.4
0.6
0.8
1
1.2
1.4
|η|MC/data
Z-boson observables
UE vs pjetT
DPS observables
Paolo Gunnellini QCD at cosmic energies May 2016 18
What about other observables? (arXiv 1512.00815)
Min. Bias observables X
bb b
b b b b bb b
ALICE datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
1
2
3
4
5
6
7Charged-particle multiplicity,
√s = 7 TeV
dNch/d
η
-1 -0.5 0 0.5 1
0.6
0.8
1
1.2
1.4
η
MC/data
Incl. jet cross sections
Forward region X
b
bb b
b bb b b
b bb
b bb b b
bTOTEM datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Charged-particle multiplicity,√s = 7 TeV
dNch/d|η|
5.4 5.6 5.8 6 6.2 6.4
0.6
0.8
1
1.2
1.4
|η|MC/data
Z-boson observables
UE vs pjetT X
b
b
b
b
b
bb b b b b
b bb
bb
b b
CMS datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1CUETHppS1
0
0.2
0.4
0.6
0.8
1
TransAVE Nch density,√s = 2.76 TeV
〈d2Nch/d
ηd
φ〉
10 20 30 40 50 60 70 80 90 100
0.6
0.8
1
1.2
1.4
pT (leading charged-particle jet) [GeV]
MC/Data
DPS observables
Paolo Gunnellini QCD at cosmic energies May 2016 19
What about other observables? (arXiv 1512.00815)
Min. Bias observables X
bb b
b b b b bb b
ALICE datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
1
2
3
4
5
6
7Charged-particle multiplicity,
√s = 7 TeV
dNch/d
η
-1 -0.5 0 0.5 1
0.6
0.8
1
1.2
1.4
η
MC/data
Incl. jet cross sections X
bb
bb
bb
bb
bb
bb
bb
bb
bbbbbbbbbbbbbbbb
b
CMS datab
PH+CUETP8M1PH+CUETP8S1-HERAPDF1.5LO
10−3
10−2
10−1
1
10 1
10 2
10 3
10 4
10 5
10 6
10 7
CMS, Inclusive jets,√s = 7 TeV, 0.5 < |y| < 1.0
d2σ/dpTdy[pb/GeV]
10 2 10 3
0.5
1
1.5
2
2.5
pT [GeV]
MC/data
Forward region X
b
bb b
b bb b b
b bb
b bb b b
bTOTEM datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Charged-particle multiplicity,√s = 7 TeV
dNch/d|η|
5.4 5.6 5.8 6 6.2 6.4
0.6
0.8
1
1.2
1.4
|η|MC/data
Z-boson observables
UE vs pjetT X
b
b
b
b
b
bb b b b b
b bb
bb
b b
CMS datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1CUETHppS1
0
0.2
0.4
0.6
0.8
1
TransAVE Nch density,√s = 2.76 TeV
〈d2Nch/d
ηd
φ〉
10 20 30 40 50 60 70 80 90 100
0.6
0.8
1
1.2
1.4
pT (leading charged-particle jet) [GeV]
MC/Data
DPS observables
Paolo Gunnellini QCD at cosmic energies May 2016 20
What about other observables? (arXiv 1512.00815)
Min. Bias observables X
bb b
b b b b bb b
ALICE datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
1
2
3
4
5
6
7Charged-particle multiplicity,
√s = 7 TeV
dNch/d
η
-1 -0.5 0 0.5 1
0.6
0.8
1
1.2
1.4
η
MC/data
Incl. jet cross sections X
bb
bb
bb
bb
bb
bb
bb
bb
bbbbbbbbbbbbbbbb
b
CMS datab
PH+CUETP8M1PH+CUETP8S1-HERAPDF1.5LO
10−3
10−2
10−1
1
10 1
10 2
10 3
10 4
10 5
10 6
10 7
CMS, Inclusive jets,√s = 7 TeV, 0.5 < |y| < 1.0
d2σ/dpTdy[pb/GeV]
10 2 10 3
0.5
1
1.5
2
2.5
pT [GeV]
MC/data
Forward region X
b
bb b
b bb b b
b bb
b bb b b
bTOTEM datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Charged-particle multiplicity,√s = 7 TeV
dNch/d|η|
5.4 5.6 5.8 6 6.2 6.4
0.6
0.8
1
1.2
1.4
|η|MC/data
Z-boson observables X
b
b bb b
bbb
b
b
b
b
b
b
b
b
b
b
CMS datab
CUETP8M1PH+CUETP8M1PH+CUETP8S1-HERAPDF1.5LO
10−6
10−5
10−4
10−3
10−2
10−1Z boson pT in leptonic decay
1/
σd
σ/dpT(Z
)[1/GeV]
1 10 1 10 2
0.6
0.8
1
1.2
1.4
pT(Z) [GeV]
MC/Data
UE vs pjetT X
b
b
b
b
b
bb b b b b
b bb
bb
b b
CMS datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1CUETHppS1
0
0.2
0.4
0.6
0.8
1
TransAVE Nch density,√s = 2.76 TeV
〈d2Nch/d
ηd
φ〉
10 20 30 40 50 60 70 80 90 100
0.6
0.8
1
1.2
1.4
pT (leading charged-particle jet) [GeV]
MC/Data
DPS observables
Paolo Gunnellini QCD at cosmic energies May 2016 21
What about other observables? (arXiv 1512.00815)
Min. Bias observables X
bb b
b b b b bb b
ALICE datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
1
2
3
4
5
6
7Charged-particle multiplicity,
√s = 7 TeV
dNch/d
η
-1 -0.5 0 0.5 1
0.6
0.8
1
1.2
1.4
η
MC/data
Incl. jet cross sections X
bb
bb
bb
bb
bb
bb
bb
bb
bbbbbbbbbbbbbbbb
b
CMS datab
PH+CUETP8M1PH+CUETP8S1-HERAPDF1.5LO
10−3
10−2
10−1
1
10 1
10 2
10 3
10 4
10 5
10 6
10 7
CMS, Inclusive jets,√s = 7 TeV, 0.5 < |y| < 1.0
d2σ/dpTdy[pb/GeV]
10 2 10 3
0.5
1
1.5
2
2.5
pT [GeV]
MC/data
Forward region X
b
bb b
b bb b b
b bb
b bb b b
bTOTEM datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Charged-particle multiplicity,√s = 7 TeV
dNch/d|η|
5.4 5.6 5.8 6 6.2 6.4
0.6
0.8
1
1.2
1.4
|η|MC/data
Z-boson observables X
b
b bb b
bbb
b
b
b
b
b
b
b
b
b
b
CMS datab
CUETP8M1PH+CUETP8M1PH+CUETP8S1-HERAPDF1.5LO
10−6
10−5
10−4
10−3
10−2
10−1Z boson pT in leptonic decay
1/
σd
σ/dpT(Z
)[1/GeV]
1 10 1 10 2
0.6
0.8
1
1.2
1.4
pT(Z) [GeV]
MC/Data
UE vs pjetT X
b
b
b
b
b
bb b b b b
b bb
bb
b b
CMS datab
CUETP6S1CUETP8S1-CTEQ6L1CUETP8S1-HERAPDF1.5LOCUETP8M1CUETHppS1
0
0.2
0.4
0.6
0.8
1
TransAVE Nch density,√s = 2.76 TeV
〈d2Nch/d
ηd
φ〉
10 20 30 40 50 60 70 80 90 100
0.6
0.8
1
1.2
1.4
pT (leading charged-particle jet) [GeV]
MC/Data
DPS observables X
b b bb b
bb
b
b
b
b
b
b
b
CMS datab
CUETP8M1CUETHppS1
10−1
1
Normalized ∆S in pp→ 4j,√s = 7 TeV
(1/
σ)d
σ/d
∆S
0 0.5 1 1.5 2 2.5 3
0.6
0.8
1
1.2
1.4
∆SMC/data
Paolo Gunnellini QCD at cosmic energies May 2016 22
Further look at UE/DPS comparisons (arXiv 1512.00815)
Tune name σeff value (mb)
CUETP8M1 26.0+0.6−0.2
CUETHppS1 15.2+0.5−0.6
Dedicated tune to DPS-sensitiveobservables in four-jet final state
CDPSTP8S2 → σeff = 19.0+4.7−3.0 mb
b
b
b
b
b
b
bbbbbb b
b bb b b b b b b b b b b b b
b
bb
b bb
ATLAS datab
CDPSTP8S2-4j
0
0.2
0.4
0.6
0.8
1
1.2
TransAVE Nch density,√s = 7 TeV
〈d2Nch/d
ηd
φ〉
2 4 6 8 10 12 14 16 18 20
0.6
0.8
1
1.2
1.4
pT (leading track) [GeV]
MC/Data
b b bb b
bb
b
b
b
b
b
b
b
CMS datab
CUETP8M1CUETHppS1
10−1
1
Normalized ∆S in pp→ 4j,√s = 7 TeV
(1/
σ)d
σ/d
∆S
0 0.5 1 1.5 2 2.5 3
0.6
0.8
1
1.2
1.4
∆S
MC/data
Not able to describe both UE and DPS observables at with the same set of tunes
Indication for need of a refinement of the current MPI model?
Paolo Gunnellini QCD at cosmic energies May 2016 23
High multiplicity scenarios
Low multiplicities → mainly softer MPI componentsHigh multiplicities → contribution of harder MPI
= 7 TeVsCMS, pp | < 1.9ch.jetη |
(GeV/c)T
p5 10 15 20 25 30 35 40 45 50
)-1
(GeV
/cT
/dp
ch.je
tdN
ch.je
t1/
N -510
-410
-310
-210
-110
1
10
80≤ ch50 < N
(GeV/c)T
p5 10 15 20 25 30 35 40 45 50
MC
/dat
a
0
1
2
3 dataPYTHIA8 MPI-offPYTHIA8 4CPYTHIA6 Z2*Herwig++ 2.5 UE_EE_3M
= 7 TeVsCMS, pp > 0.25 GeV/cch.particle
T| < 2.4, pch.particleη|
chN20 40 60 80 100 120
>(G
eV/c
)T
<p
1
1.5 dataPYTHIA8 MPI-offPYTHIA8 4CPYTHIA6 Z2*Herwig++ 2.5 UE_EE_3M
all ch. particles
chN20 40 60 80 100 120
MC
/dat
a
0.5
1
1.5
2
= 7 TeVsCMS, pp | < 1.9ch.jetη |
(GeV/c)T
p5 10 15 20 25 30 35 40 45 50
)-1
(GeV
/cT
/dp
ch.je
tdN
ch.je
t1/
N -510
-410
-310
-210
-110
1
10 data
PYTHIA8 4C
PYTHIA6 Z2*Herwig++ 2.5 UE_EE_3M
140≤ ch110 < N
(GeV/c)T
p5 10 15 20 25 30 35 40 45 50
MC
/dat
a0
1
2
3
b
b
b
b
b
b
b
b
b
b
b
CMS datab
CUETP8S1-CTEQ6L1CDPSTP8S1-4jCUETP6S1Monash
10−4
10−3
10−2
10−1
1
CMS,√s = 7 TeV, |ηch.jet| < 1.9, 110 < Nch ≤ 140
1/Nch.jetdNch.jet/dpT[GeV/c]
−1
5 10 15 20 25 30 35 40 450.60.8
11.21.41.61.8
pT [GeV/c]
MC/Data
1- Charged-particlemeasurement
2- Jet measurement
Good agreement forUE tunes at lowmultiplicities but
worse for increasingmultiplicities
→ Better descriptionat high multiplicitiesfor the DPS tunes!
Normalized jet pT spectrumat different multiplicities(top and bottom right)
Charged particle spectra pTas a function of multiplicity
(bottom left)
Eur.Phys.J. C73 (2013) 2674
Paolo Gunnellini QCD at cosmic energies May 2016 24
Diffractive-enhanced final states (I)
Diffractive dissociation cross sections as a functionof the fraction of the dissociated mass in differentranges dominated by different components→ Test of models for diffractive scenarios
Xξ
10log
-6 -5.5 -5 -4.5 -4 -3.5 -3 -2.5 -2
(m
b)Xξ
10/d
log
σd
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
< 0.5YM10
log
(7 TeV)-1bµ16.2
CMS (a)
Xξ
10log
-6 -5.5 -5 -4.5 -4 -3.5 -3 -2.5 -2 (
mb)
Xξ10
/dlo
gσd
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Data PYTHIA version:
=0.08)ε P8-MBR (=0.104)ε P8-MBR (
P8-4C P6-Z2*
< 1.1YM10
0.5 < log
(7 TeV)-1bµ16.2
CMS (c)
Best description is provided by MBR generator (arXiv.0203141) whichfully simulates the diffractive components and is tuned to Tevatron data
It uses different hadronization parameters to describe charged particle pT spectra
LEFT: SD-enhanced, RIGHT: DD-enhanced Phys. Rev. D 92 (2015) 012003
Paolo Gunnellini QCD at cosmic energies May 2016 25
Diffractive-enhanced final states (II)
η2− 1.5− 1− 0.5− 0 0.5 1 1.5 2
η/d
ch)
dNev
ents
(1/N
0.4
0.6
0.8
1
1.2
1.4
1.6CMSPreliminary
| < 2.4η 1 in |≥ chN > 0.5 GeV
Tp
(13 TeV)
SD selectiondataPYTHIA8 CUETM1PYTHIA8 CUETS1PYTHIA8 CUETM1 MBRPYTHIA8 4C MBR
bb
b
b
b
b
b
b
b
b
b
b
b
b
Datab
CUETP8M1CUETP8M1-MBR
CUETP8M1-PomFlux
10−1
1
10 1
Charged hadron p⊥ for |η| = 0.1 at√s = 7 TeV
d2Nch/d
ηdp⊥[(GeV
/c)
−1]
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
0.6
0.8
1
1.2
1.4
p⊥ [GeV/c]
MC/Data
η2− 1.5− 1− 0.5− 0 0.5 1 1.5 2
η/d
ch)
dNev
ents
(1/N
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2CMSPreliminary
| < 2.4η 1 in |≥ chN > 0.5 GeV
Tp
(13 TeV)
SD selectiondataPYTHIA8 CUETM1PYTHIA8 CUETS1PYTHIA8 CUETM1 MBRPYTHIA8 4C MBR
bb
bbbbbbb
b
b
bb
bb
bb
bb
bb
bb
b bb
bb b
bb b b
b
Datab
CUETP8M1CUETP8M1-MBR
CUETP8M1-PomFlux
10−4
10−3
10−2
10−1
1
10 1
Charged hadron p⊥ for |η| < 2.4 at√s = 7 TeV
(1/2
πp⊥)d2Nch/d
ηdp⊥[(GeV
/c)
−2]
1 2 3 4 5 6
0.6
0.8
1
1.2
1.4
p⊥ [GeV/c]
MC/Data
SD-enhancedselection: activity
in one region,veto in the other
MBR diffraction model andparameters help to improve
the SD observables, due to abetter description of low
trans.mom. region
CMS-FSQ-15-008
Need for differenthadronization parameters fordiffractive observables than
for nondiffractive?
Ongoing work!
Paolo Gunnellini QCD at cosmic energies May 2016 26
Jet observables at 13 TeV
(GeV)T
Jet p
200 300 1000 2000
dy
(pb/
GeV
)T
/ dp
σ2 d
-310
-110
10
310
510
710
910
1110
1310
1510)6|y|<0.5 (x10
)50.5<|y|<1.0 (x10)41.0<|y|<1.5 (x10)31.5<|y|<2.0 (x10)22.0<|y|<2.5 (x10)12.5<|y|<3.0 (x10)03.2<|y|<4.7 (x10
NP×CT14
(13 TeV)-172 pb
R = 0.7Tanti-k
CMSPreliminary
Inclusive jet cross section in differentrapidity bins
Various cone sizes probe different aspectsof parton evolution
Jets clustered with R = 0.7 are describedby NLO fixed-order calculations andpredictions from NLO MC generators
LO MC event generators are not able toreproduce the measurement
(GeV)T
Jet p200 300 1000 2000
Ratio
to C
T14
0.5
1
1.5
2
2.5DataHERAPDF1.5NNPDF3.0MMHT2014Exp. uncert.Theory uncert.
(13 TeV)-172 pb
CMSPreliminary
R = 0.7Tanti-k|y| < 0.5
(GeV)T
Jet p
200 300 1000 2000
Ratio
to P
H+P8
CUE
TM1
0.5
1
1.5
2
2.5DataPH+P8 CUETS1-CTEQ6L1PH+P8 CUETS1-HERAPDFP8 CUETM1Hpp CUETS1Exp. uncert.
(13 TeV)-172 pb
CMSPreliminary
R = 0.7Tanti-k|y| < 0.5
→ Fixed-order NLO ME corr. for NP effects with diff. PDF sets→ MC generators with LO or NLO dijet ME + PS and UE sim.
CMS-PAS-SMP-15-007 ATLAS-CONF-2015-034
Paolo Gunnellini QCD at cosmic energies May 2016 27
Jet observables at 13 TeV
(GeV)T
Jet p
200 300 1000 2000
dy
(pb/
GeV
)T
/ dp
σ2 d
-310
-110
10
310
510
710
910
1110
1310
1510)6|y|<0.5 (x10
)50.5<|y|<1.0 (x10)41.0<|y|<1.5 (x10)31.5<|y|<2.0 (x10)22.0<|y|<2.5 (x10)12.5<|y|<3.0 (x10)03.2<|y|<4.7 (x10
NP×CT14
(13 TeV)-172 pb
R = 0.4Tanti-k
CMSPreliminary
Inclusive jet cross section in differentrapidity bins
Various cone sizes probe different aspectsof parton evolution
Jets clustered with R = 0.4 are notdescribed by NLO fixed-order calculationsbut only by predictions from NLO MCgenerators
Effect of missing PS contributions relevantfor smaller cones
(GeV)T
Jet p200 300 1000 2000
Ratio
to C
T14
0.5
1
1.5
2
2.5DataHERAPDF1.5NNPDF3.0MMHT2014Exp. uncert.Theory uncert.
(13 TeV)-172 pb
CMSPreliminary
R = 0.4Tanti-k|y| < 0.5
(GeV)T
Jet p
200 300 1000 2000
Ratio
to P
H+P8
CUE
TM1
0.5
1
1.5
2
2.5DataPH+P8 CUETS1-CTEQ6L1PH+P8 CUETS1-HERAPDFP8 CUETM1Hpp CUETS1Exp. uncert.
(13 TeV)-172 pb
CMSPreliminary
R = 0.4Tanti-k|y| < 0.5
[GeV]T
p210×4 210×5 210×6 210×7 210×8R
atio
w.r
.t. N
LO p
QC
D (
CT
10)
0.4
0.6
0.8
1
1.2
1.4
integrated luminosity not includedRelative uncertainty of 9% in the
| < 0.5y|
PreliminaryATLAS
13 TeV
-178 pb
=0.4R jets, tanti-k
DATA (syst., total)
CT10
MMHT
NNPDF3
Non-pert. corr.×NLOJET++
maxT
p = R
µ = F
µ
Paolo Gunnellini QCD at cosmic energies May 2016 28
Multijet observables (I)
[TeV]12m
-110×3 1 2 3 4 5 6 7
* [p
b/T
eV]
yd12
m/dσ2 d
-1710
-1410
-1110
-810
-510
-210
10
410
710
1010
uncertaintiesSystematic
Non-pert. & EW corr.
)y* exp(0.3 T
p=µCT10, NLOJET++
)-0010×* < 0.5 (y ≤0.0 )0-310×* < 1.0 (y ≤0.5 )0-610×* < 1.5 (y ≤1.0 )0-910×* < 2.0 (y ≤1.5 )-1210×* < 2.5 (y ≤2.0 )-1510×* < 3.0 (y ≤2.5
ATLAS
-1 dt = 4.5 fbL∫ = 7 TeVs
= 0.4R jets, tkanti-
Inclusive dijet cross section indifferent rapidity bins as a
function of dijet mass
Jets clustered with R = 0.4 arereproduced by NLO fixed-ordercalculations
JHEP05(2014)059
Th
eo
ry/d
ata
1
1.5
2 * < 0.5y
= 0.530CTobsP = 0.306HERA
obsP
1
1.5
2 * < 1.0y ≤0.5
= 0.918CTobsP = 0.606HERA
obsP
[TeV]12m
-110×3 1 2 3 4
1
1.5
2 * < 1.5y ≤1.0
= 0.068CTobsP = 0.035HERA
obsP
Th
eo
ry/d
ata
1
2
3 * < 2.0y ≤1.5
= 0.310CTobsP = 0.338HERA
obsP
1
2
3 * < 2.5y ≤2.0
= 0.332CTobsP = 0.186HERA
obsP
[TeV]12m
-110×8 1 2 3 4 5
1
2
3 * < 3.0y ≤2.5
= 0.960CTobsP = 0.981HERA
obsP
NLOJET++)y* exp(0.3
Tp=µ
Non-pert. & EW corr.
CT10
HERAPDF1.5
exp. onlyepATLJet13
exp. onlyHERAPDF1.5
uncertaintyStatistical
uncertaintiesSystematic
ATLAS -1 dt = 4.5 fbL∫
= 7 TeVs
= 0.4R jets, tkanti-
Paolo Gunnellini QCD at cosmic energies May 2016 29
Multijet observables (II)
[TeV]12m
-110×3 1 2 3 4 5 6 7
* [p
b/T
eV]
yd12
m/dσ2 d
-1710
-1410
-1110
-810
-510
-210
10
410
710
1010
uncertaintiesSystematic
Non-pert. & EW corr.
)y* exp(0.3 T
p=µCT10, NLOJET++
)-0010×* < 0.5 (y ≤0.0 )0-310×* < 1.0 (y ≤0.5 )0-610×* < 1.5 (y ≤1.0 )0-910×* < 2.0 (y ≤1.5 )-1210×* < 2.5 (y ≤2.0 )-1510×* < 3.0 (y ≤2.5
ATLAS
-1 dt = 4.5 fbL∫ = 7 TeVs
= 0.4R jets, tkanti-
Inclusive dijet cross section indifferent rapidity bins as a
function of dijet mass
Jets clustered with R = 0.4 arereproduced by MC eventgenerators using NLO dijet MEbut sensitivity to UE tunes
JHEP05(2014)059
Th
eo
ry/d
ata
0.8
1
1.2* < 0.5y
0.8
1
1.2* < 1.0y ≤0.5
[TeV]12m
-110×3 1 2 3 4
0.8
1
1.2 * < 1.5y ≤1.0
Th
eo
ry/d
ata
0.5
1
1.5
2* < 2.0y ≤1.5
0.5
1
1.5
2* < 2.5y ≤2.0
[TeV]12m
-110×8 1 2 3 4 50.5
1
1.5
2 * < 3.0y ≤2.5
POWHEGBorn
Tp=µCT10,
EW corr.
+ PYTHIA shower
AUET2B
Perugia 2011
uncertaintyStatistical
uncertaintiesSystematic
ATLAS -1 dt = 4.5 fbL∫
= 7 TeVs
= 0.4R jets, tkanti-
Paolo Gunnellini QCD at cosmic energies May 2016 30
Multijet observables (III))
[fb/G
eV
](1
)
T / d
(pσ
d
410
310
210
110
1
10
210
310
410
510
610
Data
0.9)×HEJ (
1.0)×BlackHat/Sherpa (
1.0)×NJet/Sherpa (
>100 GeV(1)
Tp
ATLAS1 20.3 fb1=8 TeV, 95 pbs
[GeV](1)
Tp
210×23
103
10×2
Th
eo
ry/D
ata
0
0.5
1
1.5
2
systematic uncertainty
Total experimental
uncertainty
PDF)⊕NLO (scale
) [fb/G
eV
](1
)
T / d
(pσ
d
410
310
210
110
1
10
210
310
410
510
610
Data
0.6)×Pythia 8 (
1.4)×Herwig++ (
1.1)×MadGraph+Pythia (
>100 GeV(1)
Tp
ATLAS1 20.3 fb1=8 TeV, 95 pbs
[GeV](1)
Tp
210×23
103
10×2
Th
eo
ry/D
ata
0
0.5
1
1.5
2
systematic uncertainty
Total experimental
Four-jet scenarios also well describedin a wide region of the phase space
JHEP 12 (2015) 105Phys.Rev.D89(2014)092010
| < 4.7η| jet:nd, 2st1
> 50 GeVT
p
jet:th, 4rd3 > 20 GeV
Tp
4j+X→, pp -1 = 7 TeV, L = 36 pbsCMS,
SHERPAPOWHEG+P6 Z2'MADGRAPH+P6 Z2*PYTHIA8 4CHERWIG++ UE-EE-3Total Uncertainty
| < 4.7η| jet:nd, 2st1
> 50 GeVT
p
jet:th, 4rd3 > 20 GeV
Tp
4j+X→, pp -1 = 7 TeV, L = 36 pbsCMS,
SHERPAPOWHEG+P6 Z2'MADGRAPH+P6 Z2*PYTHIA8 4CHERWIG++ UE-EE-3Total Uncertainty
Jet #pt50 100 150 200 250 300 350 400 450 500
50 100 150 200 250 300 350 400 450 500
MC
/Dat
a
0.20.40.60.8
11.21.41.61.8
2Leading jet
Jet #pt50 100 150 200 250 300 350 400 450 500
50 100 150 200 250 300 350 400 450 500
MC
/Dat
a
0.20.40.60.8
11.21.41.61.8
2Subleading jet
Jet #pt50 100 150 200 250 300 350 400 450 500
MC
/Dat
a
0.20.40.60.8
11.21.41.61.8
2
Third jet
(GeV)T
Jet p50 100 150 200 250 300 350 400 450 500
MC
/Dat
a
0.20.40.60.8
11.21.41.61.8
2Fourth jet
Paolo Gunnellini QCD at cosmic energies May 2016 31
Z boson observables
Jets associated to a Z boson
Measurement of differential cross sections for different jet multiplicities→ Crucial test of our QCD predictions!
1 [GeV]≥ jets
, NTH
4−10
3−10
2−10
1−10
1
10
Data (25ns)
2j NLO + PS)≤aMC@NLO + PY8 (
CMS Preliminary (13 TeV)-12.5 fb
(R = 0.4) JetsTanti-k
| < 2.4 jet
> 30 GeV, |yjet
Tp
channelµµ →*γZ/
(jets
) [p
b/G
eV]
T/d
Hσd
1 [GeV]≥ jets
, NTH200 400 600 800 1000
aMC
@N
LO/D
ata
0.5
1
1.5
Stat. unc. (gen)
3 [GeV]≥ jets
, NTH
5−10
4−10
3−10
2−10
Data (25ns)
2j NLO + PS)≤aMC@NLO + PY8 (
CMS Preliminary (13 TeV)-12.5 fb
(R = 0.4) JetsTanti-k
| < 2.4 jet
> 30 GeV, |yjet
Tp
channelµµ →*γZ/
(jets
) [p
b/G
eV]
T/d
Hσd
3 [GeV]≥ jets
, NTH200 400 600 800 1000
aMC
@N
LO/D
ata
0.5
1
1.5
Stat. unc. (gen)
jetsN
0 1 2 3 4
) [p
b]je
ts)+
Nµµ
→(Z
(σ
1
10
210
310
410 DataSherpaMadgraph
+ jets-µ+µ →Z
-113 TeV, 85 pbPreliminary
, R=0.4tanti-k
> 30 GeVjet
Tp
| < 2.5jet
|y
ATLAS
jetsN0 1 2 3 4
Dat
a / P
red.
0.60.8
11.21.4
4≥ 3 ≥ 2 ≥ 1 ≥ 0 ≥
CMS-SMP-15-010ATLAS-CONF-2015-041
Predictions describe measurements as long as the multiplicity matchesthe number of partons in the matrix element
Paolo Gunnellini QCD at cosmic energies May 2016 32
Top observables (I)
Underlying event in top pair events
Measurement of event content in terms of charged particle multiplicity and pT
→ Test of final-state universality of UE contribution
t
t
t t W+
b
b
μ+ νμ
q
q
W-‐
CMS-TOP-15-017
CMS Preliminary
(13 TeV)-12.2 fb
[GeV]ch
Tp
0 5 10 15 20 25
MC
/Dat
a
0.5
1
1.5
) [/G
eV]
ch Tp/d
(ev
dN
ev1/
N
4−10
3−10
2−10
1−10
1
overall
away
transverse
toward
Powheg + Pythia 8tt
(CUETP8M1)
CMS Preliminary
(13 TeV)-12.2 fb
[GeV]ttT
p20 30 40 100 200 300
MC
/Dat
a
0.8
1
1.2
[GeV
]⟩
ch Tp ⟨
1.5
2
2.5
3
3.5
4
4.5
detector-level data Powheg + Pythia 8 (CUETP8M1)tt
2 Powheg + Pythia 8 (CUETP8M1) (2Q)tt2 Powheg + Pythia 8 (CUETP8M1) (Q/2)tt
Powheg + Herwig++ (EE5C) tttransverse
Predictions of POWHEG NLO matrix-element interfaced to P8 tunes from hadronicevents are able to reproduce all measured observables in each considered region
Paolo Gunnellini QCD at cosmic energies May 2016 33
Top observables (II)
Top pair in lepton + jets final states
Measurement of differential cross sections as a function of top pair pT and jetmultiplicity → Test of reliability of predictions in top sector
) [GeV]t(tT
p
]-1
[pb
GeV
)t(t
Tdp
σd
2−10
1−10
1
(13 TeV)-12.3 fbl+jets particleCMS
Preliminarydata
stat⊕sys stat.MG5_aMC@NLO P8 [FxFx]MG5_aMC@NLO P8 [MLM]Powheg P8Powheg H++MG5_aMC@NLO P8MG5_aMC@NLO H++
) [GeV]t(tT
p0 100 200 300 400 500
data
theo
ry
0.81
1.21.4
n-jets
d n-
jet
σd
10
20
30
40
50
(13 TeV)-12.3 fbl+jets particleCMS
Preliminarydata
stat⊕sys stat.MG5_aMC@NLO P8 [FxFx]MG5_aMC@NLO P8 [MLM]Powheg P8Powheg H++MG5_aMC@NLO P8MG5_aMC@NLO H++
additional jets0 1 2 3 >=4
data
theo
ry1
1.5
2
NLO predictions describe well the measurements but sensitivity tomatching scheme and PS generator
CMS-TOP-16-008
Paolo Gunnellini QCD at cosmic energies May 2016 34
Parton shower and MPI tuning
TUNING OF PYTHIA 8 TO OBSERVABLES MEASURED IN DIFFERENT PROCESSES
Study of the interplay between MPI and parton showerVarious PDF sets investigated
ObservablesTrack jet propertiesJet shapesDijet decorrelationsMultijetsZ boson pT
tt̄ gap and jet shapesTrack-jet and jet UE
Extremely important for:testing the universality of the parton shower in leptonic and hadronic collisions
testing the performance of UE simulation for different hard scattering processes
ATL-PHYS-PUB-2014-021
Paolo Gunnellini QCD at cosmic energies May 2016 35
Parton shower and MPI tuning
Significant improvement of description of observables in:Drell-Yan (left), top-antitop (center) and jet substructure (right) sectors
bb b b b b b b b b b b b b b b b b b
bb
b
b
b
b
b
ATLAS Simulation
ATLAS Datab
AU2 CTEQ6L1MonashA14-CTEQA14-MSTWA14-NNPDFA14-HERA
10−7
10−6
10−5
10−4
10−3
10−2
10−1Z pT “dressed” muons
1σ
fid.
dσ
fid.
dp T
1 10 1 10 20.70.75
0.80.85
0.90.95
1.01.05
1.1
Z pT
MC
/Dat
a
b
b
b
b
bb
bb
bb b b b b b b b b
ATLAS SimulationATLAS Datab
AU2 CTEQ6L1MonashA14-CTEQA14-MSTWA14-NNPDFA14-HERA
0.7
0.75
0.8
0.85
0.9
0.95
1.0
Gap fraction vs. Q0 for veto region: |y| < 0.8
f gap
50 100 150 200 250 300
0.96
0.98
1.0
1.02
1.04
Q0 [GeV]
MC
/Dat
ab
b
b
b
b
bb
b
b
b
b
b
b
b
ATLAS Simulation
ATLAS Datab
AU2 CTEQ6L1MonashA14-CTEQA14-MSTWA14-NNPDFA14-HERA
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Cambridge-Aachen jets, R = 1.2, 300 GeV < p⊥ < 400 GeV
1/σ
dσ
/d
τ 21
0.2 0.4 0.6 0.8 1 1.2
0.6
0.8
1
1.2
1.4
N-subjettiness τ21
MC
/Dat
a
αS values are similar for all PDF usedquite high for the hard processeslower for initial- and final-state radiationsignificantly lower than previous tunes for ISR and FSR
Damped shower needed to describe gap fraction in t̄tevents and pZ
T simultaneouslyPaolo Gunnellini QCD at cosmic energies May 2016 36
Tuning higher-order ME matched to parton showers
OBSERVABLES: gap fraction, jet shapes and jet multiplicity in t̄t events
GENERATORS: PYTHIA 8 standalone, MADGRAPH aMC@NLO andPOWHEG + PYTHIA 8
Different steps of tuning:
tuning of ISR and FSR separately, then simultaneous tune toaccount for their interplay
application of tune to matched generators
tune of the matched MADGRAPH aMC@NLO + PYTHIA 8
(Some of the) Outcomes
Significant improvement in the description of t̄t observables
Parameters of simultaneous ISR-FSR tune do not differ much fromthe separate tunes
Tune of matched MADGRAPH+PYTHIA 8 has similar parametersto standalone PYTHIA 8
ATL-PHYS-PUB-2015-007, ATL-PHYS-PUB-2015-048
Paolo Gunnellini QCD at cosmic energies May 2016 37
Summary and conclusions
High energy physics strongly relies on predictions from MonteCarlo event simulation
Very sophisticated tunes are available and are able to describe a wide range ofobservables at different collision energies
Actual measurements are sensitive to ME, UE and PS contributions and are ableto disentangle them
Tunes obtained at 7 TeV (or below) are able to reproduce data at 13 TeV in agood level of agreement
It is difficult to reproduce data relative to some corners of the phase space withina unique set of parameters (e.g. DPS-sensitive observables, diffraction)
Need for more sophisticated models/parametrizations?
New measurements, moreadvanced simulation..
Stay tuned!
Paolo Gunnellini QCD at cosmic energies May 2016 38
Summary and conclusions
High energy physics strongly relies on predictions from MonteCarlo event simulation
Very sophisticated tunes are available and are able to describe a wide range ofobservables at different collision energies
Actual measurements are sensitive to ME, UE and PS contributions and are ableto disentangle them
Tunes obtained at 7 TeV (or below) are able to reproduce data at 13 TeV in agood level of agreement
It is difficult to reproduce data relative to some corners of the phase space withina unique set of parameters (e.g. DPS-sensitive observables, diffraction)
Need for more sophisticated models/parametrizations?
New measurements, moreadvanced simulation..
Stay tuned!
THANK YOU FOR YOUR ATTENTION
Paolo Gunnellini QCD at cosmic energies May 2016 39
BACKUP SLIDES
Paolo Gunnellini QCD at cosmic energies May 2016 40
A double parton scattering (DPS)
Two different x values for thetwo partons in each proton
Impact parameter b
σDPSA,B = m
2
∫dx1dx ′1σ̂A(x1, x
′1)
∫dx2dx ′2σ̂B (x2, x
′2)
∫d2b f (x1, x2, b) f (x ′1, x
′2, b)
DPS Cross Section Double parton distribution functions
Partonic cross sections
If the partons are assumed to be uncorrelated:
f (x1, x2, b) = f (x1)f (x2)F (b)
The two scatterings factorize in the cross section formula:
σDPSAB =
m
2
∫dx1dx ′1σ̂Af (x1)f (x ′1)
∫dx2dx ′2σ̂B f (x2)f (x ′2)
∫d2b [F (b)]2
It is thus defined: σeff = 1∫d2b [F (b)]2
Paolo Gunnellini QCD at cosmic energies May 2016 41
Combined fits to whole MPI spectrum
Combined fits of observables sensitive to soft, semi-hard and hard MPI
b b bb b
bb
b
b
b
b
b
b
b
CMS datab
DPS-UE-MB Tune
10−1
1
Normalized ∆S in pp→ 4j in |η| < 4.7 at√s = 7 TeV
(1/
σ)d
σ/d
∆S[1/rad]
0 0.5 1 1.5 2 2.5 3
0.8
0.9
1.0
1.1
1.2
∆S (rad)
MC/Data
b b bb
b
bb
b
b
b
b
b
b
b
CMS datab
DPS-UE-MB-Weighted Tune
10−1
1
Normalized ∆S in pp→ 4j in |η| < 4.7 at√s = 7 TeV
(1/
σ)d
σ/d
∆S[1/rad]
0 0.5 1 1.5 2 2.5 3
0.8
0.9
1.0
1.1
1.2
∆S (rad)
MC/Data
bb b
b b b b bb b
ALICE datab
DPS-UE-MB Tune
0
1
2
3
4
5
6
7Pseudorapidity
√(s) = 7 TeV, INEL > 0
dN/d
η
-1 -0.5 0 0.5 1
0.6
0.8
1
1.2
1.4
η
MC/Data
bb b
b b b b bb b
ALICE datab
DPS-UE-MB-Weighted Tune
0
1
2
3
4
5
6
7Pseudorapidity
√(s) = 7 TeV, INEL > 0
dN/d
η
-1 -0.5 0 0.5 1
0.6
0.8
1
1.2
1.4
η
MC/Data
b
b
b
b
bb b
b b b b b b b b
bb
CMS datab
DPS-UE-MB Tune
10−2
10−1
1TransMIN charged particle density
√s = 7 TeV
(1/Neven
ts)dNch/d
ηd
φ
5 10 15 20 25
0.6
0.8
1
1.2
1.4
pTmax[GeV/c]
MC/Data
b
b
b
bbb b
b b b b b b b bb
b
CMS datab
DPS-UE-MB-Weighted Tune
10−2
10−1
1
TransMIN charged particle density√s = 7 TeV
(1/Neven
ts)dNch/d
ηd
φ
5 10 15 20 25
0.6
0.8
1
1.2
1.4
pTmax[GeV/c]
MC/Data
No hope to get the measurements well described altogether
Paolo Gunnellini QCD at cosmic energies May 2016 42
Why do we actually tune?
Not only for fun!
1 Correct description of the dataPile-up simulationEvaluation of detector effects and unfoldingEstimation of background (in MC-driven approach)Models are not ”allowed” to fail
2 Good physics predictionsCorrect evaluation of physics effectsModels are ”allowed” to fail
The danger is overtuning!
Paolo Gunnellini QCD at cosmic energies May 2016 43
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