Gian Michele Innocenti on behalf of the CMS Collaboration Massachusetts Institute of Technology
Rencontres de Moriond QCD and High Energy Interactions March 19th - 26th, 2016
1
Overview of heavy ion CMS results
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 2
Understanding hot QCD medium with CMSWe aim at creating a hot and dense deconfined medium by means of ultra-relativistic heavy ion collisions
Did we create a strongly interacting “medium”?
Have we observed quarkonia dissociation in the medium?
How partons interact with the medium?
peripheral collision
central collision
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 3
“Did we create a medium?”Time
Demonstration by an ultracold atom gas system
dN/d(ϕ-Ψ2) ~ 1+2v2cos[2(ϕ-Ψ2)]
Pressure-driven expansion
v2: elliptic flow
1
Data
0.8
1.2
0 1 2 3 -3 -2 -1 ϕ-Ψ2 [rad]
dN/d
(ϕ-Ψ
2)
Anisotropic azimuthal distribution:
y
x ϕ
In presence of a strongly interacting medium:
initial azimuthal asymmetry of the fireball
azimuthal particle anisotropy
higher particle density here
v2>0smaller particle density here
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 4
Indications of a “medium” in PbPb
4
v2>0 that increases in peripheral events.
Precise measurement of other harmonics vn→ constraint on the initial energy fluctuations
vn measurements well described by hydrodynamic models with very low shared viscosity (η/s~0.2) → We created an almost perfect fluid!
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 5
“collective effect” two particles with very different η are “connected”
PbPb
The “ridge”
single jet contribution Back-to-back jet
contributionSame structure observed in high multiplicity PbPb, pPb and pp collisions!
Do we observe “flow” phenomena in pPb and even in pp? Alternative explanation that does not involve QGP?
pPb pp
JHEP 09 (2010) 091, PLB 718 (2013) 795, CMS PAS FSQ-15-002
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 6
Long range correlation in pp at 13 TeV
YieldYield
(GeV/c)T
p0 2 4
Asso
ciat
ed y
ield
/ (G
eV/c
)
0
0.02
0.04
105≥ offline
trk = 13 TeV, Nspp
110≥ offline
trk = 7 TeV, Nspp
100, 13 TeV≈ offline
trkGlasma+BFKL, N
100, 7 TeV≈ offline
trkGlasma+BFKL, N
(a)CMS Preliminary
offline
trkN
50 100 150
Asso
ciat
ed y
ield
/ (G
eV/c
)
0
0.02
0.04
< 2.0 GeV/cT
1.0 < p
| < 4.0η∆2.0 < |
(b)Glasma+BFKL, 13 TeVGlasma+BFKL, 7 YeV
No double ridge in low multiplicity pp
Almost linear increase with multiplicity
Predictions from gluon saturation model= long-range correlation arising from initial collimated gluon emissions, NOT from “collective” phenomenal
CMS PAS FSQ-15-002
CMS PAS FSQ-15-002
Associated yield
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 7
Probing the medium with high pT particles
High pT quarks and gluons interact with the medium and lose energy
High pT hadrons and jets get “quenched” by the medium
Di-jet event in PbPb collisions: • leading jet pT = 205 GeV • subleading jet pT = 70 GeV
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results”
γJx0.5 1 1.5
γJdx
γJdN γN1
00.20.40.60.8
11.21.41.61.8
2 PbPb DataSmeared pp referencePbPb PYTHIA + HYDJET
50% - 100%
γJx0.5 1 1.5
γJdx
γJdN γN1
00.20.40.60.8
11.21.41.61.8
2| < 1.44γη > 60 GeV/c |
Tγp
| < 1.6Jetη > 30 GeV/c |TJetp
π87 >
γJφ∆
30% - 50%
γJx0.5 1 1.5
γJdx
γJdN γN1
00.20.40.60.8
11.21.41.61.8
2
10% - 30%
γJx0.5 1 1.5
γJdx
γJdN γN1
00.20.40.60.8
11.21.41.61.8
2
0% - 10%
CMS Preliminary -1, pp 5.3 pb-1bµ=2.76TeV, PbPb 150 NNs
8
Quantifying the jet quenching
pT,1 ~ pT,2
Aj~0pT,1 >>pT,2
Aj~1
large fraction of events with large Aj with respect to MC without jet quenching
Photon / ZJet
XJγ= pTJet/pTγ
~10% of the jet energy is lost. Where does the energy go?PRC 84 (2011) 024906, CMS-HIN-13-006
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 9
How transverse energy is distributed in the jet cone
Enhancement of low pT particles
inside the jet cone
Depletion of intermediate pT
particles
No modification in pPb (Observed modification in PbPb
is not from initial state effect)
PRC 90 (2014) 024908, CMS-HIN-15-004
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 10
φ1
φ2
φdijet
φ2
?
What is the angular distribution of these particles with respect to the dijet system?
Where the energy goes?
Charged particle azimuthal angle
Dijet axis
Projection on the dijet axis
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 11
φ1
φ2
φdijet
φ2
What is the angular distribution of these particles with respect to the dijet system?
Missing pT versus Δ
?
Charged particle azimuthal angle
Dijet axis
Projection on the dijet axis
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 12
φ1
φ2
φdijet
φ2
What is the angular distribution of these particles with respect to the dijet system?
?
Charged particle azimuthal angle
Dijet axis
Projection on the dijet axis
Missing pT versus Δ
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 13
Subleading jet direction
High pT imbalance at small Δ
Balanced by more low pT particles in subleading jet direction
Extends up to large Δ
Missing pT versus Δ in PbPb
JHEP 01 (2016) 006
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 14
Does the energy loss depend on parton flavour?
Hints of different suppression for D and non-prompt J/𝜓 at low pT!
Same suppression observed for inclusive jets and b-tagged jets
CMS-HIN-15-005, PRL 113 (2014) 132301
ΔEg > ΔEu,d,s > ΔEc > ΔEb
→Rlight particleAA < RDAA < RBAA ?
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 15
Heavy-Flavour production in pPb
[GeV/c]T
c jet p0 50 100 150 200 250 300 350 400
GeV
/cm
b T
dpσd pAT1
9−10
8−10
7−10
6−10
5−10
4−10
< 2labηpPb data, -2 <
PYTHIA Z2 (5.02 TeV)
CMS Preliminary (5.02 TeV)-135 nb
[GeV/c]T
c jet p0 100 200 300 400
Dat
a/PY
THIA
00.5
11.5
2
B+ production in pPb → compatible with predictions from FONLL scaled by A=208
tagged c and b-jet production → compatible with predictions from PYTHIA scaled by A=208
HF pPb production not significantly modified by cold nuclear matter effects (e.g. PDF modification in nuclei)
PRL 116 (2016) 032301, CMS-HIN-15-012 ,PLB 754 (2016) 59
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 16
Have we observed quarkonia dissociation?
in presence of a coloured medium, the qqbar potential is screened
Quarkonia melt in the QGP!
Less bounded states melts at lower temperature Tdiss (1S) > Tdiss (2S) > …
We should observe a hierarchy in the dissociation of different quarkonia states depending on their binding energies
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results”
)2 (GeV/c-µ+µm8 9 10 11 12 13 14
)2Ev
ents
/ ( 0
.1 G
eV/c
0200400600800
10001200140016001800
> 3.5 GeV/c1µ
Tp
> 4.0 GeV/c2µ
Tp
Cent. 0-100%|y| < 2.4
= 2.76 TeVNNs -1bµPbPb 166
CMSPreliminary
17
Upsilon suppression in PbPb
)2 (GeV/c-µ+µm8 9 10 11 12 13 14
)2Ev
ents
/ ( 0
.1 G
eV/c
0
500
1000
1500
2000
2500
> 3.5 GeV/c1µ
Tp
> 4.0 GeV/c2µ
Tp
|y| < 2.4
= 2.76 TeVs -1pp 5.4 pb
CMSPreliminary
Υ(1S) Υ(2S) Υ(3S)
ppPbPb
Υ(2S) slightly suppressed Υ(3S) not visible!
We observe what we call “sequential” suppression of Υ states in PbPb collisions! CMS-HIN-15-001
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 18
Conclusions
Did we create a strongly interacting “medium”? • evidences that we created in PbPb collisions a
medium that behaves like a perfect fluid • pp and pPb? did we create QGP there?
How partons interact with the medium? • high pT partons lose energy in the
deconfined medium as observed by studying jet suppression •the energy lost by the quenched partons goes to soft particles far from the initial parton direction
)2 (GeV/c-µ+µm8 9 10 11 12 13 14
)2Ev
ents
/ ( 0
.1 G
eV/c
0200400600800
10001200140016001800
> 3.5 GeV/c1µ
Tp
> 4.0 GeV/c2µ
Tp
Cent. 0-100%|y| < 2.4
= 2.76 TeVNNs -1bµPbPb 166
CMSPreliminary
Did we observe quarkonia dissociation in the medium? • evidence of sequential suppression of bottomonia states
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 19
BACKUP
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 20
Muon HCAL ECAL
Tracker |η|< 2.5
|η|< 3.0
|η|< 5.2
|η|< 2.4
Inner tracker: charged particles
EM and hadronic calorimeters Photons, Jet
Muon detectors
CMS detector
Forward Calorimeter: MB triggers, centrality
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 21
In absence of in-medium effect → RAA=1
In presence of in-medium energy loss: → pT spectra in Pb-Pb shifted at low pT → RAA<1
Charged particle RAA
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 22
How to quantify the energy loss?
Nuclear modification factor
number of binary nucleon-nucleon collisions in a PbPb collision
In absence of in-medium effect → RAA=1
In presence of in-medium energy loss: → pT spectra in Pb-Pb shifted at low pT
→ RAA<1
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 23
Flow in PbPb collisions = geometry + fluctuationsCollective behaviour in heavy-ion collisions as crucial probes to test: • hydrodynamics and transport coefficient
in the bulk • Initial state conditions • degree of freedom and particle
production mechanisms
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 24
PbPb @ 2.76 TeV
non zero elliptic flow (v2>0) suggests we created a strongly interacting medium that also exibiths properties of a almost perfect fluid!
smaller particle density here
higher particle density here
As expected due to initial geometry asymmetry!
We did create a medium in PbPb!dN/d(Φ-Ψ2) ∼1+2v2 cos[2(Φ-Ψ2)]
v2>0
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 25
v2 in peripheral PbPb collisions v2>0 in peripheral collisions is the first important evidence that the fireball exhibits a collective behaviour:
• different pressure gradient due to the geometric anisotropy of the fireball
• well predicted by hydrodynamics!
IN PL
ANE
OUT O
F PLANE
Is this really collectivity? • v2{2} can be affected by non-flow phenomena • v2{4}=v2{6}= v2{8} telling that similar results are
extracted by considering 2, 4, 8 particles.
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 26
Understanding the ridge in pPb collisions
Many evidences suggests hydro behaviour in pPb collisions:
• vn present similar behaviour vs pT as observed in PbPb collisions E.g. v2{2} > v2{4} ≃ v2{6} ≃ v2{8}
• stronger radial flow (v1) that in PbPb as a consequence of the more explosive system created in pPb collisions
• Clear mass ordering in the v2 as function of pT as expected in the HP of collective motions
Are there alternative explanations to hydro? Other explanations describes this collective phenomena as a consequence of pure initial state interactions: • Color Glass Condensate (CGC) • pQCD-based models
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 27
Testing hydrodynamics models
• In ultra-central PbPb collisions not sensitive to the geometry anisotropy
• we can test the transport coefficients of the fireball
• Hydro-models can reproduce the various harmonics vn assuming values of the shared viscosity close to KKS bound
• The medium shows collective behaviours well described by hydrodynamics calculation and presents the properties of an almost perfect fluid
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 28
“ridge” in pp collisions
offlinetrkN
0 50 100 150{4
}2c 2−
0
6−10×
= 3%{4}2v = 4%{4}2v
{4}
2c
0.00
0.02
0.04
3−10×
= 7 TeVspp
= 5.02 TeVNNspPb
< 3 GeV/cT
0.3 < p
CMS Preliminary
• similar “ridge” observed in high multiplicity pp collisions at 7 and 13 TeV
• CGC models fails at describing the ridge yield at high multiplicity
A full understanding of the pp flow-like phenomena is still far…
• in high multiplicity pp collisions, hints of non zero v2 from 4-particle correlations
• limited by low statistics
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 29
Asymmetry inside the jet cone
Integrated curve from 0 to Δ
Subleading jet direction
Missing pT versus Δ in pp
JHEP 01 (2016) 006
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 30
pT-dependence of the di-jet imbalance
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 31
di-jet angular correlation
Correlation peak is the same in data and Pythia across all values of pT
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 32
Overall momentum balance of the events is always recovered
<pT||>=0
Dijet momentum imbalance is not related to undetected activity
In MC, large components of tracks with pT 4-8 GeV/c
In both MC and Data, the large negative contribution (leading jet) with pT>8 GeV balanced by tracks with pT 0.5–8 GeV/c
In data the missing pT is compensated mostly by low pT particles < 4 GeV
Momentum balance of dijets events
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 33
Jet shapes
radial distribution of transverse momentum of tracks inside the jet cone
r track axial distance from the jet axis
Deviations from unity indicate modification of jet structure in the nuclear medium
No sizeable modification of the jet shape with respect to pp in peripheral PbPb
excess of transverse momentum fraction emitted at large radii → moderate broadening of jets!
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 34
Photon-jet correlations
partN0 100 200 300 400
γJR
0.5
0.6
0.7
0.8
0.9
1
π87 >
γJφ∆
CMS Preliminary -1, pp 5.3 pb-1bµ=2.76TeV, PbPb 150 NNs
PbPb Datapp DataSmeared pp referencePYTHIA + HYDJET
partN0 100 200 300 400
> γJ<x
0.60.650.7
0.750.8
0.850.9
0.951
1.051.1
π87 >
γJφ∆
CMS Preliminary -1, pp 5.3 pb-1bµ=2.76TeV, PbPb 150 NNs
PbPb Datapp DataSmeared pp referencePYTHIA + HYDJET
xJγ=pT,jet/pT,γ RJγ=fraction of diet events with a jet partner
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 35
Binding energy [GeV]0 0.2 0.4 0.6 0.8 1 1.2
AAR
0
0.2
0.4
0.6
0.8
1
1.2
1.4
< 30 GeV/c, |y| < 1.6)T
(2S) (6.5 < pψInclusive
(3S) (|y| < 2.4), 95% upper limitϒ
(2S) (|y| < 2.4)ϒ
< 30 GeV/c, |y| < 2.4)T
(6.5 < pψprompt J/
(1S) (|y| < 2.4)ϒ
(1S)Υ
ψJ/
(2S)Υ(3S)Υ
(2S)ψ
CMS Preliminary = 2.76 TeVNNsPbPb
0-100%
Sequential suppression
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 36
J/𝜓-𝜓(2S) suppression: a proof of recombination?
partN0 50 100 150 200 250 300 350 400
AAR
0
0.2
0.4
0.6
0.8
1
1.2
1.4 = 2.76 TeVNNsPbPb Preliminary
ψCMS: prompt J/ ψALICE: inclusive J/|y| < 2.4 2.5 < y < 4.0
< 30 GeV/cT
6.5 < p
• Low pT J/𝜓 measurements from ALICE compared to CMS results suggest a possible contribution of charm recombination in the medium
partN0 50 100 150 200 250 300 350 400
pp ]ψ
J//N
(2S)
ψ /
[ NPb
Pb ]
ψJ/
/N(2
S)ψ
[ N
0
0.5
1
1.5
2
2.5
3 = 2.76 TeVNNsCMS PbPb & pp < 30 GeV/c, 1.6 < |y| < 2.4
T3 < p
< 30 GeV/c, |y| < 1.6T
6.5 < p95% CL
0
0.5
1
1.5
2
2.5
3
0-100%Cent.
=RAA(𝜓(2S))/RAA(J/𝜓)
• 𝜓(2S) more suppressed than J/𝜓 in central PbPb collisions.
• Is recombination more relevant for excited J/𝜓 states? (sequential recombination)
• Simple kinematic effect? radial flow?
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 37
Also puzzles in pp…
total⟩|<2.4η|
tracksN⟨/|<2.4η|tracksN
0 0.5 1 1.5 2 2.5 3 3.5 4
⟩(1
S)ϒ⟨
(1S)
/ϒ
0
1
2
3
4
5
6
7
CMS| < 1.93
CM|y
⟩(1S)ϒ⟨(1S)ϒ
| < 2.4CM
|y
= 2.76 TeVspp = 5.02 TeVNNspPb
= 2.76 TeVNNsPbPb
total⟩|<2.4η|
tracksN⟨/|<2.4η|tracksN
0 0.5 1 1.5 2 2.5 3 3.5 4
⟩(2
S)ϒ⟨
(2S)
/ϒ
0
1
2
3
4
5
6
7
CMS| < 1.93
CM|y
⟩(2S)ϒ⟨(2S)ϒ
= 2.76 TeVspp = 5.02 TeVNNspPb
• Evolution (w.r.t. minbias) of the individual x-sections with the increase of track multiplicity around the state even in pp collisions:
total⟩ |<2.4η|tracks N⟨/|<2.4η|
tracksN0 0.5 1 1.5 2 2.5 3 3.5 4
⟩(3
S)ϒ⟨
(3S)
/ϒ
0
1
2
3
4
5
6
7
CMS| < 1.93
CM|y
⟩(3S)ϒ⟨(3S)ϒ
= 2.76 TeVspp = 5.02 TeVNNspPb
Other phenomena, other that color screening, could lead to a suppression sequence that depends on the binding energy!
Additional final-state effects affecting more the excited states than the ground state?
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results” 38
what happens to charmonia?
J/𝜓 RAA at higher pT (>6.5 GeV)
partN0 50 100 150 200 250 300 350 400
AAR
0
0.2
0.4
0.6
0.8
1
1.2
1.4 = 2.76 TeVNNsPbPb Preliminary
ψCMS: prompt J/ ψALICE: inclusive J/|y| < 2.4 2.5 < y < 4.0
< 30 GeV/cT
6.5 < p
J/𝜓 RAA at lower pT (<6.5 GeV)
What does it mean? At low pT there could be an antagonistic mechanism to “dissociation” or melting
Much more J/𝜓 seems to “survive” at low pT
J/𝜓 created by combining c-cbar quarks that are aboundant in the deconfined QCD medium
Gian Michele Innocenti, MIT, Moriond QCD 2016, “Overview of CMS heavy ion results”
offline
trkN
0 100 200 300As
soci
ated
yie
ld /
(GeV
/c)
0
0.1
0.2 = 2.76 TeVsPbPb
= 5.02 TeVspPb
= 13 TeVspp
= 7 TeVspp
< 2.0 GeV/cT
1.0 < p
| < 4.0η∆2.0 < |
CMS Preliminary
39
Long range correlation in pp at 13 TeV
We can extract the double ridge associated “yield” projecting the correlation plot along ΔΦ
Very similar behaviour in pp, pPb and PbPb collisions but with different magnitude
Almost no dependence on the collision energy
CMS PAS FSQ-15-002
What kind of phenomenal are we observing?
