Overview of CMS heavy -ion results

Overview of CMS heavy-ion resultsRaphaël Granier de Cassagnac

for the CMS collaborationLLR – École polytechnique / IN2P3

ERC grant “QuarkGluonPlasmaCMS”

LHC days, October 2nd 2012, Split

A wealth of results• Multiplicity and transverse energy

– dNch/dη ≈ 1600 and dET/dη ≈ 2 TeV !

• Particle correlations– Elliptic flow (incl. π0) and higher harmonics– Di-hadron correlations (the “ridge”)

• Candles: Electro+weak bosons– Z and W bosons – Isolated photons

• Jet quenching– Photon+Jet– Fully reconstructed jets – Jet fragmentation & shape

• Quarkonium suppression – Five states disappearing 02/10/2012 [email protected] - Heavy ions in CMS - Split 2

Affordable in a 20’ talk?

A wealth of results• Multiplicity and transverse energy

– dNch/dη ≈ 1600 and dET/dη ≈ 2 TeV !

• Particle correlations– Elliptic flow and higher harmonics– Di-hadron correlations (the “ridge”)

• Candles: Electro+weak bosons– Z and W bosons – Isolated photons

• Jet quenching– Photon+Jet– Fully reconstructed jets – Jet fragmentation & shape

• Quarkonium suppression – Five states disappearing 02/10/2012 [email protected] - Heavy ions in CMS - Split 3

Affordable in a 20’ talk?

All results available at https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHINOr google search “CMS heavy ions”18 submitted PbPb papers (as many as ALICE)+ 10 documented preliminary analysis (PAS)

https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsHIN



Particule detection |η|<2.4

02/10/2012 [email protected] - Heavy ions in CMS - Split 5

Silicium: pixels (3) and strips (10) |η|<2.4

EMCalPbWO4 |η|<3

HCal: Scintillators |η|<5

Muon: drift tubes + RPC |η|<2.4

+ high rapidity extension

Muons 1-2% resolutionup to 100 GeV/c

Photons 10% resolutionin central events at 20 GeV

PB-PBCOLLISION


From the key concept: centrality…

• Collision centrality related to energy deposit in (forward) calorimeters

• Then to geometrical quantities:– Ncoll = number of

elementary NN collisions– or TAA = Ncoll / σpp

• Hard probes are supposed to scale with Ncoll, in the absence of medium effect, RAA = 1 02/10/2012 [email protected] - Heavy ions in CMS - Split 7

… to the nuclear modification factor RAA

• Collision centrality related to energy deposit in (forward) calorimeters

• Then to geometrical quantities:– Ncoll = number of

elementary NN collisions– or TAA = Ncoll / σpp

• Hard probes are supposed to scale with Ncoll, in the absence of medium effect, RAA = 1 02/10/2012 [email protected] - Heavy ions in CMS - Split 8

RAA = =dNAA

dNPP Ncollx dσPP TAAx

dNAA

PbPb collisions @ √sNN = 2.76 TeV


Dec. 2010: ≈ 7 μb–1 PbPb

Dec. 2011: ≈ 150 μb–1 PbPb

x 20

+ Mar. 2011 ≈ 230 nb–1 pp @ 2.76 TeV

Binary-scaledequivalent as manyZ, W, photons…

NEW CANDLES

Electro+Weak bosons should go through the colored medium without feeling it


Z boson centrality independence (150 μb–1)

11

RAA =dNAA

dσPP TAAx

• Very low pp statistics available at 2.76 TeV≈ 20 times less Z than PbPb



12

dNAA / TAA = dσpp x RAA


(PRL 106 (2011) 212301)CMS-PAS-HIN-12-008




• Compare to POWHEG (NLO generator) instead– Well tested at Tevatron (2

TeV) and LHC (7 TeV)– 5% uncertainty from

NNLO, pdfs, etc.

13


02/10/2012 [email protected] - Heavy ions in CMS - Split

(PRL 106 (2011) 212301)CMS-PAS-HIN-12-008

13



• Compare to POWHEG (NLO generator) instead– Well tested at Tevatron (2

TeV) and LHC (7 TeV)– 5% uncertainty from

NNLO, pdfs, etc.

14


RAA = 0.95 ± 0.03 ± 0.13


(PRL 106 (2011) 212301)CMS-PAS-HIN-12-008

14

W boson centrality independence (7.2 μb–1)

15


W

W+

W–

PLB 715 (2012) 66



16


PLB 715 (2012) 66



17

dNAA / TAA = dσpp x RAA2010 PbPb ≈ pp data

RAA(W) = 1.04 ± 0.07 ± 0.12

RAA(W+) = 0.82 ± 0.07 ± 0.09

RAA(W–) = 1.46 ± 0.14 ± 0.16

Consistent with pure isospin

PLB 715 (2012) 66

More d quarks in Pb make more W+ than in pp


Photon centrality independence (7.2 μb–1)

18

PLB 710 (2012) 256( isolated photons, after large background subtraction )


Unmodified electro+weak bosons

• Within uncertainties, electro+weak bosons are not modified1. Confirm the validity of

Ncoll scaling

2. More precision may reveal nuclear PDF modifications

3. But let’s first use these calibrated probes…


JET QUENCHING

Photon+jet (150 μb–1)


Pythia+Hydjet

Area normalized to unity

Photon-jet momentum balance

arXiv:1205.0206



PbPb Pythia+Hydjet

0-10%



arXiv:1205.0206



PbPb Pythia+Hydjet

0-10%



20% lose jet partner

PbPb

PbPb

Pythia+Hydjet

pp

pp

Pythia+Hydjet

Jet-photon pT balance 14% drop

arXiv:1205.0206

Modified jet RAA (150 μb–1)


CMS-PAS-HIN-12-004

Jet RAA


CMS-PAS-HIN-12-004 Cone size R = 0.3, but does not vary a lot for R = 0.2 or 0.4

Where does the energy go?

• To large angle & low pT

– Qualitative idea in the first 2010 (di)jet paper

• Quantitatively– Low pT, fragmentation

functions– Large angle, jet shape


100 GeV inclusive jetAnti-kT R=0.3 jet in PYTHIA

95% of jet energy in r < 0.2

PRC 84 (2011) 024906 G. Roland at QM’12

95% of jet energy in pT > 4 GeV particles

Jet fragmentation

• At first sight (QM’11, run 1, pT(track) > 4 GeV & pT(jet) > 100 GeV), surviving jets are not modified

• Looking closer (QM’12, run 2, pT(track) > 1 GeV & more energetic jets), modifications appear


ξ = ln(1/z) & z=pT(track)/pT(jet)where pT(jet) is quenched already

( < pT(parton) )

arXiv:1205.5872 accepted by JHEPvs CMS-PAS-HIN-12-013

ξ

Jet fragmentation and shapes


No change at small r, high pT

Narrowing/depletion at intermediate pT

Broadening/excess at large r, low pT

CMS-PAS-HIN-12-013

PbPb/ppdistributions

Back to RAA of jets (and b-jet)


Note: first b-jet identificationin heavy-ion collisions

CMS-PAS-HIN-12-003

Back to RAA of jets, and hadrons


Charged particles with pT = 50-100 GeV/c

z = pT(track)/pT(jet) = 0.4-0.6 x < 1

Looking at the same parton pT range

PbPb fragmentation function = pp for ξ <1

Jets: CMS-PAS-HIN-12-004Hadrons: EPJC 72 (2012) 1945

G. Roland at QM’12

QUARKONIA

Quarkonium suppression

• Old predicted signature of the QGP– Quarkonia should melt one after the

other, depending on their binding energy– Recent example of melting temperatures

• @ SPS / RHIC, no / marginal access to the (yet unresolved) Upsilon family

• @ SPS, J/ψ and ψ’ studied in detail• @ RHIC, J/ψ brought up surprises…

– Though they are suppressed, the balance of various effects is not clear…


Matsui & Satz,PLB168 (1986) 415

Mocsy, EPJC61 (2009) 705BNL workshop in June

ϒ(1S)

χb

J/ψ, ϒ(2S)

χc, χ’b, ψ', ϒ(3S)

Prompt J/ψ suppression

• CMS J/ψ pT > 6.5 GeV/c– Material and B-field– Feed down from B J/ψ is

subtracted• More suppression than at RHIC

– CMS < STAR (pT > 5 GeV/c)

• More suppression than at low pT

– CMS < ALICE (all pT)

• Popular explanation: regeneration from uncorrelated and – Stronger at low pT

> 100 pairs in a central event


centrality

JHEP05 (2012) 176 & CMS-HIN-PAS-12-014

Prompt J/ψ suppression

• CMS J/ψ pT > 6.5 GeV/c– Material and B-field– Feed down from B J/ψ is

subtracted• More suppression than at

RHIC (at high pT)– CMS < STAR (pT > 5 GeV/c)

• Less suppression at low pT

– CMS < ALICE (inclusive, all pT)

• Popular explanation: regeneration from uncorrelated and – Stronger at low pT

> 100 pairs in a central event02/10/2012 [email protected] - Heavy ions in CMS - Split 34

centrality

JHEP05 (2012) 176 & CMS-HIN-PAS-12-014

First look at ψ(2S)

• Relatively less ψ(2S) than J/ψ, as expected@ midrapidity |y|< 1.6, thus high pT > 6.5 GeV

• (a hint of an opposite behaviour at lower pT, but less than 2σ)


CMS-PAS-HIN-12-007

Upsilon (1S), (2S) and (3S)


(PRL107 (2011) 052302)arXiv:1208.2826

Upsilons (1S), (2S) and (3S)

Upsilon(1S) and (2S) at LHC


Sequential disappearance of the 3 states

For minimum biasRAA (Y(1S)) = 0.56 ± 0.08 ± 0.07RAA (Y(2S)) = 0.12 ± 0.04 ± 0.02RAA (Y(3S)) < 0.10 @ 95% CL

(Remember a large part of Y(2S)comes from higher state decays,

in particular the χb)

(PRL107 (2011) 052302)arXiv:1208.2826

Five states to bind them all


Mironov at QM’12

Forgetting low pT J/ψ(regeneration)for a while…

RAA(MB) vs binding energylooks ordered…

TBD with more data vs centrality and unfolding

cold effects (pA) & feeddowncould start acting as a

thermometer?

To conclude

• Three unmodified control probes (photon, Z and W)• Detailed studies of jet quenching • Five quarkonium suppressions• And more!


1/ Being interpreted in terms of Quark-GluonPlasma properties…2/ Once we have checked what happens in pPb (2013…)

Back up

W boson production

Less W+ and more W– in PbPb than in pp (isospin effect)

– Cancels for W+ + W–

• W boosted towards the valence quark (higher rapidity)

• Spin conservation μ+ (μ–) boosted back to (away from) midrapidity

Different muon rapidity distributions (not heavy-ion specific) between W+ and W–

41

p p

u d

W+

μ+

ν

@ LO : &


Muon charge asymmetry• Less up quarks make less W+ in PbPb than in pp

• Isospin effect bringing down asymmetry by 0.2 to 0.4• (EPS09 modifications are 0.03 at most)

42

N+ – N– N+ + N–

PLB 715 (2012) 66

|ημ|


How to find photons?• Trigger on ECAL clusters

– Uncorrected ET > 15 GeV, fully efficient for ET > 20 GeV

• Subtract underlying event– From same pseudorapidity

strip, event by event• Look for isolated cluster

– Remove photons from bremsstrahlung and jet fragmentation…

• Look at shower shape in the highly segmented ECAL – Further remove isolated π0, η

43

photon-like π,η 2γ

Δϕ

Δη


Photon spectrum (2010)

44

PLB 710 (2012) 256

Consistent with JETPHOXWith unmodified pdf (CT10)


Unmodified photons

45

PLB 710 (2012) 256

• Normalised by pp!• Consistent with unity!• Uncertainties still larger than modifiedpdf uncertainties…


Jet reconstruction performances


Resolution

Responce

Modified hadrons (150 μb–1)


EPJC 72 (2012) 1945

b-jet


J/ψ at RHIC (all pT)

• Two surprises:– At midrapidity, same

suppression at RHIC and at SPS, while density must be higher

– More suppression at forward rapidity at RHIC, while density must be lower

• Two popular answers:– Cold: shadowing / saturation

brings forward yields down– Hot: recombination of

uncorrelated cc brings midrapidity yield up


PHENIX, PRL98 (2007) 232301, also 1103.6269SPS from Scomparin @ QM06

First look at ψ(2S)

• More forward 1.6 < |y| < 2.4, down to pT = 3 GeV• Relatively more ψ(2S) than J/ψ ?


CMS-PAS-HIN-12-007

(ψ’/ψ)PbPb / (ψ’/ψ)pp

• ψ(2S) more suppressed than J/ψ @ high pT

• ψ(2S) less suppressed than J/ψ @ low pT ? (<2σ)2012, May 29th Heavy Ions in CMS - [email protected] 55

HIN-12-007CMS-PAS-HIN-12-007

Overview of CMS heavy -ion results

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