outline: the cms detector and its capabilities for hard probes detection

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Outline:

• The CMS detector and its capabilities for hard probes detection

• Expected performances for jet quenching and quarkonia studies

Hard Probes of High Density QCD Physics with CMS

Carlos Lourenço, on behalf of CMS

CERN / LHCC 2007–0095 March 2007

Editor: David d’Enterriato be pub. in J. Phys. G

SQM 2007, Levoča, Slovakia, June 28, 2007

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Phase space coverage of the CMS detector

CMS (with HF, CASTOR, ZDC) + TOTEM: almost full η acceptance at the LHC ! charged tracks and muons: |η| < 2.5, full φ electrons and photons: |η| < 3, full φ jets, energy flow: |η| < 6.7 (plus η > 8.3 for neutrals), full φ excellent granularity and resolution very powerful and flexible High-Level-Trigger

CA

ST

OR

TOTEM

HFHF

= -8 -6 -4 -2 0 2 4 6 8

ZDC

5.2 < |η| < 6.6

η > 8.3 neutrals

For more details, see the slides of Ferenc SiklerCarlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007

T1/T2 CASTOR ZDC RP RP

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h±, e±, , ± measurement in the barrel (|| < 2.5)

Si Tracker + ECAL + muon-chambers

Si TrackerSilicon micro-stripsand pixels

CalorimetersECAL PbWO4

HCAL Plastic Sci/Steel sandwich

Muon BarrelDrift Tube Chambers (DT)Resistive Plate Chambers (RPC)

Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007

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QGP ?

We study the produced matter by studying how it affects well understood probes,as a function of the temperature of the system (centrality of the nuclear collisions)

Calibrated“probe source”

Matter under study

Calibrated“probe meter”

Calibratedheat source

Probe

Probing the QCD matter produced in HI collisions


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vacuum

QGP

hadronicmatter

The good probes should be:

Well understood in “pp collisions”

Only slightly affected by the hadronic matter, in a very well understood way, which can be “accounted for”

Strongly affected by the deconfined QCD medium...

Jets and heavy quarkonia (J/, ’, c, , ’, etc) are particularly good probes!

Challenge: find good probes of QCD matter


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The “probes” must be produced together with the system they probe!

They must be created very early in the collision evolution, so that they are there before the matter to be probed (the QGP): hard probes (jets, quarkonia, ...)

We must have “trivial” probes,not affected by the dense QCD matter,to serve as baseline reference: photons, Drell-Yan dimuons

We must have “trivial” collision systems,to understand how the probes are affectedin the absence of “new physics”: pp, p-nucleus, d-Au, light ions

Creating and calibrating the probes


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Tomography:Uses a calibrated probe, and a well understood interaction, to derive the3-D density profile of the mediumfrom the absorption profile of the probe.

In heavy-ion collisions:The suppression of the jets or of the quarkonia states gives the density profile and the state (hadronic or partonic) of the matter they cross

“Tomography” of the produced QCD matter


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In pp, expect twoback-to-back jets

In the QGP...expect mono jets

The away-side jetgets “absorbed” bythe dense QCDmedium

Why are the jets quenched?


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Why is jet quenching interesting?

The produced hard partons lose energy by multiple gluon radiation while traversing the dense medium

Observe parton energy loss → derive medium properties

Flavor-dependent energy loss:

Eloss

(g) > Eloss

(q) > Eloss

(Q) (color factor) (mass effect)


Suppression of high pT leading hadrons → seen at RHIC

Disappearance of “away-side” jets → indirectly seen at RHIC

Modified energy / particle flow within jet (fragmentation function) → not yet seen

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RHIC

pp

d-Au

centralAu-Au

Two-particle azimuthal correlations showback-to-back jets in pp and d-Au collisions;the jet opposite to the high-pT trigger particle

“disappears” in central Au-Au collisions

Interpretation: the produced hard partons (our probe) are “anomalously absorbed”by the dense colored medium created in central Au+Au collisions at RHIC energies

reference data

The photons are not affected by the dense medium they cross

photons reference process

Jet suppression in heavy-ion collisions at RHIC


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In a deconfined phase the QCD binding potential is Debye screened and the heavy quarkonia states are “dissolved”. In other words, the free hard gluons are energetic enough to break the bound QQ states into open charm and beauty mesons.Different heavy quarkonium states have different binding energies and, hence, are dissolved at successive thresholds in energy density or temperature of the medium. Their suppression pattern is a thermometer of the produced QCD matter.

Latti

ce Q

Qba

r fr

ee e

nerg

y

T

Why is quarkonia suppression interesting?

H. Satz, hep-ph/0512217Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007

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The feed-down from higher states leads to “step-wise” J/ and suppression patterns. It is very important to measure the heavy quarkonium yields produced in Pb-Pb collisions at the LHC energies, as a function of pT and of collision centrality.

state J/ c

' (1s) b

(2s) b' (3s)

Mass [GeV} 3.096 3.415 3.686 9.46 9.859 10.023 10.232 10.355B.E. [GeV] 0.64 0.2 0.05 1.1 0.67 0.54 0.31 0.2

Td/Tc --- 0.74 0.15 --- --- 0.93 0.83 0.74 1.10 0.74 0.15 2.31 1.13 0.93 0.83 0.74

A “smoking gun” signature of QGP formation

’

c


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Drell-Yan dimuons are not affected by the dense medium they cross

reference process

The yield of J/ mesons per DY dimuon is “slightly smaller” in p-Pb collisions than inp-Be collisions; and is strongly suppressedin central Pb-Pb collisions

Interpretation: strongly bound ccbar pairs (our probe) are “anomalously dissolved” by the deconfined medium created in central Pb-Pb collisions at SPS energies

p-Be

p-Pb

centralPb-Pb

J/ suppression in heavy-ion collisions at the SPS

J/ normal nuclear

absorption curve

exp(-L abs)

reference data


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The ’ suppression pattern in S-U and in Pb-Pb shows a significantly stronger drop than expected from the “normal extrapolation” of the p-A data

abs ~ 20 mb

’

The “change of slope” at L ~ 4 fm is quite significant and looks very abrupt...

’

’ suppression in heavy-ion collisions at the SPS


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Hard Probes at LHC energies

• Experimentally & theoretically controlled

probes of the early phase in the collision

• Very large cross sections at the LHC• CMS is ideally suited to measure them

• Pb-Pb instant. luminosity: 1027 cm-2s-1

• ∫ Lumi = 0.5 nb-1 (1 month, 50% run eff.)• Hard cross sections: Pb-Pb = A2 x pp

pp-equivalent ∫ Lumi = 20 pb-1

1 event limit at 0.05 pb (pp equiv.)

jet

Z0+jet+jet

prompt

h/h

pp s = 5.5 TeV

1 event

J

1 b

1 nb

1 pb

M. Ballitjin, C. Loizides, G. Roland, CMS note AN-2006/099Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007

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The High Level Trigger

ET reach x2

x35

x35

jets

Pb-Pb at 5.5 TeVdesign luminosity

• CMS High Level Trigger: 12 000 CPUs of 1.8 GHz ~ 50 Tflops !• Executes offline-like algorithms

• pp design luminosity L1 trigger rate: 100 kHz• Pb-Pb collision rate: 3 kHz (peak = 8 kHz) pp L1 trigger rate Pb-Pb collision rate run HLT codes on all Pb-Pb events

• Pb-Pb event size: ~2.5 MB (up to ~9 MB)• Data storage bandwidth: 225 MB/s 10–100 Pb-Pb events/s• HLT reduction factor: 3000 Hz → 100 Hz• Average HLT time budget per event: ~4 s

• Using the HLT, the event samples of hard processes are statistically enhanced by very large factors

M. Ballitjin, C. Loizides, G. Roland, CMS note AN-2006/099Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007

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Impact of the HLT on the pT reach of RAA

Nuclear modification factor = AA-yield / pp-yield = “QCD medium” / “QCD vacuum”

C. Roland et al., CMS note AN-2006/109

Pb-Pb (PYQUEN) 0.5 nb-1

Important measurement to compare with parton energy loss models and derivethe initial parton density, dN

g/dy, and the medium transport coefficient, ⟨q⟩^

HLT


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Jet reconstruction and spatial resolution

Iterative cone method plus background subtraction:1. Subtract average pile-up2. Find jets with iterative cone algorithm3. Recalculate pile-up outside the cone4. Recalculate jet energy

New developments (fast-kT algo.) under study I. Vardanyan et al., CMS note 2006/050

○ without background■ central Pb-Pb

100 GeV jet on

a Pb-Pb event,after Bg subtr.


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Jet rec. efficiency, purity and ET resolution

I. Vardanyan et al., CMS note 2006/050

The full simulation (OSCAR) is well reproduced by the fast simulation (HIROOT) after smearing


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Jet ET reach and fragmentation functions

C. Roland et al., CMS note AN-2006/109 I. Lokhtin et al., PLB567 (2003) 39

min. bias

HLT

Jet spectra up to ET ~ 500 GeV (Pb-Pb, 0.5 nb-1, HLT-triggered)

Detailed studies of medium-modified (quenched) jet fragmentation functions

Gluon radiation:large angle (out-of-cone) vs. small angle emission


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, and Z tagging of jet productiondimuon trigger

associated hadrons

g *away side

C. Mironov et al., CMS note 2007/xxx

Unique possibility to calibrate jet energy loss (and FF) with back-to-back gauge bosons (large cross sections and excellent detection capabilities).

Heavy quark dimuon (dominant) background can be rejected by a secondary vertex cut.Resolutions: 50 m in radius and 20 m in

Z0+jet


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So far, only the dimuon decay channel has been considered.The physics performance has been evaluated with the 4 T field (2 T in return yoke) and requiring a good track in the muon chambers. The good momentum resolution results from the matching of the muon tracks to the tracks in the silicon tracker.

Quarkonia studies in CMS


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Pb-Pb → + X event in CMS

dNch

/d = 3500

Pb-Pb event simulated using the official CMS software framework (developed for pp)


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→ : acceptances and mass resolutions

CMS has a very good acceptance for dimuons in the Upsilon mass region(21% total acceptance, barrel + endcaps)

The dimuon mass resolution allows usto separate the three Upsilon states:~ 54 MeV within the barrel and~ 86 MeV when including the endcaps

Barrel: both muons in || < 0.8

Barrel + endcaps: muons in || < 2.4

pT (GeV/c)

Acc

epta

nce

O. Kodolova, M. Bedjidian, CMS note 2006/089Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007

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J/ → : acceptances and mass resolutions

barrel +endcaps

O. Kodolova, M. Bedjidian, CMS note 2006/089

barrel

• The material between the silicon tracker and the muon chambers (ECAL, HCAL, magnet’s iron) prevents hadrons from giving a muon tag but impose a minimum muon momentum of 3.5–4.0 GeV/c. This is no problem for the Upsilons, given their

high mass, but sets a relatively high threshold on the pT of the detected J/’s.

• The low pT J/ acceptance is better at forward rapidities; total acceptance ~1%.

• The dimuon mass resolution is 35 MeV, in the full region.

barrel +endcaps

pT (GeV/c)

J/

Acc

epta

nce

p T (G

eV

/c)


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pT reach of quarkonia measurements (for 0.5 nb-1)

O. Kodolova, M. Bedjidian, CMS note 2006/089

J/

● produced in 0.5 nb-1

■ rec. if dN/d ~ 2500○ rec. if dN/d ~ 5000

Expected rec. quarkonia yields:J/ : ~ 180 000

: ~ 26 000’ : ~ 7300; ’’ : ~ 4400

Statistical accuracy (with HLT) of expected’ / ratio versus pT model killer...


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production in Ultra-Peripheral Pb-Pb Collisions

D. d’Enterria, A. Hees, CMS note AN-2006/107

• CMS will also study Upsilon photo-production, which occurs when the electromagnetic fields of the 82 protons of each nuclei interact with each other• This measurement (based on neutron tagging in the ZDCs) allows us, in particular, to study the gluon distribution function in the Pb nucleus• Around 500 events are expected after 0.5 nb-1,

adding the ee and decay channels


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Summary

• The CMS detector has excellent capabilities to study the dense QCD matter produced in very-high-energy heavy-ion collisions, through the use of hard probes such as high-ET (fully reconstructed) jets and heavy quarkonia

• With a high granularity inner tracker (full silicon, analog readout), a state-of-the-art crystal ECAL, large acceptance muon stations, and a powerful DAQ & HLT system, CMS has the means to measure charged hadrons, jets, photons, electron pairs, dimuons, quarkonia, Z0, etc!

• This opens the door to high-quality measurements that so far lived only in the

realm of dreams (maybe even including the study of c → J/ using the ECAL)

• However, the fantastic potential of CMS as a heavy-ion detector is presently limited by the lack of human resources; this is a golden opportunity for new collaborators: some of the most exciting physics topics (e.g., open charm and open beauty) are not yet under study within the CMS-HI team...


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An example of compressed baryonic matter


outline: the cms detector and its capabilities for hard probes detection

Documents

understood probes

hard probes jets

trivial probes

produced matter

dense qcd matter

cms detectorcms

hard probes detection

cmscarlos loureno