Transcript Slide 1

Hard Probes of High Density QCD Physics with CMS
Outline:
• The CMS detector and its capabilities
for hard probes detection
• Expected performances for
jet quenching and quarkonia studies
CERN / LHCC 2007–009
5 March 2007
Editor: David d’Enterria
to be pub. in J. Phys. G
Carlos Lourenço, on behalf of CMS
SQM 2007, Levoča, Slovakia, June 28, 2007
1/28
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
TOTEM
 excellent granularity and resolution
 very powerful and flexible High-Level-Trigger
HF
5.2 < |η| < 6.6
HF
ZD
C
h = -8
-6 -4
-2
0
2
4
6
8
η > 8.3 neutrals
T1/T2 CASTOR
ZDC RP
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
RP
 For more details, see the slides of Ferenc Sikler
2/28
h±, e±, g, m± measurement in the barrel (|h| < 2.5)
Si Tracker
+
ECAL
+
muon-chambers
Si Tracker
Calorimeters
Muon Barrel
Silicon micro-strips
and pixels
ECAL
HCAL
Drift Tube Chambers (DT)
Resistive Plate Chambers (RPC)
PbWO4
Plastic Sci/Steel sandwich
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
3/28
Probing the QCD matter produced in HI collisions
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)
Matter under study
Calibrated
“probe meter”
Calibrated
“probe source”
Probe
QGP ?
Calibrated
heat source
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
4/28
Challenge: find good probes of QCD matter
The good probes should be:
vacuum
Well understood in “pp collisions”
hadronic
matter
Only slightly affected by the hadronic
matter, in a very well understood way,
which can be “accounted for”
QGP
Strongly affected by the deconfined
QCD medium...
Jets and heavy quarkonia (J/y, y’, cc, , ’, etc) are particularly good probes!
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
5/28
Creating and calibrating the probes
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 affected
in the absence of “new physics”:
 pp, p-nucleus, d-Au, light ions
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
6/28
“Tomography” of the produced QCD matter
Tomography:
Uses a calibrated probe, and a well
understood interaction, to derive the
3-D density profile of the medium
from 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
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
7/28
Why are the jets quenched?
In pp, expect two
back-to-back jets
In the QGP...
expect mono jets
The away-side jet
gets “absorbed” by
the dense QCD
medium
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
8/28
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:
DEloss(g)
>
DEloss(q)
(color factor)
>
DEloss(Q)
(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
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
9/28
Jet suppression in heavy-ion collisions at RHIC
RHIC
pp
d-Au
photons
central
Au-Au
Two-particle azimuthal correlations show
back-to-back jets in pp and d-Au collisions;
the jet opposite to the high-pT trigger particle
“disappears” in central Au-Au collisions
The photons are not affected by
the dense medium they cross
Interpretation: the produced hard partons (our probe) are “anomalously absorbed”
by the dense colored medium created in central Au+Au collisions at RHIC energies
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
10/28
Why is quarkonia suppression interesting?
Lattice QQbar free energy
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.
T
 H. Satz, hep-ph/0512217
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
11/28
A “smoking gun” signature of QGP formation
state
Mass [GeV}
B.E. [GeV]
Td/Tc
cc
3.415
0.2
0.74
J/y
3.096
0.64
--1.10
y'
(1s)
3.686
0.05
0.15
0.15
9.46
1.1
--2.31
cb
9.859
0.67
--1.13
(2s)
10.023
0.54
0.93
0.93
cb'
10.232
0.31
0.83
0.83
(3s)
10.355
0.2
0.74
The feed-down from higher states leads to “step-wise” J/y 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.
y’
cc
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
12/28
J/y suppression in heavy-ion collisions at the SPS
p-Be
p-Pb
central
Pb-Pb
J/y normal nuclear
absorption curve
exp(-r L sabs)
The yield of J/y mesons per DY dimuon is
“slightly smaller” in p-Pb collisions than in
p-Be collisions; and is strongly suppressed
in central Pb-Pb collisions
Drell-Yan dimuons are not affected
by the dense medium they cross
Interpretation: strongly bound ccbar pairs (our probe) are “anomalously dissolved”
by the deconfined medium created in central Pb-Pb collisions at SPS energies
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
13/28
y’ suppression in heavy-ion collisions at the SPS
The y’ 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
y’
y’
sabs ~ 20 mb
The “change of slope” at L ~ 4 fm is quite significant and looks very abrupt...
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
14/28
Hard Probes at LHC energies
• Experimentally & theoretically controlled
probes of the early phase in the collision
pp s = 5.5 TeV
• Very large cross sections at the LHC
• CMS is ideally suited to measure them
1 mb
J/y
1 nb
• 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.)

h+/h-
g*+jet
jet
Z0+jet
1 pb
gprompt
1 event
 M. Ballitjin, C. Loizides, G. Roland, CMS note AN-2006/099
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
15/28
The High Level Trigger
• 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
Pb-Pb at 5.5 TeV
design luminosity
ET reach x2
jets
x35
x35
• 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-2006/099
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
16/28
Impact of the HLT on the pT reach of RAA
Nuclear modification factor = AA-yield / pp-yield = “QCD medium” / “QCD vacuum”
Pb-Pb (PYQUEN) 0.5 nb-1
HLT
Important measurement to compare with parton energy loss models and derive
^
the initial parton density, dNg/dy, and the medium transport coefficient, 〈q〉
 C. Roland et al., CMS note AN-2006/109
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
17/28
Jet reconstruction and spatial resolution
Iterative cone method plus background subtraction:
1. Subtract average pile-up
2. Find jets with iterative cone algorithm
3. Recalculate pile-up outside the cone
4. Recalculate jet energy
100 GeV jet on
a Pb-Pb event,
after Bg subtr.
○ without background
■
central Pb-Pb
Dh
New developments (fast-kT algo.) under study
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
Df
 I. Vardanyan et al., CMS note 2006/050
18/28
Jet rec. efficiency, purity and ET resolution
The full simulation (OSCAR) is well reproduced
by the fast simulation (HIROOT) after smearing
 I. Vardanyan et al., CMS note 2006/050
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
19/28
Jet ET reach and fragmentation functions
Jet spectra up to ET ~ 500 GeV (Pb-Pb, 0.5 nb-1, HLT-triggered)
 Detailed studies of medium-modified (quenched) jet fragmentation functions
min. bias
HLT
 C. Roland et al., CMS note AN-2006/109
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
Gluon radiation:
large angle (out-of-cone)
vs. small angle emission
 I. Lokhtin et al., PLB567 (2003) 39
20/28
g, g* and Z tagging of jet production
dimuon trigger
away
side
g
associated
hadrons
g*
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 mm in radius and 20 mm in f
Z0+jet
 C. Mironov et al., CMS note 2007/xxx
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
21/28
Quarkonia studies in CMS
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.
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
22/28
Pb-Pb →  + X event in CMS
dNch/dh = 3500
  m+mPb-Pb event simulated using the official CMS software framework (developed for pp)
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
23/28
 → m+m-: acceptances and mass resolutions
Acceptance
Barrel + endcaps: muons in |h| < 2.4

Barrel: both muons in |h| < 0.8
pT (GeV/c)
CMS has a very good acceptance for
dimuons in the Upsilon mass region
(21% total acceptance, barrel + endcaps)
The dimuon mass resolution allows us
to separate the three Upsilon states:
~ 54 MeV within the barrel and
~ 86 MeV when including the endcaps
 O. Kodolova, M. Bedjidian, CMS note 2006/089
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
24/28
J/y → m+m-: acceptances and mass resolutions
pT (GeV/c)
Acceptance
• 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/y’s.
• The low pT J/y acceptance is better at forward rapidities; total acceptance ~1%.
• The dimuon mass resolution is 35 MeV, in the full h region.
J/y
barrel +
endcaps
barrel +
endcaps
h
barrel
pT (GeV/c)
 O. Kodolova, M. Bedjidian, CMS note 2006/089
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
25/28
pT reach of quarkonia measurements (for 0.5 nb-1)
J/y
● produced in 0.5 nb-1
■ rec. if dN/dh ~ 2500
○ rec. if dN/dh ~ 5000
Expected rec. quarkonia yields:
J/y : ~ 180 000
 : ~ 26 000
’ : ~ 7300; ’’ : ~ 4400
Statistical accuracy (with HLT) of expected
’ /  ratio versus pT  model killer...

 O. Kodolova, M. Bedjidian, CMS note 2006/089
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
26/28
 production in Ultra-Peripheral Pb-Pb Collisions
• 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 e+e- and m+m- decay channels


 D. d’Enterria, A. Hees, CMS note AN-2006/107
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
27/28
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 cc → J/y + g 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...
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
28/28
An example of compressed baryonic matter
Carlos Lourenço, CMS, SQM 2007, Levoča, Slovakia, June 2007
29/28