2011 QCD and High Energy Interactions - Rencontres de Moriond ...

(GeV/c)
< p
> (GeV/c)
T
T
p
<
40
20
0
-20
-40
40
20
0
-20
-40
PYTHIA+HYDJET 0-30%
0.1 0.2 0.3 0.4
CMS 0-30% AJ
Pb+Pb s =2.76 TeV
NN
∫
-1
L dt = 6.7 μb
> (GeV/c)
< p
> (GeV/c)
< p
0.1 0.2 0.3 0.4
AJ
T
40
20
0
-20
-40
T
40
20
0
-20
-40
In-Cone
ΔR (GeV/c)
< p
0.1 0.2 0.3 In-Cone 0.4
AJ
ΔR (GeV/c)
< p
0.1 0.2 0.3 0.4
AJ
T
20
0
-20
-40
T
20
0
-20
-40
> 0.5 GeV/c
0.5 - 1.0 GeV/c
1.0 - 2.0 GeV/c
2.0 - 4.0 GeV/c
4.0 - 8.0 GeV/c
> 8.0 GeV/c
p > 120GeV/c
T,1
p > 50GeV/c
T,2
Δ φ >
5
π | η | < 1.6
1,2 6 1,2
Out-of-Cone
ΔR≥0.8
0.1 0.2 0.3Out-of-Cone 0.4
AJ
ΔR≥0.8
0.1 0.2 0.3 0.4
AJ
Figure 2: Average missing transverse momentum, 〈p
T 〉, for tracks with pT > 0.5 GeV/c, projected onto the
leading jet axis (solid circles). The 〈p
T 〉 values are shown as a function of dijet asymmetry AJ in 0–30% central
events, for the full event (left), inside (∆R < 0.8) one of the leading or subleading jet cones (middle) and outside
(∆R > 0.8) the leading and subleading jet cones (right). For the solid circles, vertical bars and brackets represent
the statistical and systematic uncertainties, respectively. Colored bands show the contribution to 〈p
T 〉 for five
ranges of track pT . The top and bottom rows show results for pythia+hydjet and PbPb data, respectively.
For the individual pT ranges, the statistical uncertainties are shown as vertical bars. Note that as the underlying
PbPb event in both data and MC is not φ-symmetric on an event-by-event basis, the back-to-back requirement
was tightened to ∆φ12 > 5π/6.
4 Summary and Conclusion
A strong increase in the fraction of highly unbalanced jets has been seen in central PbPb collisions
compared with peripheral collisions and model calculations, consistent with a high degree of
parton energy loss in the produced QCD medium. A large fraction of the momentum balance
of these unbalanced jets is carried by low-pT particles at large radial distance, in contrast to
pythia simulations embedded into heavy ion events. The results provide qualitative constraints
on the nature of the jet modification in PbPb collisions and quantitative input to models of the
transport properties of the medium created in these collisions.
References
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doi:10.1103/PhysRevD.33.717.
2. J. P. Blaizot and L. D. McLerran, “Jets in Expanding Quark - Gluon Plasmas”, Phys.
Rev. D34 (1986) 2739. doi:10.1103/PhysRevD.34.2739.
3. CMS Collaboration, “The CMS experiment at the CERN LHC”, JINST 3 (2008) S08004.
doi:10.1088/1748-0221/3/08/S08004.
4. O. Kodolova, I. Vardanian, A. Nikitenko et al., “The performance of the jet identification
and reconstruction in heavy ions collisions with CMS detector”, Eur. Phys. J. C50
(2007) 117. doi:10.1140/epjc/s10052-007-0223-9.
5. CMS Collaboration, “Observation and studies of jet quenching in PbPb collisions at
nucleon-nucleon center-of-mass energy = 2.76 TeV”, arXiv:1102.1957.