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

as a function of multiplicity for the transverse region. The average scalar momentum sum rises
sharply till 8 GeV due to the increase of multiple parton interaction activity, followed by a slow
increase thereafter (Fig. 3b). The distributions in Fig. 3 are well reproduced by the PYTHIA
Z1 Monte Carlo tune 7 .
The charged particle multiplicity density in the transverse region is plotted in Fig. 4a. Compared
to minimum bias there is about two times more activity in the UE. All Monte Carlo
models underestimate the multiplicity. Figs. 4b and 4c show the increase of the UE pT by about
20% from 0.9 to 7 TeV, well reproduced by a variety of models.
>
φ
d
η
/d
ch
N
2
0.1 GeV and | η|
< 2.5
T
Data 2010
PYTHIA ATLAS MC09
HERWIG+JIMMY ATLAS MC09
ATLAS
s = 7 TeV
PYTHIA DW
PYTHIA Perugia0
PHOJET
(a)
2 4 6 8 10 12 14 16 18 20
lead p [GeV]
T
> [GeV]
T
0.5 GeV and | η|
< 2.5
T
Data 2009
PYTHIA ATLAS MC09
HERWIG+JIMMY ATLAS MC09
ATLAS
s = 900 GeV
PYTHIA DW
PYTHIA Perugia0
PHOJET
(b)
1 2 3 4 5 6 7 8 9 10
lead p [GeV]
T
> [GeV]
T
0.5 GeV and | η|
< 2.5
T
Data 2010
PYTHIA ATLAS MC09
HERWIG+JIMMY ATLAS MC09
ATLAS
s = 7 TeV
PYTHIA DW
PYTHIA Perugia0
PHOJET
(c)
2 4 6 8 10 12 14 16 18 20
lead p [GeV]
T
Figure 4: Charged particle multiplicity density (a), average transverse momentum at √ s = 900 GeV (b) and
7 TeV (c)
3 Strangeness production
The production of the strange hadrons K 0 S , Λ and Ξ has been studied 8 . The mass peaks have
been reconstructed, as shown in Fig. 5a for the Ξ − . Production rates have been measured as
functions of rapidity and transverse momentum. The pT distributions extend from practically
zero to 10 GeV for K 0 S (Fig. 5b) and 6 GeV for Ξ− . The increase in production of strange particles
from 0.9 to 7 TeV is approximately consistent with results for charged particles described above,
but the rates exceed the predictions by up to a factor of three (Fig. 5c). This deficiency probably
originates from parameters regulating the frequency of s-quarks appearing in color strings.
2
Candidates / 2 MeV/c
8
6
4
2
0
3
× 10
s = 7 TeV
(a)
CMS
3
Yield: 34.4 × 10
2
Mean: 1322.1 MeV/c
2
Avg σ:
4.1 MeV/c
1300 1350 1400
2
Λπ−
invariant mass [MeV/c ]
-1
(GeV/c)
T
) dN / dp
NSD
(1/N
10
-2
10
10
1
-1
-3
-4
10
-5
10
(b)
CMS
s = 7 TeV
s = 0.9 TeV
0 2 4 6 8 10
0
KS
p [GeV/c]
T
MC / Data
0
KS
MC / Data
Λ
MC / Data
− Ξ
1
0.8
0.6
0.4
0.2
0 1 2 3 4 5 6
1
0.8
0.6
0.4
0.2
0 1 2 3 4 5 6
1
0.8
0.6
0.4
(c)
7 TeV PYTHIA6 D6T
7 TeV PYTHIA6 P0
7 TeV PYTHIA8
CMS
0.9 TeV PYTHIA6 D6T
0.9 TeV PYTHIA6 P0
0.9 TeV PYTHIA8
0.2
0 1 2 3 4 5 6
p [GeV/c]
T
Figure 5: Ξ − mass peak (a), pT distribution of K 0 S (b), hyperon pT yield ratios(c)
4 Particle correlations
Pairs of same-sign charged particles with four-momentum difference Q in the region 0.02 GeV <
Q < 2 GeV are analysed to study Bose-Einstein correlations 9 . The radius of the effective