julia velkovska
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Julia Velkovska
Selected CMS Results from pp collisions
27th Winter Workshop on Nuclear DynamicsWinter Park, Colorado
Feb 6-13, 2011
J. Velkovska 2
Motivation
To get insights into multi-particle production in high energy pp collisions soft vs hard production Description of multi-parton interactions Non-linear effects at high gluon densities Collective effects at high energy densities String fragmentation and color
neutralization
WWND 2/9/2011
J. Velkovska 3WWND 2/9/2011
Results: pp collisions at √s = 0.9, 2.36 and 7 TeV Charged Hadrons
spectra, <pT>, dN/dη, Event-by-event multiplicity Correlations
Azimuthal and Long-range rapidity correlations Bose-Einstein Correlations
Sensitive to the detailed implementation of string fragmentation, multi-parton interactions, interplay between hard-scatterings and the underlying event, collective dynamics
Strangeness production spectra, <pT>, dN/dη
Jet shapes Sensitive to parton radiation and fragmentation scheme Proposed as a model-discriminating observable in jet-quenching
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The CMS Si Tracker
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Acceptance: |η|<2.4 and full azimuthal coverage66M Pixels:● 150x100 μm2, closest to the interaction point● 3 barrel layers (4, 7 and 10 cm radii) and 2 endcaps on each side9.6 M Strips:● 10 layers: Larger silicon modules, refine momentum resolution
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Charge hadron reconstruction
Pixel hit counting (1 hit) Using the primary vertex,
calculate η for each cluster Immune to detector mis-
alignment, simplest pT > 30 MeV/c, | η | < 2
Used for dN/dη
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Tracklets (2 hits)Form hit pairs, calculate { Data-driven background subtraction{ pT > 50 MeV/c, jj < 2
Tracklets (2 hits) Form hit pair - calculate η Data-driven bg subtraction pT > 50 MeV/c, | η | < 2
Used for dN/dη
Full tracks Use all pixel and strip hits,
provide η and pT
Robust and least sensitive to bg
pT > 100 MeV/c, | η | < 2.4 Used for dN/dη and
spectra
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Charged hadron spectra and dN/dη
Spectra are well described by Tsallis fit Rapidity density nearly flat in the measured range: good
agreement between experimentsWWND 2/9/2011
JHEP 02 (2010) 041 and PRL 105 (2010) 022002Average all methods
&Symmetrize
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Comparison to models
Most PYTHIA tunes underestimate dN/dη PHOJET ( based on dual-parton model – multiple soft
strings) underpredicts dN/dη, but does well on <pT> Saturation models do relatively well
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<pT> and dN/dη vs √s compared to models
Most models fail to describe both quantities simultaneously. Why do these quantities rise and why faster than ln(√s) ? What is the role of hard processes and that of soft multi-
parton interactions ?WWND 2/9/2011
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Event-by-event multiplicity
PYTHIA 8 describes the total multiplicity well, but produces too many charged hadrons with high transverse momentum
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ArXiv: 1011.5531 JHEP, submitted
pT >0 pT >500 MeV/c
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Correlate <pT> and event-by-event multiplicity
the rise of the average transverse momentum with the multiplicity is roughly energy-independent
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ArXiv: 1011.5531 JHEP, submitted
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Di-hadron Correlations More detailed information about the dynamics
Short range rapidity correlations ~ |∆η | < 2 are sensitive to string fragmentation models
Bose-Einstein Correlations ( very short range in η and φ ) – reflect the space-time distribution of the source
Long range rapidity “Away-side” (Δφ ~ π) typically come from back-to-back jets
A new long range correlation was observed on the near side (Δφ ~ 0) in high multiplicity pp events at √s = 7 TeV
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Here is how the data look like:
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JHEP 09 (2010) 091
Min bias PT> 0.1 GeV/c
Min bias 1 < PT< 3 GeV/c
High Multiplicity PT> 0.1 GeV/c
High Multiplicity 1 < PT< 3 GeV/c
“Ridge” structure
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The Ridge Yield evolution
Associated yield is largest in 1<pT <3 GeV Increases with multiplicity
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pT
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Source parameters from BEC
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Source radii depend on multiplicity, but not much on colliding energy
Phys. Rev. Lett. 105 (2010) 032001: √s =0.9 and 2.36 TeV arXiv:1101.3518 : √s =0.9 and 7 TeV STAR: ArXiv 1004.0925
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Source radii vs pair transverse momentum kT
At high Nch the Source radius depends on kT
Radial expansion ?
Similar observation in 1.8 TeV data : Phys. Rev. D 48, 1931–1942 (1993)
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The Invariant Mass Peaks
Peak shape and S/B: Excellent agreement between data and simulation
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Strange hadron spectra
The spectra are well described by Tsallis Fit
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Mean pT and dN/dy
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CMS PAS: QCD-10-007
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Strange Hadron Rapidity Density
Yield significantly larger than MC predictions Discrepancy increases with energy and hadron mass
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Yield under predicted by a factor of 3
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Jet shapes Jets are characterized by :
charged particle multiplicity in a jet,Nch
Charge particle transverse jet shape Integrated jet shape – the fraction of
energy contained in a disk of radius r<R
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Jet Shapes in Pythia and data
Nch and transverse shape sensitive to quark/gluon fraction In PbPb events we expect different quenching of quark and
gluon jets Sensitivity to underlying jet-quenching mechanism
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CMS PAS: QCD-10-014
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Integrated Jet shapes (calo and tracking): data vs models
Agreement is worse at low pTWWND 2/9/2011
CMS PAS: QCD-10-014
Low pT
High pT
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Conclusion
Detailed CMS measurements of QCD observables in pp collisions at √s = 0.9, 2.36 and 7 TeV
Insights into soft and hard particle production mechanisms and the role of multi-parton interactions
Possible observation of collective effects in high-multiplicity pp collisions – needs further experimental and theoretical studies
Important baseline measurements for HI observables have been carried out
Stay tuned for upcoming HI results
J. Velkovska 24WWND 2/9/2011
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Identified Strange Hadrons
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Reconstruct Λ ● pair it with a negatively charged track● Secondary vertex within 5σ from primary● Require Ξ- to point to primary● Ξ- decay vertex well separated from primary
Fit pair of oppositely charged tracks to common vertex● Daughter tracks: Of good quality and Not from primary vertex● Secondary vertex– Far “enough” from the primary vertex● Require V0 point back to primary vertex
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