Julia Velkovska

Selected CMS Results from pp collisions

27th Winter Workshop on Nuclear DynamicsWinter Park, Colorado

Feb 6-13, 2011

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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

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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

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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