Transcript Slide 1

Long-Range, Near-Side Angular Correlations
in Proton-Proton Interactions in CMS
First observation in
p-p or p-pbar collisions!
Kajari Mazumdar
LHC Physics Seminar, DHEP, TIFR
October 23, 2010.
ArXiv:1009.4122 (hep-ex), CERN-PH-EP-2010-031
Submitted to Journ. of High Energy Physics
Prelude
• Trying to understand the analysis and its implications.
• We have not participated in this analysis and would like to know more
about the analysis.
• Several slides are taken from presentations by G.Tonelli (Spokesperson,
CMS collaboration) and G. Roland (leader of the analysis) at CERN
on September 21, 2010.
http://indico.cern.ch/conferenceDisplay.py?confId=10744
Additionally materials collected from various presentations by Wei Li and
private communications.
• CMS press release can be obtained from http://cms.cern.ch
• Finally, we are proud to be in CMS collaboration, to have various colleagues
with expertise in various aspects.
Summary of the seminars on 21.9.2010
•
Short-range and long-range angular collisions studied in pp collision with CMS at
LHC
• Observed long-range, near-side correlations in high-multiplicity events.
-- signal grows with event multiplicity
-- effect is maximal in transverse momentum range of 1- 3 GeV/c
• Long-range, near-side correlation is not seen in low multiplicity events and
generators, but resembles effects seen in heavy-ion collisions at high energies.
• The feature has survived all possible checks for various effects.
Collaboration decided to submit the paper to expose our findings to the
scrutiny of the scientific community at large.
Since there are a number of potential explanations, today’s presentation is
focussed on the experimental evidence in the interest of fostering a broader
discussion on the subject.
The incoming Heavy Ion run of LHC during end of the year will be an additional
important test bench.
Preliminaries
Starting November 2009, LHC delivered proton-on-proton collisions at 900 GeV,
2,360 GeV before moving on to 7000 GeV from March 30, 2010. We have few
more weeks with pp collision at 7 TeV, before heavy ion collision.
The sample of hard scattered events, till now is too small to really see anything new
at high energy scale. Integrated luminosity at present is about 3.5 /pb.
However, even with low luminosity, lower energy operation, LHC is already turning
out to be a no-loose machine, albeit, in a different sense!
• Physics commissioning studies in CMS experiment have shown that, in general, the
detector is behaving extremely well, and the results match Monte Carlo predictions
till now. The level of preparedness of the experiment is very high.
• No significant discrepancy with known theory observed.
The first physics with hadron collision: minimum bias events
 mainly soft particles (pions, kaons of average momentum few hundred MeV/c).
measurements have to be explained by mostly phenomenological models rather
than by perturbative QCD, and the description can be improved with new data.
study basic properties at the new kinematic regime offered by LHC.
 Need high resolution, highly efficient detector to “track” soft particles.
CMS All Silicon Tracker : detection of charged particles
(Pixel Detector, Tracker Inner and Tracker Outer, Tracker Endcap, ..)
r- θ view, beam parallel to horizontal axis
r-f view, beam out of/into the plane
TOB
TIB
PD
TOB
TIB
TEC
TID
PD
• Coverage up to |h|<2.5; extremely high granularity, due to the small cell size and high
longitudinal segmentation, to keep low occupancy (~ a few%) also at LHC nominal
luminosity.
• It is the largest Silicon Tracker ever built: Strips: 9.3M channels; Pixels: 66M channels.
Operational fractions: strips 98.1%; pixel 98.3%
Pixel detector: 3 barrel layers at radii between 4.4 and 10.2 cm
Single point resolution: 10 mm in r-f, 25 mm in z
Silicon detector: 10 barrel layers upto 1.1 m
Resolution of transverse momentum for particles with 1 GeV/c:
0.7% at h = 0, 2% at |h|=2.5
Primary vertex (requires atleast 3 tracks) to lie within 4.5 cm of nominal
collision point along Z and 0.15 cm in direction transverse to beam.
Track counting done with dz, dxy error ~ 100 mm
Dh  angle between two tracks in
the longitudinal plane
Df  angle between two tracks in
the transverse plane
Probing new energy frontier starts with the study of minimum bias
events in the new energy regime: charge multiplicity, average
momentum distribution, etc.
Charged hadron multiplicity in mimimum
bias events at different energies.
Strategy: make the best possible use of
the large eta coverage of CMS detector,
the redundancy of the apparatus and
the flexibility of trigger system.
High multiplicity events are rare.
Special trigger needed to collect large
statistics sample of high multiplicity
events.
For primary/online data collection implemented
event trigger condition in 2 steps:
• total transverse energy in the event > 60 GeV
• the multiplicity of charged tracks > 70/85
(above pt>400 MeV/c, |Dh|<2, within dz < 0.12 cm
of a single vertex, with z < 10 cm)
Statistics for high multiplicity
events enhanced by O(103)
Compared to min.bias
events.
High multiplicity events
The particle densities in the high
multiplicity events of proton-proton
collisions at 7TeV begin to
approach those in high-energy
collisions of nuclei such as copper.
It is natural to study the two
particle angular correlations in LHC
and compare the results with the
ones obtained in relativistic heavy
ion colliders like RHIC.
The schematization of the collision is cut into pieces and modeled in
different ways, though, actually, the pieces are correlated.
2-particle correlation probes the connection between various pieces.
clusters
The Past and The Future
Independent cluster model depicts:
1. independently produced clusters from the initial interaction.
2. Isotropic decay of these clusters in their CM system into hadrons.
Clusters: jets, resonances, strings, …, having short range correlation.
2-particle correlation probes essentially the mechanism of multiparticle
production in high energy collision of hadrons.
At high energy collisions, the mechanism of hadronisation and possible
collective effects due to high particle densities can be studied,
Various correlations have been studied extensively in previous
experiments at ISR, SPS, RHIC.
STAR, PHOBOS experiments at RHIC has reported observation of longrange (high |Dh|) angular correlation at near side (|Df| ~ 0).
Heavy Ion collisions at LHC by end of 2010, will provide more opportunity
to study the effect in detail.
Types of correlation
Angular correlation can be both short- and long-range which
characterizes QCD in the energy ranges encountered in the experiment.
Short-range correlations in minimum bias events has a typical width of
Dh ~ 1.
The correlation strength and extent can be parametrized in terms of a
simple cluster model and the strength quantified in terms of cluster size
(average no. of particles in a cluster) and width (separation of particles
in pseudorapidity)  However, this does not really lead to basic
understanding of the process.
The long range correlation, for high |Dh| values, may be significantly
affected by the presence of hot and dense matter formed in high energy
collisions of hadronic matter. Possible to study till now only in heavy-ion
facilities, but now, may be already in proton-proton collisions at LHC.
It could as well be manifestation of some jet properties at high energies.
The physical origin of long range correlation is not yet well-understood.
Steps to make 2-particle correlation for a given momentum bin
Signal: pairs from same event
• Take each event and make all possible 2-particle pair combination.
• For each pair calculate Dh and Dj and fill a histogram.
• Then average over all events
• Normalization of the histo not too important. However, the distribution is
normalised to unit integral in the present analysis.
Background: essentially product of 2 single particle distributions.
• take random combinatorial pairs, (using event mixing procedure to kill
any correlation)
• Select 2 events randomly and make pairs using one particle from each
event having similar vertex and multiplicity.
Take Dh, Dj always positive and fill other quadrants by reflection
 symmetry around Dh = 0, Dj = 0
Correlation function
Correlation function
Calculate ratio for
each multiplicity
bin first and then
weight by average
multiplicity of each
bin <N>.
R is a measure of correlated pairs/pairticle/event.
Multiplicity weighting factor: N-1 = total number of pairs/particle/event
Features of correlation plot:1
Features of correlation plot: 2
Features of correlation plot: 3
2-particle correlation in minimum bias data
Result from real data:1
Cut the region of |Dh|<0.06, |Dj|<0.06, to reduce secondary effects (tracks from
photon conversions, weak decays, event not rejected by impact parameter cut.
Result from real data: 2
Result: 3, the novel feature!
Result from Monte Carlo generated events
Multiplicity and Pt dependence: Turn on of the Ridge
High Multiplicity and Pt dependence: 2
MC has minimum at Dj = 0
Data has local maximum at Dj = 0
Systematic uncertainties
• Negligibly small statistical uncertainty.
• However the signal is subtle and unexpected.
Estimate systematic uncertainties
•Is there a way to fake the signal qualitatively?
Check for effects due to:
•Pile-up + beam background,
•Detector noise, acceptance, efficiency
•Trigger efficiency, bias
•Reconstruction efficiency, fakes
•Bugs in analysis code.
•Apply data-driven methods
No indication of effect that would fake ridge signal .
Like-sign vs. unlike sign
Correlations calculated separately for pairs of same sign and opposite signs
No dependence on relative charge sign
Analysis code
Track reconstruction code
Trigger
Events background: 1
Events background: 2
Event pileup
Pileup effects are suppressed due to excellent resolution.
Analysis with tracks paired with photons
Conclusion
CMS experiment has measured 2-dimensional angular correlations
between particles with high |Dh| over full range of Dj for proton-proton
collision at cm energies of 0.9, 2.36 and 7 TeV.
A variety of features are observed due to short- and long-range
correlations.
The most interesting feature is the first observation of near-side,
long-range correlation in high-multiplicity events at 7 TeV which resembles
the observation at RHIC experiments.
The physical origin of the correlation is not yet understood.
Further detailed studies are needed.
CMS decided to report about the finding to the HEP community.
Backup
Event and track selection
Minimum Bias Data sample at different energies
Nevent
integrated luminosity
energy
168,854
10,902
150086
3.3 mb-1
0.2 mb-1
3.0 mb-1
900 GeV
2360 GeV
7000 GeV
For high multiplicity analysis data corr. To int. lumi = 980 /nb
Data at 7 TeV
Event multiplicity corrected for all detector and reconstruction algorthm
Short range correlation vs. sqrt(s)
Quantifying cluster properties and their energy dependence
On average every 2-3 charged particles, typically pions are produced in a correlated
Fashion  cluster mass ~ 1GeV
Clusters are also getting narrower with increasing energy.
2-particle correlation
Tracking performance in high multiplicity events
pT>1.0GeV/c
|h|<1.0
o 20<N<35
• 90<N<110
o 20<N<35
• 90<N<110
pT>1.0GeV/c
The CMS Tracker feels at
home in high charged
particles multiplicity
environment.
It has been designed to
tackle thousand of tracks
per event as it will happen
with LHC running in pp at
nominal luminosity.
|h|<1.0
It is foreseen to provide
good tracking performance
also for the imminent
Heavy Ion running of LHC
where we expect to have
order of 104 tracks per
event.
Corrections
Event selection efficiency: low for low multiplicity events
Triggering
vertexing
Tracking/acceptance efficiency correction
Overall efficiency 76%, for pT ~ 100 MeV/c efficiency = 55%
No significant change after correction, since event multiplicity is high
Clusters are partially lost at the edge of
Acceptance, correlation affected by 20-25
Track Selection:
Track impact parameter significance for track pT between 100 to 200 MeV/c
Very good
agreement
between data
and monte
carlo down to
soft tracks
Use MC to
determine
efficiency of
track selection
Energy dependence of cluster properties
Data Collection in p-p collision
Dedicated high multiplicity trigger in the two steps.
Level 1 (L1): Sum of the total transverse energy ET (ECAL, HCAL, and HF) > 60 GeV.
High-level trigger (HLT): number of online tracks built from the three layers of pixel detecto
>70 (85).
Statistics for high multiplicity events enhanced by O(103).
Total datasets corresponding to 980nb-1
CMS
PHOBOS
pT>0.1GeV/c
high multiplicity pp 7TeV
comparable to ~18 nucleon
pairs, each colliding at
62.4GeV in CuCu
-2
h
+2
The particle densities in the high multiplicity events of proton-proton collisions at 7TeV
begin to approach those in high-energy collisions of nuclei such as copper.
It was considered natural to study the two particle angular correlations in LHC and
compare the results with the ones obtained in relativistic heavy ion colliders like RHIC.
Zero Yield At Minimum (ZYAM)
Strength of the near side ridge and its dependence on pt and multiplicity
Can be quantified by calculating the associated yield: number of other
Particles correlated with a specific particle.
ZYAM uses R(Dj) integrated between |Dh|=2.4 and 4.8.
First fit a polynomial in the range of 0.1 to 2.0 and find the minimum,
FZYAM.
Integrated R(Dj) between o to FZYAM and multiply with background
integrated over Dh between 2.0 to 4.8
Assume that away side jet contribution, the background, negligible.
The uncertainty in fitted minimum gives the uncertainty of the associated
yield
Detector