Single and Double-Particle Studies at CMS
Kevin Stenson for the CMS Collaboration
Kevin Stenson Single and Double-Particle Studies at CMS
QCD Physics
• QCD is the least well understood fundamental theory so there is much to learn.
• Most low energy QCD results are phenomenological models which require significant experimental input• The new energies available at the LHC allow us to test existing models
and develop new ones.
• QCD processes are responsible for much of the backgrounds in many other physics measurements and searches.
• Heavy-ion physics is also QCD physics and may tell us something about the early universe. The pp results provide a reference for heavy-ion physics.
• “Because it’s there” – Mallory’s response to “Why do you want to climb Mt. Everest?”
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Why study QCD physics at the LHC?
Kevin Stenson Single and Double-Particle Studies at CMS
CMS Detector
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• 3 barrel layers + 2 forward disks• 100 x 150 µm2 pixel size• 8-bit analog readout• 40 MHz clock (single crossing)• 66 million pixels
General purpose detector with all silicon tracker, PbWO4 EM calorimeter, and brass-scintillator hadronic calorimeter inside a superconducting solenoid providing a 3.8 T magnetic field. Muon chambers interspersed with flux return steel absorbers are inside a 2 T magnetic field.
CMS Tracker
Pixel detector• Tracks pass through ~10 barrel and
forward layers, ~40% with stereo views• 80-180 µm pitch• 8-bit analog readout• 9 million channels
Strip detectorCovers |η| < 2.4 (η = −ln[tan(θ/2)])
Kevin Stenson Single and Double-Particle Studies at CMS
Tracking performance• 98.4% of the pixels and 97.8% of the strips are active.• Strips have S/N > 20 while pixels have S/N > 50.• Pixel hit resolutions of 13µm in x and 32µm in y provides excellent
vertex resolution for b-tagging.• Energy loss measurements from the deposited charge in each
silicon layer can be used to identify particles at low momentum.
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High mass excess of data over MC is mostly due to lack of deuteron production in Pythiadeuterons
protons
kaons
http://arxiv.org/abs/1007.1988
Kevin Stenson Single and Double-Particle Studies at CMS
Tracking performance – reconstruction of particle decays
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)2 invariant mass (MeV/c-!+!420 440 460 480 500 520 540 560 580
2C
andi
date
s / 1
MeV
/c
0
200
400
600
800
1000
1200 CMS Preliminary = 900 GeV and 2360 GeVs
mass:0SPDG K
2 0.022 MeV/c±497.614
153±Yield: 17375 2 0.06 MeV/c±Mean: 497.68
2 0.12 MeV/c±: 4.53 "Core 2 0.41 MeV/c±: 11.09 "Tail
0.03±Core fraction: 0.58
)2 (+ c.c.) invariant mass (MeV/c-!p1080 1100 1120 1140 1160 1180
2C
andi
date
s / 1
MeV
/c
0
100
200
300
400
500
CMS Preliminary = 900 GeV and 2360 GeVs
mass:0"PDG 2 0.006 MeV/c±1115.683
68±Yield: 3334 2 0.06 MeV/c±Mean: 1115.97
2 0.26 MeV/c±: 1.00 #Core 2 0.14 MeV/c±: 3.25 #Tail
0.05±Core fraction: 0.15
A menagerie of weakly decaying strange particles
ct [cm]S0K
0 1 2 3 4 5 6 7 m
ass
fit y
ield
S0C
orre
cted
K
310
Corrected data
Exponential fit
CMS
τ = 90.0 ± 2.1 psStatistical uncertainties only
Lifetime result close to PDG (89.53 ± 0.05 ps) indicates
accuracy of MC.Charm decays also observed
KS
D*+D+
Λ0
Ξ−Ω−
KS
Kevin Stenson Single and Double-Particle Studies at CMS
Data sets and physics results presented
• Charged particle rate versus η and pT at √s = 0.9, 2.36, and 7 TeV
• Average pT of charged particles versus √s at √s = 0.9, 2.36, and 7 TeV
• Angular correlations between charged particles at √s = 0.9 and 2.36 TeV
• Bose-Einstein correlations between charged pions at √s = 0.9 and 2.36 TeV
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• In December, 2009 LHC provided pp collisions at √s = 0.9 and 2.36 TeV totaling ~10 µb−1.
• Since March 30, 2010 pp collisions at 7 TeV with continuously increasing luminosity (now up to ~200 nb-1)
• Plan for heavy-ion run at end of 2010, short stop, and run through 2011 before taking a long >1 year break.
LHC operations and CMS data:
Physics results presented here:
Kevin Stenson Single and Double-Particle Studies at CMS
Event corrections• Most results are reported for non-
single-diffractive (NSD) events (exclude elastic and single diffractive, include double diffractive and hard scatter events) selected by requiring:
• signal in at least one scintillation counter covering 3.2<|η|<4.7 coincident with colliding proton bunches,
• 3 GeV cluster of energy on each side of detector in forward calorimeters (2.9<|η|<5.2), and
• reconstructed primary vertex.
• Use Monte Carlo simulation to correct for missed NSD events and triggered SD events.
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Kevin Stenson Single and Double-Particle Studies at CMS
Charged hadron production at 0.9, 2.36, 7 TeV• Use three methods to measure production of charged hadrons
versus pseudorapidity (η = −ln[tan(θ/2)]):• Pixel hit counting: Efficient for pT > 30 MeV/c
• Pixel-only tracks: Efficient for pT > 50 MeV/c
• Full tracking: Efficient for pT > 100 MeV/c (also provides pT measurement)
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http://link.aps.org/doi/10.1103/PhysRevLett.105.022002http://dx.doi.org/10.1007/JHEP02(2010)041 [GeV/c]
Tp
0 1 2 3 4 5 6 ]
-2 [(
GeV
/c)
T d
p!
/dch
N2) d T
p"1/
(2 -510
-410
-310
-210
-110
1
107 TeV pp, NSD2.36 TeV pp, NSD0.9 TeV pp, NSD
Tsallis fits
CMS(b)
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xel c
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[pix
el u
nits
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0
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8
10
12
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16
18
20CMS(a)
Kevin Stenson Single and Double-Particle Studies at CMS
Charged hadron production at 0.9, 2.36, 7 TeV
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!-2 0 2
!/d
chdN
0
2
4
6
CMS NSDALICE NSDUA5 NSD
0.9 TeV
2.36 TeV
7 TeV
CMS(b)
[GeV]s10 210 310 410
0!" #"
/dch
dN
0
1
2
3
4
5
6
7 UA1 NSDSTAR NSDUA5 NSDCDF NSDALICE NSDCMS NSDE. Levin et al.PYTHIA ATLASPYTHIA D6TPHOJET
NAL B.C. inel.ISR inel.UA5 inel.PHOBOS inel.ALICE inel.
s0.161 + 0.201 ln s 2 + 0.0267 lns2.716 - 0.307 ln s 2 + 0.0155 lns1.54 - 0.096 ln
CMS(b)
[GeV]s10 210 310 410
[GeV
/c]
! Tp"
0.3
0.35
0.4
0.45
0.5
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0.6
0.65ISR inel.UA1 NSDE735 NSDCDF NSDCMS NSDTroshin et. al.PYTHIA ATLASPYTHIA D6TPHOJET
s 2 + 0.00143 lns0.413 - 0.0171 ln
CMS(a)
dN/dη shape remains the same as energy increases.
dN/dη at η≈0 versus energy has a steeper increase than predicted.
<pT> also increases with energy. Models bracket the observation.
Final results (for NSD events) are obtained by correcting the track distributions for event selection and track reconstruction efficiency.
Kevin Stenson Single and Double-Particle Studies at CMS
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Two-particle angular correlations
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R Δη,Δφ( ) = N −1( )SN Δη,Δφ( )BN Δη,Δφ( )
−1⎛
⎝⎜⎜
⎞
⎠⎟⎟
N
Two-particle correlation function:
Signal events (contains correlations)
Background (mixed events – no correlations)
Integrate over ϕ to obtain R(Δη)
Gaussian like distribution in Δη and ridge across ΔϕNarrow strong peak in near side (Δϕ ≈ 0) likely from high pT processes, e.g. jets
Broader peak in away side (Δϕ ≈ π) likely from soft processes,
e.g fragmentation
Fit to obtain correlation strength
(amplitude) and width
http://cdsweb.cern.ch/record/1267376/files/QCD-10-002-pas.pdf
Kevin Stenson Single and Double-Particle Studies at CMS
Two-particle angular correlations
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|<3
!| eff
K
1.5
2.0
2.5
3.0 (a)
(GeV)s
2103
10 410
|<3
!|"
0.4
0.6
0.8
(b)
CMS p+p, extrapolatedPHOBOS p+p
ISR p+p
pSPS-UA5 p+PYTHIA p+p, defaultPYTHIA p+p, D6T
• Can interpret results in terms of independent clusters emitted in interaction and decaying into hadrons.• More massive clusters = more hadrons
= larger size = stronger correlations
• Fit R(Δη) with
• α = strength = 〈K(K-1)〉 / 〈K〉 where K is the average cluster size.
• Γ(Δη) is a Gaussian:
• Actually measure Keff = α+1
R Δη( ) =αΓ Δη( )B Δη( )
−1⎡
⎣⎢⎢
⎤
⎦⎥⎥
exp − Δη( )2 / 4δ2( )⎡⎣
⎤⎦
Results:• Cluster size (correlation strength) increases
with √s (more jets?)
• Pythia cluster size consistent with originating from resonances (e.g. ρ); data much higher – must be other sources of correlations
• Width is well modeled and ∼flat versus √s
Kevin Stenson Single and Double-Particle Studies at CMS
Bose-Einstein Correlations
• Consider the ratio where P(p) is the probability for emitting a single particle with 4-momentum p and P(p1,p2) is the joint probability for emitting two identical particles with 4-momenta p1 and p2.
• Bose-Einstein correlations (BEC) will manifest as an enhancement when p1 ≈ p2. Use to measure how similar p1 and p2 are.
• So where ref indicates a distribution free of BEC effects.
• Obvious reference (opposite sign pairs) problematic: resonant decays
• Construct references (flipping momentum vectors, mixing events, etc.)
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R =P p1, p2( )
P p1( )P p2( )
Q = − p1 − p2( )2 = M inv2 − 4mπ
2
R = dN / dQdN / dQref
Q (GeV)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Sing
le ra
tio
0.8
1
1.2
1.4
1.6
1.8
2
(a)Ref.: Opposite charge
DataMC
= 0.9 TeVsCMS preliminary Evidence for BEC
(no BEC)
• Still have residual structure in references versus Q. Use MC to remove by constructing double ratio:
R = R / RMC =dN / dQdN / dQref
⎡
⎣⎢
⎤
⎦⎥ / dN / dQMC
dN / dQMC,ref
⎡
⎣⎢
⎤
⎦⎥
http://link.aps.org/doi/10.1103/PhysRevLett.105.032001
Kevin Stenson Single and Double-Particle Studies at CMS
Bose-Einstein Correlations
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Q (GeV)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Dou
ble
ratio
0.8
1
1.2
1.4
1.6
1.8
2
Ref.: Combined sample
= 0.9 TeVsCMS preliminary
Excluded from Fit
0.05) fm±r = (1.59 0.02± = 0.62 !
R Q( ) = C 1+ λΩ Qr( )⎡⎣ ⎤⎦ 1+δQ[ ]Fit ratio with empirical relation:
Ω(Qr) is the Fourier transform of the emission region characterized by a size r and λ gives the BEC strength. Using an exponential for Ω gives satisfactory results.
λ = 0.62 ± 0.02 ± 0.05, r = 1.59 ± 0.05 ± 0.19 fm at 0.9 TeV
λ = 0.66 ± 0.07 ± 0.05, r = 1.99 ± 0.18 ± 0.24 fm at 2.36 TeV
To compare with other measurements which fit using
a Gaussian, divide r by √π
Kevin Stenson Single and Double-Particle Studies at CMS
Conclusions
• The LHC is opening up a new energy regime which will be used to search for new physics.
• Proving the existence of new physics usually requires a good understanding of current physics
• The measurements presented probe several areas of QCD physics:
• Track multiplicity and <pT> are observed to increase as the center-of-mass energy increases from 0.9 to 2.36 to 7 TeV. Multiplicity rises faster than predicted by Pythia.
• Two particle correlations show the effects of hard processes like jets and soft processes like fragmentation – poorly modeled by Pythia.
• Observation of Bose-Einstein correlations between pairs of identical particles emitted from a region with size ~1 fm which increases by ~25% going from 0.9 TeV to 2.36 TeV pp collisions.
• These results enable a better understanding of QCD, and provide important inputs for tuning Monte Carlo simulations.
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Kevin Stenson Single and Double-Particle Studies at CMS
Backup
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Kevin Stenson Single and Double-Particle Studies at CMS
Backup
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charged particlesN5 10 15 20 25 30 35
r (fm
)
00.5
11.5
22.5
33.5 Opposite hem. same charge
charged particlesN5 10 15 20 25 30 35
!
0.20.40.60.8
11.21.4
Combined sample
= 0.9 TeVsCMS
Bose-Einstein correlation parameter (radius r and strength λ) versus charged particle multiplicity
Kevin Stenson Single and Double-Particle Studies at CMS
Bose-Einstein, separate reference samples
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Q (GeV)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Dou
ble
ratio
1
1.2
1.4
1.6
1.8
2
< 10tr N! 2 > = 5.6tr<N
0.07 (fm)±r = 1.00 0.05± = 0.89 "
= 0.9 TeVsCMS Preliminary Ref. Combined sample
Q (GeV)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Dou
ble
ratio
1
1.2
1.4
1.6
1.8
2
< 15tr N! 10 > = 12.3tr<N
0.08 (fm)±r = 1.28 0.04± = 0.64 "
= 0.9 TeVsCMS Preliminary Ref. Combined sample
Q (GeV)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
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ble
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1.2
1.4
1.6
1.8
2
< 20tr N! 15 > = 17.3tr<N
0.10 (fm)±r = 1.40 0.04± = 0.60 "
= 0.9 TeVsCMS Preliminary Ref. Combined sample
Q (GeV)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
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ble
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1
1.2
1.4
1.6
1.8
2
< 30tr N! 20 > = 24.1tr<N
0.14 (fm)±r = 1.98 0.05± = 0.59 "
= 0.9 TeVsCMS Preliminary Ref. Combined sample
Q (GeV)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Dou
ble
ratio
1
1.2
1.4
1.6
1.8
2
< 80tr N! 30 > = 36.5tr<N
0.25 (fm)±r = 2.76 0.09± = 0.69 "
= 0.9 TeVsCMS Preliminary Ref. Combined sample
Kevin Stenson Single and Double-Particle Studies at CMS
Bose-Einstein with particle ID
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Q (GeV)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
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ble
ratio
0.9
1
1.1
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candidates!!
candidates! non-!
candidates!!
candidates! non-!
candidates!!
candidates! non-!
candidates!!
candidates! non-!
Kevin Stenson Single and Double-Particle Studies at CMS
Angular correlations compared to Pythia
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Kevin Stenson Single and Double-Particle Studies at CMS
Angular correlation cluster fit (near and away side)
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