single and double- particle studies at cms · 2010-07-19 · kevin stenson single and...
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![Page 1: Single and Double- Particle Studies at CMS · 2010-07-19 · Kevin Stenson Single and Double-Particle Studies at CMS QCD Physics •QCD is the least well understood fundamental theory](https://reader034.vdocument.in/reader034/viewer/2022042114/5e9059fbc6ecb929aa465a94/html5/thumbnails/1.jpg)
Single and Double-Particle Studies at CMS
Kevin Stenson for the CMS Collaboration
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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?”
2
Why study QCD physics at the LHC?
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Kevin Stenson Single and Double-Particle Studies at CMS
CMS Detector
3
• 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)])
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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.
4
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
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Kevin Stenson Single and Double-Particle Studies at CMS
Tracking performance – reconstruction of particle decays
5
)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
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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
6
• 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:
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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.
7
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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)
8
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)
!-3 -2 -1 0 1 2 3Pi
xel c
lust
er le
ngth
alo
ng z
[pix
el u
nits
]
0
2
4
6
8
10
12
14
16
18
20CMS(a)
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Kevin Stenson Single and Double-Particle Studies at CMS
Charged hadron production at 0.9, 2.36, 7 TeV
9
!-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
0.55
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.
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Kevin Stenson Single and Double-Particle Studies at CMS
!"-4 -2 0 2 4
)!
"R
(
-2
0
2
4 (a) pp 0.9TeV
/2#<$"0<
!"-4 -2 0 2 4
)!
"R
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-2
0
2
4 (b) pp 2.36TeV
/2#<$"0<
!"-4 -2 0 2 4
)!
"R
(
-2
0
2
4 (c) pp 7TeV
/2#<$"0<
Two-particle angular correlations
10
!"-4-2
02
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2
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)#
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( 0
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5
(a) pp 0.9TeV
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(b) pp 2.36TeV
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02
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0
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4
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( 0
5
0
5
(c) pp 7TeV
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
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Kevin Stenson Single and Double-Particle Studies at CMS
Two-particle angular correlations
11
|<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
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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.)
12
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
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Kevin Stenson Single and Double-Particle Studies at CMS
Bose-Einstein Correlations
13
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
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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.
14
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Kevin Stenson Single and Double-Particle Studies at CMS
Backup
15
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Kevin Stenson Single and Double-Particle Studies at CMS
Backup
16
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
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Kevin Stenson Single and Double-Particle Studies at CMS
Bose-Einstein, separate reference samples
17
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
Dou
ble
ratio
1
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
Dou
ble
ratio
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
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Kevin Stenson Single and Double-Particle Studies at CMS
Bose-Einstein with particle ID
18
Q (GeV)0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
Dou
ble
ratio
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
= 0.9 TeVsCMS
candidates!!
candidates! non-!
candidates!!
candidates! non-!
candidates!!
candidates! non-!
candidates!!
candidates! non-!
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Kevin Stenson Single and Double-Particle Studies at CMS
Angular correlations compared to Pythia
19
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Kevin Stenson Single and Double-Particle Studies at CMS
Angular correlation cluster fit (near and away side)
20
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