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.

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

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

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)

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

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N2) d T

p"1/

(2 -510

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107 TeV pp, NSD2.36 TeV pp, NSD0.9 TeV pp, NSD

Tsallis fits

CMS(b)

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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

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CMS NSDALICE NSDUA5 NSD

0.9 TeV

2.36 TeV

7 TeV

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[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"

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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

10

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−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

11

|<3

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2.5

3.0 (a)

(GeV)s

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10 410

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0.6

0.8

(b)

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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

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1

1.2

1.4

1.6

1.8

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(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

15

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

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

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2

< 10tr N! 2 > = 5.6tr<N

0.07 (fm)±r = 1.00 0.05± = 0.89 "

= 0.9 TeVsCMS Preliminary Ref. Combined sample

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< 15tr N! 10 > = 12.3tr<N

0.08 (fm)±r = 1.28 0.04± = 0.64 "

= 0.9 TeVsCMS Preliminary Ref. Combined sample

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< 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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ratio

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1.2

1.4

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< 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

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1.4

1.6

1.8

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< 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

18

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candidates!!

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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)

20

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