single and double- particle studies at cms · 2010-07-19 · kevin stenson single and...

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?”

2

Why study QCD physics at the LHC?


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)])


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






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


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:


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


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)

http://link.aps.org/doi/10.1103/PhysRevLett.105.022002


http://livepage.apple.com/





http://dx.doi.org/10.1007/JHEP02(2010)041

http://dx.doi.org/10.1007/JHEP02(2010)041


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.


!"-4 -2 0 2 4

)!

"R

(

-2

0

2

4 (a) pp 0.9TeV

/2#<$"0<

!"-4 -2 0 2 4

)!

"R

(

-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

4

#"

0

2

4

)#

",!

"R

( 0

5

0

5

(a) pp 0.9TeV

!"-4-2

02

4

#"

0

2

4

)#

",!

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( 0

5

0

5

(b) pp 2.36TeV

!"-4-2

02

4

#"

0

2

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




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


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

⎡

⎣⎢

⎤

⎦⎥





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


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


Backup

15


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


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 "


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 "


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 "


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 "



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-!


Angular correlations compared to Pythia

19

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02

4

#"

0

2

4

)#

",!

"R

( 0246

0

2

4

6(a) PYTHIA 0.9TeV

!"-4-2

02

4#"

0

2

4

)#

",!

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( 0246

0

2

4

6(b) PYTHIA 2.36TeV

!"-4-2

02

4

#"

0

2

4

)#

",!

"R

( 0246

0

2

4

6(c) PYTHIA 7TeV

!"-4-2

02

4

#"

0

2

4

)#

",!

"R

( 0

5

0

5

(a) pp 0.9TeV

!"-4-2

02

4

#"

0

2

4

)#

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( 0

5

0

5

(b) pp 2.36TeV

!"-4-2

02

4

#"

0

2

4

)#

",!

"R

( 0

5

0

5

(c) pp 7TeV


Angular correlation cluster fit (near and away side)

20

!"-4 -2 0 2 4

)!

"R

(

-2

0

2

4 (a) pp 0.9TeV

/2#<$"0<

!"-4 -2 0 2 4

)!

"R

(

-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<

!"-4 -2 0 2 4

)!

"R

(

-2

0

2

4 (a) pp 0.9TeV

/2#<$"0<

!"-4 -2 0 2 4

)!

"R

(

-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<

single and double- particle studies at cms · 2010-07-19 · kevin stenson single and...

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