Paper on J/ and b production

Wenbin Qian, Patrick Robbe for the F-WG, Tsinghua Beijing/LAL Orsay, 2 Dec 2009

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Content « Measurement of the prompt J/ψ and b → J/ψ

production cross-sections at LHCb » based on 5 pb–1 of data collected with muon and di-muon trigger use VELO and long tracking only + muon ID measure J/ψ → μμ differential cross sections:

4 bins in pseudorapidity η, between 3 and 5 in steps of 0.5 7 bins in pT, between 0 and 7 GeV/c in steps of 1 GeV/c

2 bins for overlap with ATLAS/CMS: 2 < η < 2.5, pT > 7 GeV/c

in each phase-space bin:

extract prompt J/ψ and b→ J/ψ signal yields with combined fit of J/ψ mass and J/ψ “time” distributions

get efficiency from MC as a function of polarization (treat unknown polarization as systematics)

convert results to production cross-sections using luminosity estimate quote σ(B hadron) in LHCb’s acceptance

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Motivations (1)J/ production is a process with large cross-section: large

dataset available very soon after start of LHCb running.

Use J/ sample to measure prompt and bb cross-section from the 2 processes:

pp → X + ((2S), c0,1,2 , … → X) J/

pp → X + bb (b/b → J/ X)

These measurements are important for later analysis steps in LHCb:

Open the road to B physics with J/ and dimuon modes,

Tune B hadron spectra in Monte-Carlo,

Necessary input for absolute branching fraction measurements.

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Motivations (2)

Measurement itself is of interest:

J/ production properties not well understood: NRQCD was successful in reproducing the p

t spectrum measured at

Tevatron, but not the production polarization. The measurement of the differential cross-section at higher energies is an important observable to understand charmonium production mechanism.

Special coverage of LHCb experiment, where theoretical predictions are less accurate.

Need O(10)% error in different pt and

bins to test predictions based on FONLL computations (uncertainties between 20% and 50%)

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Some numbers

DC06:

(prompt J/14TeV) = 0.266 mb ± 0.002 mb,

(bb) = 0.698 mb ± 0.001 mb

MC09:

same prompt J/ but higher bb (~0.900 mb)

Br(b → J/) = 1.16% ± 0.10% (LEP, 1992-1994), Br(b → J/ in Monte Carlo) = 1.46%

ie (b → J/) = 0.0204 mb ± 0.0008 mb in DC06

DC06 numbers used for this analysis

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as a function of CM energyC

ross

se

ctio

n (

mb

)

Cross-sections at 7 TeV are half of cross-sections at 14 TeV

J/

Cro

ss-s

ectio

n (m

b)bb

Cro

ss-s

ectio

n (m

b)

Beam energy

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Generator Level Cross-sectionDC06 generator level differential cross-section for J/ with

3<<5.

This is what we want to measure and to re-inject in the Monte-Carlo.

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J/ Selection+, -:

Long tracks, with 2/nDOF<2

pt>0.7 GeV/c

Loose Muon PID for both tracks (StdLooseMuon, ln(L)>-1)

J/ vertex: 2/nDOF

< 6

J/ mass window: ± 400 MeV/c2

At least one primary vertex

In case of several candidates per event, take the one with smallest 2(+)+2(-).

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J/ SelectionIllustration on L0 minimum bias DC06 sample

Mass resolution: 11.2 ± 0.4 MeV/c2

S/B=17.6 ± 2.3

1.3x109 reconstructed after L0 J/ for 1 fb-1 (14 TeV)

Or 3.3x106 for 5 pb-1 at 7 TeV

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J/ Selection (MC 09, 7 TeV)

With L=1028 cm-2.s-1: 0.01 Hz

With L=1030 cm-2.s-1: 1 Hz: Requires Trigger.

Thomas Ruf

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J/ Stripping

m()>2.7 GeV/c2 p

T()>0.5 GeV/c

: long track, StdLooseMuons, 2/nDOF

<3

Vertex: 2/nDOF

<20

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Acceptance Efficiencies

Between 20% and 98% (error 1%)

Identical for prompt and J/ from B

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Reconstruction Efficiency

Between 8% and 63% (error 3%)

Identical for prompt and J/ from B

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L0 Trigger Efficiency

Between 58% and 100% (error 1%)

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Distinguish prompt J/ from J/ from b

Use: +

Primary vertex

z

-

dz

Simple approximation of « b quark » lifetime:

t distribution at generator level

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How to measure differential cross-section

Divide in J/ pt and (pseudo-rapidity) bins

Fit mass distribution to measure number of reconstructed J/

Use t distribution to measure number of reconstructed prompt J/ and J/ from b

Use efficiency per pt and bin to compute

original number of J/

Bins definition:

0pt7 GeV/c, 7 bins

35, 4 bins

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Mass distribution

Signal fitted with Crystal-Ball function:

First order polynomial function for background

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t distributiont distribution has 4 components:

Prompt J/: peak at 0 ps

J/ from B: exponential decay

J/ background

Long tail, due to association between J/ and wrong primary vertex.

Signal Only

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t background distributionBackground time distribution is determined from the data

using events in the upper sideband of the J/ mass distribution:

Upper-sideband of mass distribution on L0 Minimum Bias sample

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t signal distribution

Function:

R(t) is the resolution function obtained from data, using the negative tail of the prompt component: single Gaussian

F t = 1− f tail× 1− f b t f b×e−tb ⊗R t f tail×F tailt

Prompt J/ J/ from b tail

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t Tail Distribution Long tail because of association of the J/ to a wrong

PV when computing t: 20%: wrong choice of PV amongst the

reconstructed ones. 80%: the correct PV is not reconstructed:

Because the number of reconstructed tracks from the PV is too small.

Because 2 PV are close to each other: this lead to a distribution close to t=0, and will be included in the prompt part.

In total 1.7 % of the J/are in the tail. Small effect, but not negligible compared to J/ from B: in the region 2 < t < 15 ps, 10.2% of the J/ are tail events.

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Tail distribution from data Method has been developped to obtain the tail shape from

data. The PV of the « next event » in the J/ sample is used to

simulate the position of an un-correlated PV:

BUT:

Yuanning Gao, Wenbin Qian

Asymmetry is not reproduced correctly

t(ps)

Tail

Next event

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Why Asymmetry ?

PV reconstruction efficiency (1-) depends slightly on z of PV:

=

MC (computation #1)

MC (computation #1)

Modified next event method

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(z) from dataFor a given z(J/):

A(z) = Number of events with tz<-3 ps or t

z>10 ps (ie

only tail events) B(z) = Number of events with t

z(next event)<-3 ps or

tz(next event)>10 ps.

(z)=A(z)/B(z)

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Tail estimation from data

Then

Part at t~0 ps included in the promptdescription

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Expected number of events per bins Combined mass-t fit gives number of reconstructed J/ events

(prompt and from B) in each bin.

Procedure exercised on inclusive J/ events + toy MC background reproducing background behaviour observed on Minimum Bias.

Rescaled for 5pb-1 and 7 TeV:

3 - 3.5 3.5 - 4 4 - 4.5 4.5 - 5

0 – 1 GeV/c

1 – 2 GeV/c

2 – 3 GeV/c

3 – 4 GeV/c

4 – 5 GeV/c

5 – 6 GeV/c

6 – 7 GeV/c

4215 ± 73

143616 ± 414

122788 ± 386

38629 ± 226

12706 ± 128

4977 ± 82

2142 ± 55

41987 ± 222

232190 ± 505

134701 ± 400

36194 ± 210

10513 ± 117

3988 ± 69

1634 ± 44

90818 ± 325

236633 ± 524

107178 ± 352

24196 ± 172

6506 ± 87

2155 ± 52

799 ± 36

112309 ± 364

178834 ± 446

53316 ± 244

7236 ± 92

1315 ± 52

452 ± 37

98 ± 16

Prompt

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Expected number of events per bins

Extra Bins 2< eta<2.5, pt>7 GeV/c may be interesting because it overlaps with

ATLAS/CMS acceptance.

3 - 3.5 3.5 - 4 4 - 4.5 4.5 - 5

0 – 1 GeV/c

1 – 2 GeV/c

2 – 3 GeV/c

3 – 4 GeV/c

4 – 5 GeV/c

5 – 6 GeV/c

6 – 7 GeV/c

253 ± 21

6920 ± 101

10245 ± 119

8643 ± 123

7012 ± 91

4866 ± 75

3214 ± 60

1864 ± 53

11878 ± 128

10858 ± 121

7590 ± 96

5208 ± 78

3659 ± 64

2212 ± 50

3927 ± 76

11414 ± 128

8156 ± 105

4516 ± 76

2705 ± 57

1561 ± 43

1093 ± 36

5126 ± 87

7973 ± 105

3640 ± 69

1160 ± 39

559 ± 28

199 ± 18

126 ± 12

From B

At least 100 events per bin

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Fit result: number of events per pt bin

Corresponds to 0.145 pb-1, 14 TeV

Signal: part of the inclusive J/ sample

Background: toy Monte-Carlo reproducing behaviour seen on Minimum Bias sample.

Number of prompt J/ Fraction of J/ from b

FitMonte-Carlo input values

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Cross-section determination

From the number of reconstructed events, to obtain the cross-section:

Need the integrated luminosity.

Need the acceptance efficiency: geometrical effects only, use Monte-Carlo.

Need the trigger, reconstruction and the selection efficiencies. Monte-Carlo efficiencies are used for the moment, correct them to the real efficiencies with data.

Assume efficiencies independant of t (which is true in Monte-Carlo), only p

t and dependance.

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Fit result on MC sample

In range 0~7GeV with two μ in LHCb acceptance:

prompt J/ψ(μμ): 3104.2±2.2(fit)±7.3(eff)nb

pp bb J/ψ(μμ)X: 233.6±1.7(fit)±2.(eff)nb

prompt J/ψ, 3102.0±5.5nb

b J/ψ, 235.7±1.5nb

Input value:

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Polarization - definitions

=1

=-1

In the helicity frame, cos follows the distribution (integrating over ):

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Polarization

LHCb geometry induces fake J/ polarization

When we generate unpolarized J/

cos cos

coscos

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Polarization

Problem: production polarization of J/ is unknown and very likely different from 0 (we have 0 in the DC06 and MC09 Monte Carlo)

Tevatron measurements show disagreement with theoretical predictions.

This effect will be assigned to a systematic error on the cross-section measurement for the first paper, but a measurement of the polarization (which is one of the most interesting number for prompt J/) will be done later (in several frames, taking into account azimuthal distributions).

Weight full Monte-Carlo sample and compute the efficiencies for 3 different assumptions: =0, =-1 and =+1.

Then take difference as systematic error, per pT and bin.

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Polarization Systematics

Between 1% and 44% bias

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Systematics

Statistical error ~10 % per bin

Error on tracking, muon PID, vertexing: See Thomas talk

Error on luminosity: See Vladik's talk

Error on Br(b → J/ X): 9%, affects b cross-section measurement

Polarization: between 1% and 40%.

Fit errors: incomplete description of mass and t shapes (radiative tail, ...): 2%.

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Conclusions

Selection and fit procedures established for the measurement of prompt J/ and bb cross-sections.

Part of the distributions can be determined from data directly.

Large effect due to unknown polarization.

Note written by Wenbin is circulating in F-WG.

Skeleton of article in preparation.

A lot of work to do to be able to understand/use first data: see Thomas talk.