Prompt J/y and b➝J/y X production in pp collisions at

LHCb

Patrick Robbe, LAL Orsay & CERN, 7 Dec 2010For the LHCb Collaboration

KRUGER 2010Workshop on Discovery Physics at the LHCKruger National Park – South Africa

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• The LHCb detector • New preliminary results of J/y cross-section

measurement with 5.2 pb-1 data at LHCb.• Prospects for future LHCb measurements in

the quarkonium sector.

Outline

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• Unique acceptance amongst the LHC experiments: can explore production properties in the forward region.

The LHCb Detector

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Level 0: Hardware triggers From Muon system & calorimeters

High Level Trigger (HLT) 1: Software triggersAdd information from VELO and tracking stations to Level 0 informationFind primary vertices, etc.

HLT2: Software triggersUse all of the detector information to makeinclusive and exclusive selections

The LHCb Trigger

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Data Taking

Data used in the J/y cross-sectionmeasurement

Full data sample (37pb-1) will be used for studies for winter conferences.

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• Double differential cross-section measurement, as a function of the transverse momentum pT and of the rapidity y:

– 14 pT bins, pT < 14 GeV/c

– 5 y bins, 2<y<4.5

• Separately for:– Prompt J/y = direct J/y + J/y from cc feed-down,

– J/y from b decays.

• Use (5.2±0.5) pb-1 of data collected end of September 2010 at LHCb, with two different trigger settings, with pp collisions at center-of-mass energy of 7 TeV:– 2.2 pb-1 with standard muon trigger,

– 3 pb-1 with the lifetime unbiased muon trigger lines pre-scaled to cope with instantaneous luminosity increase.

J/y cross-section measurement

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Trigger and Selection

Selection:

L0 Trigger:Single Muon:

Di-Muon:

pT>1.4 GeV/c

pT,1>0.56 GeV/c, pT,2>0.48 GeV/c

HLT1 Trigger: Single Muon:

Di-Muon:

Confirm L0 single Muon and pT>1.8 GeV/c(Pre-scaled in Trigger 2 by 0.2)Confirm L0 Di-Muon and Mμμ>2.5 GeV/c2

HLT1 Trigger: Di-Muon: Mμμ>2.9 GeV/c2

μ tracks: ● well reconstructed tracks identified as muons in muon detector, ● pT > 0.7 GeV/c, ● Track fit quality (χ2/nDoF < 4).

Reconstructed J/y: ● mass window: 0.15 GeV/c2, ● vertex fit quality (p(χ2)>0.5%).

Event: at least one reconstructed Primary Vertex (PV): to compute proper-time

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J/y Sample

N = 564603 ± 924

Each mass distributions obtained in the 70 bins are fit with:• A Crystal Ball function for the signal to take the radiative

tail into account, • An exponential function for the background.

Crystal Ball function:

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• Computed as:• With:– N(J/y➝m+m-): number of reconstructed signal

events, separatly prompt and from b.– L: integrated luminosity (5.2 pb-1)– etot: total detection efficiency, including acceptance,

reconstruction, selection and trigger efficiencies.– B(J/y➝m+m-): (5.94 ± 0.06) %– Dy = 0.5, DpT=1: bin sizes.

Cross-section Measurement

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• Fraction of J/y from b is given by fit of tz:

Separation prompt J/y / J/y from b

PV

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tz tail• Origin of the tail are signal J/y associated with the wrong

Primary Vertex.• The shape of the tail contribution is determined directly from

data, simulating an uncorrelated PV using the one of the next event:

Sideband subtracted

With “next event” method

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tz signal and background functions• Signal:

• Convolved by resolution function: Sum of 2 Gaussians• Background: fit with function describing the shape seen in the

J/y mass sidebands: 3 positive exponentials and 2 negative exponentials.

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• Fit is performed in all analysis bins to extract number of prompt J/y and of J/y from b.

• For example, 3<pT<4 GeV/c, 2.5<y<3.

tz fit

LHCb Preliminary =7 TeVL=5 pb-1

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• Efficiencies are computed from Monte Carlo and are extensively checked on data, with control samples.

• Efficiencies are checked in Monte Carlo to be equal for prompt J/y and J/y from b in each (pT,y) bin. Small differences are treated as systematic uncertainties.

Efficiencies

LHCb Simulation LHCb Simulation

unprescaled trigger prescaled trigger

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SystematicsSource of systematic uncertainties considered:

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Polarization• J/y are not polarized in the LHCb simulation. • Different angular distributions because of different polarization lead to

very different efficiencies.• 3 extreme polarization cases studied, in the helicity frame where the

angular distribution of J/y muons is (integrating over f):

• Differences between 3% and 30% depending on the bin: quote 3 different results of the prompt J/y cross-section, one for each polarization case. 16

a=+1 a=0 a=-1

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• Integrated over the acceptance:

Results: prompt J/y cross-sectionUnpolarized J/y

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• For the 2 extreme polarization cases considered:

Results: prompt J/y cross-section

a=+1a=-1

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Results: J/y from b cross-section

• Integrated over the acceptance:

Results: bb cross-section• From the J/y from b cross-section, extrapolate to the total bb

cross-section in 4p:

• a4p: extrapolation factor computed from PYTHIA 6.4, no uncertainty associated to it.

• B(b➝J/ y X)=(1.16±0.1)%: measured at LEP, with 9% uncertainty.• With Tevatron measured hadronization fractions, we estimate

B(b➝J/ y X)=(1.08±0.05)%: assign 2% uncertainty due to hadronization fractions.

• Result:

• LHCb published value from b➝D0mnX (Phys.Lett.B694 (2010) 209)

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Results: mean pT

• Mean pT is estimated (for unpolarized prompt J/y) interpolating the measured cross-section with a cubic B-spline function.

• J/y from b mean pT 20% higher than prompt J/y.

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• With full data sample, measure polarization of prompt J/y, in bins of pT and y, taking into account the full angular distribution.

• Measurement of cc cross-section will be possible (will also allow to know proportion of J/y from feed-down)

Prospects for future measurements

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• With full data sample, measurements of y(2S) and Y(1S), Y(2S) and Y(3S) will be possible, in their decay modes to m+m-.

Other quarkonium states

Y(1S), Y(2S), Y(3S)y(2S)

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Conclusions

• New results for J/y cross-section measurement at LHCb with 5 pb-1 of data, as a function of pT and y, extending the range of the first measurement presented at ICHEP 2010.

• Large uncertainties due to unknown J/y polarization: future polarization measurement will address this issue.

• Measurement of y(2S) and Y(1S), Y(2S), Y(3S) cross-sections will allow to provide a complete picture of quarkonium production in the forward rapidity region.