p.kreuzer – bbbar at hera-b – ichep2002 amsterdam measurement of the bb production cross section...

P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam

Measurement of the bb Production Cross Section

in Proton-Nucleus Collisions at Hera-BP. Kreuzer

University of California, Los AngelesFor the Hera-B Collaboration

ICHEP 2002 Conference Amsterdam, July 27 2002

Motivations

The HERA-B spectrometer and trigger (Y2K)

The Measurement: b J/(l+l-) + X

Comparison with Data & QCD Predictions

Conclusions(hep-ex/0205106 + submit. EPJ C)


Motivations for a bb cross section measurement at Hera-B

Exp Targ p Beam (bb) nb/nucleon

Channel

E789 Au 800 (GeV/c)5.7 1.5 1.3

bJ/() X

E771 Si 800 (GeV/c)bb(++X)(-

+X)

12 (bb ) 70 nb/nucl.

at 920 GeV/c (Hera-B)

743 2717

Fixed Target Data:

Hera-B can extend the experimental panorama

Recent improvements but still large uncertainties !

by covering both bJ/( e ) and bJ/( )

max)( cmsL

cmsL

F p

px & the non-exploited negative xF

region

A test for QCD Predictions:

( )


The Hera-B Detector

Carbon

Titanium

Y2K J/-coverage: -0.25 < xF < 0.15 (now -0.4<xF<0.3 )

(920 GeV p-N interactions)


The Hera-B Di-lepton Trigger (Y2K)

Pretrigger (5 MHz)

E_t cut for e, hit coinc.for

Level I (~150 KHz)

2 leptons (ee,) requirement (no tracking ! )

Level II (~12 KHz)

2 tracks in Main Tracker and Vertex Detector

9.0 105 di-e & 4.5 105 di- events

(Farms of 240+100 PCs, ~20 Hz output)


The measurement steps

Minimize systematic errors &

Avoid luminosity dependence

X)J/ ψBr(ΔzBεRε

1

PnBn

rσ)σ(

bb

bb .

Relative detection Efficiency

Detached Vertex selection Efficiency

Select prompt J/Select bJ/

Normalize to (pN J/X)

)(P

B

ε

ε

using E789/E771 [-0.25 < xF < 0.15]


Prompt J/ selection

nP= 5710380st280sys nP= 288060

Muon Channel:Electron Channel:

Reconstruction based on Trigger tracks + Vertex + Particle ID


Isolating the b signalTotal (prompt) J/ signal Detached b J/ signal

J/

l+

l-

(z) cB(600 m) (8000 m)

IW

l+

J/Bz

X

l-

upstream | downstream

zIw


* bb -> (e+ +X)(e- +X)* combinatorial* < 0.2 prompt J/

Detached b selection (e channel)

Prompt region

Up

stre

am

Dow

nst

ream

J/peak

Main Bkgd sources:


Invariant Mass fit

3.93.2

8.6B

n

Unbinned Likelihood Fit:- Sig. shape from MC,- BKG shape data/MC

e+e- invariant mass ( GeV/c2 )


Electron channel:

Muon channel:

2.21.5

1.9B

n

Unbinned Likelihood Fit:- Sig. shape from data,- BKG shape from data

(bb ) = (stat)131130 nb/nucl (-0.25 < xF < 0.15)


Systematic Uncertainties

External (internal) sources

ChSyst %

r e 11

BR(bbJ/X) e 9

Trigger & detector sim. (R)

e 5

b production/decay modelsMRST NNLL Parton Distr. F., Peterson Fragment., Pythia)

e 5

Prompt counting J/ (np) e 5

Prompt J/ MC prod. Mod.

e 2.5

A-dependence in RzB e 1.7

Partial contribution e- 17-16

Sources dominatedby statistics

Ch Syst %

+- bkg fluctuations

+10-24

e+e- bkg shape

e 7

e+e- bkg fluctuations

e 11

Partial contribution

e-

Total systematic uncertainty

e +20 %-23 %

1024 13


R. Bonciani et al. (2002),NLO+NLL with latest MRST PDF

Nucl.Phys.B529 (1998)

Hera-B compared to other data/theory

The result shows good agreement with recent calculations beyond NLO

Hera-B Y2K @ 920 GeV:

(syst)67 (stat)

1412 32

nb/nucl

TOT(bb ) =

(92% bJ/ in our xF range)

N. Kidonakis et al. (2001),NLO+NNLL

Phys.Rev D64 (2001) 114001-1


Outlook 2002/3: O(1000) higher statistics !

Baseline Physics program - (bb ): expected error 15% (systematic limited) - Charmonium production (J/,‘, ), Atomic number dependence

ConclusionsB J/ X l+l-X observed at Hera-B

Good compatibility with recent QCD calculations

Result:

nb/nucleon (sys)(stat)32 )σ( 67

1412

bb

c


Detector characteristics (I) Large acceptance: 15-220 mrad in x (bending plane),

15-160 mrad in y (vertical plane) Target – up to 8 wires inserted into the halo of 920 GeV proton

beam (C, Ti) VDS – Vertex Detector System.

Dilepton vertex resolutions: z 600 m, x,y 70 m Dipole Magnet- field integral 2.13 Tm OTR – Outer Tracker. Honeycomb drift cells; wire pitch 5/10 mm;

spatial hit resolution 350 m; Backward hemisfere in CM (negative xF)World largerst honeycomb tracker: 1000 modules, 115000 channels

ITR – Inner Tracker: MicroStrip Gas Chambers, pitch 100 m, resolution 100 m; Forward hemisfere in CM (positive xF)World largerst (gas) micro pattern tracker


Detector characteristics (II) RICH – Ring Imaging Cherenkov Hodoscope

C4F10 radiator gas, 2 planes of PMT

4 separation: e/ p [3.4, 15] GeV/c, /K p [12,54] GeV/c ECAL – Electromagnetic CALorimeter – Sandwich sampling

calorimeter (“Shashlik”); Pb and W as converter; 3 regions MUON detector – 4 tracking stations; Gas pixel chambers,

Proportional tube chambers, some with segmented cathodes DAQ system – High bandwidth, high trigger and logging rates TRIGGER.

- Pretriggers on ECAL & MUON seeds- FLT hardware based on ITR/OTR- SLT software trigger; Tracking+Vertexing; linux farm with 240

nodes

Event reconstruction; on-line, linux farm with 200 nodes


The cross section normalization

X)J/ ψBr(ΔzBεRε

1

PnBnfrσ)σ(

bbbb

P

BR ε

εε

E789 + E771 : 356 7 25 nb/nucleon

nucleon/nb317314AA

X)J/ ψσ(pNσ sysstatr

E866 : 0.955 0.005

72%f

Relative detection efficiencies

zB Efficiency of detected

vertex selection

PB n ,n Number of observed bJ/ and prompt J/

[-0.25<xF<0.15]


x

ee

MAGNET

ECAL

Prompt J/: Particle ID / Kinem.

BR=0.340.020.02

PID: E/P + bremsstrahl

ung

Electron Channel:

PID: likelihoods from MUON and RICH detectorsMuon Channel:


Detached b J/ cuts

- z > 0.5 cm - e Imp. Param. wire Iw > 200 m , or- Min. dist. @ ZW to any other track > 250 m

REAL

MC

BKGS

Optimization procedure:

0.020.44zBR

Electron Channel Cuts:

Muon Channel Cuts: 0.010.41zBR

- z > 7.5 z

- Imp. Param. to wire Iw >45m

- “ “ to primary vtx Ip> 160m


Systematic checks (e-channelDECAY LENGTH

LIKELIHOOD FIT:

MC =0.810.03 cm

RD =1.000.30 cm

BKG=0.360.13 cm

Different bkg optimization:

Upstream

Downstream

Dz

(cm

)

Mass (ee) (GeV/c2)

Bkg

Bkg Bkg


En

trie

s / 1

00 M

eV/c

2

P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam TOT(bb ) = nb/nucleon

(bb) DeterminationSimultaneous fit to e+e- & +- (in Hera-B acceptance):

Extrapolation to the full xF range:

(syst)67(stat)

141232

e-channel (bb ) = nb/N18

15 38

-channel (bb ) = nb/N18

12 16


J/ from b decays kinematics

92% of J/ are produced in our xF range


b production modelFor the xF and pT distributions of J/ from b decays, we need a model of the b quark production and hadronizationOur b production & decay model:

• Based on HQ cross section calculation at NLO+NLL by M. Mangano, P. Nason and G. Ridolfi, Nucl. Phys. B373 (92) 295

• Latest MRST parton distribution functions (NNLL) for nucleons• Intrinsic kT of interacting partons is gaussian-distributed with <kT

2> = 0.5 GeV2

• b quarks fragmentation given by a Peterson function ( = 0.006)• The b-hadron decays to J/ is controlled by Pythia

222 pmGeV/c 4.75m Tbb

92% of J/ from b decays are produced in our xF range


b production model systematics

• Changing PDFs from MRST to CTEQ• b quark mass from 4.5 to 5.0 GeV/c2

• QCD renormalization scale from 0.5 o to 2 o

• Fragmentation functions– Peterson form with from 0.002 to 0.008

– Kartvelishvili form with from 12.4 to 15.0

• <kT2> from 0.125 to 2.0 GeV2

• Fraction of b-baryons produced in the b-hadronization process from 0 to 12%

±1.5% ±1% ±2% ±3%

±1% ±2%

Default model: MRST PDF, Peterson FF =0.006 22222 GeV/c 0.5kpmGeV/c 4.75m TTbb 0

Total: ±5%

Studied variations: Sys cont. to (bb )


Essential Bibliography

P.NASON, QCD at High Energy, Proc. Of the XX Int. Symp. on Lepton and Photon Interactions at High Energies, hep-ph/0111024P.NASON et al., Adv. Ser. Direct. High Energy Phys. 15(1998), 609

N. Kidonakis et al., Phys.Rev. D64 (2001) 114001-1

D.M.Jansen et al., Phys.Rev.Lett.74 (1995)3118

T.Alexopoulos et al., Phys.Rev.Lett.82 (1999) 41

R. Bonciani et al., Nucl.Phys.B529 (1998) 424

M.H. Schub et al., Phys.Rev.Lett. D52 (1995) 1307

P.Mangano et al., Nucl. Phys. B373 (92) 295

p.kreuzer – bbbar at hera-b – ichep2002 amsterdam measurement of the bb production cross section...

Documents