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)
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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:
( )
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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)
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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)
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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]
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
Prompt J/ selection
nP= 5710380st280sys nP= 288060
Muon Channel:Electron Channel:
Reconstruction based on Trigger tracks + Vertex + Particle ID
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
* 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:
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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 )
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)
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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]
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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:
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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
e+e- invariant mass ( GeV/c2 )
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
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
J/ from b decays kinematics
92% of J/ are produced in our xF range
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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 )
P.Kreuzer – BBbar at Hera-B – ICHEP2002 Amsterdam
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