Heavy flavour production and spectroscopy at HERA
A. Bertolin
Outline:
• HERA, H1 and ZEUS
• open beauty and charm photoproduction
• open charm spectroscopy
• charmonium production
• summary and conclusions
1
HERA, H1 and ZEUS: a brief introduction
• e p collider at high CMS energy (like having an
about 50 TeV e beam on fixed target)
• two large multipurpose experiments: H1 and ZEUS
• running ended mid 2007 after about 2500 days of
activity and 470 pb-1 of integrated luminosity per
experiment
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Open beauty and charm production at HERA
main process:
• direct g
• boson gluon fusion
• 0.8 < xg
obs
<1
key points:
• mQ (or ETjet …) >> LQCD
 region of applicability of pQCD
• collinear vs. kT parton dynamics
other processes:
• resolved g processes
• 0 < xg obs < 0.8
• photoproduction (PHP) regime  scattered electron is undetected  Q2 < 1 GeV2
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Open beauty and charm PHP at HERA
fully inclusive beauty tag with  2 jet events
Significance = d / sd
• exploits the large
lifetime of b/c hadrons
with respect to light
hadrons
• silicon microvertex
detector to have a good
measurement of d
• can measure beauty
AND charm at the same
time
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Open beauty and charm PHP at HERA
fully inclusive beauty tag with  2 jet events
• xg obs peaks at 1  BGF
is dominant
• what is called resolved-g
in the Pythia collinear
MC is also reproduced in
the Cascade kT MC
without the need of a
resolved-g component …
• NLO QCD  had is
slightly underestimating
the data for beauty but
NLO QCD is as good for
charm as for beauty …
scale set by the jets pt
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Open beauty and charm PHP at HERA
beauty tag with two jets and one associated semileptonic electron
bkg enriched
b enriched
c enriched
likelihood function based on
5 discriminant variables:
• dE/dx
• EEMC / ECAL
• ECAL / ptrack
• electron pt w.r.t. the
associated jet axis
• azimuthal angle difference
between the electron and the
missing transverse
momentum vector (// for
signal)
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Open beauty and charm PHP at HERA
beauty tag with two jets and one associated semileptonic electron
beauty
• NLO QCD is as good for charm as
for beauty
charm
• PYTHIA gives a fair description of
the shapes but the normalization is
significantly wrong … missing NLO
corrections …
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Open beauty PHP at HERA
beauty tag with D* + m (no jets)
beauty
charm
unlike-sign D* m
unlike-sign D* m
beauty
like-sign D* m
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Open beauty PHP at HERA
• many very different experimental methods in fair agreement
• can measure down to <pTb> ~ 5 GeV (~ mb) and up to ~ 25 GeV
• data are already as precise as theory … need more accurate theoretical predictions
• most recent points are right on top of the NLO predictions …
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Open charm production at HERA
charm spectroscopy at HERA:
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Open charm production at HERA
basic building blocks:
D*  K p ps
D*  K p p ps
D0  K p
D  K p p
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Open charm production at HERA: D* cross sections
• Pythia collinear
massive fails
• Pythia collinear
mass less fails
(except for h)
• Cascade kT fails
(except pt shape)
• NLO is quite
reasonable
except for h>0
 lots of beautiful data but theory is poor … may be NLO is on the right track …
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Open charm production at HERA
excited charm mesons:
D01 / D*02  D* p
excited charm strange mesons:
D s1  D* K0s
in this particular decay mode
difficult to separate D01 and D*02
D*02  D p
due to spin parity conservation
D01 can NOT decay in this mode
D s1  D0 K
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Open charm production at HERA
tabular summary:
• hopefully errors will be reduced extending the analysis to HERA II data and using
microvertex detector data (available also for ZEUS in HERA II)
• the HERA rates are in reasonable agreement with those measured in others (e+ e-)
experiments
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Charmonium production at HERA (y(1S) and y(2S))
direct g
CO model
• this particular diagram:
0.2 < z < 0.9
• more “typical” ones:
z > 0.9
direct g
CS model
0.2 < z < 0.9
p-rest frame: z = E(y)/E(g*)
open issues:
• size of the CO contributions
resolved g
CS model
z < 0.2
• collinear vs. kt parton dynamics (as in the
CASCADE MC)
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Charmonium production at HERA (y(1S) and y(2S))
• < 1/10 of the total available luminosity
• y(2S) to y(1S) cross section ratio
consistent with being flat
• y(2S)  y(1S) ( m m) X contribution
NOT subtracted from the data … not easy
experimentally !
• 15 % increase of the J/y cross section
other contributions:
• diffractive charmonium: suppressed restricting the z – pt(J/y) phase space
• charmonium from B mesons decay: much smaller than at TEVATRON
• c contribution: tiny at HERA
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J/y cross section measurements
PHP regime:
• NLO CS calculation with LARGE
scale uncertainties describes the
shapes
• LO CS + kt factorization
(CASCADE) describes the data
well
• LO CS + collinear factorization
(EPJPSI) fine in shape
DIS regime (3.6 < Q2 < 100 GeV2):
• no NLO prediction
• LO CS + kt factorization
(CASCADE) describes the data
well
• LO CS + collinear factorization
(EPJPSI) fine in shape
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J/y cross section measurements
PHP regime
DIS regime
ds/dp2T for different z rages  d2s / dp2T dz
• should be a stringent test of the matrix elements involved … would be nice to
have NLO predictions as well …
• LO CS + kt factorization (CASCADE) describes the data well
18
J/y helicity at HERA
• helicity: study of the m+ angular
distributions in the J/y rest frame 
learn about production channels
• contrary to TEVATRON at HERA
mostly “prompt” J/y … cleaner
environment
• can measure 2 out of 3 helicity
parameters (l, n)
BKV  collinear factorization
Baranov CS  LO CS kt factorization
• large differences in the predictions
(sign is different …)
• none of the predictions is able to
describe simultaneously all
measurements …
19
Conclusions
• several beauty inclusive measurements have been performed and will be refined in the
immediate future … almost fully reconstructed final states (like D*+ m ) have also been
measured
• measurements are not easy due to the small beauty cross section at HERA
• remarkably enough all measurements are giving a rather consistent picture on the
experimental side
• in general beauty production is reasonably well described by NLO even at small pTb
• charm and charmonium states are produced copiously at HERA
• inclusive as well as precise exclusive final state measurements are available … can also
perform charm spectroscopy at HERA
• in the presence of a reasonably large scale (ETjet >> mC) NLO QCD is as good for open
charm as for open beauty
• charmonium production at HERA is well described by the CASCADE MC with kT
factorization … but polarization is not well described … looking forward to better
theoretical predictions …
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Heavy flavour production and spectroscopy at HERA
• Alessandro Bertolin (Padova)
"Heavy flavour production and spectroscopy at HERA"
• Lorenzo Bellagamba (Bologna)
"Search for new physics at HERA"
• Jan Figiel (Inst. of Nucl. Physics, Krakow)
"Measurements of diffraction at HERA“
• Hans-Christian Schultz-Coulon, Heidelberg University
"Structure Function Measurements at HERA"
• Guenter Grindhammer, MPI Munich
"Jet Production and alpha_s Measurements at HERA"
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