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New Results from ZEUS at HERA
Bernd Loehr DESY
on Behalf of the ZEUS Collaboration

XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013
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HERA Operation and the ZEUS Detector

1992 - 2007

Ep 920 GeV

Ee 27.5 GeV

s 4Ep Ee 318 GeV

ZEUS Detector
p e

Three periods of data taking: HERA I : HERA II : Low proton energy: ~120
-1 -1

~380 ~65

-1

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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The Advantages of HERA II Data Taking Period
Upgrade of ZEUS detector for HERA II running - Silicon Micro Vertex Detector (MVD) HERA II running period:

Higher integrated luminosity

improved statistical precision, some analyses became feasible only then
detection of secondary vertices, new method to study heavy quark production using decay lengths of charm- and beauty mesons study of electroweak effects with polarized beams

Micro Vertex Detector

Longitudinal polarization of lepton beams

Low proton-energy running
Ongoing analyses:

measurement of longitudinal structure function

·
· ·

update previous analyses
new high statistics analyses combination of H1 and ZEUS results

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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Combination of H1 and ZEUS Results on Charm Production
Charm production in DIS in leading order:
e
'

Up to 30% of the cross-section. · test of perturbative QCD gluon content of the proton: g(x) multiple hard scale: Q2, mcharm, pT

e

· · D + X D + X D*+ X etc. ·

-> appearance of log-terms
procedures to treat heavy quark masses
Q 2 q 2 ( k k ' )2 pq Q2 ; x ; y pk 2pq Q2 x y s

Boson-gluon fusion

p pD z pq
D

H1 and ZEUS measurements of red projected onto a common (x,Q2) grid and combined. For systematic errors the correlations have been taken into account.
cc

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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Combined Results on Inclusive Charm Production

x

Combined H1+ZEUS result for reduced charm cross sections.

Comparison of individual measurements with different techniques to the combined reduced charm cross-section for the bin Q2=18 GeV2. Improved statistical and systematic errors for combined results.
Page 5

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013


Combined Results on Inclusive Charm Production, cont.

Comparison to HERAPDF1.5 NLO predictions : · derived from fit to

inclusive DIS data (HERA I and HERA II)
· using RT standard scheme with mc = 1.4 GeV. · error bands dominated

by varying 1.35 mc 1.65 GeV.

An issue: treatment of the charm mass in pQCD calculations.
Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013
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PDFs and Treatment of Charm Mass in pQCD
In pQCD, mass obeys renormalisation equation -> it is scheme dependent. Relation between pole mass and MS mass:
_____

( x, Q ) d C ( , Q 2 , , f , s ( 2 ), mc ) f ( , , f ) 0
cc red 2

1

x

DIS factorisation

hard scattering kernel: calculable in pQCD, depends on treatment of heavy quark mass

parton density function (PDF): has to be derived from fitting data, depends on treatment of heavy quark mass

Various heavy quark mass schemes exist:

Fixed flavour number scheme (FFNS): only light quarks in the proton, heavy quarks are massive and only produced in hard scattering.
Zero mass variable flavour number scheme (ZM-VFNS): heavy quark mass is set to zero, below threshold at Q2~mc2 charm cross-section is zero and only three active flavours in the proton. Above threshold charm is a massless active parton. General mass variable flavour number scheme (GM-VFNS): at low Q2 like FFNS, at high Q2 like ZM-VFNS, interpolation at intermediate scales, several interpolation procedures exist.
Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013
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Extraction of Charm Mass from pQCD Fits

Extraction of optimal charm mass values from inclusive + charm data using different heavy flavour schemes.

mc(mc)=1.26 ± 0.05exp ± 0.03mod ± 0.02param ± 0.02 GeV
s

PDG : mc(mc)= 1.275 ± 0.025 GeV

What does that mean for predictions of cross section for other processes?

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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Predictions for W and Z cross-sections at LHC

Predictions as functions of the chosen charm mass in various heavy quark schemes. Stars mark the values for the optimal charm mass in a given scheme.

Conclusion: if optimal charm mass values are chosen for the different schemes, the predictions for LHC differ for
by 2%, by 1.4 % by 1.8% .

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

Page 9


D± Production in DIS from the HERA II dataset
The HERA II dataset of 354 pb-1 was used to identify and reconstruct D± meson by their decay:

D K



in the kinematic region

5 Q 2 1000 GeV

2

; 0.02 y 0.7

1.5 pT (D ) 15 GeV 2 ; | (D ) | 1.6

Micro-Vertex-Detector used for and selection.

D



decay length measurement

Comparison to HERA I results and NLO QCD predictions.

GM-VFNS FFNS

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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D± Production from the HERA II dataset, continued
Due to the higher integrated luminosity w.r.t. previous analysis double differential cross sections can be calculated. 5 < Q2 <9 GeV2 : comparison to old analysis -> reduced statistical and systematic errors Comparison to NLO calculations (HVQDIS) These data are not included yet in the combination of H1 and ZEUS results on charm production.

Work is ongoing to include these data in the combination of results.

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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D* Production in DIS from the HERA II dataset
Process:

ep eD* X
*

Kinematic region: 5 < Q2 < 1000 GeV2 ; 0.02 < y < 0.7 ; 1.5 < pTD* < 20 GeV ; |D* | < 1.5
s

Selection of

D

:

D* D 0

; D 0 K



Charge conjugate states are implied.

Signal region :

1.80 M ( K ) 1.92 GeV ; 143.2 M 147.7 MeV ; M M ( K s ) M ( K )

Wrong sign combination : combine tracks of same charge for K and Peak below the Do position is from partly reconstructed D0-> K- + 0.
Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013



.

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D* Production in DIS from the HERA II dataset , cont.
Single differential cross-sections ZEUS results on D* production, comparison to NLO calculations (HVQDIS)

Fair agreement of data with NLO predictions

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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D* Production in DIS from the HERA II dataset , cont.
Double differential cross-sections

Comparison to NLO calculations and to H1 results. Fair agreement also for double differential cross-sections.

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

Page 14


Reduced Charm Cross-sections from the HERA II dataset
Reduced charm cross-section : with

Good agreement with ZEUS D+ and H1-ZEUS charm combination data.
Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

HERAPDF1.5 (charm data not included) describes the charm data well.
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Charm Fragmentation Functions from Photoproduction
Photoproduction at ZEUS: no scattered electron detected in the calorimeter -> Q2 0 GeV2

Charm fragmentation function:

f(c charm particle)

(charm particle) (total charm particles)

universal, independent of production process, needed to compute charm cross-section. Measured signals:
D 0 , D , D* , Ds , c

; charge conjugate states included

Range of

p

cms energy: 130 Wp 300 GeV

Acceptance region:
ET

pT 3.8 GeV and 1.6
pT
10
o

for all charm particle candidates

0.2 (0.25) for D, D* ( c )

candidates

For the selection of D o , D particles the decay lengths as measured with the vertex detector have been used.

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

Page 16


Charm Fragmentation Functions from Photoproduction, cont.

D D
0

*+

D

0

D* D0



with

D 0 K
M
tag:



Additional D* kinematical region.

D0



from

D

0

outside

0.143 M ( K ) M ( K ) 0.148 GeV

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

Page 17


Charm Fragmentation Functions from Photoproduction, cont.

D K



Appropriate mass cuts applied to remove background from D* and Ds decays.

D K



Ds
c K p




Ds



, K K



Lower peak at 1860 MeV is from D K K



c K p



Background from D , Ds 3 charged particles reflections are subtracted according to MC model.

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

Page 18


Charm Fragmentation Functions from Photoproduction, cont.
Corrections of cross-sections for 0 0 not observed states c , c , c with PYTHIA MC -> factor 1.14 Fraction of charged D mesons produced in a vector state:
P D ( D* ) /[ ( D* ) dir ( D )] V

P D = 0.595 ± 0.020 . ± 0.015(. ) ± 0.011() V
naÎve spin counting: 0.75 ; string fragmentation (LUND): 0.66 Charm fragmentation functions are universal, independent of production process. Strangeness suppression in charm production:

S 2 ( DS ) /[ ( D ) ( D 0 )]

S

0.214±0.013 .

+0.006 - 0.017

(syst.)±0.012(. )

To be compared to non-charm strange particle production: 0.22-0.30
Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013
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Inelastic J/ and ' Photoproduction

ep J / X
e
'

;
e

ep ' X
e
'

Q2 0 GeV2

Colour singlet contribution

e

g p

Colour octet contribution

g p

J/ and ' detected by their decays

J /



'



Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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Inelastic J/ and ' Photoproduction ; continued
Ratio of J/ to ' photoproduction Kinematic range: 60 < W < 190 GeV ; 0.55 < z < 0.9

Reasonable agreement with leading-order colour-singlet expectation.

W 2 ( P q)

2

Photon-proton cms energy squared

W 2 2 EP ( E p z )

( E pz )
z ( E p z ) ( E pz )

Difference between energy and momentum along the z-axis summed over all final state particles. Inelasticity: fraction of the photon energy transferred to the - particle. Transverse momentum of the - particle.
Page 21

pT
Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013


Inelastic J/ and ' Photoproduction ; continued

Differential cross-sections measured dpT2 in 5 different z ranges.

d

Comparison with nonrelativistic NLO CS and NLO CS+CO calculations. Kinematic range:
60 < W < 240 GeV pT > 1 GeV

The colour singlet contribution alone is not able to explain the data. Addition of a colour octet contribution improves the description of the data.
Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013
Page 22


Prompt Photons + Jet in Photoproduction
e jet e

jet

Direct photoproduction
p p e jet e jet



Q2 0 GeV2 Resolved photoproduction

p



p

Prompt photon emission is not affected by problems due to hadronisation. Sensitivity to quark charge, u/d separation.

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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Prompt Photons + Jet in Photoproduction, cont.
Photon signal

0

Energy weighted width of EMC cluster:

wcell

: width of EM calorimeter cell

Prompt photon signal extracted by template fit for Z.
Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013
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Prompt Photons in Photoproduction, cont.

Fraction of the exchanged photon energy in the hard interaction

FHG(Faontanaz,Guillet,Heinrich) NLO calculations give a good description of the data.
LZ(Lipatov,Zotov) kT-factorisation calculations in LO describe data reasonably well. Improved predictions will come soon.

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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Summary

·

Combination of charm results from H1 and ZEUS increased precision, provides further constraints for the description of proton structure when included in the dataset for PDF fits. Many new results on charm production to be included in the charm combination, Work is underway. Determination of the charm pole-mass and optimal charm masses for various treatments of heavy flavours in pQCD calculations. Measurements of charm fragmentation fractions support of fragmentation universality, precision is compatible with e+e- results. New data on inelastic J/ photoproduction evidence for the necessary colour- octet contributions. Updated analysis of prompt photon production provides an improved check of pQCD calculations without the complication of hadronisation corrections.

·

·

·

· ·

Bernd Loehr - XXI QFTHEP Workshop, St.Petersburg Area, June 23-30 2013

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