1. XIII LNF Spring school, 12-16 May 2008, Frascati.
Measurement of charm production in DIS at HERA
Marcello Bindi
University of Bologna
XIII LNF Spring school, May 2008, Frascati.
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

2. XIII LNF Spring School 2008, Frascati.
HERA
0.5 fb-1 per experiment
ZEUS
Collider ep; end of data taking in June 2007
2 general purpose experiments: ZEUS & H1
e± (27.6 GeV)
p (820/920 GeV)
E c.m. = 300/318 GeV
HERA I ( ): 190 pb-1
fascio e± unpolarized
HERA II ( ): 560 pb-1
e± long. polarized beam ~ 30/40 %
Lumin. Upgrade:1.57.5 x 1031cm-2s-1
Run at low Ec.m. for FL measurement
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

3. XIII LNF Spring School 2008, Frascati.
ZEUS
Calorimeter
Muons Chambers p e±
Central tracking detector
Microvertex detector (HERA II)
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

4. XIII LNF Spring School 2008, Frascati.
Deep Inelastic Scattering at HERA
(k)
(k’)
(k)
(k’)
Virtuality of the exchanged boson
Neutral Currents
(q)
(q)
Charged Currents
Bjorken variable
Inelasticity
• Kinematic determined by 2 variables
• Q2 and x mostly used
• HERA range:
Q2 = GeV2
x = ~0.5
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

5. XIII LNF Spring School 2008, Frascati.
Motivations
Heavy flavour production at HERA can be studied for different kinematic regions, from Photoproduction to DIS and for different values of transverse momentum of the heavy quark.
In DIS regime the heavy quarks are produced mainly by the Boson-Gluon-Fusion process.
This process is directly sensitive to gluon content inside the proton.
Factorization in pQCD:
s = parton distr. x hard scatt. x fragm./hadron.
Important test of pQCD at different scales (MQ, pTQ,Q2).
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

6. XIII LNF Spring School 2008, Frascati.
Charm tagging
D mesons reconstruction in the final state
D*, D0 mesons: tagged by invariant mass M (Kp) and
DM = M(Kpp) - M(Kp).
D+ meson: being a long lived particle, the
secondary vertex of the decay can be
reconstructed and used for a better
discrimination of the signal.
Both these cases, need an extrapolation factor to the full space phase to obtain the F2cc
Inclusive or Lepton Impact Parameter tagging
Measure of the impact parameter with
respect to primary vertex (beamspot)
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

7. XIII LNF Spring School 2008, Frascati.
D* production in DIS
Golden mode : can be double
tagged by the slow pion and
the D0 in the final state.
HERA II
162 pb-1
Kinematic Region
5 < Q2 < 1000 GeV2
0.02 < y < 0.7
|h (D*)| < 1.5
1.5 < pT (D*) < 15 GeV
Selection of D* candidates
D*± (2010)  D0 p±slow (BR: 67.7 %)
with D0  K± p± (BR: 3.8 %)
|h| < 1.75 (candidate tracks)
pT > 0.4 GeV (tracks from D0)
pT > 0.12 GeV (slow pion from D*)
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

8. D* differential cross sections in DIS
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

9. XIII LNF Spring School 2008, Frascati.
Muon vs D-mesons
Measurements based on reconstructed D (D*) mesons suffer from:
- low branching ratio B(cD*) x B(D*±D0 ±) x B(D0K± ±) ~ 0.6%
- angular acceptance restricted to good tracks detector region ||<1.5
- measurement limited to Q2 <~ 500 GeV2 due to limited statistics
Muons advantages :
- larger branching ratio : B(c ) ~ 10%
- larger angular acceptance of muon chamber -2 <  < 2.3
Muons disadvantages :
- background from fake muons and beauty
- need to require large muon momentum (Pt > 1.5 GeV) to reach muon chambers
Muons less precise at low Q2 ( where D* statistics is large) due to worse control of background and extrapolation in pT.
Muons should be better at large Q2 due to larger statistics.
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

10. XIII LNF Spring School 2008, Frascati.
Semi-leptonic decay to muon
Kinematic Region
20 < Q2 < GeV2
0.02 < y < 0.95
-1.6 < |hμ | < 2.3
pTμ > 1.5 GeV
HERA II
2005
135 pb-1
DIS TLTs trigger chain
≥ 1 muon from prim. vtx.
Rear/Barrel: -1.6 ≤  < 1.3, PT >1.5 GeV
Forward: ≤  < 2.3, PT >1.5 GeV
Electron finder :
- P(ele)> 0.001, Ee> 10 GeV ,
ECONE< 5 GeV
yJb > , yel < 0.95
Q2Σ>20 GeV2
|Zvtx| < 30 cm
80 > E-Pz > 40 GeV
Muon anti-isolation : Econe > 0.5 GeV
Muon-jet association; (lab. frame KTclus) massive sch. PTjet >2.5 Gev; -3 < ηjet< 3
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

11. XIII LNF Spring School 2008, Frascati.
Discriminating variables
Neutrino missing PT parallel to the muon :
PT= ( PT miss . PT ) / |PT|
PTREL (GeV)
IP (cm)
PT (GeV)
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

12. XIII LNF Spring School 2008, Frascati.
Cross sections: Q2,x,pTμ,ημ,Y1,Y2
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

13. XIII LNF Spring School 2008, Frascati.
F2cc extraction: ZEUS
Subtraction of the beauty contribution by using the RAPGAP MC
predictions for each bin.
ZEUS extracts F2cc from D meson cross sections using HVQDIS
to extrapolate to the full meson phase space:
The extrapolation is performed at the centre of gravity of the bin.
ZEUS-S PDF were used throughout.
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

14. XIII LNF Spring School 2008, Frascati.
F2cc at HERA
The measurements can distinguish between
different gluon parameterizations.
ZEUS and H1 measurements are in good agreement.
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

15. XIII LNF Spring School 2008, Frascati.
Conclusions and outlook
The charm contribution to the proton structure functions,
F2, is measured at HERA using different techniques,
based on D meson tagging and lifetime measurements.
The two collaborations, ZEUS and H1, using very
different methods for the analysis, implying different
extrapolations factors, agree on the results.
The precision of the measurement is improving, and the
measurement appears to be able to distinguish between
the different gluon parameterizations provided by theorists.
The use of the whole HERA data sample can help in
constraining the gluon parameterization in the proton
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

16. XIII LNF Spring School 2008, Frascati.
BACKUP - SLIDES
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

17. Theoretical models at NLO
Massive approach (Fixed Flavour Number Scheme):
c & b massive full massive matrix elements;
scale MQ ;
appropriate for Q2 ~ MQ2
c & b produced dinamically in the hard subprocess
(not part of the proton or photon; 3 active flavours in proton: u, d, s )
PhP : Frixione et al., FMNR
DIS : Harris & Smith, HVQDIS fully differential NLO program
Massless approach (Zero Mass Variable Flavour Number Scheme):
c & b massless  resums [aS ln (Q2/M2Q)]n ;
scale Q2, pTQ ;
appropriate for Q2 >> MQ2
c & b present in proton and photon
PhP : Kniehl et al.
DIS : only inclusive calculation of F2cc available
Combined approach (Generalized Mass Variable Flavour Number Scheme):
massive at small Q2
massless at high Q2
PhP: Cacciari et al.
DIS : only F2cc available
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

18. XIII LNF Spring School 2008, Frascati.
ZEUS: NLO QCD predictions
The HVQDIS program has been used, which evaluates
cross sections for heavy quark production at NLO in the
Fixed Flavour Number Scheme.
R=F =
Biggest uncertainty from Charm mass ~10-20 %
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

19. XIII LNF Spring School 2008, Frascati.
D± production in DIS
D± long lifetime  displaced secondary vertex (MVD).
Signed 2-D decay length significance (SDL) with respect to the beamspot
Better S/N ratio by cutting on the SDL of the decay products.
1/3 HERA II:
2005 data
135 pb-1
Kinematic Region
• 5 < Q2 < 1000 GeV2
• 0.02 < y < 0.7
• |η(D±)| < 1.6
• 3 < PT(D±) < 20 GeV
D± Selection Cuts
• PT(K) > 0.7 GeV
• PT(π,π) > 0.5 GeV
• |η(K,π,π)| < 1.6
• D* and Ds reflections
subtracted.
With SDL > 3, statistical error 7.7%  3.8%
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

20. XIII LNF Spring School 2008, Frascati.
D± cross sections
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

21. XIII LNF Spring School 2008, Frascati.
D0 production in DIS
Kinematic Region
5 < Q2 < 1000 GeV2
0.02 < y < 0.7
h |(D)| < 1.6
3 < pT (D) < 20 GeV
1/3 HERA II:
2005 data
135 pb-1
Selection of D0 candidates
|h(K,p)| < 1.6
pT (K) > 0.7 GeV ; pT (p) > 0.3 GeV
SL > 1, c2Dvtx < 8
DM tagged and untagged D0 signals are fitted simultaneously
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

22. XIII LNF Spring School 2008, Frascati.
D0 cross sections in DIS
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

23. XIII LNF Spring School 2008, Frascati.
F2cc with D±
• Previous results combined measurements of F2cc from 3 mesons to reduce uncertainty.
• F2cc determined with D± at HERA II (1/3 lumi) has comparable precision to that of the whole HERA I measurement with D0, D± and Ds mesons.
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

24. Inclusive Impact Parameter tagging
A powerful tool to separate beauty, charm and
background is the significance of the (signed) impact parameter S = d / s(d)
Kinematic Region
• 12 < Q2 < 650 GeV2
• < x < 0.032
• 0.07 < y < 0.65
HERA II:
2006 e-
54 pb-1
see talk A. Metha for technical details…
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

25. XIII LNF Spring School 2008, Frascati.
F2cc extraction: H1
H1 measures charm inclusively
Pc,b,l are the fractions of charm, beauty and light flavour from the fit and is the inclusive reduced cross section.
Structure function evaluated after small corrections for the
longitudinal structure functions FLcc
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

26. XIII LNF Spring School 2008, Frascati.
Inclusive measurements vs
D mesons
Good agreement between different techniques
Agreement with NLO QCD within the spread of different theoretical approaches
GM-VFNS MRST04
CTEQ6HQ
FFNS CCFM
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

27. XIII LNF Spring School 2008, Frascati.
H1 inclusive charm
reduced cross sections
HERA I and II agree
Combined results
agree with NLO
theoretical expectation
GM-VFNS & MRST04
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

28. XIII LNF Spring School 2008, Frascati.
12-16 May 2008
XIII LNF Spring School 2008, Frascati.

29. XIII LNF Spring School 2008, Frascati.
High Q2 HERA analyses
12-16 May 2008
XIII LNF Spring School 2008, Frascati.