Inclusive differential cross-sections for D

a where
1 Introduction
is the pseudo-rapidity de ned with respect to the proton beam direction
1
perturbation theory. Consequently in experimental data, NLO (and higher order) calculations and Monte Carlo distributions, direct and resolved enriched samples are de ned by a cut on xOBS with xOBS < 0:75 de ning the resolved enriched sample for this analysis. ZEUS 1996 + 1997 preliminary For charm photoproduction at HERA, two D → (K π) π D → (K π π π) π types of NLO calculation are available. In the \massive" scheme 1 the charm mass acts 10 to regularise initial state divergences and all charm production is assigned to the hard subprocess. In this approach the active avours in 1 the parton density functions of the proton and photon are the 3 light avours, irrespective of the hadron factorization scale. In contrast, 10 the \massless" scheme 2;3 is a variable number scheme such that above some scale 2 m2 , c charm is an active avour in the parton den- 10 sity functions, although with zero mass. 0 2 4 6 8 10 12 Photoproduction events were selected with p (GeV) photon virtuality Q2 < 1GeV2 and 0:19 < Figure 1: The inclusive y < 0:87. With the exception of the jet prof- d =dpT (D ) cross-section. iles { for which only the 10.7pb?1 subsample from 1996 was used { the data here presented corresponds to an integrated luminosity of 37pb?1 collected during the 1996 and 1997 HERA running period. Charm was tagged by identifying the decay products from D mesons using charged tracks in the Central Tracking Detector (CTD). For the inclusive crosssection measurements both the D ! D0 ! (K ) and D ! D0 ! (K ) channels were sought using the well known mass di erence method of Nussinov 4 . For the dijet analysis, only the rst channel was used. Jets were identi ed with the KT algorithm 5 from energy deposits in the calorimeter.
D* ⊥
Figure 1 shows the di erential cross-section d (ep ! D X )=dpT (D ) for D 's in the range j (D )j < 1:5. The experimental data are compared to NLO calculations in both the massive and massless schemes including Peterson fragmentation 6 . For the inclusive cross-sections the GRV-G photon parton densities 7 , charm mass mc = 1:5GeV and renormalization scale 2 = m2 = m2 + p2 have c T ? been used throughout unless otherwise stated. Figure 1 shows good agreement between both D decay channels. It is seen that the massless calculations do not agree with each other, and that, for the massive calculation based on the scheme of Frixione 1 there is large dependence on the Peterson fragmentation 2
(nb/GeV)
* * s s
dσep → D*X/dp⊥D*
Massive ε=0.015, 1.2≤mc≤1.5GeV, m⊥/2≤µR≤m⊥ Massive ε=0.06
-1
Massless Kniehl (upper full)
-2
G) (lower full)
Charm in Photoproduction at HERA
University College London, Gower Street, London WC1E 6BT, Great Britain. e-mail: sutt@desy.de
On behalf of the ZEUS Collaboration. M. Sutton
2 Inclusive D Production
ZEUS 1996 + 1997 preliminary
(nb)
15
(a) p⊥D* > 2 GeV * D → (K π) πs
(nb)
8 6 4 2 0 -1.5
(b) p⊥D* > 3 GeV
Heavy quark photoproduction at HERA can be used to examine perturbative QCD with a hard scale given by the heavy quark mass or high transverse energy ET . In photoproduction at leading order, two types of process contribute to the hard scatter: direct photoproduction where the photon enters directly into the hard subprocess, and resolved photoproduction where the photon acts as a source of partons, thereby interacting via a partonic constituent carrying some fraction, x , of the incoming photon momentum. Charm quarks present in the parton distribution functions of the photon and the proton lead to processes of the form cg ! cg and are referred to as charm excitation. At next-to-leading order, only the sum of direct and resolved processes is unambiguously de ned. To study the kinematics of the hard subprocess at anything other than leading order, it is therefore useful to consider events where at least two high ET jets are produced in the nal state. The variable Pjets ET exp(? jet ) jet OBS x (1) 2yEe can be de ned where yEe is the energy of the exchanged photon and the sum runs over the two highest ET jets in the event within the accepted jet range.a This de nition has the advantage that it is well de ned at all orders of
Inclusive di erential cross-sections for D production at HERA with respect to pT (D ) and (D ) are presented and compared to NLO QCD calculations in both the \massive" and \massless" schemes. There is signi cant disagreement between the various NLO calculations, none of which adequately describe the data, particularly for forward (D ). The di erential cross-section d (ep ! D X )=dxOBS for dijet production with charm is presented for the rst time. This illustrates direct evidence for a signi cant contribution to charm production from resolved (xOBS < 0:75) photoproduction at HERA, which is not described by a NLO calculation using the massive scheme. Comparison with LO Monte Carlo shows this is largely attributable to charm excitation below the photon factorization scale.