Production of D*±(2010) mesons in DIS diffractive interactions at HERA

SUPPLEMENTS

Nuclear Physics B (Proc. Suppl.) 99 (2001) 117-120

ELSEVIER

Production

of D**(2010)

M esons in DIS Diffractive

www.elsevier.nl/locate/npe

Interactions

at HERA

P. Thompson” %chool of Physics and Astronomy, BIRMINGHAM B15 2TT, U.K. The cross section exploring determined

u(ep +

the dynamics in QCD

University

measured

e(D**X)Y)

at HERA

The results

of the process.

analyses

of Birmingham,

of the diffractive

is presented

are compared

structure

function

differentially

with models

as a function

based

of observables

on a partonic

and with perturbative

QCD

pomeron

calculations

as

of two

gluon exchange.

momentum.

1. Introduction The rate and kinematic tive

open

charm

distributions

production

tive to the gluon content

of diffrac-

in DIS

are sensi-

of the exchange.

This

paper presents a measurement of differential distributions which explore the dynamics of diffractive charm production. Different theoretical approaches to diffractive charm production exist and are discussed 1.1.

Resolved

below.

1.2. A Two Gluon Exchange Model Complementary to the approach based pomeron teractions

parton

on

y’p in-

in terms of the elas-

from the proton

of partonic

fluctu-

ations of the photon. In the rest frame of the proton, the photon fluctuates a long time before

state scatters

Model

diffractive

are also modelled

tic scattering

the interaction parton content

Pomeron

distributions,

into a Fock state with a definite (qq, qqg, . ..). and this partonic diffractively

off the proton.

In per-

The resolved pomeron model [l] is a partonic interpretation of a factorisable pomeron. The cal-

turbative QCD diffractive open charm production can be realised by the exchange of two per-

culation

turbative

of the cross

trix elements

section

factorises

based on the parton

pomeron and a pomeron terms of the probability

into ma-

density

of the

flux factor, described in for finding a pomeron

gluons

y* +p + cC+p’

[4-71.

The lowest order process

is approximately

proportional

to

the gluon density squared [zrpG, (XP, /.J)]’ of the proton, where p is the factorisation scale. The

in the proton (xP). Within this picture, the partonic content of the pomeron has been determined by the HI experiment within a QCD anal-

contributions higher order

ysis of the inclusive

[8]. These contributions are implemented in the RID1 [9] Monte Carlo program. The lowest and

FD(3)(P,Q2,~p) th2e momentum quark

interacting

diffractive

structure

function

[a], where p is the fraction of of the pomeron carried by the with the photon.

The

parton

distributions, obtained from the fit to the data, contain a dominant gluon distribution. Similar gluon-dominated parton distributions have been obtained by Alvero et al. (ACTW) from a range of ZEUS and Hl results [3]. Open charm is produced in the resolved pomeron model by BosonGluon-Fusion, where the photon, radiated by the incoming lepton, interacts with a gluon of the pomeron, carrying a fraction z, of the pomeron 0920-5632/01/$ - see front matter 8 2001 Elsevier Science B.V PII SO920-5632(01)01318-4

of the lowest order process and the process y* + p + cCg + p’ to the

cross section have been estimated

by Ryskin

et al

higher order processes have also been calculated by Bartels at al [7]. The magnitude of the ccg contribution is sensitive to the cut-off in transverse momentum lation

of the gluon

based on this approach

the Monte Carlo generator 2.

Event

A calcu-

RAPGAP

in

[lo].

Selection

The analyses presented ples from the Hl and ZEUS All rights reserved.

(pt(g)).

is implemented

here use data samCollaborations taken

I? Thompson/Nuclear

118

during

the 1995-1997

running

periods

Physics B (Proc. Suppl.) 99 (2001) I 17-120

where the

HERA accelerator collided 820 GeV protons with Standard reconstruction 27.5 GeV positrons. methods are used to identify DIS events which that decayed through contain a D** candidate either of the following channels D*+

-_)

DOT+ -+ (ICr+)n+

D*+

-+

DOT+ -_) (A--n+0+)7r+

The overall branching nel 1 (‘3-prong’) is 2.6%

fraction

+ ( c.c.)

+ ( c.c.X2)

and 5.4%

respectively

the measurement in table

[ll].

2 (‘5-prong’)

transfer

from the positron

to the hadronic

collaboration dissociative

subtracted. This contribution is estimated from inclusive diffractive ZEUS measurements to be 31 f

15% [la].

ZEUS 4 < QL < 400 GeV2

0.05 < y < 0.7

Total

Diffractive

II**

Cross

Section

Measurements The total diffractive D** cross section determined from the 3-prong decay channel is extracted from the full 1995-1997 data sets which correspond to an integrated luminosity of

’ Pseudorapidity

is defined to be -ln(tan(8/2)), where 0 is the polar angle measured with respect to the proton beam direction.

0.02 < y < 0.7

> 2 GeV

~T(D**)

1.5 < p~(D**t)

W**)l < 1.5 xp < 0.04 _ M,

< 8.0 GeV

W**)l < 1.5 xp < 0.016 a < 0.8 i&m, -

< 1.6 GeV

It I < 1 GeV2

ranges of the Hl and ZEUS measurements for the 3-prong de-

D**

cay channel. After applying the kinematic cuts listed above, the number of D** mesons extracted from a fit to the invariant AM = M(Ii’-~+~+)

mass difference distribution - M(K-r+) was measured

to be 38 f lO(stat.) f 4(syst.) for the Hl experiment and 85 f 12 for the ZEUS experiment. The cross sections for diffractive D” production in the kinematic

region above were measured

to be

u(ep -_) e(D**X)Y) = (154 f 40 f 35)pb (Hl = preliminary [13]) and a(ep -_) e(D**X)Y) (281 f 41+Gz)pb (ZEUS preliminary [14]), where the first errors are statistical and the second systematic.

3.

of mass znergy. /3 = A.

Hl

diffractive

requires the events to be

Q'tM:,

=

2 < Q2 < 100 GeV2

system

strict the mass of the dissociating system M, transfer (M, < 1.6 GeV) and the momentum to the proton t (It 1 < 1 GeV’). The selection

2~

centre

Table 1 The kinematic

in the pseudorapidity distribution of final state hadrons adjacent to the leading proton. The Hl collaboration place requirements which re-

t is neglected

in this approximation,

energy

in the proton rest frame, y. From these events, a sample of diffractive events is obtained on the basis of a large gap

procedure of the ZEUS contribution of proton

When

is shown

the square of the hadronic Similarly,

Cuts on the

Q2, and the fractional

for both experiments

2+w2 mass of the fina“I state dissociates and W2 is

are applied to guarantee but these limit the re-

positron in the main detector is used to reconstruct the DIS kinematics defined in terms of the negative square of the four-momentum transfer at vertex,

1.

for the Hl and ZEUS exThe kinematic range of

where M, is the invariant X into which the photon

gion of phase space measured in the transverse momentum, PT, and pseudorapidity’, 7, of the of the scattered D** meson. The measurement

the leptonic

and 44 pb-’ respectively.

for the decay chan-

and decay channel

particle track quantities a reliable measurement,

(1)

20.8 pb-’ periments

tions

Comparisons

are difficult

regions covered.

between

the two cross sec-

due to the different However,

kinematic

using the Monte

Carlo

models to interpolate between the two kinematic regions suggests that the cross sections are in disagreement at the level of approximately 2a. 4.

Differential

Cross

Sections

The differential diffractive cross sections for the Hl experiment are shown in figure 1 and for the

119

P Thompson/Nuclear Physics B (Proc. Suppl.) 99 (2001) I 17-120

f ksol. IP * Ii3

-

. . . . Z-Glum (q$

F $

.-.-.

. . . .. . . . .. -2 10 7

10

f

B

f

1

7 .

. . . .._..._._...

H,Pre,imi”ary

--

10 -3 lo

0

2

6

4

8

T

. .... .............* :

I-Gluon (q;l +q&

i...............

;‘F=c--l

7

3

-1

!. . . . . . . . . . . . . _. . . . . . ~

Resol. IP * l/3 i

. . . . 2.Glum(qi) -..---.

i

Z-Glum (qt, + i q&) 0.01

10

0.02

0.04

0.03

PT’ WV)

lo-’

10“

P

XIP

measured by the Hl experiment (IS a function of p$, xp “(ep + e(D**X)Y) The data are points with error bars (inner: statistical, outer: total) and are compared with three

1. Cross sections

Figure and p. different

ZEUS

predictions,

experiment

as described

in figure

2.

in the text.

In figure

1 the

cross section is shown as a function of the transverse momentum of the D* in the y*p centre of mass system p;, of zp, and of ,!?. The resolved pomeron model gives a reasonable the shape of the cross sections.

description of However, the

model fails in the overall normalisation by a factor of 2 to 3. The two gluon model for CC and ccg fluctuations of the photon describes the normalisation of the cross section at low xp with a cut on the transverse

momentum

of the gluon of

pt(g) > 1.5 GeV. The ZEUS experiment have measured the differential D** cross sections from both decay channels

as a function of Q2, W, xp, p~(o**) Th e results are shown in figure 2. and v(D**). In each plot the results are compared with two

different theoretical predictions. The dotted line indicates the predictions made by Alvero et al (ACTW). The solid line shows predictions of the Ryskin

pQCD

implemented The

lo’J

dashed

model

for the CC contribution

as

in the RID1 Monte Carlo generator. line shows the higher

order correc-

tions which are known to cause a large normalisation uncertainty in the predictions. For this reason, the predictions of each contribution from RID1 have been normalised to the data cross section. The ACTW predictions generally describe the shape of the differential distributions well, although the magnitude is slightly overestimated. Similar conclusions can be drawn from the 5-

prong decay channel[15]. The ratio, R, of diffractive D** to total D** production has been measured by the ZEUS experiment

from the 5-prong decay channel +1.7% and from the 3-prong R = 8.9 f 2.4_,,,

to be decay

channel to be R = 6.1 5 1.9t::i%. These values are consistent with the fraction of the total cross section attributed to diffractive scattering [12]. Th e d e p en d ence of the ratio on W and Q2 has been measured from the S-prong decay channel and is shown in figure 3. The ratio is found to be consistent

5.

with being flat within the errors.

Conclusions The dynamics

of diffractive

charm

production

have been studied at HERA through the measurement of the total and differential cross section for D** meson production. In the kinematic range probed, the ZEUS experiment measure a cross section which is consistent with the expectation based on the resolved pomeron dominated QCD

parton

analysis

distributions

of inclusive

model with gluonderived from the

diffractive

DIS

data.

In contrast, the Hl experiment observe a difference of a factor 2-3 in the normalisation between the data and the predictions. The differential

have been measured

and

compared to several different calculations. resolved pomeron model is found to describe the shape of all the differential distributions.

cross sections

The well The

120

I? Thompson/Nuclear

Physics B (Proc. Suppl.) 99 (2001) 117-120

ZEUS 1995 - 1997 Prelimlnaw 4 3

~_. ....~ i...__j ~...... :

l.......

I""**.L"U

....j

FilI

loo

. iLLiJ Figure

3.

The

150

200

250

W (GeV)

ratio of diffmctive

D*&

to total

D** production as a function of Q2 and W measured by the ZEUS collaboration from the S-prong decay channel.

P

63 f g

._

,.,,.,. rmpro Rlllclp ...- Rlll~~ _._._..,

.._.. ...,i

.... ___’

1

error bars represent

the

errors added in quadrature. The solid line indicates the average ratio R = 6.1 f 1.9~~$$.

I

_

The inner

statistical error on the data points and the outer error bars represent the statistical and systematic

ZEUS Osm.III~h

.._.... .. . . . . . . . . . . . . .

1

0

-1

1

Gi

P,@> (C.G

Figure 2. The differential cross sections measured by the ZEUS experiment from the S-prong decay channel

versus

Q2, W,

xp,

p~(o**)

and

n(D**). The data are points with error bars (inner: statistical, outer: total) and are compared with three different predictions as described in the

shapes

of the distributions

turbative

two-gluon

indicate

that in per-

exchange models the data re-

quire higher order contributions der process y* + p + CF + p’.

to the leading or-

G. Ingelmann, (1985)

2. 3.

256;

5.

Bartels,

P.E. Schlein, Phys. Lett. B152 A. Donnachie,

H.

Jung,

P.V.

Landshoff,

Phys. Lett. B191 (1987). C. Adloff et al., Hl Coll., Z.Phys. C76 (1997) 613. L. Alvero et al. Phys. Rev. D59 (1999) 74022; J. Bartels, H. Lotter, M. Wiisthoff, Phys. Lett. B 379 (1996) 239. J. Bartels, C. Ewerz, H. Lotter, M. Wiisthoff,

A.

171.

Kyrieleig,

hep-

ph/0010300 E. M. Levin, A. D. Martin, M. G. Ryskin, T. Tenbner, Z. Phys. C74 (1997) 671. M. G. Ryskin and A. Solano, Proc. of the Workshop ‘Monte HERA Physics’, Ed.

Carlo Generators G. Grindhammer

for et

a1.,(1999) Hamburg (to be published). 10. H. Jung, Comput. Phys. Commun. , 86 (1995) 147.; (for update see http://wwwhl.desy.de/ jung/rapgap/rapgap.html)

12. J. Breitweg

L. Alvero et al. hep-ph/9806340. 4.

J.

11. C. Caso et al. (Particle Phys J. C3 (1998) 1.

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