- Email: [email protected]
Montecarlo studies of diffractive events at HERA
PROCEED|NGS SUPPLEMENTS
MONTECARLO
STUDIES
Nuclear Physics B (Proc. Suppl.) 25B (1992) 274-277 North-Holland
OF DIFFRACTIVE
EVENTS
AT HERA
A d a SOLANO Universit~ di Torino and INFN, Via P. Giuria 1, 1-10125 Torino P r d i m i n a r y montecarlo studies with a new diffractive event generator are presented. The experimental possibilities of the ZEUS apparatus at HERA are discussed. 1. I N T R O D U C T I O N p r <_ 1 GeV,
Hadron diffractive scattering is generally interpreted in terms of pomeron exchange [1]. The pomeron language may also be useful to describe diffractive events
with p r the transverse momentum of the scattered proton with respect to the incoming b e a m direction.
in Deep Inelastic Scattering (DIS).
~2
The pomeron (lP) is a Regge trajectory with the vacuum quantum numbers, which dominates the elasis still unclear.
p;
p, I \
tic amplitude I2]. The physical nature of the pomeron
q 7"
Different theoretical approaches ex-
Mx
ist [3] offering rather diverse physical pictures, ranging from a strictly non perturbat]ve treatment to a fuiiy perturbative QCD one; the expectations of these models for yields, flavour content and angular distribution of the produced jets differ significantly. Many models
P ' ~ , t 7 P'
consider the pomeron as a system of two or more ghions, but its partonic structure is far from being well established. RERA ~'i~ offer the opportunity te probe the microscopic structure of the pomeron with an electromag-
Figure I: Diffractive interaction in DIS. For the kinematical variables: Q2 = _q2 = (pt - p t , ) 2, zBj = ~.q, t = ( p - p,)2
netic current. Diffractive events will occur in the DIS of electrons and protons with invariant masses up to x/~ = 314 GeV. In these c';cnts (fig.l) the ckt~al photon does not interact directly with one of the proton's constituents but rather with a pomeron emitted by the proton. Diffractive events at HERA will be identified by the following conditions: p' zL = -- > 0.9, P where p and p' are the incoming and outcoming proton momenta, respectively, and
In these events the proton emerges from the interaction isolated in rapidity, with its m o m e n t u m very close to that of the beam. The study of the properties of the associated hadronic system m a y help in investigating the dynamics of diffractive scattering. Diffractive central clusters with masses up to M x ~ 100 GeV are expected.
2. E X P E R I M E N T A L
APPARATUS
t!ERA is the first e(30 GeV)-p(820 GeV) collider. Tw,) experiments will operate in the first phase: ZEUS and Hi.
0920-5632/92/$05.00 © !932 - Elsevier Science Publishers B.V. All rights reserved.
A. Solano / Monte Carlo studies of diffractive events at HERA In the ZEUS apparatus [4] diffractive events will be recorded where [5]:
275
If however the electron is not detected by the LUMI, the calorimeter information from the ZEUS main ap-
® the outgoing proton is tagged and measured by
paratus is needed in order to recover M x .
the Leading Proton Spectrometer (LPS); * the associated hadron activity is measured by the ZEUS central apparatus;
3. M O N T E C A R L O
STUDIES
Montecarlo studies are necessary to evaluate the ca-
* the outgoing electron is detected by the Lumi-
pabilities of the ZEUS detector and in particular of the LvS . "- physics. . . . . .in . . -~:~. . .~.blve
nosity Monitor (LUMI) or by the ZEUS central
A first a t t e m p t has been made in developing a MC
apparatus.
generator for diffractive events in e-p collisions. The
T h e aim of the LPS is to detect and m o m e n t u m
lnode! recently proposed by N.N.Nikoleev [6] has been
analyse very small angle protons which escape detec-
cons: J~rt 2[. In this model the properties of the diffrac-
tion by the ZEUS central a p p a r a t u s and remain inside
tive dissociation of virtual photons in DIS are s~.ud-
the beampipe. The LPS uses six stations of siliccn mi-
led, treating the pomeron perturhatively as the Low-
crostrip detectors located in "Roman pots" along the
Nussinov [7] exchange of two non interacting and seem-
b e a m p i p e and the fields of the beamline magnets. De-
ingly uncorrelated gluons. Diffractive dissociation of
tectors and pots are shaped so as to match the 10 ¢
photons is predicted to contribute -,~ 15% of the total
profile of the proton beam, in order to maximize accep-
DIS rate at small z s j and large Q2. The model makes
tance. The front-end electronics is mounted very close
detailed predictions for the mass spectrum and the an-
to the detectors, inside the pots. T[~e zL,pT ranges to
gular distribution of the diffractively produced jets (for
which the LPS is sensitive are:
the process 3" + lIP --- q + q ), which axe used as specific ingredients of the MC simulation. The variables
0.35 < xL < 1
ZBj and Q: are generated according to the usual inclusive DIS differential cross section, within the limits
and
10 -s < zBj < 10 -2 and Q2 _> 1 GeV z. For the It[ dis-
O < PT < 1.4 GeV, with an expected resolution of ~ 1% in xL and 30 MeV in PT. The LPS will also be able to trigger on events with zL > 0.96, that is in the diffractive region. Photoproduction events with electrons scattered at very low angles can be analysed using the LUMI in the
tribution a simple exponential is adopted in the region It I < 1 GeV 2, with the slope parameter b = 4.5 GeV -2 derived from diffractive dissociation of real photons on hydrogen [8]. For the hadronization 3ETSET7.2 in the default configuration is used [9]. The mass spectrum given by
range
d~,T( D D ) I M2 dtdM]- it=0 = E v v ( Q 2 + M~r) s
16
• [1 + 47r2ae"AslPF~p(z~O'Q'2)M}] ~" - 4 m r a d < 0 < ~r
(the polar angle 0 is measured with resp~=ct to the pro-
L
~D
is shown in fig.2. The values used for EDD are
ton direction). For this kind of events the LPS and the LUMI information is sufficient to reconstruct the mass
EDD(u~ + d d ) = 35 pb
of the hadronic system and M ] = (1 - ~ , ) ¢ w 2 + Q~) - Q~ + t.
Q" J
EDD(c6) = I #b
27~
A. Solano / Monte Carlo studies of diffractive events at HERA
reflect ng the characteristic strong heavy flavour sup-
pression of this m o d e l The triple-pomeron coupling
Fig.3 shows the distribution of the xL variable for diffractive events, which are indeed concentrated in the high x r region, where the LPS will be able to trigger.
AaW = 0.16
(GeV/c)-
The mean values for the scattered protons variables are--
is derived from photoproduction d a t a [8].
< 0v > ~ 0.5 m r a d and
{/1
< PT > ~ 0.4 GeV,
104
both well within the sensitivity of the LPS. The sharp separation between the outgoing proton
0 ---10 3
and the central hadronic system can be clearly seen in the rapidity distribution (fig.4) of all the particles produced in the interaction.
I0 102 i i
200 400 600 800 1000
0
M,, =
(,GeV=)
20000 .6 t,0 16000
Figure 2: Mass spectrum of the hadronic system associated with diffractive events Imposing a lower cut on M x to select large hadronic masses, more visible in the central apparatus, reduces the event sample considerably. For exanaple only ~ 30% ( ~ 15%) of the events survive the Mx > 10 GeV (ll~t- > 20 GeV) cut.
0
-o 12000 I b "o 8000 4000 0
,,I, -12 -8
S -4
0
4
8
12 ¢/
Figure 4: Rapidity distrib,tion. The well separated peak is due to the scattered protons.
0,~ 10000 e-
. i
.6 I,,.
¢7 24000
. i
8000
Finally in fig.5 tile distribution in the polar angle 0 of all the particles produced is shown. After applying
6000
the cut 4 ° < 0 < 176 ° in order to fall inside the geo-
x" "o 4000 1::) "o
/
metrical acceptance of the ZEUS main apparatus, the average charged multiplicity is 7 + 8 and the average
2000~
visible energy is around 15 GeV. 'ib conclude, the preliminary studies presevLted here
0.9
0.92 0.94 0.96 0.98
clearly show that the LPS at ZEUS has a chance to XL
Figure .% xc distribution
explore the diffractive region in DIS and t9 measure the pe'ueron s t r u c t , r e function.
A. Solano /Monte Carlo studies o~ diffractJve events at HERA
A. Donnachie and P.B. Landshoff, in: Proceedings of the HERA Workshop, ed. R.D. Peccei (DESY, Hamburg, 1988) p. 351;
6000
5000~ 0
4000~
K.H. Streng, in: Proceedings of the HERA Workshop, ed. R.D. Peccei (DESY, Hamburg, 1988) p. 365; J. Bartels and G. Ingelman, Phys. Lett. B235 (1990) 175;
V
2000 1000 0
IIZ
0
277
20 40
60
80 10012C 140 160 180
(deg) Figure 5: Polar angle distribution of all the particles produced with ~he cug 4° < 0 < 176°
REFERENCES [1] K. Goulianos, Phys. Rep. 101 (1983) 169. [2] See, for example, P.D.B. Collins, An Introduction to Regge Theory and High Energy Physics, (Cambridge University Press, Cambridge, 1977). [3] See, for instance: G. lngelman and P. Schlein, Phys. Lett. 152B (1985) 256;
M.G. Ryskin, DESY Report, DESY 90-050 (1990); N.N. Nikolaev and B.G. Zakharov, Univ. of Torino preprint DFTT-5/91 (1991).
[4] The ZEUS Detector, PRC-89-01 (1989). [5] M. Arneodo and C. Peroni, Nucl. Phys. B (Proc. Suppl.) 12 (1990) 149; D. Kisielewsl~, ZEUS-Note 89-26 (1989). [6] N.N. Nikolaev and B.G. Zakhaxov, Univ. of Torino preprint DFTT-5/91 (!991). See also these proceedings. [71 F.E. Low, Phys. Rev. D12 (1975) 163; S. Nussinov, Phys. Rev. Lett. 34 (1975) 1286; Phys. Rev. D14 (1976) 246.
N. Arteaga-Romero et al., Mod. Phys. Lett. A1 (1986) 211;
[8] T.J. Chapin et al., Phys. Rev. D3i (1985) 17.
E.L. Berger, J.C. Collins, D.E. Soper, G. Sterman, Nucl. Phys. B286 (1987) 704;
[9] T. Sjostrand, JETSET version 7.2, CERN Program Pool long writeup, program W5035 (1989).
MONTECARLO
STUDIES
Nuclear Physics B (Proc. Suppl.) 25B (1992) 274-277 North-Holland
OF DIFFRACTIVE
EVENTS
AT HERA
A d a SOLANO Universit~ di Torino and INFN, Via P. Giuria 1, 1-10125 Torino P r d i m i n a r y montecarlo studies with a new diffractive event generator are presented. The experimental possibilities of the ZEUS apparatus at HERA are discussed. 1. I N T R O D U C T I O N p r <_ 1 GeV,
Hadron diffractive scattering is generally interpreted in terms of pomeron exchange [1]. The pomeron language may also be useful to describe diffractive events
with p r the transverse momentum of the scattered proton with respect to the incoming b e a m direction.
in Deep Inelastic Scattering (DIS).
~2
The pomeron (lP) is a Regge trajectory with the vacuum quantum numbers, which dominates the elasis still unclear.
p;
p, I \
tic amplitude I2]. The physical nature of the pomeron
q 7"
Different theoretical approaches ex-
Mx
ist [3] offering rather diverse physical pictures, ranging from a strictly non perturbat]ve treatment to a fuiiy perturbative QCD one; the expectations of these models for yields, flavour content and angular distribution of the produced jets differ significantly. Many models
P ' ~ , t 7 P'
consider the pomeron as a system of two or more ghions, but its partonic structure is far from being well established. RERA ~'i~ offer the opportunity te probe the microscopic structure of the pomeron with an electromag-
Figure I: Diffractive interaction in DIS. For the kinematical variables: Q2 = _q2 = (pt - p t , ) 2, zBj = ~.q, t = ( p - p,)2
netic current. Diffractive events will occur in the DIS of electrons and protons with invariant masses up to x/~ = 314 GeV. In these c';cnts (fig.l) the ckt~al photon does not interact directly with one of the proton's constituents but rather with a pomeron emitted by the proton. Diffractive events at HERA will be identified by the following conditions: p' zL = -- > 0.9, P where p and p' are the incoming and outcoming proton momenta, respectively, and
In these events the proton emerges from the interaction isolated in rapidity, with its m o m e n t u m very close to that of the beam. The study of the properties of the associated hadronic system m a y help in investigating the dynamics of diffractive scattering. Diffractive central clusters with masses up to M x ~ 100 GeV are expected.
2. E X P E R I M E N T A L
APPARATUS
t!ERA is the first e(30 GeV)-p(820 GeV) collider. Tw,) experiments will operate in the first phase: ZEUS and Hi.
0920-5632/92/$05.00 © !932 - Elsevier Science Publishers B.V. All rights reserved.
A. Solano / Monte Carlo studies of diffractive events at HERA In the ZEUS apparatus [4] diffractive events will be recorded where [5]:
275
If however the electron is not detected by the LUMI, the calorimeter information from the ZEUS main ap-
® the outgoing proton is tagged and measured by
paratus is needed in order to recover M x .
the Leading Proton Spectrometer (LPS); * the associated hadron activity is measured by the ZEUS central apparatus;
3. M O N T E C A R L O
STUDIES
Montecarlo studies are necessary to evaluate the ca-
* the outgoing electron is detected by the Lumi-
pabilities of the ZEUS detector and in particular of the LvS . "- physics. . . . . .in . . -~:~. . .~.blve
nosity Monitor (LUMI) or by the ZEUS central
A first a t t e m p t has been made in developing a MC
apparatus.
generator for diffractive events in e-p collisions. The
T h e aim of the LPS is to detect and m o m e n t u m
lnode! recently proposed by N.N.Nikoleev [6] has been
analyse very small angle protons which escape detec-
cons: J~rt 2[. In this model the properties of the diffrac-
tion by the ZEUS central a p p a r a t u s and remain inside
tive dissociation of virtual photons in DIS are s~.ud-
the beampipe. The LPS uses six stations of siliccn mi-
led, treating the pomeron perturhatively as the Low-
crostrip detectors located in "Roman pots" along the
Nussinov [7] exchange of two non interacting and seem-
b e a m p i p e and the fields of the beamline magnets. De-
ingly uncorrelated gluons. Diffractive dissociation of
tectors and pots are shaped so as to match the 10 ¢
photons is predicted to contribute -,~ 15% of the total
profile of the proton beam, in order to maximize accep-
DIS rate at small z s j and large Q2. The model makes
tance. The front-end electronics is mounted very close
detailed predictions for the mass spectrum and the an-
to the detectors, inside the pots. T[~e zL,pT ranges to
gular distribution of the diffractively produced jets (for
which the LPS is sensitive are:
the process 3" + lIP --- q + q ), which axe used as specific ingredients of the MC simulation. The variables
0.35 < xL < 1
ZBj and Q: are generated according to the usual inclusive DIS differential cross section, within the limits
and
10 -s < zBj < 10 -2 and Q2 _> 1 GeV z. For the It[ dis-
O < PT < 1.4 GeV, with an expected resolution of ~ 1% in xL and 30 MeV in PT. The LPS will also be able to trigger on events with zL > 0.96, that is in the diffractive region. Photoproduction events with electrons scattered at very low angles can be analysed using the LUMI in the
tribution a simple exponential is adopted in the region It I < 1 GeV 2, with the slope parameter b = 4.5 GeV -2 derived from diffractive dissociation of real photons on hydrogen [8]. For the hadronization 3ETSET7.2 in the default configuration is used [9]. The mass spectrum given by
range
d~,T( D D ) I M2 dtdM]- it=0 = E v v ( Q 2 + M~r) s
16
• [1 + 47r2ae"AslPF~p(z~O'Q'2)M}] ~" - 4 m r a d < 0 < ~r
(the polar angle 0 is measured with resp~=ct to the pro-
L
~D
is shown in fig.2. The values used for EDD are
ton direction). For this kind of events the LPS and the LUMI information is sufficient to reconstruct the mass
EDD(u~ + d d ) = 35 pb
of the hadronic system and M ] = (1 - ~ , ) ¢ w 2 + Q~) - Q~ + t.
Q" J
EDD(c6) = I #b
27~
A. Solano / Monte Carlo studies of diffractive events at HERA
reflect ng the characteristic strong heavy flavour sup-
pression of this m o d e l The triple-pomeron coupling
Fig.3 shows the distribution of the xL variable for diffractive events, which are indeed concentrated in the high x r region, where the LPS will be able to trigger.
AaW = 0.16
(GeV/c)-
The mean values for the scattered protons variables are--
is derived from photoproduction d a t a [8].
< 0v > ~ 0.5 m r a d and
{/1
< PT > ~ 0.4 GeV,
104
both well within the sensitivity of the LPS. The sharp separation between the outgoing proton
0 ---10 3
and the central hadronic system can be clearly seen in the rapidity distribution (fig.4) of all the particles produced in the interaction.
I0 102 i i
200 400 600 800 1000
0
M,, =
(,GeV=)
20000 .6 t,0 16000
Figure 2: Mass spectrum of the hadronic system associated with diffractive events Imposing a lower cut on M x to select large hadronic masses, more visible in the central apparatus, reduces the event sample considerably. For exanaple only ~ 30% ( ~ 15%) of the events survive the Mx > 10 GeV (ll~t- > 20 GeV) cut.
0
-o 12000 I b "o 8000 4000 0
,,I, -12 -8
S -4
0
4
8
12 ¢/
Figure 4: Rapidity distrib,tion. The well separated peak is due to the scattered protons.
0,~ 10000 e-
. i
.6 I,,.
¢7 24000
. i
8000
Finally in fig.5 tile distribution in the polar angle 0 of all the particles produced is shown. After applying
6000
the cut 4 ° < 0 < 176 ° in order to fall inside the geo-
x" "o 4000 1::) "o
/
metrical acceptance of the ZEUS main apparatus, the average charged multiplicity is 7 + 8 and the average
2000~
visible energy is around 15 GeV. 'ib conclude, the preliminary studies presevLted here
0.9
0.92 0.94 0.96 0.98
clearly show that the LPS at ZEUS has a chance to XL
Figure .% xc distribution
explore the diffractive region in DIS and t9 measure the pe'ueron s t r u c t , r e function.
A. Solano /Monte Carlo studies o~ diffractJve events at HERA
A. Donnachie and P.B. Landshoff, in: Proceedings of the HERA Workshop, ed. R.D. Peccei (DESY, Hamburg, 1988) p. 351;
6000
5000~ 0
4000~
K.H. Streng, in: Proceedings of the HERA Workshop, ed. R.D. Peccei (DESY, Hamburg, 1988) p. 365; J. Bartels and G. Ingelman, Phys. Lett. B235 (1990) 175;
V
2000 1000 0
IIZ
0
277
20 40
60
80 10012C 140 160 180
(deg) Figure 5: Polar angle distribution of all the particles produced with ~he cug 4° < 0 < 176°
REFERENCES [1] K. Goulianos, Phys. Rep. 101 (1983) 169. [2] See, for example, P.D.B. Collins, An Introduction to Regge Theory and High Energy Physics, (Cambridge University Press, Cambridge, 1977). [3] See, for instance: G. lngelman and P. Schlein, Phys. Lett. 152B (1985) 256;
M.G. Ryskin, DESY Report, DESY 90-050 (1990); N.N. Nikolaev and B.G. Zakharov, Univ. of Torino preprint DFTT-5/91 (1991).
[4] The ZEUS Detector, PRC-89-01 (1989). [5] M. Arneodo and C. Peroni, Nucl. Phys. B (Proc. Suppl.) 12 (1990) 149; D. Kisielewsl~, ZEUS-Note 89-26 (1989). [6] N.N. Nikolaev and B.G. Zakhaxov, Univ. of Torino preprint DFTT-5/91 (!991). See also these proceedings. [71 F.E. Low, Phys. Rev. D12 (1975) 163; S. Nussinov, Phys. Rev. Lett. 34 (1975) 1286; Phys. Rev. D14 (1976) 246.
N. Arteaga-Romero et al., Mod. Phys. Lett. A1 (1986) 211;
[8] T.J. Chapin et al., Phys. Rev. D3i (1985) 17.
E.L. Berger, J.C. Collins, D.E. Soper, G. Sterman, Nucl. Phys. B286 (1987) 704;
[9] T. Sjostrand, JETSET version 7.2, CERN Program Pool long writeup, program W5035 (1989).