Luminosity determination at HERA-B
Introduction
A precise determination of the luminosity is required for the measurement of absolute cross-sections. The integrated luminosity () is defined bywhere is the number of events of a given process and is the corresponding cross-section. In the case of HERA-B, which is a forward spectrometer [1], [2] experiment, operated at the 920 GeV proton beam of the HERA accelerator at the DESY Laboratory in Hamburg, the proton beam is bunched and interacts with a nuclear target placed on the halo of the beam. The number of proton–nucleus (pA) interactions per bunch crossing is subject to statistical fluctuations. For HERA-B, as for all other experiments having a bunched beam, the luminosity can be expressed aswhere is the average number of interactions per bunch crossing BX, is the number of beam bunches crossing the apparatus and is the interaction cross-section (for a more detailed discussion, see 4 The cross-sections, 5 General remarks on the luminosity determination). As a consequence, given the cross-section of proton–nucleus interactions, the luminosity can be measured by determining and . The average number of interactions per BX can be obtained from a fully unbiased sample of events in various ways: by looking at inclusive quantities which are proportional to the number of interactions in one event (such as the number of tracks or the energy released in a calorimeter), by counting the number of primary vertices or by counting the number of empty events. The first method has the advantage of entailing only a rather straightforward analysis of the data, but the signal corresponding to a single interaction must be evaluated precisely and detector stability becomes a relatively critical issue. In the second method, the vertex reconstruction efficiencies must be known precisely as well as the probability of erroneously merging or splitting primary vertices during reconstruction. In the third method, the distribution of the number of interactions per bunch crossing must be either known or assumed and the efficiency for detecting non-empty events and the impact of noise events must be evaluated.
After careful studies the HERA-B Collaboration has decided to exploit the method based on counting events with evidence of at least one interaction (which is equivalent to the third method listed above), since this method minimizes the systematic error on the luminosity determination allowing to achieve a final precision of about 5%.
The paper is organized in the following way. In 2 The HERA accelerator and the target, 3 The HERA-B detector and the data sample the main features of the HERA accelerator relevant for this analysis and the HERA-B detector are briefly described. Section 4 summarizes all of the published proton–nucleus cross-section measurements which are used for the luminosity determination. In 5 General remarks on the luminosity determination, 6 The determination of, the relevant relations for the determination of the luminosity are described. In Section 7 we discuss the systematic uncertainties and comment on delta ray production, while in Section 8 we report the results obtained for the interaction trigger (defined below) data sample.
Section snippets
The HERA accelerator and the target
HERA is a double storage ring designed for colliding a 920 GeV proton beam with a 27.5 GeV electron beam. Four interaction regions exist: two of them house the general purpose ep detectors H1 and ZEUS, while the other two accommodate the fixed target experiments HERA-B and HERMES. In the following we describe the beam parameters and the filling scheme used during the HERA-B data taking period 2002–2003.
The typical proton current is 80 mA, distributed over 180 bunches with a typical bunch length
The HERA-B detector and the data sample
The HERA-B experiment is a forward magnetic spectrometer with an acceptance extending from 15 to 220 mrad horizontally and to 160 mrad vertically. This large angular coverage allows studies in kinematic regions not accessible to previous fixed-target high energy experiments. A top view of the detector is shown in Fig. 2.
The first part of the spectrometer is devoted to tracking and vertex measurements and consists of the target, a silicon vertex detector, a magnet and a tracking system. The second
The cross-sections
The total pA cross-section can be divided into elastic and inelastic contributions:In this context, the cross-section is regarded as the sum of the elastic and quasielastic contribution . The inelastic cross-section includes a minimum bias part (mb) and a diffractive part which can be further subdivided into target single diffractive (tsd, ), beam single diffractive (bsd, ) and double diffractive (dd, )
General remarks on the luminosity determination
In the following, the luminosity given by Eq. (2) will be expressed in terms of the total number of events satisfying the IA trigger , the average number of interactions per bunch crossing , the trigger efficiency per single interaction and the total hadronic cross-section . In order to do this, two assumptions are made:
- •
the number of interactions per filled bunch can be described by a single Poisson distribution , for all bunch crossings in a given data run:
The determination of
The determination of relies on the pseudo random trigger data sample acquired in parallel to the IA trigger and the large-acceptance detectors which constitute the spectrometer. Specifically, is obtained by combining the information from a variety of subdetectors to also provide a cross-check of the stability of the result and the systematic uncertainties due to the detector response and the event model of the Monte Carlo. Only the filled bunches of HERA were considered.
The average
General considerations
According to Eq. (11) and the assumption on which Eq. (6) is based, the following systematic uncertainties must be taken into account:
- •
the uncertainty on , arising from the Monte Carlo (MC) event model and the poorly known observation probability of diffractive processes;
- •
the uncertainty on the determination of ;
- •
the uncertainty associated with deviations of the interaction probability distribution from the assumed Poisson distribution (e.g. due to the uneven filling of bunches);
- •
the
Summary and conclusions
As previously noted the uncertainties affecting the total luminosity measurement are dominated by the systematic contribution, since each IA trigger run contains enough random trigger events to make the contribution from statistics negligible. In Table 6 we summarize the overall relative uncertainty on the total luminosity calculation . In the second column the uncertainty on is given. This contribution depends on the present knowledge of the cross-sections (see Table 1) and
Acknowledgments
We are grateful to the DESY laboratory and to the DESY accelerator group for their strong support since the conception of the HERA-B experiment. The HERA-B experiment would not have been possible without the enormous effort and commitment of our technical and administrative staff.
References (24)
Nucl. Instr. and Meth. A
(2000)Nucl. Instr. and Meth. A
(2001)Nucl. Instr. and Meth. A
(2003)Nucl. Instr. and Meth. A
(2000)Nucl. Instr. and Meth. A
(2002)Nucl. Instr. and Meth. A
(2005)Nucl. Instr. and Meth. A
(2005)Nucl. Instr. and Meth. A
(2004)Nucl. Instr. and Meth. A
(2000)Nucl. Instr. and Meth. A
(2001)Nucl. Instr. and Meth. A
(2001)Nucl. Instr. and Meth. A
(2002)Nucl. Phys. B Proc. (Suppl.)
(2003)Nucl. Instr. and Meth. A
(2004)Nuclear Physics A
(2003)
Nucl. Phys. B
Phys. Lett. B
Cited by (3)
FV <sup>0</sup> production in p+A collisions at √s=41.6 GeV
2009, European Physical Journal CInvestigation of heavy-quark production in proton-nucleus collisions with the HERA-B detector
2009, Physics of Atomic NucleiKinematic distributions and nuclear effects of J/ψ production in 920 GeV fixed-target proton-nucleus collisions
2009, European Physical Journal C
- 1
Supported by the Foundation for Fundamental Research on Matter (FOM), 3502 GA Utrecht, The Netherlands.
- 2
Supported by the CICYT Contract AEN99-0483.
- 3
Supported by the German Research Foundation, Graduate College GRK 271/3.
- 4
Supported by the Bundesministerium für Bildung und Forschung, FRG, under contract numbers 05-7BU35I, 05-7DO55P, 05-HB1HRA, 05-HB1KHA, 05-HB1PEA, 05-HB1PSA, 05-HB1VHA, 05-HB9HRA, 05-7HD15I, 05-7MP25I, 05-7SI75I.
- 5
Also from Fondazione Giuseppe Occhialini, I-61034 Fossombrone(Pesaro Urbino), Italy.
- 6
Supported by the U.S. Department of Energy (DOE).
- 7
Supported by the Portuguese Fundação para a Ciência e Tecnologia under the program POCTI.
- 8
Supported by the Danish Natural Science Research Council.
- 9
Supported by the National Academy of Science and the Ministry of Education and Science of Ukraine.
- 10
Supported by the Ministry of Education, Science and Sport of the Republic of Slovenia under contracts number P1-135 and J1-6584-0106.
- 11
Supported by the U.S. National Science Foundation Grant PHY-9986703.
- 12
Supported by the Russian Ministry of Education and Science, Grant SS-1722.2003.2, and the BMBF via the Max Planck Research Award.
- 13
Supported by the Norwegian Research Council.
- 14
Supported by the Swiss National Science Foundation.
- 15
Visitor from Dipartimento di Energetica dell’ Università di Firenze and INFN Sezione di Bologna, Italy.
- 16
Visitor from P.N. Lebedev Physical Institute, 117924 Moscow B-333, Russia.
- 17
Visitor from Moscow Physical Engineering Institute, 115409 Moscow, Russia.
- 18
Visitor from Moscow State University, 119992 Moscow, Russia.
- 19
Visitor from Institute for High Energy Physics, Protvino, Russia.
- 20
Visitor from High Energy Physics Institute, 380086 Tbilisi, Georgia.
- ✠
Deceased.