Strangeness Production in DeepInelastic ep Scattering at HERA ^{§}
Khurelbaatar Begzsuren^{*}The Institue of Physics and Technology of the Mongolian Academy of Sciences, Ulaanbaatar, Mongolia
Abstract
The production of neutral strange hadrons is studied using deepinelastic events measured with the H1 detector
at HERA. The measurements of Ks0 and ΛΛ¯ productions are made in two regions of phase space defined by the negative fourmomentum transferred squared of the photon, 7 < Q^{2} < 100GeV^{2} and the inelasticity 0.1 < y < 0.6 for the Ks0 and
145 < Q^{2} < 20000GeV^{2} and 0.2 < y < 0.6 for the Λ . Ks0 and ΛΛ¯ production cross sections are determined.
Differential ratios of Ks0 production to charged hadron production are measured. Differential ΛΛ¯ yields per event are
determined. The Λ − Λ¯ asymmetry is measured and found to be consistent with zero. Predictions of leading order Monte
Carlo programs are compared to data.
PACS number(s): 04.40.Nr, 04.70.Bw, 11.27.+d.
Keywords: Quantum chromodynamics, strange quarks.
Article Information
Article History:
Received Date: 25/11/2013
Revision Received Date: 25/12/2013
Acceptance Date: 26/12/2013
Electronic publication date: 31/12/2014
Collection year: 2014
© Khurelbaatar Begzsuren; Licensee Bentham Open.
openaccess license: This is an open access article licensed under the terms of the Creative Commons Attribution NonCommercial License (
http://creativecommons.org/licenses/bync/3.0/) which permits unrestricted, noncommercial use, distribution and reproduction in any medium, provided the work is properly cited.
^{*} Address correspondence to this author at the The Institue of Physics and
Technology of the Mongolian Academy of Sciences, Ulaanbaatar,
Mongolia; Tel: 49 40 89984215, Fax: 49 40 89983093; Email: hurel@mail.desy.de^{§} Presented at the Low x workshop, May 30  June 4 2013, Rehovot and
Eilat, Israel.
Open Peer Review Details 
Manuscript submitted on 25112013 
Original Manuscript 
Strangeness Production in DeepInelastic ep Scattering at HERA § 
1. INTRODUCTION
The measurement of strange particle production in high energy collisions provides valuable information for understanding Quantum Chromodynamics (QCD) in the perturbative and nonperturbative regime. In neutral current deepinelastic ep scattering (DIS) at HERA four different processes depicted in Fig. (1) contribute to strange hadron production.

Fig (1)
Schematic diagrams for the processes contributing to strangeness production in ep scattering: (a) direct production from the
strange sea, (b) BGF, (c) heavy hadron decays and (d) fragmentation. The diagrams relevant for K^{0} production are shown.

Strange quarks may originate directly from the strange sea of the proton (Fig. 1a), from bosongluonfusion (BGF, Fig. 1b), from the decays of heavy flavoured hadrons (Fig. 1c) and from the creation of ss¯
pairs in the nonperturbative fragmentation process (Fig. 1d). The latter process is the dominant source for strange hadron production. In the modelling of the fragmentation process the suppression of ss¯
pairs due to the mass of the strange quark is generally controlled by the strangeness suppression factor λ_{s} [1Sjöstrand T. Highenergy physics event generation with PYTHIA 5.7 and JETSET 7.4 Comput Phys Commun 1994; 82: 7489., 2(a) Sjöstrand T. The Lund Monte Carlo for jet fragmentation and E+ E Physics Jetset Version 6.2 Comput Phys Commun 1986; 39: 347407.; (b) Sjöstrand T, Bengtsson M. The Lund Monte Carlo for jet fragmentation and E+ E physics. Jetset Version 6.3: An Update Comput Phys Commun 1987; 43: 36779.; (c) Andersson B, Gustafson G, Ingelman G, Sjöstrand T. Parton fragmentation and string dynamics Phys Rept 1983; 97: 31145.]. Especially, the ratio of Ks0 to charged particles should strongly depends on this quark mass effect.
This paper presents new measurements of Ks0 production at low Q^{2} and Λ production at high Q^{2}. Results are presented on Ks0 and production cross sections, on the ratio of Ks0 production to charged particles production measured in the same phase space region, on Λ yields normalised to DIS cross sections, and on the Λ−Λ¯ asymmetry. The measurements are shown as a function of several observables characterising the DIS kinematics and the strange particles production dynamics in the laboratory frame. The results are compared with predictions obtained from leading order Monte Carlo calculations, based on matrix elements with parton shower simulation. The rôle of strangeness suppression on hadrons with strangeness is investigated.
2. SELECTION OF HADRON CANDIDATES
The data used in the analyses correspond to an integrated luminosity of 109pb^{1} in case of Ks0 production and 340p1 in case of Λ production and were collected with the H1 detector [3(a) Abt I, Ahmed T, Aid S, et al. [H1 Collaboration]. The H1 detector at HERA Nucl Instrum Meth A 1997; 386: 31047.; (b) Abt I, Ahmed T, Aid S et al. [H1 Collaboration]. The tracking, calorimeter and muon detectors of the H1 experiment at HERA Nucl Instrum Meth A 1997; 386: 34896.] in the years 2004 to 2007 when protons with an energy of 920 GeV collided with electrons^{1} with an energy of 27.6 GeV producing a centreofmass energy of s = 319 GeV. The kinematics of the scattering process at HERA are described using the Lorentz invariant variables Q^{2} denoting the square of the photon virtuality, the inelasticity in the proton rest frame y and Bjorken scaling variable x. At fixed s only two of these variables are independent because of Q^{2} = xys. The following kinematic ranges are used in the analyses: 7 < Q^{2} < 100GeV^{2} and 0.1 < y < 0.6 for the Ks0 and 145 < Q^{2} < 20000GeV^{2} and 0.2 < y < 0.6 for the Λ (see Table 1).
Table 1Phase space regions explored in the analyses of Ks0
and Λ production, respectively.
The Ks0 mesons and Λ baryons^{2} are measured by the kinematic reconstruction of their decays Ks0→π+π− and Λ→pπ−, respectively. The number of Ks0 mesons and Λ baryons is obtained by fitting the invariant mass spectra with the sum of a signal and background function. For the signal function the skewed tstudent function is used while the background shape is described by a threshold function with exponential damping. In total approximately 290000 Ks0 mesons and 7000 ΛΛ¯ baryons are reconstructed in the phase space given in Table1. The fitted Ks0and Λ masses agree with the world average [4Nakamura K. (Particle Data Group). Review of particle physics J Phys G 2010; 37: 075021.].
3. RESULTS AND DISCUSSION
3.1. Inclusive Cross Sections
The visible inclusive production cross sections measured in the kinematic region defined in Table1, are
σvisep→eKs0X=10.66±0.02stat.−8.5+9.4syst.nb,σvisep→eΛX=144.7±0.04stat.−8.5+9.4syst.pb,
using a strangeness suppression factor of λ_{s} the models RAPGAP [5Jung H. Hard diffractive scattering in highenergy e p collisions and the Monte Carlo generator RAPGAP Comp Phys Commun 1995; 86: 14761.] and DJANGOH [6Buchmüller W, Ingelman G, Schuler GA, Siesberger H. DJANGO Proceedings of physics at HERA DESY Hamburg 1992; 1419] predict Ks0 cross sections of 10.93 nb and 9.88 nb, respectively, in reasonable agreement with the measurement. The cross section predictions for Λ+Λ¯ production from the MEPS and CDM [7(a) Andersson B, et al. Coherence effects in deep inelastic scattering Z Phys C 1989; 43: 62532.; (b) Lönnblad L. Rapidity gaps and other final state properties in the colour dipole model for deep inelastic scattering. Z Phys C 1995; 65: 28591.]models are shown in Table2 for two values of the strangeness suppression parameter λ_{s}. The measured inclusive Λ+Λ¯ cross section is close to the CDM prediction with λ_{s} = 0.22 and λ_{s} = 0.286 to the MEPS prediction with .
Table 2Monte Carlo predictions for different settings of the
strangeness suppresion factor λ^{s}
3.3. Ratio of Ks0Production to Charged Particle Production
By normalising the Ks0 production cross section to the cross section of charged particle production many model dependent uncertainties, like the cross section dependence on proton PDFs, cancel thus enhancing the sensitivity to details of the fragmentation process. In Fig. (2c) the ratio of Ks0 production to the cross section charged particle procduction is shown as a function of η in comparison to the expectations from DJANGOH using three different values of λ_{s} ranging from 0.220 to 0.35. The ratio in η is well described by the model in shape and a high sensitivity on λ_{s} is observed in the absolute value of this ratio, demonstrating the clear potential of using this ratio for extracting the strangeness suppression factor λ_{s}.
3.4. Λ Production to DIS Cross Section Ratio
In Fig. (3b) the ratio ofproduction to DIS cross section is shown as a function of Q^{2} in comparison to the expectations from RAPGAP and DJANGOH both using λ_{s} = 0.286 and λ_{s} = 0.220. The DJANGOH prediction with λ_{s} = 0.286 yields the worst description of the data by overshooting them significantly independent of Q^{2}. For the same strangeness suppression factor also RAPGAP tends to yield ratios larger than observed in data for Q^{2} < GeV^{2}. The best description is provided by DJANGHO using λ_{s} = 0.220.
3.5. Λ+Λ¯ Asymmetries
The Λ+Λ¯ asymmetry is defined as:
1
AΛ=σvisep→eΛX−σvisep→eΛ¯Xσvisep→eΛX+σvisep→eΛ¯X.
This observable could shed light on the mechanism of baryon number transfer in ep scattering. A significant positive asymmetry would be an indication for the baryon number transfer from the proton to the Λ baryon. If present such an effect should be more pronounced in the positive region in the laboratory frame. For the kinemaic region defined in table 1 the asymmetry is measured to be
AΛ=0.002±0.022stat.±0.018syst..
In Fig. (3c) the asymmetry A_{Λ} is shown as a function of Q_{2}. The data show no evidence for a nonvanishing asymmetry in the phase space region investigated.
CONCLUSION
This paper presents a study of inclusive production of Ks0 and Λ in DIS at low Q_{2} and Λ high Q_{2} measured with the H1 detector at HERA. The cross sections of Ks0 and Λ production are measured as a function of the DIS kinematic variable Q_{2} and of strange hadron production variables in the laboratory frame. In addition results on the ratio of production cross section to the charged particle cross section, the Λ production to DIS cross section ratio and the Λ=Λ¯ asymmetry are presented. The measurements are compared to model predictions of DJANGOH, based on the colourdipol model (CDM) and RAPGAP based on DGLAP matrix element calculations supplemented with parton showers (MEPS). Within the uncertainties both models provide a reasonable description of the data. The sensitivity of the ratio of Ks0 to charged particle production cross sections on the strangeness suppression factor λ_{s} is demonstrated, however, a detailed understanding of concurrent processes of Ks0 production is mandatory prior to the determination of λ_{s}. The measured visible Λ cross section is found to be described best by the CDM using and the MEPS model using λ_{s} = 0.220 . When investigating theproduction to DIS cross section ratio the best agreement is observed for the CDM with λ_{s} = 0.220. The Λ=Λ¯ asymmetry is found to be consistent with zero.
CONFLICT OF INTEREST
The author confirms that this article content has no conflicts of interest.
NOTES
^{1 }In this paper "electron" is used to denote both electron and positron.
^{2 }Unless otherwise noted, charge conjugate states are always implied.
ACKNOWLEDGEMENS
Declared none.
REFERENCES
[1] 
Sjöstrand T. Highenergy physics event generation with PYTHIA 5.7 and JETSET 7.4 Comput Phys Commun 1994; 82: 7489. 
[2] 
(a) Sjöstrand T. The Lund Monte Carlo for jet fragmentation and E+ E Physics Jetset Version 6.2 Comput Phys Commun 1986; 39: 347407.; (b) Sjöstrand T, Bengtsson M. The Lund Monte Carlo for jet fragmentation and E+ E physics. Jetset Version 6.3: An Update Comput Phys Commun 1987; 43: 36779.; (c) Andersson B, Gustafson G, Ingelman G, Sjöstrand T. Parton fragmentation and string dynamics Phys Rept 1983; 97: 31145. 
[3] 
(a) Abt I, Ahmed T, Aid S, et al. [H1 Collaboration]. The H1 detector at HERA Nucl Instrum Meth A 1997; 386: 31047.; (b) Abt I, Ahmed T, Aid S et al. [H1 Collaboration]. The tracking, calorimeter and muon detectors of the H1 experiment at HERA Nucl Instrum Meth A 1997; 386: 34896. 
[4] 
Nakamura K. (Particle Data Group). Review of particle physics J Phys G 2010; 37: 075021. 
[5] 
Jung H. Hard diffractive scattering in highenergy e p collisions and the Monte Carlo generator RAPGAP Comp Phys Commun 1995; 86: 14761. 
[6] 
Buchmüller W, Ingelman G, Schuler GA, Siesberger H. DJANGO Proceedings of physics at HERA DESY Hamburg 1992; 1419 
[7] 
(a) Andersson B, et al. Coherence effects in deep inelastic scattering Z Phys C 1989; 43: 62532.; (b) Lönnblad L. Rapidity gaps and other final state properties in the colour dipole model for deep inelastic scattering. Z Phys C 1995; 65: 28591. 