New results from the NA49 experiment on hadron production in p+p and p+C interactions and survey of backward hadrons in p+C collisions

Recent results on proton, anti-proton, neutron and charged kaon production in proton-proton and proton, anti-proton, neutron, deuteron and triton production in proton-carbon collisions at 158 GeV/c beam momentum are presented. Data samples of 4.8 million and 385 734 inelastic events in p+p and p+C, respectively, are obtained with the NA49 detector at the CERN SPS accelerator. The charged particles are identified by energy loss measurement in a system of four TPC chambers, while the neutrons are detected in a forward hadronic calorimeter. The data cover a major fraction of the phase space, ranging from 0 to 1.9 GeV in pT and in Feynman x variable from -0.8 to 0.95 for protons, from -0.2 to 0.3 for anti-protons, from 0.1 to 0.95 for neutrons and from 0 to 0.5 for kaons. The comparison of the results on proton and neutron production in p+p interactions and deep inelastic e+p collisions at HERA reveals an independence of target fragmentation on the projectile type. Using the charged kaon data in p+p collisions as a reference, a new evaluation of the energy dependence of kaon production, including neutral kaons, is conducted over a range from 3 GeV to p+p− collider energies. A survey of backward production of protons and pions in p+C collisions in the range of lab angles from 10 to 180 degrees, from 0.2 to 1.2 GeV/c in lab momentum and from 1 to 400 GeV/c in projectile momentum has been performed.


Introduction
In the absence of quantitative prediction in the nonperturbative sector of QCD, the study of soft hadronic physics has to rely on self-consistent high statistics data sets, including a variety of projectile and target combinations. In addition full phase space coverage with complete particle identification and small systematic uncertainty is required.
The NA49 experiment aims at providing such data sets ranging from elementary hadron-proton collisions to hadron-nucleus and nucleus-nucleus interactions, obtained using the same detector layout combining wide acceptance, systematic uncertainty of less than 5% and complete particle identification via measurement of specific energy loss. It is therefore well suited for the comparison of the different processes and to a detailed scrutiny of the evolution from elementary to nuclear hadronic phenomena and thus providing a basis for a model independent study to the underlying production mechanisms.
In the framework of this extensive experimental program the NA49 experiment has already published papers concerning pion [1], baryon [2] and kaon [3] production in p+p collisions, and pion [4,5] production in p+C collisions. Two new articles have been submitted for publication regarding baryon and light ion production [6] and a survey of backward production of protons and pions from 1 to 400 GeV/c beam momentum [7] in p+C interactions. a e-mail: makariev@inrne.bas.bg In this paper the recent results from [2,3,6,7] will be discussed.

Data sets
The data sets obtained by the NA49 experiment are summarized in Fig. 1.

Variables and cross section definition
The NA49 experiment establishes the invariant double differential cross sections Here p T is the transverse momentum and x F is the reduced longitudinal momentum: will be presented, with σ inel being total inelastic cross section.

Baryons
It total 333 cross sections for protons and 143 for antiprotons have been derived, with systematic errors of 2.5% for protons and 3.3% for anti-protons, respectively, see [2]. The data cover the range in x F from -0.05 to 0.95 (-0.05 to 0.4) and in p T from 0 to 1.9 GeV/c (0 to 1.5 GeV/c) for protons (anti-protons). An example of x F distributions of protons at fixed p T is shown in Fig. 1.  As the NA49 detector acceptance is limited at low p T and large x F the results from other measurements have been used to supplement the NA49 data, for a detailed discussion see [2]. The dense coverage allows for precise determination of integrated quantities which are also available in [2].
The p T integrated density distribution dn/dx F for neutrons is presented in Fig. 2 as a function of x F . The K 0 L and n distributions, which have been subtracted from the total measured yields, are also shown in the figure.

Comparison to e+p collisions at HERA
Data from the ZEUS collaboration at HERA concerning proton [8] and neutron [9] production in the target hemisphere provide results at mean energies of about 130 GeV in the photon-proton cms. These data allow for a rather detailed comparison of the e+p and p+p interactions. The ZEUS data cover a range from 0.1-0.7 GeV/c in p T and from 0.6 to 0.99 in x F for protons [8], and from 0.05 to 0.6 GeV/c in p T and from 0.26 to 0.97 in x F for neutrons [9]. The comparison of the shape of the p T distributions with the proton results from NA49 (full lines), normalized to the ZEUS data, is shown in Figs  . . neutrons from [9]. The data were successively divided by 3 for different x F values for better separation. The dashed lines represent the parametrization used in [9] This comparison reveals that the shape of the p T distributions is similar between the protons and neutrons as well as between the different interactions. Also the absolute proton and neutron densities have been shown to be equal in e+p and p+p collisions [2], corresponding to hadronic factorization.

Kaons
The results on kaon production consist of 158 data points for each K + and K − . They cover a range from 0 to 1.7 GeV/c in p T and from 0 to 0.4 (0.5) in x F for K + (K − ) with a systematic uncertainty of 2.2% at x F < 0.25 and up to 10% for K + and 6% for K − at larger x F . As in the case of baryons the integrated quantities have been established with high precision.
As the interaction energy of √ s = 17.2 GeV is located at a strategical point between threshold-dominated and scaling phenomena at lower and higher energies, respectively, a new and complete study of the s-dependence of kaon production including K 0 S has been attempted using the new NA49 data as a reference. This study covers the energy range 3 < √ s < 1800 GeV and aims at establishing an internally consistent picture of kaon production. The √ s dependence of the total kaon yields n K is presented in Fig. 5.
This new evaluation should replace the data from [10] which show an unphysical behaviour of charged kaon yield [3]. These results also prove the validity of the relation K 0 S = 0.5(K + + K − ) for √ s > 5 GeV.

Baryons
The data from p+C collisions cover a phase space area ranging from 0 to 1.9 GeV/c in transverse momentum and from -0.80 to 0.95 for protons and from -0.2 to 0.3 for anti-protons in x F [6]. This results in 491 and 121 cross s. The full line through the K 0 S is the result of an eyeball fit, the lines through the K + and K − data are derived from [3]. The full circles in the K 0 S data correspond to 0.5( n K + + n K − ) established at the corresponding √ s values section values for protons and anti-protons, respectively. Existing data [11], which are complementary to the NA49 measurements, in the far backward hemisphere are used to extend the coverage for protons into the region of intranuclear cascading, see [6]. The corresponding proton x F distributions, which cover the phase space from x F =-2 up to the kinematic limit at x F ∼ +1, are shown in Fig. 6. Both the NA49 data (full circles) and [11] (open circles) are plotted. Several interesting features should be mentioned: • The kinematic limit extends far below x F = -1.
• There is no indication of diffractive peak at x F ∼ -1.
• The maximum of the distributions in backward region is located at x F ∼ -0.92 rather than x F = -1.
The p T integrated dn/dx F distributions for protons and neutrons are presented in Fig. 7 in the projectile hemisphere together with the neutron results from p+p collisions (dotted lines). Protons and neutrons show within errors the same transfer towards the center when passing from p+p to p+C reaction. The densities are equal at x F ∼ 0.3. Densities from p+C are bigger at lower x F , reaching increase of about 1.3 at x F = 0.1, and smaller at larger x F decreasing to 0.6 of the p+p densities at x F towards +1. The latter value corresponds to the expected fraction of single projectile collisions in p+C interactions derived from the nuclear density distribution and from the inelastic cross sections [5].

Deuterons and tritons
Deuterons and tritons can be identified via specific energy loss measurement by the NA49 detector at lab momenta below 2 GeV/c for deuterons and 3 GeV for tritons. The density ratio of deuterons to protons R d and tritons to protons R t are shown in Fig. 5.2 as a function of x F , where the transformation from lab to cms system is performed assuming the proton mass. In the figure are also presented the results from [12] (open symbols) which show good agreement as in the case of the proton data [11]. The ratio R t is suppressed with respect to R d by a factor of 0.1 to 0.2 depending on p T but independent on x F . These ratios are for the first time available at low p T and in the approach to x F = 0, where R d decreases to values of about 1%.

Three component mechanism
The final state of p+A collisions consists of three basic components: • The fragmentation of the projectile particle • The fragmentation of the target nucleons • The intra-nuclear cascading, generated by the interaction of the participating nucleons and secondary produced hadrons inside the nucleus.  Figure 8. Deuteron and triton to proton density ratios R d and R t as a function of x F for fixed values of p T between 0.1 and 0.9 GeV/c. The full lines give the two-dimensional data interpolation established for R d , the broken lines are the same multiplied by the suppression factors, assumed x F independent, see [6] These components have been separated and quantified for pion production [5,7] and baryon production [6] in p+C collisions. An example of the net proton p T integrated dn/dx F distribution is shown in Fig. 9 where the total yield and the three components are indicated with different lines.
It has to be noted that this separation relies essentially on experimental input. There is only one assumption made concerning the target component and no other free parameters are used in the analysis. The target component is predicted from p+p collisions, assuming that the hadronization of the hit target nucleus is equivalent to the hadronization of the elementary nucleon-nucleon interactions, taking into account the isospin symmetry and multiplying by the mean number of collisions ν that the projectile suf- fers on its way through the target nucleus, see [6]. For the p+C interaction ν is equal to 1.6, see [4]. The projectile and nuclear components follow, without additional assumptions, from the subtraction of the target fragmentation from the measured total yield.

Survey of backward production of protons and pions in p+C interactions
In Sect. 5.1, Fig. 6, it was shown that backward proton production is peaked at around x F = -0.92 and not at x F = -1.
One mechanism for this baryon transfer can be pion production in intra-nuclear cascading [2]. To study this phenomenon a survey of backward production of protons and pions in p+C collisions has been performed [7]. This survey covers the region of Θ lab from 10 to 180 degrees, p lab from 0.2 to 1.2 GeV/c and beam momentum p beam from 1 to 400 GeV/c. About 3500 data points coming from 19 experiments have been localized [7]. As these experiments use in general different variables and binning, all results were transformed to invariant cross sections (1) and expressed in the three variables Θ lab , p lab and 1/ √ s. A three dimensional interpolation in these three variables was performed which uses the physics constraints of smoothness, continuity and regularity at the phase space boundaries, for details see [7]. An example of π − distributions as a function of 1/ √ s at fixed p lab and Θ lab = 67 degrees together with the global data interpolation is presented in Fig. 10.
The data interpolation establishes a strong constraint for possible experimental deviations. In fact 4 of the 19 data sets have been shown to be inconsistent with all other results.

Conclusions
New inclusive data on kaon and baryon production in p+p interactions and baryon and light ion production in p+C collisions at 158 GeV/c beam momentum have been presented. These results are part of the systematic study of soft hadronic interactions by the NA49 experiment, combining wide phase space coverage and systematic errors below 5%. The comparison of the results on proton and neutron production in p+p interactions and deep inelastic e+p collisions at HERA reveals hadronic factorization in the target fragmentation region.
New evaluation of kaon yields in p+p collisions has been established in the range 3 < √ s < 1800 GeV where the relation K 0 S = 0.5(K + + K − ) is shown to be valid for √ s > 5 GeV. A survey of backward production of protons and pions in the range of lab angles from 10 to 180 degrees, from 0.2 to 1.2 GeV/c in lab momentum and from 1 to 400 GeV/c in projectile momentum has been performed. Over 3500 data points from 19 experiments have been used in order to establish a consistent description of proton and pion production in the backward hemisphere.