E n erg y d e p e n d e n c e o f h ad ron s p e c tr a an d y ie ld s in p + p an d 7B e + 9B e c o llisio n s from th e N A 6 1 /S H I N E e x p e r im e n t at th e C E R N S P S
D .T . L arsen for th e N A 6 1 /S H I N E C o lla b o ra tio n
U n iw e rsy te t Jag iello ń sk i, Ł o jasiew icza 11, 30-348 K ra k ó w , P o la n d E -m ail: d la r s e n @ c e r n .c h
A b s t r a c t . T h e N A 6 1 /S H IN E p ro g ra m m e o n s tro n g in te ra c tio n s covers th e s tu d y o f th e o n se t o f d e co n fin em en t a n d aim s to discover th e c ritic a l p o in t o f s tro n g ly in te ra c tin g m a t te r by p e rfo rm in g a n e n erg y a n d sy stem -size sc a n over th e full C E R N S P S m o m e n tu m ran g e. So fa r th e sc a n s o f p + p , 7B e + 9B e, as w ell as A r+ S c h ave b e e n c o m p le te d . R e su lts fro m p + p a n d B e + B e collisions are now e m erg in g , in p a r tic u la r th e e n erg y d e p e n d e n c e o f h a d ro n s p e c tr a a n d yields. S ta tu s a n d p re lim in a ry re s u lts fro m th is effort w ill b e p re se n te d .
1. In tro d u ctio n
NA61/SHINE [1] is a fixed target experiment at the CERN SPS. Tracking is provided by five tim e projection chambers. Additionally, particle identification is aided by time-of-flight detectors. A m odular calorimeter is used to determ ine the collision centrality for nucleus-nucleus collisions. B oth prim ary and secondary beams are available to the experiment, allowing data taking w ith projectile sizes ranging from proton to lead, as well as with pions and kaons. Besides studying strong interactions, the experiment also performs precise hadron production reference measurements for neutrino (Fermilab and T2K) and cosmic-ray (KASCADE and Pierre Auger Observatory) physics.
2. A n a ly sis o f id en tified h ad ron sp ectra
More th an 90% of prim ary negative particles produced in inelastic interactions at SPS energies are n- . Thus, the n- spectra may be obtained by subtracting the small non-pion contribution from the overall spectra for negatively charged hadrons. This non-pion contribution is taken from the EPOS model [2]. Since particle identification is not required, a very large phase-space acceptance is obtained. In addition to the h - method [3], d E / d x and time-of-flight may be used for proper identification. The d E /d x method is based on the particle energy loss in the TPC gas, while the particle mass can be identified using the time-of-flight information. The acceptances for these methods are shown in Figure 1 for p + p interactions at 158 GeV/c. The results are corrected for detector inefficiencies, feed-down from weak decays and secondary interactions, contribution from non-target interactions, as well as trigger and event selection biases.
Content from this w ork m ay be used under the term s o f the Creative Com m ons A ttribution 3.0 licence. Any further distribution
15th International Conference on Strangeness in Quark Matter (SQM2015) IOP Publishing Journal of Physics: Conference Series 668 (2016) 012020 doi:10.1088/1742-6596/668/1/012020
F igu re 1. Acceptance for various d ata analysis methods. Blue, solid area: h - ; m agenta lines: d E / d x ; yellow lines: time-of-flight.
F ig u re 2. Transverse mass spectra at mid-rapidity.
Left: NA61/SHINE inelastic p + p interactions at 158 GeV/c; right: NA49 central P b + P b collisions at 158A GeV/c.
3. p + p resu lts
The results presented below were obtained for identified hadrons produced in inelastic p + p interactions at 20, 31, 40, 80 and 158 G eV /c [4].
Figure 2 shows spectra of transverse mass of negatively and positively charged n, K, p and A produced in inelastic p + p interactions at mid-rapidity. Corresponding NA49 measurements [5, 6, 7] for central P b + P b collisions are also shown. The d ata was fitted using a blast wave model param etrisation [8] dy = Ajm TK 1 ( mT T°s h Io ( PT sTnhp) . The param eter p is related to the transverse flow velocity f T by p = ta n h -1 f T. One finds th a t fix is significantly smaller in p + p th an P b + P b collisions. While the spectra are approximately exponential in p + p reactions, this exponential dependence is modified in P b + P b interactions by the transverse flow.
Figure 3 shows rapidity spectra of n - produced in inelastic p + p interactions (left), the energy dependence of the rms w idth a of those spectra divided by beam rapidity (centre) and divided by predictions (aLS) from the hydro-dynamical model [9, 10] (right). Although the shapes of the rapidity spectra are approxim ately Gaussian, the best fit was obtained using a sum of two Gaussians. It can be seen in Figure 3 (centre) th a t a / y beam for n- decreases with increasing collision energy. While a / a LS and a / y beam are smaller in p + p th an P b + P b collisions, no qualitative difference is observed for their energy dependence.
The “kink” , “horn” , and “step” [6, 14] are im portant signals for the onset of deconfinement observed in central P b + P b interactions. Figure 4 shows th a t the energy dependence of mean n multiplicity increases slower in p + p th an in P b + P b collisions ( “kink”). Hence, the two dependences cross each other at around 40A GeV/c.
The new measurements of NA61/SHINE significantly improve the world d ata for the inverse-slope param eter T of m T spectra of Kaons [16] and for yields of mesons at m id-rapidity [17, 18]. The m T spectra were fitted to the exponential function dd^ndy =
t2++nTKt exp ^ — Vt+m+ m . S is the yield integral and m K is the K mass. Figure 5 shows the result of the fit to the spectra of K+ and K - at mid-rapidity.
Figure 6 shows the energy dependence of T for K+ and K - . Surprisingly, the results from inelastic p + p interactions exhibit rapid changes similar to those observed in central P b + P b
F igu re 3. Left: n- rapidity spectra in inelastic p + p interactions. Centre: energy dependence of the scaled w idth of rapidity spectra. Right: energy dependence of the ratio of the measured w idth to th a t predicted by the hydro-dynamical model [9, 10]. World d ata from Refs. [11, 12, 13].
Not corrected for isospin effects.
F igu re 4. Energy depen
dence of mean n multiplicity per wounded nucleon in inelas
tic p + p interactions and cen
tral P b + P b (A u+A u) collisions.
World d ata from Refs. [12, 15].
F igu re 5. Transverse momen
tu m spectra of K mesons at mid
rapidity in inelastic p + p inter
actions at CERN SPS energies.
F igu re 6. Energy dependence of the inverse slope param eter T of transverse mass spectra of K+ and K - in inelastic p + p and central P b + P b /A u + A u in
teractions. World d ata from Refs. [16, 19, 20, 21].
collisions ( “step”).
While the h - method [3] can be used to measure spectra for n- , a similar approach cannot be used for n+ because of the large K and p contam ination. Instead, the ratio of the measured n+
15th International Conference on Strangeness in Quark Matter (SQM2015) IOP Publishing Journal of Physics: Conference Series 668 (2016) 012020 doi:10.1088/1742-6596/668/1/012020
F igu re 7. Left: energy depen
dency of n + /n - ratio in inelastic p + p interactions compared with EPOS predictions. Right: cor
rection factors used to obtain n+
yields from n - yields.
F igu re 8. Energy dependence of the K + /n + and K - / n - ra
tios in inelastic p + p and cen
tral P b + P b and A u+ A u in
teractions. World d ata from Refs. [15, 17, 18, 21, 22].
F igu re 9. Energy dependence of the K + /n + and K - / n - ratios in inelastic p + p interactions compared [23] with theoretical models EPO S [2], UrQMD [24, 25], P y th ia 8 [26] and HSD [27].
and n- yields was calculated within the acceptance of the time-of-flight an d /o r d E / d x methods, and compared with model predictions. As can be seen in Figure 7 (left), the agreement between the measurements and the EPOS model [2] is b etter th an 0.1%. Finally, the mid-rapidity yield was calculated as the product of the measured n- yield at y = 0, the measured n + /n - ratio within the time-of-flight and d E /d x acceptance, and the EPOS correction factor Cm c. This is shown in Figure 7 (right): n+(y = 0) = n - (y = 0)n+ / n - (to f — d E / d x ) C MC, where Cm c
M C
=[n+Zn-a 1 -
L ( tof )J m c 5%.Figure 8 shows the energy dependence of the K + /n + and K - / n - ratios observed in inelastic p + p interactions, as well as world d ata for central P b + P b and A u+A u interactions as reference.
Again surprisingly, the d ata suggests th a t even inelastic p + p interactions exhibit a step structure in the energy dependence of the K + /n + ratio, which may be considered a precursor of the “horn”
found in central P b + P b collisions.
Furtherm ore, the measured ratios were compared to theoretical models, as shown in Figure 9.
It is evident th a t none of the models adequately describes the data.
Figure 10 shows for protons from inelastic p + p interactions the spectra of pT (left) and
(
mT}
at m id-rapidity in inelastic p + p interactions. Right: comparison of (m y) — mp calculated using the spectra from the left plot with predictions of models [23].
F igu re 11. Inelastic cross section of 7B e+ 9Be collisions as well as to tal cross section for p + p interactions as a function of beam momentum.
The curve shows the prediction of the G lauber model.
(right) as a function of collision energy. N either the m agnitude nor the energy dependence of (m y) is reproduced by th e UrQMD or th e HSD model.
4. B e + B e resu lts
Previously, the inelastic cross section for 7B e+ 9Be was only measured at 1.45A G eV /c [28].
The new measurements of NA61/SHINE now extend this to 13A, 20A and 30A G eV /c [2 9].
A scintillator counter placed behind th e target measures th e square of the charge of a particle passing through it. Inelastic interactions were selected by requiring a signal below th a t expected for an intact beam nucleus. D ata was also taken with the target removed to be able to subtract the background caused by beam interactions with detector material. The interaction probability is given by Pi n t = PiI—]p R, where P j and Pr are th e interaction probabilities when th e target is inserted and removed, respectively. Using Pi n t, th e cross section can be calculated from
°i n e l = p L ef f N A / AP int, L e // = ^a b si 1 — e L/Aabs) and Xa bs = . Na , p L and A are the Avogadro constant, the target density, length and atomic number, respectively. Figure 11 shows th a t the new measurements are in agreement w ith the previous measurement as well as the G lauber model [30] prediction.
Prelim inary results on spectra were obtained for 7B e+ 9Be interactions at beam mom enta of 20A, 30A, 40A, 75A and 150A G eV /c in th e centrality classes 0-5%, 5-10%, 10-15% and 15
20% [31]. Unless otherwise stated, only statistical errors are shown. The centrality was derived from the energy deposited in the forward calorimeter PSD. Figure 12 shows the relationship between deposited energy and the centrality classes. Due to the m odularity of this detector, it is possible to change th e acceptance during off-line analysis. A smaller acceptance will cause some of th e projectile spectators to be lost, but at th e same tim e reduces the contribution from particles produced in the collision. Thus, the result will depend on the selected acceptance. This effect is largest for low beam momenta. For example for the to tal n- multiplicity at 20A GeV/c, the spread for different acceptances is up to 5%.
15th International Conference on Strangeness in Quark Matter (SQM2015) IOP Publishing Journal of Physics: Conference Series 668 (2016) 012020 doi:10.1088/1742-6596/668/1/012020
F ig u re 12. Left: distribu
tion of forward energy E p mea
sured in the PSD calorime
ter in 7B e+ 9Be collisions at 150A GeV/c. Right: definition of forward energy event classes.
F ig u re 13. Transverse mass spectra of n- mesons for 7B e+ 9Be, p + p and P b + P b interactions. Left: 20A GeV/c;
right: 150 A G eV /c beam momentum.
Figure 13 shows for 7B e + 9Be n- collisions the m T spectra for the different centrality classes for the beam mom enta 20A and 150A G eV/c, as well as for inelastic p + p and central P b + P b interactions. The p + p d ata is described very well by an exponential function, while both 7B e+ 9Be and P b + P b spectra deviate from this function at low and high values of m T.
Calculating normalised n- m T spectra as the ratios (7B e+ 9B e)/(p + p ) and (P b + P b )/(p + p ) facilitates the comparison of the shapes of the spectra produced in these different size systems.
Figure 15 shows qualitatively similar behaviour for both 7B e+ 9Be and P b + P b reactions. The high-mT regions of both 7B e+ 9Be and P b + P b exhibit an increase of the ratio, while for the interm ediate regions a decrease is seen. This effect is stronger for central P b + P b collisions.
Usually, this is a ttrib u ted to collective flow. Also, for 7B e+ 9Be reactions the increase of the ratio at high values of m T appears to be larger at higher beam momenta. This may suggest an increase of the m agnitude of collective effects in 7B e+ 9Be collisions w ith increasing beam momentum. The low-mT regions of both 7B e+ 9Be and P b + P b reactions exhibit an increase of the ratio. Possible explanations include isospin asym m etry of p + p d ata or electromagnetic effects.
Figure 15 shows the n- rapidity spectra from 7B e+ 9Be collisions in the different centrality classes for beam m om enta 20A and 150A G eV/c, as well as for inelastic p + p reactions. The d ata were fitted to two Gaussians symmetrically displaced w ith respect to mid-rapidity. Although their widths are the same, the am plitudes are different due to the asym m etry of 7B e+ 9Be collisions. By extending the fit range into the backward rapidity region, it was possible to obtain a stable fit.
The rms w idth a y of the spectra can be obtained from the fitted function. Figure 16 (left) shows the a y/ybeam for 7B e+ 9Be, as well as inelastic p + p and central P b + P b interactions. The widths of the spectra for all systems decrease monotonically w ith respect to collision energy.
However, the widths of the spectra do not behave monotonically with respect to the system
F igu re 14. R atio of normalised transverse mass spectra of n- mesons for different system sizes. Left: (7B e + 9B e)/(p + p );
right: (P b + P b )/(p + p ).
F igu re 15. Rapidity spectra of n- mesons for 7B e+ 9Be and inelastic p + p interactions. Left:
20A GeV/c; right: 150A GeV/c.
F igu re 16. Left: energy de
pendence of scaled w idth of 7B e + 9Be n- rapidity spectra;
right: effect of isospin asymme
try in p + p interactions. World d ata from Refs. [7].
size for a given collision energy. The w idth of the spectra from P b + P b lies between those of p + p and 7B e + 9Be collisions. W hen comparing the rapidity spectra from different systems, one must consider th a t p + p has larger isospin asym m etry th an 7B e+ 9Be and P b + P b . This isospin asym m etry can be accounted for by calculating the average spectra for n- and n+. The only large-acceptance d a ta available for production in p + p in the relevant energy domain is from NA49 at 158 G eV /c [32]. Unfortunately, this is also the energy with the smallest differences for the rapidity widths of the different system sizes. Still, Figure 16 (right) shows for 158 G eV /c beam mom entum th a t by taking into account the different isospin asymmetries, the dependence of the rapidity w idth with respect to system size becomes monotonic.
5. C on clu sion
The analysis of inelastic p + p and 7B e + 9Be interactions at CERN SPS energies showed many interesting effects. In particular, the p + p d ata exhibited step-like structures in the energy region where the onset of deconfinement was found in central P b + P b collisions. Also, many of the measurements could not be explained well by theoretical models. For 7B e+ 9Be reactions
15th International Conference on Strangeness in Quark Matter (SQM2015) IOP Publishing Journal of Physics: Conference Series 668 (2016) 012020 doi:10.1088/1742-6596/668/1/012020
new results on the inelastic cross section were obtained at several energies. Transverse mass and rapidity spectra were measured in the SPS energy range for three centrality classes. An indication of a transverse flow effect was found at the highest beam momenta.
A ck n o w led g em en ts
Supported by the N ational Science Centre of Poland (grants U M O -2014/13/N /ST2/02565, U M O -2014/12/T/ST2/00692, U M O -2013/11/N /ST2/03879 and UMO-2012/04/m/sT2/00816).
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