Abstract (data abstract)

CERN-LHC. The yields of strange (${\rm K}^{0}_{S}$, $\Lambda$, $\bar{\Lambda}$) and multi-strange ($\Xi^{-}$, $\bar{\Xi}^{+}$, $\Omega^{-}$, $\bar{\Omega}^{+}$) hadrons are measured at midrapidity in proton-proton (pp) collisions at $\sqrt{s}$ = 7 TeV as a function of the charged-particle multiplicity density (${\rm d}N_{\rm ch}/{\rm d}\eta$).The production rate of strange particles increases faster than that of non-strange hadrons, leading to an enhancement of strange particles relative to pions, similar to that found in nucleus-nucleus collisions as well as in proton-nucleus collisions at the LHC. This is the first observation of an enhanced production of strange particles in high-multiplicity pp collisions. The magnitude of this strangeness enhancement increases with the event activity, quantified by ${\rm d}N_{\rm ch}/{\rm d}\eta$, and with hadron strangeness. It reaches almost a factor of two for the $\Omega$ at the highest multiplicity presented. No enhancement is observed for particles with no strange quark content, demonstrating that the observed effect is strangeness rather than mass related. The results are not reproduced by any of the Monte Carlo models commonly used at the LHC, suggesting that further developments are needed for a complete microscopic understanding of strangeness production and indicating the presence of a phenomenon novel in high-multiplicity pp collisions.

Table 1

Data from Figure 1

10.17182/hepdata.77284.v1/t1

$K^{0}_{S}$ transverse momentum spectrum: V0M Class I (pp at $\sqrt{s}=7$ TeV).
Table 2

Data from Figure 1

10.17182/hepdata.77284.v1/t2

$K^{0}_{S}$ transverse momentum spectrum: V0M Class II (pp at $\sqrt{s}=7$ TeV).
Table 3

Data from Figure 1

10.17182/hepdata.77284.v1/t3

$K^{0}_{S}$ transverse momentum spectrum: V0M Class III (pp at $\sqrt{s}=7$ TeV).
Table 4

Data from Figure 1

10.17182/hepdata.77284.v1/t4

$K^{0}_{S}$ transverse momentum spectrum: V0M Class IV (pp at $\sqrt{s}=7$ TeV).
Table 5

Data from Figure 1

10.17182/hepdata.77284.v1/t5

$K^{0}_{S}$ transverse momentum spectrum: V0M Class V (pp at $\sqrt{s}=7$ TeV).
Table 6

Data from Figure 1

10.17182/hepdata.77284.v1/t6

$K^{0}_{S}$ transverse momentum spectrum: V0M Class VI (pp at $\sqrt{s}=7$ TeV).
Table 7

Data from Figure 1

10.17182/hepdata.77284.v1/t7

$K^{0}_{S}$ transverse momentum spectrum: V0M Class VII (pp at $\sqrt{s}=7$ TeV).
Table 8

Data from Figure 1

10.17182/hepdata.77284.v1/t8

$K^{0}_{S}$ transverse momentum spectrum: V0M Class VIII (pp at $\sqrt{s}=7$ TeV).
Table 9

Data from Figure 1

10.17182/hepdata.77284.v1/t9

$K^{0}_{S}$ transverse momentum spectrum: V0M Class IX (pp at $\sqrt{s}=7$ TeV).
Table 10

Data from Figure 1

10.17182/hepdata.77284.v1/t10

$K^{0}_{S}$ transverse momentum spectrum: V0M Class X (pp at $\sqrt{s}=7$ TeV).
Table 11

Data from Figure 1

10.17182/hepdata.77284.v1/t11

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class I (pp at $\sqrt{s}=7$ TeV).
Table 12

Data from Figure 1

10.17182/hepdata.77284.v1/t12

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class II (pp at $\sqrt{s}=7$ TeV).
Table 13

Data from Figure 1

10.17182/hepdata.77284.v1/t13

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class III (pp at $\sqrt{s}=7$ TeV).
Table 14

Data from Figure 1

10.17182/hepdata.77284.v1/t14

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class IV (pp at $\sqrt{s}=7$ TeV).
Table 15

Data from Figure 1

10.17182/hepdata.77284.v1/t15

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class V (pp at $\sqrt{s}=7$ TeV).
Table 16

Data from Figure 1

10.17182/hepdata.77284.v1/t16

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class VI (pp at $\sqrt{s}=7$ TeV).
Table 17

Data from Figure 1

10.17182/hepdata.77284.v1/t17

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class VII (pp at $\sqrt{s}=7$ TeV).
Table 18

Data from Figure 1

10.17182/hepdata.77284.v1/t18

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class VIII (pp at $\sqrt{s}=7$ TeV).
Table 19

Data from Figure 1

10.17182/hepdata.77284.v1/t19

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class IX (pp at $\sqrt{s}=7$ TeV).
Table 20

Data from Figure 1

10.17182/hepdata.77284.v1/t20

$\Lambda+\bar{\Lambda}$ transverse momentum spectrum: V0M Class X (pp at $\sqrt{s}=7$ TeV).
Table 21

Data from Figure 1

10.17182/hepdata.77284.v1/t21

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class I (pp at $\sqrt{s}=7$ TeV).
Table 22

Data from Figure 1

10.17182/hepdata.77284.v1/t22

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class II (pp at $\sqrt{s}=7$ TeV).
Table 23

Data from Figure 1

10.17182/hepdata.77284.v1/t23

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class III (pp at $\sqrt{s}=7$ TeV).
Table 24

Data from Figure 1

10.17182/hepdata.77284.v1/t24

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class IV (pp at $\sqrt{s}=7$ TeV).
Table 25

Data from Figure 1

10.17182/hepdata.77284.v1/t25

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class V (pp at $\sqrt{s}=7$ TeV).
Table 26

Data from Figure 1

10.17182/hepdata.77284.v1/t26

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class VI (pp at $\sqrt{s}=7$ TeV).
Table 27

Data from Figure 1

10.17182/hepdata.77284.v1/t27

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class VII (pp at $\sqrt{s}=7$ TeV).
Table 28

Data from Figure 1

10.17182/hepdata.77284.v1/t28

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class VIII (pp at $\sqrt{s}=7$ TeV).
Table 29

Data from Figure 1

10.17182/hepdata.77284.v1/t29

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class IX (pp at $\sqrt{s}=7$ TeV).
Table 30

Data from Figure 1

10.17182/hepdata.77284.v1/t30

$\Xi^{-}+\bar{\Xi}^{+}$ transverse momentum spectrum: V0M Class X (pp at $\sqrt{s}=7$ TeV).
Table 31

Data from Figure 1

10.17182/hepdata.77284.v1/t31

$\Omega^{-}+\bar{\Omega}^{+}$ transverse momentum spectrum: V0M Class I+II (pp at $\sqrt{s}=7$ TeV).
Table 32

Data from Figure 1

10.17182/hepdata.77284.v1/t32

$\Omega^{-}+\bar{\Omega}^{+}$ transverse momentum spectrum: V0M Class III+IV (pp at $\sqrt{s}=7$ TeV).
Table 33

Data from Figure 1

10.17182/hepdata.77284.v1/t33

$\Omega^{-}+\bar{\Omega}^{+}$ transverse momentum spectrum: V0M Class V+VI (pp at $\sqrt{s}=7$ TeV).
Table 34

Data from Figure 1

10.17182/hepdata.77284.v1/t34

$\Omega^{-}+\bar{\Omega}^{+}$ transverse momentum spectrum: V0M Class VII+VIII (pp at $\sqrt{s}=7$ TeV).
Table 35

Data from Figure 1

10.17182/hepdata.77284.v1/t35

$\Omega^{-}+\bar{\Omega}^{+}$ transverse momentum spectrum: V0M Class IX+X (pp at $\sqrt{s}=7$ TeV).
Table 36

Data from Figure 2

10.17182/hepdata.77284.v1/t36

$2K^{0}_{S}/(\pi^{+}+\pi^{-})$ ratio for pp collisions at $\sqrt{s}=7$ TeV as a function of the average charged particle multiplicity density at midrapidity.
Table 37

Data from Figure 2

10.17182/hepdata.77284.v1/t37

$(\Lambda+\bar{\Lambda})/(\pi^{+}+\pi^{-})$ ratio for pp collisions at $\sqrt{s}=7$ TeV as a function of the average charged particle multiplicity density at midrapidity.
Table 38

Data from Figure 2

10.17182/hepdata.77284.v1/t38

$(\Xi^{-}+\bar{\Xi}^{+})/(\pi^{+}+\pi^{-})$ ratio for pp collisions at $\sqrt{s}=7$ TeV as a function of the average charged particle multiplicity density at midrapidity.
Table 39

Data from Figure 2

10.17182/hepdata.77284.v1/t39

$(\Omega^{-}+\bar{\Omega}^{+})/(\pi^{+}+\pi^{-})$ ratio for pp collisions at $\sqrt{s}=7$ TeV as a function of the average charged particle multiplicity density at midrapidity.
Table 40

Data from Figure 2

10.17182/hepdata.77284.v1/t40

$\pi^{+}+\pi^{-}$ integrated yields, pp at $\sqrt{s}=7$ TeV (V0M Multiplicity Classes).
Table 41

Data from Figure 2

10.17182/hepdata.77284.v1/t41

$p+\bar{p}$ integrated yields, pp at $\sqrt{s}=7$ TeV (V0M Multiplicity Classes).
Table 42

Data from Figure 2

10.17182/hepdata.77284.v1/t42

$K^{0}_{S}$ integrated yields, pp at $\sqrt{s}=7$ TeV (V0M Multiplicity Classes).
Table 43

Data from Figure 2

10.17182/hepdata.77284.v1/t43

$\Lambda+\bar{\Lambda}$ integrated yields, pp at $\sqrt{s}=7$ TeV (V0M Multiplicity Classes).
Table 44

Data from Figure 2

10.17182/hepdata.77284.v1/t44

$\Xi^{-}+\bar{\Xi}^{+}$ integrated yields, pp at $\sqrt{s}=7$ TeV (V0M Multiplicity Classes).
Table 45

Data from Figure 2

10.17182/hepdata.77284.v1/t45

$\Omega^{-}+\bar{\Omega}^{+}$ integrated yields, pp at $\sqrt{s}=7$ TeV (V0M Multiplicity Classes).
Table 46

Data from Figure 3

10.17182/hepdata.77284.v1/t46

$(\Lambda+\bar{\Lambda})/2K^{0}_{S}$ ratio for pp collisions at $\sqrt{s}=7$ TeV as a function of charged particle multiplicity density at midrapidity.
Table 47

Data from Figure 3

10.17182/hepdata.77284.v1/t47

$(p+\bar{p})/(\pi^{+}+\pi^{-})$ ratio for pp collisions at $\sqrt{s}=7$ TeV as a function of charged particle multiplicity density at midrapidity.
Table 48

Data from Figure 4

10.17182/hepdata.77284.v1/t48

$(2K^{0}_{S})/(\pi^{+}+\pi^{-})$ double-ratio vs multiplicity for pp collisions at $\sqrt{s}=7$ TeV.
Table 49

Data from Figure 4

10.17182/hepdata.77284.v1/t49

$(\Lambda+\bar{\Lambda})/(\pi^{+}+\pi^{-})$ double-ratio vs multiplicity for pp collisions at $\sqrt{s}=7$ TeV.
Table 50

Data from Figure 4

10.17182/hepdata.77284.v1/t50

$(\Xi^{-}+\bar{\Xi}^{+})/(\pi^{+}+\pi^{-})$ double-ratio vs multiplicity for pp collisions at $\sqrt{s}=7$ TeV.
Table 51

Data from Figure 4

10.17182/hepdata.77284.v1/t51

$(\Omega^{-}+\bar{\Omega}^{+})/(\pi^{+}+\pi^{-})$ double-ratio vs multiplicity for pp collisions at $\sqrt{s}=7$ TeV.
Table 52

Data from Table 1

10.17182/hepdata.77284.v1/t52

Event multiplicity classes, their corresponding fraction of the INEL>0 cross-section and their corresponding average charged particle density at midrapidity ($|\eta|<0.5$)....

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