Σ(1385) production in proton-proton collisions at s =7 TeV

Transcription

1 Σ(1385) production in proton-proton collisions at s =7 TeV Enrico Fragiacomo, Massimo Venaruzzo, Giacomo Contin, Ramona Lea July 16, Introduction Objective of this note is to support the Σ(1385) analysis of data from proton-proton collisions at s =7 TeV. Software Data analysis processed on the grid with Root v b and AliRoot v an. Made use of the AliRsnMiniPackage framework available in AliRoot. The analysis has been performed on ESDs. Data sample Periods: LHC10b: runs , , , , , , , , , , , , , , , , , , , , , , , , , , LHC10c: runs , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , LHC10d: runs , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , LHC10d1, LHC10d2, LHC10f6a Pythia Perugia-0 anchor runs for efficiency. LHC12a4. For the study of the Λ(1520) contamination, the dedicated enhanced-signal Monte Carlo simulation LHC12a4 was used (3.45x10 6 accepted events) Statistics 211 million accepted events (170 million for the anchor runs) 1

2 Detectors involved ITS, TPC - main ALICE tracking detectors, TPC for de/dx References List of latest Indico presentations on the Σ(1385) analysis 02/07/2012: 15/06/2012: 04/06/2012: 16/04/2012: 2 Decay channels All four Σ(1385) species ( Σ +,Σ, Σ, Σ + ) have been analyzed separately. The main decay channel (branching ratio 88%) Σ Λ + π was used. In particular, 1. Σ + Λ + π + 2. Σ Λ + π 3. Σ Λ + π 4. Σ + Λ + π + Note that channels 2. and 4. are the same as Ξ Λ + π and Ξ + Λ + π +. A residual peak at around GeV/c 2 from the Ξ is indeed present in the invariant mass distributions of Σ and its antiparticle Σ +. No topological reconstruction is possible for the Σ(1385) due to its strong decay. The signal peak has to be extracted from the invariant mass distribution of the candidates Λ and π pairs. Accurate candidates selection is mandatory to reduce the combinatorial background under the signal peak. For each particle specie, the analysis has been repeated for 11 bins of the transverse momentum p T : , , , , , , , , , , GeV/c. A window of y < 0.8 for the candidates Λ π pairs has been selected at midrapidity. The procedure for the signal extraction will be discussed for the Σ +. The same procedure was used for its antiparticle Σ. For Σ and Σ +, the peak from the Ξ has been fitted with a Gaussian. Examples of invariant mass distributions and signal distributions after the extraction procedure described in section 4 are shown in figures 1 and 2. 3 Candidates selection See tables 1 and 2. 2

3 Figure 1: Examples of invariant mass distribution, on the left for the Σ + and on the right for the Σ, and event-mixed background for the Λπ candidate pairs. Figure 2: Examples of signal distribution, on the left for the Σ + and on the right for the Σ, after the extraction procedure described in section 4. Table 1: Track selection criteria Selection Value Number of TPC crossed rows > 70 TPC Refit flag ktrue Kink daughters kfalse χ 2 per cluster < 4 SPD clusters 1 DCA z < 2 cm DCA r < 7 σ DCA (p T ), σ DCA (p T ) = ( GeV/c p 1 T ) cm p T > 0.15 GeV/c Track η < Signal extraction 4.1 Invariant mass distribution Figure 3 shows, for the Σ + and for all p T -bins, the invariant mass distribution (in black) for the Λπ candidate pairs. Event mixing (EM) was used to calculate the combinatorial background. Λ s from a good i-th event have been paired with π s from up to five 3

4 Table 2: Selection criteria for the Λ Selection Value DCA r Λ to PV <0.5 DCA r π to PV <0.05 DCA Λ Daughters <0.5 Λ Cosine of Pointing Angle >0.99 subsequent good events, with characteristics similar to the i-th event (tracks multiplicity difference not greater than 10 and vertexz difference not greater than 1 cm). The eventmixing background is shown in red in figure 3. Figure 3: Invariant mass distribution and event-mixed background for the Λπ candidate pairs. 4.2 Normalization of the EM distribution The EM distribution contains by definition more entries than the same-event invariant mass distribution and needs therefore to be normalized to it. Problems arise since the EM distribution has a different shape with respect to the same-event invariant mass distribution not only in the signal region but also in an extended region on both sides of the signal peak. Only in the rightmost part of the invariant mass window both distributions have the same shape, providing a criterium (a region) were the EM distribution 4

5 can be normalized to the invariant mass distribution. This criterium holds for the first nine p T -bins but fails for the last two ones due to the limited invariant mass window. For the latter the very last part of the invariant mass window was used as normalization region. This is seen in figure 3 and in table 3 which summarizes the normalization region for each p T -bin and the normalization constants. Not surprisingly, the normal- Table 3: Event mixing normalization p T -bin (GeV/c) Range (GeV/c 2 ) Constant ± ± ± ± ± ± ± ± ± ± ±0.20 ization constants are compatible with 5 (the number of mixed events) for all the p T -bins but the last two ones. For each p T -bin different normalization regions have been tried out to check the effect on the final results (see section 7 where the contributions to the systematic errors are discussed). 4.3 Residual background Figure 4 shows the invariant mass distribution after the subtraction of the normalized EM background. The distribution is fitted with a constant straight line in the EM normalization region as a cross check. Table 4 shows the results of the straight line fit. The different shape of the invariant mass distribution and the EM background around the signal region produce a residual background after the EM subtraction. The residual background takes contributions from two different effects: correlated Λ-π pairs from the kinematics of the event. The extend of this residual background is p T -dependent and is reproduced in the Monte Carlo. (see section 4.4). The four lower p T -bins are mostly unaffected by this contribution. correlated Λ-π pairs from the Λ Λππ channel. (see section 4.5). This contribution affects all p T -bins with comparable intensity. 4.4 Fitting the residual background from Monte Carlo Figure 5 shows the invariant mass distribution for Monte Carlo (MC) simulated data after the EM-background subtraction (EM-background also from MC data) and after 5

6 Figure 4: Invariant mass distribution after the subtraction of the normalized EM background. The distribution is fitted with a constant straight line in the EM normalization region as a cross check. Table 4 shows the results of the fit. Table 4: Constant straight line fit values in the EM normalization region. p T -bin (GeV/c) χ 2 /ndf p /64 14± /59 10± /54 0± /49-5± /44-5± /39-8± /24-10± /19-5± /5-11± /11-5± /2 2±11 the further subtraction of the MC-true signal. The MC residual background is fitted with a polynomial of third order in the region from 1.26 GeV/c 2 (just left of the signal region) and the lower edge of the normalization region (see table 3). The χ 2 probability is as low as 4% for the p T -bin but not less than 20% for all the other p T - bins. A second order polynomial could not reproduce well the shape of the residual background in all the region considered, and higher orders are producing an overfit of 6

7 Figure 5: Residual background from Monte Carlo data. The distribution is fitted with a third order polynomial the data. Slight changes in the fitting regions have been tried out to check the effect on the final results (see section 7 where the contributions to the systematic errors are discussed). The fitting polynomial from the MC residual background in figure 5 is scaled (one parameter) to fit the residual background from real data in figure 4. The fit is performed in the region from 1.46 (just right of the signal region) and the lower edge of the normalization region. Figure 6 shows the polynomial fit to the residual background. The polynomial is extrapolated in the signal region providing a model-guided way to describe the background below the signal. Note that, as already noticed, the residual background is almost absent in the lower p T -bins. 4.5 Contamination from the Λ(1520) Figure 7 shows the invariant mass after the residual background subtraction. The MCtrue invariant mass distribution for Λπ pairs coming from the Λ Λππ decay has been calculated for events from the Λ(1520) dedicated enhanced-signal Monte Carlo simulation and fitted with a Gaussian (see figure 8). The mean and the σ of the Gaussian have then been used to constrain the Gaussian peak (mean and σ) in the combined fit (Gaussian plus Breit-Wigner) of the distribution in figure 7. Table 5 summarizes the results of the combined fit. Both the Gaussian means and the widths are in agreement within errors with the MC corresponding values. The Gaussian parameters (mean and width) have also been kept fixed and the difference accounted for as systematic uncertainty (see paragraph 7). Masses and widths from the Breit-Wigner function in the combined fit are presented in figures 9 and 10. Values for the Σ are also shown in the figures. Finally, the signal functions have been integrated to extract the raw yields. The 7

8 Figure 6: The polynomial from the fit of the residual background in MC data (figure 6) is scaled to fit the EM-background-subtracted invariant mass distribution from figure 4. Only one parameter is used. Table 5: Results of the combined fit (Gaussian for the Λ -peak, BW for the signal). Statistical errors only. p T (GeV/c) χ 2 /ndf Prob (%) BW mean (GeV/c 2 ) BW Γ (GeV/c 2 ) GA mean (GeV/c 2 ) GA σ (GeV/c 2 ) / ± ± ± ± / ± ± ± ± / ± ± ± ± / ± ± ± ± / ± ± ± ± / ± ± ± ± / ± ± ± ± / ± ± ± ± / ± ± ± ± / ± ± ± ± / ± ± ± ± 0.01 statistical errors in the signal distributions have been propagated through the integrals to provide the statistical uncertainties on the raw yields. Table 6 summarizes the raw yields for the Σ +, together with their relative statistical uncertainties. 8

9 Figure 7: The invariant mass distribution after the subtraction of the residual background. The peak from the Λ(1520) is contaminating the distribution in the lefthand side of the signal. 5 Efficiency computation In order to extract the absolute yields, the raw yields (N RAW ) were corrected for the decay branching ratio (0.88) and for losses due to in-flight decays, geometrical acceptance, and detector efficiency (N cor = N RAW /ɛbr, where BR indicates the decay branching ratio). The global efficiencies ɛ Σ were determined from Monte Carlo simulations with PYTHIA 6.4 (tune Perugia0) event generator and with a GEANT3-based simulation of the ALICE detector response. 6 Normalization Finally, corrections for the trigger efficiency and the vertex requirement were applied in order to obtain the absolute yields per inelastic collision: d 2 N = Ncor (p T ) 1 ɛ trigger (1) dydp T y p T ɛ vert N MB here N cor is the number of reconstructed Σ, N MB is the total number of minimum bias triggers, ɛ trigger = % 3.5% is the trigger selection efficiency for inelastic collisions and ɛ vert = is the vertex efficiency. 9

10 Figure 8: MC-true invariant mass distribution for Λπ pairs coming from the Λ Λππ decay. The peaks are fitted with a Gaussian. Figure 9: Masses as function of p T (left for the Σ + and right for the Σ ). 10

11 Figure 10: Widths as function of p T (left for the Σ + and right for the Σ ). Table 6: Raw yields for the Σ +, together with their relative statistical uncertainties. p T (GeV/c) Raw yield ( 10 4 ) Stat error (%) Table 7: Efficiency corrections as a function of p T for all four Σ species. The errors are statistical. p T correction Σ + correction Σ correction Σ correction Σ ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

12 7 Systematic uncertainties The investigated systematic uncertainties are listed below together with their maximum value. Table 8 shows a summary of the systematic uncertainties. Table 8: Sources of systematic uncertainties and their relative contribution (minimum and maximum values as a function of p T ) to the yields and to the spectrum fit parameters Source Yields dn/dy Tracks 3% Topological selections 2%-7% TPC de/dx selection for PID 2%-6% Signal extraction: EM normalization 2%-5% Signal extraction: Residual background 3%-10% Signal extraction: Λ contamination peak 1%-10% Material budget 4% Geant3/Fluka correction 1% +7% Normalization 3.5% Spectrum fit function 15% Total 10%-18% 15% Tracks. From other analysis, an uncertainty of 3% has been taken for the π. Topological selections. Each selection cut (DCA Λ to PV, DCA π to PV, DCA Λ daughters, Λ Cosine of Pointing Angle, m Λ ) has been varied by ±10% around the standard value. The overall uncertainty ranges from 2 to 7%. TPC de/dx selection for PID. The cut has been varied from the default 3 to 2.5 or 4.5 and the maximum seen deviation was taken to be the systematic uncertainty due to this selection. Up to 6%. Signal extraction. procedure Contributions to the systematic errors from the signal extraction Event mixing normalization region. Several regions chosen. Less than 5%. f inal checks ongoing Range of the polynomial fit to the MC residual background. Varied by ±10%. Less than 3%. require further check Fit to real data of the residual background extracted from MC data. Up to 10%. Fit of the Λ contamination peak from MC data to real data. The Gaussian peak from MC data was used either to set the mean and the width of the combined (Gaussian+Breit-Wigner) fit or to fix them. The difference accounted for as systematic uncertainty. Up to 10%. 12

13 Material Budget. From previous analysis the material budget uncertanties are conservatively estimated at 4% constant throughout p T. Uncertainty due to the Geant3/Fluka correction. From the Λ analysis an uncertainty due to the Geant3/Fluka correction of 1% is taken. Normalization Uncertanties. The uncertainty on the normalization is provided internally by the collaboration and is estimated to be asymmetrical, +7% and 3.5% acting on all yield values. Spectrum fit function. The extrapolated part of the spectrum has been fitted using different fitting functions and an uncertainty of 15% was found (see figure 14). Summary of the Uncertainties Figure 11 summarizes all the uncertainties considered for the preliminary spectra except the normalization uncertainty, taken to be asymmetrical. Figure 11: Statistical and systematic uncertainties in the yields for the Σ +. Note that the normalization uncertainty, being asymmetric, was not included in these plots. 13

14 8 Physics results Figures 12 and 13 show the corrected spectrum fitted with a Lévy-Tsallis function. The overall uncertainties (sum in quadrature of statistical and systematical errors) is shown. In order to separate the two contributions to the total uncertainty in the fit parameters, the fit has been performed also with the statistical errors only. Figure 15 shows the mean p T versus particle mass. The <p T > for the Σ + from this analysis has been inserted. Figures 16 and 17 show the ratios of Σ and Ξ, π and K for ALICE and STAR, and compare the ratios with a theoretical calculation from F. Becattini et al. The theoretical values for the Ξ, the π and the K has been extracted from Predictions of hadron abundances in pp collisions at the LHC, F. Becattini, P. Castorina, A. Milov and H. Satz, J. Phys. G38 (2011) The model is a statistical hadronization model with T = 170 MeV and γ S = 0.6. The values for the Σ has been provided by the author as private communication and was obtained with the same model. Note that the figure 17 is a derived preliminary and the final one with separate syst and stat will be presented later in the week. The STAR value for the Σ was published in Phys Rev Lett 97, (2006). The ALICE value for the Ξ was published in Phys. Lett. B 712 (2012) , the one for STAR in Phys. Rev. C 75, (2007). Finally, the STAR values for pions and kaons were extracted from Phys. Rev. C 71, (2005) and ref. therein. Figure 12: Corrected spectrum for both Σ + and Σ with Lévy-Tsallis fit. Statistical and systematical uncertainties are summed in quadrature. 14

15 Figure 13: Corrected spectrum for both Σ and Σ + with Lévy-Tsallis fit. Statistical and systematical uncertainties are summed in quadrature. Figure 14: Computation of extrapolation systematics by using an alternative m t - exponential to represent the spectrum. 15

16 Figure 15: Mean p T versus particle mass Figure 16: Ratio Σ /Ξ 16

17 Figure 17: Ratio Σ /Ξ, π and K. Note that the figure is a derived preliminary, the final one with separate syst and stat will be presented later in the week. 17