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Measurement of the pion form factor at KLOE with photons at small angle
Nuclear Physics B (Proc. Suppl.) 162 (2006) 90–94 www.elsevierphysics.com
Measurement of the pion form factor at KLOE with photons at small angle KLOE collaboration∗ presented by Stefan E. M¨ uller a
a
& Federico Nguyen
b
Laboratori Nazionali di Frascati, Via Enrico Fermi 40, 00044 Frascati (RM), Italy
b
Universit` a Roma TRE, Dipartimento di Fisica e Sezione INFN, Via della Vasca Navale 84, 00146 Roma, Italy The KLOE experiment at the DAΦNE φ-factory in Frascati performs a measurement of the cross section e+ e− → π + π − using Initial State Radiation events, known as radiative return method. Here we present work in progress to improve the published result which used photons emitted at small polar angles without requiring their explicit detection (no photon tagging). In particular, a possibility to normalize the cross section using μμγ events is discussed. The knowledge of the μμγ cross section also allows to test the initial state radiator function.
1. Introduction The main source of uncertainty in the value of the muon magnetic anomaly predicted [1] in the Standard Model is from the hadronic contribution, ahlo μ , to the lowest order. This quantity is estimated with a dispersion integral of the hadronic cross section measurements. In particular, the pion form factor, Fπ , defined via σππ ≡ σe+ e− →π+ π− ∝ s−1 βπ3 (s)|Fπ (s)|2 , accounts for ∼ 70% of the central value and for ∼ 60% of the uncertainty in ahlo μ . Furthermore, Fπ estimated from the differen∗ F.
Ambrosino, A. Antonelli, M. Antonelli, C. Bacci, P. Beltrame, G. Bencivenni, S. Bertolucci, C. Bini, C. Bloise, S. Bocchetta, V. Bocci, F. Bossi, D. Bowring, P. Branchini, R. Caloi, P. Campana, G. Capon, T. Capussela, F. Ceradini, S. Chi, G. Chiefari, P. Ciambrone, S. Conetti, E. De Lucia, A. De Santis, P. De Simone, G. De Zorzi, S. Dell’Agnello, A. Denig, A. Di Domenico, C. Di Donato, S. Di Falco, B. Di Micco, A. Doria, M. Dreucci, G. Felici, A. Ferrari, M. L. Ferrer, G. Finocchiaro, S. Fiore, C. Forti, P. Franzini, C. Gatti, P. Gauzzi, S. Giovannella, E. Gorini, E. Graziani, M. Incagli, W. Kluge, V. Kulikov, F. Lacava, G. Lanfranchi, J. LeeFranzini, D. Leone, M. Martini, P. Massarotti, W. Mei, S. Meola, S. Miscetti, M. Moulson, S. M¨ uller, F. Murtas, M. Napolitano, F. Nguyen, M. Palutan, E. Pasqualucci, A. Passeri, V. Patera, F. Perfetto, L. Pontecorvo, M. Primavera, P. Santangelo, E. Santovetti, G. Saracino, B. Sciascia, A. Sciubba, F. Scuri, I. Sfiligoi, T. Spadaro, M. Testa, L. Tortora, P. Valente, B. Valeriani, G. Venanzoni, S. Veneziano, A. Ventura, R. Versaci, G. Xu.
0920-5632/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2006.09.069
tial rate in the decay τ − → π − π 0 ντ [2] differs by ∼ 15% from that measured in e+ e− → π + π − annihilations, above the ρ meson peak. DAΦNE √ is an e+ e− collider running from 1999 to 2005 at s = Mφ , the φ meson √ mass. In 2006 DAΦNE delivered collisions at s = 1 GeV. In
Year Ldt (pb √ s
−1
)
2001 2002 2004 2005
2006
170
240
320
730 1260
Mφ ∼ 1.0195 GeV
1 GeV
Table 1 Luminosity integrated by the KLOE experiment, from 2001 to 2006.
Table 1 the integrated luminosity collected by the KLOE experiment is shown. The KLOE detector consists of a drift chamber with excellent momentum (σp /p ∼ 0.4% for tracks with polar angles larger than 45◦ ) and vertex (σvtx ∼ 3 mm) resolution and an electromagnetic calorimeter with good energy ∼ 5.7%/ E [GeV]) and high time (σt ∼ (σE /E 57 ps/ E [GeV] ⊕ 100 ps) resolution.
KLOE collaboration / Nuclear Physics B (Proc. Suppl.) 162 (2006) 90–94
91
2. Extraction of Fπ At DAΦNE, Fπ is extracted measuring the differential cross section in the ππ invariant mass, s , from Initial State Radiation (ISR) events [3,4], e+ e− → π + π − γ: dσππγ = σππ (s ) H(s ) , (1) ds H(s ) being the radiator function. An independent way to extract σππ is to normalize bin-by-bin the measured ππγ differential cross section with the same for e+ e− → μ+ μ− γ events. Neglecting Final State Radiation effects we have: dσππγ /ds σππ = σμμ . (2) dσμμγ /ds s
In this way most of the theoretical uncertainties present in the published analysis [6] cancel out either partially (radiator function, 0.5%) or exactly (luminosity, 0.6% and vacuum polarization, 0.2%). From the experimental point of view also efficiencies related to track and vertex reconstruction are expected to partially compensate in the ratio of the differential spectra in eq.(2). From the statistical point of view, this case is less favoured because there are two sources of event counting, compared to eq.(1). 3. Fiducial volume Figure 1 shows the fiducial volume used in the present analysis. Events are selected with two and only two charged tracks with a polar angle 50◦ < θ < 130◦ , connected to a vertex of transverse radius ρ < 8 cm and |z| < 7 cm. The photon is not explicitly detected, so its direction is reconstructed from the tracks by closing the kinematics: p γ = −( pπ+ + p π− ). The requirement of a small angle photon, θγ < 15◦ , exhibits the following advantages: • the collinear divergence of ISR events allows high statistics; • Final State Radiation events are suppressed to ∼ 0.3% level; • the contamination from the resonant process e+ e− → φ → π + π − π 0 – where at least
Figure 1. Fiducial volume for the small angle photon (narrow cones) and for the the pion tracks (wide cones).
one of the 2 photons coming from the π 0 is lost – is reduced to the level of 5%. This untagged photon analysis, based on an integrated luminosity of 140 pb−1 of data taken in 2001, led to the published result [6]. However, this selection does not allow to cover the ππ threshold region, that requires a different selection, with different systematic effects [7]. 4. Improvements with respect to the published analysis The analysis of data taken since 2002 has many advantages: • cleaner and more stable running conditions of the machine result in less machine background as well as smaller fluctuations in the collision energy; • an additional trigger level was implemented at the end of 2001 to eliminate a 30% loss due to pions penetrating up to the outer calorimeter plane and being misidentified as
92
Evts/0.5 MeV
KLOE collaboration / Nuclear Physics B (Proc. Suppl.) 162 (2006) 90–94
10 5
10 4
10
3
80
100
120
140
160
180 200 mtrk (MeV)
Figure 2. Left: the correlation between mtrk and Mππ , for data and π + π − π 0 Monte Carlo events, and the curve used in the analysis to suppress the π + π − π 0 contamination. Data and Monte Carlo populations are not to scale. Right: definitions of the μμγ (filled dark histogram) and of the ππγ (filled light histogram) events, after the selection discussed in the previous sections. The residual contamination of π + π − π 0 events is evident at high mtrk values.
cosmic rays events; from 2002 on this inefficiency decreases down to 0.2% on ππγ events, also allowing an efficient selection of μμγ events; • the offline background filter, which contributed the largest experimental systematic uncertainty to the published work [6], has been improved and includes now a downscale algorithm providing an unbiased control sample. This greatly facilitates the evaluation of the filter efficiency which increased from 95% to 98.5%, with negligible systematic uncertainty; • in the event classification during the data reconstruction, the lower limit in the track mass variable 2 is moved from 90 MeV to 80 MeV to increase the selection efficiency for μμγ events, and a cut inthe missing mass variable 3 , mmiss = E 2 − |P X |2 X
is applied selecting events with mmiss <
2 Defined under the hypothesis that the final state consists of two charged tracks with the same mass, mtrk , and one photon. 3 Defined assuming that the process is e+ e− → π + π − X.
120 MeV, to provide a higher rejection of π + π − π 0 events with respect to the previous analysis; • assuming an overall efficiency of 50% for both ππγ and μμγ, the analysis of only 500 pb−1 of the available data sets is sufficient to achieve a statistical uncertainty of the order of 1% in the region above the ρ peak, using the ratio of the differential spectra as in eq.(2). In addition to the aforementioned items, much work has been spent to improve the knowledge of the detector response and of the KLOE simulation program [8]. 5. Pion and muon identification Discrimination of ππγ and μμγ from e+ e− → e e γ events is performed using a function [5] based on approximate likelihood estimators. They are based on the time of flight, and on the shape and the energy of the clusters associated to tracks. For instance, electrons deposit most of the energy in the first plane of the calorimeter while minimum ionizing muons and pions release + −
93
KLOE collaboration / Nuclear Physics B (Proc. Suppl.) 162 (2006) 90–94
Evts/0.01 GeV
2
of π + π − π 0 is shown, due to the curve in the left plot. 10 5
Data
6. Preliminary results
10 4
MC μμγ 10 3
MC eeγ 10
2
MC πππ 10
1
0.4
0.5
0.6
0.7
0.8
2
M
0.9 2 ππ (GeV )
almost the same energy in each plane. An event is selected if at least one of the two tracks is not identified as an electron. This criterion leads to a rejection power of 97% for eeγ events, while keeping a selection efficiency of 99% for ππγ and μμγ events. At this selection level, we define as μμγ and ππγ, respectively, those events with 80 MeV < mtrk < 115 MeV, or with mtrk > 130 MeV and below the curve shown in the left plot of Figure 2. In this plot, for illustrating purposes, π + π − π 0 Monte Carlo events are superimposed together with the curve used for rejecting this background 2 plane, Mππ being the process in the mtrk -Mππ ππ invariant mass. The right plot shows the regions in mtrk chosen to define the two signal processes, after the fiducial volume, event classification and eeγ rejection requirements, applied on 242.2 pb−1 of data taken in 2002. It consists of a clean subsample of the 320 pb−1 (see Table 1),√selected on the basis of more stable values for s and position of the interaction point and better trigger conditions. At high mtrk values, the further reduction
Evts/0.01 GeV
2
Figure 3. Number of ππγ events from 242.2 pb−1 of 2002 data, plus expected background contributions from μμγ, π + π − π 0 and eeγ.
Applying all the selection steps from the previous sections on the same 2002 data sample, the invariant mass spectra are shown in Figure 3 and Figure 4. 2 specIn more detail, Figure 3 shows the Mππ trum after the pion identification requirement, corresponding to 3.4 × 106 events, while Figure 4 2 spectrum after the muon identifishows the Mμμ cation requirement, corresponding to 8.7 × 105 events. Also shown in both plots are the expected residual background contributions from other processes, as evaluated from Monte Carlo. The μμγ and ππγ processes have been simulated using the PHOKHARA generator [3,9]; while
Data
10 4
10 3
MC ππγ
MC eeγ
10 2
MC πππ 10
1
0.4
0.5
0.6
0.7
0.8
2
M
0.9 2 (GeV )
μμ
Figure 4. Number of μμγ events from 242.2 pb−1 of 2002 data, plus expected background contributions from ππγ, π + π − π 0 and eeγ.
the eeγ process has been generated with the BABAYAGA code [10]. π + π − π 0 events are produced assuming [11] that the reaction is dominated by e+ e− → ρπ → π + π − π 0 , using a Breit-Wigner
94
KLOE collaboration / Nuclear Physics B (Proc. Suppl.) 162 (2006) 90–94
shape with a mass dependent width for the ρ resonance. Nevertheless, the largest remaining background in both cases is due to the leakage of ππγ events into the μμγ mtrk region and vice versa. Work is in progress to improve the performance of the particle ID algorithm to separate pions and muons. 7. Summary and Outlook In March 2006 KLOE finished the data taking campaign. The available statistics will allow to perform the selection of μμγ events with relative statistical accuracy better than 1%. The benefits amount to the possibility of: • extracting Fπ with minimum impact from systematic – either theoretical or experimental – uncertainties, • testing the radiator function with the measurement of the e+ e− → μ+ μ− γ differential cross section. From the experimental point of view, several improvements have been presented: more efficient trigger criteria and offline selection algorithms under better control. We are finalizing the estimate of the corrections and of the systematic uncertainties related to the 2002 data sample. Also from the theory side, further improvements are expected, e.g. more accurate Bhabha generators [12,13], reducing the fractional theoretical error down to 0.2% in the luminosity measurement. REFERENCES 1. M. Passera, these proceedings. 2. M. Davier, S. Eidelman, A. H¨ocker and Z. Zhang, Eur. Phys. J. C 31 (2003) 503. 3. H. Czy˙z, these proceedings. 4. A. Denig, these proceedings. 5. B. Valeriani, “Measurement of the pion form factor at KLOE via radiative return”, PhD Thesis, Universit¨ at Karlsruhe, IEKPKA/2005-14, 2005. 6. A. Aloisio et al. [KLOE Collaboration], Phys. Lett. B 606 (2005) 12. 7. D. Leone, these proceedings.
8. F. Ambrosino et al. [KLOE Collaboration], Nucl. Instrum. Meth. A 534 (2004) 403. 9. H. Czy˙z, A. Grzeli´ nska, J. H. K¨ uhn and G. Rodrigo, Eur. Phys. J. C 39 (2005) 411. 10. C. M. Carloni Calame et al., Nucl. Phys. B 584 (2000) 459. 11. A. Aloisio et al. [KLOE Collaboration], Phys. Lett. B 561 (2003) 55; [Erratum-ibid. B 609 (2005) 449]. 12. C. M. Carloni Calame, these proceedings. 13. G. Fedotovich, these proceedings.
Measurement of the pion form factor at KLOE with photons at small angle KLOE collaboration∗ presented by Stefan E. M¨ uller a
a
& Federico Nguyen
b
Laboratori Nazionali di Frascati, Via Enrico Fermi 40, 00044 Frascati (RM), Italy
b
Universit` a Roma TRE, Dipartimento di Fisica e Sezione INFN, Via della Vasca Navale 84, 00146 Roma, Italy The KLOE experiment at the DAΦNE φ-factory in Frascati performs a measurement of the cross section e+ e− → π + π − using Initial State Radiation events, known as radiative return method. Here we present work in progress to improve the published result which used photons emitted at small polar angles without requiring their explicit detection (no photon tagging). In particular, a possibility to normalize the cross section using μμγ events is discussed. The knowledge of the μμγ cross section also allows to test the initial state radiator function.
1. Introduction The main source of uncertainty in the value of the muon magnetic anomaly predicted [1] in the Standard Model is from the hadronic contribution, ahlo μ , to the lowest order. This quantity is estimated with a dispersion integral of the hadronic cross section measurements. In particular, the pion form factor, Fπ , defined via σππ ≡ σe+ e− →π+ π− ∝ s−1 βπ3 (s)|Fπ (s)|2 , accounts for ∼ 70% of the central value and for ∼ 60% of the uncertainty in ahlo μ . Furthermore, Fπ estimated from the differen∗ F.
Ambrosino, A. Antonelli, M. Antonelli, C. Bacci, P. Beltrame, G. Bencivenni, S. Bertolucci, C. Bini, C. Bloise, S. Bocchetta, V. Bocci, F. Bossi, D. Bowring, P. Branchini, R. Caloi, P. Campana, G. Capon, T. Capussela, F. Ceradini, S. Chi, G. Chiefari, P. Ciambrone, S. Conetti, E. De Lucia, A. De Santis, P. De Simone, G. De Zorzi, S. Dell’Agnello, A. Denig, A. Di Domenico, C. Di Donato, S. Di Falco, B. Di Micco, A. Doria, M. Dreucci, G. Felici, A. Ferrari, M. L. Ferrer, G. Finocchiaro, S. Fiore, C. Forti, P. Franzini, C. Gatti, P. Gauzzi, S. Giovannella, E. Gorini, E. Graziani, M. Incagli, W. Kluge, V. Kulikov, F. Lacava, G. Lanfranchi, J. LeeFranzini, D. Leone, M. Martini, P. Massarotti, W. Mei, S. Meola, S. Miscetti, M. Moulson, S. M¨ uller, F. Murtas, M. Napolitano, F. Nguyen, M. Palutan, E. Pasqualucci, A. Passeri, V. Patera, F. Perfetto, L. Pontecorvo, M. Primavera, P. Santangelo, E. Santovetti, G. Saracino, B. Sciascia, A. Sciubba, F. Scuri, I. Sfiligoi, T. Spadaro, M. Testa, L. Tortora, P. Valente, B. Valeriani, G. Venanzoni, S. Veneziano, A. Ventura, R. Versaci, G. Xu.
0920-5632/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2006.09.069
tial rate in the decay τ − → π − π 0 ντ [2] differs by ∼ 15% from that measured in e+ e− → π + π − annihilations, above the ρ meson peak. DAΦNE √ is an e+ e− collider running from 1999 to 2005 at s = Mφ , the φ meson √ mass. In 2006 DAΦNE delivered collisions at s = 1 GeV. In
Year Ldt (pb √ s
−1
)
2001 2002 2004 2005
2006
170
240
320
730 1260
Mφ ∼ 1.0195 GeV
1 GeV
Table 1 Luminosity integrated by the KLOE experiment, from 2001 to 2006.
Table 1 the integrated luminosity collected by the KLOE experiment is shown. The KLOE detector consists of a drift chamber with excellent momentum (σp /p ∼ 0.4% for tracks with polar angles larger than 45◦ ) and vertex (σvtx ∼ 3 mm) resolution and an electromagnetic calorimeter with good energy ∼ 5.7%/ E [GeV]) and high time (σt ∼ (σE /E 57 ps/ E [GeV] ⊕ 100 ps) resolution.
KLOE collaboration / Nuclear Physics B (Proc. Suppl.) 162 (2006) 90–94
91
2. Extraction of Fπ At DAΦNE, Fπ is extracted measuring the differential cross section in the ππ invariant mass, s , from Initial State Radiation (ISR) events [3,4], e+ e− → π + π − γ: dσππγ = σππ (s ) H(s ) , (1) ds H(s ) being the radiator function. An independent way to extract σππ is to normalize bin-by-bin the measured ππγ differential cross section with the same for e+ e− → μ+ μ− γ events. Neglecting Final State Radiation effects we have: dσππγ /ds σππ = σμμ . (2) dσμμγ /ds s
In this way most of the theoretical uncertainties present in the published analysis [6] cancel out either partially (radiator function, 0.5%) or exactly (luminosity, 0.6% and vacuum polarization, 0.2%). From the experimental point of view also efficiencies related to track and vertex reconstruction are expected to partially compensate in the ratio of the differential spectra in eq.(2). From the statistical point of view, this case is less favoured because there are two sources of event counting, compared to eq.(1). 3. Fiducial volume Figure 1 shows the fiducial volume used in the present analysis. Events are selected with two and only two charged tracks with a polar angle 50◦ < θ < 130◦ , connected to a vertex of transverse radius ρ < 8 cm and |z| < 7 cm. The photon is not explicitly detected, so its direction is reconstructed from the tracks by closing the kinematics: p γ = −( pπ+ + p π− ). The requirement of a small angle photon, θγ < 15◦ , exhibits the following advantages: • the collinear divergence of ISR events allows high statistics; • Final State Radiation events are suppressed to ∼ 0.3% level; • the contamination from the resonant process e+ e− → φ → π + π − π 0 – where at least
Figure 1. Fiducial volume for the small angle photon (narrow cones) and for the the pion tracks (wide cones).
one of the 2 photons coming from the π 0 is lost – is reduced to the level of 5%. This untagged photon analysis, based on an integrated luminosity of 140 pb−1 of data taken in 2001, led to the published result [6]. However, this selection does not allow to cover the ππ threshold region, that requires a different selection, with different systematic effects [7]. 4. Improvements with respect to the published analysis The analysis of data taken since 2002 has many advantages: • cleaner and more stable running conditions of the machine result in less machine background as well as smaller fluctuations in the collision energy; • an additional trigger level was implemented at the end of 2001 to eliminate a 30% loss due to pions penetrating up to the outer calorimeter plane and being misidentified as
92
Evts/0.5 MeV
KLOE collaboration / Nuclear Physics B (Proc. Suppl.) 162 (2006) 90–94
10 5
10 4
10
3
80
100
120
140
160
180 200 mtrk (MeV)
Figure 2. Left: the correlation between mtrk and Mππ , for data and π + π − π 0 Monte Carlo events, and the curve used in the analysis to suppress the π + π − π 0 contamination. Data and Monte Carlo populations are not to scale. Right: definitions of the μμγ (filled dark histogram) and of the ππγ (filled light histogram) events, after the selection discussed in the previous sections. The residual contamination of π + π − π 0 events is evident at high mtrk values.
cosmic rays events; from 2002 on this inefficiency decreases down to 0.2% on ππγ events, also allowing an efficient selection of μμγ events; • the offline background filter, which contributed the largest experimental systematic uncertainty to the published work [6], has been improved and includes now a downscale algorithm providing an unbiased control sample. This greatly facilitates the evaluation of the filter efficiency which increased from 95% to 98.5%, with negligible systematic uncertainty; • in the event classification during the data reconstruction, the lower limit in the track mass variable 2 is moved from 90 MeV to 80 MeV to increase the selection efficiency for μμγ events, and a cut inthe missing mass variable 3 , mmiss = E 2 − |P X |2 X
is applied selecting events with mmiss <
2 Defined under the hypothesis that the final state consists of two charged tracks with the same mass, mtrk , and one photon. 3 Defined assuming that the process is e+ e− → π + π − X.
120 MeV, to provide a higher rejection of π + π − π 0 events with respect to the previous analysis; • assuming an overall efficiency of 50% for both ππγ and μμγ, the analysis of only 500 pb−1 of the available data sets is sufficient to achieve a statistical uncertainty of the order of 1% in the region above the ρ peak, using the ratio of the differential spectra as in eq.(2). In addition to the aforementioned items, much work has been spent to improve the knowledge of the detector response and of the KLOE simulation program [8]. 5. Pion and muon identification Discrimination of ππγ and μμγ from e+ e− → e e γ events is performed using a function [5] based on approximate likelihood estimators. They are based on the time of flight, and on the shape and the energy of the clusters associated to tracks. For instance, electrons deposit most of the energy in the first plane of the calorimeter while minimum ionizing muons and pions release + −
93
KLOE collaboration / Nuclear Physics B (Proc. Suppl.) 162 (2006) 90–94
Evts/0.01 GeV
2
of π + π − π 0 is shown, due to the curve in the left plot. 10 5
Data
6. Preliminary results
10 4
MC μμγ 10 3
MC eeγ 10
2
MC πππ 10
1
0.4
0.5
0.6
0.7
0.8
2
M
0.9 2 ππ (GeV )
almost the same energy in each plane. An event is selected if at least one of the two tracks is not identified as an electron. This criterion leads to a rejection power of 97% for eeγ events, while keeping a selection efficiency of 99% for ππγ and μμγ events. At this selection level, we define as μμγ and ππγ, respectively, those events with 80 MeV < mtrk < 115 MeV, or with mtrk > 130 MeV and below the curve shown in the left plot of Figure 2. In this plot, for illustrating purposes, π + π − π 0 Monte Carlo events are superimposed together with the curve used for rejecting this background 2 plane, Mππ being the process in the mtrk -Mππ ππ invariant mass. The right plot shows the regions in mtrk chosen to define the two signal processes, after the fiducial volume, event classification and eeγ rejection requirements, applied on 242.2 pb−1 of data taken in 2002. It consists of a clean subsample of the 320 pb−1 (see Table 1),√selected on the basis of more stable values for s and position of the interaction point and better trigger conditions. At high mtrk values, the further reduction
Evts/0.01 GeV
2
Figure 3. Number of ππγ events from 242.2 pb−1 of 2002 data, plus expected background contributions from μμγ, π + π − π 0 and eeγ.
Applying all the selection steps from the previous sections on the same 2002 data sample, the invariant mass spectra are shown in Figure 3 and Figure 4. 2 specIn more detail, Figure 3 shows the Mππ trum after the pion identification requirement, corresponding to 3.4 × 106 events, while Figure 4 2 spectrum after the muon identifishows the Mμμ cation requirement, corresponding to 8.7 × 105 events. Also shown in both plots are the expected residual background contributions from other processes, as evaluated from Monte Carlo. The μμγ and ππγ processes have been simulated using the PHOKHARA generator [3,9]; while
Data
10 4
10 3
MC ππγ
MC eeγ
10 2
MC πππ 10
1
0.4
0.5
0.6
0.7
0.8
2
M
0.9 2 (GeV )
μμ
Figure 4. Number of μμγ events from 242.2 pb−1 of 2002 data, plus expected background contributions from ππγ, π + π − π 0 and eeγ.
the eeγ process has been generated with the BABAYAGA code [10]. π + π − π 0 events are produced assuming [11] that the reaction is dominated by e+ e− → ρπ → π + π − π 0 , using a Breit-Wigner
94
KLOE collaboration / Nuclear Physics B (Proc. Suppl.) 162 (2006) 90–94
shape with a mass dependent width for the ρ resonance. Nevertheless, the largest remaining background in both cases is due to the leakage of ππγ events into the μμγ mtrk region and vice versa. Work is in progress to improve the performance of the particle ID algorithm to separate pions and muons. 7. Summary and Outlook In March 2006 KLOE finished the data taking campaign. The available statistics will allow to perform the selection of μμγ events with relative statistical accuracy better than 1%. The benefits amount to the possibility of: • extracting Fπ with minimum impact from systematic – either theoretical or experimental – uncertainties, • testing the radiator function with the measurement of the e+ e− → μ+ μ− γ differential cross section. From the experimental point of view, several improvements have been presented: more efficient trigger criteria and offline selection algorithms under better control. We are finalizing the estimate of the corrections and of the systematic uncertainties related to the 2002 data sample. Also from the theory side, further improvements are expected, e.g. more accurate Bhabha generators [12,13], reducing the fractional theoretical error down to 0.2% in the luminosity measurement. REFERENCES 1. M. Passera, these proceedings. 2. M. Davier, S. Eidelman, A. H¨ocker and Z. Zhang, Eur. Phys. J. C 31 (2003) 503. 3. H. Czy˙z, these proceedings. 4. A. Denig, these proceedings. 5. B. Valeriani, “Measurement of the pion form factor at KLOE via radiative return”, PhD Thesis, Universit¨ at Karlsruhe, IEKPKA/2005-14, 2005. 6. A. Aloisio et al. [KLOE Collaboration], Phys. Lett. B 606 (2005) 12. 7. D. Leone, these proceedings.
8. F. Ambrosino et al. [KLOE Collaboration], Nucl. Instrum. Meth. A 534 (2004) 403. 9. H. Czy˙z, A. Grzeli´ nska, J. H. K¨ uhn and G. Rodrigo, Eur. Phys. J. C 39 (2005) 411. 10. C. M. Carloni Calame et al., Nucl. Phys. B 584 (2000) 459. 11. A. Aloisio et al. [KLOE Collaboration], Phys. Lett. B 561 (2003) 55; [Erratum-ibid. B 609 (2005) 449]. 12. C. M. Carloni Calame, these proceedings. 13. G. Fedotovich, these proceedings.