- Email: [email protected]
Recent FOCUS results on charm mixing and CP violation
ELSEVIER
Nuclear Physxs
B (Proc. Suppl.) 93 (2001) 272-275
Recent FOCUS results on charm mixing and CP violation Doris Yangsoo Kim’* aLoomis Lab of Physics, University of Illinois, 1110 W. Green St., Urbana, IL. 61801, U.S. We eummarise recent results on CP violation and mixing based on a high etatimtica photo-produced charm sample collected by the FOCUS experiment. We compare the lifetimes of two neutral D meson decayr: Do -t K-d and Do + K-K+. A mixing parameter for the neutral D system (yip) ie obtained under the suumption that K-r+ is an equal mixture of CP odd and even eigenstatee. In addition, we searched for a CP width asymmetry in Dt --, K-K+r+,D' --, K-K+ and Do *r-r '. We found no evidence for CP violation by comparing the decay rater for the particle and antiparticle.
1. INTRODUCTION The interests in CP and heavy flavor physics are ever increasing. Here we summarize two recent reports published by the FOCUS experiment: a measurement of lifetime differences in the neutral D-meson system and search for CP violation in Do and D+ decays [1,2]. 1.1. Basic mixing phenomenology The neutral charm mesons, Do and @J, are not CP eigenstates. In charm sector, experimental limits [3,4] suggest that the off-diagonal element in the decay matrix is much smaller than the average width of eigenstates, contrary to the case of neutral K and B mesons. The nominal variables in charm mixing studies are defined as 2 = AM/I’, y = AI’/2I’. The precise physical meaning of these variables depends on the level of CP violation assumed. See references [5] for the details. Traditionally, two different methods are employed in measurements of x and y. In the first method, one studies the time evolution of such wrong sign D’ decays as D*+ + r+(K+a-) which occurs at a much lower level than the tight sign D’ decays as D*+ -+ (K-r+). Mixing (implying non-zero values of x and y) creates a non-exponential time evolution for the WS component. If the neutral charm mesons decay via
r+
‘This work was rupported by Univumity of Illinoi8 under contract DEFG02-SlER40677 with the U.S. Department of Energy. 0920-5632/01/$ - see front matier 0 2001 Elsevler Science B V PI1 SO920-5632(00)01117-8
hadronic channels, there is an interference term between the mixing and the doubly Cabibbe suppressed paths, which provides a particularly sensitive measurement of 6, a rotational tram+ formation of x and y that depends on a strong phase shift. In the second method, one compares lifetimes from several CP eigenstate decay channels of neutral charm mesons. For example, Do + K-K+ is a CP even eigenstate while Do + K-r+ is half CP even and half odd. If CP is an approximate symmetry of the decay, then y = ycp where WP
=
~(0 + Kr) ~(0 +
KK)
-1
1.2. The FOCUS detector The FOCUS (E831) is the successor of the E687 experiment [6] with a significantly upgraded spectrometer and consists of a hundred members from USA, Italy, Brazil, Mexico and Korea. The data taking was done during the 19961997 fixed target run and we’ve reconstructed more than 1 million golden mode charm particles, D + Kr, K2r, K3r. The most relevant detector components to this analysis are the silicon vertexing detectors and Cerenkov Counters. We use a segmented Be/Be0 target to minimize the absorption correction to charm particle lifetimes. Roughly 62% of charm particles decay in the air gap between the target segments. Our silicon microstrip vertex detector produces an extremely good proper time resoluAll nghts resewed
D Y Kim/Nuclear
Physics B (Proc
tion, 8% of the De lifetime. This level of reso lution allows us to fit the lifetime evolution histogram with a binned likelihood, suppressing the need for any resolution convolution systematics or error inflation. Charged particles are identified by 300 cells of gas threshold Cerenkov Counters. The variable for the assumed particle ID is a sum of log-likelihood of cell responses. Our Cerenkov algorithm provides the flexibility to significantly vary the level of misidentification reflections to gauge background systematics.
I.. 8.8
.
#II. 8.1
Figure 1. (a) Reconstructed mass distribution of 00 + K-r+ and its conjugate candidates. The yield is 119 738 K-r+ and K+rsignal events. (b) That of Do + K-K+ candidates. The yield is 10331 K-K+ signal events. The vertical and dashed lines indicate the signal and sideband regions used for the lifetime and ycp fits [l].
Suppl.) 93 (2001) 272-275
2. COMPARING LIFETIMES Kr, KK DECAYS
213
OF
D +
2.1. Event selection The cuts used to obtain a clean signal were designed to produce a nearly flat efficiency in re duced proper time. The reduced proper time (t’) is a fit variable developed by fixed target experiments, defined as proper time subtracted by the minimum amount of detachment required between primary and secondary vertices. Our quoted result was based on requiring a minimum ul detachment between primary and secondary vertices and kaon hypothesis over pion hypothesis in Cerenkov response favored by a factor of e’ for kaon candidates [l]. Then we either require D’ tag (the reconstructed mass difference between D’ and D is required to be less then 3 MeV around the central value) or require a set of inclusive cuts, which consists of more stringent Cerenkov requirement on kaons and pions, momenta of decay particles balanced each other, requirement of primary vertex inside the target material and resolution of proper time less than 60 fs. The D’ tagged sample has a better signal to noise ratio while the inclusive sample accommodates larger sample size. From the combination of two samples, we obtain 119738 D + Kr and 10331 D + KK events. 2.2. Fitting technique As shown in Figure 1 (b), there is a prominent reflection background coming from misidentified Dc + K-a+ decays in the Ds + K-K+ sample. We accommodate the reflection effect by using a modified version of the mass sideband subtraction fitting technique used in the E687 experiment [7]. The amount of the Dc -B K-d reflection is obtained by a mass fit to the K-K+ sample and the shape of the reflection is deduced from a high statistics Monte Carlo sample. We assume that time evolution of the reflection is described by the lifetime of Ds + K-r+ and fit the reduced proper time distributions of the De 4 K-r+ and the Do -+ K-K+ samples at the same time. The fit parameters are: the D + Kr lifetime ~(0 + Km), the lifetime difference ycp, and each number of background events
214
D. Y Kim/Nuclear
Physrcs B (Pnw. Suppl) 93 (2001) 272-275
under the signal region of Do + K-x+ and that of Do + K-K+. The signal contributions for the LY’ + K-r+, Do -+ K-K+ and the reflection from the misidentified De -+ K-r+ in the reduced proper time histograms are described by f(t’) exp(-t’/r) in the fit likelihood. f(t’) is a function which corrects the detector acceptance and the absorption of particles in matter. The background number parameters are either floated or fixed to the number of events in mass sidebands using a Poisson penalty term in the fit likelihood. We choose 200 fs as the bin size of the reduced proper time, large enough compared to our proper time resolution. 20 bins are used in the fit (see Figure 2).
ferent number of histogram bins and two options of background handling as stated in the previous subsection are tried. The differences in fitted yap is added quadratic8hy in 8 conservative manner. We tried other variations of selection and fitting and found that the results are consistent with our number. We obtained yap and ~(0 + Kr) as follows: ycp = (3.42 f 1.39 f 0.74)%
(2)
~(0 + K+) = 409.2 f 1.3fs
(3)
The result on ~(0 -+ K+) has a statistical error only. Detailed systematics study including absolute distance scale is needed to obtain the final number. Our gap value is consistent with the E791 one, (0.8 f 2.9 f l.O)%, which is compiled from their lifetime papers [4]. Recently the CLEO experiment published a measurement -5.8% < y’ < 1% by studying the time evolution of wrong sign D’ decays as the D’+ -+ r+(K+r-) [8]. The theoretical expectation for the strong phase involved in this process is varied [9] and caution is needed in combining the ycp and the d into one y parameter. 3. SEARCH CHARM
FOR CP ASYMMETRY DECAY
IN
Figure 2. Signal versus reduced proper time for the selected D -, Kr and K-K+ events. The data is background subtracted and includes the (very small) Monte Carlo correction [l].
In the Standard Model scheme, the most likely place to observe direct CP violation in D decay rates is the singly Cabibbo-suppressed channels. Duccella et d. predicted the size of the CP violation between 0.002% and 0.14% [lo]. Here we selected three major D decays modes to search for the violation effect, D+ 4 K-K+&, Do + K-K+ and De --* z-z+.
2.3. Rerults gnd discussion on ycp The systematic errors are estimated by changing the selection cuts and by trying different fitting methods. We tested the Cerenkov identification hypothesis for kaon candidates and the minimum detachment required between primary and secondary vertices. The former affects the level of reflection backgrounds and the later affects the amount of non-charm backgrounds. Dif-
3.1. Defining asymmetry a la fixed target experiment Unlike the situation in the collider experiments, there is a charge asymmetry in production of charm particles in fixed target experiments. With the FOCUS experiment, the order of the production asymmetry is about -3%, which arises most likely due to fragmentation dynamics, for example, quark-diquark asymmetry in target material. We scaled the decay rate of singly Cabibbe suppressed channels by Cabibbc+favored onea to
D Y Km/Nuclear
Physics B (Proc Suppl) 93 (2001) 272-275
Table 1 The CP asymmetry in singly Cabibbo-suppressed Decay Mode
D+ --+ K-K+r+ DO + K-K+ DO --) r-r +
E791 [ll]
+0.006 f +O.Oll f 0.005
-0.014 f 0.029
-0.001 f +0.022 f 0.015
-0.010 f 0.049 f 0.012
+0.048 f +0.039 f 0.025
-0.049 f 0.078 f 0.025
3.2. CP asymmetry results The event selection method is explained in [2]. The statistical error of the neutral decay channels is not as good as the charged one, since the D* tag is required to determine the charm of the parent Do. Besides, the Do --, s-a + channel suffers from a large Ks reflection background. Table 1 shows the asymmetry numbers obtained by us and by the E791 experiment. There is no clear evidence for CP violation in our numbers. The new limits is 2-3 times better than the previous measurements by the E791 [ll]. REFERENCES 1. 2.
3. 4. 5.
D decay modes
FOCUS [2]
accommodated the production asymmetry. If we see a clear deviation in our CP asymmetry variables, it may be due to either the suppressed channel or the favored channel. We choose D+ + K-n+@, Do + K-R+ as the normalization mode for the three-prong and the two-prong decay channels, respectively.
E831 Collab., J.M. Link et OZ., Phys. Lett. B485 (2000) 62. E831 Collab., J.M. Link et al., submitted to Phys. Lett. B. e-Print Archive: hepex/0005037 Particle Data Group, C. Case et al., Eur. Phys. J. C3 (1998) 1. E791 Collab., Aitala et al., Phys. Rev. Lett. 83 (1999) 32. Harry Nelson, UCSB HEP 99-08 (Aug. 1999), e-Print Archive: hepex/9908021 and references therein; E. Golowich, Proceedings of the Conference on B Physics and CP Violation, Honolulu, HI (March 1997) and references therein; A.A. Petrov, Phys. Rev. D56 (1997)1685; E. Golowich and A.A. Petrov, Phys. Lett. B427 (1998) 172; J.F. Donoghue,
275
E. Golowich,B.R. Holstein and J. Trampetic, Phys. Rev. D33 (1986) 179; T. Oh1 et al., Nucl. Phys. B403 (1993) 605; S. Bergmann, Y. Grossman, Z. Ligeti, Y. Nir, A. A. Petrov, FERMILAB-Pub-00/102 (May 2000), e-Print Archive: hepph/0005181. 6. E687 Collab., P.L. Frabetti et al., Nucl. Instr. Meth. A320 (1992) 519. 7. See for example E687 Collab., P.L. Frabetti et aZ., Phys. Rev. Lett. 70 (1993) 1755; Phys. Rev. Lett. 71 (1993) 827; Phys. Lett. B323 (1994) 459 8. CLEO Collab., R. Godang et al., Phys. Rev. Lett. 84 (2000) 5038. 9. A.F. Falk, Y. Nir, A.A. Petrov, JHEP 9912 (1000) 019. 10. F. Buccella et d., Phys. Rev. D51 (1995) 3478. 11. E791 Collab., Aitala et al., Phys. Lett. B403(1997) 377; Phys. Lett. B421(1998) 405.
Nuclear Physxs
B (Proc. Suppl.) 93 (2001) 272-275
Recent FOCUS results on charm mixing and CP violation Doris Yangsoo Kim’* aLoomis Lab of Physics, University of Illinois, 1110 W. Green St., Urbana, IL. 61801, U.S. We eummarise recent results on CP violation and mixing based on a high etatimtica photo-produced charm sample collected by the FOCUS experiment. We compare the lifetimes of two neutral D meson decayr: Do -t K-d and Do + K-K+. A mixing parameter for the neutral D system (yip) ie obtained under the suumption that K-r+ is an equal mixture of CP odd and even eigenstatee. In addition, we searched for a CP width asymmetry in Dt --, K-K+r+,D' --, K-K+ and Do *r-r '. We found no evidence for CP violation by comparing the decay rater for the particle and antiparticle.
1. INTRODUCTION The interests in CP and heavy flavor physics are ever increasing. Here we summarize two recent reports published by the FOCUS experiment: a measurement of lifetime differences in the neutral D-meson system and search for CP violation in Do and D+ decays [1,2]. 1.1. Basic mixing phenomenology The neutral charm mesons, Do and @J, are not CP eigenstates. In charm sector, experimental limits [3,4] suggest that the off-diagonal element in the decay matrix is much smaller than the average width of eigenstates, contrary to the case of neutral K and B mesons. The nominal variables in charm mixing studies are defined as 2 = AM/I’, y = AI’/2I’. The precise physical meaning of these variables depends on the level of CP violation assumed. See references [5] for the details. Traditionally, two different methods are employed in measurements of x and y. In the first method, one studies the time evolution of such wrong sign D’ decays as D*+ + r+(K+a-) which occurs at a much lower level than the tight sign D’ decays as D*+ -+ (K-r+). Mixing (implying non-zero values of x and y) creates a non-exponential time evolution for the WS component. If the neutral charm mesons decay via
r+
‘This work was rupported by Univumity of Illinoi8 under contract DEFG02-SlER40677 with the U.S. Department of Energy. 0920-5632/01/$ - see front matier 0 2001 Elsevler Science B V PI1 SO920-5632(00)01117-8
hadronic channels, there is an interference term between the mixing and the doubly Cabibbe suppressed paths, which provides a particularly sensitive measurement of 6, a rotational tram+ formation of x and y that depends on a strong phase shift. In the second method, one compares lifetimes from several CP eigenstate decay channels of neutral charm mesons. For example, Do + K-K+ is a CP even eigenstate while Do + K-r+ is half CP even and half odd. If CP is an approximate symmetry of the decay, then y = ycp where WP
=
~(0 + Kr) ~(0 +
KK)
-1
1.2. The FOCUS detector The FOCUS (E831) is the successor of the E687 experiment [6] with a significantly upgraded spectrometer and consists of a hundred members from USA, Italy, Brazil, Mexico and Korea. The data taking was done during the 19961997 fixed target run and we’ve reconstructed more than 1 million golden mode charm particles, D + Kr, K2r, K3r. The most relevant detector components to this analysis are the silicon vertexing detectors and Cerenkov Counters. We use a segmented Be/Be0 target to minimize the absorption correction to charm particle lifetimes. Roughly 62% of charm particles decay in the air gap between the target segments. Our silicon microstrip vertex detector produces an extremely good proper time resoluAll nghts resewed
D Y Kim/Nuclear
Physics B (Proc
tion, 8% of the De lifetime. This level of reso lution allows us to fit the lifetime evolution histogram with a binned likelihood, suppressing the need for any resolution convolution systematics or error inflation. Charged particles are identified by 300 cells of gas threshold Cerenkov Counters. The variable for the assumed particle ID is a sum of log-likelihood of cell responses. Our Cerenkov algorithm provides the flexibility to significantly vary the level of misidentification reflections to gauge background systematics.
I.. 8.8
.
#II. 8.1
Figure 1. (a) Reconstructed mass distribution of 00 + K-r+ and its conjugate candidates. The yield is 119 738 K-r+ and K+rsignal events. (b) That of Do + K-K+ candidates. The yield is 10331 K-K+ signal events. The vertical and dashed lines indicate the signal and sideband regions used for the lifetime and ycp fits [l].
Suppl.) 93 (2001) 272-275
2. COMPARING LIFETIMES Kr, KK DECAYS
213
OF
D +
2.1. Event selection The cuts used to obtain a clean signal were designed to produce a nearly flat efficiency in re duced proper time. The reduced proper time (t’) is a fit variable developed by fixed target experiments, defined as proper time subtracted by the minimum amount of detachment required between primary and secondary vertices. Our quoted result was based on requiring a minimum ul detachment between primary and secondary vertices and kaon hypothesis over pion hypothesis in Cerenkov response favored by a factor of e’ for kaon candidates [l]. Then we either require D’ tag (the reconstructed mass difference between D’ and D is required to be less then 3 MeV around the central value) or require a set of inclusive cuts, which consists of more stringent Cerenkov requirement on kaons and pions, momenta of decay particles balanced each other, requirement of primary vertex inside the target material and resolution of proper time less than 60 fs. The D’ tagged sample has a better signal to noise ratio while the inclusive sample accommodates larger sample size. From the combination of two samples, we obtain 119738 D + Kr and 10331 D + KK events. 2.2. Fitting technique As shown in Figure 1 (b), there is a prominent reflection background coming from misidentified Dc + K-a+ decays in the Ds + K-K+ sample. We accommodate the reflection effect by using a modified version of the mass sideband subtraction fitting technique used in the E687 experiment [7]. The amount of the Dc -B K-d reflection is obtained by a mass fit to the K-K+ sample and the shape of the reflection is deduced from a high statistics Monte Carlo sample. We assume that time evolution of the reflection is described by the lifetime of Ds + K-r+ and fit the reduced proper time distributions of the De 4 K-r+ and the Do -+ K-K+ samples at the same time. The fit parameters are: the D + Kr lifetime ~(0 + Km), the lifetime difference ycp, and each number of background events
214
D. Y Kim/Nuclear
Physrcs B (Pnw. Suppl) 93 (2001) 272-275
under the signal region of Do + K-x+ and that of Do + K-K+. The signal contributions for the LY’ + K-r+, Do -+ K-K+ and the reflection from the misidentified De -+ K-r+ in the reduced proper time histograms are described by f(t’) exp(-t’/r) in the fit likelihood. f(t’) is a function which corrects the detector acceptance and the absorption of particles in matter. The background number parameters are either floated or fixed to the number of events in mass sidebands using a Poisson penalty term in the fit likelihood. We choose 200 fs as the bin size of the reduced proper time, large enough compared to our proper time resolution. 20 bins are used in the fit (see Figure 2).
ferent number of histogram bins and two options of background handling as stated in the previous subsection are tried. The differences in fitted yap is added quadratic8hy in 8 conservative manner. We tried other variations of selection and fitting and found that the results are consistent with our number. We obtained yap and ~(0 + Kr) as follows: ycp = (3.42 f 1.39 f 0.74)%
(2)
~(0 + K+) = 409.2 f 1.3fs
(3)
The result on ~(0 -+ K+) has a statistical error only. Detailed systematics study including absolute distance scale is needed to obtain the final number. Our gap value is consistent with the E791 one, (0.8 f 2.9 f l.O)%, which is compiled from their lifetime papers [4]. Recently the CLEO experiment published a measurement -5.8% < y’ < 1% by studying the time evolution of wrong sign D’ decays as the D’+ -+ r+(K+r-) [8]. The theoretical expectation for the strong phase involved in this process is varied [9] and caution is needed in combining the ycp and the d into one y parameter. 3. SEARCH CHARM
FOR CP ASYMMETRY DECAY
IN
Figure 2. Signal versus reduced proper time for the selected D -, Kr and K-K+ events. The data is background subtracted and includes the (very small) Monte Carlo correction [l].
In the Standard Model scheme, the most likely place to observe direct CP violation in D decay rates is the singly Cabibbo-suppressed channels. Duccella et d. predicted the size of the CP violation between 0.002% and 0.14% [lo]. Here we selected three major D decays modes to search for the violation effect, D+ 4 K-K+&, Do + K-K+ and De --* z-z+.
2.3. Rerults gnd discussion on ycp The systematic errors are estimated by changing the selection cuts and by trying different fitting methods. We tested the Cerenkov identification hypothesis for kaon candidates and the minimum detachment required between primary and secondary vertices. The former affects the level of reflection backgrounds and the later affects the amount of non-charm backgrounds. Dif-
3.1. Defining asymmetry a la fixed target experiment Unlike the situation in the collider experiments, there is a charge asymmetry in production of charm particles in fixed target experiments. With the FOCUS experiment, the order of the production asymmetry is about -3%, which arises most likely due to fragmentation dynamics, for example, quark-diquark asymmetry in target material. We scaled the decay rate of singly Cabibbe suppressed channels by Cabibbc+favored onea to
D Y Km/Nuclear
Physics B (Proc Suppl) 93 (2001) 272-275
Table 1 The CP asymmetry in singly Cabibbo-suppressed Decay Mode
D+ --+ K-K+r+ DO + K-K+ DO --) r-r +
E791 [ll]
+0.006 f +O.Oll f 0.005
-0.014 f 0.029
-0.001 f +0.022 f 0.015
-0.010 f 0.049 f 0.012
+0.048 f +0.039 f 0.025
-0.049 f 0.078 f 0.025
3.2. CP asymmetry results The event selection method is explained in [2]. The statistical error of the neutral decay channels is not as good as the charged one, since the D* tag is required to determine the charm of the parent Do. Besides, the Do --, s-a + channel suffers from a large Ks reflection background. Table 1 shows the asymmetry numbers obtained by us and by the E791 experiment. There is no clear evidence for CP violation in our numbers. The new limits is 2-3 times better than the previous measurements by the E791 [ll]. REFERENCES 1. 2.
3. 4. 5.
D decay modes
FOCUS [2]
accommodated the production asymmetry. If we see a clear deviation in our CP asymmetry variables, it may be due to either the suppressed channel or the favored channel. We choose D+ + K-n+@, Do + K-R+ as the normalization mode for the three-prong and the two-prong decay channels, respectively.
E831 Collab., J.M. Link et OZ., Phys. Lett. B485 (2000) 62. E831 Collab., J.M. Link et al., submitted to Phys. Lett. B. e-Print Archive: hepex/0005037 Particle Data Group, C. Case et al., Eur. Phys. J. C3 (1998) 1. E791 Collab., Aitala et al., Phys. Rev. Lett. 83 (1999) 32. Harry Nelson, UCSB HEP 99-08 (Aug. 1999), e-Print Archive: hepex/9908021 and references therein; E. Golowich, Proceedings of the Conference on B Physics and CP Violation, Honolulu, HI (March 1997) and references therein; A.A. Petrov, Phys. Rev. D56 (1997)1685; E. Golowich and A.A. Petrov, Phys. Lett. B427 (1998) 172; J.F. Donoghue,
275
E. Golowich,B.R. Holstein and J. Trampetic, Phys. Rev. D33 (1986) 179; T. Oh1 et al., Nucl. Phys. B403 (1993) 605; S. Bergmann, Y. Grossman, Z. Ligeti, Y. Nir, A. A. Petrov, FERMILAB-Pub-00/102 (May 2000), e-Print Archive: hepph/0005181. 6. E687 Collab., P.L. Frabetti et al., Nucl. Instr. Meth. A320 (1992) 519. 7. See for example E687 Collab., P.L. Frabetti et aZ., Phys. Rev. Lett. 70 (1993) 1755; Phys. Rev. Lett. 71 (1993) 827; Phys. Lett. B323 (1994) 459 8. CLEO Collab., R. Godang et al., Phys. Rev. Lett. 84 (2000) 5038. 9. A.F. Falk, Y. Nir, A.A. Petrov, JHEP 9912 (1000) 019. 10. F. Buccella et d., Phys. Rev. D51 (1995) 3478. 11. E791 Collab., Aitala et al., Phys. Lett. B403(1997) 377; Phys. Lett. B421(1998) 405.