- Email: [email protected]
Radiative hyperon decays at KTeV
lIlllml~mmlmimlm~
ELSEVIER
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 75B (1999) 41--44
Radiative Hyperon Decays at KTeV Erik J. Ramberg a aFermilab P.O. Box 500 Batavia, Illinois, USA 60510 Experiment 799 at Fermilab was the rare decay run of the KTeV apparatus. Besides collecting a large number of neutral kaon decays, the apparatus was used to trigger for rare decays of neutral hyperons. In this paper we present the status of our measurements of the hyperon radiative decays E ° -4 ~°7, .--2 --4 A~, A -4 p~r-7, as well as evidence for the first observation of E ° -4 ATr°'r and ~o ..~ Ae+e -.
1. I n t r o d u c t i o n The KTeV experiment at Fermilab was designed for the observation of neutral KL decays and control of their regeneration into Ks.[1] As such, the detector starts approximately 90 meters distance from the production target. The neutral beam at this point is predominately neutrons and neutral kaons. However, previous neutral kaon experiments at Fermilab have shown that there is a significant enough flux of neutral hyperons at this distance so that investigations of the properties of their production and subsequent decay could be made.J2] As well, the high energy and spatial resolution of the KTeV electromagnetic calorimeter provides a singular laboratory for the investigation of radiative decays of neutral hyperons.[3] The hyperon group at K T E V has observed many such radiative hyperon decays. The physically interesting decays fall into several categories. The first is weak radiative hyperon decays, where a weak transition occurs and the emitted photon is the only particle recoiling against the final state baryon. Examples discussed in this note are -_o --r ~°7 and E ° --+ AT. The second class of radiative decays is three body radiative decays, where the normal weak transition is accompanied by a photon (A --r pzr- V and E ° -+ ATr°7). Finally, there is the pure electromagnetic transition Z ° ~ Ae+e - . We report here on the status of our analysis of these modes. 0920-5632/99/$ - see front matter © 1999 Elsevier Science B.V.
PII S0920-5632(99)00320-5
2. H y p e r o n Run
Triggers
During
the
KTeV
Because hadroproduced hyperons are typically created with large XF, and because in their decays they transfer most of their momentum to the final state baryon, then there is usually a high momentum track going in the very forward direction. We took advantage of this fact by designing a trigger for decays where there is a high momentum track going in the neutral beam direction, and thus through the beam holes in the calorimeter. The trigger was optimized for our search and subsequent discovery of ~° beta decay.[4] This trigger required the following: 1) a hit in either of the beam hole scintillators, 2) hits in the drift chambers in the beam hole regions corresponding to the hit scintillator, 3) at least two clusters in the calorimeter and energy totaling 15 GeV or greater, and 4) a loose veto on hadronic showers after the calorimeter. We also took prescaled triggers that contained no requirements on the calorimeter. Because the decay modes discussed here all contain a final state lambda decaying into a proton and pion, then the hadronic veto of our trigger does affect the collection of these modes, but this is true for the normalization modes as well and cancels in the calculation of branching ratios. In the major run of the KTeV detector, there were approximately 10 s --2 decays in our detector. The acceptance for hyperon decays varies with the specific mode, of course, but is in the range All rights reserved.
42
E.J. RamberglNuclear Physics B (Proc. Suppl.) 75B (1999) 41-44
of 1-10%.
ing ratio calculations compare favorably with the previous observation.[7]
3. W e a k R a d i a t i v e H y p e r o n D e c a y s Despite their seeming simplicity, weak radiative hyperon decays are both experimentally and theoretically unknown.J5] Experimentally, there are only two quantities relevant to the measurement of these decays: their branching fraction and the a s y m m e t r y of the photon emission with respect to the initial spin direction. Despite this, there is only one accurate measurement of a W R H D a s y m m e t r y - the decay E + ~ p% where the a s y m m e t r y is unexpectedly large and negative.[6] Theoretically, these decays are difficult because of the involvement of weak, electromagnetic and strong forces. One recent a t t e m p t has indicated that experimental measurements of the two E ° W R H D decays may resolve some of the questions surrounding this topic.J8] 3.1. -~° -+ E°7 This is a very interesting decay to study. The final state particles are the same as in the normalization mode, E ° -+ A:r °, and the two can be distinguished by their invariant mass combinations. We select events where there is a good A decay and two neutral clusters in the calorimeter. We then determine a neutral decay vertex by finding the point along the A flight p a t h where the invariant mass of all the final state particles matches the ~o mass. This vertex is required to be u p s t r e a m of the A vertex and within the decay volume of the KTeV apparatus, and the total transverse m o m e n t u m of the decay products is required to be small. If the invariant mass of the two photons is within 10 MeV of the ro mass, then the decay was a normalization mode. We saw approximately 2.5 million normalization m o d e events in our d a t a with this analysis. To find the signal, a cut was made for the 2 photons to be outside of the r ° mass. Then the invariant A7 mass was determined for each of the two photons. If the closest combination was within 8 MeV of the E ° mass, then this was a candidate --'° - + E ° 7 event. Figure 1 shows this invariant mass for our data. Approximately 5000 events lie on top of a background of 1400. Initial branch-
1600 14()C)
Z°-.-> y°7
t 20O 1000 800 6OO 400 2OO
I 17
1 18
m,.
1 19
I 2
1 21
1 22
(CeV/c')
Figure 1. Best invariant mass combination between A and one photon. The peak is at the Z ° mass and has about 5000 events. The analysis of the a s y m m e t r y in this decay is difficult in that it involves three stages of decay: the E °, the S ° and then the A. There are polarization transfers at each stage. In the E ° -~ A 7 decay, the A retains only the component of the polarization along its line of flight, and this is inverted because of the unit of spin that the photon carries. This means t h a t for a detector with perfect acceptance the average polarization at this stage becomes -1/3 of the previous stage. This factor was neglected in reference 7. In our data, we have made a two-dimensional plot of the relevant cosines in the decay: the cosine between the =° and the A in the ~° rest frame, and the cosine between the ~° and the proton in the A rest frame. This two dimensional plot for the data and for two different Monte Carlo simulations using different a s y m m e t r y values is shown in Figure 2. As can be seen, preliminary results from the data favor a significant negative a s y m m e t r y for this decay. E ° --+ A7 This is another weak radiative hyperon decay of interest that is evident in our data. Reference 8 cites the a s y m m e t r y in this particular mode as determining whether vector dominance plays 3.2.
E.J. Ramberg/Nuclear Physics B (Proc. Suppl.) 75B (1999) 41-44
a strong role in this class of decays. In about 1/4 of our data, we triggered on events with only one electromagnetic cluster in the calorimeter. In this data, we choose events with a good A candidate and a high energy neutral cluster. We then determine a decay vertex by finding the point along the A line of flight t h a t minimizes the transverse m o m e n t u m with respect to the production target. Again, this decay vertex is required to be upstream of the A decay point and within the KTeV vacuum decay vessel. In the E ° peak for the invariant mass plot of all final state particles there is approximately 1100 events. There were 1.13 million normalization mode events in the data sample used. The acceptance ratio between these two modes was determined to be approximately 1.04, and this yields a preliminary branching ratio determination of (0.94 + 0.03) x 10 -3, with no systematic error determined as yet. This agrees well with th e on!y previous measurement.[9 ]
"~ M o
Figure 2. H i s t o g r a m of the correlation between the two cosine values described in the text for the decay E ° -~ E°% a) is for Monte Carlo d a t a with an a s y m m e t r y value of a = - 1 . 0 , b) is for Monte Carlo d a t a with an a s y m m e t r y value of a = 0.0, and c) is for the data. 4. T h r e e cays
Body
and
Other
Radiative De-
Besides the weak radiative decays, KTeV can do exceedingly well on the other hyperon decays where photons are emitted. Three decay modes
43
are discussed in this section, one for each of the neutral hyperons. Two of these modes have not been previously observed. 4.1. A --+ pTr-~ This decay mode is simply the radiative correction to the normal decay m o d e of the A. As such, the branching ratio for this mode is predictable. The previous measurement saw 72 events.[10] We searched for this decay in a one day sample of KTeV data in the early p a r t of the run and approximately 70 events were seen passing all the cuts. Using this yield, we estimate that about 5000 of these events will be seen in the entire KTeV data set. 4.2. _-o __+ ATrO7 This decay contrasts with the previous one in that both the final state particles are neutral and thus the inner b r e m m s t r a h l u n g process is presumably not applicable. This means that the branching rate is dominated by direct emission processes. No observation or limit has been placed on this decay and little theoretical work has been performed on it. In the KTeV data, this mode is very distinctive. We choose events where there are 3 neutral clusters in the calorimeter. We determine a neutral vertex by assuming 2 of the clusters come from photons in a 7r° decay. This vertex is required to be upstream of the A vertex. The transverse m o m e n t u m of the combination of charged tracks and 3 clusters is then required to be small. All 3 combinations of pairs of neutral clusters are used. We then plot the invariant mass of the A and the 3 photons assuming this vertex. We find a peak in this plot of about 10-20 events at the E ° mass. We have not investigated as yet potential background sources to this decay mode. 4.3. E ° -+ Ae+e Finally, KTeV is even sensitive to decays of the E ° neutral hyperon. None will survive from production at the primary target, since they decay at that point and the final state products will not pass all of the filtering, sweeping and collimation of our beamline. However, because we have seen more than 5000 events of the type =o __4 ~o.r, we can use this decay chain to tag rare decays of the
44
Eft. Ramberg /Nuclear Physic~ B (Proc. Suppl.) 75B (1999) 41-44
E ° down to the 10 -3 level. One interesting decay to look for is E ° -+ Ae+e -. Again, there is no experimental observation or limit on this decay mode. There is one QED prediction from 1958 of the branching ratio to be 0.00545.[11] This decay is interesting in that it can be used to measure the relative parity between the E ° and the A. At KTeV, with our very precise tracking and calorimetry, we can identify electrons very well. We looked for events that contained two electrons and a photon, as well as a decaying A in them. The vertex for the two electrons had to be upstream of the decay vertex for the A, and all of the charged particles had to point to this original vertex. With the photon added in, we then choose those events with a total transverse momentum from the primary production that is small and where all the final state products form a F-° invariant mass. We then look at the invariant mass of the A e - e + combination. That mass plot is shown in Figure 3. As can be seen, there is a peak at the E ° mass, with about 20 events in it. We preliminarily identify that mass peak with the observation of a new decay mode of the E °. Work will continue on the analysis of this mode.
6
'J eJS~.
~$
Events Fon~n 9
~4
. °111
~Ae*e'-f)
of o ~ moss.
.
. . 1.1~
.
. I~
.
. . ,_~
. I~
, .... , .... ,... I.~ 14 1.45
1.5
Figure 3. Histogram of the invariant Ae+e mass for events satisfying conditions of the E°7 decay of the Eo. A peak at the E ° mass indicates the likely presence of a new decay mode of that particle.
5. C o n c l u s i o n We have found that although KTeV is not optimized for observation of hyperons, it is nontheless a very powerful detector for doing so. The combination of cleanly defined neutral beam, precise electromagnetic calorimetry and very accurate tracking give us a great opportunity to study radiative decays of the neutral hyperons. We have described many of the exciting opportunities for this kind of physics in this note. A new run of the KTeV detector and beamline is planned to occur in 1999. We expect on the order of 3 times the statistics for'most of the decay modes cited here. REFERENCES 1. KTeV design report; Arisaka, et.al.; Fermilab-FN-580; (1992) 2. Ramberg, et.al.; Physics Letters B 338; p. 403; (1994) 3. For a description of an analysis using the calorimeter, see Adams, et.al.; Physical Review Letters 80; p. 4123; (1998). 4. See Monnier's contribution to these proceedings. 5. For a review of weak radiative hyperon decays, see Lach, Zenczykowski; Int. Journal of Modern Physics A 10; p. 3817; (1995). 6. Timm, et.al.; Physical Review D 51; p.4638; (1995) 7. Teige, et.al.; Physical Review Letters 63; p. 2717; (1989) 8. Zenczykowski, Physical Review D 44; p. 1485; (1991). See also Zenczykowski's contribution to these proceedings. 9. James, et.al.; Physical Review Letters 64; p. 843; (1990) 10. Baggett, et.al.; Physics Letters B42; p. 379; (1972) 11. Feinberg; Physical Review 109; p. 1019; (1958)
ELSEVIER
PROCEEDINGS SUPPLEMENTS
Nuclear Physics B (Proc. Suppl.) 75B (1999) 41--44
Radiative Hyperon Decays at KTeV Erik J. Ramberg a aFermilab P.O. Box 500 Batavia, Illinois, USA 60510 Experiment 799 at Fermilab was the rare decay run of the KTeV apparatus. Besides collecting a large number of neutral kaon decays, the apparatus was used to trigger for rare decays of neutral hyperons. In this paper we present the status of our measurements of the hyperon radiative decays E ° -4 ~°7, .--2 --4 A~, A -4 p~r-7, as well as evidence for the first observation of E ° -4 ATr°'r and ~o ..~ Ae+e -.
1. I n t r o d u c t i o n The KTeV experiment at Fermilab was designed for the observation of neutral KL decays and control of their regeneration into Ks.[1] As such, the detector starts approximately 90 meters distance from the production target. The neutral beam at this point is predominately neutrons and neutral kaons. However, previous neutral kaon experiments at Fermilab have shown that there is a significant enough flux of neutral hyperons at this distance so that investigations of the properties of their production and subsequent decay could be made.J2] As well, the high energy and spatial resolution of the KTeV electromagnetic calorimeter provides a singular laboratory for the investigation of radiative decays of neutral hyperons.[3] The hyperon group at K T E V has observed many such radiative hyperon decays. The physically interesting decays fall into several categories. The first is weak radiative hyperon decays, where a weak transition occurs and the emitted photon is the only particle recoiling against the final state baryon. Examples discussed in this note are -_o --r ~°7 and E ° --+ AT. The second class of radiative decays is three body radiative decays, where the normal weak transition is accompanied by a photon (A --r pzr- V and E ° -+ ATr°7). Finally, there is the pure electromagnetic transition Z ° ~ Ae+e - . We report here on the status of our analysis of these modes. 0920-5632/99/$ - see front matter © 1999 Elsevier Science B.V.
PII S0920-5632(99)00320-5
2. H y p e r o n Run
Triggers
During
the
KTeV
Because hadroproduced hyperons are typically created with large XF, and because in their decays they transfer most of their momentum to the final state baryon, then there is usually a high momentum track going in the very forward direction. We took advantage of this fact by designing a trigger for decays where there is a high momentum track going in the neutral beam direction, and thus through the beam holes in the calorimeter. The trigger was optimized for our search and subsequent discovery of ~° beta decay.[4] This trigger required the following: 1) a hit in either of the beam hole scintillators, 2) hits in the drift chambers in the beam hole regions corresponding to the hit scintillator, 3) at least two clusters in the calorimeter and energy totaling 15 GeV or greater, and 4) a loose veto on hadronic showers after the calorimeter. We also took prescaled triggers that contained no requirements on the calorimeter. Because the decay modes discussed here all contain a final state lambda decaying into a proton and pion, then the hadronic veto of our trigger does affect the collection of these modes, but this is true for the normalization modes as well and cancels in the calculation of branching ratios. In the major run of the KTeV detector, there were approximately 10 s --2 decays in our detector. The acceptance for hyperon decays varies with the specific mode, of course, but is in the range All rights reserved.
42
E.J. RamberglNuclear Physics B (Proc. Suppl.) 75B (1999) 41-44
of 1-10%.
ing ratio calculations compare favorably with the previous observation.[7]
3. W e a k R a d i a t i v e H y p e r o n D e c a y s Despite their seeming simplicity, weak radiative hyperon decays are both experimentally and theoretically unknown.J5] Experimentally, there are only two quantities relevant to the measurement of these decays: their branching fraction and the a s y m m e t r y of the photon emission with respect to the initial spin direction. Despite this, there is only one accurate measurement of a W R H D a s y m m e t r y - the decay E + ~ p% where the a s y m m e t r y is unexpectedly large and negative.[6] Theoretically, these decays are difficult because of the involvement of weak, electromagnetic and strong forces. One recent a t t e m p t has indicated that experimental measurements of the two E ° W R H D decays may resolve some of the questions surrounding this topic.J8] 3.1. -~° -+ E°7 This is a very interesting decay to study. The final state particles are the same as in the normalization mode, E ° -+ A:r °, and the two can be distinguished by their invariant mass combinations. We select events where there is a good A decay and two neutral clusters in the calorimeter. We then determine a neutral decay vertex by finding the point along the A flight p a t h where the invariant mass of all the final state particles matches the ~o mass. This vertex is required to be u p s t r e a m of the A vertex and within the decay volume of the KTeV apparatus, and the total transverse m o m e n t u m of the decay products is required to be small. If the invariant mass of the two photons is within 10 MeV of the ro mass, then the decay was a normalization mode. We saw approximately 2.5 million normalization m o d e events in our d a t a with this analysis. To find the signal, a cut was made for the 2 photons to be outside of the r ° mass. Then the invariant A7 mass was determined for each of the two photons. If the closest combination was within 8 MeV of the E ° mass, then this was a candidate --'° - + E ° 7 event. Figure 1 shows this invariant mass for our data. Approximately 5000 events lie on top of a background of 1400. Initial branch-
1600 14()C)
Z°-.-> y°7
t 20O 1000 800 6OO 400 2OO
I 17
1 18
m,.
1 19
I 2
1 21
1 22
(CeV/c')
Figure 1. Best invariant mass combination between A and one photon. The peak is at the Z ° mass and has about 5000 events. The analysis of the a s y m m e t r y in this decay is difficult in that it involves three stages of decay: the E °, the S ° and then the A. There are polarization transfers at each stage. In the E ° -~ A 7 decay, the A retains only the component of the polarization along its line of flight, and this is inverted because of the unit of spin that the photon carries. This means t h a t for a detector with perfect acceptance the average polarization at this stage becomes -1/3 of the previous stage. This factor was neglected in reference 7. In our data, we have made a two-dimensional plot of the relevant cosines in the decay: the cosine between the =° and the A in the ~° rest frame, and the cosine between the ~° and the proton in the A rest frame. This two dimensional plot for the data and for two different Monte Carlo simulations using different a s y m m e t r y values is shown in Figure 2. As can be seen, preliminary results from the data favor a significant negative a s y m m e t r y for this decay. E ° --+ A7 This is another weak radiative hyperon decay of interest that is evident in our data. Reference 8 cites the a s y m m e t r y in this particular mode as determining whether vector dominance plays 3.2.
E.J. Ramberg/Nuclear Physics B (Proc. Suppl.) 75B (1999) 41-44
a strong role in this class of decays. In about 1/4 of our data, we triggered on events with only one electromagnetic cluster in the calorimeter. In this data, we choose events with a good A candidate and a high energy neutral cluster. We then determine a decay vertex by finding the point along the A line of flight t h a t minimizes the transverse m o m e n t u m with respect to the production target. Again, this decay vertex is required to be upstream of the A decay point and within the KTeV vacuum decay vessel. In the E ° peak for the invariant mass plot of all final state particles there is approximately 1100 events. There were 1.13 million normalization mode events in the data sample used. The acceptance ratio between these two modes was determined to be approximately 1.04, and this yields a preliminary branching ratio determination of (0.94 + 0.03) x 10 -3, with no systematic error determined as yet. This agrees well with th e on!y previous measurement.[9 ]
"~ M o
Figure 2. H i s t o g r a m of the correlation between the two cosine values described in the text for the decay E ° -~ E°% a) is for Monte Carlo d a t a with an a s y m m e t r y value of a = - 1 . 0 , b) is for Monte Carlo d a t a with an a s y m m e t r y value of a = 0.0, and c) is for the data. 4. T h r e e cays
Body
and
Other
Radiative De-
Besides the weak radiative decays, KTeV can do exceedingly well on the other hyperon decays where photons are emitted. Three decay modes
43
are discussed in this section, one for each of the neutral hyperons. Two of these modes have not been previously observed. 4.1. A --+ pTr-~ This decay mode is simply the radiative correction to the normal decay m o d e of the A. As such, the branching ratio for this mode is predictable. The previous measurement saw 72 events.[10] We searched for this decay in a one day sample of KTeV data in the early p a r t of the run and approximately 70 events were seen passing all the cuts. Using this yield, we estimate that about 5000 of these events will be seen in the entire KTeV data set. 4.2. _-o __+ ATrO7 This decay contrasts with the previous one in that both the final state particles are neutral and thus the inner b r e m m s t r a h l u n g process is presumably not applicable. This means that the branching rate is dominated by direct emission processes. No observation or limit has been placed on this decay and little theoretical work has been performed on it. In the KTeV data, this mode is very distinctive. We choose events where there are 3 neutral clusters in the calorimeter. We determine a neutral vertex by assuming 2 of the clusters come from photons in a 7r° decay. This vertex is required to be upstream of the A vertex. The transverse m o m e n t u m of the combination of charged tracks and 3 clusters is then required to be small. All 3 combinations of pairs of neutral clusters are used. We then plot the invariant mass of the A and the 3 photons assuming this vertex. We find a peak in this plot of about 10-20 events at the E ° mass. We have not investigated as yet potential background sources to this decay mode. 4.3. E ° -+ Ae+e Finally, KTeV is even sensitive to decays of the E ° neutral hyperon. None will survive from production at the primary target, since they decay at that point and the final state products will not pass all of the filtering, sweeping and collimation of our beamline. However, because we have seen more than 5000 events of the type =o __4 ~o.r, we can use this decay chain to tag rare decays of the
44
Eft. Ramberg /Nuclear Physic~ B (Proc. Suppl.) 75B (1999) 41-44
E ° down to the 10 -3 level. One interesting decay to look for is E ° -+ Ae+e -. Again, there is no experimental observation or limit on this decay mode. There is one QED prediction from 1958 of the branching ratio to be 0.00545.[11] This decay is interesting in that it can be used to measure the relative parity between the E ° and the A. At KTeV, with our very precise tracking and calorimetry, we can identify electrons very well. We looked for events that contained two electrons and a photon, as well as a decaying A in them. The vertex for the two electrons had to be upstream of the decay vertex for the A, and all of the charged particles had to point to this original vertex. With the photon added in, we then choose those events with a total transverse momentum from the primary production that is small and where all the final state products form a F-° invariant mass. We then look at the invariant mass of the A e - e + combination. That mass plot is shown in Figure 3. As can be seen, there is a peak at the E ° mass, with about 20 events in it. We preliminarily identify that mass peak with the observation of a new decay mode of the E °. Work will continue on the analysis of this mode.
6
'J eJS~.
~$
Events Fon~n 9
~4
. °111
~Ae*e'-f)
of o ~ moss.
.
. . 1.1~
.
. I~
.
. . ,_~
. I~
, .... , .... ,... I.~ 14 1.45
1.5
Figure 3. Histogram of the invariant Ae+e mass for events satisfying conditions of the E°7 decay of the Eo. A peak at the E ° mass indicates the likely presence of a new decay mode of that particle.
5. C o n c l u s i o n We have found that although KTeV is not optimized for observation of hyperons, it is nontheless a very powerful detector for doing so. The combination of cleanly defined neutral beam, precise electromagnetic calorimetry and very accurate tracking give us a great opportunity to study radiative decays of the neutral hyperons. We have described many of the exciting opportunities for this kind of physics in this note. A new run of the KTeV detector and beamline is planned to occur in 1999. We expect on the order of 3 times the statistics for'most of the decay modes cited here. REFERENCES 1. KTeV design report; Arisaka, et.al.; Fermilab-FN-580; (1992) 2. Ramberg, et.al.; Physics Letters B 338; p. 403; (1994) 3. For a description of an analysis using the calorimeter, see Adams, et.al.; Physical Review Letters 80; p. 4123; (1998). 4. See Monnier's contribution to these proceedings. 5. For a review of weak radiative hyperon decays, see Lach, Zenczykowski; Int. Journal of Modern Physics A 10; p. 3817; (1995). 6. Timm, et.al.; Physical Review D 51; p.4638; (1995) 7. Teige, et.al.; Physical Review Letters 63; p. 2717; (1989) 8. Zenczykowski, Physical Review D 44; p. 1485; (1991). See also Zenczykowski's contribution to these proceedings. 9. James, et.al.; Physical Review Letters 64; p. 843; (1990) 10. Baggett, et.al.; Physics Letters B42; p. 379; (1972) 11. Feinberg; Physical Review 109; p. 1019; (1958)