- Email: [email protected]
Recent progress in Bs mixing measurements
,immqmmimmmam
Nuclear Physics B (Proc. Suppl.) 66 (1998) 510-513
ELSEVIER
PROCEEDINGS SUPPLEMENTS
Recent Progress in Bs Mixing Measurements G. Crawforda aStanford Linear Accelerator Center, Stanford, CA 94309, USA We review the status of Bs mixing measurements from LEP experiments, including new results from DELPHI. We briefly review the various experimental techniques used and their limitations and discuss the possibilities for improving these measurements in the near future. Including the new DELPHI results the combined world average limit on the Bs oscillation frequency is Ares > 8.4ps-1 at 95% CL.
1. M O T I V A T I O N In the conventional formalism used to describe B ° - B 0 mixing, the prob_aability to observe an initial B ° state as a final B ° state at time t is
F~-Bo(t) = 1e-Dr(1 - cos(Amt))
(1)
where the oscillation frequency Am is a function of QCD factors and CKM matrix elements involving the top quark [1]. The precision measurements of time-dependent Bd mixing now being performed [2] could lead to clean extraction of the important CKM matrix element Vtd were it not for large theoretical uncertainties in the calculation of the QCD factors. A measurement of the Bs oscillation frequency Ams combined with the Bd mixing measurements can lead to a much smaller theoretical uncertainty in extracting V~d using the relation
~s1____2 vI
Amd
IVdl
(2)
since most of the QCD factors cancel out in the ratio. If we assume unitarity of the CKM matrix then ]V~sl N ]Vcbl = 0.041 -4- 0.003 and IV~dl could be extracted with a theoretical uncertainty of only 10-20% [1]. Equation 2 also makes clear the great experimental challenge in measuring Bs mixing." the oscillation frequency is
Ares ,,~ sin-2OcAmd ,,, 10 -- 30ps -1,
(3)
where Oc is the Cabibbo angle. This gives an oscillation time scale ~- ~ It~Ares ,,, 0.1 -- 0.3 ps. The 0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All fights reserved. PIt S0920-5632(98)00097-8
typical proper time error in b decay reconstruction for the LEP experiments is ar "~ 0.2 - 0.4 ps, barely enough to resolve the "slow" end of the frequency spectrum. Moreover, the best proper time errors come from events where the Bs decay has been almost completely reconstructed, but here the sample size is very small. Thus experimenters have tried many techniques to try to optimize event reconstruction efficiency, decay length resolution, and Bs tagging purity to maximize their "reach" for resolving Bs oscillations. So far these considerable efforts have only yielded lower limits on the oscillation frequency.
2. EXPERIMENTAL T E C H N I Q U E S Experimental techniques to measure Bs mixing basically follow those used to measure timedependent Bd mixing [2] with the additional requirement that one wants to enhance the small (10%) Bs component of b-hadrons produced in Z ° decays without sacrificing too much in statistics. This is usually done by reconstructing quasiexclusive final states distinctive of Bs decays, but has also been attempted using identified fragmentation kaons in the initial state as a signature of the spectator s quark in the B meson. In all cases one must tag both the initial and final state flavor of the b hadron and estimate its decay length. The decay proper time is calculated as v = L/,yl3 where L is the measured decay length and ~F/~ is the relativistic boost of the B. The decay length is the distance from the primary vertex to the B decay vertex projected along the estimated B flight direction (e.g., the jet direction). The B
G. Crawford/NuclearPhysics B (Proc. Suppl.) 66 (1998) 510-513
decay vertex can be found by either an explicit decay reconstruction or by more inclusive techniques. The proper time resolution cr~ is then given by =
+
(4)
where the first term corresponds to the boost resolution (10-20% typical) and dominates for large proper times (~- > r s -- 1.5ps); the second term is the decay length resolution (aL "~ 200 -- 400#m typical), which dominates the proper time resolution at the short proper times relevant for B, mixing. The sensitivity S to a B, mixing signal of frequency A m , given a proper time resolution of av is given approximately by [3]
511
[4]. The basic concept is to "Fourier transform" the analysis by fixing the input value of Am8 and fitting for the amplitude A of oscillations observed at this frequency. In the convention of Eqn. (1), this is the replacement of 1 - cos(Amt) -+ 1 - Acos(Amt).
(6)
At each frequency Am, if A = 0 there is no evidence of mixing, whereas if A = 1 mixing is occuring at this frequency with a significance 1/O'A. To set a 95% CL limit, one must exclude A > 1 at this level, hence one requires A + 1.65aA < 1. The significance of any single experiment can then be combined with other experiments treating the correlated and uncorrelated pieces in the appropriate manner. 4. R E C E N T R E S U L T S
S = x/-N(1 - 2~)exp(-(Amsa~.)2),
(5)
where N is the sample size and ~7is the B, mistag rate. Clearly even large, pure event samples (if they existed) would lose any power to resolve fast B, oscillations when a~ > 2~Am,; this is why the best Bs mixing limits have so far come from quasi-exclusive B, reconstruction with good decay length resolution. 3. T H E A M P L I T U D E
METHOD
Early limits on time-dependent B, mixing were extracted from the data by maximum likelihood techniques, varying Am8 and fitting to map out the distribution of lnE vs Ares. All timedependent quantities, such as proper time resolution, reconstruction efficiency, mistag rates and b hadron fractions were parameterized from Monte Carlo simulations and convolved in the likelihood function for each experiment. Unfortunately individual experiments could not be combined to improve the overall limit mainly because of difficulty in extracting the correlated statistical and systematic uncertainties from the individual likelihood functions. Moser and Roussarie [3] have proposed an elegant solution to the problem of combining different B, mixing experiments which has been adopted by the LEP B Oscillations working group
Here we will briefly review the recent published results from ALEPH and OPAL and the new results from DELPHI released for this conference. 4.1. A L E P H
ALEPH pioneered many of the B8 mixing analyses techniques and currently quotes results from five different analyses. They have analyzed most of their data from 1991-95 and last gave an update at ICHEP96 [5]. Three of the analyses use a high momentum lepton to inclusively tag the final state, and they examine either a identified kaon or lepton or the jet charge (Qjet) opposite the tagged jet to infer the initial state flavor. These analyses so far only use data from 199194. They have high statistics (5K-60K events) but suffer from rather poor average proper time resolution (at "~ 0.3 - 0.4 ps) and thus have sensitivity to oscillations only up to Am8 ,~ 3ps-1; due to statistical fluctuations they set better limits. The other two analyses rely on reconstructing a Ds meson in the exclusive KKTr(3r) final states (e.g., ¢~r,K*K, etc.) or the ¢/'P mode; they then combine the Ds vertex with a large impact parameter track(s) to form a candidate Bs vertex; these techniques offer low statistics (3001500 events) but much improved proper time resolution (a~ ,-, 0.2 ps) due to explicit vertex reconstruction. These analyses have sensitivity to
512
G. Crawford/Nuclear Physics B (Proc. Suppl.) 66 (1998) 510--513
oscillations up to Ares = 5 - 7ps -1 and set comparable limits. Combining all of their analyses using the amplitude method described above, ALEPH sets an overall limit of Ares > 7.8ps -1 at 95% CL. 4.2. O P A L The OPAL collaboration has analyzed data from 1991-94 using four different analysis techniques. Two of these use an inclusive lepton to tag the final state and compare to another lepton or the jet charge of the opposite jet to infer the initial state. Their event statistics and proper time resolution are comparable to the inclusive ALEPH analyses. Both inclusive analyses were updated this summer [7] but the limits did not change significantly: Am, > 2.2ps -1 at 95% CL (dflepton), Am, > 2.9ps -1 at 95% CL (l- Qjet). The latter result has been extracted using the amplitude method and is so far the only result combined into the world averages. In 1996, OPAL reported [6] results from two quasi-exclusive analyses which reconstructed either D, mesons or ¢ mesons and vertexed these with an identified lepton to achieve a high purity B, sample with good proper time resolution. Unfortunately these analyses (using a maximum likelihood method) observed a large signal for B, mixing at frequencies near Am, = 0.5ps -1 which is highly disfavored by the other results quoted here; thus these analyses don't set useful limits on Bs mixing. These analyses have not yet been updated. OPAL does not yet combine their analyses to quote an overall limit. 4.3. D E L P H I DELPHI has also analysed data from 1991-94 to perform two inclusive B, mixing analyses using leptons as a final state tag and leptons or jet charge opposite the tagged jet to infer the initial state. They obtain limits comparable to the similar OPAL analyses quoted above. The major update released for this conference was a re-analysis by DELPHI of their full dataset (1991-5) using an improved tracking algorithm in a quasi-exclusive analysis which reconstructs D, mesons and leptons to form candidate Bs vertices. The D, mesons are reconstructed in both
'~ 3.5
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PRELIMINARY .2., ' ' "
'
"
11.1~'~
.i~ili~i::
2.5 2 1.5 1 0.5
-0.5 0
2
4
6
8
10
12
14
am. (ps'b
Figure 1. LEP combined B8 oscillation amplitude as a function of Am,.
hadronic and semileptonic modes. Most of the improvement in the analysis comes from much higher vertexing efficiency achieved with full 3D information provided by the upgraded vertex detector in 1994-5 [8]. They have also optimized their initial state tag to use all available information (i.e., identified kaons, leptons, jet charge) in the hemisphere opposite the Bs decay to correctly identify the initial state with high probability. This analysis is comparable to the ALEPH Ds - 1 analysis in statistics and proper time resolution, and sets a limit of Am8 > 8.3ps -1 at 95% CL, slightly better than their expected sensitivity. DELPHI has also combined all of their analyses to set a limit of Am, > 8.1ps -1 at 95% CL. 4.4. LEP Average All ALEPH, DELPHI analyses and the OPAL l - Qjet analysis have been combined in a grand average by the LEP B oscillation working group
G. Crawford/Nuclear Physics B (Proc. Suppl.) 66 (1998) 510-513 [4]. This reduces the impact of statistical fluctuations which can generate "lucky" or "unlucky" individual results and should improve the overall limit on average. The combined result is
A m s > 8.4ps -1 at 95%CL
(7)
The results of the combined amplitude fit are shown in Fig. 1. 5. F U T U R E P R O S P E C T S As seen from the above results, the analyses using quasi-exclusive Bs reconstruction have dominated the Bs mixing limits and are likely to continue to do so for the near future. Improvements in vertexing and reconstruction code to take full advantage of the upgraded LEP vertex detectors (a la DELPHI) is likely to drive any improvement in limits since the most of the data has been analyzed [9]. New and updated results from OPAL and L3 are eagerly awaited and could also have significant impact. In the near future, the SLD experiment at SLC will start to set Ares limits with their existing data (about 200k Z °, compared to 5M Z ° per LEP experiment). SLD can compete in these analyses because of very efficient and pure initial state tagging using the beam polarization (,,~ 80%) and the excellent decay length resolution of their new vertex detector (aL ~" 100 -- 200#m typical). This translates to a typical proper time resolution of ar "" 0.1 ps using inclusive vertexing techniques, and can improve with more exclusive reconstruction. This improvement in vertexing technology allows SLD much better "reach" in Ares with far fewer Z's than LEP. SLD can achieve a limit of Ares > 15ps-1 at 95% CL with a data sample of ~ 500k Z°'s, which they expect to acquire in their 1997-98 run. Many experiments at colliders and fixed target machines have been proposed to measure the Bs mixing frequency due to the experimental challenge and theoretical interest. Here we quote only the expectations from experiments which are relatively certain to run soon: HERA-B, a gas-jet target experiment in the HERA ring dedicated to b physics, expects to run in 1998 and set limits of Ares > 11 - 13ps -1 at 95% CL[10]. Both
513
CDF and DO experiments at Fermilab have expressed plans for measuring Bs mixing with upgraded vertex detectors during Tevatron Run II which is scheduled to start in 1999; recent estimates claim they can measure Ares > 20ps -~ at 95% CL with the full Run II dataset [11]. In the long-term future this measurement can be done "easily" at LHC-B [12]. 6. A C K N O W L E D G M E N T S I would like to thank my colleagues at LEP and SLD for their assistance in preparing this report, and in particular P. Roudeau, O. Schneider, and S. Willocq. I should also thank S. Adler, E. Barberio, K. Berkelman, R. Ben-David, F. Filthaut, T. Paul, V. Sharma, D. Stickland, and A. Turcot for making the Conference memorable. REFERENCES 1. Particle Data Group, Phys. Rev. D54 (1996) 95. 2. See for example the review by E. Barberio in these Proceedings. 3. H. G. Moser and A. Roussarie, Nucl. Inst. Meth. A384 (1997) 491. 4. LEP B Oscillations Working Group, preprint LEPBOSC 97/3. See also their Web site at http ://wwwcn. cern. ch/~ offline/lepwgs 5. K. Jakobs, Proc. XXVIIIth ICHEP, Warsaw, Eds. Z. Adjuk and A. K. Wroblewski. World Scientific, 1997. 6. S. Tarem, op. cit. 7. OPAL Collaboration, CERN-PPE/97-036 and CERN-PPE/97-064. 8. DELPHI Collaboration, private communication. 9. As of this writing, ALEPH has just announced a much improved inclusive lepton analysis with better decay length resolution: see EPS-HEP contribution 612. 10. HERA-B Collaboration, Nucl. Inst. Meth. A368 (1995) 124. 11. D. Amidei et. al. (Tev2000 Study Group), FERMILAB-PUB-96-082 12. See the report by S. Conetti in these Proceedings and references therein.
Nuclear Physics B (Proc. Suppl.) 66 (1998) 510-513
ELSEVIER
PROCEEDINGS SUPPLEMENTS
Recent Progress in Bs Mixing Measurements G. Crawforda aStanford Linear Accelerator Center, Stanford, CA 94309, USA We review the status of Bs mixing measurements from LEP experiments, including new results from DELPHI. We briefly review the various experimental techniques used and their limitations and discuss the possibilities for improving these measurements in the near future. Including the new DELPHI results the combined world average limit on the Bs oscillation frequency is Ares > 8.4ps-1 at 95% CL.
1. M O T I V A T I O N In the conventional formalism used to describe B ° - B 0 mixing, the prob_aability to observe an initial B ° state as a final B ° state at time t is
F~-Bo(t) = 1e-Dr(1 - cos(Amt))
(1)
where the oscillation frequency Am is a function of QCD factors and CKM matrix elements involving the top quark [1]. The precision measurements of time-dependent Bd mixing now being performed [2] could lead to clean extraction of the important CKM matrix element Vtd were it not for large theoretical uncertainties in the calculation of the QCD factors. A measurement of the Bs oscillation frequency Ams combined with the Bd mixing measurements can lead to a much smaller theoretical uncertainty in extracting V~d using the relation
~s1____2 vI
Amd
IVdl
(2)
since most of the QCD factors cancel out in the ratio. If we assume unitarity of the CKM matrix then ]V~sl N ]Vcbl = 0.041 -4- 0.003 and IV~dl could be extracted with a theoretical uncertainty of only 10-20% [1]. Equation 2 also makes clear the great experimental challenge in measuring Bs mixing." the oscillation frequency is
Ares ,,~ sin-2OcAmd ,,, 10 -- 30ps -1,
(3)
where Oc is the Cabibbo angle. This gives an oscillation time scale ~- ~ It~Ares ,,, 0.1 -- 0.3 ps. The 0920-5632/98/$19.00 © 1998 Elsevier Science B.V. All fights reserved. PIt S0920-5632(98)00097-8
typical proper time error in b decay reconstruction for the LEP experiments is ar "~ 0.2 - 0.4 ps, barely enough to resolve the "slow" end of the frequency spectrum. Moreover, the best proper time errors come from events where the Bs decay has been almost completely reconstructed, but here the sample size is very small. Thus experimenters have tried many techniques to try to optimize event reconstruction efficiency, decay length resolution, and Bs tagging purity to maximize their "reach" for resolving Bs oscillations. So far these considerable efforts have only yielded lower limits on the oscillation frequency.
2. EXPERIMENTAL T E C H N I Q U E S Experimental techniques to measure Bs mixing basically follow those used to measure timedependent Bd mixing [2] with the additional requirement that one wants to enhance the small (10%) Bs component of b-hadrons produced in Z ° decays without sacrificing too much in statistics. This is usually done by reconstructing quasiexclusive final states distinctive of Bs decays, but has also been attempted using identified fragmentation kaons in the initial state as a signature of the spectator s quark in the B meson. In all cases one must tag both the initial and final state flavor of the b hadron and estimate its decay length. The decay proper time is calculated as v = L/,yl3 where L is the measured decay length and ~F/~ is the relativistic boost of the B. The decay length is the distance from the primary vertex to the B decay vertex projected along the estimated B flight direction (e.g., the jet direction). The B
G. Crawford/NuclearPhysics B (Proc. Suppl.) 66 (1998) 510-513
decay vertex can be found by either an explicit decay reconstruction or by more inclusive techniques. The proper time resolution cr~ is then given by =
+
(4)
where the first term corresponds to the boost resolution (10-20% typical) and dominates for large proper times (~- > r s -- 1.5ps); the second term is the decay length resolution (aL "~ 200 -- 400#m typical), which dominates the proper time resolution at the short proper times relevant for B, mixing. The sensitivity S to a B, mixing signal of frequency A m , given a proper time resolution of av is given approximately by [3]
511
[4]. The basic concept is to "Fourier transform" the analysis by fixing the input value of Am8 and fitting for the amplitude A of oscillations observed at this frequency. In the convention of Eqn. (1), this is the replacement of 1 - cos(Amt) -+ 1 - Acos(Amt).
(6)
At each frequency Am, if A = 0 there is no evidence of mixing, whereas if A = 1 mixing is occuring at this frequency with a significance 1/O'A. To set a 95% CL limit, one must exclude A > 1 at this level, hence one requires A + 1.65aA < 1. The significance of any single experiment can then be combined with other experiments treating the correlated and uncorrelated pieces in the appropriate manner. 4. R E C E N T R E S U L T S
S = x/-N(1 - 2~)exp(-(Amsa~.)2),
(5)
where N is the sample size and ~7is the B, mistag rate. Clearly even large, pure event samples (if they existed) would lose any power to resolve fast B, oscillations when a~ > 2~Am,; this is why the best Bs mixing limits have so far come from quasi-exclusive B, reconstruction with good decay length resolution. 3. T H E A M P L I T U D E
METHOD
Early limits on time-dependent B, mixing were extracted from the data by maximum likelihood techniques, varying Am8 and fitting to map out the distribution of lnE vs Ares. All timedependent quantities, such as proper time resolution, reconstruction efficiency, mistag rates and b hadron fractions were parameterized from Monte Carlo simulations and convolved in the likelihood function for each experiment. Unfortunately individual experiments could not be combined to improve the overall limit mainly because of difficulty in extracting the correlated statistical and systematic uncertainties from the individual likelihood functions. Moser and Roussarie [3] have proposed an elegant solution to the problem of combining different B, mixing experiments which has been adopted by the LEP B Oscillations working group
Here we will briefly review the recent published results from ALEPH and OPAL and the new results from DELPHI released for this conference. 4.1. A L E P H
ALEPH pioneered many of the B8 mixing analyses techniques and currently quotes results from five different analyses. They have analyzed most of their data from 1991-95 and last gave an update at ICHEP96 [5]. Three of the analyses use a high momentum lepton to inclusively tag the final state, and they examine either a identified kaon or lepton or the jet charge (Qjet) opposite the tagged jet to infer the initial state flavor. These analyses so far only use data from 199194. They have high statistics (5K-60K events) but suffer from rather poor average proper time resolution (at "~ 0.3 - 0.4 ps) and thus have sensitivity to oscillations only up to Am8 ,~ 3ps-1; due to statistical fluctuations they set better limits. The other two analyses rely on reconstructing a Ds meson in the exclusive KKTr(3r) final states (e.g., ¢~r,K*K, etc.) or the ¢/'P mode; they then combine the Ds vertex with a large impact parameter track(s) to form a candidate Bs vertex; these techniques offer low statistics (3001500 events) but much improved proper time resolution (a~ ,-, 0.2 ps) due to explicit vertex reconstruction. These analyses have sensitivity to
512
G. Crawford/Nuclear Physics B (Proc. Suppl.) 66 (1998) 510--513
oscillations up to Ares = 5 - 7ps -1 and set comparable limits. Combining all of their analyses using the amplitude method described above, ALEPH sets an overall limit of Ares > 7.8ps -1 at 95% CL. 4.2. O P A L The OPAL collaboration has analyzed data from 1991-94 using four different analysis techniques. Two of these use an inclusive lepton to tag the final state and compare to another lepton or the jet charge of the opposite jet to infer the initial state. Their event statistics and proper time resolution are comparable to the inclusive ALEPH analyses. Both inclusive analyses were updated this summer [7] but the limits did not change significantly: Am, > 2.2ps -1 at 95% CL (dflepton), Am, > 2.9ps -1 at 95% CL (l- Qjet). The latter result has been extracted using the amplitude method and is so far the only result combined into the world averages. In 1996, OPAL reported [6] results from two quasi-exclusive analyses which reconstructed either D, mesons or ¢ mesons and vertexed these with an identified lepton to achieve a high purity B, sample with good proper time resolution. Unfortunately these analyses (using a maximum likelihood method) observed a large signal for B, mixing at frequencies near Am, = 0.5ps -1 which is highly disfavored by the other results quoted here; thus these analyses don't set useful limits on Bs mixing. These analyses have not yet been updated. OPAL does not yet combine their analyses to quote an overall limit. 4.3. D E L P H I DELPHI has also analysed data from 1991-94 to perform two inclusive B, mixing analyses using leptons as a final state tag and leptons or jet charge opposite the tagged jet to infer the initial state. They obtain limits comparable to the similar OPAL analyses quoted above. The major update released for this conference was a re-analysis by DELPHI of their full dataset (1991-5) using an improved tracking algorithm in a quasi-exclusive analysis which reconstructs D, mesons and leptons to form candidate Bs vertices. The D, mesons are reconstructed in both
'~ 3.5
-
i3
.
l
i
d
,
'
' "
t64sa
'
'
'
'
'
"
.e- .,~,~,
PRELIMINARY .2., ' ' "
'
"
11.1~'~
.i~ili~i::
2.5 2 1.5 1 0.5
-0.5 0
2
4
6
8
10
12
14
am. (ps'b
Figure 1. LEP combined B8 oscillation amplitude as a function of Am,.
hadronic and semileptonic modes. Most of the improvement in the analysis comes from much higher vertexing efficiency achieved with full 3D information provided by the upgraded vertex detector in 1994-5 [8]. They have also optimized their initial state tag to use all available information (i.e., identified kaons, leptons, jet charge) in the hemisphere opposite the Bs decay to correctly identify the initial state with high probability. This analysis is comparable to the ALEPH Ds - 1 analysis in statistics and proper time resolution, and sets a limit of Am8 > 8.3ps -1 at 95% CL, slightly better than their expected sensitivity. DELPHI has also combined all of their analyses to set a limit of Am, > 8.1ps -1 at 95% CL. 4.4. LEP Average All ALEPH, DELPHI analyses and the OPAL l - Qjet analysis have been combined in a grand average by the LEP B oscillation working group
G. Crawford/Nuclear Physics B (Proc. Suppl.) 66 (1998) 510-513 [4]. This reduces the impact of statistical fluctuations which can generate "lucky" or "unlucky" individual results and should improve the overall limit on average. The combined result is
A m s > 8.4ps -1 at 95%CL
(7)
The results of the combined amplitude fit are shown in Fig. 1. 5. F U T U R E P R O S P E C T S As seen from the above results, the analyses using quasi-exclusive Bs reconstruction have dominated the Bs mixing limits and are likely to continue to do so for the near future. Improvements in vertexing and reconstruction code to take full advantage of the upgraded LEP vertex detectors (a la DELPHI) is likely to drive any improvement in limits since the most of the data has been analyzed [9]. New and updated results from OPAL and L3 are eagerly awaited and could also have significant impact. In the near future, the SLD experiment at SLC will start to set Ares limits with their existing data (about 200k Z °, compared to 5M Z ° per LEP experiment). SLD can compete in these analyses because of very efficient and pure initial state tagging using the beam polarization (,,~ 80%) and the excellent decay length resolution of their new vertex detector (aL ~" 100 -- 200#m typical). This translates to a typical proper time resolution of ar "" 0.1 ps using inclusive vertexing techniques, and can improve with more exclusive reconstruction. This improvement in vertexing technology allows SLD much better "reach" in Ares with far fewer Z's than LEP. SLD can achieve a limit of Ares > 15ps-1 at 95% CL with a data sample of ~ 500k Z°'s, which they expect to acquire in their 1997-98 run. Many experiments at colliders and fixed target machines have been proposed to measure the Bs mixing frequency due to the experimental challenge and theoretical interest. Here we quote only the expectations from experiments which are relatively certain to run soon: HERA-B, a gas-jet target experiment in the HERA ring dedicated to b physics, expects to run in 1998 and set limits of Ares > 11 - 13ps -1 at 95% CL[10]. Both
513
CDF and DO experiments at Fermilab have expressed plans for measuring Bs mixing with upgraded vertex detectors during Tevatron Run II which is scheduled to start in 1999; recent estimates claim they can measure Ares > 20ps -~ at 95% CL with the full Run II dataset [11]. In the long-term future this measurement can be done "easily" at LHC-B [12]. 6. A C K N O W L E D G M E N T S I would like to thank my colleagues at LEP and SLD for their assistance in preparing this report, and in particular P. Roudeau, O. Schneider, and S. Willocq. I should also thank S. Adler, E. Barberio, K. Berkelman, R. Ben-David, F. Filthaut, T. Paul, V. Sharma, D. Stickland, and A. Turcot for making the Conference memorable. REFERENCES 1. Particle Data Group, Phys. Rev. D54 (1996) 95. 2. See for example the review by E. Barberio in these Proceedings. 3. H. G. Moser and A. Roussarie, Nucl. Inst. Meth. A384 (1997) 491. 4. LEP B Oscillations Working Group, preprint LEPBOSC 97/3. See also their Web site at http ://wwwcn. cern. ch/~ offline/lepwgs 5. K. Jakobs, Proc. XXVIIIth ICHEP, Warsaw, Eds. Z. Adjuk and A. K. Wroblewski. World Scientific, 1997. 6. S. Tarem, op. cit. 7. OPAL Collaboration, CERN-PPE/97-036 and CERN-PPE/97-064. 8. DELPHI Collaboration, private communication. 9. As of this writing, ALEPH has just announced a much improved inclusive lepton analysis with better decay length resolution: see EPS-HEP contribution 612. 10. HERA-B Collaboration, Nucl. Inst. Meth. A368 (1995) 124. 11. D. Amidei et. al. (Tev2000 Study Group), FERMILAB-PUB-96-082 12. See the report by S. Conetti in these Proceedings and references therein.