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almost since the discovery of charm mesons , mixing of @xmath0 mesons have been sought in analogy to the well - known @xmath1 mixing .
due to very effective gim suppression , the expected mixing rate in the charm system is much smaller than for kaons . only very recently ,
the babar @xcite and belle @xcite collaborations have reported the first evidence of charm mixing .
these results have renewed the interest from the theory community as the observed mixing rate could be caused by physics beyond the standard model or at least provide additional constraints on new physics .
none of the mixing measurement have a significance above four standard deviations , but several have similar precision for the mixing parameters . by combining the measurements we therefore obtain more precise values for the mixing parameters and exclude the no - mixing hypothesis with larger confidence . combining the different mixing measurements is not completely straightforward , since not all measurements are sensitive to the same charm mixing parameters .
the heavy flavor averaging group ( hfag ) in 2006 created a subgroup with the responsibility of providing averages of charm physics measurements .
one of the high priority tasks of this group is to combine the charm mixing measurements into world - average values for the fundamental mixing parameters .
the first average assuming cp conservation was shown at fpcp @xcite . besides those results
, we here report the first results of combining mixing measurements where we allow for cp violation .
mixing is present in the @xmath0 system if the mass eigenstates , @xmath2 and @xmath3 , differ from the flavor eigenstates , @xmath4 and @xmath5 .
generally one can write @xmath6 .
the variables of fundamental interest are the mass difference , @xmath7 and decay width difference , @xmath8 between the two mass eigenstates .
traditionally , in charm mixing one uses the dimensionless variables , @xmath9 and @xmath10 , where @xmath11 is the average decay width .
cp violation in mixing or in the interference between mixing and decay would manifest itself as @xmath12 and @xmath13 , respectively is for the moment assumed to be independent of decay mode . ] .
in addition cp violation could show up in the decay itself giving rise to decay mode dependent parameters .
most measurements do not directly measure @xmath14 .
for instance in mixing measurements using @xmath15 decays there is a unknown strong phase , @xmath16 , so the results obtained are for @xmath17 and @xmath18 . in the averaging procedure , we first combine measurements of the same parameters to obtain the more precise observables .
most measurements are performed using likelihood fits and the combination is therefore performed by multiplying likelihood functions from each measurement and finding the new maximum . by combining likelihoods , correlations between observables and possible non - gaussian tails are taken into account . for measurements which are not using likelihoods ,
we construct a likelihood using symmetrized , gaussian uncertainties . to combine different types of measurements ,
the different combined likelihoods are recalculated as a function of @xmath19 minimizing over any other variables .
@xmath16 is included since there is both a direct measurement @xcite and by combining the @xmath15 measurement with the other measurement of @xmath20 and @xmath21 , one can also get a precise measurement of @xmath16 .
when plotting confidence contours for @xmath14 we minimize the likelihood over @xmath16 .
the combining of likelihood functions is currently only done for the cp conserving case . in principle
it can be done also for the cp violating case by simply having two more variables , @xmath22 and @xmath23 , in the final likelihood function .
unfortunately not all likelihoods are currently available for the measurements which allow for cp violation .
a simple combination is therefore performed by forming a @xmath24 of all measurements expressed in terms of the fundamental mixing parameters .
the @xmath24 assumes gaussian errors , but correlations between observables in each individual measurement is taken account by using the full covariance matrix for each result .
the following averages were performed by adding log likelihoods from fits where cp conservation was assumed .
one can observe charm mixing by finding a difference in the lifetime measured in decays to cp eigen states such as @xmath25 and @xmath26 and the mixed - cp decay @xmath27 .
we combine six results @xcite from such analyzes .
all of these measure @xmath28 . in the limit of cp conservation one has @xmath29 .
the average of the six measurements is @xmath30 .
this is @xmath31 from the no - mixing hypothesis .
as can be seen from figure  [ fig : ycp ] , this average is mainly driven by the recent belle measurement .
values and the hfag average.,width=302 ]      wrong - signed semileptonic decays provide a clean way of searching for charm mixing , but the measurements are only sensitive to the integrated mixing rate @xmath32 .
four measurements @xcite are combined and give an average of @xmath33 .
in addition to the semileptonic decays , @xmath34 can also measured in the analysis of fully hadronic decays .
the semileptonic result is therefore combined from two hadronic analyzes @xcite and in addition an analysis of tagged decays at the @xmath35 @xcite .
the combination is illustrated in figure  [ fig : rm ] and gives an average value of @xmath36 . in the transformation to a likelihood in @xmath14
, we ignore the non - physical region of @xmath37 .
decays are averaged with results from multi - body hadronic charm decays.,width=302 ]      one can measure @xmath20 and @xmath21 directly using a time - dependent dalitz plot analysis of @xmath38 decays .
two measurements @xcite have been published and these have been averaged by hfag and gives @xmath39 and @xmath40 . combining this average with the averages above for @xmath34 and @xmath41 using likelihoods mapped as a function of @xmath14 we obtain @xmath42 and @xmath43 .
contours of the combined likelihood function at the levels corresponding to @xmath44 to @xmath45 confidence levels are shown in figure  [ fig : xy ] .
note that the confidence levels shown correspond to two - dimensional coverage probabilities of 68.27% , 95.45% , etc .
, and therefore @xmath46 etc .     from the combination of @xmath41 , @xmath47 and @xmath14 from the time - dependent dalitz analysis of @xmath48.,width=302 ]
as mentioned above , one can measure @xmath50 and @xmath51 using the doubly - cabibbo suppressed ( dcs ) decay @xmath49 .
the likelihood functions are available for two measurements @xcite of this type .
these are combined and gives the averages @xmath52 and @xmath53 .
the corresponding likelihood contours are shown in figure  [ fig : kpi ] .    .
the wiggles in the 4 and @xmath45 contours for @xmath54 is a binning effect .
, width=302 ]      the combined likelihood for @xmath49 decays can be expressed as a function of @xmath19 ignoring the part with @xmath54 .
this likelihood can be combined with the likelihood from the combination of the other mixing results in section  [ sec : nonkpi ] which do not depend on @xmath16 .
an additional constraint comes from a cleo - c measurement @xcite of @xmath55 , where a small dependence on @xmath20 and @xmath21 is ignored in the combination .
figure  [ fig : all ] shows the likelihood contours in @xmath14 after minimizing over @xmath16 .
the region around the central value is almost unchanged with respect to the result without the @xmath49 decays ( figure  [ fig : xy ] ) .
this is also reflected in the over all average for @xmath20 and @xmath21 which are @xmath56 the @xmath49 measurements do not contribute much to the central value , because of the poorly known phase @xmath16 .
however they do help exclude the no - mixing hypothesis and cause the dip seen in the contours close to @xmath57 .
at @xmath57 we obtain @xmath58 with respect to the minimum .
this corresponds to a significance of the combined mixing signal of @xmath59 .
the combination also gives an improved value for @xmath16 .
this can be seen in the projection of the likelihood after minimizing over @xmath20 and @xmath21 in figure  [ fig : delta ] .
the combination gives @xmath60 . without the cleo - c measurement of @xmath16
, there would be an equally good second minimum at @xmath61 .
, from combining all mixing measurements .
, width=302 ]
measurements of charm mixing can be done without assuming cp conservation by fitting @xmath62 and @xmath63 mesons as separate samples .
most of the measurements above have done that and we therefore can combine those to also provide constraints on the cp violating parameters . when allowing for cp violation , the measured parameters are related slightly differently to the mixing parameters . for the lifetime ratio measurements , one has @xmath64 where @xmath65 is the measured relative lifetime difference for @xmath66 and @xmath67 . for @xmath49 decays ,
the @xmath50 and @xmath51 measured for @xmath62 and @xmath63 are related as follows @xmath68 where @xmath69 . for @xmath38
decays the measurement directly gives @xmath20 , @xmath21 , @xmath22 and @xmath23 , while for the @xmath34 analysis the results are not separated and therefore just measure @xmath32 .
the measurement of @xmath16 from cleo - c is not done separately for @xmath62 and @xmath63 mesons and is not included in the combined result allowing for cp violation .    in total 22 measurements
are combined in a @xmath24-fit to extract seven parameters , the four mixing and cp violation parameters , @xmath20 , @xmath21 , @xmath22 and @xmath23 , and three characterizing @xmath49 , namely @xmath16 , the dcs rate @xmath70 , and the direct decay rate asymmetry @xmath71 .
the fit gives @xmath72 and the following mixing parameters @xmath73 the mixing parameters are almost unchanged with respect to the cp conserving average .
this is also seen from the confidence levels shown in figure  [ fig : xycpv ] .
one can also draw the @xmath44 to @xmath45 confidence level contour for @xmath23 versus @xmath22 using @xmath74 .
this is shown in figure  [ fig : qp ] .
the no - cp violation hypothesis is seen to lie well within the @xmath75 contour .
the combined results for the @xmath49 parameters are @xmath76 there is little change in @xmath16 with respect to the cp conserving average and no evidence for direct cp violation as @xmath71 is consistent with zero .     from combining mixing measurements with cp violation allowed .
the dashed blue curve shows the @xmath75 contour from the cp conserving case for comparison.,width=302 ]     from combining mixing measurements with cp violation allowed.,width=302 ]
evidence of charm mixing has been reported from several experiments in the last year .
a new subgroup of hfag has performed an average of these and other existing charm mixing results .
the combined result has a signal significance in excess of 5 standard deviations and gives the mixing parameters @xmath77 cp violation parameters have also been combined and gives @xmath78 this is fully consistent with no cp violation being present in charm mixing .
hfag intends to periodically update these averages as new results become available in order to provide the most precise mixing parameters to the community .
99 b.  aubert _ et al . _ [ babar collaboration ] , phys .
lett .   * 98 * , 211802 ( 2007 ) [ arxiv : hep - ex/0703020 ] . m.  staric _ et al . _ [ belle collaboration ] , phys .
lett .   * 98 * , 211803 ( 2007 ) [ arxiv : hep - ex/0703036 ] .
a.  j.  schwartz , _ in the proceedings of 5th flavor physics and cp violation conference ( fpcp 2007 ) , bled , slovenia , 12 - 16 may 2007 , pp 024 _ [ arxiv:0708.4225 [ hep - ex ] ] .
d.  m.  asner _
[ cleo collaboration ] , int .
j.  mod .
a * 21 * , 5456 ( 2006 ) [ arxiv : hep - ex/0607078 ] .
e.  m.  aitala _ et al .
_ [ e791 collaboration ] , phys .  rev .
* 83 * , 32 ( 1999 ) [ arxiv : hep - ex/9903012 ] .
j.  m.  link _ et al . _
[ focus collaboration ] , phys .
b * 485 * , 62 ( 2000 ) [ arxiv : hep - ex/0004034 ] .
s.  e.  csorna _ et al .
_ [ cleo collaboration ] , phys .  rev .
d * 65 * , 092001 ( 2002 ) [ arxiv : hep - ex/0111024 ] .
b.  aubert _ et al .
_ [ babar collaboration ] , phys .
lett .   * 91 * , 121801 ( 2003 ) [ arxiv : hep - ex/0306003 ] .
k.  abe _ et al . _ [ belle collaboration ] , phys .  rev
.  lett .   * 88 * , 162001 ( 2002 ) [ arxiv : hep - ex/0111026 ] .
e.  m.  aitala _ et al .
_ [ e791 collaboration ] , phys .  rev .
lett .   * 77 * , 2384 ( 1996 ) [ arxiv : hep - ex/9606016 ] . c.  cawlfield _ et al . _
[ cleo collaboration ] , phys .
d * 71 * , 077101 ( 2005 ) [ arxiv : hep - ex/0502012 ] .
u.  bitenc _ et al . _ [ belle collaboration ] , phys .
d * 72 * , 071101 ( 2005 ) [ arxiv : hep - ex/0507020 ]
. b.  aubert _ et al . _ [ babar collaboration ] , phys .
d * 76 * , 014018 ( 2007 ) [ arxiv:0705.0704 [ hep - ex ] ] .
b.  aubert _ et al .
_ [ babar collaboration ] , phys .
* 97 * , 221803 ( 2006 ) [ arxiv : hep - ex/0608006 ] .
b.  aubert _ et al .
_ [ babar collaboration ] , arxiv : hep - ex/0607090 .
l.  m.  zhang _
[ belle collaboration ] , phys .  rev .
* 99 * , 131803 ( 2007 ) [ arxiv:0704.1000 ]
. l.  m.  zhang _
[ belle collaboration ] , phys .
lett .   * 96 * , 151801 ( 2006 ) [ arxiv : hep - ex/0601029 ] .
recently the first evidence for charm mixing has been reported by several experiments . to provide averages of these mixing results and other charm results , 
 a new subgroup of the heavy flavor averaging group has been formed . 
 we here report on the method and results of averaging the charm mixing results .