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recently , a lot of new experimental results are reported on charmed baryons  @xcite .
babar and belle give very accurate results based on their precise analysis .
in addition , new experiments such as j - parc , panda , lhcb , and belle ii are coming and expected to give further informations for charmed baryons .
mass spectrum of singly charmed baryons has been determined experimentally in high accuracy .
experimental status for masses of the ground state is evaluated as three or four - star by the particle data group .
the excited states are also investigated fairly well .
in contrast to singly charmed baryons , experimental data for doubly and triply charmed baryons are not well established . a candidate for the doubly charmed baryon , @xmath4 ,
has been reported only by the selex collaboration  @xcite , while @xmath4 has not been confirmed yet by other experimental groups such as babar  @xcite and belle  @xcite .
further experimental and theoretical confirmations are required to establish @xmath4 .
other doubly charmed baryons and any triply charmed baryons have not been observed by experiments . in this situation ,
theoretical predictions for the doubly and triply charmed baryon masses may give useful information for the experimental discovery of these states .
charmed baryon spectrum has been mainly investigated using gauge configurations generated with 2 + 1 flavors dynamical staggered quarks  @xcite . in this case
, a choice for light valence quarks requires a special care : one may take other fermion formulations for the valence light quarks ( the mixed action ) to avoid a problem due to a complicated flavor structure of the staggered quarks .
alternatively one may construct charmed baryon operators with the valence naive quark and then rewrite the correlation function of these operators in terms of the staggered propagators  @xcite .
both approaches violate the unitarity of the theory at finite lattice spacing , in addition to the rooting problem of dynamical staggered quarks .
therefore it is necessary to check their results using other combinations of dynamical and valence quarks which maintain the unitarity at non - zero lattice spacing .
another issue in the existing calculations on dynamical staggered configurations is that their chiral extrapolations suffer from large higher order corrections .
nlo su(2 ) heavy baryon chiral perturbation theory is employed to extrapolate their data to the physical point from 220  290 mev pion masses , but the result shows a bad convergence even at @xmath5  mev . a calculation directly at the physical quark masses without chiral extrapolation is the best way to remove this uncertainty .
there are a few investigations with other fermion formulations .
s.  drr _ et al _ have calculated charmed omega baryon masses using smeared improved wilson and brillouin fermions for valence strange and charm quarks  @xcite .
though their calculation is performed only at one quark mass ( @xmath6  mev ) and one lattice spacing ( @xmath7  fm ) on the @xmath8 @xmath9-improved wilson quark ensemble , they obtained a result which is consistent with the experimental value for the singly charmed omega baryon ( @xmath10 ) mass .
the etmc has studied charmed baryons on ensembles generated with @xmath8 dynamical twisted - mass quarks , employing the same twisted - mass fermion for degenerate up and down valence quarks and osterwalder - seiler fermions for strange and charm valence quarks  @xcite . for a doubly charmed baryon
, they have found @xmath11 gev .
this is the only result that is consistent with the selex experimental value @xmath12  gev , while other lattice qcd results deviate from this value .
reasons for this disagreement among lattice qcd results must be understood and should be eventually resolved .
one possible source for systematic uncertainties in the etmc calculation is a lattice artifact caused by the heavy charm quark mass at their lattice spacings , @xmath13  fm . indeed their results for charmed baryon masses , especially for @xmath14
, do not show clear scaling behaviors . to reduce this uncertainty ,
one must employ a heavy quark action that handles mass dependent lattice artifacts in the formulation , such as the fermilab action  @xcite , the relativistic heavy quark action  @xcite , or highly improved actions .
chiral extrapolation of etmc data from @xmath15  mev using the nlo heavy baryon chiral perturbation theory is another source of systematic uncertainties , as in the case of staggered quarks .    in ref .
@xcite , the present authors have shown that the charm quark mass corrections are under control at @xmath2  gev by adopting the relativistic heavy quark action of ref .
it removes the leading cutoff errors of @xmath16 and the next to leading effects of @xmath17 for arbitrary order @xmath18 by tuning finite number of parameters . employing this action for the charm quark
, we have investigated properties of mesons involving charm quarks with the 2 + 1 dynamical flavor pacs - cs configurations on @xmath0 lattice  @xcite reweighted to the physical point for up , down and strange quark masses .
we have found our results for charmed meson masses are consistent with experiment at a percent level , and so are those for the decay constants with a few percent accuracy , though our results are obtained at a single lattice spacing .    encouraged by this result , we have extended our investigation to the charmed baryon sector and report the result in this paper .
one of the big advantage in our investigation over previous calculations is that the chiral extrapolation is no more necessary , since our calculations are performed at the physical point .
we are free from the convergence problem of the heavy baryon chiral perturbation theory .
we first compare our masses of singly charmed baryons with the corresponding experimental values , to check if our method works also for the baryon sector . we then evaluate the doubly and triply charmed baryon spectra as our predictions .
a part of this work has been reported in ref .
@xcite .
this paper is organized as follows .
section ii explains our method and simulation parameters .
section iii describes our results for the singly charmed baryon spectrum , and comparison with experiments . in sec .
iv , we present our results for doubly and triply charmed baryon masses . a conclusion is given in sec .
our investigation is based on a set of @xmath19 flavor dynamical lattice qcd configurations generated by the pacs - cs collaboration  @xcite on a @xmath20 lattice using the nonperturbatively @xmath9-improved wilson quark action with @xmath21  @xcite and the iwasaki gauge action  @xcite at @xmath22 .
the aggregate of 2000 md time units were generated at the hopping parameter given by @xmath23 , and 80 configurations separated by 25 md time units were selected for our calculations .
we then reweight those configurations to the physical point given by @xmath24 .
the reweighting shifts the masses of @xmath25 and @xmath26 mesons from @xmath27  mev and @xmath28  mev to @xmath29  mev and @xmath30  mev , with the cutoff at the physical point estimated to be @xmath2  gev from the @xmath1 baryon mass .
our parameters and statistics at the physical point are given in table  [ table : statistics ] .
.simulation parameters .
md time is defined as the number of trajectories multiplied by the trajectory length . [ cols="^,^,^,^,^",options="header " , ]
our results for the singly charmed baryon spectrum at the physical point are summarized in fig .
[ figure : mass_singly_charmed_experiment ] .
all our values for the charmed baryon masses are predictions from lattice qcd , since the physical charm quark mass has already been fixed by the mass for the spin - averaged @xmath31 charmonium state and no other experimental inputs for charmed baryon masses are required .
as can be seen from the figure , our prediction for the singly charmed baryon spectrum in lattice qcd at a single lattice spacing is in reasonable agreement with the experimental one . in fig .
[ figure : mass_lambda_c_lattice ] , we also compare our value for @xmath32 with other results obtained in recent lattice qcd simulations using the dynamical staggered quarks  @xcite , and the twisted mass quarks  @xcite .
all results are consistent with each other , though the statistical error is larger for our result due to the conservative choice of our fitting interval .    figure  [ figure : mass_sigma_c_lambda_c_lattice ] displays mass differences .
our results are consistent with experimental values within 2 @xmath33 uncertainty .
these agreements indicate that the decomposition of @xmath34 @xmath35-type and @xmath36-type baryons , as well as that of @xmath37 and @xmath34 charmed baryons , have been made successfully in our calculation .
mass . , width=283 ]     types ( left panel ) , @xmath38 types ( right panel ) . , title="fig:",width=283 ]   types ( left panel ) , @xmath38 types ( right panel ) . ,
title="fig:",width=283 ]    we note that several systematic errors have not been fully evaluated yet for these results .
first , finite size effects are not taken into account .
although the nlo heavy baryon chiral perturbation theory predicts that finite size effects for charmed baryons are less than 1 % , higher order terms may give significant contributions .
a direct confirmation in lattice qcd by comparing spectra among different lattice volumes is desirable .
second , strong decays such as @xmath39 are not taken into account in our analysis , since @xmath39 is kinematically prohibited on our lattice volume .
last but not least , our results are obtained at a single lattice spacing without continuum extrapolation . although a naive order counting gives a percent level of cutoff effects from @xmath40 terms in the relativistic heavy quark action , the continuum extrapolation is necessary to remove this uncertainty .
additional calculations should be performed in the future to remove all systematic errors mentioned above .
for doubly and triply charmed baryons , an experimental value has been reported only for @xmath4 , though the experimental status is controversial . in the other channels ,
lattice qcd result gives predictions before experimental mass measurements .
figure  [ figure : mass_doubly_charmed_experiment ] shows our results for the doubly charmed baryons .
our estimate for @xmath41 clearly deviates from the experimental value by selex collaboration  @xcite .
the difference is @xmath42 , as shown in the right figure .
our result for @xmath41 is consistent with results from other lattice qcd calculations except etmc , as shown in fig .
[ figure : mass_xi_cc_lattice ] .
this discrepancy is need to be understood and should be resolved .
similarly , fig .
[ figure : mass_omega_ccc_lattice ] displays lattice qcd results for the triply charmed baryon from several groups .
our prediction agrees with that by others except etmc .
a marginal discrepancy is observed between our value and that of ref .
@xcite for @xmath14 @xmath43 @xmath44 @xmath45 mass difference ( the right figure ) .    for a more detailed comparison ,
precise evaluations of all systematic errors are required . in particular ,
the largest source of systematic errors for our calculations is the lattice artifact , which should be removed by the continuum extrapolation .        .
, width=283 ]     ( left panel ) , and @xmath46 @xmath43 @xmath44 @xmath47 ( right panel ) . , title="fig:",width=283 ]   ( left panel ) , and @xmath46 @xmath43 @xmath44 @xmath47 ( right panel ) .
, title="fig:",width=283 ]
we have studied charmed baryon masses in @xmath48 dynamical lattice qcd at a lattice spacing of @xmath2  gev .
the reweighting technique allows us to perform a measurement directly at the physical point .
it removes a systematic error associated with chiral extrapolations of charmed baryon masses , which had prevented previous lattice qcd calculations from predicting precise values for charmed baryon masses .
our results for the mass spectrum of singly charmed baryons are consistent with experiments within 2 @xmath33 uncertainty .
this confirms that we are able to control charm quark mass corrections successfully , not only in the meson sector  @xcite but also in the baryon sector , by use of the relativistic heavy quark action of ref .
@xcite .
we then extract predictions for doubly charmed baryons .
our result for @xmath41 is consistent with values of other lattice qcd calculations employing the dynamical staggered quarks , but disagree with the estimation by the etmc .
moreover , our @xmath4 mass is different from the selex experimental value , approximately by 85 mev , which corresponds to @xmath42 .
a similar deviation between etmc and us is also observed in the triply charmed baryon mass , @xmath14 .
precise estimations of all systematic errors , especially the lattice artifacts , are required to resolve these discrepancies .
y.n . thanks yasumichi aoki , carleton detar and nilmani mathur for valuable discussions , and stephan drr for his comment on this manuscript . numerical calculations for the present work
have been carried out on the pacs - cs computer under the `` interdisciplinary computational science program '' of the center for computational sciences , university of tsukuba .
this work is supported in part by grants - in - aid of the ministry of education , culture , sports , science and technology(mext)-japan ( nos .  18104005 , 20340047 , 20540248 , 21340049 , 22244018 , and 24540250 ) , the grants - in - aid for scientific research on innovative areas ( no .
2004 : 20105001 , 20105002 , 20105003 , and 20105005 ) , and spire ( strategic program for innovative research ) .
we investigate the charmed baryon mass spectrum using the relativistic heavy quark action on 2 + 1 flavor pacs - cs configurations previously generated on @xmath0 lattice . 
 the dynamical up - down and strange quark masses are tuned to their physical values , reweighted from those employed in the configuration generation . at the physical point , 
 the inverse lattice spacing determined from the @xmath1 baryon mass gives @xmath2  gev , and thus the spatial extent becomes @xmath3  fm . 
 our results for the charmed baryon masses are consistent with experimental values , except for the mass of @xmath4 , which has been measured by only one experimental group so far and has not been confirmed yet by others . in addition , we report values of other doubly and triply charmed baryon masses , which have never been measured experimentally .    uthep-655 + utccs - p-69 +