+ cc ts are harming orkshop. y therefore ). Estimates ery c d u cc , etraquarks kgrounds. This pa- ons and tetraquarks tal requiremen y ons considered are: t CHARM2000 w .tau.ac.il y h for ing the bac iz harmed bary UP 2255-95, 21 June 1995 ons and T ysik A, Hadrons and Nuclei A y of Exact Sciences, tT View metadata,citationandsimilarpapersatcore.ac.uk v - 9506405, (HEPPH-9506405) ym@tauph acult , 69978 Ramat Aviv, Israel, ysics and Astronom y A. Moinester ternal structure, pro duction cross sections, tal and theoretical review. It ma 〉 y
PostScript processed by the SLAC/DESY Libraries on 26 Jun 1995. Abstract HEP-PH-9506405 urra hrift fur Ph w to Searc es, in hes of doubly c kler F ersit y Preprin (ccs); and the tetraquark is T ( ts with high energy hadron b eams and a high in- Murra + cc Ho ersit ho ol of Ph hing ratios, and yields. Exp erimen tal searc Sc E-mail: m e [email protected] R.&B.Sac el Aviv Univ hiv (ccu), and T t, in the spirit of the aims of the recen el Aviv Univ T Submitted to Zeitsc Arc ++ cc en of masses, lifetim y sp ectrometer are considered here. The bary y mo des, branc en for optimizing the signal and minim Doubly Charmed Bary ossible exp erimen p er is designed as an exp erimen exp erimen giv b e of assistance in the planning for a future state-of-the-art v deca (ccd), are giv at xed target exp erimen tensit P provided byCERNDocumentServer brought toyouby CORE
Intro duction
The Quantum Chromo dynamics hadron sp ectrum includes doubly charmed
+ ++ +
baryons: (ccd), (ccu); and (ccs), as well as ccc and ccb. Prop erties
cc cc cc
of ccq baryons were discussed by Bjorken [1], Richard [2], Fleck and Richard
and Martin [3], Savage and Wise and Springer [4, 5 ], Kiselev et al. [6, 7 ],
Falk et al. [8], Bander and Subbaraman [9], and Stong [10 ]. Singly charmed
baryons are an active area of current research [11, 12 , 13 , 14 , 15 , 16 ], but there
are no exp erimental data on the doubly charmed variety. A dedicated double
charm state of the art exp eriment is feasible and required to observe and to
investigate such baryons. The required detectors and data acquisition system
would need very high rate capabilities, and therefore would also serveasa
testing ground for LHC detectors. Double charm physics is in the mainstream
and part of the natural development of QCD research. This pap er is an
exp erimental and theoretical review, as part of the planning for a state-of-
the-art very charming exp eriment, in the spirit of the aims of the recent
CHARM2000 workshop [17]. The presentwork is an expanded version of a
workshop contribution [18] dealing with a CHarm Exp eriment with Omni-
Purp ose Setup (CHEOPS) at CERN [19].
The ccq baryons should b e describ ed in terms of a combination of p er-
turbative and non-p erturbative QCD. For these baryons, the light q orbits a
tightly b ound cc pair. The study of such con gurations and their weak decays
can help to set constraints on phenomenological mo dels of quark-quark forces
[3, 20 ]. Hadron structures with size scales much less than 1/ should b e
qcd
well describ ed by p erturbative QCD. This is so, since the small size assures
that is small, and therefore the leading term in the p erturbative expansion
s
is adequate. The tightly b ound (cc) diquark in ccq may satisfy this condi-
3
tion. For ccq, on the other hand, the radius is dominated by the low mass q,
and is therefore large. The relative (cc)-(q) structure may b e describ ed sim-
ilar to mesons Qq , where the (cc) pair plays the role of the heavy antiquark.
Savage and Wise [4] discussed the ccq excitation sp ectrum for the q degree
of freedom (with the cc in its ground state) via the analogy to the sp ectrum
of Qq mesons. Fleck and Richard [3] calculated excitation sp ectra and other
prop erties of ccq baryons for a variety of p otential and bag mo dels, which
describ e successfully known hadrons. Stong [10] emphasized how the QQq
excitation sp ectra can b e used to phenomenologically determine the QQ p o-
tential, to complement the approach taken for QQ quarkonium interactions. 1
The ccq calculations contrast with ccc or ccb or b-quark physics, which are
closer to the p erturbative regime. As p ointed out by Bjorken [1], one should
strive to study the ccc baryon. Its excitation sp ectrum, including several
narrow levels ab ove the ground state, should b e closer to the p erturbative
regime. The ccq studies are a valuable prelude to such ccc e orts.
A tetraquark (ccu d) structure (designated here byT)was describ ed by
Richard, Bander and Subbaraman, Lipkin, Tornqvist, Ericson and Karl,
Nussinov, Chow, Maonohar and Wise, Weinstein and Isgur, Carlson and
Heller and Tjon, and Ja e, [2, 9, 21 , 22 , 23, 24 , 25 , 26 , 27 , 28 , 29 ]. Tetraquarks
with only u,d,s quarks have also b een extensively studied [2, 30, 31 ]. The
doubly charmed tetraquark is of particular interest, as the calculations of
these authors indicate that it may b e b ound. Some authors [2, 9, 24 , 25]
compare the tetraquark structure to that of the antibaryon Qu d, which has
the coupling Q (ud) . In the T, the tightly b ound (cc) then plays the role
3
3 3
of the antiquark Q. The tetraquark may also have a deuteron-like meson-
+ 0 +
meson weakly b ound D D comp onent, coupled to 1 , and b ound bya
long range one-pion exchange p otential [22, 24 ], which corresp onds to light
quark exchanges in the quark picture. Such a structure has b een referred
to as a deuson byTornqvist [22]. The deuson is analogous with the H
2
molecule; where the heavy and light quarks play the roles of protons and
electrons, resp ectively. The discovery of such an exotic hadron would have
far reaching consequences for QCD, for the concept of con nement, and for
sp eci c mo dels of hadron structure (lattice, string, and bag mo dels). De-
tailed discussions of exotic hadron physics can b e found in recent reviews
[36]. Some other exotics that can b e investigated in CHEOPS are: Pen-
taquarks uudcs; uddcs; udscs; uudcc; uddcc; udscc [32], Hybrid q qg [33], usd d
+
U (3100) [34], uuddss H hexaquark [35], uuddcc H hexaquark [25], q qs s or
cc
q qg C(1480) [36 ], andc cqqqqq heptaquark [9]. But we do not discuss these
various exotic hadrons in detail in this rep ort.
+ 0 0
Should only the ccu d (D D ) b e b ound; or should the ccd u (D D )
+ 0
also b e b ound? The D D state, if ab ove the DD threshold, can only
decay strongly to doubly charmed systems. But it is easier to pro duce only
0
one cc pair, as in D D . However, this state has numerous op en strong
decaychannels. These include charmonium plus one or two pions and all
the multipion states and resonances b elow 3.6 GeV, and it is therefore not
0
strong interaction stable. One may argue that a D D state is unlikely to
b e b ound. In a deuson, b ound by pion-exchange, the sign of the p otential 2
which binds the two D mesons dep ends on the pro duct of the sign of the two
vertices asso ciated with the pion exchange. The sign of the D vertex dep ends
on T , the z-comp onent of isospin, whichchanges from +1 to 1inchanging
z
from p ositive to negativeD . Therefore, if the p otential is attractive in the
+ 0 0
case of D D , it will b e repulsive in the case of D D . Consequently,
+ 0 0
the calculations [22, 24 ] for a b ound D D suggest that the D D maybe
unb ound. Shmatikov [37 ] explicitly studied the widths and decay mechanisms
0 + 0
of D D , including some b ound p ossibilities. Therefore, in the D D
0
search, it would b e of value to also lo ok at D D data. Even if no p eak
is observed, the combinatoric backgrounds may help understand those for
+ 0
D D .
Mass of ccq Baryons and T
++
Bjorken [1] suggests mass ratios M( = ) = 1.60 and M( ==1.57),
ccc bbb
++
which follows from the extrap olation of M( =; ! ) and M( = ). He as-
sumes the validity of the "equal-spacing" rule for the masses of all the J=3/2
baryons, which gives the p ossibilitytointerp olate b etween ccc, bbb, and
ordinary baryons. The masses of ccq baryons with J=1/2 were estimated
relative to the central J=3/2 value. The cc diquark is a color antitriplet with
spin S=1. The spin of the third quark is either parallel (J=3/2) or anti-
parallel (J=1/2) to the diquark. The magnitude of the splitting is in inverse
prop ortion to the pro duct of the masses of the light and heavy quarks. These
are taken as 0.30 GeV for u and d, 0.45 GeV for s, 1.55 GeV for c, and 4.85
GeV for b. The equal spacing rule for J=3/2, with n the numb er of quarks
i
of a given avor, is then [1]:
M =1=3[1:23(n + n )+1:67n +4:96n +14:85n ]: (1)
u d s c b
For ccq, the J=1/2 states are lower than the J=3/2 states by ab out 0.1 GeV
[2]. This approach for ccq gives results close to those of Richard et al. [2, 3 ].
Fleck and Richard [3] also estimate the tetraquark mass. Fleck and Richard,
and Nussinov [24] have shown that ccq and ccu d masses near 3.7 GeV are
consistent with exp ectations from QCD mass inequalities.
The estimates lead to masses [1, 2, 3 ]:
(ccs), 1/2+, 3.8 GeV;
(ccu), 1/2+, 3.7 GeV; 3
(ccd), 1/2+, 3.7 GeV;
(ccu d), 1/2+, 3.6 GeV;
Lifetime of ccq Baryons and T.
++ +
The and decays should probably b e dominated by sp ectator dia-
cc cc
grams [1, 3, 11 , 38 , 39 , 40] with lifetimes ab out 200fs, roughly half of the
0 +
D or . Fleck and Richard [3] suggest that p ositiveinterference will o ccur
c
between the s-quark resulting from c-decay, and the pre-existing s-quark in
+ ++
. Its lifetime would then b e less than that of . Bjorken [1] and also
cc c
+
Fleck and Richard [3] suggest that internal W exchange diagrams in the
cc
decay could reduce its lifetime to around 100fs, roughly half the lifetime of
+
the . The lifetime of the T should b e much shorter, according to the pat-
c
+
tern set by the D lifetime. These estimates are consistent with the present
understanding of charmed hadron lifetimes [11, 38, 39 , 40 ]. One exp ects that
predominantly doubly charmed hadrons are pro duced with small momentum
in the center of mass of the colliding hadrons. They are therefore suciently
fast in the lab oratory frame. The lifetime b o ost in the lab oratory frame for
q
a ccq baryon is roughly [41] p =2M , if it is pro duced at the center
in N
of rapidity with a high energy hadron b eam of momentum p .For a CERN
in
exp eriment with p 400 GeV/c, this corresp onds to 15, with ccq
in
energies near 55 GeV.
Pro duction Cross Section of ccq Baryons
One can consider pro duction of doubly charmed hadrons by proton and Sigma
and pion b eams. Pion b eams are more e ective in pro ducing high-X D
F
mesons, as compared to b eams. Here, X designates the Feynman X -
F F
value, X = p =P ,evaluated with lab oratory momenta. And baryon
F D beam
b eams are likely more e ective than pion b eams in pro ducing ccq and cqq
baryons at high X .
F
Consider a hadronic interaction in whichtwo ccpairs are pro duced. The
two c's combine and then form a ccq baryon. Calculations for ccq pro duction
via suchinteractions have not yet b een published. Even if they are done, they 4
will have large uncertainties. Some ingredients to the needed calculations
can b e stated. For ccq pro duction, one must pro duce two c quarks (and
asso ciated antiquarks), and they must join to a tightly b ound, small size
anti-triplet pair. The pair then joins a light quark to pro duce the nal ccq.
The two c-quarks may arise from two parton showers in the same hadron-
hadron collision, or even from a single parton shower, or they may b e present
as an intrinsic charm comp onent of the incident hadron, or otherwise. The
two c-quarks may b e pro duced (initial state) with a range of separations and
relative momenta (up to say tens of GeV/c). In the nal state, if they are
tightly b ound in a small size cc pair, they should have relative momentum
lower than roughly 1. GeV/c. The overlap integral b etween initial and
nal states determines the probability for the cc-q fusion pro cess. For cqq
pro duction, a pro duced c quark may more easily combine with a (pro jectile)
di-quark to pro duce a charmed baryon. A ccq pro duction calculation in this
framework, based on two parton showers in the same hadron-hadron collision,
is in progress by Levin [42].
As an aid in comparing di erent p ossible calculations, one may parame-
terize the yield as:
(ccq )= (cq q ) (ccq )= (cq ) k [ (cc)= (in:)] kR: (2)
Here, (cc) is the charm pro duction cross section, roughly 25 b; (in:)is
the inelastic scattering cross section, roughly 25 mb; and R is their ratio,
3
roughly 10 [43]. Here, k is the assumed "suppression" factor for join-
ing two c's together with a third light quark to pro duce ccq; compared to
cqq or cq pro duction, where the c quark combines with a light diquark to
give cqq or a light quark to give cq. Eq. 2 do es not represent a calcula-
tion, and has no comp elling theoretical basis. It implicitly factorizes ccq
pro duction into a factor (R) that accounts for the pro duction of a second c-
quark, and a factor (k) describing a subsequent ccq baryon formation prob-
ability. Considering the overlap integral describ ed in the preceding para-
graph, one may exp ect k values less than unity for simple mechanisms of
ccq formation. It is p ossible to have a factor k>1, if there is some enhance-
ment correlation in the pro duction mechanism. Reliable theoretical cross
section calculations are needed, including the X -dep endence of ccq pro duc-
F
tion. In the absence of such a calculation, we will explore the exp erimen-
tal consequences of a ccq search for the range k=0.1-1.0, corresp onding to 5
4 3
(ccq )= (cq q ) (ccq )= (cq ) 10 10 . Assuming (cc)charm pro-
duction cross sections of 25 microbarns, this range corresp onds to ccq cross
sections of 2.5-25. nb/N.
p
Aoki et al. [44 ] rep orted a low statistics measurementat s =26 GeV for
2
double to single op en charm pair pro duction, of 10 . This D DDD to D D
ratio was for all central and di ractiveevents. This high ratio is encouraging
for ccq searches, compared to the value from NA3 [45] of ( )= ( )
4
3 10 .We assume that the result is relevant, even though pro duction
is only a small part ( 0.4%) of the charm pro duction cross section, with most
of the cross section leading to op en charm. For double or double charm
pair hadropro duction, the suppression factor k for two c-quarks to join into
the same ccq is missing. These two results for double charm pro duction
therefore establish a range of values for R in Eq. 2, consistent with the value
3
10 estimated ab ove in the discussion of Eq. 2. Robinett [46] discussed
pro duction and Levin [42] discussed ccq pro duction in terms of multiple
parton interactions. Halzen et al. [47 ] discussed evidence for multiple parton
interactions in a single hadron collision, from data on the pro duction of two
lepton pairs in Drell-Yan exp eriments.
It will b e of interest to compare ccq pro duction in hadron versus electron-
p ositron collisions, even if CHEOPS deals with hadron interactions. Follow-
ing pro duction of a single heavy quark from the decay of a Z or W b oson
pro duced in an electron-p ositron collision, Savage and Wise [4] discussed
the exp ected suppression for the the pro duction of a second heavy quark by
string breaking e ects or via a hard gluon. Kiselev et al. [6] calculated low
cross sections for double charm pro duction at an electron-p ositron collider B
p
5
factory, for s= 10.6 GeV. They nd (ccq )= (cc)=7:10 . Although
this result is inapplicable to hadronic interactions as in CHEOPS; the work
describ es some imp ortant calculational steps, and also demonstrates the con-
tinued wide interest in this sub ject.
Anumber of works [7, 48 , 49, 50 , 51 , 52 , 53 , 54, 55 ] consider the pro duction
and decay of doubly heavy hadrons (b cq, bc, etc.) at future hadron collider
exp eriments at the FNAL Tevatron or CERN LHC. Kiselev et al. [7] givea
p
preliminary estimate of (ccq ) 10: nb/N in hadronic pro duction at s=
100. GeV. This corresp onds to k=0.4 in the parameterization of Eq. 2. In
hadronic pro duction, the pro cess gg ! bb or q q ! bb may b e followed by
gluon bremsstrahlung and splitting b ! bg ! bcc to yield B (bc) mesons
c
[8, 5 , 56 , 57 , 58, 59 , 60 , 61 , 62 , 63]. Doubly charmed baryon pro duction may 6
then p ossibly pro ceed via the weak decay b ! ccs. This quark pro cess has
recently b een claimed [64] to dominate charm baryon pro duction in B decay.
A CHEOPS xed target study for ccq (p ossibly including some B mesons)
c
can b e a valuable prelude to collider studies of doubly heavy hadrons.
Bro dsky and Vogt [65 ] suggested that there may b e signi cantintrinsic
charm (IC) cc comp onents in hadron wave functions, and therefore also cccc
comp onents. The IC probabilitywas obtained from the measurements of
charm pro duction in deep inelastic scattering. The Ho mann and Mo ore
analysis [66] of EMC data yields 0.3% IC probability in the proton. Theo-
retical calculations of the IC comp onenthave also b een rep orted [67]. The
double intrinsic charm comp onent can lead to ccq pro duction, as the cc pairs
pre-exist in the incident hadron.
Bro dsky and Vogt [65] discussed double pro duction [45 ] in the frame-
work of IC. The data o ccur mainly at large X , while pro cesses induced by
F
gluon fusion tend to b e more central. They claim that the data (transverse
momentum, X distribution, etc.) suggest that pro duction is highly cor-
F
related, as exp ected in the intrinsic charm picture. A recent exp erimentof
Ko dama et al. [68] searched for soft di ractive pro duction of op en charm in
D D pairs with a 800 GeV proton b eam and a Silicon target. The exp eriment
set a 90% con dence level upp er limit of 26 microbarns p er Silicon nucleus for
di ractivecharm pro duction. Ko dama et al. estimated that the total di rac-
tive cross section p er Silicon nucleus, ab ove the charm threshold, is 12.2 mb.
The ratio of these values gives an upp er limit of 0.2% for the probability
that ab ove the charm threshold, a di ractiveevent contains a charm pair.
Ko dama et al. interpreted this as the upp er limit on the IC comp onentof
the proton. Bro dsky et al [69] discuss the probability for the intrinsic charm
in an incident high energy hadron to b e freed in a soft di ractiveinteraction
in a high energy hadronic collision. In their formalism, the IC probabilityis
2 2 2
multipliedby a resolution factor =m , where is an appropriate soft mass
c
scale [69 ]. If we take the soft scale to b e of order =0.2 GeV or the mass,
qcd
one obtains a signi cant resolution factor suppression for charm pro duction
in a soft pro cess. Thus, the charm fraction that should b e observed in a soft
hadronic or di ractive cross section should b e considerably smaller than the
intrinsic charm probability. If the suppression factor is for example 10, that
would change the upp er limit of the Ko dama et al. exp eriment from 0.2% to
2%. The data would not therefore place a useful limit on the IC comp onent.
In the case of hard reactions such as the deep inelastic lepton scattering of 7
the EMC exp eriment, the suppression factor is not present.
Despite the small IC probability and the suppression factor, Bro dsky
and Vogt [65] found that the large X charmed hadropro duction data is
F
consistent with the IC picture. This includes the A dep endence and X
F
dep endence of J/ hadropro duction (NA3) and the leading particle e ect
+
seen in the observed pro duction asymmetry for D =D mesons at large X
F
for an incident b eam [65]. Explanations of the leading D data requires that
the charm quark coalesce with a valence quark. This happ ens automatically
when one frees the IC Fock state, since the charm quark and valence quark
are already moving at approximately the same velo city.
When one frees a double charm IC state in a soft collision, b oth charm
quarks will b e moving at approximately the same velo city as the valence
quark. Thus, coalescence into a ccq state is likely. With gauge interac-
tions, particles may coalesce into b ound states primarily when they are at
low relativevelo city. One may exp ect that aside from the IC mechanism,
ccq pro duction will b e predominantly central. Intrinsic charm ccq pro duc-
tion, with its exp ected high X distribution, would therefore b e esp ecially
F
attactive. An IC ccq pro duction cross section calculation would b e of great
interest.
We can also refer to an empirical formula which reasonably describ es
the pro duction cross section of a mass M hadron in central collisions. The
transverse momentum distribution at not to o large p follows a form given
t
as [70]:
q
2
2
2
d =dp exp( B M + p ); (3)
t
t
1
where B is roughly a universal constant 5 - 6 (GeV) . The exp onential
q
2
2
(Boltzmann) dep endence on the transverse energy E = M + p has in-
t
t
1
spired sp eculation that particle pro duction is thermal, at a temp erature B
160 MeV [70]. We assume that this equation is applicable to ccq pro duc-
tion. To illustrate the universalityofB,weevaluate it for a few cases. For
0 2 2
and , empirical ts to data give exp(-bp ), with b=1.1 GeV and b=2.0
c
t
2
GeV , resp ectively [71 , 72]. With B 2Mb, this corresp onds to B= 5.0
1 1 0
GeV for , and B= 5.3 GeV for . For inclusive pion pro duction,
c
1
exp eriment gives exp(-bp ) with b = 6 GeV [73]; and B b, since the pion
t
1 0
mass is small. Therefore, B= 5-6 GeV is valid for , hyp eron, and pion
c
2
pro duction. After integrating over p , including a (2J+1) statistical factor
t
to account for the spin of the pro duced ccq, and taking the mass of ccq and 8
D to b e 3.7 and 2.0 GeV resp ectively; we estimate the ratio as:
4
(ccq )= (D ) (2J +1)exp[ 5[M (ccq ) M (D )]] 4 10 : (4)
This result corresp onds to k=0.4 in the parameterization of Eq. 2. In apply-
ing Eq. 4 to ccq pro duction, we assume that the suppression of cross section
for the heavy ccq pro duction (for q = u,d,s) as compared to the lightD (cq )
pro duction is due to the increased mass of ccq. However, this formula ig-
nores imp ortant dynamical input, including threshold e ects and a p ossible
suppression factor for the extra charm pro duction in ccq, and therefore can
b e considered an upp er limit. One may apply Eq. 4 with appropriate masses
to estimate yield ratios of other particles. For the T, we assume the same
pro duction cross section as for the ccq, based on the mass dep endence of Eq.
4.
Decay Mo des and Branching Ratios of ccq
Baryons
The semileptonic and nonleptonic branching ratios of ccq baryons have b een
estimated by Bjorken [1] in unpublished notes of 1986. He uses a statisti-
cal approach to assign probabilities to di erent decay mo des. He rst con-
siders the most signi cant particles in a decay, those that carry baryon or
strangeness numb er. Pions are then added according to a Poisson distribu-
tion. The Bjorken metho d and other approaches for charm baryon decay
mo des are describ ed by Klein [13]. Savage and Springer [5] examined the
avor SU(3) predictions for the semileptonic and nonleptonic ccq weak de-
cays. They give tables of exp ected decay mo des, where the rates for di erent
mo des are given in terms of a few reduced matrix elements of the e ective
hamiltonian. In this way, they also nd many relationships b etween decay
rates of di erent mo des. Savage and Springer discuss the fact that the SU(3)
predictions for the decay of the D-mesons can b e understo o d only by includ-
ing the e ects of nal state interactions [74]. They suggest that FSI e ects
should b e much less imp ortant for very charming baryons (ccq) compared to
charmed mesons.
+
The c decays weakly, for example by c ! s + ud + n( ), with n=0,1,
+ + + +
etc. In that case, for example, ccs ! css +( or ). The event 9
top ology contains two secondary vertices. In the rst, a css baryon and 3
mesons are pro duced. This vertex may b e distinguished from the primary
vertex, if the ccs lifetime is suciently long. The css baryon now propagates
some distance, and decays at the next vertex, in the standard mo des for a
css baryon. The exp erimentmust identify the two secondary vertices.
++
We describ e some decaychains considered by Bjorken [1]. For the ,
cc
++ ++ 0 ++ + + 0 +
one mayhave ! K followed by ! and K ! K .
cc c c c
+ + +
A K nal state was estimated by Bjorken [1] to haveasmuch
c
++
as 5% branching ratio. Bjorken also estimated a 1.5% branch for !
cc
+ + + + +
; and 1.5% for ! K . Bjorken nds that roughly 60% of
c cc c
the ccq decays are hadronic, with as many as one-third of these leading to
nal states with all charged hadrons. The decay top ologies should satisfy a
suitable CHEOPS charm trigger, with reasonable eciency. There are also
predicted 40% semi-leptonic decays. However, with a neutrino in the nal
state, it is not feasible to obtain the mass resolution required for a double
charm search exp eriment.
Decay Mo des and Branching Ratios of the T
One can search for the decayofT ! DD,orT! D D, as discussed
by Nussinov [24 ]. The pion or gamma are emitted at the primary inter-
action p oint, where the D* decays immediately. The two D mesons decay
downstream. The D* decayto -D is useful for a search, since the charged
pion momentum can b e measured very well. One can get very go o d reso-
lution for the reconstruction of the T mass. For the gamma decaychannel,
the exp erimental resolution is worse. There will therefore b e relatively more
background in this channel, since the gamma multiplicity from the target is
high, and one must reconstruct events having two D mesons, with all gam-
mas.
Signal and Background Considerations
High energies are needed for studies of high mass, and short lifetime baryons.
Thereby, one pro duces high energy doubly charmed baryons. The resulting
large lifetime b o ost improves separation of secondary and primary vertices, 10
and improves track and event reconstruction. CHEOPS with 450 GeV pro-
tons or other 350-450 GeV hadrons [19] has this high energy advantage.
One can identify charm candidates by requiring that one or more decay
particles from a short lived parenthave a suciently large impact parameter
or transverse miss distance relative to the primary interaction p oint. This
transverse miss distance (S) is obtained via extrap olation of tracks that are
measured with a high resolution detector close to the target. This quantityis
a quasi-Lorentz invariant. Consider a relativistic unp olarized parent baryon
or a spin zero meson that decays into a daughter that is relativistic in the
parent's center of mass frame. Co op er [75 ] has shown that the average trans-
verse miss distance is S c =2. For example, with c 60 microns
c
should haveS 90 microns. The E781 on-line lter cut is on the sum of
the charged decay pro ducts of the doubly charmed baryon and the singly
charmed baryon daughter's decay pro ducts. Any one of these with P>15
GeV/c and S>30 microns generates a trigger [76]. Events from the primary
vertex are typically rejected by the cut on S. With a vertex detector with
20 micron strips, the E781 resolution in S is ab out 4 microns for very high
momenta tracks. For events in E781 with a 15 GeV track, the transverse
miss-distance resolution deteriorates to ab out 9 microns, due to multiple
scattering [77 ]. And the resolution gets even worse for yet lower momenta
tracks. As this resolution b ecomes worse, backgrounds increase, since the
S-cut no longer adequately separates charm events from the primary interac-
tion events. The backgrounds are not only events from the primary vertex,
but also from the decays of the hadrons asso ciated with the two asso ciatedc
quarks pro duced together with the two c quarks. One may exp ect that the
requirement to see two related secondary vertices may provide a signi cant
reduction in background levels.
Some b qq pro duction and decay, with two secondary vertices, maybe
observed in CHEOPS, and must b e considered at least as background to ccq
pro duction. The b qq and ccq events may b e distinguished by the larger b qq
lifetime, and the higher transverse energy released in the b decay. It is not
the aim of CHEOPS to study b qq baryons. Exp eriments at CERN gave only
a small numb er of reconstructed b qq baryons, at a center of mass energy
around 30 GeV [78].
CHEOPS considers using a multiplicity jump trigger [79], whichisin-
tended to b e sensitive to an increase in the number of charged tracks fol-
lowing a charm decay. Such a trigger for high rate b eams has not yet b een 11
used in a complete exp eriment, and still requires research and development.
Backgrounds are p ossible with such a trigger, due to secondary interactions in
targets and the interaction detector (Cerenkov, p ossibly [19]) following each
target. Also, gamma rays from a primary interaction may convert afterwards
to electron-p ositron pairs, and falsely re the trigger. If the rejection ratio of
such non-charmed events is not suciently high, the trigger may not achieve
its needed purp ose of reducing the accepted event rate to manageable values.
This trigger would b e sensitivetoevents with X > -.1, and therefore has
F
e ectively an "op en" trigger X -acceptance. Most of the charm events ac-
F
cepted will then b e mainly asso ciated with charm mesons near X =0, since
F
these dominate the cross section in hadronic pro cesses. The decay of ccq to
a singly charmed hadron may trigger, or the charmed hadron's decaymay
re the trigger. The event also has twoanticharmed quarks, asso ciated with
charmed hadrons, and they may also re the trigger. However, low-X events
F
mayhave high backgrounds, since it is more dicult to separate them from
non-charmed events, due to the p o or miss distance resolution. For higher
X events, one obtains a sample of doubly charmed baryons with improved
F
reconstruction probability b ecause of kinematic fo cussing and lessened mul-
tiple scattering and improved particle identi cation. The multiplicity-jump
trigger for CHEOPS could b e supplemented by a momentum condition trig-
ger P > 15 GeV/c, similar to this requirement in E781. This could enhance
the high-X acceptance, and give higher qualityevents.
F
For double charm, the target design is imp ortant. Toachieve a high inter-
action rate and still have small multiple scattering e ects, one maycho ose ve
400 micron Copp er targets, separated by 1 mm. The total target thickness
is limited to 2% interaction length in order to keep multiple scattering under
control. With di erent target segments, one requires a longitudinal tracking
resolution of 200-300 microns, in order to identify the target segment asso-
ciated with a given interaction. The knowledge of the target segment allows
the on-line pro cessor to reconstruct tracks, and identify a charm event. The
tracking detectors would then b e placed as close as p ossible to the targets, to
achieve the b est p ossible transverse miss-distance resolution. The optimum
target design and thickness for double charm requires study via Monte Carlo
simulation.
One may require separation distances of secondary from primary vertices
of 1-4 , dep ending on the backgrounds. The requirement for twocharm
vertices in ccq decays may reduce backgrounds suciently, so that this sep- 12
aration distance cut is less imp ortant than in the case of cqq studies. For a
lifetime of 100fs, with a lab oratory lifetime b o ost of 15, the distance from
the pro duction p oint to the decay p oint is around 450 microns. E781 can
attain roughly 300 micron b eam-direction resolution for X =0.2, with a
F
650 GeV b eam, and 20 micron strip silicon detectors. For lower X events,
F
the resolution deteriorates due to multiple scattering, and there is little gain
in using narrower strips. CHEOPS aims to achieve 150 micron resolution
for the high X events. Signal and background and trigger simulations and
F
target design developmentwork are in progress for CHEOPS [19].
Pro jected Yields for CERN CHEOPS
For CHEOPS with a Baryon b eam, one may rely on previous measurements
done with similar b eams. The op en charm pro duction cross section at SPS
energies is roughly 25 b. Taking Eq. 2 with a reduction factor of kR=4.
4
10 , with k=0.4, wehave(ccq) 10: nb/N. This kR value follows from
Kiselev et al. [7] and from Eq. 4. We assume a measured branching ratio B=
10% for the sum of all ccq decays; this b eing 50% of all the decays leading to
only charged particles. We also assume a measured B = 20% for the sum of
all cqq decays, this b eing roughly the value achieved in previous exp eriments.
With these branching ratios, we estimate BB =10:0:20:1=0:2nb=N .
For CHEOPS, wenowevaluate the rate of reconstructed ccq events. The
7
exp ectations are based on a b eam of 5. 10 p er spill, assuming 240 spills
10
p er hour of e ective b eam, or 1.2 10 /hour. For a 4000 hour run (2 years),
11
anda2%interaction target, one achieves 9.5 10 interactions p er target
nucleon. We assume that (charm) = 25 b and (in) = 25 mb for a proton
1=3
target, and takea charm pro duction enhancement p er nucleon of A (with
5
mass A 64 for CHEOPS). One then obtains a high sensitivity of 1.5 10
charm events for eachnb p er nucleon of e ective cross section (for nucleons
in A 64 nuclei), where = BB". Here " is the overall eciency
ef f
for the exp eriment. Fermilab E781 with 650 GeV pion and b eams is
scheduled for 1996-97. This exp erimentmay therefore observe ccq baryons
b efore CHEOPS, as describ ed in recent rep orts [80 , 81 ]. The CHEOPS Letter
of Intent [19 ] describ es plans to achieve roughly ten times more reconstructed
charm events than Fermilab E781. However, the CHEOPS exp eriment is not
yet scheduled. The charm sensitivity of E781 is describ ed in detail elsewhere 13
[32, 76 ]
We consider also the exp ected CHEOPS eciency for the charm events,
by comparison to E781 estimated [76] eciencies. The E781 eciencies for
cqq decays include a tracking eciency of 96% p er track, a trigger eciency
averaged over X of roughly 18%, and a signal reconstruction eciency of
F
roughly 50%. The CHEOPS trigger eciency for cqq should b e higher than
E781, if lowX events are included. However, the signal reconstruction
F
eciency is low for lowX events. The reconstruction eciency should
F
be lower for double charm events, since they are more complex than single
charm events. Yet, using the prop osed typeofvertex detector, multivertex
events can b e reconstructed with go o d eciency [78]. In a sp ectator decay
mo de, the nal state from ccq decay will likely b e a csq charm baryon plus
+ +
a W decay, either semileptonic (40% total B) or hadronic (25% , 75%
most likely). One may exp ect the vertex to b e tagged more often (roughly
a factor of two) for double charm compared to single charm. There should
therefore b e a higher trigger eciency and a lower reconstruction eciency
for double compared to single charm. We assume here however that the
pro duct of these two eciencies remains roughly the same. Therefore, the
overall average ccq eciency is taken to b e " ' 8%, comparable to the
exp ected E781 value for cqq detection. The exp ected yield given ab oveis1.5
5
10 charm events/(nb/N) of e ective cross section. For BB = 0.2 nb/N,
one has =0:016nb=N , and therefore N(ccq) 2400 events for CHEOPS.
ef f
This is the total exp ected yield for ccu,ccd,ccs pro duction for ground and
excited states. For k > 0.4, the yields are yet higher.
Conclusions
The observation of doubly charmed baryons or T would make p ossible a de-
termination of their lifetimes and other prop erties. The exp ected low yields
and short lifetimes make double charm hadron research an exp erimental chal-
lenge. The discovery and subsequent study of the ccq baryons or T should
lead to a deep er understanding of the heavy quark sector. 14
Acknowledgements
Thanks are due to J. App el, S. Bro dsky,P. Co op er, F. Dropmann, L. Frank-
furt, S. Gavin, J. Grunhaus, K. Konigsmann, B. Kop eliovich, E. Levin, H.
J. Lipkin, B. Muller, S. Nussinov, S. Paul, B. Povh, J. Russ, M. A. Sanchis-
Lozano, M. Savage, R. Vogt, R. Werding, and M. Zavertiev for stimulating
discussions. This work was supp orted in part by the U.S.-Israel Binational
Science Foundation (B.S.F.), Jerusalem, Israel.
References
[1] J. D. Bjorken, FERMILAB-CONF-85/69, Is the ccc a New Deal
for Baryon Sp ectroscopy?, Int. Conf. on Hadron Sp ectroscopy, Col-
lege Park, MD, Apr. 1985, Published in College Pk. Hadron. Sp ect.,
(QCD161:I407:1985, P. 390.);
Unpublished Draft, "Estimates of Decay Branching Ratios for Hadrons
Containing Charm and Bottom Quarks", July 22, 1986;
Unpublished Draft, "Masses of Charm and Strange Baryons", Aug. 13,
1986.
[2] J.M. Richard, in Pro c. of the CHARM2000 Workshop, see Ref. [17 ],
(Archive: [email protected], - HEPPH-9407224),
E. Bagan, H. G. Dosch, P. Gosdzinsksy, S. Narison, J. M. Richard,
Z. Phys. C64 (1994) 57,
S. Zouzou, J. M. Richard, Few-Bo dy Systems 16 (1994) 1,
HEPPH-9309303
J. M. Richard, Nucl. Phys. B 21 (Pro c. Suppl.) (1991) 254,
J. P. Adler, J. M. Richard, P.Taxil, Phys. Rev. D25 (1982) 2370.
[3] S. Fleck, J. M. Richard, Prog. Theor. Phys. 82 (1989) 760;
S. Fleck, J. M. Richard, Particle World 1 (1990) 67.
A. Martin, J. M. Richard, CERN-TH/95-86,HEPPH-9504276 .
[4] M. J. Savage, M. B. Wise, Phys. Lett B248 (1990) 177.
[5] M. J. Savage, R. P. Springer, Int. J. Mo d. Phys. A6 (1991) 1701. 15
[6] V. V. Kiselev et al., Phys. Lett. 332 (1994) 411;
V. V. Kiselev et al., Preprint IHEP 94-10 (1994);
V. V. Kiselev et al., Yad. Fiz. 57 (1994) 733;
V. V. Kiselev, Int. J. Mo d. Phys. 10A (1995) 465,
S. S. Gershtein et al., Preprint IHEP 94-81, (1994), HEPPH-9504319.
[7] V. V. Kiselev et al., Sov. J. Nucl. Phys. 46 (1987) 535; HEPPH-9504302.
[8] A. F. Falk, M. Luke, M. J. Savage, M. B. Wise, Phys. Rev. D49 (1994)
555.
[9] M. Bander, A. Subbaraman, Phys. Rev. D50 (1994) 5478; University
of California (Irvine) Preprint UCI-TR-95-9, HEPPH-9503341, March
1995.
[10] M. L. Stong, U. Karlsruhe Preprint TTP 95-02, HEPPH-9505217.
[11] J. A. App el, Ann. Rev. Nucl. Part. Sci. 42 (1992) 367.
[12] S. P.K.Tavernier, Rep. Prog. Phys. 50 (1987) 1439.
[13] S. R. Klein, Int. Jour. Mo d. Phys. A5 (1990) 1457.
[14] Particle Data Group, Phys. Rev. D50 (1994) 1171.
[15] H. W. Sieb ert, Nucl. Phys. B21 (Pro c. Suppl.) (1991) 223.
[16] J. G. Korner, H. W. Sieb ert, Ann. Rev. Nucl. Part. Sci., 41 (1992) 511.
[17] D. M. Kaplan and S. Kwan, Editors, Pro c. of the CHARM2000 Work-
shop, The Future of High Sensitivity Charm Exp eriments, Fermilab,
June 1994, FERMILAB-CONF-94/190.
[18] M. A. Moinester, Doubly Charmed Baryons and Tetraquarks, Tel Aviv
U. Preprint 2238-95, HEPPH-9503289, Contribution to Nov. 1994 Work-
shop at CERN on "Physics with Hadron Beams with a High Intensity
Sp ectrometer". Workshop Organizer: S. Paul, CERN PPE, CH1211
Geneva 23, Switzerland.
[19] S. Paul et al., Letter of Intent, CHEOPS, CHarm Exp eriment with
Omni-Purp ose Setup, CERN/SPSLC 95-22, SPSLC/I202, March 28,
1995; CHEOPS Prop osal in preparation, March 1996 target date. 16
[20] J. L. Rosner, Enrico Fermi Institute and U. Chicago preprint EFI-95-02,
HEPPH-9501291, Jan. 1995.
[21] H. J. Lipkin, Phys. Lett. B45 (1973) 267; Phys. Lett. 172B (1986) 242.
[22] N. A. Tornqvist, Z. Phys. C61 (1994) 525; Phys. Rev. Lett. 67 (1991)
556; Il Nuovo Cimento 107A (1994) 2471.
[23] T. E. O. Ericson, G. Karl, Phys. Lett. B309 (1993) 426.
[24] S. Nussinov, Unpublished Draft, Private Communication, 1995.
[25] Chi-Keung Chow, Preprint CALT-68-1964, HEPPH-9412242, 1994.
[26] A. V. Maonohar, M. B. Wise, Nucl. Phys. B399 (1993) 17.
[27] J. Weinstein, N. Isgur, Phys. Rev. D41 (1990) 2236.
[28] J. Carlson, L. Heller, J. A. Tjon, Phys. Rev. D37 (1988) 744;
L. Heller and J. A. Tjon, Phys. Rev. D35 (1987) 969.
[29] R. L. Ja e, Phys. Rev. D15 (1967) 267; 281.
[30] M. W. Beiner, B. C. Metsch, H. R. Petry,
HEPPH-9505215, submitted to Zeit. fur Phys. A, 1995.
[31] A. E. Dorokhov, N. I. Ko chelev, Yu. A. Zub ov, Z. Phys. C65 (1995) 667.
[32] M. A. Moinester, D. Ashery, L. G. Landsb erg, H. J. Lipkin,
in Pro c. CHARM2000 Workshop, ibid., Fermilab, June 1994,
Tel Aviv U. PreprintTAUP 2179-94, HEPPH-9407319.
[33] M. Zielinsky et al., Z. Phys. C31 (1986) 545; C34 (1987) 255.
[34] K. Martens, Dr. Rer. Nat. thesis, U. Heidelb erg,
+ + +
"Die Suche nach dem Zerfall U (3100) ! p in dem
Hyp eronenstrahlexp erimentWA89", Nov. 1994.
[35] M. A. Moinester, C. B. Dover, H. J. Lipkin, Phys. Rev. C46 (1992) 1082. 17
[36] L. G. Landsb erg, Surveys in High Energy Phys. 6 (1992) 257;
K. Peters, Pro ceed. of LEAP-92 Conf., Courmayeur, Aosta Valley, p.93,
Sep. 14-19, 1992, Eds. C. Guaraldo et al., 1993, North-Holland;
L. G. Landsb erg, Yad.Fiz. 57 (1994) 47;
L. G. Landsb erg, M. A. Moinester, M. A. Kubantsev,
Rep ort IHEP 94-19, TAUP 2153-94, Protvino and Tel Aviv, 1994.
[37] M. Shmatikov, HEPPH-9503471.
[38] G. Bellini, Int. Workshop "Heavy Quarks in Fixed Target", Char-
lottesville, Virginia, Oct. 1994.
[39] M. B. Voloshin, M. A. Shifman, Sov. Phys. JETP 64 (1986) 698.
[40] R. Forty, "Lifetimes of Heavy Flavor Particles", CERN-PPE/94-144,
Sep. 1994, Invited talk, XIV Int. Conf. on Physics in Collision,
Tallahassee, Florida, June 1994.
[41] L. Frankfurt, Private Communication, 1994.
[42] E. Levin, Private Communication, 1995.
[43] B. D'Almagne, Int. Symp osium on the Pro duction and Decayof
Heavy Flavors, Stanford, Cal., Sep. 1987, SLAC Heavy Flavors 1987
(QCD161:I732:1987);
K. Ko dama et al., E653 Coll., Phys. Lett. B263 (1991) 579;
M. L. Mangano, P. Nason, G. Ridol , Nucl. Phys. B405 (1993) 507.
[44] S. Aoki et al., CERN WA75 collab oration, Phys. Lett. B187 (1987) 185.
[45] J. Badier et al., CERN NA3 collab oration, Phys. Lett. B158 (1985) 85,
Phys. Lett. B114 (1982) 457, Phys. Lett. B124 ( 1983) 535.
[46] R. W. Robinett, Phys. Lett. B230 (1989) 153.
[47] F. Halzen, P.Hoyer, W. Y. Stirling, Phys. Lett. B188 (1987) 375.
[48] ATLAS internal note, Phys-NO-041 (1994);
ATLAS internal note Phys-NO-058 (1994).
[49] A. Fridman, B. Margolis, CERN preprint CERN-TH 6878/93 (1993). 18
[50] P. Lebrun, R.J. Oakes, Preprint FERMILAB-Conf-93/303 (1993).
[51] M. J. White, M. J. Savage, Phys. Lett. B271 (1991) 410.
[52] M. A. Sanchis-Lozano, Phys. Lett. B321 (1994) 407.
[53] K. Kolo dziej, A. Leike, R. Rueckl, Pro duction of B Mesons in Hadronic
c
Collisions, Max-Planck-Institut (Muenchen) preprint MPI-PhT/95-36,
U. Muenchen preprint LMU-08/95, HEPPH-9503206, (1995).
[54] T.E. Browder, K. Honscheid, U. Hawaii Preprint UH 511-816-95 , Ohio
State preprint OHSTPY-HEP-E-95-010, HEPPH-9503414.
[55] A.V. Berezhnoy, A.K. Likho ded, O.P. Yushchenko, IHEP-95-59,
HEPPH-9504302, (1995).
[56] P. Nason et al., CERN preprint CERN-TH.7134/94 (1994).
[57] E. Braaten, K. Cheung, T.C. Yuan, Phys. Rev. D48 (1993) 5049.
[58] C-H Chang, Y-Q Chen, Phys. Rev. D48 (1993) 4086.
[59] M. Lusignoli, M. Masetti, S. Petrarca, Phys. Lett. B266 (1991) 142.
[60] S. S. Gershtein, A.K. Likho ded, S.R. Slab ospitsky,Int. J. Mo d. Phys.
A13 (1991) 2309.
[61] M. A. Sanchis-Lozano, submitted to Nucl. Phys. B, HEPPH-9502359,
Mar. 15, 1995.
[62] K. Cheung, HEPPH-9503286, (1995).
[63] M. Masetti, F. Sartogo, Roma Preprint 1099-1995, HEPPH-9503491,
(1995).
[64] I. Dunietz, P. S. Co op er, A, F. Falk, M. B. Wise,
Phys. Rev. Lett. 73 (1994) 1075.
[65] R. Vogt, S. J. Bro dsky,Phys. Lett. 349B (1995) 569; R. Vogt, S. J.
Bro dsky, SLAC-PUB-6468, LBL-35380, HEPPH-9405236, Apr. 1994,
R.V. Gavai, S. Gupta, P.L. McGaughey, E. Quack, P.V. Ruuskanen, R.
Vogt, Xin-Nian Wang, GSI-94-76, HEPPH-9411438, Nov. 1994, 19
R. Vogt, Nucl. Phys. A553 (1993) 791c,
R. Vogt, S. J. Bro dsky,P.Hoyer, Nucl. Phys. B383 (1992) 643,
R. Vogt, S. J. Bro dsky,P.Hoyer, Nucl. Phys. B360 (1991) 67,
R. Vogt, LBL-37655, HEPPH-9503207, Jan. 1995.
[66] E. Ho mann, R. Mo ore, Z. Phys. C20 (1983) 71.
[67] F. S. Navarra, M. Nielsen, C. A. A. Nunes, M. Teixeira, HEPPH-
9504388; J. F. Donoghue, E. Golowich, Phys. Rev. D15 (1977) 3421.
[68] K. Ko dma et al., Phys. Lett. B316 (1993) 188.
[69] S.J. Bro dsky,P.Hoyer, A.H. Mueller, W. Tang, Nucl.Phys. B369 (1992)
519; and S. J. Bro dsky, private communication.
[70] R. Hagedorn, "The Long Way to the Statistical Bo otstrap Mo del",
CERN-TH-7190-94, Mar. 1994;
R. Hagedorn, in Quark Matter 84, ed. K. Ka jantie, Lecture Notes in
Physics Vol. 221 (Springer-Verlag, New York, 1985);
H. Grote, R. Hagedorn, J. Ranft, "Atlas of Particle Pro duction Sp ectra",
CERN Rep ort, 1970.
[71] A. Simon, CERN WA89, Rencontres de Moriond, (1994); F. Dropmann,
CERN WA89, Rencontres de Moriond, (1995); edited byJ.Tran Thanh
Van (Editions Frontieres);
R. Werding et al., CERN WA89, 27th Int. Conf. on High Energy Physics
(ICHEP ), Glasgow, Scotland, July 1994.
[72] F. S. Rotondo, Phys. Rev. D47, (1993) 3871.
[73] S. D. Ellis and R. Stroynowski, Rev. Mo d. Phys. 49 (1977) 753.
[74] L. Chau, H. Cheng, Phys. Rev. D36 (1987) 137, Phys. Lett. B222 (1989)
285;
A. Kamal, R. Verma, Phys. Rev. D35 (1987) 3515, D36 (1987) 3527 (E).
[75] P. Co op er, "Miss Distance Distribution", unpublished rep ort,
Aug. 5, 1981, private communication.
[76] J. Russ, in Pro c. CHARM2000 Workshop, ibid., Fermilab, June 1994. 20
[77] J. Russ, E781 Internal Rep ort, Oct. 29, 1993;
P. Co op er, E781 Internal Rep ort, Feb. 27, 1994;
"Multiple Coulomb Scattering in Silicon Tracking".
[78] D. Barb eris, CERN WA92 BEATRICE collab oration,
in Pro c. CHARM2000 Workshop, ibid., Fermilab, June 1994.
[79] A. M. Halling, S. Kwan, Nucl. Inst. Meth. A333 (1993) 324.
[80] M. A. Moinester, Tel Aviv U. Internal Rep ort, Oct. 1994.
[81] J. Grunhaus, P. Co op er, J. Russ,
Fermilab E781 Rep ort H718, Dec. 1994. 21