Large production of the $\ccbar$ pairs and high integrated luminosity make
the PEPII B-Factory, an excellent place for studying the charm hadrons. In this
paper, we present a few of the most recent results from the BaBar collaboration
in the charm sector.Comment: To appear in the proceedings of the XIII International Workshop on
  Deep Inelastic Scattering - DIS 2005, April 27-May 1, 2005, Madison,
  Wisconsi

Chen, Chunhui

English

arXiv

Chunhui Chen

Crossref

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Charm Physics at BABAR
Chunhui Chen
Department of Physics, University of Maryland
College Park, Maryland 20742-4111, U.S.A
(for the BABAR Collaboration)
Abstract. Large production of thecc pairs and high integrated luminosity make the PEP-IIB
Factory an excellent place for studying the charm hadrons. In this paper, we present a few of the
most recent results from theBABAR collaboration in the charm sector.
INTRODUCTION
TheBABAR detector [1] is a general purpose detector designed to collect data at PEP-II
asymmetrice+e− collider, operating at the center-of-mass energy corresponding to the
ϒ(4S) resonance or∼ 40 MeV below it. With copious production ofcc pairs from the
continuum and high integrated luminosity,BABAR is not only aB Factory, it is also an
excellent laboratory to study the charm production and decays. In this paper, we present
a few of the most recent charm analysis results fromBABAR.
D0 −D0 MIXING
Charm mixing is characterized by two dimensionless parameters,x ≡ ∆m/Γ andy ≡
∆Γ/2Γ, where ∆m (∆Γ) is the mass (width) difference between the two neutralD
mass eigenstates, andΓ is the average width. If eitherx or y is nonzero, then the
D0−D0 mixing will occur. In the Standard Model (SM),D0−D0 mixing rate is heavily
suppressed by the Glashow-Iliopoulos-Maiani (GIM) mechanism[2]. However, the SM
mixing rate can be enhanced by the non-perturbative effectsand possible new physics
beyond SM.
Based on a sample of 87fb−1 data,BABAR performed a search ofD0−D0 mixing [3]
to measure the overall time-integrated mixing rateRmix = (x2+ y2)/2 using the decay
chainD∗+ → D0π+, D0 → K±e∓ν [4]. The charge of the pion daughter of the charged
D∗ identifies the production flavor of the neutralD, while the charge of the electron
identifies the decay flavor. These charges are equal for unmixed decays and opposite for
mixed decay. Using event selection and reconstruction based on neural networks and
charged kaon and electron particle identification, we obtained 49620± 265 unmixed
events and 114±61 mixed events. This results in
Rmix = 0.0024±0.0012(stat)±0.0004(syst)
Rmix < 0.0042 at 90% CL.
(1)
SEARCH FOR D0 → ℓ+ℓ−
In the SM, the flavor-changing neutral current (FCNC) decaysD0 → e+e− andD0 →
µ+µ− are highly suppressed by the GIM mechanism. The lepton-flavor violating (LFV)
decayD0 → e±µ∓ is strictly forbidden in the SM. Some extensions to the Standard
Model [5], such as theR-parity violating supersymmetry, can enhance the FCNC pro-
cesses by many orders of magnitude and can also permit the LFVdecays.
BaBar performed a search for the decays ofD0 → e+e−, D0 → µ+µ−, andD0 →
e±µ∓ based on a sample of 122fb−1 data [6]. To ensure as clean a sample as possible,
the reconstructedD0 → ℓ+ℓ− candidate is required to originate from aD∗+ → D0π+
decay. A minimum value of 2.4GeV/c is imposed on the center-of-mass momentum of
eachD0 candidate to further reduce the combinatorial background involving the decay
products ofB mesons. Tight particle selection criteria are also appliedto the daughters
of D0 → ℓ+ℓ− decays.
We observed no significant signals in all three decay modes. As a result, the branching
fraction upper limits (UL) have been calculated using theD0 → π+π− decay as the
normalization mode. We obtain
Br(D0 → e+e−) < 1.2×10−6 at 90% CL.,
Br(D0 → µ+µ−) < 1.3×10−6 at 90% CL.,
Br(D0 → µ±e∓) < 8.1×10−7 at 90% CL.
(2)
These results represent significant improvements on the previous limits [7, 8].
SEARCH FOR DSJ(2632)+
The SELEX Collaboration at FNAL has recently reported the observation of a narrow
state [9] at a mass of 2632 MeV/c2 that decays toD+s η. Evidence for the same state in the
D0K+ mass spectrum was also presented. BaBar has searched for this resonance in the
final statesD+s η, D0K+ andD∗+K0S [10] produced ine
+e− → cc events, using 125fb−1
data. As shown in Fig. 1, no signal is observed inD+s η decay channel; similarly, no
evidence ofDsJ(2632)+ was found in theD0K+ andD∗+K0S final states, although large
and clean signals for the decayDs2(2573)+ → D0K+ andDs1(2536)+ → D∗+K0S are
seen.
Ξ0C PRODUCTION AND DECAYS
Using a sample of 116fb−1 data, BABAR has performed a branching fraction ratio
measurement of theΞ0c decaying toΩ−K+ andΞ−π+ [11]. The result
Br(Ξ0c → Ω−K+)
Br(Ξ0c → Ξ−π+)
= 0.294±0.018(stat)±0.016(syst) (3)
is a significant improvement over the previous measurement by CLEO [12] and is
consistent with a spectator quark model prediction [13].
   A   ARB B
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2.45 2.5 2.55 2.6 2.65 2.7 2.75
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BABAR
preliminary
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*+
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preliminary
FIGURE 1. (Left:) TheD+s η invariant mass distribution after background subtraction. The arrow indi-
cates the mass location of the expectedDsJ(2632)+ state. (Middle:) TheD0K+ invariant mass distribution.
The red histogram is the invariant mass distribution ofD0K− combinations, and the blue line indicates
the mass location of the expectedDsJ(2632)+ state. (Right:) The distribution of the difference in invariant
mass of theD∗+K0S combination andD
∗+ candidate. The blue line indicates the mass location of the
expectedDsJ(2632)+ state.
Although copious production ofΞ0c in B decays has been predicted, such process has
been only observed by CLEO [14] with a significance of∼ 3σ in theΞ0c → Ξ−π+ decay
mode and∼ 4σ in theΞ+c → Ξ−π+π+ decay mode. We studied theΞ0c production by
measuring the spectrum of theΞ0c momentump∗ in thee+e− center-of-mass frame. The
Ξ0c produced by theB decays tends to have a smaller momentum; itsp∗ distribution
peaks below 1.5 GeV/c and has a kinematic limit ofp∗ = 2.135 GeV/c atBABAR. As for
theΞ0c from continuum production, its momentum distribution peaks t a much higherp∗
value. By examining thep∗ distribution of theΞ0c from on-resonance and off-resonance
sample, we found
Br(B → Ξ0cX)×Br(Ξ
0
c → Ξ
−π+) = 2.11±0.19(stat)±0.25(syst)×10−4, (4)
and
σ(e+e− → Ξ0cX)×Br(Ξ0c → Σ−π+) = 388±39(stat)±41(syst) fb, (5)
where bothΞ0c andΞ̄0c are included in the cross-section.
MEASUREMENT OF Λ+C MASS
The invariant masses of the charm hadron ground states are currently reported by the
Particle Data Group (PDG) with a precision of about 0.5–0.6MeV/c2 [15]. The best
individual measurements have a statistical and systematicprecision of about 0.5MeV/c2
and use data samples of a few hundred events. TheBABAR data sample contains a large
amount of different charm hadron decays and, due to the excell nt momentum and vertex
resolution inBABAR, many of the decay modes can be reconstructed with an event-
by-event mass uncertainty of a few MeV/c2. We can therefore provide significantly
improved estimate of the charm hadron masses.
With a sample of 232fb−1 data,BABAR performed a precision measurement of the
Λ+c mass. The measurement is based on the reconstruction of the decay modesΛ+c →
ΛK̄0K+ andΛ+c →Σ0K̄0K+. Because almost all of theΛ+c invariant mass in these decays
tied to the well-known rest masses of theΛ+c decay products, the systematic uncertainty
in the reconstructed mass is significantly reduced comparedto the measurements that try
to use the more common decay modes. Combining the results from th se two modes,
the measuredΛ+c mass is
m(Λ+c ) = 2286.46±0.14 MeV/c
2. (6)
This result is in agreement with the mass values measured in other much large sample of
Λ+c decays, includingΛ+c → pK−π+ andΛ+c → pK0S decays, although these are subject
to large systematic uncertainties.
ThisΛ+c mass measurement is the most precise measurement of an open charm hadron
mass to date and is an improvement in precision by more than a fctor of four over the
current PDG value of 2284.9± 0.6MeV/c2. Our result is about 2.5σ higher than the
PDG value, which is based on several highQ-value decay modes, mainlyΛ+c → pK−π+
decays.
CONCLUSION
BABAR has a very rich and active charm physics program. In this paper we discussed
only a few most recent results fromBABAR. Given the excellent luminosity achieved by
PEP-II, much more high precision charm physics results are exp cted in the near future.
REFERENCES
1. BABAR Collaboration, B. Aubertet al., Nucl. Instr. Methods Phys. Res., Sect. A479, 1 (2002).
2. S. L. Glashow, J. Iliopoulos and L. Maiani, Phys. Rev. D2, 1285 (1970).
3. BABAR Collaboration, B. Aubertet al., Phys. Rev. D70, 091102 (2004).
4. We imply charged conjugate modes throughout the paper.
5. G. Burdman, E. Golowich, J. Hewett and S. Pakvasa, Phys. Rev. D 66, 014009 (2002).
6. BABAR Collaboration, B. Aubertet al., Phys. Rev. Lett.93, 191801 (2004).
7. E791 Collaboration, E. M. Aitalaet al., Phys. Lett. B462, 401 (1999).
8. HERA-B Collaboration, I. Abtet al., Phys. Lett. B596, 173 (2004).
9. SELEX Collaboration, A. V. Evdokimovet al., Phys. Rev. Lett.93, 242001 (2004).
10. BABAR Collaboration, B. Aubertet al., arXiv:hep-ex/0408087.
11. BABAR Collaboration, B. Aubertet al., arXiv:hep-ex/0504014.
12. CLEO Collaboration, S. Hendersonet al., Phys. Lett. B283, 161 (1992).
13. J. G. Korner and M. Kramer, Z. Phys. C55, 659 (1992).
14. CLEO Collaboration, B. Barishet al., Phys. Rev. Lett.79, 3599 (1997).
15. Particle Data Group, S. Eidelmanet al., Phys. Lett. B592, 1 (2004).


Charm Physics at BABAR

Large production of the c{bar c} pairs and high integrated luminosity make the PEPII B Factory an excellent place for studying the charm hadrons. In this paper, we present a few most recent results from BaBar collaboration in charm sector

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Work supported by Department of Energy contract DE-AC02-76SF00515Charm Physics at BABARChunhui ChenDepartment of Physics, University of MarylandCollege Park, Maryland 20742-4111, U.S.A(for the BABAR Collaboration)Abstract. Large production of the cc pairs and high integrated luminosity make the PEP-II BFactory an excellent place for studying the charm hadrons. In this paper, we present a few mostrecent results from BABAR collaboration in charm sector.Keywords: BABAR, charm, Standard ModelPACS: 13.20.Fc,13.25.Ft,13.30.Eg,14.20.LqINTRODUCTIONThe BABAR detector [1] is a general purpose detector that collect data at PEP-II asym-metric e+e− collider, operating at the center-of-mass energy of ϒ (4S) resonance or∼ 40 MeV below it. With copious production of cc pairs from the continuum and highintegrated luminosity, BABAR is not only a B Factory, it is also an excellent laboratoryto study the charm production and decays. In this paper, we present a few most recentcharm analysis results from BABAR.D0 −D0 MIXINGCharm mixing is characterized by two dimensionless parameters, x ≡ ∆m/Γ and y ≡∆Γ/2Γ, where ∆m (∆Γ) is the mass (width) difference between the two neutral Dmass eigenstates, and Γ is the average width. If either x or y is nonzero, then theD0−D0 mixing will occur. In the Standard Model (SM), D0−D0 mixing rate is heavilysuppressed by the Glashow-Iliopoulos-Maiani (GIM) mechanism[2]. However, the SMmixing rate can be enhanced by the non-perturbative effects, and possible new physicsbeyond SM.Based on a sample of 87fb−1 data, BABAR performed a search of D0 −D0 mixing [3]to measure the overall time-integrated mixing rate Rmix = (x2 + y2)/2 using the decaychain D∗+ → D0π+, D0 → K±e∓ν [4]. The charge of the pion daughter of the chargedD∗ identifies the production flavor of the neutral D, while the charge of the electronidentifies the decay flavor. These charges are the same for unmixed decays and differentfor mixed decay. Combining with the a neural network and charged kaon and electronparticle identification, we obtained 49620± 265 unmixed events and 114± 61 mixedevents. This gives us the final resultRmix = 0.0024±0.0012(stat)±0.0004(syst)Rmix < 0.0042 at 90% CL.(1)SLAC-PUB-11308BABAR-PROC-05/029June 2005Invited talk presented at International Workshop on Deep Inelastic Scattering, April 27-May14, 2005, Madison, WI, USASEARCH FOR D0 → `+`−In the SM, the flavor-changing neutral current (FCNC) decays D0 → e+e− and D0 →µ+µ− are highly suppressed by the GIM mechanism. The lepton-flavor violating (LFV)decay D0 → e±µ∓ is strictly forbidden in the SM. Some extensions to the StandardModel [5], such as the R-parity violating supersymmetry, can enhance the FCNC pro-cesses by many orders of magnitude, and also allow the LFV decays.BaBar performed a search for the decays of D0 → e+e−, D0 → µ+µ−, and D0 →e±µ∓ based on a sample of 122fb−1 data [6]. To ensure as clean a sample as possible,the reconstructed D0 → `+`− candidate is required to originate from a D∗+ → D0π+decay. A minimum value of 2.4GeV/c is imposed on the center-of-mass momentum ofeach D0 candidate to further reduce the combinatorial background involving the decayproducts of B mesons. Tight particle selection criteria are also applied to the daughtersof D0 → `+`− decays.We observed no significant signals in all three decay modes. As a result, the branchingfraction upper limits (UL) have been calculated using the D0 → π+π− decay as thenormalization mode. We obtainBr(D0 → e+e−) < 1.2×10−6 at 90% CL.,Br(D0 → µ+µ−) < 1.3×10−6 at 90% CL.,Br(D0 → µ±e∓) < 8.1×10−7 at 90% CL..(2)These results represent significant improvements on the previous limits [7, 8], andfurther restrict possible new physics contribution to these decay processes.SEARCH FOR DSJ(2632)+The SELEX Collaboration at FNAL has recently reported the existence of a narrowstate [9] at a mass of 2632 MeV/c2 that decays to D+s η . Evidence for the same state inthe corresponding D0K+ mass spectrum was also presented. BaBar has searched for thisresonance in the final states D+s η , D0K+ and D∗+K0S [10] produced in e+e− → cc events,using 125fb−1 data. As shown in Fig. 1, we don’t see any signals in the D+s η decaychannel; similarly, no evidence of DsJ(2632)+ was found in the D0K+ and D∗+K0S finalstates, although we have seen large and clean signals for the decay Ds2(2573)+ →D0K+and Ds1(2536)+ → D∗+K0S .Ξ 0C PRODUCTION AND DECAYSUsing a sample of 116fb−1 data, BABAR performed a branching fraction ratio measure-ment of the Ξ 0c decayed to Ω−K+ and Ξ−π+ [11]. The resultBr(Ξ 0c → Ω−K+)Br(Ξ 0c → Ξ−π+)= 0.294±0.018(stat)±0.016(syst) (3)   A   ARB B2) GeV/c+K0m(D2.45 2.5 2.55 2.6 2.65 2.7 2.752event/5 MeV/c0100200300400500600BABARpreliminary2) GeV/c*+)-m(DsK*+m(D0.45 0.5 0.55 0.6 0.65 0.7 0.752event/2.5 MeV/c020406080100120140160180200220BABARpreliminaryFIGURE 1. (Left:) The D+s η invariant mass distribution after background subtraction. The arrow indi-cates the mass location of the expected DsJ(2632)+ state. (Middle:) The D0K+ invariant mass distribution.The red histogram is the invariant mass distribution of D0K− combination, and the blue line indicatesthe mass location of expected the DsJ(2632)+ state. (Right) The distribution of invariant mass differencebetween D∗+K0S combination and D∗+ candidate. The blue line indicates the mass location of the expectedDsJ(2632)+ state.is a significant improvement over the previous measurement by CLEO [12], and isconsistent with a spectator quark model prediction [13].Although copious production of Ξ 0c in B decays has been predicted, such process hasbeen only observed by CLEO [14], with a significance of ∼ 3σ in the Ξ 0c →Ξ−π+ decaymode and ∼ 4σ in the Ξ+c → Ξ−π+π+ decay mode. We studied the Ξ 0c production bymeasuring the spectrum of the Ξ 0c momentum p∗ in the e+e− center-of-mass frame. TheΞ 0c produced by the B decays tends to have a smaller momentum, its p∗ distributionpeaks below 1.5 GeV/c and has a kinematic limit of p∗ = 2.135 GeV/c at BABAR. As forthe Ξ 0c from continuum production, its momentum distribution peaks at a much higher p∗value. By examining the p∗ distribution of the Ξ 0c from on-resonance and off-resonancesample, we foundBr(B → Ξ 0c X)×Br(Ξ0c → Ξ−π+) = 2.11±0.19(stat)±0.25(syst)×10−4, (4)andσ(e+e− → Ξ 0c X)×Br(Ξ 0c → Σ−π+) = 388±39(stat)±41(syst) fb, (5)where both Ξ 0c and Ξ̄ 0c are included in the cross-section.MEASUREMENT OF Λ+C MASSThe invariant masses of the charm hadron ground states are currently reported by theParticle Data Group (PDG) with a precision of about 0.5–0.6MeV/c2 [15]. The bestindividual measurements have a statistical and systematic precision of about 0.5MeV/c2and use data samples of a few hundred events. The BABAR data sample contains a largeamount of different charm hadron decays and, due to the excellent momentum and vertexresolution in BABAR, many of the decay modes can be reconstructed with an event-by-event mass uncertainty of a few MeV/c2. We can therefore significantly improve theprecision on the charm hadron mass measurements.With a sample of 232fb−1 data, BABAR performed a precision measurement of theΛ+c mass. The measurement is based on the reconstruction of the decay modes Λ +c →Λ K̄0K+ and Λ+c → Σ 0K̄0K+. Because almost all of the Λ+c invariant mass in thesedecays results from the well-known rest mass values of the Λ +c decay products, thesystematic uncertainty in the reconstructed mass is significantly reduced compared tothe precision obtained in other decay modes. Combining the results from those twomodes, the final result of the Λ+c mass ism(Λ+c ) = 2286.46±0.14 MeV/c2. (6)This result is in agreement with the mass values measured in other much large sample ofΛ+c decays, including Λ+c → pK−π+ and Λ+c → pK0S decays, although these are subjectto large systematic uncertainty.This Λ+c mass measurement is the most precise measurement of an open charm hadronmass to date and is an improvement in precision by more than a factor of four over thecurrent PDG value of 2284.9± 0.6MeV/c2. Our result is about 2.5σ higher than thePDG value, which is based on several high Q-value decay modes, mainly Λ +c → pK−π+decays.CONCLUSIONBABAR has a very rich and active charm physics program. In this paper we discussedonly a few most recent results from BABAR. Given the excellent luminosity achieved byPEP-II, much more high precision charm physics results are expected in the near future.REFERENCES1. BABAR Collaboration, B. Aubert et al., Nucl. Instr. Methods Phys. Res., Sect. A 479, 1 (2002).2. S. L. Glashow, J. Iliopoulos and L. Maiani, Phys. Rev. D 2, 1285 (1970).3. BABAR Collaboration, B. Aubert et al., Phys. Rev. D 70, 091102 (2004).4. We imply charged conjugate modes throughout the paper.5. G. Burdman, E. Golowich, J. Hewett and S. Pakvasa, Phys. Rev. D 66, 014009 (2002).6. BABAR Collaboration, B. Aubert et al., Phys. Rev. Lett. 93, 191801 (2004).7. E. M. Aitala et al. [E791 Collaboration], Phys. Lett. B 462, 401 (1999).8. I. Abt et al. [HERA-B Collaboration], Phys. Lett. B 596, 173 (2004).9. A. V. Evdokimov et al. [SELEX Collaboration], Phys. Rev. Lett. 93, 242001 (2004).10. BABAR Collaboration, B. Aubert et al., arXiv:hep-ex/0408087.11. BABAR Collaboration, B. Aubert et al., arXiv:hep-ex/0504014.12. S. Henderson et al. [CLEO Collaboration], Phys. Lett. B 283, 161 (1992).13. J. G. Korner and M. Kramer, Z. Phys. C 55, 659 (1992).14. B. Barish et al. [CLEO Collaboration], Phys. Rev. Lett. 79, 3599 (1997).15. S. Eidelman et al. [Particle Data Group], Phys. Lett. B 592, 1 (2004).

Charm Physics at BaBar

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Charm Physics at BaBar

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