We discuss recent experimental results on the modification of hadron
properties in a nuclear medium. Particular emphasis is placed on an $\omega$
production experiment performed by the CBELSA/TAPS collaboration at the ELSA
accelerator. The data shows a smaller $\omega$ meson mass together with a
significant increase of its width in the nuclear medium.Comment: Proceedings for CIPANP 2006 Puerto Ric

Martin Kotulla

null null

English

arXiv

Crossref

Mesons in the Nuclear Medium

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Mesons in the nuclear Medium
Martin Kotulla, for the CBELSA/TAPS collaboration
II. Physikalisches Institut, Heinrich-Buff Ring 16, 35392 Giessen, Germany
Abstract. We discuss recent experimental results on the modification of hadron properties in a
nuclear medium. Particular emphasis is placed on anω production experiment performed by the
CBELSA/TAPS collaboration at the ELSA accelerator. The data shows a smallerω meson mass
together with a significant increase of its width in the nuclear medium.
Keywords: omega meson, medium modification, mass
PACS: 13.60.Le, 14.40.-n, 25.20.-x
INTRODUCTION
Hadrons are composite systems bound by the strong interaction. At short ranges (r ≤
0.1 fm) or high energies, the theory of strong interaction, QCD, is very well tested and
confirmed by perturbative methods. However, at larger distances or at energies in the
range of the lowest lying hadrons (e.g. the nucleon mass≈ 1 GeV), the perturbative
picture breaks down and the full complexity manifests itself in a many body structure
of gluons, valence quarks and sea quarks. Consequently, themass of a hadron con-
sisting of light quarks (u,d,s) can not be deduced from the mass of the elementary
valence quarks alone. The mass balance for the proton, e. g.,is 938 MeV/c2 compar-
ing with ≈ 12 MeV/c2 for all three current quark masses. Therefore, almost the entire
mass must be generated by the strongly interacting binding field or the properties of
the QCD vacuum. Embedding a hadron into a nucleus, another strongly interacting en-
vironment, should necessarily affect (and alter) its mass.Moreover, chiral symmetry is
another player which is at the very heart of QCD. For masslessquarks, which is very
close to nature in case ofu,d,s quarks, right and left handed quark fields decouple. A
consequence in the hadron spectrum would be the degeneracy of opposite parity states,
which is neither realized for baryons nor for mesons. The reason is the spontaneous
breakdown of chiral symmetry, indicated by its measure or order parameter, the non-
zero vacuum expectation value of the ¯qq operator (chiral condensate). However, model
calculations suggest a significant temperature and densitydependence of the chiral con-
densate pointing to a partial restauration of chiral symmetry. The experimental observ-
able consequence would be a change of hadron masses towards degeneracy of opposite
parity states.
The change of hadron properties when embedded in the nuclearm dium is an inten-
sively debated field in hadron physics. Today, the qualitative and quantitative behaviour
of the mass of vector mesons is at hand, triggered by a few nextgeneration experiments
[1, 2, 3]. The general method to measure a modification of a hadron mass is to produce
a sufficiently short lived meson in a nuclear environment either via a heavy ion collision
or in an elementary reaction on a nucleus. Theρ meson is a possible probe, since its
FIGURE 1. After background subtraction and forpω ≤ 500 MeV/c. Left: Theω line-shape (blue solid
histogram) on C in comparison with a LH2 reference signal (dashed histogram) and a Geant response
simulation (dashed line). A BUU transport calculation witha 16% downward mass shift reproduces the
data (red solid line) [4]. Right: The same analysis for Nb.
cτ = 1.3 fm is smaller than the size of a nucleus (a few fm). Otherwise, th ρ meson is
very broad (150 MeV), which makes its distinction from aππ background difficult. The
ω meson is an alternative probe, itscτ = 23 fm is small enough to have a sufficiently
high probability for a decay inside a nucleus and its vacuum width (8.5 MeV) promises
a well defined signal.
THE ω MESON IN THE NUCLEAR MEDIUM
The experiment was performed at the electron stretcher accelerator (ELSA) in Bonn
using the Crystal Barrel and TAPS calorimeters (see [1] for me details and [5] for a
facility overview). Theω mesons were produced via the reactionγA→ ωX and detected
through theirω → π0γ decay. This channel offers a 1000 times higher branching ratio
than e+e− measurements and a unique isolated signal since theρ meson decay is
suppressed by a factor 100, whereas di-lepton decays are of th same order for theρ
andω. Otherwise, a distortion of theπ0γ invariant mass by a possible re-scattering of
the strongly interactingπ0 meson in the nucleus has to be investigated very carefully
and it has been shown that the effect in theω mass region is negligible [6, 7]. The
resulting spectrum ofπ0γ events shows a pronouncedω peak on a smooth background,
which is mainly originating from incompletely measuredπ0π0 events when one photon
escaped detection. The background subtracted signal is shown in fig. 1 for C and Nb
targets. Details of the background subtraction can be foundin [1, 8]. The line-shape
on the nuclear targets shows a significant tail on the low invar ant mass side when
compared to the reference shapes on LH2 or a GEANT simulation. The extraction of
an in-medium signal is less straight forward, since the decay of theω meson occurs at
different densities of the nucleus resulting in an average density of≈ 60%ρ0 for Nb and
a little less for the C target. Therefore, models need to be used to separate quantitatively
the integrated mass distribution m(ρ) and to unfold the spectral shape at normal nuclear
matter densityρ0. As an example, fig. 1 shows the expected line-shape within a BUU
calculation with anω mass lowered by 16% atρ0. Calculations with an 8% dropping
FIGURE 2. PRELIMINARY: The transparency ratio normalized on12C for the measured data (solid
red line) in comparison with calculations. The best fit line corresponds to a width ofΓ(ρ0, p= 0 MeV/c) =
40 MeV for the Giessen model [9] andΓ(ρ0, p = 750 MeV/c) = 95 MeV for the Valencia model [10].
mass describe the measured data similarily well. Although,the experimental resolution
of FWHM=55 MeV together with the theoretical models limit the extraction of a precise
in-medium signal, a clear evidence of a dropping mass scenario for theω meson can be
concluded.
The extraction of theω width in the nuclear medium from the measurement of the
mass distribution is limited by the experimental resolution and the unfolding of the
nuclear density distribution at which theω decays. Fortunately, the measurement of the
ω absorption as a function of the nuclear size allows a straight forward way to extract the
(inelastic) cross section of theω meson which can be related to its width [9, 10]. Here,
the transparency ratioTA is introduced, which is a measure of the probability for theω
meson to leave the nuclear without absorption. Fig. 2 showsTA for the measured data in
comparison to two model calculations. As a preliminary result, ΓGi(ρ0, p= 0 MeV/c) =
40 MeV [9] andΓVal(ρ0, p = 750 MeV/c) = 95 MeV [10] can found. The width in the
Giessen calculation has been computed as a function of theω momentum and agrees
at higher momenta with the Valencia value (ΓGi(ρ0, p = 750 MeV/c) = 105 MeV).
The intuitive expectation of a significant broadening of theω in nuclear matter as
a consequence of the opening of additional partial channels(e.g. ωN → NN) can be
confirmed. This topic will be elaborated in a forthcoming publication.
REFERENCES
1. D. Trnka, et al.,Phys. Rev. Lett. 94, 192303 (2005).
2. M. Naruki, et al.,Phys. Rev. Lett. 96, 092301 (2006).
3. S. Damjanovic, et al.,Phys. Rev. Lett. 96, 162302 (2006).
4. P. Muehlich,priv. communication (2005).
5. F. Klein,these preoceedings (2006).
6. J. Messchendorp, et al.,Eur. Phys. Jour. A 11, 95 (2001).
7. P. Muehlich, et al.,Eur. Phys. Jour. A 20, 449 (2004).
8. D. Trnka,PhD thesis (2006).
9. P. Muehlich, et al.,Nucl. Phys. A 773, 156 (2006).
10. M. Kaskulov, and E. Oset,priv. communication,preliminary (2006).


arXiv:nucl-ex/0609012v1  11 Sep 2006Mesons in the nuclear MediumMartin Kotulla, for the CBELSA/TAPS collaborationII. Physikalisches Institut, Heinrich-Buff Ring 16, 35392 Giessen, GermanyAbstract. We discuss recent experimental results on the modification of hadron properties in anuclear medium. Particular emphasis is placed on anω production experiment performed by theCBELSA/TAPS collaboration at the ELSA accelerator. The data shows a smallerω meson masstogether with a significant increase of its width in the nuclear medium.Keywords: omega meson, medium modification, massPACS: 13.60.Le, 14.40.-n, 25.20.-xINTRODUCTIONHadrons are composite systems bound by the strong interaction. At short ranges (r ≤0.1 fm) or high energies, the theory of strong interaction, QCD, is very well tested andconfirmed by perturbative methods. However, at larger distances or at energies in therange of the lowest lying hadrons (e.g. the nucleon mass≈ 1 GeV), the perturbativepicture breaks down and the full complexity manifests itself in a many body structureof gluons, valence quarks and sea quarks. Consequently, themass of a hadron con-sisting of light quarks (u,d,s) can not be deduced from the mass of the elementaryvalence quarks alone. The mass balance for the proton, e. g.,is 938 MeV/c2 compar-ing with ≈ 12 MeV/c2 for all three current quark masses. Therefore, almost the entiremass must be generated by the strongly interacting binding field or the properties ofthe QCD vacuum. Embedding a hadron into a nucleus, another strongly interacting en-vironment, should necessarily affect (and alter) its mass.Moreover, chiral symmetry isanother player which is at the very heart of QCD. For masslessquarks, which is veryclose to nature in case ofu,d,s quarks, right and left handed quark fields decouple. Aconsequence in the hadron spectrum would be the degeneracy of opposite parity states,which is neither realized for baryons nor for mesons. The reason is the spontaneousbreakdown of chiral symmetry, indicated by its measure or order parameter, the non-zero vacuum expectation value of the ¯qq operator (chiral condensate). However, modelcalculations suggest a significant temperature and densitydependence of the chiral con-densate pointing to a partial restauration of chiral symmetry. The experimental observ-able consequence would be a change of hadron masses towards degeneracy of oppositeparity states.The change of hadron properties when embedded in the nuclearm dium is an inten-sively debated field in hadron physics. Today, the qualitative and quantitative behaviourof the mass of vector mesons is at hand, triggered by a few nextgeneration experiments[1, 2, 3]. The general method to measure a modification of a hadron mass is to producea sufficiently short lived meson in a nuclear environment either via a heavy ion collisionor in an elementary reaction on a nucleus. Theρ meson is a possible probe, since itsFIGURE 1. After background subtraction and forpω ≤ 500 MeV/c. Left: Theω line-shape (blue solidhistogram) on C in comparison with a LH2 reference signal (dashed histogram) and a Geant responsesimulation (dashed line). A BUU transport calculation witha 16% downward mass shift reproduces thedata (red solid line) [4]. Right: The same analysis for Nb.cτ = 1.3 fm is smaller than the size of a nucleus (a few fm). Otherwise, th ρ meson isvery broad (150 MeV), which makes its distinction from aππ background difficult. Theω meson is an alternative probe, itscτ = 23 fm is small enough to have a sufficientlyhigh probability for a decay inside a nucleus and its vacuum width (8.5 MeV) promisesa well defined signal.THE ω MESON IN THE NUCLEAR MEDIUMThe experiment was performed at the electron stretcher accelerator (ELSA) in Bonnusing the Crystal Barrel and TAPS calorimeters (see [1] for me details and [5] for afacility overview). Theω mesons were produced via the reactionγA→ ωX and detectedthrough theirω → π0γ decay. This channel offers a 1000 times higher branching ratiothan e+e− measurements and a unique isolated signal since theρ meson decay issuppressed by a factor 100, whereas di-lepton decays are of th same order for theρandω. Otherwise, a distortion of theπ0γ invariant mass by a possible re-scattering ofthe strongly interactingπ0 meson in the nucleus has to be investigated very carefullyand it has been shown that the effect in theω mass region is negligible [6, 7]. Theresulting spectrum ofπ0γ events shows a pronouncedω peak on a smooth background,which is mainly originating from incompletely measuredπ0π0 events when one photonescaped detection. The background subtracted signal is shown in fig. 1 for C and Nbtargets. Details of the background subtraction can be foundin [1, 8]. The line-shapeon the nuclear targets shows a significant tail on the low invar ant mass side whencompared to the reference shapes on LH2 or a GEANT simulation. The extraction ofan in-medium signal is less straight forward, since the decay of theω meson occurs atdifferent densities of the nucleus resulting in an average density of≈ 60%ρ0 for Nb anda little less for the C target. Therefore, models need to be used to separate quantitativelythe integrated mass distribution m(ρ) and to unfold the spectral shape at normal nuclearmatter densityρ0. As an example, fig. 1 shows the expected line-shape within a BUUcalculation with anω mass lowered by 16% atρ0. Calculations with an 8% droppingFIGURE 2. PRELIMINARY: The transparency ratio normalized on12C for the measured data (solidred line) in comparison with calculations. The best fit line corresponds to a width ofΓ(ρ0, p= 0 MeV/c) =40 MeV for the Giessen model [9] andΓ(ρ0, p = 750 MeV/c) = 95 MeV for the Valencia model [10].mass describe the measured data similarily well. Although,the experimental resolutionof FWHM=55 MeV together with the theoretical models limit the extraction of a precisein-medium signal, a clear evidence of a dropping mass scenario for theω meson can beconcluded.The extraction of theω width in the nuclear medium from the measurement of themass distribution is limited by the experimental resolution and the unfolding of thenuclear density distribution at which theω decays. Fortunately, the measurement of theω absorption as a function of the nuclear size allows a straight forward way to extract the(inelastic) cross section of theω meson which can be related to its width [9, 10]. Here,the transparency ratioTA is introduced, which is a measure of the probability for theωmeson to leave the nuclear without absorption. Fig. 2 showsTA for the measured data incomparison to two model calculations. As a preliminary result, ΓGi(ρ0, p= 0 MeV/c) =40 MeV [9] andΓVal(ρ0, p = 750 MeV/c) = 95 MeV [10] can found. The width in theGiessen calculation has been computed as a function of theω momentum and agreesat higher momenta with the Valencia value (ΓGi(ρ0, p = 750 MeV/c) = 105 MeV).The intuitive expectation of a significant broadening of theω in nuclear matter asa consequence of the opening of additional partial channels(e.g. ωN → NN) can beconfirmed. This topic will be elaborated in a forthcoming publication.REFERENCES1. D. Trnka, et al.,Phys. Rev. Lett. 94, 192303 (2005).2. M. Naruki, et al.,Phys. Rev. Lett. 96, 092301 (2006).3. S. Damjanovic, et al.,Phys. Rev. Lett. 96, 162302 (2006).4. P. Muehlich,priv. communication (2005).5. F. Klein,these preoceedings (2006).6. J. Messchendorp, et al.,Eur. Phys. Jour. A 11, 95 (2001).7. P. Muehlich, et al.,Eur. Phys. Jour. A 20, 449 (2004).8. D. Trnka,PhD thesis (2006).9. P. Muehlich, et al.,Nucl. Phys. A 773, 156 (2006).10. M. Kaskulov, and E. Oset,priv. communication,preliminary (2006).

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