Evidence for narrow resonant structures at $W \approx 1.68$ and $W \approx 1.72$ GeV in real Compton scattering off the proton

0.19 MB·English

by  V. Kuznetsov

Evidence for narrow resonant structures at W ≈ 1.68 and W ≈ 1.72 GeV in real
Compton scattering off the proton
V. Kuznetsov1,∗, F.Mammoliti2,3, V.Bellini2,3, G.Gervino4,5, F.Ghio6,7,
G.Giardina8, W. Kim9, G. Mandaglio2,8, M.L. Sperduto2,3, and C.M. Sutera2,3
1Petersburg Nuclear Physics Institute, 188300 Gatchina, Russia
2INFN - Sezione di Catania, via Santa Sofia 64, I-95123 Catania, Italy
3Dipartimento di Fisica ed Astronomia, Universita´ di Catania, I-95123 Catania, Italy
4Dipartamento di Fisica Sperimentale, Universita´ di Torino, via P.Giuria, I-00125 Torino, Italy
5 INFN - Sezione di Torino, I-10125 Torino, Italy
6INFN - Sezione di Roma, piazzale Aldo Moro 2, I-00185 Roma, Italy
7Instituto Superiore di Sanit´a, viale Regina Elena 299, I-00161 Roma, Italy
5 8Dipartimento di Fisica e di Scienze della Terra - Universita` di Messina, salita Sperone 31, 98166 Messina, Italy and
1 9Kyungpook National University, 702-701 Daegu, Republic of Korea∗
0 (Dated: April28, 2015)
2
FirstmeasurementofthebeamasymmetryΣforComptonscatteringofftheprotonintheenergy
pr range Eγ = 0.85−1.25 GeV is presented. The data reveals two narrow structures at Eγ = 1.036
A and Eγ =1.119 GeV. They may signal narrow resonances with masses near 1.68 and 1.72 GeV, or
they may be generated by the sub-threshold KΛ and ωp production. Their decisive identification

7 requiresadditional theoretical and experimental efforts.
2
PACSnumbers: 14.20.Gk,13.60.Rj,13.60.Le

]
x
e TheobservationofanarrowenhancementatW ∼1.68 ference with other resonances.
-
l GeV in η photoproduction [1–5] and Compton scatter- Therecenthigh-precisionandhigh-resolutionmeasure-
c
u ingoffthe neutron[6](the so-called“neutronanomaly”) ment of the γp → ηp cross section by the A2@MaMiC
n is of particular interest because it may signal a nu- Collaboration [20] made it possible to retrieve a small
[ cleon resonance with unusual properties: a mass near dip at W ≈ 1.69 GeV which was not resolved in pre-
M ∼1.68 GeV, a narrow (Γ≤25 MeV) width, a strong vious experiments. At the same time the revision of the
2
v photoexcitation on the neutron, and a suppressed decay GRAALbeamasymmetryΣforγp→ηprevealedareso-
3 to πN final state [7–11]. Such resonance was never pre- nantstructureatW =1.685GeV [21,22](see also[23]).
3 dicted by the traditional Constituent Quark Model [12]. The bump in the Compton scattering off the neutron
3 On the contrary,its properties coincide surprisingly well at W = 1.685 GeV was observed at GRAAL [6]. The
4 with those expected for an exotic state predicted in the motivation for this work was to search, in analogy with
0 framework of the chiral soliton model [13–17]. η photoproduction, a resonant structure in polarization
.
1 On the other hand, several groups [18] explained the observables for Compton scattering on the proton.
0
bump in the γn → ηn cross section in terms of the in- In this Rapid Communication, we report on the first
5
terference of well-known wide resonances. Although this measurement of the beam asymmetry Σ for Compton
1
: assumption was challenged by the results on Compton scattering off the proton in the range of incident-photon
v scattering off the neutron [6] (this reaction is governed energies E =0.85−1.25 GeV. The data were collected
i γ
X by different resonances), it is widely discussed in liter- attheGRAALfacility[25]from1998to2003inanumber
ature. Another explanation was proposed by M. Dor- of data-taking periods. The main difference between the
r
a ing and K. Nakayama [19]. They explained the neutron periodswastheusageofeitherUVorgreenlaserlight. A
anomalyasvirtualsub-thresholdKΛandKΣphotopro- highly-polarized and tagged photon beam was produced
duction (“cusp effect”). At present, the decisive identi- bymeansofbackscatteringofthislighton6.04GeVelec-
fication of the narrow peculiarity at W ∼ 1.68 GeV is a tronscirculatinginthestorageringoftheEuropeanSyn-
challenge for both theory an experiment. chrotron Radiation Facility (ESRF,Grenoble, France).
∗ The tagged photon-energy range was ∼ 0.8−1.5 GeV
One benchmark signature of the N (1685) resonance
with the UV laser and ∼0.65−1.1 GeV with the green
(ifitdoesexist)isstrongphotoexcitationontheneutron
one. The linear beam polarizationvaried from∼40%at
and weak (but not zero) photoexcitation on the proton.
the lower energy limits up to ∼ 98% at the upper ones.
Suchresonancewouldappearincrosssectiononthepro-
Theresultsobtainedwithtwodifferenttypesofrunswere
ton as a minor peak(dip) structure which might be not
then used for cross-checks.
(or poorly) seen in experiment. However its signal may
be amplified inpolarizationobservablesdue to the inter- Scatteredphotonsweredetectedinacylindricallysym-
metrical BGO ball [26]. The ball provided the detection
of photons emitted at θ =25−155◦ with respect to a
lab
beamaxis. Recoilprotonsemittedatθ ≤25◦ werede-
lab
∗Electronicaddress: [email protected] tectedinanassemblyofforwarddetectors. Itconsistedof
2
twoplanarmultiwirechambers,adoublehodoscopescin- 3000 157o 3000 143o 3500 131o
tillator wall, and a lead-scintillator time-of-flight (TOF) 2500
wall [27]. 2500 3000
2000 2500
The data analysis was similar to that used in the pre- 2000
1500 2000
vious measurement on the neutron [6]. At first, the γp 1500
1500
final states were identified using the criterion of copla- 1000 1000
1000
narity, cuts on the proton and photon missing masses, 500 500 500
and comparing the measured TOF and the polar angle
0 0 0
of the recoil proton with the same quantities calculated 0 0.1 0 0.1 0 0.1
6000
assuming the γp→γp reaction. 4500
4000 5000 5000
The sample of the selected events was still populated
3500
by events from the π0 photoproduction. Two types of 3000 4000 4000
the π0 background were taken into consideration: 2500 3000 3000
i) Symmetric π0 → 2γ decays. The pion decays in two 2000
1500 2000 2000
photonsofnearlyequalenergies. Beingemittedinanar-
1000
1000 1000
rowcone alongthe piontrajectory,suchphotons imitate 500
a single-photon hit in the BGO ball; 0 0 0
0 0.1 0 0.1 0 0.1
ii) Asymmetric π0 → 2γ decays. One of the photons E [GeV] E [GeV] E [GeV]
mis mis mis
takes the main part of the pion energy. It is emitted
FIG. 1: Spectra of missing energy. Upper panels show the
nearly along the pion trajectory. Such photon and the
results of simulations. Solid lines correspond to Compton
recoil proton mimic Compton scattering. The second events. Dashed areas are the events from γp → π0p. Dark
photonissoftandisemittedintoabackwardhemisphere
areas are the yields of other reactions. Lower panels show
relativetothepiontrack. Itsenergydependsonthepion the spectra obtained in experiment. Dashed areas are the
energy and may be as low as 6−10 MeV. estimated contamination of π0 events. Solid lines indicate
The symmetric events were efficiently rejected by an- thecutusedtoselectthemixtureofComptonandπ0 events.
alyzing the distribution of energies deposited in crystals Dashed lines are theside-band cuts used to select π0 events.
attributed to the correspondingcluster in the BGO ball.
Theefficiencyofthisrejectionwasverifiedinsimulations thebackwardgapintheGRAALdetector. Thedistribu-
and found to be 99%. tions of Compton and π0 events get closer being almost
Theasymmetricπ0 →2γ decayswerethe majorprob- unresolved at 131◦ (the angular bin 122−137◦).
lem. The GRAAL detector provides the low-threshold The experimental spectra (lower panels of Fig. 1) are
(5MeV)detectionofphotonsinthenearly4πsolidangle. quite similar to the simulated ones. Solid lines show the
If one (high-energy) photon would be emitted at back- cut −0.04 ≤ E ≤ 0.025. This cut was used to select
mis
ward angles, an the second (low-energy) photon could the mixture of Compton and π0 events. The events in
thenbe detected inthe BGOballorinthe forwardlead- the region above E = 0.035 GeV are mostly from π0
mis
scintillator wall. This feature made it possible to sup- photoproduction. Dashed lines in Fig. 1 indicate side-
press the π0 photoproduction. band cuts. These cuts select mostly π0 events.
For the further selection of events the missing energy
Fig.2showsthe beamasymmetryΣ ofeventsselected
E was employed
mis using the mainand side-bandcuts. The resultsobtained
with the UV and green lasers are statistically indepen-
Emis =Eγ −Eγ′ −Tp(θp), (1) dent. They are in good agreement. The data points
obtained with the side-band cuts (right panels of Fig. 2)
whereEγ denotestheenergyoftheincomingphoton,Eγ′
are close to the SM11 solution of the SAID partial-wave
is the energy of the scattered photon, and T (θ ) is the
p p analysis (PWA) for π0 photoproduction. The minor dis-
kinetic energy of the recoil proton.
crepancy is due to the contamination of Compton and
Thesimulatedspectraofthemissingenergyareshown
in the upper panels of Fig. 1. π0 events form wide dis- other events.
tributions. Compton events generate narrow peaks cen- ThebeamasymmetriesofthemixtureofComptonand
tered around E = 0. The events in this region be- π0 events (the main cut, left panels of Fig. 2) deviate
mis
long to both Compton scattering and π0 photoproduc- fromtheSM11solution. Therearetwonarrowstructures
tion. The contamination of events from other reactions which are not seen with the side-band cuts.
(mostly double neutral pion photoproduction) does not The validity of this observation was verified by means
exceed2%. AtlargerE the spectraaredominatedby of different cuts of the missing energy in the overall an-
mis
π0 events. gular range 122−165◦, namely −0.04≤E <0 GeV,
mis
The Compton peak is clearly seen at 157◦ (the angu- 0. ≤ E < 0.025 GeV, and 0.025≤ E < 0.05 GeV.
mis mis
lar bin 151−165◦). At these angles soft photons from The first two cuts selected the mixture of Compton and
asymmetric π0 decays are efficiently detected in either π0 events. Bothstructureswereseenwiththesecutsand
the BGO Ball or in the forward shower wall. At more dissapeared with the third one which selected mostly π0
forwardanglespart ofsuch photons escapes outthrough events.
3
0.6 o 0.6 o 0.6 0.4
131 131
S 0.4 0.4 S 0.4 131 deg 0.3 143 deg
0.2 0.2 0.2
0.2
-0 -0 0.1
0
-0.2 -0.2 -0
-0.2
-0.4 -0.4 -0.1
0.9 1 1.1 1.2 0.9 1 1.1 1.2
-0.4
0.6 o 0.6 o -0.2
143 143
S 0.4 0.4 -0.6 -0.3
0.2 0.2 -0.8 -0.4
0.9 1 1.1 1.2 0.9 1 1.1 1.2
-0 -0 0.4 0.4
-0.2 -0.2 S 0.3 157 deg 0.3 Sum
-0.4 -0.4 0.2 0.2
0.9 1 1.1 1.2 0.9 1 1.1 1.2
0.6 o 0.6 o 0.1 0.1
157 157
0.4 0.4 -0 -0
0.2 0.2 -0.1 -0.1
-0 -0 -0.2 -0.2
-0.2 -0.2 -0.3 -0.3
-0.4 -0.4 -0.4 -0.4
0.9 1 1.1 1.2 0.9 1 1.1 1.2 0.9 1 1.1 1.2 0.9 1 1.1 1.2
E [GeV] E [GeV] E [GeV] E [GeV]
g g g g
FIG. 2: On the left: Beam asymmetry Σ for the mixture of FIG. 3: Beam asymmetry Σ for Compton scattering on the
Compton and π0 events. On the right: Beam asymmetry Σ proton. Dark(open) circles are the results obtained with
obtainedusingside-bandcuts(mostlyπ0events). Dark(open) UV(green) laser.
circles are the results obtained with UV(green) laser. Solid
lines are the SAID SM11 solution for the γp → π0p beam lackoftheoreticalpredictionsthedatawasfitinasimple
asymmetry. way: the results from three angular bins were summed
with weights proportional to inverse squares of their er-
ThebeamasymmetryshownintheleftpanelsofFig.2
isthe combinationofbothComptonandπ0 beamasym- rors (lower right panel of Fig. 3) and fit either by the 4-
order polynomial (the background hypothesis) or by the
metries (the minor contribution of events from other re-
4-order polynomial-plus-two modified Breit-Wigner dis-
actions can be neglected)
tributions (the background-plus-signal hypothesis). The
Σtot =αΣcomp+(1−α)Σπ0 (2) formula for the Breit-Wigner distributions
where α = Ncomp denotes the fraction of Compton (Eγ −ERi)cos(φi)+Γisin(φi)
events. Ncomp+Nπ0 Ai (Eγ −ERi)2+ Γ42i ,i=1,2 (3)
The contamination of π0 events was determined by
normalizing the simulated π0 spectrum in the angular was suggested in Ref. [29] to describe the interference
bin of 157◦ to the experimental one in the region of the between a narrow resonance and background. The mass
side-bandcut(Fig.1). Thenthesamenormalizationwas centers of the distributions were extracted as ER1 =
usedtodetermine the π0 contaminationintwootheran- 1.036 ± 0.002 GeV (W1 = 1.681 GeV) and ER2 =
gular bins. 1.119±0.002 GeV (W2 = 1.726 GeV). The widths were
The fraction of Compton events α varied from ∼90% Γ1 = 25±10 and Γ2 = 35±12 MeV ( Γ1 = 18±6 and
to ∼ 40% at 0.85 to 1.25 GeV in the angular bin 157◦, Γ2 =21±7MeVintheunitsofthecenter-of-massenergy
from ∼75%to ∼35%in the angular bin 143◦, and from W). The χ-squares of the fits were 75.7/39(background
from ∼ 60% to ∼ 30% in the angular bin 131◦. The π0 hypothesis)and29.7/31(signal-plusbackgroundhypoth-
beam asymmetry Σπ0 was taken from the SAID SM11 esis). The log likelihood ratio of these two hypotheses
solution. Then Compton beam asymmetry Σcomp was (p2ln(LB+S/LB) corresponded to the confidence level
derived using Eq. 2 in which the π0 beam asymmetry of ≈4.8σ.
Σπ0 was set equal to the SAID SM11 solution. The errors shown in Fig. 3 are only statistical. The
TheresultsareshowninFig.3. Attheenergiesbelow1 systematic uncertaintymainly originatesfromthe deter-
GeVthe Comptonbeamasymmetryis closeto 0. Above mination of α. One may see from the Eq. 2 that it less
1 GeV there are two narrow structures. They are better affects Σ if (i) α is largeand(ii) Σ ≈Σπ0. This
comp comp
◦ ◦
pronouncedat131 andalmostdegenerateat157 . This uncertainty mostly affects Σ in the regions of the
comp
is a typical trend for the beam asymmetry Σ which a observed structures. It results in the additional ≈ 20%
priori approaches 0 at 180◦. errors in the extraction of the amplitudes A in Eq. 3.
i
Compton scattering was calculated by A. L’vov et The observation of the narrow structure at W ≈ 1.68
al. [28] on the base of dispersion relations. The range of GeV correlates with the previous results on η photopro-
modelvalidityisbelow 1GeV.NocalculationsofComp- duction [1–5], Compton scattering off the neutron [6],
ton scattering at higher energies is available. Because of and η photoproduction on the proton [21, 22]. The sec-
4
ondstructureatW ≈1.73GeVwasnotseeninthemen- not seen in η photoproduction on the neutron.
tionedexperiments. HoweverthemodifiedSAIDpartial- The observation of these structures may signal one or
wave analysis [10] hinted two narrow P11 resonances at two narrow resonances. Their masses and width which
W = 1.68 GeV and W = 1.73 GeV. Both structures stem from our simple fit are M1 = 1.681±0.002stat ±
were also seen in the preliminary data on πN scattering 0.005syst, M2 =1.726±0.002stat±0.005syst GeV, Γ1 =
by the EPECUR Collaboration [30]. The preliminary 18±6 and Γ2 =21±7 MeV. The systematic errors∆M
evidence for the peak at W = 1.72 GeV in KΛ invari- areduetothe accuracyofthe calibrationoftheGRAAL
ant mass was reported by the STAR Collaboration [31] tagging system.
but remained unpublished. The structure at W ≈ 1.68 The decisive identification of both structures requires
GeVisonemorechallengefortheexplanationoftheneu- a common fit of Compton and η photoproduction data.
tron anomaly in terms of the interference of well-known Accurate calculations of Compton scattering are needed
resonances [18]. This hypothesis cannot explain all ex- for that. One particular task is to determine the waves
perimental findings. and quantum numbers. Cusp is a priori an S-wave phe-
TheenergiesW ≈1.68andW ≈1.73GeVcorrespond nomenon. The Chiral Soliton Model predicts one exotic
to the KΛ and ωp photoproduction thresholds. This P11 state with the mass near 1.7 GeV [13].
favors the cusp effect as an explanation of the neutron It is our pleasure to thank the staff of the Euro-
anomaly. Furthermore, a narrow step-like structure was pean Synchrotron Radiation Facility (Grenoble, France)
∗
also observed at the K Λ threshold [32]. On the other for the stable beam operation. This work was sup-
hand it still remains unclear as to (i) why this effect is ported by INFN Sezione di Cataniaand by High Energy
not seen in πN photoproduction, (ii) whether it could Physics Department of Petersburg Nuclear Physics In-
occur inComptonscattering,and(iii) whythe structure stitute. Discussions with Profs. B. Krusche, A. L’vov,
at W ≈ 1.72 GeV is seen in Compton scattering and is M. Polyakov,and H. Schmieden were quite stimulating.
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