The Forward Tagger for CLAS12 at Jefferson Laboratory and the MesonEx experiment

Transcription

1 The Forward Tagger for CLAS12 at Jefferson Laboratory and the MesonEx experiment A. Celentano, Ciclo XXVI Advisors: External Advisor: Prof. Mauro Taiuti Dott. Marco Battaglieri Prof. Adam Szczepaniak

2 The MesonEx experiment at Jefferson Laboratory Meson Spectroscopy program with quasi-real photons Goals: Measure the light-quarks mesons spectrum in the mass range GeV/c2 Determine masses and properties of rare qq states. Search for exotics. Experimental technique: low Q2 electron scattering on a hydrogen target. Provides a high-flux of high-energy, linearly polarized, quasi-real photons. Complementary and competitive to real photoproduction. JLab approved experiment, foreseen in 2016 Kinematic ranges

3 Meson Spectroscopy Mesons are the simplest hadronic bound state, the ideal laboratory to study the interaction between quarks, to understand the role of gluons, and to investigate the origin of color confinement. Meson spectroscopy is a powerful tool to investigate how QCD partons manifest themselves under the strong interaction at the energy scale of the nucleon mass. In this regime, the theory is non-perturbative. Numerical methods QCD-inspired models Constituent Quark Model (CQM) Total angular momentum Parity Charge conjugation Not all combinations are allowed: 0--, 0+-, 1-+, are explicitly forbidden. SU(3) flavor symmetry: mesons are organized in nonets of particles with same JPC and similar masses.

4 Exotic mesons QCD does not prohibit the existence of unconventional meson states such as hybrids (qqg), tetraquarks (qqqq), and glueballs. The presence of states with manifest gluonic component, behind the CQM, would be the opportunity to directly look inside hadron dynamics. Exotic quantum numbers would provide an unambiguous evidence of these states. Lattice QCD calculations provided a first hint on the spectrum and mass range of exotics. Mass range: 1.4 GeV GeV Lightest exotic is a 1-+ state Standard Mesons J. J. Dudek et al, Phys. Rev. D82, (2010) Exotics

5 Meson Spectroscopy with EM probes Photoproduction: exotic mesons are most likely produced by a spin-1 probe. Peripheral production with beams: Peripheral photo-production: Excite the glue Need spin-flip for exotic quantum numbers No spin-flip required for exotic quantum numbers Low cross-sections require an high photon flux: low Q2 electron scattering. Provides an high flux of high-energy, linearly polarized, quasi real photons. Equivalent photon flux in MesonEx: s Competitive and complementary to real photoproduction.

6 Jefferson 6 GeV Continuous Electron Beam Accelerator Facility E = GeV Imax = 100 A - Hall A, C Imax = 800 na Hall B Duty Cycle ~ 100% (E)/E ~ 2.5 x 10-5 Polarization ~ 80% Beam is delivered simultaneously to the 3 experimental halls.

7 Jefferson 12 GeV

8 The CLAS12 detector in Hall JLab High acceptance (~4 ) detector, designed to luminosity 1035 cm-2 s-1 Forward Detector: 5 < θ < 35 Central Detector: θ > 35 Good energy resolution Good particle ID High angular acceptance Excellent detector for a meson spectroscopy experiment: measure exclusive reaction channels and determine the JPC quantum numbers of produced states via PWA. Angular acceptance needs to be extended do detect low angle scattered electrons. Need a new, dedicated facility: the Forward Tagger.

9 The Forward Tagger Facility for the MesonEx experiment The MesonEx experiment: CLAS12: measure final state hadrons. Forward Tagger: measure the scattered electron. Scattered electron measurement: Energy Ee', to determine E and T Scattering angle, to determine Q2 and the pol. plane Forward Tagger Facility: FT-Cal: PbWO4 calorimeter FT-Hodo: Scintillation hodoscope FT-Trck: Micromegas tracker

10 The FT Cal Forward Tagger Facility core component. It measures the energy of scattered electrons with few % resolution, and provides a fast coincidence signal. Requirements: Strong radiation hardness Good energy and timing resolution Small radiation length and Moliere Radius Compatible with high magnetic field Technology: 332 PbWO4 crystals, 15x15x200 mm3 LA-APD readout, custom FEE 0 operating temperature Foreseen energy resolution σ E / E=2.3% / E 0.5 %

11 The FT Cal Forward Tagger Facility core component. It measures the energy of scattered electrons with few % resolution, and provides a fast coincidence signal. Requirements: Strong radiation hardness Good energy and timing resolution Small radiation length and Moliere Radius Compatible with high magnetic field Technology: 332 PbWO4 crystals, 15x15x200 mm3 LA-APD readout, custom FEE 0 operating temperature R&D: Extensive characterization of all components Single crystal assembly: crystal + APD + amplifier

12 The FT Cal Forward Tagger Facility core component. It measures the energy of scattered electrons with few % resolution, and provides a fast coincidence signal. Requirements: Strong radiation hardness Good energy and timing resolution Small radiation length and Moliere Radius Compatible with high magnetic field Technology: 332 PbWO4 crystals, 15x15x200 mm3 LA-APD readout, custom FEE 0 operating temperature R&D: Extensive characterization of all components Realization of small-scale prototypes to validate technical choices

13 FT Cal crystals FT-Cal crystals properties: Radiation hardness Dimensions Light yield Optical transmission GIESSEN UNIVERSITY CERN I tuned the measurement procedure characterizing few samples in Genova: light yield vs temperature. This procedure was exported to the massive characterization. Measure at +18 C ACCOS: Semi-automatic facility for PbWO4 crystals room temperature, built for CMS ECAL and currently being used for PANDA ECAL crystals. All the FT-Cal crystals were ACCOS during 2013

14 FT Cal APDs FT-Cal APDs characterized in the temperature range 0 25 Gain vs Vb and T Dark current vs V and T b Gain stability Gain measured with a DC-technique : 1) 2) 3) 4) Measure I vs Vb under constant illumination (LED) Subtract dark current Re-normalize to G=1 (Vb < 50 V) Repeat this procedure at different temperatures I developed a semi-automatic facility to characterize 24 sensors at time. Built and commisioned in Genova in Spring 2013 Installed in Roma Tor Vergata and used to characterize all the FT-Cal APDs All the APDs were characterized during summer 2013 G=1

15 FT Cal APDs FT-Cal APDs characterized in the temperature range 0 25 Gain vs Vb and T Dark current vs V and T b Gain stability Gain measured with a DC-technique : 1) 2) 3) 4) Measure I vs Vb under constant illumination (LED) Subtract dark current Re-normalize to G=1 (Vb < 50 V) Repeat this procedure at different temperatures I developed a semi-automatic facility to characterize 24 sensors at time. Built and commisioned in Genova in Spring 2013 Installed in Roma Tor Vergata and used to characterize all the FT-Cal APDs All the APDs were characterized during summer 2013 G=1

16 FT Cal prototype Objectives: Test the technical choices. Demonstrate detector operation feasibility. Validate MonteCarlo simulations. Design: 4x4 matrix of PbWO4 matrix, each 15x15x200 mm3 LA-APD readout (10x10mm2) and FT FEE Copper shield for thermal stabilization Custom motherboard for signals, LV, HV Measurements: Energy resolution and linearity between few MeV (cosmic rays) to 4 GeV (e- beam test) Energy resolution vs temperature Noise in realistic conditions

17 FT Cal prototype LNF BTF BTF: Facility for detectors test and commissioning. Electron beam with energy 500 MeV, ~2 electrons/bunch Results: Excellent linearity up to 4 GeV Good agreement with MC results for Ee>1.5 GeV: E/E = 2 GeV Low-energy noise contribution underestimated in MC, now corrected. DETECTOR BEAM

18 FT Cal prototype LNF BTF BTF: Facility for detectors test and commissioning. Electron beam with energy 500 MeV, ~2 electrons/bunch Results: DETECTOR Excellent linearity up to 4 GeV Good agreement with MC results for Ee>1.5 GeV: E/E = 2 GeV Low-energy noise contribution underestimated in MC, now corrected. Results confirmed the FT-Cal design validity and the detector operation feasibility. BEAM

19 Moving toward MesonEx A detailed preparatory work is required for the different reaction that will be measured in MesonEx. Need to study the measurement feasibility in MesonEx trough MonteCarlo simulations. Given the foreseen CLAS12+FT performances: Evaluate the acceptance and the event rate. Verify if a Partial Wave Analysis is feasible. Study the sensitivity to exotics. Prepare the tools and the framework for the real data analysis. Strategy: Use pseudo-data as input, and project them on the detector trough MonteCarlo. Apply reconstruction algorithms and perform PWA on reconstructed events. Use lower energy data: As guidance for pseudo-event generation. To validate the analysis procedure. I studied the reaction extrapolating results at higher energy. in MesonEx, analyzing CLAS-g12 data and

20 0 channel: motivations The final state is one of the MesonEx golden channels. Good candidate to search for exotic mesons: the JPC quantum numbers of resonances are constrained. A P-wave resonance would be a 1-+ exotic signal! The channel was investigated by past experiments (VES, E852, Crystal Barrel): a possible exotic signal - 1 (1400) - has been seen but still a definite answer is missing. Reaction dynamics: Signal: resonance production via Regge t-channel exchange mechanism. Known resonances: a0(980), a2(1320), a2(1700) 1(1400) contribution included with different strength. Background: double-regge exchange (Deck mechanism) X

21 0 channel partial wave analysis of two pseudoscalars Partial wave analysis: Measure the angular distribution of decay products to disentangle different resonances Each resonance has a specific signature depending on its spin For the decay in two pseudoscalars: X Technique: maximum likelihood fit Parametrize the reaction intensity as a coherent sum of different waves. Free parameters are the wave intensities complex numbers! Fits are performed in bins of M to derive the production parameter dependence on this variable. A resonance appears as a structure usually a peak in the corresponding wave. PWA configuration employed in the analysis: S0 (a0), P+ ( 1), D+ (a2) waves + background wave. Data were divided in 72 M bins, from 0.7 to 2.5 GeV/c2, 25 MeV/c2 width.

22 0 channel CLAS analysis g12 run I analyzed the reaction CLAS g12 run (April 1, June 9, 2008): 44 days of Bremsstrahlung photon beam, Emax=5.45 GeV. 40 cm-long LH2 target billion triggers (68 pb-1) recorded. Initial data sample (1 proton, 4 measured): 4.42M I developed a set of selection cuts to identify a clean sample of events.

23 0 channel CLAS analysis g12 run I analyzed the reaction CLAS g12 run (April 1, June 9, 2008): 44 days of Bremsstrahlung photon beam, Emax=5.45 GeV. 40 cm-long LH2 target billion triggers (68 pb-1) recorded. Initial data sample (1 proton, 4 measured): 4.42M I developed a set of selection cuts to identify a clean sample of events. identification: Order the 4 photons according to their relative angle are those with the smallest relative angle. Cut on the two pairs invariant masses. Selection

24 0 channel CLAS analysis g12 run I analyzed the reaction CLAS g12 run (April 1, June 9, 2008): 44 days of Bremsstrahlung photon beam, Emax=5.45 GeV. 40 cm-long LH2 target billion triggers (68 pb-1) recorded. Initial data sample (1 proton, 4 measured): 4.42M I developed a set of selection cuts to identify a clean sample of events. I ended with 30.3k events for the reaction Statistics is too low to perform a PWA Results can be compared with the theoretical amplitude to validate it and fix free parameters

25 0 channel CLAS analysis results First measure of the differential cross section with respect to invariant mass Fit performed with the theoretical calculation from the reaction amplitude. Validates the amplitude expression. Free parameters are experimentally determined.

26 0 channel MesonEx simulation invariant mass Expected statistics in the MesonEx 80 day beam time: GENERATED Reconstruction algorithm: the same employed in the CLAS analysis Overall detection acceptance: ~ 3% Nmeas ~ 15 M I checked that the angular distributions are not too distorted by the detector, as a pre-requisite for PWA invariant mass VS angle

27 0 channel MesonEx simulation invariant mass Expected statistics in the MesonEx 80 day beam time: RECONSTRUCTED Reconstruction algorithm: the same employed in the CLAS analysis Overall detection acceptance: ~ 3% Nmeas ~ 15 M I checked that the angular distributions are not too distorted by the detector, as a pre-requisite for PWA invariant mass VS angle

28 0 channel MesonEx simulation 5% exotic included Exotic signal included in pseudo-data Good agreement between fit results and calculation from theoretical amplitude. P-wave structure clearly identified, although significant detector-induced effects are visible. MesonEx can measure and identify a (1400) exotic signal via PWA, for a production strength greater than 5 % of the a2(1320) signal

29 Conclusions My PhD thesis work was done within the MesonEx experiment in Hall B at JLab. Experiment goal: comprehensive study of the meson spectrum in the mass range 1 3 GeV/c 2, searching for rare qq states and exotics, via low Q2 electron scattering on a hydrogen target. Measure final state hadrons with the CLAS12 detector. Measure scattered electron with the new Forward Tagger Facility. FT-Cal: the facility core. All the components were characterized with dedicated measurement campaigns. Small scale prototypes were developed and tested with cosmic rays and electron beams to validate technical choices and demonstrate detector operation feasibility. The FT-Cal is currently been built. First tests are foreseen in Summer 2014 in Genova. In parallel to the hardware activity, and in preparation to MesonEx, I performed a feasibility study of the reaction, to evaluate the experiment sensitivity to a 1(1400) exotic state. Project pseudo-data on the detector and perform a full PWA of the reconstructed events. Use low energy data to validate the amplitude, extrapolating results to the MesonEx range. The MesonEx sensitivity to 1(1400) is 5% of the a2(1320) cross-section.

30 Backup Slides

31 Constituent Quark Model In the Constituent Quark Model (CQM), mesons are quark antiquark bound states, with defined JPC quantum numbers. Total angular momentum Parity Charge conjugation Not all combinations are allowed: 0--, 0+-, 1-+, are explicitly forbidden. SU(3) flavor symmetry: mesons are organized in nonets of particles with same JPC and similar masses.

32 Kinematics: Low Q2 electron scattering Equivalent photon flux: At 1035 cm-2 s-1, in the MesonEx kinematic range, with ~ 100 barn: R ~ 7 KHz. Corresponds to on a 40 cm LH2 target. Virtual photon polarization, defined event by event: Transverse linear polarization Longitudinal polarization Q2 vs E vs E

33 CLAS12 specifications

34 The Forward Tagger Facility

35 FT Cal crystals Crystals radiation hardness: Evaluated trough a 30 Gy 60Co source. Longitudinal transmission measured before and after the exposure, using a spectrophotometer. Radiation hardness evaluated trough 420 nm 100 FT-Cal crystals were Giessen during November 2013 More than 50% of the crystals were out of specifications. No correlation was seen between data and manifacturer specifications. A full measurement campaign is planned in February 2014

36 FT Cal APDs APD characterization facility: All the APDs were characterized during summer sensors were found outside specifications and were sent back to the factory for replacement Measures 24 sensors at time, in ~ 20 hours. Employs a custom-designed current multiplexer, to have readout via a single picoammeter. Cooling provided by an external chiller. System inserted in a gas and vacuum-tight box Handled trough a Labview program.

37 FT Cal prototype test with cosmic rays Goals: Demonstrate operational principles Provided a first estimate of cal. constants Tune MC simulations Setup: Detector placed in between 3 plastic scintillators counters Scintillators hits positions provide cosmic track Trigger given by the 3 counters coincidence Results: Data-MonteCarlo agreement within 6% Cal. constants in agreement with high energy results within 10% Cal. constants ratio Cosmic rays 500 MeV e- beam

38 FT Cal prototype LNF BTF DA NE The BTF test beam is obtained attenuating the primary LINAC electron beam delivered to the DAFNE machine. BTF Variable intensity: e-/bunch Variable electron energy: MeV LINAC BTF beam properties: DETECTOR BEAM

39 CLAS12 analysis: pseudo data generation Event generator: Generate events according to a given photo-production amplitude ( hit or miss technique). Electron low Q 2 scattering is included inside the generator. In the one-photon-exchange approximation: Summing over virtual photon helicity (vphoton pol. vectors completeness relation + curr. conservation): Resulting amplitude is the product of two terms: : emission of a quasi-real photon (m ~ 0) by the electron : photoproduction on the proton of the final state px A for production on the proton: incoherent sum of two terms, i.e. signal (resonances at low M ) and bck (high-mass, non-resonant mechanism).

40 CLAS12 analysis: reaction amplitude Signal: resonance production via Regge t-channel exchange mechanism. Known resonances: a0(980), a2(1320), a2(1700), as indicated from the low-energy data analysis. 1(1400) contribution included with different strength to study the experiment sensitivity Resonance-photon-reggeon coupling is a free parameter to be determined from the low-energy data. Background: double-regge exchange (Deck mechanism) Dominant mechanism at high invariant mass. Used also to parametrize low mass residual non-resonant background. X

41 0 channel reaction amplitude Resonant amplitude in the low-mass resonant region: Regge description of the resonance production on the proton. Breit-Wigner description of the resonance propagation and decay. Spherical harmonics for resonance products angular distribution. Proton-proton-reggeon coupling from other reactions (factorization) Decay coupling g from resonances width and branching ratios. Reggeon-photon-resonance coupling from CLAS analysis

42 0 channel reaction amplitude Non-resonant amplitude in the high-mass resonant region: Regge description of the process, trough a double-regge exchange (Deck-effect) Two amplitudes, with different particle ordering, and thus different exchanged trajectories combinations. In principle, this amplitude describes the reaction dynamics at sufficiently high invariant mass. I parametrized the residual, non-resonant background at low invariant mass using the same amplitude, scaled trough a smooth function, as determined and suggested from the CLAS data analysis. The expression I employed converges to the Double-Regge formula for high (>2 GeV) invariant mass.

43 CLAS12 analysis: PWA PWA of reconstructed events, in M independent bins: Data were divided in 72 M bins, from 0.7 to 2.5 GeV/c2, 25 MeV/c2 width. I considered the t-range between -0.1 and -0.3 GeV2, and the full E range from 6.5 to 10.5 GeV Intensity formula for the fit: Contributions with J < 3, M < 2 included. For each J, I considered only the term requiring minimum photon spin-flip. Embeds the electron scattering, i.e. the virtual photon polarization. Includes parity invariance. Incoherent bck parametrized trough the SAME double Regge expression used to generate pseudo-data. Not correct to fit with the above formula, at least in the low mass region, since the background wave is not orthogonal to the other waves. Need another strategy. Fit the high mass range with that formula, evaluate the background there, and FIX IT in the low mass region. The invariant mass dependence is embedded in A bck, therefore Vbck is a constant. The fits in the low mass region have less free parameters. The background there is FIXED.

44 PWA fits: background wave Background: At high invariant mass (>2.2 GeV), the background intensity was free and determined by the PWA fit (Vbck is a free parameter) BLACK POINTS At lower invariant mass Vbck is fixed. background IS NOT FITTED, but constrained BLUE POINTS I changed the mass-cut down to 2.0 GeV. Results were compliant within the errors. Errors: Vbck is determined from the high mass fits, performing an average of the different bins there. It has an error (negligible for GENERATED events, but not for RECONSTRUCTED) At lower invariant mass, Vbck is fixed. An error in Vbck determination translates to a systematic shift for low mass points. To evaluate the effect on the other waves, I repeated fits by fixing Vbck to its nominal value, + its error.

45 0 channel MesonEx simulation no exotic included First case: no exotic signal included in pseudo-data Setup the analysis framework Test the PWA procedure Study the detector-induced effects (finite acceptance and resolution) The leakage to the P-wave leakage from other waves is ~ 1%. It is only due to detector effects