Search for Exotics at Jefferson Lab

R. De Vita Search for Exotics at Jefferson LabMarrakesh, 27 September 2011

Search for Exotics at Jefferson Lab

Raffaella De VitaINFN –Genova

For the CLAS Collaboration

PINAN11Partons in Nucleons and Nuclei Marrakesh, 27 September 2011


Why Hadron Spectroscopy• QCD is responsible for most of the visible mass of the

universe• Understanding the origin of this mass, i.e. the mass of

hadrons, is a necessary step to reach a deep understanding of QCD

- Revealing the nature of the mass of the hadrons- Identify the relevant degrees of freedom- Understand the origin of confinement- Validate LQCD predictions

• Meson spectroscopy is a key tool to investigate these issues

Mesons & Baryons

pp

> 1 fm

Effective Degrees of FreedomQuarks and Gluons

0.1 – 1 fm<< 0.1 fm


Meson SpectroscopyMesons are the simplest quark bound state, i.e. the best benchmark to understand how quarks interact to form hadrons and what the role of gluons is

Historically, the study of meson properties led to some of the most relevant discoveries in particles physics

• in 1947 the discovery of the pion by Powell, Occhialini and Lattes

• in the same year, the discovery of strange particles by Rochester and Butler

• the interpretation of the φdecay to KK by Zweig and others in 1963

• the discovery of the J/ψ in 1974• …

C.F.Powell, Nobel Lecture, 11th December 1950

K0

p-

p+

G.D. Rochester and C.C. Butler, Nature 160 (1947) 855.

The new frontier in meson spectroscopy is the search for unconventional states with quark-gluon configuration different from regular quark-antiquark states


Unconventional States and Exotics Known hadron configurations are baryons, made of three quarks, and

mesons, made by a quark and antiquark pair QCD does not prohibit the existence of other configuration and other color

singlet states, such as tetraquarks, glueballs and hybrids can also exist These unconventional configurations can have the same quantum numbers

as regular mesons or have “exotic” quantum number Finding a proof of their existence is a both a fundamental validation of the

theory and a precious source of information Many experiments in the world have searched and will search for these

states …• proton-antiproton annihilation: Crystal Barrel at

CERN, LHC, Panda at GSI, ...

• e+ e- annihilation: LEP, Babar at SLAC, DAΦNE at Frascati, CLEO at Cornell, BES at Beijing, …

• proton-proton scattering: WA experiments at CERN, GAMS at Protvino, …

• pion beams on fixed target: E852 at BNL, COMPASS at Cern, VES …

• photoproduction experiments: CLAS at Jefferson Lab, GlueX and CLAS12 at Jefferson Lab


Search for Hybrid MesonsUnderstanding the role of gluons and the origin of confinement is crucial to complete our picture of strong interaction

• At high energy experimental evidence is found in jet production

• At lower energies the hadron spectrum carries information about the gluons that bind quarks

• Can we find hints of the glue in the meson spectrum?

Search for non-standard states with explicit gluonic degrees of freedom

q q

q q

Regular meson

Hybrid meson

• Predicted by various models as the Flux Tube model, the Bag model, QCD Sum Rules, ..

• Now predicted by lattice QCD calculations


Hybrids and Exotics

Normal meson:flux tube in ground state

m=0, PC=(-1)S+1 qq

Hybrid meson:flux tube in excited state

m=1, PC=(-1)S qq

Excitation of the flux tube leads to a new spectrum of hadrons that can have exotic quantum numbers JPC = 0+- , 1-+ , 2+- ...

Masses are predicted to be around 2 GeV, a range that can be explored at Jefferson Lab

The best option to identify unambiguously a meson as an hybrid state is to look for exotic quantum numbers

Normal mesons (qq) are classified according to their JPC where P=(-1)L+1 C=(-1)L+S

JPC= 0-+ (π,K,η,η’)1-- (ρ,K*,ω,Φ)1+- (b1,K1,h1,h1’)...

S1

S2

L

Some combinations of JPC are not allowed for conventional qq systems but can exist for “unconventional” states as hybrids


Lattice QCDJ. Dudek et al., Phys. Rev. D82, 034508 (2010)

1S

2S

3S?

1D

1P

2P1F 1-+

Existence of exotics is supported by LQCD

Fully dynamical calculation by the JLab Hadron Spectrum Collaboration: two flavors of light

quarks and an heavier (strange) quark

two lattice volumes large set of operators stable dependence on

quark masses• Good agreement of regular meson spectrum with known states• Exotic multiplets with quantum numbers 1-+,0+- and 2-+ are predicted


Exotic Candidates: Experimental Evidence The lightest exotic hybrids are expected in the 1-+ wave:

• π1(1400): observed in ηπ final states by several experiments( GAMS, KEK, E852, Crystal Barrel) with a width of few hundreds MeV. Mass lower than expectation and interpretation still unclear

• π1(1600): observed in several decay modes (ρπ, η’π, f1π, b1π) by different experiments (E852,VES,COMPASS) but with different

production mechanism; not observed in reanalysis of E852 data and by CLAS . Evidence of the state still controversial

• π1(2000): seen by E852 in f1π and b1π final states produced in peripheral πp scattering. Further confirmation is desirable

M. G. Alekseev et al.(COMPASS),PRL 104, 241803 (2010)

M. G. Alekseev et al.(COMPASS),

PRL 104, 241803 (2010)π1(1600)

• Most of experimental evidence comes from pion (S=0) beam experiments (E852, VES, COMPASS)

→ collecting more data with different production mechanism (different probes) is crucial • Controversial evidence may arise from

PWA ambiguities → larger data sets, polarization

information, more sophisticated analysis tools, large

computing resources


Exotics in Photoproduction

Pion BeamQuark spins anti-alignedJPC = 1-- , 1++

Quark spinsalready alignedJPC = 0+- , 1-+ , 2+-

Photon Beam

regular mesons @ Eg = 5GeVX = a2

Exotic meson @ Eg = 8GeVX = p1(1600)

Photoproduction: exotic JPC are more likely produced by S=1 probe

Few data (so far) but expected similar production rate as regular mesons

Production rate for exotics is expected to be comparable to regular mesons

A. Szczepaniak and M. Swat, Phys. Lett. B516 (2001) 72

Knowledge of the photon polarization can be used as a filter in the PWA


Jefferson Laboratory

A B C

Continuous Electron Beam Accelerator Facility

® E: 0.75 –6 GeV® Imax: 200mA® RF: 1499 MHz® Duty Cycle: 100%® s(E)/E: 2.5x10-5

® Polarization: 80%® Simultaneous

distribution to 3 experimental Halls

Injector

LIN

AC LINAC

Experimental Halls


6 GeV CEBAF

CHL-2

Upgrade of the arc magnets

The 12 GeV Upgrade

Construction of the new Hall D

Beam Power: 1MWBeam Current: 90 µAMax Pass energy: 2.2 GeVMax Enery Hall A-C: 10.9 GeVMax Energy Hall D: 12 GeV

Upgrade of the instrumentation of the existing Halls


CLAS12 and GLUEXMeson spectroscopy is one of the main topics that will be studied with the Jlab 12 GeV upgrade. Key elements are:• High intensity tagged photon beams• Detectors with large acceptance and good particle identification

capabilities

GlueX in Hall D CLAS12 in Hall B


The GlueX Detector

TOFtime of flight

SCstart counter

• 2.2T superconducting solenoidal magnet• Fixed target (LH2)• 108 tagged g/s (8.4-9.0GeV)• hermetic

2.2 TeslaSolenoid

Calorimetry• Barrel Calorimeter (lead, fiber sandwich)• Forward Calorimeter (lead-glass blocks)

PID• Time of Flight wall (scintillators)• Start counter• Barrel Calorimeter

Charged particle tracking• Central drift chamber (straw tube)• Forward drift chamber (cathode strip)


The Hall D Photo Beam

1.5 T dipole

magnet

12m long vacuum

chamber

e-

20mm diamond radiator

coherent bremsstrahlung spectrum

Microscope:• Movable to cover different energy ranges• 100 x 5 scintillating fibers (2mm x 2mm)• 800MeV covered by whole microscope• 100MHz tagged g/sec on target• ~8MeV energy bite/column

Fixed array hodoscope:• 190 scintillators• 50% coverage below 9GeV g• 100% coverage above 9GeV g• Tags 3.0-11.7 GeV g• ~30MeV energy bite/counter• 3.5 – 17 MHz/counter

Photon Polarization:• 20 mm diamond radiator• Coherent peak is linearly polarized

• ~40% polarization with peak @ 9GeV

• Peak location tunable with diamond angle

•Crucial to simplify PWA!!


The Hall D Complex3

~100 meters

electron beam

Groundbreaking, April 2009 In the Hall, February 2011


GlueX: expected performance

Design Goal: high and uniform acceptanceComparison of acceptance plots between BNL E852 and GlueX in a sample channel:high and uniformacceptance in invariant mass and Gottfried-Jackson angles.


CLAS12 1Forward Detector:- TORUS magnet- Forward SVT tracker- HT Cherenkov Counter- Drift chamber system- LT Cherenkov Counter- Forward ToF System- Preshower calorimeter - E.M. calorimeter (EC)

Central Detector:- SOLENOID magnet - Barrel Silicon Tracker- Central Time-of-Flight

Proposed upgrades:- Micromegas (CD)- Neutron detector (CD)- RICH detector (FD)- Forward Tagger (FD)


The CLAS12 Forward Tagger 1

Forward TaggerE’ 0.5-4.5 GeVn 7-10.5 GeVq 2.5-4.5 degQ2 0.007 – 0.3 GeV2

W 3.6-4.5 GeVPhoton Flux 5 x 107 g/s @

Le=1035

Photon Polarization

10-70%

e-γ*

p

e-CLAS12Forward

Tagger

Electron detection via Calorimeter+Tracker+Veto• calorimeter to determine the

electron energy with few % accuracy → homogenous PbWO4 crystals

• tracker to determine precisely the electron scattering plane and the photon polarization → MicroMegas

• veto to distinguish photons from electrons → scintillator tiles with WLS fiber readoutPbWO4

Calorimeter424 Crystals


Experiment Layout

Moller Shield

Calorimeter

Tracker

Scintillation

Hodoscope


PWA in CLAS12

Test for 2 t bins:- line: generated

wave- |t|=0.2 GeV2

- |t|=0.5 GeV2

As a function of M3πThe CLAS12 detector system is intrinsically capable of meson spectroscopy measurements

In preparation for the experiment, PWA tools are being developed and tested on pseudo data (Monte Carlo) for different reactions as γp→nπ+π+π-

a2→ρπ D-wave a1→ρπ S-wave a1→ρπ S-wave

π2→ρπ P-wave π2→ρπ F-wave π2→f2π S-wave

π2→f2π D-wave π2→f2π P-wave 3π all wave


SummaryMeson spectroscopy is a key field for the understanding of fundamental questions in hadronic physics as what is the origin of the nucleon mass and what is the role of gluons

Jefferson Lab has launched a broad and comprehensive program to study meson spectroscopy via photoproduction:

• Study of meson spectroscopy in the light –quark sector for masses around 2 GeV• Two complementary experiments will run in Hall B and D• GlueX: bremsstrahlung tagged photon beam, hermetic detector with excellent

coverage for neutral an charged particles• CLAS12: quasi-real photon beam, high resolution and large acceptance detector

with excellent Pid capabilities• Analysis tools are being developed and usage of GPUs to reduce computing time

is being explored• A series of workshops on Partial Wave Analysis involving theorists and

experimentalists was started (INT Nov. 2009, ECT* Jan. 2011, Jlab June 2011,…)

Detector construction is in progress and first 12 GeV beam is expected in 2014


Summary


Quasi-Real PhotoproductionStudies at large W (~100 GeV) show a smooth transition between Q2=0 and Q20

Technique used in high energy experiments:

Q2 < W2

COMPASS: <1 GeV2 <Q2> ~ 10-1

GeV2

ZEUS: 10-7 – 0.02 GeV2 <Q2> ~5 10-5 GeV2

H1: <2 GeV2


Quasi-Real PhotoproductionAnalysis of existing CLAS data shows clear peaks associated to known mesons in

ep®(e’)pp0p0

ep®(e’)pp0h

a0(980)f2(1270)f0(980)

The Technique works!!


KinematicsElectron kinematics:

• E=0.5-4 GeV• q=2-5 deg.

• Q2=0.007-0.33 GeV2

• Eg=7-10.5 GeV• Photon Polarization:

10-65% (determined on an event-by-event basis)

Documents

Search for Exotics at Jefferson Lab