Study of the processes π+πβ β π²π² (π)π with
the CMD-3 detector at VEPP-2000 collider
Vyacheslav Ivanov
phd student, CMD-3 collaboration
Novosibirsk, Russia
Pion-Kaon Interactions Workshop
JLab, 14.02.2018 1
Outline
1. VEPP-2000 collider and CMD-3 detector
2. π+πβ β πΎπΎ (π)π processes: charged kaons/pions identification
3. π+πβ β πΎπΎ (π)π processes
3.1 πΎ+πΎβπ+πβ final state
3.2 π+πβ β πΎ+πΎβπ and πΎ+πΎβπ(782) processes
3.3 πΎ+πΎβπ0 final state
4. Plans & Conclusion
2
3
β’ π+πβ symmetric beams machine for the energy scan
in range π β (2ππ; 2.005 GeV)
β’ Round beams technology used
β’ The maximum luminosity is 1032 cmβ2sβ1 at
2.0 GeV
β’ Compton backscattering beam energy measurement
(Β±60 keV precision)
β’ ~ 120 pb-1 is collected by each detector, the goal is to
collect ~1 fb-1 in ~5 years
1. VEPP-2000 collider & CMD-3 detector
SND
CMD-3
4
CMD-3 detector
Drift chamber
Z chamber
LXe calorimeter CsI calorimeter
BGO calorimeter
TOF
π veto system
4
1.3 T solenoid
Physics program of CMD-3
β’ (ππβ2) puzzle: precise measurement of π =π(π+πβββππππππ )
π(π+πββπ+πβ) - the goal is to achieve <1%
systematic for major channels
β’ Study of exclusive hadronic channels of π+πβ annihilation, test of isotopic relations
β’ Study of the βexcitedβ vector mesons: πβ², πβ²β², πβ², πβ²β¦
β’ CVC tests: comparison of isovector part of π(π+πβ β βππππππ ) with π βdecay spectra
β’ Study of GE/GM for nucleons near threshold
β’ Diphoton physics (e.g. πβ² production)
5
CMD-3 data analysis status
Conversion decays
Dark photon search
- Preliminary results reported at conferences
- Final results are published
- Is being studied, not reported
+ also π²+π²βππ π,
π²πΊπ²Β±π βπ π etc.
2. π+πβ β π²π² (π)π processes: kaon/pion separation β’ πΎ/π separation is the starting point and key issue for such processes
π²+π²βπ +π β events (MC)
β’ We use the ππΈ/ππ₯ in drift chamber
(DC), but for single kaons and pions it
works reliably only up to π <450 MeV/c
β’ For the final states πΎ+πΎβ, πΎ+πΎβπ0, πΎ+πΎβ2π0 the use of ππΈ/ππ₯π·πΆ is
insufficient
β’ We are developing the PID procedure
with the use of ππΈ/ππ₯πΏππ in 14 layer
LXe-calorimeter
MC MC
LXe-calorimeter of CMD-3
πππ
πππ
DC
CsI
BGO
CsI
BGO
BGO
BGO
The PID with LXe β’ For each DC-track we calculate the 10 values of the responses of the boosted decision trees (BDT)
classifiers, trained for the separation of particular pair of charged particles in particular ranges of
momenta and path length in LXe-layer (2200 classifiers in total):
πΒ± πΒ± πΒ± πΎΒ±
πΒ± π ππ ππππ πππ(πΒ±/πΒ±) - - -
πΒ± π ππ ππππ πππ(πΒ±/πΒ±) π ππ ππππ πππ(π
Β±/πΒ±) - -
πΎΒ± π ππ ππππ πππ(πΎΒ±/πΒ±) π ππ ππππ πππ(πΎ
Β±/πΒ±) π ππ ππππ πππ(πΎΒ±/πΒ±) -
πΒ± π ππ ππππ πππ(πΒ±/πΒ±) π ππ ππππ πππ(π
Β±/πΒ±) π ππ ππππ πππ(πΒ±/πΒ±) π ππ ππππ πππ(π
Β±/πΎΒ±)
β’ The training samples: MC for πΎΒ±, MC &
experimental events for others
β’ Input variables: ππΈ/ππ₯π·πΆ , 14 values of
ππΈ/ππ₯πΏππ
β’ Sophisticated detector response simulation is
required: determination of the layers
transparency coefficients, correlative and
anticorrelative redistribution of induced
charge between upper and lower strips etc.,
βgeometric effectβ, recombination etc.
rotation in magnetic field
β’ Main problem: bad simulation of πβ, πΎβ interactions with nuclei (the PID only for π+, πΎ+
will be used at first)
π π¬/π ππ³πΏπ in 14 layers: MC/exp comparison for π +
β’ π+ sample is selected from 2π+2πβ events, π β (1.6 GeV; 2 GeV)
EXP MC
π π¬/π ππ³πΏπ in 14 layers: MC/exp comparison for π β
β’ πβ sample is selected from 2π+2πβ events, π β (1.6 GeV; 2 GeV)
EXP MC
MC/exp comparison for π β & π +
π β π +
π β π +
LXe cluster energy / path, MeV/c LXe cluster energy / path, MeV/c
total cluster energy / path, MeV/c total cluster energy / path, MeV/c
π π¬/π ππ³πΏπ in 14 layers: MC/exp comparison for π²+
β’ πΎ+ sample is selected from πΎ+πΎβπ+πβ events, π β (1.6 GeV; 2 GeV)
EXP MC
π π¬/π ππ³πΏπ in 14 layers: MC/exp comparison for π²β
β’ πΎβ sample is selected from πΎ+πΎβπ+πβ events, π β (1.6 GeV; 2 GeV)
EXP MC
β’ Background suppression via cuts on BDT responses:
electrons muons pions
Example: selection of π+πβ β π²+π²β(πΈ) at π > π. π πππ
Example: selection of π+πβ β π²+π²β(πΈ) at π > π. π πππ
πΒ± suppression πΒ± & πΒ± suppression
πΒ± & πΒ± & πΒ± suppression
3.1 Study of π+πβ β π²+π²βπ +π β process
β’ The major intermediate states were found to be
β’ π0(500)π & π0(980)π β’ π 770 πΎπΎ πβπ€ππ£π β’ (πΎ1 1270 πΎ)πβπ€ππ£πβ (πΎβ(892)π)πβπ€ππ£ππΎ β’ (πΎ1 1400 πΎ)πβπ€ππ£πβ (πΎβ(892)π)πβπ€ππ£ππΎ β’ (πΎ1 1400 πΎ)πβπ€ππ£πβ (π 770 πΎ)πβπ€ππ£ππΎ
β’ Their relative amplitudes were found from the unbinned fit
of the data (relative phases were fixed at 0)
β’ 2011-2012 energy scan, 23 pbβ1
β’ Event selection: 4 or 3 tracks
β’ Kaon/pion separation using log-likelihood function
based on ππΈ/ππ₯π·πΆ:
β’ 4-tracks events: signal events selection using energy-
momentum conservation
β’ 3-tracks events: signal/background separation by fitting
energy disbalance distribution
β’ => ~24k of signal events selected
MC for signal
data
Study of the dynamics of π²+π²βπ +π β production
Ec.m. = 1.95 GeV
πΎβ(892)
π 770 π(1020)
Ec.m. = 1.95 GeV Ec.m. = 1.95 GeV
Cross section of the π+πβ β π²+π²βπ +π β process
CMD-3
CMD-3 β’ The results for 2011-2012 scan were
published, but new 2017 scan with ~60
ππβ1 of integrated luminosity provided
~3 times more signal events
β’ First look: we see a systematic drop near
ππ threshold
β’ But the result is too preliminary to make
any statement
ππ π¦π π‘ = 6 Γ· 12%
3.2 Study of π+πβ β π²+π²βπΌ & π²+π²βπ(πππ) processes
β’ π and π(782) are treated as the recoil particles
β’ Only ππ intermediate state is seen in πΎ+πΎβπ
β’ Event classes with 2, 3 and 4 tracks are considered. πΎ/π separation is performed using log-
likelihood function. For instance, in 2-track class the distribution is:
Study of π+πβ β π²+π²βπΌ & π²+π²βπ(πππ) processes
β’ πΎ+πΎβπ+πβ is the major background in 3- and 4-track classes:
β’ But it can be suppressed by the cuts on 2πΎπ and 2πΎ2π missing masses:
Study of π+πβ β π²+π²βπΌ & π²+π²βπ(πππ) processes β’ ππ selection:
β’ Signal/background separation for both processes is performed by fitting of ππππ π ,2πΎ distribution
π
π(782)
π+πβ β ππΌ cross section fitting β’ Fitting of the π+πβ β ππ cross section is the one of the best way for π(1680) parameters extraction
β’ Cross section of π+πβ β ππ is parametrized by quasi-two body formula
or, more precisely, as three-body expression
with
β’ New data of 2017 scan is being
analyzed and new π(1680) -
dedicated scans are planned for
near future => we are at good
disposition to perform the most
precise study of πβ²
π+πβ β π²+π²βπ(πππ) cross section fitting
where
and the components of hadronic current π½π₯,π¦ are
3.3 π²+π²βπ π final state
β’ Events selection: 2 tracks, >= 2 photons
β’ 4C kinematic fit with all the photon pairs (energy-momentum conservation required), π2 < 35
β’ The main backgrounds: πΎ+πΎβπΎ , πΎ+πΎβ2π0 ,
πΎΒ±πΎπ,πΏπβ, π+πβπ0, π+πβ2π0
β’ The background suppression is performed by the
training of BDT classifier. The input variables are:
1) ππΈ/ππ₯π·πΆ, 2) momenta and angles of charged
particles & photons 3) π2πππ π ,2πΎ
Conclusion & Plans
Thank you for attention!
β’ CMD-3 collaboration is progressing in datataking and data analysis for the π+πβ β πΎπΎ (π)π
processes
β’ πΎ/π separation is the starting point and key issue for such processes => the charged PID
procedure using ππΈ/ππ₯πΏππ is being developed
β’ π+πβ β πΎπΎ (π)π processes has reach dynamics, provide a case for isotopic relations test and,
especially, for π 1680 parameters measurement