Measurement of the hadronic cross sections at Novosibirsk

Abstract

Abstract. Two experiments, CMD-3 and SND, collected over 100 pb −1 in total taking data in 2011-2013 at the VEPP-2000 electron-positron collider. The data sample covers the whole available c.m. energy range from 0.32 GeV to 2.0 GeV. An upgrade of VEPP-2000 started in the second half of 2013, aimed at the luminosity increase up to 10 times at 2.0 GeV. Data taking is expected to resumed in 2015. The current results of analysis of the collected data for various modes of e + e − → hadrons are discussed. VEPP-2000The electron-positron collider VEPP-2000[1, 2] was commissioned at Budker Institute of Nuclear Physics (Novosibirsk, Russia) in 2010. The machine covers the c.m. energy range from √ s = 0.32 GeV to 2.0 GeV and employs the novel technique of round beams to reach a design luminosity up to 10 32 cm −2 s −1 at 2 GeV.Two detectors, CMD-3[3] and SND [4], are installed in the interaction regions of VEPP-2000.CMD-3 (Cryogenic Magnetic Detector) is a generalpurpose detector (Fig. 1). The cylindrical drift chamber with hexagonal cells is surrounded by the Z-chamber, a MWPC with a dual anode and cathode readout, used for precise determination of the fiducial volume for charged particles. The barrel electromagnetic calorimeter, placed outside the superconducting solenoid (0.13X 0 , 13 kGs), is composed of two systems: the Liquid Xenon calorimeter (about 5.4X 0 ), surrounded by the CsI crystal calorimeter (about 8.1X 0 ). The LXe calorimeter has 7 layers and utilizes dual readout: the anode signals are used for a measurement of the total energy deposition, while signals from the cathode strips provide information about a shower proa e-mail: I.B.Logashenko@inp.nsk.su file and are used for a measurement of photon coordinates with high precision (about 1-2 mm). The endcap BGO crystal calorimeter (about 13.4X 0 ) operates in the main magnetic field. The time-of-flight system, designed to identify slow moving particles, is placed between the two layers of the barrel calorimeter. The detector is surrounded by the muon range system. The SND (Fig.2) is a general-purpose nonmagnetic detector. It consists of the 9-layer drift chamber, the aerogel Cherenkov counters, the three-layer spherical electromagnetic calorimeter with 1680 NaI(Tl) crystals and the muon system.The physics program of experiments includes the high precision measurement of cross sections of various modes of e + e − → hadrons, studies of known and searches for new vector mesons, measurement of nn and pp production cross sections near threshold and searches for exotic hadrons.Measurement of the total cross section of e + e − → hadrons in the VEPP-2000 energy range is very interesting in relation with the problem of anomalous magnetic moment of muon, a μ .The most recent measurement of a μ done at BNL with 0.54 ppm precision [5] gave a very interesting outcome:

Measurement of the e+e− → π+π− process cross section with the SND detector at the VEPP-2000 collider in the energy region 0.525 < $$ \sqrt{s} $$ < 0.883 GeV

“…These measurements are largely motivated by the need for high-precision calculation of the hadronic contribution to the anomalous magnetic moment of the muon (g − 2)/2 [4]. In particular, the e + e − → π + π − cross section in the energy region below 1 GeV gives the dominant contribution to this value and should be measured with accuracy better than 1% [5].…”

Separation ofe+e−→e+e−ande+e−→ π+π−events using SND detector calorimeter

“…These measurements are largely motivated by the need of high-precision calculation of the hadronic contribution to the anomalous magnetic moment of the muon (g − 2)/2 [3]. In particular, the e + e − → π + π − cross section at the energy region below 1 GeV gives the dominant contribution to this value and should be measured with accuracy higher than 1% [4].…”