The Pentaquark Q+ Search At Hermes Weilin Yu (Peking University) for Hermes Collaboration 2019/2/24 Weilin Yu QCD和轻强子暑期班 Weilin Yu QCD和轻强子暑期班

Outline Particle Identification Search For pKS Resonance Systematic Studies Summary and Conclusions 2019/2/24 Weilin Yu QCD和轻强子暑期班

The HERMES Experiment 2019/2/24 Weilin Yu QCD和轻强子暑期班

The HERMES Spectrometer Beam: 27.6 GeV e+/e- from HERA accelerator Track reconstruction: Dp/p < 2%, Dq < 0.6 mrad Particle ID: TRD, Preshower, Calorimeter (hadron/lepton sep.) dual radiator RICH (p, K, p separation) 2019/2/24 Weilin Yu QCD和轻强子暑期班

Particle Identification hadron/positron separation combining signals from: TRD, calorimeter, preshower, RICH hadron separation Dual radiator RICH for p, K, p 2019/2/24 Weilin Yu QCD和轻强子暑期班

What is a Pentaquark Q+ Composed of 4 quarks and one antiquarks . Has strangeness S=+1 . Allowed by the Standard Quark Model. Decay Mode: Q+  Ks p or K+n 2019/2/24 Weilin Yu QCD和轻强子暑期班

Event Reconstruction Define appropriate event topology Hadron identification: RICH p: 1 - 15 GeV p: 4 – 9 GeV Lambda is well identified  protons are cleanly identified Reject events from L(1116)  pp- within ±2s of ML invariant mass calibration ±1.9MeV direct reconstruction: detection of each decay particle, invariant mass reconstruction 2019/2/24 Weilin Yu QCD和轻强子暑期班

Identification after all cuts 2019/2/24 Weilin Yu QCD和轻强子暑期班

Results Peak at: M= 1528 ± 2.6 MeV = 8 ± 2 MeV Significance: 3.7 Unbinned fit is used: result independent of bin size and starting point 2019/2/24 Weilin Yu QCD和轻强子暑期班

The Signal and its Background Peak at: 1527 ± 2.3 MeV = 9.2 ± 2 MeV Significance: 4.3 Known S*s (excluded in Pythia6) mixed event background PYTHIA6 2019/2/24 Weilin Yu QCD和轻强子暑期班

A Non-Zero Width for Q+? HERMES intrinsic width: Γ = 17 ± 9 ± 3 MeV Observed width FWHM: 19 – 24 MeV Detector resolution (from MC) FWHM: 10 – 14.6 MeV re-fit spectra with Breit-Wigner convoluted with a Gaussian (fixed by MC) HERMES intrinsic width: Γ = 17 ± 9 ± 3 MeV 2019/2/24 Weilin Yu QCD和轻强子暑期班

Q+ Isospin Well established L(1520)  pK- with acceptance:1.5% No peak structure for Q++  pK+ zero counts at 91% CL Q+ not isotensor probably isoscalar 2019/2/24 Weilin Yu QCD和轻强子暑期班

Summary of Positive Results hep-ex/0301020, PRL hep-ex/0304040, ? http://na49info.cern.ch hep-ex/0404003 hep-ex/0403051 hep-ex/0403044 hep-ex/0401024 hep-ex/0403011 hep-ex/0312044, PLB Proc. Mong. Acad Sci. hep-ex/0307018, PRL hep-ex/0311046, PRL hep-ex/0307083, PLB hep-ex/0309042, ? 2019/2/24 Weilin Yu QCD和轻强子暑期班

Summary of Null Results 0 null results published, only 3 on arXiv so far (7-18-04) ⇒ need null results to be published 2019/2/24 Weilin Yu QCD和轻强子暑期班

How real is the Peak? check for kinematical reflections detector acceptance and cuts (PYTHIA6 MC / Toy MC) Q+ vs S*+ is Q+ a pentaquark or a previously unobserved S*+ add a forth hadron is the peak still there can we guess the production process for the Q+ can we supress background 2019/2/24 Weilin Yu QCD和轻强子暑期班

Kinematical Reflection particle miss-identification ghost tracks PID “leaks” expect peak in M(p-p) if p+ is p and KS is L(1116) Spectrum of events associated with L(1116) remove L(1116) contribution 2019/2/24 Weilin Yu QCD和轻强子暑期班

The Mass Spectrum relatively large BG, although good PID for proton and KS what if we require one additional hadron? could additional hadron help remove KS from other process? 2019/2/24 Weilin Yu QCD和轻强子暑期班

Mass Spectrum after requiring 4th hadron as p Black requires 4th hadron (all species) only p Red specifies 4th hadron as p K & p Green specifies 4th hadron as not p 2019/2/24 Weilin Yu QCD和轻强子暑期班

Why does additional p help? remove remove add new cut: |M(KSp)-892|<75 MeV high statistics K* 2019/2/24 Weilin Yu QCD和轻强子暑期班

Mass Spectrum After Removing K* standard cuts applied + K* veto + L veto  background is greatly reduced signal/noise: from 1:3 to 2:1 2019/2/24 Weilin Yu QCD和轻强子暑期班

Conclusions and Outlook Direct reconstruction of Q+ invariant mass Mass: Intrinsic Width: Significance: ~ 4 s Q+ is probably an isosinglet Requiring additional p improves signal/background, it eliminates KS contamination from various processes Anticipate doubling statistics this year Will soon report on X-- search and Q+ from TOF (low p) 2019/2/24 Weilin Yu QCD和轻强子暑期班

Detector Mass Calibration HERMES Mass[MeV] 496.8±0.2 1115.70±0.01 1522.7±1.9 1321.5 ±0.3 PDG Mass[MeV] 497.67 1115.68 1519.5±1.0 1321.31±0.13 s width (data)[MeV] 6.2±0.2 2.6±0.1 4.4±3.7 3.1 ±0.3 s width (MC)[MeV] 5.4 2.1 3.5 2.5 Decay Pcm[MeV/c] 206 101 244 139(Lp-) HERMES can precisely measure the above known particles The MC well reproduce the data The systematic error estimated is about 1.9MeV 2019/2/24 Weilin Yu QCD和轻强子暑期班