Transverse momentum and pseudorapidity distributions with minimum bias events in CMS at the LHC

Transcription

1 Transverse momentum and pseudorapidity distributions with minimum bias events in CMS at the LHC Christof Roland/ MIT For the CMS Collaboration Rencontres de Moriond QCD Session 14 th March, 2010 Moriond 2010 QCD Session 1

2 Introduction The majority of the pp collisions are soft no hard parton scattering Modeling soft hadron production is done phenomenologically hadronization, fragmentation, Various event classes: Elastic single-diffractive (SD), double-diffractive (DD) non-diffractive: ND dn/dη and dn/dp T distributions of primary charged hadrons are measured, per Non-Single Diffractive (NSD) event 900GeV and 2.36 TeV the first physics results at the highest-ever collision energy Important for high-luminosity LHC runs with event pileup Also important as a reference for heavy ion collisions Moriond 2010 QCD Session 2

3 The CMS Detector EM and HAD calorimeters Muon detectors magnet yoke Hadronic Forward Calorimeter aka HF inner tracker solenoid BSC HF Pixel detector: 53.3cm long, 3 layers with radii: 4.4, 7.3, 10.2 cm Moriond 2010 QCD Session 3

4 Start of the LHC: First Collisions CMS Experiment at the LHC, CERN Date Recorded: :21 CET Run/Event: / Candidate Collision Event Moriond 2010 QCD Session 4

5 Trigger and event selection Data taking: 12 and 14 Dec. ( 2 2 hours) 10 Hz collision rate, i.e. no pileup Trigger: One hit in the Beam Scintillator Counters (BSC) in COINCIDENCE with beam bunches crossing: beam pickups (BPTX) Event selection: One hit of >3 GeV total energy on BOTH sides in the Forward Calorimeter (HF) One charged particle of p T > 0.2GeV in η < 2.5 Beam Halo rejection (BSC time coincidence) Beam background rejection High multiplicity events rejected by clustervertex compatibility Valid collision vertex -2.5 < η < < η < -3 3 < η < 5 NSD η < 2.5 Generated particles after event selection η < 2.5 Efficiencies: NSD: ~86 % SD: ~19 % DD: ~34 % Moriond 2010 QCD Session 5

6 Detector performance The CMS silicon pixel and strip tracker detectors were used Pixels: three 53.3 cm long layers with radii 4.4, 7.3, 10.2 cm >97% of all channels operational, hit efficiency optimized For details of the tracker performance see talk of D. Giordano The energy loss in the tracker layers well described by MC The vertex position distributions are clean Gaussians, with no tails Moriond 2010 QCD Session 6

7 Three analysis methods Cluster Counting Clusters per layer in η <2 3 measurements of dn/dη (p T > 30 MeV/c) Immune to mis-alignment Background rejection by cluster shape cut Sensitive to noise Tracklet Counting 2 of 3 pixel layers in η <2 3 measurements of dn/dη (p T > 75 MeV/c) Background rejection by 2 layer coincidence Sensitive to mis-alignment Tracking Method Full tracks (pixel and strips) in η <2.4 dn/dη and dn/dp T Sensitive to mis-alignment Over 50% Efficient for p T > 0.1, 0.2, 0.3 GeV/c for π, K, p All: Corrected for efficiency, weak decays, secondaries and extrapolated to p T =0 Moriond 2010 QCD Session

8 Results: p T -spectrum Fit with the Tsallis-function: Limits: exponential at low p T power-law at high p T Only a little change in the parameters as a function of η Differential yield of charged hadrons in the range η <2.4 The η bins are shifted by four units vertically. Moriond 2010 QCD Session 8

9 Results: p T -distribution The transverse-momentum distribution of charged hadrons was measured up to 4 GeV/c. Well described by the Tsallis-function With increasing energy, the pt-spectrum gets harder (as expected) Measured yield of charged hadrons for η < 2.4, fit with the Tsallis function. Moriond 2010 QCD Session 9

10 Results: dn/dη dn ch /dη distributions obtained from the three methods at 0.9 TeV and 2.36 TeV. The error bars represent systematic uncertainties excluding those common to all the methods. dn ch /dη distributions averaged over the cluster counting, tracklet and global track methods and symmetrized in η. The shaded band represents systematic uncertainties. The error bars on the UA5 and ALICE data points are statistical only. Moriond 2010 QCD Session 10

11 <p T > energy dependence Collision energy dependence of average transverse momentum. Moriond 2010 QCD Session 11

12 <p T > energy dependence S.M. Troshin, N.E. Tyurin Collision energy dependence of average transverse momentum. S.M. Troshin, N.E. Tyurin: arxiv: Suggest this dependence is a result of collective rotation of the transient state. Moriond 2010 QCD Session March

13 dn/dη energy dependence pp data Charged particle pseudorapidity density as a function of collision energy. Moriond 2010 QCD Session 13

14 dn/dη energy dependence pp data Charged particle pseudorapidity density as a function of collision energy. => Steeper rise than Pythia, Phojet Moriond 2010 QCD Session 14

15 dn/dη energy dependence pp data Levin et al. arxiv: Priv. Comm. Describe data quite well Moriond 2010 QCD Session 15

16 dn/dη energy dependence pp data? Levin et al. arxiv: Priv. Comm. Describe data quite well => Give prediction for 7TeV! Moriond 2010 QCD Session 16

17 dn/dη energy dependence pp data pp compared to AA Charged particle pseudorapidity density as a function of collision energy. N. Armesto et al.phys.rev.lett.94:022002,2005 Prediction of charged particle multiplicity evolution with collision energy in HI and pp based on Geometrical Scaling and Q 2 Sat Moriond 2010 QCD Session 17

18 dn/dη energy dependence pp data pp compared to AA?? Charged particle pseudorapidity density as a function of collision energy. N. Armesto et al.phys.rev.lett.94:022002,2005 Prediction of charged particle multiplicity evolution with collision energy in HI and pp based on Geometrical Scaling and Q 2 Sat Moriond 2010 QCD Session 18

19 Conclusions We have completed the first analysis of collision data recorded by CMS published by JHEP The first published data at 2.36 TeV The results follow the trend indicated by earlier experiments The energy-dependence of the multiplicity density is steeper than predicted by the PYTHIA and PHOJET model tunes used The paper demonstrates the readiness of the CMS detector in the LHC startup in 2009 Excellent detector performance is shown and high quality data is taken This is the start of the long and exciting physics program of CMS at the LHC! Moriond 2010 QCD Session 19

20 Outlook All major CMS detector components are ready for data Tracker very good understanding of the detector Photons/Electrons strategies to commission electrons and photons with the first data are ready Jets Jets and MET are performing well in CMS on first data Muons The Muon System has been commissioned with Cosmic Rays data CMS is ready to explore the high energy domain with the incoming data at 7 TeV Moriond 2010 QCD Session 20

21 Backup Slides Moriond 2010 QCD Session 21

22 Statistics Moriond 2010 QCD Session 22

23 Efficiency, Event Fractions Moriond 2010 QCD Session 23

24 Errors Moriond 2010 QCD Session 24