1. Measurement of J/y, Y and b-hadron production in proton-proton collisions at √s = 7 TeV
Roberto Covarelli (CERN)
on behalf of the CMS collaboration
Heavy Quarks & Leptons 2010, Frascati, 11/10/2010
Frascati, 11/10/2010
Roberto Covarelli

2. Physics motivations (I)
Prompt production:
Including feed-down from (ccJ, y’ → J/y and cbJ → Y)
Several theoretical mechanisms contributing:
Color Singlet (new developments in recent years)
Color Octet
No model predicting successfully both cross-section and polarization at TeVatron
Y(1S)
prompt
J/y
J.P. Lansberg arXiv: [hep-ph]
Frascati, 11/10/2010
Roberto Covarelli

3. Physics motivations (II)
bb cross-section:
NLO contributions important at LHC
At TeVatron good theory-data agreement:
FONLL approach
Improved b- fragmen tation function (LEP Z0 → bb data)
as2
as3
M . Cacciari et al.,
JHEP 0404, 068 (2004)
as3
Frascati, 11/10/2010
Roberto Covarelli

4. The QQ cross-section = integrated luminosity
measures bb cross-section times BR(B → J/y) in bins of J/y pT-y
= integrated luminosity
A = average signal acceptance/efficiency in the bin *
Acceptance: from MC simulation
Efficiency: determined with a data-driven method from J/y events
= pT bin size *
* = function of pT, y
Frascati, 11/10/2010
Roberto Covarelli

5. The CMS detector CALORIMETERS SOLENOID 3.8 T B-field TRACKER MUON
ECAL
Scintillating PbWO4
CALORIMETERS
Crystals
SOLENOID
3.8 T B-field
HCAL
Plastic scintillator/
brass
sandwich
TRACKER
MUON
ENDCAPS
MUON BARREL
Silicon Strips
Pixels
Drift Tubes
(DT)
Resistive Plate
Chambers (RPC)
Cathode Strip Chambers (CSC)
Resistive Plate Chambers (RPC)
Frascati, 11/10/2010
Roberto Covarelli

6. Muon reconstruction in CMS
HCAL
Large rapidity coverage:
|η| < 2.4
Excellent muon momentum resolution:
matching between m-chambers and in the silicon tracker (only using the latter for momentum determination at low pT)
strong solenoidal magnetic field
MUON SYSTEM
(3.8 T)
Because of the increasing material thickness traversed and the different lever arm, the resolution changes with pseudo-rapidity
CALORIMETERS
PIXELS +
TRACKER
pT resolution
in silicon
tracker (muons)
Frascati, 11/10/2010
Roberto Covarelli
CMS Phys. TDR, 2006 6 6

7. Muon triggers in CMS L1: hardware muon system and calorimeters only
2 trigger levels
Trigger requirements changing with increasing luminosity:
Single muons:
pT > 3 GeV threshold at the startup
Gradually increasing (pT > 7 GeV at L ~ 1031 Hz cm-2)
Double muons:
L1 requirements only at the startup, no pT threshold
At L ~ 1031 Hz cm-2 ad-hoc strategies adopted for quarkonia (combination of L1 and HLT muons, or HLT muon and track in specific invariant mass regions… etc.)
L1: hardware
muon system and
calorimeters only
HLT: software
matching of different
subdetectors.
Fast local tracker
reconstruction for muons
allowing us to go down to 0 quarkonium pT in the forward region
Frascati, 11/10/2010
Roberto Covarelli

8. The di-muon mass spectrum
Total integrated
luminosity
all opposite-sign
Used for the analyses
shown here:
updates in progress!
The CMS collab., PAS BPH
The CMS collab., PAS BPH
Frascati, 11/10/2010
Roberto Covarelli

9. Event selection and yields
Selections based on:
Muons in acceptance
Track quality (nhits, nhits in pixels, c2, |dxy|, |dz|)
Muon quality (global fit c2, track-muon segment angular matching)
Di-muon vertex probability
|y| < 1.4
1.4 < |y| < 2.4
~ J/y (100 nb-1)
~ 950 Y’s (280 nb-1)
|y| < 2.4
Frascati, 11/10/2010
Roberto Covarelli

10. Acceptance Acceptance is determined from MC:
Strongly dependent on polarization assumptions for the prompt component (polarization not well known)
Agreement to give result in 5 scenarios:
Isotropic
Extreme values of lq (= ±1) in the helicity frame (along the QQ momentum)
Extreme values of lq (= ±1) in the Collins-Soper frame (along the collision axis)
Main systematic uncertainties coming from:
pT smearing and calibration
uncertainty on final state radiation spectrum
J/y acceptance
in isotropic scenario
Frascati, 11/10/2010
Roberto Covarelli

11. Efficiency Muon efficiency from data: the “tag-and-probe” method:
Require one well-identified muon in the event (“tag”)
single muon trigger efficiency |h| < 1.2
Another candidate muon, with looser criteria L, is paired to it (“probe”)
Compare resonance yields for all tag-probe pairs and for pairs where the probes pass a given selection S  determine e (S | L)
Corrections for binning and not exact efficiency factorization (“muon correlations”) taken from MC
Main systematic uncertainties from:
Limited statistics
Correlations
Frascati, 11/10/2010
Roberto Covarelli

12. J/y b-fraction
Measurement of prompt/non-prompt component with a 2D fit to mass and “pseudo”-proper decay length lJ/y = Lxy ∙ mJ/y / pT:
Decay length parameterization:
For prompt events, d-function
For non-prompt events, MC templates
For background events a generic superposition of different contributions (symmetric + asymmetric with effective lifetimes)
all convoluted with 3-Gaussian resolution
u = pT/|pT|
x = SVxy - PVxy
Main systematic uncertainties:
Pixel misalignment
Resolution model
Primary vertex estimation
Frascati, 11/10/2010
Roberto Covarelli

13. J/y cross-sections Prompt (isotropic scenario) Non-prompt
Frascati, 11/10/2010
Roberto Covarelli

14. Y cross-sections
Integrated and pT differential cross-section for Y(1S):
Due to the limited yield of Y(2S) and Y(3S), only total cross-section ratio to Y(1S) is measured:
Y(1S) cross-section
Frascati, 11/10/2010
Roberto Covarelli

15. Conclusions
First measurements of prompt J/y, b-hadron and Y(nS) production cross sections have been presented, using nb-1 of data collected by CMS in the first months of LHC operation:
Relying on excellent muon reconstruction/triggering performances of CMS down to low pT
Muon efficiencies effectively determined from data, with typical uncertainties of 5-10%
Prompt/non-prompt J/y separation achieved using vertex information
Measurements already dominated by uncertainties on luminosity/polarization
Prompt J/y, total and differential in pT and |y|
Non-prompt J/y, total and differential in pT and |y|
Y(1S), total and differential in pT
[Y(2S) + Y(3S)]/Y(1S) ratio, total
Frascati, 11/10/2010
Roberto Covarelli