W,Z,H + jets at LHCb
Marı́a Vieites Dı́az, Xabier Cid Vidal
Universidade de Santiago de Compostela
Spanish LHC network meeting,
Madrid, May 9th , 2016
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
Introduction
CMS
LHCb
Complementary
⌘ coverage!
LHCb takes less data than ATLAS/CMS
but in return gets:
Precise integrated luminosity computation
Stable trigger conditions
Excellent vertexing, particle ID,
momentum resolution...
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
2
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
Jets at LHCb
How?
! Particle Flow approach, with neutral recovery
! Reconstructed using anti-kT (R = 0.5)
! Calibration in data, using Z ! µµ + jets
! Efficiency above 90% for jets with pT
above 20 GeV/c
! Reconstructed both o✏ine and online.
JHEP01 (2014) 033
Opens many new topics for LHCb!
QCD: constraints in proton PDFs and probe hard QCD in a
unique environment
Direct searches of detached particles decaying to jets
Higgs physics: bb and cc final states
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
3
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
b and c jets tagging
! Requires jet with a secondary vertex (SV) close enough to be
reconstructed
! SV properties (displacement, kinematics, multiplicity,...) and jet
properties combined in two BDTs.
! BDT(bc|udsg ) trained to separate (b, c) jets from light-parton jets
! BDT(b|c) trained to discriminate b jets from c jets
! "b ⇠ 65%, "c ⇠ 25%, light jet mistag probability ⇠ 0.3%.
JINST 10 (2015) P06013
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
4
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
Final states studied in Santiago: lepton+2b/c jets
Aims:
Proof of concept! Can we measure
W + bb, W + cc and tt
production cross sections?
(H! bb(cc) main backgrounds)
Analysed sample: 2 fb
1
at
p
s = 8 TeV
Events with high pT isolated lepton (electron or muon) and two
heavy flavour tagged jets
For the jets: pT > 12.5 GeV/c and 2.2< ⌘ <4.2
For the lepton: pT > 20 GeV/c and 2.< ⌘ <4.5
4 data categories: `flavour ⇥ `charge
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
5
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
Fit strategy
Simultaneous 4D fit to four data categories (µ+ , µ , e+ , e )
! Fit variables:
MVA (uGB, JINST 10 (2015) T03002) to separate W+bb from tt: Use
topology and kinematic variables and sub-combination masses
BDT(b|c) for both jets
Di-jet mass
Phys. Lett. B767 (2017) 110
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
6
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
lepton+2b/c jets results
Cross sections and theoretical predictions in LHCb fiducial region
NLO theory prediction: MCFM with PDF set CT10 interleaved with
Pythia8
MCFM:
Phys. Rev. D62 (2000) 114012
M. Vieites Dı́az
(USC)
CT10:
Spanish LHC network meeting
Phys. Rev. D82 (2010) 074024
7
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
Towards H! bb and H! cc
Higgs searched for in association to W or Z
Same selection as before, except for tighter
pT cut on the jets (20 GeV/c)
Background yields fixed to theory predictions
Limit extracted using the CLs method
First exploratory study on H! cc!
Use BDT(b|c) to separate from H! bb.
LHCb-CONF-2016-006
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
8
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
Conclusions
LHCb has proven to be able to cope with jets and perform
competitive measurements in the forward region.
b vs. c tagging available: BDT(b|c), allows for measurements such as
W + cc production cross section.
We expect several improvements for Run II and beyond:
Will take ⇠5 fb
1
in Run II (3 fb
1
in Run I)
Increased cross sections and better acceptance for physics with jets
(higher boost)
Many new ideas for an improved BDT against tt background
After Run II, much better tagging (upgraded vertex detector, only
5mm from the beams)
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
9
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
Backup
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
10
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
The LHCb detector
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
11
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
Instrumented regions of the LHC main detectors
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
12
Introduction
lepton+2b/c jets
Higgs searches
Conclusions
uniform Gradient Boost
uGB: JINST 10 (2015) T03002
Allows to build a discriminator variable uncorrelated with a given spectator
variable (di-jet mass in the analysis in Phys. Lett. B767 (2017) 110 ).
M. Vieites Dı́az
(USC)
Spanish LHC network meeting
13