Transcript LHCb status and plans Roger Forty (CERN) on behalf of the LHCb Collaboration 1.

LHCb status and plans
Roger Forty (CERN)
on behalf of the LHCb Collaboration
1. LHCb status
2. Physics highlights
3. Plans
Physics at the LHC, Vancouver, 4–9 June 2012
1. LHCb status
• LHCb is the dedicated flavour physics experiment at the LHC
• ATLAS & CMS search for the direct production of new states
LHCb is designed to see their indirect effect on charm and beauty decays
via virtual production in loop diagrams:
e+e- (4S)  B0B0
• Such an indirect approach can be very powerful:
e.g. B0–B0 mixing discovered at ARGUS (1987)
→ top quark unexpectedly heavy: m(t) > 50 GeV/c2
• Key topics for LHCb include: to check whether CP violation is due to a
single phase in the quark mixing (CKM) matrix, as in the Standard Model
Study rare decays: FCNC decays (e.g. Bs → m+m-) are strongly suppressed
in SM, may be enhanced by Supersymmetry, or other new physics
Roger Forty
LHCb status and plans
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Forward spectrometer
• Forward-peaked B production → LHCb is a forward spectrometer
(operating in collider mode)
10 – 300 mrad
p
p
[PYTHIA]
• bb cross-section = 284 ± 53 mb at the LHC (s = 7 TeV)
→ ~ 100,000 bb pairs produced/second (104  B factories)
Charm production factor ~20 higher! [CONF-2010-013]
Roger Forty
LHCb status and plans
[PLB 694 209]
See talk of
Marco Adinolfi
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Advantages
• Large boost: B decay lengths ~ O (1 cm)
• Complementary coverage for other physics
– Electroweak, QCD, exotics, …
Roger Forty
LHCb status and plans
W± charge
L * asymmetry vs h
b
Bc+
ATLAS/CMS
LHCb
William Barter & Marianna Fontana
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[CONF-2011-039]
[arXiv:1205.3452]
[arXiv:1204.0079]
• All b-hadron species are produced at the
high energy of the LHC
– New states discovered
e.g. Lb*(5912/5920) orbitally-excited states
– New decay modes discovered
e.g. Bc+  J/y p+p-p+
– Bs physics is rich and little explored
B+  p+m+m-
[CONF-2012-006]
• Enormous production rate  have overtaken
B factories even for B0 and B+ decays
– BR (B+ p+m+m-) = (2.4 ± 0.6stat ± 0.2sys) ×10-8
Previous limit < 6.9×10-8 (Belle PRD 78 011101)
Rarest B decay ever observed!
Collaboration
804
16
55
Muon
detector
RICH
Calorimeters
Magnet
VELO
Tracker
Added since PLHC-2011:
Birmingham, Cincinnati,
Lahore, Rostock
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LHCb status and plans
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Tracking performance
• Dipole magnet, polarity regularly
switched to cancel systematic effects
• New this year: beam optics changed
to decouple crossing angles from
LHC (V) and spectrometer magnet (H)
• Momentum resolution:
Dp/p = 0.4 – 0.6 % (5–100 GeV/c)
Real data!
[CONF-2012-002]
Bs  J/y f
Beam optics at interaction point
s(mB) = 8 MeV/c2
cf
Roger Forty
LHCb status and plans
~ 16 MeV/c2
22 MeV/c2
[CMS DPS-2010-040]
[ATLAS CONF-2011-050]
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Vertex detection
• VELO (Vertex Locator)
21 modules of r-f silicon sensor disks
Retracted for safety during beam injection
• Reconstructed beam-gas vertices
(used for luminosity measurement)
• Impact parameter resolution ~ 20 mm
Proper-time resolution: st = 45 fs
cf CDF: st = 87 fs [PRL 97 242003]
Prompt J/y
Bs  J/y f
z
Roger Forty
[PLB 693 69]
r
7 mm
[CONF-2012-002]
VELO sensors
LHCb status and plans
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Particle identification
• Charged hadrons identified with two
Ring-imaging Cherenkov detectors
covering 2 < p < 100 GeV/c
• Hybrid Photon Detectors (HPDs)
500 tubes each with 1024 pixels
High efficiency, low noise
RICH-1: dual
dualradiator
radiator
RICH-1:
Allows strong suppression of
combinatorial background in
hadronic decays e.g. f  K+K-
• New this year: gas-tight box for aerogel
to avoid contamination by C4F10 gas
Without RICH
With RICH
eKK > 90% for epK < 5%
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LHCb status and plans
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Calorimeters + Muon
• ECAL: Shashlik Pb-scintillator
s(E)/E = 10% /√E  1%
Bs → f g
[arXiv:1202.6267]
• HCAL: Tile Fe-scintillator
allows triggering on hadronic final states
• Muon system: 5 stations MWPCs/Fe
 m+m-
Giacomo Graziani
[arXiv: 1202.6579]
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Data taking
• Nominal LHCb luminosity = 2 ×1032 cm-2 s-1
Precision physics depending on vertex structure:
easier in a low-pileup environment
pp collisions/crossing (25ns)
LHCb
• Continuous (automatic) adjustment of offset of
colliding beams allows luminosity to be levelled
Thanks to LHC team for excellent collaboration!
• Data taken with high efficiency > 90%
Offline data quality rejects < 1%
Detectors all with > 98% active channels
2011
2012
2011 was a fantastic year!  L dt = 1 fb-1
(used for most results shown here)
~ 30 × more data than at PLHC-2011
Data taking in 2012 at 4 × 1032 cm-2 s-1
~ 0.4 fb-1 integrated so far
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LHCb status and plans
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Data processing
Detector
Output rate of single server vs time
• Trigger in two steps: Level-0 in hardware
pT of e, m, and hadron (thresholds ~ 1–3 GeV)
 reduce rate to 1 MHz
• Then all detectors read out into large CPU farm
(~1500 servers) High Level Trigger in software
• New this year:
– Output rate increased to 4.5 kHz to provide
data sample for analysis during shutdown
(events are relatively small ~ 60 kB)
– Deferred triggering: fraction of events written
to local storage of CPUs and processed during
inter-fill gap ~10% increase in effective power
4.5 kHz
Storage
• O(1010) events recorded per year: centralized “stripping” selection to reduce
to samples of < ~107 events for individual analysis: ~ 800 selections!
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2. Physics highlights
• 56 physics publications to date, more in pipeline
> 80 preliminary results submitted as Conference Papers
all available at www.cern.ch/lhcb
[LHCb-CONF-xxx]
• Can only give a selective taste of LHCb’s physics output
— for the full feast see the contributed talks and posters
• Tagging of production flavour (B/B)
important for mixing & CP analyses
Performance calibrated using
control channels such as B+→ J/y K+
Miriam Calvo Gomez
• Tagging power: eeff = e (1-w)2
determined from mixing signals
•
eeff = (3.2 ± 0.8) % (Opposite side)
(1.3 ± 0.4) % (Same side)
Roger Forty
[CONF-2011-050]
LHCb status and plans
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Particle-antiparticle mixing
• Studied for all neutral mesons B0 : now well-established
• Bs0 : studied using Bs0  Ds+ p- decays
Dms = 17.725 ± 0.041 ± 0.026 ps-1 (world-best)
cf : 17.77 ± 0.10 ± 0.07 ps-1 (CDF PRL 97 242003)
D0
[CONF-2011-029]
• D0 : “wrong-sign” decays D0  K+p- measured
Time-dependent analysis in progress to
separate mixing from DCS contribution
B0
[arXiv:1202.4979]
Bs0
[CONF-2011-050]
B0  D*-m+n
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Right-sign
Wrong-sign
LHCb status and plans
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CP violation
Olivier Leroy
• Phase of B0 mixing is well known: sin 2b = 0.67 ± 0.02
[PDG]
• Analogous phase in the Bs system is denoted fs [arXiv:1106.4041]
Expected to be very small, precisely predicted: fs = -0.036 ± 0.002 rad (SM)
• First Tevatron results hinted at large value (discrepancy with SM up to ~3 s)
• Golden mode for this study is Bs  J/y f
• VV final state: mixture of CP-odd and -even components separated using
angular analysis
Transversity angle distributions
CP-even
CP-odd
[LHCb-CONF-2012-002]
Roger Forty
[CONF-2012-002]
LHCb status and plans
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CPV in Bs mixing
SM
Update with 10 × data
Result presented at PLHC-2011
Sean Benson
Results correlated with
DGs = width difference of
the Bs mass-eigenstates
 plotted as contours in
(fs vs DGs) plane
Ambiguous solution
excluded by study of
phase vs KK mass
[arXiv:1202.4717]
• LHCb result consistent with Standard Model
First significant direct measurement of DGs = 0.116 ± 0.018 ± 0.006 ps-1
•
fs also measured in a second mode: Bs  J/y f0
Combined result: fs = -0.002 ± 0.083 ± 0.027 rad [CONF-2012-002]
Still room for new physics: increased precision required!
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CPV in B decays
Daniel Johnson
• Using the particle ID capability of LHCb, can isolate clean samples of the
various decays that contribute to 2-body B → h+h- (h = p, K, p)
• B0 → K+p-: direct CP violation (in decay) clearly visible in raw distributions
Bs0 → pK--Kp++
[PRL 108 201601]
0
+ pB
Bs →
→K
p+ K
- contribution
•• Corrections
required
fortodetector
Adjusting the
selection
enhanceand
theproduction
Bs → p+ Kasymmetries
controlled using
D0 → K-p+, B0 → J/y K*0 samples: percent-level effects
+
ACP (Bs → p K ) = 0.27 ± 0.08 ± 0.02
ACP = G(B0 → K- p+) – G(B0 → K+ p-) / sum = -0.088 ± 0.011 ± 0.008
→ First 3 s evidence for CP asymmetry in Bs decays
0.012
in good agreement with world average:
-0.098 ± 0.011
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Rare decays
Mitesh Patel
• Bs  m+m- strongly suppressed in SM
Mmm in sensitive region of BDT
+
-9
*
Predicted BR = (3.2 ± 0.2)
10 m m candidate
Bs 
very sensitive to new physics [JHEP 1010 009]
• Analysis based on multivariate estimator
(BDT, combining vertex and geometrical
information) & dimuon mass Mmm
• Their distributions calibrated using data:
B → hh and dimuon resonances
• World-best limit set:
Cosme Adrover
BR < 4.5 × 10-9 (at 95% CL)
cf < 7.7 × 10-9 (CMS arXiv:1203.3976)
< 22 × 10-9 (ATLAS CONF-2012-010)
Setting limit on BR
[arXiv:1203.4493]
• Sensitivity (slightly) greater than CMS
from 5 less integrated luminosity
* Experimental BR is time-integrated, so prediction
mmm =should
5.357 GeV, BDT = 0.90, Decay length = 11.5 mm
be scaled by ×1.1 for comparison [arXiv:1204.1737]
Tracks shown for pT > 0.5 GeV
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Rare decays
• Bs  m+m- strongly suppressed in SM
Predicted BR = (3.2 ± 0.2)  10-9 *
very sensitive to new physics [JHEP 1010 009]
Mitesh Patel
Mmm in sensitive region of BDT
• Analysis based on multivariate estimator
(BDT, combining vertex and geometrical
information) & dimuon mass Mmm
• Not enough candidates to provide
significant measurement of BR
• World-best limit set:
Cosme Adrover
BR < 4.5 × 10-9 (at 95% CL)
cf < 7.7 × 10-9 (CMS arXiv:1203.3976)
< 22 × 10-9 (ATLAS CONF-2012-010)
Setting limit on BR
[arXiv:1203.4493]
• Large enhancement of BR relative to
SM expectation is ruled out
* Experimental BR is time-integrated, so prediction should
be scaled by ×1.1 for comparison [arXiv:1204.1737]
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B0  K*m+m-
Cosme Adrover
[arXiv:1101.0470]
• Rare decay via
b → s penguin:
• Forward-backward asymmetry sensitive
to modification of the helicity structure
Previous results hinted at discrepancy
• LHCb has largest sample in the world:
900 events, as clean as the B factories!
SM
[CONF-2012-008]
• Zero-crossing point precisely predicted
q2 (AFB= 0) = 4.0 – 4.3 GeV2/c4
+1.1 GeV2/c4
• First measurement: 4.9 -1.3
Earlier discrepancies not confirmed
• However, evidence seen for different BR
between B+  K+mm & B0  K0mm modes
( Additional slides)
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Impact of results
• LHCb results provide strong constraints on possible models for new physics
Complementary to the direct searches at ATLAS/CMS
• Recent examples: limit on Bs  m+m- constraining SUSY at high tan b
and combination of Bs  m+m- and fs restricting various models:
[N. Mahmoudi, Moriond QCD]
[D. Straub, arXiv:1107.0266]
Direct exclusion
(CMS 4.4 fb-1)
(fs)
• And then something unexpected…
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CPV of charm
• Expected to be small in the SM (< 10-3)
•
D0  K+K1.4 × 106 signal
DACP = difference in CP asymmetry
for D0  K+K- and D0  p+pRobust: detection and production
asymmetries cancel (at first order)
DACP = (-0.82 ± 0.21 ± 0.11)%
Zero CPV is excluded at 3.5 s
• Before the LHCb result: “CP violation…
at the percent level signals new physics”
[Y. Grossman, arXiv:hep-ph/0609178] (and many others)
After: “We have shown that it is plausible that the SM accounts for the measured
value… Nevertheless, new physics could be at play”
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LHCb status and plans
[J.Brod et al, arXiv:1111.5000]
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[PRL 108 111602]
• Enormous statistics available:
> 106 D0  K+K- from D*+  D0 p+
Charge of p from D* determines D0 /D0
Silvia Borghi
3. Plans
• New physics has not yet shown itself
clearly at the LHC
• Essential to improve measurements of
precisely-predicted quantities:
fs, BR(Bs→ m+m), q2 (AFB= 0) …
[PLB 712 203]
• Clean determination using B  DK tree
decays with theoretical uncertainty < 1º
g (rad)
Another example: the CP-angle g
is the least well measured UT angle
(depends on rare b → u decays)
Uncertainty 10–12º [UTfit/CKMfitter]
• First constraints already achieved:
Can profit from much higher statistics
 long term programme at LHCb
Daniel Johnson & Sean Benson
Roger Forty
LHCb status and plans
rB
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LHCb upgrade
Silvia Borghi
• Expect to double data-set by end of this year
After long shutdown, further doubling of data-set in 2015–17
(plus increase of cross-sections with higher energy): total of 5–7 fb-1
• Main limitation that currently prevents exploiting higher luminosity is the
hardware trigger: keeping output rate < 1 MHz requires raising of thresholds
 hadronic yields reach plateau:
• Propose to remove the hardware trigger
Read out LHCb at 40 MHz crossing rate
Flexible software trigger in CPU farm
 increase in yields by factor 10–20
at 1–2 × 1033 cm-2 s-1 (25 ns is required)
• Requires replacing front-end electronics
Planned for the long shutdown in 2018
Running for ~10 years will give 50 fb-1
→ General-purpose detector for the forward region
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Detector modifications
• Baseline detector modifications to allow 40 MHz readout
R&D on possible detector upgrades
e.g. Scintillating-fibre tracker
TORCH time-of-flight
Pixel VELO
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Upgrade status
• Letter of Intent for upgrade submitted to LHCC last year
 Encouraged to proceed to Technical Design Report
• Framework TDR just submitted (25 May) [LHCC-2012-007]
 schedules & cost of subsystems, and institute interests
• Update of physics case and expected performance:
Timeline (tight!)
2011
2012
Letter of Intent
Framework TDR
R&D ongoing
2013 Subsystem TDRs
2014-16 Tender & prodn
2017 Acceptance testing
2018 Installation
2019 Data taking
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Conclusions
• LHCb taking data with high efficiency and excellent detector performance
– Luminosity above design, 1.4 fb-1 recorded so far
– Excellent mass and decay-time resolution, particle ID, etc.
• World-best measurements of many physics parameters
– Dms, DGs, fs, BR(Bs  m+m-), masses, lifetimes, etc.
First observations of new decays, evidence for CP violation of Bs
• So far almost all are in good agreement with the Standard Model
→ strong constraints on new physics in the flavour sector
• Possible hints of physics beyond the Standard Model require further study:
– Evidence seen for CP violation in charm, unexpected
– Isospin asymmetry for B → K m+m- is also puzzling
• Upgrade of LHCb in preparation for 2018: 10 × yield + software trigger
→ Much more to come: new collaborators welcome!
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Additional slides
• Signal for B0 → K∗0μ+μ• SM prediction for isospin
asymmetry AI of K∗μ+μ[Feldmann &Maas, JHEP 01 074]
• LHCb result: consistent with
SM expectation in this mode
AI (%)
[arXiv:1205.3422]
q2 (GeV2/c4)
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Isospin asymmetry
• Compare B0  K0m+m- and
B+  K+m+m- decays
• Isospin asymmetry AI defined as:
G(B0  K0m+m-) - G(B+  K+m+m-)
G(B0  K0m+m-) + G(B+  K+m+m-)
• Expect AI ~ zero in SM (< few %)
Results from other experiments
tended toward negative values
• No asymmetry seen in closely-related
B  K*m+m- mode (K*+  K0p+)
No clear interpretation, so far
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LHCb status and plans
[arXiv:1205.3422]
• Previous discrepancy with SM is
supported by new LHCb result:
AI < 0 with over 4s significance
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g determination
• Various methods used
Inputs mostly from B factories
gcomb = (75.5 ±10.5)° [UTfit]
• Impact of LHCb results on the
combination, for the ADS method
(B+ → D0K+, D0 →K+p-)
[D. Derkach, LHCb-TALK-2012-077]
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ASL
• Strong interest in semileptonic (flavour-specific) asymmetry due to
D0 result for dimuon asymmetry (comparing # of m+m+ and m-m- events)
ASL = (−7.87 ± 1.72 ± 0.93) × 10−3 [PRD 84 052007] (expect < 10-3 in SM)
• Same approach difficult at pp machine due to production asymmetries
Instead use semileptonic decays, B(s) → D+(s) (K+K-p+) m- X
Result from LHCb expected soon
• Note: if ASL is large, expected to
see large fs in most models
Projected LHCb precision
(statistical, 1 fb-1)
[arXiv:0910.1032]
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