B (and D) physics at LHC

B (and D) physics at LHC

✗ pp collider at 14 TeV (7 TeV in 2010-12) with bunch crossings at 40 MHz;

✗ Luminosity up to 10³⁴ cm^-2s^-1 → 10 nb^-1/s;

✗ At 7 TeV Inelastic pp cross-section about 60 mb.

✗ At 7 TeV σ_bb ≈ 500 μb and σ_cc ≈ 4.7 mb (one every 120/12 inelastic interactions);

✗ Pythia 6.4 predicts more than 1(8) mb at 14 TeV;

✗ LHC will be the most powerfull B and D factory!

✗ Direction of b and b-bar (and c and c-bar) very correlated;

✗ A 4π coverage not optimal → build a forward spectrometer

CKM matrix

✗ CKM matrix has shown to be the main mechanism of flavor mixing and CP violation in the weak interactions;

✗ Possible new physics (NP) effects could appear as small corrections to the current

phenomenological scheme;

✗ This goal can be achieved by high precision indirect

measurements in the flavor sector, and in particular in the rare decays of heavy mesons as D and B;

The Bs → J/Ψ Φ channel

✗ For the B_s mixing phase a combined measurements at Tevatron showed a 2.1σ deviation from the SM;

✗ Is it a hint on NP? ^✗ LHCb would reach a statistical

precision of about 0.07 rad with 1 fb^-1;

✗ If Φ_s is close to value measured at Tevatron a measurement at 5σ would be possible at LHCb next year;

✗ More time is needed to reach the SM value;

B _s → μμ

✗ Very rare process well predicted in the SM:

BR (SM) = (3.35 ± 0.32) 10^-9

✗ Sensitive to NP that could enhance the BR;

✗ Present limit set by CDF → BR < 4.3 10^-8 (95% CL);

✗ With the SM BR LHCb expects 8 signal and 12 background events in 2 fb^-1;

A typical B event

Main apparatus requirements

✗ High pseudo-rapidity coverage (forward detector);

✗ B mesons have a long lifetime cτ = 0.5 mm with = O(10-100);

✗ Good vertex resolution to individuate separated vertices of B and to make measurements of the fast B oscillations;

✗ B mesons have a not-so-large mass (5 GeV) → look for particles with a not-so- large transverse momentum p_T = O(1) GeV;

✗ 20% of B mesons decay to leptons: need good muon and electron-ID;

✗ A lot of background is expected:

✗ Good particle ID to distinguish between π/p/K to reject processes topologically similar;

✗ Good mass reconstruction → momentum resolution;

✗ A selective trigger able to find rapidly interesting events at a interaction rate of 40 MHz (i.e. 25 ns inter-bunch interval) with hadrons or leptons in the final state;

Luminosity

✗ A “clean” interaction point is needed → take under control the number of interactions;

✗ Working at 2 10³² cm^-2s^-1 ensures 1 interaction per bunch crossing →8 10⁴ bb/s;

✗ This luminosity should be reached already in the current LHC phase before the 2012 stop;

✗ 2009 run conditions (√s = 900 GeV);

✗ Integrated luminosity ~ 7 μb^-1;

✗ Expected conditions next run 2010/11 (√s = 7 TeV)

✗ Expected integrated luminosity ~1 fb^-1 in 12-18 months

Current Luminosity of LHC

End of may:

a luminosity of 10²⁹ Hz/cm² at 7 TeV with squeezed beams and about 2 10¹⁰ p/bch;

The aim is to reach ~10³² Hz/cm² by end of 2010;

LHCb hopes to integrate 0.1 fb^-1 in 2010 and 1 fb^-1 in 2011!

Event rate in LHCb

✗ In the current conditions of beam, LHCb is having 100 Hz rate of events;

The apparatus

✗ LHCb is a single-arm spectrometer with an

acceptance of: 15-250 mrad (V) 15-300 mrad (H)

✗ 1.9 < η < 5



p p

~ 300 mrad

10 mrad

The VErtex LOcator

✗ A vertex locator with a proper time resolution of 40 fs;



The VErtex LOcator

✗ Made of silicon 21 sensors (r and Ф) can be opened for safety during non- stable beam conditions;

The VErtex LOcator

✗ Closed for the first time on the beam on April 1^st with a procedure that lasts about 15 minutes;

✗ Stability in (X; Y ; Z) : (10; 4; 10) μm;

✗ PV resolution for track multiplicity of 25:

x: ~16 μm y: ~15 μm z: ~90 μm

Primary vertices

VErtex LOcator

✗ Resolutions on impact parameter to the primary verex:

Data-MC comparison has improved with latest Velo alignment: residual difference due to a “bug” in the simulation of RF foil (wrong, smaller, thickness used in the MC)!

Being rechecked with a new MC production..

The tracking system

✗ A tracking system with a resolution of 0.4% on the particle momenta and a B-mass resolution of 20 MeV



The tracking system

✗ Similar sensors for TT and IT: micro-strips with 200 µm pitch;

✗ TT + 3 Stations (T1, T2 and T3) each with 4 detection planes (0º, +5º, -5º, 0º);

Trigger Tracker

✗ 200 µm pitch silicon sensors;

✗ Signal to noise ratio in line with expectations;

Unbiased residuals larger than MC

→ improvement expected after better alignment;

Inner Tracker

✗ Aligned with TED runs: SPS beam splashed on collimators 300 m downstream LHCb → “muon beam” with high multiplicity (2/cm²);

✗ alignment to ~15μm from TED data;

✗ efficiencies O(98%);

✗ 99.5 % of detector channels working;

✗ S/N ~ 15.5 in line with expectations;

✗ Aligned with TED runs: SPS beam splashed on collimators 300 m downstream LHCb → “muon beam” with high multiplicity (2/cm²);

Outer Tracker

✗ Made by straw 3mm diameter tubes operated as drift tubes with TDC: track position is determined by the drift time;

✗ Performance in agreement with expectations;

✗ Room for improvement after a better alignment with data;

O2 was added to the gas mixture in order to

mitigate ageing effects.

No effect on hit

efficiency is observed.

Tracking performance

✗ Tracking on long tracks was

performed by using all detectors;

✗ Impressive results already obtained on Ks, Λ on 65µb^-1;

Ks with 65µb^-1

Λ with 65µb^-1

Tracking performance

✗ Tracking on long tracks was

performed by using all detectors;

✗ Impressive results already obtained on

Ω,Ξ on 800 µb^-1;

Ξ on 800

µb^-1 Ω on 800 µb^-1

The RICHES

✗ A RICH system that provides a 3σ K/π separation in a 3-100 GeV p range;



The RICHES

✗ LHCb is equipped with two RICH systems to measure velocity of particles with low and high momentum (5-500 GeV);

✗ 3 radiators with different refraction indices are needed;

The RICHES

✗ bla

RICH performance

✗ Very important is the alignment with respect to the tracking system;

Δθ before aligment RICH1

Orange points → photon hits

Continuous lines are expected distribution for each particle hypothesis: K or π (proton below threshold)

RICH performance

✗ Particle identification, and in particular K/π is a crucial tool for LHCb;

✗ Can you see the Φ peak in the KK invariant mass?

Without RICH

RICH performance

✗ By using RICH info for a better K identification wonderful peaks appear:

Φ → KK LHCb is also ready for D physics D⁰ → πK

D⁰ → KK

The calorimeters

✗ A fast calorimetry system providing precious information for the first level trigger;



e

h

The calorimeters

- ECAL: Pb/Scintillator with WLS fibers in Shashlik configuration+PMT, 25 X₀

- HCAL: Fe(16mm)/Scintillator tiles (4mm) 5.6 λ_I

- High radiation level at small angle

Calorimeters

✗ The calorimeters systems work very effectively, providing one of the principal trigger at LHCb;

✗ Time alignment now 1 ns;

✗ ECAL Energy calibration ongoing;

✗ Need 50M events to achieve 1% with π⁰;

✗ Not only pions. First evidence of J/ψ → ee

Calorimeters

✗ 3 body decays using π⁰;

The apparatus

✗ A robust muon system providing information for the first level trigger and for off- line muon identification;



µ

Muon System

✗ Five detection stations interleaved with iron wall:

✗ 4 downstream the calorimeters equipped with four-gap MWPC;

✗ 1 upstream the calorimeters equipped with light chambers:

✗ Two-gap MWPC;

✗ Gas Electron Multiplier (GEM) based detectors in the central region;

✗ Provides the first level trigger together with the Calo system;

✗ Provides crucial info for the muon identification;

✗ Well aligned and timed with cosmic rays;

Muon system detectors

✗ MWPC and GEM detectors were chosen for their extremely high efficiencies (well above 95%) and time resolutions of the order of 3 ns;

MWPC

GEM detector

Time distribution of cosmic hits after time alignment performed with cosmics

Time alignment

✗ After a first fine alignment performed with cosmic rays the detector was aligned in time and space by means of particles coming from LHC interactions:

Station misalignment with respect to the second one

measured with a Kalman fit on the reconstructed tracks

Time resolution of MWPC with cathode readout or wire readout;

Muon System efficiency

✗ The time resolutions of the different sub-region of the detector were find to be lower than 4.5 ns;

✗ Detection efficiency higher than 99.7%

(except M1)

✗ An important parameter for properly trigger is represented by the detection efficiency between two bunch crossings (Requirement was 95%):

The J/ψ (family)

✗ Not only the J/Ψ was reconstructed, but already the Ψ(2S);

Muon Identification

✗ Muon System provide unique information for the off-line muon-ID;

✗ J/ψ very useful for calibrating the muon identification;

✗ The two decay muons can be used as “tag” and “probe”:

Mip peak in Hcal for tag µ

Muon Identification

Comparison of the Muon ID efficiency with MC evaluated using J/ψ (tag and probe) as a function of the momentum of the muon.

Although the number of J/ψ is still small, a good agreement is found between real life and Monte Carlo;

An identification efficiency higher than 95% was found for high muon momenta;

Muon mis-identification

✗ By using the decays of K_s and Λ a first measurement of the rate of muon mis-ID was performed;

✗ The plots show the probability of mis-identify the pion from the K_s or the proton from the Λ as a muon;

✗ For high momenta this probability is below 1% as expected;

Muon tracking

✗ The Muon System is able to perform a fast measurement of the muon P_T with a resolution of about 20%;

✗ This is enough for triggering, but not for physics;

✗ The momentum of the muons is precisely measured with the tracking system;

Plot shows the squared distance between hits in the stations of the muon system and tracks extrapolated from the

reconstructed ones.

A very good agreement with the expectation was found.

Muon background

✗ Same sign muon events to evaluate the J/ψ background;

This plot is using data only. The fact that the invariant mass distribution of same sign muons describes fairly well the background underneath the Jpsi peak confirms the dominance of combinatorial dimuon background

Trigger system

✗ The experiment foresees two trigger levels:

✗ Level-0, hardware trigger, looks for particles with high P_T in the

calorimeters and the muon system (40 MHz → 1 MHz);

✗ High Level Trigger (HLT), completely software (500 CPUs), subdivided in two steps:

✗ HLT-1: associates signals from L0, with high IP tracks in the vertex detector (1MHz → 10 kHz);

✗ HLT-2: exploits all the information to perform a full reconstruction and a loose selection on fundamental channels (10 kHz → 2 kHz);

Trigger efficiency

✗ The expected efficiencies in the different channels are listed below:

We can write up to 2 kHz. No needs of HLT2 for the moment;

The apparatus at work

Finally the B mesons!

✗ B mesons are entering the game...

Neutral semileptonic Dsµν (Ds→πKK) decay

Neutral semileptonic B^-→D⁰π^- (D⁰→π⁺K^-) decay

...more B mesons

✗ B⁺ → J/ψK⁺ (J/ψ→µµ) decay also seen!

Primary vertex

B decay vertex

+

K⁺ -

J/ψ

B⁺

XY Projection

[mm]

[mm]

Tracks from primary vertex

First invariant mass distributions

✗ For B mesons, but also for D mesons, we started to have the possibility of reconstructing their invariant mass:

...and the W too!

✗ The first signals of W → νμ seen!

Conclusion

✗ LHCb is performing in an excellent way

✗ Some alignment and calibration is still going on;

✗ Fantastic agreement between MC and Real data so far;

✗ A lot of particles already reconstructed and first B-peak observed;

✗ About 20 nb^-1 integrated up to now;

✗ Hopefully on the 2010/2011 run we’ll have 1 fb^-1 of data;

✗ Most of the LHCb physics program can be done with that statistics;

It is never too late to join us!

Some expected sensitivity

A clean measurement of can be done with a statistical precision of 5-7^o; Current measurements from Babar-Belle is 73⁺²²_-25

B → μμK* is a rare decay in the SM with a BR of 10^-6; The forward backward asymmetry A_FB is an

interesting observable that gave non “Standard”

results at Babar and Belle.

More statistics is needed!

We are going to have the biggest D sample in the HEP world!

We can test: D rare decays, CP violation and D mixing;